Coal power unit flue gas recirculation system and control method

Abstract

The invention discloses a coal power unit flue gas recirculation system and a control method, and relates to the technical field of coal power unit boilers, and the coal power unit flue gas recirculation system comprises a first flue, an economizer, a denitration device, a bypass flue and a recirculation flue. The first circulating flue can enable flue gas about to flow into the denitration device to flow back into the first flue so as to be mixed with the heated flue gas again, and the second circulating flue leads the heated and mixed flue gas back to the combustor area, so that the hearth temperature of the combustor area is increased, the low-load stable combustion capacity of the coal power unit is improved, and the combustion efficiency of the coal power unit is improved. The first circulating flue and the second circulating flue can be used together with the bypass flue, so that the temperature of flue gas flowing into the denitration device can be increased to ensure the denitration effect of the denitration device, part of air can be replaced by recirculated flue gas, the convection heat exchange intensity is improved, meanwhile, the boiler operation oxygen amount is reduced, and the boiler operation efficiency is improved. The NOx emission concentration of a hearth outlet and the smoke exhaust heat loss of the boiler are effectively reduced, and the boiler efficiency is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of coal power unit boiler, in particular to a coal power unit flue gas recirculation system and control method. BACKGROUND

[0002] At present, coal power gradually changes from the main power supply providing power to the supporting and adjusting power supply providing power, and coal power unit deep peak shaving and ultra-low load peak shaving operation will become the norm.

[0003] When the coal power unit is in deep peak shaving operation, the main and reheat steam temperature is generally lower than the design value. In order to maintain the main and reheat steam temperature, the convective heat transfer intensity needs to be improved, and a higher oxygen content operation mode deviating from the design value is often used in operation. This will cause the NOx (mainly thermal NOx) emission concentration at the furnace outlet to be significantly higher than that under normal load conditions, posing a severe challenge to NOx emission control under low load. At the same time, high oxygen content operation increases the excess air ratio and improves the exhaust gas heat loss, reducing the boiler thermal efficiency and having a negative impact on the economy of the unit. The conventional flue gas recirculation technology sends part of the flue gas at the tail of the boiler into the furnace again through a recirculation fan, thereby achieving the purpose of adjusting the reheat steam temperature, reducing the oxygen content and NOx emission concentration.

[0004] On the other hand, in order to solve the problem of SCR denitration device operation during unit startup, there is currently a full-condition denitration technology solution in China. This solution places a bypass flue gas heating device in the bypass flue, and during unit startup, the bypass flue gas heating device heats the bypass flue gas, and then the bypass flue gas is mixed with the flue gas in the main flue to increase the inlet flue gas temperature of the denitration device to the minimum allowable temperature of the denitration catalyst.

[0005] During the startup phase of the coal power unit or ultra-low load operation, the boiler stable combustion faces great challenges, and at this time the flue gas temperature at the tail of the boiler is also very low. If the recirculation flue gas with too low temperature is injected into the furnace, it will significantly worsen the combustion conditions and exacerbate the instability of the boiler combustion, leading to the risk of poor boiler combustion and even flameout. At the same time, in order to ensure the operation of the denitration system, the bypass flue gas heating device of the full-condition denitration system needs to be adjusted to a large output and maintained in this state for a long time, which requires high equipment and large fuel consumption. SUMMARY

[0006] The present application aims to provide a coal power unit flue gas recirculation system and control method to alleviate the technical problems in the prior art that during the startup phase of the coal power unit or ultra-low load operation, the method of injecting a certain amount of recirculation flue gas with low temperature into the furnace easily leads to the risk of poor boiler combustion and even flameout, and requires high equipment and large fuel consumption to ensure the operation of the denitration system.

[0007] The application provides a coal-fired unit flue gas recirculation system, comprising a first flue, an economizer, a denitration device, a bypass flue and a recirculation flue.

[0008] The front end of the first flue is used for connecting the overfire air output end of a boiler in the coal-fired unit and can introduce overfire air.

[0009] The economizer has a flue gas import end and a flue gas export end, and the flue gas import end is connected with the tail end of the first flue.

[0010] The front end of the second flue is connected with the flue gas export end.

[0011] The denitration device is connected with the tail end of the second flue.

[0012] One end of the bypass flue is connected with the first flue, and the other end is connected with the second flue in communication, and the bypass flue is provided with a heating assembly, and the heating assembly is used for heating the flue gas in the bypass flue.

