Chapter 4 Results and Discussion
4.4 Economic Benefits
In this section, the economic benefits are estimated by the data obtained by this research. With this estimation, we can have a vision to know whether it is worth to build a biogas power generation plant for a swine farm, how much clean energy it can produce, and how much economic benefits it can bring.
From the study of Tseng and his colleagues [15], the average biogas produced is around 0.1 m3 per head pig per day. The resultant erergy is 1.7 kWeh per m3 biogas according to the obtained data of section 4.1.
Based on the current tested data , the economy benefits in a scale of 1000 swine farm can be estimated and summarized in the following table.
Table 4.4 Economy Benefits for 1000 Scale Swine Farm per year
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Biogas Production 36500 m3
Electricity Generated 62050 kWh
Electricity Charge Saved 186150 NT$
CO2 Emission Reduction 39.5 ton
The electric energy can reach about 62000kWh per year, and it can save 186000NT$ of electricity charge per year, providing that the present electricity purchase charge is 3NT$ per kWeh).
The carbon dioxide emission coefficient, from Taiwan Power Company, shows how many kilogram carbon dioxide produced per kWh electricity produced from power plant. According to the data of 2008, the carbon dioxide emission coefficient is 0.636 kg per kWhe. Bioengery is a kind of green energies because its carbon source is from the carbon dioxide in the air by photosynthesis, so the net carbon dioxide emission is zero in a cycle. The utilization of bioenergy from biogas can reduce the power generation from fossil fuel and also reduce the carbon dioxide emssion.
As a consequence, each kWhe elecricity generated by biogas can reduce 0.636 kg carbon dioxide emission in Taiwan. Therefore, the carbon dioxide reduction is around 40 tons per year for a 1000 scale swine farm.
If the heated water from waste heat recovery system is properly utilized, such as heating the anaerobic digester, it can save the heating energy from elecricity or nature gas.
The heating value from heat recovery can reduce the amount of nature gas used with same heating value. According to the data from CPC, the price of nature gas has been increased 86.3% within decade, from 8.05 NT$/m3 at 2001 to 15 NT$/m3 at 2010. With this trend, the waste heat recovery is expected to become more and more important in the near future. For the 1000-swine farm, the estimated energy recovery is around
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1.5 108 kJ per year from Table 3.3. Converting the heating value into nature gas charge, it is around 60,000 NT$/ year.
Table 3.5 shows the statistics on swine farms in Taiwan from Council of Agriculture. In this table, the swine number in the farm scale over 1000 heads, which have economic potential for installing biogas electric generators, is around 4.3 million heads, which is about 66% of total swine population in Taiwan. Based on the data in this study, the overall economic benefits in Taiwan from biogas for the swine farms over 1000 pigs can be estimated as following:
z Electricity generation: 2.67 108 kWeh per year z Electricity charge saved: 800 million NT$ per year z Nature gas charge saved: 260 million NT$ per year z Carbon dioxide reduction: 170 thousand ton per year
Table 4.5 Statistics on Swine Farms in Taiwan
Swine farm scale (head) Numbers of swine farm Head on farm
1 – 19 2580 (22.89%) 18,563 (0.28%)
Total 11,271 (100.00%) 6,515,792 (100.00%)
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Comparing the above estimated data with those of Tsai [1], the amount of electricity generated in this study is 3.7 times greater. This discrepancy mainly comes from two reasons. First, the amount of biogas product is different. Tsai [1] used the IPCC recommended coefficient for methane generation, which is 5kg head-1 year-1. In this study, the amount of biogas product was measured as 0.1m3 head-1 day-1, equivalent to about the methane generation of 15.33kg head-1 year-1. Second, Tsai [1] assumed the thermal efficiency of engine was 25%, but it was measured as 28.7%
in this study.
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Chapter 5
Conclusions and Recommendations
5.1 Conclusions
This study carries out the 30kW-generator experiments on a small biogas plant in a swine farm to collect data to serve as a preliminary study for constructing a 300-KW power plant of a bigger scale biogas plant in the near future. It is divided into three parts. Firstly, the effects of fuel supply rate together with corresponding different excess air ratio on power generation were studied. Secondly, the effect of oxygen-enriched combustion for engine was tested. Finally, a heat exchanger was installed to recover waste heat from the engine exhaust gas to increase the use of the energy. At the end, the energy balance for overall biogas into the energy cycle was tested and calculated. The economic benefits were estimated by the data obtained by this research.
