Ash chemistry in chemical looping process for biomass valorization: A review

Chemical looping process (CLP) is a novel carbon capture technology for biomass valorization. Low-cost and robust oxygen carrier (OC) is crucial for industrialization of CLP. However, ash in solid fuels will unavoidably deposit on OC and even react with OC, leading to severe agglomeration and sintering which could not only degrade the full functionality of OC but also make the complete separation of ash and OC challenging. In order to holistically elucidate the ash chemistry in CLP, this review has systematically analyzed the effect of ash compositions in biomass on CLP efficiency, the impact of ash components on physicochemical properties and oxygen transfer capacity of OC, as well as the melting and agglomeration behaviors of ash components. Specifically, both inhibition and enhancement effects of various ash components have been illustrated. Particularly, the influence of alkali and alkaline earth metals in biomass ash on agglomeration of OC has been analyzed in detail. Four mechanisms are summarized to explain the agglomeration and melting process, including coating-induced, melting-induced, ash deposition-melting, the layer joint and bridge joint mechanisms. Ultimately, strategies are proposed to effectively mitigate adverse impacts of ash and recycle useful metals for industry use and re-synthesis of OC. To promote future development of CLP, perspectives are provided to guide the novel design of next generation OC in terms of structural and compositional optimization.