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Performance evaluation of adding ethanol production into an existing combined heat and power plant
Bioresource Technology 101 (2010) 613–618
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Performance evaluation of adding ethanol production into an existing combined heat and power plant F. Starfelt a,*, E. Thorin a, E. Dotzauer a, J. Yan a,b a b
Mälardalen University, SE-721 23 Västerås, Sweden Royal Institute of Technology, SE-100 44 Stockholm, Sweden
a r t i c l e
i n f o
Article history: Received 21 April 2009 Received in revised form 28 July 2009 Accepted 31 July 2009 Available online 15 September 2009 Keywords: Polygeneration Combined heat and power Bio-ethanol Efficiency improvement Bioenergy
a b s t r a c t In this paper, the configuration and performance of a polygeneration system are studied by modelling the integration of a lignocellulosic wood-to-ethanol process with an existing combined heat and power (CHP) plant. Data from actual plants are applied to validate the simulation models. The integrated polygeneration system reaches a total efficiency of 50%, meeting the heating load in the district heating system. Excess heat from the ethanol production plant supplies 7.9 MW to the district heating system, accounting for 17.5% of the heat supply at full heating load. The simulation results show that the production of ethanol from woody biomass is more efficient when integrated with a CHP plant compared to a stand-alone production plant. The total biomass consumption is reduced by 13.9% while producing the same amounts of heat, electricity and ethanol fuel as in the stand-alone configurations. The results showed that another feature of the integrated polygeneration system is the longer annual operating period compared to existing cogeneration. Thus, the renewable electricity production is increased by 2.7% per year. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The use of renewable vehicle fuels (biofuel) represents one of the key options for reducing pressure on the levels of atmospheric carbon. The fossil fuels utilized in the transportation sector are estimated to account for 21% of the greenhouse gas emissions in the EU. Measures have been taken to reduce these emissions and reach the EU goal of 5.75% biofuels in the year 2010 (Commission of the European Communities, 2006). In 2005, the usage of biofuels in Sweden reached 2.2%, mainly due to mixing ethanol with gasoline. Domestic ethanol production is low and the majority of the ethanol used in Sweden is imported from Brazil. The feedstock in Brazil is sugarcane, which can easily be fermented to ethanol with yeast. Sweden’s cold climate is not favourable for cultivation of sugarcane, and thus agro-feedstock has been considered. However, approximately 50–75% of the costs for producing ethanol from agro-feedstock such as wheat, corn or sugar beets are feedstock costs (Reith et al., 2001; Balat and Balat, 2009). Studies have shown that wood has a significantly better energy yield than wheat as an energy crop (Sims et al., 2006). In Sweden, woody biomass is used in cogeneration plants for residence heating and simultaneous electricity production. Due to the large heat load variation, the availability of
* Corresponding author. Address: Mälardalen University, P.O. Box 883, SE-721 23 Västerås, Sweden. Tel.: +46 21 101366; fax +46 21 101370. E-mail addresses: [email protected] (F. Starfelt), [email protected] (E. Thorin), [email protected] (E. Dotzauer), [email protected] (J. Yan). 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.07.087
cogeneration plants is low. This calls for an increase in operation and effective use of low-temperature heat from cogeneration plants. The objective of this paper was to investigate integration synergies by integrating ethanol production with an existing CHP plant with focus on system performance. Such a system with multiple products is referred to as a polygeneration system, and might come to play an important role of a regional bioenergy system (Dahlquist et al., 2007). CHP plants can supply energy for biofuel production where excess heat released from the biofuel production process can be utilized in district heating systems. Pfeffer et al. (2005, 2007) and Reith et al. (2001) have studied simultaneous ethanol, electricity and heat production with little attention on the technology of fermenting cellulosic wood or the performance of the complete detailed process. In this paper, detailed and complete integration of the CHP and ethanol processes are studied by integration of a lignocellulose-to-ethanol process and an existing CHP plant. The results are significant for the retrofit or extension of an existing cogeneration plant to a polygeneration system. 2. Biofuel production The technology of fermenting hydrocarbons from wood is a promising technology that has been studied for the last two decades, see for example Reith et al. (2001) and Gnansounou et al. (2009). Wood consists mainly of cellulose, hemicellulose and lignin. The lignin is loosened by steam pre-treatment, exposing the cellulose and hemicellulose for hydrolysis to mainly glucose and
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F. Starfelt et al. / Bioresource Technology 101 (2010) 613–618
