Ammonia-coal coupling ultra-low nitrogen combustion method for vertical coal powder boiler

Abstract

The application discloses an ammonia-coal coupling ultra-low nitrogen combustion method for a vertical pulverized coal boiler, which is applied to a boiler body, and the boiler body is sequentially provided with a lower combustion zone, a middle combustion zone and an upper combustion zone from bottom to top; the side wall and / or the four corners of the lower combustion zone are provided with a lower burner, the side wall and / or the four corners of the middle combustion zone are provided with a middle burner, and the side wall and / or the four corners of the upper combustion zone are provided with an upper burner; the lower burner and the middle burner are ammonia-coal co-combustion burners, and the upper burner is a pure ammonia burner; the middle burner and the lower burner adopt the ammonia-coal co-combustion staged oxygen supply and staged combustion principle, utilize the rapid ignition and stable combustion characteristics of ammonia to form a stable ammonia flame, the flame is rich in active reducing components and residual ammonia, can effectively reduce the nitrogen oxides generated in the coal combustion process, and the upper burner reduces the NOx generated by the middle and lower layers, further reducing the content of NOx in the flue gas.

Description

Technical Field

[0001] This invention relates to the field of ammonia-coal combustion technology, and in particular to a method for coupled ammonia-coal ultra-low nitrogen combustion in a vertical pulverized coal boiler. Background Technology

[0002] my country is a major coal producer, and coal has a long history of being used as fuel to provide heat for industrial and domestic use. Grinding coal into pulverized coal increases its specific surface area, facilitating mixing with combustion air, which is beneficial for ignition and complete combustion, thus improving energy efficiency. Pulverized coal combustion is commonly used in thermal power plants or boilers to heat water or steam. As a solid fuel, coal is less flammable than liquid or gaseous fuels. When boiler load and temperature are low, liquid or gaseous fuels are often used to assist ignition and flame stabilization. For example, diesel or plasma igniters are used for boiler ignition or peak-shaving requirements.

[0003] Since coal is a high-carbon fuel, it produces a large amount of carbon dioxide when burned. Therefore, there is an urgent need to develop efficient and stable combustion technologies that can be used to mix coal with low-carbon and zero-carbon fuels.

[0004] This invention focuses on coal-ammonia co-combustion technology in thermal power plants and boilers, particularly in vertical boilers. Traditional pulverized coal combustion in boilers often results in significant nitrogen oxide (NOx) emissions, and ammonia combustion also produces substantial NOx emissions, causing significant environmental impacts. To address this issue, various low-NOx combustion technologies have been developed, but these technologies still have shortcomings in terms of combustion stability and fuel burnout rate. Therefore, developing a novel combustion method that can effectively reduce NOx emissions while ensuring the stability of coal-ammonia co-combustion and complete fuel combustion has become an urgent problem to be solved. Summary of the Invention

[0005] The purpose of this invention is to provide a method for ultra-low nitrogen combustion of ammonia and coal coupled in a vertical pulverized coal boiler, so as to solve one or more technical problems existing in the prior art, and at least provide a beneficial option or create conditions.

[0006] The technical solution adopted to solve the above-mentioned technical problems is as follows:

[0007] This invention provides a vertical pulverized coal boiler ammonia-coal coupled ultra-low nitrogen combustion system, including a boiler body with an ash discharge outlet at the bottom and a flue gas discharge outlet at the top. The boiler body, from bottom to top, has a lower combustion zone, a middle combustion zone, and an upper combustion zone. Lower combustion zone burners are located on the side walls and / or at the four corners of the lower combustion zone; middle combustion zone burners are located on the side walls and / or at the four corners of the middle combustion zone; and upper combustion zone burners are located on the side walls and / or at the four corners of the upper combustion zone. The lower and middle combustion zones... All burners are ammonia-coal co-fired burners, with the upper burner being a pure ammonia burner. The ammonia-coal co-fired burner is equipped with an ammonia gas channel, a primary combustion air channel, a pulverized coal air channel, a secondary combustion air channel, an ammonia combustion channel, an ammonia-coal mixed combustion channel, and a first ignition gun. The ammonia gas channel and the primary combustion air channel are both connected to the ammonia combustion channel. The first ignition gun is located in the ammonia combustion channel. The ammonia combustion channel, the pulverized coal air channel, and the secondary combustion air channel are all connected to the ammonia-coal mixed combustion channel.

