Preheating system for assisting desulfurization wastewater bypass drying

Abstract

A preheating system for assisting desulfurization wastewater bypass drying comprises a flue gas discharge pipeline, and a dust remover, a flue gas channel of a preheater, a desulfurization tower and a flue gas discharge chimney are sequentially arranged on the flue gas discharge pipeline according to the flow direction of flue gas. The preheater further comprises a wastewater bypass channel which is arranged at an interval with the flue gas channel; a wastewater channel is arranged at the bottom of the desulfurization tower, and a drying tower is arranged at the end part of the wastewater channel; and a bypass passing through the preheater is arranged on the wastewater channel to form a wastewater bypass channel. The flue gas waste heat can be used for increasing the temperature of the desulfurization concentrated waste liquid, and pipeline crystallization blockage accidents are prevented; the use amount of high-temperature bypass flue gas is reduced, the drying performance is improved, and an effective way is provided for comprehensive removal of flue gas pollution and comprehensive utilization of waste heat of a power station; the system is combined with a dust removal system, and comprehensive utilization of waste liquid, waste solid and waste heat is facilitated.

Description

Technical Field

[0001] The present application relates to a preheating system for assisting desulfurization wastewater bypass drying. Background Art

[0003] Coal-fired power units emit a large amount of pollutants. With the development demand for in-depth pollution control, the demand for comprehensive pollution treatment in thermal power plants is becoming increasingly strong, which requires strict control of SO2 emission concentration. Power plants have put into operation flue gas desulfurization systems, of which more than 90% are wet desulfurization.

[0004] Among the many wet flue gas desulfurization technologies, limestone-gypsum flue gas desulfurization has been widely used due to its good economic efficiency of absorbents and by-products, accounting for as high as 92.9%. However, wet flue gas desulfurization will produce a large amount of desulfurization wastewater, which contains a large number of harmful soluble ions and insoluble particles. The most widely used chemical precipitation + membrane concentration process currently includes acid-base neutralization coagulation clarification + softening + membrane concentration steps. The water quality treated by this method can reach a higher standard, but it also has the disadvantages of high investment and the inability to remove pollutants such as Cl and selenium, and cannot truly achieve zero discharge of desulfurization wastewater.

[0005] In the context of zero wastewater discharge, the state encourages the use of waste heat evaporation drying, crystallization and other treatment processes to achieve zero wastewater discharge. Desulfurization wastewater flue gas evaporation technology uses tail flue gas to absorb desulfurization wastewater, and at the same time, it converts Cl which is difficult to treat by chemical precipitation into - The soluble salts and heavy metal pollutants are dispersed into the ash, which in a certain sense realizes the zero emission of desulfurization wastewater from the power plant. However, a large amount of high-quality flue gas waste heat is consumed to ensure the drying effect, and more flue gas before the air preheater needs to be extracted, which affects the operation of the air preheater. Summary of the invention

[0006] In order to solve the above problems, the present application proposes a preheating system for auxiliary desulfurization wastewater bypass drying, including a flue gas exhaust pipe, on which a dust collector, a flue gas channel of a preheater, a desulfurization tower and a smoke exhaust chimney are arranged in sequence according to the flow direction of the flue gas; the preheater also includes a wastewater bypass channel arranged at intervals from the flue gas channel; a wastewater channel is arranged at the bottom of the desulfurization tower, and a drying tower is arranged at the end of the wastewater channel; a bypass through the preheater is arranged on the wastewater channel to form a wastewater bypass channel. The present application can use the waste heat of flue gas to increase the temperature of desulfurization concentrated waste liquid and prevent pipeline crystallization blockage accidents; reduce the use of high-temperature bypass flue gas, improve drying performance, and provide an effective way for comprehensive removal of flue gas pollution and comprehensive utilization of waste heat in power plants; the present application is combined with a dust removal system to help the comprehensive utilization of waste liquid-waste solid-waste heat.

[0007] Preferably, a first control valve is provided on the wastewater bypass channel and the parallel part of the wastewater channel, a second control valve is provided on the wastewater bypass channel relative to the rear end of the preheater, and a power pump is provided relative to the front end of the preheater.

[0008] Preferably, a wastewater storage tank is provided on the wastewater channel between the desulfurization tower and the wastewater bypass channel; a high-temperature buffer tank is provided on the wastewater channel between the wastewater bypass channel and the drying tower; and a driving pump is provided on the wastewater channel between the high-temperature buffer tank and the drying tower.

