The Kinetics of Oxidation of Chars From Three Different Coals, as Measured in Fluidized Beds

There is substantial disagreement over the order of the reaction between a coal char and O₂. This is chiefly because of the difficulties of making good measurements when burning is kinetically controlled; thus heat and mass transfer can often obscure the true kinetics of the reaction. Consequently, a fluidized bed is an attractive reactor in which to measure the rate of this reaction. Chars from three different coals (of high, medium and low ranks) were prepared. The rate of oxidation of a char was measured by adding a small mass (∼ 3 mg) of tiny particles (sieved to 106–150 μm) of the char to a fluidized bed of silica sand (90–126 μm), which was electrically heated to a fixed temperature between 700 and 950°C. The fluidizing gas was a mixture of O₂ and N₂ at 1 bar; its composition varied between 0 and 100% O₂. The concentrations of CO and CO₂ were continuously measured in the off-gases from the bed, using i.r. analysers, thus enabling the rate of oxidation of the added batch of char to be measured. Interestingly, it was found necessary to modify the quartz cylinder housing the fluidized bed to prevent backmixing of ambient air into the freeboard. The char particles were sufficiently small that mass transfer to their exteriors did not control the rate of reaction. Furthermore, the rapid rates of heat transfer from the bed to the particles of char ensured that the char burned at the temperature of the bed. Likewise, the mass of added char was small enough for the rate of burning not to be controlled by inter-phase mass transfer. The maximum rate of oxidation was taken to be the initial rate, which could, at least in principle, be correlated with the initial properties of a char, such as its BET area, pore diameters, etc. The char of the low-rank coal burned with kinetics which were zeroth order in O₂ and with a negligible activation energy. The other two coals had orders of reaction which were 0.7–0.8 at 700°C, but 0.2–0.25 at 950°C. These orders are discussed in detail.