Coal particle chemical transformational behaviour after thermochemical conversion in a fixed bed

Abstract

This paper examines the chemical transformational behaviour that coal particles undergo during conversion in deep packed beds, typically found in industrial combustion and gasification operations. Experiments were performed in a transient overfeed packed bed laboratory scale reactor fed with 4, 6 and 8 mm coal particles. The reactor was operated at atmospheric pressure and is 1.2 m in length and has an inner diameter of 104 mm. The coal loading was 3.3 kg and resulted in a bed height of 520 mm. A typical inertinite-rich South African seam four coal from the Highveld region was used as fuel in the reactor. The transient temperature profile in the descending bed as well as the exit gas composition is presented here. A post experiment dissection of the bed contents along with full chemical characterisation was undertaken. Care was taken to ensure that the dissection method, specifically designed for minimal disturbance to the fixed bed structure, could provide an accurate representation of the characteristic reaction zones. The residual volatile matter and the overlap in the reduction and pyrolysis zones were insensitive to particle size variation and mainly determined by the maximum temperature in each zone. Where it was found useful, these reaction zone profiles are compared to industrial and pilot operations. The reaction front velocity and heating rates that particles experience in the different reaction zones was obtained and showed significant variation during the transient start-up stage, but are remarkably comparable once the stable reaction front is formed. This current mode of operation represents fixed bed combustion and gasification operations particularly during the transient start-up stages