A natural gas boiler low nitrogen combustion system and process

Abstract

The present invention relates to a natural gas boiler low-nitrogen combustion system and process, in which part of the flue gas is taken from an opening on a smoke exhaust duct between an outlet and a chimney of the natural gas boiler, and is sent into a combustion-supporting duct after being pressurized by a circulating fan, and is evenly mixed with the combustion-supporting air through a flue gas mixing device, and then is sent together through a burner nozzle into the natural gas boiler for combustion. The present invention provides a natural gas boiler low-nitrogen combustion system and process, and the low-nitrogen combustion process can reduce the NOx concentration of the natural gas boiler flue gas, and the system is simple and easy to implement modification.

Description

Technical Field

[0001] The present invention belongs to the technical field of natural gas combustion pollutant emission reduction control, and in particular relates to a natural gas boiler low-nitrogen combustion system and process. Background Art

[0002] With the development of economy and science and technology in recent years, people's requirements for quality of life and living environment are increasing day by day, and the government's requirements for pollutant emissions are also gradually increasing. For example, for boiler flue gas in the power industry, on the basis of achieving standard emissions, ultra-low emissions have been proposed in recent years. For example, NOx emission concentration is required to be less than 50mg / Nm3. Natural gas is a clean fuel, and natural gas boilers themselves are environmentally friendly boilers with low pollutant emissions. However, due to the increase in environmental protection requirements, they are also facing the requirements of nitrogen reduction and denitrification transformation. Boiler methods to reduce nitrogen oxide emissions include reducing NOx generation during combustion and performing flue gas denitrification. Flue gas denitrification includes SCR and SNCR processes. Measures to reduce NOx generation during combustion include burning high-quality fuels (such as natural gas) and adopting new low-nitrogen combustion processes. Summary of the invention

[0003] Based on this, it is necessary to provide a natural gas boiler low-nitrogen combustion system and process to address the above problems. The low-nitrogen combustion process can reduce the NOx concentration in the flue gas of the natural gas boiler, and the system is simple and easy to implement.

[0004] The technical solution adopted by the present invention is: a part of the flue gas is taken from the opening of the exhaust air duct from the outlet of the natural gas boiler to the front of the chimney, and is sent into the combustion-supporting air duct after being pressurized by the circulating fan, and is evenly mixed with the combustion-supporting air through the flue gas mixing device, and then sent into the furnace through the burner nozzle. Since the oxygen content of the flue gas is lower than that of the air, the oxygen content of the mixed combustion-supporting gas after mixing with the flue gas is also reduced, and the oxygen content of the local combustion area is reduced without reducing the combustion-supporting air volume and changing the mixing and fluidization of the gas flow field and the fuel in the furnace, and the local combustion temperature is reduced simultaneously. Since the amount of NOx generated during combustion is closely related to the local temperature, the higher the temperature, the greater the amount of NOx generated. Reducing the local combustion temperature can effectively reduce the amount of nitrogen oxides generated. At the same time, due to the reduction in oxygen content, an oxygen-deficient reducing gas is formed in the local combustion area, which is conducive to the reduction of NOx and further reduces the amount of NOx generated.

[0005] A burnt air nozzle is arranged on the end of the natural gas boiler opposite to the burner nozzle to draw in a part of the combustion-supporting air that is not mixed with the flue gas and send it into the furnace after preheating, so that the gas that is not completely burned after reducing the oxygen content in the combustion area can continue to burn completely, realizing staged combustion and preventing the reduction of the boiler thermal efficiency.

[0006] A low-nitrogen combustion system for a natural gas boiler comprises a natural gas boiler, an air preheater, a combustion-supporting air blower, a combustion-supporting air duct, a circulating fan, a flue gas mixing device and a burner nozzle, wherein the burner nozzle is installed on the natural gas boiler, the air inlet of the combustion-supporting air blower is connected to the external environment, the air outlet of the combustion-supporting air blower is connected to the air inlet of the air preheater, the air outlet of the air preheater is connected to the burner nozzle through the combustion-supporting air duct and then enters the natural gas boiler, the air inlet of the circulating fan is connected to the smoke exhaust duct located between the outlet of the natural gas boiler and the chimney, the air outlet of the circulating fan is connected to the air inlet of the flue gas mixing device, and the air outlet of the flue gas mixing device is connected to the combustion-supporting air duct.