[0009] Preferably, the flue gas exhaust duct is led out from a boiler provided with an air preheater; and an auxiliary flue gas duct is provided between the boiler before the air preheater and the drying tower.

[0010] Preferably, it also includes a dry fume outlet pipe connected to the outlet end of the drying tower, and the outlet end of the dry fume outlet pipe is connected to the fume exhaust pipe in front of the dust collector.

[0011] Preferably, the preheater is a heat pipe preheater.

[0012] Preferably, the heat pipe preheater includes an upper heat exchange cavity and a lower heat exchange cavity, a plurality of inner heat exchange tubes are arranged between the upper heat exchange cavity and the lower heat exchange cavity, and a phase change medium is arranged in the inner heat exchange tubes; the upper heat exchange cavity is connected to the wastewater bypass channel, and the lower heat exchange cavity is connected to the flue gas channel. The present application utilizes the waste heat of the flue gas after dust removal to preheat the desulfurized concentrated wastewater through the heat pipe preheater to increase the wastewater temperature, on the one hand to prevent pollutant crystallization and blockage, on the other hand to improve the evaporation efficiency, save the amount of high-temperature flue gas bypass, and improve energy utilization efficiency.

[0013] Preferably, a spacer insulation section is provided between the upper heat exchange cavity and the lower heat exchange cavity, and the inner heat exchange tube is provided through the spacer insulation section;

[0014] The interval heat insulation section comprises an outer shell body arranged outside the inner heat exchange tube, and a heat insulation layer is arranged between the outer shell body and the inner heat exchange tube.

[0015] Preferably, a plurality of transverse partitions are arranged in the upper heat exchange cavity, the inlet end of the wastewater bypass channel is arranged at the top of the upper heat exchange cavity, and the outlet end of the wastewater bypass channel is arranged at the bottom of the upper heat exchange cavity; the transverse partitions are staggered so that the interior of the upper heat exchange cavity forms staggered internal circulation channels.

[0016] Preferably, the heat pipe preheater is designed as follows:

[0017] Step S1: obtaining the inlet temperature and physical property parameters of the wastewater, the inlet flue gas of the heat exchanger and the temperature of the heat transfer medium of the heat exchanger, and obtaining the ratio F of the heat dissipation section length to the heat absorption section length of the heat exchanger;

[0018] Step S2: According to the law of conservation of energy and the heat balance equation, the heat absorbed by the heat pipe is equal to the heat dissipated by the heat pipe. The Newton cooling formula for convective heat transfer can be obtained as follows:

[0019]

[0020] Where Q h is the heat pipe's heat absorption, unit W, Q c is the heat dissipation of the heat pipe, unit W, T0 is the flue gas temperature, unit K, T1 is the temperature of the heat transfer medium in the pipe, unit K, T m is the arithmetic mean temperature of the wastewater, in K, d0 is the outer diameter of the pipe, in m, d i is the inner diameter of the tube, in m, l1 is the length of the heat absorption section, in m, l2 is the length of the heat dissipation section, in m, h h0 It is the heat transfer coefficient between the outer wall and the flue gas, in W / (m 2 ·K), h hi It is the heat transfer coefficient between the inner wall of the heat absorption section and the heat transfer medium, with the unit of W / (m 2 ·K), h c0 is the heat transfer coefficient between the outer wall and the wastewater, in W / (m 2 ·K), h ci It is the heat transfer coefficient between the inner wall of the heat dissipation section and the heat transfer medium, with the unit of W / (m 2 ·K), λ is the thermal conductivity of the pipe wall material, unit is W / (m·K), the heat transfer resistance between the inner wall and the heat transfer medium and the heat conduction resistance of the pipe wall are neglected, and formula (1) is simplified to:

[0021] h h0 (T0-T1)=h c0 F(T1-T m ) (2)

[0022] Step S3: The Reynolds number is calculated as follows:

[0023]

[0024] Among them, Re h0 is the Reynolds number on the flue gas side, Re c0 is the Reynolds number on the wastewater side, u h0 is the flue gas velocity in m / s, u c0 is the wastewater flow rate in m / s, V h0 is the kinematic viscosity coefficient of the flue gas, in m 2 / s,V c0 is the kinematic viscosity coefficient of the wastewater, in m 2 / s,q h0 is the flue gas flow rate, in m 3 / s,q c0is the wastewater flow rate, in m 3 / s, l3 is the flue width, unit is m;

[0025] Step S4: The calculation formula of the Nusselt number is as follows:

