基于生命周期的户用沼气系统可用能核算——以广西恭城瑶族自治县为例

发展沼气生态农业可以实现资源的综合利用,带来经济效益与生态效益,同时解决我国农村地区能源短缺和环境污染问题。明确沼气系统内部的物质能量转化利用情况,可为沼气农业系统优化和效益提升提供科学依据。提出基于生命周期的户用沼气系统可用能核算方法,并以全国生态农业示范县——广西恭城瑶族自治县为例,核算了该县典型户用沼气系统建设、运行和利用单元投入产出的可用能流,分析了整个系统的可用能转化与利用效率。结果表明:系统的可用能投入为(1.06×108)kJ/a,可用能产出为(5.00×107)kJ/a,主要产出形式为沼渣;可用能转化率为48.82%,利用率为21.60%,其中沼气利用效率最高;系统产生的环境排放为(3.42×105)kJ/a,主要形式为系统利用单元沼气燃烧产生的CO2。由此可见,沼气生态农业可通过增加转化环节实现农业废弃物的再利用,系统可用能效率具备极大的提升空间,系统可持续性有待加强。可以考虑从改进工艺技术和改善发酵环境两方面提高户用沼气系统能量转化的能力,通过沼渣沼液综合利用技术方面的创新提高户用沼气系统的可用能利用效率。生命周期可用能核算方法可以更全面的反映系统的能量利用效率,便于诊断薄弱环节,为系统优化提供依据。

Exergy-based life cycle accounting of household biogas system: a case study of Gongcheng, Guangxi

Biogas-linked agro-ecosystem can bring in economic and environmental benefits via integrated resource utilization, which provides an energy consuming pattern that has prominent advantages in relieving energy shortage and reducing environmental pollution. It is important to make clear how materials and energy convert in the agro-ecosystem, because the facts could suggest us how to improve the performance of the agro-ecosystem and get more benefits. Exergy of a system in a certain environment is defined as the amount of work that can be extracted from the system, which can be regarded as a uniform measure for different forms of energy and different states of matter. This paper presents an exergy-based life cycle accounting method for household biogas system. The proposed method is applied to a case study of Gongcheng as a "National Eco-agriculture Demonstration County" due to its demonstration status of biogas-linked agro-ecosystem in China. It has a high popularization rate of household biogas system. The typical household biogas system of Gongcheng is supposed to have three stages, i.e., construction, operation and utilization. We calculate the exergy input - output of each subsystem and hereby analyze the exergy efficiency of the whole agro-ecosystem. The results show that, the exergy input of the system is 1.06×10⁸kJ/a while the exergy output (mainly in the form of biogas residue) is 5.00×10⁷kJ/a, with the exergy conversion rate being 48.82%. The exergy utilization rate is 21.60%, and biogas contributes more than biogas residue and biogas slurry. The exergy of system emission reaches 3.42×10⁵kJ/a, which mainly comes from the biogas combustion. Major conclusions are as follows: (1) Compared with the traditional agro-ecosystem, biogas-linked agro-ecosystem has a longer utilization chain for agricultural wastes, which takes in nonrenewable resources and outputs renewable resources. Therefore, household biogas system is an effective measure in relieving energy shortage and reducing environmental pollution in rural area and realizing the sustainable development. (2) The exergy efficiency of the household biogas system is at a low level, and still needs to be promoted. Improving the process technology and optimizing the fermentation condition may be a choice to increase the exergy conversion rate. Meanwhile, technological innovation in the multi-purpose use of biogas residue and biogas slurry can increase the exergy utilization rate. (3) The exergy-based life cycle accounting method of household biogas system in this paper extends the application of exergy accounting, and fills in the gap of household biogas research field. The method may help us understand the household biogas system more clearly and directly in energy conversion and utilization mode. As a result, it is convenient to find out the weak points of the agro-ecosystem that waste more energy and seek for optimal solutions accordingly.