Environmental impacts of biogas deployment – Part I: life cycle inventory for evaluation of production process emissions to air

A Life Cycle Inventory (LCI) was developed to identify the unit processes in the life-cycle of biogas production and utilization offering the greatest opportunities for emission to air reduction, hence potential for environmental improvement. The systems investigated included single feedstock digestion and multiple feedstock co-digestion, small (<500 kWel) and large-scale (≥500 kWel) biogas plants, and selected biogas utilization pathways and digestate management options. Analysis was performed in accordance with ISO 14040 and 14044 standards, using SimaPro 7.2 software and Ecoinvent® v2.1 database. The analysis is based on published data considering primarily conditions for Germany. Results indicated significant variation of emission levels for all unit processes related to biogas production and utilization. Emissions from the feedstock supply logistics were highly influenced by the origin of feedstock used. For example, the fossil fuel related carbon dioxide (CO2,fossil) emissions associated with feedstock supply were over 50 times higher for Municipal Solid Waste (MSW) compared to cattle manure. The higher value for MSW was associated with the requisite collection, transport and pre-treatment, whereas only transportation was required for cattle manure. Emissions from unit processes in biogas plant operation and biogas utilization depended on combined efficiency of energy generation (electricity and thermal), potential substitution of fossil fuels with biogas and utilization of the heat by-product of electricity generation. For example, the results indicated that upgrading of biogas to biomethane, with almost 100% conversion efficiency, caused 6 times less non-methane volatile organic compounds (NMVOC) emissions if plant heating was supplied from coupled small-scale CHP unit as opposed to heating with natural gas. Harnessing of the residual biogas from digestate storage areas was estimated to reduce methane emission by a factor up to 14. Overall, this study provides basic data required for identification and mitigation of emission ‘hot-spots’ in biogas production and utilization, including the evaluation of environmental and public health impacts of biogas technology options by attributional Life Cycle Assessment (LCA) methodology.