[0026]

[0027] Where Nu is the Nusselt number and Pr is the Planck number;

[0028] Step S5: The calculation formula of the heat transfer coefficient is as follows:

[0029]

[0030] where λ h0 is the thermal conductivity of flue gas, in W / (m·K), λ c0 is the thermal conductivity of wastewater, unit W / (m·K). Substituting formula (3), (4), (5) into formula (2), we can get:

[0031]

[0032] Where C1 is the constant before the Reynolds number in the flue gas Nusselt number calculation formula, C2 is the constant before the Reynolds number in the wastewater Nusselt number calculation formula, n1 is the power value of the Reynolds number in the flue gas Nusselt number calculation formula, and n2 is the power value of the Reynolds number in the wastewater Nusselt number calculation formula;

[0033] Step S6: Calculate the wastewater outlet temperature, which can be calculated from the wastewater arithmetic mean temperature:

[0034] T2″=2T m -T2′ (7)

[0035] Where T2′ is the wastewater inlet temperature, unit K, T2″ is the wastewater outlet temperature, unit K. Substituting them into formula (6) can obtain the relationship between the wastewater outlet temperature and the flue gas temperature:

[0036]

[0037] Step S7: Check the wastewater outlet temperature. If the wastewater outlet temperature is lower than the design requirement, increase the ratio F of the heat dissipation section length to the heat absorption section length of the heat exchanger, and return to step S1 to re-calculate the design until the wastewater outlet temperature reaches the design requirement. If the wastewater outlet temperature is higher than the design requirement, reduce the ratio F of the heat dissipation section length to the heat absorption section length of the heat exchanger, and return to step S1 to re-calculate the design until the wastewater outlet temperature reaches the design requirement. If the wastewater outlet temperature reaches the design requirement, continue to calculate in step S8.

[0038] Step S8: Calculate the heat to be provided by the heat exchanger and the heat recovered from the flue gas by the single tube of the heat exchanger according to the set wastewater heating temperature:

[0039]

[0040] Where Q is the total heat provided by the heat exchanger, in kJ, C p is the specific heat capacity of wastewater at constant pressure, in kJ / (kg·K), M c is the mass flow rate of wastewater, in kg / s;

[0041] Step S9: Calculate the number of tubes required for the heat exchanger, that is, the ratio of the total heat recovered from the flue gas by the heat exchanger to the heat recovered from the flue gas by a single tube of the heat exchanger:

[0042]

[0043] Where n is the number of tubes required for the heat pipe preheater. The present application provides a specific heat exchange method for a heat pipe heat exchanger suitable for this heat exchange condition, which can effectively complete the heating of desulfurization wastewater and the utilization of exhaust flue gas heat.

[0044] This application can bring the following beneficial effects:

[0045] 1. The present application can utilize flue gas waste heat to increase the temperature of desulfurization concentrated waste liquid, prevent pipeline crystallization blockage accidents; reduce the use of high-temperature bypass flue gas, improve drying performance, and provide an effective way for comprehensive removal of power station flue gas pollution and comprehensive utilization of waste heat; the present application is combined with a dust removal system to facilitate the comprehensive utilization of waste liquid, waste solid and waste heat.

[0046] 2. This application utilizes the waste heat of flue gas after dust removal to preheat the desulfurization concentrated wastewater through a heat pipe preheater to increase the wastewater temperature. On the one hand, it prevents pollutant crystallization and blockage, and on the other hand, it improves the evaporation efficiency, saves the amount of high-temperature flue gas bypass, and improves energy utilization efficiency.

[0047] 3. The present application provides a specific heat exchange method for a heat pipe heat exchanger suitable for this type of heat exchange condition, which can effectively complete the heating of desulfurization wastewater and the utilization of exhaust flue gas heat. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0049] Figure 1 A schematic diagram of this application.

[0050] Figure 2 Schematic diagram of the heat pipe preheater.

[0051] Figure 3 This is a three-dimensional diagram of a heat pipe preheater. DETAILED DESCRIPTION

[0052] In order to clearly illustrate the technical features of the present solution, the present application is described in detail below through specific implementation methods and in conjunction with the accompanying drawings.

