基于能值分析的规模化沼气工程沼液回流工艺生态效益评价

为评价沼气工程沼液回流以及不回流2种模式的优劣,该研究引入相对能值转换系数和生态投入能值的概念,给出了三沼分配产出能值的方法,并选取D(CSTR工艺)和L(USR工艺)2座具有代表性的沼气工程实例应用,每个工程均设定沼液回流和不回流2种模式相对应,利用能值评价指标对2座工程进行了详细的分析。结果表明D工程各项评价指标优于L工程,沼液不回流模式优于沼液回流模式;但当沼气工程无途径消纳产生的沼液时,回流模式可以降低沼液对周边环境的污染,此时沼液回流模式优于不回流模式,L工程周边土地较少,采用回流工艺使环境负载率由2.15降低为1.05,能值可持续指标由0.41增加到0.93。采用何种模式要根据工程实际情况而定,建议沼气工程每处理每吨VS(鸡粪)需配备的土地为0.5 hm~2/t,处理每吨VS(牛粪)需配备土地0.2 hm~2/t,当周边土地小于需配备土地时,沼气工程应适当采用沼液回流。

Ecological evaluation of large-scale biogas engineering with biogas slurry recycle mode based on emergy analysis method

Biogas project is a kind of powerful tool for dealing with and utilizing the waste of livestock, and it is crucial to the development of China where the livestock production has been up to 85.378 million tons, while A large amount of biogas slurry will be produced in the process of biogas project operation. If excessive quantities of biogas slurry directly apply to the land, it will be harmful to the environment. How to properly dispose the biogas slurry is the problem which must be solved in the biogas project development process and the technical operation mode should be studied in order to improve its efficiency. At present, recirculation is a common way to reduce the influence of biogas slurry to environment, and appropriate biogas slurry recirculation can improve the stability of fermentation system, reduce the use of groundwater and avoid secondary pollution, but the biogas slurry perennial recirculation will cause lower gas production and choke the pipeline. Emergy analysis was selected in this research in order to compare these two technology modes which were biogas slurry recirculation and no slurry recirculation, however, the application of emergy analysis in biogas project was not mature, there was no difference of transformity between livestock manures and no appropriate emergy distribution method for multi-products output system. In view of this, ''relative emergy transformity'' had been introduced to differentiate the emergy transformity of chicken and cow manures, ecological input emergy had been introduced to measure the influence of biogas slurry excessive application on the environment, and a method to distribute the output emergy of biogas, slurry and digestate also had been given in this paper. Moreover, two representative biogas plants were selected as cases for study, researchers had observed those two plants for one year, and had done some relative experiments to get data and monitor its status. Each plant, in particular, was designed in two kinds of modes, slurry recirculation or not, to use for contrast and the two plants were analyzed and compared in details by emergy evaluation index system. The results showed that the values of evaluation indexes of biogas plant D were better than L and the mode of no slurry recirculation was better than recirculation, but the recirculation mode can reduce the pollution to the surrounding environment where the biogas slurry had no way to be used. Through the comparative analysis of the two kinds of modes, the value of ELR(environment load rate) could be decreased dramatically from 2.15 to 1.05 by using the recirculation mode in plant L, and the value of ESI(emergy sustainability index) could be increased significantly from 0.41 to 0.93, so the technical mode should be chosen according to conditions and environment of the biogas plant. It is suggested that the dispose of one ton of chicken manure VS by biogas plant should be equipped with 0.5 hm2/t land, and the dispose of one ton of cow manure VS is 0.2 hm2/t, thus the biogas plant should properly use the recirculation mode of the slurry when the surrounding land is less than its need.