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Sweet sorghum Opportunities for a new, renewable fuel and food industry in Australia

AUGUST 2013 RIRDC Publication No. 13/087

Sweet sorghum Opportunities for a new, renewable fuel and food industry in Australia

by Ian O’Hara, Geoff Kent, Peter Alberston, Mark Harrison, Phillip Hobson, Neil McKenzie, Lalehvash Moghaddam, David Moller, Thomas Rainey, Wanda Stolz, Heng-Ho Wong, and Brendon Ellett

August 2013 RIRDC Publication N. 13/087 RIRDC Project No. PRJ-005254

© 2013 Rural Industries Research and Development Corporation. All rights reserved.

ISBN 978-1-74254-580-6 ISSN 1440-6845 Sweet sorghum: Opportunities for a new renewable fuel and food industry in Australia Publication No. 13/087 Project No. PRJ005254 The information contained in this publication is intended for general use to assist public knowledge and discussion and to help improve the development of sustainable regions. You must not rely on any information contained in this publication without taking specialist advice relevant to your particular circumstances. While reasonable care has been taken in preparing this publication to ensure that information is true and correct, the Commonwealth of Australia gives no assurance as to the accuracy of any information in this publication. The Commonwealth of Australia, the Rural Industries Research and Development Corporation (RIRDC), the authors or contributors expressly disclaim, to the maximum extent permitted by law, all responsibility and liability to any person, arising directly or indirectly from any act or omission, or for any consequences of any such act or omission, made in reliance on the contents of this publication, whether or not caused by any negligence on the part of the Commonwealth of Australia, RIRDC, the authors or contributors. The Commonwealth of Australia does not necessarily endorse the views in this publication. This publication is copyright. Apart from any use as permitted under the Copyright Act 1968, all other rights are reserved. However, wide dissemination is encouraged. Requests and inquiries concerning reproduction and rights should be addressed to RIRDC Communications on phone 02 6271 4100. Researcher Contact Details Ian O’Hara Centre for Tropical Crops and Biocommodities Queensland University of Technology PO Box 2434 BRISBANE QLD 4001

Geoff Kent Centre for Tropical Crops and Biocommodities Queensland University of Technology PO Box 2434 BRISBANE QLD 4001

Email:

Email:

[email protected]

[email protected]

In submitting this report, the researchers have agreed to RIRDC publishing this material in its edited form. RIRDC Contact Details Rural Industries Research and Development Corporation Level 2, 15 National Circuit BARTON ACT 2600 PO Box 4776 KINGSTON ACT 2604 Phone: Fax: Email: Web:

02 6271 4100 02 6271 4199 [email protected] http://www.rirdc.gov.au

Electronically published by RIRDC in August 2013 Print-on-demand by Union Offset Printing, Canberra at www.rirdc.gov.au or phone 1300 634 313

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Foreword With increasing global concerns over greenhouse gas emissions and the future supply of food and energy products, there is an urgent need to develop new technologies that expand provision of renewable energy and increase food production. Sweet sorghum is receiving significant global interest because of its capacity to co-produce energy, food, and feed products in integrated biorefineries. This report deals with the opportunities to develop a sweet sorghum industry in Australia, demonstrating the production of energy, food, and feed products and assessing the potential economic and sustainability benefits of sweet sorghum biorefineries in the Australian context. The outcomes of this project will be of benefit to agricultural producers and agro-industry companies seeking new opportunities for value-creation in the bioeconomy. Throughout the project, trial crops of sweet sorghum were cultivated to provide samples for the production of energy, food and feed products at the laboratory and pilot-scales. Techno-economic modelling and life cycle assessment were undertaken for a range of sweet sorghum biorefinery options. The results showed that there are significant opportunities for the development of sweet sorghum industries in Australia, particularly through integration with the sugarcane production and processing industries. This project was funded by industry partner AgriFuels Ltd and the Australian Government through the Rural Industries Research and Development Corporation (RIRDC). This report is an addition to RIRDC’s diverse range of over 2000 research publications and it forms part of our Bioenergy, Bioproducts and Energy R&D program, which aims to meet Australia’s research and development needs for the development of sustainable and profitable bioenergy and bioproducts industries and to develop an energy cross-sectoral R&D plan. Most of RIRDC’s publications are available for viewing, free downloading or purchasing online at www.rirdc.gov.au. Purchases can also be made by phoning 1300 634 313.

Craig Burns Managing Director Rural Industries Research and Development Corporation

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About the Authors Ian M. O’Hara Associate Professor Ian O’Hara is a chemical engineer with extensive experience in the sugar industry in research and consultation, policy development, management, process design, and supervision. Ian undertakes research and consultation projects within the Australian and overseas sugar industries including design of green field sugar factories, integration of refining, ethanol, and cogeneration facilities, and other work in the fields of energy efficiency, sugar recovery, sugarcane analysis, and process improvement. Ian is Theme Leader (Bioenergy) within the Queensland University of Technology (QUT) Centre for Tropical Crops and Biocommodities and manages the Mackay Renewable Biocommodities Pilot Plant, a unique, publicly-accessible, pilot-scale research facility in Australia for the development and demonstration of processing technologies for biofuels production. Geoff Kent Associate Professor Geoff Kent has over 25 years experience in raw sugar factory research, consulting, and training, specialising in sugarcane transport and milling. He is recognised as a leading expert in milling technology. Geoff is Team Leader - Cane Supply and Processing within the QUT Centre for Tropical Crops and Biocommodities. He is responsible for much of QUT´s research, consulting, and training activities in sugarcane supply, transport and, milling. Peter Albertson Mr Peter Albertson has 16 years experience conducting laboratory and field research in Queensland and New South Wales. His previous research at CSIRO Plant Industry focused on the key enzymes for sucrose accumulation in sugarcane and the biochemical basis for after-roast darkening in Macadamia kernels. As the Pilot Plant Technologist for the Mackay Renewable Biocommodities Pilot Plant, he provides operational support for academic and commercial projects undertaken in the facility. Mark D. Harrison Dr Mark Harrison is a Senior Research Fellow with 20 years of experience in protein science, sugarcane biotechnology, and enzymatic hydrolysis of biomass. Mark was awarded a Queensland State Government Smart State Fellowship to produce cell-wall degrading enzymes in transgenic sugarcane and his current research involves the expression of recombinant proteins in sugarcane to improve the economics of cellulose conversion to biofuels (such as cellulosic ethanol) on an industrial-scale. The ongoing development of this technology is a key part of the collaboration between Syngenta and QUT, and Mark is Team Leader (Enzymology) within the QUT Syngenta Centre for Sugarcane Biofuel Development. Phillip Hobson Dr Phil Hobson is a Principal Research Fellow with over 25 years experience in engineering project management, research, and consulting in the commercial, academic, and public sectors. Phil manages a diverse range of projects in the areas of biomass processing. His core skills include process modelling of practical thermal systems as well as supply chain, life cycle, and financial analyses relating to the production of biocommodities. Phil is the Theme Leader (Industrial Bioprocessing) within the QUT Centre for Tropical Crops and Biocommodities. Neil McKenzie Mr Neil McKenzie is a Research Fellow at QUT with 30 years experience in raw sugar manufacturing research. Neil has an instrument and electrical background, and has extensive experience in research iv

and consulting from harvesting and milling through to sugar production, steam generation, and generation of by-products. Neil also manages development and production of specialised sugar factory instrumentation for sale to the global sugar industry. Lalehvash Moghaddam Dr Lalehvash Moghaddam is a post-doctoral research fellow with expertise in polymer chemistry and processing. She has extensive experience in thermochemical conversion of sugarcane bagasse to biofuels and other high-value products. She completed her PhD at QUT in 2009 and currently works in the QUT Centre for Tropical Crops and Biocommodities on research projects that include thermochemical conversion of biomass, algal biotechnology, and biodiesel production. David Moller Mr David Moller is a Senior Research Fellow with over 20 years experience in raw sugar manufacturing and refined sugar processing factories. David has previously served as factory operations manager responsible for all aspects of raw sugar milling and cogeneration activities at a number of different Australian sugar factories. His current areas of research, consulting, and training include clarification, evaporation, crystallisation, and boiler operation. Thomas Rainey Dr Thomas Rainey is an early career researcher with 10 years of research experience in biomass processing with a focus on sugar, pulp & paper, and biorefining. He has considerable experience in the manufacture of highly value–added products from sugarcane bagasse. Tom has experience with a wide range of biomass fractionation and pretreatment technologies as a precursor to value–added lignin products and ethanol, as well as paper pulp. His research interests include production of nanofibres and assessment of agricultural crops for the production of biorefinery/biofuel products and consumer materials. Wanda Stolz Ms Wanda Stolz is a member of the Bioprocessing group within the QUT Centre for Tropical Crops and Biocommodities and has over 15 years of experience in analytical research laboratories. Wanda provides high-quality technical support to research staff and consultants in the CTCB. Heng-Ho Wong Dr Heng-Ho Wong has considerable research experience in bioprocess engineering (fermentation and bioproduct recovery) and industrial oleochemical processes. Currently, he is the Manager of the Mackay Renewable Biocommodities Pilot Pant, a unique facility owned by the QUT Centre for Tropical Crops and Biocommodities. He is responsible for maintaining and operating biorefinery research equipment in the facility. Brendon Ellett Mr Brendon Ellet holds a BSc in Maths and Physics, a post-graduate Diploma in E-commerce Management, and is a founder of AgriFuels Ltd. For the past eight years, Brendon has been involved in planning and early stage development of national and international energy, resource, and biofuels opportunities, with a particular focus on Asia and new food and fuel feedstock crops. Brendon has a diverse practical and corporate background, including ten years in agricultural farming operations and over fifteen years in corporate marketing and business management for local and international companies.

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Acknowledgments The authors gratefully acknowledge the Rural Industries Research and Development Corporation and AgriFuels Ltd for funding the research. In addition, the authors sincerely acknowledge the support of AgriFuels Ltd in undertaking sweet sorghum field trials, providing samples for use in the project, and for their cooperation throughout the project. In particular, the assistance of Robert Smallwood and Bill Trevor are gratefully acknowledged. The authors also acknowledge the assistance of Peter Hatfield (Arachis Australia Pty Ltd) who assisted with agronomic support and oversight of the field trials. We would also like to thank VAFF Pty Ltd for assistance with the manufacture of trial quantities of food and feed products from sweet sorghum. The authors also acknowledge the assistance of Marguerite Renouf from Life Cycle Strategies for assistance with the life cycle analysis. Finally, the authors acknowledge Dr Anthony Mann and Dr Floren Plaza for assistance with sample preparation and compression tests during sweet sorghum milling.

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Contents Foreword ............................................................................................................................................... iii About the Authors ................................................................................................................................ iv Acknowledgments................................................................................................................................. vi Tables...................................................................................................................................................... x Figures .................................................................................................................................................. xii Executive Summary ............................................................................................................................ xv Introduction ........................................................................................................................................... 1 Objectives ............................................................................................................................................... 2 Methodology .......................................................................................................................................... 3 Chapter 1: Sweet sorghum – a multi-product crop ........................................................................... 4 1.1 Agronomy of sweet sorghum ...................................................................................................... 5 1.2 Sweet sorghum processing models ............................................................................................. 8 1.3 Food and feed products ............................................................................................................. 10 1.4 Fuel and energy products .......................................................................................................... 11 1.5 Other products........................................................................................................................... 13 1.6 Integration with sugarcane production ..................................................................................... 13 1.7 Economic modelling ................................................................................................................. 16 1.8 Conclusion ................................................................................................................................ 16 Chapter 2: Sweet sorghum field trials............................................................................................... 18 2.1 Introduction ............................................................................................................................... 18 2.2 Pre-project field trials ............................................................................................................... 18 2.3 Multi-cultivar field trial (2010–11) .......................................................................................... 22 2.4 AFL Rcv27751 winter field trial (2012) ................................................................................... 26 2.5 AFL Rcv27751 seed and ratoon crop field trial (2013) ............................................................ 31 2.6 Overall conclusions from the field trials .................................................................................. 36 Chapter 3: Energy products from sweet sorghum ........................................................................... 38 3.1 Electricity generation from combustion of sweet sorghum bagasse......................................... 38 vii

3.2 Ethanol production from sweet sorghum .................................................................................. 40 3.3 Conclusions ............................................................................................................................... 61 Chapter 4: Food and feed products from sweet sorghum ............................................................... 62 4.1 Introduction ............................................................................................................................... 62 4.2 Grain ......................................................................................................................................... 62 4.3 Juice .......................................................................................................................................... 70 4.4 Bagasse ..................................................................................................................................... 72 4.5 Mixed Products ......................................................................................................................... 74 4.6 Summary and Conclusions ....................................................................................................... 77 Chapter 5: Techno-economic assessment of sweet sorghum opportunities in Australia ............. 78 5.1 Introduction ............................................................................................................................... 78 5.2 Methodology ............................................................................................................................. 78 5.3 Products assessed ...................................................................................................................... 79 5.4 Sweet sorghum biorefinery process options assessed .............................................................. 80 5.5 Basis of assessment ................................................................................................................... 84 5.6 Results of techno-economic assessment ................................................................................... 95 5.7 Conclusion .............................................................................................................................. 103 Chapter 6: Life cycle assessment of sweet sorghum biorefinery product options in Australia ............................................................................................................................................. 105 6.1 Introduction ............................................................................................................................. 105 6.2 Methodology ........................................................................................................................... 105 6.3 Results and discussion ............................................................................................................ 116 6.4 Conclusions ............................................................................................................................. 122 Chapter 7: Assessment of the potential for integration of sweet sorghum and sugarcane processing ........................................................................................................................................... 124 7.1 Introduction ............................................................................................................................. 124 7.2 Overview of the Australian sugar industry ............................................................................. 124 7.3 Assumptions............................................................................................................................ 126 7.4 Sweet sorghum planting and crop cycles ................................................................................ 127 7.5 Harvesting sweet sorghum for processing in sugar factories ................................................. 128 7.6 Transporting sweet sorghum to existing sugar factories ........................................................ 129 7.7 Extraction of juice from sweet sorghum ................................................................................. 130 7.8 Analysis of sweet sorghum juice and fibre ............................................................................. 132 7.9 Clarification and evaporation of sweet sorghum juice ........................................................... 132 7.10 Other effects.......................................................................................................................... 133 viii

7.11 Conclusions ........................................................................................................................... 134 Chapter 8: Milling properties of sweet sorghum ........................................................................... 135 8.1 Introduction ............................................................................................................................. 135 8.2 Sample harvesting, delivery, and preparation ......................................................................... 136 8.3 Characterisation of sweet sorghum ......................................................................................... 136 8.4 Assessment of the milling characteristics of sweet sorghum ................................................. 137 8.5 Analysis of first expressed juice from sweet sorghum ........................................................... 140 8.6 Analysis of sweet sorghum bagasse after extraction of juice ................................................. 142 8.7 Total sweet sorghum analysis ................................................................................................. 142 8.8 Assessing extraction of sugars from sweet sorghum in a simulated conventional sugar milling process .............................................................................................................................. 143 8.9 Outcomes and conclusions...................................................................................................... 145 Results ................................................................................................................................................ 146 Implications........................................................................................................................................ 149 Recommendations ............................................................................................................................. 149 References .......................................................................................................................................... 151

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Tables Table 1.1

Characteristics of the Rio sweet sorghum cultivar from crop trials in Australia ................................8

Table 1.2

Composition (%) of sweet sorghum juice and syrup........................................................................10

Table 1.3

Milling parameters of sweet sorghum obtained during a factory trial ..............................................14

Table 1.4

Sweet sorghum juice measurements from a factory trial ..................................................................14

Table 2.1

Summary of sweet sorghum field trials ............................................................................................18

Table 2.2

Field data from the first planting of the 2010–11 sweet sorghum field trials ...................................23

Table 3.1

Proximate analysis, calorific value, and ultimate analysis1 ..............................................................39

Table 3.2

Sugar analysis of sugarcane and sweet sorghum juice .....................................................................40

Table 3.3

Fermentations conditions .................................................................................................................41

Table 3.4

Sugar, alcohol, and organic acid concentrations (mg/mL) in the 48 h samples ...............................42

Table 3.5

Ethanol yields from laboratory fermentation trials ..........................................................................43

Table 3.6

Composition of feedstocks ...............................................................................................................48

Table 3.7

Biomass composition of water-extractives free materials ................................................................49

Table 3.8

Composition of sweet sorghum and sugarcane juice........................................................................49

Table 3.9

Process quantities from pretreatment ...............................................................................................53

Table 3.10

Average composition of pretreatment hydrolysates from sweet sorghum and sugarcane pretreatment .....................................................................................................................................53

Table 3.11

Composition of sweet sorghum and sugarcane solid residues from pretreatment ............................54

Table 3.12

Biomass composition of water-extractives free pretreated sweet sorghum and sugarcane fibre ......54

Table 3.13

Process quantities for enzymatic hydrolysis ....................................................................................55

Table 3.14

Composition of hydrolysates from enzymatic hydrolysis of pretreated sweet sorghum and sugarcane fibres ...............................................................................................................................57

Table 3.15

Fermentation of hydrolysed fibre - analysis of fermentable sugars, fermentation products, and fermentation inhibitors. ....................................................................................................................60

