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 O-2 with syngas (CO and H-2) 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 26.3 wt.% and the deformation temperature (DT) is 1460 degrees C. It is found that at 1300 degrees C the ash layer does not reduce the O-2 mass transfer until it is thicker than 1.3 mm. The ash layer, however, allows O-2 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 O-2 in the absence of an ash layer to the Boudouard reaction, i.e. between char and CO2. (C) 2015 Elsevier B.V. All rights reserved.