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www.elsevier.com/locate/renene. Technical note. Efficient energy systems with CO2 capture and storage from renewable biomass in pulp and paper mills.
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Renewable Energy XX (2004) XXX–XXX www.elsevier.com/locate/renene

Technical note

Efficient energy systems with CO2 capture and storage from renewable biomass in pulp and paper mills Kenneth Mo¨llersten a,b,, Lin Gao b,c, Jinyue Yan b, Michael Obersteiner a a

b

International Institute for Applied Systems Analysis, A-2361 Laxenburg, Austria Division of Energy Engineering, Department of Mechanical Engineering, Lulea˚ University of Technology, SE-971 87 Lulea˚, Sweden c Institute of Engineering Thermophysics, Chinese Academy of Sciences, P.O. Box 2706, Beijing 100080, PR China Received 1 July 2003; accepted 4 January 2004

Abstract This paper investigates the impact of combining CO2 capture and storage with alternative systems for biomass-based combined heat and power production (CHP) in Kraft pulp and paper mills. We compare heat, power, and CO2 balances of systems with alternative configurations of the CHP and CO2-capture systems. Because the captured CO2 comes from renewable biomass, the studied systems yield negative CO2 emissions. It is shown that pulp mills and integrated pulp and paper mills have the potential to become net exporters of biomass-based electricity while at the same time removing CO2 from the atmosphere on a net basis. The study shows that that the overall best CO2 abatement is achieved when CO2 capture is carried out within a biomass integrated gasifier with combined cycle where the syngas undergoes a CO-shift reaction. This configuration combines efficient energy conversion with a high CO2 capture efficiency. Furthermore, cost curves are constructed, which show how the cost of CO2 capture and storage in pulp and paper mills depends on system configuration and the CO2 transportation distance. # 2004 Elsevier Ltd. All rights reserved.

 Corresponding author. Present address: Swedish Energy Agency, Climate Change Division, P.O. Box 310, SE-631 04 Eskilstuna, Sweden. Tel.: +46-16-544 20 94; fax+46-16-544 22 62. E-mail address: [email protected] (K. Mo¨llersten).

0960-1481/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2004.01.003

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Keywords: CO2 capture; Pulp and paper mills; Biomass; Black liquor; Electricity production; Carbonnegative production

1. Introduction The third assessment report of the Intergovernmental Panel on Climate Change (IPCC) [1] provides the strongest evidence so far that the global warming of the last 50 years is due largely to human activity, especially the CO2 emissions that arise when fossil fuels are burned. In the same report, the IPCC further concludes that the stabilisation of the atmospheric CO2 concentration requires CO2 emissions to eventually drop well below current levels. Industrial companies acting in a greenhouse gas (GHG)-constrained future will be exposed to new environmental requirements that are imposed by both institutions and the environmentally aware consumers. In order to stay competitive in such an environment, companies must consider the development of effective CO2 management strategies. Due to the large CO2 emissions inherent in most energy production, the implications of GHG constraints will be far-reaching for energy-intensive industries. Biomass-based industries rich in self-generated biomass residues are uniquely equipped to implement low-carbon production. The Kraft pulp industry, which accounts for around 70% of pulp production world-wide [2] belongs to this group of industries. In the Kraft pulp process, a mixture of lignin and inorganic chemicals known as black liquor is a by-product of fibre extraction from wood. Slightly more than half of the biomass entering a Kraft pulp mill is dissolved in the black liquor. Black liquor is burned in recovery boilers which recover important pulping chemicals and feed steam to the mill combined heat and power (CHP) system. In modern Kraft market pulp mills, the fuel requirement for the CHP system is typically covered through black liquor and internally generated bark, whereas integrated pulp and paper mills need to import fuels to satisfy the process demand for medium pressure (MP) and low pressure (LP) steam. Most pulp mills and all integrated mills rely on electricity import to cover part of their electricity demand. This paper investigates opportunities for energy efficient low-carbon production of Kraft pulp and paper. The paper evaluates and compares several configurations of low-carbon CHP systems and develops the new concept of ‘‘carbon-negative pulp and paper production’’, which are products manufactured with negative CO2 emissions. Carbon-negative production is a technological opportunity based on renewable biomass-based energy conversion with CO2 capture and permanent storage (BECS). Today, most R&D and demonstration efforts concerning CO2 capture and storage technologies have been focused on achieving nearly zero emissions from fossil fuel-based systems such as, for example, coal-fired power plants. Because BECS involves capture and storage of biotic CO2 from renewable biomass, the technology can yield negative CO2 emissions—i.e. CO2 is removed from the atmosphere on a net basis [3]. Swedish analyses have shown that on the national level BECS may play an important role in least-cost CO2 mitigation paths [4,5]. Fig. 1 illustrates the basic principle of BECS in a forest industry environment.

