A 1000MW ultra-supercritical secondary reheat unit Energy consumption analysis of thermodynamic system

: Taking the thermodynamic system of a 1000MW ultra-supercritical secondary reheating unit as the research object, the modeling energy consumption analysis was carried out, and the spatial distribution of energy consumption was determined, so as to provide a scientific basis for the operation optimization and energy-saving transformation of the unit. The analysis results show that: Boiler is one of the largest equipment factory irreversible loss, make up the loss by 85.1% of the total loss of the system, although the number of its external loss is not big but internal loss is very big, and the loss is high grade of energy, the number of the steam turbine cold source loss although large irreversible loss is not big but the grade is not high, its loss make up only 2.32% of the total loss system.


Introduction
Under the guidance of national energy conservation and emission reduction policy, high-efficiency and lowpollution power generation technology has become the mainstream. Improving steam parameters and increasing reheating times are effective ways to improve unit efficiency, and ultra-supercritical secondary reheating technology is an important technical means to improve thermal economy of generating units [1].Compared with the supercritical unit, the thermal efficiency of the ultrasupercritical unit can be increased by 3%-4%, and the thermal efficiency of the whole plant is higher than 50% by adopting the secondary reheating technology [2][3][4].The waste heat recovery system is generally designed for the secondary reheating ulra-supercritical units, which further optimizes the thermal system, boiler efficiency, pipeline efficiency, turbine flow efficiency and plant power consumption rate [5]. The traditional method of heat balance analysis is based on the first law of thermodynamics. The inner efficiency of boiler and the absolute inner efficiency and cycle efficiency of steam turbine are used to describe the closeness of the actual steam cycle to the ideal cycle.However, the traditional thermal balance analysis method only focuses on the quantity balance of energy without considering the difference of energy in "quality", only calculates the external loss of the system but ignores the internal loss, and ignores the internal loss caused by the irreversibility of the process.Therefore, this method only shows the result of energy conversion, but cannot reveal the essential cause of energy loss, cannot accurately point out the location of energy saving potential, and sometimes even gives false appearance [6].For example, in the analysis of the condenser as a circulating cold source, the traditional heat balance analysis method thinks that most of the system loss occurs here, and it seems that the main energy saving space of the power plant is also here,But exergy analysis, based on the second law of thermodynamics, suggests that the loss here is not great, because the amount of heat emitted from the condenser is large but the energy of that heat is not high.In contrast, the conventional thermal balance analysis method calculates that the loss of the boiler is not large, and the efficiency of the boiler in the power plant can reach 90% or more, but the exergy analysis method considers the loss of the boiler to be large, because the amount of energy lost by the boiler is small but the loss of high grade energy [7][8][9][10].In this paper, a 1000MW ultra-supercritical secondary intermediate reheating unit is taken as the research object. Based on the first and second laws of thermodynamics, the loss of work capacity due to irreversibility in the cycle process is analyzed, and the location of energy loss is accurately located, so as to provide a reference for targeted energy saving and potential exploration.

Unit energy consumption analysis evaluation index
2.1 Purpose (exergy) efficiency of a device or system: Where, ΔEout is the net income related to the purpose exergy and ΔEin is the net consumption to achieve the purpose exergy.The above equation is a general expression for exergy efficiency,The expression of specific exergy efficiency varies depending on the purpose for which the system or device is used.For example, ΔEout of the boiler plant is the feed water and reheated steam obtained in the boiler exergy, ΔEin is the fuel chemistry exergy.For the engine island, ΔEout is the output shaft work and ΔEin is the net input exergy.ΔEout of the heat exchanger device is the cold fluid obtained in the heat exchanger exergy, ΔEin is the hot fluid reduced in the heat exchanger exergy.

Exergy loss factor of a device or system:
The thermal system is divided into several subsystems, in which the ratio of the exergy loss of any subsystem to the total exergy consumption of the system is the exergy loss coefficient of the subsystem.For a power plant, the total exergy consumption of the system is the chemistry exergy of the fuel.

Maching series Data
The boiler of a 1000MW ultra-supercritical secondary reheating unit in China adopts Single furnace tower arrangement, Tangential burning at four corners, swing and tail flue baffle temperature regulation, balanced ventilation, steel frame suspension structure, semi-open air arrangement, and AIR-cooled dry slag removal system.The steam turbine adopts a single-axis fivecylinder four-row steam mode, and a two-stage external steam cooler is used in the high-pressure heater side to make cross-stage utilization of the superheat energy.The main design parameters of boilers and steam turbines are as follows:

