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ScienceDirect Procedia Materials Science 10 (2015) 513 – 518

2nd International Conference on Nanomaterials and Technologies (CNT 2014)

Energy Generation from Municipal Solid Waste by Innovative Technologies – Plasma Gasification

M. Rajasekhara*, N.Venkat Raob, G.Chinna Raoc, G. Priyadarshinid, N. Jeevan Kumare a,c,d

Vardhaman College of Engineering (Autonomous), shamshabad - 501218, Hyderabad, India Narasimha Reddy Engineering College, Kompally, Medchal – 500014, Ranga Reddy, India

b

Abstract

The growing population all around the world has increased the quantum of solid waste the disposal of which has become a complex task to the municipal authorities. In search of a suitable remedy to this problem we have arrived at a number of varieties of waste management methods and strategies for dealing with municipal solid waste. These technologies help to generate energy in the form of heat, electricity and fuel and also remarkably reduce the diversion of biomass waste from landfill. The process of plasma gasification is one among those successful proven technologies of producing energy from waste. Gasification is a process in which a solid material containing carbon, such as coal or biomass, is converted into a gas called syngas which contains hydrogen and carbon monoxide. The focus of the paper is to analyze the process of plasma gasification and also to propose a critical assessment of MSW. It also aims to compare the performance of the process to that of the conventional energy recovery methods. © Published by Elsevier Ltd. This © 2015 2015The TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the International Conference on Nanomaterials and Technologies (CNT 2014). Peer-review under responsibility of the International Conference on Nanomaterials and Technologies (CNT 2014) Keywords: Municipal Solid Waste; Pyrolysis; Gasification; Hydrocarbon; Energy; Syngas; Plasma Arc * Corresponding author. Tel.: +919985024240 E-mail address: [email protected]

1.

Introduction

Higher standards of living of ever increasing population have resulted in an increase in the quantity and variety of solid waste generated. It is now identified that if generation of waste continues indiscriminately then very soon it would be beyond rectification. Hence, management of solid waste has become very important in order to minimize the adverse impacts of municipal solid waste on the environment. Waste other than liquid or gaseous is called solid waste, it can be classified as municipal, industrial, agricultural, medical and sewage sludge. Solid waste also can be classified as garbage, rubbish, pathological waste, industrial waste and agricultural waste. Garbage refers to the putrescible solid waste generated during meat preparation, fruit and vegetables. The moisture content of these

2211-8128 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the International Conference on Nanomaterials and Technologies (CNT 2014) doi:10.1016/j.mspro.2015.06.094

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wastes is about 70 % and heating value of around 6x10 6 J/Kg. Rubbish, the another type of solid waste consists of 25% of moisture and 15x106 J/kg of heating value. The examples of rubbish wastes are paper, rubber, leather, metals, glass, ceramics etc. Dead animals and human wastes are called pathogenic waste containing 85 % of moisture and 2.5x106 J/Kg heating value. Refuse is very heterogeneous in composition and the geographical, temporal, and seasonal variations in its composition make it difficult to define a typical refuse. The solid refuse generated in urban areas contains particles of various sizes and types and consists of dust, vegetable leaves, waste paper, large paper board cartons, glass bottles, worn out tyres and night soil. Table 1: composition of city refuse (% by weight) Composition Kanpur

New Delhi

Calcutta

Bangalore

Paper

1.35

5.88

0.14

1.5

3.20

Putrescible matter

53.34

57.71

47.25

75.2

59.37

Dust and ash

25.93

22.95

33.58

12.0

15.9

Metals

0.18

0.59

0.66

0.1

0.13

Glass

0.38

0.31

0.24

0.2

0.52

Textiles

1.57

3.56

0.28

3.1

3.26

Plastics, leather

0.66

1.46

1.54

0.9

1.1

Stones and wooden matter

18.59

6.4

16.98

18.9

16.4

500

520

540

578

580

Density (Kg/m3)

Mumbai

The amount of refuse collected from urban areas in India is of the order of 0.3 kg to 0.5 kg per person per day excluding night soil. Improper handling of solid waste leads to health hazard and cause damage to the environment. The waste can be collected efficiently before the waste disposed effectively. 2.

Collection and Transportation of Solid Waste

Efficient collection and transportation are essential parts of the overall solid waste management programme. The basic mode of refuse collection in India is from communal storage points. The waste is collected in bins with capacity of 100 to 500 liters and placed at intervals of 50 to 200 meters. Daily collection of solid waste from bins is essential because the organic matter in the bins may tend to decompose rapidly. Another kind of waste collection methods block collection and kerbside collection are practiced in developed countries. Block collection is the process of conveying of waste by individuals to the waiting vehicles with the help of containers. In kerbside collection, the containers are placed on the footway in advance of the collection time and to be retrieved later. Transportation of the collected waste constitutes a key stage in the overall waste management system. In India, no single mode of collection is effective, economical and efficient due to congested and narrow roads. Hand cart collection is the best method in narrow and congested roads where the waste can be brought by individuals and transferred to the main transport vehicle. 3.

