Low-grade energy gradient utilization system of coal-fired power generation unit and control method

Abstract

The invention discloses a low-grade energy gradient utilization system of a coal-fired generator set and a control method, the low-grade energy gradient utilization system comprises a first-stage air heater, a second-stage air heater and a low-temperature economizer, a water inlet of the first-stage air heater is communicated with a water outlet of a low-pressure heater, and a water outlet of the second-stage air heater is communicated with a water outlet of a low-pressure heater; low-temperature condensed water is used for heating low-temperature air input from the fan side through the first-stage air heater, and a water outlet of the first-stage air heater is communicated with a water outlet of the condensed water pump; an air outlet of the first-stage air heater is communicated with an air inlet of the second-stage air heater, and air is further heated by condensed water through the second-stage air heater and is fed into the air preheater. And a water inlet of the low-temperature coal economizer communicates with a water inlet of the low-pressure heater, condensed water is heated through high-temperature flue gas output by the air preheater side, and air is heated through the high-temperature condensed water. The flue gas waste heat gradient utilization is realized by reasonably arranging the flue gas waste heat exchange temperature area, and the technical guarantee is provided for realizing the full-working-condition safety investment of the system on the basis of reducing the loss in the flue gas waste heat utilization process.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of coal-fired generating units, in particular to a coal-fired generating unit low-grade energy cascade utilization system and control method. BACKGROUND

[0002] In the prior art, the flue gas heat loss is the highest part of the various heat losses of the boiler of the coal-fired generating unit. As known, the most important factor affecting the flue gas heat loss is the boiler flue gas temperature, and the effective utilization of the flue gas waste heat of the coal-fired generating unit has become one of the key directions for improving energy utilization efficiency and promoting the sustainable development of coal-fired power generation technology.

[0003] A typical scheme for reducing the flue gas temperature is to form a low-temperature economizer arranged in front of the dust collector to transfer the flue gas heat to the heat recovery system, thereby increasing the condensate temperature and reducing the corresponding low-temperature heater steam extraction amount, and improving the thermal efficiency of the unit. However, this scheme mainly stays at the existing 90℃-130℃ flue gas utilization level, and fails to realize deep-level cascade utilization of flue gas waste heat.

[0004] In addition, the flue gas waste heat of the coal-fired generating unit under low load conditions is greatly reduced, and the recoverable waste heat energy is limited, and part of the low load conditions does not have system operation conditions. In addition, when the low-temperature economizer is used for flue gas waste heat utilization, the condensate temperature as the heat exchange medium will be reduced synchronously with the reduction of the unit load, and the reduction of the condensate temperature will cause part of the flue gas temperature inside the low-temperature economizer to be lower than the acid dew point, aggravate the low-temperature corrosion, and even endanger the safety of the equipment.

[0005] Therefore, it is urgent to provide an effective solution for recovering low-grade energy for coal-fired generating units to overcome the above-mentioned defects. SUMMARY

[0006] To solve the above technical problems, the present application provides a coal-fired generating unit low-grade energy cascade utilization system and control method, which realizes cascade utilization of flue gas waste heat by reasonably arranging the flue gas waste heat exchange temperature zone, and provides a good technical guarantee for realizing safe operation of the system under all conditions on the basis of reducing the flue gas waste heat utilization process loss.

[0007] The application provides a low-grade energy cascade utilization system of a coal-fired power generating unit, the coal-fired power generating unit comprising a condenser, a condensate pump, a low-pressure heater, a deaerator and an air preheater; the low-grade energy cascade utilization system comprising a first-stage warm air heater, a second-stage warm air heater and a low-temperature coal economizer; a water inlet of the first-stage warm air heater being communicated with a water outlet of the low-pressure heater, and a water outlet of the first-stage warm air heater being communicated with a water outlet of the condensate pump; an air inlet of the first-stage warm air heater being communicated with an outlet of a fan, and an air outlet of the first-stage warm air heater being communicated with an air inlet of the second-stage warm air heater; a flue gas inlet of the low-temperature coal economizer being communicated with a flue gas outlet of the air preheater, and a flue gas outlet of the low-temperature coal economizer being communicated with a dust removal system side; a water inlet of the low-temperature coal economizer being communicated with a water inlet of the low-pressure heater, and a water outlet of the low-temperature coal economizer being communicated with a water inlet of the second-stage warm air heater; a water outlet of the second-stage warm air heater being communicated with a water outlet of the low-pressure heater, and an air outlet of the second-stage warm air heater being communicated with an air inlet of the air preheater.