[0008] The beneficial effects of this invention are:

[0009] The nitrogen combustion system is arranged in three layers: upper, middle, and lower. Each layer is equipped with fuel and air nozzles, and sufficient vertical distance is maintained between the layers. The middle and lower combustion zones use ammonia-coal co-combustion burners, while the upper combustion zone uses a pure ammonia burner, ensuring the independence of the combustion process and the stability of the flame structure in each layer. Both the middle and lower burners employ the principle of ammonia-coal co-combustion with staged oxygen supply and phased combustion, utilizing the rapid ignition and stable combustion characteristics of ammonia to form a stable ammonia flame. This flame is rich in active reducing components and residual ammonia, effectively reducing nitrogen oxides generated during coal combustion. By controlling the temperature and combustion air volume, the combustion process is kept in an environment with an excess air coefficient below 1 and a suitable reaction temperature, further suppressing NOx formation. The upper burner, by adjusting the combustion air volume, keeps the flame in a reducing atmosphere, reducing the NOx generated in the middle and lower layers, further lowering the NOx content in the flue gas.

[0010] As a further improvement to the above technical solution, the ammonia-coal co-combustion burner includes a first shell, a second shell, a third shell, and a fourth shell. The third shell is fitted inside the fourth shell, the second shell is fitted inside the third shell, the first shell is fitted inside the second shell, the first ignition gun is located inside the first shell, the ammonia gas passage is formed between the side wall of the first ignition gun and the first shell, the primary combustion air passage is formed between the first shell and the second shell, the ammonia combustion passage is formed inside the second shell, the pulverized coal air passage is formed between the second shell and the third shell, the secondary combustion air passage is formed between the third shell and the fourth shell, and the ammonia-coal mixed combustion passage is formed inside the fourth shell.

[0011] As a further improvement to the above technical solution, the ammonia gas channel, the primary combustion air channel, the pulverized coal air channel, and the secondary combustion air channel are all annular channels.

[0012] As a further improvement to the above technical solution, the first housing is provided with a plurality of first ammonia outlets that spray ammonia radially along the first housing and a plurality of second ammonia outlets that spray ammonia axially along the first housing at one end near the ammonia combustion channel.

[0013] As a further improvement to the above technical solution, the primary combustion air channel is provided with a swirling air outlet structure.

[0014] As a further improvement to the above technical solution, the first ignition gun is located on the central axis of the first housing.

[0015] As a further improvement to the above technical solution, a diffusion section is provided at one end of the second shell near the ammonia-coal mixed combustion channel, and the diameter of the diffusion section gradually increases along the jet direction of the first ignition gun flame.

[0016] As a further improvement to the above technical solution, the pure ammonia burner includes a fifth housing, a sixth housing, and a second ignition gun. The fifth housing is fitted inside the sixth housing, and the second ignition gun is located inside the fifth housing. An ammonia delivery channel is formed between the side wall of the second ignition gun and the fifth housing. A combustion air channel is formed between the fifth housing and the sixth housing. The sixth housing is provided with a pure ammonia combustion channel. Both the ammonia delivery channel and the combustion air channel are connected to the pure ammonia combustion channel.

[0017] As a further improvement to the above technical solution, the second ignition gun is located on the central axis of the fifth housing.