[0013] The recirculation flue comprises a first recirculation flue and a second recirculation flue, one end of each of the first recirculation flue and the second recirculation flue is connected with the second flue, and the connection position is located behind the air outlet of the bypass flue along the flue gas flow direction, the other end of the first recirculation flue is connected with the air outlet of the boiler, and the other end of the second recirculation flue is connected with the burner area of the boiler.

[0014] Further, the flue gas export end is provided with a main flue valve body for adjusting the flue gas flow rate of the economizer into the second flue.

[0015] The bypass flue is provided with a first valve body assembly for adjusting the flue gas flow rate of the bypass flue.

[0016] The recirculation flue is provided with a second valve body assembly for adjusting the flue gas flow rate in the recirculation flue.

[0017] Further, the second valve body assembly comprises a second shutoff valve and a second flow rate adjusting valve.

[0018] The second shutoff valve is arranged at the connection position of the recirculation flue and the second flue.

[0019] The second flow rate adjusting valve is arranged in the recirculation flue and is used for adjusting the flue gas flow rate between the recirculation flue and the second flue.

[0020] Further, the second valve body assembly further comprises a first mixing adjusting valve and a second mixing adjusting valve.

[0021] The first mixing adjusting valve is arranged in the first recirculation flue.

[0022] The second mixing adjusting valve is arranged in the second recirculation flue.

[0023] Further, the coal power unit flue gas recirculation system further comprises a dust removal device and an air induction device.

[0024] The dust removal device is arranged in the recirculation flue.

[0025] The air induction device is arranged in the recirculation flue and behind the dust removal device in the flue gas flow direction.

[0026] Further, the coal power unit flue gas recirculation system further comprises a first flow meter, a second flow meter and a third flow meter.

[0027] The first flow meter is arranged in the recirculation flue and used for measuring the flue gas flow between the recirculation flue and the second flue.

[0028] The second flow meter is arranged in the first recirculation flue.

[0029] The third flow meter is arranged in the second recirculation flue.

[0030] Further, the first valve assembly comprises a first shut-off valve and a first flow regulating valve.

[0031] The first shut-off valve is arranged at the joint of the bypass flue and the first flue.

[0032] The first flow regulating valve is arranged in the bypass flue and behind the heating assembly in the flue gas flow direction.

[0033] Further, the first valve assembly further comprises an expansion joint.

[0034] The expansion joint is arranged at the joint of the bypass flue and the second flue.

[0035] The purpose of the present application is also to provide a control method of the coal power unit flue gas recirculation system, the method comprising: Obtaining the operation condition of the coal power unit.

[0036] Judging whether the operation condition is a high load condition.

[0037] If not, turning on the bypass flue, turning on the heating assembly, turning off the second recirculation flue, and at the same time, turning on the first recirculation flue to introduce the flue gas in the second flue into the first flue.

[0038] Obtaining the current temperature of the flue gas entering the denitration device; Based on the current temperature, adjusting the operation power of the heating assembly. Wherein, the operation power is inversely proportional to the current temperature.

[0039] Further, after the step of judging whether the operation condition is a high load condition, the method further comprises: If yes, the heating assembly is turned off, the bypass flue is bypassed, and the first circulating flue and the second circulating flue are turned on to introduce the flue gas in the second flue to the exhaust end of the boiler and the combustion zone of the boiler.

[0040] Advantages: In the coal-fired unit flue gas recirculation system provided by the application, a recirculation flue is additionally arranged on the second flue connected to the flue gas outlet end of the economizer. During the startup stage of the coal-fired unit or the ultra-low load operation, part of the flue gas in the first flue can be bypassed to the second flue through the bypass flue, and the flue gas in the bypass flue is heated by the heating assembly and mixed with the flue gas at the outlet of the economizer before entering the second flue, so as to increase the temperature of the flue gas flowing into the denitration device. The first circulating flue can recirculate the flue gas flowing into the denitration device to the first flue to be mixed with the heated flue gas again, so as to further increase the temperature of the flue gas at the inlet of the denitration device. During the low load operation of the coal-fired unit, the heated and mixed flue gas can be introduced back to the burner zone of the boiler through the second circulating flue, so as to increase the temperature of the burner zone of the boiler and improve the low load stable combustion capability of the coal-fired unit. The first circulating flue and the second circulating flue can be used simultaneously with the bypass flue, which can not only increase the temperature of the flue gas flowing into the denitration device to ensure the denitration effect of the denitration device, but also replace part of the air with the recirculated flue gas to increase the convective heat transfer intensity and reduce the oxygen content during the operation of the boiler, thereby effectively reducing the NOx emission concentration at the outlet of the furnace and the heat loss of the flue gas of the boiler and improving the efficiency of the boiler.

