Effect of Steam on the Release of K and Cl during Biomass and Black Liquor Combustion

The supercritical steam temperatures achieved in biomass and kraft recovery boilers are limited by alkali and chlorides in the fuel because these elements contribute to fouling and corrosion in the convective passes. There is a lack of experimental data on the effect of steam in the combustion gases on the release of alkali and chloride from biomass and black liquor. In this work, the release of K and Cl from three biomass fuels in different gas environments is measured. The gas environments studied were balanced with N-2 and contained 11% CO2, 1% O-2, and either 8 or 17% steam. The mass loss and particle temperature throughout combustion were measured. Measurements show that the presence of steam slows the release of K and Cl during char oxidation in wood. Temperature measurements during char conversion experiments show that the particle temperature drops in the presence of steam as a result of endothermic gasification reactions on the char surface by steam and CO2. The decrease in the char temperature results in a lower KCl vapor pressure, lowering the KCl vaporization rate; however, experiments show that this lower temperature alone does not explain the decrease in KCl release. For black liquor, steam and CO2 did not affect the mass loss or the release of K and Cl, but this appears to be due to the small black liquor droplets fully burning before reaching a high enough temperature for significant K and Cl release.