Flue gas condensation waste heat recovery system and control method

Abstract

The invention discloses a flue gas condensation waste heat recovery system and a control method, the system comprises a boiler, an air duct, a waste heat recovery flue, a flue gas and air mixer and a chimney, an outlet of the boiler is connected with an inlet of the waste heat recovery flue through a flue, an outlet of the waste heat recovery flue is connected with a wet flue, an outlet of the wet flue is connected with the flue gas and air mixer, and the flue gas and air mixer is connected with the chimney. An outlet of the flue gas and air mixer is connected with a chimney through a dry flue; the air duct is communicated with the waste heat recovery flue and connected with a high-temperature air pre-heater through a mixed air duct, and an outlet of the high-temperature air pre-heater is connected with the flue gas and air mixer through a high-temperature air duct. The hot flue gas is used for indirectly heating air, the hot air is mixed with the wet flue gas after condensation and heat release, the water vapor concentration of the mixed flue gas and the corresponding saturated vapor temperature are reduced, and the temperature of the mixed flue gas is ensured to be higher than the water vapor concentration of the mixed flue gas and the corresponding saturated vapor temperature by controlling the temperature and flow of the hot air; and the problem of corrosion of wet flue gas to equipment is solved.

Description

Technical Field

[0001] The invention belongs to the field of waste heat recovery and relates to a flue gas condensation waste heat recovery system and a control method. Background Art

[0002] During the fuel combustion process, fuels with high moisture content and rich hydrogen content, or the injection of ammonia and desulfurization slurry into the flue gas to control air pollution often lead to the generation of water vapor in the flue gas. These water vapors contain high enthalpy values, including latent heat of vaporization, and are discharged into the atmosphere with the flue gas, resulting in significant energy waste. In order to solve this problem, a low-temperature cooling medium is usually used to absorb heat from the flue gas, thereby reducing the flue gas temperature to reach the dew point temperature, so that the water vapor in the flue gas condenses into water and releases the latent heat of vaporization. This process not only improves the thermal efficiency, but also uses the increased cooling medium temperature to heat boiler feed water, condensed water in steam power generation, air required for fuel combustion, and heat sources for regional heating and industrial production, ultimately achieving reduced energy consumption and reduced emissions of carbon dioxide and other air pollutants.

[0003] However, the flue gas after condensation waste heat recovery is in a saturated state, and condensed water will precipitate during the subsequent cooling process, which may cause serious corrosion problems in the flue, fan and chimney, thus threatening safe production. If anti-corrosion measures are taken for these equipment, the investment cost will increase significantly, and the installation technology and cost will also increase accordingly. When using resin or glass flakes as anti-corrosion materials, potential fire risks must also be considered, which may result in casualties and property losses. In addition, despite the implementation of anti-corrosion treatment, the equipment is still facing serious corrosion because the problem of flue gas condensate water precipitation has not been fundamentally solved, which in turn increases the cost of inspection and maintenance. Therefore, how to effectively deal with the corrosion problem in flue gas condensation waste heat recovery is still a technical problem that needs to be solved urgently in this field. Summary of the invention

[0004] The object of the present invention is to provide a flue gas condensation waste heat recovery system and a control method, which can solve the corrosion problem existing in the flue gas condensation waste heat recovery and the resulting cost problem and safety risk problem.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] A flue gas condensation waste heat recovery system comprises a boiler, an air duct, a waste heat recovery flue, a smoke-air mixer and a chimney, wherein the outlet of the boiler is connected to the inlet of the waste heat recovery flue through the flue, the outlet of the waste heat recovery flue is connected to the wet flue, a smoke measuring system for measuring the wet flue gas data at the outlet of the waste heat recovery flue is arranged on the wet flue, the outlet of the wet flue is connected to the smoke-air mixer, the outlet of the smoke-air mixer is connected to the chimney through a dry flue, a dry flue thermometer and an induced draft fan are arranged on the dry flue; the air duct is communicated with the waste heat recovery flue, and is connected to a high-temperature air preheater through a mixing air duct, and the outlet of the high-temperature air preheater is connected to the smoke-air mixer through the high-temperature air duct.

[0007] Furthermore, a medium-temperature air preheater, a high-temperature condenser, a low-temperature air preheater and a low-temperature condenser are sequentially arranged in the waste heat recovery flue along the flue gas flow direction; the inlet of the low-temperature condenser is connected to a condensation water pipe, and the outlet is connected to the high-temperature condenser through a condenser connecting pipe, and the outlet of the high-temperature condenser is connected to a condenser water outlet pipe; the air duct is connected to the low-temperature air preheater and the medium-temperature air preheater through a low-temperature air duct and a medium-temperature air duct respectively; a low-temperature air gate is arranged on the low-temperature air duct, and a medium-temperature air gate is arranged on the medium-temperature air duct; the low-temperature air duct and the medium-temperature air duct merge into a mixing air duct and are connected to the high-temperature air preheater; the inlet of the high-temperature air preheater is connected to a high-temperature heat source pipe provided with a control valve, the outlet of the high-temperature air preheater is connected to the smoke-air mixer through the high-temperature air duct, and a high-temperature air duct thermometer is arranged on the high-temperature air duct.

[0008] Furthermore, the air duct inlet is connected to a blower and an air flow meter, a hygrometer and an air duct thermometer are arranged on the air duct.

[0009] Furthermore, the boiler is provided with a fuel inlet and a working fluid outlet pipe, the fuel inlet is provided with a fuel analysis system, and the working fluid outlet pipe is provided with a device for real-time monitoring of the boiler load L B Boiler load meter.

[0010] The present invention also provides a control method for a flue gas condensation waste heat recovery system, which controls the target air volume V in the air duct. A , so that: V AIM-A =COE A ×L B , where V AIM-A is the target air flow rate, in Nm 3 / h; L B is the load of the boiler (100), in MW; COE A is the air volume boiler load factor, in Nm 3 / (h×MW), is the target air volume V A With boiler load L B The ratio range is 100~300Nm 3 / (h×MW); adjust the load of the blower during operation so that the air flow rate V measured by the air flow meter is T-A =V AIM-A ; Adjust the test temperature T of dry flue gas in the dry flue T-DG , so that the dry flue gas temperature T measured by the dry flue thermometer T-DG =T AIM-DG , where dry flue gas temperature T T-DG Measured by high temperature air duct thermometer, unit is ℃, T AIM-DG is the target temperature of dry flue gas, in °C.

