Assessment of waste derived gases as a renewable energy source – Part 2

Abstract

UK government requires local authorities to develop Municipal Waste Management Strategies, to put in place policies to reduce greenhouse gas emissions and contribute to meeting both the EU Landfill Directive and UK government recycling targets.To assist the decision making process, a study was commissioned to compare the energy produced by treating 100,000tpa residual municipal solid waste to provide a fuel derived from treating the waste using anaerobic digestion (AD) and gasification processes used on-site, with the equivalent off-site use. Comparison of technology scenarios was based on a number of parameters, comprising efficiency of energy production, energy capacity, avoided CO2 emissions and capital and operating costs.The study examined a total of 41 technology combinations that produce/use waste-derived gases based on gasification and AD, against a base case of on-site power production only. Scenarios considered for using the gases derived included on/off-site use in CHP units based on gas engines and fuel cells and use as a transport fuel for vehicles. The relative performance of on/off site options was influenced by a trade-off between a reduction in efficiency caused by supply chain losses and parasitic requirements that tend to favour on-site options and improvements in efficiency achieved by delivering a greater quantity of recovered heat to end users.Based on the assumptions applied to the study, all scenarios involving gasification and on-site fuel use achieved higher overall energy efficiencies than the equivalent off-site use, despite the greater use of waste heat assumed for off-site scenarios. In contrast to gasification, off-site scenarios using biogas from AD via injection into the gas grid, had the highest efficiency. Off-site use as a liquid fuel in a fuel cell gave a lower energy efficiency than the base case of power only for MBT AD, while for source segregated (SS) AD the base case of power only had the marginally lowest energy efficiency.The magnitude of the energy produced by gasification/combustion gave the highest outputs, ranging from 44GWh/y to 94GWh/y. AD technologies gave net energy outputs between 10GWh/y to 22GWh/y for MBT AD and 40GWh/y to 53GWh/y for SS AD. The energy from waste power only/CHP cases produced energy outputs of 65 and 73GWh/y respectively.Part 1 of the paper set out the background to the study, assumptions, methodology used and references for the 41 scenarios assessed, while in Part 2 the outputs generated by the assessment process are presented and discussed.