Research On Waste Heat Cascade Utilization Of High Back-pressure Combined Heating With Large-scale Coal-fired Units

High back pressure(HBP)heating technology recoveries turbine waste heat for heating,which reduces the heating loss,expands the heating capacity of unit and improves the cogeneration energy saving.In this thesis,based on the second law of thermodynamics,the exergy analysis method is adopted to study the heating energy consumption of large-scale coal-fired HBP cogeneration units.Aiming at the limitation of HBP heating,the HBP exhaust steam and extraction steam cascade heating method is adopted,and a super high back pressure(SHBP)cascade heating system combined multiple units is proposed further to improve the energy saving of cogeneration system.The xingneng thermal power plant in Shanxi Gujiao is taken as the case study,and the simulation model is built by the Ebsilon professional software.Through running simulation,the heating capacity and the energy saving effect of SHBP heating system combined multiple units are analyzed.On this basis,according to the heat load change of different heating periods and the peak regulation requirement of power grid,the system operation optimization is studied to improve the energy saving of heating system and to tap the potential of peak regulation of cogeneration.By the exergy analysis of cogeneration,the energy saving potential of HBP heating is explored from the overall thermal performance of unit and the heating process.The results indicate that,compared with the adjustable extraction unit,the turbine loss of the HBP unit is reduced because of avoiding the throttling of the regulating valve on the medium and low pressure cylinder connecting pipe.And as the heat source parameter is closer to the thermal network,the heating loss is obviously reduced.The overall exergy efficiency of cogeneration unit is increased by 4.2 percent,and the heating consumption rate is reduced by 52.6 percent.However,limited to the temperature of exhaust steam,the HBP heating has some application limitations.Limited to the utilization rate of exhaust steam,the energy saving potential also needs to be deeply excavated.In view of the limitation of HBP heating,the cascade heating method with exhaust steam and extraction steam is further studied in this thesis,and the SHBP heating system combined multiple units is proposed for the cascade utilization of HBP exhaust steam waste heat.According to the actual case,different HBP series cascade heating schemes are designed,and the heating proportion of exhaust steam is gradually increased.The results show that with the increase of the number of series units,the proportion of exhaust steam heating and the waste heat recovery increases,which raise the power generation load and the heating capacity of system.However,when the designed heating load,the exhaust steam heat of SHBP units cannot be recovered completely.Therefore,the operation optimization is needed to give full play to the energy saving of the SHBP combined heating system.The four-stage series SHBP heating scheme is adopted,and the optimized operation strategy is put forward for SHBP combined heating system.Based on the change of thermal network boundary conditions in different heating periods,the regulation process of full heat load condition of the system is simulated to study the energy saving of the whole heating period.The results show the steam extraction flow and the unit back pressure could be adjusted according to the change of heating demand,and by joint adjustment of multiple units the energy saving of HBP heating system can be improved in a larger range of heat load.According to the simulation of system state in heating period,the average heating consumption rate of SHBP combined heating system is only 5.19kg/GJ,which saves 74,000 tons of standard coal compared with conventional HBP heating system.The safe operation range of the HBP extraction unit is analyzed in this thesis,and the peak shaving optimization strategy of the SHBP combined heating system is proposed.On this basis,the peak shaving process of system in different heating periods is simulated,and the peak shaving capacity of system in the whole heating period is calculated.The results show that the SHBP combined heating system regulates the generated power by changing the main steam flow,the steam flow of air cooling island and the heating steam flow of extraction steam or exhaust steam.By replacing part of the heat load of exhaust steam with extraction steam,the main steam flow can be reduced further for depth peak shaving.The peak regulation calculation of heating period shows that the participation of system in the peak regulation of power grid can free up 2.715 billion kWh of network space for renewable energy generation,and the peak regulation pressure of power grid is alleviated.In conclusion,the waste heat cascade utilization combined multiple HBP units is a high efficiency heating technology with the prospect of engineering application.