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Fig. 1. Biomass energy with CO2 capture and permanent storage. The relationship between the fraction of carbon released and the fraction of carbon captured varies with the capture technology applied.

Section 2 of this paper provides a background covering opportunities for increased overall energy efficiency and CO2 reductions in Kraft pulp and paper mills’ CHP systems and, furthermore, the main conclusions drawn from previous studies on the incorporation of BECS in Kraft pulp mills are summarised. In the main part of the paper, (Sections 3–5), alternative CHP systems based on black liquor gasification (BLG) incorporating BECS in Kraft pulp mills are defined and simulated on a consistent basis and in greater detail compared to previous studies. The impact of BECS on energy efficiency and the CO2 balances of the systems is evaluated. The analysis is also extended to include integrated production of pulp and paper. In Section 6, an economic evaluation of the studied systems is carried out presenting the costs of CO2 capture and storage. The results are discussed in Section 7 and main conclusions from the paper are drawn in Section 8. 2. Combined heat and power systems in Kraft mills In existing Kraft pulp mills with modern CHP systems based on recovery boilers and biomass boilers, electrical efficiencies are low (up to 15%) [6]. Significantly improved overall energy efficiency and increased electrical efficiency could be accomplished by the introduction of black liquor integrated gasifier with combined

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cycles (BLGCC) [6–8], which is, however, not a commercially available technology today. Increasing the electrical efficiency of CHP systems often leads to a reduction in steam production, which means that the attractiveness of introducing gasification-based systems with combined cycles partly relies on a development towards reduced process steam demand in the pulp and paper mills. Alternatively, additional fuel could be imported to the mills. An indication of the medium-term potential for reductions in process steam demand is provided by the Swedish ‘‘KAM’’ research programme which has defined a reference market Kraft pulp mill [9] based on the best respective technologies available for commercial use in the late 1990s. In the KAM reference mill, the required process steam is 11 GJ/ADt pulp (air-dry tonne pulp), which is a reduction by 24% compared to the 1994 Swedish average. Berglin et al. [10] analysed the performance of recovery boilers and BLG in a KAM reference pulp mill environment. The results show that electrical efficiencies around 20% could be achieved with recovery boilers whereby surplus steam was used to generate additional electricity with condensing steam turbines whereas 28–31% could be achieved with BLGCC. It has been shown that BECS increases the technical CO2 reduction potential of Kraft pulp mill CHP systems [11,12]. However, it should be noted that although capture and transportation of CO2 is feasible and technically proven, further investigation regarding the reliability and safety of long-term storage remains necessary [13,14]. The IPCC predicts that these technologies could give major contributions to CO2 abatement by 2020 [13]. Mo¨llersten et al. [12] compared the CO2 abatement potential of alternative BECS technologies in Kraft pulp mills. The study took into account changes in primary emissions (emissions on-site in the pulp mills) and secondary emissions (emissions elsewhere in the energy system). The largest reduction potential was found for post-combustion capture of CO2 from recovery boiler and bark boiler flue gases in that case study. However, CO2 capture from the flue gases of recovery boilers, which have fairly low CO2 concentration is done by chemical absorption. The regeneration of chemical absorbents is heat-consuming, which leads to a rather large reduction in the already low electrical efficiency. Accordingly, the mills’ dependence on imported electricity would increase. At higher pressures and CO2 concentrations, physical absorption can be used whereby the solvent is regenerated by pressure reduction. The main energy requirement for physical absorption is for compression and pumping of physical absorbent, which results in a smaller energy penalty compared to when chemical absorption is used. Mo¨llersten et al. [12] found that pressurised BLGCC with pre-combustion CO2 capture by physical absorption resulted in a higher electrical efficiency but lower overall CO2 reduction compared to post-combustion CO2 capture in recovery boilers. This was due to the significantly lower amount of CO2 produced per unit fuel when black liquor is gasified compared to when the black liquor is combusted in a recovery boiler. The potential for CO2 capture in BLGCC can be enhanced with a water-gas shift reaction (CO shift), whereby CO is reacted with water to form H2 1

