CN205878220U - Coal fired boiler combination formula air heater - Google Patents

Abstract

The utility model provides a combined air preheater for a coal-fired boiler, which includes an evaporation tube bundle, a connecting tube, a heat regulator, a liquid level gauge, a connecting tube group, a temperature measuring device, a controller, a control valve, a water supply pipe, a first A first-stage air preheater and an exhaust valve; the heat regulator is located above the evaporator bundle, and the first-stage air preheater is located above the heat regulator; the upper end of the evaporator bundle is connected to the heat regulator through a pipe, and the heat regulator It communicates with the first-stage air preheater through a communication tube group; the evaporation tube bundle is arranged in the flue. The utility model improves the problem of acid condensation and ash accumulation in the air preheater of the coal-fired boiler, solves the problem that the air preheater cannot be used in the small coal-fired boiler, and greatly improves the efficiency of the small coal-fired boiler.

Description

A coal-fired boiler combined air preheater

technical field

The utility model relates to an air classification heating system and device, in particular to a combined air preheater for a coal-fired boiler.

Background technique

Context 1: Threats to coal-fired boiler air preheaters

SO ₃ in the flue gas of coal-fired boilers combines with water vapor to form H ₂ SO ₄ steam. When the temperature of the heating surface is lower than the acid dew point, sulfuric acid solution appears on the heating surface, and the results are: (1) corrosion and perforation of the heating surface; ( 2) The acid liquid on the heating surface adsorbs the fly ash in the flue gas, forming "cohesive" ash deposits, which affects heat transfer. In severe cases, acid corrosion can cause damage to the structure and performance of the heat exchanger, affect normal production, and even cause safety accidents.

Background 2: The use status of the existing coal-fired boiler air preheater

Coal-fired boilers with small capacity and low parameters basically do not use air preheaters.

Large-capacity coal-fired boilers use tubular air preheaters.

Power plant boilers use rotary air preheaters.

Background 3: Analysis of problems existing in the use of air preheaters for existing coal-fired boilers

(1) Small-capacity and low-parameter coal-fired boilers do not use air preheaters. Since the air preheater is facing corrosion, dust accumulation, blockage and operation management problems, the small capacity and low parameter coal-fired boiler uses the air preheater to recover low-temperature heat energy, which is cost-effective. It is the most cost-effective to exhaust the flue gas at high temperature. At present, for boilers of 10t/h and below, the exhaust gas temperature is above 180°C. my country has a large number of users of small-capacity and low-parameter boilers, and the high-temperature discharge of flue gas is extremely unfavorable to the energy-saving and emission-reduction of the entire country.

(2) Large-capacity coal-fired boilers use shell-and-tube air preheaters. Large-capacity coal-fired boilers use shell-and-tube air preheaters to preheat air and recover heat energy, which significantly increases the thermal efficiency of the boiler and improves the combustion conditions of the boiler. Ideally, the benefits of energy saving are significantly higher than equipment investment and maintenance costs , and the problem of corrosion damage can be controlled within the boiler maintenance period, and the corroded preheater components can be quickly replaced during the maintenance period.

However, the shell-and-tube air preheater is realized by using cold (air) and hot (flue gas) fluids through partition heat exchange. According to the principle of heat transfer, the wall temperature at the inlet of the air side of the tubular air preheater is the lowest. , it will inevitably lead to the lowest wall temperature on the flue gas side corresponding to the countercurrent heat exchange method (often used in design). (higher than the acid dew point) is the first design parameter, and the exhaust gas temperature is subject to the minimum wall temperature. The result of this design concept is: under standard conditions, the minimum wall temperature and the exhaust gas temperature often need to differ by 60°C to 80°C. If the ambient temperature changes, it will inevitably lead to a change in the minimum wall temperature of the heat exchanger. For example: in the long winter, the air temperature in the south will be as low as 0°C, and the air temperature in the north will be as low as -30°C. The minimum wall temperature of the tubular air preheater must be lower than the original design value, and acid condensation is inevitable. A difference of more than 100°C between the minimum wall temperature and the exhaust gas temperature cannot solve the problem of acid condensation corrosion on the wall surface. Once acid condensation occurs, the heating surface will begin to corrode and accumulate dust, until the perforation is damaged and air leaks, causing a large amount of cold air to enter the flue gas, and the temperature of the exhaust gas will be low; at the same time, due to the large amount of cold air entering the flue gas, It is also necessary to make up the air required by the boiler, increasing the induced air volume of the blower and the induced draft fan, resulting in a substantial increase in the energy consumption of the fan.

