- Email: [email protected]
Biomass fuelled trigeneration system in selected buildings
Energy Conversion and Management 52 (2011) 2448–2454
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Biomass fuelled trigeneration system in selected buildings Y. Huang a,⇑, Y.D. Wang b, S. Rezvani a, D.R. McIlveen-Wright a, M. Anderson a, N.J. Hewitt a a b
Centre for Sustainable Technologies, School of Built Environment, University of Ulster, Newtownabbey, BT37 0QB, UK The Sir Joseph Swan Institute for Energy Research, Newcastle University, Newcastle Upon Tyne, NE1 7RU, UK
a r t i c l e
i n f o
Article history: Available online 21 February 2011 Keywords: Trigeneration Computational simulation Biomass Gasifier Techno-economic analyses
a b s t r a c t Many buildings require simultaneous electricity, heating and cooling. Biomass is one of the renewable energy sources which is not intermittent, location-dependent or very difficult to store. If grown sustainably, biomass can be considered to be CO2 neutral. A trigeneration system consisting of an internal combustion (IC) engine integrated with biomass gasification may offer a combination for delivering heat, electricity and cooling cleanly and economically. The producer gas generated by the gasifier is used to provide electricity for building use via the IC engine. The waste heat is recovered from the engine cooling system and exhaust gases to supply hot water to space heating, excess heat is also used to drive an absorption cooling system. The proposed system is designed to meet the energy requirements for selected commercial buildings and district heating/cooling applications. This work focuses on the modeling and simulation of a commercial building scale trigeneration plant fuelled by a biomass downdraft gasifier. In order to use both energy and financial resources most efficiently, technical and economic analyses were carried out, using the ECLIPSE process simulation package. The study also looks at the impact of different biomass feedstock (willow, rice husk and miscanthus) on the performance of a trigeneration plant. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The emission of carbon dioxide from fossil fuel-fired power generation plants and the consequent effect on the global environment is a major concern. Given the global challenges related to climate, methods for managing and reducing these emissions must be found and implemented. The replacement of all or part of these fossil fuels by renewable energy sources, such as biomass and waste, is the obvious alternative. Biomass is available in abundance and practically everywhere. If grown in a sustainable manor the use of biomass, which is considered to produce no net CO2 emissions in its life cycle, and as a replacement for fossil fuels in power generation systems is one of the most attractive applications of reducing CO2 emissions. There are a range of technologies for providing heat and electricity from biomass [1], but most of these technologies have reduced system efficiency at small scale [2]. Trigeneration, as an efficient solution, offers simultaneous generation of electrical power, heating and refrigeration/cooling utilizing desirable feedstock combinations from a single primary energy source [3,4]. Typical applications can be found in many commercial buildings, such as hotels, hospitals or multi-residential communities. Instead of aiming towards a high electricity output, as an innovative type of renewable energy application, the goal of trigen⇑ Corresponding author. E-mail address: [email protected] (Y. Huang). 0196-8904/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2010.12.053
eration powered by biomass is to improve the overall energy utilization efficiency. In this study a trigeneration system, based on an engine genset, which produces electricity from the producer gas generated in a biomass gasifier and which simultaneously supplies heating and cooling by making use of waste heat has been examined. In order to substitute conventional energy and to use financial resources [5] most efficiently, the proposed trigeneration system, which is based on the previous experimental work [6], a biomass gasifier is modelled and simulated using the ECLIPSE [7] process simulation package, and a technical and economic analysis is carried out. The ECLIPSE simulation is validated against this experimental work, which has been standard procedure to ensure the reliability of the modeling process. 2. The trigeneration system and the simulation software 2.1. The proposed trigeneration system The proposed trigeneration system contains three main units: an internal combustion engine genset, which is the basic primary mover of the system; a heat recovery and storage system; and an absorption refrigeration/cooling system, as seen in Fig. 1. The system is operated in the following way: A biomass gasifier with an integrated gas cleaning system generates producer gas as a fuel.
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Fig. 1. Schematic diagram of the proposed trigeneration system integrated a biomass gasifier.
The fuel is utilized in an engine genset to generate electricity for the use in the building. The engine cooling system and the exhaust gases are used to recover the waste heat for the thermal storage and supply of hot water/central heating in the building. An ammonia absorption system, which is run by a part of the waste heat from the engine exhaust, is used to supply the cooling for the building when necessary. 2.2. Type of gasifier Gasification is a process developed to convert carbon based solid fuels to a gaseous form containing carbon monoxide and hydrogen as main constituents. Being a renewable, low cost and environmentally friendly energy alternative, various biomass gasification systems have been developed worldwide [8], as demonstration systems and as commercial plants to utilize different feedstock types. Since the type of gasifier technology used and the oxidant employed determine the quality of the gas produced, some important considerations should be given to matching the trigeneration system with the selected gasification process [9]. In the above context, the producer gas should be suitable for efficient operation of the IC engine. A range of gasification technologies is available for different scale applications. Downdraft gasifiers are suitable for small and medium-sized applications. This technology offers a relatively efficient biomass to gaseous fuel conversion and produces a gas with sufficiently low tar content to operate an internal combustion engine. In this case, a fixed bed, downdraft gasifier is considered to be most suitable, as shown in Fig. 2. 2.3. The properties of the fuels In order to evaluate the technical, environmental and economic performance of the proposed trigeneration system, especially the impact of different fuels three common biomass or biomass waste species are chosen for the simulation. The ultimate, proximate analyses and calorific values of the fuels used in the simulation are shown in Table 1. The biomass and biomass waste types
Fig. 2. Schematic of the proposed downdraft biomass gasifier.
