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

EXECUTIVE SUMMARY: Life cycle assessment (LCA) is a powerful tool that may be used to quantify the environmental impacts of products and services. It includes all processes, from cradle-to-grave, along the supply chain of the product. When analysing energy systems, greenhouse gas (GHG) emissions (primarily CO2, CH4 and N2O) are the impact of primary concern. In using LCA to determine the climate change mitigation benefits of bioenergy, the life cycle emissions of the bioenergy system are compared with the emissions for a reference energy system. The selection of reference energy system can strongly affect the outcome.
When reviewing the literature one finds large ranges of GHG emissions per unit of energy from LCA studies of similar bioenergy systems. The differences occur for a multitude of reasons including differences in technologies, system boundaries, and reference systems. Some studies may be incomplete in that the bioenergy system and reference system provide different services. Others may omit some sources of emissions (e.g. land use change).
This paper discusses key criteria for comprehensive LCAs based on IEA Bioenergy Task 38 case studies. LCAs of the GHG balance of four different bioenergy systems and their counterpart reference system are highlighted using the case study examples.
The first example investigates heat production from woody biomass and grasses. This study shows that the emissions saved for the same type of service can vary due to the source of the biomass. The bioenergy systems studied reduce GHG emissions by 75-85% as compared to the counterpart reference systems.
In the second example, electricity is produced from woody biomass using two different technologies with different efficiencies. Depending on the technology, the biomass must
be transported different distances. The example illustrates the importance of the efficiency of the system and the small impact of soil organic carbon (SOC) decline in comparison
with emissions saved. Since the bioenergy systems include carbon sequestration, they reduce GHG emissions by 108-128% as compared to the counterpart reference systems.
A biogas plant providing combined heat and power is analysed in the third example, which illustrates the importance of finding a beneficial use for the heat produced, and of controlling fugitive emissions. In the optimal configuration of closed storage and maximised use of heat, the biogas system reduces emissions by 71% as compared to the counterpart reference system. This reduction decreases to 44% when the heat is not fully used and to only 27% if fugitive emissions are not controlled.
In the final example the bioenergy system provides biodiesel for transport. This example demonstrates the importance of the use of co-products, as the same bioenergy chain produces very different emissions savings per kilometre depending on whether the co-product is used as a material or combusted for energy. Compared to the reference system, the bioenergy system reduce GHG emissions by 18% and 42% when the co-products are used for energy or materials respectively.
Similar to the case studies presented here, published studies find that GHG mitigation is greater where biomass is used for heat and electricity applications rather than for liquid transport fuels. Overall, the emissions savings from bioenergy systems tend to be similar to that of other renewable energy sources.