Abstract
Agricultural woody residues are available in massive quantities and provide a considerable potential for energy production. However, to encourage environmentally sustainable bioenergy strategies, it is necessary to assess the environmental performance of each specific bioenergy chain. Life cycle assessment (LCA) is recognized to be one of the best methodologies to evaluate the environmental burdens of bioenergy chains. The application of LCA to bioenergy from agricultural residues requires practitioners to make choices on how to interpret agricultural residues (i.e. by-products or co-products) and on how to allocate emissions among the different products generated along the bioenergy chain. These are among the most debated issues in the LCA community, given their potentially large influence on final LCA outcomes. A uniform consensus on these issues is still lacking, and no single method is equally suitable for all solutions. The aim of this paper is to assess how different ways of agricultural residue interpretation and different allocation methods (both of upstream and downstream emissions), affect the environmental performance of bioenergy production fed by agricultural residues. In order to address the issue, we perform a full attributional LCA of the electricity production in a combustion combined heat and power plant (CHP) fed with woody residues from apple orchards (AWRs), as a case study. Bioelectricity production from CHP fed with agricultural residues is a good example of a multifunctional process, since multiple
products (e.g. grain, fruit, straw, wood, etc.) and energy (e.g. heat and power) are co-produced along the whole chain. We solve the bioenergy system into two different ways, depending on the interpretation of AWRs as by-products or co-products, and we apply different allocation methods, partitioning the impacts according to different features of the co-products (mass, energy, exergy and economic value). The study focuses on greenhouse gas (GHG) emissions and cumulative energy demand, but other impact categories are investigated as well. The environmental impacts from two conventional fossil fuel systems are used to benchmark the environmental performance of the agricultural bio-energy chain. Our results show how different allocations, and especially allocation choices on upstream processes, can turn a benefit (i.e. a positive saving) to a disadvantage (i.e. an increased impact) and vice versa. Indeed, when AWRs are interpreted as a by-product or when upstream emissions are allocated on an economic base, the bioenergy chain guarantees significant GHG saving (up to 97%), primary energy demand reduction (up to 97%) and lower environmental impacts compared to the Italian non-renewable electricity grid mix. But, in the case of mass allocation of upstream emissions, the savings of GHG emissions and energy requirements drop to only 53% and 56%, respectively, and the figures of the bioenergy systems are similar or even worse than the fossil reference system for the majority of the other impact categories.