xylose, respectively. Glucose can be fermented with regular baker’s yeast whereas xylose requires specially selected or genetically modified micro-organisms (Reith et al., 2001). The non-fermentables, mainly lignin, are separated before or after fermentation depending on the configuration of the process. In a polygeneration process, this by-product could be used as fuel in a CHP plant to provide steam for the biofuel process and the steam turbine whilst the excess heat is utilized for district heating. Biogas, consisting primarily of methane and carbon dioxide, is produced by anaerobic digestion of organic material. In order to increase the efficiency of polygeneration systems, anaerobic digestion of the stillage from the distillation in the ethanol production process is possible. The by-products could be used to produce animal feed which might be more economically rewarding (Morgen and Henriksen, 2006). It is vital that the by-products are utilized efficiently (Börjesson, 2009). Börjesson (2006) states that the energy required to dry the by-products so that they become suitable for animal feed is substantial and the production of biogas is more energy-efficient. With polygeneration technology, required heat for biogas production can be provided from a CHP plant with profitable outcomes such as increased electricity production. Many studies have been done on the mechanism and process of fermentation using woody biomass as feedstock. However, the integration of ethanol production with cogeneration plants has not been studied in detail. 3. From combined heat and power to polygeneration Sweden has about 220 district heating systems, accounting for more than 40% of the total heat supply (Sahlin et al., 2003). The district heating is primarily produced from renewable energy resources. In order to increase the efficiency of ethanol production from woody biomass, it is vital to utilize the excess heat from the process. CHP plants that provide heat in district heating systems often reach efficiencies up to 90%. In order to integrate biofuel production with CHP plants it is of importance, if the efficiencies are to be kept high, to make use of the low-temperature excess heat in the district heating system or in the process itself. Since the electricity production is related to the amount of heat provided by the heat condenser after the steam turbine, it is of interest to analyze the impact the integration would have on the electricity and heat production. A polygeneration system integrates biofuel production with existing CHP plants to produce multiple energy products including, for example, electricity, heat, ethanol and biogas. Such a system could preferably use local arable land and forest by-products to supply the feedstock. 4. Methodology and assumptions This paper investigates the feasibility of integrating bio-ethanol production with an existing CHP plant in Enköping, Sweden. The technology for ethanol production used in Örnsköldsvik is applied. Computer-based simulation tools are used to perform this case study. The methodology includes the following steps: 1. Field study and data collection: Data collection was conducted by field studies at the existing cogeneration plant in Enköping and ethanol production pilot plant in Örnsköldsvik. The heat and power production as well as supply load was determined, and the data for validation of the modelling was collected. 2. Modelling of the processes: Computer-based simulation models were developed and validated with the collected data. 3. Integration of bio-ethanol production into the existing CHP plant: The system configuration was designed by integrating a bioethanol production process into the existing CHP plant.
Table 1 System input data. Data
Design case
Summer case
Isentropic turbine efficiency Steam inlet to the turbine Temperature Pressure District heating Temperature supply Temperature return Flow Feedstock to ethanol process Saccharification Steam in Temperature Pressure Hydrolysis Temperature Steam in Pressure
80%
75%
540 °C 100 bar
500 °C 100 bar
87 °C 42 °C 240 kg/s 25 kg/s
87 °C 42 °C 55 kg/s 25 kg/s
375 °C 30 bar
348 °C 30 bar
190 °C
190 °C
15 bar
15 bar
Following the above, a systematic analysis was carried out for evaluation of the performance of the systems studied in this paper. Feedstock composition for Salix was used in the modelling of the ethanol process (Sassner et al., 2006). The hemicellulose in wood is mainly made up of xylan. The other hemicellulose carbohydrates (galactan, arabinan and mannan) were assumed to have the same reactions and yields as xylan in the model (Aden et al., 2002). The conversion yield of the pre-treatment hydrolyser for hemicellulose carbohydrates was assumed to be an average of 0.9 (Aden et al., 2002) (mole based). For cellulosic carbohydrates the yield was assumed to be 0.1. In the saccharification stage, the yield was assumed to be 0.9 for conversion from the cellulose carbohydrates left in the mash. The total hydrolysis reactions were modelled to take place in the hydrolysis reactors. Treatment of the ethanol from the 94% (mass based) concentration after distillation is not considered in this paper. The dewatering can be done with adsorption using a molecular sieve (Pfeffer et al., 2007; Cardona and Sanchez, 2007). The developed model for the ethanol plant includes the assumptions that the energy requirement for hydrolysis and pre-treatment is a linear function of feedstock input. Functions for upscaling and downscaling of the plant were declared in the model. The energy consumption of the distillation stage follows the theoretical energy consumption as a function of ethanol concentration in the broth (Öhgren et al., 2006). The steam introduced in the stand-alone ethanol plant is considered to be produced without losses and the reference temperature used is 20 °C. Auxiliary power consumption in the ethanol plant is not considered in this paper. The composition of by-products, mainly lignin, was assumed to be the same as for wood chips. Eriksson et al. (2003) describe the wood hydrolysis residues to have low ash content and a Lower Heating Value (LHV) of 22.8 MJ/kg at 4.4% moisture content. An equivalent LHV of 8.89 MJ/kg based on 57% moisture content of the residues was used in this paper. Energy needed to dry the lignin fuel after the membrane filter was not considered. The flue gas condensation system and other cooling arrangements in the district heating system were not included in the simulations. The system input data are summarized in Table 1.