CO + H2Ovap ! CO2 + H2 + 44.5 MJ/molco.

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and CO21. In the cited study, however, this option was not considered. The impact of including CO shift will be analysed in the present paper.

3. System modelling The modelling of alternative CHP systems based on BLG in this study is carried out in two different mill environments; the KAM market pulp mill (MPM) described in Section 2 and an integrated pulp and paper mill (IPPM). The IPPM, an extension of the KAM market pulp mill, was defined by Berglin et al. [10]. The IPMM has a steam consumption which is approximately 5% lower than the average Swedish 1994 fine paper mill. In Table 1, the process steam and electricity requirements of the MPM and IPPM are presented. Simulations of energy conversion systems (CHP plants) were carried out by using the ASPEN PLUS process simulator [15]. Table 2 summarises the alternative CHP system configurations which have been considered. Fig. 2 provides an overview of the analysed systems by showing the Table 1 Process energy requirements in the modelling mill environments Energy requirement (GJ/ADt end product)

Electricity Medium pressure steam (12 bar) Low pressure steam (4 bar) a

Market pulp mill

Integrated pulp and paper milla

2.5 4.3 5.7

4.8 7.5 8.3

1.2 tonnes paper are produced for every ADt pulp produced.

Table 2 Summary of analysed CHP system configurations Case

Biomass conversiona Boiler

Gasifier

CO2 capture No capture

CO2 capture from syngasb No CO shift unit

MPM1 MPM2 MPM3 IPPM1 IPPM2 IPPM3 IPPM4 a

N.A. N.A. N.A.    

With CO shift unit

      

Black liquor is gasified in all cases. When additional fuel is required to meet process steam demands either a biomass boiler or a biomass gasifier is considered. b Capture of CO2 from the syngas of both black liquor and biomass gasifiers is considered when applicable.

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Fig. 2. Simplified chart of analysed system. The IPPM4 case was chosen for this illustration.

IPPM4 case. All cases are based on a pressurised (approximately 30 bar) hightemperature, oxygen-blown black liquor gasifier such as the one used in the Chemrec process [16]. The syngas is cooled in a quenching bath using the weak wash as coolant whereby the weak wash is evaporated using the sensible heat of the syngas. The quenching adjusts the fraction of steam in the syngas to ensure an adequate amount of water for a water-gas shift reaction to proceed in a downstream CO-shift reactor. In most cases of integrated gasification, a waste heat boiler is used to recover heat for steam production instead of an energetically less efficient quenching bath. However, we have opted to use a quench bath due to its simpler and more reliable operation and lower capital cost. This is justified as power generation is not the main objective in our case of industrial CHP. The CO shift takes place in a high-temperature reactor and a low-temperature reactor in series. CO2 is absorbed in a physical absorption unit using a selective liquid solvent (such as dimethylether of polyethylene used in the Selexol process)2. The work consumed for CO2 absorption depends on the partial pressure of the CO2 in the gas mixture. In our studied cases, the work varies from 0.21 MJ/kg CO2 for the lowest partial 2 We consider H2S removal through physical absorption prior to the CO2 capture. H2S and CO2 could be removed together in a single absorption unit. However, this could be disadvantageous in the subsequent storage of captured CO2.