[0018] The present invention also provides a combustion method applied to the ammonia-coal coupled ultra-low nitrogen combustion system of a vertical pulverized coal boiler as described in any one of the above claims, the combustion method comprising:

[0019] Control the supply of corresponding combustion gas and combustion air to the burners of each layer, and simultaneously control the ignition of the burners of each layer;

[0020] By adjusting the delivery ratio of primary combustion air, ammonia, secondary combustion air, and pulverized coal air in the lower burner, the temperature in the lower combustion zone is kept below 800℃, and the excess air coefficient is kept below 1.

[0021] By adjusting the delivery ratio of primary combustion air, ammonia, secondary combustion air, and pulverized coal air in the intermediate burner, the temperature in the intermediate combustion zone is maintained above 1000℃, and the excess air coefficient is maintained between 1.1 and 1.3.

[0022] By adjusting the ratio of combustion air and ammonia delivery in the upper-layer pure ammonia burner, the temperature of the upper combustion zone is kept slightly lower than that of the middle combustion zone, and the excess air coefficient is between 0.5 and 0.8. Attached Figure Description

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments;

[0024] Figure 1 This is a structural cross-sectional schematic diagram of an embodiment of the vertical pulverized coal boiler ammonia-coal coupled ultra-low nitrogen combustion system provided by the present invention, wherein the two arrows represent upward and downward directions, respectively;

[0025] Figure 2 This is a side view of an embodiment of the vertical pulverized coal boiler ammonia-coal coupled ultra-low nitrogen combustion system provided by the present invention, wherein the two arrows indicate upward and downward directions respectively;

[0026] Figure 3 This is a cross-sectional schematic diagram of an ammonia-coal co-fired burner in an embodiment of the vertical pulverized coal boiler ammonia-coal coupled ultra-low nitrogen combustion system provided by the present invention.

[0027] Figure 4 This is a cross-sectional schematic diagram of a pure ammonia burner in one embodiment of the vertical pulverized coal boiler ammonia-coal coupled ultra-low nitrogen combustion system provided by the present invention.

[0028] Figure label:

[0029] Boiler body 100, lower combustion zone 101, middle combustion zone 102, upper combustion zone 103, ash and slag discharge outlet 110, flue gas discharge outlet 120;

[0030] Ammonia-coal co-combustion burner 200, ammonia gas passage 201, primary combustion air passage 202, pulverized coal air passage 203, secondary combustion air passage 204, ammonia combustion passage 205, ammonia-coal mixed combustion passage 206, first shell 210, first ammonia outlet 211, second ammonia outlet 212, second shell 220, diffuser section, third shell 230, fourth shell 240, first ignition gun 250, swirl exhaust structure 260;

[0031] Pure ammonia burner 300, ammonia delivery channel 301, combustion air channel 302, pure ammonia combustion channel 303, fifth housing 310, sixth housing 320, second ignition gun 330. Detailed Implementation

[0032] This section will describe in detail specific embodiments of the present invention. Preferred embodiments of the present invention are shown in the accompanying drawings. The purpose of the drawings is to supplement the textual description with graphics, so that people can intuitively and vividly understand each technical feature and overall technical solution of the present invention, but they should not be construed as limiting the scope of protection of the present invention.

[0033] In the description of this invention, it should be understood that the orientation descriptions, such as up, down, front, back, left, right, etc., are based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0034] In the description of this invention, if there are words such as "several", they mean one or more, "multiple" means two or more, "greater than", "less than", "exceeding" etc. are understood to exclude the number itself, and "above", "below", "within" etc. are understood to include the number itself.

[0035] In the description of this invention, unless otherwise explicitly defined, terms such as "set up," "install," and "connect" should be interpreted broadly, and those skilled in the art can reasonably determine the specific meaning of the above terms in this invention in conjunction with the specific content of the technical solution.