[0053] like Figure 1-3 As shown, a preheating system for auxiliary desulfurization wastewater bypass drying includes a flue gas exhaust pipe 1, on which a dust collector 2, a flue gas channel 6 of a preheater 3, a desulfurization tower 4 and a smoke exhaust chimney 5 are arranged in sequence according to the flow direction of the flue gas; the preheater 3 also includes a wastewater bypass channel 7 arranged at intervals from the flue gas channel 6; a wastewater channel 8 is arranged at the bottom of the desulfurization tower 4, and a drying tower 9 is arranged at the end of the wastewater channel 8; a wastewater bypass channel 7 is formed by bypassing the preheater 3 on the wastewater channel 8.

[0054] A first control valve 11 is provided at the parallel part of the wastewater bypass channel 7 and the wastewater channel 8 , a second control valve 12 is provided at the wastewater bypass channel 7 relative to the rear end of the preheater, and a power pump 14 is provided relative to the front end of the preheater 3 .

[0055] A wastewater storage tank 15 is provided on the wastewater channel 8 between the desulfurization tower 4 and the wastewater bypass channel 7; a high-temperature buffer tank 16 is provided on the wastewater channel 8 between the wastewater bypass channel 7 and the drying tower 9; and a driving pump 17 is provided on the wastewater channel 8 between the high-temperature buffer tank 16 and the drying tower 9.

[0056] The flue gas exhaust duct 1 is led out from a boiler 19 provided with an air preheater 18 ; an auxiliary flue gas duct 20 is provided between the boiler 19 before the air preheater 18 and the drying tower 9 .

[0057] It also includes a dry fume outlet pipe 21 connected to the outlet end of the drying tower 9, and the outlet end of the dry fume outlet pipe 21 is connected to the fume exhaust pipe 1 in front of the dust collector 2.

[0058] The preheater 3 is a heat pipe preheater.

[0059] The heat pipe preheater includes an upper heat exchange cavity 22 and a lower heat exchange cavity 23, and a plurality of internal heat exchange tubes 24 are arranged between the upper heat exchange cavity 22 and the lower heat exchange cavity 23, and a phase change medium is arranged in the internal heat exchange tubes 24; the upper heat exchange cavity 22 is connected to the wastewater bypass channel 7, and the lower heat exchange cavity 23 is connected to the flue gas channel 6.

[0060] A spacer insulation section 25 is provided between the upper heat exchange cavity 22 and the lower heat exchange cavity 23, and the inner heat exchange tube 24 is provided through the spacer insulation section 25;

[0061] The spacer heat insulation section 25 includes an outer shell 26 disposed outside the inner heat exchange tube 24 , and a heat insulation layer 27 is disposed between the outer shell 26 and the inner heat exchange tube 24 .

[0062] A plurality of transverse partitions 28 are arranged in the upper heat exchange cavity 22, the inlet end of the wastewater bypass channel 7 is arranged at the top of the upper heat exchange cavity 22, and the outlet end of the wastewater bypass channel 7 is arranged at the bottom of the upper heat exchange cavity 22; the transverse partitions 28 are staggered so that staggered internal circulation channels 29 are formed inside the upper heat exchange cavity 22.

[0063] The heat pipe preheater is designed as follows:

[0064] Step S1: obtaining the inlet temperature and physical property parameters of the wastewater, the inlet flue gas of the heat exchanger and the temperature of the heat transfer medium of the heat exchanger, and obtaining the ratio F of the heat dissipation section length to the heat absorption section length of the heat exchanger;

[0065] Step S2: According to the law of conservation of energy and the heat balance equation, the heat absorbed by the heat pipe is equal to the heat dissipated by the heat pipe. The Newton cooling formula for convective heat transfer can be obtained as follows:

[0066]

[0067] Where Q h is the heat pipe's heat absorption, unit W, Q c is the heat dissipation of the heat pipe, unit W, T0 is the flue gas temperature, unit K, T1 is the temperature of the heat transfer medium in the pipe, unit K, T m is the arithmetic mean temperature of the wastewater, in K, d0 is the outer diameter of the pipe, in m, d i is the inner diameter of the tube, in m, l1 is the length of the heat absorption section, in m, l2 is the length of the heat dissipation section, in m, h h0 It is the heat transfer coefficient between the outer wall and the flue gas, in W / (m 2 ·K), h hi It is the heat transfer coefficient between the inner wall of the heat absorption section and the heat transfer medium, with the unit of W / (m 2 ·K), h c0 is the heat transfer coefficient between the outer wall and the wastewater, in W / (m 2 ·K), h ci It is the heat transfer coefficient between the inner wall of the heat dissipation section and the heat transfer medium, with the unit of W / (m 2 ·K), λ is the thermal conductivity of the pipe wall material, unit is W / (m·K), the heat transfer resistance between the inner wall and the heat transfer medium and the heat conduction resistance of the pipe wall are neglected, and formula (1) is simplified to:

