Research On Integration Mechanism And Operation Optimization Of Multi-heat Sources Complementary And Flexible Heating System Based On CHP Units

Combined heat and power(CHP)units nowadays should not only improve the heating capacity to satisfy the demand of newly increased district heat load,but also improve the operation flexibility to satisfy the need of renewable energy accommodation.However,due to the characteristic of thermoelectric coupling,there is an incompatibility in simultaneously improving its energy utilization efficiency and flexibility.Exploring available heat sources,carrying out research on the integration mechanism and operation optimization of multi-heat sources complementary and flexible heating system based on the CHP units,is an effective way for decreasing the high peak-shaving energy consumption and improve the heating capacity of the traditional extraction condensing(EC)CHP units.Taking the large-scale coal-fired CHP units as the research object,this paper first reveals the problem of high peak-shaving energy consumption of the traditional ECCHP units.Then,to realize the coordinated improvement of high-efficiency and lowcarbon heating and flexibility of the CHP units,a thermodynamic evaluation method is proposed for the waste heat resources that can be used as heat sources for district heating inside the units.Considering the constraints of energy balance between the heat source and the heating network,the integration mechanism and operation optimization of multi-heat source complementary and flexible heating system based on the CHP units are studied based on the principle of "temperature counterpart and cascade utilization".Moreover,by integrating the CHP units with internal heat sources or external heat sources respectively,two types of multi-heat sources complementary flexible heating systems at the plant level and regional level are proposed and studied.An evaluation method of peak-shaving energy consumption of CHP units based on the second law of thermodynamics is proposed,which can clarify the changes in energy consumption for generating heat and power production by using different thermoelectric decoupling technologies under different operation conditions.Taking a 350 MW EC-CHP unit as an example,the peak-shaving energy consumption of the unit was evaluated after adopting four different flexibility enhancement technologies.The change in power regulation capacity and the limiting factors of the unit under different heating loads are analyzed,and the comprehensive evaluation is carried out from the perspectives of operation strategy,thermoelectric operation characteristics,and peak-shaving energy consumption.The results show that these flexibility improvement technologies can effectively increase the peak-shaving capacity of the unit,but it will cause significant peak-shaving energy consumption due to "high quality but low utilization ".It reveals the current situation that the EC unit obtains flexibility by sacrificing energy efficiency,and it is urgent to develop a multi-heat sources complementary CHP flexible heating system.The basic principles of multi-heat sources complementary system integration,the quantitative calculation and evaluation methods of waste heat resources,and the whole process guidance theory of system integration mechanism and construction are proposed.The quantitative calculation method of waste heat resources of CHP units is systematically explained from the laws of thermodynamics,and the coefficient of availability of waste heat resources of units is proposed for district heating,which is used to evaluate the availability of waste heat resources and guide the improvement of the waste heat resource recovery and utilization process.Meanwhile,considering the supply-demand balance and energy grade constraint between the heat source and the heat network,the concepts of maximum heat supply temperature rise ratio and heat supply ratio of waste heat sources are defined from the heat network side and the heat source side respectively,which guide the matching and arrangement process among different heat sources and propose the integration mechanism and construction process of CHP multi-heat sources heat heating system.Taking the utilization of flue gas waste heat as an example,a low-temperature/high-temperature flue gas waste heat classification and efficient heating system is constructed,which takes into account flexible peak-shaving and low-carbon and efficient heating.Based on the above theory,aiming at the problems of large heating irreversible loss of the EC-CHP units,incomplete utilization of waste heat and poor flexibility of the high-back pressure(HBP)CHP units,the steam jet pump(SJP)is introduced to construct a plant-level multi-heat sources complementary CHP cascade heating system combined of exhausted steam waste heat source,mixed heat source and extraction steam heat source.The system design concept of process-component-system coupling collaborative optimization is proposed to design the structure of the SJP,and its variable working characteristics and operation regulation strategy are obtained by numerical simulation.A study of the applicability of this multi-heat sources heating system was carried out to compare the thermodynamic performance and flexibility of the old and new systems under full operating conditions.The results show that in the new system constructed by the integrated S JP,the proportion of exhausted steam waste heat source heating is high,the heat transfer temperature difference is small,the heating loss is reduced,and the average temperature of the heat source is even lower than the water supply temperature of the heat network.Compared with the single extraction heat source system,the new system has significant energy saving and flexibility advantages.Since the heating capacity and flexibility of a single CHP unit cannot meet the needs of district heating load growth and renewable energy accommodation,the system is further integrated with other external heat sources in addition to the unit’s internal heat source to build a regional-level multi-source complementary flexible energy supply system with the CHP unit as the core.Based on the refined modeling of CHP units and considering the operation constraints of other electric/heat sources in the process of electrothermal energy transportation,a scheduling model of the proposed system is established with the optimization objective of minimizing carbon emissions and operating costs.The operation strategy of the system after integrating three types of external heat sources is proposed,and the energy consumption and wind power accommodation effect of the system are compared and analyzed based on a case study.Taking into account the advantages and disadvantages of a single heat supply method,a multi-heat sources complementary CHP flexible energy supply system coupled with the electric heat pump and unit waste heat supply retrofit is further proposed.The example shows that,compared with the single extraction steam heat source heating system,the multi-heat sources complementary energy supply system integrated with different heat sources leads to a better system decarbonization effect and the ability to consume the wind power through the coordinated operation of multi-heat sourcess and multi-power sources,achieving a synergistic improvement of efficient low-carbon heat supply and flexibility of CHP units at the regional level.