5. System description The cogeneration plant in Enköping is 100% biomass based and supplies 98% of the city’s heat demand and nearly half of the city’s electricity demand. The grate-fired boiler produces steam at 540 °C and 100 bars. At full load the steam flow is 27 kg/s and the CHP
F. Starfelt et al. / Bioresource Technology 101 (2010) 613–618
plant produces 55 MW heat (including flue gas condenser) and 22 MW net of electricity. The boiler has two economizers used to preheat incoming feed water before it is vaporized in the walls of the boiler (Fig. 1). The steam is super heated in three steps before expanding in the turbine. The turbine has four extractions at different pressure levels and one insertion after the first turbine stage. The extractions are used for feed water preheaters, deaerator and condensers. The pilot scale plant in Örnsköldsvik is owned by two universities and operated by a company, Sekab. The plant utilizes the separate hydrolysis and fermentation (SHF) process configuration (Fig. 2). Steam is provided to the existing plant by electrical boilers. High-pressure steam is used for the hydrolysis stage of the ethanol production and low-pressure steam is used for distillation and evaporation. Steam from the saccharification stage is re-used in the pre-treatment hydrolysis stage. The integrated system configuration of the bio-ethanol production process and the existing CHP plant is shown in Fig. 3. Steam is extracted from the turbine at 30, 15 and 6 bars for the ethanol production process. Outgoing condensate is used to preheat the district heating water in the return pipeline. 6. System modelling and validation The polygeneration system was modelled with the heat and mass balance software IPSEproTM (Simtech, 2003), designed for energy and chemical engineering. IPSEproTM is a steady-state model-
615
ling software that has the advantage, compared to most other simulation tools, that the user is allowed to develop and modify components in the Model Development Kit (MDK). Existing equations in the power plant library (APP_Lib) provided by Simtech, designer of IPSEproTM, can be modified to fit specific configurations. Furthermore, new components and libraries can be developed in MDK. The process simulation environment (PSE) is used to simulate and choose the configuration of the system. IPSEproTM applies a gradient-based solver using a two-method solving technique (Häggståhl and Dahlquist, 2003). The equations are divided into smaller groups before they are solved simultaneously with a numerical solution method. Measured data from operation of the Enköping CHP plant were used for modelling and validating the CHP plant model. The validation was done on-site at the plant jointly with the plant staff. The essential simulated data are compared with measured plant data in Table 2, where it is shown that the deviations between measured and simulated data are small. The configuration of the pilot scale plant in Örnsköldsvik was used for modelling the ethanol production. The plant configuration design includes steam pre-treatment and the SHF configuration (Table 3). The CHP and ethanol plant models were integrated to a polygeneration system (Fig. 3). The stand-alone models are corresponding to real plant operation data. Steam for hydrolysis is extracted between the first and second stages of the steam turbine, at 30 and 15 bars, respectively. Steam extracted after the third stage of the turbine is providing energy to the distillation unit at 6 bars. Cooling water is used to cool the heat
Fig. 1. Simplified flow scheme of the Enköping CHP plant with steam data for turbine extractions.
Fig. 2. Simplified flow scheme of the ethanol from wood process.
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Fig. 3. Polygeneration system with CHP and ethanol production.
Table 2 Validation of the essential data for the stand-alone CHP simulation model.
7. Results and discussion
Point
Measured
Simulated
Unit
Steam flow Electricity Pressure steam Pressure steam Pressure steam Pressure steam Pressure steam
27.0 22.0 16.2 6.3 2.2 0.6 0.3
27.1 22.0 15.0 6.2 2.2 0.6 0.2
kg/s MW bar bar bar bar bar
extraction 1 extraction 2 extraction 3 extraction 4 condenser
Table 3 Simulated stand-alone ethanol production plant. Reference case ethanol Feedstock in Steam in distillation Steam in hydrolysis 1 Steam in hydrolysis 2 Ethanol Efficiency ethanol plant
222.5 7.9 12.8 9.1 62.4 25%
MW MW MW MW MW –
from the exothermic fermentation reactions in the fermentation vessel and to cool the mash before fermentation. Excess heat from the hydrolysis, distillation and fermentation vessel is used to preheat incoming district heating water before the main condenser.
The models of the stand-alone plants (Figs. 1 and 2) and the polygeneration model shown in Fig. 3 were simulated to evaluate the performance of the systems, including efficiency, power-toheat ratio, fuel supply and operation possibilities. A design case where the district heating load is set to the same amount of heat as the existing CHP plant at full load was simulated initially. The results are shown in Table 4. The stand-alone ethanol plant produces ethanol with an efficiency of 25% and the CHP plant model has a total efficiency of 88%. The ratio between produced electricity and heat (power-to-heat ratio) is 0.48 in the existing CHP plant. The two stand-alone plants together without any integration have a total efficiency of 39%. The integrated polygeneration system gives a total efficiency of 50%, set to supply the same amount of heat to the district heating system as in the single-standing CHP plant. The total biomass consumption for the production of electricity, heat and biofuels can be reduced in the integrated system compared to stand-alone systems as shown in the Sankey diagrams in Fig. 4. In the polygeneration configuration, the stillage from the distillation unit could be digested to biogas. Heat released from the ethanol production plant contributes by 7.9 MW to the district heating system, accounting for 17.5% of the heat demand at the design case. Annual operation of the existing Enköping CHP plant varies due to different heating needs during summer and winter. The results of the polygeneration design case (winter) and summer case are
Table 4 Simulation results of the different plant configurations and load cases. Type of plant Reference CHP Polygeneration Design Summer Incl. biogas
Fuel for combustion (MW)
Electricity (MW)
District heating (MW)
Total efficiency (%)
Ethanol (MW)
Biogas (MW)
Feedstock for hydrolysis (MW)
75.5
22.0
45.1
88
–
–
–
34.1 35.5 34.1
21.9 5.4 21.9
45.1 10.4 45.1
50 42 64
62.4 62.4 62.4
– – 33.9
222.5 222.5 222.5
F. Starfelt et al. / Bioresource Technology 101 (2010) 613–618
617
Fig. 4. Sankey diagrams of the two stand-alone plants and the polygeneration system. EtOH is the ethanol plant. Stillage from the distillation is digested to biogas. The figures are in MW.
