Coal based cogeneration system for synthetic/substitute natural gas and power with CO₂ capture after methanation: Coupling between chemical and power production

Cogeneration of synthetic natural gas (SNG) and power from coal efficiently and CO₂ capture with low energy penalty during coal utilization are very important technical paths to implement clean coal technologies in China. This paper integrates a novel coal based cogeneration system with CO₂ capture after chemical synthesis to produce 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 chemical synthesis traditionally, CO₂ 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 product 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 CO ₂ separation, and thus zero energy/efficiency penalties for CO ₂ capture can be realized through system integration when the chemicals to power output ratio (CPOR) varies in the range of 1.0-4.6. Sensitivity analysis hints that an optimized recycle ratio of the unconverted gas and CPOR can maximize system performance (The optimized R_u for ESR maximum is around 9, 4.2, and 4.0, and the corresponding CPOR is around 4.25, 3.89, and 3.84, at τ=4.94, 5.28 and 5.61), and minimize the effi-ciency penalty for CO₂ capture (The optimized Ru for minimization of CO₂ capture energy penalty is around 6.37 and the corresponding CPOR is around 3.97 at τ=4.94, ε=16.5). The polygeneration plant with CO₂ capture after chemical synthesis has a good thermodynamic and environmental performance and may be an option for clean coal technologies and CO₂ emission abatement.