Exergy analysis and the energy saving mechanism for coal to synthetic/substitute natural gas and power cogeneration system without and with CO₂ capture

The energy saving mechanism and the potential of efficiency improvement for coal to synthetic/substitute natural gas and power plant with different schemes and CO₂ capture is disclosed through exergy analysis, and the effects of key parameters on exergy losses and system performance are investigated. Scheme A represents the system without CO₂ capture but with a full syngas component adjustment and partial recycle of the chemical unconverted gas, Scheme B represents the system without CO₂ capture and syngas component adjustment but with partial recycle of the chemical unconverted gas, and Scheme C represents the SNG and power cogeneration with CO₂ capture and partial recycle of the chemical unconverted gas but without syngas component adjustment. Results show that the exergy efficiencies of Scheme A, B and C range from 56% to 62%, 57% to 67%, 52% to 62%, respectively. Coal gasification, water-gas-shift process, SNG methanation, and fuel combustion in combined cycle are identified as the main sources of exergy losses. Compared with Scheme A, the exergy efficiency of Scheme B is higher due to the avoidance of exergy losses from syngas adjustment. Scheme C is less energy efficient than Scheme B because of the CO₂ capture. Compared with single product systems, the total exergy input of Scheme A, B and C can be reduced by 7.0-11.0%, 14.0-19.0%, 15.0-21.0%, respectively assuming the same product output. The chemical to power output ratio (CPOR) will impact the exergy losses of the whole plant greatly. For all schemes, with the increasing CPOR, the exergy losses for chemical synthesis island will increase whereas the exergy losses for power island will decrease. Especially high CPOR will cause sharp exergy losses of chemical synthesis island. The coupling between exergy losses for chemical synthesis and power islands leads to an optimal CPOR making the total exergy losses of the plant minimal and the system efficiency maximized. The results presented in this paper can help to confirm the potential of system integration and can be a guide for system integration.