A flue gas waste heat recovery air preheating system and method

Abstract

The present invention discloses a flue gas waste heat recovery air preheating system and method. The system includes an air preheater, which comprises a housing and heat exchange tubes arranged inside the housing. The housing includes a first preheating system and a second preheating system that are connected in sequence. The first preheating system includes an air preheater inlet section, an air preheating section I, and an air preheating section I outlet section. The second preheating system includes an air preheating section II outlet section, an air preheating section II, and an air preheating section II inlet section. An air pipeline is provided between the air preheating section I outlet section and the air preheating section II inlet section. Air is discharged after being heated successively through the first preheating system, the air pipeline, and the second preheating system of the air preheater. The flue gas waste heat recovery air preheating system proposed by the present invention is compact in structure and small in floor area. The two-stage layout can effectively reduce the risk of low-temperature corrosion at the air preheater inlet section, reduce the comprehensive cost of waste heat recovery, and achieve better energy conservation and emission reduction effects.

Description

Technical Field

[0001] The present invention relates to the technical field of waste heat recovery and utilization, and particularly relates to a flue gas waste heat recovery air preheating system and method. Background Art

[0002] An air preheater is a common auxiliary equipment for furnaces that recovers the waste heat of flue gas, increases the temperature of the air entering the furnace, reduces the fuel consumption of the furnace, and improves the combustion condition. The traditional tubular air preheater has the characteristics of being less prone to leakage and being easy to replace and overhaul compared with the rotary air preheater. However, due to the low air inlet temperature, the traditional tubular air preheater often encounters the problem of low-temperature corrosion during operation, which seriously affects the service life and safe operation of the equipment. On the other hand, the traditional tubular air preheater generally uses straight round tubes, with low heat transfer efficiency and large volume. Moreover, since most of the heat exchangers are arranged in a cross-flow pattern, in order to reduce the flow resistance on the flue gas side outside the tubes, a relatively low flue gas flow rate is generally adopted. On the one hand, this reduces the overall heat transfer coefficient of the heat exchanger. On the other hand, limited by the end temperature difference of heat transfer in the cross-flow arrangement, the flue gas temperature at the outlet of the air preheater is generally relatively high, and the maximum recovery of flue gas waste heat cannot be achieved. Therefore, the existing technology needs to be improved and developed. Summary of the Invention

[0003] The present invention provides a flue gas waste heat recovery air preheating system and method. The structural design of the flue gas waste heat recovery air preheating system proposed by the present invention adopts a two-stage co-current flow. The first stage of air preheating adopts a co-current flow anti-low-temperature corrosion arrangement method, and the second stage of air preheating adopts a counter-current flow arrangement. The heat exchange tubes all adopt spiral twisted high-efficiency heat exchange tubes, which have a good heat transfer enhancement effect, reduce the pressure drop while ensuring the gas flow rate, are not prone to vibration, and have a strong anti-fouling ability.

[0004] The object of the present invention is to propose a flue gas waste heat recovery air preheating system, including an air preheater. The air preheater includes a housing and heat exchange tubes arranged inside the housing. The housing is of a vertical structure. The housing includes a first preheating system and a second preheating system that are sequentially connected. In the first preheating system, the flow directions of air and flue gas are the same, and in the second preheating system, the flow directions of air and flue gas are opposite. The first preheating system includes an air preheater inlet section, a first stage of air preheating section, and a first stage of air preheating section outlet section. The second preheating system includes a second stage of air preheating section outlet section, a second stage of air preheating section, and a second stage of air preheating section inlet section. An air pipeline is arranged between the first stage of air preheating section outlet section and the second stage of air preheating section inlet section. The flue gas discharged from the boiler tail flue gas passage enters the flue gas purification system after releasing heat through the tube passes of the first stage of air preheating section and the second stage of air preheating section. The air pressurized by the blower is discharged after being heated successively through the first preheating system, the air pipeline, and the second preheating system of the air preheater.

[0005] The air at a lower temperature first enters the first stage of air preheating and flows in the same direction as the flue gas. The air at a lower temperature first indirectly contacts the flue gas at a higher temperature. At the same time, the inlet air enters the heat exchange section circumferentially. The heat exchange tubes are arranged in a rectangular shape, leaving a relatively large flow space outside the tubes. The air velocity at the inlet section is controlled at a relatively low flow rate, which is beneficial to increasing the outer wall temperature of the heat exchange tubes (controlling the wall temperature above the acid dew point temperature).