Table 4.1

Nutritional analysis of sweet sorghum grain1 ...................................................................................63

Table 4.2

Comparison between sweet sorghum grain and other whole grains .................................................64

Table 4.3

Nutritional analysis of sweet sorghum grain and other grains used for animal feed ........................65

Table 4.4

Nutritional analysis of sweet sorghum flour (cultivar AFL Rcv27751) and comparison with other grain flours..............................................................................................................................67

Table 4.5

Nutritional analysis of sweet sorghum breakfast cereal and comparison with other commercial products ...........................................................................................................................................69

Table 4.6

Nutritional analysis of sweet sorghum juice and syrup samples, and comparison with equivalent sugarcane products .........................................................................................................71

Table 4.7

Nutritional analysis of sweet sorghum bagasse ................................................................................73

Table 4.8

Nutritional analysis of fish feed pellets with 32% and 38% fishmeal protein ..................................76

Table 4.9

Nutritional analysis of cattle feed pellets prepared with sweet sorghum processing residues and commercial cattle feed products ................................................................................................77

Table 5.1

Feedstock data for techno-economic assessment .............................................................................89

Table 5.2

Revenue data for techno-economic modelling .................................................................................90

Table 5.3

Key assumptions for mass and energy balance ................................................................................91

Table 5.4

Capital cost assumption data ............................................................................................................92

Table 5.5

Biorefinery operating labour requirements and labour costs data ....................................................93

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Table 5.6

Biorefinery operating materials usage rates and costs data ..............................................................94

Table 5.7

Biorefinery process low pressure steam requirements .....................................................................94

Table 5.8

Biorefinery process electricity requirements....................................................................................95

Table 5.9

Key economic and financial data .....................................................................................................95

Table 5.10

Product quantities for the sweet sorghum biorefinery process options for a 1.0 mt capacity facility ..............................................................................................................................................96

Table 5.11

Total revenue for biorefineries with capacities of 0.01, 0.1 and 1.0 mt ...........................................96

Table 5.12

Total capital costs for biorefineries with capacities of 0.01, 0.1 and 1.0 mt ....................................98

Table 5.13

Total annual production costs for biorefineries with capacities of 0.01, 0.1 and 1.0 mt ................100

Table 6.1

Sweet sorghum crop yield and composition used in the LCA (base case) .....................................106

Table 6.2

Key crop production, harvesting, and transport inputs used in the LCA analysis ..........................108

Table 6.3

Net carbon emissions from two land change scenarios ..................................................................109

Table 6.4

Biorefinery input and production data: animal feed – grain and juice extraction plant .................110

Table 6.5

Biorefinery input and production data: animal feed – syrup, utilities – steam and power generation, and bagasse pelletisation .............................................................................................110

Table 6.6

Biorefinery input and production data: ethanol production ...........................................................111

Table 6.7

GWP of biorefinery products in kg of CO2-eq for process options 1 to 6 (base case crop yield of 69 t/ ha) .....................................................................................................................................117

Table 6.8

Displaced products used in the consequential system LCA ...........................................................120

Table 7.1

Possible sweet sorghum cropping cycles for integration with sugarcane processing .....................127

Table 8.1

Analysis of disintegrator extract from prepared sweet sorghum ....................................................137

Table 8.2

Fibre content in sweet sorghum samples determined by two different methods ............................137

Table 8.3

Mass balances for sweet sorghum juice extraction ........................................................................141

Table 8.4

Sugar content and composition in sweet sorghum first-expressed juice ........................................141

Table 8.5

Analysis of sugar content and composition in sweet sorghum bagasse ..........................................142

Table 8.6

Estimates of the constituents of the sweet sorghum samples .........................................................143

Table 8.7

Analysis of sugar content and composition in sweet sorghum final bagasse..................................143

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Figures Figure 1.1

Images of sweet sorghum from AgriFuels Pty Ltd crop trials in Childers, Queensland .................... 6

Figure 1.2

Stages of growth of the sorghum plant (NSW Department of Primary Industries 2005) .................. 7

Figure 1.3

Typical product options for a sweet sorghum biorefinery................................................................. 9

Figure 1.4

Processing model for sweet sorghum production (AgriFuels Pty Ltd) ............................................. 9

Figure 1.5

Theoretical ethanol yields from sweet sorghum based on QUT modelling..................................... 12

Figure 2.1

2007-08 sweet sorghum crop trial (a) 19 October 2007. (b) 9 November 2007. (c) 20 December 2007). (d) 22 January 2008............................................................................................ 19

Figure 2.2

Seed planting during the 2009–10 field trial ................................................................................... 20

Figure 2.3

The stalk from an M81-E plant ....................................................................................................... 21

Figure 2.4

Varieties under trial at maturity ...................................................................................................... 24

Figure 2.5

Analysis of sweet sorghum plants generated in the 2010–11 field trial (a) Average stalk weight (kg). (b) Average sucrose % in juice. (c) Average total fermentables (as hexose) in juice. (d) Mass of total fermentable sugars per stalk (kg). .............................................................. 25

Figure 2.6

AFL Rcv27751 31 May 2012. DAP = 14. ...................................................................................... 27

Figure 2.7

AFL Rcv27751 14 August 2012. DAP = 89. .................................................................................. 28

Figure 2.8

AFL Rcv27751 14 September 2012. DAP = 120. .......................................................................... 28

Figure 2.9

AFL Rcv27751 3 October 2012. DAP = 139. ................................................................................ 29

Figure 2.10

AFL Rcv27751 1 November 2012. DAP = 168. ............................................................................ 30

Figure 2.11

AFL Rcv27751 15 November 2012. DAP = 182. .......................................................................... 30

Figure 2.12

Comparison between seed crop (left) and ratoon crop (right) 21 February 2013. DAP = 35. DAR = 42. ...................................................................................................................................... 32

Figure 2.13

Ratoon crop AFL Rcv27751 14 March 2013. DAR = 63. .............................................................. 33

Figure 2.14

Ratoon crop AFL Rcv27751 14 March 2013. DAR = 63. .............................................................. 33

Figure 2.15

Seed crop AFL Rcv27751 14 March 2013. DAP = 56. .................................................................. 34

Figure 2.16

Ratoon crop AFL Rcv27751 27 March 27 2013. DAR = 76. ......................................................... 35

Figure 2.17

Seed crop AFL Rcv27751 27 March 2013. DAP = 69. .................................................................. 35

Figure 3.1

Mackay Renewable Biocommodities Pilot Plant located at the Mackay Sugar Ltd Racecourse Mill ................................................................................................................................................. 44

Figure 3.2

Flow diagram of the processes conducted at the pilot plant for sweet sorghum and sugarcane biomass ........................................................................................................................................... 46

Figure 3.3

Sweet sorghum upon arrival at the MRBPP.................................................................................... 47

Figure 3.4

Chipped stalk material from (a) sweet sorghum and (b) sugarcane ................................................ 48

Figure 3.5

Biomass feeding and linear weighing systems ................................................................................ 50

Figure 3.6

Andritz horizontal pretreatment reactor .......................................................................................... 50

Figure 3.7

Hydrolysate collected from a sweet sorghum pretreatment run ...................................................... 51

Figure 3.8

Key reactor components (a) Andritz vertical reactor. (b). Solid residue blow down tank. ............. 52

Figure 3.9

Pretreated fibre post steam explosion of sweet sorghum (left) and sugarcane (right) stalk ............ 52

Figure 3.10

Stirred bio-reactor used for enzymatic hydrolysis and some fermentation trials ............................ 55

Figure 3.11

Enzyme hydrolysis of pretreated, steam-exploded sweet sorghum fibre at 0 h (top left), 1.6 h (top right), and 19 h (bottom left). .................................................................................................. 56

Figure 3.12

10 L stirred fermenter ..................................................................................................................... 58

Figure 3.13

100 L stirred fermenter ................................................................................................................... 59

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Figure 4.1

Major sweet sorghum components and their application to food and feed products ...................... 62

Figure 4.2

Sweet sorghum flour ....................................................................................................................... 66

Figure 4.3

Sweet sorghum breakfast cereal ...................................................................................................... 68

Figure 4.4

Juice samples from sweet sorghum ................................................................................................. 70

Figure 4.5

Ground sweet sorghum bagasse ...................................................................................................... 73

Figure 4.6

Fish feed pellets prepared using sweet sorghum processing residues fishmeal protein contents of (a) 32% and (b) 38% .................................................................................................................. 75

Figure 4.7

Cattle feed pellets prepared using 16% wheat gluten and sweet sorghum processing residues ...... 76

Figure 5.1

Process option 1 .............................................................................................................................. 81

Figure 5.2

Process option 2 .............................................................................................................................. 81

Figure 5.3

Process option 3 .............................................................................................................................. 82

Figure 5.4

Process option 4 .............................................................................................................................. 83

Figure 5.5

Process option 5 .............................................................................................................................. 83

Figure 5.6

Process option 6 .............................................................................................................................. 84

Figure 5.7

Stalk crushing and juice extraction ................................................................................................. 85

Figure 5.8

Syrup production ............................................................................................................................ 85

Figure 5.9

Steamed-flaked grain production .................................................................................................... 86

Figure 5.10

Grain pre-hydrolysis ....................................................................................................................... 86

Figure 5.11

Ethanol production.......................................................................................................................... 86

Figure 5.12

Fibre processing .............................................................................................................................. 87

Figure 5.13

Animal feed (stillage) production ................................................................................................... 87

Figure 5.14

Cogeneration and electricity production ......................................................................................... 87

Figure 5.15

Bagasse pellet production ............................................................................................................... 88

Figure 5.16

Revenue by product for 1.0 million tonne capacity biorefinery process options ............................ 97

Figure 5.17

Capital costs by process operation for 1.0 million tonne capacity biorefinery process options ...... 99

Figure 5.18

Production costs breakdown for 1.0 million tonne capacity biorefinery process options ............. 101

Figure 5.19

IRR (a) and NPV (b) of sweet sorghum biorefinery process options ............................................ 102

Figure 5.20

Sensitivity of IRR to feedstock (a) grain and (b) stalk prices ....................................................... 103

Figure 6.1

Processing stages and mass allocation factors for process option 1 .............................................. 113

Figure 6.2

Processing stages and mass allocation factors for process option 2 .............................................. 113

Figure 6.3

Processing stages and mass allocation factors for process option 3 .............................................. 114

Figure 6.4

Processing stages and mass allocation factors for process option 4 .............................................. 114

Figure 6.5

Processing stages and mass allocation factors for process option 5 .............................................. 115

Figure 6.6

Processing stages and mass allocation factors for process option 6 .............................................. 115

Figure 6.7

GWP of sweet sorghum and sugarcane delivered to the factory gate ........................................... 116

Figure 6.8

The relative contribution of crop production, harvest, transport and processing to the overall GWP of ethanol produced from sweet sorghum ........................................................................... 118

Figure 6.9

Relative GWP of ethanol compared with ULP for process options and sweet sorghum yield scenarios ....................................................................................................................................... 119

Figure 6.10

Results of a consequential whole of system LCA of sweet sorghum for the six process options for the base case (69 t/ ha) crop yield scenario ............................................................................. 121

Figure 6.11

Net avoided GWP determined for the consequential whole of system LCA of sweet sorghum for the six process options and three crop yield scenarios ............................................................ 122

Figure 7.1

Existing sugar process flow diagram ............................................................................................ 125

Figure 7.2

Sugarcane being harvested using a harvester and in-field transport.............................................. 128

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Figure 7.3

Sugarcane transported to a sugarcane factory by train .................................................................. 129

Figure 7.4

Option for a sweet sorghum transport system using existing railway infrastructure ..................... 130

Figure 7.5

Option for a sweet sorghum road transport system ....................................................................... 130

Figure 7.6

Roller mills in a sugarcane factory................................................................................................ 131

Figure 7.7

Washing sugarcane juice from mud using a rotary drum vacuum filter ........................................ 133

Figure 8.1

Schematic illustration of a typical Australian sugar industry six-roller mill ................................. 135

Figure 8.2

Sweet sorghum. (a) Sweet sorghum as delivered. (b) Billeted sweet sorghum with stalk and leaf. ............................................................................................................................................... 136

Figure 8.3

Sweet sorghum fibre prepared from leaf and stalk........................................................................ 138

Figure 8.4

Soluble sugars extraction from sweet sorghum ............................................................................. 139

Figure 8.5

Pressure compression testing of sweet sorghum and sugarcane at (a) low and (b) high pressure . 140

Figure 8.6

Total soluble solids extraction (°Bx) from sweet sorghum ........................................................... 144

Figure 8.7

Total sugar extraction from sweet sorghum .................................................................................. 144

Figure 8.8

Moisture content of sweet sorghum samples................................................................................. 145

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Executive Summary What the report is about Sweet sorghum is receiving significant global interest as an agro-industrial crop because of its capacity to co-produce energy, food, and feed products in integrated biorefineries. This report assesses the opportunities to develop a sweet sorghum industry in Australia, reports on research demonstrating the production of energy, food, and feed products, and assesses the potential economic and sustainability benefits of sweet sorghum biorefineries in the Australian context. Who is the report targeted at? This report will be of broad interest to agricultural producers and agro-industrial companies across rural and regional Australia seeking new opportunities for value-creation in the bioeconomy. The report will also be of interest to researchers investigating new biomass feedstocks for the production of biofuels, biochemicals, and renewable bioproducts. Where are the relevant industries located in Australia? The characteristics of sweet sorghum agronomy suggest that sweet sorghum has a wide potential cropping area from tropical and sub-tropical Queensland, Northern Territory, and Western Australia and in temperate regions of New South Wales, Victoria and Western Australia. The sweet sorghum industry may offer value in small scales producing higher value products to large agro-industrial complexes built around broad-acre cropping areas. In particular, there are significant short term opportunities for co-location of sweet sorghum integrated with sugarcane production, to increase biomass availability in sugarcane processing systems throughout Queensland. Background Sweet sorghum is a rapidly-maturing, photoperiod-sensitive plant. It produces a stalk up to five metres tall with a high concentration of fermentable sugars at a level similar to that of sugarcane, and produces a large panicle of nutritionally-valuable grain similar to that of grain sorghum. Unlike many other crops used for renewable energy production, sweet sorghum can simultaneously produce food and feed products. Considerable research is underway globally into the use of sweet sorghum for coproducing energy, food, and feed products. This research includes studies on enhancing sweet sorghum crop traits and investigating a range of potential products from sweet sorghum feedstocks. While this international research is continuing, there have been few studies undertaken in Australia on the potential opportunities for sweet sorghum in the Australian context, and most of these studies primarily focussed on crop trials of major sweet sorghum varieties. In this context, Queensland University of Technology (QUT), with our industry partner AgriFuels Ltd, commenced a significant research project to understand the potential opportunities for sweet sorghum in Australia and to investigate sweet sorghum biorefinery options. This work provided information for the proposed development by AgriFuels Ltd of a sweet sorghum biorefinery in the Childers region of south-eastern Queensland. The project aimed to demonstrate the production of trial energy, food, and feed products at the laboratory and pilot-scales, and to assess the economic and sustainability benefits of sweet sorghum biorefineries. One opportunity for reducing the initial investment cost in a new sweet sorghum industry is to integrate sweet sorghum production with existing sugarcane or grain sorghum processing xv

infrastructure. There are significant opportunities to integrate sweet sorghum crop production and processing operations within existing underused sugarcane production and processing industries. This would provide a low cost entry for sweet sorghum producers into the Australian market and improve the economics of sugarcane processing through better use of sugarcane industry infrastructure. The potential of such integration has been assessed within this project. Aims/objectives The objectives of the project were to assess and demonstrate the commercial feasibility and sustainability of sweet sorghum as a feedstock for renewable energy and food production in Australia, by: •

assessing the productivity, product quality, and other significant agronomic indicators of commercially-relevant sweet sorghum varieties in field trials in south-eastern Queensland



optimising the fermentation process for ethanol production from sweet sorghum juice, including the co-fermentation of sweet sorghum juice with sugarcane juice or molasses at both the laboratory and pilot-scales



producing trial quantities of sweet sorghum products for end-use product testing, including grain for animal feed and concentrated liquor for the food industry



undertaking an economic evaluation and life cycle assessment of the proposed sweet sorghum cropping, harvesting, and processing system



evaluating the opportunities for using existing sugarcane industry infrastructure for harvesting, transport, and processing of sweet sorghum, including use of transport and processing infrastructure outside of the sugarcane crushing season.

Methods used The project was undertaken as a collaborative project between researchers at QUT and project partner AgriFuels Ltd. The project was undertaken in five main phases. Phase 1 involved the cultivation, sampling, and analysis of multi-variety sweet sorghum crop trials grown by AgriFuels Ltd in Childers, Queensland. Key agronomic factors were assessed in a series of crop trials over three years. The various components of the crop were assessed for their end-use value. Phase 2 analysed potential energy products from sweet sorghum with a focus on electricity products from combustion and ethanol production. This phase investigated the fermentation of sweet sorghum juice and fibre into ethanol at laboratory and pilot scales. Phase 3 demonstrated the production of trial quantities of food and feed products from sweet sorghum grain, juice, and fibre. These products were analysed for nutritional composition, micronutrient composition, energy value, and other parameters. Phase 4 analysed the techno-economic potential of integrated sweet sorghum biorefinery options. In this phase, a detailed life cycle assessment (carbon footprint analysis) of each of the conceptual process options was also undertaken. Phase 5 assessed the opportunities for integrated sugarcane and sweet sorghum industries, and the potential benefits of using existing sugarcane infrastructure for harvesting, transportation and processing of sweet sorghum. A detailed analysis of the specific challenges required for processing sweet sorghum through a sugarcane factory milling train was undertaken.

