Life Cycle Environmental and Economic Assessment of Different Biogas and Biogas Residue Operation Models

The utilization of biogas and biogas residues is an important objective of animal manure resource management. Regarding the environmental friendliness and economic suitability of utilization modes, previous studies have evaluated and compared biogas or biogas residue individually, which may lead to incomplete environmental and economic assessments. In this study, the environmental and economic aspects of six integrated biogas and digestate operation modes, i.e., biogas utilization (direct combustion, cogeneration, and purification) and biogas residue utilization (field return and composting), were analyzed via life cycle assessment (LCA) and the net present value (NPV) method, with an animal manure biogas plant in Tangshan City as the study object. The results of LCA showed that biogas cogeneration + biogas residue composting disposal was superior to other models in most environmental indicators. NPV analysis showed that direct biogas combustion + biogas residue composting was the best, breaking even in the 3rd year with a cumulative profit of 250 million CNY. With increased environmental costs and benefits, the biogas cogeneration + biogas residue composting model had the best performance in terms of efficiency ratio at 2.58; the NPV of both operating models of biogas cogeneration increased, while the NPV of the other operating models decreased about 20%.