Effects of the water washing pretreatment on the ash formation during biomass combustion

The widespread combustion of agricultural residues is limited by ash-related slagging and fouling, mainly caused by alkali and alkaline earth metals (AAEMs). This study systematically investigated water washing pretreatment on ash formation and combustion behavior of corn straw (CS), soybean straw (SS), and rice husk (RH). Washing parameters, including solid-to-liquid ratio (S/L), duration and mode (stirring, soaking, rinsing) were optimized. A multi-technique characterization approach, encompassing proximate/ultimate analysis, higher heating value (HHV) measurement, XRF, XRD, TGA, and SEM, was employed to elucidate the underlying mechanisms. The results demonstrated that optimal washing conditions (stirring, 12 h, S/L = 1:30) substantially reduced the ash content of CS from 14.63% to 6.94% and increased its HHV from 15.81 to 16.66 MJ·kg−1. Significant removal of problematic inorganics was achieved, with K2O in CS ash decreasing from 14.52% to 3.29% and chlorine being almost eliminated. Consequently, the base-to-acid ratio and slagging index for CS ash dropped from 1.09 to 0.47 and 4.91 to 0.77, respectively, indicating a markedly reduced slagging and fouling propensity. Washing increased ignition and burnout temperatures, confirming enhanced thermal stability due to catalytic AAEM removal. These findings provide fundamental insights and practical guidelines for optimized water washing to enable cleaner, more efficient combustion of diverse agricultural residues.