Table 1 Analysis of biomass species. Biomass
Willow chip
Miscanthus
Rice husk
Water (%ar) Ash (%ar) VM and FC (%ar)
33.51 0.57 65.92
11.29 2.59 86.12
11.25 14.26 74.49
19.26 17.73 47.96 6.75 0.52 0.11 0.25 44.41
17.93 16.56 45.81 6.11 0.42 0.14 0.00 47.52
Biomass ultimate analysis (wt.%, daf) HHV (MJ/kg) 18.73 LHV (MJ/kg) 17.37 Carbon 51.00 Hydrogen 6.00 Nitrogen 0.05 Sulphur 0.05 Chlorine 0.00 Oxygen 42.90
selected during the case studies are willow chips, miscanthus and rice husk [10–12], which have calorific values ranging from 17.93 MJ kg 1 to 19.26 MJ kg 1 (HHV, dry and ash free). The carbon contents range from 45.81% to 51% (daf) and the moisture contents
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from 11.25% to 33.51% (ar). The percentage of ash in the feedstock is between 0.57% and 14.26% (ar). 2.4. Energy consumption of commercial systems
simulated using the function of the energy and mass balance of ECLIPSE. 3. Simulation results and discussion
The objective of this study is to assess the trigeneration system integrated with a biomass gasifier for its suitability in supplying electricity, space heating and cooling to buildings. As mentioned, the applications of trigeneration would be beneficial to buildings when there are an electrical demand and a simultaneous requirement of heating and cooling loads. The energy demand profiles of commercial buildings depend on building types and also vary with geographical location [13,14]. An energy demand curve for a building has been assumed, the economic benefits and when the system should run in heating or cooling mode are judged on the graph shown in Fig. 3. 2.5. The software—a brief introduction of ECLIPSE Following the modeling of the trigeneration system and the definition of the design parameters, a simulation software called ECLIPSE, as shown in Fig. 4, is used to simulate the working process of the proposed trigeneration system to provide a consistent basis for evaluation and comparison [7]. ECLIPSE was developed for the European Commission and has been used by the Northern Ireland Centre for Energy Research and Technology at the University of Ulster since 1986 [15]. ECLIPSE was successfully used for many European and international projects to implement techno-economic analysis of power systems. ECLIPSE is a personal-computer-based package containing all of the program modules necessary to complete rapid and reliable step-by-step technical, environmental and economic evaluations of chemical and allied processes. ECLIPSE uses generic chemical engineering equations and formulae and includes a high-accuracy steam–water thermodynamics package for steam cycle analysis. It has its own chemical industry capital costing program covering over 100 equipment types. The chemical compound properties database and the plant cost database can both be modified to allow new or conceptual processes to be evaluated. A techno-economic assessment study is carried out in stages; initially a process flow diagram is prepared, technical design data can then be added and amass and energy balance completed. Consequently, the system’s environmental impact is assessed, capital and operating costs are estimated and an economic analysis performed. The proposed trigeneration system is
The process flow diagrams of the proposed trigeneration system integrated with a low pressure biomass gasifier in ECLIPSE are illustrated in Figs. 5–7. The system modelled has a 250 kW electricity output at the full load [16,17]. If the engine electrical output at peak load is more than the demands from users the excess electricity from the engine would be sent to the grid with the same electricity selling price, this assumption is made to allow the article to be comparable for varying prices and incentives schemes. The system is scaled to more than cover the peak electricity demand of the building and runs at full load. Excess electricity is sold to the Grid. Heat is stored in water tanks, in order to quickly cater for variations in demand for heating and/or cooling. For the technical performance of the system the efficiency for both electricity and trigeneration is used. Although sustainably-grown biomass is assumed to be used as the fuel, CO2 is emitted and the amount of CO2 emissions is used as a monitor of the environmental impact of the system. The economic evaluation of the selected cases was carried out using the net present value accounts (NPV). As the main indicator, the breakeven electricity selling price (BESP) of each trigeneration system was calculated using the financial modelling of ECLIPSE. This computational method uses the estimated capital cost investment requirements along with the fixed and variable operating and maintenance cost (O&M). Conventional fossil fuel power generation systems usually have life spans between 20 and 25 years. The gasification and ancillary equipment in the trigeneration system would be expected to have similar lifetimes. Therefore the project life of the trigeneration system was set to 20 years. The discounted cash flow rate is set to 8%. Table 2 shows the main technical, environmental and economic results for the simulations of the power only (PO) and trigeneration (TG) options. 3.1. Case study 1: Dried willow chip gasification The willow from a nearby farm is harvested, chipped and transported to the plant. Considering fresh willow chips with a high moisture content [18], the biomass has to be dried and stored before gasification using preheated air. In case 1, the dried willow
350
Energy Use (kW)
300 250 200 150 100 50 0 Jan
Feb
Mar
Apr
May
Electricity kW
Jun
Jul
Heating kW
Aug
Sep
Oct
Cooling kW
Fig. 3. Energy profile of the selected building.
Nov
Dec
Y. Huang et al. / Energy Conversion and Management 52 (2011) 2448–2454
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Fig. 4. ECLIPSE process modelling and simulation.
