The Integration And Optimization Studies Based On The Theory Of The Energy Cascade Utilization For The System Of Coal-based Liquid Fuel-power Polygeneration

Fossil fuels, which were rich in coal, poor in oil and little in nature gas, had beenregarded as the major China’s energy resources. Based on this situation, the overalloptimization for the process of coal conversion and transfer, from material to energy, wasneeded. From a perspective of feasibility study, the system of coal-based chemical-powerpolygeneration made it reality for the utilization of coal in a way of high-efficiency andlow-carbon. It was also a way, what’s important, to solve the compatible and coordinativedevelopment among energy shortages, environmental pollution and social benefits.The integration of polygeneration system follows the cascade utilization theory ofchemical energy and physical energy, along with the keys of energy-saving potential andessential regularity could been actually reflected by the process of material conversion andenergy transfer. The model of polygeneration system, which involved chemical field andpower generation field, was established with the basic idea: the GSP coal gasification asthe source process of system, the systems of Fischer-Tropsch Synthesis (FTS) andIntegrated Gasification Combined Cycle (IGCC) were rationally merged in order tointegrate the polygeneration system of coal-based FT syncrude and IGCC. This systemmostly include: air separation unit (ASU), GSP coal gasification unit (GSP), water gas shiftunit (WGS), rectisol unit, FTS unit, refining unit, Claus sulfur recovery unit (CSR) and thepower generation of combined cycle unit (PGCC). Through the software of Aspen Plus, thesubsystems were modeled and simulated.The study on the system integration emphasized on the thermodynamic point, whichis based on the cascade utilization theory of chemical energy and physical energy, andestablished the exergy efficiency as the optimization objective function. The value ofexergy efficiency determines the system degree of thermodynamic perfection as theperformance indicator of actual process, and the analysis of local system, especially thesize and distribution of exergy loss, is the key to indicate the energy-saving potential. Theresults revealed that as the proportion of syngas used to FTS was increased, the efficiency of energy utilization was gradually enhanced, and it could be illustrated by the exergyefficiency of polygeneration system, which were64.41%(FT-25)，66.65%(FT-75) and69.89%(FT-100), respectively. In terms of the production of FT synfuels and emission ofCO2, the most yield of synfuels comes from the polygeneration system in series such asliquefied petroleum gas (LPG)39.23t/h, naphtha70.44t/h and diesel197.34t/h, yet theleast emission of CO2comes from the polygeneration system in parallel such as carboncapture rates reduced from81.48%(FT-25) to30.61%(FT-100), and carbon emissionrates increased from8.62%(FT-25) to14.78%(FT-100). In terms of the electricitygeneration, the quantity of electricity generation obviously presents a decreasing trend thatare1517.19MW (FT-25),422.96MW (FT-75) and-134.43MW (FT-100), respectively.The series system, that is to say, could not meet the need of power consumption. For theabove reasons, the polygeneration system has the character of variable operatingconditions.Through the calculation and analysis of the polygeneration subsystems, the value anddistribution of exergy loss was researched. The results showed that exergy loss of thepolygeneration system mainly attributed to the processes of separation and chemicalreaction coincidence with the conversion of chemical exergy, even the more extensive thechemical reaction, the greater the exergy loss. The major distributions of exergy lossfocused on four subsystems: ASU, GSP, CSR and PGCC, which accounted for89.83%(FT-25)，83.46%(FT-75) and78.17%(FT-100) of the total exergy loss. In addition, theexergy efficiency of WGS and FTS subsystems was increased, especially the FTS adding7.72%, because the application of reaction heat to yield the saturation steam realized thecascade utilization of physical exergy with the conversion process of chemical exergy. Thecoal gasification, including Shell with the waste heat boiler process and GSP with quenchprocess, was evaluated by the sensitivity analysis tool. For the process of dry pulverizedcoal gasification, the waste heat boiler process was more exergy-efficient than the quenchprocess, and the effect of different parameters on exergy efficiency was great.For the inforementioned reasons, for the system of coal-based FT syncrude and IGCC polygeneration, the key factors of the energy-saving potential wasn’t only depended on thelevel of the chemical exergy of syngas converting into more high-exergy such as chemicalenergy of fuels and electrical energy, but the process, which could maximized the cascadeutilization of physical exergy coincidence with the conversion process of chemical exergyof syngas.