[0007] The circulating fan is a variable frequency fan, with two fans in total. Depending on the boiler load, the two fans can be operated simultaneously, or only one fan can be operated, with one fan in normal use and the other in standby. The total maximum air volume of the two fans is approximately 20% of the combustion-supporting air volume, which is adjusted in real time according to the operating conditions; its pressure is calculated based on the pressure of the combustion-supporting air system at the mixing point and the total resistance loss of the recirculating flue gas, ensuring that the recirculating flue gas pressure at the mixing point is higher than the combustion-supporting air pressure; and appropriate margins are left for each.

[0008] In one embodiment, a burnout air nozzle is provided on the side wall of the natural gas boiler at an end opposite to the burner nozzle, and the burnout air nozzle is connected to the air outlet of the air preheater through a pipeline.

[0009] In one embodiment, the flue gas mixing device includes a recirculation air duct and an injection pipe. The recirculation air duct is arranged inside the combustion-supporting air duct. The recirculation air duct is perpendicular to the inner wall of the combustion-supporting air duct. The injection pipe is arranged on the side wall of the leeward side of the recirculation air duct.

[0010] The flue gas mixing device is inserted into the combustion-supporting air duct from one side of the straight pipe section of the combustion-supporting air duct after the diameter of the recirculation flue is reduced, and multiple injection pipes are installed in the opening to allow the recirculation flue gas to be evenly mixed into the combustion-supporting air.

[0011] In one embodiment, the inlet end of the recirculation air duct is welded and sealed to the inner wall of the combustion-supporting air duct, and the outlet end of the recirculation air duct is welded and sealed by a steel plate and fixed by a steel support welded to the inner wall of the combustion-supporting air duct.

[0012] In one embodiment, the cross-section of the recirculation air duct is an octagon, the size of the windward vertical surface of the recirculation air duct cross-section is smaller than the size of another vertical surface, and the upper and lower surfaces adjacent to the windward vertical surface of the recirculation air duct cross-section are both inclined surfaces and are respectively tangent to the rear end of the combustion-supporting air duct.

[0013] In one of the embodiments, jet pipes are provided on both the vertical surface and the inclined surface of the leeward side of the recirculation air duct, and the diameter of the jet pipe on the vertical surface of the leeward side of the recirculation air duct is smaller than the diameter of the jet pipe on the inclined surface, and the angle between the jet pipe on the vertical surface of the leeward side of the recirculation air duct and the jet pipe on the inclined surface ranges from 45 degrees to 80 degrees.

[0014] In one of the embodiments, differential pressure transmitters are provided on the recirculation air duct and the combustion-supporting air duct located at the rear end of the recirculation air duct, and a differential pressure transmitter is provided at the inlet and outlet of the flue gas mixing device. The flow rate of the circulating flue gas can be calculated through the differential pressure value and a fixed empirical formula for easy adjustment; a muffler and an expansion joint are installed in sequence on the pipeline of the combustion-supporting air blower inlet, an expansion joint and an electric regulating damper are installed in sequence on the pipeline of the combustion-supporting air blower outlet, an expansion joint, an isolation valve, a muffler and an expansion joint are installed in sequence on the pipeline of the circulation fan inlet, an expansion joint and an isolation valve are installed in sequence on the pipeline of the circulation fan outlet, an electric regulating damper is installed on the pipeline of the air preheater inlet, an electric regulating damper is installed on the pipeline of the flue gas mixing device inlet, and an expansion joint is installed on the pipeline of the flue gas mixing device outlet.

[0015] In order to prevent the thermal expansion of the flue from damaging the system when the boiler is started and stopped, expansion joints are installed on the flue; to facilitate the maintenance of the circulating fan, isolation dampers are installed on the inlet and outlet pipes of the circulating fan; to reduce the noise of the circulating fan, a muffler is installed on the inlet pipe of the circulating fan.

