Hydrogen-rich syngas production from chemical looping gasification of lignite by using NiFe2O4 and CuFe2O4 as oxygen carriers

•Chemical looping gasification of lignite was promoted by NiFe2O4 and CuFe2O4.•Pure cubic spinel structure is beneficial for the mobility of lattice oxygen.•NiFe2O4 exhibits the highest gas yield of 24.47 mol/kg during lignite gasification.•1.65 times increase of total syngas yield was obtained by addition of steam. Chemical looping gasification provides a novel technology to enable clean conversion of coal with inherent CO2 capture. In this study, four types of NiFe2O4 and CuFe2O4 synthesized by sol–gel method and solid-phase method were used as oxygen carriers to promote the H2 production from gasification of lignite. The effects of the types of oxygen carrier and oxygen carrier to coal (O/C) ratio on chemical looping gasification of lignite were first investigated by a thermogravimetric analyzer (TGA) combing with various analytical methods. The results show that, among four types of oxygen carrier, the NiFe2O4 synthesized by sol–gel method exhibits the highest gasification reactivity with lignite char at an O/C ratio of 1, since its highly cubic spinel structure is beneficial to improve the mobility of lattice oxygen. Furthermore, the operating conditions for chemical looping gasification of lignite were optimized and the gasification performance of four types oxygen carriers was compared by using a fixed bed reactor. It is found that NiFe2O4 synthesized by sol–gel method exhibits the highest total gas yield of 24.47 mol/kg and H2/CO ratio of 0.77 during chemical looping gasification of lignite under the optimal operating conditions, ascribed to Fe-Ni synergistic effect which provided higher gasification reactivity with lignite char. The addition of steam drastically improves the total gas yield and H2/CO ratio of syngas from 24.47 mol/kg and 0.77 to 64.98 mol/kg and 2.79, suggesting that the H2/CO ratio of syngas can be flexibly adjusted to meet the need of downstream process via controlling the amount of steam added. These findings provide an efficient method to obtain H2-rich syngas via chemical looping gasification of lignite.