车用生物燃气工程范例余热定量评估及可利用性分析

针对车用生物燃气工程能耗高、余热利用率低的问题,该文以国内4个典型工程为基础,构建了产气规模为1万m3/d的示例工程,并对其进行余热分析.分析结果显示,此类工程用能量大,占总产能的30.01%~36.44%;余热利用率低,只有部分贫液余热得以回收;系统余热主要由脱碳塔顶气余热、脱碳贫液余热、压缩机余热、沼液余热和锅炉尾气余热5部分组成,其多为低品位余热、量大稳定.余热计算表明,在最冷月和最热月系统余热潜力分别为5.87×104、4.79×104MJ/d,最大节能潜力分别为74.81%和73.92%,节能潜力降序排列为沼液余热>贫液余热>塔顶气余热>压缩机余热>锅炉余热.余热可利用性分析认为工程余热可利用性较高,回收价值较大.

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.