According to above experiment results, this study can obtain the following conclusions:
1. The maximum allowable total volume flow rate (sum of biogas and air flow rates) into the engine is about 1800 L/min, so it restricts the maximum excess air ratio for biogas supply rates 240L/min at λ=1.13 and for 260L/min at λ=1.01.
2. The optimum biogas flow rate to the present engine is around 240 to 260L/min. The maximum power generation and thermal efficiency are 26.8 kW and 28.7% at biogas supply rate of 260L/min. The biogas supply rate below 220L/min cannot let the engine operate
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normally. From the results of estimated CH4 consumption ratio, it shows a higher power output and better thermal efficiency need to be accomplished by a greater conversion of CH4 in the combustion process.
3. The engine performances, such as power generation, thermal efficiency, exhaust temperature and percentage of CH4 used, by 1%
oxygen-enriched air are not improved much. By 3%
oxygen-enriched air, the maximum power generation and thermal efficiency are increased up to 28.2kW and 30.2%, respectively, for 260 L/min of fuel supply rate. Most importantly, under this condition, the engine can operate normally at a lower limiting fuel supply rate, such as 220 L/min.
4. For biogas supply rate of 240 L/min with λ=1.13, the heat exchanger can recover 923kJ/min of heat with a 94% of heat exchanger effectiveness, leading to an overall efficiency of 47.3%.
The estimated overall economic benefits in Taiwan by using biogas for the swine farms over 1000 pigs can be reached as following:
z Electricity generation: 2.67 108 kWeh per year (Electricity charge saved: 800 million NT$ per year)
z Heat recovery: 1.5 108 kJ per year (Nature gas charge saved: 260 million NT$ per year)
z Carbon dioxide reduction: 170 thousand ton per year
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5.2 Recommendations
Based on this study, the recommendations to solve the problems of the limit excess air ratio at high biogas supply rate and the lower limit of biogas supply to engine, and the future works are suggested:
1. Redesign the engine to increase the volume limit of gas into the engine.
2. Use the biogas with higher concentration of methane to increase the lower limiting biogas supply rate.
3. Use oxygen-enriched air to increase the lower limit biogas supply rate.
4. Continue the engine tests with higher concentration of methane in a swine farm of 10,000 pigs to identify the scale-up effect.
5. Construct the biogas and pure methane supply switch system. After the engine runs with biogas in an operation duration and is going to stop, switch the fuel into pure methane to maintain the combustion for 1 to 2 minutes to clean up H2S residue in the engine. Measure the SOX in the waste gas for knowing the time to clean H2S.
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References
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[5] N. Tippayawong, A. Promwungkwa, P. Rerkkriangkrai, “Long-term operation of a small biogas/diesel dual-fuel engine for on-farm electricity generation”, Biosystems Engineering, 98, pp. 26-32, 2007.
[6] Ivan Dario Bedoya , Andres Amell Arrieta, Francisco Javier Cadavid,
“Effects of mixing system and pilot fuel quality on diesel–biogas dual fuel engine performance”, Bioresource Technology, 100, pp.
6624-6629, 2009.
[7] J.B. Holm-Nielsen, T. Al Seadi, P. Oleskowicz-Popiel, “The future of anaerobic digestion and biogas utilization”, Bioresource Technology, 100, pp. 5478-5484, 2009.
[8] Uffe Jorgensena, Tommy Dalgaarda, Erik Steen Kristensen,
“Biomass energy in organic farming—the potential role of short
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rotation coppice”, Biomass and Bioenergy, 28, pp. 237-248, 2005.
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239-248, 2003.
[10] Pal Borjesson, Maria Berglund, “Environmental systems analysis of biogas systems—Part I: Fuel-cycle emissions”, Biomass and Bioenergy, 30, pp. 469-485, 2006.
[11] A. Rodriguez Andara, J.M. Lomas Esteban, “Kinetic study of the anaerobic digestion of the solid fraction of piggery slurries”, Biomass and Bioenergy, 17, pp. 435-443, 1999.
