Heat exchange system of low-heating-value gas boiler

Abstract

The invention discloses a low-heating-value gas boiler heat exchange system which comprises a horizontal flue and a vertical flue, a high-temperature reheater and a low-temperature reheater are arranged in the horizontal flue, and an upper-stage air preheater and a lower-stage air preheater are arranged in the vertical flue. A left side connecting flue and a right side connecting flue are arranged on the left side and the right side of the middle position of the upper-stage air pre-heater and the lower-stage air pre-heater, a bypass flue is connected between the left side connecting flue and the right side connecting flue and provided with a steam flue gas heat exchanger, and the bypass flue is connected with the two ends of the flue gas heat exchange side of the steam flue gas heat exchanger. The two ends of the steam heat exchange side of the steam flue gas heat exchanger communicate with a steam bypass, and the steam bypass communicates with an outlet of the low-temperature reheater and an inlet of the high-temperature reheater. The smoke temperature of the lower-stage air pre-heater is increased, the condensation possibility of ammonium hydrogen sulfate is reduced, the corrosion risk of the ammonium hydrogen sulfate to the heating surface of the lower-stage air pre-heater is reduced, and the service life of equipment is remarkably prolonged; and the overtemperature risk of the reheater is reduced, and the flexibility of reheating steam temperature adjustment is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of boiler systems, and in particular to a heat exchange system for a low calorific value gas boiler. Background Art

[0002] Low-calorific-value gas boiler systems are commonly used in industrial production. They generate heat by burning low-calorific-value gas, providing a heat source for industrial production. They are widely used in fields such as steel mill gas-fired power generation and coal chemical plant lignite tail gas power generation. To improve the boiler's thermal efficiency, low-calorific-value gas boiler systems typically incorporate flue gas heat recovery devices, such as air preheaters and economizers, in the boiler's tail flue. These devices recover waste heat from the flue gas, reducing exhaust temperatures and improving the boiler's thermal efficiency.

[0003] Because the exhaust gas produced by the combustion of low-calorific-value coal gas contains sulfur oxides and nitrogen oxides, gas-fired boilers often feed excessive ammonia at low loads to meet nitrogen oxide emission requirements. This escaped ammonia reacts with sulfur oxides and water vapor to form ammonium bisulfate, which exists in gaseous form in the denitrification system. The flue gas temperature decreases along the flue gas flow path, especially during low-load operation, where the flue gas temperature at the cold end of the air preheater falls below the dew point of ammonium bisulfate. Ammonium bisulfate exists in liquid form. Liquid ammonium bisulfate is highly viscous and deposits fly ash on the flue gas side of the air preheater, clogging the heating surface tubes and, in severe cases, causing corrosion of the heating surface.

[0004] The low-temperature reheater and low-temperature superheater are placed in parallel in the flue duct, typically using flue gas dampers to regulate the reheat steam temperature. Under low-load operating conditions, steam flow within the reheater decreases, and the flue gas damper on the reheater side is only 5% open, practically fully closed, with no margin for adjustment. The metal wall temperature of some tubes in the high-temperature reheater is excessively high, creating overheating and posing a serious safety hazard to reheater operation. To ensure unit safety, emergency water spraying is used to regulate the reheater temperature. However, the use of reheater water spraying reduces the unit's economic efficiency. Due to high high-pressure cylinder exhaust temperature, mismatched boiler heating surfaces, and excessive reheater heating surface layout, the reheater emergency water spraying used in a particular ultra-high-pressure gas-fired unit with reheat exceeded the design value at all loads, resulting in low unit economic efficiency. Studies have found that the impact of reheat steam desuperheating water on the heat consumption rate of the steam turbine is greater than that of superheater desuperheating water. At the same time, the frequent addition of desuperheating water will cause the temperature of the desuperheater pipe to change repeatedly, and frequently be subjected to large thermal shocks, which may form thermal fatigue cracks and eventually lead to cracking and bursting of the heating surface pipes, affecting the safe operation of the unit. Summary of the Invention

[0005] In order to improve the economic efficiency of the unit, a flue gas damper is usually used to adjust the reheat steam temperature during the temperature reduction of the reheater of a low calorific value gas boiler. Considering the limitations of the flue gas damper door adjustment mentioned above, and in order to solve the technical problem that sulfur oxides in the exhaust gas of the low calorific value gas furnace react with ammonia and water vapor to generate ammonium bisulfate, which causes blockage and corrosion of the heating surface of the air preheater, a low calorific value gas boiler heat exchange system is provided.