[0006] In order to reduce the resistance on the air side, the circumferential air inlet and outlet methods are adopted at both the inlet section and the outlet section of the air preheater, reducing the air velocity and resistance at the inlet section and the outlet section, and then reducing the overall air flow resistance of the air preheater.

[0007] Preferably, the heat exchange tubes are spiral twisted heat exchange tubes. The heat exchange tubes of the first preheating system are arranged in a rectangular shape, and the heat exchange tubes of the second preheating system are arranged in a triangular shape. The heat exchange tubes adopt spiral twisted high specific surface heat exchange tubes that can enhance heat transfer on both sides. By strengthening the external flow disturbance of the tubes and the internal swirl flow, the convective heat transfer coefficients inside and outside the tubes are increased. The spiral twisted high specific surface heat exchange tubes are formed by twisting a straight circular tube, and 150 - 200 mm straight tube sections are still reserved at both ends of the heat exchange tubes.

[0008] Based on the deficiencies of traditional tubular air preheaters, the present invention adopts spiral twisted high specific surface heat exchange tubes as heat exchange elements. At the same time, the flow and heat transfer form of the flue gas and air in the air preheater adopts a co - flow form, and the second stage of air preheating in the main heat exchange section adopts a counter - flow arrangement. As a result, not only the heat exchange performance of the air preheater is greatly improved, but also the risk of low - temperature corrosion of traditional tubular air preheaters is overcome. Ordinary carbon steel tubes can be used as the heat exchange tube material, reducing the material cost of the air preheater. Moreover, the maximum temperature drop of the flue gas can be achieved, thus obtaining the maximum energy - saving benefit.

[0009] The air preheater adopts spiral twisted high specific surface heat exchange tubes. Due to the spiral flow inside the tubes, it is more difficult for dust to accumulate on the inner wall surface of the tubes. The flue gas passes through the tube side and flows directly through the heat exchange tubes, avoiding the transverse erosion of the flue gas flowing through the shell side in traditional shell - and - tube air preheaters, greatly reducing the flue gas resistance. Or under the condition of the same flue gas resistance drop, the flue gas velocity is higher, which is more conducive to the cleaning of the heat exchange tube surface and improving the comprehensive heat transfer performance.

[0010] The structural design of the air preheating system adopts a two - stage layout. The tube bundles in the first stage of air preheating are arranged in a rectangular shape to reduce the air velocity outside the tubes. The first stage of air preheating preheats the air to a relatively safe air temperature t a2 and then enters the second stage of air preheating. The tube bundles in the second stage of air preheating are arranged in a triangular shape to increase the air velocity outside the tubes, thereby improving the heat transfer performance on the air side. The safe temperature of t a2 is controlled as the outer wall temperature t of the inlet section of the second stage of air preheating w2Higher than the acid dew point temperature.

[0011] Preferably, an air return pipeline for preventing the heat exchange tubes at the inlet section of the first stage of air preheating from being corroded by low-temperature dew point is provided between the outlet section of the first stage of air preheating and the inlet section of the air preheater. In order to avoid the corrosion of the heat exchange tube wall temperature at the inlet section of the air preheater being lower than the low-temperature dew point, an air return pipeline is adopted in which part of the intermediate air is returned to the inlet section of the air preheater, so as to increase the inlet air temperature t a1 in such a way that t a1 The safety temperature control is that the outer wall temperature t of the pipe at the inlet section of the first stage of air preheating w1 is higher than the acid dew point temperature. By increasing the heat exchange tube wall temperature at the inlet section, the entire heat exchange element can be protected from low-temperature corrosion, and ordinary carbon steel materials can be used, thereby reducing the overall cost of the air preheater and improving the equipment reliability.

[0012] Preferably, a control valve for controlling the air return flow rate is provided on the air return pipeline.

