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.