Energy Analysis Of PolygenerationSystems Based On CoalGasification

Polygeneration system is a complex, high efficient and environmentally friendly system with the chemical fuel and power productions. Methanol synthesis and power generation are the typical sections to form the simplest cogeneration system. Thermodynamic methods were used to optimize gasifier, shift reaction and CO2 removal, and methanol synthesis processes. Polygeneration is a promising system to make the best use of thermal energy with different qualities to achieve high environmental and economic benefits. The equilibrium working temperature of the gasifier and the composition of the outlet syngas (short for synthesis gas) at this temperature were calculated by solving the chemical equilibrium and the energy equilibrium equations. The analysis was used to study the effect of gasifying agent mixture ratio on the outlet syngas composition, the cold gas efficiency, the thermal efficiency and the exergy efficiency. The optimum O2/coal ratio was found for a reasonable vapor/coal ratio (the vapor/coal ratio had little effect on the efficiencies) for various kinds of coal to maximum the thermal efficiency and the exergy efficiency. These results can be used as a reference for optimization of polygeneration systems. The syngas generated from coal needs shifting reaction to adjust H2/CO ratio to fit the methanol synthesis requirement. The shifting reaction consumed energy but increased the methanol synthesis ratio and the system efficiency. The energy method was used to analyse the effect of methanol synthesis ratio on system efficiency to show the advantages of polygeneration system. Furthermore, the exergy analysis considered both the physical exergy and the chemical exergy were used to analyse the chemical synthesis and the power generation. The results when methanol synthesis ratio was 0.9 to the equilibrium state were compared with the separate electrical and chemical systems and showed that polygeneration was 3.5% more efficient and less polluted. Polygeneration, which based on IGCC, was able to achieve the highest efficiency with the least environmental impacts. The pollutions were nearly zero emission after clean up of syngas. The cogeneration system also gave a good chance to mitigate CO2 emission. In the methanol and electricity cogeneration systems, CO2 mitigation was not merely an accessional process that consumed energy and reduced the overall efficiency. The CO2 mitigation process was integrated with the methanol generation. Consuming some energy, but the methanol synthesis ratio was growing and CO2 was partly captured. As further development to achieve CO2 zero emission, unlike IGCC, the cogeneration system will greatly reduced CO2 mitigation costs. Polygeneration system will be highly competitive when environmental impacts were considered.