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pressure (without CO shift) to 0.14 MJ/kg CO2 for the highest partial pressure (with CO shift). The captured CO2 is compressed to 80 bar in a two-stage intercooled compressor. After the clean-up section the syngas is used to fuel a gas turbine for power generation. The exhaust gas from the gas turbine is recovered in a heat recovery steam generator (HRSG) and steam is fed to a steam turbine which generates additional electricity and process heat for the pulp and paper mills. Part of the process demand for MP and LP steam is generated in the quenching bath and shift reactors. When additional fuel is required to satisfy the process steam demand either a supplemental biomass boiler or biomass integrated gasifier with combined cycle (BGCC) is considered (as illustrated in Table 2). Table 2 shows that for each mill type one case includes a CO-shift reactor prior to the CO2 capture for an enhanced capture rate. The main assumptions of the studied millintegrated CHP systems are given in Table 3. Table 4 presents the main characteristics of the CO2-lean fuel gas which is fed to the gas turbine after CO2 absorption. Table 3 Main assumptions for CHP system Gasifiers

Cold gas efficiency (%)

Black liquor

Biomass

77

77

Syngas properties Raw gas

After quench

Raw gas

After quench

Temperature ( C) Pressure (bar) Composition (mol%) N2 CO CO2 H2O H2 H2S

950 32

211 25

900 27

209 25

0.2 29.5 14.6 22.0 31.1 1.5

0.1 13.5 6.7 64.3 14.2 0.7

0.2 30.0 24.2 15.9 24.1 0.0

0.1 13.0 10.4 63.7 10.4 0.0

CH4

1.1

0.5

5.6

2.4

v

Gas turbine v Turbine inlet temperature ( C) Pressure ratio Mechanical efficiency (%) Isentropic efficiency, expander (%)

1250 17 98 92

Isentropic efficiency, compressor (%)

87

Steam cycle v Turbine inlet temperature ( C) Turbine inlet pressure (bar) Mechanical efficiency (%) Isentropic efficiency, expander (%) high pressure/medium pressure v Pinch temperature difference of HRSG ( C) v Feed water temperature ( C)

440 66 98 85/87 15 120

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Table 4 Characteristics of CO2-lean fuel gas to the gas turbine

v

Temperature ( C) Pressure (bar) Composition (mol%) N2 CO CO2 H2O H2 H2S CH4

Without CO shift

With CO shift

110 20

110 20

0.4 47.6 0.2 0.0 50.1 0.0 1.8

0.4 0.4 0.7 0.0 96.8 0.0 1.8

The composition is based on cases MPM2 and MPM3 and would vary slightly when supplemental biomass is gasified too.

4. System performance The performance of the studied systems is presented in Tables 5 and 6. Please note that the lower heating value (LHV) is used throughout the paper. LP steam is 4.5 bar and MP steam is 12 bar. In all studied cases, a net electricity surplus is obtained which allows for power export to the grid. Table 5 shows that process steam requirements are satisfied through the black liquor-based CHP system in all

Table 5 Performance of the market pulp mill CHP systems MPM1 Black liquor (MW) Bark and woody biomass (MW) CO2 recovery (%) CO2 capture rate (kg CO2/s) MP steam to mill (t/h) LP steam to mill (t/h) CO2 separation power consumption (MW) CO2 compressor power consumption (MW) Air separation unit power consumption (MW) Other internal power consumption (MW) GT output (MW) ST output (MW) Powerhouse net output (MW) Mill electricity consumption (MW) Electricity surplus (MW) Electricity surplus (MW h/ADt pulp) Electrical efficiency (%) Total efficiency (%) Pulp production 1550 ADt/d.

