The Energy Saving Mechanism And Characterization Study On Coal Poly-generation Systems

Coal-based poly-generation is one of the clean, high-efficient coal technologies. Coal to alternative fuels through poly-generation technology is also an important path to enforce energy security. In this essay, we analyze the exergy destruction regularity of gasification unit which is the fundamental unit of poly-generation systems. We also analyze the possibility of reducing exergy destruction of the gasification unit by alternating the gasification reaction route. The thermodynamic performances of coal to methanol and coal to substitute natural gas (SNG) poly-generation systems are also compared to reveal the inner reasons of the performance difference, and to make up for the optimization principle of poly-generation system.We analyze how the exergy destruction of gasification process changes through different reaction route, reaction temperature and pressures, etc. by studying the Gibbs free energy change regularities of gasification reactions. We analyze the composition of exergy destruction of traditional coal-water-slurry (CWS) gasification, and study the influence regularity of CWS concentration and oxygen-to-coal ratio. Results show that by increasing the concentration of CWS we can reduce the exergy destruction and improve the exergy efficiency of gasification process.In order to compare the performance difference of coal to methanol and coal to SNG poly-generation systems, we first compare the traditional coal to methanol and coal to SNG single production system. Results show that the exergy efficiency of traditional coal to methanol of which the purge gas is combusted in a torch system and a distillation unit is utilized to purify the product is 48.7%, while the exergy efficiency of traditional coal to SNG system is 56.7%. It is indicated that three key factors impact the performance enhancement of coal to alternative fuel process:(1) whether the fuel is distillated, (2) the synthesis temperature and the amount of heat release from reactions, and (3) whether the chemical purge gases from synthesis and distillation units are recovered. By cancelling the distillation unit and re-combust the purge gas in the captive power plant, we can increase the exergy efficiency of traditional coal to methanol system.For non-adjustment, partial cycle poly-generation systems of coal to methanol and coal to SNG, we compare the energy efficiency, exergy efficiency and energy saving ratio of these two systems, and compare the energy penalty of capturing CO2 of these two systems. Results show that the exergy efficiency of coal to methanol and coal to SNG poly-generation system are 53.8% and 60.9%, respectively, under the optimized recycle ratio of unreacted syngas. The energy saving ratio are 15.0% and 14.2%, respectively. The energy saving mechanisms of the two poly-generation systems are different. For coal to methanol poly-generation system, more energy is saved in synthesis subsystem by re-combusting purge gas in the power-generation subsystem. For coal to SNG poly-generation system, more energy is saved in the water-gas-shift reaction unit and purification unit by cancelling the water-gas-shift reaction. For the two poly-generation systems with CO2 capture, as the capture unit of coal to SNG system is set after synthesis reaction unit, the processed syngas is less in amount and the CO2 concentration is higher in the syngas, so the energy penalty of capturing CO2 is lower for coal to SNG poly-generation system.By using CO2 instead of O2 as gasification oxidants, we can alternate the gasification reaction route to reduce the exergy destruction in gasification unit. Besides, the air separation unit is also cancelled to avoid the massive energy consumption in producing pure oxygen. The energy efficiency and exergy efficiency are also increased. We compare the performance differences of coal to methanol system with novel gasification process and traditional coal to methanol system, the exergy efficiency of these two systems are 61.3% and 48.7%, respectively. The novel gasification method can achieve a massive exergy destruction reduction in gasification unit, and by utilizing the captured CO2 as oxidants, the system overall exergy efficiency is improved.Besides, in order to verify the feasibility of this novel gasification method, we proceed the coal coking and coke-CO2 gasification reaction experiment. And we analyze the influence regularity on coke-CO2 gasification reaction kinetic characteristics of coal type, coking conditions and gasification conditions. Results show that the experimental brown coal, long flame coal and bituminous coal are all fit for the novel gasification process, the coking conditions have little influence on gasification reaction rate. Gasification temperature of 1000℃ can ensure adequate reaction rate. Besides, by adding alkali catalysts can also help to accelerate the gasification reaction rate.