Cogeneration of substitute natural gas and power from coal by moderate recycle of the chemical unconverted gas

The thermodynamic analysis and the coupling and optimization between chemical synthesis and power generation in a polygeneration system are presented. Unlike full conversion of syngas into chemicals in the traditional SNG (synthetic natural gas) production system, by moderate conversion the sharp increase in energy consumption for SNG synthesis can be avoided in the new system. Also, by recovering the chemical unconverted gas for combined cycle, electricity is cogenerated efficiently. Results show that the overall efficiency of the novel system can be as high as 59%–65%. And compared to single production systems, the (energy saving ratio) ESR of the new system is over 11.0% and the energy consumption for SNG production can be decreased by around 12%. Sensitivity analysis shows that an optimized conversion ratio of SNG, (chemicals to power output ratio) CPOR, recycle ratio of the unconverted gas Ru, and pressure ratio of gas turbine can lead to the maximum of ESR. Abolishing the syngas composition adjustment and improving the inlet temperature of gas turbine both can help to enhance the system efficiency. Under low Ru, improving the H2/CO mole ratio in the syngas helps to improve system efficiency, while under high Ru, an optimized H2/CO can lead to the maximum of ESR.