Advanced exergy analysis on supercritical water gasification of coal compared with conventional O2-H2O and chemical looping coal gasification

Abstract

Supercritical water gasification (SCWG) that converting coal into hydrogen-rich syngas is a promising cascade utilization technique of coal. Advanced exergy analysis of SCWG is conducted to evaluate avoidable exergy destructions, and results are compared with conventional (CG) and chemical looping gasification (CLG). In the benchmark condition, the exergy efficiency of SCWG process is 68.63%, which is higher than CG and CLG. The biggest irreversibility of gasification process occurs in gasifier caused by gasification reaction, and SCWG gasifier has high avoidable exergy destruction potential. The modified exergy efficiency of SCWG process is 82.38% under unavoidable conditions. However, avoidable exergy destruction portion of syngas self-heating recovery is only 3.55% due to heat transfer mismatch between supercritical water and syngas. Suitable external heat sources should be adopted, such as the variable heat source with constant cp, which can reduce exergy destruction of supercritical water heating to 44.86%. The effects of parameters on SCWG is performed with single factor analysis method. Coal water slurry concentration (CWSC) is the key factor affecting the performance of SCWG. With gasification temperature < 750 °C, gasification pressure > 23 MPa, CWSC > 17% and oxygen-coal ratio < 0.46 kg kg−1, the exergy efficiency of SCWG process is higher than CG and CLG.