Devolatilization times of mm-size coal particles

Abstract

The devolatilization time of mm-size coal particles affects the in-bed combustion efficiency of volatiles in fluidized bed combustors and has been studied by numerous workers. Conflicting effects of the various parameters on devolatilization time have been reported, when different rigs, measurement methods and conditions were used. Therefore, this work aimed to study the effects of experimental parameters using the same rig, procedure and measurement method to eliminate these differences. Four coals, ranging from lignite to bituminous, from the Penn State coal sample bank were studied in a thermogravimetric analysis (TGA) apparatus. Higher heating rates, closer to those in a fluidized bed combustor, were obtained by inserting the coal particles directly into the already hot furnace, rather than using the relatively slow heating rate programmed into the apparatus. The devolatilization time was measured from the weight loss curve for individual coal particles in the 1–5 mm size range. The effect of particle size was correlated by equations of the form, t v = Ad p 1.5, consistent with a heat transfer-controlled regime. No effects of coal type or gas flow rate on the devolatilization time were found. The effects of atmosphere (inert or air) and devolatilization time definition (90 or 95% weight loss) were expressed as ratios of the values of the correlation parameter, A, and the effect of temperature was presented as an Arrhenius-type plot for A. The decreases in devolatilization time with increasing temperature or oxygen concentration again reflect the controlling nature of the heat transfer process. No fragmentation of the coal was observed in this work, but the effect of primary fragmentation was deduced by comparison with literature data. It is suggested that an effect of coal type on devolatilization time is only observed when different degrees of primary fragmentation occur for different coals, and this may explain the differing observations for coal type effect in the literature.