[0041] The control method of the coal-fired unit flue gas recirculation system provided by the application is applied to the coal-fired unit flue gas recirculation system provided above, and the technical effects described above can be achieved, which will not be described here. BRIEF DESCRIPTION OF DRAWINGS

[0042] In order to more clearly illustrate the specific embodiments of the application or the technical solutions in the prior art, the drawings needed in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings in the following description are some embodiments of the application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0043] Figure 1 The structure diagram of the coal-fired unit flue gas recirculation system provided by the embodiment of the application is shown in the figure. Figure 2 The flowchart of the control method of the coal-fired unit flue gas recirculation system provided by the embodiment of the application is shown in the figure.

[0044] FIG. 100 - first flue; 200 - economizer; 210 - second flue; 220 - main flue valve body; 300 - denitration device; 400 - bypass flue; 410 - heating assembly; 420 - first shutoff valve; 430 - first flow regulating valve; 500 - recirculation flue; 510 - first circulation flue; 511 - first mixing regulating valve; 512 - second flow meter; 520 - second circulation flue; 521 - second mixing regulating valve; 522 - third flow meter; 530 - second shutoff valve; 540 - second flow regulating valve; 550 - dust removal device; 560 - induced draft device; 570 - first flow meter; 600 - burner zone. DETAILED DESCRIPTION

[0045] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some but not all of the embodiments of the present application. The components of the embodiments of the present application described and shown in the drawings can be arranged and designed in various different configurations.

[0046] Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative labor fall within the scope of protection of the present application.

[0047] It should be noted that: similar reference numerals and letters represent similar items in the following drawings, therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0048] In the description of the present application, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship when the product of the present application is usually placed, and are only for the convenience of describing the present application and simplifying the description, and therefore cannot be understood as indicating or implying that the indicated device or element must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application. In addition, the terms "first", "second", "third" and the like are only used for differentiation in description, and cannot be understood as indicating or implying relative importance.

[0049] In addition, the terms "horizontal", "vertical", and the like do not mean that the components must be absolutely horizontal or vertical, but can be slightly inclined. For example, "horizontal" only means that it is more horizontal than "vertical", and does not mean that the structure must be completely horizontal, but can be slightly inclined.

[0050] In the description of the present application, it should be noted that, unless otherwise explicitly specified and limited, the terms "arrangement", "installation", "connection", "connection" should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium, or it can be connected inside two elements. For those skilled in the art, the specific meanings of the above terms in the present application can be understood according to the specific circumstances.

[0051] The present application will be further described in detail below by specific embodiments and in conjunction with the accompanying drawings.

[0052] Embodiment one Reference Figure 1 The coal-fired unit flue gas recirculation system provided in the embodiment comprises a first flue 100, an economizer 200, a second flue 210, a denitration device 300, a bypass flue 400, and a recirculation flue 500.

[0053] The front end of the first flue 100 is used to connect the overfire air output end of the boiler in the coal-fired unit and can introduce overfire air. The economizer 200 has a flue gas inlet end and a flue gas outlet end, and the flue gas inlet end is connected to the end of the first flue 100. The front end of the second flue 210 is connected to the flue gas outlet end. The denitration device 300 is connected to the end of the second flue 210. One end of the bypass flue 400 is connected to the first flue 100, and the other end is connected to the second flue 210 in communication, and the bypass flue 400 is provided with a heating assembly 410 for heating the flue gas in the bypass flue 400. The recirculation flue 500 comprises a first recirculation flue 510 and a second recirculation flue 520, one end of each of the first recirculation flue 510 and the second recirculation flue 520 is connected to the second flue 210, and the connection position is located behind the air outlet of the bypass flue 400 in the flue gas flow direction, the other end of the first recirculation flue 510 is connected to the air outlet of the boiler, and the other end of the second recirculation flue 520 is connected to the burner area of the boiler.

[0054] In the embodiment, the coal-fired unit comprises a plurality of boilers, and each boiler has a burner area and an overfire air output end, and the flue gas generated by the boiler is discharged from the overfire air output end through the burner area. The coal-fired unit flue gas recirculation system provided in the embodiment is arranged correspondingly with the boilers of the coal-fired unit.