Efficiency of a device or system:
The boiler, steam turbine and pipeline efficiencies are shown in the following table: The plant efficiency calculated by the exergy analysis method is slightly lower than the plant efficiency calculated by the thermal balance method. The difference between the two is due to the characteristics of the fuel itself, namely, the low calorific value of the fuel and its chemistry exergy.Although the total plant efficiency calculated by the two methods is approximately the same, the loss distribution calculated by the two methods is quite different.The boiler efficiency calculated by heat balance method is 95.03% and that calculated by exergy analysis method is 55.1%.The thermal efficiency of the turbine island calculated by the thermal balance method is 51.15%, while the thermal efficiency of the turbine island calculated by the exergy method is 87.24%.The reason is that the two efficiencies stand on different ground. Thermal balance calculation focuses on the quantity of each loss, while exergy analysis focuses on the quality of each loss, that is, the loss of the work capacity of the working medium.The working medium absorbs the heat transferred by the flue gas in the boiler. Due to the huge heat transfer temperature difference, the heat transfer process is inevitably accompanied by a huge irreversible loss.Such processes as combustion, heat transfer with finite temperature differences, logistic mixing of different parameters, and frictional resistance are not considered losses in traditional thermal equilibrium calculations because they obey the first law of thermodynamics, which theoretically conserved energy in quantity.But in exergy analysis these are all losses, because the capacity of energy to do work is reduced.A large amount of effective energy in the energy is converted into invalid energy in the boiler, which cannot be utilized in the later energy conversion process and can only be released to the environment through the condenser and cooling tower to form waste heat. Therefore, the heat balance analysis method believes that the main energy loss of the system occurs at the turbine side. The exergy analysis says that the main energy loss of the system has occurred on the boiler side but that the emission of this energy is on the turbine side.The energy consumption distribution of the whole plant thermal system is shown in the following Through calculation and analysis, the irreversible loss of the system mainly occurs at the boiler side, and its loss accounts for 85.1% of the total loss of the system.The loss is mainly caused by the irreversibility of combustion and heat transfer.In the process of fuel combustion, chemical exergy is transformed into the physical fire of flue gas, which will be accompanied by a large amount of available energy loss. The furnace is the place with the highest working temperature of the boiler, and the heat transfer temperature difference can reach thousands of degrees. The huge heat transfer temperature difference leads to a great heat transfer loss.The incomplete combustion of fuel, heat transfer loss, heat dissipation loss, smoke exhaust loss and other factors combine to cause a great loss of available energy at the boiler side. The irreversible loss of the turbo-generator body is the second largest loss source in the power plant, and its irreversible loss accounts for 7.52% of the total loss of the system.This loss includes nozzle, rotor blade, blade height, sector, impeller friction, partial steam intake, steam leakage, wet steam, residual speed loss and other losses. The turbine cold source loss is the third largest loss source in power plant, and its irreversible loss accounts for 2.32% of the total system loss.Although the amount of steam turbine cold source loss is large, the steam exhaust temperature of the steam turbine is only 28.1℃, which is not big difference with the ambient temperature, so the grade of this part of heat is very low. When the working medium flows through the main steam pipeline, water supply pipeline, primary reheating cold and hot section pipeline and secondary cold and hot section pipeline, the irreversible loss caused by the heat dissipation and pressure loss of the pipeline accounts for 1.755% of the total loss of the system, which is the fourth largest loss source in the power plant.
In the high pressure heater system, the design end difference of #1 high pressure heater and #3 high pressure heater is the same. The temperature rise of water supply in #1 high pressure heater and #3 high pressure heater is 39.8℃ and 36.5℃, respectively. According to the design data, the heat transfer temperature difference between #1 high pressure heater and #3 high pressure heater is 26.83℃ and 30℃, respectively.Although the heat exchange of #3 high pressure heater is less than that of #1 high pressure heater, the irreversible loss of #3 high pressure heater is greater than that of #1 high pressure heater due to the large heat exchange temperature difference. Since the use of an external steam cooler reduces the heat transfer temperature difference between the #2 and #4 high pressure heaters, the irreversible loss of the #2 and #4 high pressure heaters is lower than that of the #1 and #3 high pressure heaters. The irreversible loss of the low pressure heater system is slightly higher than that of the high pressure heater system. In the low-pressure heater system, the design end difference of the #6 low-pressure heater and the #7 lowpressure heater is the same. The temperature rise of the condensate in the #6 low-pressure heater and the #7 lowpressure heater is 23℃ and 36.7℃, respectively. According to the design data, the heat transfer temperature difference between the #6 low-pressure heater and the #7 low-pressure heater is 31.26℃ and 29.34℃, respectively. Although the heat transfer temperature difference of the #7 low-pressure heater is less than that of the #6 low-pressure heater, the irreversible loss of the #7 low-pressure heater is greater than that of the #6 low-pressure heater due to the large heat transfer. The water drain of the #8 low-pressure heater flows into the outlet of the #8 low-pressure heater through the water drain pump. This method reduces the heat transfer temperature difference of the heater and is conducive to reducing the irreversible loss of heat transfer.

Conclusion
Through modeling and analyzing the thermal system of the unit, the following conclusions are drawn: The boiler is the equipment with the largest irreversible loss in the power plant. Although the amount of lung heat emitted by the condenser is very large, the heat temperature carried by the exhaust steam of the steam turbine is not different from the ambient temperature. The taste of the exhaust steam of the steam turbine is very low, and the loss of work capacity is not large. The difference between the whole-plant exergy efficiency calculated by exergy analysis and the whole-plant thermal efficiency of the first law depends on the characteristics of the fuel consumed in the plant. It depends on the difference between the low calorific value of the fuel and the chemical exergy. Based on the second law of thermodynamics of exergy analysis takes into account not only the traditional heat balance analysis method have noticed in the system caused by the external loss but also considers the internal process within the irreversible loss, help us to understand the nature of the energy loss reasons, focus on how to reduce all kinds of irreversible factors in the system of debasement of energy, In order to make better use of energy.