[0008] Optionally, the low-grade energy cascade utilization system further comprises a condensate recirculation pump, and the water outlet of the first-stage warm air heater is communicated with the water outlet of the condensate pump through the condensate recirculation pump.

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

[0010] Optionally, the water inlet of the low-temperature coal economizer is communicated with the water inlet of the low-pressure heater.

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

[0012] Optionally, the water outlet of the second-stage warm air heater is further communicated with the water inlet of the first-stage warm air heater.

[0013] The application further provides a control method based on the aforementioned low-grade energy cascade utilization system, the control method comprising the following steps: presetting a temperature difference threshold value of an inlet temperature of a condensate recirculation pump and an outlet temperature of a condensate pump, and a flue gas temperature threshold value of an outlet of a low-temperature coal economizer; collecting the inlet temperature of the condensate recirculation pump, the outlet temperature of the condensate pump and the flue gas temperature of the outlet of the low-temperature coal economizer; determining a first control instruction of adjusting a frequency of the condensate recirculation pump according to the temperature difference threshold value of the inlet temperature of the condensate recirculation pump and the outlet temperature of the condensate pump, and the inlet temperature of the condensate recirculation pump and the outlet temperature of the condensate pump; and determining a second control instruction of adjusting an opening degree of a bypass regulating valve according to the flue gas temperature threshold value of the outlet of the low-temperature coal economizer and the flue gas temperature of the outlet of the low-temperature coal 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 the optional scheme of the present application, the water outlet of the secondary air heater is also communicated with the water inlet of the primary air heater. In this way, the condensed water heated and warmed further in the secondary air heater can increase the water inlet temperature of the primary air heater, and after mixing with the condensed water flowing out of the low-pressure heater, the heat exchange capacity of the primary air heater can be effectively improved.

[0020] In another optional scheme of the present application, the low-grade energy cascade utilization system further comprises a condensed water recirculation pump and a bypass regulating valve, and the water outlet of the primary air heater is communicated with the water outlet of the condensed water pump through the condensed water recirculation pump. The water inlet of the low-temperature economizer is communicated with the water inlet of the low-pressure heater, and the bypass regulating valve is arranged in the condensed water bypass of the low-pressure heater. Based on the adjustment and control of the condensed water recirculation pump and the bypass regulating valve, the condensed water is heated by the flue gas at the tail of the boiler, and then the cold air is heated by the condensed water in different temperature intervals, so that the flue gas waste heat energy can be fully recovered, and the system can be put into operation under low load conditions. Under full load conditions, the standard coal consumption of power generation is effectively reduced, and the economic efficiency of the whole plant is improved. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 A schematic diagram of a low-grade energy cascade utilization system of a coal-fired generating unit is provided for the embodiments of the present application;

[0022] Figure 2 A control block diagram of a low-grade energy cascade utilization control method of a coal-fired generating unit is provided for the embodiments of the present application;

[0023] Figure 3 A frequency control flowchart of a condensed water recirculation pump is provided for the embodiments of the present application;

[0024] Figure 4 An opening degree control flowchart of a bypass regulating valve is provided for the embodiments of the present application.

[0025] In the drawings:

[0026] boiler 1, steam turbine 2, high-pressure cylinder 21, medium-pressure cylinder 22, low-pressure cylinder 23, condenser 3, condensed water pump 41, feed water pump 42, condensed water recirculation pump 43,

[0027] low-pressure heater 5, deaerator 6, high-pressure heater 7, air preheater 8,

[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 DESCRIPTION

[0029] In order for those skilled in the art to better understand the technical solutions of the present application, the present application will be further described in detail below in combination with the drawings and specific embodiments.

[0030] With the further promotion of coal-fired power energy saving and emission reduction upgrading technical transformation, the coal consumption level of thermal power plants is continuously reduced. As is known to all, the flue gas heat loss in the active thermal power unit is the highest part of the boiler heat loss of the coal-fired power generating unit, and the main factor affecting the flue gas heat loss is the boiler flue gas temperature. A typical scheme for reducing flue gas temperature is to form a low-temperature economizer arranged in front of the dust collector to transfer the flue gas heat to the heat recovery system, thereby increasing the condensate temperature, reducing the corresponding low-temperature heater steam extraction amount, and improving the thermal efficiency of the unit.