presented in Table 4. The CHP plant is usually shut down for a period during summer due to the low heat load in the district heating system, and for maintenance purposes. The existing plant configuration can operate down to 21% of full district heating load. Simulations of a summer case with low heat demand shows that the polygeneration system can operate at loads down to 15% of full load. This shows that the by-products from the ethanol production can supply all required fuel to the CHP plant and an additional 35.5 MW of lignin fuel (negative value in Table 4). The polygeneration system can increase electricity production by 2.7% per year. The findings of the simulations are that it would be theoretically possible to replace 48% of all the gasoline and diesel sold in the city of Enköping with ethanol. Enköping is a transit city, which might imply that more fuel per inhabitant is sold than in other cities of the same size. Thus, other cities could utilize polygeneration systems of this type to provide biofuel to all or most vehicles in the area. The possibility of operating a flexible system where the polygeneration plant could vary its production depending on price setting and other input data would be of interest, though a higher investment cost would be incurred. The economical aspects of the polygeneration system compared to stand-alone plants have to be investigated. To find a usage for the low-temperature water from the condensation of the ethanol after distillation is a challenging task. This large amount of low-temperature excess heat could be used with a profitable outcome. This is included in the objective for future work. Whether it is concerning short term, where it is regarding implementation of biofuel production in existing CHP plants, or in the long term, where new full-scale polygeneration systems are developed, regional systems all have individual criteria of inputs and outputs. Case studies of the use of biomass for simultaneous vehicle fuel production and cogeneration should be analyzed regarding arable land, different feedstock, distribution system, etc., to establish a pathway for a fossil fuel free region (Dahlquist et al., 2007). Leduc et al. (2008) used a computer based model to find economic optimal locations and sizes for methanol production sites regarding feedstock supply, transpor-
tation and distribution. Similar studies would be of interest to investigate for the existing CHP plants in Sweden. The outcome from such analysis would be the geographical locations where the polygeneration design should be implemented with respect to existing district heating systems as well as the chain from farming land or forest to distribution and gas filling stations for the biofuel. In order to increase renewable CHP based electricity production, useful heat loads for low-temperature excess heat is essential. Such heat loads could be found within integration possibilities with biofuel production. One option would be to use lower temperatures and pressures in the hydrolysis and distillation stages. Stillage from the distillation unit can be digested to biogas under anaerobic conditions. If a yield of 60 m3 of methane per ton of total stillage feed is assumed, the total efficiency would reach about 64% (Table 4). The required electricity and thermal energy for anaerobic digestion in the process is not taken into account in the calculations. Utilization of the stillage and other by-products are essential in a polygeneration system to reach a sufficient efficiency. 8. Conclusions The simulation results show that the production of ethanol from wood is more efficient when the process is integrated with a CHP plant, compared to stand-alone plants. Synergies of the integration are that biomass consumption is reduced by 13.9% while producing the same amounts of heat, electricity and ethanol as in the stand-alone configurations. The total efficiency is improved by 11% points in a polygeneration system compared to stand-alone plants. Simulations show that the polygeneration system can be operated more hours annually, thus increasing renewable electricity production over the year. Acknowledgement The Swedish Energy Agency and ENA Energy in Enköping is acknowledged for their funding.
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References Aden, A., Ruth, M., Ibsen, K., Jechura, J., Neeves, K., Sheehan, J., Wallace, B., 2002. Lignocellulosic biomass to ethanol process design and economics utilizing cocurrent dilute acid prehydrolysis and enzymatic hydrolysis for corn stover. Technical Report, National Renewable Energy Laboratory, US Department of Energy Laboratory. Balat, M., Balat, H., 2009. Recent trends in global production and utilization of bioethanol fuel. Applied Energy 86, 2273–2282. Börjesson, P., 2006. Energibalans för bioetanol – en kunskapsöversikt [Energy balance of bioethanol – a review]. Report No. 59. Department of Environmental and Energy Systems Studies, Lund University (in Swedish). Börjesson, P., 2009. Good or bad bioethanol from a greenhouse gas perspective – What determines this? Applied Energy 86, 589–594. Cardona, C.A., Sanchez, O.J., 2007. Fuel ethanol production: process design trends and integration opportunities. Bioresource Technology 98, 2415–2457. Commission of the European Communities, 2006. Communication from the Commission – An EU Strategy for Biofuels, SEC (2006), p. 142. Dahlquist, E., Thorin, E., Yan, J., 2007. Alternative pathways to a fossil-fuel free energy system in the Mälardalen Region of Sweden. International Journal of Energy Research 3, 1226–1236. Eriksson, G., Kjellström, B., Lundqvist, B., Paulrud, S., 2003. Combustion of wood hydrolysis residue in a 150 kW powder burner. Fuel 83, 1635–1641. Gnansounou, E., Dauriat, A., Villegas, J., Panichelli, L., 2009. Life cycle assessment of biofuels: energy and greenhouse gas balances. Bioresource Technology. doi:10.1016/j.biortech.2009.05.067. Häggståhl, D., Dahlquist, E., 2003. Evaluation of Prosim and IPSEpro, two heat and mass balance simulation softwares. In: Conference Proceedings of SIMS, 18–19 September, Västerås, Sweden.
