| China has abundant biomass resources, developing and utilizing biomass actively is an effective way to solve the energy crisis and environmental problems. Biogas power generation project is one of the most effective way in utilizing biomass, among which the cogeneration unit is most widely used, biogas power units exhaust has characteristic of high temperature and lots of heat, it would p roduce energy quality loss if supplying heat directly. With the development of the biogas warming methods, especially the solar warming method, providing a possibility for biogas power units waste heat further utilization. A combined biogas generator-ORC system was proposed based on the original biogas cogeneration system in this paper, then analyzed the performance combined with the practical biogas project. The main results of this study are as followings:1.The actual operation situation of the biogas plant were analyzed. When the biogas project operated normally in 2008, the annual biogas production is 414.4 thousand m3, the annual electricity production is 451.8 thousand k W·h, the annual capacity of handling pure cow dung is 2370m3; The daily average biogas production of the two tower is 855m3 and 823m3 respectively of the May, the daily biogas production is 1174m3 and 1413m3 respectively, the volume biogas productivity is 1.97m3/(m3·d) and 2.36m3/(m3·d) respectively on May 2; The total biogas production is 50.9 thousand m3 in August, which was used to produce electricity is 45.5 thousand m3 and the power production is 61.5 thousand k W·h, means the discharged biogas directly is 5411m3; Exergy analysis of the waste heat utilization system results showed that, the least exergy loss is 14.6k W and the maxim um exergy loss is only 18.8k W in the plate heat exchanger, while in the shell-and-tube heat exchanger the least exergy loss is still 33.8k W and the maximum exergy loss is up to 36.2k W, the exergy efficiency of the plate heat exchanger is above 72%, and the maximum can reach 77%, while the maximum value of the shell-and-tube heat exchanger is 27%, the biggest exergy loss occur when the main circulating water and the exhaust transferred heat, so it has the greatest potential for energy savings.2.Determined the exhaust organic rankine cycle based on characteristics of the biogas generator exhaust. Analyzed the organic rankine cycle with two-stage condenser by simulating on Aspen plus. When the pressure is 2.73 MPa, toluene organic rankine cycle has maximum outp ut power and efficiency, is 12.15 k W and 19.84% respectively, hexane organic rankine cycle could recover the most heat, is 15.54 k W, the operating temperature is lowest when propane as working fluid; Increasing operating pressure, the output power and effici ency increased 0.64 k W, 0.69 k W, 1.14 k W, 1.96 k W, 2.41 k W and 0.73%, 0.79%, 1.27%, 2.27%, 3.3% respectively; Increasing degree of superheat, the output power and efficiency decreased 0.12 k W and 0.13% respectively toluene as working fluid, the output power and efficiency decreased 0.29 k W and 0.38% respectively hexane as working fluid, the maximum output power is 7.13 k W and the maximum efficiency is 11.08% for R245 fa organic rankine cycle, the maximum output power is 4.7k W and 4.31 k W for R134 a and propane organic rankine cycle respectively.3.Evaluated the performance of the combined biogas generator-ORC system and the project. output increased from 68 k W for the combined system to 79.74 k W for the original cogeneration system, the heat output decreased from 133.83 k W for the original cogeneration system to 87.17 k W for the combined system; Energy utilization rate and electricity efficiency is 67.3% and 32.15% respectively of the combined system, it is 81.38% and 27.42% respectively for the original system; The exergy efficiency is 33.21% of the original cogeneration system and 33.97% of the combined system; The primary energy economic ratio is 35.44% of the original system and 32.67 of the combined system. The project with combined system produce electricity 78 thousand k W·h than the original one, equal 28.87 t standard coal. |
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