Flammability Limits Of Gaseous Fuel Air Mixtures Under Flow Conditions

Experimental studies of lean flammability limits (LFLs) for methane, hydrogen, carbon monoxide, in addition to mixtures of these gases (i.e. CH 4/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 at these elevated conditions, 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 in 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 are fairly consistent with the experimentally determined values near ambient conditions, however it is not recommended for use at elevated pressure and/or temperature. 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.

Flammability limit is a most significant property of substances to ensure safety of chemical processes and fuel application. Although there are numerous flammability literature data available for pure substances, for fuel mixtures these are not always available. Especially, for fuel mixture storage, operation, and transportation, inert gas inerting and blanketing have been widely applied in chemical process industries while the related date are even more scarce. Lower and upper flammability limits of hydrocarbon mixtures in air with and without additional nitrogen were measured in this research. Typically, the fuel mixture lower flammability limit almost keeps constant at different contents of added nitrogen. The fuel mixture upper flammability limit approximately linearly varies with the added nitrogen except mixtures containing ethylene. The minimum added nitrogen concentration at which lower flammability limit and upper flammability limit merge together is the minimum inerting concentration for nitrogen, roughly falling into the range of 45 plus/minus 10 vol % for all the tested hydrocarbon mixtures. Numerical analysis of inert gas dilution effect on lower flammability limit and upper flammability limit was conducted by introducing the parameter of inert gas dilution coefficient. Fuel mixture flammability limit can be quantitatively characterized using inert gas dilution coefficient plus the original Le Chatelier's law or modified Le Chatelier's law. An extended application of calculated adiabatic flame temperature modeling was proposed to predict fuel mixture flammability limits at different inert gas loading. The modeling lower flammability limit results can represent experimental data well except the flammability nose zone close to minimum inerting concentration. Le Chatelier's law is a well-recognized mixing rule for fuel mixture flammability limit estimation. Its application, unfortunately, is limited to lower flammability limit for accurate purpose. Here, firstly a detailed derivation was conducted on lower flammability limit to shed a light on the inherent principle residing in this rule, and then its application was evaluated at non-ambient conditions, as well as fuel mixture diluted with inert gases and varied oxygen concentrations. Results showed that this law can be extended to all these conditions.

Combustion, Flames, and Explosions of Gases, Second Edition focuses on the processes, methodologies, and reactions involved in combustion phenomena. The publication first offers information on theoretical foundations, reaction between hydrogen and oxygen, and reaction between carbon monoxide and oxygen. Discussions focus on the fundamentals of reaction kinetics, elementary and complex reactions in gases, thermal reaction, and combined hydrogen-carbon monoxide-oxygen reaction. The text then elaborates on the reaction between hydrocarbons and oxygen and combustion waves in laminar flow. The manuscript tackles combustion waves in turbulent flow and air entrainment and burning of jets of fuel gases. Topics include effect of turbulence spectrum and turbulent wrinkling on combustion wave propagation; ignition of high-velocity streams by hot solid bodies; burners with primary air entrainment; and description of jet flames. The book then takes a look at detonation waves in gases; emission spectra, ionization, and electric-field effects in flames; and methods of flame photography and pressure recording. The publication is a valuable reference for readers interested in combustion phenomena.