Using Life-Cycle Assessments to Demonstrate the Impact of Using Wood Waste as a Renewable Fuel in Urban Settings for District Heating

The use of wood waste for heating in urban settings provides an opportunity for communities to reduce annual fossil emissions by directly reducing the amount of fossil fuel used. Life-cycle assessments (LCA) comparing the environmental impacts of alternative processes or products provide the essential information to better understand opportunities for improvement. An LCA was performed on a Seattle, Washington, district heating system that provides thermal energy to a large number of buildings in downtown Seattle. This study presents annual impacts in terms of carbon emissions for heat production generated using a new boiler design fuel mix including wood wastes as well as natural gas. Results are compared with the results from the 100 percent natural gas boiler that was previously used. The LCA includes results from both a life-cycle inventory of all inputs and outputs and a life-cycle impact assessment comparing alternatives. Results show that global warming potential (GWP) was reduced by 57 percent for the mix fuel design boiler compared with an all natural gas boiler. When 100 percent woody biomass is used, the reduction increases to 104 percent. Transportation and collection of feedstocks contributed minimally (8%) to the overall impact, while the combustion life-cycle stage accounted for 92 percent of the total GWP.