[0068] h h0 (T0-T1)=h c0 F(T1-T m) (2)

[0069] Step S3: The Reynolds number is calculated as follows:

[0070]

[0071] Among them, Re h0 is the Reynolds number on the flue gas side, Re c0 is the Reynolds number on the wastewater side, u h0 is the flue gas velocity in m / s, u c0 is the wastewater flow rate in m / s, V h0 is the kinematic viscosity coefficient of the flue gas, in m 2 / s,V c0 is the kinematic viscosity coefficient of the wastewater, in m 2 / s,q h0 is the flue gas flow rate, in m 3 / s,q c0 is the wastewater flow rate, in m 3 / s, l3 is the flue width, unit is m;

[0072] Step S4: The calculation formula of the Nusselt number is as follows:

[0073]

[0074] Where Nu is the Nusselt number and Pr is the Planck number;

[0075] Step S5: The calculation formula of the heat transfer coefficient is as follows:

[0076]

[0077] where λ h0 is the thermal conductivity of flue gas, in W / (m·K), λ c0 is the thermal conductivity of wastewater, unit W / (m·K). Substituting formula (3), (4), (5) into formula (2), we can get:

[0078]

[0079] Where C1 is the constant before the Reynolds number in the flue gas Nusselt number calculation formula, C2 is the constant before the Reynolds number in the wastewater Nusselt number calculation formula, n1 is the power value of the Reynolds number in the flue gas Nusselt number calculation formula, and n2 is the power value of the Reynolds number in the wastewater Nusselt number calculation formula;

[0080] Step S6: Calculate the wastewater outlet temperature, which can be calculated from the wastewater arithmetic mean temperature:

[0081] T2″=2T m -T2′ (7)

[0082] Where T2′ is the wastewater inlet temperature, unit K, T2″ is the wastewater outlet temperature, unit K. Substituting them into formula (6) can obtain the relationship between the wastewater outlet temperature and the flue gas temperature:

[0083]

[0084] Step S7: Check the wastewater outlet temperature. If the wastewater outlet temperature is lower than the design requirement, increase the ratio F of the heat dissipation section length to the heat absorption section length of the heat exchanger, and return to step S1 to re-calculate the design until the wastewater outlet temperature reaches the design requirement. If the wastewater outlet temperature is higher than the design requirement, reduce the ratio F of the heat dissipation section length to the heat absorption section length of the heat exchanger, and return to step S1 to re-calculate the design until the wastewater outlet temperature reaches the design requirement. If the wastewater outlet temperature reaches the design requirement, continue to calculate in step S8.

[0085] Step S8: Calculate the heat to be provided by the heat exchanger and the heat recovered from the flue gas by the single tube of the heat exchanger according to the set wastewater heating temperature:

[0086]

[0087] Where Q is the total heat provided by the heat exchanger, in kJ, C p is the specific heat capacity of wastewater at constant pressure, in kJ / (kg·K), M c is the mass flow rate of wastewater, in kg / s;

[0088] Step S9: Calculate the number of tubes required for the heat exchanger, that is, the ratio of the total heat recovered from the flue gas by the heat exchanger to the heat recovered from the flue gas by a single tube of the heat exchanger:

[0089]

[0090] Where n is the number of tubes required for the heat pipe preheater.

[0091] When carrying out specific design, in combination with a usage scenario and specific parameters of this application, follow the following steps:

[0092] Step S1: The wastewater inlet temperature is preset to be 20°C, the flow rate is 5t / h, the flue gas temperature at the heat exchanger inlet is preset to be 120°C, the flow rate is preset to be 192m 3 / s, the flue length and width are both 4m, the outer diameter of the heat exchanger tube is 38mm, the inner diameter of the tube is 32mm, the temperature of the heat transfer medium is 100℃, and the ratio of the heat dissipation section length to the heat absorption section length of the heat exchanger is set to F, and F=0.65;

[0093] Step S2: According to the law of conservation of energy and the heat balance equation, the heat absorbed by the heat pipe is equal to the heat dissipated by the heat pipe. The Newton cooling formula for convective heat transfer can be obtained as follows:

[0094]

