In this paper we study the characteristics of combustion in a 40 MW pulverised wood fired boiler in order to find measures that would enhance its performance in regards to the emissions of carbon monoxide (CO) and nitrogen oxides (NOx), as well as the amount of unburned carbon in the ash. The latter has historically been observed to be high, which led to this study. The main flows in the furnace were studied with a Plexiglas model. The residence time, temperature, oxygen level, mixing rate and the fuel’s particle size distribution were measured and correlated to the responses of the boiler performance. NOx was found to be formed mainly by conversion of fuel nitrogen as is common for biomass combustion. We conclude that CO burnout was limited by insufficient mixing and CO production by entrained particles in the combustion chamber exhaust. The reason for the high amount of unburned carbon in the ash was that the fuel particles were too large. Our results are in agreement with the flow studied in the Plexiglas model. To improve mixing and the staging of the combustion, we recommend changing the design of the secondary air inlets. Our study predicts that both NOx and CO emissions can be lowered by taking this measure.