[0013] The present invention also provides a method for recovering flue gas waste heat and preheating air using the above-mentioned flue gas waste heat recovery air preheating system, which includes the following steps: The flue gas discharged from the boiler tail flue gas passage passes through the tube passes of the first stage of air preheating and the second stage of air preheating, releases heat and then enters the flue gas purification system. The air pressurized by the blower sequentially passes through the inlet section of the air preheater, the first stage of air preheating and the outlet section of the first stage of air preheating for the first-stage preheating. During the first-stage preheating process, the air and the flue gas in the heat exchange tube are in a co-current heat exchange mode. The air passing through the outlet section of the first stage of air preheating then enters the inlet section of the second stage of air preheating, the second stage of air preheating and the outlet section of the second stage of air preheating through the first air pipeline for the second-stage preheating and then is discharged for the next operation. During the second-stage preheating process, the air and the flue gas in the heat exchange tube are in a counter-current heat exchange mode.

[0014] Preferably, the first stage of air preheating is also provided with a bypass return flue. When the ambient air temperature is relatively low, part of the air passing through the outlet section of the first stage of air preheating is returned to the inlet section of the air preheater, and is mixed with the blown air to increase the inlet temperature of the air preheater, so as to prevent the heat exchange tubes at the inlet section of the first stage of air preheating from being corroded by low-temperature dew point.

[0015] Compared with the prior art, the beneficial effects of the present invention are:

[0016] 1. The structural design of the flue gas waste heat recovery air preheating system of the present invention adopts a two-stage co-current type, in which the first stage of air preheating adopts a co-current type anti-low-temperature corrosion layout, and the second stage of air preheating adopts a counter-current type large temperature difference layout.

[0017] 2. The heat exchange tubes in the flue gas waste heat recovery air preheater are all spirally twisted high-efficiency heat exchange tubes, which have good enhanced heat transfer effect, ensure gas flow rate while reducing pressure drop, are not prone to vibration and have strong anti-ash deposition ability.

[0018] 3. In order to protect the heat exchange tubes at the inlet section of the air preheater from low-temperature corrosion, an intermediate air extraction reflux method is adopted.

[0019] 4. Due to the characteristics of enhanced heat exchange on both sides of the spiral twisted tube and (air preheater section II), a larger heat exchange temperature difference can be obtained, and the required heat exchange area will be greatly reduced. The entire flue gas waste heat recovery air preheating system is 30-50% smaller in volume and 30-50% lighter than the traditional air preheating device, achieving efficient and energy-saving operation of flue gas waste heat recovery.

[0020] 5. The air preheater adopts a two-stage flue pipe layout, a vertical layout, a compact structure and a small footprint. The two-stage layout can effectively reduce the risk of low-temperature corrosion at the inlet section of the air preheater. The air preheating II section adopts a countercurrent layout and all heat exchange tubes use spiral twisted high-efficiency heat exchange tubes and a parallel flow layout, which can increase the air side flow rate and enhance the heat exchange inside and outside the tube, thereby improving the overall heat exchange effect and heat recovery level, reducing the comprehensive cost of waste heat recovery, and achieving better energy saving and emission reduction effects.

[0021] 6. Compared with the traditional single-stage co-current air preheater, the two-stage air preheater arrangement of the present invention adopts a counter-current type, so the smoke temperature can be lowered to a lower level (even lower than the air outlet temperature), thereby obtaining more low-temperature waste heat of the smoke. Compared with the traditional transverse flushing air preheater, the parallel flow arrangement adopted by the present invention can control the air flow velocity outside the tube at a higher level under the same air flow pressure drop outside the tube, thereby reducing the space requirement outside the tube, and then reducing the volume of the heat exchanger. The transverse flushing air preheater is also subject to the heat exchange end difference temperature limitation that the outlet smoke temperature must be higher than the outlet air temperature. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a schematic diagram of the structure of the flue gas waste heat recovery air preheating system of Example 1;

[0023] Figure 2 Schematic diagram of the heat exchange tube structure in the air preheater in Example 1;

[0024] Figure 3 Schematic diagram of the arrangement of heat exchange tubes in Example 1, wherein: Figure a is a rectangular arrangement, and Figure b is a triangular arrangement;

[0025] Description of reference numerals: 1. Blower; 2. Inlet section of air preheater; 3. First stage of air preheating; 4. Outlet section of the first stage of air preheating; 5. Air pipeline; 6. Inlet section of the second stage of air preheating; 7. Second stage of air preheating; 8. Outlet section of the second stage of air preheating; 9. Control valve; 10. Air return pipeline; 11. Flue gas passage.