[12] JIANG Yao-hua et al., “Research of Biogas as Fuel for Internal Combustion Engine”, IEEE Xplore, 2009.
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[14] Bui Van Ga, Tran Van Nam, Tran Thanh Hai Tung, Truong Le Bich Tram, “Biogas-Petroleum Conversion Kit for Stationary Engines”
Environment Protection Research Center, 2009
[15] Chung, Y.C., K.L. Ho, C.P. Tseng, “Two-stage biofilter for effective NH3 removal from waste gases containing high concentrations of H2S”, J. Air Waste Manag. Assoc. 57: 337-347, 2007
[16] Sheng-Yi Chiu, Chien-Ya Kao, Chiun-Hsun Chen, Tang-Ching Kuan, Seow-Chin Ong, Chih-Sheng Lin, “Reduction of CO2 by a high-density culture of Chlorella sp. in a semicontinuous
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photobioreactor”, Bioresource Technology, Volume 99, Issue 9, June, Pages 3389-3396, 2008
[17] 2006 IPCC guidelines for national greenhouse gases inventories.
Paris (France): IPCC, 2006
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Fig. 1.1 Simple Carbon Cycle for Biogas
Fig. 1.2 Scope of this Research
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Fig. 2.1 Range of Capacities for the Power Generators
Fig. 2.2 Values of Power Generators
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Fig. 3.1a Experiment Layout
Fig. 3.1b Waste Heat Recovery Layout
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Fig. 3.1c Oxygen-Enriched Combustion Layout
Fig. 3.2 Four stroke diesel engine
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Fig. 3.3a VA-400 flow sensor
Fig. 3.3b VA-400 flow sensor data
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Fig. 3.4a TF-4000 Flow meter
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Fig. 3.4b TF-4000 Flow Meter Data
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Fig. 3.5a K-Type Thermocouple
Fig. 3.5b J-Type Thermocouple
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Fig. 3.6a VF-2000 Vortex Flow Sensor
Fig. 3.6b VF-2000 Detailed Data Model Code : VF-2032-F01
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Fig. 3.7 IMR 1400 Gas Analyzer
Fig. 3.8 Water Pump
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Fig. 3.9a Heat Exchanger
Fig. 3.9b Slab with Fins and Pipes
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Fig. 3.10a CompactDAQ Chassis
Fig. 3.10b NI 9203 Analog Input Module
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Fig. 3.10c NI 9211 Analog Input Module
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0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 0
Fig. 4.1 Power generation v.s. excess air ratio at different biogas supply rates
0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 0.10
Fig. 4.2 Thermal efficiency v.s. excess air ratio at different biogas supply rates
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0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 400
Fig. 4.3 Waste gas temperature v.s. excess air ratio with different biogas supply rates
0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 0
Fig. 4.4 O2 concentration in waste gas v.s. excess air ratio with different biogas supply rates
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0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 150
Fig. 4.5 CO concentration v.s. excess air ratio with different biogas supply rates
0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 6
Fig. 4.6 CO2 concentration in waste gas v.s. excess air ratio with different biogas supply rates
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0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 0.70
Figure 4.7 Estimated CH4 consumption ratios in combustion with different biogas supply rates as a function excess air ratio
0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 10
Fig. 4.8 Power generation v.s. excess air ratio with different biogas supply rates with normal one and 1% O2 addition
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0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 0.18
Fig. 4.9 Thermal efficiency v.s. excess air ratio with different biogas supply rates with normal one and 1% O2 addition
0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 0.70
Fig. 4.10 Estimated CH4 consumption ratios in combustion with different biogas supply rates as a function excess air ratio for normal one and
1%oxygen addition
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0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 10
Fig. 4.11 Power generation v.s. excess air ratio with different biogas supply rates with normal one and 3% O2 addition
0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 0.18
Fig. 4.12 Thermal efficiency v.s. excess air ratio at different biogas supply rates with normal one and 3% O2 addition
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0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 0.65
0.70 0.75 0.80 0.85 0.90 0.95 1.00
Used CH 4 (%)
Excess Air Ratio
3% oxygen 260L/min 240L/min 220L/min Normal
260L/min 240L/min 220L/min
Fig. 4.13 Estimated CH4 consumption ratios in combustion with different biogas supply rates as a function excess air ratio for normal one and
3%oxygen addition