Effects of aluminosilicate-based additives on potassium release and ash melting during biomass combustion

The release of alkali compounds during the combustion of solid biomass causes fine particle emissions and deposition formation on heat exchanger surfaces. These depositions drastically reduce the efficiency and durability of biomass-fired power plants. The main driver of fine particle emissions and deposition formation is the released potassium content of the fuel. Mineral sorbents can shift the potassium release behavior and the ash melting towards significantly higher temperatures. As a result, less potassium enters the gas phase, and fine particle and deposition formation are reduced. This work analyzes the effect of kaolin and coal fly ash on the potassium release behavior and ash stability of miscanthus, torrefied wood, and beech wood. The ash melting behavior is analyzed and the ash composition is determined by X-ray fluorescence analysis. A novel electrothermal evaporation unit connected to an inductively coupled plasma optical emission spectrometer is used to determine the temperature-resolved potassium release from the samples. The experimental data is compared to the most common fuel indices for the prediction of the slagging and fouling tendencies of biofuels. Conflicting fuel suitability predictions are made by the fuel indices studied. Kaolin and coal fly ash shift the potassium release of miscanthus and torrefied wood by up to 300 °C and increase their ash stability. The additives do not improve the ash characteristics of beech wood. A comparison of the different methods shows that potassium release analysis is more accurate than fuel indices and ash melting analysis in predicting ash-related problems and the effect of additives.