Fig. 5. The flow diagram of the trigeneration system: biomass gasifier system.
chip consumption is 227.8 kg/h (dry basis). The electricity generated from the TG system is 250 kW; the heat consumption of drying process is 82 kW; the heat recovered from the cooling system and the exhaust gas of the engine is 240.8 kW; the cooling effect generated from the absorption refrigerator driven by the waste heat from the engine is 92.1 kW. The thermal efficiency for the electricity generation is 22.9% at lower heating value (LHV) with the CO2 emissions of 1544 g/kWh; the efficiency for the trigeneration is 53.5% (LHV) with the CO2 emissions of 658 kg/kWh including electricity, heat and cooling. It can be seen that the efficiency of TG is more than doubled and the CO2 emissions reduced is more than half compared to that of the PO option. In Table 2, the minimum specific capital investment is given at £2579/kWe. The relatively high cost is mainly due to the small size of plant. Consid-
ering willow chip costs of £55/per dry tonnes, a BESP of £107.8/ MWh is estimated based on the power only option. For the trigeneration configuration the BESP is reduced to £102.3/MWh. 3.2. Case study 2: Miscanthus gasification In case 2, the fuel used is miscanthus. With the same electrical output, the fuel consumption in this scenario is 234.0 kg/h (dry basis). The heat recovered from the cooling system and the exhaust gas of the engine is 314.1 kW, higher than that of willow chips. This is because miscanthus has the low moisture content and does not need a drier in the process. The cooling effect generated from the absorption refrigerator is 92.0 kW. The thermal efficiency for PO is 22.3% (LHV) with the CO2 emissions of 1514 kg/kWh; the
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Fig. 6. The flow diagram of the trigeneration system: IC engine system.
Fig. 7. The flow diagram of the trigeneration system: absorption system.
efficiency for TG is 58.6% (LHV) with the CO2 emissions of 576 g/ kWh. It can be seen that since the efficiency of TG is more than doubled the CO2 emission reduction is more than half compared to that of PO. Similarly the minimum specific capital investment is calculated at £2520/kWe. As the miscanthus fuel cost taken £55/per dry tonnes, this gives an electricity BESP of £107.7/MWh based on the power only option. For the trigeneration option the BESP falls from £107.7/MWh to £96.9/MWh.
3.3. Case study 3: Rice husk gasification In the third case, with the same level of electrical output, when rice husk is fed to the system, the fuel consumption is 315.9 kg/h (db). The heat recovered from the cooling system and the exhaust gas of the engine is 318.6 kW; the cooling effect generated from the absorption refrigerator is 92.2 kW. The thermal efficiency for PO is 20.5% (LHV) with the CO2 emissions of 1594 g/kWh compared
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Biomass
Willow chips Case 1
Miscanthus Case 2
Rice husk Case 3
Biomass input, kg/h (dry basis) CV (LHV), kJ/kg (dry basis) Total thermal input, kWth Product gas heating value, MJ/m3 Engine electric output, kWe (gross) Auxiliary power usage (kWe) Engine electric output, kWe (net) Overall plant electricity efficiency (net),% Heat recovered from water jacket kWth Heat recovered from gas/ water HEX, kWth Total available waste heat, kWth (CHP) Total available waste heat, kWth (TG) Hot water temperature, °C Power/Heat Ratio (CHP) Power/Heat Ratio (TG) Heat consumption of refrigeration, kWth Cooling effect, kW COP Overall plant efficiency (CHP)% Overall plant efficiency (TG)% Gaseous emissions CO2 (g/kWh) (electrical) CO2 (g/kWh) (CHP) CO2 (g/kWh) (TG) Biomass cost, £/db (dry basis) Minimum specific investment, £/kWe (CHP) Minimum specific investment, £/kWe (TG) BESP, £/MWh (PO) BESP, £/MWh (CHP) BESP, £/MWh (TG)*
227.8 17,220 1089.9 5.14 266.1 16.1 250.0 22.9
234.0 17,215 1119.0 5.08 264.9 14.9 250.0 22.3
315.9 13,905 1220.2 5.06 264.0 14.0 250.0 20.5
95.4
84.2
76.9
325.9
410.2
422.7
421.3
494.4
499.6
240.8
314.1
318.6
75.0 0.59 0.76 180.5
75.0 0.51 0.62 180.3
75.0 0.50 0.61 181.0
92.1 0.51 61.6 53.5
92.0 0.51 66.5 58.6
92.2 0.51 61.4 54.1
1544 571 658 55 2198
1514 507 576 55 2142
1594 532 604 35 2142
2579
2520
2520
107.8 83.6 102.3
107.7 78.2 96.9
100.5 70.6 89.2
* The unit cost for selling heating was taken as £5.0/GJ. The unit cost for selling cooling was taken as £4.0/GJ.
120 100
(£/MWh)
Break-even electricity selling price
to TG with an overall efficiency of 54.1% (LHV) and specific CO2 emissions of 604 g/kWh. From the simulation results, the minimum specific capital investment is estimated at £2520/kW which is identical to case 2. Using a biomass fuel price of £35/per dry tonnes of rice husk, this gives a BESP of £100.5/MWh based on the power only option. For the trigeneration configuration the BESP is reduced to £89.2/MWh, as shown Figs. 8 and 9. Considering a good construction material for high performance concrete if rice husk ash from the gasifier can be sold at the price of £50/tonne, some reduction in BESP can be made up to £10.1/MWh.
100%
Breakdowns of BESP (TG)
Table 2 Technical, environmental and economic results for the trigeneration systems.
80% 60% 40% 20% 0% Willow chips
Fuel Cost
Miscanthus
Operating Expenditure
Rice husk
Capital Expenditure
Fig. 9. Breakdowns of breakeven electricity selling price (TG).
