On fluidising a heated bed of sand with a mixture of methane and air: When do the bubbles ignite and when does combustion occur between the sand particles?

摘要

? 2022 The Combustion InstituteCombustion in a heated bed of sand, fluidised by mixtures of CH4 and air, is inhibited. It was recently suggested that this is partly because a bubble only ignites, when the longest residence time of the fluidising gas passing through this bubble exceeds the ignition delay, tig, of the gas. This idea led to: (db/3Umf) ≥ tig as the condition for a bubble of diameter, db, to ignite. To test this, beds of sand (3 different sizes) were fluidised by different mixtures of CH4 and air. The bubbling bed was electrically heated slowly up to ～ 1100 °C, whilst monitoring the bed's temperature and the composition of the gas at a fixed point in the bed. The temperature, at which bubbles ignited at this observation point, was identified as that of the bed, when the local concentration of CO reached a maximum. The mean size of the igniting bubbles was calculated from the flowrate of gas through the bed and the height above the distributor. Using tig (from the literature) at the measured ignition temperature, the above equation described the observations very satisfactorily. Thus, the fuel-richness of the fluidising gas and its temperature affect tig, Umf and db. Combustion of CO (or CH4) in the gas passing interstitially between fluidised sand particles was found to be unlikely, but can be promoted by fluidising large particles in a very hot bed, in which CO has a concentration larger than ≈ 10 vol.% at 1 at.