CN114877315A

CN114877315A - Multistage preheating combustion method for ammonia coal

Info

Publication number: CN114877315A
Application number: CN202210547447.5A
Authority: CN
Inventors: 牛芳; 刘鹏中; 王鹏涛; 段璐; 龚艳艳; 魏琰荣; 李美军; 杨石; 底一; 刘刚; 马慧艳
Original assignee: China Coal Research Institute Ccri Energy Saving Technology Co ltd
Current assignee: China Coal Research Institute Ccri Energy Saving Technology Co ltd
Priority date: 2022-05-19
Filing date: 2022-05-19
Publication date: 2022-08-09

Abstract

The invention discloses a multistage preheating combustion method for ammonia coal, which changes a primary preheating mode of a traditional self-sustaining preheating combustor for coal powder into three-stage preheating, on one hand, the retention time of the coal powder in a high-temperature reduction region is prolonged, so that a large amount of high-temperature combustible reducing gas is contained in high-temperature gas-solid mixed fuel generated after the ammonia coal is mixed, thus being beneficial to promoting the heterogeneous reduction of NO, inhibiting the generation of nitrogen oxide in the combustion process of the ammonia coal fuel, being beneficial to increasing the propagation speed of flame, enhancing the stability of the flame and ensuring that the ignition performance of the ammonia coal fuel is better; on the other hand, the heat released by the combustion of the combustible gas in the reaction front of the high-temperature gas-solid mixed fuel is increased, the heat radiation between a flame area and an unburned area is enhanced, and the flame combustion flame of the ammonia-coal mixed fuel is further stable. Therefore, the ammonia coal multi-stage preheating combustion method provided by the embodiment of the invention has the advantages of good ignition performance, stable flame, effective inhibition of generation of nitrogen oxides and the like.

Description

Multistage preheating combustion method for ammonia coal

Technical Field

The invention relates to the technical field of ammonia coal combustion, in particular to an ammonia coal multi-stage preheating combustion method.

Background

Ammonia, as a zero-carbon fuel, can be used directly as a coal-fired boiler fuel, and can greatly reduce carbon dioxide emissions compared to conventional fossil fuels. Under the national double-carbon target, ammonia gas as a hydrogen carrier is a novel renewable zero-carbon fuel and is one of high-quality alternative fuels of a pulverized coal boiler. However, the low reactivity and high nitrogen content of ammonia molecules cause the problems of difficult ignition, difficult stable combustion and high NOx emission, so the ammonia coal co-combustion is one of effective technical paths for reducing carbon dioxide emission of the pulverized coal boiler. At present, pulverized coal combustion generally adopts self-sustaining preheating combustion, which means that a combustion method designed by two or more stable combustion mechanisms such as a rotational flow mechanism, a bluff body mechanism, a pre-combustion/preheating chamber and the like is adopted before pulverized coal enters a hearth for combustion, so that high-temperature flue gas is enabled to flow back, self-sustaining preheating combustion is realized, but the self-sustaining preheating combustion has the problems of poor combustion effect of ammonia coal and the like.

Disclosure of Invention

The present invention is made based on the discovery and recognition of the following facts and problems by the inventors.

In the related technology, the self-sustaining preheating combustion technology is primary preheating, the retention time of primary preheating fuel is short, and reducing pyrolysis gas generated by coal powder pyrolysis is less. For the ammonia coal fuel, the reductive pyrolysis gas generated by coal powder pyrolysis is less, on one hand, the high-temperature gas-solid mixed fuel formed by mixing ammonia coal not only has poorer inhibition effect on the generation of nitrogen oxides in the combustion process, but also has slower flame propagation speed and poorer ignition performance; on the other hand, the high-temperature gas-solid mixed fuel formed by mixing ammonia and coal releases less heat when combustible gas is combusted in a reaction front (the boundary between the combustion zone and the preheating zone is called as the reaction front), carbon smoke formed by the high-temperature gas-solid mixed fuel is less, the heat radiation effect between the flame zone and the unburned zone is poor, and the flame combustion is unstable.

The present invention is directed to solving, at least to some extent, one of the technical problems in the related art.

Therefore, the embodiment of the invention provides an ammonia coal multi-stage preheating combustion method which has good ignition performance, stable flame and effective inhibition of nitrogen oxide generation.

