The Mechanism Of Minimal Energy Penalty For CO₂Capture And The Study On Coal-based Polygeneration System For Cogenerating Substitute Natural Gas And Power

It is an important issue to coordinate the energy utilization and evironment protection. To solve the the energy utilizaion problem in the traditional coal to SNG sytem and to explore the low energy penalty and low cost for CO2capture, this essay studied the coupling mechanism the cascade utilization of chemical energy in fuel and CO2separation, then proposed a polygeneration system with CO2capture which produces SNG and power, and also analyzed the cost reduction potential of the polygeneration system.First, the law of Gibbs free energy loss and the mechanism of CO2enrichment during fuel conversion were explored, the relationship between the utilization of Gibbs free energy and CO2separation work was built and illustrated, and the mechanism of minimal energy penalty for CO2capture by utilizing Gibbs free energy was disclosed.Based on the mechanism for minimal energy penalty for CO2capture, this work proposed a new polygneration system (PG) for SNG and power, simulated the thermal performance of this system, analyzed its energy-saving mechanism and made performance sensitivities. Results show that, at designed condition, the thermal efficiency of the new polygeneration system with CO2capture is as high as59.7%, and the energy saving ratio is19.5%compared with IGCC+CC and traditional coal to SNG systems. The optimal thermal efficiency can be62%, and optimal energy saving ratio is21.5%. Through moderate recycle of the off gas in the new polygeneration system, the sharp increase of the energy consumption for unit SNG production can be avoided and the CO2can be enriched during chemical synthesis. The optimal recycle ratio of the off gas, which yields best system thermal efficiency and ESR, is observed. Compared to IGCC and traditional coal to SNG system, the benefit from Gibbs free energy utilization can make up or even can be greater than the work for CO2separation in the polygeneration system, which makes the zero or even negative energy penalty for CO2capture possible. Compared to the polygeneration system which recover CO2and produces methanol and power, the CO2concentration before separation in the proposed system can be5percentage higher, and the unit energy penalty for CO2capture is less.Also, the cost reduction potential of the polygeneration system is analyzed, and the roles of localization, efficiency improvement and capacity scale in cost reduction are disclosed. Currently, the unit investment of this PG system with CO2capture is around1700$/kW. The investment of air separation unit, coal gasification unit and the combined cycle unit takes over60%portion of the whole investment, and especially the investment of gasification unit takes over30%portion. The SNG production cost varies from0.18-0.36$/Nm3with different coal price. Compared with IGCC+CC and traditional coal to SNG system, when the product outputs are the same, the total investment of the polygeneration system is slightly higher but the total annual cost is around3.8%lower. Compared to IGCC and SNG system, the CO2capture cost of the PG is around7.9$/t. Through localization, the unit investment of the polygeneration system can be decreased by29.4%currently. The current localization levels of coal gasification, SNG synthesis and gas turbine technologies are relatively low, and these technologies are the key of further localization. The installed capacity, technology further localization and efficiency upgrade will take an important role in cost reduction, which will make the unit investment of the PG decrease to700~1100$/kW that is possibly lower than IGCC+CC or even than PC+CC system.