[0095] Where Q h is the heat pipe absorption (W), Q c is the heat dissipation of the heat pipe (W), T0 is the flue gas temperature (K), T1 is the temperature of the heat transfer medium in the pipe (K), T m is the arithmetic mean temperature of the wastewater (K), d0 is the outer diameter of the pipe (m), d i is the inner diameter of the tube (m), l1 is the length of the heat absorption section (m), l2 is the length of the heat dissipation section (m), h h0 is the heat transfer coefficient between the outer wall and the flue gas W / (m 2 ·K), h hi is the heat transfer coefficient between the inner wall of the heat absorption section and the heat transfer medium W / (m 2 ·K), h c0 is the heat transfer coefficient between the outer wall and the wastewater W / (m 2 ·K), h ci is the heat transfer coefficient between the inner wall of the heat dissipation section and the heat transfer medium W / (m 2 ·K), λ is the thermal conductivity of the pipe wall material W / (m·K), due to h hi 、h ci ,λratioh h0 、h c0 Big 10 3 -10 5 Therefore, the heat transfer resistance between the inner wall and the heat transfer medium and the heat conduction resistance of the tube wall can be ignored, and formula (1) can be simplified to:

[0096] h h0 (T0-T1)=h c0 F(T1-T m ) (2)

[0097] Step S3: The calculation formula of Reynolds number is as follows:

[0098]

[0099] Among them, Re h0 is the Reynolds number on the flue gas side, Re c0 is the Reynolds number on the wastewater side, u h0 is the flue gas velocity (m / s), u c0 is the wastewater flow rate (m / s), v h0 is the kinematic viscosity coefficient of the flue gas (m 2 / s), v c0 is the kinematic viscosity coefficient of the wastewater (m 2 / s), q h0 is the flue gas flow rate (m 3 / s), qc0 is the wastewater flow rate (m 3 / s), l3 is the flue width (m), calculate Re h0 =19159.66, Re c0 Equal to 9.965.

[0100] Step S4: The calculation formula of the Nusselt number is as follows:

[0101]

[0102] Where Nu is the Nusselt number and Pr is the Planck number. Calculate Nu h0 Equal to 75.425, Nu c0 Equal to 3.379.

[0103] Step S5: The calculation formula of the heat transfer coefficient is as follows:

[0104]

[0105] where λ h0 is the thermal conductivity of flue gas W / (m·K), λ c0 is the thermal conductivity of wastewater W / (m·K), h h0 Equal to 83.166W / (m·K), h c0 Equal to 56.395W / (m·K), substituting formula (3)(4)(5) into formula (2) yields:

[0106]

[0107] Among them, C1 is the constant before the Reynolds number in the flue gas Nusselt number calculation formula, C2 is the constant before the Reynolds number in the wastewater Nusselt number calculation formula, n1 is the power value of the Reynolds number in the flue gas Nusselt number calculation formula, and n2 is the power value of the Reynolds number in the wastewater Nusselt number calculation formula.

[0108] Step S6: Calculate the wastewater outlet temperature, which can be calculated from the wastewater arithmetic mean temperature:

[0109] T2″=2T m -T2′ (7)

[0110] Where T2′ is the wastewater inlet temperature (K), T2″ is the wastewater outlet temperature (K), and substituting them into formula (6) can obtain the relationship between the wastewater outlet temperature and the flue gas temperature:

[0111]

[0112] Step S7: Check the wastewater outlet temperature. If the wastewater outlet temperature is lower than the design requirement, increase the ratio F of the heat dissipation section length to the heat absorption section length of the heat exchanger, and return to step S1 to re-calculate the design until the wastewater outlet temperature reaches the design requirement. If the wastewater outlet temperature is higher than the design requirement, reduce the ratio F of the heat dissipation section length to the heat absorption section length of the heat exchanger, and return to step S1 to re-calculate the design until the wastewater outlet temperature reaches the design requirement. If the wastewater outlet temperature reaches the design requirement, continue to calculate in step S8. The wastewater outlet temperature is 89.24°C, which meets the design requirement, and continue to calculate in step S8.

[0113] Step S8: Calculate the heat to be provided by the heat exchanger and the heat recovered from the flue gas by the single tube of the heat exchanger according to the set wastewater heating temperature:

[0114]

[0115] Where Q is the total heat (kJ) to be provided by the heat exchanger, C p is the specific heat capacity of wastewater at constant pressure kJ / (kg·K), M c is the mass flow rate of wastewater (kg / s), Q is calculated to be 401.72 kJ / s, Q h It is 794.27W.

