An experimental and numerical study of the combustion of landfill gas

Landfill gas (LFG) results from the biological decomposition of municipal waste and consists typically of equal amounts of CO2 and CH4, as well as of trace amounts of a variety of other organic compounds. Upon removal of most of the trace organic compounds, LFG can be used as a fuel in internal combustion engines and gas turbines for generation of heat and electricity. Combustion of LFG besides producing energy has the additional beneficial effect of preventing its release into the atmosphere, where it results into significant air pollution. The large quantity of non-combustible CO2 (40--50%) LFG contains presents problems with flame stability. In order to attain stable and efficient combustion, 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 were experimentally determined in the stagnation-flow configuration. The effect of CO2 concentration in the fuel feed on flame propagation, extinction, and structure was evaluated for a wide range of conditions. As expected, it was indeed found that the presence of CO2 significantly decreases the laminar flame speeds and extinction strain rates. The simulations were conducted along the stagnation streamline in a counter flow configuration using a detailed description of molecular transport and the detailed GRI 2.11 chemical kinetic mechanism. The simulations have shown that as the CO 2 concentration in the fuel feed increases, the combustion of LFG results in larger amounts of NO emissions per gr of consumed CH4. They have also helped to provide physical insight into the controlling mechanism.;The experimental results from laminar flame speed, extinction strain rates, species structure and thermal structure are compared with the simulation results. The experimental results agree fairly well with the optically thin model and not so well with the optically thick model because the GRI2.11 mechanism is optimized by using optically thin model. The effect of radiation model on laminar flame speed, extinction strain rate and flame thermal structure are carefully studied numerically. The optically thick model has higher laminar flame speed, extinction strain rate and maximum flame temperature than optically thin model. At high CO2 concentration, those values of optically thick model are even higher than adiabatic model because of the strong radiation feedback heat transfer. Flammability simulations show that as CO2 concentration increases, the flammable range noticeably decreases. Further detailed numerical study on CO2 effect show that CO2 does not affect the flame kinetically although it does participate the reactions. CO2 affect the flame through thermal dilution and radiation.