Life cycle assessment of carbon emission from a household biogas digester: Implications for policy

摘要

The rural household biogas program in China has been regarded as an effective approach to address energy shortage issues and to reduce carbon emission in rural areas. After more than fifty years of development, it has neither succeeded in rural energy substitution nor achieved other social and environmental goals, although available studies have shown otherwise. The reason behind this disparity largely lies in inaccurate adoption of the operational cycle, i.e., the fact that the actual operational time of the biogas digester was often overestimated. Presented In this paper, an EIO-LCA model using the Chinese Economic Input-Output table of 2007 was built to estimate carbon-dioxide (CO2) emission during the life cycle of a typical 8 m~3 household biogas digester and their sources with regard to economic sectors. The life cycle of a biogas digester can be divided into three stages: construction (including pond building and pen, toilet and stove alterations), operation (including maintenance and biogas combustion) and disposal. The results show that the net CO2 emission of a biogas digester is 1558.91 kg after one year of operation, which corresponds to an actual CO2 emission of 2878.30 kg (of which direct and indirect emissions are 1558.90 and 1319.40 kg, respectively) and an emission reduction by fuel substitution and fertilizer conservation of 1078.59 and 240.80 kg, respectively. Therefore, the model indicates that a biogas digester should be run for at least 2.77 years to have a net CO2 emission reduction benefit. The model also indicates that comprehensive utilization of biogas and its byproducts is very important for carbon emission in rural areas. Most of the indirect emissions were derived from construction materials, accounting for 88.18%. Moreover, the smelting and pressing of ferrous metals in the 43-step production chain of construction materials accounted for 22.48% of the indirect emissions. This gives us a novel perspective to develop emission reduction policies. Taken together, the results obtained in this study provide substantial information for policy-making on biogas promotion and management as well as renewable energy development in rural China.