Of course, the biggest advantage of the tubular air preheater is its good sealing performance. Once corrosion damage occurs, this advantage will disappear; Not good for emission reduction. Practice has shown that due to changes in ambient temperature, when the shell-and-tube air preheater has not reached the maintenance period, the heating surface will be severely damaged and serious cross-wind phenomena will occur, which will affect the performance of the boiler and the thermal efficiency of the boiler.

(3) The utility boiler adopts a rotary air preheater. The rotary air preheater uses cold (air) and hot (flue gas) fluids to alternately pass through the heat accumulator element (corrugated plate) for contact heat exchange. Its advantages are: fast heat transfer speed, high heat transfer intensity, and small size ;Although acid condensation and dust accumulation are unavoidable, the main frame structure is corrosion-resistant, and the modular design and layout of the heat accumulator elements make it convenient and quick to replace after being damaged by corrosion.

However, the disadvantages of the rotary air preheater are significant.

Leakage. This is a structural defect of the rotary air preheater. The huge heat accumulator or the flue duct rotates relative to each other, and the gap between the flue gas side and the air side has direct air leakage (fluid with high pressure flows to fluid with low pressure through the gap, that is, air enters the flue gas) and carries Air leakage phenomenon. Under normal circumstances, the primary air and secondary air leak into the atmosphere and flue gas through the gaps of the fan-shaped sealing plates. When the processing quality of the device is good, the air leakage rate is 6% to 8%. The air leakage rate is 8% to 12%. When the processing quality or maintenance is not in place, the air leakage rate is 15% to 30%. The result of air leakage is that the flue gas is mixed with the leaked cold air, and the heat recovery efficiency is reduced; at the same time, in order to ensure the normal operation of the boiler, it is necessary to increase the blast air volume and induced air volume, which greatly increases the energy consumption of the blower and induced draft fan.

Literature research and theoretical calculations show that for every 1% reduction in the air leakage rate of the rotary air preheater, the power supply coal consumption of the coal-fired generating set will be reduced by about 0.15g/(kWh), calculated based on the average air leakage rate of the rotary air preheater being 10% , to solve the air leakage problem of the air preheater, and the national annual power generation can save about 10 million tons of standard coal.

Acid condensation, fouling and clogging. When the air and flue gas periodically flow through the corrugated plate to release heat, the temperature of the corrugated plate will periodically change from high to low and then from low to high, and acid condensation and acid liquid evaporation will appear on the surface of the corrugated plate. The acid liquid on the heating surface absorbs the dust in the flue gas, and then the acid liquid evaporates to produce "cohesive" dry ash, which gradually blocks the airflow channel and forms ash blockage. The result of dust accumulation and blockage is: the heat transfer rate decreases gradually, the flow resistance of flue gas increases, and the energy consumption of the fan increases; "cohesive" dust accumulation is difficult to clean up, and in serious cases, it needs to be shut down to clean the dust.

It can be seen that although the application of the rotary air preheater solves the problem of the tubular air preheater, it also exposes its own serious shortage.

At present, the design level and operation level of power plant boilers are very high. To further improve the thermal efficiency of the boiler unit, improving the preheating method of the air preheater is an important direction to improve thermal efficiency.

In short, the form of air preheaters for coal-fired boilers is not uniform, indicating that the air preheating technology is not perfect, and there are insurmountable problems: acid condensation on the heating surface, corrosion and dust accumulation, which lead to a decrease in boiler thermal efficiency and poor performance of the boiler system. unstable.

At present, there are few domestic and foreign literatures on coal-fired boilers adopting a unified model and air preheaters that can further improve the thermal efficiency of boilers.

