High-Quality Syngas Production by Chemical Looping Gasification of Bituminite Based on NiFe2O4 Oxygen Carrier

Abstract

Chemical looping gasification (CLG) is an effective coal utilization technology. In this work, the CLG of bituminite was investigated through fixed-bed batch experiments using NiFe2O4 oxygen carrier (OC) to achieve high-quality syngas. The changes in the phase of the oxygen carrier during the chemical looping reaction and the reaction mechanism were explored. The results show that elevated temperature and adding a fraction of steam facilitate the gasification reaction. Adding an appropriate amount of ZrO2 into the NiFe2O4 and modification with alkali metal can enhance the performance of the oxygen carrier. A carbon conversion of 95% and a syngas (CO and H2) selectivity of 86% were obtained under the optimized reaction conditions of 950 °C, an oxygen-carrier-to-bituminite (O/B) ratio of 7:3, a NiFe2O4/ZrO2 ratio of 7:3, and a steam rate of 0.08 mL/min. Modification of the NiFe2O4 by doping alkali metal can significantly facilitate the CLG process. Alkali lignin ash has a more pronounced modifying effect on oxygen carriers than K2CO3. The NiFe2O4 OC underwent a gradual reduction in Ni2+ → Ni and Fe3+ → Fe8/3+ → Fe2+ → Fe processes during the gasification reaction phase. In addition, 20 redox cycles were conducted to demonstrate the oxygen carriers’ good cyclic reaction performance in the CLG process. After 20 redox cycles, the carbon conversion rate was maintained at about 90%, and the syngas selectivity was stably kept at over 80%. This work laid the theoretical foundation for the clean and efficient use of bituminite.