Low-grade energy cascade utilization system and control method of coal-fired generating unit

Abstract

The application discloses a low-grade energy cascade utilization system and a control method of a coal-fired generating unit, and the low-grade energy cascade utilization system comprises a primary air heater, a secondary air heater and a low-temperature economizer, a water inlet of the primary air heater is communicated with a water outlet of a low-pressure heater, low-temperature air input on the fan side is heated by the primary air heater through low-temperature condensate water, and a water outlet of the primary air heater is communicated with a water outlet of a condensate water pump; an air outlet of the primary air heater is communicated with an air inlet of the secondary air heater, air is further heated by the secondary air heater through condensate water, and the air is sent into an air preheater. A water inlet of the low-temperature economizer is communicated with a water inlet of the low-pressure heater, condensate water output on the air preheater side is heated by high-temperature flue gas, and air is heated by high-temperature condensate water. By reasonably arranging flue gas waste heat exchange temperature zones, flue gas waste heat cascade utilization is realized, technical support is provided for realizing system full-working-condition safe investment on the basis of reducing flue gas waste heat utilization process loss.

Description

Technical Field

[0001] This invention relates to the field of coal-fired power generation technology, specifically to a system and control method for the cascade utilization of low-grade energy in coal-fired power generation units. Background Technology

[0002] In existing technologies, flue gas heat loss accounts for the largest proportion of all heat losses in coal-fired power plant boilers. It is well known that the most significant factor affecting flue gas heat loss is the boiler flue gas temperature. Therefore, the effective utilization of waste heat from the flue gas in coal-fired power plants has become one of the key directions for improving energy efficiency and promoting the sustainable development of coal-fired power generation technology.

[0003] A typical scheme to reduce flue gas temperature relies on a low-temperature economizer positioned before the dust collector, which transfers heat from the flue gas to the regenerative system. By increasing the condensate temperature, the corresponding amount of steam extracted by the low-temperature heater is reduced, thereby improving the unit's thermal efficiency. However, this scheme primarily utilizes flue gas heat at the existing 90℃–130℃ temperature level, failing to achieve deeper, tiered utilization of flue gas waste heat.

[0004] Furthermore, the waste heat from flue gas decreases significantly under low-load conditions in coal-fired power generating units, resulting in limited recoverable waste heat energy and making system operation impossible under certain low-load conditions. Additionally, when utilizing waste heat from flue gas using a low-temperature economizer, the condensate temperature, acting as the heat exchange medium, decreases synchronously with the unit load. This decrease in condensate temperature leads to some flue gas temperatures inside the low-temperature economizer falling below the acid dew point, exacerbating low-temperature corrosion and potentially jeopardizing equipment safety.

[0005] In view of this, there is an urgent need to provide effective solutions for recovering low-grade energy from coal-fired power generating units in order to overcome the above-mentioned shortcomings. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a low-grade energy cascade utilization system and control method for coal-fired power generating units. By rationally arranging the flue gas waste heat exchange temperature zone, the cascade utilization of flue gas waste heat is realized. While reducing losses during the flue gas waste heat utilization process, it provides a good technical guarantee for the safe operation of the system under all operating conditions.

[0007] This invention provides a low-grade energy cascade utilization system for a coal-fired power generation unit. The coal-fired power generation unit includes a condenser, a condensate pump, a low-pressure heater, a deaerator, and an air preheater. The low-grade energy cascade utilization system includes a primary air heater, a secondary air heater, and a low-temperature economizer. The inlet of the primary air heater is connected to the outlet of the low-pressure heater, and the outlet of the primary air heater is connected to the outlet of the condensate pump. The air inlet of the primary air heater is connected to the outlet of a blower. The air outlet of the low-temperature economizer is connected to the air inlet of the secondary air heater; the flue gas inlet of the low-temperature economizer is connected to the flue gas outlet of the air preheater, and the flue gas outlet of the low-temperature economizer is connected to the dust removal system side; the water inlet of the low-temperature economizer is connected to the water inlet of the low-pressure heater, and the water outlet of the low-temperature economizer is connected to the water inlet of the secondary air heater; the water outlet of the secondary air heater is connected to the water outlet of the low-pressure heater, and the air outlet of the secondary air heater is connected to the air inlet of the air preheater.