[0036] Reference Figures 1 to 4 The following are embodiments of the vertical pulverized coal boiler ammonia-coal coupled ultra-low nitrogen combustion method of the present invention:

[0037] In some embodiments, refer to Figure 1 and Figure 2 A vertical pulverized coal boiler ammonia-coal coupled ultra-low nitrogen combustion system includes:

[0038] The boiler body 100 has an ash discharge port 110 at the bottom for discharging ash and slag. The boiler body 100 has a flue gas discharge port 120 at the top, which is connected to a flue gas discharge pipe for discharging flue gas. The boiler body 100 has a lower combustion zone 101, a middle combustion zone 102, and an upper combustion zone 103 from bottom to top. Each layer is designed with fuel and air nozzles. The three combustion zones maintain a vertical distance of at least 1m to ensure the independence of the combustion process in each layer and form a stable flame structure. This design helps to achieve uniform combustion of fuel and improve combustion efficiency.

[0039] The lower combustion zone 101 has multiple lower burners on its sidewall, the middle combustion zone 102 has multiple middle burners on its sidewall, and the upper combustion zone 103 has multiple upper burners on its sidewall. The lower and middle burners are ammonia-coal co-fired burners 200, while the upper burners are pure ammonia burners 300. Temperature sensors are installed in the upper combustion zone 103, middle combustion zone 102, and lower combustion zone 101. In other embodiments, such as in a tangentially shaped boiler, multiple lower burners can be located at the corners of the lower combustion zone 101, multiple middle burners can be located at the corners of the lower-middle combustion zone 102, and multiple upper burners can be located at the corners of the upper combustion zone 103, achieving stable combustion and low NOx performance. Alternatively, in some ultra-large boilers, burners corresponding to each combustion zone are installed at the corners and sidewalls of the boiler body 100.

[0040] The middle and lower burners employ the staged combustion principle of ammonia-coal co-combustion, utilizing the rapid ignition and stable combustion characteristics of ammonia to form a stable ammonia flame. This flame is rich in active reducing components and residual ammonia, effectively reducing nitrogen oxides (NOx) generated during coal combustion. By controlling the temperature and combustion air volume, the combustion process is kept in an environment with an excess air coefficient below 1 and a suitable reaction temperature, further suppressing NOx formation and reducing nitrogen oxide emissions.

[0041] The upper burner adjusts the amount of combustion air to keep the ammonia burner flame in a reducing atmosphere, thereby reducing the NOx produced by the middle and lower combustion layers and further reducing the NOx content in the flue gas.

[0042] Reference Figure 3The ammonia-coal co-combustion burner 200 includes a first shell 210, a second shell 220, a third shell 230, a fourth shell 240, and a first ignition gun 250. The first shell 210, second shell 220, third shell 230, and fourth shell 240 are all cylindrical shells. The first ignition gun 250 is located on the central axis of the first shell 210. The first ignition gun 250 uses a high-energy electronic ignition gun to ensure reliable ignition. The outer peripheral walls of the first shell 210 and the first ignition gun 250 form an ammonia gas channel 201. The cross-section of the ammonia gas channel 201 is annular to ensure uniform ammonia gas output. The second shell 220 is sleeved on the outside of the first shell 210, forming a primary combustion air channel 202 between the first shell 210 and the second shell 220. The third shell 230 is sleeved on the outside of the second shell 220. A pulverized coal air passage 203 is formed between the second shell 220 and the third shell 230. A fourth shell 240 is fitted outside the third shell 230. A secondary combustion air passage 204 is formed between the third shell 230 and the fourth shell 240. The second shell 220 is also provided with an ammonia combustion passage 205. The fourth shell 240 is also provided with an ammonia-coal mixed combustion passage 206. The primary combustion air passage 202 and the ammonia passage 201 are both connected to the ammonia combustion passage 205. The ammonia combustion passage 205, the secondary combustion air passage 204, and the pulverized coal air passage 203 are all connected to the ammonia-coal mixed combustion passage 206. The primary combustion air and ammonia are mixed in the ammonia combustion passage 205 and ignited by the first igniter to form an ammonia combustion flame. Then, it is mixed with the pulverized coal air and the secondary combustion air in the ammonia-coal mixed combustion passage 206 and ignites the pulverized coal to form a pulverized coal burner flame. To address the requirements of ammonia-coal co-combustion in vertical boilers, a combustion method is adopted where ammonia flames ignite pulverized coal, instead of the existing method of pulverized coal flames igniting ammonia. A high-energy electronic ignition gun for ammonia fuel installed at the center of the burner ensures reliable ammonia burner ignition, and the ammonia burner flame ensures stable ignition and combustion of low-temperature pulverized coal, resulting in faster and more stable boiler start-up. The layered layout and staged combustion design enhance the boiler's adaptability to different loads and fuel conditions, improving operational flexibility and reliability.