[0011] Furthermore, the target temperature value T of the dry flue gas is AIM-DG Determined by the following formula:

[0012] T AIM-DG =T S-DG +T SH-DG

[0013] Among them, T S-DG is the saturation temperature corresponding to the water vapor concentration in the dry flue gas, in °C; T SH-DG is the superheat of water vapor in dry flue gas, that is, the target temperature value of water vapor T AIM-DG The saturation temperature T corresponding to the water vapor concentration in the dry flue gas S-DG The higher temperature value, the control range of this value is 1 to 10℃;

[0014] The saturation temperature of water vapor in dry flue gas is T S-DG Determined by fitting the following polynomial based on the water vapor concentration in dry flue gas:

[0015]

[0016] Among them, W DG is the water vapor concentration in dry flue gas, in %; T s0-DG , T s1-DG , T s2-DG , T s3-DG , T s4-DG , T s5-DG and T s6-DG are the corresponding coefficients in the polynomial, and their ranges are: (-6.5 to -7.0), (17.0 to 17.5), (-3.3 to -3.5), (0.42 to 0.45), (-3.1 to -3.5)×10 -2 , (1.0 to 1.2) × 10 -3 and (1.5 to 2.0)×10 -5 ;

[0017] The concentration of water vapor in dry flue gas is W DGDetermined according to the amount of water vapor contained in the air in the air duct, the amount of water vapor contained in the wet flue gas in the wet flue, and the corresponding amount of air and wet flue gas:

[0018]

[0019] Among them, M w-A is the amount of water vapor brought into the dry flue gas along with the air in the air duct, Nm 3 / h; M w-SG is the amount of water vapor in the wet flue gas, Nm 3 / h; V A-T is the actual air flow in the air duct, measured by an air flow meter, in Nm 3 / h; V SG It is the wet flue gas flow rate in the wet flue, measured by the flue gas measurement system, in Nm 3 / h;

[0020] The amount of water vapor M carried into the dry flue gas along with the air in the air duct w-A Determined by the following formula:

[0021] M w-A =V T-A ×W A

[0022] Among them, V T-A The actual air volume in the air duct is measured by an air flow meter in Nm 3 / h; W A is the volume concentration of water vapor in the air, in %;

[0023] The volume concentration of water vapor in the air is W A Determined by the following formula:

[0024] W A =W S-A ×RH A ÷100

[0025] Among them, W S-A is the saturated vapor concentration of air, in %; RH A is the relative humidity of the air, measured by a hygrometer, in %;

[0026] Saturated vapor concentration of air W S-A Determined according to the polynomial fitting formula of air temperature:

[0027]

[0028] Among them, T CA is the cold air temperature at the blower inlet, measured by the duct thermometer; W s0-A , W s1-A , Ws2-A , W s3-A , W s4-A , W s5-A Calculate the air saturated vapor concentration W S-A The coefficients of the polynomials are in the range of (0.6 to 0.65), (0.04 to 0.05), (1.4 to 1.5)×10 -3 , (2.8 to 2.9)×10 -5 , (2.5 to 2.6) × 10 -7 and (2.8 to 2.9)×10 -9 ;

[0029] The amount of water vapor in the wet flue gas M w-SG Determined by the following formula:

[0030] M w-SG =V SG ×W SG

[0031] Among them, V SG It is the wet flue gas temperature in the wet flue, measured by the flue gas measurement system, in Nm 3 / h; W SG It is the concentration of water vapor contained in the wet flue gas in the wet flue, in Nm 3 / h;

[0032] The concentration of water vapor in the wet flue gas is W SG Determined by polynomial fitting of flue gas temperature:

[0033]

[0034] Among them, T SG is the wet flue gas temperature in the wet flue (450), measured by the flue gas measurement system (455), in °C; W s0-SG , W s1-SG , W s2-SG , W s3-SG , W s4-SG , W s5-SG The water vapor concentration W in the wet flue gas is calculated respectively SG The coefficients of the polynomial, in which the water vapor contained reaches saturation, are in the range of (0.6 to 0.65), (0.04 to 0.05), (1.4 to 1.5)×10 -3 , (2.8 to 2.9)×10 -5 , (2.5 to 2.6) × 10 -7 and (2.8 to 2.9)×10 -9 .

[0035] Furthermore, the test temperature T of the dry flue gas in the dry flue isT-DG There are two types of adjustments:

[0036] If the temperature value of dry flue gas test is T T-DG T AIM-DG =T S-DG +T SH-DG Calculated dry flue gas target temperature value T AIM-DG If the temperature is low, the flue gas condensation waste heat recovery system needs to be adjusted to increase the test temperature T of the dry flue gas. T-DG ; If the dry flue gas test temperature value T T-DG T AIM-DG =T S-DG +T SH-DG Calculated dry flue gas target temperature value T AIM-DG If the temperature is too high, the flue gas condensation waste heat recovery system needs to be adjusted to reduce the test temperature T of the dry flue gas. T-DG .

[0037] Furthermore, the test temperature T of the dry flue gas is increased T-DG , the specific adjustment methods are as follows:

[0038] Step 1: Ensure that the control valve on the high-temperature heat source pipe and the medium-temperature damper set on the medium-temperature air duct remain closed, and the blower is in operation, then gradually increase the opening of the low-temperature damper, and at the same time, the dry flue gas temperature value T measured by the dry flue thermometer T-DG The target temperature of dry flue gas is T AIM-DG For comparison, the air flow rate V measured by the air flow meter T-A and target air flow V AIM-A Perform the following operations based on the comparison results:

[0039] (a) If V T-A Less than V AIM-A , and T T-DG Less than T AIM-DG , then further increase the low temperature damper opening;

[0040] (b) If V T-A Less than V AIM-A , T T-DG In (0.99~1.01)×T AIM-DG If the temperature is within the range, the low temperature damper opening is maintained unchanged;

[0041] (c) If V T-A Equal to V AIM-A , and T T-DG Less than T AIM-DG , start step 2;

[0042] Step 2: Open the medium temperature damper and gradually increase the opening to increase the amount of air flowing through the medium temperature air preheater, while reducing the opening of the low temperature damper. In this process, maintain the air volume V T-A unchanged and equal to the target air volume V AIM-A In addition, the dry flue gas temperature value T T-DG The target temperature of dry flue gas is T AIM-DG Compare; judge the opening of the low-temperature damper, and perform the following different operations according to different comparison and judgment results:

[0043] (a) If the low temperature damper has not reached the closed state and T T-DG Less than T AIM-DG , then further increase the opening of the medium temperature damper and reduce the opening of the low temperature damper;

[0044] (b) If the low temperature damper does not reach the closed state, T T-DG In (0.99~1.01)×T AIM-DG Within the range, keep the opening of the medium temperature damper and the low temperature damper unchanged;

[0045] (c) If the low temperature damper reaches the closed state and T T-DG Less than T AIM-DG , start step 3;

[0046] Step 3: Keep the low temperature damper closed and the medium temperature damper open to maintain the air flow V in the duct. T-A unchanged and equal to the target air volume V AIM-A , open the control valve to use the high temperature air preheater to further heat the hot air from the medium temperature air preheater, gradually increase the opening of the control valve, and adjust the dry flue gas temperature value T measured by the dry flue thermometer. T-DG The target temperature of dry flue gas is T AIM-DG Perform the following operations based on the comparison results:

[0047] (a) If T T-DG Less than T AIM-DG , then further increase the opening of the control valve;

[0048] (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, maintain the control valve opening unchanged.

