Kaolin additivation in pulverized wood firing systems – potassium capture, emissions and ash behavior

Pulverized fuel (PF) combustion systems based on solid biofuels are a key in sustainable energy transition, especially when retrofitting coal-fired plants. Nonetheless, challenges persist due to alkali metal-induced emissions and ash-related operational issues. This paper investigates the effect of kaolin additivation during pulverized biomass combustion. Industrial wood pellets were milled and mixed with kaolin at concentrations of 0.5, 1.0, and 1.5 wt% and combusted in a laboratory-scale PF furnace. Prior laboratory analyses showed that 1.5 wt% kaolin additivation increased potassium retention in ashes from 34 % to 96 % at 1100 °C and raised ash fusion temperatures (e.g., flow temperature from 1277 °C to 1432 °C), indicating the formation of high temperature-stable potassium(K)-aluminum(Al)-silicate compounds. Combustion tests revealed alterations in gaseous emissions (nitrogen oxides (NO_x) and carbon dioxide (CO₂)) as well as changes in the composition of the resulting ashes. Chemical and X-ray diffraction analyses suggest a shift of potassium compounds within the ash fractions and a formation of stable K-Al-silicate compounds within the bottom ash. However, high ignition losses were reported for the bottom ashes, attributable to incomplete combustion linked to short residence times. This was also reflected in high CO emissions, which did not show a significant improvement with additivation. Although the results from the PF combustion experiments do not allow a generalization of emissions trends due to incomplete burnout, the observed trends, along with laboratory results, confirm and highlight the potential of kaolin as an additive to mitigate K-related detrimental effects in PF biomass combustion systems.