AUTO-IGNITION OF CH4 AIR, C3H8 AIR, CH4 C3H8 AIR AND CH4 CO2 AIR USING AN IGNITION BOMB

The distinction between auto-ignition and hot-surface ignition of a given gas is emphasized. In ideal auto-ignition there is no diffusion of heat or matter. Published information on auto-ignition temperatures (AIT) of multi-component fuels in air is scarce. This also applies to North Sea natural gas, of which CH4, higher alkanes and CO2 are essential components. In the present experimental laboratory-scale study, AIT of four types of hydrocarbon mixtures (CH4/air, C3H8/air, CH4/C3H8/air and CH4/air/CO2) have been measured using a 11 ignition bomb. The experimental method ensured that the gas mixtures studied were of known composition and homogeneous in concentration. The gas mixture was admitted to the pre-evacuated ignition bomb in the form of a turbulent jet when the bomb wall had reached the desired temperature. Ignition was recognized as a sudden pressure rise in the bomb a few seconds after the gas flow into the bomb had stopped. The minimum AITs for CH4/air and C3H8/air were found to be 640-degrees-C and 500-degrees-C, respectively. The AIT of CH4/C3H8/air decreased with increasing propane content and total fuel concentration. A fuel concentration region was discovered for which CH4/C3H8/air and C3H8/air with the same ratio of propane to oxygen gave the same AIT. Reducing the oxygen content of a CH4/air mixture by adding CO2 gave, under the present experimental conditions, a systematic increase of AIT with increasing CO2 content. The role of the CO2 was probably essentially that of an inert diluent. It has been known for a long time that the 'minimum hot-surface ignition temperature' is not a constant for a given gas mixture, but highly dependent, by several hundred degrees centigrade, on the dynamic state of the gas, the geometry and material of the ignition surface, and the mode of heat supply to the surface. The direct application of AIT values to assess industrial hot-surface ignition risks may therefore be unduly conservative. Consequently there is a need for general mathematical models that can predict minimum ignition temperatures for various practical situations in industry. Such models will have to contain sub-models of ignition chemistry, fluid mechanics and heat and mass transfer.