[0008] Optionally, the low-grade energy cascade utilization system further includes a condensate recirculation pump, and the outlet of the primary air heater is connected to the outlet of the condensate pump through the condensate recirculation pump.

[0009] Optionally, the condensate recirculation pump is a variable frequency pump.

[0010] Optionally, the inlet of the low-temperature economizer is connected to the inlet of the low-pressure heater.

[0011] Optionally, the low-grade energy cascade utilization system further includes a bypass regulating valve, which is located in the condensate bypass of the low-pressure heater.

[0012] Optionally, the outlet of the secondary air heater is also connected to the inlet of the primary air heater.

[0013] The present invention also provides a control method based on the aforementioned low-grade energy cascade utilization system. The control method includes the following steps: presetting a temperature difference threshold between the inlet temperature and the outlet temperature of the condensate recirculation pump, and a flue gas temperature threshold at the outlet of the low-temperature economizer; acquiring the inlet temperature and outlet temperature of the condensate recirculation pump, and the flue gas temperature at the outlet of the low-temperature economizer; determining a first control command to adjust the frequency of the condensate recirculation pump based on the temperature difference threshold between the inlet temperature and the outlet temperature of the condensate recirculation pump, and the inlet temperature and outlet temperature of the condensate recirculation pump; and determining a second control command to adjust the opening degree of the bypass regulating valve based on the flue gas temperature threshold at the outlet of the low-temperature economizer and the flue gas temperature at the outlet of the low-temperature economizer.

[0014] Optionally, determining the first control command for adjusting the frequency of the condensate recirculation pump includes: using the sum of the outlet temperature of the condensate pump and the temperature difference threshold as the setpoint of the PID controller, and generating the first control command based on feedback from the inlet temperature of the condensate recirculation pump; obtaining corresponding feedforward compensation amounts by multiplying the flue gas flow rate by the first feedforward coefficient and the opening degree of the bypass regulating valve by the second feedforward coefficient, respectively, and generating corresponding feedforward signals which are superimposed on the first control command to adjust the frequency of the condensate recirculation pump.

[0015] Optionally, the step of generating the first control command based on the inlet temperature feedback of the condensate recirculation pump includes: if the inlet temperature of the condensate recirculation pump is lower than the sum of the outlet temperature of the condensate pump and the temperature difference threshold, then outputting a first control command to increase the frequency of the condensate recirculation pump.

[0016] Optionally, the second control command for determining the opening degree of the bypass regulating valve includes: if the current flue gas temperature at the outlet of the low-temperature economizer is higher than the flue gas temperature threshold, then outputting a second control command to increase the opening degree of the bypass regulating valve.

[0017] Compared with existing technologies, this solution offers a novel approach by providing a low-grade energy cascade utilization system for coal-fired power generating units. Specifically, it mainly consists of a primary air heater, a secondary air heater, and a low-temperature economizer. The inlet of the primary air heater is connected to the outlet of the low-pressure heater, using low-temperature condensate to heat the low-temperature air input from the fan side. The outlet of the primary air heater is connected to the outlet of the condensate pump. The outlet of the primary air heater is connected to the inlet of the secondary air heater, where condensate further heats the air before it is fed into the air preheater. The inlet of the low-temperature economizer is connected to the inlet of the low-pressure heater, using high-temperature flue gas output from the air preheater to heat the condensate, which in turn heats the air.

[0018] This configuration, where condensate is heated from the boiler's exhaust gas and then used to heat cold air in stages through condensate at different temperature ranges, reduces heat transfer losses caused by the large temperature difference in the air preheater, effectively minimizing cold source losses on the turbine side. It achieves deep coupling of the boiler-turbine heat and mass transfer process, conforming to the energy-saving principle of "temperature matching and tiered utilization." Simultaneously, the system can control the exhaust gas temperature through condensate flow regulation, effectively preventing low-temperature corrosion of the heating surfaces caused by flue gas temperatures falling below the acid dew point. Furthermore, while effectively enhancing the utilization of waste heat from the flue gas, it provides a solid technical guarantee for safe operation of the system under all conditions.