However, for the high-temperature flue gas discharge of large boilers, there are many documents on recovering waste heat, such as patent document CN1619246A (2004.10.28) "A method for avoiding corrosion of boiler air preheater at low temperature", using deaerator or water supply tank The high-temperature water is used to heat the air. After the water heats the air, the temperature is still relatively high, and then the water with a relatively high temperature is sent to the flue gas cooler to absorb heat and heat up, and the flue gas releases heat to cool down. Because the water temperature in the flue gas cooler is relatively high , will not cause the wall temperature outside the cooler to be lower than the acid dew point, avoiding the corrosion of the heating surface; such as the patent document CN102705861A (2012.06.15) "A coal-fired industrial boiler air preheating system", in the boiler tail flue and Heat exchangers are arranged at the outlet of the blower, and the heat absorbed in the flue is transferred to the air in the air duct by using the heat transfer medium water to realize the waste heat of the flue gas to preheat the air. Due to the strict control of the water temperature and the temperature of the heat exchange wall, it can avoid The occurrence of acid condensation; such as the patent document CN100402964C (2004.12.14) "Heat pipe heat exchanger avoids dew point corrosion method and a heat pipe heat exchanger", adopts heat pipe technology, and consists of many independent heat pipes to form an air preheater, It has good heat transfer performance and anti-corrosion performance. This type of technology is aimed at waste heat recovery, and the effect is obvious. If it is to replace the air preheater, it can solve problems such as air preheaters for small-capacity boilers, tubular air preheaters with better anti-corrosion performance, and rotary air preheaters. There is still a big gap in solving the air leakage problem of the device.

Contents of the invention

The purpose of this utility model is to improve and innovate the existing problems and deficiencies in the existing coal-fired boiler air preheating technology, to provide a combined air preheater for coal-fired boilers, to prevent corrosion and "bonding" of the air preheating device "Protection" dust accumulation, lower exhaust gas temperature, and improve boiler thermal efficiency.

The technical scheme of the utility model is as follows:

A combined air preheater for a coal-fired boiler, including an evaporator tube bundle, a connecting tube, a heat regulator, a liquid level gauge, a connecting tube group, a temperature measuring device, a controller, a control valve group, a water supply pipe, and a first-stage air preheater. Heater, exhaust valve; the heat regulator is located above the evaporator tube bundle, and the first-stage air preheater is located above the heat regulator; the upper end of the evaporator tube bundle is connected to the heat regulator through a pipe, and the heat regulator is connected to the first stage The air preheater is connected through the connecting tube group; the evaporation tube bundle is arranged in the flue;

The evaporating tube bundle is composed of at least 2 groups of evaporating tube rows, any group of evaporating tube rows is composed of at least 2 flue gas heat exchange tubes; the lower end of the flue gas heat exchange tubes on any group of evaporating tube rows passes through the lower header The lower tube header is provided with a communication port, and the communication ports on two adjacent groups of evaporator tubes are connected through the communication tube; the upper end of the flue gas heat exchange tube on any group of evaporator tubes passes through the upper tube header The upper surface of the upper pipe header is provided with a steam exhaust port, and the steam exhaust port is connected with the heat regulator;

The heat regulator includes a steam drum and a serpentine tube group, the lower wall of the steam drum communicates with the exhaust port on the evaporator tube bundle through a pipeline; the serpentine tube group is arranged in the steam drum, and the serpentine tube group is provided with at least 2 The water inlet ends of all the serpentine pipes are connected in parallel to a water inlet header pipe, and the water inlet header pipe is connected with the water supply pipe; the water outlet ends of all the serpentine pipes are connected in parallel to an outlet water header pipe. The water outlet header pipe is connected with the water outlet pipe; a control valve group is connected through a pipeline between the water inlet header pipe and the water outlet header pipe, and the control valve group is composed of at least two control valves; the control valve group is electrically connected The signal output terminal of the controller and the signal input terminal of the controller are electrically connected to the temperature measuring device, and the temperature measuring device is installed on the steam drum;

The first-stage air preheater is provided with at least 2 sets of air heat exchange tube rows, any set of air heat exchange tube rows includes a steam lower header, a steam upper header and at least 2 air heat exchange tubes, any one The upper ends of all the air heat exchange tubes on a group of air heat exchange tube rows are connected through the steam upper header, and the lower ends of all the air heat exchange tubes on any group of air heat exchange tube rows are connected through the steam lower header; all steam The lower header is connected to the steam drum through the connecting pipe group; the first-stage air preheater is provided with an exhaust pipe, and the exhaust valve is arranged on the exhaust pipe;

The upper end of the liquid level gauge communicates with the steam drum through the pipeline, and the lower end of the liquid level gauge communicates with the communication port on the evaporating tube bundle through the pipeline.

Further, it also includes a third-stage air preheater, a first air duct, a second-stage air preheater, and a second air duct, and the third-stage air preheater and the second-stage air preheater are all arranged in the flue , the second-stage air preheater is located upstream of the evaporator bundle, the third-stage air preheater is located between the evaporator bundle and the second-stage air preheater; the air inlet end of the second-stage air preheater passes through the second air The pipeline communicates with the air outlet end of the first-stage air preheater, and the air outlet end of the second-stage air preheater communicates with the inlet end of the third-stage air preheater through the first air pipe.