[0006] The present invention adopts the following technical solutions: A low calorific value coal gas boiler heat exchange system includes a horizontal flue and a vertical flue at the rear of the boiler. A high-temperature reheater and a low-temperature reheater are arranged in the horizontal flue. An economizer, an upper air preheater, and a lower air preheater are arranged in the vertical flue at the rear of the boiler. A left connecting flue and a right connecting flue are respectively provided on the left and right sides of a middle position between the upper air preheater and the lower air preheater in the vertical flue at the rear of the boiler. A bypass flue is connected between the left connecting flue and the right connecting flue. A steam flue heat exchanger is provided on the bypass flue. The bypass flue connects the two ends of the flue heat exchange side of the steam flue heat exchanger. The two ends of the steam heat exchange side of the steam flue heat exchanger are connected to the steam bypass. The steam bypass connects the low-temperature reheater outlet and the high-temperature reheater inlet.

[0007] Preferably, an electric isolating door and an electric regulating door are installed on the left connecting flue and the right connecting flue respectively.

[0008] Preferably, the upper air preheater and the lower air preheater are respectively of tubular structure, wherein the flue gas flows inside the tube and the air flows outside the tube.

[0009] Preferably, the steam-flue gas heat exchanger is of a tube-type or shell-and-tube structure, with steam flowing inside the tube and flue gas flowing outside the tube or on the shell side.

[0010] Preferably, electric shut-off valves are respectively provided on the steam bypasses at both ends of the steam heat exchange side of the steam-to-flue gas heat exchanger.

[0011] Preferably, a low-temperature reheater and high-temperature reheater connecting pipeline is installed between the high-temperature reheater and the low-temperature reheater, and an electric regulating valve is installed on the low-temperature reheater and high-temperature reheater connecting pipeline.

[0012] Preferably, a temperature sensor and a pressure sensor are provided on the steam heat exchange side of the steam-to-flue gas heat exchanger.

[0013] The beneficial effects of the present invention are as follows: the present invention increases the flue gas temperature of the lower-stage air preheater by adopting a steam heating flue gas system, reduces the possibility of condensation of ammonium bisulfate, reduces the risk of corrosion of the heating surface of the lower-stage air preheater by ammonium bisulfate, and significantly extends the service life of the equipment; at the same time, through heat exchange between part of the low-temperature reheater steam and the flue gas, the risk of overheating of the reheater is reduced, and the flexibility of reheat steam temperature regulation is increased. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a structural schematic diagram of the present invention; In the figure: 1. Upper air preheater; 2. Lower air preheater; 3. Left side connecting to flue; 4. Right side connecting to flue; 5. Left side connecting to flue electric isolation door; 6. Left side connecting to flue electric regulating door; 7. Right side connecting to flue electric isolation door; 8. Right side connecting to flue electric regulating door; 9. Bypass flue; 10. Steam-flue gas heat exchanger; 11. Steam bypass; 12. Connecting pipes between low-temperature reheater and high-temperature reheater; 13. Low-temperature reheater; 14. High-temperature reheater; 15. Electric regulating valve; 16. Bypass steam inlet electric shut-off valve; 17. Bypass steam outlet electric shut-off valve. DETAILED DESCRIPTION

[0015] The technical solution of the present invention is further described in detail below through specific embodiments and in conjunction with the accompanying drawings: Example: Figure 1 As shown, a low calorific value coal gas boiler heat exchange system includes a horizontal flue and a vertical flue at the rear of the boiler. A high-temperature reheater 14 and a low-temperature reheater 13 are arranged in the horizontal flue. An economizer, an upper air preheater 1, and a lower air preheater 2 are arranged in the vertical flue at the rear of the boiler. A left connecting flue 3 and a right connecting flue 4 are respectively provided on the left and right sides of the middle position between the upper air preheater and the lower air preheater in the vertical flue at the rear of the boiler. The left connecting flue is connected to the inlet of the flue gas bypass 9, and the right connecting flue is connected to the outlet of the flue gas bypass. A steam flue heat exchanger 10 is provided on the bypass flue. The bypass flue connects the two ends of the flue gas heat exchange side of the steam flue gas heat exchanger. The two ends of the steam heat exchange side of the steam flue gas heat exchanger are connected to the steam bypass 11, and the steam bypass is connected to the outlet of the low-temperature reheater and the inlet of the high-temperature reheater.