[0016] To adjust the circulating flue gas flow, you can start one circulating fan alone or two circulating fans at the same time. You can also adjust the working conditions of the circulating fans by frequency conversion. Small flow adjustment is achieved through the electric regulating damper on the circulating air duct. The burning air volume is adjusted by the electric regulating damper on the pipeline of the burning air outlet.

[0017] In one embodiment, the combustion system is automatically controlled by DCS (PLC) and manually supervised, and the DCS (PLC) automatic control system is incorporated into the original control system of the boiler.

[0018] The beneficial effects achieved by the present invention are as follows: under the premise of ensuring the stability of natural gas ignition in the boiler, part of the flue gas is drawn out from the opening in the exhaust flue in front of the chimney, pressurized by the flue gas circulation fan, and the air volume is adjusted by the circulating air volume regulating door, and evenly mixed with the combustion-supporting air through the flue gas mixing device, and sprayed into the furnace through the burner nozzle for combustion. Since the oxygen content of the flue gas is lower than that of the air, the oxygen content of the mixed combustion-supporting gas after mixing with the recycled flue gas is also reduced, and the oxygen content of the local combustion area is reduced without reducing the combustion-supporting air volume and changing the mixing and fluidization of the gas flow field and the fuel in the furnace, and the local combustion temperature is simultaneously reduced. Since the amount of NOx generated during combustion is closely related to the local temperature, the higher the temperature, the greater the amount of NOx generated. Reducing the local combustion temperature can effectively reduce the amount of nitrogen oxides generated. At the same time, due to the reduction in oxygen content, an oxygen-deficient reducing gas is formed in the local combustion area, which is conducive to the reduction of NOx and further reduces the amount of NOx generated. A burnt air nozzle is arranged on the side wall of the natural gas boiler at the end opposite to the burner nozzle, which draws a part of the combustion-supporting air that is not mixed into the circulating flue gas and sends it into the furnace after preheating, so that the gas that is not completely burned after reducing the oxygen content in the combustion area can continue to burn completely, realizing staged combustion and preventing the reduction of boiler thermal efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a structural schematic diagram of an embodiment of the present invention;

[0020] Figure 2 is a schematic structural diagram of a flue gas mixing device in one embodiment of the present invention;

[0021] Figure 3 It is a schematic diagram of the side structure of a flue gas mixing device in one embodiment of the present invention. DETAILED DESCRIPTION

[0022] like Figure 1-Figure 3 As shown, part of the flue gas is taken from the opening of the exhaust air duct from the outlet of the natural gas boiler 1 to the front of the chimney, and is sent into the combustion-supporting air duct 16 after being pressurized by the circulating fan 9, and is evenly mixed with the combustion-supporting air through the flue gas mixing device 13, and then sent into the furnace through the burner nozzle 15. Since the oxygen content of the flue gas is lower than that of the air, the oxygen content of the mixed combustion-supporting gas after mixing with the flue gas is also reduced, so that the oxygen content of the local combustion area is reduced without reducing the combustion-supporting air volume and changing the mixing and fluidization of the gas flow field and the fuel in the furnace, and the local combustion temperature is reduced simultaneously. Since the amount of NOx generated during combustion is closely related to the local temperature, the higher the temperature, the greater the amount of NOx generated. Reducing the local combustion temperature can effectively reduce the amount of nitrogen oxides generated. At the same time, due to the reduction in oxygen content, an oxygen-deficient reducing gas is formed in the local combustion area, which is conducive to the reduction of NOx and further reduces the amount of NOx generated.

[0023] A burnt air nozzle 21 is provided at one end of the natural gas boiler 1 opposite to the burner nozzle 15, to introduce a part of the combustion-supporting air which is not mixed into the circulating flue gas, and to send it into the furnace after preheating, so that the gas which is not completely burned after reducing the oxygen content in the combustion area can continue to burn completely, thereby realizing staged combustion and preventing the reduction of the thermal efficiency of the boiler.