[0019] In an optional embodiment of the present invention, the outlet of the secondary air heater is also connected to the inlet of the primary air heater. In this way, the condensate, which is further heated in the secondary air heater, can increase the inlet temperature of the primary air heater. After mixing with the condensate flowing out of the low-pressure heater, the heat exchange capacity of the primary air heater can be effectively enhanced.

[0020] In another optional embodiment of the present invention, the low-grade energy cascade utilization system further includes a condensate recirculation pump and a bypass regulating valve. The outlet of the primary air heater is connected to the outlet of the condensate pump via the condensate recirculation pump. The inlet of the low-temperature economizer is connected to the inlet of the low-pressure heater, and the bypass regulating valve is located in the condensate bypass of the low-pressure heater. Based on the regulation and control of the condensate recirculation pump and the bypass regulating valve, the condensate is heated by the flue gas at the boiler tail end, and then the cold air is heated in a cascade manner by the condensate in different temperature ranges. This allows for the full recovery of the waste heat energy from the flue gas, enabling the system to be put into operation under low-load conditions. Under full load conditions, this effectively reduces the standard coal consumption for power generation and improves the overall economic efficiency of the plant. Attached Figure Description

[0021] Figure 1 A schematic diagram of a low-grade energy cascade utilization system for a coal-fired power generating unit provided in this application embodiment;

[0022] Figure 2 This application provides a control block diagram of a method for cascade utilization of low-grade energy in a coal-fired power generating unit.

[0023] Figure 3 A frequency control flowchart for a condensate recirculation pump is provided in an embodiment of this application;

[0024] Figure 4 This is a flowchart illustrating the opening control of a bypass regulating valve, as provided in an embodiment of this application.

[0025] In the picture:

[0026] Boiler 1, Steam Turbine 2, High-Pressure Cylinder 21, Intermediate-Pressure Cylinder 22, Low-Pressure Cylinder 23, Condenser 3, Condensate Pump 41, Feedwater Pump 42, Condensate Recirculation Pump 43

[0027] 5. Low-pressure heater; 6. Deaerator; 7. High-pressure heater; 8. Air preheater.

[0028] Primary air heater 91, secondary air heater 92, low-temperature economizer 10, bypass regulating valve 11, low-grade energy cascade utilization system 100. Detailed Implementation

[0029] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] With the deepening of technological transformation and upgrading of coal-fired power plants to improve energy conservation and emission reduction, the coal consumption level of power generation in thermal power plants has been continuously reduced. As is well known, flue gas heat loss in existing thermal power units accounts for the largest proportion of all heat losses in boilers of coal-fired power generating units, and the most significant factor affecting flue gas heat loss is the boiler flue gas temperature. A typical solution to reduce flue gas temperature is based on the formation of a low-temperature economizer arranged before the dust collector, which transfers heat from the flue gas to the regenerative system. By increasing the condensate temperature, the corresponding amount of steam extracted by the low-temperature heater is reduced, thereby improving the unit's thermal efficiency.

[0031] However, this proposed solution primarily utilizes flue gas waste heat at the existing 90℃~130℃ level, failing to achieve deeper, tiered utilization of flue gas waste heat. Furthermore, the flue gas waste heat decreases significantly under low-load conditions in coal-fired power units, resulting in limited recoverable waste heat energy, and some low-load conditions do not meet the requirements for system operation. In addition, when utilizing flue gas waste heat based on a low-temperature economizer, the condensate temperature, acting as the heat exchange medium, decreases synchronously with the unit load. This decrease in condensate temperature leads to some flue gas temperatures inside the low-temperature economizer falling below the acid dew point, exacerbating low-temperature corrosion and potentially jeopardizing equipment safety.

[0032] Based on this, this application provides a low-grade energy cascade utilization system for coal-fired power generating units. Through the coupling of a complete working fluid system (flue gas, air, steam, and water), and the configuration of flue gas waste heat exchange temperature zones, it achieves cascade utilization of flue gas waste heat, reduces losses during the utilization process, and ensures safe system operation under all conditions while further enhancing the utilization capacity of flue gas waste heat. Please refer to [link to relevant documentation]. Figure 1 The figure is a schematic diagram of a low-grade energy cascade utilization system for a coal-fired power generation unit provided in an embodiment of this application.