Further, partitions are provided between two adjacent sets of air heat exchange tube rows in the first-stage air preheater, and all partitions are arranged alternately.

The principle of combined air preheater: the evaporator tube bundle is pre-injected with saturated water, and the water in the evaporator tube bundle absorbs the heat of the flue gas to generate steam. After the steam enters the heat regulator, it enters the first-stage air preheater along the connecting tube group. The air is heated in the first-stage air preheater, and the steam releases latent heat and turns into condensed water, which flows back to the evaporator bundle through the connecting tube group, and the air absorbs latent heat and then heats up; the heated air enters the second-stage air preheater in the high-temperature section for heating, The temperature is further raised, and then enters the third-stage air preheater to continue heating, and the temperature rises again. The heat regulator adjusts the amount of steam entering the first-stage air preheater by adjusting the amount of water flowing through the serpentine tube (specifically: by pre-writing the program in the controller, the controller receives the temperature sent by the temperature measuring device) After the temperature signal is obtained, the temperature signal is judged to control the opening of the control valve group to achieve the purpose of regulation), so that the heat absorbed by the cold air and the water in the serpentine tube are equal to the latent heat released by the steam, ensuring The temperature of the saturated water in the evaporator tube bundle (or the temperature of the wall surface of the heat exchange tube) is higher than the acid dew point.

Compared with the existing air preheater technology, the utility model has the following advantages:

(1) The heating surface can avoid acid condensation, eliminate the corrosion problem of the heating surface, and the problem of air leakage and "bonding" ash accumulation due to the damage of the heat exchange tube caused by acid condensation, which reduces the boiler exhaust gas temperature;

(2) The safe operation coefficient of boiler auxiliary equipment induced draft fan, blower and boiler system is greatly improved, and the maintenance workload is reduced.

(3) Solved the problem that small coal-fired boilers cannot use air preheaters, and greatly improved the efficiency of small coal-fired boilers.

Description of drawings

Fig. 1 is the front view structure schematic diagram of system and device of the present utility model;

Fig. 2 is a schematic top view of the structure of the first-stage air preheater;

Fig. 3 is a structural schematic diagram of any group of evaporating tube rows.

Description of the drawings: 1-evaporation tube bundle, 2-communication tube, 3-heat regulator, 4-liquid level gauge, 5-steam drum, 6-serpentine tube group, 7-communication tube group, 8-temperature measuring device, 9-controller, 10-control valve group, 11-water supply pipe, 12-third stage air preheater, 13-first air pipe, 14-second stage air preheater, 15-second air pipe, 16-first-stage air preheater, 17-exhaust valve, 101-flue gas heat exchange tube, 102-lower header, 103-upper header, 104-exhaust port, 105-communication port, 1601 -Air heat exchange tube, 1602-steam lower header, 1603-steam upper header, 1604-baffle

detailed description

For small-capacity and low-parameter boilers, the capacity is below 10t/h, the pressure is 0.98MPa, the steam temperature is 183°C, the exhaust gas temperature is 165°C, and there is no air preheater. After the boiler is running, the actual exhaust gas temperature is 175°C to 200°C. The first-stage air heating system and device composed of the utility model coal-fired boiler air preheating system and device are used for transformation, and the exhaust gas temperature of the boiler can be reduced to below 140°C. which is:

As shown in Figure 1, a combined air preheater for a coal-fired boiler includes an evaporation tube bundle 1, a connecting tube 2, a heat regulator 3, a liquid level gauge 4, a connecting tube group 7, a temperature measuring device 8, and a controller 9 , control valve group 10, water supply pipe 11, first stage air preheater 16, exhaust valve 17; The upper end of the evaporating tube bundle 1 communicates with the heat regulator 3 through a pipeline, and the heat regulator 3 communicates with the first-stage air preheater 16 through the connecting tube group 7; the evaporating tube bundle 1 is arranged in the flue;

As shown in Figure 3, the evaporating tube bundle 1 is composed of at least 2 groups of evaporating tube rows, and any group of evaporating tube rows is composed of at least 2 flue gas heat exchange tubes 101; The lower ends of the tubes 101 are connected through the lower header 102, and the lower header 102 is provided with a communication port 105, and the communication ports 105 on two adjacent groups of evaporation tube rows are connected through the communication tube 2; any group of evaporation tube rows The upper end of the upper flue gas heat exchange tube 101 is connected through the upper tube header 103, and the upper surface of the upper tube header 103 is provided with a steam exhaust port 104, and the steam exhaust port 104 is connected with the heat regulator 3;

