The role of ash layer in syngas combustion in underground coal gasification

Abstract

Underground coal gasification (UCG) proceeds generally in the presence of an ash layer on the coal (or char) surface. The ash layer would not only increase the mass transfer resistance to gases as reported in the literature but also lead to reactions of O2 with syngas (CO and H2) generated from the ash–char interface. This paper studies the effects of ash layer in a specially designed one-dimension gasification assembly using a bituminous coal char by analyzing the ash and the gases components. The ash content of the char is 26.3wt.% and the deformation temperature (DT) is 1460°C. It is found that at 1300°C the ash layer does not reduce the O2 mass transfer until it is thicker than 1.3mm. The ash layer, however, allows O2 to react with CO generated from the ash–char interface to form CO2, which raises the local temperature in the ash layer leading to local ash melting and transfers the char reaction from partial oxidation by O2 in the absence of an ash layer to the Boudouard reaction, i.e. between char and CO2.