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Results/key findings The major outcomes of the project for each objective were: Assessing the productivity, product quality, and other significant agronomic indicators for crop trials in South-East Queensland of commercially-relevant sweet sorghum varieties Field trials of sweet sorghum were undertaken to assess the agronomic performance of the crop and to generate trial quantities of products for further analysis. Comparative assessments of eight sweet sorghum varieties were undertaken under common growing conditions. The AgriFuels variety AFL Rcv27751 was one of the top performers with good stalk and grain yields and high levels of total fermentable sugars in the stalk juice. Stalk height and diameter (and hence yield) were affected by water availability throughout the trial, but reached a maximum height at ~120–140 DAP for the seed crop and ~90–110 DAP for the ratoon crop. Profiles of the concentration of sugars in the stalk juice were determined. For plant crops, maximum total fermentable sugars concentrations occurred about ~140 DAP for a December plant crop. Optimising the fermentation process for ethanol production from sweet sorghum juice, including the co-fermentation of sweet sorghum juice with sugarcane juice or molasses at both laboratory and pilot scale Laboratory scale trials of sweet sorghum juice fermentation were undertaken to optimise fermentation conditions. High ethanol yields from the fermentation of sweet sorghum were achieved with efficiencies as high as ~94% under optimum conditions. Sugarcane and sweet sorghum juices were both readily fermented with the microorganisms used in this project. The addition of sweet sorghum juice to sugarcane juice resulted in higher ethanol yields than the fermentation of sugarcane juice alone. Optimum ethanol yields were obtained at high inoculum loadings, a fermentation temperature of 33 °C and a nutrient loading of 40 mg/100 mL . Pilot scale trials of the conversion of sweet sorghum fibre into ethanol were undertaken at the Mackay Renewable Biocommodities Pilot Plant. These trials included pretreatment of fibre using two-stage mild acid pretreatment, enzymatic hydrolysis of the pretreated fibre and fermentation of the hydrolysed cellulose into ethanol. The results showed that sweet sorghum fibre can be readily converted into ethanol using these processing techniques. Similar results were obtained in the bioconversion of sweet sorghum fibre to ethanol in comparison to sugarcane fibre. Producing trial quantities of sweet sorghum products for end-use product testing, including grain for animal feed and concentrated liquor for food industry uses Sweet sorghum products were generated including sweet sorghum flour, syrup, sweet sorghum-based breakfast cereal, fish and animal feed pellets and human dietary fibre products. The results of the analyses show that there are significant opportunities for manufacturing food and feed products from sweet sorghum in Australia, including livestock feed from sweet sorghum grain, supplementation of livestock feed with sweet sorghum syrup, stock feed/roughage from sweet sorghum bagasse, and the production of mixed animal and fish feed products incorporating all three residues. Undertaking an economic evaluation and life cycle assessment (LCA) of the proposed cropping, harvesting, and processing system The techno-economic potentials of six sweet sorghum biorefinery processes were assessed in the Australian context. Five of the process options assessed were determined to have IRRs that exceeded the typical project hurdle rate of 15% and achieved positive NPVs, given the assumptions used in the assessment. The economic feasibility of ethanol production from sweet sorghum was shown to xvii

increase with ethanol production capacity. Integration of sweet sorghum juice, fibre, and grain for ethanol production was shown to deliver benefits through greater economies of scale. A carbon footprint analysis of six biorefinery process options for the conversion of sweet sorghum to fuel and animal feed products under Australian conditions was also undertaken. A whole of system consequential Life Cycle Assessment determined that the net reduction in Global Warming Potential (GWP; per hectare) due to the aggregated effects of all biorefinery products resulted in a strong net reduction in GWP. This was true even for the lowest sweet sorghum crop yield scenario. Process options in which significant resources (fibre to produce electricity and LP steam) were required to process the fibre into fermentable sugars (process options 3 and 5) gave reduced system GWP benefits relative to the other process options. Evaluating the opportunities for using existing sugarcane industry infrastructure for harvesting, transportation, and processing of sweet sorghum crops, including use of transport and processing infrastructure in the non-crushing season An assessment of integration of sweet sorghum with sugarcane production and processing showed that there are potential effects across the harvesting, transport, and processing sectors with changed practices required for efficient integration. However, there are also significant opportunities to reduce the costs of start-up for a sweet sorghum industry, and to better use sugarcane processing infrastructure to improve the profitability and viability of the sugarcane growing, harvesting, and milling sectors. Milling trials were undertaken at the QUT Pilot Plant Precinct, Brisbane, to assess the potential impacts of crushing sweet sorghum through conventional sugarcane factory milling trains. The trials showed that no changes to a sugarcane factory shredder will be necessary to process sweet sorghum containing stalk only or stalk and leaves. Sugarcane factory milling stations will operate at a marginally lower speed with sweet sorghum than with sugarcane for the same fibre rate, but sweet sorghum will likely achieve similar or higher compaction levels to sugarcane. Implications for relevant stakeholders The establishment of a sweet sorghum industry in Australia offers significant potential to grow Australia’s gross production of energy, food and feed products from agriculture. Techno-economic assessment of sweet sorghum biorefinery options in Australia undertaken as a part of this project have shown very attractive returns on investment for new biorefinery facilities co-producing bioethanol, electricity and animal feed products. Human food products also offer niche opportunities from sweet sorghum biorefinery production. Sweet sorghum is increasingly being seen as a valuable biorefinery crop with global potential and technology for efficient production is rapidly developing. Biofuels are one of the major options for reducing our reliance on fossil fuels for transportation energy. Increasing crude oil prices and global warming associated with anthropogenic carbon emissions will continue to create a demand for renewable fuels. The co-production of energy, food and feed products improves the sustainability of potential products by reducing the greenhouse gas footprint of individual products. As shown in this report, the use of grain from sweet sorghum for animal feed production offers the opportunity to reduce the carbon emissions associated with grain fed cattle products. Such products may allow beef producers to provide a point of difference for their products in the market. Australia is well placed to establish integrated biorefineries producing products for domestic use and for export into the Asian market. Sweet sorghum offers one crop that has significant potential for contributing to the development of Australia’s bioeconomy.

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Recommendations Further research and commercialisation activities are required before sweet sorghum will become commercially cultivated and processed at large scale in Australia. The following research and commercialisation activities are critical to capturing the potential economic benefits of this investment in Australia: 1. assessment and validation of sweet sorghum cultivar performance across varying climatic regions of Australia 2. further development of improved sweet sorghum cultivars optimised for Australian climatic conditions 3. further development of sweet sorghum cultivars with improved traits such as increased biomass yields 4. large-scale demonstration of continuous processing of sweet sorghum through sugar factories or demonstration-scale biorefineries 5. detailed assessment of sweet sorghum biorefinery product and market opportunities with a focus on the Australian and Asian markets 6. large-scale animal feeding trials are required to confirm the digestible and metabolisable energy benefits and palatability of sweet sorghum animal feed products proposed in this report.

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Introduction Sweet sorghum (S. bicolor) is a rapidly-maturing, photoperiod-sensitive plant. It produces a stalk up to five metres tall with a high concentration of fermentable sugars at a level similar to that of sugarcane, and produces a large panicle of nutritionally-valuable grain similar to that of grain sorghum. Unlike many other crops used for renewable energy production, sweet sorghum can simultaneously produce food and feed products. Like sorghum, the grain from sweet sorghum can be used as an animal feed or processed into ethanol through mashing and fermentation. When processed, the sweet sorghum grain can also be used as a gluten free human food product and the flour from sweet sorghum grain is widely consumed in many countries. Although sweet sorghum juice has a similar total fermentable sugar content to sugarcane juice, sweet sorghum juice has a much higher concentration of glucose and fructose, making it a particularly suitable for fermentation, but generally unsatisfactory for crystal sugar production. However, sweet sorghum syrup is promising for production of gluten-free beer and other food products. High purity ethanol from the fermentation process can also be refined into high-grade, extra-neutral alcohol, used for the manufacture of food-grade products, including flavourings. The residual fibre (bagasse) from sweet sorghum can be used to produce electricity, paper, and cattle fodder. Fermentable sugars from the biomass can also be used as a feedstock for ethanol production, with a proportion of the biomass used to provide steam and power for the process, and to generate renewable electricity. Other renewable products (such as green chemicals and polymers) can also be produced from components of the sweet sorghum crop, many of which can replace comparable items produced from fossil fuel feedstocks. Considerable research is underway globally into the use of sweet sorghum for coproducing energy, food and feed products. This research includes studies on enhancing sweet sorghum crop traits and investigating a range of potential products from sweet sorghum feedstocks. While this international research is continuing, there have been few studies undertaken in Australia on the potential opportunities for sweet sorghum in the Australian context and most of these studies primarily focussed on crop trials of major sweet sorghum varieties. In this context, Queensland University of Technology, with our industry partner AgriFuels Ltd, commenced a significant research project to understand the potential opportunities for sweet sorghum in Australia and to investigate sweet sorghum biorefinery options. This work provided information for the proposed development by AgriFuels Ltd of a sweet sorghum biorefinery in the Childers region of south-eastern Queensland. The project aimed to demonstrate the production of trial energy, food and feed products at the laboratory and pilot-scales, and to assess the economic and sustainability benefits of sweet sorghum biorefineries. One opportunity for reducing the initial investment cost in a new sweet sorghum industry is to integrate sweet sorghum production with existing sugarcane or grain sorghum processing infrastructure. There are significant opportunities to integrate sweet sorghum crop production and processing operations within existing underused sugarcane production and processing industries. This would provide a low cost entry for sweet sorghum producers into the Australian market and improve the economics of sugarcane processing through better use of sugarcane industry infrastructure. The potential of such integration has been assessed within this project.

1

Objectives The objectives of the project were to assess and demonstrate the commercial feasibility and sustainability of sweet sorghum as a feedstock for renewable energy and food production in Australia, by: •

assessing the productivity, product quality, and other significant agronomic indicators of commercially-relevant sweet sorghum varieties in field trials in south-eastern Queensland



optimising the fermentation process for ethanol production from sweet sorghum juice, including the co-fermentation of sweet sorghum juice with sugarcane juice or molasses at both the laboratory and pilot-scales



producing trial quantities of sweet sorghum products for end-use product testing, including grain for animal feed and concentrated liquor for the food industry



undertaking an economic evaluation and life cycle assessment of the proposed sweet sorghum cropping, harvesting, and processing system



evaluating the opportunities for using existing sugarcane industry infrastructure for harvesting, transport, and processing of sweet sorghum, including use of transport and processing infrastructure outside of the sugarcane crushing season.

2

Methodology The project was undertaken as a collaborative project between researchers at the Queensland University of Technology (QUT) and project partner AgriFuels Ltd. The project was undertaken in five main phases. During Phase 1, a comprehensive literature review was undertaken detailing the state-of-the-art relating to sweet sorghum agronomy and products with a focus on previous work conducted in Australia. This literature review is presented in Chapter 1. In addition, Phase 1 involved the cultivation, sampling, and analysis of multi-variety sweet sorghum crop trials grown by AgriFuels Ltd in Childers, Queensland. Eight sweet sorghum cultivars were grown including one promising commercial variety licensed by AgriFuels Ltd. Key agronomic factors were assessed in a series of crop trials over three years. These trials assessed the effect of growing conditions on major crop traits and selected promising commercial varieties under the conditions tested. Factors assessed included crop and product yield, productivity, product quality, effect of pests, weeds and disease, and crop composition. The various components of the crop were assessed for their end-use value. A report on the crop trials and the results of the assessment of the crop samples are presented in Chapter 2 of this report. Phase 2 consisted of an analysis of potential energy products from sweet sorghum with a focus on electricity products from combustion and ethanol production. Juice extracted from sweet sorghum stalk was fermented at the laboratory-scale to provide information on optimum fermentation conditions, and compare sweet sorghum juice and sugarcane juice fermentation. This phase also investigated the co-fermentation of sweet sorghum and sugarcane juice, and the opportunities this provides for integration of ethanol production assets at co-located facilities. Pilot-scale fermentation trials were undertaken at the QUT Mackay Renewable Biocommodities Pilot Plant in Mackay, Queensland. These trials compared the efficacy of ethanol production from sweet sorghum juice and fibre compared to sugarcane juice and fibre. The results of these trials are detailed in Chapter 3. Phase 3 consisted of the production of trial quantities of food and feed products from sweet sorghum grain, juice, and fibre. The sweet sorghum products were made with the assistance of VAFF Pty Ltd and included sweet sorghum flour, breakfast cereal, concentrated syrup, dietary fibre, fish pellets, and cattle pellets. These products were subsequently analysed for nutritional composition, micronutrient composition, energy value, and other parameters. The comparison between sweet sorghum products and equivalent, existing products are detailed in Chapter 4. In Phase 4, a detailed techno-economic model was developed to assess the economic potential of sweet sorghum biorefinery options. The model consisted of mass and energy balances, capital, operating and material costs models, EBIT model and discounted cash flow analyses. Six conceptual process options were assessed for a range of energy and feed products. A detailed life cycle assessment (LCA) of each of the conceptual process options was also undertaken using SimaPro LCA software. The LCA model provided information on the sustainability benefits of each of the sweet sorghum biorefinery processes. The results of the techno-economic assessment are provided in Chapter 5 and the results of the life cycle assessment are provided in Chapter 6 of this report. Phase 5 assessed the opportunities for integrated sugarcane and sweet sorghum industries and the potential benefits of using existing sugarcane infrastructure for harvesting, transportation and processing of sweet sorghum. In this phase, an analysis of the opportunities for integration was conducted and followed with more detailed analysis of the specific challenges required for processing sweet sorghum through a sugarcane factory milling train. The results of these assessments are provided in Chapters 7 and 8 of this report.

3

Chapter 1: Sweet sorghum – a multiproduct crop T.J. Rainey and I.M. O’Hara Sweet sorghum (Sorghum bicolor L. Moench var. bicolor) is a rapidly-maturing, C4, monocotyledonous plant from the family Poaceae. The Poaceae are the most economically-important plant family and include maize (corn), wheat, millet, and rice, as well as bamboo. Sweet sorghum produces a stalk which can be up to five metres tall containing high concentrations of fermentable sugars and a large panicle of grain similar to that of grain sorghum. Unlike many other crops used for renewable energy production, sweet sorghum can simultaneously produce energy, food, and feed products. The sweet sorghum plant has three basic components which can be harvested and used to produce valuable products: grain, juice from the stalk, and fibre from the stalk and leaves. Sweet sorghum produces a nutritionally-valuable grain which can be used directly as an animal feed. Alternatively, the grain can be used as a feedstock for biofuels production, with the starch component converted into ethanol, and the residual protein and other matter converted to a high protein distiller’s grain for animal feed. In many parts of the world, sweet sorghum grain is ground into flour and used for baking into bread and other human food products. The juice from the sweet sorghum stalk can be extracted and used to produce a wide variety of products. Although the juice from some varieties of sweet sorghum have a total fermentable sugar concentration comparable to that of sugarcane, sweet sorghum juice has a much higher concentration of glucose and fructose, making it particularly suitable for fermentation but generally unsatisfactory for crystal sugar production. While ethanol is a key product from fermentation, other fermentation products include other alcohols and organic acids which may be used in chemical and polymer applications. The extracted juice from sweet sorghum can be purified and concentrated to produce food-quality syrup for the production of gluten-free beer and as a sweetener in a variety of food products. Ethanol from fermentation processes can also be purified into high-grade, extra-neutral alcohol and used for the manufacture of food flavouring, pharmaceutical, and industrial products. Sweet sorghum fibre can be used for renewable electricity generation and to provide steam and power for the production process. Other renewable products, such as green chemicals and polymers, can also be produced from the components of sweet sorghum fibre, many of which can replace comparable items produced from fossil fuel feedstocks. Sweet sorghum fibre can also be used for the production of paper products, textile products, and composite building materials. While there are multiple products that can be produced from sweet sorghum, the product of principle interest globally is ethanol for use as a renewable transportation fuel. There are three major fermentative pathways to producing ethanol from sweet sorghum. These pathways are: •

saccharification of the starch in sweet sorghum grain into glucose and the fermentation of the glucose into ethanol



extraction of the juice containing fermentable sugars (sucrose, glucose, and fructose) from the sweet sorghum stalk and the fermentation of these sugars into ethanol



pretreatment and hydrolysis of the lignocellulosic fibre from the stalk and leaves into fermentable sugars (primarily glucose and xylose) and the fermentation of these sugars into ethanol.