4. Conclusions and recommendations From the results and discussions, it can be concluded that: It is economically feasible to use willow chips, miscanthus and rice husk as the fuel to operate the downdraft gasifier and IC engine based TG plant, as there is little difference between the BESP from one fuel to the other; The process efficiency of TG is much higher than that of PO, but is lower than that of the combined heat and power (CHP) configuration; The TG system with a biomass gasifier would be beneficial to the building system if the power/heat ratio is in the range of 0.5 and 0.75; The trigeneration option (TG) has significant reduction in CO2 emissions over the power generation option, even when the carbon neutrality of using sustainably-grown biomass is not assumed. This small scale TG option emits similar CO2 emissions as that of conventional large scale power generation technologies. Since small scale systems are generally less efficient than those of larger scale, their CO2 emissions would also be greater. Hence this TG system offers a promising method of small scale power generation, with heating and cooling, but without increasing emissions above those of conventional PG technologies. The waste heat recovered from the system fuelled by willow chips is lower than that fuelled by miscanthus and rice husk as the willow fuel requires additional drying. Specific investment, SI, is founded to be very high for the TG system with a small biomass fuelled system, ranging from £2520/kWe to £2579/kWe; The breakeven electricity selling price (BESP) of the TG system is better than that of the PO option with the CHP option producing the cheapest electricity. This study shows that the high capital cost of the TG plant reduces the economic viability for small scale systems, the PO also suffers as the heat energy of the system is not recovered. The TG system would perform much better economically in a building with a higher cooling load spread over a 12 month period. Acknowledgement
80
This research outcome is from a joint UK–China research Program funded by the Engineering and Physical Sciences Research Council of the UK.
60 40 20
References
0 Willow chips
Miscanthus PO
CHP
Rice husk
TG
Fig. 8. Breakeven electricity selling price vs biomass.
[1] Thornley P, Upham P, Huang Y, Rezvani S, Brammer J, Rogers J. Integrated assessment of bioelectricity technology options. Energy Policy 2009;37(3):890–903. [2] McIlveen-Wright DR, Williams BC, McMullan JT. Options for small and medium scale power generation from biomass. Dev Chem Eng Mineral Process 1999;2:85–114.
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[3] Lin Lin, Yaodong Wang, Tarik Al-Shemmeri, Tom Ruxton, Stuart Turner. An experimental investigation of a household size trigeneration. Appl Therm Eng 2007;27:576–85. [4] Wang YD, Al-Shemmeri T. A theoretical and experimental investigation of a small scale trigeneration system, — a comparison between trigeneration and separate generation systems. In: Proceedings of the fall technical conference of ASME internal combustion engine division; 2003. p. 41–6 [ISSN 1066-5048]. [5] Rentizelas A, Tolis A, Tatsiopoulos I. Biomass district energy trigeneration systems: emissions reduction and financial impact. Water Air Soil Pollut: Focus 2009;9:139–50. doi 10.1007/s11267-008-9202. [6] Wang YD, Al-Shemmeri T. A theoretical and an experimental investigation of a small scale trigeneration system, — a comparison between trigeneration and separate generation systems. In: Proceedings of the fall technical conference of the ASME internal combustion engine division; 2003. p 41–6 [ISSN 10665048]. [7] Williams BC, McMullan JT. Techno-economic analysis of fuel conversion and power generation systems—the development of a portable chemical process simulator with capital cost and economic performance analysis capabilities. Int J Energy Res 1996;20:125–42. [8] McIlveen-Wright DR, Williams BC, McMullan JT. Wood gasification integrated with fuel cells. Renew Energy 2000;19:223–8. [9] Beenackers AACM, Maniatis K. Gasification technologies for heat and power from biomass. In: Kaltschmitt M, Bridgwater AV, editors. Biomass gasification
[10]
[11] [12] [13] [14] [15] [16]
[17] [18]
and pyrolysis state of the art and future prospects. UK: CPL Press; 1997. p. 24–52. Huang Y, McIlveen-Wright D, Rezvani S, Wang YD, Hewitt N, Williams BC. Biomass co-firing in a pressurized fluidized bed combustion (PFBC) combined cycle power plant: a techno-environmental assessment based on computational simulations. Fuel Process Technol 2006;87(10):927–34. Tsai WT, Lee MK, Chang YM. Fast pyrolysis of rice husk: product yield and compositions. Bioresource Technol 2007;98:22–8. Sun S, Zhao Y, Ling F, Su F. Experimental research on air cyclone gasification of rice husk. Fuel Process Technol 2009;90:465–71. McIlveen-Wright DR, McMullan JT, Guiney DJ. Wood-fired fuel cells in selected buildings. Power Source 2003;118:393–404. Yu Philip CH, Chow WK. Energy use in commercial buildings in Hong Kong. Appl Energy 2001;69:243–55. The university of Ulster, ECLIPSE process simulator. Jordanstown, Northern Ireland: Energy Research Centre; 1992. Wang YD, Huang Y, Roskilly AP. Trigeneration integrated with absorption enhanced reforming of lignite and biomass. FUEL 2009; 88(10):2004–10. Wang YD, Huang Y, Roskilly AP, Ding YL, Hewitt NJ. Trigeneration running with raw jatropha oil. Fuel Process Technol 2010;91(3):348–53. Ledin S. Willow wood properties, production and economy. Biomass Bioenergy 1996;11(2/3):75–83.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Biomass fuelled trigeneration system in selected buildings Y. Huang a,⇑, Y.D. Wang b, S. Rezvani a, D.R. McIlveen-Wright a, M. Anderson a, N.J. Hewitt a a b
Centre for Sustainable Technologies, School of Built Environment, University of Ulster, Newtownabbey, BT37 0QB, UK The Sir Joseph Swan Institute for Energy Research, Newcastle University, Newcastle Upon Tyne, NE1 7RU, UK
a r t i c l e
i n f o
Article history: Available online 21 February 2011 Keywords: Trigeneration Computational simulation Biomass Gasifier Techno-economic analyses
