Boiler waste heat utilization system based on different loads and control strategy

Abstract

The invention relates to the technical field of flue gas waste heat recovery, and discloses a boiler waste heat utilization system based on different loads, which comprises a boiler, an air pre-heater, a secondary air heater and a No.7 low-pressure heater, the bottom of the boiler is connected with an air pre-heater, a bottom inlet of the air pre-heater is connected with a secondary air heater, and the bottom of the secondary air heater is connected with an air feeder; a bottom outlet of the air pre-heater is connected with the low-temperature economizer. An outlet of the low-temperature economizer is connected with the electric precipitator. A water inlet pipeline is arranged on one side of the seventh low-pressure heater, a water outlet pipeline is arranged on the other side of the seventh low-pressure heater, and a water inlet and outlet bypass is arranged between the water inlet pipeline and the water outlet pipeline and used for feeding water to the low-temperature economizer in a mixed mode through an inlet and an outlet of the seventh low-pressure heater. The seventh low-pressure heater is connected with the sixth low-pressure heater through a water outlet pipeline, a heat exchange pipeline is arranged between the low-temperature economizer and the secondary air heater, a conveying pipeline is arranged between the secondary air heater and the sixth low-pressure heater, and an air heater bypass is arranged between the heat exchange pipeline and the conveying pipeline. The low-pressure heater, the low-temperature coal economizer, the air pre-heater and the air heater are connected in series through a condensed water pipeline, and secondary air is heated by recovering waste heat of flue gas passing through the low-temperature coal economizer.

Description

Technical Field

[0001] The present invention relates to the technical field of flue gas waste heat recovery, and in particular to a boiler waste heat utilization system and control strategy based on different loads and different ambient temperatures. Background Art

[0002] With the development of the global and domestic economic, energy and environmental protection situation, coal-fired power plants, especially those with poor economic efficiency, will face more stringent environmental protection requirements and severe market operation situations. Energy conservation and emission reduction have become two binding indicators for the development of coal-fired power generation enterprises. Energy conservation and consumption reduction in thermal power plants are of great social significance for low-carbon development and are also of great significance for improving the market competitiveness of enterprises. The heat loss of boiler exhaust is the biggest factor affecting boiler efficiency. If the exhaust is too high and the flue gas cannot be effectively utilized, it will cause greater energy loss.

[0003] At present, the boiler load capacity of the main units of power plants is 660MW, and most of the newly built units are 1000MW ultra-supercritical boilers. Therefore, in order to solve the problem of waste heat utilization of flue gas of 1000MW boilers, a low-temperature economizer can be installed behind the air preheater, and the waste heat of flue gas can be recovered by linking the heater. The feed water of the high-pressure heater passes through the low-temperature economizer, absorbs the waste heat of flue gas, and then heats the secondary air through the heater, and finally re-enters the steam turbine heat recovery system to improve the cycle efficiency and reduce the coal consumption of power generation. Summary of the invention

[0004] One of the purposes of the present invention is to provide a boiler waste heat utilization system and control strategy for a 1000MW boiler under different loads and different ambient temperatures, so as to solve the problem that the opening of each valve in the existing waste heat system needs manual response during operation.

[0005] The technical solution of the present invention is as follows:

[0006] A boiler waste heat utilization system based on different loads, comprising a boiler, an air preheater, a secondary air heater and a No. 7 low-pressure heater; the bottom of the boiler is connected to the air preheater, the bottom inlet of the air preheater is connected to the secondary air heater, and the bottom of the secondary air heater is connected to the blower; the bottom outlet of the air preheater is connected to a low-temperature economizer, and the outlet of the low-temperature economizer is connected to an electric precipitator;

