Nonpremixed and Premixed Combustion of Mixtures of Producer Gas and Methane

Abstract

Experimental and kinetic modeling studies are carried out to characterize nonpremixed and premixed combustion of producer gas and mixtures of producer gas and methane. The producer gas, employed in the study, is made up of 55.00% carbon monoxide, 2.34% hydrogen, 12.72% methane, 5.24% ethene, and 24.7% carbon dioxide by mass. The primary focus is on characterizing the chemical influence of addition of producer gas on combustion of methane. The kinetic modeling studies are carried out employing a detailed chemical-kinetic mechanism, called the San Diego Mechanism, a skeletal mechanism, and a reduced mechanism made up of five global steps. Experiments on nonpremixed combustion are carried out employing the counterflow configuration. Critical conditions of extinction are measured for producer gas, methane, and mixtures of producer gas and methane. They are compared with predictions obtained using the detailed, skeletal, and reduced mechanism. Critical conditions of autoignition are measured for producer gas and compared with the predictions obtained employing the detailed mechanism. Experimental data and predictions show that with increasing amounts of producer gas in the mixture, the flame is more difficult to extinguish. Flame structures show that at a fixed value of the strain rate, leakage of oxygen from the reaction zone decreases with increasing amounts of producer gas in the combustible mixture. This is attributed to enhanced consumption of oxygen in an overall chain branching step that consumes hydrogen. Thus, the increase in the overall reactivity of the combustible mixture is attributed to presence of hydrogen in producer gas. Computations are performed, using the detailed mechanism and the skeletal mechanism, to investigate aspects of premixed combustion of stoichiometric mixtures of producer gas, methane, oxygen, and nitrogen at fixed values of adiabatic temperature. Burning velocities are calculated. They are found to be less sensitive to the amount of producer gas in the mixture. This is qualitatively different from that observed for nonpremixed combustion. It is attributed to complete consumption of all reactants including oxygen and combined influences of H₂ and CO in producer gas on overall combustion of methane. A third set of computations was performed on strained premixed flames stabilized in counterflow between a stream of a stoichiometric mixture of producer gas, methane, oxygen, and nitrogen, and a stream of nitrogen at fixed adiabatic temperature. Critical conditions of extinction were obtained. The strain rate at extinction increased with increasing amounts of producer gas. The increase was comparable to that observed for nonpremixed combustion of these fuels.