a b s t r a c t Many buildings require simultaneous electricity, heating and cooling. Biomass is one of the renewable energy sources which is not intermittent, location-dependent or very difficult to store. If grown sustainably, biomass can be considered to be CO2 neutral. A trigeneration system consisting of an internal combustion (IC) engine integrated with biomass gasification may offer a combination for delivering heat, electricity and cooling cleanly and economically. The producer gas generated by the gasifier is used to provide electricity for building use via the IC engine. The waste heat is recovered from the engine cooling system and exhaust gases to supply hot water to space heating, excess heat is also used to drive an absorption cooling system. The proposed system is designed to meet the energy requirements for selected commercial buildings and district heating/cooling applications. This work focuses on the modeling and simulation of a commercial building scale trigeneration plant fuelled by a biomass downdraft gasifier. In order to use both energy and financial resources most efficiently, technical and economic analyses were carried out, using the ECLIPSE process simulation package. The study also looks at the impact of different biomass feedstock (willow, rice husk and miscanthus) on the performance of a trigeneration plant. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The emission of carbon dioxide from fossil fuel-fired power generation plants and the consequent effect on the global environment is a major concern. Given the global challenges related to climate, methods for managing and reducing these emissions must be found and implemented. The replacement of all or part of these fossil fuels by renewable energy sources, such as biomass and waste, is the obvious alternative. Biomass is available in abundance and practically everywhere. If grown in a sustainable manor the use of biomass, which is considered to produce no net CO2 emissions in its life cycle, and as a replacement for fossil fuels in power generation systems is one of the most attractive applications of reducing CO2 emissions. There are a range of technologies for providing heat and electricity from biomass [1], but most of these technologies have reduced system efficiency at small scale [2]. Trigeneration, as an efficient solution, offers simultaneous generation of electrical power, heating and refrigeration/cooling utilizing desirable feedstock combinations from a single primary energy source [3,4]. Typical applications can be found in many commercial buildings, such as hotels, hospitals or multi-residential communities. Instead of aiming towards a high electricity output, as an innovative type of renewable energy application, the goal of trigen⇑ Corresponding author. E-mail address: [email protected] (Y. Huang). 0196-8904/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2010.12.053
eration powered by biomass is to improve the overall energy utilization efficiency. In this study a trigeneration system, based on an engine genset, which produces electricity from the producer gas generated in a biomass gasifier and which simultaneously supplies heating and cooling by making use of waste heat has been examined. In order to substitute conventional energy and to use financial resources [5] most efficiently, the proposed trigeneration system, which is based on the previous experimental work [6], a biomass gasifier is modelled and simulated using the ECLIPSE [7] process simulation package, and a technical and economic analysis is carried out. The ECLIPSE simulation is validated against this experimental work, which has been standard procedure to ensure the reliability of the modeling process. 2. The trigeneration system and the simulation software 2.1. The proposed trigeneration system The proposed trigeneration system contains three main units: an internal combustion engine genset, which is the basic primary mover of the system; a heat recovery and storage system; and an absorption refrigeration/cooling system, as seen in Fig. 1. The system is operated in the following way: A biomass gasifier with an integrated gas cleaning system generates producer gas as a fuel.
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Y. Huang et al. / Energy Conversion and Management 52 (2011) 2448–2454
Fig. 1. Schematic diagram of the proposed trigeneration system integrated a biomass gasifier.
The fuel is utilized in an engine genset to generate electricity for the use in the building. The engine cooling system and the exhaust gases are used to recover the waste heat for the thermal storage and supply of hot water/central heating in the building. An ammonia absorption system, which is run by a part of the waste heat from the engine exhaust, is used to supply the cooling for the building when necessary. 2.2. Type of gasifier Gasification is a process developed to convert carbon based solid fuels to a gaseous form containing carbon monoxide and hydrogen as main constituents. Being a renewable, low cost and environmentally friendly energy alternative, various biomass gasification systems have been developed worldwide [8], as demonstration systems and as commercial plants to utilize different feedstock types. Since the type of gasifier technology used and the oxidant employed determine the quality of the gas produced, some important considerations should be given to matching the trigeneration system with the selected gasification process [9]. In the above context, the producer gas should be suitable for efficient operation of the IC engine. A range of gasification technologies is available for different scale applications. Downdraft gasifiers are suitable for small and medium-sized applications. This technology offers a relatively efficient biomass to gaseous fuel conversion and produces a gas with sufficiently low tar content to operate an internal combustion engine. In this case, a fixed bed, downdraft gasifier is considered to be most suitable, as shown in Fig. 2. 2.3. The properties of the fuels In order to evaluate the technical, environmental and economic performance of the proposed trigeneration system, especially the impact of different fuels three common biomass or biomass waste species are chosen for the simulation. The ultimate, proximate analyses and calorific values of the fuels used in the simulation are shown in Table 1. The biomass and biomass waste types
Fig. 2. Schematic of the proposed downdraft biomass gasifier.