The ammonia coal multi-stage preheating combustion method provided by the embodiment of the invention comprises the following steps: conveying pulverized coal to a first position through primary air; primary preheating is carried out on the primary air and the pulverized coal at the first position, and the primary air and the pulverized coal are preheated to a first preset temperature; conveying the primary air and the pulverized coal which are preheated at the first stage to a second position;

performing secondary preheating on the primary air and the pulverized coal at the second position, and preheating to a second preset temperature to enable the pulverized coal to be combusted to generate primary flame, exhaust oxygen in the primary air, and generate pyrolysis reaction and generate pyrolysis gas when the pulverized coal is subjected to secondary preheating, wherein the second preset temperature is higher than the first preset temperature; conveying the pyrolysis gas and the pulverized coal subjected to secondary preheating to a third position;

carrying out three-stage preheating on the pulverized coal and the pyrolysis gas at the third position, and preheating to a third preset temperature, wherein the third preset temperature is higher than the second preset temperature; conveying the pulverized coal and the pyrolysis gas subjected to the three-stage preheating to a fourth position through secondary air, and combusting at the fourth position;

admixing ammonia gas into the pulverized coal in at least one of the first location prior to the primary preheating, the second location prior to the secondary preheating, the third location prior to the tertiary preheating, and the fourth location.

Therefore, the ammonia coal multi-stage preheating combustion method provided by the embodiment of the invention has the advantages of good ignition performance, stable flame, effective inhibition of generation of nitrogen oxides and the like.

In some embodiments, ammonia is admixed into the pulverized coal prior to the primary preheating at the first location, the primary preheating is carried out for a time period of 20ms to 100ms, and the first preheating temperature is 150 ℃ to 200 ℃.

In some embodiments, ammonia is admixed with the coal fines prior to the primary preheating at the first location, and the second location

The primary preheating temperature is 175-200 ℃, and the coal powder is preheated to the primary preheating direction

Ammonia gas is mixed, and the first preheating temperature is 150-175 ℃.

In some embodiments, ammonia gas is admixed into the pulverized coal before the secondary preheating at the second location, the secondary preheating is performed for a time of 1ms to 10ms, and the second preset temperature is 600 ℃ to 800 ℃.

In some embodiments, ammonia gas is admixed into the pulverized coal prior to the secondary preheating at the second location, the second predetermined temperature being 700 ℃ to 800 ℃; and at the second position, ammonia gas is mixed into the pulverized coal after the secondary preheating, and the second preset temperature is 600-700 ℃.

In some embodiments, ammonia gas is admixed into the pulverized coal before the tertiary preheating at the third location, the tertiary preheating time is 100ms to 1000ms, and the third preset temperature is 900 ℃ to 1100 ℃.

In some embodiments, ammonia gas is admixed into the pulverized coal before the tertiary preheating at the third location, the third predetermined temperature is 1000 ℃ to 1100 ℃, and ammonia gas is admixed into the pulverized coal after the tertiary preheating at the third location, the third predetermined temperature is 900 ℃ to 1000 ℃.

In some embodiments, the primary wind has a wind speed of 15m/s to 25 m/s; and/or the wind speed of the secondary wind is 15m/s-40 m/s.

In some embodiments, ammonia is admixed into the coal fines prior to the primary preheating at the first location in a proportion of 0% to 80%.

In some embodiments, ammonia is admixed into the coal fines prior to the secondary preheating at the second location in a proportion of 0% to 70%.

In some embodiments, ammonia is admixed into the pulverized coal before the tertiary preheating at the third location in a proportion of 0% to 50%.

In some embodiments, ammonia is blended into the pulverized coal in the fourth position in a proportion of 0% to 20%.

In some embodiments, the coal fines have a particle size of 75 μm or less.

Drawings

FIG. 1 is a flow chart of a multistage preheating combustion method for ammonia coal according to an embodiment of the present invention.

FIG. 2 is a schematic diagram of one-stage preheating of the ammonia coal multi-stage preheating combustion method according to the embodiment of the invention.

FIG. 3 is a schematic diagram of the two-stage preheating of the ammonia coal multi-stage preheating combustion method according to the embodiment of the invention.

FIG. 4 is a schematic diagram of three-stage preheating of the ammonia coal multi-stage preheating combustion method according to the embodiment of the invention.

FIG. 5 is a combustion schematic diagram of the ammonia coal multi-stage preheating combustion method according to the embodiment of the invention.

Reference numerals:

a swirler 1; a cylinder body 2; a primary air duct 3; a return cap 4; a preheating chamber 5; a high temperature reflux zone 6 and a combustion zone 7.

Detailed Description

Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the drawings are illustrative and intended to be illustrative of the invention and are not to be construed as limiting the invention.

The technical solution of the present application is described in detail below with reference to the accompanying drawings.

As shown in fig. 1, the ammonia coal multi-stage preheating combustion method of the embodiment of the invention comprises:

the first step is as follows: the pulverized coal is conveyed to a first position through primary air, primary air and the pulverized coal are preheated at the first position to a first preset temperature, and the primary air and the pulverized coal after primary preheating are conveyed to a second position.