[0055] The recirculation flue 500 is additionally arranged on the second flue 210 connected with the flue gas outlet end of the economizer 200 of the flue gas recirculation system of the coal-fired unit.

[0056] During the start-up stage of the coal-fired unit or the ultra-low load operation, part of the flue gas in the first flue 100 can be bypassed to the second flue 210 through the bypass flue 400. The heating assembly 410 heats the flue gas in the bypass flue, and the heated flue gas is mixed with the flue gas at the outlet of the economizer and then enters the second flue 210, thereby increasing the temperature of the flue gas flowing into the denitration device 300.

[0057] The first circulating flue 510 can recirculate the flue gas flowing into the denitration device 300 to the first flue 100 to be mixed with the heated flue gas again, thereby further increasing the temperature of the flue gas at the inlet of the denitration device 300.

[0058] During the low load operation of the coal-fired unit, the heated and mixed flue gas can be introduced back to the burner area 600 through the second circulating flue 520, thereby increasing the furnace temperature of the burner area 600, and improving the low load combustion stability of the coal-fired unit.

[0059] In addition, in the embodiment, the first circulating flue 510 and the second circulating flue 520 can be used simultaneously with the bypass flue 400. The temperature of the flue gas flowing into the denitration device can be increased to ensure the denitration effect of the denitration device, and part of the air can be replaced by the recirculated flue gas to increase the convective heat transfer intensity while reducing the oxygen content of the boiler operation, thereby effectively reducing the NOx emission concentration at the outlet of the furnace and the heat loss of the boiler exhaust, and improving the efficiency of the boiler.

[0060] In the embodiment, the flue gas outlet end of the economizer is provided with a main flue valve 220 for adjusting the flow of flue gas from the economizer 200 into the second flue 210. The bypass flue 400 is provided with a first valve assembly for adjusting the flow of flue gas in the bypass flue 400. The recirculation flue 500 is provided with a second valve assembly for adjusting the flow of flue gas in the recirculation flue 500.

[0061] Specifically, in the embodiment, the flue gas outlet end of the economizer 200 is provided with the main flue valve 220, the bypass flue 400 is provided with the first valve assembly, and the recirculation flue 500 is provided with the second valve assembly. The main flue valve 220, the first valve assembly, and the second valve assembly can accurately adjust the flow of flue gas.

[0062] In the start-up stage or the state of ultra-low load operation of the coal-fired unit, the main flue valve body 220 can control the flue gas flow rate of the flue gas flowing from the economizer 200 into the second flue 210, so as to avoid that the excessive low-temperature flue gas directly enters the denitration device 300. The first valve body assembly can dynamically adjust the flue gas flow rate of the bypass flue 400, so as to ensure that the heating assembly 410 only performs efficient heating on the necessary proportion of flue gas, thereby reducing the heating power requirement and fuel consumption. The second valve body assembly can flexibly control the flue gas flow rate in the recirculation flue 500, so as to optimize the amount of flue gas flowing back to the first flue 100 or the burner area 600 when the burner area 600 is in low-load operation, so as to stabilize the furnace temperature and improve the combustion efficiency.

[0063] Meanwhile, the synergistic effect of the main flue valve body 220, the first valve body assembly and the second valve body assembly enhances the adaptability of the system to the deep peak-shaving working condition, reduces the dependence on the high-power long-time operation of the bypass flue gas heating device, reduces the equipment requirement, and further improves the inlet flue gas temperature stability of the denitration device 300 and the stable combustion capability of the burner area 600, so as to finally realize the efficient and safe operation of the coal-fired unit in the low-load working condition.

[0064] In the embodiment, the second valve body assembly includes a second shutoff valve 530 and a second flow regulating valve. The second shutoff valve 530 is arranged at the joint of the recirculation flue 500 and the second flue 210. The second flow regulating valve is arranged in the recirculation flue 500 and is used to regulate the flue gas flow rate between the recirculation flue 500 and the second flue 210.

[0065] In the embodiment, the second shutoff valve 530 is arranged at the joint of the recirculation flue 500 and the second flue 210, and the second flow regulating valve is additionally arranged in the recirculation flue 500, so as to realize the staged and accurate control of the flue gas backflow path.