Table 1 Analysis of biomass species. Biomass
Willow chip
Miscanthus
Rice husk
Water (%ar) Ash (%ar) VM and FC (%ar)
33.51 0.57 65.92
11.29 2.59 86.12
11.25 14.26 74.49
19.26 17.73 47.96 6.75 0.52 0.11 0.25 44.41
17.93 16.56 45.81 6.11 0.42 0.14 0.00 47.52
Biomass ultimate analysis (wt.%, daf) HHV (MJ/kg) 18.73 LHV (MJ/kg) 17.37 Carbon 51.00 Hydrogen 6.00 Nitrogen 0.05 Sulphur 0.05 Chlorine 0.00 Oxygen 42.90
selected during the case studies are willow chips, miscanthus and rice husk [10–12], which have calorific values ranging from 17.93 MJ kg 1 to 19.26 MJ kg 1 (HHV, dry and ash free). The carbon contents range from 45.81% to 51% (daf) and the moisture contents
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Y. Huang et al. / Energy Conversion and Management 52 (2011) 2448–2454
from 11.25% to 33.51% (ar). The percentage of ash in the feedstock is between 0.57% and 14.26% (ar). 2.4. Energy consumption of commercial systems
simulated using the function of the energy and mass balance of ECLIPSE. 3. Simulation results and discussion
The objective of this study is to assess the trigeneration system integrated with a biomass gasifier for its suitability in supplying electricity, space heating and cooling to buildings. As mentioned, the applications of trigeneration would be beneficial to buildings when there are an electrical demand and a simultaneous requirement of heating and cooling loads. The energy demand profiles of commercial buildings depend on building types and also vary with geographical location [13,14]. An energy demand curve for a building has been assumed, the economic benefits and when the system should run in heating or cooling mode are judged on the graph shown in Fig. 3. 2.5. The software—a brief introduction of ECLIPSE Following the modeling of the trigeneration system and the definition of the design parameters, a simulation software called ECLIPSE, as shown in Fig. 4, is used to simulate the working process of the proposed trigeneration system to provide a consistent basis for evaluation and comparison [7]. ECLIPSE was developed for the European Commission and has been used by the Northern Ireland Centre for Energy Research and Technology at the University of Ulster since 1986 [15]. ECLIPSE was successfully used for many European and international projects to implement techno-economic analysis of power systems. ECLIPSE is a personal-computer-based package containing all of the program modules necessary to complete rapid and reliable step-by-step technical, environmental and economic evaluations of chemical and allied processes. ECLIPSE uses generic chemical engineering equations and formulae and includes a high-accuracy steam–water thermodynamics package for steam cycle analysis. It has its own chemical industry capital costing program covering over 100 equipment types. The chemical compound properties database and the plant cost database can both be modified to allow new or conceptual processes to be evaluated. A techno-economic assessment study is carried out in stages; initially a process flow diagram is prepared, technical design data can then be added and amass and energy balance completed. Consequently, the system’s environmental impact is assessed, capital and operating costs are estimated and an economic analysis performed. The proposed trigeneration system is
The process flow diagrams of the proposed trigeneration system integrated with a low pressure biomass gasifier in ECLIPSE are illustrated in Figs. 5–7. The system modelled has a 250 kW electricity output at the full load [16,17]. If the engine electrical output at peak load is more than the demands from users the excess electricity from the engine would be sent to the grid with the same electricity selling price, this assumption is made to allow the article to be comparable for varying prices and incentives schemes. The system is scaled to more than cover the peak electricity demand of the building and runs at full load. Excess electricity is sold to the Grid. Heat is stored in water tanks, in order to quickly cater for variations in demand for heating and/or cooling. For the technical performance of the system the efficiency for both electricity and trigeneration is used. Although sustainably-grown biomass is assumed to be used as the fuel, CO2 is emitted and the amount of CO2 emissions is used as a monitor of the environmental impact of the system. The economic evaluation of the selected cases was carried out using the net present value accounts (NPV). As the main indicator, the breakeven electricity selling price (BESP) of each trigeneration system was calculated using the financial modelling of ECLIPSE. This computational method uses the estimated capital cost investment requirements along with the fixed and variable operating and maintenance cost (O&M). Conventional fossil fuel power generation systems usually have life spans between 20 and 25 years. The gasification and ancillary equipment in the trigeneration system would be expected to have similar lifetimes. Therefore the project life of the trigeneration system was set to 20 years. The discounted cash flow rate is set to 8%. Table 2 shows the main technical, environmental and economic results for the simulations of the power only (PO) and trigeneration (TG) options. 3.1. Case study 1: Dried willow chip gasification The willow from a nearby farm is harvested, chipped and transported to the plant. Considering fresh willow chips with a high moisture content [18], the biomass has to be dried and stored before gasification using preheated air. In case 1, the dried willow
350
Energy Use (kW)
300 250 200 150 100 50 0 Jan
Feb
Mar
Apr
May
Electricity kW
Jun
Jul
Heating kW
Aug
Sep
Oct
Cooling kW
Fig. 3. Energy profile of the selected building.
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Fig. 4. ECLIPSE process modelling and simulation.
