Raw coal is burned in furnaces or chain grate boilers. In this work, we studied the chamber combustion mechanism in a chain grate boiler (CGB) with a capacity of 20 t/hr. A mathematical model was built using Computational Fluid Dynamics (CFD). It was found that chamber combustion with swirl burners instead of grate firing has a positive effect on NO emissions. NO emissions dropped from 700 ppm in case 5 to 250 ppm in case 1. The calculated and measured NO emissions agreed well, with a discrepancy of less than 15%, except on plane 2 for four calculated cases. For the temperature field, the discrepancy between the measured and simulated values was below 12% for the four cases. These discrepancies between calculated and measured results are acceptable. This study provides a good idea for the design and reconstruction of CGBs. To reduce NO emissions, the reconstruction method can be used for CGBs instead of the grate firing system.