Fundamental and environmental aspects of landfill gas utilization for power generation

Landfill gas (LFG) results from the biological decomposition of municipal waste and consists of mostly equal amounts of CO2 and CH4, as well as trace amounts of a variety of other organic compounds. Upon removal of most of the trace organic compounds, LFG can be used as fuel in internal combustion engines and gas turbines for generation of heat and electricity. Producing energy from LFG has the additional benefit of preventing its release into the atmosphere, where it results into significant air pollution. The large quantity of CO2 in landfill gas (typically 40-50%) presents problems with its utilization for energy production, since it negatively impacts combustion efficiency and stability. To improve the economics of LFG utilization for energy production, it is important to develop a better fundamental knowledge base about its burning characteristics. This has been the goal of this combined experimental and numerical investigation. Laminar flame speeds, extinction strain rates, temperature, and species concentrations profiles, including NOx, were experimentally determined. We have used a stagnation-flow experimental configuration, which makes it possible to simulate the experiments using a complete description of molecular transport and the detailed GRI 2.11 chemical kinetic mechanism. The experimental results from laminar flame speeds, extinction strain rates, species structure, and thermal structures compare generally well with the simulation results. As expected, it was found that the presence of CO2 in LFG significantly decreases the laminar flame speeds and extinction strain rates. The study indicates that increased CO2, concentrations in LFG increase the amount of NO emissions per gram of consumed CH4. Considering a number of detailed (DRM) and semi-detailed radiation models (SRM), we also assessed the effect of thermal radiation on laminar flame speeds, extinction strain rates, and flame structure. The optically thick (DRM) model resulted in higher laminar flame speeds, extinction strain rates, and maximum flame temperatures compared to the optically thin (SRM) model. Fundamental flammability limits were also calculated, and it was found that as the CO2 concentration increases, the flammable range noticeably decreases. Analysis of the flame structure revealed that the effect of CO2 on the flame response is of thermal rather than kinetic nature. (C) 2001 Elsevier Science B.V. All rights reserved.