[0007] A water inlet pipeline is provided on one side of the No. 7 low-pressure heater, and a water outlet pipeline is provided on the other side, and an inlet and outlet bypass is provided between the water inlet pipeline and the water outlet pipeline, which is used for the inlet and outlet mixed water supply of the No. 7 low-pressure heater to the low-temperature economizer; the No. 7 low-pressure heater is connected to the No. 6 low-pressure heater through the water outlet pipeline, a heat exchange pipeline is provided between the low-temperature economizer and the secondary air heater, a delivery pipeline is provided between the secondary air heater and the No. 6 low-pressure heater, and a heater bypass is provided between the heat exchange pipeline and the delivery pipeline;

[0008] The high-temperature flue gas generated by the boiler drops in temperature after heat exchange with the mixed feed water of the No. 7 low-pressure heater in the low-temperature economizer, and then enters the subsequent flue gas system after entering the electrostatic precipitator through a pipeline; the mixed feed water at the inlet and outlet of the No. 7 low-pressure heater is transported to the low-temperature economizer through a pipeline, where it exchanges heat with the flue gas, and the water temperature increases due to the absorbed heat; part of the heated water is transported to the heater for heat exchange with the cold air, and then transported to the No. 6 low-pressure heater, and the other part is transported to the No. 6 low-pressure heater through the heater bypass.

[0009] Furthermore, a No. 7 low-pressure heater inlet bypass regulating valve is provided on the inlet and outlet water bypass on the side close to the water inlet pipeline.

[0010] Furthermore, an outlet regulating valve of low-pressure heater No. 7 is provided on the pipeline between low-pressure heater No. 7 and low-pressure heater No. 6.

[0011] Furthermore, a heater bypass regulating valve is provided on the heater bypass.

[0012] Furthermore, the low-temperature economizer is 6 parallel flue gas heat exchangers, and the flue gas heat exchangers are heat pipes for heat exchange.

[0013] A control strategy for a boiler waste heat utilization system under different loads includes the following steps:

[0014] Step 1: According to the real-time ambient temperature T and the unit load L, determine the opening K1 of the No. 7 low-pressure inlet bypass regulating valve, and obtain the fitting curve 1 by fitting the data points:

[0015] K1=aL+bT+c (1)

[0016] Step 2: According to the real-time ambient temperature T, the unit load L and the set heater outlet secondary air temperature S, the heater bypass regulating valve opening K2 is determined, and the fitting curve 2 is obtained by fitting the data points:

[0017] K2=dL+eT+fS+g (2)

[0018] When the heater bypass regulating valve opening K2 is greater than 100, it indicates that the set heater outlet secondary air temperature S cannot be achieved under the unit load L and ambient temperature T, and the actual secondary air heater outlet air temperature is greater than the set value;

[0019] According to the real-time ambient temperature T, unit load L and the set heater outlet secondary air temperature S, the valve opening of the No. 7 low-pressure heater inlet bypass regulating valve and the heater bypass regulating valve is determined to realize the flow control of the boiler water side itself.

[0020] Step 3: Based on the fitting curves in step 1 and step 2, a strategy for changing the valve opening with the set temperature value is formulated.

[0021] Furthermore, the step 1 is specifically as follows:

[0022] Select 0℃, 5℃, 10℃, 15℃, 20℃, 25℃, 30℃, 35℃ ambient temperature, when the unit load is 1000MW, 800MW, 600MW, 400MW; by changing the opening of the No. 7 low-pressure inlet bypass regulating valve, keep the mixed feed water temperature between 68℃ and 72℃, and record the data points of the opening K1 to obtain the fitting curve 1.

[0023] Furthermore, the step 2 is specifically as follows:

[0024] When the ambient temperature is 0℃, 5℃, 10℃, 15℃, 20℃, 25℃, 30℃ and 35℃ and the unit load is 1000MW, 800MW, 600MW and 400MW, the secondary air temperature at the heater outlet is set to 50℃, 60℃, 70℃ and 80℃; the data points of the heater bypass regulating valve opening K2 are determined through experiments to obtain the fitting curve 2.