0 0 0

0 0 4.5 10.1 99.7 20.5 105.6 66.5 1.0 31 75

MPM2 338 0 31 9.7 101.1 137.6 1.9 4.2 4.5 10.1 98.5 16.0 93.8 39.2 54.6 0.9 28 72

MPM3 0 90 26.6

4.0 12.5 4.5 10.1 92.7 9.7 71.3 32.1 0.4 21 65

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Table 6 Performance of the integrated pulp and paper mill CHP systems IPPM1 Black liquor (MW) Bark and woody biomass (MW) CO2 recovery (%) CO2 capture rate (kg CO2/s) MP steam to mill (t/h) LP steam to mill (t/h) CO2 absorption power consumption for (MW) CO2 compressor (MW) Air separation unit power consumption (MW) Other internal power consumption (MW) GT output (MW) ST output (MW) Powerhouse net output (MW) Mill electricity consumption (MW) Electricity surplus (MW) Electricity surplus (MWh/ADt paper) Electrical efficiency (%) Total efficiency (%)

97 0 0

IPPM2 114 0 0

0 0 4.5 13.1 99.7 31.8 114.0

0 0 6.2 13.6 135.4 0 115.5

40.0 0.5 26 80

41.8 0.5 26 77

IPPM3 338 114 33 13.7 175.6 199.6 2.8 6.1 6.2 13.6 135.4 0 106.7 73.8 32.9 0.5 24 76

IPPM4 184 90 44.6

6.1 20.2 6.8 15.7 146.3 16.4 113.9 40.1 0.6 22 67

Paper production 1860 ADt/d

MPM cases. For the IPPM cases, process steam requirements are not satisfied by the black liquor alone (Table 6). Internally generated bark is generated at the rate 2.9 GJ/ADt pulp which is equivalent to 52 MW. Around half of this bark is required to cover the lime kiln fuel demand and the remaining half is assumed to be available for internal consumption as biomass fuel or to be sold. Thus, additional biomass fuel must be supplied in all IPPM cases. The results illustrate how the electricity surplus decreases with an increasing CO2 capture rate. We can observe that the CO2 capture leads to a larger drop in electrical efficiency for the MPM (see cases MPM1 and MPM3) than for the IPPM (see cases IPPM2 and IPPM4). This can be regarded as an effect of the system optimisation with respect to the mills’ steam requirements. Note that cases MPM1 and MPM3 both use steam turbines. In contrast, IPPM2 uses no steam turbine as to satisfy the mill steam demand whereas IPPM4 uses a steam turbine, which leads to a lower drop in electrical efficiency compared to the MPM system. The impact of CO2 capture on the gas turbine performance can be seen clearly in the results for the MPM. There are two main reasons for the drop in power production from the gas turbine in cases MPM1–MPM2–MPM3. CO2 plays a role as coolant in the combustion. With the capture of CO2 more coolant (air) is needed, which leads to a larger work requirement in the compressor. In addition, when CO is converted to CO2 and H2 some of the chemical energy is converted to reaction heat which means that the total energy content of the fuel to the combustor decreases. Some of the reaction heat can be recovered and thus made useful in the process.