Fig. 5. The flow diagram of the trigeneration system: biomass gasifier system.
chip consumption is 227.8 kg/h (dry basis). The electricity generated from the TG system is 250 kW; the heat consumption of drying process is 82 kW; the heat recovered from the cooling system and the exhaust gas of the engine is 240.8 kW; the cooling effect generated from the absorption refrigerator driven by the waste heat from the engine is 92.1 kW. The thermal efficiency for the electricity generation is 22.9% at lower heating value (LHV) with the CO2 emissions of 1544 g/kWh; the efficiency for the trigeneration is 53.5% (LHV) with the CO2 emissions of 658 kg/kWh including electricity, heat and cooling. It can be seen that the efficiency of TG is more than doubled and the CO2 emissions reduced is more than half compared to that of the PO option. In Table 2, the minimum specific capital investment is given at £2579/kWe. The relatively high cost is mainly due to the small size of plant. Consid-
ering willow chip costs of £55/per dry tonnes, a BESP of £107.8/ MWh is estimated based on the power only option. For the trigeneration configuration the BESP is reduced to £102.3/MWh. 3.2. Case study 2: Miscanthus gasification In case 2, the fuel used is miscanthus. With the same electrical output, the fuel consumption in this scenario is 234.0 kg/h (dry basis). The heat recovered from the cooling system and the exhaust gas of the engine is 314.1 kW, higher than that of willow chips. This is because miscanthus has the low moisture content and does not need a drier in the process. The cooling effect generated from the absorption refrigerator is 92.0 kW. The thermal efficiency for PO is 22.3% (LHV) with the CO2 emissions of 1514 kg/kWh; the
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Fig. 6. The flow diagram of the trigeneration system: IC engine system.
Fig. 7. The flow diagram of the trigeneration system: absorption system.
efficiency for TG is 58.6% (LHV) with the CO2 emissions of 576 g/ kWh. It can be seen that since the efficiency of TG is more than doubled the CO2 emission reduction is more than half compared to that of PO. Similarly the minimum specific capital investment is calculated at £2520/kWe. As the miscanthus fuel cost taken £55/per dry tonnes, this gives an electricity BESP of £107.7/MWh based on the power only option. For the trigeneration option the BESP falls from £107.7/MWh to £96.9/MWh.
3.3. Case study 3: Rice husk gasification In the third case, with the same level of electrical output, when rice husk is fed to the system, the fuel consumption is 315.9 kg/h (db). The heat recovered from the cooling system and the exhaust gas of the engine is 318.6 kW; the cooling effect generated from the absorption refrigerator is 92.2 kW. The thermal efficiency for PO is 20.5% (LHV) with the CO2 emissions of 1594 g/kWh compared
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Biomass
Willow chips Case 1
Miscanthus Case 2
Rice husk Case 3
Biomass input, kg/h (dry basis) CV (LHV), kJ/kg (dry basis) Total thermal input, kWth Product gas heating value, MJ/m3 Engine electric output, kWe (gross) Auxiliary power usage (kWe) Engine electric output, kWe (net) Overall plant electricity efficiency (net),% Heat recovered from water jacket kWth Heat recovered from gas/ water HEX, kWth Total available waste heat, kWth (CHP) Total available waste heat, kWth (TG) Hot water temperature, °C Power/Heat Ratio (CHP) Power/Heat Ratio (TG) Heat consumption of refrigeration, kWth Cooling effect, kW COP Overall plant efficiency (CHP)% Overall plant efficiency (TG)% Gaseous emissions CO2 (g/kWh) (electrical) CO2 (g/kWh) (CHP) CO2 (g/kWh) (TG) Biomass cost, £/db (dry basis) Minimum specific investment, £/kWe (CHP) Minimum specific investment, £/kWe (TG) BESP, £/MWh (PO) BESP, £/MWh (CHP) BESP, £/MWh (TG)*
227.8 17,220 1089.9 5.14 266.1 16.1 250.0 22.9
234.0 17,215 1119.0 5.08 264.9 14.9 250.0 22.3
315.9 13,905 1220.2 5.06 264.0 14.0 250.0 20.5
95.4
84.2
76.9
325.9
410.2
422.7
421.3
494.4
499.6
240.8
314.1
318.6
75.0 0.59 0.76 180.5
75.0 0.51 0.62 180.3
75.0 0.50 0.61 181.0
92.1 0.51 61.6 53.5
92.0 0.51 66.5 58.6
92.2 0.51 61.4 54.1
1544 571 658 55 2198
1514 507 576 55 2142
1594 532 604 35 2142
2579
2520
2520
107.8 83.6 102.3
107.7 78.2 96.9
100.5 70.6 89.2
* The unit cost for selling heating was taken as £5.0/GJ. The unit cost for selling cooling was taken as £4.0/GJ.
120 100
(£/MWh)
Break-even electricity selling price
to TG with an overall efficiency of 54.1% (LHV) and specific CO2 emissions of 604 g/kWh. From the simulation results, the minimum specific capital investment is estimated at £2520/kW which is identical to case 2. Using a biomass fuel price of £35/per dry tonnes of rice husk, this gives a BESP of £100.5/MWh based on the power only option. For the trigeneration configuration the BESP is reduced to £89.2/MWh, as shown Figs. 8 and 9. Considering a good construction material for high performance concrete if rice husk ash from the gasifier can be sold at the price of £50/tonne, some reduction in BESP can be made up to £10.1/MWh.
100%
Breakdowns of BESP (TG)
Table 2 Technical, environmental and economic results for the trigeneration systems.
80% 60% 40% 20% 0% Willow chips
Fuel Cost
Miscanthus
Operating Expenditure
Rice husk
Capital Expenditure
Fig. 9. Breakdowns of breakeven electricity selling price (TG).