[0116] Step S9: Calculate the number of tubes required for the heat exchanger, that is, the ratio of the total heat recovered from the flue gas by the heat exchanger to the heat recovered from the flue gas by a single tube of the heat exchanger:

[0117]

[0118] Where n is the number of tubes required for the heat pipe preheater, which is 506.

[0119] In summer, when the exhaust temperature is higher than 120℃, the wastewater outlet temperature can be kept constant by increasing the wastewater flow rate. In winter, when the exhaust temperature is lower than 120℃, the wastewater outlet temperature can be kept constant by reducing the wastewater flow rate.

[0120] The above are only embodiments of the present application and are not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included within the scope of the claims of the present application.

Claims

1. A preheating system for auxiliary desulfurization wastewater bypass drying, characterized in that: It comprises a flue gas exhaust duct, on which a dust collector, a flue gas passage of a preheater, a desulfurization tower and a smoke exhaust chimney are arranged in sequence according to the flow direction of the flue gas; the preheater also comprises a wastewater bypass passage arranged at intervals from the flue gas passage; a wastewater passage is arranged at the bottom of the desulfurization tower, and a drying tower is arranged at the end of the wastewater passage; a wastewater bypass passage is formed by a bypass passing through the preheater on the wastewater passage.

2. A preheating system for auxiliary desulfurization wastewater bypass drying as claimed in claim 1, characterized in that: The wastewater bypass channel and the parallel part of the wastewater channel are provided with a first control valve, a second control valve is provided on the wastewater bypass channel relative to the rear end of the preheater, and a power pump is provided relative to the front end of the preheater.

3. A preheating system for auxiliary desulfurization wastewater bypass drying as claimed in claim 1, characterized in that: A wastewater storage tank is arranged on the wastewater channel between the desulfurization tower and the wastewater bypass channel; a high-temperature buffer tank is arranged on the wastewater channel between the wastewater bypass channel and the drying tower; and a driving pump is arranged on the wastewater channel between the high-temperature buffer tank and the drying tower.

4. A preheating system for auxiliary desulfurization wastewater bypass drying as claimed in claim 1, characterized in that: The flue gas exhaust duct is led out from a boiler provided with an air preheater; an auxiliary flue gas duct is provided between the boiler in front of the air preheater and the drying tower.

5. A preheating system for auxiliary desulfurization wastewater bypass drying as claimed in claim 1, characterized in that: It also includes a dry fume outlet pipe connected to the outlet end of the drying tower, and the outlet end of the dry fume outlet pipe is connected to the fume exhaust pipeline in front of the dust collector.

6. A preheating system for auxiliary desulfurization wastewater bypass drying as claimed in claim 1, characterized in that: The preheater is a heat pipe preheater.

7. A preheating system for auxiliary desulfurization wastewater bypass drying as claimed in claim 6, characterized in that: The heat pipe preheater includes an upper heat exchange cavity and a lower heat exchange cavity, a plurality of inner heat exchange tubes are arranged between the upper heat exchange cavity and the lower heat exchange cavity, and a phase change medium is arranged in the inner heat exchange tubes; the upper heat exchange cavity is connected to the wastewater bypass channel, and the lower heat exchange cavity is connected to the flue gas channel.

8. A preheating system for auxiliary desulfurization wastewater bypass drying as claimed in claim 7, characterized in that: A spacer insulation section is provided between the upper heat exchange cavity and the lower heat exchange cavity, and the inner heat exchange tube is provided through the spacer insulation section; The interval heat insulation section comprises an outer shell body arranged outside the inner heat exchange tube, and a heat insulation layer is arranged between the outer shell body and the inner heat exchange tube.

9. A preheating system for auxiliary desulfurization wastewater bypass drying as claimed in claim 8, characterized in that: A plurality of transverse partitions are arranged in the upper heat exchange cavity, the inlet end of the wastewater bypass channel is arranged at the top of the upper heat exchange cavity, and the outlet end of the wastewater bypass channel is arranged at the bottom of the upper heat exchange cavity; the transverse partitions are staggered so that staggered internal circulation channels are formed inside the upper heat exchange cavity.

