| Combined heat and power(CHP),which is an important technology for energy cascade utilization and pollutant concentration control,has a diversified development trend that concerns energy conservation,emission reduction,heating expansion as well as flexibility.The high back-pressure(HBP)mode is outstanding even among all the CHP technologies for its high energy utilization efficiency.However,the application of this mode is not as mature as the traditional,such as the extraction condensing(EC)mode.And its energy saving potential,application limitations,and performance enhancement are still not clear enough for the application,which becomes barriers for its development.This dissertation gives detailed research on the adaption of the large-scale coal-fired high back-pressure CHP plant,mainly from the following aspects:Firstly,the thermodynamic performance of the HBP unit was studied and compared with the traditional EC unit.And the advantages and limitations of the HBP unit were discussed theoretically.After that,research on the CHP plants with dual units was conducted,which was the most common form of the HBP unit.Finally,the optimization of the CHP plant was presented and discussed from both energy conservation and flexibility.The superiority and optimization direction of CHP plant were revealed through the energy consumption dispatch based on the Second law of thermodynamics,which indicated that the efficiency of the CHP plant could be further improved through the optimization of the heat source for district heating(DH).Compared with the EC unit,the energy-saving potential of the HBP unit was discussed from both overall and partial thermodynamic performance.The HBP unit had no exhausted steam loss and its exergy consumption of per unit heat is 34.73%lower than the EC unit,and the exergy-based generation efficiency is 5.18%higher.The two common types of the HBP unit were also analyzed,which had better performance on the flexibility and DH adaption.Based on the above research,the operation strategy and thermodynamic performance of the CHP plants with dual EC units and with HBP-EC units were compared.The indexes of the maximum temperature rise ratio of the heating condenser and the heating ratio of the HBP unit were proposed and used to measure the influence of the DH network condition on the heating capacity and feasible operational region of the CHP plant.Results showed that the temperature rise of the return and supply water could reduce the maximum heat load of the HBP unit and narrow its feasible operational region.Then the nonlinear optimization method was used to solve the CHP economic dispatch problem,in which the energy consumption characteristics and feasible region were considered.The case study of the whole heating period showed that the HBP unit provided most of the DH demand and nearly half electric demand in the HBP-EC unit.And the coal consumption of the HBP-EC unit was 8.55%lower than the dual EC unit,and the generation efficiency and fuel utilization factor increased by 6.85%and 5.78%,respectively,leading to a 32.79 g/kWh decrement of the standard coal consumption rate.The energy saving potential of the CHP plants with HBP-EC units was explored.And two novel optimization systems were proposed,including the combined waste heat recovery between the flue gas and exhausted steam and the CHP plant integrated with the turbine using non-reheated steam.Thermodynamics and economic analysis showed that the exhausted steam utilization ratio of the former novel system was increased.In the given condition,the utilization ratio of the novel system was 22.74%higher than that of the reference system,and the heating load was increased by 16.26%,leading to a 19.65 g/kWh decrement of the standard coal consumption rate.And the novel system also brought $1.80 million per year,and the dynamic payout period of the equipment was 2.06 years.The later novel system(CHP plant integrated with the turbine using non-reheated steam)significantly reduced the exergy loss in the heating process.And the energy-saving and economic advantages of the novel system were compared with the traditional system through a case study in a typical heating period.The flexibility enhancement technologies for the CHP plant were summarized,including the coal-fired boiler,electric boiler,electric heat pump,thermal energy storage system,absorption heat pump,and other waste heat recovery process.These technologies were further classified into power to heat(P2H)mode and auxiliary heat source(AHS)mode based on the technical principle,operational strategy.After that,the coal-saving potential and techno-economic analysis were performed,which provided a logic for the technology selection.With the AHS mode,the power range of the CHP plant expanded from 78.72%-54.87%to 100%-25.20%,which expanded to 78.72%-0 with the P2H mode.In the given case,the waste heat recovery technology had the best coal-saving potential,and the thermal energy storage system(Using extraction steam as the heat source)had the best techno-economic performance.The chosen of the flexibility improvement technology for the CHP plant should comprehensively concern the peaking demand,operation strategy,coal-saving potential as well as the techno-economic performance. |
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