4

Potential crop feedstocks for bioethanol production globally include sugarcane, sugar beet, maize, rice, wheat, and potato, and cellulosic energy crops, such as switchgrass, sweet sorghum, and Miscanthus (Hattori and Morita 2010). Hattori and Morita reviewed the suitability of many current and potential crops in both tropical and temperate climates for ethanol production taking into account each crop’s Net Energy Benefit (NEB). NEB is an indicator of the renewable energy generated after accounting for the fossil fuel required for harvesting, transport, fertiliser usage, and optimal land usage. For tropical and subtropical regions, the results showed that sugarcane and sweet sorghum were the best feedstocks for bioethanol production. This conclusion was based on the high NEB of sweet sorghum and sugarcane and, to a lesser extent, their high biomass yields. Compared to sugarcane, sweet sorghum has a higher tolerance to salt and drought (Almodares and Hadi 2009; Smith and Buxton 1993; Gnansounou, Dauriat and Wyman 2005; Nan and Ma 1989; Sutherland 2002; Rooney et al. 2007) while producing greater amounts of biomass (Wu et al. 2010; Türe, Uzun and Türe 1997; Mamma et al. 1996; Mamma et al. 1995; Rooney et al. 2007). Sweet sorghum requires less water than sugarcane and requires less fertiliser to produce significant biomass (Almodares and Hadi 2009). Sweet sorghum produces a comparable amount of fermentable sugars to sugarcane (Wu et al. 2010; Mamma et al. 1995) and sweet sorghum juice is more suitable for fermentation to ethanol than sugarcane juice (Almodares and Hadi 2009). Sweet sorghum is also highly adaptable to different climates (Smith and Buxton 1993). Although sweet sorghum is believed to have originally developed in tropical regions (Curt, Fernandez and Martinez 1995; Gnansounou, Dauriat and Wyman 2005), it also grows well in temperate climates (Smith and Buxton 1993; Türe, Uzun and Türe 1997; Curt, Fernandez and Martinez 1998; Gnansounou, Dauriat and Wyman 2005).

1.1 Agronomy of sweet sorghum Commercially grown sorghum varieties can be classified as either sweet, grain, or forage sorghum (Almodares and Hadi 2009). Grain sorghum is the fifth most widely grown cereal crop globally (Rooney et al. 2007). From 2004–09, Australia’s average production of sorghum grain was 2.3 million tonnes (t) from about 769 000 hectares (ha) at an average yield of 2.96 t/ha (ABARE 2010). Grain sorghum usually produces a larger grain panicle with higher starch content than sweet sorghum whereas sweet sorghum, while producing a grain panicle, produces more biomass and a thicker stalk with a higher fermentable sugar content. Images of sweet sorghum from the crop trials in the present study are presented in Figure 1.1. At maturity, up to 75% of the plant biomass is contained in the plant stalk, 10–15% in the leaves, up to 7% in the grains, and ~10% in the roots (Grassi 2001). The stages of growth for sweet sorghum are similar to the stages of growth for grain sorghum (Figure 1.2). Emergence of from the soil typically occurs 3–10 days after planting and effective germination requires soil temperatures of >16 °C and good soil moisture. The yield potential of the plant is set during the vegetative stage, however dry matter production occurs at a constant rate from stage 2 through to plant maturity (NSW Department of Primary Industries 2005). Following anthesis, the seed develops through a soft dough stage and hard dough stage as the water in the seed is displaced by starch until the plant reaches physiological maturity. The time from sowing to harvest is typically 115–140 days (NSW DPI 2005). Sweet sorghum grains can be brown, red, or white, with grain colour depending upon the presence of phenolic compounds (Wall and Blessin 1970). White or pale yellow grain are most suitable for starch production with food applications (Subramanian, Hoseney and Bramel-Cox 1994). Sweet sorghum grain yields are typically 3–7 t/ha (Almodares and Hadi 2009) and mature grain contain ~12% water, 10% protein, ~4% lipids, 73% carbohydrates (mostly starch), 2.2% fibre, and 1.5% ash (Grassi 2001). Sweet sorghum stalk yields vary but are typically 50–100 t/ha (Woods 2000; Sutherland 2002; Almodares and Hadi 2009). Stalk composition also varies but sugar concentrations of 12–21% are typically reported (Almodares and Hadi 2009). The majority of soluble sugar in the stalk is sucrose but significant amounts of glucose and fructose are also present. Woods (Woods 2000) reported 7– 5

13% sugar in the stalk, 12–17% fibre, and moisture around 75%. Brown mid-rib varieties of sorghum are reported to have generally lower lignin contents than other varieties which improve their digestibility for animal feed (Pedersen 1996). Although sweet sorghum is well adapted to temperate climates, growth is maximised at high temperatures for most cultivars (Almodares and Hadi 2009).

Figure 1.1 Images of sweet sorghum from AgriFuels Pty Ltd crop trials in Childers, Queensland

Drought tolerance characteristics of sweet sorghum derive from the heavy wax layer and small stomata on its leaves, and an extensive root structure. Reports of water usage for optimal growth vary significantly but suggest that sweet sorghum requires between 30% less water than sugarcane (Sutherland 2002) and 67% less water than sugarcane (Almodares and Hadi 2009) for comparable 6

yields. Smith and Buxton (1993) concluded that sweet sorghum produces more biomass in temperate climates when irrigated. Biomass yield was ~90 t/ha for the irrigated crop and ~65 t/ha for the nonirrigated crop. However, the total sugar yield (the mean yield was 6.0 t/ha) and theoretical ethanol production (based on total starch and sugars) were not significantly affected by irrigation.

Figure 1.2 Stages of growth of the sorghum plant (NSW Department of Primary Industries 2005)

It has been reported that nitrogen fertilisation has little discernible effect on sugar production and total biomass yield from sweet sorghum (Smith and Buxton 1993). This finding was contradicted by other studies that reported improvements in sucrose content and biomass yield with nitrogen fertilisation (Almodares et al. 2008; Rooney et al. 2007) up to 140 kg N/ha (Rooney et al. 2007). This apparent contradiction between the effects of nitrogen fertiliser addition on yield may have been the result of varying residual nitrogen levels in the soils at the various sites were the work was undertaken. Common grain and forage sorghum pests in Australia include wireworm, cutworm, armyworms, sorghum midge, and Heliothis. Sorghum midge and Heloithis can have a significant effect on grain development and yield (Collett 2004b; DPI 2005). Common sorghum diseases in Australia include leaf rust, head smut, sorghum ergot, seedling blight, and stalk rot. Climatic factors influence the likelihood of disease becoming significant and well managed crops may be less susceptible (Collett 2004b; DPI 2005). Herbicide applications for grain sorghum in Australia may include pre-plant application, post-emergent application, and an application after grain harvest, depending upon the farming method (Wylie 2008). Previous investigations into the agronomy of sweet sorghum in Australia were undertaken in the 1980s by Ferraris and co-workers in Queensland (Ferraris 1981a, 1981b; Ferraris and CharlesEdwards 1986b, 1986a) and Martin and Kelleher in New South Wales (Martin and Kelleher 1984). In 1981, Ferraris reported the results of a field evaluation of sugar, grain, fibre, and leaf production from 37 sweet sorghum cultivars at Ayr in North Queensland. Of the cultivars trialled, the Rio cultivar 7

showed the most promise and for this reason was used in a study in New South Wales (Martin and Kelleher 1984). The sugar yield was 3.6 t/ha for the plant crop and 1.6 t/ha for the ratoon crop. The cultivar Italian had the highest soluble solids yields (7.6 t/ha and 4.8 t/ha for plant and ratoon crops) and FS26 cultivar had the highest grain yield (5.7 t/ha). Stalk fibre yields on a dry basis were around 9 t/ha for the higher yielding varieties. Significant variability was evident between the cultivars tested in this trial. For the plant crop, fresh stalk yield varied between 2.9 and 57.8 t/ha, fibre content varied between 9.2 and 25.7%, and sucrose content in juice varied from 0.4 to 14.9%. Soluble solids in the juice, sucrose in the juice, stalk fibre percentage, and grain yields all decreased in the ratoon crop. Ferraris (Ferraris 1981a) reported this information for 11 different varieties. The properties of the Rio cultivar measured in these trials are provided in Table 1.1. The Wray cultivar was extensively investigate in the mid–90s (Ferraris and Charles-Edwards 1986b, 1986a) because of its (alleged) superiority for sugar production. The total biomass yield from these trials only averaged 18.5 t/ha and the authors could not measure sugar production. A similar study was undertaken in New South Wales (Martin and Kelleher 1984) to assess the effect of row spacing and plant population on sugar and biomass production. Doubling the plant density increased the biomass yield and water soluble carbohydrate yield by around 20% (Table 1.1).

Table 1.1 Characteristics of the Rio sweet sorghum cultivar from crop trials in Australia Ferraris, 1981a

Martin and Kelleher, 1984

Ratoon crop

8 plants/m2

16 plants/m2

Stalk fibre content, % 12.2 10.3 Juice soluble solids, % 16.6 14.7 Juice sucrose, % 11.8 9.3 Juice purity, % 71.0 63.0 CCS 7.6 5.5 Growth period, days 145.0 79.0 Stalk height, cm 258.0 .. Stalk diameter, mm 16.0 .. Crop dry matter content, % 31.0 27 Fresh stalk yield, t/ha 47.9 29.2 Stalk dry matter yield, t/ha 12.0 7.0 Leaf dry matter yield, t/ha 1.1 0.9 Panicle dry matter yield, t/ha 3.4 0.9 Trash dry matter yield, t/ha 1.0 0.7 Total dry matter yield, t/ha 17.5 9.4 Water soluble carbohydrate, t/ha .. .. .., not available. Source: (Ferraris 1981a; Martin and Kelleher 1984).

.. .. .. .. .. 86.0 220-255.0 12.9 .. .. 7.8 1.4 2.8 .. 12.0 4.26

.. .. .. .. .. 86.0 .. 10.6 .. .. 10.0 1.9 3.0 .. 14.8 5.2

Plant crop

1.2 Sweet sorghum processing models One of the major advantages of sweet sorghum cultivation is that the major components of the crop can be used to generate multiple, value-added products: food and feed products, fuel and energy products, and other products. The generation of multiple products from biomass in an integrated processing facility is known as biorefining and there has been significant recent research in biorefinery processes (Edye et al. 2005; Pye 2005; Kim and Day 2010). Product options for sweet sorghum biorefineries are presented in Figure 1.3 and a processing model for a sweet sorghum biorefinery process is presented in Figure 1.4.

8

Grain

• Animal feed • Gluten-free flour • Alcoholic liquors

Juice

• • • • •

Ethanol Biodiesel Edible oils Chemicals and polymers Sweeteners - crystal or liquid

Fibre

• • • • • • • •

Heat and power Ethanol Bio-crude Chemicals and polymers Lignin products Biomass pellets Animal feed Dietary fibre

Figure 1.3 Typical product options for a sweet sorghum biorefinery

Figure 1.4 Processing model for sweet sorghum production (AgriFuels Pty Ltd)

9

1.3 Food and feed products 1.3.1 Crystal sugar production Sweet sorghum was used for commercial crystal sugar (sucrose) production in the USA in the 1880s, although production had effectively stopped by 1900 (Lime 1979). Further research into crystal sugar production from sweet sorghum was also undertaken in the 1960s and 1970s. The major problem associated with crystal sugar production from sweet sorghum is that the relatively high proportion of reducing sugars (glucose and fructose) in the juice inhibits crystallisation of sucrose, making production uneconomic. Limitations in sweet sorghum storage time and the high levels of starch and aconitic acid in sweet sorghum juice also hindered commercial crystal sugar production from sweet sorghum juice. The pH and temperature for efficient clarification of sweet sorghum juice are different to those of sugarcane juice and an extra clarification stage is required to improve starch removal (Smith et al. 1973). In a factory trial in 1974 (Smith et al. 1973), raw sweet sorghum juice was limed to pH 7.7–7.9, heated to 50–55 °C, and the clarified juice was then concentrated to syrup of 35 brix. Brix (°Bx) is a measure of the percentage of total soluble solids in a water solution. The syrup was then limed to pH 7.1–7.3 and heated to 60–70 °C. After further concentration of the syrup to 60–65 °Bx, aconitic acid was removed by liming to pH 8.3, heating, and precipitation with calcium chloride (Lime 1979). A summary of the composition of sweet sorghum juice in this factory trial is provided in Table 1.2. The resulting sugar crystals had a starch content of 0.003% and an aconitic acid content of 0.04%. Woods (2000) also reported on factory trials in Zimbabwe where sweet sorghum juice was blended with sugarcane juice to produce crystal sugar.

Table 1.2 Composition (%) of sweet sorghum juice and syrup Dry solids True purity

Reducing substances

Ash Starch Aconitic acid Gums

Raw juice

13.3

78.74

5.11

6.68

2.120

1.96

0.24

Clarified juice

12.6

82.47

4.96

7.08

0.210

..

0.25

Clarified semi-syrup

34.3

82.62

5.07

7.11

0.040

2.01

0.27

Finished syrup

64.3

83.42

5.26

6.43

0.014

0.55

0.24

.., not available. Source: (Smith et al. 1973)

1.3.2 Liquid sugar production Despite the challenges associated with producing a crystal sugar from sweet sorghum, it is used in many countries as a liquid sweetener or table syrup known as Jaggery (Lime 1979; Worley, Vaughan and Cundiff 1992). The typical process for producing sweet sorghum syrup involves crushing the sweet sorghum stalk to extract juice from the fibre, clarification of the juice to remove impurities, and concentration by evaporation to >75 °Bx. The addition of enzymes may be necessary to prevent gelling of the starch or crystallisation of the sucrose at high concentrations (Bitzer and Fox).

1.3.3 Gur production Indian ‘gur’ is a low purity, solid sugar product made from sugarcane molasses or sweet sorghum syrup (Ghosh 1972). Gur from sweet sorghum is produced by extracting the juice and clarifying the 10

juice to remove impurities. Water from the clarified juice is evaporated to its striking point and allowed to cool. Gur produced from sweet sorghum juice has similar moisture content to gur produced from sugarcane juice, lower sucrose content, and higher reducing sugars content.

1.3.4 Alcoholic beverage production Alcoholic beverages have been produced from sweet sorghum syrups and grains throughout Asia and Africa for centuries. In China, Baijiu liquors such as Mao Tai are distilled from fermented sorghum grains. Beer has also been produced from sweet sorghum grains (Reddy et al. 2006; Nguyen 1984; Mesta 2005).

1.3.5 Sorghum flour and baked products Sweet sorghum grain has been used for centuries to produce flour for cooking and baking in Africa and India (Peabody 2004; Nguyen 1984). Sorghum grains don’t contain gluten and baked products prepared using sorghum flour, while possessing a relatively dry or crumbly texture, are an acceptable part of gluten-free diets.

1.3.6 Animal feed products Sweet sorghum has been widely used for animal feed and forage (e.g. Nan and Ma 1989). Although there are detailed studies on the effect of forage sorghum crops on animals (e.g. Miron et al. 2007), literature relating to the effect of feeding sweet sorghum to animals is more limited (Nan and Ma 1989; Ferraris and Charles-Edwards 1986b; Agro Products 2008; Peabody 2004) for animal fodder. The relatively low lignin content and high sugar content in sweet sorghum make it particularly suitable for animal fodder and increase its palatability relative to other agricultural crops or crop residues.

1.4 Fuel and energy products 1.4.1 Ethanol Ethanol as a transportation fuel can be produced from sweet sorghum juice (Mamma et al. 1995; Wu et al. 2010), stalk (Nan and Ma 1989), stalk and grain (Nan and Ma 1989), and the whole plant (Sipos et al. 2009). Sweet sorghum has been reported to produce ethanol yields of around 3100 L/ha (Almodares and Hadi 2009; Smith and Buxton 1993) from the stalk. Nan and Ma (1989) achieved 2500–3200 kg/ha from the stalk but also fermented the grain to yield a further 600–1700 kg/ha. Modelling at QUT showed that a total ethanol yield of 8130 L/ha was achievable at moderate processing efficiencies from the sweet sorghum plant (Figure 1.5). The model assumed a total productivity of 60 t/ha of sweet sorghum consisting of 3 t/ha of grain (73% starch), 50 t/ha of stalk (15% total sugars and 15% dry fibre), and 7 t/ha of leaves (40% dry fibre).

1.4.2 Biodiesel production Sweet sorghum has also been investigated as a source of sugars for the production of oils via microbial fermentation and subsequent chemical transformation of those oils into biodiesel (Economou et al. 2010; Gao et al. 2010). Several microorganisms including heterotrophic algae and oleogenic yeasts are known to produce oils directly from sugars through fermentation processes. Such organisms can produce oils in high quantities suitable for biodiesel production or nutritional oils human consumption.

11

Figure 1.5 Theoretical ethanol yields from sweet sorghum based on QUT modelling

1.4.3 Hydrogen and methane production Research into the production of hydrogen from sweet sorghum juice and fibre has been conducted using anaerobic bacteria (Antonopoulou et al. 2008; Antonopoulou et al. 2007; Ntaikou et al. 2008; Panagiotopoulos et al. 2010; Saraphirom and Reungsang 2010; Shi et al. 2010). Methane can also be produced from anaerobic digestion of sweet sorghum residues (Stamatelatou, Dravillas and Lyberatos 2003) or as a by-product of the hydrogen production process (Antonopoulou et al. 2008; Shi et al. 2010). The methane can then be combusted to provide process energy or generate electricity.

