Biomass ash chemistry in chemical looping combustion: Interaction mechanism of potassium-ash and iron-based oxygen carriers

Chemical looping combustion (CLC) of biomass presents a promising technology for CO2 negative emission. Considering the release of K-containing species in the fuel reactor, the interaction between K and oxygen carrier (OC) is one of the crucial concerns during biomass CLC process. Inorganic-K (K2CO3, K2SO4, and KCl) and organic-K (CH3COOK) are commonly found in biomass. Cellulose, a model compound for biomass, is used to simulate the reductive atmosphere. By combining characterization (ICP-OES, XRD, SEM/EDS) and thermodynamic calculation, the effects of different K-sources, reaction atmosphere (N2 and reductive atmosphere), and support material (Al2O3 and SiO2) on the performance of Fe-based OCs were investigated, elucidating the underlying mechanisms. The results indicated that both CH3COOK and K2CO3 directly reacted with Fe-based OCs in N2 atmosphere, while K2SO4 and KCl did not exhibit such direct reactivity with OCs. The presence of melting K2CO3 and K–Si–O components caused agglomeration of particles. In the long-term redox experiments, all K-sources significantly enhanced the reactivity of OCs, but the enhancement diminished as the number of cycles increased. When Al2O3 was used as support material, the Fe-based OCs displayed favorable resistance to K-ash erosion. With SiO2 as support material, the addition of K led to severe agglomeration, particularly in case of CH3COOK and K2CO3, resulting in hindered gas-solid reactions and decreased reactivity of OCs. Numerous irregular cavities were formed inside the agglomeration, due to the expansion and consolidation of pores influenced by the inward diffusion of K-containing gaseous, and the coalescence of particles by the outer viscous layer.