[0026] Figure 4 It is a schematic diagram showing the variation of flue gas and air temperatures with the process in the two-stage air preheater in Example 1, where: t a represents the air temperature, and t w represents the outer wall temperature of the heat exchange tube, and t f represents the flue gas temperature. Detailed implementation mode

[0027] The following embodiments are further illustrations of the present invention rather than limitations thereof.

[0028] Example 1

[0029] As Figures 1 to 4 shown, a flue gas waste heat recovery air preheating system includes an air preheater, and the air preheater includes a housing and heat exchange tubes ( Figure 1 not shown) arranged inside the housing. The housing is of a vertical structure and includes a first preheating system and a second preheating system that are connected in sequence. The first preheating system and the second preheating system are connected in series by the flue gas passage 11. The flow directions of air and flue gas in the first preheating system are the same, and the flow directions of air and flue gas in the second preheating system are opposite. The first preheating system includes the inlet section 2 of the air preheater, the first stage 3 of air preheating, and the outlet section 4 of the first stage of air preheating. The second preheating system includes the outlet section 8 of the second stage of air preheating, the second stage 7 of air preheating, and the inlet section 6 of the second stage of air preheating. An air pipeline 5 is arranged between the outlet section 8 of the first stage of air preheating and the inlet section 6 of the second stage of air preheating. The flue gas discharged from the boiler tail flue gas passage passes through the tube passes of the first stage and the second stage of air preheating, releases heat, and then enters the flue gas purification system. The air pressurized by the blower 1 passes through the first preheating system, the air pipeline 5, and the second preheating system of the air preheater in sequence, is heated, and then discharged.

[0030] The lower-temperature air first enters the first stage of air preheating and flows in the same direction as the flue gas. The lower-temperature air first indirectly contacts the higher-temperature flue gas. At the same time, the inlet air enters the heat exchange section circumferentially. The heat exchange tubes in the first preheating system are arranged in a rectangular shape, leaving a relatively large flow space outside the tubes. The air velocity at the inlet section is controlled at a relatively low flow rate, which is beneficial to increasing the outer wall temperature of the heat exchange tubes (controlling the wall temperature above the acid dew point temperature).

[0031] In order to reduce the air-side resistance, the circumferential inlet and outlet air flow modes are adopted for the inlet section 2 and outlet section 4 of the air preheater, and the inlet section 6 and outlet section 8 of the second section of the air preheater, so as to reduce the air flow velocity and resistance in the inlet and outlet sections, and then reduce the overall air flow resistance of the air preheater.

[0032] The heat exchange tubes are spiral twisted heat exchange tubes. The spiral twisted high specific surface heat exchange tubes are formed by twisting a straight circular tube. 150 - 200 mm straight tube sections are still reserved at both ends of the heat exchange tubes. The heat exchange tubes of the first preheating system are arranged in a rectangular shape, and the heat exchange tubes of the second preheating system are arranged in a triangular shape. The heat exchange tubes adopt spiral twisted high specific surface heat exchange tubes that can enhance heat transfer on both sides. By strengthening the external flow disturbance and internal swirl of the tubes, the convective heat transfer coefficients inside and outside the tubes are improved.

[0033] Based on the deficiencies of traditional tubular air preheaters, the present invention adopts spiral twisted high specific surface heat exchange tubes as heat exchange elements. At the same time, the flow and heat transfer form of flue gas and air in the air preheater adopts a co-flow form, and the second section of the main heat exchange section of air preheating adopts a counter-flow and triangular tube arrangement. Therefore, not only the heat transfer performance of the air preheater is greatly improved, but also the risk of low-temperature corrosion of traditional tubular air preheaters is overcome. Ordinary carbon steel tubes can be used as the heat exchange tube material, reducing the material cost of the air preheater. Moreover, the maximum temperature drop of the flue gas can be achieved, thus obtaining the maximum energy-saving benefit.

