Fundamental and environmental aspects of landfill gas utilization for power generation

Landfill gas (LFG) results from the biological decomposition of municipal w aste 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 th e trace organic compounds, LFG can be used as fuel in internal combustion e ngines and gas turbines for generation of heat and electricity. Producing e nergy from LFG has the additional benefit of preventing its release into th e atmosphere, where it results into significant air pollution. The large qu antity 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 e nergy production, it is important to develop a better fundamental knowledge base about its burning characteristics. This has been the goal of this com bined experimental and numerical investigation. Laminar flame speeds, extin ction strain rates, temperature, and species concentrations profiles, inclu ding NOx, were experimentally determined. We have used a stagnation-flow ex perimental configuration, which makes it possible to simulate the experimen ts using a complete description of molecular transport and the detailed GRI 2.11 chemical kinetic mechanism. The experimental results from laminar fla me speeds, extinction strain rates, species structure, and thermal structur es compare generally well with the simulation results. As expected, it was found that the presence of CO2 in LFG significantly decreases the laminar f lame 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 rad iation models (SRM), we also assessed the effect of thermal radiation on la minar flame speeds, extinction strain rates, and flame structure. The optic ally 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 noti ceably 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.