The ignition and combustion of mixtures of a gaseous hydrocarbon fuel in air when fluidising a bed of sand

Abstract

Previous work has been examined here to arrive at a proper criterion for when a mixture of a hydrocarbon gas and air ignites, when it is used to fluidise a bed of e.g. silica sand. It was concluded that, even when the bed has been warmed up to ≈ 900 °C, combustion of such a mixture of gases is usually prevented in the bed's particulate phase, i.e. in the gas moving interstitially between particles of sand. This is caused by free radicals recombining on the surfaces of the fluidised particles. As for the bubble phase, combustion in a bubble of a hydrocarbon gas, pre-mixed with air, is inevitably delayed by the chemistry involving a sequence of straight and branched chain reactions. Also, there can be a toroidal motion of gas inside a bubble; this enhances the transport of heat and radicals from the torus to the sand and so inhibits ignition of a bubble's contents. The result is that for a stoichiometric mixture of propane and air at atmospheric pressure, ignition occurs when a bubble's diameter has grown by coalescence to:db ∼ 3 Umf tig ∼ 2.7 × 10–9Umf exp (158 kJ mol–1 / RT)Here Umf is the superficial velocity of the fluidising gas for incipient fluidisation; tig is the autoignition delay for the particular fluidising gas mixture. This approximate statement is for both slow and fast bubbles; it is very similar to a criterion deduced previously by a different route. The above relationship is based on ignition occurring in a bubble, whenever the residence time in the gas passing up through the bubble along its vertical axis exceeds the ignition delay.