Unveiling the pivotal role of Ni doping in ilmenite as oxygen carrier to realize simultaneous enhanced oxygen release and inhibited phase segregation in chemical looping process

Abstract

Biomass chemical looping gasification (BCLG) has demonstrated great potential in tackling global climate challenges through green energy transition. However, CH₄ and tar generation are still significant obstacles for the commercialization of BCLG. In this study, we have developed a cost-effective Ni-modified ilmenite oxygen carrier (OC) for BCLG to greatly reduce the CH₄ content and simultaneously increase the syngas generation. Several industrial wastes were investigated and screened based on their syngas and CH₄ reactivity. Results show that ilmenite exhibits excellent syngas selectivity and potential reactivity with CH₄. However, the reaction of ilmenite with CH₄ proceeds slowly owing to the phase transformation process of TiFe₂O₅ - TiFeO₃ - Fe being the rate-limiting step. Thus, various metallic dopants (i.e., Ni, Co, and Ca) were applied as promoters to reinforce its CH₄ reactivity. Interestingly, Ni exhibited a higher promoting effect than Ca, whereas Co had little promotion on ilmenite reactivity. The superior performance of Ni doping could be attributed to the incorporation of Ni²⁺ element in Fe-O-Ti structure rather than Ni⁰, which was validated by pre-activation and cyclic experiments, and density functional theory calculations. Modulated electronic structure by Ni²⁺ in Fe-O-Ti lattice was responsible for significantly promoted oxygen release capacity and enhanced Fe/Ti interactions, thereby activating the reactivity of ilmenite with CH₄ and suppressing Ti/Fe phase segregation. Therefore, this as-prepared 5Ni-ilmenite could be a promising cost-effective OC in BCLG for high quality syngas production.