A mathematical modeling of combustion mechanism of Korean high-ash anthracite briquette is developed and analysed to show its applicability to the actual system. The shrinking core model is chosen. With the assumption of 2C+O₂=2CO reaction only at the moving boundary surface and 2CO+O₂=2CO₂ reaction in the ash layer, the whole mechanism can be described by two reaction moduli representing above reactions, respectively. The actual briquette burning characteristics are shown to be predicted without considering the oxidation reaction of carbon monoxide in the ash layer, in other words, with assuming oxygen diffusion control. For carbon monoxide concentration profile, ash layer reaction should be included.