Experimental Determination of Lean Flammability Limits of Renewable and Process Gas Mixtures at Elevated Temperature and Pressure Conditions

Abstract

Elevated pressure and temperature conditions are widely encountered during gas turbine operation. To avoid unexpected ignition and explosion of mixtures of fuel and air under these conditions, it is imperative to identify the flammability limits of relevant fuel mixtures. Common fuels include process gases such as natural gas, coke oven gas and IGCC syngas fuel. The flammability limits of pure fuels and common gas/air mixtures have been widely reported, however a significant lack of flammability data for fuel mixtures relevant for use in gas turbines as well as data at elevated pressure and temperature conditions is available. The objective of this study is to characterize the flammability limits of fuel/air mixtures and their dependence on initial temperature and pressure. Experimental studies of lean flammability limits (LFLs) for methane, hydrogen, and carbon monoxide, in addition to mixtures of these gases (i.e. CH4/H2, H2/CO, and CH4/CO2) were performed at temperatures up to 200 °C and pressures up to 9 bar. ASTM Standard E918 (1983) provided the framework for tests using a one-liter pressure-rated test cylinder in which the fuel-air mixtures were prepared and then ignited. Flammability is determined using a 7% and 5% pressure rise criterion per the ASTM E918 and European EN 1839 standards, respectively. The LFLs for each gas and gas mixture are found to decrease linearly with increasing temperature for the temperature range tested. The LFLs of hydrogen and mixtures containing hydrogen are observed to increase with an increase in the initial pressure, whereas the LFLs of all other mixtures exhibit a negligible dependence on pressure. For mixtures, predicted LFL values obtained using Le Chatelier’s mixing rule (LC) are fairly consistent with the experimentally determined values near ambient conditions, however it is not recommended for use at elevated pressure and/or temperature. Finally, the experimental data presented in this study are compared with previous experimental studies, flammability limits calculated using numerical methods, and past studies of predicted LFL values for similar fuel/air mixtures. The purpose for characterizing the flammability limits for these gaseous mixtures is to extend the results to developing appropriate procedures for the safe industrial use of renewable gases, such as bio-derived methane, biogas composed mainly of methane and carbon dioxide, and renewably derived syngas which contains large quantities of hydrogen and carbon monoxide gas.