Thermodynamic Process Splitting Analytical Method And Integration Research Of The Coal-based Poly Generation Systems With Low Carbon Emission

To secure energy supply and achieve the target of"carbon emission peak in 2030,carbon neutrality in 2060" in China,it is necessary to promote the efficient and clean utilization of fossil energy,e.g.coal.pay attention to the development of renewable energy and effectively reduce carbon emission.This paper focuses on the efficient,low-carbon and multi-functional utilization of coal-based energy.Firstly,a thermodynamic process splitting analytical method is proposed to reveal the energy saving mechanism of the process modification measures for thermal cycles.Furthermore,the material complementarity and energy substitution are investigated among the thermodynamic processes.Finally,the superior thermodynamic performance is illustrated for the advanced zero-carbon emission power cycle(coalbased Allam cycle),and the novel coal-based polygeneration systems are constructed with the integration of the Allam cycle for chemical synthesis and power generation.This study might provide theoretical supports and various possible technical options for development of the coal-based clean and low-carbon emission systems with coordination of energy,resources and environment.A new general thermodynamic analytical method,termed as cycle splitting method,is proposed for the process modification of complex thermodynamic cycles.The analytical method“splits" the process modification measure from the complex thermal cycle and formalizes it as closed curve that can be expressed on T-s diagram,which is named as equivalent"simple cycle”.It can directly explain the energy saving mechanism of different process modification measures and guide the process optimization.The split simple cycles can also be used as computing tool to accurately evaluate the energy saving effectiveness of various process modification measures.The cycle splitting method is applied to analyzing the process modifiecation of the steam Rankine cycle.The energy saving mechanism of reheating and regeneration is reexplained using the equivalent simple cycles.The efficiency improvement mechanism of"turbine-boiler coupling" process modification measure for coal-fired power plant can also be understood as efficient coordination among equivalent simple cycles.The cycle splitting method is also applied to analyzing the process modification of the supercritical carbon dioxide(S-CO2)cycle,and a novel integrated cycle was proposed.By changing the split position of the stream for the recompression,the recompression and the intercooling are discoupled,and the cycle efficiency is further increased by 0.88 percentage points compared with the conventional integrated cycle.The "cycle splitting" is developed to the "process splitting" for analysis of the integrated coal gaification Allam cycle zero-carbon emission power generation system,which is split into four processes:the closed S-CO2 cycle.steam Rankine cycle,open process and air separation unit.Based on the thermodynamic conversion characteristics of the four processes above,the process splitting analytical models are obtained and the process optimization is carried out in three aspects:the thermal integration of S-CO2 cycle recuperation unit and air separation unit.the steam extraction of steam Rankine cycle,and the heat redistribution of the syngas heat recovery.The energy saving mechanism of the modification meausures can be clearly illustrated by the split thermal cycles/equivalent thermodynamic processes:the highefficiency S-CO2 cycle should replace the low-efficiency air intercooling compression and steam Rankine cycle for power generation as much as possible,while taking into account of the amount of steam required for pre-drying and gasification and the pinch temperature difference in the heat recovery process,so as to increase the net power output of the system.Finally,the net efficiency of the optimized coal-based Allam cycle is 42.48%,which is 3.68 percentage points higher than that of the basic configuration.Integrating Allam cycle with methanol synthesis process based on coal gasification,a novel coal-based polygeneration system is constructed for methanol production,power generation and carbon capture.Based on the "cycle splitting" and"process splitting",the "system splitting" analytical method is proposed,which splits the polygeneration system into"chemical subsystem" and"power subsystem".The system splitting analytical method can clearly illustrate that the introduction of Allam cycle effectively avoids the carbon capture energy consumption of chemical subsystem:the chemical subsystem obtains part of H2 from the power subsystem for methanol synthesis,and unreacted CO enters Allam cycle to drive S-CO2 cycle for efficient power generation,which does not affect the oxygen consumption of the power subsystem.In addition,the process waste heat and purge gas of the chemical subsystem are also efficiently recovered by the power subsystem.Further,a new method for product distribution adjustment is proposed by combining syngas component adjustment and unreacted syngas recycle,and the optimal combination modes are obtained to meet requirements of different product distributions.Through system integration and product distribution adjustment,the fuel saving ratio of polygeneration system can soar to 8.79%.The solar gasification is introduced into the proposed polygeneration system above,and a solar thermochemical hybrid coal-based polygeneration system with carbon capture is proposed.At the designate point,the methanol production and net power generation are increased by 94.05%and 19.58%,respectively,and the fuel saving ratio can reach 35.10%.The splitting analytical method is applied to the field of multi-energy complementarity to analyze the conversion performance of solar energy in the novel polygeneration system.Comparing the system performances with and without solar integration,the changes of different grade energy flows in each process are obtained and then these changes are split from the coal-based polygeneration system to make up a "net solar energy conversion system".Results show that the made-up system has several processes with energy grade upgrading.It is visually illustrated that the solar energy can be upgraded and utilized efficiently in the proposed polygeneration system.In addition,for the solar hybrid system,solar variation and intermittence should be considered.The polygeneration system adopts the solar/autothermal hybrid gasification mode to maintain the continuous operation.Under different solar conditions,the daily solar contribution ratio,energy saving rate and solar energy conversion efficiency are in ranges of 5.9%～10.3%,12.7%～17.6%and 29.46%～31.40%,respectively.