Leduc, S., Schwab, D., Dotzauer, E., Schmid, E., Obersteiner, M., 2008. Optimal location of wood gasification plants for methanol production with heat recovery. International Journal of Energy Research 32 (12), 1080–1091. Morgen, C., Henriksen, N., 2006. Integrated Biomass Utilisation System. Publishable Final Report. Doc. No. 486762. Öhgren, K., Rudolf, A., Galbe, M., Zacchi, G., 2006. Fuel ethanol production from steam-pretreated corn stover using SSF at higher dry matter content. Biomass and Bioenergy 30, 863–869. Pfeffer, M., Wukovits, W., Friedl, A., 2005. Optimization of the energy demand of bioethanol production by process integration. Chemical Engineering Transactions 7, 115–120. Pfeffer, M., Wukovits, W., Beckmann, G., Friedl, A., 2007. Analysis and decrease of the energy demand of bioethanol-production by process integration. Applied Thermal Engineering 27, 2657–2664. Reith, J., Veenkamp, J., van Ree, R., de Laat, W., Niessen, J., de Jong, E., Elbersen, H., Claassen, P., 2001. Co-production of bio-ethanol, electricity and heat from biomass wastes: potential and R&D issues. In: Contribution to the First European Conference on Agriculture and Renewable Energy, 6–8 May, Amsterdam, the Netherlands. Sahlin, J., Knutsson, D., Ekvall, T., 2003. Effects of planned expansion of waste incineration in the Swedish district heating systems. Resources, Conservation and Recycling 41, 279–292. Sassner, P., Galbe, M., Zacchi, G., 2006. Bioethanol production based on simultaneous saccharification and fermentation of steam-pretreated Salix at high dry-matter content. Enzyme and Microbial Technology 39, 756–762. Sims, R., Hastings, A., Schlamadinger, B., Taylor, G., Smith, P., 2006. Energy crops: current status and future prospects. Global Change Biology 12, 2054–2076. Simtech, 2003. IPSEproTM Users Guide.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Performance evaluation of adding ethanol production into an existing combined heat and power plant F. Starfelt a,*, E. Thorin a, E. Dotzauer a, J. Yan a,b a b
Mälardalen University, SE-721 23 Västerås, Sweden Royal Institute of Technology, SE-100 44 Stockholm, Sweden
a r t i c l e
i n f o
Article history: Received 21 April 2009 Received in revised form 28 July 2009 Accepted 31 July 2009 Available online 15 September 2009 Keywords: Polygeneration Combined heat and power Bio-ethanol Efficiency improvement Bioenergy
a b s t r a c t In this paper, the configuration and performance of a polygeneration system are studied by modelling the integration of a lignocellulosic wood-to-ethanol process with an existing combined heat and power (CHP) plant. Data from actual plants are applied to validate the simulation models. The integrated polygeneration system reaches a total efficiency of 50%, meeting the heating load in the district heating system. Excess heat from the ethanol production plant supplies 7.9 MW to the district heating system, accounting for 17.5% of the heat supply at full heating load. The simulation results show that the production of ethanol from woody biomass is more efficient when integrated with a CHP plant compared to a stand-alone production plant. The total biomass consumption is reduced by 13.9% while producing the same amounts of heat, electricity and ethanol fuel as in the stand-alone configurations. The results showed that another feature of the integrated polygeneration system is the longer annual operating period compared to existing cogeneration. Thus, the renewable electricity production is increased by 2.7% per year. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The use of renewable vehicle fuels (biofuel) represents one of the key options for reducing pressure on the levels of atmospheric carbon. The fossil fuels utilized in the transportation sector are estimated to account for 21% of the greenhouse gas emissions in the EU. Measures have been taken to reduce these emissions and reach the EU goal of 5.75% biofuels in the year 2010 (Commission of the European Communities, 2006). In 2005, the usage of biofuels in Sweden reached 2.2%, mainly due to mixing ethanol with gasoline. Domestic ethanol production is low and the majority of the ethanol used in Sweden is imported from Brazil. The feedstock in Brazil is sugarcane, which can easily be fermented to ethanol with yeast. Sweden’s cold climate is not favourable for cultivation of sugarcane, and thus agro-feedstock has been considered. However, approximately 50–75% of the costs for producing ethanol from agro-feedstock such as wheat, corn or sugar beets are feedstock costs (Reith et al., 2001; Balat and Balat, 2009). Studies have shown that wood has a significantly better energy yield than wheat as an energy crop (Sims et al., 2006). In Sweden, woody biomass is used in cogeneration plants for residence heating and simultaneous electricity production. Due to the large heat load variation, the availability of
* Corresponding author. Address: Mälardalen University, P.O. Box 883, SE-721 23 Västerås, Sweden. Tel.: +46 21 101366; fax +46 21 101370. E-mail addresses: [email protected] (F. Starfelt), [email protected] (E. Thorin), [email protected] (E. Dotzauer), [email protected] (J. Yan). 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.07.087
cogeneration plants is low. This calls for an increase in operation and effective use of low-temperature heat from cogeneration plants. The objective of this paper was to investigate integration synergies by integrating ethanol production with an existing CHP plant with focus on system performance. Such a system with multiple products is referred to as a polygeneration system, and might come to play an important role of a regional bioenergy system (Dahlquist et al., 2007). CHP plants can supply energy for biofuel production where excess heat released from the biofuel production process can be utilized in district heating systems. Pfeffer et al. (2005, 2007) and Reith et al. (2001) have studied simultaneous ethanol, electricity and heat production with little attention on the technology of fermenting cellulosic wood or the performance of the complete detailed process. In this paper, detailed and complete integration of the CHP and ethanol processes are studied by integration of a lignocellulose-to-ethanol process and an existing CHP plant. The results are significant for the retrofit or extension of an existing cogeneration plant to a polygeneration system. 2. Biofuel production The technology of fermenting hydrocarbons from wood is a promising technology that has been studied for the last two decades, see for example Reith et al. (2001) and Gnansounou et al. (2009). Wood consists mainly of cellulose, hemicellulose and lignin. The lignin is loosened by steam pre-treatment, exposing the cellulose and hemicellulose for hydrolysis to mainly glucose and
