(480d) Life Cycle Assessment of Waste-to-Energy Technologies

The United States generated 243 million tons of municipal solid waste (MSW) in 2009. With household waste generation continuing to grow at a rate of 3% per annum (Baidoo, Ferguson et al. 2009), the annual United States MSW generation is expected to reach 400 million tons by the year 2030. The current trend for the treatment of MSW is for 54% to be landfilled, 33% recycled and 12% combusted with energy recovery (United States Environmental Protection Agency 2010).

 The disposal of MSW through landfilling causes direct and indirect environmental harm through increased greenhouse gas (GHG) emissions. For example, U.S. methane emissions from landfills were close to 130 TgCO₂e in 2007 or 32% of all human generated methane emissions for that year (Jaramillo and Matthews 2005). In the larger context, such emissions contribute to climate change, smog formation and human health degradation. In light of these findings, a number of experts within the municipal solid waste management community are calling for a change in the waste management paradigm.  For example, the City of Austin, Texas recently ratified a zero-waste master plan that aims to achieve a 20% reduction in solid waste disposal to landfills by 2012 and 90% reduction by 2040.  Such dramatic shifts in the MSW waste flow will likely call for new, or different, MSW treatment options.

One of the primary options under consideration is waste-to-energy (WTE).  In general terms, WTE technologies seek to convert post-recyclable MSW into a useful form of energy.  These approaches range from “tried and true” technologies such as advanced thermal recycling (i.e. incineration with a highly effective emissions control system) to edge-of-the envelope technologies such as plasma-arc reduction.  Many opinion leaders in the solid waste management community have lauded praise on WTE technologies claiming that they offer the dual benefits of decreasing net GHG emissions and helping ameliorate the United States’ present energy crisis. However, a realistic understanding of the potential benefits of WTE technologies demands an accurate accounting of each technology’s environmental impact over its entire life cycle. Moreover, those impacts must be reported in such a way as to promote meaningful comparison with the current landfill-based approach for solid waste disposal. 

We present a Life Cycle Assessment study of a gasification WTE technology and compare its environmental impact with other waste to energy technologies and landfill emissions. Although individual life cycle assessment studies have been performed for most or all of the viable WTE technologies (Kalogo, Habibi et al. 2007; Velumani and Meenakshi 2007; Datiz 2008; Chester and Martin 2009; Ducharme 2010; Zaman 2010) each of these LCAs are based on different process boundaries (in terms of number of upstream and downstream processes included), geographical differences (e.g. European data vs. U.S. data), different emission factors and varying sets of assumptions.  Such disparities disallow meaningful comparison between the various MSW management strategies.  This study’s LCA encompasses emissions from multiple upstream processes thereby “capturing” the majority of energy use and emissions.  For example, it considers energy use and emissions associated with a facility’s construction and demolition averaged over its productive life.  The LCA begins with a detailed process flow diagram for each technology that leads to the determination of relevant unit process data modules.  Next an inventory  life cycle impact assessment is carried out.  This assessment focuses on mid-point indicators that fall under four major categories: climate change impact, human health impact, ecosystem impact, and non-renewable energy used. Special emphasis is placed on the global warming potential indicator since it accurately reflects greenhouse gas (GHG) emissions represented as CO₂ equivalents. Finally, the paper explores the usefulness of LCA studies in informing policy decisions on the adoption of waste-to-energy technologies.