[0049] Furthermore, the test temperature T of the dry flue gas is reduced T-DG , the specific adjustment methods are as follows:

[0050] In the first case, the control valve is in the open state and the high-temperature air preheater is in the state of heating the air. The following operations are required:

[0051] Step 1: Reduce the opening of the control valve and adjust the dry flue gas temperature T measured by the dry flue thermometer. T-DG The target temperature of dry flue gas is T AIM-FG Perform the following operations based on the comparison results:

[0052] (a) If T T-DG Greater than T AIM-DG , then further reduce the opening of the control valve;

[0053] (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, maintain the control valve opening unchanged;

[0054] (c) If the control valve reaches the closed state, T T-DG Still greater than T AIM-DG , maintain the control valve in a closed state and proceed to step 2;

[0055] Step 2: Reduce the opening of the medium temperature damper, increase the opening of the low temperature damper, and maintain the air volume V T-A unchanged and equal to the target air volume V AIM-A During this adjustment process, the following comparisons and operations are performed:

[0056] (a) If T T-DG Greater than T AIM-DG , further reduce the opening of the medium-temperature air door of the medium-temperature air preheater, and increase the opening of the low-temperature air door of the low-temperature air preheater;

[0057] (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, maintain the opening of the medium temperature damper unchanged, and maintain the opening of the low temperature damper unchanged;

[0058] (c) If T T-DG Greater than T AIM-DG , the medium temperature damper reaches the closed state, and the third step is performed;

[0059] The third step is to keep the medium temperature damper closed, gradually reduce the opening of the low temperature damper, and perform the following comparisons and operations:

[0060] (a) If T T-DG Greater than T AIM-DG , reduce the opening of the low-temperature damper;

[0061] (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, maintain the opening of the low-temperature damper unchanged;

[0062] In the second case, if the control valve is in the closed state, the high-temperature air preheater is in the shutdown state, and the low-temperature damper is in the closed state, perform the following operations:

[0063] Step 1: Reduce the opening of the medium temperature air door of the medium temperature air preheater, increase the opening of the low temperature air door of the low temperature air preheater, and maintain the air volume V T-A unchanged and equal to the target air volume V AIM-A During this adjustment process, the following comparisons and operations are performed:

[0064] (a) If T T-DG Greater than T AIM-DG , further reduce the opening of the medium-temperature air door of the medium-temperature air preheater, and increase the opening of the low-temperature air door of the low-temperature air preheater;

[0065] (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, maintain the opening of the medium temperature damper unchanged, and maintain the opening of the low temperature damper unchanged;

[0066] (c) If T T-DG Greater than T AIM-DG , the medium temperature damper reaches the closed state, and the second step is performed;

[0067] Step 2: Keep the medium temperature damper closed, gradually reduce the opening of the low temperature damper, and make the following judgments, comparisons, and operations:

[0068] (a) If T T-DG Greater than T AIM-DG , reduce the opening of the low-temperature damper;

[0069] (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, maintain the opening of the low-temperature damper unchanged;

[0070] In the third case, if the control valve is in the closed state, the high-temperature air preheater is in the shutdown state, and the medium-temperature damper is in the closed state, perform the following operations:

[0071] Keep the medium temperature damper closed and gradually reduce the opening of the low temperature damper. Perform the following comparisons and operations:

[0072] (a) If T T-DG Greater than T AIM-DG , reduce the opening of the low-temperature damper;

[0073] (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, maintain the opening of the low temperature damper unchanged.

[0074] Compared with the prior art, the present invention has the following beneficial effects:

[0075] The flue gas condensation waste heat recovery system proposed by the present invention is that when the flue gas flows through the condensation system, water vapor will encounter a cooling medium, thereby condensing to form water droplets, releasing a large amount of latent heat. This part of latent heat can be used to heat the air entering the system. With the help of indirect heating of the hot flue gas, the temperature of the heated air is effectively increased. This process reduces energy loss and makes the system more efficient. The hot air is mixed with the condensed wet flue gas. During the mixing process, the water vapor concentration in the wet flue gas will decrease. By adjusting the temperature and flow rate of the hot air, it can be ensured that the temperature of the mixed flue gas is always higher than the saturated steam temperature corresponding to the mixed gas. This control method not only improves the energy recovery efficiency, but also avoids the condensation of water in the wet flue gas due to the low temperature, further reducing the water vapor concentration in the mixed flue gas.

[0076] The present invention effectively solves the corrosion problem of wet flue gas after flue gas condensation waste heat recovery on subsequent flue ducts, valves, induced draft fans and chimneys. By controlling the temperature of the mixed flue gas to keep it within a reasonable range, the risk of corrosion can be significantly reduced, thereby extending the service life of the equipment and reducing maintenance costs. It not only improves the energy efficiency of the flue gas condensation waste heat recovery system and reduces pollutant emissions, but also optimizes the overall operating efficiency by reducing the risk of corrosion. BRIEF DESCRIPTION OF THE DRAWINGS

[0077] Figure 1 It is a schematic diagram of the process flow of the present invention;

[0078] In the figure: 100- boiler; 110- working medium outlet pipe; 115- boiler load meter; 120- fuel inlet; 125- fuel analysis system; 200- air duct; 205- air flow meter; 206- hygrometer; 207- air duct thermometer; 210- blower; 220- low temperature air preheater; 222- low temperature damper; 224- low temperature air duct; 230- medium temperature air preheater; 232- medium temperature damper; 234- medium temperature air duct; 240- mixing air duct; 250- high temperature air preheater; 2 53-high-temperature heat source pipe; 255-control valve; 260-high-temperature air duct; 270-high-temperature air duct thermometer; 300-condensate pipe; 310-low-temperature condenser; 320-condenser connecting pipe; 330-high-temperature condenser; 340-condenser outlet pipe; 400-flue; 410-waste heat recovery flue; 420-induced draft fan; 430-dry flue; 435-dry flue thermometer; 440-smoke-air mixer; 450-wet flue; 455-flue gas measurement system; 500-chimney. DETAILED DESCRIPTION

