Economic and environmental impacts of an integrated-state anaerobic digestion system to produce compressed natural gas from organic wastes and energy crops

Anaerobic Digestion (AD) is a well-developed sustainable technology to convert organic waste streams and energy crops to produce renewable gaseous biofuels, while recycling nutrients and mitigating greenhouse gas emissions. In this study, the environmental and economic impacts of an integrated-state AD technology (i-AD) producing Compressed Natural Gas (noted as BioCNG) were investigated from dairy-manure, food-wastes, and miscanthus biomass feedstocks, and compared with that of stand-alone liquid-state (LS-AD) and solid-state (SS-AD) AD technologies. A coupled life-cycle assessment and techno-economic analysis (LCA-TEA) approach was used to estimate the Global Warming Potential (GWP) and the Minimum Selling Price (MSP) of BioCNG - a renewable alternative to fossil-CNG. The results illustrated that the Fossil Energy Ratios (FERs) for BioCNG were between 2.3 and 3.3 in the increasing order as LS-AD < i-AD < SS-AD. The life-cycle GWPs to produce BioCNG via LS-AD, SS-AD, and i-AD were -5.1, -15.1, and -12.0 kgCO(2)eq/GGE (Gasoline-Gallon-Equivalent) respectively. The MSP of BioCNG (without incentives) via LS-AD ($2.9/GGE) was lower than that of both SS-AD ($4.1/GGE) and i-AD ($4.9/GGE). When the tipping-fee ($44/Mg), RIN(Renewable Index Number)-credit ($0.46/RIN), and carbon-credit($13.6/MgCO(2)eq) were considered, the MSP of BioCNG dropped by up to 70%, 45%, and 25% for LS-AD, SS-AD, and i-AD, respectively. Fungal-pretreatment of miscanthus had negligible impacts on the environmental and economic performances of BioCNG. Backhauling of solid-digestate for miscanthus cultivation may reduce the MSP, energy usage, and GWP by up-to 5%, 16%, and 7%, respectively. The commercial production of BioCNG from energy crops can potentially be competitive at a higher BioCNG market price or with favorable energy policies, financial support, and tax-benefits.