A literature survey and an experimental study of coal devolatilization at mild and severe conditions: influences of heating rate, temperature, and reactor type on products yield and composition

Coal devolatilization studies to maximize the yield of condensable products by operating at elevated temperatures and heating rates have been published. The objectives of this study were to investigate the influences of relatively mild operating conditions (e.g. relatively low temperature and pressure) on product quality, by comparing devolatilized products obtained at various temperatures and heating rates. Fixed bed, fluid bed, and entrained flow reactor units were used to obtain pyrolysis products. In addition, literature data on tar yields in various reactor units at a range of temperatures and residence times were surveyed and compared with experimental data. The liquids were characterized by a number of techniques, including field ionization mass spectroscopy (f.i.m.s.), sequential elution solvent chromatography (s.e.s.c.) and elemental analysis. The results demonstrate that the quality and yield of liquids obtained at a rapid heating rate are functions of peak pyrolysis temperature. It was shown that at a rapid heating rate, the yields of heavier polyfunctional groups (i.e. hydrocarbons with greater mean molecular weight) are greater than those obtained in the fixed bed slow heating rate reactor. The liquids generated at a slow heating rate are of lower molecular weight, viscosity, and sulphur content, and of higher H/C atomic ratios compared with the liquids obtained in a rapid heating rate unit. The effect of increasing the maximum pyrolysis temperature (at a constant slow heating rate) was to increase the yield of light gases (mainly H) at the expense of char hydrogen content and char reactivity. The tar yield is not markedly influenced when the peak devolatilization temperature is increased at a relatively slow heating rate. However, the quality (as defined by the H/C (atomic) ratio) of the liquids, and the reactivity (in air) of char, was reduced when the peak pyrolysis temperature was increased. At a rapid heating rate, the primary products, which have many structural characteristics of the parent coal, are devolatilized. The quality of the liquids obtained at a rapid heating rate is, therefore, determined by the devolatilized primary coal fragments evolved at the devolatilization temperature. In a slow heating rate fixed bed unit, however, the primary coal fragments undergo additional cracking reactions which involve stabilization of free radicals by donatable hydrogen. This leads to the formation of low molecular weight hydrocarbons of relatively higher quality. In-situ (both intraparticle or extraparticle) stabilization of reactive coal fragments by donatable hydrogen may lead to a significant improvement in the overall quality of the pyrolysis liquids in a fixed bed system in which time-temperature history is conducive for such reactions.