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5. The CO 2 impact of BECS To calculate the overall CO2 impact when introducing CO2 capture and storage in the production, we consider changes in direct and indirect net emissions compared to the reference cases MPM1 and IPPM1. Direct emissions are emissions onsite in the mills and indirect emissions are emissions which arise elsewhere. Based on the performance of the systems (presented in Tables 5 and 6) and following the principles described below, Table 7 was produced, which displays the systems’ overall CO2 impact compared to the reference cases. The only change in primary emissions compared to the reference cases is the reduction due to capture and storage of CO2 from black liquor and biomass fuel. Regarding secondary emissions the following approach was applied: . The mills’ electricity surplus decreases when CO2 capture is applied. We assume that natural gas-fired combined cycle power plants with an electrical efficiency of 60%3 produce the marginal electricity which compensates for the decreased electricity export from mill to grid. . Extraction of biomass requires energy, which leads to net emissions of CO2 if fossil fuels are used. The internally generated bark and biomass in black liquor available at the mills are by-products of the pulp production, so the emissions due to their extraction should be allocated to the production of pulp and paper. Therefore, only emissions from the extraction of biomass required in addition to the internally generated fuels are allocated to the BECS systems in our analysis. It is reasonable to consider two alternative levels of CO2 emissions for biomass extraction. As a lower value, we considered data for unrefined forestry residues. Bo¨rjesson and Gustavsson [17] estimate that 2.9 kg CO2 is emitted per GJ forestry residue extracted. The figure, based on Swedish conditions, includes 50 km transportation of the fuel. As a value on the high end, we used data for dedicated biomass plantations. The result of a comprehensive environmental life cycle assessment of fuel supply from dedicated eucalyptus plantations shows that 21 kg CO2 is emitted per GJ biomass extracted [18]. Our analysis reveals that the impact on the CO2 balances of the systems in our study is small. . CO2 transportation by pipeline requires work for pressurisation. The initial pressurisation is considered in our analysis in that compression penalises the net power output of the mill CHP systems. Emissions due to work required for booster compressors along the pipelines is regarded as negligible. . CO2 capture and sequestration requires additional infrastructure such as pipelines. It is important to ensure that emissions are not to any significant extent moved from the tailpipe to the construction process. According to Strømman [19], a life cycle assessment of large-scale hydrogen production with CO2 capture

3

Representing the best available technology for natural gas-fired combined cycle power plants today and in the near future.

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Table 7 Impact on CO2 emissions compared to reference case Case

MPM2b MPM3b IPPM2d IPPM3d IPPM4d

Direct emissions compared to reference (tCO2/ADt)a

Indirect emissions compared to reference (tCO2/ADt)a

CO2 capture and storage

Impact from change in electricity production

Biomass fuel extraction

0.5 1.6 0 0.6 2.2

0.1 0.2 0.0 0.0 0.0

N.A.c N.A.c 0.0/0.0e 0.0/0.0e 0.0/0.1e

CO2 emissions compared to reference (tCO2/ADt)a

0.4 1.4 0f 0.6f 2.1f

a

For MPM1-3, the CO2 accounting is per ton pulp and for IPPM1-4 the accounting is per ton paper. MPM1 is reference case. c No additional biomass fuel needs to be extracted. d IPPM1 is reference case. e Higher extraction emission value/lower extraction emission value. f The higher biomass extraction emission value was applied to calculate the overall CO2 emission impact. b

and storage has shown that the CO2 emissions due to the construction of additional infrastructure are negligible. Tables 5, 6 and 7 show that Kraft pulp mills and integrated pulp and paper mills have potential to be net producers of biomass-based electricity while at the same time removing substantial amounts of CO2 from the atmosphere for each tonne of pulp or paper produced. Including the CO shift option has a quite favourable impact on the overall CO2 budget of the production.

6. BECS system economics A simple economic analysis was carried out for guidance regarding the costs of CO2 capture in pulp and paper mills. Capital cost estimates for the system components were taken from several detailed studies in the literature4 [6,9,20–22]. Table 8 presents the estimated incremental capital costs for the CO2 capture systems relative to the base cases without CO2 capture (MPM1 and IPPM1). IPPM2 is not included in the analysis as it does not feature CO2 capture. A scaling factor of 0.7 was used to adjust capital costs for size. An estimated initial accuracy of the source cost data is 30%. The cost of transporting CO2 by pipeline and storing it permanently was estimated using a model developed by International Energy Agency [23]. The model calculates capital cost, fixed and variable operating costs for the pipelines, booster 4

Year 2000 USD are used throughout the paper.

