A shrinking particle model with apparent reaction rate parameters is frequently applied in the modeling of burning of coal particles in boilers. In oxy-fuel combustion, the nitrogen in the gas atmosphere is replaced by carbon dioxide and water vapor, enhancing the burning rate by gasification. A shrinking particle model, including the gasification taking place inside the particles, is presented. Boundary layer combustion of CO reduces the burning rate of char because of consumption of oxygen, allowing for less to reach the particle surface. This is accounted by an effective Sherwood number. Analytical and numerical solutions for combustion of char and char inventory in the bed are extended to include the effect of internal gasification and boundary layer combustion of CO. Measured char inventories in air and oxy-fuel conditions in pilot-scale circulating fluidized-bed combustion are compared to calculated char inventories. The modeling results are also applicable for pulverized fuel combustion.