Interaction of ilmenite oxygen carrier with wheat straw ash during chemical looping combustion: Mechanisms and performance variation

Abstract

Chemical looping combustion (CLC) offers a promising approach for biomass utilization. The utilization of biomass as the primary fuel in CLC (bio-CLC) is a way to generate negative CO2 emissions and thus interesting for climate change mitigation. However, the presence of ash in biomass raises concerns about the performance variation of the oxygen carrier during CLC. This study investigated the impact of adding wheat straw ash (WSA) to the ilmenite oxygen carrier throughout the CLC process. The effects of WSA addition ratio (0–15 %) and cycle number on the performance of oxygen carrier were evaluated. The results showed a progressive enhancement followed by a subsequent reduction in the performance of ilmenite with the increase of WSA addition ratio, and the peak reactivity of ilmenite was attained with 10 % WSA addition. Addition of 15 % WSA resulted in severe particle sintering, causing a slight diminishment in ilmenite reactivity. Furthermore, the mechanisms of the performance variation of ilmenite with WSA addition were explored. The superior redox reactivity and oxygen release capability of the ilmenite with 10 % WSA addition were evidenced by X-ray photoelectron spectroscopy and hydrogen temperature-programmed reduction analysis, respectively. Notably, X-ray diffraction and thermodynamic calculation confirmed the formation of KFeO2, which significantly enhanced the reactivity of ilmenite. The specific surface area of the ilmenite was greatly enlarged with 10 % WSA addition, and a porous structure was evidenced by scanning electron microscope. The findings suggest the interaction between the WAS and ilmenite enhances the reactivity of the ilmenite with an optimal WAS addition ratio of 10 %, providing a guidance for the long-term operation of CLC.