This study was performed to investigate the combustion characteristics of coal and rice husk and their co-combustion in a 100 kW circulating fluidized bed combustor (diam : 150 mm, height: 6000 mm). Gaining insights into the effects that the primary and secondary excess air ratios (lambda(PA) and lambda(SA)), biomass fraction, and secondary air (SA) position have on CO and NOx emissions was also an aim of this study. In contrast to coal combustion, and co-combustion of coal and rice husk, the combustion of rice husk could not be sustained without the aid of an external heat source, and the released volatiles were found to be fired chiefly above the bottom bed. Air-staging substantially decreased NOx (40-75% reduction), but with a high CO penalty, compared to no air-staging. However, the effect of the SA position which was profoundly engaged with the char inventory in the bed was pronounced for coal combustion but not for rice husk combustion. Increase in lambda(PA) and lambda(SA) tended to reduce CO levels but enhanced NOx formation. The devolatilization and combustion of small eluding rice husk particles were found in the upper zone, despite being fed in pellet form. For co-combustion, the use of more rice husk resulted in higher CO emissions. Although rice husk contains half the nitrogen content of coal, co-firing it with coal gave higher NOx emissions when a higher fraction of rice husk was included in the mixed fuel.