[0066] In the deep peak-shaving working condition of the coal-fired unit, the second shutoff valve 530 can quickly cut off or open the recirculation flue 500, so as to avoid that the low-temperature flue gas flows into the recirculation flue 500 when it is not necessary to flow into the burner area 600 or the first flue 100, thereby affecting the normal combustion.

[0067] The second flow regulating valve can regulate the flow rate of the backflow flue gas, so as to prevent that the excessive flue gas backflow dilutes the combustion concentration while stabilizing and improving the furnace temperature. The synergistic effect of the second shutoff valve 530 and the second flow regulating valve improves the response speed and safety of the flue gas recirculation system of the coal-fired unit.

[0068] In the embodiment, the second valve body assembly further includes a first mixing regulating valve 511 and a second mixing regulating valve 521. The first mixing regulating valve 511 is arranged in the first recirculation flue 510. The second mixing regulating valve 521 is arranged in the second recirculation flue 520.

[0069] Specifically, the first mixing adjusting valve 511 dynamically controls the amount of flue gas flowing back to the first flue 100, thereby increasing the denitration inlet flue gas temperature, ensuring sufficient mixing with the bypass heating flue gas, and stabilizing the denitration device inlet temperature in a reasonable range.

[0070] The second mixing adjusting valve 521 precisely adjusts the amount of recirculated flue gas injected into the burner zone 600 to regulate the furnace temperature and avoid excessive low-temperature flue gas entering the burner zone 600 to dilute the fuel concentration.

[0071] In this embodiment, the coal-fired power unit flue gas recirculation system further includes a dust removal device 550 and an induced draft device 560. The dust removal device 550 and the induced draft device 560 are arranged in the recirculation flue 500, and the induced draft device 560 is arranged behind the dust removal device in the flue gas flow direction. The recirculated flue gas flows into the induced draft device after being cleaned by the dust removal device.

[0072] The dust removal device 550 efficiently removes fly ash from the recirculated flue gas to prevent the nozzle of the burner zone 600 from being blocked, the wear being intensified, or the combustion being fluctuated due to the backflow of dust, and to reduce the wear of the induced draft device 560.

[0073] In addition, the induced draft device 560 in this embodiment is specifically an induced draft fan, which can introduce the flue gas in the second flue 210 into the recirculation flue 500 and actively regulate the flow rate of the flue gas in the recirculation flue 500, ensuring that the flue gas can be stably injected into the burner zone 600 through the recirculation flue 500 at low load.

[0074] In this embodiment, the coal-fired power unit flue gas recirculation system further includes a first flow meter 570, a second flow meter 512, and a third flow meter 522.

[0075] The first flow meter 570 is arranged in the recirculation flue 500 and is used to measure the flue gas flow between the recirculation flue 500 and the second flue 210. The second flow meter 512 is arranged in the first circulation flue 510. The third flow meter 522 is arranged in the second circulation flue 520.

[0076] In this embodiment, by arranging the first flow meter 570, the second flow meter 512, and the third flow meter 522 in the recirculation flue 500, the first circulation flue 510, and the second circulation flue 520 respectively, the monitoring and control of the flue gas recirculation system are realized.

[0077] Specifically, the first flow meter 570 can monitor the total flow of the recirculation flue 500 in real time and is linked with the second valve body assembly to form a dynamic adjusting mechanism, thereby controlling the total amount of flue gas backflow and avoiding the lag of denitration temperature control or combustion fluctuation caused by relying on experience for adjustment.

[0078] The second flow meter 512 is arranged in the first circulating flue 510 to monitor the amount of flue gas flowing back to the inlet of the economizer 200, and is combined with the first mixed regulating valve 511 to adjust the flue gas entering the denitration device 300, so that the inlet temperature of the denitration device is stabilized at a preset value.

[0079] The third flow meter 522 is arranged in the second circulating flue 520 to monitor the flue gas flow injected into the burner zone 600, and is combined with the second mixed regulating valve 521 to respond to the adjustment of the furnace temperature, so as to ensure the stable combustion state of the burner zone 600.

[0080] In the embodiment, the first valve body assembly includes a first shut-off valve 420 and a first flow regulating valve 430. The first shut-off valve 420 is arranged at the joint of the bypass flue 400 and the first flue 100. The first flow regulating valve 430 is arranged in the bypass flue 400 and behind the heating assembly 410 in the flue gas flow direction.