1.4.4 Cogeneration and electricity production Sweet sorghum fibre is able to be burnt in boilers to generate heat and steam for the operation of processing equipment and to generate electricity for both the process and export to the electricity network. In Australia, electricity generated from sweet sorghum combustion may be eligible for Renewable Energy Certificates (RECs) which are generated under the Australian Government Renewable Energy Scheme which aims to deliver 20% of Australia’s electricity production by 2020. Alternatively, the sweet sorghum fibre can be gasified and used to generate heat and power for the process. Sweet sorghum fibre can also be pelleted and exported as an energy product.

12

1.5 Other products 1.5.1 Chemical products Many chemical and fuel products can be produced from sweet sorghum, either from fermentation of the juice, hydrolysis and fermentation of starch from the grain, or from the cellulosic sugars in the fibre. Lignin from sweet sorghum has potential as a source of aromatic chemicals (Beauchet, MonteilRivera and Lavoie 2012; Varanasi et al. 2013). Sweet sorghum has been investigated for use as a feedstock for lactic acid production (Hetenyi et al. 2010; Richter and Trager 1994). However, compared to other crops, such as sugarcane, corn, or cassava, little research has been conducted on production of chemicals from sweet sorghum feedstocks.

1.5.2 Fibre and textile products Unlike other agricultural fibres, there have been few reports describing the application of sweet sorghum fibre to production of paper, structural materials, and textiles. Although several references propose that sweet sorghum fibre can be used for paper production (Sipos et al. 2009; Thomas 2009) the only major published works in the research area are from Belayachi and Delmas (Belayachi and Delmas 1995, 1997).

1.5.3 Compost products As a raw material for compost, sweet sorghum bagasse is naturally low in nutrients and nitrogen (Negro et al. 1999). Negro and co-workers studied the effect of mixing sweet sorghum bagasse with other wastes including pig slurry and sewage sludge. Co-composting resulted in a product that contained sufficient nitrogen, phosphorous, and potassium to meet EU requirements.

1.6 Integration with sugarcane production The similarities in crop physiology between sweet sorghum and sugarcane suggest that there may be options for integrating sweet sorghum and sugarcane production, and using existing sugarcane harvesting, transporting, and processing infrastructure. The use of existing infrastructure potentially reduces the investment costs associated with the establishment of a sweet sorghum industry. Earlystage research on integration (such as varietal evaluation and economic modelling) is currently being undertaken in the USA, Brazil, and China (Kim and Day 2011; Li et al. 2010; Nielsen 2011). Establishment of a sweet sorghum industry at the fringes of an existing sugarcane industry, using land marginal for sugarcane production and/or integrating into fallow/break crops on land currently used for sugarcane production, presents opportunities to improve sugar processing infrastructure use, thereby improving the economic viability of the sugar industry.

1.6.1 Processing sweet sorghum in a sugar factory There are only a small number of reports describing the processing of sweet sorghum in sugarcane factories. The only reported trial in the last decade in Australia was by Webster and co-workers (2004). In other countries, two factory processing trials have been reported; Smith, Lime and coworkers, a research team at the US Department of Agriculture in the early 1970s and Woods (Woods 2000) in Zimbabwe. 1.6.1.1 Webster and co-workers Webster and co-workers (Webster et al. 2004) studied harvesting, transporting, and crushing of ~475 t (15 ha) of sweet sorghum at Mossman Central Factory, North Queensland, in November 2002. The 13

goal of the research was to produce ethanol from sweet sorghum using existing sugarcane facilities. Sweet sorghum was harvested with a conventional sugarcane harvester and transported to Mossman Factory by road for processing. The ratoon crop was harvested and processed into silage with a silage harvester. The authors investigated the effect of harvesting on the bulk density of the product. Specifically, sweet sorghum was harvested with (i) a forage harvester, (ii) a sugarcane harvester with the extraction fans operating, and (iii) a sugarcane harvester with the extraction fans not operating. In the case of the forage harvester, sorghum billets were cut to lengths of 15–20 mm, which increased the bulk density. Moisture, brix, fibre, and extraction efficiency were all measured in comparison to sugarcane crushed immediately before the trial with sweet sorghum (Table 1.3). The moisture content of both prepared sweet sorghum and sweet sorghum bagasse were very similar to sugarcane. The brix content of the prepared sweet sorghum was 8.5–9.5, compared to 18.5 for sugarcane. The fibre content of the prepared sweet sorghum was 23.7–27.6, compared to 16.0 for sugarcane, although the final bagasse had very similar fibre content. Brix extraction was 75–78% for sweet sorghum compared to 88% for sugarcane. Although efficiency of Brix extraction was lower from sweet sorghum, the authors suggested that altering the mill settings could improve extraction efficiency.

Table 1.3 Milling parameters of sweet sorghum obtained during a factory trial Harvest method

Sampling location

Forage harvester

“Fans on” sweet sorghum

“Fans off” sweet sorghum

Moisture

Brix

Fibre

Brix extraction

Prepared sorghum

63.8

8.60

27.6

na

No.5 mill

54.0

3.25

42.8

75.6

Prepared sorghum

66.8

9.60

23.7

na

No.5 mill

56.8

3.60

39.6

77.3

Prepared sorghum

65.5

18.05

16.0

na

No.5 mill

53.5

5.6

40.8

88.0

na, not applicable. Source: (Webster et al. 2004).

Samples of sweet sorghum juice were analysed during the trials (Table 1.4). The juice produced from the first mill (first expressed juice, FEJ) and second (second mill juice, SMJ) mill in the tandem were analysed for both the ‘fans-on’ and ‘fans-off’ scenarios. Sweet sorghum juice was evaporated to produce syrup of 60 °Bx without liming or clarification of the juice. Mixed juice, evaporator supply juice and syrup were also analysed.

Table 1.4 Sweet sorghum juice measurements from a factory trial pH

Brix

Temperature (° C)

Specific gravity

1

5.33

11.4

23

1.055

1

FEJ , “fans on” FEJ , “fans off”

5.35

13.0

23

1.060

2

5.43

4.7

29

1.020

2

SMJ , “fans off”

5.49

5.7

29

1.024

Mixed juice

5.42

7.8

29

1.033

Evaporator supply juice

5.40

7.9

32

1.034

Syrup

4.67

60.4

29

1.320

SMJ , “fans on”

1

2

First expressed juice. Second mill juice. Source: (Webster et al. 2004).

14

Finally, the authors conducted analyses of juice deterioration for the three types of harvested sorghum (i.e. ‘fans-on’, ‘fans-off’ and ‘forage harvested’). They found that the method which was used to harvest the sugarcane affected the decline in brix of the juice, but not the extraction efficiency. The brix content of forage-harvested sorghum juice declined rapidly after 2 hours, eliminating any other benefits of forage harvesting. 1.6.1.2 Smith, Lime and co-workers Research by Smith, Lime and co-workers focussed on the production of crystal sugar from sweet sorghum. The project goal was to supplement the crystal sugar produced from sugarcane with crystal sugar from sweet sorghum. Research commenced in 1969 with a series of studies into starch removal from sweet sorghum juice, followed by pilot-plant trials reported in 1972 and 1973 (Smith et al. 1972; Smith et al. 1973). Factory-scale crushing trials of ~40 acres of sweet sorghum occurred in 1974 (Lime 1979; Smith and Lime 1975). The key findings were (i) a ‘short-chop’ sugarcane harvester could be used for harvesting and trash removal from sweet sorghum stalks, (ii) mat formation and milling performance were satisfactory for processing through a sugarcane factory, (iii) the starch content of the sugar could be adequately reduced by a low-temperature clarification process, and (iv) the levels of aconitic acid could be reduced and the sugar crystallised. 1.6.1.3 Woods The PhD project entitled “Integrating sweet sorghum and sugarcane for bioenergy: modelling the potential for electricity and ethanol production in SE Zimbabwe” (Woods 2000) included a two year research program into the processing of sweet sorghum stalks through a Zimbabwean sugar factory. In 1998, 20 t of sweet sorghum stalks were processed through a milling train. In 1999, 202 t of sweet sorghum stalks were processed through a sugarcane diffuser. All of the sweet sorghum was manually harvested. Laboratory fermentation trials were conducted using the sweet sorghum juice. The author claimed that sweet sorghum juice was more suitable as a substrate for fermentation to ethanol by yeast than ‘C’ molasses derived from sugarcane. Woods’ reported that, while the fermentation of sweet sorghum juice would be unlikely to produce a greater ethanol yield, there were no foreseeable challenges in converting sweet sorghum juice to ethanol. Woods’ concluded that sweet sorghum could be grown to supply sugar factories with an alternative sugar crop outside of the sugarcane crushing season. The author’s main concerns about integration of sweet sorghum and sugarcane were mainly systemic or logistic, rather than technical. In particular, the author highlighted the risk-adverse nature of much of the sugar industry in Zimbabwe. Other key challenges identified were: •

a shorter ‘off-crop’ period will reduce the time available for factory maintenance



variability in seasonal sweet sorghum yield due to sweet sorghum’s shorter growing season



harvesting of sweet sorghum in the ‘off-crop’ season coincided with periods of high rainfall potentially affecting in-field equipment movements and stillage disposal



sweet sorghum is quite ‘pithy’ compared to sugarcane, potentially affecting milling performance.

It was found that manually stripping the leaves assisted in maintaining the harvested sugar levels in the sweet sorghum stalks. Woods (2000) suggested that this may be an option to improve the storage properties of sorghum stalks.

15

1.6.2 Transport costs Webster and co-workers (Webster et al. 2004) studied harvesting options for sweet sorghum to maximise the bulk density of the harvested material. Using a sugarcane harvester with the extractor fans on (as for conventional sugarcane harvesting), some stalk material was lost with the extracted trash. This occurred because sorghum billets were generally of smaller diameter and lighter than sugarcane billets. With the extractor fans off, the loss of stalk billets to the trash was minimised but trash carried over with the billets, resulting in lower bulk density in sugarcane bins. Bins of sweet sorghum billets from the forage harvester had a bulk density of 400 kg/m3, while the bins from the sugarcane harvester with the extractor fans on were around 200 kg/m3 or less with the fans off. This compared to the bulk density of 300 kg/m3 for sugarcane billets. The lower bulk density of the sugarcane harvester sweet sorghum billets would increase transport costs compared to sugarcane billets. Although the forage-harvested billets had a higher bulk density than sugarcane billets, the brix of the extracted juice declined rapidly after just 2 h for forage harvested sweet sorghum, potentially limiting the time window for crushing forage harvested billets. Another critical parameter determining the economics of sweet sorghum integration into the sugarcane cropping/processing system is the transport distance to the factory. In their analysis, Keating and co-workers (Keating et al. 2002) studied the effect of transport distance on the production costs of ethanol from sweet sorghum juice in the Australian context. In their analysis, transporting the sweet sorghum more than 50 km resulted in transport costs of greater than $0.20 /litre of ethanol, increasing the total production cost to $0.60 /litre. Using the assumptions from this study, they suggested that this restricted the economical transport distance for sweet sorghum to 50 km if it was to be used for ethanol production. In another study it was proposed that, for a green field sweet sorghum to ethanol facility, a series of smaller crude ethanol plants (producing 60% (v/v) ethanol) could be distributed through sweet sorghum farmland (Guo et al. 2010). This would reduce the transportation cost between the farm and the crude ethanol plant. The crude ethanol plants would in turn be circumferentially located around a pure ethanol plant. The economics were modelled as a function of crude ethanol capacity for a region in the Chinese context.

1.7 Economic modelling Nguyen and Prince (1996) modelled the effect of extending the sugar processing season with sweet sorghum on the economics of a juice to ethanol plant in Queensland. Using their assumptions, the cost of producing ethanol from sweet sorghum and sugarcane was 8% less than by using sugarcane alone. Gnansounou and co-workers (2005) conducted an economic analysis for the use of sweet sorghum to make combinations of ethanol, sugar, and electricity in the Chinese context. Their study included a comprehensive sensitivity analysis. Four cases were presented: using the juice to make sugar or ethanol and the bagasse to make ethanol or produce electricity. Their conclusion was that, in the Chinese context, the juice was better used for sugar production than for ethanol and that the bagasse was better used for ethanol production than for generation of electricity.

1.8 Conclusion This chapter has reviewed the key literature associated with research on the cultivation of sweet sorghum in Australia, and the production of food, feed, and energy products from the harvestable components. The literature has shown that sweet sorghum has significant potential as a multi-product bioenergy crop, but that only limited research work has been undertaken on potential applications in Australia.

16

The following chapters of this report will investigate these opportunities by further assessing sweet sorghum field trials, the production of food, feed, and energy products, and report on technoeconomic and life cycle modelling. The potential opportunities and challenges associated with integration of sweet sorghum and sugarcane processing in Australia will also be investigated, and preliminary integration experiments will be described.

17

Chapter 2: Sweet sorghum field trials B. Ellet, M.D. Harrison, and I.M. O’Hara

2.1 Introduction Sweet sorghum field trials were undertaken to compare the agronomic properties and productivity of sweet sorghum varieties in the Childers region of south-east Queensland (Table 2.1). Sweet sorghum variety Rcv27751 was imported by AgriFuels Ltd with the approval of the Australian Quarantine and Immigration Services and was used in all field trials, either alone or with other globally-recognised, commercially-available sweet sorghum varieties. Two field trials (2007–08 and 2009–10) were undertaken by AgriFuels Ltd prior to the present project. Further sweet sorghum field trials in collaboration with AgriFuels Ltd were undertaken during the three years of this RIRDC project in 2010–11, 2012, and 2013. Given their direct relevance, the results of the field trials undertaken prior to and during the RIRDC project are presented in the following section.

Table 2.1 Summary of sweet sorghum field trials Pre-RIRDC project

RIRDC project

2007-08

2009-10

2010-11

Planted

3 October 2007

17 September 2009

17 December 2010 17 May 2012

Harvest

22 January 2008

13 February 2010

26 May 2011

4 December 2012 May 2013

Cultivars

Rcv27751

Rcv27751

Rcv27751

Rcv27751

Keller

Keller

Top-76-6

Top-76-6

Dale

Dale

M81-E

M81-E

2012

20131 17 January 2013

Rcv27751

Rio Wray Italian 1

The 2013 field trial compared the ratoon crop from the 2012 field trial and a freshly planted crop of the same variety

2.2 Pre-project field trials 2.2.1 Field trial (2007–08) The field trial was planted in the Red Ridge region of Childers, Queensland (25º10' S, 152º20' E, elevation 70 m). The soil at the field trial site was volcanic, red clay loam. Prior to the field trial, the Red Ridge site had been used for sugarcane cultivation (2003–06) and subsequently fallowed for 18 months. The trial was conducted over the spring and summer seasons of 2007–08 using the AgriFuels variety Rcv27751. Photographs of the field trial are presented in Figure 2.1. The key objectives of the trial were the monitoring of crop growth and measurement of the sugar content in the leaves and stalk at maturity. 18

Following a pre-planting fertiliser program, the plots were planted using a three row Earthway precision seeder, a common sorghum grain air seeder, on 3 October 2007. Dynamic Lifter chicken pellets were broadcast post-planting. Conditions at planting were mild and dry. The majority of seedlings emerged within 7–12 days (Figure 2.1a). Where the press wheel created a firm bed and improved seed contact with soil, the plants emerged more quickly and showed vigorous early growth. Irrigation water (30 mm) was applied immediately after planting and irrigation intervals and water volumes were dictated by daily evapotranspiration rates and crop growth. Maximum crop irrigation was ~7 mm/day and the total amount of irrigation water used was ~2.2 ML/ha. Major weeds included broadleaf weeds wild turnip (Brassica tournefortii) and pig weed (Portulaca Sp.) and the grass species crows foot (Eleusine indica), african love grass (Eragrostis curvula), and nut grass (Cyperus rotundus L.). Herbicides were used in the early vegetative period to control weeds. There was some evidence of phytotoxicity in sweet sorghum one week after application. However, after three weeks any evidence of phytotoxicity in sweet sorghum had disappeared and all the weeds were destroyed. Once the sweet sorghum canopy closed, the weeds had minimal effect on the plot.

Figure 2.1 2007-08 sweet sorghum crop trial (a) 19 October 2007. (b) 9 November 2007. (c) 20 December 2007). (d) 22 January 2008.

There was no evidence of sweet sorghum diseases during the field trial. Heliothis catapillar (Helicoverpa armigera) and green vegetable bugs (Nezara viridula) were the dominant species that colonised sweet sorghum as the weather became warmer and more humid. They were consistent pests over the warmer months and regular control with insecticide (Lorsban™) was required.

19

Sweet sorghum reached a maximum height and canopy area ~80 days after planting (Figure 2.1d). The maximum height was 2.38 m including the height of the grain head. As the grain head matured, the bottom leaves on the sweet sorghum went brown (senesced). Ten plants were harvested on 22 January 2008 and the weight of the grain heads and sugar content of the leaves and stalk were measured. The average total soluble solids content in leaves was 8.0 °Bx and the average sugar content in the stalk was 8.6 °Bx. The average weight of the grain heads was 133.4 g. It was established that the success of sweet sorghum plantings in this region of Australia would be improved through careful management of key agronomic factors. Key factors identified were: • good contact with the soil to allow for maximum germination and root growth • regular nutritional monitoring and control of the fertiliser program to maintain optimum nutrient balance while ensuring key nutrients are in good supply at critical growth stages • soil moisture monitoring and water management to allow plant stress to be manipulated. Plants with insufficient water reach their reproductive stage too early and accumulate sugar prematurely while over-irrigation towards harvesting significantly lowers sugar accumulation.