[0025] Furthermore, the step 3 is specifically as follows:

[0026] Determine the temperature difference between the set temperature and the original temperature. If the temperature difference between the set temperature and the original temperature is less than 5°C, according to the fitting curves 1 and 2, obtain the new opening K1 of the No. 7 low-pressure inlet bypass regulating valve and the opening K2 of the heater bypass regulating valve.

[0027] If the temperature difference between the set temperature and the original temperature is greater than 5°C, the opening of the No. 7 low-pressure inlet bypass regulating valve K1 and the opening of the heater bypass regulating valve K2 are adjusted step by step at a rate of 5°C, or the temperature is adjusted step by step at a rate of 3°C.

[0028] Compared with the prior art, the beneficial effects of this solution are:

[0029] 1) This system connects the low-pressure heater, low-temperature economizer, air preheater and air heater in series through the condensate pipe, and heats the secondary air with the flue gas waste heat recovery from the low-temperature economizer. This can improve the boiler efficiency and reduce the coal consumption of the unit. At the same time, by increasing the inlet secondary air temperature, the comprehensive temperature of the cold end of the air preheater is increased, achieving the effect of anti-wear and anti-blocking treatment of the air preheater;

[0030] 2) Through fitting curves, the key valve openings, such as the opening of the No. 7 low-pressure inlet bypass regulating valve and the heater bypass regulating valve, are associated with the real-time ambient temperature, unit load, and the set heater outlet secondary air temperature, and corresponding fitting curves are established to solve the problem that the opening of each valve in the existing waste heat system needs to be manually responded to during operation. This can increase the unit response rate, improve boiler efficiency, and make the entire system run more efficiently. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a principle block diagram of an embodiment of a boiler flue gas waste heat recovery system of the present invention;

[0032] In the figure, 1, boiler; 2, air preheater; 3, secondary air heater; 4, No. 6 low-pressure heater; 5, No. 7 low-pressure heater; 6, low-temperature economizer; 7, electrostatic precipitator; 8, heater bypass regulating valve; 9, No. 7 low-pressure heater inlet bypass regulating valve; 10, blower; 11, No. 7 low-pressure heater outlet regulating valve; 12, No. 8 low-pressure heater; 13, No. 5 low-pressure heater. DETAILED DESCRIPTION

[0033] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0034] The present invention has a total of 5, 6, 7, and 8 low-pressure heaters, among which the water temperature gradually increases from the condenser through the 8, 7, 6, and 5 low-pressure heaters in sequence. The outlet temperature of the 8 low-pressure heater, that is, the inlet temperature of the 7 low-pressure heater, is about 44°C, the outlet temperature of the 7 low-pressure heater is about 75°C, the outlet temperature of the 6 low-pressure heater is about 100°C, and the outlet temperature of the 5 low-pressure heater is about 130°C. In order to ensure the heat exchange efficiency of the heater and the reliability of the equipment, the 7 low-pressure heater with an inlet water temperature of 44°C and an outlet water temperature of 75°C is selected, and a bypass regulating valve is added to control the mixed water temperature between 68 and 72°C.

[0035] like Figure 1 As shown, a boiler waste heat utilization system under different loads includes a boiler 1, an air preheater 2 connected to the bottom outlet of the boiler 1, the outlet of the air preheater 2 is connected to the low-temperature economizer 6 through a pipeline, and the flue gas heat is recovered by arranging a heat pipe assembly in the low-temperature economizer 6.

[0036] A No. 7 low-pressure heater inlet bypass regulating valve 9 and a No. 7 low-pressure heater outlet regulating valve 11 are provided at both ends of the No. 7 low-pressure heater 5. The No. 7 low-pressure heater inlet bypass regulating valve 9 is located between the No. 7 low-pressure heater 5 and the low-temperature economizer 6. The outlet end of the low-temperature economizer 6 is connected to the secondary air heater 3 through a heat exchange pipe, and the secondary air heater 3 is connected to a blower 10.