[0079] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0080] like Figure 1 As shown, a flue gas condensation waste heat recovery system described in this embodiment includes a boiler 100, an air duct 200, a waste heat recovery flue 410, a smoke-air mixer 440, and a chimney 500. The outlet of the boiler 100 is connected to the inlet of the waste heat recovery flue 410 through the flue 400, and the outlet of the waste heat recovery flue 410 is connected to the wet flue 450. The wet flue 450 is provided with a flue gas measurement system 455 for measuring the wet flue gas data at the outlet of the waste heat recovery flue 410, including the wet flue gas flow rate V LNG , wet flue gas temperature T LNG , wet flue gas oxygen content O2 LNG , wet flue gas water vapor concentration H2O LNG and wet flue gas sulfur dioxide concentration SO2 LNG The outlet of the wet flue 450 is connected to the smoke-air mixer 440 , and the outlet of the smoke-air mixer 440 is connected to the chimney 500 through the dry flue 430 , on which a dry flue thermometer 435 and an induced draft fan 420 are provided.

[0081] The air duct 200 is communicated with the waste heat recovery flue duct 410 , and is connected to the high-temperature air preheater 250 through the mixing air duct 240 , and the outlet of the high-temperature air preheater 250 is connected to the smoke-air mixer 440 through the high-temperature air duct 260 .

[0082] In the waste heat recovery flue 410, a medium-temperature air preheater 230, a high-temperature condenser 330, a low-temperature air preheater 220 and a low-temperature condenser 310 are arranged in sequence along the flue gas flow direction. The inlet of the low-temperature condenser 310 is connected to a condensation water pipe 300, and the outlet is connected to the high-temperature condenser 330 through a condenser connecting pipe 320. The outlet of the high-temperature condenser 330 is connected to a condenser water outlet pipe 340. The air duct 200 is connected to the low-temperature air preheater 220 and the medium-temperature air preheater 234 through the low-temperature air duct 224 and the medium-temperature air duct 234 respectively; a low-temperature damper 222 is provided on the low-temperature air duct 224; a medium-temperature damper 232 is provided on the medium-temperature air duct 234; the low-temperature air duct 224 and the medium-temperature air duct 234 merge into the mixing air duct 240 and are connected to the high-temperature air preheater 250; the inlet of the high-temperature air preheater 250 is connected to a high-temperature heat source pipe 253 provided with a control valve 255, and the outlet of the high-temperature air preheater 250 is connected to the smoke-air mixer 440 through the high-temperature air duct 260, and a high-temperature air duct thermometer 270 is provided on the high-temperature air duct 260.

[0083] The inlet of the air duct 200 is connected to a blower 210 and the air duct 200 is provided with an air flow meter 205 , a hygrometer 206 and an air duct thermometer 207 .

[0084] The boiler 100 is provided with a fuel inlet 120 and a working fluid outlet pipe 110. The fuel inlet 120 is provided with a fuel analysis system 125 to provide information such as fuel composition and calorific value, which is used to adjust the fuel ratio, ensure the best combustion state, and improve the thermal efficiency of the boiler 100. The working fluid outlet pipe 110 is provided with a boiler load meter 115 to monitor the load L of the boiler 100 in real time. B , to ensure the stable operation of the system. The boiler load L B According to the flow rate F of the working fluid heated by the boiler WM , and the temperature of the working fluid T WM , working fluid pressure P WM Or the enthalpy of the working fluid H WM Calculate other parameters.

[0085] A control method for a flue gas condensation waste heat recovery system includes the following processes:

[0086] Control the target air volume in the air duct 200 so that: V AIM-A =COE A ×L B , where V AIM-A is the target air flow rate, in Nm 3 / h; L B is the load of boiler 100, in MW; COE A is the air volume boiler load factor, in Nm 3 / (h×MW), is the target air volume V A With boiler load L B The ratio range is 100~300Nm 3 / (h×MW).

[0087] During operation, the load of the blower 210 is adjusted so that the air flow rate V measured by the air flow meter 205 is T-A =V AIM-A .

[0088] Adjust the test temperature T of the dry flue gas in the dry flue 430 T-DG , so that the dry flue gas temperature T measured by the dry flue thermometer 435 T-DG =T AIM-DG , where dry flue gas temperature T T-DG Measured by high temperature air duct thermometer 270, unit is ℃, T AIM-DG is the target temperature of dry flue gas, in °C.

[0089] Target temperature of dry flue gas T AIM-DG Determined by the following formula:

[0090] T AIM-DG =T S-DG +TSH-DG

[0091] Among them, T S-DG is the saturation temperature corresponding to the water vapor concentration in the dry flue gas, in °C; T SH-DG is the superheat of water vapor in dry flue gas, that is, the target temperature value of water vapor T AIM-DG The saturation temperature T corresponding to the water vapor concentration in the dry flue gas S-DG The higher the temperature value, the control range of which is 1 to 10°C.

[0092] The saturation temperature of water vapor in dry flue gas is T S-DG Determined by fitting the following polynomial based on the water vapor concentration in dry flue gas:

[0093]

[0094] Among them, W DG is the water vapor concentration in dry flue gas, in %; T s0-DG , T s1-DG , T s2-DG , T s3-DG , T s4-DG , T s5-DG and T s6-DG are the corresponding coefficients in the polynomial, and their ranges are: (-6.5 to -7.0), (17.0 to 17.5), (-3.3 to -3.5), (0.42 to 0.45), (-3.1 to -3.5)×10 -2 , (1.0 to 1.2) × 10 -3 and (1.5 to 2.0)×10 -5 .

[0095] The concentration of water vapor in dry flue gas is W DG Determine according to the amount of water vapor contained in the air in the air duct 200, the amount of water vapor contained in the wet flue gas in the wet flue duct 450, and the corresponding amount of air and wet flue gas:

[0096]

[0097] Among them, M w-A is the amount of water vapor carried into the dry flue gas along with the air in the air duct 200, Nm 3 / h; M w-SG is the amount of water vapor in the wet flue gas in the wet flue 450, Nm 3 / h; V A-T is the actual air flow in the air duct 200, measured by the air flow meter 205, in Nm 3 / h; V SG is the wet flue gas flow rate in the wet flue 450, measured by the flue gas measurement system 455, in Nm3 / h.