[0024] A low-nitrogen combustion system for a natural gas boiler comprises a natural gas boiler 1, an air preheater 2, a combustion-supporting air blower 3, a combustion-supporting air duct, a circulating fan 9, a flue gas mixing device 13 and a burner nozzle 15. The burner nozzle 15 is installed on the natural gas boiler 1. The air inlet of the combustion-supporting air blower 3 is connected to the external environment, the air outlet of the combustion-supporting air blower 3 is connected to the air inlet of the air preheater 2, the air outlet of the air preheater 2 is connected to the burner nozzle 15 through the combustion-supporting air duct, and then connected to the natural gas boiler 1. The air inlet of the circulating fan 9 is connected to the smoke exhaust duct located between the outlet of the natural gas boiler 1 and the chimney, the air outlet of the circulating fan 9 is connected to the air inlet of the flue gas mixing device 13, and the air outlet of the flue gas mixing device 13 is connected to the combustion-supporting air duct.

[0025] The circulating fan 9 is a variable frequency fan, with two fans in total. Depending on the boiler load, two fans can be operated simultaneously, or only one fan can be operated, one for normal use and the other for standby. The total maximum air volume of the two fans is about 20% of the combustion-supporting air volume, which is adjusted in real time according to the operating conditions; its pressure is calculated based on the pressure of the combustion-supporting air system at the mixing point and the total resistance loss of the recirculating flue gas, ensuring that the recirculating flue gas pressure at the mixing point is higher than the combustion-supporting air pressure; and appropriate margins are left for each.

[0026] The outlet air ducts of the two circulating fans are merged and then divided into two air ducts to enter the two combustion-supporting air ducts respectively. After being mixed with the combustion-supporting air through the flue gas mixing device 13, they enter the burners on the left and right sides of the boiler respectively. An expansion joint 8 and an electric regulating damper 12 are installed on each air duct.

[0027] For example, for a natural gas boiler with a smaller tonnage, only one circulating fan 9 and one circulating flue need to be provided corresponding to the air supply system.

[0028] A burnout air nozzle 21 is provided on the side wall of the natural gas boiler 1 at one end opposite to the burner nozzle 15 , and the burnout air nozzle 21 is connected to the air outlet of the air preheater 2 through a pipeline.

[0029] The flue gas mixing device 13 includes a recirculation air duct 17 and an injection pipe 18 . The recirculation air duct 17 is arranged inside the combustion-supporting air duct 16 . The recirculation air duct 17 is perpendicular to the inner wall of the combustion-supporting air duct 16 . The injection pipe 18 is arranged on the side wall of the recirculation air duct 17 on the leeward side.

[0030] The flue gas mixing device 13 is a recirculation air duct 17 with a reduced diameter, which is inserted into the combustion-supporting air duct 16 from one side of the straight pipe section of the combustion-supporting air duct 16, and a plurality of injection pipes 18 are installed at the opening, so that the recirculated flue gas can be evenly mixed into the combustion-supporting air.

[0031] The inlet end of the recirculation air duct 17 is welded and sealed to the inner wall of the combustion-supporting air duct 16 , and the outlet end of the recirculation air duct 17 is welded and sealed by a steel plate, and supported and fixed by a steel section 19 welded to the inner wall of the combustion-supporting air duct 16 .

[0032] The cross section of the recirculation air duct 17 is an octagon, the windward vertical surface of the recirculation air duct 17 is smaller than the other vertical surface, and the upper and lower surfaces adjacent to the windward vertical surface of the recirculation air duct 17 are both inclined surfaces, and are tangent to the rear end of the combustion-supporting air duct 16 respectively.

[0033] The vertical surface and the inclined surface of the leeward side of the recirculation air duct 17 are both provided with a jet pipe 18, and the diameter of the jet pipe 18 on the vertical surface of the leeward side of the recirculation air duct 17 is smaller than the diameter of the jet pipe 18 on the inclined surface, and the angle between the jet pipe on the vertical surface of the leeward side of the recirculation air duct 17 and the jet pipe on the inclined surface ranges from 45 degrees to 80 degrees, preferably 60 degrees.

