Life cycle assessment of energy generation from biogas—Attributional vs. consequential approach

Many studies that apply life cycle assessment methodology avoid a strict differentiation between attributional (aLCA) and consequential (cLCA) life cycle assessment. The main distinction that can be made is that an aLCA approach describes a state of average production systems of an economic system while in contrast the consequential approach describes changes (induced by political decisions) in production systems within the economic system. The task of this study was to analyze a biogas system from an environmental point of view and thereby to work out the methodological differences of aLCA and cLCA approaches. The Life cycle inventory quantity primary energy demand (PED) as well as the impact categories global warming potential (GWP), eutrophication (EP), acidification (AP) and photochemical ozone creation potential (POCP) were analyzed. The aLCA approach was split into three scenarios, a physical, an economic and a core product focused one (with focus on the main product) to show the impact of by-product handling. The cLCA approach was split into a local scenario using on-site data and a general scenario using higher aggregated data to show the effects of substitution caused by the introduction of a new technology. The results of the two approaches were compared with the environmental impact of the current average and marginal German electricity mix. Global warming potential per functional unit varied between 3.8 g and 12.5 g of CO2 equivalent in the biogas scenarios. Compared to the average and marginal German electricity mix savings in PED, GWP and partly in AP and POCP can be achieved. However, high variations in the proportion to the reference electricity system, the total quantity results as well as the contribution of single processes to the total result were found. This makes it indispensable to distinguish accurately between the aLCA and the cLCA approach.