The second step is that: and carrying out secondary preheating on the primary air and the coal powder at a second position, preheating to a second preset temperature to enable the coal powder to be combusted to generate primary flame, exhausting oxygen in the primary air, carrying out pyrolysis reaction on the coal powder during secondary preheating to generate pyrolysis gas, and conveying the pyrolysis gas and the coal powder subjected to secondary preheating to a third position, wherein the second preset temperature is higher than the first preset temperature.

The third step: and carrying out three-stage preheating on the pulverized coal and the pyrolysis gas at a third position, preheating to a third preset temperature, wherein the third preset temperature is higher than the second preset temperature, conveying the pulverized coal and the pyrolysis gas subjected to three-stage preheating to a fourth position through secondary air, and burning at the fourth position.

Wherein ammonia gas is admixed into the pulverized coal in at least one of the first location prior to the primary preheating, the second location prior to the secondary preheating, the third location prior to the tertiary preheating, and the fourth location. That is, ammonia can be mixed with coal gas at any position in the first position before primary preheating, the second position before secondary preheating, the third position before tertiary preheating, or the fourth position during combustion, or can be mixed with coal gas at four positions simultaneously, so that the ammonia coal fuel forms high-temperature gas-solid mixed fuel.

The first preset temperature is lower during primary preheating of the ammonia coal multi-stage preheating combustion method, and the purpose is to prepare for primary flame generated by primary air and coal dust at a second position. Because the third preset temperature is higher than the second preset temperature, the pulverized coal is further subjected to pyrolysis reaction during the three-stage preheating, main gas components of the generated pyrolysis gas are hydrogen, oxygen, carbon monoxide, methane, acetylene, ethane, propane and carbon dioxide, and the type and the quantity of the pyrolysis gas after the three-stage preheating are more than those of the pyrolysis gas generated during the two-stage preheating.

According to the ammonia coal multi-stage preheating combustion method, the coal powder is preheated in three stages, the preheating mode (primary preheating) of a traditional self-maintaining preheating burner is changed, and the preheating path of the coal powder is reconstructed from the combustion organization principle. On one hand, the retention time of the coal powder in the high-temperature reduction area is prolonged, so that the high-temperature gas-solid mixed fuel generated after the ammonia coal is mixed contains a large amount of high-temperature combustible reducing gas, which is beneficial to promoting the heterogeneous reduction of NO, inhibiting the generation of nitrogen oxides in the combustion process of the ammonia coal fuel, increasing the propagation speed of flame, enhancing the stability of the flame and ensuring that the ignition performance of the ammonia coal fuel is better; on the other hand, the heat released by the combustion of the combustible gas in the reaction front of the high-temperature gas-solid mixed fuel formed by the ammonia coal is increased, so that more soot is formed in the flame zone, the heat radiation between the flame zone and the unburned zone is enhanced, and the flame combustion flame of the ammonia coal mixed fuel is further stable.

Alternatively, the ammonia coal multi-stage preheating combustion method of the embodiment of the invention can adopt a reverse direct flow mode to enter the combustor to realize three-stage preheating, and as shown in fig. 1 to 5, the combustor comprises a cyclone 1, a cylinder 2, a primary air pipe 3 and a backflow cap 4. Wherein, the cylinder 2 adopts a gradually expanding cone shape, the cylinder 2 limits a preheating chamber 5, the outlet end of the cyclone 1 is connected with the inlet end of the cylinder 2, the outlet end of the cylinder 2 is connected with the inlet end of a hearth (not shown in the figure), the primary air pipe 3 is inserted on the cyclone 1 and is coaxially arranged with the cylinder 2, the inlet end of the primary air pipe 3 is arranged outside the cylinder 2, and the outlet end of the primary air pipe 3 is arranged in the cylinder 2. The reflux cap 4 is positioned in the cylinder 2 and is arranged at the outlet end of the primary air pipe 3.

It should be noted that a high-temperature reflux zone 6 (shown in fig. 5) is formed between the combustion zone 7 and the preheating zone in the burner, that is, the high-temperature heat generated in the combustion zone 7 is radiated and refluxed to the preheating zone.