[0031] However, the flue gas waste heat of this scheme mainly stays at the existing 90℃-130℃ flue gas utilization level, and fails to realize the cascade utilization of deep-level flue gas waste heat. In addition, the flue gas waste heat of the coal-fired power generating unit under low load conditions is greatly reduced, and the recoverable waste heat energy is limited, and part of the low load conditions does not have system operation conditions. In addition, when the low-temperature economizer is used for flue gas waste heat utilization, the condensate temperature as the heat exchange medium will be reduced synchronously with the reduction of the unit load, and the reduction of the condensate temperature will cause part of the flue gas temperature in the low-temperature economizer to be lower than the acid dew point, aggravate the low-temperature corrosion, and even endanger the safety of the equipment.

[0032] Based on this, the embodiment of the present application provides a low-grade energy cascade utilization system of a coal-fired power generating unit, which realizes flue gas waste heat cascade utilization and reduces the exergy loss of the flue gas waste heat utilization process through the coupling of the flue gas, air, steam and water whole working medium system, and configures a flue gas waste heat heat exchange temperature zone, thereby further improving the flue gas waste heat utilization capacity while realizing the safe operation of the system under all working conditions. Please refer to Figure 1 , which is a principle schematic diagram of a low-grade energy cascade utilization system of a coal-fired power generating unit provided by the embodiment of the present application.

[0033] The low-grade energy cascade utilization system 100 provided by the embodiment is applied to Figure 1 a supercritical coal-fired power generating unit formed by a boiler 1 and a steam turbine 2 as shown in the figure. Exemplarily, the boiler 1 is a once-reheated, supercritical pressure coal-fired boiler, and the steam turbine 2 is a three-cylinder, three-exhaust, single-shaft, condensing steam turbine.

[0034] Among them, the steam turbine 2 includes a high-pressure cylinder 21, a medium-pressure cylinder 22 and a low-pressure cylinder 23 arranged in sequence, the exhaust port of the low-pressure cylinder 23 is communicated with the steam inlet port of the condenser 3, and the low-pressure steam completing work on the steam turbine 2 side directly enters the condenser 3 along the exhaust pipe and is condensed into condensate water. The condensate pump 41 is arranged on the water outlet pipeline of the condenser 3 and forms a condensate water system with the condensate pump 41. The condensate water output by the condensate pump 41 passes through the low-pressure heater 5, the deaerator 6 and the high-pressure heater 7 in sequence, and is heated by the steam extraction of the high-pressure cylinder 21, the medium-pressure cylinder 22 and the low-pressure cylinder 23 respectively, and is returned to the economizer on the boiler 1 side.

[0035] The high-pressure cylinder 21 can extract steam to the high-pressure heater 7, the medium-pressure cylinder 22 can extract steam to the oxygen remover 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 the condensate water.

[0036] The high-temperature flue gas at the outlet of the economizer on the boiler 1 side exchanges heat with the air input on the air blower side through the air preheater 8, so as to reduce the temperature of the flue gas discharged and increase the temperature of the air delivered to the furnace and the coal pulverizing system on the boiler 1 side.

[0037] In the embodiment, the low-grade energy cascade utilization system 100 includes a first-stage air heater 91, a second-stage air heater 92, and a low-temperature economizer 10. The flue gas cooled through heat exchange in the air preheater 8 is further cooled in the low-temperature economizer 10 and delivered to a dust removal system (not shown in the figure).

[0038] In a specific implementation, the first-stage air heater 91 is a water / air heater, that is, a heat exchanger that heats cold air into hot air using water (condensate water) as a heat source.

[0039] The water inlet of the first-stage air heater 91 is in communication with the water outlet of the low-pressure heater 5, and the water outlet of the first-stage air heater 91 is in communication with the water outlet of the condensate water pump 41 through a condensate water recirculation pump 43, that is, the water outlet of the condensate water recirculation pump 43 is in communication with the water outlet pipeline of the condensate water system. The air inlet of the first-stage air heater 91 is in communication with the outlet of the air blower, and the air outlet of the first-stage air heater 91 is in communication with the air inlet of the second-stage air heater 92. In this way, the high-temperature condensate water taken from the low-pressure heater 5 side is recirculated to the water outlet of the condensate water pump 41 after exchanging heat to the air delivered by the air blower in the first-stage air heater 91. The low-temperature air taken from the air blower side is heated in the first-stage air heater 91 and then enters the next-stage heater (the second-stage air heater 92).