[0033] The low-grade energy cascade utilization system 100 provided in this implementation plan is applied to... Figure 1 The supercritical coal-fired power generation unit shown is formed by the boiler 1 and the steam turbine 2. For example, the boiler 1 is a single-reheat, supercritical pressure coal-fired boiler, and the steam turbine 2 is a three-cylinder, three-exhaust, single-shaft, condensing steam turbine.

[0034] The steam turbine 2 includes a high-pressure cylinder 21, an intermediate-pressure cylinder 22, and a low-pressure cylinder 23 arranged sequentially. The exhaust port of the low-pressure cylinder 23 is connected to the steam inlet of the condenser 3. The low-pressure steam that has completed its work on the turbine 2 side enters the condenser 3 directly along the exhaust pipe and is condensed into condensate. A condensate pump 41 is installed on the outlet pipe of the condenser 3, forming a condensate system with the condensate pump 41. The condensate output by the condensate pump 41 passes sequentially through the low-pressure heater 5, the deaerator 6, and the high-pressure heater 7. The condensate is then heated stage by the extraction steam from the high-pressure cylinder 21, the intermediate-pressure cylinder 22, and the low-pressure cylinder 23, and returned to the economizer on the boiler 1 side.

[0035] Among them, the high-pressure cylinder 21 can extract steam to the high-pressure heater 7, the medium-pressure cylinder 22 can extract steam to the oxygenator 6, and the low-pressure cylinder 23 can extract steam to the low-pressure heater 5, so as to realize the step-by-step heating of condensate.

[0036] The high-temperature flue gas from the economizer outlet on boiler 1 and the air input from the blower side exchange heat through the air preheater 8 to reduce the temperature of the exhaust flue gas and increase the temperature of the air delivered to the furnace and pulverizing system on boiler 1.

[0037] In this embodiment, the low-grade energy cascade utilization system 100 includes a primary air heater 91, a secondary air heater 92, and a low-temperature economizer 10. The flue gas, which has been cooled by heat exchange in the air preheater 8, is further cooled by the low-temperature economizer 10 and then sent to the dust removal system (not shown in the figure).

[0038] In its specific implementation, the primary air heater 91 is a water / air heater, that is, a heat exchanger that uses water (condensate) as a heat source to heat cold air into hot air.

[0039] The inlet of the primary air heater 91 is connected to the outlet of the low-pressure heater 5. The outlet of the primary air heater 91 is connected to the outlet of the condensate pump 41 via the condensate recirculation pump 43, meaning the outlet of the condensate recirculation pump 43 is connected to the outlet pipe of the condensate system. The air inlet of the primary air heater 91 is connected to the fan outlet, and the air outlet of the primary air heater 91 is connected to the air inlet of the secondary air heater 92. Thus, the high-temperature condensate taken from the low-pressure heater 5 side, after being heated by the primary air heater 91 and transferred to the air supplied by the fan, is recirculated back to the outlet of the condensate pump 41. The low-temperature air taken from the fan side, after being heated by the primary air heater 91, enters the next stage heater (secondary air heater 92).

[0040] In its specific implementation, the low-temperature economizer 10 is a flue gas / water heat exchanger, that is, a heat exchanger that uses flue gas as a heat source to heat water (condensate) into hot water.

[0041] The flue gas inlet of the low-temperature economizer 10 is connected to the flue gas outlet of the air preheater 8, and the flue gas outlet of the low-temperature economizer 10 is connected to the dust removal system. The water inlet of the low-temperature economizer 10 is connected to the water inlet of the low-pressure heater 5. The water outlet of the low-temperature economizer 10 is connected to the water inlet of the secondary air heater 92. Thus, the high-temperature flue gas taken from the air preheater 8 exchanges heat with the condensate in the low-temperature economizer 10, and the cooled flue gas enters the dust removal system. The low-temperature condensate taken from the condensate bypass exchanges heat with the high-temperature flue gas in the low-temperature economizer 10, and the heated condensate enters the secondary air heater 92.