[0043] Since the first shell 210, second shell 220, third shell 230, and fourth shell 240 are all cylindrical, and the ammonia gas channel 201, primary combustion air channel 202, pulverized coal air channel 203, and secondary combustion air channel 204 are all annular channels, they facilitate uniform gas mixing. Each channel is equipped with valves to control the airflow rate and on / off state, regulating the flow rate of each channel. The second shell 220 has a diffuser section at one end near the ammonia-coal mixing combustion channel 206, with the diameter of the diffuser section gradually increasing along the direction of the flame from the first ignition gun 250. The diffuser section facilitates the diffusion of flame combustion into the ammonia-coal mixing combustion channel 206.

[0044] To improve ammonia combustion efficiency, a swirling air outlet structure 260 is provided between the primary combustion air channels 202. The swirling air outlet structure 260 includes a swirling base and swirling fan blades. The swirling fan blades ensure more uniform mixing of air and ammonia. The outlet end of the first housing 210 has multiple first ammonia outlets 211 that inject ammonia radially and multiple second ammonia outlets 212 that inject ammonia axially. Ammonia from the first ammonia outlets 211 is premixed with air from the primary combustion air channels 202 and enters the ammonia combustion channel 205. A portion of the ammonia from the second ammonia outlets 212 is ignited by the first igniter. The ammonia from the second ammonia outlets 212 directly enters the ammonia combustion channel 205 for combustion.

[0045] By controlling the amount of combustion air in the lower burner to maintain an excess air coefficient of less than 1, the amount of nitrogen oxides generated during combustion is very small, and there are a large amount of active ingredients and residual ammonia. The high-temperature ammonia flame with these components can easily ignite the pulverized coal flame, and the active reducing components in the ammonia combustion flue gas can be used to greatly reduce the nitrogen oxides generated during coal combustion.

[0046] Both the middle and lower burners utilize the ammonia-coal co-fired burner 200, designed based on the staged combustion principle of ammonia-coal co-fired combustion. It leverages the rapid ignition and stable combustion characteristics of ammonia to form a stable ammonia flame, then uses an active reducing ammonia flame to ignite pulverized coal and suppress fuel-type and thermal nitrogen oxides generated during coal combustion. Because the combustion air is staged and mixed with the fuel, especially the ammonia combustion flame in the middle of the burner which is oxygen-deficient, it significantly reduces nitrogen oxide formation. Furthermore, hydrogen molecules, residual ammonia, and carbon monoxide formed during the initial incomplete combustion phase are mixed with these by the secondary combustion air entering the combustion zone at the flame tail and ultimately burned completely at a suitable temperature.

[0047] In addition, the furnace temperature at the flame outlet of the lower-level ammonia-coal co-combustion burner 200 is controlled at a low temperature, i.e., below 800°C, and the excess air coefficient is kept below 1. This allows for further utilization of low-temperature, low-oxygen ammonia-coal co-combustion staged combustion, which helps to further reduce the generation of fuel-type and thermal NOx during the combustion of ammonia and pulverized coal.

