Migration and Transformation of Alkali and Alkaline Earth Metals during Co-gasification of Biomass and Coal

This study investigated the co-gasification process of subbituminous coal with two typical biomass feedstocks (elm and cotton stalk) in a fixed-bed reactor. Stepwise leaching experiments were conducted on gasification residual char under different gasifying agents (air, CO2) and residence times to extract alkali/alkaline earth metals (AAEMs) in various occurrence forms. The research revealed that AAEMs volatilization occurred in two distinct stages: (1) Pyrolysis and devolatilization stage: This stage accounted for 40.74~93.10% of total AAEMs volatilization, where massive AAEMs were entrained into gas phase by instantaneous gas flow. Within 0-5 min under air atmosphere: 30% K and 32% Na volatilized in coal/cotton stalk blends; 49% K volatilized in coal/elm blends. Elevated temperature significantly enhanced volatilization, while prolonged residence time increased K volatilization to 44% and Na to 63%. Ca in coal/cotton stalk showed higher volatility than in coal/elm, with CO2 atmosphere notably altering Ca dispersion patterns. (2) Gas-solid reaction and ash formation stage: The reaction rate in this stage was relatively slow, during which the occurrence forms of alkali (earth) metal elements such as K, Ca, Na, Mg in semi-coke underwent significant changes when contacting with air (or CO2). Under air atmosphere during 5-120 min residence time: water-soluble K, insoluble K, and ammonium acetate-soluble K in cotton stalk transformed into HCl-soluble and volatile phases; In coal/elm blends, the HCl-soluble Ca content decreased from 35% to 13% under air atmosphere, while the volatile phase increased from 11% to 27%. Water-soluble, ammonium acetate-soluble, and insoluble Na all transformed into volatile phases; Mg mainly existed in ammonium acetate-soluble form (55%) with little change during residence time. Prolonged residence time caused K and Ca to combine with S, Al, Si under air atmosphere forming CaSO4 and KAlSiO4; CO2 atmosphere increased components such as CaCO3, CaS, MgS.