[0042] To regulate the flow of low-temperature condensate into the low-temperature economizer 10 to meet the actual operating requirements of the thermal system, a bypass regulating valve 11 can be added to the low-grade energy cascade utilization system 100. The bypass regulating valve 11 can be installed in the condensate bypass of the low-pressure heater 5 to regulate the flow rate of low-temperature condensate into the low-temperature economizer 10 as needed.

[0043] In its specific implementation, the secondary air heater 92 is a water / air heater, that is, a heat exchanger that uses water (condensate) as a heat source to heat cold air into hot air.

[0044] The inlet of the secondary air heater 92 is connected to the outlet of the low-temperature economizer 10, and the outlet of the secondary air heater 92 is connected to the outlet of the low-pressure heater 5. The air inlet of the secondary air heater 92 is connected to the air outlet of the primary air heater 91, and the air outlet of the secondary air heater 92 is connected to the air inlet of the air preheater 8. Thus, the high-temperature condensate taken from the low-temperature economizer 10 exchanges heat in the secondary air heater 92 with the low-temperature air taken from the primary air heater 91. The cooled condensate flows back to the condensate system side and enters the deaerator 6. The low-temperature air taken from the primary air heater 91 is heated by the secondary air heater 92 and then enters the air preheater 8.

[0045] Furthermore, to enhance the heat exchange capacity of the primary heater 91, the outlet of the secondary heater 92 is connected to the inlet of the primary heater 91. That is, the outlet of the secondary heater 92 is simultaneously connected to the outlet of the low-pressure heater 5 and the inlet of the primary heater 91. In this way, the condensate from the secondary heater 92, which is further heated, increases the inlet temperature of the primary heater 91. When mixed with the condensate flowing from the low-pressure heater 5, this effectively enhances the heat exchange capacity of the primary heater 91.

[0046] Based on the low-grade energy cascade comprehensive utilization system provided in this implementation plan, the air drawn in by the fan is heated sequentially by a primary air heater 91 and a secondary air heater 92 before entering the air preheater 8. This two-stage preheating significantly increases the air temperature entering the air preheater 8, reducing the consumption of high-temperature flue gas by the air preheater 8. Simultaneously, the flue gas exiting the air preheater 8 passes through a low-temperature economizer 10 for heat exchange with the condensate bypass. Because the condensate temperature in the bypass is relatively low, the flue gas temperature is significantly reduced, effectively ensuring the efficient utilization of waste heat energy. After being cooled to a reasonable temperature, the flue gas enters the unit's dust removal system.

[0047] The flue gas discharged from the boiler tail end first heats the condensate in the low-temperature economizer 10. The condensate then heats the air in the secondary air heater 92. The condensate flowing out of the secondary air heater 92 can be further mixed with the condensate flowing out of the low-pressure heater 5 through the condensate bypass, and then heats the low-temperature air flowing into the fan side through the primary air heater 91, before flowing back to the condensate system side through the condensate recirculation pump 43. Overall, through the deep coupling of the boiler-turbine heat and mass transfer process, following the basic energy-saving principle of "temperature matching and tiered utilization," the waste heat of the flue gas can be comprehensively utilized in a graded manner. In addition, based on the effective conversion of heat from the flue gas side to the condensate side, the extraction of steam from the high-pressure cylinder 21 and intermediate-pressure cylinder 22 of the turbine can be further reduced, reducing coal consumption and providing a good technical guarantee for improving the overall economic efficiency of the unit.

[0048] Based on the aforementioned low-grade energy cascade utilization system for coal-fired power generating units, this application also provides a control method for the cascade utilization of low-grade energy in coal-fired power generating units. Please refer to... Figure 2 This figure is a control block diagram of a method for cascade utilization of low-grade energy in a coal-fired power generating unit, provided in an embodiment of this application. The method includes the following steps:

[0049] Step S201: Set a temperature difference threshold between the inlet temperature of the condensate recirculation pump 43 and the outlet temperature of the condensate pump 41, and a flue gas temperature threshold at the outlet of the low-temperature economizer 10.

[0050] Step S202: Collect the inlet temperature of the condensate recirculation pump 43, the outlet temperature of the condensate pump 41, and the flue gas temperature at the outlet of the low-temperature economizer 10.