614
F. Starfelt et al. / Bioresource Technology 101 (2010) 613–618
xylose, respectively. Glucose can be fermented with regular baker’s yeast whereas xylose requires specially selected or genetically modified micro-organisms (Reith et al., 2001). The non-fermentables, mainly lignin, are separated before or after fermentation depending on the configuration of the process. In a polygeneration process, this by-product could be used as fuel in a CHP plant to provide steam for the biofuel process and the steam turbine whilst the excess heat is utilized for district heating. Biogas, consisting primarily of methane and carbon dioxide, is produced by anaerobic digestion of organic material. In order to increase the efficiency of polygeneration systems, anaerobic digestion of the stillage from the distillation in the ethanol production process is possible. The by-products could be used to produce animal feed which might be more economically rewarding (Morgen and Henriksen, 2006). It is vital that the by-products are utilized efficiently (Börjesson, 2009). Börjesson (2006) states that the energy required to dry the by-products so that they become suitable for animal feed is substantial and the production of biogas is more energy-efficient. With polygeneration technology, required heat for biogas production can be provided from a CHP plant with profitable outcomes such as increased electricity production. Many studies have been done on the mechanism and process of fermentation using woody biomass as feedstock. However, the integration of ethanol production with cogeneration plants has not been studied in detail. 3. From combined heat and power to polygeneration Sweden has about 220 district heating systems, accounting for more than 40% of the total heat supply (Sahlin et al., 2003). The district heating is primarily produced from renewable energy resources. In order to increase the efficiency of ethanol production from woody biomass, it is vital to utilize the excess heat from the process. CHP plants that provide heat in district heating systems often reach efficiencies up to 90%. In order to integrate biofuel production with CHP plants it is of importance, if the efficiencies are to be kept high, to make use of the low-temperature excess heat in the district heating system or in the process itself. Since the electricity production is related to the amount of heat provided by the heat condenser after the steam turbine, it is of interest to analyze the impact the integration would have on the electricity and heat production. A polygeneration system integrates biofuel production with existing CHP plants to produce multiple energy products including, for example, electricity, heat, ethanol and biogas. Such a system could preferably use local arable land and forest by-products to supply the feedstock. 4. Methodology and assumptions This paper investigates the feasibility of integrating bio-ethanol production with an existing CHP plant in Enköping, Sweden. The technology for ethanol production used in Örnsköldsvik is applied. Computer-based simulation tools are used to perform this case study. The methodology includes the following steps: 1. Field study and data collection: Data collection was conducted by field studies at the existing cogeneration plant in Enköping and ethanol production pilot plant in Örnsköldsvik. The heat and power production as well as supply load was determined, and the data for validation of the modelling was collected. 2. Modelling of the processes: Computer-based simulation models were developed and validated with the collected data. 3. Integration of bio-ethanol production into the existing CHP plant: The system configuration was designed by integrating a bioethanol production process into the existing CHP plant.
Table 1 System input data. Data
Design case
Summer case
Isentropic turbine efficiency Steam inlet to the turbine Temperature Pressure District heating Temperature supply Temperature return Flow Feedstock to ethanol process Saccharification Steam in Temperature Pressure Hydrolysis Temperature Steam in Pressure
80%
75%
540 °C 100 bar
500 °C 100 bar
87 °C 42 °C 240 kg/s 25 kg/s
87 °C 42 °C 55 kg/s 25 kg/s
375 °C 30 bar
348 °C 30 bar
190 °C
190 °C
15 bar
15 bar
Following the above, a systematic analysis was carried out for evaluation of the performance of the systems studied in this paper. Feedstock composition for Salix was used in the modelling of the ethanol process (Sassner et al., 2006). The hemicellulose in wood is mainly made up of xylan. The other hemicellulose carbohydrates (galactan, arabinan and mannan) were assumed to have the same reactions and yields as xylan in the model (Aden et al., 2002). The conversion yield of the pre-treatment hydrolyser for hemicellulose carbohydrates was assumed to be an average of 0.9 (Aden et al., 2002) (mole based). For cellulosic carbohydrates the yield was assumed to be 0.1. In the saccharification stage, the yield was assumed to be 0.9 for conversion from the cellulose carbohydrates left in the mash. The total hydrolysis reactions were modelled to take place in the hydrolysis reactors. Treatment of the ethanol from the 94% (mass based) concentration after distillation is not considered in this paper. The dewatering can be done with adsorption using a molecular sieve (Pfeffer et al., 2007; Cardona and Sanchez, 2007). The developed model for the ethanol plant includes the assumptions that the energy requirement for hydrolysis and pre-treatment is a linear function of feedstock input. Functions for upscaling and downscaling of the plant were declared in the model. The energy consumption of the distillation stage follows the theoretical energy consumption as a function of ethanol concentration in the broth (Öhgren et al., 2006). The steam introduced in the stand-alone ethanol plant is considered to be produced without losses and the reference temperature used is 20 °C. Auxiliary power consumption in the ethanol plant is not considered in this paper. The composition of by-products, mainly lignin, was assumed to be the same as for wood chips. Eriksson et al. (2003) describe the wood hydrolysis residues to have low ash content and a Lower Heating Value (LHV) of 22.8 MJ/kg at 4.4% moisture content. An equivalent LHV of 8.89 MJ/kg based on 57% moisture content of the residues was used in this paper. Energy needed to dry the lignin fuel after the membrane filter was not considered. The flue gas condensation system and other cooling arrangements in the district heating system were not included in the simulations. The system input data are summarized in Table 1.
