Potassium precipitation and transformation during the combustion of torrefied wheat straw-effect of additives

Recent advances in torrefaction pretreatment technologies have the potential to improve biomass storage, transportation, and combustion. Torrefaction pretreatment, on the other hand, does not address the issue of high alkali metal content in biomass, which can easily lead to ash deposition, slagging, and corrosion on the boiler heating surface. NH4H2PO4, Ca-3(PO4)(2), diatomite (primarily SiO2), and calcined kaolin (Al2O3 center dot 2SiO(2)) were added to torrefied wheat straw to control the release and transformation of potassium in the process of biomass combustion, and the effects of additives on K precipitation and migration were explored. The results demonstrated that all four additives might limit the release of K from torrefied wheat straw during combustion. Phosphoric acid additives had a better effect than silicon-aluminum additives, with the best result being Ca-3(PO4)(2), followed by calcined kaolin. The amount of phosphoric acid additives should be controlled at 10wt%, and the amount of silicon aluminum additives should be maintained at 5wt%, based on the economy and retention impact of K. XRD and chemical fractionation analysis revealed that phosphoric acid additives primarily control the K transformation by forming K-Ca-P salt and ammonium acetate state K, silica-aluminum additives primarily control it by forming K silicates and aluminosilicates, and diatomite also improved the retention of water-soluble K through physical adsorption.