[0037] The outlets of No. 6 low-pressure heater 4 and No. 7 low-pressure heater 5 are connected, the secondary air heater 3 and No. 6 low-pressure heater 4 are connected through a conveying pipeline, a heater bypass is provided between the conveying pipeline and the heat exchange pipeline, and the heater bypass regulating valve 8 is located on the heater bypass on the side of the heater main pipeline.

[0038] The high-temperature flue gas generated by boiler 1 is cooled down after mixing with feed water in low-temperature economizer 6 and low-pressure heater 7 for heat exchange, and then enters the subsequent flue gas system through the pipeline into the electrostatic precipitator 7;

[0039] The mixed feed water at the inlet and outlet of No. 7 low-pressure heater 5 is transported to the low-temperature economizer 6 through a pipeline, where it exchanges heat with the flue gas and transports the absorbed heat to the secondary air heater 3 through the medium (water) to heat the cold air at the inlet of the air preheater;

[0040] By adjusting the heater bypass regulating valve 8, a part of the heated condensed water is transported to the secondary air heater 3, and after heat exchange with the cold air in the secondary air heater 3, it is transported to the No. 6 low-pressure heater 4 through the pipeline; the other part of the heated condensed water is directly transported back to the No. 6 low-pressure heater 4.

[0041] A control strategy for a boiler waste heat utilization system under different loads includes the following steps:

[0042] Step 1, select 0℃, 5℃, 10℃, 15℃, 20℃, 25℃, 30℃, 35℃ ambient temperature, when the unit load is 1000MW, 800MW, 600MW, 400MW, change the opening K1 of the No. 7 low-pressure inlet bypass regulating valve, keep the mixed feed water temperature between 68℃ and 72℃, record the opening, and obtain the regulating valve opening value under various ambient temperatures and loads; determine the data points of the opening K1 of the No. 7 low-pressure inlet bypass regulating valve by experiment to obtain the fitting curve 1;

[0043] When the real-time ambient temperature is 0℃ and the unit load is 1000MW, the opening of the No. 7 low-pressure inlet bypass regulating valve (K1) is opened to 83%;

[0044] When the real-time ambient temperature is 0℃ and the unit load is 750MW, open the No. 7 low-pressure inlet bypass regulating valve (K1) to 60%;

[0045] When the real-time ambient temperature is 0℃ and the unit load is 500MW, the opening of the No. 7 low-pressure inlet bypass regulating valve (K1) is opened to 36%;

[0046] When the real-time ambient temperature is 30°C and the unit load is 1000MW, the opening of the No. 7 low-pressure inlet bypass regulating valve (K1) is opened to 86%;

[0047] When the real-time ambient temperature is 30°C and the unit load is 750MW, the opening of the No. 7 low-pressure inlet bypass regulating valve (K1) is opened to 63%;

[0048] When the real-time ambient temperature is 30°C and the unit load is 500MW, the opening of the No. 7 low-pressure inlet bypass regulating valve (K1) is opened to 40%;

[0049] K1=aL+bT+c (1)

[0050] For the data of this unit, the fitting results are a=0.0938; b=0.086; c=-10.46.

[0051] Step 2, select 0℃, 5℃, 10℃, 15℃, 20℃, 25℃, 30℃, 35℃ ambient temperature, when the unit load is 1000MW, 800MW, 600MW, 400MW, set the heater outlet secondary air temperature to 50℃, 60℃, 70℃, 80℃; by changing the heater bypass regulating valve opening K2, make the heater outlet secondary air temperature close to the set value, and record the opening; determine the data point of the heater bypass regulating valve opening K2 through experiments to obtain fitting curve 2.