5. System description The cogeneration plant in Enköping is 100% biomass based and supplies 98% of the city’s heat demand and nearly half of the city’s electricity demand. The grate-fired boiler produces steam at 540 °C and 100 bars. At full load the steam flow is 27 kg/s and the CHP
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plant produces 55 MW heat (including flue gas condenser) and 22 MW net of electricity. The boiler has two economizers used to preheat incoming feed water before it is vaporized in the walls of the boiler (Fig. 1). The steam is super heated in three steps before expanding in the turbine. The turbine has four extractions at different pressure levels and one insertion after the first turbine stage. The extractions are used for feed water preheaters, deaerator and condensers. The pilot scale plant in Örnsköldsvik is owned by two universities and operated by a company, Sekab. The plant utilizes the separate hydrolysis and fermentation (SHF) process configuration (Fig. 2). Steam is provided to the existing plant by electrical boilers. High-pressure steam is used for the hydrolysis stage of the ethanol production and low-pressure steam is used for distillation and evaporation. Steam from the saccharification stage is re-used in the pre-treatment hydrolysis stage. The integrated system configuration of the bio-ethanol production process and the existing CHP plant is shown in Fig. 3. Steam is extracted from the turbine at 30, 15 and 6 bars for the ethanol production process. Outgoing condensate is used to preheat the district heating water in the return pipeline. 6. System modelling and validation The polygeneration system was modelled with the heat and mass balance software IPSEproTM (Simtech, 2003), designed for energy and chemical engineering. IPSEproTM is a steady-state model-
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ling software that has the advantage, compared to most other simulation tools, that the user is allowed to develop and modify components in the Model Development Kit (MDK). Existing equations in the power plant library (APP_Lib) provided by Simtech, designer of IPSEproTM, can be modified to fit specific configurations. Furthermore, new components and libraries can be developed in MDK. The process simulation environment (PSE) is used to simulate and choose the configuration of the system. IPSEproTM applies a gradient-based solver using a two-method solving technique (Häggståhl and Dahlquist, 2003). The equations are divided into smaller groups before they are solved simultaneously with a numerical solution method. Measured data from operation of the Enköping CHP plant were used for modelling and validating the CHP plant model. The validation was done on-site at the plant jointly with the plant staff. The essential simulated data are compared with measured plant data in Table 2, where it is shown that the deviations between measured and simulated data are small. The configuration of the pilot scale plant in Örnsköldsvik was used for modelling the ethanol production. The plant configuration design includes steam pre-treatment and the SHF configuration (Table 3). The CHP and ethanol plant models were integrated to a polygeneration system (Fig. 3). The stand-alone models are corresponding to real plant operation data. Steam for hydrolysis is extracted between the first and second stages of the steam turbine, at 30 and 15 bars, respectively. Steam extracted after the third stage of the turbine is providing energy to the distillation unit at 6 bars. Cooling water is used to cool the heat
Fig. 1. Simplified flow scheme of the Enköping CHP plant with steam data for turbine extractions.
Fig. 2. Simplified flow scheme of the ethanol from wood process.
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Fig. 3. Polygeneration system with CHP and ethanol production.
Table 2 Validation of the essential data for the stand-alone CHP simulation model.
7. Results and discussion
Point
Measured
Simulated
Unit
Steam flow Electricity Pressure steam Pressure steam Pressure steam Pressure steam Pressure steam
27.0 22.0 16.2 6.3 2.2 0.6 0.3
27.1 22.0 15.0 6.2 2.2 0.6 0.2
kg/s MW bar bar bar bar bar
extraction 1 extraction 2 extraction 3 extraction 4 condenser
Table 3 Simulated stand-alone ethanol production plant. Reference case ethanol Feedstock in Steam in distillation Steam in hydrolysis 1 Steam in hydrolysis 2 Ethanol Efficiency ethanol plant
222.5 7.9 12.8 9.1 62.4 25%
MW MW MW MW MW –
from the exothermic fermentation reactions in the fermentation vessel and to cool the mash before fermentation. Excess heat from the hydrolysis, distillation and fermentation vessel is used to preheat incoming district heating water before the main condenser.