[0051] In step S203, based on the temperature difference threshold between the inlet temperature of the condensate recirculation pump 43 and the outlet temperature of the condensate pump 41, and the inlet temperature of the condensate recirculation pump 43 and the outlet temperature of the condensate pump 41, a first control command for adjusting the frequency of the condensate recirculation pump 43 is determined; based on the flue gas temperature at the outlet of the low-temperature economizer 10 and the flue gas temperature threshold, a second control command for adjusting the opening degree of the bypass regulating valve 11 is determined.

[0052] In a specific implementation, the condensate recirculation pump 43 can be a variable frequency pump, so that the frequency of the condensate recirculation pump 43 can be controlled according to the inlet temperature of the condensate recirculation pump 43.

[0053] For example, the temperature difference threshold between the inlet temperature of the condensate recirculation pump 43 and the outlet temperature of the condensate pump 41 is 5°C, which means that the inlet temperature of the condensate recirculation pump 43 is set to be 5°C higher than the outlet temperature of the condensate pump 41. If the inlet temperature of the condensate recirculation pump 43 is lower than the sum of the outlet temperature of the condensate pump 41 and the temperature difference threshold, a first control command to increase the frequency of the condensate recirculation pump 43 is output. That is, the frequency of the condensate recirculation pump 43 is increased based on the condition that the current temperature difference between the inlet temperature of the condensate recirculation pump 43 and the outlet temperature of the condensate pump 41 is lower than the temperature difference threshold, so as to make full use of the temperature of the condensate to heat the air supplied by the fan.

[0054] For example, the flue gas temperature threshold at the outlet of the low-temperature economizer 10 is 90°C, that is, the flue gas temperature at the outlet of the low-temperature economizer 10 is set to 90°C. If the current flue gas temperature at the outlet of the low-temperature economizer 10 is 95°C, which is higher than the flue gas temperature threshold of 90°C, a second control command is output to increase the opening of the bypass regulating valve 11. In other words, the opening of the bypass regulating valve 11 is increased based on the condition that the flue gas temperature at the outlet of the low-temperature economizer 10 is higher than the flue gas temperature threshold, so as to make full use of the waste heat of the flue gas and the heat exchange capacity of the low-temperature economizer 10 to heat the air supplied by the fan.

[0055] Based on the conventional operation equipment control of the unit, and based on the low-grade energy cascade utilization control method of coal-fired power generation units, the condensate recirculation pump 43 and the bypass regulating valve 11 are adjusted to utilize the condensate heated by the flue gas at the tail of the boiler to achieve cascade heating of cold air through condensate in different temperature ranges, thereby meeting the operation requirements of full-load conditions.

[0056] For the frequency control of the condensate recirculation pump 43, a combination of feedback and feedforward methods can be used based on the inlet temperature of the condensate recirculation pump. Please refer to [link / reference]. Figure 3 This figure is a flowchart illustrating the frequency control of a condensate recirculation pump according to an embodiment of this application. Exemplarily, the controller can be a PID (Proportional-Integral-Derivative) controller. In other possible implementations, other types of controllers may also be used, which can be determined according to the overall system design requirements; this embodiment does not impose any limitations.

[0057] like Figure 3As shown, the sum of the outlet temperature of the condensate pump 41 and the temperature difference threshold is used as the setpoint of the PID controller, and feedback control of the condensate recirculation pump 43 is achieved accordingly. The feedforward control of the condensate recirculation pump 43 includes feedforward of the flue gas flow rate and feedforward of the opening degree of the bypass regulating valve 11. Specifically, the feedforward compensation amounts are the first feedforward compensation amount obtained by multiplying the flue gas flow rate by a first feedforward coefficient, and the second feedforward compensation amount obtained by multiplying the opening degree of the bypass regulating valve by a second feedforward coefficient. Based on the first and second feedforward compensation amounts, corresponding feedforward signals are formed and superimposed on the first control command output by the PID controller to adjust the frequency of the condensate recirculation pump 43.

[0058] Here, the first and second feedforward coefficients can be determined based on empirical values ​​to adapt to the control of equipment during normal unit operation. Specifically, they can be determined according to the overall system design requirements; this application does not limit this.