[0034] A differential pressure transmitter 14 is provided on the recirculation air duct 17 and the combustion-supporting air duct located at the rear end of the recirculation air duct 17, and a differential pressure transmitter 14 is provided at the inlet and outlet of the flue gas mixing device 13. The flow rate of the circulating flue gas can be calculated through the differential pressure value and a fixed empirical formula, which is convenient for adjustment; a muffler 7 and an expansion joint 8 are installed in sequence on the pipeline of the air inlet of the combustion-supporting air blower 3, an expansion joint 8 and an electric regulating damper 12 are installed in sequence on the pipeline of the air outlet of the combustion-supporting air blower 3, an expansion joint 8, an isolation valve 6, a muffler 7 and an expansion joint 8 are installed in sequence on the pipeline of the air inlet of the circulation fan 9, an expansion joint 8 and an isolation valve 6 are installed in sequence on the pipeline of the air outlet of the circulation fan 9, an electric regulating damper 12 is installed on the pipeline of the air inlet of the air preheater 2, an electric regulating damper 12 is installed on the pipeline of the air inlet of the flue gas mixing device 13, and an expansion joint 8 is installed on the pipeline of the air outlet of the flue gas mixing device 13.

[0035] In order to prevent the system from being damaged by the thermal expansion of the flue when the boiler 1 is started and stopped, expansion joints 8 are respectively arranged on the flue; in order to facilitate the maintenance of the circulating fan 9, an isolation valve 6 is provided on the inlet and outlet pipelines of the circulating fan 9; in order to reduce the noise of the circulating fan 9, a muffler 7 is arranged on the inlet pipeline of the circulating fan 9.

[0036] To adjust the circulating flue gas flow rate, one circulating fan 9 can be turned on alone or two circulating fans 9 can be turned on at the same time. The working condition of the circulating fan 9 can also be adjusted by frequency conversion. Small flow rate adjustment is achieved by the electric regulating damper 12 on the circulating fan 9. The ember air volume is adjusted by the electric regulating damper 12 on the pipeline of the ember air nozzle 21.

[0037] The combustion system provided by the present invention is automatically controlled and manually supervised by DCS or PLC, and the DCS or PLC automatic control system is incorporated into the original control system of the boiler to achieve integrated control.

[0038] Preferably, the natural gas boiler low-nitrogen combustion process provided by the present invention can also be used for low-nitrogen combustion of other furnace-type boilers or kilns, and can also be used for the mixing of any two gaseous fluids with different parameters. For example, in a circulating fluidized bed boiler, the combustion-supporting air mixed with the recycled flue gas is sent to the bottom of the air distribution plate at the bottom of the furnace, and enters the furnace evenly through the air distribution plate and the gas cap to fluidize and burn the fuel in the furnace. The same principle can also reduce the NOx concentration in the flue gas of a circulating fluidized bed boiler. Layer-burning furnaces such as chain furnaces also send the combustion-supporting air mixed with the recycled flue gas to the bottom of the furnace under the air distribution plate, enter the furnace through the air distribution plate, and pass through the fuel layer to burn the fuel. Similarly, since the local combustion temperature is reduced and the reducing gas is generated after the oxygen content is reduced, the NOx generation is reduced, thereby achieving the purpose of reducing the NOx concentration in the flue gas.

[0039] Preferably, the natural gas boiler includes a combustion system, a boiler drum for supplying water to the boiler is provided above the combustion system, the combustion system includes a furnace, a combustion device and a water-cooled wall cavity are provided in sequence from bottom to top in the furnace, an upper header connected to the boiler drum is provided above the furnace, a lower header connected to the boiler drum is provided on the outer side of the front wall of the furnace, and two cold water screens are provided in the water-cooled wall cavity, the upper and lower ends of the two water-cooled screens respectively pass through the furnace top and the furnace front wall, and are respectively connected to the upper header and the lower header correspondingly. In order to better reduce the bed temperature and the oxygen content, and thus reduce the original emission of NO2, the present invention adds two water-cooled screens in the furnace, and the water-cooled screens and the water-cooled walls work together to cool the fluidized bed, and the steam-water mixture generated by the water-cooled screens and the water-cooled walls is sent to the upper header through the steam-water connecting pipe.