[0098] The amount of water vapor M brought into the dry flue gas along with the air in the air duct 200 w-A Determined by the following formula:

[0099] M w-A =V T-A ×W A

[0100] Among them, V T-v is the actual air volume in the air duct 200, measured by the air flow meter 205, in Nm 3 / h; W A It is the volume concentration of water vapor in the air, in %.

[0101] The volume concentration of water vapor in the air is W A Determined by the following formula:

[0102] W A =W S-A ×RH A ÷100

[0103] Among them, W S-A is the saturated vapor concentration of air, in %; RH A is the relative humidity of the air, measured by the hygrometer 206, in %.

[0104] Saturated vapor concentration of air W S-A Determined according to the polynomial fitting formula of air temperature:

[0105]

[0106] Among them, T CA is the cold air temperature at the inlet of the blower 210, measured by the air duct thermometer 207; W s0-A , W s1-A , W s2-A , W s3-A , W s4-A , W s5-A Calculate the air saturated vapor concentration W S-A The coefficients of the polynomials are in the range of (0.6 to 0.65), (0.04 to 0.05), (1.4 to 1.5)×10 -3 , (2.8 to 2.9)×10 -5 , (2.5 to 2.6)×10 -7 and (2.8 to 2.9)×10 -9 .

[0107] The amount of water vapor in the wet flue gas in the wet flue 450 is M w-SG Determined by the following formula:

[0108] M w-SG =V SG ×W SG

[0109] Among them, V SG is the wet flue gas temperature in the wet flue 450, measured by the flue gas measurement system 455, in Nm 3 / h; W SG The concentration of water vapor in the wet flue gas in the wet flue 450, in Nm 3 / h.

[0110] The concentration of water vapor contained in the wet flue gas in the wet flue 450 is W SG Determined by polynomial fitting of flue gas temperature:

[0111]

[0112] Among them, T SG is the wet flue gas temperature in the wet flue 450, measured by the flue gas measurement system 455, in °C; W s0-SG , W s1-SG , W s2-SG , W s3-SG , W s4-SG , W s5-SG The water vapor concentration W in the wet flue gas is calculated respectively SG The coefficients of the polynomial, in which the water vapor contained reaches saturation, are in the range of (0.6 to 0.65), (0.04 to 0.05), (1.4 to 1.5)×10 -3 , (2.8 to 2.9)×10 -5 , (2.5 to 2.6)×10 -7 and (2.8 to 2.9)×10 -9 .

[0113] The test temperature T of the dry flue gas in the dry flue 430 is T-DG There are two main types of adjustments:

[0114] 1. Increase the test temperature T of dry flue gas T-DG

[0115] If the temperature value of dry flue gas test is T T-DG T AIM-DG =T S-DG +T SH-DG Calculated dry flue gas target temperature value T AIM-DG If the temperature is low, the flue gas condensation waste heat recovery system needs to be adjusted to increase the test temperature T of the dry flue gas. T-DG , the specific adjustment methods are as follows:

[0116] Step 1: Ensure that the control valve 255 on the high-temperature heat source pipe 253 and the medium-temperature damper 232 set on the medium-temperature air duct 234 remain closed, and the blower 210 is in operation, then gradually increase the opening of the low-temperature damper 222, and at the same time, the dry flue gas temperature value T measured by the dry flue thermometer 435 is T-DG The target temperature of dry flue gas is T AIM-DG For comparison, the air flow rate V measured by the air flow meter 205 is T-A and target air flow V AIM-A Perform the following operations based on the comparison results:

[0117] (a) If V T-A Less than V AIM-A , and T T-DG Less than T AIM-DG , then further increase the opening of the low temperature damper 222;

[0118] (b) If V T-A Less than V AIM-A , T T-DG In (0.99~1.01)×T AIM-DG If the temperature is within the range, the opening of the low temperature damper 222 is maintained unchanged;

[0119] Explanation of dry flue gas temperature T T-DG The target value is reached, and the water vapor in the dry flue gas in the dry flue 430 has a certain degree of superheat T SH-DG , ensuring that the water vapor contained in the dry flue gas will not condense into water in the dry flue 430, the induced draft fan 420 and the chimney 500, thereby avoiding corrosion.

[0120] (c) If V T-A Equal to V AIM-A , and T T-DG Less than T AIM-DG , start step 2.

[0121] Step 2: Open the medium temperature air door 232 and gradually increase the opening degree, so that the amount of air flowing through the medium temperature air preheater 230 increases, and at the same time reduce the opening degree of the low temperature air door 222. In this process, the air volume V of the air duct 200 is maintained. T-A unchanged and equal to the target air volume V AIM-A In addition, the dry flue gas temperature value T measured by the dry flue thermometer 435 is T-DG The target temperature of dry flue gas is T AIM-DG Compare; judge the opening of the low-temperature damper 222, and perform the following different operations according to different comparison and judgment results:

[0122] (a) If the low temperature damper 222 is not closed, and T T-DGLess than T AIM-DG , then further increase the opening of the medium temperature damper 232 and reduce the opening of the low temperature damper 222;

[0123] (b) If the low temperature damper 222 is not in the closed state, T T-DG In (0.99~1.01)×T AIM-DG Within the range, the openings of the medium temperature damper 232 and the low temperature damper 222 are maintained unchanged;

[0124] Explanation of dry flue gas temperature T T-DG The target value is reached, and the water vapor in the dry flue gas in the dry flue 430 has a certain degree of superheat T SH-DG , ensuring that the water vapor contained in the dry flue gas will not condense into water in the flue 430, the induced draft fan 420 and the chimney 500, thereby avoiding corrosion;

[0125] (c) If the low temperature damper 222 reaches the closed state, and T T-DG Less than T AIM-DG , start step three.

[0126] Step 3: Keep the low temperature air door 222 closed and keep the medium temperature air door 232 open to maintain the air flow rate V in the air duct 200. T-A unchanged and equal to the target air volume V AIM-A , open the control valve 255 to use the high temperature air preheater 250 to further heat the hot air from the medium temperature air preheater 203, gradually increase the opening of the control valve 255, and adjust the dry flue gas temperature value T measured by the dry flue thermometer 435. T-DG The target temperature of dry flue gas is T AIM-DG Perform the following operations based on the comparison results:

[0127] (a) If T T-DG Less than T AIM-DG , then further increase the opening of the control valve 255;

[0128] (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, the opening of the control valve 255 is maintained unchanged.

