Integrated Optimization And Exergy Economic Analysis Of Coal And Biomass Co-gasification-FT Liquids-Electricity Polygeneration System

With the rapid development of the national economy and the development requirements of the current era of carbon peaking and carbon neutrality,energy utilization and environmental issues are the current research focus.Our country’s fossil energy utilization is mainly coal,as a non-renewable energy,it cannot be used indefinitely.The development of new energy in a short period of time is not enough to replace the status of fossil energy.Therefore,coal is still the main energy source before the arrival of the new energy boom era.The use of coal energy not only has high carbon emissions but also causes great pollution to the atmospheric environment,which does not conform to the requirements of green and low carbon development.However,the coal-based polygene ration system realizes the efficient use of coal and reduces carbon emissions.At the same time,the chemical synthesis part and the organic coupling of the power generation part realizes the comprehensive cascade utilization of chemical energy and physical energy.Biomass is a clean and economical renewable energy,the complementary use of coal resources and biomass resources can not only save coal resources,it can also effectively reduce CO₂.Therefore,blending and co-gasification of coal and biomass blending is the core of syngas production in the polygonation system,a polygeneration system that organically couples the FT synthesis process with the combined cycle power generation technology can improve energy utilization efficiency,reducing environmental pollution,and it is also one of the solutions to realize the coordinated development of energy and environment.Based on the principle of comprehensive cascade utilization of physical energy and chemical energy in the polygeneration system,this research established a tandem FT synthetic oil-electricity polygeneration system for the co-gasification of coal and biomass.Through the analysis of the thermodynamic performance of the polygeneration system,the effect of the co-gasification of coal and biomass on the thermal efficiency and exergy efficiency of the polygeneration system was mainly explored,the effect of the FT synthesis cycle ratio on the thermal performance of the series polygeneration system is also analyzed.On this basis,the economics of coal and biomass co-gasification polygeneration system and the influence of FT synthesis cycle ratio on the unit exergy cost of system output were analyzed by the unit exergy economic cost of system output.The subunits of the integrated polygeneration system mainly include:coal and biomass co-gasification unit,syngas conversion and purification unit,FT synthesis unit,oil refining unit,and IGCC power generation unit.A simulation model and a logistics balance model were established,and the thermal efficiency,exergy efficiency,and unit exergy economic cost of system output under different biomass mass percentages and different FT synthesis cycle ratios were calculated and analyzed,the result shows:（1）Under the condition that the total feed rate of the co-gasification unit of the polygeneration system is 1200 t/d and the biomass mass percentage in the raw material is from 0 to 100%,the cold gas efficiency varies with the biomass mass percentage.The increase first increased and then decreased,and reached a maximum of 82.1%when the biomass percentage was 20%.At this time,the co-gasification characteristics were the best,which also reflected the synergistic effect of coal and biomass co-gasification.（2）With the increase of the percentage of blended biomass,the CO₂ capture decreased by 34.2%and the emission decreased by 45.4%based on the coal-based polygeneration system;the system thermal efficiency and exergy efficiency both increased first and then decreased,when the biomass percentage was 20%,the system thermal efficiency reached a maximum of 44.7%,and the system exergy efficiency also reached a maximum of 49.5%.Taking the coal-based polygeneration system as the benchmark,the biomass percentage increases from 0 to 100%,and the unit exergy economic cost of the system output decreases first and then increases,among them,the unit energy cost of the system is reduced by 14.90 RMB/GJ,and the unit non-energy cost is increased by 31.41RMB/GJ,when the biomass percentage is 20%,the unit exergy economic cost of the total output of the system is the minimum 94.05 RMB/GJ.It shows that a certain proportion of coal and biomass co-gasification polygeneration system can not only reduce the loss of system energy,but also reduce the loss of effective energy,and also reduce carbon emissions,and the economy is relatively good.（3）By adjusting the FT synthesis cycle ratio,it was found that with the increase of the FT synthesis cycle ratio of the polygeneration system,the CO₂ capture amount of the system increases and the emission decreases.When the FT synthesis cycle ratio is increased from 0 to 1,as more syngas is used to synthesize higher-grade liquid fuels,chemical energy losses are reduced and the thermodynamic performance of the polygeneration system is improved,among them,the thermal efficiency of the pure coal polygeneration system has increased by 8.6%,and the exergy efficiency has increased by9.6%;the thermal efficiency of the system with a biomass percentage of 20%has increased by 8.8%,and the exergy efficiency has increased by 9.8%;the thermal efficiency of the pure biomass polygeneration system has increased by 7.8%,and the exergy efficiency has increased by 8.7%.Meanwhile,the unit exergy economic cost of the system output decreases with the increase of the FT synthesis cycle ratio,the cycle ratio increases from 0to 1,the pure coal polygeneration system reduced by 22 RMB/GJ,and the co-gasification polygeneration system with 20%biomass decreased by 21.6 RMB/GJ.It shows that the increase of the unreacted gas circulation ratio of the FT synthesis unit not only improves the energy utilization level and effective energy conversion of the system,but also reduces the exergy economic cost and makes the system economy more suitable.