The intrinsic combustion reactivity of pulverised coal chars: The use of experimental pore diffusion coefficients

Abstract

This paper reports the intrinsic reactivity to oxygen (the rate of reaction per unit surface of pore area, ρi) of fine particles of three coal chars. The chars were prepared from a swelling low-volatile bituminous coal, a non-swelling low-volatile bituminous coal and a brown coal. Rate measurements were made under two conditions: in a low-temperature fixed-bed reactor where chemical processes alone controlled burning rates; and in a high-temperature entrained flow reactor, where pore diffusion had a strong effect on rates. The intrinsic reactivities at high temperature were determined using measured values of the pore diffusion coefficient (De) and other measured properties without the need to use any structural model of the chars' pore systems. The diffusion coefficients were measured using a non-isobaric chromatographic technique, the resulting coefficients being much smaller and having a higher temperature dependence than the corresponding Knudsen diffusion coefficients calculated using Wheeler's ‘average pore size’ structural model. It is concluded that these independent measurements of pore diffusivity, at moderately high temperatures give values which are more relevant to the kinetic processes involved in coal combustion than those determined using simplistic structural models. For the samples considered here, intrinsic reactivities, determined from measured diffusion coefficients and reaction rates, were in close agreement with those predicted on the basis of earlier models.