4. Conclusions and recommendations From the results and discussions, it can be concluded that: It is economically feasible to use willow chips, miscanthus and rice husk as the fuel to operate the downdraft gasifier and IC engine based TG plant, as there is little difference between the BESP from one fuel to the other; The process efficiency of TG is much higher than that of PO, but is lower than that of the combined heat and power (CHP) configuration; The TG system with a biomass gasifier would be beneficial to the building system if the power/heat ratio is in the range of 0.5 and 0.75; The trigeneration option (TG) has significant reduction in CO2 emissions over the power generation option, even when the carbon neutrality of using sustainably-grown biomass is not assumed. This small scale TG option emits similar CO2 emissions as that of conventional large scale power generation technologies. Since small scale systems are generally less efficient than those of larger scale, their CO2 emissions would also be greater. Hence this TG system offers a promising method of small scale power generation, with heating and cooling, but without increasing emissions above those of conventional PG technologies. The waste heat recovered from the system fuelled by willow chips is lower than that fuelled by miscanthus and rice husk as the willow fuel requires additional drying. Specific investment, SI, is founded to be very high for the TG system with a small biomass fuelled system, ranging from £2520/kWe to £2579/kWe; The breakeven electricity selling price (BESP) of the TG system is better than that of the PO option with the CHP option producing the cheapest electricity. This study shows that the high capital cost of the TG plant reduces the economic viability for small scale systems, the PO also suffers as the heat energy of the system is not recovered. The TG system would perform much better economically in a building with a higher cooling load spread over a 12 month period. Acknowledgement
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This research outcome is from a joint UK–China research Program funded by the Engineering and Physical Sciences Research Council of the UK.
60 40 20
References
0 Willow chips
Miscanthus PO
CHP
Rice husk
TG
Fig. 8. Breakeven electricity selling price vs biomass.
[1] Thornley P, Upham P, Huang Y, Rezvani S, Brammer J, Rogers J. Integrated assessment of bioelectricity technology options. Energy Policy 2009;37(3):890–903. [2] McIlveen-Wright DR, Williams BC, McMullan JT. Options for small and medium scale power generation from biomass. Dev Chem Eng Mineral Process 1999;2:85–114.
2454
Y. Huang et al. / Energy Conversion and Management 52 (2011) 2448–2454
[3] Lin Lin, Yaodong Wang, Tarik Al-Shemmeri, Tom Ruxton, Stuart Turner. An experimental investigation of a household size trigeneration. Appl Therm Eng 2007;27:576–85. [4] Wang YD, Al-Shemmeri T. A theoretical and experimental investigation of a small scale trigeneration system, — a comparison between trigeneration and separate generation systems. In: Proceedings of the fall technical conference of ASME internal combustion engine division; 2003. p. 41–6 [ISSN 1066-5048]. [5] Rentizelas A, Tolis A, Tatsiopoulos I. Biomass district energy trigeneration systems: emissions reduction and financial impact. Water Air Soil Pollut: Focus 2009;9:139–50. doi 10.1007/s11267-008-9202. [6] Wang YD, Al-Shemmeri T. A theoretical and an experimental investigation of a small scale trigeneration system, — a comparison between trigeneration and separate generation systems. In: Proceedings of the fall technical conference of the ASME internal combustion engine division; 2003. p 41–6 [ISSN 10665048]. [7] Williams BC, McMullan JT. Techno-economic analysis of fuel conversion and power generation systems—the development of a portable chemical process simulator with capital cost and economic performance analysis capabilities. Int J Energy Res 1996;20:125–42. [8] McIlveen-Wright DR, Williams BC, McMullan JT. Wood gasification integrated with fuel cells. Renew Energy 2000;19:223–8. [9] Beenackers AACM, Maniatis K. Gasification technologies for heat and power from biomass. In: Kaltschmitt M, Bridgwater AV, editors. Biomass gasification
[10]
[11] [12] [13] [14] [15] [16]
[17] [18]
and pyrolysis state of the art and future prospects. UK: CPL Press; 1997. p. 24–52. Huang Y, McIlveen-Wright D, Rezvani S, Wang YD, Hewitt N, Williams BC. Biomass co-firing in a pressurized fluidized bed combustion (PFBC) combined cycle power plant: a techno-environmental assessment based on computational simulations. Fuel Process Technol 2006;87(10):927–34. Tsai WT, Lee MK, Chang YM. Fast pyrolysis of rice husk: product yield and compositions. Bioresource Technol 2007;98:22–8. Sun S, Zhao Y, Ling F, Su F. Experimental research on air cyclone gasification of rice husk. Fuel Process Technol 2009;90:465–71. McIlveen-Wright DR, McMullan JT, Guiney DJ. Wood-fired fuel cells in selected buildings. Power Source 2003;118:393–404. Yu Philip CH, Chow WK. Energy use in commercial buildings in Hong Kong. Appl Energy 2001;69:243–55. The university of Ulster, ECLIPSE process simulator. Jordanstown, Northern Ireland: Energy Research Centre; 1992. Wang YD, Huang Y, Roskilly AP. Trigeneration integrated with absorption enhanced reforming of lignite and biomass. FUEL 2009; 88(10):2004–10. Wang YD, Huang Y, Roskilly AP, Ding YL, Hewitt NJ. Trigeneration running with raw jatropha oil. Fuel Process Technol 2010;91(3):348–53. Ledin S. Willow wood properties, production and economy. Biomass Bioenergy 1996;11(2/3):75–83.