[0040] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0041] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A natural gas boiler low nitrogen combustion system, characterized in that: It includes a natural gas boiler, an air preheater, a combustion-supporting air blower, a combustion-supporting air duct, a circulating fan, a flue gas mixing device and a burner nozzle, wherein the burner nozzle is installed on the natural gas boiler; The air inlet of the combustion-supporting air blower is connected to the external environment, and the air outlet of the combustion-supporting air blower is connected to the air inlet of the air preheater; The air inlet of the circulating fan is connected to the smoke exhaust duct between the outlet of the natural gas boiler and the chimney; The air inlet of the flue gas mixing device is connected with the air outlet of the circulating fan and the air outlet of the air preheater; The air outlet of the flue gas mixing device is connected to the burner nozzle through the combustion-supporting air duct, and further connected to the natural gas boiler; A burnout air nozzle is provided on the side wall of the natural gas boiler at one end opposite to the burner nozzle, and the burnout air nozzle is connected to the air outlet of the air preheater through a pipeline; The flue gas mixing device comprises a recirculation duct and an injection pipe, wherein the recirculation duct is arranged inside the combustion-supporting duct, and the recirculation duct is inserted into the combustion-supporting duct from one side of the straight pipe section of the combustion-supporting duct after the diameter is reduced, the recirculation duct is perpendicular to the inner wall of the combustion-supporting duct, and the injection pipe is arranged on the side wall of the leeward side of the recirculation duct; The vertical surface and the inclined surface of the leeward side of the recirculation air duct are both provided with an air jet pipe, and the diameter of the air jet pipe on the vertical surface of the leeward side of the recirculation air duct is smaller than the diameter of the air jet pipe on the inclined surface, and the angle between the air jet pipe on the vertical surface of the leeward side of the recirculation air duct and the air jet pipe on the inclined surface is in the range of 45 degrees to 80 degrees; The inlet end of the recirculation air duct is welded and sealed to the inner wall of the combustion-supporting air duct, and the outlet end of the recirculation air duct is welded and sealed by a steel plate and fixed by a steel support welded to the inner wall of the combustion-supporting air duct; The cross section of the recirculation air duct is an octagon, the size of the windward vertical surface of the recirculation air duct cross section is smaller than the size of another vertical surface, and the upper and lower surfaces adjacent to the windward vertical surface of the recirculation air duct cross section are both inclined surfaces and are respectively tangent to the rear end of the combustion-supporting air duct.

2. A natural gas boiler low nitrogen combustion system according to claim 1, characterized in that: A differential pressure transmitter is provided on the recirculation air duct and the combustion-supporting air duct located at the rear end of the recirculation air duct, a muffler and an expansion joint are installed in sequence on the pipeline of the combustion-supporting air blower air inlet, an expansion joint and an electric regulating damper are installed in sequence on the pipeline of the combustion-supporting air blower air outlet, an expansion joint, an isolation valve, a muffler and an expansion joint are installed in sequence on the pipeline of the circulation fan air inlet, an expansion joint and an isolation valve are installed in sequence on the pipeline of the circulation fan air outlet, an electric regulating damper is installed on the pipeline of the air preheater air inlet, an electric regulating damper is installed on the pipeline of the flue gas mixing device air inlet, and an expansion joint is installed on the pipeline of the flue gas mixing device air outlet.

3. A natural gas boiler low nitrogen combustion system according to claim 2, characterized in that: The low-nitrogen combustion system is automatically controlled and manually supervised by DCS or PLC, and the DCS or PLC automatic control system is incorporated into the original control system of the boiler.

4. A process method using the natural gas boiler low nitrogen combustion system according to any one of claims 1 to 3, characterized in that: Part of the flue gas is taken from the opening of the exhaust duct between the outlet of the natural gas boiler and the chimney, and is sent into the combustion-supporting air duct after being pressurized by the circulating fan. It is evenly mixed with the combustion-supporting air through the flue gas mixing device, and then sent together through the burner nozzle into the natural gas boiler for combustion.

5. The process according to claim 4, characterized in that: A burnout air nozzle is arranged at one end of the natural gas boiler opposite to the burner nozzle, and a part of the combustion-supporting air preheated by the air preheater and before being mixed into the flue gas is introduced into the natural gas boiler through the burnout air nozzle to realize staged combustion.

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