Disposal of solid waste

The process of selecting the right waste disposal method is a complex one due to the heterogeneity in the urban waste. Appropriate method of waste disposal can save money and avoids future problems. The most predominant methods used for treating municipal solid waste are open dumping, sanitary land filling, incineration and composting. Open dumping of solid waste is practiced mostly in India because it is cheap and requires no planning. In general, low lying areas and out skirts of the city are the places for dumping the waste. Open dumping create nuisance to the public by breeding of the flies, rats and mosquitoes. It is also a source of objectionable odors and causing air pollution but it helps to decrease the volume of the waste. Sanitary land filling is the method of disposing of solid waste according to the standards. The waste is dumped in the pits covered with High Density Polyethylene liner (HDPE) to avoid ground water contamination and the waste is compacted in thin layers within small area. The end products of the solid waste after digestion are Carbon dioxide and methane. The only disadvantage of the sanitary land filling is ground water pollution from leachates. Incineration involves burning of solid waste at high


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temperatures where 75 % of the waste can be converted in to ashes, metals and unburned combustibles. This residue must still be disposed in different manner. Incineration may lead to the contamination of the air unless designed the plant properly. 4.

Plasma Gasification

Gasification is a process in which a solid material containing carbon, such as coal or biomass, is converted into a gas. It is a thermo chemical process, means that the feedstock is heated to high temperatures, producing gases which can undergo chemical reactions to form a synthesis gas called syngas. Syngas mainly contains hydrogen and carbon monoxide, and can then be used to produce energy. Pyrolysis vaporizes the volatile component of the feedstock as it is heated. The volatile vapors are mainly hydrogen, carbon monoxide, carbon dioxide, methane, hydrocarbon gases, tar, and water vapor. Since biomass feedstock tend to have more volatile components (70-86% on a dry basis) than coal (around 30%), pyrolysis plays a major role in biomass gasification than in coal gasification. Gasification further breaks down the pyrolysis products with the provision of additional heat, some of the tars and hydrocarbons in the vapors are thermally cracked to give smaller molecules, with higher temperatures resulting in fewer remaining tars and hydrocarbons. Steam gasification - this reaction converts the char into gas through various reactions with carbon dioxide and steam to produce carbon monoxide and hydrogen. Higher temperatures favor hydrogen and carbon monoxide production, and higher pressures favor hydrogen and carbon dioxide production over carbon monoxide. The heat needed for all the above reactions to occur is usually provided by the partial combustion of a portion of the feedstock in the reactor with a controlled amount of air, oxygen, or oxygen enriched air. Heat can also be provided from external sources using superheated steam, heated bed materials and by burning some of the chars or gases separately. This choice depends on the gasifier technology. There are then further reactions of the gases formed, with the reversible water-gas shift reaction changing the concentrations of carbon monoxide, steam, carbon dioxide and hydrogen within the gasifier. The result of the gasification process is a mixture of gases.

Fig.1. Process of Converting Trash to Gas

5.

Introduction to Gasifier types There are different gasifiers for conversion of biomass in to gas, are mentioned below.

5.1. Updraft fixed bed The biomass is fed in at the top of the gasifier, and the air, oxygen or steam intake is at the bottom, hence the biomass and gases move in opposite directions. Some of the resulting char falls and burns to provide heat. The

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methane and tar-rich gas leaves at the top of the gasifier, and the ash falls from the grate for collection at the bottom of the gasifier. 5.2. Plasma Untreated biomass is dropped into the gasifier, coming into contact with electrically generated plasma, usually at atmospheric pressure and temperatures of 1,500-5,000°C. Organic matter is converted into very high quality syngas, and inorganic matter is vitrified into inert slag. 5.3. Downdraft fixed bed The biomass is fed in at the top of the gasifier and the air, and oxygen or steam intake is also at the top or from the sides, hence the biomass and gases move in the same direction. Some of the biomass is burnt, falling through the gasifier throat to form a bed of hot charcoal which the gases have to pass through. 5.4. Entrained Flow Powdered biomass is fed into a gasifier with pressurized oxygen and/or steam. A turbulent flame at the top of the gasifier burns some of the biomass, providing large amounts of heat, at high temperature (1200-1500°C), for fast conversion of biomass into very high quality syngas. The ash melts onto the gasifier walls, and is discharged as molten slag 5.5. Bubbling Fluidized bed A bed of fine inert material sits at the gasifier bottom with air, oxygen or steam being blown upwards through the bed just fast enough (1-3m/s) to agitate the material. Biomass is fed in from the side, mixes, and combusts or forms syngas which leaves upwards and operates at temperature below 900°C to avoid ash melting and sticking. 6.