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

[0041] The flue gas inlet of the low-temperature economizer 10 is in communication with the flue gas outlet of the air preheater 8, and the flue gas outlet of the low-temperature economizer 10 is in communication with the dust removal system side. The water inlet of the low-temperature economizer 10 is in communication with the water inlet of the low-pressure heater 5. The water outlet of the low-temperature economizer 10 is in communication with the water inlet of the second-stage air heater 92. In this way, the high-temperature flue gas taken from the air preheater 8 side exchanges heat with the condensate water in the low-temperature economizer 10, and the low-temperature flue gas after being cooled enters the dust removal system. The low-temperature condensate water taken from the condensate water bypass exchanges heat with the high-temperature flue gas in the low-temperature economizer 10, and the condensate water after being heated enters the second-stage air heater 92.

[0042] In order to adjust the low-temperature condensate water flowing into the low-temperature economizer 10 to meet the actual needs of the operation condition of the thermal system, the low-grade energy cascade utilization system 100 can be additionally provided with a bypass regulating valve 11, which can be arranged at the condensate water bypass of the low-pressure heater 5 to adjust the flow of the low-temperature condensate water flowing into the low-temperature economizer 10 according to needs.

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

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

[0045] Further, in order to improve the heat exchange capacity of the primary air heater 91, the water outlet of the secondary air heater 92 is also in communication with the water inlet of the primary air heater 91, that is, the water outlet of the secondary air heater 92 is in synchronous communication with the water outlet of the low-pressure heater 5 and the water inlet of the primary air heater 91. In this way, the condensate water further heated and warmed in the secondary air heater 92 can improve the water inlet temperature of the primary air heater 91, and after mixing with the condensate water flowing out of the low-pressure heater 5, the heat exchange capacity of the primary air heater 91 can be effectively improved.

[0046] Based on the low-grade energy cascade comprehensive utilization system provided in the embodiment, the air sucked by the fan is heated in the primary air heater 91 and the secondary air heater 92 in sequence and then enters the air preheater 8. After two-stage preheating, the air temperature entering the air preheater 8 can be greatly improved, and the consumption of the air preheater 8 for high-temperature flue gas can be reduced. At the same time, the flue gas after the outlet of the air preheater 8 exchanges heat with the condensate water bypass of the low-temperature economizer 10. Since the condensate water temperature of the condensate water bypass is relatively low, the flue gas temperature will be significantly reduced, which can effectively ensure that the flue gas waste heat energy is effectively utilized and reduced to a reasonable temperature before entering the unit dust removal system.

[0047] The flue gas discharged from the boiler tail passes through the low-temperature economizer 10 to heat the condensate, the condensate heats the air through the secondary warm air heater 92, the condensate flowing out of the secondary warm air heater 92 can be further mixed with the condensate flowing out of the low-pressure heater 5 through the condensate bypass, and then the mixed condensate is heated by the primary warm air heater 91 to heat the low-temperature air flowing into the air fan side, and the mixed condensate is flowed 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, the basic principle of energy saving of "temperature matching and gradient utilization" can be followed to realize the gradient comprehensive utilization of the flue gas waste heat. In addition, based on the effective conversion of the heat on the flue gas side to the condensate side, the extraction of the high-pressure cylinder 21 and the medium-pressure cylinder 22 of the steam turbine can be further reduced, the coal consumption can be reduced, and a good technical guarantee for improving the overall economy of the unit is provided.

