Parametric studies on devolatilization of a subbituminous coal in a reactive gas environment

Abstract

A laboratory scale fixed bed coal gasifier was set up to simulate the conditions existing in the devolatilization zone of an air-blown, fixed-bed coal gasifier. Devolatilization behaviour of a subbituminous coal was evaluated in the temperature range 350 °C to 550 °C and at pressures 30, 300 and 375 psig. Three feed coal particle sizes, (−2, +1), (−4, +3) and (−9, +6)mm, were studied. The gas feed was a synthetic mixture of composition similar to that leaving the gasification zone of a fixed bed gasifier and contained 30% by volume of steam. Devolatilization runs were conducted over coal residence times of 5, 10, 20 and 30 min durations. The gas evolution rates showed a peak around 5 min from the start of a run and most of the gas evolution tapered off just under 30 min. Thirty key components in the tars were quantified and these included aliphatic and aromatic homologues, as well as sulphur and nitrogen substituted structures. The molecular weights of the tar samples showed a maximum between 300 and 500. A first order kinetic model applied to the total weight loss data yielded activation energies in the range 4 to 11 kcal mol −1. Differential equations for obtaining concentration profiles for tar and gas inside the coal particle were solved numerically. From these calculations it was concluded that the pressure buildup (due to evolution of tar and gas) inside the coal particle was higher for larger particles, at a given external pressure, but decreased with external pressure. The concentration of tar inside the particle did not appear to be sensitive to low pressures (around 1 atm), but increased in the higher range of pressure (above 20 atm) and also with particle size.