[0059] In practical implementation, the first and second feedforward coefficients can be adaptively adjusted according to system operation. For the first feedforward coefficient, when the inlet temperature of the condensate recirculation pump 43 is lower than the sum of the outlet temperature of the condensate pump 41 and the temperature difference threshold, the first feedforward coefficient can be increased to increase the frequency of the condensate recirculation pump 43. Similarly, for the second feedforward coefficient, when the inlet temperature of the condensate recirculation pump 43 is lower than the sum of the outlet temperature of the condensate pump 41 and the temperature difference threshold, the second feedforward coefficient can be increased to increase the frequency of the condensate recirculation pump 43.

[0060] To improve the stability of the low-grade energy cascade utilization system, the first feedforward coefficient can be adjusted first. When the current temperature difference between the inlet temperature of the condensate recirculation pump 43 and the outlet temperature of the condensate pump 41 is lower than the temperature difference threshold, the first feedforward coefficient is first increased. If the current temperature difference between the inlet temperature of the condensate recirculation pump 43 and the outlet temperature of the condensate pump 41 is still lower than the temperature difference threshold, the second feedforward coefficient is then increased. In this way, the potential impact on the stability of the cascade utilization effect caused by simultaneously adjusting the first and second feedforward coefficients can be avoided.

[0061] For the opening control of bypass regulating valve 11, feedback control can be used. Please refer to [link / reference]. Figure 4 The figure is a flowchart illustrating the opening control of a bypass regulating valve according to an embodiment of this application. For example, the controller can be a PID controller.

[0062] like Figure 4 As shown, the sum of the system-set allowable exhaust gas temperature and the flue gas temperature threshold is used as the setpoint for the PID controller, and this is used to achieve feedback control of the bypass regulating valve 11. This generates the second control command output by the PID controller, which adjusts the opening of the bypass regulating valve 11.

[0063] After trial production and operation, the low-grade energy cascade utilization system for coal-fired power generating units provided in the embodiments of this application has the following beneficial technical effects:

[0064] First, the inlet flue gas temperature of the dust collector on the dust removal system side can be reduced to 90℃, the resistivity of flue gas dust decreases, and the efficiency of electrostatic precipitator is effectively improved.

[0065] Second, by reducing the outlet flue gas temperature of the low-temperature economizer 10 to below the acid dew point, more than 80% of the sulfuric acid vapor in the flue gas can be condensed and agglomerated by the dust collector, and the low-temperature corrosion of the dust collector outlet equipment can be reasonably controlled.

[0066] Third, after the exhaust gas temperature is reduced, the volumetric flow rate of the flue gas through the induced draft fan is reduced by 10%, the pressure drop on the flue gas side of the air preheater 8 is reduced, and the power saving effect of the induced draft fan is obvious.

[0067] Fourth, according to calculations, for coal-fired power generating units, the low-grade energy cascade utilization system constructed in this invention can increase the air-side inlet temperature of the air preheater 8 by approximately 100°C, and reduce the standard coal consumption for power generation by 0.5~2.5 g / kW·h under full operating load. Overall, the economic efficiency of the entire plant operation is effectively improved.

[0068] It should be understood that the specific implementation of the functions of the air preheater 8, the primary air heater 91 and the secondary air heater 92, as well as the bypass regulating valve 11 and the condensate recirculation pump 4 described in this embodiment are not the core inventive points of this application. Those skilled in the art can implement them based on existing technology, so they will not be described in detail here.

[0069] Furthermore, the ordinal numbers "first" and "second," etc., used herein are only for describing the composition or structure of the same function in the technical solution. It is understood that the use of the aforementioned ordinal numbers does not constitute a limitation on the understanding of the technical solution for which protection is sought in this application.