[0048] Based on the foregoing low-grade energy cascade utilization system of the coal-fired power generation unit, the embodiment of the present application further provides a low-grade energy cascade utilization control method of the coal-fired power generation unit. Please refer to Figure 2 , which is a control block diagram of a low-grade energy cascade utilization control method of a coal-fired power generation unit provided by the embodiment of the present application. The low-grade energy cascade utilization control method of the coal-fired power generation unit comprises the following steps:

[0049] Step S201, presetting the temperature difference threshold of the inlet temperature of the condensate recirculation pump 43 and the outlet temperature of the condensate pump 41, and the flue gas temperature threshold at the outlet of the low-temperature economizer 10;

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

[0051] Step S203, determining the first control instruction for adjusting the frequency of the condensate recirculation pump 43 according to the temperature difference threshold of 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; determining the second control instruction for adjusting the opening of the bypass adjusting valve 11 according to the flue gas temperature at the outlet of the low-temperature economizer 10 and the flue gas temperature threshold.

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

[0053] Exemplarily, the temperature difference threshold of the inlet temperature of the condensate water recirculation pump 43 and the outlet temperature of the condensate water pump 41 is 5℃, that is, the inlet temperature of the condensate water recirculation pump 43 is set to be 5℃ higher than the outlet temperature of the condensate water pump 41. If the inlet temperature of the condensate water recirculation pump 43 is lower than the sum of the outlet temperature of the condensate water pump 41 and the temperature difference threshold, the first control instruction for increasing the frequency of the condensate water recirculation pump 43 is output, that is, the current temperature difference between the inlet temperature of the condensate water recirculation pump 43 and the outlet temperature of the condensate water pump 41 is lower than the temperature difference threshold, and the frequency of the condensate water recirculation pump 43 is increased to sufficiently utilize the temperature of the condensate water to heat the air sent by the fan.

[0054] Exemplarily, the flue gas temperature threshold at the outlet of the low-temperature economizer 10 is 90℃, that is, the flue gas temperature at the outlet of the low-temperature economizer 10 is set to be 90℃. If the current flue gas temperature at the outlet of the low-temperature economizer 10 is 95℃, which is higher than the flue gas temperature threshold 90℃, the second control instruction for increasing the opening of the bypass adjusting valve 11 is output, that is, the flue gas temperature at the outlet of the low-temperature economizer 10 is higher than the flue gas temperature threshold, and the opening of the bypass adjusting valve 11 is increased to sufficiently utilize the waste heat of the flue gas and the heat exchange capacity of the low-temperature economizer 10 to heat the air sent by the fan.

[0055] On the basis of the conventional equipment control of the unit, the condensate water recirculation pump 43 and the bypass adjusting valve 11 are adjusted based on the low-grade energy cascade utilization control method of the coal-fired generating unit, the condensate water heated by the flue gas at the tail of the boiler is utilized, and the cold air is cascade heated by the condensate water in different temperature intervals to meet the operation requirements of the full-load operation.

[0056] For the frequency control of the condensate water recirculation pump 43, the feedback and feedforward combination mode can be adopted based on the inlet temperature of the condensate water recirculation pump. Please refer to Figure 3 , which is a frequency control flowchart of a condensate water recirculation pump provided in the embodiment of the present application. Exemplarily, the controller can be a PID (Proportional-Integral-Derivative) controller. In other possible implementation schemes, the controller can also adopt other types of controllers, which can be determined according to the overall design requirements of the system, and the embodiment of the present application is not limited.

[0057] As shown in 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 production commissioning, the low-grade energy cascade utilization system of the coal-fired generating unit provided in the embodiments of the present application has the following beneficial technical effects:

[0064] First, the dust collector inlet flue gas temperature on the dust removal system side can be reduced to 90 DEG C, the flue gas dust specific resistance is reduced, and the electric dust removal efficiency is effectively improved.

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

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

[0067] Fourth, according to the calculation, for the coal-fired generating unit, the low-grade energy cascade utilization system constructed by the present application can increase the air side inlet temperature of the air preheater 8 by about 100 DEG C, and can reduce the standard coal consumption of power generation by 0.5~2.5g / kW·h under full load conditions. Overall, the overall plant operation economy is effectively improved.

[0068] It should be understood that the specific implementation of the air preheater 8, the first and second warm air heaters 91 and 92, and the bypass regulating valve 11 and the condensate recirculation pump 4 described in the embodiments, is not the core of the present application. The skilled person can implement it based on the prior art, so it will not be described again.

[0069] In addition, the ordinal numbers "first" and "second" used in this paper are only used to describe the same function or structure in the technical scheme. It can be understood that the use of the ordinal numbers does not constitute a limitation on the technical scheme claimed by the present application.

