Mechanism of lignite-to-pure syngas low temperature chemical looping gasification synergistic in situ S capture

Phosphogypsum (PG) is a by-product produced in phosphate fertilizer industry, and can be used as the oxygen carrier for the process of chemical looping gasification (CLG) of lignite to produce syngas. However, the temperature for this process is around 1173 K, and consumes a lot of energy. Additionally, during the process, a lot of H2S is yielded, which seriously affects the quality of syngas. Therefore, in order to save energy and improve the quality of syngas, this paper proposes a new oxygen carrier to lower the reaction temperature of the CLG to produce syngas while simultaneously capturing sulfur (S) from the gas product. Meanwhile, thermodynamic calculations and laboratory experiments have been used to investigate the characteristics of proposed Ni/PG material along with the mechanism of the process. The results showed that the Ni element existed in the form of small NiO crystals on the surface of PG while acting as the oxygen carrier, and possessed excellent reactivity and recyclability characteristics. Furthermore, the use of the material can reduce the reaction temperature of the CLG for syngas production and purify syngas through the chemical reaction: 3Ni+2H2S→Ni3S2+2H2. During the reduction process, the main reactions for syngas production consisted of solid-solid and gas-solid reactions, and therefore, the Shrinking Core Model (SCM) was the most probable mode of this process. Furthermore, the main chemical reaction for the regeneration of Ni/PG was a gas-solid reaction.