[0070] The above are merely preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A low-grade energy cascade utilization system for a coal-fired power generation unit, the coal-fired power generation unit comprising a condenser, a condensate pump, a low-pressure heater, a deaerator, a high-pressure heater, and an air preheater, wherein the condensate pump outputs condensate water that sequentially passes through the low-pressure heater, the deaerator, and the high-pressure heater; characterized in that, The low-grade energy cascade utilization system includes a primary air heater, a secondary air heater, and a low-temperature economizer. The inlet of the primary air heater is connected to the outlet of the low-pressure heater, and the outlet of the primary air heater is connected to the outlet of the condensate pump; the air inlet of the primary air heater is connected to the fan outlet, and the air outlet of the primary air heater is connected to the air inlet of the secondary air heater. The flue gas inlet of the low-temperature economizer is connected to the flue gas outlet of the air preheater, and the flue gas outlet of the low-temperature economizer is connected to the dust removal system side; the water inlet of the low-temperature economizer is connected to the water inlet of the low-pressure heater, and the water outlet of the low-temperature economizer is connected to the water inlet of the secondary air heater. The outlet of the secondary air heater is connected to the outlet of the low-pressure heater, and the air outlet of the secondary air heater is connected to the air inlet of the air preheater.

2. The low-grade energy cascade utilization system for coal-fired power generating units according to claim 1, characterized in that, The low-grade energy cascade utilization system also includes a condensate recirculation pump, and the outlet of the first-stage heater is connected to the outlet of the condensate pump through the condensate recirculation pump.

3. The low-grade energy cascade utilization system for coal-fired power generating units according to claim 2, characterized in that, The condensate recirculation pump is a variable frequency pump.

4. The low-grade energy cascade utilization system for coal-fired power generating units according to claim 3, characterized in that, The inlet of the low-temperature economizer is connected to the inlet of the low-pressure heater.

5. The low-grade energy cascade utilization system for coal-fired power generating units according to claim 4, characterized in that, The low-grade energy cascade utilization system also includes a bypass regulating valve, which is located in the condensate bypass of the low-pressure heater.

6. The low-grade energy cascade utilization system for coal-fired power generating units according to claim 4, characterized in that, The outlet of the secondary air heater is also connected to the outlet of the low-pressure heater.

7. A low-grade energy cascade utilization control method based on the low-grade energy cascade utilization system of the coal-fired power generating unit according to claim 5, characterized in that, The control method includes the following steps: The temperature difference threshold between the inlet temperature of the condensate recirculation pump and the outlet temperature of the condensate pump, and the flue gas temperature threshold at the outlet of the low-temperature economizer are preset. The inlet temperature of the condensate recirculation pump and the outlet temperature of the condensate pump, as well as the flue gas temperature at the outlet of the low-temperature economizer, are collected. Based on the temperature difference threshold between the inlet temperature and the outlet temperature of the condensate recirculation pump, as well as the inlet temperature and the outlet temperature of the condensate recirculation pump, a first control command for adjusting the frequency of the condensate recirculation pump is determined. Based on the flue gas temperature threshold at the outlet of the low-temperature economizer and the flue gas temperature at the outlet of the low-temperature economizer, a second control command is determined to adjust the opening of the bypass regulating valve.

8. The low-grade energy cascade utilization control method according to claim 7, characterized in that, The first control command for determining the frequency of the condensate recirculation pump includes: The sum of the outlet temperature of the condensate pump and the temperature difference threshold is used as the set value of the PID controller, and the first control command is generated based on the feedback of the inlet temperature of the condensate recirculation pump. The corresponding feedforward compensation is obtained by multiplying the flue gas flow rate by the first feedforward coefficient and the opening degree of the bypass regulating valve by the second feedforward coefficient, and the corresponding feedforward signals are generated and superimposed on the first control command to adjust the frequency of the condensate recirculation pump.

9. The low-grade energy cascade utilization control method according to claim 8, characterized in that, The process of generating the first control command based on the inlet temperature feedback of the condensate recirculation pump includes: If the inlet temperature of the condensate recirculation pump is lower than the sum of the outlet temperature of the condensate pump and the temperature difference threshold, a first control command is output to increase the frequency of the condensate recirculation pump.

10. The low-grade energy cascade utilization control method according to any one of claims 7 to 9, characterized in that, The second control command for determining the opening degree of the bypass regulating valve includes: If the current flue gas temperature at the outlet of the low-temperature economizer is higher than the flue gas temperature threshold, a second control command is output to increase the opening of the bypass regulating valve.

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