Influence of Ammonium Dihydrogen Phosphate Addition on the Behavior of Potassium During Biomass Combustion

The effect of ammonium dihydrogen phosphate (NH4H2PO4) on the migration and transformation of potassium (K) during rice straw combustion was carried out in a laboratory-scale tube furnace at temperatures of 750 °C, 850 °C, and 950 °C. The characteristics of ashes were analyzed by chemical fractionation, scanning electron microscopy, X-ray fluorescence and X-ray diffraction. Results indicated that with the temperature increasing, the higher PO43−/K molar ratio, the stronger the ability of K-retention. When the molar ratio of PO43−/K was 1:1, the change rate of K retention increased by 4.95%, 23.12% and 47.99% compared to 1:2 molar ratio of PO43−/K at 750 °C, 850 °C and 950 °C, respectively. Water-soluble potassium of RS reacted with PO43− actively and was converted to insoluble potassium, which could avoid potassium migrating into gaseous phase. The addition of NH4H2PO4 can combine with K and Ca to generate the ternary complex salts of Ca–P–K. The compounds are mainly in the form of K2CaP2O7 that has a high melting point, which can improve the ash melting characteristics effectively.