2.2.2 Field trial (2009–10) This field trial and all subsequent field trials were undertaken on AgriFuels Ltd property at Kevin Livingston Drive, Isis Central, Queensland (25º12' S, 152º11' E, elevation 135m). Like the previous field trial site at Red Ridge, the soil at the field trial site was a volcanic, red clay loam. Prior to the start of the field trial, the site had been used for tomato cultivation. The objectives of the 2009-10 field trials were to test mechanical planting of heat-treated Rcv27751 seed using a modified carrot seeder, compare growth of four popular USA varieties (M81-E, Keller, Top-76-6, and Dale) and AgriFuels Ltd variety Rcv27751, and generate seed stock for later trials. A carrot seed planter was used to plant the sweet sorghum seed on 17 September 2009 (Figure 2.2). Seed was planted in 380 mm spaced rows with seeds planted 100 mm apart. The crops were monitored at regular intervals during growth.

Figure 2.2 Seed planting during the 2009–10 field trial

20

During the period immediately following planting, the crop site was very dry and daily temperatures were high. Sugarcane adjacent to the field trial site showed significant drought stress. These conditions had a noticeable effect on germination and reduced overall biomass growth rates. M81-E is regarded globally as one of the leading sweet sorghum varieties for syrup production. However, at 78 days after planting, M81-E had the lowest biomass and poorest germination rate of the four USA varieties. In contrast, Keller produced the highest biomass of the four USA varieties and had the most uniform growth, as well as producing strong stalks of consistent thickness. Unfortunately, no Rcv27751 seeds germinated. Subsequent testing determined that the method used to heat treat and store the seeds had rendered them sterile. All four USA varieties were flowering by 90 days after planting, with Top-76-6 being the first to commence flowering (late November) and all USA varieties had rapidly grown (Figure 1.2) by 132 days after planting. Despite a poor start, the M81-E variety produced stalks of comparable thickness to sugarcane and ~30% thicker than the stalks from the Keller variety (Figure 2.3).Grain head size and shape from the M81-E, Dale, and Top-76-6 varieties were similar, but Keller produced lighter grain heads with smaller seeds. The average total soluble solids in the juice from Keller, M81-E, Top-76-6, and Dale were 15.5%, 14.2%, 13.4%, and 15.3%, respectively.

Figure 2.3 The stalk from an M81-E plant

All the plants were harvested on 13 February 2010, by chopping the plants with a sugarcane knife at ground level, and left to ratoon. Conditions were very dry and high temperatures (35–40 °C) were consistently recorded at the field trial site. The percentage of Keller, M81-E, Top-76-6, and Dale plants that successfully ratooned under these conditions were 30%, 50%, 95%, and 95%, respectively.

21

2.3 Multi-cultivar field trial (2010–11) 2.3.1 Trial overview and objectives The 2010–11 field trials were the first undertaken within the RIRDC project and the aims of the trials were to: •

compare the agronomic performance and productivity of eight sweet sorghum varieties in the Childers region



assess the stalk soluble solids contents



generate additional seed for subsequent trials.

Heavy rain throughout the Childers region of Queensland in late 2010 delayed planting of the trial crops, which had been originally scheduled to occur during October 2010. The field trials were eventually planted in December 2010, and progressively harvested to assess stalk weight and juice composition from March–May 2011. Varieties assessed during the trial were Top 76-6, Dale, Keller, M81-E, Rio, Wray, Italian, and AgriFuels variety AFL rcv27751. There were three additional plantings of AFL rcv27751 between December and March 2011 to provide information about optimum conditions for crop establishment. Heavy rain associate with major flooding throughout Queensland occurred several days after planting of the seed. Water runoff washed out a small amount of seed in some areas of the paddock, although the majority of the seeds remained intact.

2.3.2 Crop management Diammonium phosphate was broadcast at a rate of 250 kg/ha and incorporated into the soil with a rotary hoe two weeks prior to planting. The first planting was carried out on 17 December 2010 using a single row, hand-operated, small plot seeder with one seed planted every 15 cm. There was poor emergence of seedlings from the trial with low seedling populations and erratic emergence. It was likely that heavy and persistent rain immediately after planting and/or poor quality seed were the cause of this poor germination. Full seedling emergence was delayed until nearly ten days after planting. AFL Rcv27751 resulted in the most consistent germination. The second planting of two rows of AFL Rcv27751 occurred on 1 February 2011. The seed bed was rough and seed contact with soil was less than desirable. Further, rain did not fall until eight days after planting, resulting in sub-optimal conditions for crop establishment. The third planting of four rows of AFL Rcv27751 occurred on 18 March 2011. The soil was freshly rotary hoed following 50 mm of rain and the planter was set to plant at a depth of 40 mm. This trial was a winter trial to test grain head production with limited biomass growth. The fourth planting of eight rows of AFL Rcv27751 occurred on 31 March 2011. In this case, soil moisture was optimal for planting. The field data (average plant height, stalk diameter, and number of leaves) from the first planting are presented in Table 2.2. The best seed yields were obtained from the AFL Rcv27751, Rio, and M81-E varieties.

22

Table 2.2 Field data from the first planting of the 2010–11 sweet sorghum field trials 6 Jan

18 Jan

1 Feb

Days after planting

20

32

46

Rcv27751 Italian Wray Rio M81-E Keller Dale Top76-6

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

0.5 0.5 0.4 0.4 0.4 0.5 0.35 0.5

Rcv27751 Italian Wray Rio M81-E Keller Dale Top 76-6

5 4 4 5 3 5 4 4

15 6 12 14 13 12 12 14

3 3 3 3 3 3 3 3

8 7 8 8 7 8 7 8

Sample date

Rcv27751 Italian Wray Rio M81-E Keller Dale Top 76-6 .., not available.

17 Feb

3 Mar

18 Mar

31 Mar

12 Apr

27 Apr

10 May

24 May

62 76 91 Plant height (m) 1.1 1.8 2.2 2.3 0.7 0.7 0.7 0.8 0.8 1.3 2.1 2.6 1.2 1.8 2.7 2.4 0.8 1.5 2.1 2.5 1.2 1.4 1.8 2.4 0.7 1.3 2.1 2.2 0.8 1.4 1.8 2.5 Stalk diameter (mm) 23 28 36 32 8 9 9 9 19 20 28 21 17 18 21 19 19 23 30 30 17 19 20 20 17 19 28 26 18 20 40 40 Number of leaves 9 9 11 12 7 7 5 8 9 10 12 10 9 11 12 11 6 12 13 13 9 11 9 11 4 9 11 10 6 11 12 16

104

116

131

144

158

2.5 .. 2.5 2.8 2.8 2.7 2.8 2.6

2.7 .. 2.8 2.9 3.3 2.2 3.2 2.8

2.9 .. 2.9 3.2 3.2 2.3 3.3 3

2.9 .. 2.9 3.2 3.4 2.5 3.4 3.3

2.6 .. 3.2 2.9 3.4 2.7 3.1 3.2

45 .. 25 19 37 25 25 40

34 .. 26 24 28 18 22 35

38 .. 25 21 22 18 28 30

32 .. 28 23 26 21 25 32

30 .. 29 21 32 25 28 32

16 .. 11 10 15 8 10 15

11 .. 8 9 9 7 11 13

10 .. 8 11 8 6 12 13

14 .. 10 11 15 10 9 14

14 .. 10 11 15 10 9 4

2.3.3 Analysis of sweet sorghum plants Sampling of plants to assess sweet sorghum maturity (Figure 2.4) commenced on 18 March 2011 and continued every two weeks until 26 May 2011. For each test, six stalks from each variety were cut, stripped of their leaves, tied, tagged, and shipped overnight to QUT in Brisbane. At QUT, grain heads were removed from the stalks, the stalks were weighed, and the length and diameter of each stalk (top, middle, and bottom) were measured. Stalks were then fibrated in a Jeffco cutter grinder. Juice was extracted from the fibre in a hydraulic press at a pressure of 6 MPa and the masses of bagasse and juice were determined. Sweet sorghum juice samples were analysed for sucrose, glucose, and fructose content using high performance liquid chromatography (HPLC). A Shodex SP 810 carbohydrate column was used to resolve the sugars with water as the mobile phase and a refractive index detector was used to identify the sugars as they eluted from the column. Samples were also analysed for dissolved solids (Brix) using a Bellingham Stanley precision digital refractometer.

23

Figure 2.4 Varieties under trial at maturity

The Italian variety exhibited very poor growth characteristics and produced relatively short plants (~16 °C) for good germination. Plant crops mature in ~4–4.5 months from planting and ratoon crops mature in ~3–3.5 months from harvesting of the plant crop. Sweet sorghum can be grown in areas marginal for sugarcane production due to the reduced water requirements of the crop or used as a break crop during sugarcane fallow. Six planting and harvesting scenarios for sweet sorghum integration with sugarcane processing are presented in Table 7.1.

Table 7.1 Possible sweet sorghum cropping cycles for integration with sugarcane processing Planting

Harvesting

Comments

Dec–Jan

Apr–May

This period is suitable for harvesting and processing sweet sorghum prior to the start of the sugarcane crushing season.

Jun–Aug

This period is generally unsuitable for sweet sorghum processing as sugarcane infrastructure is being used for sugarcane harvesting and sugar production. Planting in February may be possible if sugarcane harvesting is due to commence later than June.

May–Jul

Sep–Nov

This period is unsuitable for sweet sorghum processing as sugarcane infrastructure is being used for sugarcane harvesting and sugar production. In addition, soil temperatures are likely to be too low for good sweet sorghum germination in most areas.

Aug–Sep

Dec–Jan

This period allows harvesting at the completion of the crushing season, however in some areas soil temperatures may be too low for good sweet sorghum germination.

Feb–Mar

This period may be suitable for sweet sorghum production as harvesting is outside the sugarcane crushing season and soil temperatures should be suitable for good germination. Maintenance requirements in the sugarcane factory may limit factory availability through this period.

Feb–Apr

Oct–Nov

From this analysis, it appears that the best cropping cycle for integration of sweet sorghum into the sugarcane industry requires planting in December/January to allow harvesting just prior to the sugarcane processing season (Table 7.1). In some areas, a longer processing period is possible but will depend on adequate soil temperatures and factory maintenance requirements. Unlike sugarcane, which can be effectively harvested over an extended period of several months, the effective harvesting period for sweet sorghum is very narrow (3–4 weeks). For this reason, a coordinated approach to planting is required to achieve continuous supply of sweet sorghum with optimal grain filling and sugar content in stalk juice. Such a coordinated program will require resources for scheduling planting of sweet sorghum seed, followed by a scheduled and coordinated harvesting and transport system.

127

7.5 Harvesting sweet sorghum for processing in sugar factories Options for harvesting of sweet sorghum stalk include the use of sugarcane or forage harvesters. As sweet sorghum processing occurs outside of the sugarcane harvesting season, sugarcane harvesters would be potentially available for sweet sorghum harvesting depending upon the equipment owners’ participation in this activity. This approach will use harvesting equipment across a period when it is normally idle and increasing the use of harvesting equipment. Harvesting of sweet sorghum grain separately for the stalk (where desired) is likely to require specialised equipment to enable effective harvesting at the height of the sweet sorghum head. Alternatively, sweet sorghum grain heads can be harvested with the stalks using a conventional sugarcane harvester and transported together with the stalk to the sugarcane factory for processing. Because the crushing rate of sweet sorghum is only ~75% of the crushing rate of sugarcane for a constant fibre processing rate, the harvesting rate of sweet sorghum is only required to be 75% that if sugarcane. Assuming constant harvester throughput, less harvesters are required for sweet sorghum harvesting to ensure continuous supply to the factory. If the sweet sorghum farm has short rows, long hauls to the siding, it poor crop yields and harvester throughput cannot be maintained, then additional harvesters will be required to maintain a continuous supply of sweet sorghum to the factory. In-field transport of the sweet sorghum can also be carried out using existing sugarcane transport equipment (Figure 7.2). No changes are likely to be required to these transport systems to allow them to handle sweet sorghum.

Figure 7.2 Sugarcane being harvested using a harvester and in-field transport

128

7.6 Transporting sweet sorghum to existing sugar factories The cost-effectiveness of sweet sorghum transport will depend upon how effectively the sweet sorghum cultivation areas are able to be serviced with existing transport infrastructure. New cultivation areas more distant from existing sugarcane areas are unlikely to be readily supported by existing rail infrastructure (Figure 7.3) but may be able to be adequately serviced with road transportation, either to the factory or to railway sidings. Sweet sorghum grown in existing sugarcane areas as a break crop may be more readily by existing rail systems. The cost-effectiveness of this will depend upon designing harvesting and transport schedules that allow efficient collection and transport runs.

Figure 7.3 Sugarcane transported to a sugarcane factory by train

The integration of an existing road transport system into rail systems has been recently examined (Kent 2013). In the scenarios examined, one or more rail sidings at convenient locations would be used as depots for road transport operations. Trucks transport empty bins to remote “pads” or hardstands near the new sweet sorghum farm areas, where they will be filled by harvesters and returned full to the pads (Figure 7.4). This would require additional trucks and trailers if no existing road transport of sugarcane is available. An alternative approach would be to use a road transport fleet (Figure 7.5) to pick up the sweet sorghum from “pads” or hardstands adjacent to the field and deliver it directly to the sugar factory. Road bins typically hold ~20 t of sweet sorghum and a transport fleet of prime movers with specially adapted trailers would be required. At the factory, the bins will be emptied directly into the sugarcane tip and empty bins returned to the fields for re-filling. Such a system may require significant modifications at the sugar factory to allow the road bins to be weighed and tipped. A road weighbridge is also likely to be required.

129

Figure 7.4 Option for a sweet sorghum transport system using existing railway infrastructure

Figure 7.5 Option for a sweet sorghum road transport system

The cost of transporting raw materials for processing is a significant proportion of the total cost of processing. Therefore, the costs associated with either a new transport system for sweet sorghum or adaptation of existing systems will need to be carefully examined as this will have a major effect on the overall process economics.

7.7 Extraction of juice from sweet sorghum Extraction of juice from sugarcane has been described above (Section 7.2). The milling properties of sweet sorghum produced in our field trials (Chapter 2) have been evaluated at the bench-scale using equipment previously used for sugarcane milling research and this is reported in the following chapter (Chapter 8). Continuous operation of the extraction plant is required for optimal efficiency. Each stop 130

and restart of the extraction process results in a decrease in efficiency and higher sugar losses. As described above (Section 7.5), coordination will be required to maintain sufficient supply of sweet sorghum to the sugar factory for continuous operation of the extraction plant. If insufficient sweet sorghum is available, then it may be preferable to operate continuously for a portion of every 24 hour period or for a continuous period for a portion of every week. Such a mode of operation will be more expensive but enables effective processing where full supply of sweet sorghum is not available. Once sweet sorghum has been harvested it must be processed as quickly as possible. If sweet sorghum cannot be processed within 16 h of harvest, then significant deterioration of the sugars will occur through microbial activity. The water added to the extraction station (Figure 7.6) is typically applied at a rate proportional to the fibre rate to ensure that high levels of sugar extraction are maintained. Using this assumption, the same amount of water will be required for sweet sorghum as is required for sugarcane at the same fibre rate although the actual crushing rate of sweet sorghum is likely to be ~75% of the sugarcane crushing rate.

Figure 7.6 Roller mills in a sugarcane factory

As previously shown in this report, sweet sorghum has a similar composition to sugarcane bagasse. Sweet sorghum bagasse is likely to be able to be combusted in the boiler station to generate steam and electricity with no modifications to the bagasse handling and boiler equipment.

131

7.8 Analysis of sweet sorghum juice and fibre Sugarcane payment is based on analysis of the two main components of sugarcane, fibre and juice. Each sample of first expressed juice is analysed for its sugar content in an on-site laboratory. Prepared sugarcane is also sampled and analysed for fibre content. Fibre analysis is time-consuming and without on-line analytical methods (discussed below) it is not possible to sample and analyse each delivery. An important consideration when analysing samples for payment is ensuring the integrity of samples taken from the process. This is achieved using a sample tracking system that monitors the flow of material from the sugarcane tip to where the samples of juice and prepared sugarcane are taken. The collection of samples for sweet sorghum payment can occur using the same techniques. Unlike sugarcane, which has very low levels of reducing sugars (glucose and fructose), sweet sorghum has higher levels of these sugars which will affect the analytical technique currently used for juice analysis (polarimetry). Instead, it would be preferable for each sample to be paid for using a formula that takes into account total fermentable sugars (glucose, sucrose, and fructose) in the juice and fibre components of the stalk. The analysis of total fermentable sugars can be carried out by high pressure liquid chromatography (HPLC). However, each analysis takes 10–15 min. Multiple HPLCs may be required to process the number of samples required per hour. Alternatively, near-infrared (NIR) spectroscopy, either on-line or laboratory-based, may be used for analysis of these components in sweet sorghum juice. NIR is currently used in the Australian sugar industry for sugarcane analysis and payment (Staunton et al. 1999). NIR instruments scan the sugarcane as it enters the first roller mill in the extraction station. NIR correlation equations are in use for sugarcane payment and are used in many factories throughout the Australian industry. While NIR is a much more rapid analytical technique than HPLC, it is necessary to maintain laboratory capability to undertake HPLC analysis on a subset of the samples delivered to the factory. The laboratory analysis of the subsets of samples is then used to maintain NIR calibration on a daily basis. Fibre analysis of sweet sorghum prepared cane can be carried out using existing sugarcane fibre machines. These analyses take ~45 min per sample. NIR calibrations for measurement of fibre in sugarcane bagasse should be able to be readily modified for application to sweet sorghum bagasse or new calibrations for sweet sorghum fibre developed.

