Flammability Limits of Low BTU Gases: Computations in a Perfectly Stirred Reactor and Experiments

Abstract

The demand for gas turbines suitable for Low Btu gases is increasing worldwide. This paper presents a theoretical and experimental investigation of the flammability limits of Low Btu gases for gas turbine applications. Most modern gas turbines utilize premixed combustion, making it important to know at which fuel-air ratio the flame extinguishes. The flammability limit for a gaseous fuel is a property, which is coupled to both thermodynamic quantities and the shape of the combustion chamber. Consequently, this property is characteristic for each combustor and for each fuel. The experiments were made in an atmospheric pressure premixed combustor at Alstom Power Technology Ltd. Switzerland, adapted for Low Btu gaseous fuels. Five different residual gases from chemical factories were investigated. The gases consisted of methane, carbon monoxide, hydrogen and nitrogen, with lower heating values about 2-3.5 MJ/kg for all examined gases (Table 1). A steady state Perfectly Stirred Reactor (PSR) was used as a model for the primary combustion zone. The reactions were modeled by a detailed mechanism for methane with 61 species and 667 reactions, developed by Warnatz [1]. The PSR calculations were done by decreasing the residence time until the combustion in the PSR extinguished. These calculations were repeated for different equivalence ratios to obtain the relation between the residence time and the limit of flammability. The calculations showed a relationship between the residence time in the PSR and the extinction point. It was found that the computed values of the flammability limits, or more correctly called stability limits, qualitatively follow the experimental results. However, since the computational results are strongly dependent on the residence time, a comparison with the experiments must include the residence time of the real burner, which is difficult to define.