Principal feedstock preparation steps for biomass gasification

6.1. Sizing Smaller particles have a larger surface area to volume ratio, and the gasification reaction occurs faster when there is a larger biomass surface area. Smaller particles can also be suspended in gas flows more readily, and if very small, the particles may act like a fluid. Achieving the correct feedstock sizing for the gasifier is important. Crude sizing operations include chipping, cutting and chopping, but in order to get very small ground particles, pulverizing milling equipment is needed, this is an energy intensive process. A screening process is often used to ensure any remaining larger particles and extraneous materials are removed. 6.2. Drying The removal of moisture contained within the biomass by evaporation, typically using temperatures between 100°C and 120°C. Drying requires a significant amount of energy in order to evaporate the large mass of water. This heat can be provided externally, or extracted from the gasifier syngas or other plant process steps. Gasification efficiency increases with drier biomass, but drying costs also increased quickly below 10% moisture. 6.3. Torre faction A mild thermal treatment (approximately 30 minutes at between 200°C and 300°C, in the absence of oxygen) resulting in a low-oxygen content, dry and relatively brittle product. Torrefied wood is much easier to grind than untreated wood, using 80% less energy for a given sizing, and with a significant increase in milling plant capacity.


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6.4. Pyrolysis The thermal degradation of biomass in the absence of oxygen, whereby the volatile parts of a feedstock are vaporized by heating. The reaction forms three products: a vapor that can be condensed into a liquid (pyrolysis oil), other gases, and a residue consisting of char and ash. Fast pyrolysis processes are designed and operated to maximize the liquid fraction (up to 75% by mass), and require rapid heating to temperatures of 450°C to 600°C, and rapid quenching of the vapors to minimize undesirable secondary reactions. The resulting liquids and solids can be ground together to form a bio-slurry for gasification 6.5. Plasma Gasifiers Plasma gasification usually takes place in the absence of a gasification oxidant, with some gas (e.g. air, oxygen, nitrogen, noble gases) only present to produce the plasma in the jet or arc, for the provision of heat. Extremely high temperatures (greater than 5,000°C) ensure that the feedstock is broken down into its main component atoms of carbon, hydrogen and oxygen. These quickly re-combine to form hydrogen and carbon monoxide gases, thereby producing a very high quality syngas, with no methane, hydrocarbons or tars. Other plasma gasifiers work at lower temperatures (from 1,500°C to 5,000°C, but still well above EF conditions), producing some tars and hydrocarbons, which are then immediately cracked. Plasma torches have highly adjustable power outputs, hence temperatures and syngas components can be controlled. Since plasma gasification usually uses waste feedstock, chlorides levels can be high, which can lead to high levels of impurities (such as dioxins and metals) in the syngas, although many of the heavier elements are vitrified and hence safely removed. Conclusion As it has been one of the best proven methods for treating municipal solid waste it may be employed in every municipal corporation. Treatment of solid waste by various other methods may involve certain difficulties for the complete disposal of residues. As far as plasma gasification is concerned the total disposal of residues is possible hence there will be no need of leaving any residues that may cause further problem. In order to make it more accessible and affordable to all, the equipment related to the process may be made available at reasonable expenses by innovative design. References Advanced Plasma Power 2007 “Gas plasma Outputs – clean syngas: the effects of plasma treatment on the reduction of organic species in the syn-gas” Ahmed I, Gupta AK. 2009, Characteristics of cardboard and paper gasification with CO2. Appl Energy, 86(12):2626–34. Anna P, Yang W, Lucas C. 2006, Development of a thermally homogeneous gasifier system using high-temperature agents. Clean Air, 7(4):363– 79. Anh NP, Changkook R, Vida NS, Jim S. 2008, Characterisation of slow pyrolysis products from segregated wastes for energy production. J Anal Appl Pyrol, 81(1):65–71. Anthony D, Sylvie V, Pierre C, Sebastien T, Guillaume B, Andre Z, et al. 2009, Mechanisms and kinetics of methane thermal conversion in a syngas. Ind Eng Chem Res, 48(14):6564–72. Blasiak W, Szewczyk D, Lucas C. 2002, Reforming of biomass wastes into fuel gas with high temperature air and steam. In Pyrolysis & Gasification of Biomass & Waste. Strasbourg, France. 81(3):291–7. Boroson ML, Howard JB, Longwell JP, Peters WA. 1989, Product yield and kinetics from the vapour phase cracking of wood pyrolysis tars. AIChE J, 35(1):120–8. Colomba DB. 2004, Modeling wood gasification in a countercurrent fixed-bed reactor. AIChE J, 50(9):2306–19. Consonni, S., Giugliano, M., Grosso, M., 2005. Alternative strategies for energy recovery from municipal solid waste. Part A: Mass and energy balances. Waste Management 25, 123–135. Ecke H, Sakanakura H, Matsuto T, Tanaka N, Lagerkvist A. 2000, State-of-the-art treatment processes for municipal solid waste incineration residues in Japan. Waste Manage Res;18(1):41–51. Gomez E, Amutha RD, Cheeseman CR, Deegan D, Wise M, Boccaccini AR. 2009, Thermal plasma technology for the treatment of wastes: a critical review. J Hazard Mater, 161(2–3):614–26. Grace, J., 1986. Contacting modes and behaviour classification of gas-solid and other two-phase suspensions. Canadian Journal of Chemical Engineering 64, 353–363.

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