Modeling alkali condensation and mitigation in biomass combustion: Effects of water leaching and sulfur addition

Biomass combustion provides renewable heat and power, but agricultural residues often contain high concentrations of potassium and chlorine that promote the formation of condensable KCl, KOH, and K 2 SO 4 . This study applies detailed chemical-kinetic modeling in a plug flow reactor, coupled with saturation-based condensation, to quantify the combined effects of water leaching and sulfur addition on alkali speciation. Seven biomass fuel samples, comprising two reference wood-pellet samples and five alternative biomasses, were evaluated using alkali-release scenarios under boiler-relevant conditions. Alkali release strongly controlled downstream speciation, increasing condensed KOH and, in high-chlorine fuels, KCl. Water leaching reduced condensed KCl by more than an order of magnitude in fuels rich in soluble potassium and chlorine. Sulfur addition redirected potassium toward K 2 SO 4 and reduced condensed KCl and KOH by 40%–100%, depending on fuel and dose. Combined leaching and sulfur addition provided the strongest mitigation for wheat straw and empty fruit bunch and reduced their stoichiometric sulfur requirements by 85%–96%. Sensitivity analyses showed that the saturation-ratio criterion and a uniform ± 50 K shift in the temperature profile affected condensation onset and axial location, while fuel rankings and the principal water-leaching effects remained stable. A ± 20 % residence-time variation had negligible effects on final condensed amounts. Treating sodium as potassium affected absolute predictions for high-sodium fuels, highlighting the need to interpret these results as potassium-equivalent alkali trends. Overall, the framework supports comparative fuel screening and evaluation of alkali-mitigation strategies, while predicted condensed concentrations should not be interpreted as direct estimates of surface deposition or corrosion.