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Table 8 Estimated capital costs for CO2 capture Component

Incremental capital cost relative to base casea [MUSD] MPM2b

BLG island Biomass gasification island (with ASU) Biomass boiler Shift reactor CO2 absorber Gas turbine HRSG Steam turbine CO2 compressor Total

MPM3b

IPPM3c

IPPM4c

0 –

0 –

0 53

0 74

– – 7 0 0 0 4

– 14 14 4d 0 2 8

11 – 8 9 3 8 6

11 20 20 18d 7 2 11

11

38

60

137

a

Capital costs include overnight capital, installations and indirect costs 8% of OC, and interest during construction. b Incremental cost is relative to MPM1. c Incremental cost is relative to IPPM1. d This turbine is fuelled predominantly with H2. No commercial gas turbines exist that run on H2. Future options include commercial gas turbines modified for H2 through N2 injection into the combustion chamber, and so called hydrogen combustion turbines. A 10% increase of the specific capital cost was assumed for the gas turbine of case 3.

compressors, and injection wells. CO2 injection is assumed to take place in CO2retaining deep saline aquifers (water-containing layers) with negligible seepage back to the atmosphere and the depth of the injection wells was set to 1000 m. On-shore injection wells were assumed. Aquifers were chosen because they are generally more widespread than hydrocarbon fields. Other suitable candidate underground CO2 storage locations not considered in this study are exhausted natural gas and oil fields, not exhausted oil fields (so-called enhanced oil recovery), and unminable coal layers. Fig. 3 illustrates the importance of economies of scale in CO2 storage by showing the cost of transportation and storage calculated for a set of CO2 flow rates and transportation distances. Capital costs were annualised using an 11% capital charge rate (based on 10% interest rate and a plant life of 25 years). In addition to pipelines, the use of large tankers might be economically attractive for long-distance transportation of compressed/liquefied CO2 over water. Pulp mills are generally located near harbours, which facilitates economically feasible tanker transportation. Ekstro¨m et al. [24] estimated a 17 USD/tCO2 cost for transporting and injecting CO2 from pulp mills using tanker transportation. Seven hundred kilometre transportation, infrastructure for loading and intermediate storage of the CO2, and a CO2 production rate of approximately 20 kg/s were considered. Since the cost of CO2 transportation by tanker is quite insensitive to the transportation distance and the rate of CO2 output, this value may be considered as a cap

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Fig. 3. Economies of scale of CO2 transportation and storage.

on the transportation and storage cost for projects allowing for transportation over water. In the present paper, the cost of CO2 capture and storage [USD/tCO2] was calculated according to Eq. (1). Cost of CO2 capture and storage ¼

Annual incremental cost Annually captured CO2

ð1Þ

The Annual incremental cost was calculated according to Eq. (2). DCinv þ DCO&M þ DE  pe þ DFb  pfuel þ Ctr

ð2Þ

where DCinv denotes annualised incremental capital cost, DCO&M incremental fixed annual operation and maintenance (O&M) cost, DE annual powerhouse net outputannual powerhouse net output in base case, pe electricity price, DFb annual bark and woody biomass fuel consumptionannual bark and woody biomass fuel consumption in base case, pfuel price of biomass fuel, Ctr cost for CO2 transportation and storage. Incremental capital costs were taken from Table 8 and were annualised using an 11% capital charge rate. The incremental fixed annual O&M cost is 10% of the incremental capital cost. Annual powerhouse net output, annual bark and woody biomass fuel consumption, and annually captured CO2 were calculated using the items bark and woody biomass, powerhouse net output, and CO2 capture rate, respectively, from Tables 5 and 6 and an assumed annual operating time of 330 days per year. Electricity and biomass fuel prices were set to 50 USD/MW h and 4 USD/GJ, respectively. Fig. 4 shows the cost of CO2 capture and storage depending on case and the transportation distance. A 20 USD/tCO2 cost cap for long-distance transportation and storage was assumed, which considers the possibility to transport CO2 over