10. A preheating system for auxiliary desulfurization wastewater bypass drying as claimed in claim 7, characterized in that: The heat pipe preheater is designed as follows: Step S1: obtaining the inlet temperature and physical property parameters of the wastewater, the inlet flue gas of the heat exchanger and the temperature of the heat transfer medium of the heat exchanger, and obtaining the ratio F of the heat dissipation section length to the heat absorption section length of the heat exchanger; Step S2: According to the law of conservation of energy and the heat balance equation, the heat absorbed by the heat pipe is equal to the heat dissipated by the heat pipe. The Newton cooling formula for convective heat transfer can be obtained as follows: Where Q h is the heat pipe's heat absorption, unit W, Q c is the heat dissipation of the heat pipe, unit W, T0 is the flue gas temperature, unit K, T1 is the temperature of the heat transfer medium in the pipe, unit K, T m is the arithmetic mean temperature of the wastewater, in K, d0 is the outer diameter of the pipe, in m, d i is the inner diameter of the tube, in m, l1 is the length of the heat absorption section, in m, l2 is the length of the heat dissipation section, in m, h h0 It is the heat transfer coefficient between the outer wall and the flue gas, in W / (m 2 ·K), h hi It is the heat transfer coefficient between the inner wall of the heat absorption section and the heat transfer medium, with the unit of W / (m 2 ·K), h c0 is the heat transfer coefficient between the outer wall and the wastewater, in W / (m 2 ·K), h ci It is the heat transfer coefficient between the inner wall of the heat dissipation section and the heat transfer medium, with the unit of W / (m 2 ·K), λ is the thermal conductivity of the pipe wall material, unit is W / (m·K), the heat transfer resistance between the inner wall and the heat transfer medium and the heat conduction resistance of the pipe wall are neglected, and formula (1) is simplified to: h h0 (T0-T1)=h c0 F(T1-T m ) (2) Step S3: The Reynolds number is calculated as follows: Among them, Re h0 is the flue gas side Reynolds number, Re c0 is the Reynolds number on the wastewater side, u h0 is the flue gas velocity in m / s, u c0 is the wastewater flow rate in m / s, V h0 is the kinematic viscosity coefficient of the flue gas, in m 2 / s,V c0 is the kinematic viscosity coefficient of the wastewater, in m 2 / s,q h0 is the flue gas flow rate, in m 3 / s,q c0 is the wastewater flow rate, in m 3 / s, l3 is the flue width, unit is m; Step S4: The calculation formula of the Nusselt number is as follows: Where Nu is the Nusselt number and Pr is the Planck number; Step S5: The calculation formula of the heat transfer coefficient is as follows: where λ h0 is the thermal conductivity of flue gas, in W / (m·K), λ c0 is the thermal conductivity of wastewater, unit W / (m·K). Substituting formula (3), (4), (5) into formula (2), we can get: Where C1 is the constant before the Reynolds number in the flue gas Nusselt number calculation formula, C2 is the constant before the Reynolds number in the wastewater Nusselt number calculation formula, n1 is the power value of the Reynolds number in the flue gas Nusselt number calculation formula, and n2 is the power value of the Reynolds number in the wastewater Nusselt number calculation formula; Step S6: Calculate the wastewater outlet temperature, which can be calculated from the wastewater arithmetic mean temperature: <h2 style=";text-align:left;direction:ltr">T2″=2T<h2 style=";text-align:left;direction:ltr"> m <h2 style=";text-align:left;direction:ltr"> -T2′ (7) Where T2′ is the wastewater inlet temperature, unit K, T2″ is the wastewater outlet temperature, unit K. Substituting them into formula (6) can obtain the relationship between the wastewater outlet temperature and the flue gas temperature: Step S7: Check the wastewater outlet temperature. If the wastewater outlet temperature is lower than the design requirement, increase the ratio F of the heat dissipation section length to the heat absorption section length of the heat exchanger, and return to step S1 to re-calculate the design until the wastewater outlet temperature reaches the design requirement. If the wastewater outlet temperature is higher than the design requirement, reduce the ratio F of the heat dissipation section length to the heat absorption section length of the heat exchanger, and return to step S1 to re-calculate the design until the wastewater outlet temperature reaches the design requirement. If the wastewater outlet temperature reaches the design requirement, continue to calculate in step S8. Step S8: Calculate the heat to be provided by the heat exchanger and the heat recovered from the flue gas by the single tube of the heat exchanger according to the set wastewater heating temperature: Where Q is the total heat provided by the heat exchanger, in kJ, C p is the specific heat capacity of wastewater at constant pressure, in kJ / (kg·K), M c is the mass flow rate of wastewater, in kg / s; Step S9: Calculate the number of tubes required for the heat exchanger, that is, the ratio of the total heat recovered from the flue gas by the heat exchanger to the heat recovered from the flue gas by a single tube of the heat exchanger: Where n is the number of tubes required for the heat pipe preheater.

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