沼气发电厂余热利用的研究-毕业论文外文翻译.docx

翻译部分外文文献Biogas Power Plants Waste Heat Utilization ResearchesAbstract-This paper discussed the significance of developing biogas power plants in large breeding farms firstly. Then the system energy balance and the produced process of waste heat were studied respectively. The status of making use of waste heat in biogas power plants were also discussed in China. Through studies, two main schemes of making use of waste heat in biogas power plants of large breeding farms was proposed, which were providing residents with heating or cooling load for the requirements of indoor environment according to seasons and constructing cold storages for poultry product processing and storage. In this paper, the methods of making use of waste heat in a biogas power plant of an agricultural demonstration zone in a coastal city were discussed from both sides of economic and environmental benefits and we adopted the second method at last. The results showed that the comprehensive utilization rate of biogas energy was nearly 80% after making full use of waste heat. The project had good economic and environmental benefits. The schemes can give some references in the utilization of waste heat in biogas power plants of large breeding farms and other similar projects.Keywords- biogas power plant; anaerobic digestion; biogas internal combustion engine; waste heat; cold storageI. INTRODUCTIONWith the continuously increasing of livestock breeding in various parts of China, how to dispose feces and sewage produced by livestocks became very urgent. Livestock feces and sewage in fact belonged to biomass energy within the scope of renewable energy. The best way of making use of biomass energy was to develop biogas power plants 1. The principle was that the organic garbage of livestock breeding and cropstraw were treated by anaerobic digestion and then produced biogas. Biogas was converted into green electricity and thermal energy. The residues of anaerobic digestion could be used as efficient green organic fertilizer in improving the ecological environment of soil. For this reason, biogas power generation had been paided wide attention and actively popularized all over the world.Until 2002, the number of large breeding farms was up to 8,241 and the total amount of livestock feces added up to 1.8 billion tons every year in China. Many breeding farms could directly change livestock feces into biagas through anaerobic digestion and biogas was transited into electricity and heat, but the utilization rate of heat was very low actually. Only in a small number of plants was waste heat used for the heat demanding of production process or buildings. For other plants, the waste heat was released directly into air. Until now, there had not a demonstration of biogas power plant that the waste heat could be used efficiently which utilized agricultural waste as raw materials in China 2. For example, the biogas power plant in Mengniu Aoya Model Pasture was the largest one in China. The raw materials of anaerobic digestion were cow feces and sewage. The technologies and main equipments were imported from Germany. There was about 6×1012J waste heat produced every year which was mainly used to produce hot water for cow houses or heating system in buildings. The direct emission of waste heat could not only result the waste of energy, but also cause thermal pollution of environment. Therefore, how to improve the utilization rate of waste heat in biogas power plants was very urgent.II. ENERGY BALANCE AND WASTE HEAT PRODUCTIONA. System Energy Balance1. Energy balance of biogas internal combustion engines The energy distribution of biogas power process is shown in Fig. 1: From Fig. 1, we know that the energy balance of biogas internal combustion engines can be summarized and be shown with (1):Q = Q + Q +Q (1)Where, Z Q total system energy;D Q power output;H Q recoverable energy, including the recoverable heat of the flus gas, the cooling water, the air cooler and ubricating oil cooler and so on;S Q engine heat discharge.From (1), we know that the available energy in the system mainly includs electricity energy and recoverable heat. Generally, the energy conversion rate of power in biogas power plants cannot easily increase significantly at present. In order to improve the energy utilization rate of biogas, waste heat should be made full use of. In theory, 90% of the waste heat could be effectively used.2. Energy balance of anaerobic digester tanks The reaction in anaerobic digester tanks was anaerobic digestion reaction. Anaerobic digestion was particularly suited for wet biomass materials 3. In order to achieve more biogas, we must create a suitable temperature environment for the function of anaerobic micro-organisms. The results of laboratory researches and application experiences showed that if the temperature in the anaerobic digestion tanks increased by 10 , the reaction rate