A novel coal based cogeneration system for substitute natural gas and power with CO₂ capture and moderate recycle of the chemical unconverted gas

Cogeneration of substitute natural gas (SNG) and power from coal efficiently and CO₂ capture with low energy penalty during coal utilization are very important technical paths to clean coal technologies for China which is rich in coal but lack of natural gas resources. This paper integrates a novel coal based cogeneration system with CO₂ capture for SNG and power, and presents the energetic and exergy analysis based on the thermodynamic formulas and the use of ASPEN PLUS 11.0. In the novel system, instead of separation from the gas before synthesis traditionally, CO ₂ will be removed from the unconverted gas after synthesis, whose concentration can reach as high as 55% before separation and is much higher than 30% in traditional SNG production system. And by moderate recycle instead of full recycle of chemical unconverted gas back into SNG synthesis, the sharp increase in energy consumption for SNG synthesis with conversion ratios will be avoided, and by using part of the chemical unconverted gas, power is cogenerated efficiently. Thermodynamic analysis shows that the benefit from both systematic integration and high CO₂ concentration makes the system have good efficiency and low energy penalty for CO₂ capture. The overall efficiency of the system ranges from 53%-62% at different recycle ratios. Compared to traditional single production systems (IGCC with CO₂ capture for power, traditional SNG system for SNG production), the energy saving ratio (ESR) of the novel system is 16%-21%. And compared to IGCC and traditional SNG system, the energy saving benefit from cogeneration can even offset the energy consumption for CO₂ separation and realize zero energy penalties for CO₂ capture systematically. Sensitivity analysis hints that an optimized recycle ratio of unconverted gas and chemicals to power output ratio (CPOR) can maximize system performance and minimize the energy penalty for CO₂ capture.