[0081] The first shut-off valve 420 in the embodiment is arranged at the joint of the bypass flue 400 and the first flue 100, and the first flow regulating valve 430 is arranged behind the heating assembly 410. The first shut-off valve 420 and the first flow regulating valve 430 are electrically connected to realize the safe isolation of the bypass heating flue gas and the control of the temperature and flow.

[0082] The first shut-off valve 420 can cut off the bypass flue 400 when the boiler has abnormal conditions such as combustion fluctuation, to prevent low-temperature flue gas from flowing back to the first flue 100 to aggravate the combustion deterioration. The first flow regulating valve 430 is arranged behind the heating assembly 410 to directly regulate the flow of the heated high-temperature flue gas, avoiding dry burning damage of the heating assembly 410 due to sudden flow changes, and avoiding temperature fluctuations caused by mixing of cold and hot flue gases.

[0083] In the embodiment, the first valve body assembly further includes an expansion joint. The expansion joint is arranged at the joint of the bypass flue 400 and the second flue 210.

[0084] In the embodiment, the expansion joint is arranged at the joint of the bypass flue 400 and the second flue 210 to realize the self-dissipation of thermal stress and vibration isolation of the high-temperature flue system. The expansion joint can absorb the axial thermal displacement difference between the bypass flue 400 and the second flue 210, avoid weld cracking, and inhibit the transmission of boiler combustion vibration to the first flow regulating valve 430.

[0085] When the load changes rapidly, the expansion joint can compensate for the thermal deformation of the flue to ensure that the flow regulating valve is always in a normal working position, and can avoid metal fatigue cracks caused by thermal stress to prolong the maintenance period of the flue.

[0086] Embodiment two In combination with Figure 2The control method of the coal-fired unit flue gas recirculation system provided by the embodiment specifically comprises the following steps. S100, acquiring an operation condition of the coal-fired unit.

[0087] S200, judging whether the operation condition is a high-load condition.

[0088] S300, if not, turning on the heating assembly, turning on the second flue, turning off the second recirculation flue, and simultaneously, turning on the first recirculation flue to introduce the flue gas in the second flue into the first flue.

[0089] S400, acquiring a current temperature of the flue gas entering the denitration device.

[0090] S500, adjusting the operation power of the heating assembly based on the current temperature, wherein the operation power is inversely proportional to the current temperature.

[0091] Specifically, in the embodiment, during the unit startup stage, the first shutoff valve 420 of the bypass flue 400 is opened, the heating assembly 410 heats the bypass flue gas in the bypass flue 400, the main flue adjusting valve, the first shutoff valve 420 and the first flow adjusting valve 430 are controlled, the heated bypass flue gas is mixed with the flue gas in the second flue 210, so that the inlet flue temperature of the denitration device is increased to the minimum allowable temperature (about 300 DEG C) of the denitration catalyst.

[0092] Subsequently, the second shutoff valve 530 is opened and the induced draft device 560 is started, the power of the induced draft device 560 and the second flow adjusting valve 540, the first mixing adjusting valve 511 are controlled, and the high-temperature flue gas of 300 DEG C is re-injected into the first flue 100 for recycling. Since the temperature of the injected recirculated flue gas (about 300 DEG C) is higher than the temperature of the secondary hot air (about 100 DEG C when the unit is started), the temperature of the burner area of the boiler can be increased, thereby shortening the unit startup time by about 15%. With the increase of the temperature at the outlet of the burnout air of the boiler, the power of the heating assembly 410 in the bypass flue 400 can be gradually reduced, and the energy saving rate can reach about 30%.

[0093] In the embodiment, the judgment basis is that the unit power generation power ≤ 50% rated load is a low-load condition, and the unit power generation power > 50% rated load is a high-load condition.

[0094] When the inlet flue gas temperature of the denitration device is less than 270℃ (i.e. the coal-fired unit is in an ultra-low load state), the second shutoff valve 530 is opened, and the heating assembly 410 in the bypass flue 400 is controlled to heat the bypass flue gas in the bypass flue 400. The main flue damper 220, the first shutoff valve 420 and the first flow regulating valve 430 are controlled to mix the flue gas in the bypass flue 400 with the flue gas in the second flue 210, so as to increase the inlet flue gas temperature of the denitration device to the minimum allowable temperature (about 300℃) of the denitration catalyst, thereby ensuring that the denitration treatment of the flue gas can be normally implemented.