could be doubled 2. Therefore, biogas power plants which took agricultural organic waste as raw resource often adopted the technology based on medium temperature digestion (38-45 ) in Europe. The temperature environment in anaerobic digester tanks was maintenanced by the waste heat recovered from biogas internal combustion engines under most cases. After the combustion of biogas in engines, electricity was transmited to the State Grid or to local users after transformers. Part of waste heat was sent into the anaerobic digester tanks in order to maintain the optimum fermentation temperature. The method of installing waste heat pipes in concrete walls was used in Germany 4. The energy balance in anaerobic digester tanks is shown in (2): Q + Q = Q + Q + Q (2)Where, J Q heat input with raw materials;W Q waste heat for heating the anaerobic digester tanks;L Q heat output with liquid;S1 Q energy conversion losses when heating;S 2 Q fermentation devices heat loss.3. Waste heat of comprehensive utilizationThe recoverable heat eliminated the heat required for the heating of anaerobic digester tanks was the energy that can be comprehensive utilizated. This can be shown in (3): Q = Q Q (3)where􀋈Q waste heat of comprehensive utilization B. Waste Heat GenerationThe main form of biogas power plants in China was biogas internal combustion engines and mainly dual-fuel units 5. Biogas was combusted in the units and electricity was produced. Then electricity was transmitted out after transformers. The waste heat in cooling water was mainly sent to anaerobic digester tanks to satisfy the actual heat needs of anaerobic digestion. The temperature of flus gas was about 550 6 and the waste heat in the flus gas was recovered through heat exchangers which were installed in the exports of the flue gas ducts. The principle of waste heat generating in biogas power plants was shown in Fig. 2.C. Common Methods of Waste Heat UtilizationThe utilization of waste heat was not just determined by the quantity of the waste heat, it was also determined by the quality of waste heat and economy. The higher of the waste heat temperature, the better of its quality. Many types of heat recovery units were used at present. There were about four mainly methods of waste heat utilization 7: 1. Waste heat was changed into low temperature heat through heat exchangers and then was used for heating, hot water, drying and so on. 2. Waste heat was used to generate power through gas turbines. This method mainly applied to high temperature waste heat recovery like the waste heat from engine vent-pipes of vehicles 8. 3. Waste heat was used as the power of absorption refrigeration systems for residents, such as LiBr absorption refrigeration units. 4. The temperature of waste heat was raised to a higher lever by heat pump, so the application ranges were widened.III. PROJECT OVERVIEWThe large breeding farm was in a modern agriculture demonstration zone in a coastal city. Except for breeding farm 􀋈the zone also included slaughterhouse, meat processing plant, organic fertilizer plant, edible fungi plant and other enterprisesbased on ecological agriculture. Due to the large scale, anaerobic digester was used in innocuous treatment of livestock feces and sewage. In this paper, the waste heat utilization schemes were ditermined according to actual situations.IV. COMPREHENSIVE UTILIZATION TECHNOLOGYRESEARCH OF WASTE HEATThis paper mainly studied the utilization technology of waste heat of biogas power plants. Part of the waste heat was sent into the anaerobic digester tanks in order to maintain the best temperature environment of anaerobic digestion process. The comprehensive utilization of the other part of waste heat was determined in accordance with actual situations. The two main schemes for making use of waste heat in large breeding farms were as follows: Scheme A: On condition that the biogas power plants were near to the residential areas or the breeding farms had the actualneeds of heating or air-conditioning themselves. The waste heat could be used as cooling load of building through LiBr absorption refrigeration units in summer and as heating load through heat exchangers in winter. In transitional seasons, waste heat could be heated by waste heat boilers to provide hot water or steam to residents or factories. Waste heat also could be used as the energy of dehumidifiers and drying systems 9. For agriculture demonstration zones, the temperature in greenhousesneeded to maintain at a certain value in order to promote the growth of plants. The common method was to install heating system in greenhouses by radiators or hot water pipes. So the residual heat could be used as heating load of greenhouse heating system in agricultural zones. The schematic diagram of scheme A is shown in Fig. 3. Scheme B: As for large breeding farms, there had the actual needs of poultry product processing industry. This would promote local