[0129] 2. Reduce the test temperature T of dry flue gas T-DG

[0130] If the dry flue gas test temperature value T T-DG T AIM-DG =T S-DG +T SH-DG Calculated dry flue gas target temperature value T AIM-DTIf the temperature is too high, the flue gas condensation waste heat recovery system needs to be adjusted to reduce the test temperature T of the dry flue gas. R-DG , the specific adjustment methods are as follows:

[0131] In the first case, the control valve 255 is in the open state, and the high temperature air preheater 250 is in the state of heating the air. Then the following operations need to be performed:

[0132] Step 1: Reduce the opening of the control valve 255, and adjust the dry flue gas temperature T measured by the dry flue thermometer 435. T-DG The target temperature of dry flue gas is T AIM-DG Perform the following operations based on the comparison results:

[0133] (a) If T T-DG Greater than T AIM-DG , the opening of the control valve 255 is further reduced;

[0134] (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, the opening of control valve 255 is maintained unchanged;

[0135] (c) If the control valve 255 reaches the closed state, T T-DG Still greater than T AIM-DG , maintain the control valve 255 in a closed state and proceed to step 2.

[0136] Step 2: Reduce the opening of the medium temperature air door 232, increase the opening of the low temperature air door 222, and maintain the air volume V of the air duct 200 T-A unchanged and equal to the target air volume V AIM-A During this adjustment process, the following comparisons and operations are performed:

[0137] (a) If T T-DG Greater than T AIM-DG , further reducing the opening of the medium-temperature damper 232 of the medium-temperature air preheater 230, and increasing the opening of the low-temperature damper 222 of the low-temperature air preheater 220;

[0138] (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, the opening of the medium temperature damper 232 is maintained unchanged, and the opening of the low temperature damper 222 is maintained unchanged;

[0139] (c) If T T-DG Greater than T AIM-DG , the medium temperature air door 232 reaches the closed state, and the third step is performed.

[0140] The third step is to maintain the closed state of the medium temperature air door 232, gradually reduce the opening of the low temperature air door 222, and perform the following comparison and operation:

[0141] (a) If T T-DG Greater than T AIM-DG , reduce the opening of the low temperature damper 222;

[0142] (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, the opening of the low temperature damper 222 is maintained unchanged.

[0143] In the second case, if the control valve 255 is in the closed state, the high temperature air preheater 250 is in the shutdown state, and the low temperature damper 222 is in the closed state, the following operations are performed:

[0144] Step 1: Reduce the opening of the medium temperature air door 232 of the medium temperature air preheater 230, increase the opening of the low temperature air door 222 of the low temperature air preheater 220, and maintain the air volume V T-A unchanged and equal to the target air volume V AIM-A During this adjustment process, the following comparisons and operations are performed:

[0145] (a) If T T-DG Greater than T AIM-DG , further reducing the opening of the medium-temperature damper 232 of the medium-temperature air preheater 230, and increasing the opening of the low-temperature damper 222 of the low-temperature air preheater 220;

[0146] (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, the opening of the medium temperature damper 232 is maintained unchanged, and the opening of the low temperature damper 222 is maintained unchanged;

[0147] (c) If T T-DG Greater than T AIM-DG , the medium temperature air door 232 reaches the closed state and proceeds to the second step.

[0148] Step 2: Keep the medium temperature damper 232 closed, gradually reduce the opening of the low temperature damper 222, and perform the following comparison and operation:

[0149] (a) If T T-DG Greater than T AIM-DG , reduce the opening of the low temperature damper 222;

[0150] (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, the opening of the low temperature damper 222 is maintained unchanged.

[0151] In the third case, if the control valve 255 is in the closed state, the high temperature air preheater 250 is in the shutdown state, and the medium temperature damper 232 is in the closed state, the following operations are performed:

[0152] Keep the medium temperature damper 232 closed, gradually reduce the opening of the low temperature damper 222, and perform the following comparison and operation:

[0153] (a) If T T-DG Greater than T AIM-DG , reduce the opening of the low temperature damper 222;

[0154] (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, the opening of the low temperature damper 222 is maintained unchanged.

Claims

1. A flue gas condensation waste heat recovery system, characterized in that: The invention comprises a boiler (100), an air duct (200), a waste heat recovery flue (410), a smoke-air mixer (440) and a chimney (500); the outlet of the boiler (100) is connected to the inlet of the waste heat recovery flue (410) through the flue (400); the outlet of the waste heat recovery flue (410) is connected to a wet flue (450); a smoke measuring system (455) for measuring wet smoke data at the outlet of the waste heat recovery flue (410) is provided on the wet flue (450); the outlet of the wet flue (450) is connected to the smoke-air mixer (440); the outlet of the smoke-air mixer (440) is connected to the chimney (500) through a dry flue (430); a dry flue thermometer (435) and an induced draft fan (420) are provided on the dry flue (430); The air duct (200) is in communication with the waste heat recovery flue (410), and is connected to a high-temperature air preheater (250) via a mixing air duct (240). The outlet of the high-temperature air preheater (250) is connected to a smoke-air mixer (440) via a high-temperature air duct (260).

2. The flue gas condensation waste heat recovery system according to claim 1, characterized in that: The waste heat recovery flue (410) is provided with a medium-temperature air preheater (230), a high-temperature condenser (330), a low-temperature air preheater (220) and a low-temperature condenser (310) in sequence along the flue gas flow direction; the inlet of the low-temperature condenser (310) is connected to a condensation water pipe (300); the outlet of the low-temperature condenser (310) is connected to the high-temperature condenser (330) via a condenser connecting pipe (320); the outlet of the high-temperature condenser (330) is connected to a condenser water outlet pipe (340); the air duct (200) is connected to the low-temperature air preheater (220) and the medium-temperature air preheater (220) via a low-temperature air duct (224) and a medium-temperature air duct (234), respectively. 34); a low-temperature air door (222) is provided on the low-temperature air duct (224), and a medium-temperature air door (232) is provided on the medium-temperature air duct (234); the low-temperature air duct (224) and the medium-temperature air duct (234) merge into a mixing air duct (240) and are connected to a high-temperature air preheater (250); a high-temperature heat source pipe (253) provided with a control valve (255) is connected to the inlet of the high-temperature air preheater (250), and the outlet of the high-temperature air preheater (250) is connected to a smoke-air mixer (440) via a high-temperature air duct (260), and a high-temperature air duct thermometer (270) is provided on the high-temperature air duct (260).