[0070] The above is only the preferred embodiment of the present application, and it should be pointed out that for those skilled in the art, without departing from the principle of the present application, a number of improvements and refinements can be made, which should be considered as the protection scope of the present application.

Claims

1. A low-grade energy cascade utilization system of a coal-fired power generating unit, the coal-fired power generating unit comprising a condenser, a condensate pump, a low-pressure heater, a deaerator, and an air preheater; characterized in that, The low-grade energy cascade utilization system comprises a first-stage air heater, a second-stage air heater and a low-temperature economizer; The water inlet of the first-stage air heater is communicated with the water outlet of the low-pressure heater, and the water outlet of the first-stage air heater is communicated with the water outlet of the condensate pump; the air inlet of the first-stage air heater is communicated with the outlet of the fan, and the air outlet of the first-stage air heater is communicated with the air inlet of the second-stage air heater; The flue gas inlet of the low-temperature economizer is communicated with the flue gas outlet of the air preheater, and the flue gas outlet of the low-temperature economizer is communicated with the dust removal system side; the water inlet of the low-temperature economizer is communicated with the water inlet of the low-pressure heater, and the water outlet of the low-temperature economizer is communicated with the water inlet of the second-stage air heater; The water outlet of the second-stage air heater is communicated with the water outlet of the low-pressure heater, and the air outlet of the second-stage air heater is communicated with the air inlet of the air preheater.

2. The low-grade energy cascade utilization system of a coal-fired power generating unit according to claim 1, characterized in that, The low-grade energy cascade utilization system further comprises a condensate recirculation pump, and the water outlet of the first-stage air heater is communicated with the water outlet of the condensate pump through the condensate recirculation pump.

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

4. The low-grade energy cascade utilization system of a coal-fired power generating unit according to any one of claims 1 to 3, characterized in that, The water inlet of the low-temperature economizer is communicated with the water inlet of the low-pressure heater.

5. The low-grade energy cascade utilization system of a coal-fired power generating unit according to claim 4, characterized in that, The low-grade energy cascade utilization system further comprises a bypass regulating valve arranged in the condensate bypass of the low-pressure heater.

6. The low-grade energy cascade utilization system of a coal-fired power generating unit according to claim 4, characterized in that, The water outlet of the second-stage air heater is further communicated with the water outlet of the low-pressure heater.

7. The low-grade energy cascade utilization control method for the low-grade energy cascade utilization system of the coal-fired power generating unit according to any one of claims 1 to 6, characterized by, The control method comprises the following steps: presetting a temperature difference threshold value of the inlet temperature of the condensate recirculation pump and the outlet temperature of the condensate pump and a flue gas temperature threshold value of the outlet of the low-temperature economizer; collecting the inlet temperature of the condensate recirculation pump, the outlet temperature of the condensate pump and the flue gas temperature of the outlet of the low-temperature economizer; determining a first control instruction for adjusting the frequency of the condensate recirculation pump according to the temperature difference threshold value of the inlet temperature of the condensate recirculation pump and the outlet temperature of the condensate pump and the inlet temperature of the condensate recirculation pump and the outlet temperature of the condensate pump; determining a second control instruction for adjusting the opening degree of the bypass regulating valve according to the flue gas temperature threshold value of the outlet of the low-temperature economizer and the flue gas temperature of the outlet of the low-temperature economizer.

8. The low-grade energy cascade utilization control method according to claim 7, characterized by, The determination of the first control instruction for adjusting the frequency of the condensate recirculation pump comprises: taking the sum of the outlet temperature of the condensate pump and the temperature difference threshold value as the set value of a PID controller, and forming the first control instruction according to the feedback of the inlet temperature of the condensate recirculation pump; and multiplying the flue gas flow by a first feedforward coefficient and multiplying the opening degree of the bypass regulating valve by a second feedforward coefficient to obtain corresponding feedforward compensation amounts, respectively, and forming corresponding feedforward signals and superimposing them on the first control instruction to adjust the frequency of the condensate recirculation pump.

9. The low-grade energy cascade utilization control method according to claim 8, characterized by, The formation of the first control instruction according to the feedback of the inlet temperature of the condensate recirculation pump comprises: 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 value, outputting the first control instruction for increasing 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 by, The second control command for adjusting 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, output a second control command for increasing the opening degree of the bypass regulating valve.

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