[0048] The intermediate burner, located above the flue gas layer of the lower burner, can utilize a high-temperature, low-oxygen ammonia-coal co-combustion staged combustion method within the existing flue gas environment of the furnace. For example, the combustion flame environment of the intermediate burner ammonia-coal co-combustion burner 200 is maintained at a high temperature, exceeding 1000℃, with an excess air coefficient between 1.1 and 1.3. This further utilizes the ignition advantage of ammonia to promote the complete combustion of pulverized coal at high temperatures. This not only improves the fuel burnout rate but also helps to simultaneously reduce the formation of both fuel-type and thermal NOx.

[0049] Reference Figure 4The upper combustion zone 103 uses a pure ammonia burner 300. The pure ammonia burner 300 includes a fifth housing 310, a sixth housing 320, and a second ignition gun 330. Both the fifth housing 310 and the sixth housing 320 are cylindrical housings. The fifth housing 310 is fitted inside the sixth housing 320. The second ignition gun 330 is located inside the fifth housing 310 and is situated on the central axis of the fifth housing 310, which is conducive to the complete ignition of ammonia. An ammonia delivery channel 301 is formed between the side wall of the second ignition gun 330 and the fifth housing 310. A combustion air channel 302 is formed between the fifth housing 310 and the sixth housing 320. The sixth housing 320 is provided with a pure ammonia combustion channel 303. Both the ammonia delivery channel 301 and the combustion air channel 302 are connected to the pure ammonia combustion channel 303.

[0050] The structure of the pure ammonia burner 300 is the same as that of the ammonia combustion structure in the lower and middle layers of the burner. However, during operation, the amount of combustion air needs to be reduced so that the flame of the ammonia burner is in a reducing atmosphere. The combustion flame environment of the upper pure ammonia burner 300 is kept at a medium to high temperature, which is slightly lower than the flame environment temperature of the middle layer burner. The excess air coefficient is less than 1, between 0.5 and 0.8, which facilitates the reduction of NOx in the flue gas of the middle and lower layers of combustion flame.

[0051] The upper-level burners can adjust the overall heat distribution of the boiler to make it more reasonable, and further reduce NOx in the flue gas to achieve a good NOx reduction effect.

[0052] Furthermore, the combustion system in this embodiment can also optimize heat distribution and improve fuel burnout rate:

[0053] The intermediate burner utilizes the flue gas environment generated by the lower burner and employs a high-temperature, low-oxygen ammonia-coal co-combustion staged combustion method, promoting the complete combustion of pulverized coal at high temperatures and improving fuel burnout rate. The arrangement of the upper burner not only regulates the overall heat distribution of the boiler, making it more rational, but also further reduces NOx in the flue gas through its reducing flame, achieving a dual improvement in heat utilization and environmental benefits.

[0054] The present invention also provides an embodiment of a combustion method applied to an ammonia-coal coupled ultra-low NOx combustion system for a vertical pulverized coal boiler according to any of the above-mentioned methods. The combustion method includes:

[0055] Control the supply of corresponding combustion gas and combustion air to the burners of each layer, and simultaneously control the ignition of the burners of each layer;

[0056] By adjusting the delivery ratio of primary combustion air, ammonia, secondary combustion air, and pulverized coal air in the lower burner, the temperature of the lower combustion zone 101 is kept below 800℃, and the excess air coefficient is kept below 1.

[0057] By adjusting the delivery ratio of primary combustion air, ammonia, secondary combustion air, and pulverized coal air in the intermediate burner, the temperature of the intermediate combustion zone 102 is maintained above 1000℃, and the excess air coefficient is maintained between 1.1 and 1.3.

[0058] By adjusting the ratio of combustion air and ammonia delivery in the upper ammonia burner 300, the temperature of the upper combustion zone 103 is kept slightly lower than that of the middle combustion zone 102, and the excess air coefficient is between 0.5 and 0.8.

