Using a Life Cycle Assessment Approach to Estimate the Net Greenhouse Gas Emissions of Bioenergy

Abstract

Life Cycle Assessment (LCA) is used to quantify the environmental impacts of products or services. It includes all processes, from cradle-to-grave, along the supply chain of the product or service. When analysing the global warming impact of energy systems, greenhouse gas (GHG) emissions (particularly CO₂, CH₄, and N₂O) are of primary concern. To determine the comparative GHG impacts of bioenergy, the bioenergy system being analysed should be compared with a reference energy system, e.g. a fossil energy system. A reference energy system should be chosen that is realistically likely to be displaced by the bioenergy system. If this reference system is not certain, then one option is to use as the reference energy system the average fossil energy for that region. Another option is to make a conservative evaluation by comparing the bioenergy system with the best available fossil energy technology. Alternatively, a non-fossil option may be selected as the relevant reference energy system. Depending on the context of the study, this might be another renewable option or nuclear power. The scope of the analysis (system boundary) should include all processes along the value chain with significant GHG emissions, including, where relevant, upstream processes of extraction or biomass production, and end-of-life processes. The system boundary should be defined so that the bioenergy and reference fossil systems provide equivalent products and services. If it is not possible to achieve this through expansion of the system boundary then the GHGs can be allocated amongst energy and non-energy co-products of the bioenergy system (such as biodiesel and rapeseed cake, from processing of rapeseed oil), based on their share of physical (for example energy) or financial contributions. Changes in carbon stocks in biomass, soil, and landfill can cause GHG emissions (or removals). These can be very important and should be included in the analysis. In general, LCA is not concerned with the time at which the environmental impacts occur. However, in some cases bioenergy systems cause short-term GHG emissions due to the accelerated oxidation of carbon stocks through combustion as compared to natural decay. While this can affect short-term GHG targets, over a long-term perspective sustainable bioenergy causes less GHG emissions than comparable fossil energy systems. Use of agricultural residues may affect GHG emissions through either changes in soil organic carbon (SOC) or land use changes that occur indirectly, in order to provide the equivalent services that the residues were providing. Exploitation leading to soil productivity losses may require compensating fertilisation (causing GHG emissions) to maintain yield levels and can also cause cropland expansion elsewhere to compensate for yield losses if these occur. The type of technology, scale of plant, and co-products in both the bioenergy and reference energy system can influence the GHG mitigation benefits of the bioenergy system. Since small changes in methodological assumptions and input parameters can have large effects on the estimated environmental impacts, the bioenergy and reference systems should be described and assumptions listed in a transparent manner.