3. The flue gas condensation waste heat recovery system according to claim 2, characterized in that: The inlet of the air duct (200) is connected to a blower (210), and an air flow meter (205), a hygrometer (206) and an air duct thermometer (207) are arranged on the air duct (200).

4. The flue gas condensation waste heat recovery system according to claim 3, characterized in that: The boiler (100) is provided with a fuel inlet (120) and a working fluid outlet pipe (110), the fuel inlet (120) is provided with a fuel analysis system (125), and the working fluid outlet pipe (110) is provided with a device for real-time monitoring of the boiler (100) load L B Boiler load meter (115).

5. A control method for a flue gas condensation waste heat recovery system according to claim 4, characterized in that: The target air volume in the air duct (200) is controlled so that: V AIM-A =COE A ×L B , where V AIM-A is the target air flow rate, in Nm 3 / h; L B is the load of the boiler (100), in MW; COE A is the air volume boiler load factor, in Nm 3 / (h×MW), is the target air volume V A With boiler load L B The ratio range is 100~300Nm 3 / (h×MW); During operation, the load of the blower (210) is adjusted so that the air flow rate V measured by the air flow meter (205) is T-A =V AIM-A ; Adjust the test temperature T of the dry flue gas in the dry flue (430) T-DG , so that the dry flue gas temperature T measured by the dry flue thermometer (435) T-DG =T AIM-DG , where dry flue gas temperature T T-DG Measured by the high temperature air duct thermometer (270), the unit is ℃, T AIM-DG is the target temperature of dry flue gas, in °C.

6. The control method of a flue gas condensation waste heat recovery system according to claim 5, characterized in that: The target temperature value T of the dry flue gas AIM-DG Determined by the following formula: T AIM-DG =T S-DG +T SH-DG Among them, T S-DG is the saturation temperature corresponding to the water vapor concentration in the dry flue gas, in °C; T SH-DG is the superheat of water vapor in dry flue gas, that is, the target temperature value of water vapor T AIM-DG The saturation temperature value TS- corresponding to the water vapor concentration in dry flue gas DG The higher temperature value, the control range of this value is 1 to 10℃; The saturation temperature of water vapor in dry flue gas TS- DG Determined by fitting the following polynomial based on the water vapor concentration in dry flue gas: Among them, W DG is the water vapor concentration in dry flue gas, in %; T s0-DG , T s1-DG , T s2-DG , T s3-DG , T s4-DG , T s5-DG and T s6-DG are the corresponding coefficients in the polynomial, and their ranges are: (-6.5 to -7.0), (17.0 to 17.5), (-3.3 to -3.5), (0.42 to 0.45), (-3.1 to -3.5)×10 -2 , (1.0 to 1.2) × 10 -3 and (1.5 to 2.0)×10 -5 ; The concentration of water vapor in dry flue gas is W DG Determine according to the amount of water vapor contained in the air in the air duct (200), the amount of water vapor contained in the wet flue gas in the wet flue (450), and the corresponding amount of air and wet flue gas: Among them, M w-A is the amount of water vapor carried into the dry flue gas along with the air in the air duct (200), Nm 3 / h; M w-SG is the amount of water vapor in the wet flue gas in the wet flue (450), Nm 3 / h; V A-T is the actual air flow in the air duct (200), measured by the air flow meter (205), in Nm 3 / h; V SG is the wet flue gas flow rate in the wet flue (450), measured by the flue gas measurement system (455), in Nm 3 / h; The amount of water vapor M carried into the dry flue gas along with the air in the air duct (200) w-A Determined by the following formula: M w-A =V T-A ×W A Among them, V T-A is the actual air volume in the air duct (200), measured by the air flow meter (205), in Nm 3 / h; W A is the volume concentration of water vapor in the air, in %; The volume concentration of water vapor in the air is W A Determined by the following formula: IN A =In S-A ×RH A ÷100 Among them, W S-A is the saturated vapor concentration of air, in %; RH A is the relative humidity of the air, measured by a hygrometer (206), in %; Saturated vapor concentration of air W S-A Determined according to the polynomial fitting formula of air temperature: Among them, TC A is the cold air temperature at the inlet of the blower (210), measured by the air duct thermometer (207); W s0-A , W s1-A , W s2-A , W s3-A , W s4-A , W s5-A Calculate the air saturated vapor concentration W S-A The coefficients of the polynomials are in the range of (0.6 to 0.65), (0.04 to 0.05), (1.4 to 1.5)×10 -3 , (2.8 to 2.9)×10 -5 , (2.5 to 2.6)×10 -7 and (2.8 to 2.9)×10 -9 ; The amount of water vapor in the wet flue gas in the wet flue (450) M w-SG Determined by the following formula: M w-SG =V SG ×W SG Among them, V SG is the wet flue gas temperature in the wet flue (450), measured by the flue gas measurement system (455), in Nm 3 / h; W SG is the concentration of water vapor contained in the wet flue gas in the wet flue (450), in Nm 3 / h; The concentration of water vapor in the wet flue gas (450) is W SG Determined by polynomial fitting of flue gas temperature: Among them, T SG is the wet flue gas temperature in the wet flue (450), measured by the flue gas measurement system (455), in °C; W s0-SG , W s1-SG , W s2-SG , W s3-SG , W s4-SG , W s5-SG The water vapor concentration W in the wet flue gas is calculated respectively SG The coefficients of the polynomial, in which the water vapor contained reaches saturation, are in the range of (0.6 to 0.65), (0.04 to 0.05), (1.4 to 1.5)×10 -3 , (2.8 to 2.9)×10 -5 , (2.5 to 2.6) × 10 -7 and (2.8 to 2.9)×10 -9 .

7. The control method of a flue gas condensation waste heat recovery system according to claim 5, characterized in that: The test temperature T of the dry smoke in the dry smoke duct (430) is T-DG There are two types of adjustments: If the temperature value of dry flue gas test is T T-DG T AIM-DG =T S-DG +T SH-DG Calculated dry flue gas target temperature value T AIM-DG If the temperature is low, the flue gas condensation waste heat recovery system needs to be adjusted to increase the test temperature T of the dry flue gas. T-DG ; If the dry flue gas test temperature value T T-DG T AIM-DG =T S-DG +T SH-DG Calculated dry flue gas target temperature value T AIM-DG If the temperature is too high, the flue gas condensation waste heat recovery system needs to be adjusted to reduce the test temperature T of the dry flue gas. T-DG .