[0052] When the set secondary air temperature at the heater outlet is 50°C, the real-time ambient temperature is 0°C and the unit load is 1000MW, the heater bypass regulating valve is opened to 75%;

[0053] When the set secondary air temperature at the heater outlet is 70°C, the real-time ambient temperature is 0°C and the unit load is 1000MW, the heater bypass regulating valve is opened to 63%;

[0054] When the set secondary air temperature at the heater outlet is 50°C, the real-time ambient temperature is 20°C and the unit load is 1000MW, the heater bypass regulating valve is opened to 105% (indicating that the actual secondary air temperature is greater than the set value);

[0055] When the set secondary air temperature at the heater outlet is 70℃, the real-time ambient temperature is 20℃ and the unit load is 1000MW, the heater bypass regulating valve is opened to 93%;

[0056] When the set secondary air temperature at the heater outlet is 50°C, the real-time ambient temperature is 0°C and the unit load is 500MW, the heater bypass regulating valve is opened to 38%;

[0057] When the set secondary air temperature at the heater outlet is 70°C, the real-time ambient temperature is 0°C and the unit load is 500MW, the heater bypass regulating valve is opened to 27%;

[0058] When the set secondary air temperature at the heater outlet is 50°C, the real-time ambient temperature is 20°C and the unit load is 500MW, the heater bypass regulating valve is opened to 68%;

[0059] When the set secondary air temperature at the heater outlet is 70°C, the real-time ambient temperature is 20°C and the unit load is 500MW, the heater bypass regulating valve is opened to 57%;

[0060] K2=dL+eT+fS+g (2)

[0061] For the data of this unit, the fitting results are d=0.0737; e=1.482; f=-0.564; g=29.695.

[0062] When the heater bypass regulating valve opening (K2) is greater than 100, it indicates that the set heater outlet secondary air temperature cannot be reached under the unit load and ambient temperature, and the actual heater outlet air temperature is greater than the set value.

[0063] Step 3: Based on the fitting curves in steps 1 and 2, formulate a strategy for changing the valve opening with the set temperature value.

[0064] The initial temperature value is T1, and the set temperature value is T2. If the temperature change is less than 5°C, that is, the absolute value of the temperature difference between the two is less than 5°C;

[0065] |T1-T2|≤5

[0066] According to the fitting curves 1 and 2, the new opening of the No. 7 low-pressure inlet bypass regulating valve (K1) and the opening of the heater bypass regulating valve (K2) are obtained. The valve opening can be directly changed remotely through the electric head.

[0067] If the temperature change is greater than 5°C, that is, the absolute value of the temperature difference between the two is less than 5°C;

[0068] |T1-T2|>5

[0069] Do the following:

[0070] 1) When the set temperature T2 is higher than the initial temperature T1, first set the temperature to:

[0071] T2=T1+3

[0072] According to fitting curves 1 and 2, the opening of the No. 7 low-pressure inlet bypass regulating valve K1' and the opening of the heater bypass regulating valve K2' are obtained. The valve opening can be directly changed remotely through the electric head. After waiting for the electric head to open in place, based on the set temperature of T2', repeat the above operation at a heating rate of 3℃ until the set temperature is raised to T2.

[0073] 2) When the set temperature T2 is lower than the initial temperature T1, first set the temperature to:

[0074] T2=T1-5

[0075] According to the fitting curves 1 and 2, the opening of the No. 7 low-pressure inlet bypass regulating valve K1" and the opening of the heater bypass regulating valve K2" are obtained. The valve opening can be directly changed remotely through the electric head. After waiting for the electric head to open in place, based on the set temperature of T2, repeat the above operation at a cooling rate of 5℃ until the set temperature is reduced to T2.

[0076] Based on different temperature conditions and typical unit load conditions, the openings of the heater bypass regulating valve and the No. 7 low-pressure heater inlet bypass regulating valve are adjusted differently. This can accurately and evenly transmit heat according to the heat required by the boiler itself, allowing the boiler to utilize the corresponding heat and reduce heat waste. At the same time, it solves the problem of manual response to the opening of each valve in the existing waste heat system during operation.

