Kraft black liquor is a by-product of the Kraft chemical pulping process. It is burned in a special boiler, called a Kraft recovery boiler, to recover energy and chemicals. One proposed concept is the mixing of biomass and black liquor for co-combustion in the recovery boiler. This laboratory scale study was done as a first step in evaluating this concept by comparing the combustion behavior of different levels of biomass addition and the fate of the fuel nitrogen. A unique feature of black liquor combustion is that up to 40% of the fuel nitrogen can be found in the molten salts after combustion, as sodium cyanate. This study found that part of the biomass nitrogen can also be found in the ash from the biomass-black liquor mixtures. Combustion experiments were carried out with black liquor-biomass mixtures made with wood or bark of up to 50% biomass. Single droplets were burned in a laboratory furnace at 900 degrees C and 10% O(2) and at 1100 degrees C and 3.3% O(2). Cyanate formation was studied by pyrolysis and gasification of individual droplets at 800 degrees C in 13% CO(2)/87% N(2). The laboratory tests showed that there is an increase in combustion times, mostly the char burning time and an increase in NO formation with increased biomass addition. Cyanate formation tests indicated that black liquor promotes the conversion of biomass nitrogen to cyanate.