Effect of Gasifying Medium on the Coal Chemical Looping Gasification with CaSO4 as Oxygen Carrier

The chemical looping gasification uses an oxygen carrier for solid fuel gasification by supplying insufficient lattice oxygen. The effect of gasifying medium on the coal chemical looping gasification with CaSO4 as oxygen carrier is investigated in this paper. The thermodynamical analysis indicates that the addition of steam and CO2 into the system can reduce the reaction temperature, at which the concentration of syngas reaches its maximum value. Experimental result in thermogravimetric analyzer and a fixed-bed reactor shows that the mixture sample goes through three stages, drying stage, pyrolysis stage and chemical looping gasification stage, with the temperature for three different gaseous media. The peak fitting and isoconversional methods are used to determine the reaction mechanism of the complex reactions in the chemical looping gasification process. It demonstrates that the gasifying medium (steam or CO2) boosts the chemical looping process by reducing the activation energy in the overall reaction and gasification reactions of coal char. However, the mechanism using steam as the gasifying medium differs from that using CO2. With steam as the gasifying medium, parallel reactions occur in the beginning stage, followed by a limiting stage shifting from a kinetic to a diffusion regime. It is opposite to the reaction mechanism with CO2 as the gasifying medium.

Graphical AbstractGraph 1. Chemical looping gasification using CaSO4 as oxygen carrier is illustrated above. Coal gasification first occurs while oxygen carrier reacts with coal or syngas and is reduced to CaS. Then the as-reduced oxygen carrier is transferred to the air reactor and re-oxidized by air. The strong exothermic oxidation reaction can provide sufficient heat for coal gasification, which is an endothermic reaction.Graph 2. In a CO2 atmosphere, the gasification of coal char and the reaction between coal char and CaSO4 start at approximately the same temperature. However, the gasification of coal char in steam occurs at a lower temperature, with the reaction between coal char and CaSO4 divided into two evident stages.Figure optionsDownload full-size imageDownload as PowerPoint slide