[0077] The invention is merely illustrative of the principles and applications of the invention. It will therefore be appreciated that many modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the invention as defined by the appended claims. It will be appreciated that the various dependent claims and features described herein may be combined in ways other than as described in the original claims. It will also be appreciated that features described in conjunction with individual embodiments may be used in other described embodiments.

[0078] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. It should therefore be understood that many modifications may be made to the exemplary embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in a manner different from that described in the original claims. It should also be understood that features described in conjunction with individual embodiments may be used in other described embodiments.

Claims

1. A boiler waste heat utilization system based on different loads, characterized in that: It comprises a boiler (1), an air preheater (2), a secondary air heater (3) and a No. 7 low-pressure heater (5); the bottom of the boiler (1) is connected to the air preheater (2), the bottom inlet of the air preheater (2) is connected to the secondary air heater (3), and the bottom of the secondary air heater (3) is connected to a blower (10); the bottom outlet of the air preheater (2) is connected to a low-temperature economizer (6), and the outlet of the low-temperature economizer (6) is connected to an electric precipitator (7); The No. 7 low-pressure heater (5) is provided with a water inlet pipeline on one side and a water outlet pipeline on the other side, and an inlet and outlet bypass is provided between the water inlet pipeline and the water outlet pipeline, which is used for the inlet and outlet mixed water of the No. 7 low-pressure heater (5) to be supplied to the low-temperature economizer (6); the No. 7 low-pressure heater (5) is connected to the No. 6 low-pressure heater (4) through the water outlet pipeline, a heat exchange pipeline is provided between the low-temperature economizer (6) and the secondary air heater (3), a delivery pipeline is provided between the secondary air heater (3) and the No. 6 low-pressure heater (4), and a heater bypass is provided between the heat exchange pipeline and the delivery pipeline; The high-temperature flue gas generated by the boiler (1) is cooled down after heat exchange with the mixed feed water of the No. 7 low-pressure heater (5) in the low-temperature economizer (6), and then enters the subsequent flue gas system after entering the electrostatic precipitator (7) through the pipeline; the mixed feed water of the inlet and outlet of the No. 7 low-pressure heater (5) is transported to the low-temperature economizer (6) through the pipeline, and heat is exchanged with the flue gas in the low-temperature economizer (6), and the water temperature is increased by the absorbed heat; a part of the heated water is transported to the secondary air heater (3) to exchange heat with the cold air, and then transported to the No. 6 low-pressure heater (4), and the other part is transported to the No. 6 low-pressure heater (4) through the heater bypass; The inlet water temperature of the No. 7 low-pressure heater (5) is 44°C, and the outlet water temperature is 75°C. At the same time, the inlet and outlet water bypass controls the mixed water temperature to be between 68 and 72°C.

2. A boiler waste heat utilization system based on different loads according to claim 1, characterized in that: A No. 7 low-pressure heater inlet bypass regulating valve (9) is provided on the water inlet and outlet bypass on the side close to the water inlet pipeline.

3. The boiler waste heat utilization system under different loads according to claim 2 is characterized in that: A No. 7 low-pressure heater outlet regulating valve (11) is provided on the pipeline (4) between the No. 7 low-pressure heater (5) and the No. 6 low-pressure heater.

4. The boiler waste heat utilization system under different loads according to claim 3 is characterized in that: A heater bypass regulating valve (8) is arranged on the heater bypass.

5. The boiler waste heat utilization system based on different loads according to claim 1 is characterized in that: The low-temperature economizer (6) is composed of six flue gas heat exchangers arranged in parallel, and the flue gas heat exchangers are heat pipes for heat exchange.

