Comparative study between fluidized-bed and fixed-bed operation modes in pressurized chemical looping combustion of coal

The performance differences between the fluidized-bed and the fixed-bed operation modes were comparatively studied in a pressurized coal-fuelled chemical looping combustion (CLC) process. Pressurized thermogravimetric (TG) analysis was applied to elucidate the findings observed in pressurized fluidized-bed and fixed-bed experiments. The results showed that the performance of the reaction between coal and oxygen carrier was strongly dependent on the operating pressure and the employed operation mode. Elevated pressure lower than 0.5 MPa promoted the conversion of coal gasification products to CO2 and H2O but pressure beyond 0.5 MPa resulted in a negative effect for both operation modes. TG and characterization analyses confirmed that the negative effect of pressure had little to do with oxygen carrier property but was mainly ascribed to the suppression of coal pyrolysis at high pressure which leads to a decrease in char reactivity. Fixed-bed mode showed superiority over fluidized-bed mode in enhancing the CO2 concentration and the carbon conversion, but was unfavourable for the pore structural properties of oxygen carrier, and thereby undesirable for long-term stable operation.