Quantitive estimation and availability analysis of waste heat from vehicle biogas plant

Abstract: Vehicle biogas, the product deriving from the organic waste anaerobic digestion accompanying with the purification and compression process, has the advantages of higher energy efficiency, environmentally friendliness, sustainability, and so on. The vehicle biogas plant has aroused attention from all walks of life and owned a broad prospect, because it can not only dispose organic waste, but also produce clean vehicle biogas. However, there were still several problems in its operation process, such as high operating costs, high energy consumption and low utilization rate of waste heat. In order to solve these problems, this paper establishes a model of vehicle biogas plant which produces 10 000 m3 biogas daily. We firstly introduce the general situation of this model and calculate the potential of waste heat. What's more, the availability of waste heat is evaluated. Finally, combined with the requirement of heat, the suggestion of the waste heat utilization is put forward. Results of analysis show that this plant needs a lot of thermal energy, approximately accounting for 30.01%-36.44% of biogas energy. Moreover, merely recycling a part of the CO2-poor MEA liquid waste heat after decarburization results in low utilization rate of waste heat. It also reveals that the main parts of the waste heat in the system are made up of 5 types, i.e. waste heat from stripper top gas for decarburization, CO2-poor MEA liquid waste heat after decarburization, waste heat of cooling water from compressor, waste heat in biogas slurry and waste heat of boiler exhaust gas. Besides, the low-grade waste heat has the characteristics of enormous quantity and stabilization. The main parts of heat required include the heat of the fermentation liquid, the heat of maintaining high-temperature anaerobic digestion and the heat of decarburization. The calculation of requirement of heat shows that the quantity of total heat required is 7.85×104 MJ/d in the coldest month, and 6.48×104 MJ/d in the hottest month. The calculation of waste heat indicates that the potential of total waste heat is respectively 5.87×104 MJ/d in the coldest month, and 4.79×104 MJ/d in the hottest month. The corresponding maximum energy-saving rate is 74.81% and 73.92%, respectively. The energy-saving potential of each part of waste heat in descending order of quantity is: waste heat of biogas slurry > waste heat of CO2-poor MEA liquid after decarburization > waste heat of stripper top gas for decarburization > waste heat of cooling water from compressor > waste heat of boiler exhaust gas. Additionally, the analysis of waste heat proves that waste heat from this project can be more effectively utilized and preferably collected. Based on the analysis above, we propose some suggestions about the utilization of waste heat: 1) It is recommended that the waste heat of stripper top gas is collected to drive heat pump rather than cycle in system. 2) Waste heat of CO2-poor MEA liquid can be used to warm the low-temperature CO2-rich MEA liquid via the heat exchanger. 3) We recommend the waste heat of compressor cooling water is adopted to produce hot water by the heat pump, which will be regarded as domestic hot water or heating hot water. 4) Waste heat of biogas slurry can be used to heat low-temperature fermentation liquid by heat exchanger. 5) Waste heat of boiler exhaust gas can produce stream by heat exchanger, which is applied into system itself.