[0095] Subsequently, the second shutoff valve 530 is opened, the induced draft device 560 is started, and the power of the induced draft device 560 and the second flow regulating valve 540, the first mixing regulating valve 511 and the second mixing regulating valve 521 are adjusted, so as to re-inject the high-temperature flue gas at 300℃ into the first flue 100 and the burner area 600 for recycling. Since the temperature (about 300℃) of the flue gas injected into the recirculation flue 500 is higher than the temperature (about 250℃) of the secondary hot air, the furnace temperature of the burner area 600 can be increased, which is beneficial to the stable combustion of the unit in a low load state.

[0096] With the injection of the flue gas in the recirculation flue 500, the boiler efficiency can be improved, the NOx emission concentration can be reduced, and the main and reheat steam temperatures can be ensured while the air volume entering the furnace is reduced to reduce the oxygen content of the flue gas and the NOx emission concentration. When the combustion of the burner area 600 is unstable, the operating power of the heating assembly 410 in the bypass flue 400 can be increased, the flue gas temperature at the inlet of the denitration device is further increased to not higher than the maximum allowable temperature (about 420℃) of the denitration catalyst, the high-temperature flue gas is re-injected into the furnace of the burner area 600 for recycling, the furnace temperature of the burner area 600 is further increased, and the low load stable combustion capability of the coal-fired unit is improved.

[0097] In addition, when the inlet flue gas temperature of the denitration device is between 270℃ and 300℃ (i.e. the coal-fired unit is in a low load state), the heating assembly 410 in the bypass flue 400 can be closed.

[0098] At this time, the first shutoff valve 420 is opened, and the main flue damper 220, the first shutoff valve 420 and the first flow regulating valve 430 are controlled to mix the flue gas in the bypass flue 400 with the flue gas in the second flue 210, so as to increase the inlet flue gas temperature of the denitration device to the minimum allowable temperature (about 300℃) of the denitration catalyst.

[0099] Subsequently, the second shutoff valve 530 is opened, the air induction device 560 is started, the power of the air induction device 560 and the second shutoff valve 530, the first mixing adjustment valve 511 and the second mixing adjustment valve 521 are adjusted, and the high-temperature flue gas at 300 DEG C is re-injected into the first flue 100 and the furnace of the burner area 600, so as to be recycled. Since the temperature of the re-injected recycled flue gas (about 300 DEG C) is higher than the temperature of the secondary hot air (about 275 DEG C), the furnace temperature of the burner area 600 can be increased, which is beneficial to the stable combustion of the coal power unit at low load. Moreover, with the input of the recycled flue gas, the furnace air volume can be reduced to reduce the oxygen content of the exhaust flue gas (for example, the oxygen content is reduced from 12.28% to 8.6% at 18% load), the passive mode of the traditional "high oxygen content for maintaining the steam temperature" is broken, the stable control of the steam temperature at low oxygen content is realized, the NOx emission concentration is reduced, and the boiler efficiency is improved.

[0100] In the step S300, if no, the heating assembly is opened, the second flue is conducted, the second circulating flue is closed, and meanwhile, the first circulating flue is conducted to introduce the flue gas in the second flue into the first flue. Specifically, the first valve body assembly is opened to conduct the bypass flue 400; the heating assembly 410 is opened to heat the flue gas in the bypass flue 400, so that the heated flue gas in the bypass flue 400 flows into the second flue 210. The second valve body assembly and the first mixing adjustment valve 511 are opened to conduct the recycled flue 500 and the first circulating flue 510, and to introduce part of the flue gas in the second flue 210 into the furnace through the recycled flue 500 and the first circulating flue 510.

[0101] In the embodiment, after the step S200 of judging whether the operating condition is the high-load condition, the step S210 of closing the heating assembly and the bypass flue is further included. S210, if yes, the heating assembly and the bypass flue are closed, and meanwhile, the first circulating flue and the second circulating flue are conducted to introduce the flue gas of the second flue into the burner area and the exhaust air output end of the boiler.

[0102] In the high-load condition, the first shutoff valve 420 and the heating assembly 410 are closed, the air induction device 560 is started, and the second mixing adjustment valve 521 is adjusted, so that the flue gas in the recycled flue 500 is injected into the burner area 600, the flue gas amount of the furnace is increased, and the main steam temperature and the reheat steam temperature are increased.

[0103] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than limit them. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that the technical solutions recorded in the foregoing embodiments can be modified, or some or all of the technical features can be replaced by equivalents. Such modifications or replacements do not change the essence of the corresponding technical solutions out of the scope of the technical solutions of the embodiments of the present application.

