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Environmental impacts of biogas deployment – Part II: life cycle assessment of multiple production and utilization pathways

Poeschl, Martina, Ward, Shane, Owende, Philip
Journal of cleaner production 2012 v.24 pp. 184-201
agricultural land, alternative fuels, anaerobic digestion, attributional life cycle assessment, biogas, cattle manure, climate change, computer software, corn silage, electricity, energy balance, energy crops, energy density, environmental impact, environmental performance, feedstocks, fertilizers, food industry, fossil fuels, greenhouse gas emissions, heat, municipal solid waste, nutrient content, primary energy, straw
Energy security concerns and the need for mitigation of environmental impacts associated with energy generation from fossil fuels (e.g., greenhouse gas emissions), has accelerated the deployment of renewable fuels such as biogas. The objective of this study was to conduct an attributional Life Cycle Assessment (LCA) of multiple biogas production and utilization pathways in order to identify areas where further mitigation of potential environmental impacts could be realized to enhance environmental sustainability of biogas deployment. The LCA of pre-defined small (<500 kWₑₗ) and large-scale (≥500 kWₑₗ) biogas systems was conducted in accordance with the ISO 14040 standards, using SimaPro 7.2 computer software. The functional unit was the anaerobic digestion of 1 tonne of feedstock mixture to produce biogas with the digestate as process end product with multiple utilization options. The analyses quantified the impacts of feedstock type (both single feedstock and co-digestion), biogas utilization pathways, and the digestate processing and handling unit processes. Analyses also considered the replacement of fossil fuels and chemical fertilizer with equivalent energy value of the biogas and nutrient content of the digestate, respectively. The recorded variations in life-cycle impact categories for the scenarios compared indicated the importance of judicious selection of biogas pathways for environmental impact mitigation. The LCA and life-cycle energy analyses for single feedstock scenarios considered indicated that straw and corn silage as most efficient feedstocks for biogas. For example, straw mixture improved the environmental performance by almost 830% compared to the base scenario of cattle manure feedstock. This was mainly ascribed to the higher energy density, which exceeded the primary energy inputs for feedstock supply logistics. In order to minimize the environmental damage associated with feedstock type in all impact categories considered, and simultaneously maintain a positive energy balance, the analyses suggest that co-digestion of Municipal Solid Waste (MSW) with agricultural and food industry residues are most appropriate for both small and large-scale biogas plants; co-digestion of waste and residues accounted for just 1% of the estimated impacts on agricultural land occupation, compared to the co-digestion of predominantly energy crop feedstock, and also reduced the climate change impacts by up to 30%. The results also indicated for the small-scale plants, the most promising pathway for sustainable biogas utilization would be in tri-generation; compared to electricity only generation in Combined Heat and Power (CHP), tri-generation could reduce the overall environmental impact by almost 200%. For the scenarios that included purification and upgrading biogas to biomethane for gas grid injection (arguably the most promising technology that could support rapid utilization expansion), it was noted that only the scenario with coupled small-scale CHP unit covering internal heat demands was capable of reducing the overall impact on fossil fuel depletion, compared to electricity generation alone. This was explained by the higher potential for fossil fuel substitution with biomethane, due to higher conversion efficiency (ca. 100%). It was also found that, the recovery of residual biogas from digestate storage reduced the environmental impacts of digestate management process by ca. tenfold, due to combined reduction of the potential biogas leakage to the atmosphere and subsequent use of the extra yield for energy generation.