[0034] The air preheater adopts spiral twisted high specific surface heat exchange tubes. Due to the spiral flow inside the tubes, it is more difficult for ash to accumulate on the inner wall surface of the tubes. The flue gas passes through the tube side and flows directly through the heat exchange tubes, avoiding the transverse scouring of the flue gas in the shell side of traditional shell-and-tube air preheaters, greatly reducing the flue gas resistance. Or under the condition of the same flue gas resistance drop, the flue gas flow velocity is higher, which is more conducive to the cleaning of the heat exchange tube surface and improves the comprehensive heat transfer performance.

[0035] The structural design of the air preheating system adopts a two-stage layout. The tube bundle of the first preheating system is arranged in a rectangular shape to reduce the air flow velocity outside the tubes. The first preheating system preheats the air to a relatively safe air temperature t a2 and then enters the second preheating system. The tube bundle of the second preheating system is arranged in a triangular shape to increase the air flow velocity outside the tubes, thereby improving the heat transfer performance on the air side. The safe temperature of t a2 is controlled such that the outer wall temperature t w2 of the inlet section of the second section of air preheating is higher than the acid dew point temperature.

[0036] A heat exchange tube (outer wall temperature t w1)The air return pipeline 10 is subject to low-temperature dew point corrosion. A control valve 9 for controlling the air return flow rate is provided on the air return pipeline 10. The air return flow rate is determined by adjusting the size of the control valve according to the inlet air temperature. To avoid the heat exchange tube wall temperature t of the inlet section 2 of the air preheater w1 from being lower than the low-temperature dew point and corroding, an air return pipeline is adopted in which part of the middle air is returned to the inlet section 2 of the air preheater to increase the inlet air temperature, ensuring that the entire equipment is also protected from low-temperature corrosion, and ordinary carbon steel materials can be used to reduce the overall cost of the air preheater.

[0037] From Figure 4 it can be seen that under normal circumstances, the control valve 9 (bypass valve) is closed. The air pressurized by the blower 1 enters from the inlet section 2, absorbs the waste heat of the flue gas in the air preheating section I, and is discharged from the outlet section 4. The air temperature rises from t a1 to t a2 , and then after being preheated in the second preheating stage, the air enters the air preheater section II from the inlet section 6 of the air preheater section II, continues to absorb the waste heat of the flue gas, and is discharged from the outlet section 8. The air temperature rises from ta2 to t a3 ; at the same time, the flue gas in the heat exchange tube enters the air preheating section I from the inlet section 2 of the air preheater, releases part of the heat, is discharged from the outlet section 4 and passes through the flue gas channel 11, and then enters the air preheater section II to further release heat, and then enters the flue gas purification system. The flue gas temperature drops from t f1 at the inlet of the air preheater section I to t f2 at the outlet of the air preheater section I and then enters the air preheater section II and drops to t f3 and then is discharged to the purification system. When the tube wall temperature t w1 is lower than the acid dew point temperature, the control valve 9 (bypass valve) is opened. Part of the air discharged from the outlet section 4 of the air preheating section I is returned through the air return pipeline 10 and mixed with the inlet air of the air preheater to increase the inlet air temperature t a1 , so that the outer wall temperature t of the heat exchange tube w1 rises above the acid dew point to ensure that the inlet section of the air preheater is not corroded by low temperature.

[0038] A method for recovering waste heat from flue gas and preheating air using the above system specifically includes the following steps: The flue gas discharged from the flue gas channel at the tail of the boiler passes through the tube passes of the first-stage air preheater and the second-stage air preheater, releases heat, and then enters the flue gas purification system. The air pressurized by the blower 1 sequentially passes through the inlet section 2 of the air preheater, the first-stage air preheater 3, and the outlet section 4 of the first-stage air preheater for the first-step preheating. During the first-step preheating, the air and the flue gas in the heat exchange tubes exchange heat in a co-current manner. The air passing through the outlet section 4 of the first-stage air preheater then enters the inlet section 6 of the second-stage air preheater, the second-stage air preheater 7, and the outlet section 8 of the second-stage air preheater through the first air pipeline for the second-step preheating and is then discharged for the next operation. During the second-step preheating, the air and the flue gas in the heat exchange tubes exchange heat in a counter-current manner. When the ambient air temperature is relatively low, part of the air passing through the outlet section of the first-stage air preheater flows back to the inlet section of the air preheater and mixes with the blown air to increase the inlet temperature of the air preheater and prevent the heat exchange tubes at the inlet section of the first-stage air preheater from being corroded by low-temperature dew point.

