Alkali metal release during coal-biomass co-gasification causes severe ash deposition, slagging and corrosion, threatening gasifier reliability. While extensive work has focused on the final amount of alkali released, the dynamic competition between vaporisation and in-situ capture by ash matrix remains unclear, particularly regarding how the initial K chemical form influences this balance over time. In this work, three synthetic ash samples were prepared: pure coal ash (Coal), coal ash blended with 30 wt% corn stover ash (CS-blend, KCl-dominated), and coal ash blended with 30 wt% torrefied corn stover ash (TCS-blend, acetate-dominated). Based on in-situ morphological observations, 1000 °C and 1100 °C were selected for isothermal treatment under reducing atmosphere for varying holding times, corresponding to partial melting and extensive melting of the blended ashes, respectively. The Fe internal standard method was used to quantify retention ratios, with residual K and Na contents in quenched samples determined by ICP-OES. Coal ash captured gaseous K up to 261 % after 120 min at 1000 °C, confirming strong aluminosilicate trapping. For CS-blend, K retention decreased with increasing holding time and temperature, indicating volatilization dominates; Na retention declined with temperature but showed little variation with holding time. For TCS-blend, both K and Na exhibited fluctuating retentions, implying acetate-derived species introduce additional fixation that competes with release. At 1000 °C after 120 min of holding, Na retentions approached 100 % in both blends, with XRD identifying labradorite/bytownite as Na hosts, whereas no K-bearing crystals were found, suggesting K resides in the amorphous aluminosilicate network. These results reveal that ash matrix is an efficient in-situ trap, and that K speciation governs the release-capture balance. Na and K follow distinct fixation pathways – Na into crystalline feldspars, K into non-crystalline aluminosilicates. Shifting K from Cl-bound to organic-dominated forms via feedstock pretreatment like torrefaction can effectively reduce alkali release, offering a practical strategy for slagging mitigation.

Co-gasification,Alkali release,Potassium speciation,Retention kinetics