[0059] The steps to maintain the temperature of the lower combustion zone 101 below 800℃ and the excess air coefficient below 1 by adjusting the delivery ratio of primary combustion air, ammonia, secondary combustion air, and pulverized coal air in the lower burner specifically include:

[0060] Limiting the flow rate of primary combustion air causes ammonia to undergo oxygen-deficient pre-combustion within the ammonia combustion channel 205, reducing the initial combustion intensity;

[0061] By adjusting the valves or baffles in the secondary combustion air passage 204, the total oxygen supply is reduced, ensuring that the overall excess air coefficient λ < 1.

[0062] After the ammonia is partially burned in the combustion channel, it is mixed with the pulverized coal in the pulverized coal air channel 203. The environment of the partially consumed oxygen is used to suppress the subsequent combustion temperature. Temperature control and excess air coefficient control are achieved through staged oxygen supply and staged fuel combustion.

[0063] The input ratio of ammonia to pulverized coal is controlled by a flow meter. The high nitrogen content of ammonia dilutes the oxygen concentration in the combustion zone, thereby reducing the rate of heat release during combustion.

[0064] Ammonia and primary combustion air are premixed and combusted in the ammonia combustion channel 205. Then, pulverized coal is introduced into the mixed combustion channel to form a diffused combustion environment with local oxygen deficiency.

[0065] Furthermore, the system is equipped with a temperature sensor, enabling temperature monitoring and dynamic adjustment. The sensor sends real-time temperature signals to the controller, dynamically adjusting the flow rates of ammonia, pulverized coal, or secondary combustion air. An oxygen concentration sensor monitors the oxygen content in the flue gas, and combined with the fuel input, calculates the λ value to automatically adjust the combustion air supply.

[0066] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to the embodiments. Those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of the present invention, and these equivalent modifications or substitutions are all included within the scope defined by the claims of this application.

Claims

1. A method for ammonia-coal coupling ultra-low nitrogen combustion in a vertical pulverized coal boiler, characterized in that, The combustion method is applied to an ammonia-coal coupled ultra-low nitrogen combustion system of a vertical pulverized coal boiler, comprising a boiler body provided with an ash and slag discharge port at the bottom and a flue gas discharge port at the top, and sequentially provided with a lower combustion zone, a middle combustion zone and an upper combustion zone from bottom to top; the lower combustion zone is provided with a lower burner at the side wall and / or the four corners, the middle combustion zone is provided with a middle burner at the side wall and / or the four corners, and the upper combustion zone is provided with an upper burner at the side wall and / or the four corners; the lower burner and the middle burner are ammonia-coal co-combustion burners, and the upper burner is a pure ammonia burner; the ammonia-coal co-combustion burner is provided with an ammonia gas passage, a primary combustion air passage, a pulverized coal wind passage, a secondary combustion air passage, an ammonia gas combustion passage, an ammonia-coal mixed combustion passage and a first ignition gun; the ammonia gas passage and the primary combustion air passage are communicated with the ammonia gas combustion passage, and the first ignition gun is arranged in the ammonia gas combustion passage; the ammonia gas combustion passage, the pulverized coal wind passage and the secondary combustion air passage are communicated with the ammonia-coal mixed combustion passage; The combustion method comprises: controlling the burners of each layer to be supplied with corresponding combustion gas and combustion air, and igniting the burners of each layer at the same time; by adjusting the delivery ratio of the primary combustion air, ammonia gas, secondary combustion air and pulverized coal wind of the lower burner, the temperature of the lower combustion zone is kept below 800 DEG C, and the air excess coefficient is kept below 1; by adjusting the delivery ratio of the primary combustion air, ammonia gas, secondary combustion air and pulverized coal wind of the middle burner, the temperature of the middle combustion zone is kept above 1000 DEG C, and the air excess coefficient is kept between 1.1 and 1.3; by adjusting the delivery ratio of the combustion air and ammonia gas of the upper pure ammonia burner, the temperature of the upper combustion zone is kept slightly lower than that of the middle combustion zone, and the air excess coefficient is between 0.5 and 0.8.

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