The models of the stand-alone plants (Figs. 1 and 2) and the polygeneration model shown in Fig. 3 were simulated to evaluate the performance of the systems, including efficiency, power-toheat ratio, fuel supply and operation possibilities. A design case where the district heating load is set to the same amount of heat as the existing CHP plant at full load was simulated initially. The results are shown in Table 4. The stand-alone ethanol plant produces ethanol with an efficiency of 25% and the CHP plant model has a total efficiency of 88%. The ratio between produced electricity and heat (power-to-heat ratio) is 0.48 in the existing CHP plant. The two stand-alone plants together without any integration have a total efficiency of 39%. The integrated polygeneration system gives a total efficiency of 50%, set to supply the same amount of heat to the district heating system as in the single-standing CHP plant. The total biomass consumption for the production of electricity, heat and biofuels can be reduced in the integrated system compared to stand-alone systems as shown in the Sankey diagrams in Fig. 4. In the polygeneration configuration, the stillage from the distillation unit could be digested to biogas. Heat released from the ethanol production plant contributes by 7.9 MW to the district heating system, accounting for 17.5% of the heat demand at the design case. Annual operation of the existing Enköping CHP plant varies due to different heating needs during summer and winter. The results of the polygeneration design case (winter) and summer case are
Table 4 Simulation results of the different plant configurations and load cases. Type of plant Reference CHP Polygeneration Design Summer Incl. biogas
Fuel for combustion (MW)
Electricity (MW)
District heating (MW)
Total efficiency (%)
Ethanol (MW)
Biogas (MW)
Feedstock for hydrolysis (MW)
75.5
22.0
45.1
88
–
–
–
34.1 35.5 34.1
21.9 5.4 21.9
45.1 10.4 45.1
50 42 64
62.4 62.4 62.4
– – 33.9
222.5 222.5 222.5
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Fig. 4. Sankey diagrams of the two stand-alone plants and the polygeneration system. EtOH is the ethanol plant. Stillage from the distillation is digested to biogas. The figures are in MW.
presented in Table 4. The CHP plant is usually shut down for a period during summer due to the low heat load in the district heating system, and for maintenance purposes. The existing plant configuration can operate down to 21% of full district heating load. Simulations of a summer case with low heat demand shows that the polygeneration system can operate at loads down to 15% of full load. This shows that the by-products from the ethanol production can supply all required fuel to the CHP plant and an additional 35.5 MW of lignin fuel (negative value in Table 4). The polygeneration system can increase electricity production by 2.7% per year. The findings of the simulations are that it would be theoretically possible to replace 48% of all the gasoline and diesel sold in the city of Enköping with ethanol. Enköping is a transit city, which might imply that more fuel per inhabitant is sold than in other cities of the same size. Thus, other cities could utilize polygeneration systems of this type to provide biofuel to all or most vehicles in the area. The possibility of operating a flexible system where the polygeneration plant could vary its production depending on price setting and other input data would be of interest, though a higher investment cost would be incurred. The economical aspects of the polygeneration system compared to stand-alone plants have to be investigated. To find a usage for the low-temperature water from the condensation of the ethanol after distillation is a challenging task. This large amount of low-temperature excess heat could be used with a profitable outcome. This is included in the objective for future work. Whether it is concerning short term, where it is regarding implementation of biofuel production in existing CHP plants, or in the long term, where new full-scale polygeneration systems are developed, regional systems all have individual criteria of inputs and outputs. Case studies of the use of biomass for simultaneous vehicle fuel production and cogeneration should be analyzed regarding arable land, different feedstock, distribution system, etc., to establish a pathway for a fossil fuel free region (Dahlquist et al., 2007). Leduc et al. (2008) used a computer based model to find economic optimal locations and sizes for methanol production sites regarding feedstock supply, transpor-
tation and distribution. Similar studies would be of interest to investigate for the existing CHP plants in Sweden. The outcome from such analysis would be the geographical locations where the polygeneration design should be implemented with respect to existing district heating systems as well as the chain from farming land or forest to distribution and gas filling stations for the biofuel. In order to increase renewable CHP based electricity production, useful heat loads for low-temperature excess heat is essential. Such heat loads could be found within integration possibilities with biofuel production. One option would be to use lower temperatures and pressures in the hydrolysis and distillation stages. Stillage from the distillation unit can be digested to biogas under anaerobic conditions. If a yield of 60 m3 of methane per ton of total stillage feed is assumed, the total efficiency would reach about 64% (Table 4). The required electricity and thermal energy for anaerobic digestion in the process is not taken into account in the calculations. Utilization of the stillage and other by-products are essential in a polygeneration system to reach a sufficient efficiency. 8. Conclusions The simulation results show that the production of ethanol from wood is more efficient when the process is integrated with a CHP plant, compared to stand-alone plants. Synergies of the integration are that biomass consumption is reduced by 13.9% while producing the same amounts of heat, electricity and ethanol as in the stand-alone configurations. The total efficiency is improved by 11% points in a polygeneration system compared to stand-alone plants. Simulations show that the polygeneration system can be operated more hours annually, thus increasing renewable electricity production over the year. Acknowledgement The Swedish Energy Agency and ENA Energy in Enköping is acknowledged for their funding.
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