economic development and partly resolve the employment problems of local farmers. This must use cold storages for poultry product storage. So on the condition that it was not economic for the long-distance transportation of cold and heat, waste heat could be used as the power resources of cold storages and the remaining heat could also be used as the power of production links such as slaughterhouses and so on. Ammonia absorption refrigeration units that used waste heat as power had the advantages like low greenhouse gas emission, environment protection, energy efficiency high and so on. So they had been widely used in cold storage refrigeration systems. Because the charge of waste heat was nearly zero, economic benefits were very obvious. The schematic diagram of scheme B is shown in Fig. 4.V. DISCUSSIONA. Biogas Power Plant Construction Because the breeding farm was especially larger than other ones, a large number of livestock feces and sewage would produced every day. In order to harmless dispose those livestock feces and sewage, the scheme of constructing biogas power plant was adopted. The form of anaerobic digester tanks was vertical ones on the ground. There were eight biogas internal combustion engines in the power plant and the power of each unit was 500kW. The basic information of the biogas power plant is shown in Table 1. Because of the relatively large-scale, the quantity of waste heat in that biogas power plant was very large. If it was not madegood use of, this would result seriously waste. To this plant, the quantity of waste heat was about 4,800kW. According to the actual situation, anaerobic digester tanks themselves required about 50% of the waste heat to maintain a relatively high fermentation temperature to ensure gas production. This part of waste heat was about 2,400kW and it also changed with seasons and ambient temperature. That part of heat was the heat that could be comprehensive utilizated.B. Waste Heat Comprehensive Utilization StudyAccording to actual situations, there was a long distance away from residential areas, so the transportion process of heat and cold energy could result too much loss. Taking into account the fact that it was a large-scale breeding farm and poultry product processing was required, so we decided to make use of waste heat by the scheme of cold storage at last. In this paper, the utilization scheme of waste heat was systematically demonstrated and planned. For the unstable supply of waste heat, a waste heat boiler was used as auxiliary heat source in order to ensure the normal working of cold storage throughout the year.Ammonia absorption refrigeration units in cold storage took waste heat as theirs power. Thermal coefficient of the refrigeration units was between 0.3-0.4 10. Here we selected 0.35. The total cooling capacity was 840kW. According to commonly used experience indicators, the productive capacity of the cold storage was about 2,000t with the storage capacity of 10,000 pigs. The cold storage can satisfied the practical requirements of the breeding farm. According to the actual situation, the remaining waste heat was delivered to other heat users, such as slaughterhouses and so on.C. Economic and Social Benefits EvaluationBy calculating, the comprehensive utilization rate of biogas energy was almost 80% after comprehensive utilization of waste heat. At the same time, heat pollution by the direct emissions of waste heat into the atmosphere could be avoided. A systematicstudy about the economic and environmental impacts was also carried out.After calculation, the mechanical load of this cold storage was about 630kW. If piston ammonia refrigeration compressors were used in the cold storage and the coefficient of performance (COP) was assumed to be 3, the power of the motors was about 240kW. So the electricity consumption was about 2.1×106kW·h every year. In accordance with the present power price of 0.71 CNY/kW·h, the charge of electricity was about 1,492,700 CNY each year. We could save powerfare about 1,500 thousand CNY each year because the cost of waste heat can be neglected in biogas power plants. So the project of comprehensive utilization of waste heat had very good economic benefitsThe method that waste heat was used as the power of cold storage could not only save running costs, but also reduce the emissions of CO2 and SO2 indirectly. Biomass was a renewable energy source with zero or low carbon dioxide emissions whenit was produced in a sustainable way 11. Through calculations, the method could save coal about 850t each year indirectly. If we assumed that the combustion of coal per ton would produce 0.58t CO2 at the same time, we could reduce the emissions of CO2 about 500t. This would avoid the impacts of greenhouse gas emissions on climate. Furthermore, the SO2 emissions could be very low because of the inherently low sulphur content of biomass. So the scheme had obviously economic and