6. A control strategy for boiler waste heat utilization system under different loads according to claim 4, characterized in that: The steps include: Step 1: According to the real-time ambient temperature T and unit load L, by changing the opening K1 of the No. 7 low-pressure inlet bypass regulating valve, keep the mixed feed water temperature of the inlet and outlet water bypass between 68°C and 72°C, record the opening, and obtain the regulating valve opening value under various ambient temperatures and loads; at the same time, obtain the fitting curve 1 by fitting the data points: K1=aL+bT+c (1) a, b, and c are constants obtained from the fitting curve; Step 2: According to the real-time ambient temperature T and the unit load L, the heater bypass regulating valve opening K2 is changed to make the heater outlet secondary air temperature reach the set value S, and the heater bypass regulating valve opening K2 is recorded. At the same time, the fitting curve 2 is obtained by fitting the data points: K2=dL+eT+fS+g (2) d, e, f, and g are constants obtained from the fitting curve; Step 3: Based on the fitting curves in steps 1 and 2, formulate a strategy for changing the valve opening with the set temperature value.

7. A control strategy for boiler waste heat utilization system under different loads according to claim 6, characterized in that: The step 1 is specifically as follows: When the ambient temperature is selected as 0℃, 5℃, 10℃, 15℃, 20℃, 25℃, 30℃ and 35℃ and the unit load is 1000MW, 800MW, 600MW and 400MW; by changing the opening K1 of the No. 7 low-pressure inlet bypass regulating valve, the mixed feed water temperature is maintained between 68℃ and 72℃, and the opening is recorded to obtain the regulating valve opening value under various ambient temperatures and loads.

8. The control strategy of boiler waste heat utilization system under different loads according to claim 6 is characterized in that: The step 2 is specifically as follows: When the ambient temperature is selected as 0℃, 5℃, 10℃, 15℃, 20℃, 25℃, 30℃ and 35℃ and the unit load is 1000MW, 800MW, 600MW and 400MW, set the secondary air temperature at the heater outlet to 50℃, 60℃, 70℃ and 80℃; by changing the opening K2 of the heater bypass regulating valve, make the secondary air temperature at the heater outlet reach the set value and record the opening.

9. The control strategy of boiler waste heat utilization system under different loads according to claim 6 is characterized in that: In step 2, when the heater bypass regulating valve opening K2 is greater than 100, it indicates that the set heater outlet secondary air temperature S cannot be reached under the unit load L and ambient temperature T, and the actual secondary air heater outlet air temperature is greater than the set value; According to the real-time ambient temperature T, unit load L and the set heater outlet secondary air temperature S, the valve opening of the No. 7 low-pressure heater inlet bypass regulating valve and the heater bypass regulating valve is determined to realize the flow control of the boiler water side itself.

10. The control strategy of boiler waste heat utilization system under different loads according to claim 6, characterized in that: The step 3 is specifically as follows: The initial temperature value is T1, and the set temperature value is T2. If the temperature change is less than 5°C, that is, the absolute value of the temperature difference between the two is less than 5°C; |T1-T2|≤5 According to the fitting curves 1 and 2, the new opening degree K1 of the inlet bypass regulating valve of No. 7 low-pressure heater and the opening degree K2 of the bypass regulating valve of the heater are obtained; If the temperature change is greater than 5°C, that is, the absolute value of the temperature difference between the two is greater than 5°C; |T1-T2|>5 Do the following: 1) When the set temperature T2 is higher than the initial temperature T1, first set the temperature to: T2=T1+3 According to the fitting curves 1 and 2, the opening of the No. 7 low-pressure inlet bypass regulating valve K1' and the opening of the heater bypass regulating valve K2' are obtained. After the electric head opening is in place, based on the set temperature of T2, the above operation is repeated at a heating rate of 3°C until the set temperature is raised to T2; 2) When the set temperature T2 is lower than the initial temperature T1, first set the temperature to: T2=T1-5 According to the fitting curves 1 and 2, the opening degree K1" of the inlet bypass regulating valve of No. 7 low-pressure heater and the opening degree K2" of the heater bypass regulating valve are obtained. After the electric head opening is in place, based on the set temperature of T2, the above operation is repeated at a cooling rate of 5°C until the set temperature is reduced to T2.

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