7.9 Clarification and evaporation of sweet sorghum juice In a sugarcane factory, sugarcane juice is processed to remove insoluble impurities and as many of the soluble impurities as possible. In this process, juice is heated to >100 °C and solids are separated from the juice through settling in a clarifier. The solid impurities sink to the bottom of the clarifier and are removed as mud. The mud is washed to recover any residual sugar and dewatered via filtration (Figure 7.7) prior to recycling of the mud to sugarcane fields The clear juice from the clarifier is then concentrated in a multiple-effect evaporator system from a concentration of 15 °Bx to 66–70 °Bx. Few changes will be required to process sweet sorghum juice using existing heaters, clarifier, filters, and evaporators in sugar factories; however, if the flow rates of the streams are considerably different to sugarcane juice processing operations there may be effects on heater and evaporator scaling rates. Concentrated sweet sorghum syrup would be stored for use within the ethanol distillery, rather than for production of crystal sugar. Sweet sorghum syrup will be used alone or blended with molasses from the sugar factory as a feedstock for production of bioethanol.

132

Figure 7.7 Washing sugarcane juice from mud using a rotary drum vacuum filter

The optimum factory operating conditions for crushing sweet sorghum and processing sweet sorghum juice are yet to be determined and there may be requirements for further research in the following areas: • • • • •

effects of sweet sorghum processing on heater and evaporator scaling optimum clarification conditions clarification additives for sweet sorghum processing (e.g. flocculent type and additive rate) mud processing (porosity, retention, pH control, and handling) optimum sugar concentration in sweet sorghum syrup for safe storage.

7.10 Other effects One of the major changes likely to arise from integration of sweet sorghum processing in a sugar factory will be the reduced length of the period available for factory maintenance. The typical maintenance period for a sugarcane factory is between December and June, enabling the existing workforce to carry out most of the maintenance activities. The shortening of this maintenance period (e.g. from December to April) may require a different approach to maintenance activities including greater use of contract maintenance labour and different process materials to reduce maintenance requirements The reduction in time available for maintenance will be offset by the increased usage of process equipment. An additional operating period of 2 months will bring the total operational period for the 133

sugar factory from 5–6 months per year to 7–8 months per year. This will allow skilled operators to carry out their core activities for a greater proportion of the year.

7.11 Conclusions This chapter has explored the key considerations relating to the potential integration of sweet sorghum and sugarcane processing using existing sugarcane processing infrastructure. There are potential effects across the harvesting, transport, and processing sectors and changes to existing practices will be required for efficient integration. However, there are significant opportunities to reduce the cost of start-up for a sweet sorghum industry and to better use sugarcane processing infrastructure to improve the profitability of the sugarcane growing, harvesting, and milling sectors. In this project, an economic analysis of these opportunities has not been undertaken as there are many potential scenarios and the economic cost and opportunities of these scenarios are very dependent upon regional effects and the installed capital available in existing sugar factories. The next chapter reports on research that was undertaken to assess in more detail the particular effects of sweet sorghum processing on the sugarcane extraction system and will inform future factory trials for the processing of sweet sorghum in sugarcane factories.

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Chapter 8: Milling properties of sweet sorghum N.J. McKenzie, G.A. Kent, W. Stolz, and I.M. O’Hara

8.1 Introduction Establishing a sweet sorghum industry in Australia will be more rapid and cost-effective if sweet sorghum can be processed in existing sugarcane factories. A broad overview of issues surrounding sweet sorghum production and processing into the Australian sugarcane industry was presented in Chapter 7. In the present chapter, we focused specifically on the milling properties of sweet sorghum and describe the results of milling trials undertaken at the QUT Pilot Plant Precinct, Brisbane. The extraction of juice from sugarcane in Australian sugar factories takes place in two stages: • The harvested material falls into a shredder where rotating hammers with hardened tips smash the sugarcane to break open the cells. • The fibrous material is compressed under high pressure in a six-roller mill (Figure 8.1) to release the juice. Extra water is added and the fibrous material is washed (a process called imbibition) and squeezed through three to five more six-roller mills.

Figure 8.1 Schematic illustration of a typical Australian sugar industry six-roller mill

135

8.2 Sample harvesting, delivery, and preparation Sweet sorghum cultivar AFL Rcv27751 (~100 kg) was hand cut at ground level on 8 and 9 December 2012, and delivered to QUT on 10 December 2012. Three samples were prepared: stalk only (leaves and grain heads removed), stalk and leaf (grain heads removed), and whole plant. The sweet sorghum plants appeared to be in good health and had an average stalk diameter from 25–35 mm (Figure 8.2). Sweet sorghum was cut into billets ~ 250 mm long with a drop saw and any attached leaf or grain was also cut to length and included in the appropriate samples.

Figure 8.2 Sweet sorghum. (a) Sweet sorghum as delivered. (b) Billeted sweet sorghum with stalk and leaf.

Samples for juice extraction were shredded in a hammer mill at 1390 r/min for 30 s to simulate disruption in a shredder in an Australian sugar factory. The shredded sorghum is called prepared sorghum in this report in a similar way to the shredded sugarcane being called prepared sugarcane.

8.3 Characterisation of sweet sorghum 8.3.1. Analysis of the moisture content in prepared sweet sorghum The moisture content in stalk only, stalk and leaf, and whole plant samples were determined (BSES Method 7 – Moisture – Determination in bagasse and cane by drying). Samples (1 kg) were dried to a constant mass in an oven at 105 °C. The results showed that stalk only and stalk and leaf samples had similar moisture contents (79.97% and 77.2%, respectively), while whole plant samples had a significantly lower moisture content (65.8%).

8.3.2. Analysis of the disintegrator extract of prepared sweet sorghum Prepared sweet sorghum samples (2 kg) were analysed for total sucrose, glucose, fructose, and total soluble solids using the wet disintegrator method. The samples were mixed with 6 L of water and disintegrated in a high-speed blender for 40 min. The resulting extracts were analysed for sugar content and composition. Sucrose, glucose, and fructose concentrations were measured using HPLC 136

and a refractive index detector. Total soluble solids (brix) were determined using a precision refractometer and a sugarcane industry method (BSES Method 17 – Brix (total solids) – Determination in sugar solutions by Abbe refractometer). The results of these analyses are presented in Table 8.1.

Table 8.1 Analysis of disintegrator extract from prepared sweet sorghum Sucrose (%)

Glucose (%)

Fructose (%)

Brix (°Bx)

Moisture (%)

Fibre (%)

Stalk only



1.42

1.65

6.47

79.67

13.86

Stalk and leaf



1.33

1.48

7.07

77.15

15.78



0.73

0.99

6.65

65.82

27.53

Sample

Whole plant –, zero.

8.3.3. Analysis of the fibre content in sweet sorghum The fibre content of prepared sorghum samples was determined using BSES Method 4A – Fibre – Determination in cane by SRI can fibre machine. The results of these analyses are presented in Table 8.2. Samples containing the grain heads could not be analysed because the fine particles produced from the grain blocked the filter gauze, reduced flow rates, and pressured the sample container. Therefore the fibre content for this sample was determined by difference. The fibre content by difference is included for the other samples for comparison. The results showed that the fibre contents of the stalk only and stalk and leaf samples were similar to that if sugarcane, with the inclusion of the leaf raising fibre content by ~2%. The sample containing grain heads (whole plant) had ~double the fibre content of stalk only and stalk and leaf samples.

Table 8.2 Fibre content in sweet sorghum samples determined by two different methods Sample

Fibre (%) - cane fibre machine method

Fibre (%) - difference method

Stalk only

14.3

13.9

Stalk and leaf

16.6

15.8

Whole plant .. not available.

..

27.5

8.4 Assessment of the milling characteristics of sweet sorghum 8.4.1 Measuring cell breakage The integration of sweet sorghum processing into a sugarcane factory requires that sweet sorghum is efficiently disrupted in the milling station. As described in section 8.2, sweet sorghum was shredded by hammer milling to simulate the disruption process in a raw sugar factory shredder. Visually, the prepared sorghum looked very similar to commercially-shredded sugarcane (Figure 8.3).

137

Figure 8.3 Sweet sorghum fibre prepared from leaf and stalk

The conventional measurement of cell disruption during sugarcane processing is the pol in open cells. Pol is the percentage of sucrose in liquid. Pol in open cells is the ratio between the pol that can be easily washed out of sugarcane and the total pol in the sugarcane. Given that there was no soluble sucrose detected in the prepared sweet sorghum (Table 8.1), it was not possible to determine pol in open cells. Therefore, total soluble solids (°Bx) in open cells and total sugars in open cells were used as substitute methods to assess efficiency of cell breakage. Total soluble solids (°Bx) in open cells (BSES 2001) and total sugars (the sum of the sucrose, glucose, and fructose measured by HPLC) in open cells were determined and the results are presented in Figure 8.4. The °Bx in open cells for the stalk only and stalk and leaf sweet sorghum samples are the same, and similar to those values expected for sugarcane (~90%). Cell disruption measured using total soluble solids was significantly lower for whole plant sweet sorghum samples, suggesting that the grain head is absorbing some of the energy from the hammers and reducing the breakage of cells containing sugar (stalk and leaf). Alternatively, it may be simply that the amount of sugar easily available is reduced as a result of the increased fibre content due to the presence of the grain heads. The total sugars measure of open cells was similar to the brix measure, except that the measurement for stalk only was lower.

138

Figure 8.4 Soluble sugars extraction from sweet sorghum

8.4.2 Roller mill feeding performance of sweet sorghum The behaviour of the prepared sugarcane in the roller mills of the extraction station in a sugarcane factory dictates the processing rate (t/h) for the entire factory. The density of prepared sugarcane at the entrance to the first pair of rollers (Figure 8.1) in the first roller mill in the milling train is the key parameter that determined this behaviour. This density is usually defined in terms of compaction, which is the ratio of the mass of fibre to the total volume (in contrast to density which is defined as the ratio of total mass to total volume). To estimate the mill feeding performance, compression behaviour of the prepared sweet sorghum was measured. In a low pressure experiment, two 6 kg samples from each batch of prepared sweet sorghum were compressed to a pressure of 50 kPa. Since the pressure acting on the material at the base of the feed chute in a sugar factory is typically 3 kPa, only the compression up to a pressure of 10 kPa was analysed. Prepared sugarcane samples from several sugarcane districts are presented as controls and the results are presented in Figure 8.5a. The sweet sorghum stalk only and stalk and leaf samples achieved compactions similar to those of sugarcane over the 10 kPa pressure range, indicating that these samples should mill at similar milling speeds to sugarcane for the same fibre rate. The whole plant showed higher compactions but were still within the range of compactions measured for sugarcane. Higher compaction results in a lower mill speed to process a given fibre rate.

139

Figure 8.5 Pressure compression testing of sweet sorghum and sugarcane at (a) low and (b) high pressure

8.4.3 Crushing performance of sweet sorghum The extraction performance of a six roller mill in a mill train depends largely on the compaction that can be achieved in the nip of the final pair of rollers (Figure 8.1). Since the torque on a roller mill is controlled at a constant level, the achievable compaction varies with compression characteristics of the material passing through the roller. To gain some insight into the achievable compaction for sweet sorghum at this position in the roller mill, compression tests at high pressures were conducted. Figure 8.5b presents the results of compression tests with each of the three sample types. All three types of sweet sorghum samples achieved higher compactions than those published for sugarcane, indicating that sweet sorghum can potentially achieve similar or lower bagasse moisture contents. Sweet sorghum stalk only and stalk and leaf samples had similar compaction responses; however, while higher than that of sugarcane, the whole plant sample compactions were markedly different. It is likely that the difference between the response curves for sweet sorghum arise from the blockage of pore spaces between larger fibrous particles by the fine particles in the grain heads. Further testing is required to examine the processing of sweet sorghum samples containing grain.

8.5 Analysis of first expressed juice from sweet sorghum The crushing performance tests described in Section 8.4.3 resulted in the expression of first-expressed sweet sorghum juice (FEJ), again using similar terminology to that used for sugarcane. The sorghum material remaining after the juice was expressed is termed bagasse, again using the sugarcane analogy. The juice and bagasse from the tests were collected separately. The masses of juice collected, bagasse generated, and the mass balances are presented in Table 8.3.

140

Table 8.3 Mass balances for sweet sorghum juice extraction Sample no.

Initial mass (g)

Mass FEJ (g)

Mass of bagasse (g)

Juice extracted (%)

Total % of original sample collected

Stalk only

1

2000

1298

649

75

97

2

2000

1256

692

73

97

Stalk and leaf

3

2000

1148

795

68

97

4

2000

1171

793

70

97

Whole plant

5

2000

669

1286

46

98

6

2000

617

1341

43

98

Sample

Significantly less juice was extracted from samples containing whole plant (stalk, leaf, and grain). Approximately 70% of the juice in the stalk only and stalk and leaf samples were extracted while only 45% of the juice was extracted from the whole plant samples. It should be noted that samples containing grain (whole plant) had approximately twice the fibre content of the other samples. The FEJ was analysed for sugar content and composition. Sucrose, glucose, and fructose concentrations were measured using HPLC and a refractive index detector. Total soluble solids (brix, °Bx) were determined using a precision refractometer and a sugarcane industry method (BSES Method 17 – Brix (total solids) – Determination in sugar solutions by Abbe refractometer). The results of these analyses are presented in Table 8.4.

Table 8.4 Sugar content and composition in sweet sorghum first-expressed juice Sample no.

Sucrose (%)

Glucose (%)

Fructose (%)

Total sugars (%)

Brix (°Bx)

Stalk only

1

-

2.28

2.04

4.32

7.70

2

-

1.35

2.03

3.38

7.37

Stalk and leaf

3

-

0.75

1.66

2.41

7.55

4

-

1.57

1.81

3.38

8.21

5

-

1.02

1.78

2.8

9.20

6

-

0.46

1.20

1.66

8.46

Sample

Whole plant -, zero.

The key findings from the analyses were as follows: •

sucrose was undetectable in the FEJ



the principal fermentable sugars in FEJ were glucose and fructose



fructose to glucose ratios in FEJ juice from stalk only , stalk and leaf, and whole plant samples were 0.94, 2.23, and 1.65, respectively



total soluble solids in FEJ from stalk only , stalk and leaf, and whole plant samples were significantly lower than would be typically observed for sugarcane FEJ (14–18 °Bx)

141



total reducing sugars in FEJ from stalk only, stalk and leaf, and whole plants samples were 3.57, 1.94, and 1.64%, respectively. This was significantly higher than the total reducing sugars typically observed in sugarcane FEJ (0.5–1.2%).

8.6 Analysis of sweet sorghum bagasse after extraction of juice Pressed bagasse from sweet sorghum milling performance trials (Samples 1, 3, and 5 from Table 8.3) containing a small amount of unexpressed juice was analysed by the wet disintegrator method to determine the residual soluble sucrose, glucose, fructose, and total soluble solids (°Bx) contents (Table 8.5). The bagasse was mixed with 6 L of water and disintegrated for 40 min to maximise extraction of the residual sugars and solids. The moisture contents, total soluble solids, and total sugars in the samples were analysed using the methods detailed in Section 8.3. Fibre content in bagasse was calculated using the difference method; fibre (%) was the difference between 100 and the combined brix and water content (%).

Table 8.5 Analysis of sugar content and composition in sweet sorghum bagasse Sample no.

Sucrose (%)

Glucose (%)

Fructose (%)

Total sugars (%)

Brix (°Bx)

Water (%)

Fibre1 (%)

Stalk only

1

-

0.97

1.10

2.07

4.55

52.11

43.34

Stalk and leaf

3

-

0.37

0.87

1.25

4.99

52.64

42.37

Whole plant

5

-

0.42

0.63

1.05

4.39

51.76

43.85

Sample

-, zero. 1 Fibre % in bagasse = 100 – brix % in bagasse – water % in bagasse.

8.7 Total sweet sorghum analysis The total constituents of the sweet sorghum samples determined from the wet disintegrator and bagasse moisture methods (as detailed in Table 8.1) can also be estimated by combining the average FEJ results from Table 8.4 and the average bagasse results from Table 8.5. The results of the total constituent analysis by both methods are presented in Table 8.6.

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Table 8.6 Estimates of the constituents of the sweet sorghum samples Method

Sucrose (%)

Glucose (%)

Fructose (%)

Brix (°Bx)

Water (%)

Fibre (%)

Stalk only

-

1.84

1.73

6.65

79.09

14.26

Stalk and leaf

-

0.60

1.34

6.50

76.88

16.62

Whole plant

-

0.62

1.02

6.04

..

..

Stalk only

-

1.42

1.65

6.47

79.67

13.86

Stalk and leaf

-

1.33

1.48

7.07

77.15

15.78

Whole plant

-

0.73

0.99

6.65

65.82

27.53

Sample

FEJ, bagasse, and fibre

Wet disintegration and bagasse moisture -, zero. .., not available.