[0039] The above is only the preferred embodiment of the present invention. It should be noted that the above preferred embodiment should not be regarded as a limitation of the present invention. The protection scope of the present invention should be subject to the scope defined by the claims. For those of ordinary skill in the art of this technology, without departing from the spirit and scope of the present invention, several improvements and refinements can still be made, and these improvements and refinements should also be regarded as within the protection scope of the present invention.

Claims

1. A method for recovering waste heat from flue gas and preheating air using a waste heat recovery air preheating system for flue gas, characterized in that The described flue gas waste heat recovery air preheating system includes an air preheater. The air preheater includes a housing and heat exchange tubes disposed inside the housing. The housing is of a vertical structure. The housing includes a first preheating system and a second preheating system that are sequentially connected. In the first preheating system, the flow directions of air and flue gas are the same. In the second preheating system, the flow directions of air and flue gas are opposite. The first preheating system includes an air preheater inlet section, an air preheating section I, and an air preheating section I outlet section. The second preheating system includes an air preheating section II outlet section, an air preheating section II, and an air preheating section II inlet section. An air pipeline is provided between the air preheating section I outlet section and the air preheating section II inlet section. The flue gas discharged from the boiler tail flue gas passage passes through the tube passes of the air preheating section I and the air preheating section II, releases heat, and then enters the flue gas purification system. The air pressurized by the blower sequentially passes through the first preheating system, the air pipeline, and the second preheating system of the air preheater, is heated, and then discharged. The method includes the following steps: The flue gas discharged from the boiler tail flue gas passage passes through the tube passes of the air preheating section I and the air preheating section II, releases heat, and then enters the flue gas purification system. The air pressurized by the blower sequentially passes through the air preheater inlet section, the air preheating section I, and the air preheating section I outlet section to complete the first-step preheating. During the first-step preheating, the air and the flue gas in the heat exchange tubes are in a co-current heat exchange mode. The air passing through the air preheating section I outlet section then passes through the first air pipeline to enter the air preheating section II inlet section, the air preheating section II, and the air preheating section II outlet section for the second-step preheating and then is discharged for the next operation. During the second-step preheating, the air and the flue gas in the heat exchange tubes are in a counter-current heat exchange mode. The heat exchange tubes are spiral twisted heat exchange tubes. The heat exchange tubes of the first preheating system are arranged in a rectangular shape, and the heat exchange tubes of the second preheating system are arranged in a triangular shape. The lower-temperature air first enters the air preheating section I and flows in a co-current direction with the flue gas. The lower-temperature air first indirectly contacts the higher-temperature flue gas. At the same time, the inlet air enters the heat exchange section circumferentially. The heat exchange tubes are arranged in a rectangular shape, leaving a relatively large flow space outside the tubes. The air velocity at the inlet section is controlled at a relatively low flow rate, which is beneficial to increasing the outer wall temperature of the heat exchange tubes and controlling the wall temperature to be higher than the acid dew point temperature. In order to reduce the air-side resistance, both the air preheater inlet section and the air preheater outlet section adopt a circumferential air inlet and outlet mode, reducing the air velocity and resistance at the inlet section and the outlet section, and then reducing the overall air flow resistance of the air preheater.

2. The method for recovering waste heat of flue gas and preheating air according to claim 1, wherein An air return pipeline for preventing the heat exchange tubes at the air preheating section I inlet section from being corroded by low-temperature dew point is provided between the air preheating section I outlet section and the air preheater inlet section.

3. The method for recovering waste heat from flue gas and preheating air according to claim 2, wherein A control valve for controlling the air return flow rate is provided on the air return pipeline.

4. The method for recovering waste heat from flue gas and preheating air according to claim 1, characterized in that Part of the air passing through the air preheating section I outlet section returns to the air preheater inlet section to prevent the heat exchange tubes at the air preheating section I inlet section from being corroded by low-temperature dew point.

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