Claims

1. A flue gas recirculation system for a coal-fired power plant, characterized in that: include: A first flue (100), the front end of which is used to connect to the overburnt air output end of the boiler in the coal-fired power unit and is capable of introducing the overburnt air; an economizer (200) having a flue gas inlet end and a flue gas outlet end, wherein the flue gas inlet end is connected to the end of the first flue (100); a second flue (210), the front end of which is connected to the flue gas outlet; a denitrification device (300) connected to the end of the second flue (210); A bypass flue (400), one end of which is connected to the first flue (100) and the other end of which is connected to the second flue (210); a heating component (410) is provided in the bypass flue (400); the heating component (410) is used to heat the flue gas in the bypass flue (400); The recycling flue (500) includes a first recycling flue (510) and a second recycling flue (520), wherein one end of the first recycling flue (510) and the second recycling flue (520) are both connected to the second flue (210), and the connection point is located behind the air outlet of the bypass flue (400) along the direction of flue gas flow, the other end of the first recycling flue (510) is connected to the air outlet of the boiler, and the other end of the second recycling flue (520) is connected to the burner area of ​​the boiler.

2. The flue gas recirculation system of a coal-fired power plant according to claim 1, characterized in that: The flue gas outlet end is provided with a main flue valve body (220) for regulating the flow of flue gas discharged from the economizer (200) into the second flue (210); A first valve body assembly for regulating the flue gas flow of the bypass flue (400) is provided in the bypass flue (400); A second valve body assembly for regulating the flue gas flow in the recirculation flue (500) is provided in the recirculation flue (500).

3. The flue gas recirculation system of a coal-fired power plant according to claim 2, characterized in that: The second valve body assembly includes: a second shutoff valve (530) provided at the junction of the recirculation flue (500) and the second flue (210); The second flow regulating valve (540) is provided in the recirculation flue (500) and is used to regulate the flue gas flow between the recirculation flue (500) and the second flue (210).

4. The flue gas recirculation system of a coal-fired power plant according to claim 2, characterized in that: The second valve body assembly further includes: a first mixing regulating valve (511), disposed in the first circulating flue (510); The second mixing regulating valve (521) is provided in the second circulating flue (520).

5. The flue gas recirculation system of a coal-fired power plant according to claim 2, characterized in that: The coal-fired power unit flue gas recirculation system further includes: A dust removal device (550) is provided in the recycling flue (500); The induced draft device (560) is arranged in the recycling flue (500) and is arranged behind the dust removal device along the flue gas flow direction.

6. The flue gas recirculation system of a coal-fired power plant according to claim 2, characterized in that: The coal-fired power unit flue gas recirculation system further includes: a first flow meter (570), disposed in the recirculation flue (500) and used to measure the flue gas flow between the recirculation flue (500) and the second flue (210); a second flow meter (512) disposed in the first circulating flue (510); The third flow meter (522) is arranged in the second circulation flue (520).

7. The flue gas recirculation system of a coal-fired power plant according to claim 2, characterized in that: The first valve body assembly includes: a first shutoff valve (420) provided at the junction of the bypass flue (400) and the first flue (100); The first flow regulating valve (430) is arranged in the bypass flue (400) and is arranged behind the heating component (410) along the flue gas flow direction.

8. The flue gas recirculation system of a coal-fired power plant according to claim 7, characterized in that: The first valve body assembly further includes: An expansion joint is provided at the junction of the bypass flue (400) and the second flue (210).

9. A method for controlling a flue gas recirculation system of a coal-fired power plant, characterized in that: The method comprises: Obtain the operating conditions of coal-fired power units; Determining whether the operating condition is a high-load condition; If not, the bypass flue is opened, the heating component is turned on, the second circulation flue is closed, and at the same time, the first circulation flue is opened to introduce the flue gas in the second flue into the first flue; Obtain the current temperature of the flue gas entering the denitrification device; Based on the current temperature, the operating power of the heating component is adjusted; wherein the operating power is inversely proportional to the current temperature.

10. The control method according to claim 9, characterized in that: After the step of determining whether the operating condition is a high-load condition, the method further includes: If so, the heating component and the bypass flue are closed, and at the same time, the first circulation flue and the second circulation flue are opened to introduce the flue gas from the second flue into the overburnt air output end of the boiler and the combustion zone of the boiler.

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