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 lifecycle 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. District heating systems distribute heat generated in a centralized location to local users. They are capable of producing steam, hot water, or chilled water at a central point and piping it underground to individual buildings, where it is used for heating, hot water consumption, or air conditioning. Some of the environmental and economic benefits to the community are improved energy efficiency, fuel flexibility, ease of operation and maintenance, reliability, and decreased life-cycle and environmental costs. Andrews (2009) reported that district heating systems with combined heat and power have one of the lowest carbon footprints. Several boiler systems—Seattle, Washington, and St. Paul, Minnesota, for example—have converted from fossil fuels to renewable biomass. Using wood wastes to generate heat reduces fossil emissions from natural gas or other fossil fuels. In a recent study, substituting wood-based fuels with fossil fuels in sawmill operating boilers resulted in a 66 percent reduction in carbon emissions (Puettmann and Lippke 2012). Burning wood residue in boilers for heat generation has historically been restricted to those industries where biomass or ‘‘hogged fuel’’ is readily available as a byproduct, primarily in the forest products industry. There is recognition that biomass resources in the United States have been largely underutilized for the production of fuels (Gan and Smith 2006). Reports have suggested that over 1.3 billion bone dry tons (BDT) are available for fuel alone in the United States (Fernholz et al. 2009). With increasing fossil fuel prices and rapid technological advances in biomass conversion processes the interest in biofuels is