Gasification Performance and Mechanism of High-Silicon Phosphogypsum Oxygen Carrier in Chemical Looping Gasification

The phosphogypsum (PG) stockpile in Yunnan, China, is one of the largest accumulations of PG. Our group has focused on the application of chemical looping gasification (CLG) with a PG oxygen carrier characterized by high silicon content. FactSage 7.1 software was used for the thermodynamic simulation analysis of the reaction mechanism and reaction characteristics at different temperatures. Experiments were conducted at a laboratory scale in a fixed-bed facility with lignite as fuel using PG as an oxygen carrier and N-2/H2O as gasifying agents at a temperature of 1173 K. We investigated the gasification performance and silicon transfer behavior during chemical looping gasification using X-ray fluorescence spectrometry, X-ray diffractometry, X-ray photoelectron spectroscopy, and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. The results indicated that the silicon in PG participated in the production of syngas through a series of parallel and tandem side reactions (R10-R15) during the reduction process. The high-silicon PG was more conducive to syngas production than the desilicated PG during the reduction process of CLG under the investigated experimental conditions. At 1173 K, the sequence of the parallel reactions was R15 > R8 > R12 > R9, suggesting that an increase in the silicon content inhibited the production of H2S (R8) by increasing the silicon content, which is highly desirable for a clean production process. In CLG, the silicon in the fresh oxygen carrier PG was transformed from SiO2 into CaSiO3, Ca3SiO7, and Ca2SiO4; distributed unevenly; and attached to the surface of CaSO4 after eight cycles. The theory of the Zener effect was developed to understand the mechanism of the interaction between the inert support SiO2 and PG oxygen carrier.