[0016] The left connecting flue is equipped with a left connecting flue electric isolating door 5 and a left connecting flue electric regulating door 6, and the right connecting flue is equipped with a right connecting flue electric isolating door 7 and a right connecting flue electric regulating door 8.

[0017] The upper and lower air preheaters are tubular structures, with flue gas flowing inside the tubes and air flowing outside. The steam-flue gas heat exchanger is a tubular or shell-and-tube structure, with steam flowing inside the tubes and flue gas flowing outside the tubes or on the shell side.

[0018] A bypass steam inlet electric shut-off valve 16 is provided on the steam bypass inlet at both ends of the steam heat exchange side of the steam-to-flue gas heat exchanger, and a bypass steam outlet electric shut-off valve 17 is provided on the steam bypass outlet.

[0019] A low-temperature reheater and high-temperature reheater connecting pipeline 12 is installed between the high-temperature reheater and the low-temperature reheater, and an electric regulating valve 15 is installed on the low-temperature reheater and high-temperature reheater connecting pipeline.

[0020] A temperature sensor and a pressure sensor are provided on the steam heat exchange side of the steam-to-flue gas heat exchanger.

[0021] When using this invention, the electric regulating valve is adjusted to allow some low-temperature reheater steam to be drawn from the low-temperature reheater outlet. After passing through the steam flue gas heat exchanger, the steam is introduced into the high-temperature reheater inlet pipeline, reducing the risk of reheater overheating and increasing the flexibility of reheat steam temperature regulation. Simultaneously, flue gas from the left connecting flue, after passing through the bypass flue and exchanging heat with steam in the steam flue gas heat exchanger, is introduced into the right connecting flue and enters the lower-stage air preheater. This increases the flue gas temperature in the lower-stage air preheater, reduces the possibility of ammonium bisulfate condensation, mitigates the risk of ammonium bisulfate corrosiveness on the heating surfaces of the lower-stage air preheater, and significantly extends the service life of the equipment.

[0022] The embodiment described above is only a preferred solution of the present invention and does not limit the present invention in any form. Other variations and modifications are possible without exceeding the technical solution described in the claims.

Claims

1. A heat exchange system for a low calorific value gas boiler, comprising a horizontal flue and a vertical flue at the rear of the boiler, wherein a high-temperature reheater and a low-temperature reheater are arranged in the horizontal flue, and an economizer, an upper air preheater, and a lower air preheater are arranged in the vertical flue at the rear of the boiler, characterized in that: A left connecting flue and a right connecting flue are respectively provided on the left and right sides of the middle position between the upper air preheater and the lower air preheater in the vertical flue at the rear of the boiler. A bypass flue is connected between the left connecting flue and the right connecting flue. A steam flue gas heat exchanger is provided on the bypass flue. The bypass flue connects the two ends of the flue gas heat exchange side of the steam flue gas heat exchanger. The two ends of the steam heat exchange side of the steam flue gas heat exchanger are connected to the steam bypass. The steam bypass is connected to the low-temperature reheater outlet and the high-temperature reheater inlet.

2. A low calorific value gas boiler heat exchange system according to claim 1, characterized in that: The left connecting flue and the right connecting flue are respectively equipped with an electric isolating door and an electric regulating door.

3. The low calorific value gas boiler heat exchange system according to claim 1, characterized in that: The upper air preheater and the lower air preheater are respectively of tubular structure, wherein the flue gas flows inside the tube and the air flows outside the tube.

4. The low calorific value gas boiler heat exchange system according to claim 1, characterized in that: The steam-gas heat exchanger is of a tube or shell-and-tube structure, with steam flowing inside the tubes and flue gas flowing outside the tubes or on the shell side.

5. The low calorific value gas boiler heat exchange system according to claim 1, characterized in that: Electric shut-off valves are respectively provided on the steam bypasses at both ends of the steam heat exchange side of the steam-to-gas heat exchanger.

6. The low calorific value gas boiler heat exchange system according to claim 1, characterized in that: A low-temperature reheater and high-temperature reheater connecting pipeline is installed between the high-temperature reheater and the low-temperature reheater, and an electric regulating valve is installed on the low-temperature reheater and high-temperature reheater connecting pipeline.

7. The low calorific value gas boiler heat exchange system according to claim 1, characterized in that: The steam heat exchange side of the steam-to-gas heat exchanger is provided with a temperature sensor and a pressure sensor.

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