An experimental and kinetic study of syngas/air combustion at elevated temperatures and the effect of water addition

Laminar flame speeds of premixed syngas/air mixtures were measured at various fuel equivalence ratios (0.6-3.0), H2 content of the fuel, and preheat temperatures (298-500 K) using a spherically expanding flame configuration. The measured laminar flame speeds were compared with simulations using with three existing chemical kinetic models - GRI Mech 3.0, H2/CO Davis Mechanism and San Diego mechanism. Reasonable agreement between computations and measurements was achieved at room temperature that validated the new experimental configuration. However, at higher preheat temperatures discrepancies between computed and measured values were large, especially for fuel rich mixtures. Addition of H2O to two fuels (H2/CO = 5/95 and 50/50) up to 40% in the fuel-air mixture was studied to understand the effect of moisture in coal derived syngas. For the H2/CO = 5/95 fuel, flame speed was observed to increase with up to 20% H2O addition and then to decrease with any further water addition. However, the higher H2 content fuel (H2/CO = 50/50) only showed a decrease in flame speed with an increasing water concentration in the fuel-air mixture. The different trends have been explained as a result of the competing chemical and physical (dilution and thermal) effects of H2O addition on the syngas flames using sensitivity analyses and by analyzing reaction rates and radical concentrations.