This study is continous effort of Lin’s work [3], which carried out the electricity generation of the subproject 2 in a small swine farm in Miaoli. In Lin’s research, 60% methane concentration of biogas was used. Later, the whole energy research program was moved to Taiwan Sugar swine farm in Taichung, whose size was about 1.5 times of that in Miaoli. The present experiments used 73% methane concentration of biogas. This study consisted three parts. Firstly, the effect of biogas supply rate together with different excess air ratio on generator performance was investigated. Secondly, a comparison with Lin’s [3] results was made. Finally, a waste heat recovery system was applied to preheat the inlet gas (the mixture of biogas and air) under different temperatures and the preheating influence on the generator performance was analyzed.
According to the experiment results, this study can obtain the following conclusions:
1. At a given excess air ratio, the higher the biogas supply rate, the higher the power generation. With 73% CH4 of biogas, the maximum power output is 26.7 kW at biogas supply of 260 L/min with λ=0.84. The maximum thermal efficiency and maximum CH4 consumption ratio are 0.27 and 96.03% occurred at the biogas supply rate of 200 L/min with λ= 1.1. For the biogas supply rates above 180L/min, the power generation and thermal efficiency increase with the increase of excess air ratio.
2. With 73% CH4 of biogas, when the biogas flow rate is 140L/min, it
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provides a complete flammability domain between the upper and lower limits, which are = 0.8 and = 1.58, respectively.
3. When the methane concentration rises from 60% to 73%, the trends of power generation and thermal efficiency change, the corresponding lean misfire limit can be widened from λ=1.13 to λ=1.27 for 180L/min.
4. The power generation with 73% CH4 of biogas are higher than the ones with 60% CH4 of biogas, except the region around λ<0.85. However, the thermal efficiency increases with the increasing methane concentration just in the region of λ>0.95. For the mixture on the relatively rich side (λ<0.95), there is no benefit.
5. The effect of increasing inlet gas temperature on power generation and thermal efficiency is obvious when excess air ratios are relatively high (λ>1.3).
6. When the inlet gas temperature increases from 40℃ to 120℃, for biogas supply rate of 140 L/min with λ=1.58. There is an obvious improvement, the power generation increases from 7.3 kW to 11.1 kW, and the thermal efficiency increases from 0.119 to 0.181.
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 future works are suggested:
1. Build dehumidifying system to reduce humidity of biogas.
2. Construct turbocharger to increase the pressure of air entering the engine.
3. Consider to use turbo engine instead of IC engine.
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Fig. 1.1 Carbon Dioxide Emissions Avoided via the Use of Renewable Energy Sources in Germany 2009. [26]
Fig. 1.2 Simple Carbon Cycle for Biogas [3]
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Fig. 1.3 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. 3.2 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. 3.5 K-Type Thermocouple
Fig. 3.6 HM5000 Gas Analyzer
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Fig.3.7 Guardian Plus Infra-Red Gas Monitor
Fig 3.8 Heat Exchanger
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Fig. 3.9a CompactDAQ Chassis
Fig. 3.9b NI 9203 Analog Input Module
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Fig. 3.9c NI 9211 Analog Input Module
Fig.3.10 Temperature Monitor
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Fig.3.11 Center 311 Humidity Temperature Meter
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Fig. 4.1 Power generation v.s. excess air ratio at different biogas supply rates
Fig. 4.2 Thermal efficiency v.s. excess air ratio at different biogas supply rates
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Fig. 4.3 Waste gas temperature v.s. excess air ratio with different biogas supply rates
Fig. 4.4 O2 concentration in waste gas v.s. excess air ratio with different biogas supply rates
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Fig. 4.5 CO2 concentration in waste gas v.s. excess air ratio with different biogas supply rates
Fig. 4.6 NOx concentration in waste gas v.s. excess air ratio with different biogas supply rates
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Fig. 4.7 Estimated CH4 consumption ratios in combustion v.s. excess air ratio with different biogas supply rates
Fig. 4.8 Power generation with different methane concentrations of biogas
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Fig. 4.9 Thermal efficiency with different methane concentrations of biogas
Fig. 4.10 Waste gas temperature with different methane concentrations of biogas
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Fig. 4.11 Power generation of biogas supply rate 180 L/min v.s. excess air ratio with different inlet gas temperatures
Fig. 4.12 Thermal efficiency of biogas supply rate 180 L/min v.s. excess air ratio with different inlet gas temperatures
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Fig. 4.13 Power generation of biogas supply rate 160 L/min v.s. excess air ratio with different inlet gas temperatures
Fig. 4.14 Thermal efficiency of biogas supply rate 160 L/min v.s. excess air ratio with different inlet gas temperatures
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Fig. 4.15 Power generation of biogas supply rate 140 L/min v.s. excess air ratio with different inlet gas temperatures
Fig. 4.16 Thermal efficiency of biogas supply rate 140 L/min v.s. excess air ratio with different inlet gas temperatures