8. The control method of a flue gas condensation waste heat recovery system according to claim 7, characterized in that: The test temperature T of the dry flue gas is increased T-DG , the specific adjustment methods are as follows: Step 1: Ensure that the control valve (255) on the high-temperature heat source pipe (253) and the medium-temperature air damper (232) provided on the medium-temperature air duct (234) remain closed, and the blower (210) is in operation, and then gradually increase the opening of the low-temperature air damper (222), and at the same time, the dry flue gas temperature value T measured by the dry flue thermometer (435) is T-DG The target temperature of dry flue gas is T AIM-DG For comparison, the air flow rate V measured by the air flow meter (205) is T-A and target air flow V AIM-A Perform the following operations based on the comparison results: (a) If V T-A Less than V AIM-A , and T T-DG Less than T AIM-DG , the opening of the low temperature air door (222) is further increased; (b) If V T-A Less than V AIM-A , T T-DG In (0.99~1.01)×T AIM-DG If the temperature is within the range, the opening of the low temperature air door (222) is maintained unchanged; (c) If V T-A Equal to V AIM-A , and T T-DG Less than T AIM-DG , start step 2; Step 2: Open the medium temperature air door (232) and gradually increase the opening degree, so that the amount of air flowing through the medium temperature air preheater (230) increases, while reducing the opening degree of the low temperature air door (222). In this process, the air volume V of the air duct (200) is maintained at T-A unchanged and equal to the target air volume V AIM-A In addition, the dry flue gas temperature value T measured by the dry flue thermometer (435) T-DG The target temperature of dry flue gas is T AIM-DG Compare; judge the opening of the low-temperature air door (222), and perform the following different operations according to different comparison and judgment results: (a) If the low temperature damper (222) has not reached the closed state, and T T-DG Less than T AIM-DG , the opening of the medium temperature air door (232) is further increased, and the opening of the low temperature air door (222) is reduced; (b) If the low temperature damper (222) has not reached the closed state, T T-DG In (0.99~1.01)×T AIM-DG Within the range, the openings of the medium temperature air door (232) and the low temperature air door (222) are maintained unchanged; (c) If the low temperature damper (222) reaches the closed state, and T T-DG Less than T AIM-DG , start step 3; Step 3: Maintain the low temperature air door (222) in a closed state and maintain the opening of the medium temperature air door (232) unchanged to maintain the air flow rate V of the air duct (200) T-A unchanged and equal to the target air volume V AIM-A , open the control valve (255) to use the high temperature air preheater (250) to further heat the hot air from the medium temperature air preheater (203), gradually increase the opening of the control valve (255), and adjust the dry flue gas temperature value T measured by the dry flue thermometer (435) T-DG The target temperature of dry flue gas is T AIM-DG Perform the following operations based on the comparison results: (a) If T T-DG Less than T AIM-DG , the opening of the control valve (255) is further increased; (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, the opening of the control valve (255) is maintained unchanged.

9. The control method of a flue gas condensation waste heat recovery system according to claim 7, characterized in that: The test temperature T of the dry flue gas is reduced T-DG , the specific adjustment methods are as follows: In the first case, the control valve (255) is in the open state and the high temperature air preheater (250) is in the state of heating the air. The following operations need to be performed: Step 1: Reduce the opening of the control valve (255) to adjust the dry flue gas temperature T measured by the dry flue thermometer (435) T-DG The target temperature of dry flue gas is T AIM-DG Perform the following operations based on the comparison results: (a) If T T-DG Greater than T AIM-DG , the opening of the control valve (255) is further reduced; (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, the opening of the control valve (255) is maintained unchanged; (c) If the control valve (255) reaches the closed state, T T-DG Still greater than T AIM-DG , maintain the control valve (255) in a closed state, and proceed to step 2; Step 2: Reduce the opening of the medium temperature air door (232), increase the opening of the low temperature air door (222), and maintain the air volume V of the air duct (200) T-A unchanged and equal to the target air volume V AIM-A During this adjustment process, the following comparisons and operations are performed: (a) If T T-DG Greater than T AIM-DG , further reducing the opening of the medium-temperature air damper (232) of the medium-temperature air preheater (230), and increasing the opening of the low-temperature air damper (222) of the low-temperature air preheater (220); (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, the opening of the medium temperature air door (232) is maintained unchanged, and the opening of the low temperature air door (222) is maintained unchanged; (c) If T T-DG Greater than T AIM-DG , the medium temperature air door (232) reaches the closed state, and the third step is performed; The third step is to maintain the medium temperature air door (232) in a closed state, gradually reduce the opening of the low temperature air door (222), and perform the following comparison and operation: (a) If T T-DG Greater than T AIM-DG , reducing the opening of the low temperature air door (222); (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, the opening of the low temperature air door (222) is maintained unchanged; In the second case, if the control valve (255) is in a closed state, the high-temperature air preheater (250) is in a shutdown state, and the low-temperature damper (222) is in a closed state, the following operations are performed: Step 1: Reduce the opening of the medium-temperature air door (232) of the medium-temperature air preheater (230), increase the opening of the low-temperature air door (222) of the low-temperature air preheater (220), and maintain the air volume V of the air duct (200) T-A unchanged and equal to the target air volume V AIM-A During this adjustment process, the following comparisons and operations are performed: (a) If T T-DG Greater than T AIM-DG , further reducing the opening of the medium-temperature air damper (232) of the medium-temperature air preheater (230), and increasing the opening of the low-temperature air damper (222) of the low-temperature air preheater (220); (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, the opening of the medium temperature air door (232) is maintained unchanged, and the opening of the low temperature air door (222) is maintained unchanged; (c) If T T-DG Greater than T AIM-DG , the medium temperature air door (232) reaches the closed state, and the second step is performed; Step 2: Maintain the closed state of the medium temperature air door (232), gradually reduce the opening of the low temperature air door (222), and perform the following judgment, comparison and operation: (a) If T T-DG Greater than T AIM-DG , reducing the opening of the low temperature air door (222); (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, the opening of the low temperature air door (222) is maintained unchanged; In the third case, if the control valve (255) is in a closed state, the high temperature air preheater (250) is in a shutdown state, and the medium temperature damper (232) is in a closed state, the following operations are performed: The medium temperature air door (232) is kept closed, and the opening of the low temperature air door (222) is gradually reduced. The following comparison and operation are performed: (a) If T T-DG Greater than T AIM-DG , reducing the opening of the low temperature air door (222); (b) If T T-DG In (0.99~1.01)×T AIM-DG Within the range, the opening of the low temperature air door (222) is maintained unchanged.

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