Combustion of hydrogen in a bubbling fluidized bed

The combustion of hydrogen in a hot, bubbling bed of quartz sand fluidized by air has been studied for the first time, by injecting hydrogen just above the distributor. via six horizontal fine tubes of Cr/Ni. Overall the fluidizing gas was oxygen-rich, with the composition varying from nearly stoichiometric to very lean Mixtures. With the bed initially fluidized at room temperature, combustion (after ignition by a pilot flame) occurs in a premixed flame sitting on top of the bed. When the sand warms up, combustion becomes explosive in bubbles leaving the bed, exactly as with a hydrocarbon as fuel. However, in contrast to hydrocarbons, it is clear that when the bed reaches 500-600 degrees C, heat is produced both above the top of the bed (because of H-2 bypassing the bed) and very low down in the bed. In fact, with hydrogen as fuel, the location of where bubbles ignite descends abruptly to low in the sand; furthermore, the descent occurs at similar to 500 degrees C, which is similar to 100 K below the ignition temperature predicted by well-established kinetic models. However, the kinetic models do reproduce the observations, if it is assumed that the Cr/Ni hypodermic tubes, through which the fuel was injected, exert a catalytic effect, producing free H atoms, which then give rise to HO2 radicals, In this situation, kinetic modeling indicates that bubbles ignite When they become sufficiently large and few enough to have a lifetime (i.e. the interval between their collisions) longer than the ignition delay for the temperature of the sand. The amounts of NO found in the off-gases were at a maximum (24 ppm), when the bed was at similar to 500 degrees C for lambda = [O-2]/[O-2](stoich) = 1.05. The variations of [NO] with [air]/[H-2] and also temperature indicate that NO is produced, at least partly, via the intermediate N2H. In addition, the air-afterglow emission of green light (from NO + O -> NO2 + hv) was observed in the freeboard, indicating the presence there of both NO and free atoms of oxygen for 1.05 < lambda < 1.1. (C) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.