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Fig. 4. Cost of CO2 capture and storage for the studied BECS systems.

long distances by tanker as outlined above. The results suggest that enhanced CO2 capture through CO shift decreases the cost of CO2 capture and storage. Our cost assessment assumes dedicated single pipelines for each project. If a CO2 grid with trunk pipelines becomes a reality, similar to the case for natural gas, allowing numerous CO2-emitting point sources to be connected to a CO2 transport network, the average scale of the transportation system would increase thus decreasing the average cost. For example, the cost of transporting CO2 5000 km in large-diameter pipelines has been estimated at 25 USD/tCO2 [23].

7. Discussion The analysis of energy systems in this paper assumes predicted future pulp and paper mills with a considerably lower process steam demand than today’s mills. The estimated CO2 reductions of pulp and paper mills with BECS would be higher than calculated in this paper if mills of today’s performance were used as reference. For example, the average electrical efficiency of Swedish Kraft pulp mills, which are energy efficient in an international perspective [25], is approximately 9%. As a comparison, MPM3 in this study has an electrical efficiency of 20% in spite of the penalties due to CO2 capture. There are issues that call for more detailed examination than is contained in this paper. For example, the paper discusses fuelling gas turbines with hydrogen-rich feed gas. We have considered using near conventional gas turbines with dilution of the fuel with inert gas (air) as coolant in the combustion for NOx control. Other strategies for power generation from hydrogen or hydrogen-rich fuels that could be considered to get a more complete picture include fuel cells with combined cycles and hydrogen/oxygen combustion turbine cycles. However, we regarded the detail

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of this paper sufficient for our level of systems analysis. Moreover, it should also be noted that there is considerable experience accumulated in the chemical and petroleum industries for operating shift reactors, physical absorption units, and the liquefaction and transportation of CO2. The attractiveness of carbon-negative production depends on the benefits to the manufacturer, both tangible and intangible, which would have to compensate for production cost increases. Tangible benefits could include credits given for CO2 reductions and a price increase of branded products featuring carbon-negativity. Credits for CO2 reductions could be based on either a tax approach with a ‘‘negative tax’’ or an emissions trading system whereby owners of plants that remove CO2 from the atmosphere are awarded credits corresponding to the amount of CO2 removed. Intangible benefits include the goodwill value that companies implementing carbon-negative production could earn. It is clear, however, that biomassbased industries rich in self-generated biomass residues will be uniquely equipped to generate carbon-negative production in the future provided the development of safe CO2 storage technologies. 8. Conclusions We have evaluated the energy efficiency and CO2 balances of biomass-based CHP systems with CO2 capture and storage (BECS) in pulp and paper mills. It was shown that the CO2 reduction potential of CHP systems in pulp and paper mills could be leveraged with BECS. The commercialisation of technologies for black liquor and biomass gasification and permanent CO2 storage would make it possible for pulp and paper mills to manufacture pulp or paper while being a net exporter of biomass-based electricity and, at the same time, remove substantial amounts of CO2 from the atmosphere for each tonne pulp or paper produced. Thus, carbonnegative production can become economically feasible provided sufficient incentives to generate negative CO2 emissions, e.g. through a ‘‘negative CO2 tax’’ or emission trading permits paid to owners of plants that remove CO2 from the atmosphere. Acknowledgements Financial support from the Kempe Foundation, the Swedish Energy Agency, and the Chinese Academy of Sciences is gratefully acknowledged. References [1] Intergovernmental Panel on Climate Change (IPCC). Climate change 2001: the scientific basis. Cambridge: Cambridge University Press; 2001. [2] FAO. Statistical databases. 2002. Available from: http://www.fao.org. [3] Obersteiner M, Azar Ch, Kauppi P, Mo¨llersten K, Moreira J, Nilsson S, et al. Managing climate risk. Science 2001;294(5543):786–7.

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