The results from the FEJ and bagasse analyses (Table 8.6) were similar to the results from the wet disintegrator and bagasse moisture methods (Table 8.1).

8.8 Assessing extraction of sugars from sweet sorghum in a simulated conventional sugar milling process Conventional sugar milling involves passing the bagasse through a series of six-roller mills in the milling train with water added to aid extraction. This process was simulated in the QUT Pilot Plant Precinct, Brisbane, to assess the overall extraction efficiency of sugars in sweet sorghum. Samples (2 kg) were pressed four times and washed between each pressing with water (520 g). This resulted in water addition rates of ~190%, 170%, and 95%, respectively to stalk only, stalk and leaf, and whole plant samples. While these water addition rates were lower than what is typically used in sugar factories (200–250%), the use of water rather than recirculated juice at each stage partially compensated for this difference. The water addition rate for the sample containing grain (whole plant) was lower because we did not anticipate that the fibre content would increase significantly with the inclusion of the grain head. The final bagasse analysis results are presented in Table 8.7.

Table 8.7 Analysis of sugar content and composition in sweet sorghum final bagasse Sample no.

Sucrose (%)

Glucose (%)

Fructose (%)

Total sugars (%)

Brix (°Bx)

Water (%)

Fibre1 (%)

Stalk only

2

-

0.15

0.15

0.37

1.54

47.93

50.53

Stalk and leaf

4

-

0.14

0.14

0.27

2.06

47.31

50.64

Whole plant

6

-

0.42

0.10

0.52

2.39

48.20

49.42

Sample

-, zero. 1 Fibre % in bagasse = 100 – brix % in bagasse – water % in bagasse.

The comparison between the extraction results from the first pressing and all pressings for brix and total sugars are presented in Figure 8.6 and Figure 8.7, respectively.

143

Figure 8.6 Total soluble solids extraction (°Bx) from sweet sorghum

The efficiencies of total soluble solids (°Bx) extractions in the first pressing from all samples were similar (Figure 8.6). After a total of four pressings, the extraction efficiencies increased to similar levels. The highest extraction efficiency was observed for stalk only samples (94%) followed by stalk and leaf (91%) and whole plant (89%) samples.

Figure 8.7 Total sugar extraction from sweet sorghum

The efficiencies of total sugar extractions were higher for both stalk only and stalk and leaf samples than for whole plant samples (Figure 8.7). While significantly less water was added to whole plant samples, relative to the other samples, such a reduction in extraction efficiency was unexpected. Further testing is required to optimise the processing of sweet sorghum samples containing grain. While the extraction efficiencies measured using these two methods were marginally less than observed in the sugarcane crushing process, pressing is intrinsically less efficient than the rolling process in a milling unit and the results give us confidence that the conventional compound imbibition process between milling units will suffice for the extraction of sugars from sweet sorghum samples containing stalk only or stalk and leaf. The sugarcane bagasse expelled from the final six-roller mill in the milling train is used as fuel in the boiler to generate high-pressure steam and electricity to run the sugar factory. The final bagasse moisture content is an important fuel quality parameter and the results are presented in Figure 8.8. The results indicate that the moisture content in all bagasse samples decreased from the first to the fourth pressing. Further, all sweet sorghum bagasse samples contained less than 50% moisture after 144

the fourth pressing. This is similar to the moisture content of sugarcane bagasse after it exits the last six-roller mill in the milling train.

Figure 8.8 Moisture content of sweet sorghum samples

8.9 Outcomes and conclusions The outcomes from the sweet sorghum milling trials undertaken at the QUT Pilot Plant Precinct, Brisbane were as follows: • the shredding of sorghum and sugarcane in a small hammer mill gave similar results • low pressure compression tests indicated differences between the behaviour of sweet sorghum retaining leaf or leaf and grain but these differences were within the expected range for sugarcane • high pressure compression tests demonstrated superior compaction of sweet sorghum, relative to sugarcane • simulated extraction of sugars from sweet sorghum samples gave encouraging results • inclusion of sweet sorghum grain heads has a significant, negative effect on milling performance. The conclusions from the milling trials are as follows: •

no changes to a sugarcane factory shredder will be necessary to process sweet sorghum containing stalk only or stalk and leaves



sugarcane factory milling stations will operate at a marginally lower speed with sweet sorghum than with sugarcane for the same fibre rate but sweet sorghum will likely achieve similar or higher compaction levels to sugarcane



the inclusion of grain heads in the sweet sorghum harvest significantly affects fibre levels, feeding, and milling performance



if grain heads are to be a significant component of the sweet sorghum harvest, then more detailed research is required to optimise the milling process. 145

Results This project undertook a comprehensive assessment of sweet sorghum opportunities in Australia including field trials, the production of trial quantities of food, and feed products, and technoeconomic and LCA analysis. This section compares the major outcomes of the project against the project objectives. Assessing the productivity, product quality, and other significant agronomic indicators for crop trials in South-East Queensland of commercially-relevant sweet sorghum varieties Within this project, field trials of sweet sorghum were undertaken to assess the agronomic performance of the crop and to generate trial quantities of products for further analysis. Comparative assessments of eight sweet sorghum varieties were undertaken under common growing conditions. The AgriFuels variety AFL Rcv27751 was one of the top performers with good stalk and grain yields and high levels of total fermentable sugars in the stalk juice. Seedling emergence was affected by soil preparation, good seed contact with the soil, water availability, and soil temperature. Good seedling emergence was determined to be essential for crop establishment. Very few problems were experienced with pests and diseases during the trial conditions. Weeds were easily controlled with cultivation and herbicide use. Stalk height and diameter (and hence yield) were affected by water availability throughout the trial but reached a maximum height at ~120–140 DAP for the seed crop and ~90–110 DAP for the ratoon crop. Profiles of the concentration of sugars in the stalk juice were determined. For plant crops, maximum total fermentable sugars concentrations occurred about ~140 DAP for a December plant crop. Optimising the fermentation process for ethanol production from sweet sorghum juice, including the co-fermentation of sweet sorghum juice with sugarcane juice or molasses at both laboratory and pilot scale Laboratory scale trials of sweet sorghum juice fermentation were undertaken to optimise fermentation conditions. High ethanol yields from the fermentation of sweet sorghum were achieved with efficiencies as high as ~94% under optimum conditions. Sugarcane and sweet sorghum juices were both readily fermented with the microorganisms used in this project. The addition of sweet sorghum juice to sugarcane juice resulted in higher ethanol yields than the fermentation of sugarcane juice alone. Optimum ethanol yields were obtained at high inoculum loadings, a fermentation temperature of 33 °C and a nutrient loading of 40 mg/100 mL. Pilot scale trials of the conversion of sweet sorghum fibre into ethanol were undertaken at the Mackay Renewable Biocommodities Pilot Plant. These trials included pretreatment of fibre using two-stage mild acid pretreatment, enzymatic hydrolysis of the pretreated fibre and fermentation of the hydrolysed cellulose into ethanol. The results showed that sweet sorghum fibre can be readily converted into ethanol using these processing techniques. Similar results were obtained in the bioconversion of sweet sorghum fibre to ethanol in comparison to sugarcane fibre. Producing trial quantities of sweet sorghum products for end-use product testing, including grain for animal feed and concentrated liquor for food industry uses Sweet sorghum generates three major components (grain, juice, and bagasse) that have application to the production of food and feed products. These components were generated from two sweet sorghum cultivars (Dale and AFL Rcv27751) during the 2011 growing season in the field and analysed for their nutritional content. The nutritional contents of these residues were comparable to similar residues 146

already used in feed and food products from other crops. Further, in conjunction with VAFF Pty Ltd and AgriFuels Ltd, trial batches of manufactured feed and food products were generated from these residues and their nutritional content compared to existing similar commercial products. The results of the analyses show that there are significant opportunities for manufacturing food and feed products from sweet sorghum in Australia, including livestock feed from sweet sorghum grain, supplementation of livestock feed with sweet sorghum syrup, stock feed/roughage from sweet sorghum bagasse, and the production of mixed animal and fish feed products incorporating all three residues. Undertaking an economic evaluation and life cycle assessment (LCA) of the proposed cropping, harvesting, and processing system The techno-economic potentials of six sweet sorghum biorefinery processes were assessed in the Australian context. While there are many potential scales, product options, and locations for a new sweet sorghum industry and associated biorefinery in Australia, this assessment focused on a small number of product opportunities and assumed a generic, regional location. The assessment compared a range of product options and facility scales for a standalone sweet sorghum biorefinery and identified the key economic challenges and opportunities for proponents of sweet sorghum production in Australia. Five of the process options assessed were determined to have IRRs that exceeded the typical project hurdle rate of 15% and achieved positive NPVs, given the assumptions used in the assessment. The economic feasibility of ethanol production from sweet sorghum was shown to increase with ethanol production capacity. Integration of sweet sorghum juice, fibre, and grain for ethanol production was shown to deliver benefits through greater economies of scale. Sensitivity analysis of the results of the techno-economic assessment against feedstock price showed that, while feedstock price will affect project profitability, a significant variation in the price of one component only (sweet sorghum stalk or grain) did not change the economic viability of any of the project options with the variation range assessed. This analysis has highlighted potential opportunities for commodity biorefinery products from sweet sorghum in Australia. A carbon footprint analysis of six biorefinery process options for the conversion of sweet sorghum to fuel and animal feed products under Australian conditions was also undertaken. The carbon footprint of the sweet sorghum crop (stalk and grain panicle) as a feedstock harvested and transported to the biorefinery gate was similar to that of sugarcane at the same point in the supply chain. This was despite the reduced farming inputs (primarily nitrogen and water) required relative to those required for sugarcane. Low reported yields, rather than high farming inputs, for sweet sorghum were the primary reason why the carbon footprint for this crop was not significantly lower than that of sugarcane. A sensitivity analysis involving yields at the higher values reported in the literature indicated sweet sorghum can have a significantly lower carbon footprint relative to sugarcane. The contribution of harvesting related GHG emissions were relatively high compared with sugarcane because of the need to harvest two crops (seed and ratoon) in a year to achieve crop sizes which are comparable or greater than those for sugarcane. The allocation of GWP across the full range of products available from the sweet sorghum plant delivered low unit emissions associated with any given product. The allocated GWP of ethanol production in all process options and for the base case crop yield were significantly lower than that reported for ethanol production from sugarcane in Australia. The GWPs associated with electricity production in most process options were lower than those reported in the literature for the sugar industry. These benefits could be attributed to the fact that while the effects of sweet sorghum and 147

sugarcane farming have similar effects per unit of crop, more products can be generated from sweet sorghum than from sugarcane. Therefore the effect of farming attributed to each individual product is lower. Sweet sorghum grain animal feed had a significantly lower GWP than that reported for comparable wheat or barley feeds. A whole of system consequential LCA determined that the net reduction in GWP (per hectare) due to the aggregated effects of all biorefinery products resulted in a strong net reduction in GWP. This was true even for the lowest sweet sorghum crop yield scenario. Process options in which significant resources (fibre to produce electricity and LP steam) were required to process the fibre into fermentable sugars (process options 3 and 5) gave reduced system GWP benefits relative to the other process options. Evaluating the opportunities for using existing sugarcane industry infrastructure for harvesting, transportation, and processing of sweet sorghum crops, including use of transport and processing infrastructure in the non-crushing season The key considerations relating to the potential integration of sweet sorghum and sugarcane processing using existing sugarcane processing infrastructure have been explored. The integration of sweet sorghum and sugarcane processing was considered to be attractive as: •

there is considerable capital investment in sugar industry infrastructure that is only in operation for up to six month per year (June–December)



sugarcane is grown on fertile soils where sufficient rainfall or irrigation is available. Sweet sorghum may be able to be grown in adjacent areas where either the soil or climate do not support efficient sugarcane production



for a new standalone sweet sorghum venture, considerable capital will be required to install green-field processing facilities. If existing equipment from sugar mills was available, the initial investment required would be significantly reduced and the start-up of this new industry would be accelerated



with increased international competition in the raw sugar market, it is essential for sugarcane processors in Australia to seek alternative ways of operating that add value to their existing infrastructure.

There are potential effects across the harvesting, transport, and processing sectors with changed practices required for efficient integration. However, there are significant opportunities also to reduce the costs of start-up for a sweet sorghum industry and to better use sugarcane processing infrastructure to improve the profitability and viability of the sugarcane growing, harvesting, and milling sectors. The reduction in time available for maintenance will be offset by the increased usage of process equipment. An additional operating period of 2 months will bring the total operational period for the sugar mill from 5–6 months per year to 7–8 months per year. Milling trials were undertaken at the QUT Pilot Plant Precinct, Brisbane, to assess the potential impacts of crushing sweet sorghum through conventional sugarcane factory milling trains. The trials showed that no changes to a sugarcane factory shredder will be necessary to process sweet sorghum containing stalk only or stalk and leaves. Sugarcane factory milling stations will operate at a marginally lower speed with sweet sorghum than with sugarcane for the same fibre rate but sweet sorghum will likely achieve similar or higher compaction levels to sugarcane. The inclusion of grain heads in the sweet sorghum harvest significantly affects fibre levels, feeding, and milling performance and if grain heads are to be a significant component of the sweet sorghum harvest, then more detailed research is required to optimise the milling process. 148

Implications The establishment of a sweet sorghum industry in Australia offers significant potential to grow Australia’s gross production of energy, food and feed products from agriculture. Techno-economic assessment of sweet sorghum biorefinery options in Australia undertaken as a part of this project have shown very attractive returns on investment for new biorefinery facilities co-producing bioethanol, electricity and animal feed products. Human food products also offer niche opportunities from sweet sorghum biorefinery production. Sorghum is currently grown as a summer grain crop in Australia and forage sorghum is also widely grown throughout Queensland, New South Wales, Victoria and Western Australia. Sweet sorghum is expected to also grow well in many of these same areas, although total productivity will depend on suitable growing conditions for high biomass yield. The opportunities for establishment of new sweet sorghum industries include both the tropical regions of northern Australia and the sub-tropical and temperature regions of south-eastern and western Australia. This report will provide vital information to agricultural producers and agro-industrial companies considering investments in biorefinery technologies. Sweet sorghum is increasingly being seen as a valuable biorefinery crop with global potential and technology for efficient production is rapidly developing. Biofuels are one of the major options for reducing our reliance on fossil fuels for transportation energy. Increasing crude oil prices and global warming associated with anthropogenic carbon emissions will continue to create a demand for renewable fuels. While this report has focussed on the opportunities associated with ethanol production, other fuel products including aviation fuels and marine diesel products are also being considered and may offer attractive market opportunities. The co-production of energy, food, and feed products improves the sustainability of potential products by reducing the greenhouse gas footprint of individual products. As shown in this report, the use of grain from sweet sorghum for animal feed production offers the opportunity to reduce the carbon emissions associated with grain fed cattle products. Such products may allow beef producers to provide a point of difference for their products in the market. Australia is well placed to establish integrated biorefineries producing products for domestic use and for export into the Asian market. Sweet sorghum is one crop that has significant potential for contributing to the development of Australia’s bioeconomy.

149

Recommendations This project has assessed the opportunities associated with sweet sorghum cultivation, and the production of food and energy products in sweet sorghum biorefineries, in Australia. The economic and sustainability benefits of potential biorefinery processes have been assessed. Further research and commercialisation activities are required, however, before sweet sorghum will become commercially cultivated and processed at large scale in Australia. The following research and commercialisation activities are critical to capturing the potential economic benefits of this investment in Australia: 1. assessment and validation of sweet sorghum cultivar performance across varying climatic regions of Australia 2. further development of improved sweet sorghum cultivars optimised for Australian climatic conditions 3. further development of sweet sorghum cultivars with improved traits such as increased biomass yields 4. large scale demonstration of continuous processing of sweet sorghum through sugar factories or demonstration-scale biorefineries 5. detailed assessment of sweet sorghum biorefinery product and market opportunities with a focus on the Australian and Asian markets 6. large-scale animal feeding trials are required to confirm the digestible and metabolisable energy benefits and palatability of sweet sorghum animal feed products proposed in this report.

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Sweet sorghum -Opportunities for a new, renewable fuel and food industry in Australia By Ian O’Hara, Geoff Kent, Peter Alberston, Mark Harrison, Phillip Hobson, Neil McKenzie, Lalehvash Moghaddam, David Moller, Thomas Rainey, Wanda Stolz, Heng-Ho Wong, and Brendon Ellett Pub. No. 13/087 With increasing global concerns over greenhouse gas emissions and the future supply of food and energy products, there is an urgent need to develop new technologies that expand provision of renewable energy and increase food production. Sweet sorghum is receiving significant global interest because of its capacity to co-produce energy, food, and feed products in integrated biorefineries. This report deals with the opportunities to develop a sweet sorghum industry in Australia, demonstrating the production of energy, food, and feed products and assessing the potential economic and sustainability benefits of sweet sorghum biorefineries in the Australian context.. RIRDC is a partnership between government and industry to invest in R&D for more productive and sustainable rural industries. We invest in new and emerging rural industries, a suite of established rural industries and national rural issues. Most of the information we produce can be downloaded for free or purchased from our website . RIRDC books can also be purchased by phoning 1300 634 313 for a local call fee.

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