The heat regulator 3 includes a steam drum 5 and a serpentine tube group 6, the lower wall of the steam drum 5 communicates with the exhaust port 104 on the evaporation tube bundle 1 through a pipeline; the serpentine tube group 6 is arranged in the steam drum 5, The serpentine pipe group 6 is provided with at least two serpentine pipes, and the water inlet ends of all the serpentine pipes are connected in parallel to a water inlet header pipe, and the water inlet header pipe is connected with the water supply pipe 11; the water outlet of all the serpentine pipes The ends are connected in parallel on a water outlet header pipe, and the water outlet header pipe is connected with the water outlet pipe; a control valve group 10 is connected through a pipeline between the water inlet header pipe and the water outlet header pipe, and the control valve group 10 consists of at least Composed of two control valves; the control valve group 10 is electrically connected to the signal output end of the controller 9, and the signal input end of the controller 9 is electrically connected to the temperature measuring device 8, which is installed on the steam drum 5;

The first-stage air preheater 16 is provided with at least 2 sets of air heat exchange tube rows, and any set of air heat exchange tube rows includes a steam lower header 1602, a steam upper header 1603 and at least 2 air heat exchange tubes 1601, the upper ends of all the air heat exchange tubes 1601 on any set of air heat exchange tubes are connected through the steam upper header 1603, and the lower ends of all the air heat exchange tubes 1601 on any set of air heat exchange tubes are connected through the steam lower header. The headers 1602 are connected; all the lower steam headers 1602 are connected with the steam drum 5 through the connecting tube group 7, that is, the number of connecting tubes included in the connecting tube group 7 is at least equal to the number of air heat exchange tube rows; the first An exhaust pipe is arranged on the primary air preheater 16, and the exhaust valve 17 is arranged on the exhaust pipe;

The upper end of the liquid level gauge 4 communicates with the steam drum 5 through a pipeline, and the lower end of the liquid level gauge 4 communicates with the communication port 105 on the evaporation tube bundle 1 through a pipeline.

When the boiler capacity is greater than 10t/h, it also includes a third-stage air preheater 12, a first air pipeline 13, a second-stage air preheater 14, a second air pipeline 15, a third-stage air preheater 12 and The second-stage air preheaters 14 are all arranged in the flue, and the second-stage air preheater 14 is located upstream of the evaporator bundle 1. Therefore, the temperature at the flue where the second-stage air preheater 14 is located is relatively higher than that of the evaporator bundle 1. The temperature at the flue where it is located is higher; the third-stage air preheater 12 is located between the evaporation tube bundle 1 and the second-stage air preheater 14; the air inlet port of the second-stage air preheater 14 passes through the second The air pipe 15 communicates with the air outlet end of the first-stage air preheater 16, and the air outlet end of the second-stage air preheater 14 is connected with the inlet end of the third-stage air preheater 12 through the first air pipe 13 Pass.

As shown in Figure 2, partitions 1604 are arranged between two adjacent groups of air heat exchange tube rows in the first-stage air preheater 16, and all partitions are arranged in a staggered manner, that is, each partition 1604 divides the first stage The passages in the air preheater 16 are divided into "Z" shaped passages, which prolongs the flow time of the air in the first stage air preheater 16, which is conducive to fully heating and utilizing the heat of the steam.

The working process of the combined air preheater is as follows:

After the evaporating tube bundle 1 absorbs the heat of the flue gas, the generated steam enters the heat regulator 3 through the exhaust port 104, and then most of the steam enters the first-stage air preheater 16 through the connecting tube group 7, and the first-stage air preheats In the device 16, the steam condenses and flows back to the evaporator bundle 1 under the action of gravity, the air absorbs heat and heats up, and the heated air enters the second-stage air preheater 14 and the third-stage air preheater 12 to continue to heat up.

In order to ensure the normal operation of the system device, add water to the evaporation tube bundle 1 to the liquid level specified by the liquid level gauge 4 before operation. After the boiler is running, the exhaust valve 17 is fully opened to exhaust steam for a certain period of time, and the liquid level of the liquid level gauge 4 returns to the design position, close the exhaust valve 17.

In order to ensure that the temperature of the heat exchange tube wall surface of the evaporator tube bundle 1 is higher than the acid dew point, the temperature is measured by the temperature measuring device 8 on the steam drum 5, and the signal value is transmitted to the controller 9, and the control valve group 10 is controlled according to the program set by the controller 9. The valves are switched on and off to adjust the amount of boiler feed water entering the serpentine tube group 6 of the heat regulator 3 .

