Comparative Assessment of Kaolin Addition and Acid Washing for Fouling Mitigation in Alkali-Rich Kenaf Biomass

Herbaceous biomass is a promising renewable energy resource, but its use in thermochemical systems is often limited by severe fouling and ash agglomeration resulting from alkali-rich ash chemistry. This study directly compares two practical fouling mitigation strategies, kaolin addition and acid washing, for alkali-rich torrefied kenaf biomass under identical experimental conditions. The study quantitatively distinguishes aluminosilicate-based alkali stabilization from pretreatment-based alkali removal as two distinct pathways for controlling ash transformation. Kenaf exhibited severe ash agglomeration and contained high levels of K2O (17.38 wt.%), CaO (31.52 wt.%), MgO (14.98 wt.%), SO3 (9.43 wt.%), and P2O5 (6.90 wt.%). Kaolin addition progressively shifted the ash composition toward a SiO2–Al2O3-rich system. From KA-10 to KA-30, SiO2 increased from 22.86 to 33.58 wt.%, while Al2O3 increased from 7.65 to 15.43 wt.%. X-ray diffraction (XRD) analysis further showed that increasing kaolin addition suppressed alkali-salt phases and promoted the formation of aluminum-silicate phases. In contrast, acid washing directly reduced alkali species, decreasing K2O to 5.66–7.83 wt.% and eliminating detectable Na2O. The acid-washed samples were characterized by calcium-rich sulfate and silicate phases, indicating a distinct ash transformation pathway. Kaolin addition primarily reduced fouling by promoting aluminosilicate-based alkali stabilization, whereas acid washing reduced alkali–metal contents before thermal treatment. This distinction clarifies the different roles of additive-based and pretreatment-based strategies for fouling control in alkali-rich herbaceous biomass.