Claims (3)

1. A combined air preheater for a coal-fired boiler, characterized in that it comprises an evaporation tube bundle (1), a communication tube (2), a heat regulator (3), a liquid level gauge (4), a communication tube group (7 ), temperature measuring device (8), controller (9), control valve group (10), water supply pipe (11), first-stage air preheater (16), exhaust valve (17); The device (3) is located above the evaporating tube bundle (1), and the first-stage air preheater (16) is located above the heat regulator (3); the upper end of the evaporating tube bundle (1) is connected to the heat regulator (3) through a pipe, The heat regulator (3) communicates with the first-stage air preheater (16) through the connecting tube group (7); the evaporation tube bundle (1) is arranged in the flue; The evaporating tube bundle (1) is composed of at least 2 groups of evaporating tube rows, any group of evaporating tube rows is composed of at least 2 flue gas heat exchange tubes (101); the flue gas heat exchanging tubes on any group of evaporating tube rows ( The lower end of 101) is connected through the lower pipe header (102), and the lower pipe header (102) is provided with a communication port (105), and the communication ports (105) on the adjacent two groups of evaporation tube rows pass through the communication pipe (2 ) are connected; the upper ends of the flue gas heat exchange tubes (101) on any group of evaporator tubes are connected through the upper header (103), and the upper surface of the upper header (103) is provided with a steam outlet (104 ), the exhaust port (104) is connected with the heat regulator (3); The heat regulator (3) includes a steam drum (5) and a serpentine tube group (6), and the lower wall of the steam drum (5) communicates with the exhaust port (104) on the evaporation tube bundle (1) through a pipe; The serpentine tube group (6) is arranged in the steam drum (5), and the serpentine tube group (6) is provided with at least two serpentine tubes, and the water inlet ends of all the serpentine tubes are connected in parallel on a water inlet header pipe, The water inlet header pipe is connected with the water supply pipe (11); the water outlet ends of all serpentine pipes are connected in parallel on a water outlet header pipe, and the water outlet header pipe is connected with the water outlet pipe; the water inlet header pipe and the water outlet pipe The header pipes are connected with a control valve group (10) through pipelines, and the control valve group (10) is composed of at least 2 control valves; the control valve group (10) is electrically connected to the signal output end of the controller (9) to control The signal input end of the device (9) is electrically connected to the temperature measuring device (8), and the temperature measuring device (8) is arranged on the steam drum (5); The first-stage air preheater (16) is provided with at least 2 sets of air heat exchange tube rows, and any set of air heat exchange tube rows includes a steam lower header (1602), a steam upper header (1603) and at least 2 air heat exchange tubes (1601), the upper ends of all the air heat exchange tubes (1601) on any set of air heat exchange tubes are connected through the upper steam header (1603), and any set of air heat exchange tubes on the top The lower ends of all the air heat exchange tubes (1601) are connected through the steam lower header (1602); all the steam lower headers (1602) are connected with the steam drum (5) through the communication tube group (7); the first The first-stage air preheater (16) is provided with an exhaust pipe, and the exhaust valve (17) is arranged on the exhaust pipe; The upper end of the liquid level gauge (4) communicates with the steam drum (5) through a pipeline, and the lower end of the liquid level gauge (4) communicates with the communication port (105) on the evaporation tube bundle (1) through a pipeline. 2. A combined air preheater for a coal-fired boiler according to claim 1, characterized in that it also includes a third-stage air preheater (12), a first air duct (13), a second-stage air The preheater (14), the second air duct (15), the third stage air preheater (12) and the second stage air preheater (14) are all arranged in the flue, and the second stage air preheater ( 14) Located upstream of the evaporation tube bundle (1), the third-stage air preheater (12) is located between the evaporation tube bundle (1) and the second-stage air preheater (14); the second-stage air preheater (14 ) of the air inlet end communicates with the air outlet end of the first-stage air preheater (16) through the second air pipe (15), and the air outlet end of the second-stage air preheater (14) passes through the first air pipe (13) communicates with the inlet end of the third-stage air preheater (12). 3. A combined air preheater for a coal-fired boiler according to claim 1 or 2, characterized in that, in the first-stage air preheater (16), between two adjacent groups of air heat exchange tube rows Partitions (1604) are arranged in between, and all the partitions are arranged in a staggered manner.