Specifically, in the first position, primary preheating: as shown in fig. 2, the primary air carries the pulverized coal into the primary air duct 3 through the inlet end of the primary air duct 3, and the pulverized coal in the primary air duct 3 is preheated at one stage. Secondary preheating at a second position: as shown in fig. 3 and 5, the primary air carrying the pulverized coal to the backflow cap 4 is ejected through a backflow channel formed by the outer wall of the primary air pipe 3 and the backflow cap 4, a strong vortex region is formed at the backflow cap 4 due to the action of the annular reverse jet flow, heat in the high-temperature backflow region 6 is absorbed to generate primary flame, and the ejected pulverized coal enters the primary flame to complete secondary preheating. In the secondary preheating process of the pulverized coal, oxygen in primary air is exhausted, the small-particle pulverized coal is pyrolyzed, and volatile components (mainly carbon monoxide, methane and hydrogen) are separated out. Tertiary preheating at the third position: as shown in fig. 4, by the primary flame, the pulverized coal enters a nested high-temperature reflux zone 6 formed by the swirler 1 and the barrel 2 together to complete three-stage preheating, the pulverized coal further undergoes a pyrolysis reaction, and the volatile components (mainly hydrogen, oxygen, carbon monoxide, methane, acetylene, ethane, propane and carbon dioxide) are further separated out. And combustion at the fourth position: as shown in fig. 5, the pulverized coal is mixed with the secondary air entering the cyclone 1 and starts to burn to form a main flame, and the main flame is sprayed into a hearth through the rectifying chamber.

The ammonia gas can be mixed with the primary air before the primary air enters the primary air pipe 3, or the primary air outlet reflux cap 4 generates primary flame, or the high-temperature reflux area 6, or the combustion area 7, and the ammonia gas and the pulverized coal preheated by three stages form high-temperature gas-solid mixed fuel to be sprayed into a hearth for combustion.

In other embodiments, the coal powder can be preheated by a preheater at the first position, the second position and the third position.

For example, the preheater includes a primary preheater, a secondary preheater, and a tertiary preheater, wherein the tertiary preheater is in communication with the burner. The first step is as follows: the pulverized coal is conveyed to a first preheater (a first position) through primary air, primary air and the pulverized coal are preheated in a first stage in the first preheater to a first preset temperature, and the primary air and the pulverized coal after the primary preheating are conveyed to a second preheater (a second position).

The second step is that: the primary air and the pulverized coal are preheated in the second preheater in a second stage, the pulverized coal is preheated to a second preset temperature so that the pulverized coal is combusted to generate primary flame, oxygen in the primary air is exhausted, the pulverized coal is subjected to pyrolysis reaction in the second preheater to generate pyrolysis gas, the second preset temperature is higher than the first preset temperature, and the pyrolysis gas and the pulverized coal preheated by the second preheater are conveyed to a third-stage preheater (a third position).

The third step: and (3) carrying out three-stage preheating on the pulverized coal and the pyrolysis gas in a third preheater to a third preset temperature, wherein the third preset temperature is higher than the second preset temperature, conveying the pulverized coal and the pyrolysis gas preheated by the third preheater to a burner (a fourth position) through secondary air, and spraying the pulverized coal and the pyrolysis gas into a hearth after the pulverized coal and the pyrolysis gas are combusted in the burner.

Wherein ammonia gas is admixed into the pulverized coal in at least one of the primary preheater, the secondary preheater, the tertiary preheater, and the burner. That is, the ammonia gas can be mixed with the coal gas at any position before the preheating of the primary preheater, or before the preheating of the secondary preheater, or before the preheating of the tertiary preheater, or during the combustion in the combustor, or can be simultaneously mixed with the coal gas at four positions, so that the ammonia coal fuel forms high-temperature gas-solid mixed fuel for combustion.

Wherein, each of the first-stage preheater, the second-stage preheater and the third-stage preheater can adopt preheating modes including but not limited to electric heating, high-temperature steam heating or high-temperature flue gas heating, and the details are not repeated in this embodiment.

In some embodiments, the primary preheating time is 20ms to 100ms and the first preheating temperature is 150 ℃ to 200 ℃. For example, the primary preheating time is 60ms and the first preset temperature is 180 ℃.

Optionally, ammonia gas is blended into the pulverized coal before primary preheating at a first position, the first preheating temperature is 175-200 ℃, and ammonia gas is blended into the pulverized coal after primary preheating at a first preheating temperature of 150-175 ℃.

Because the ammonia is mixed into the pulverized coal before primary preheating at the first position, when the primary preheating is carried out on the pulverized coal, the ammonia absorbs part of heat, the primary preheating efficiency of the pulverized coal can be influenced, and the first preheating temperature can be set to be higher, for example, 190 ℃. If ammonia gas is mixed into the pulverized coal after primary preheating, only the pulverized coal is preheated when primary preheating is carried out at the first position, the pulverized coal has higher heat absorption efficiency, and the first preheating temperature can be set to be lower, such as 160 ℃.

In some embodiments, the secondary preheating is for a time period of 1ms to 10ms and the second predetermined temperature is 600 ℃ to 800 ℃. For example, the secondary preheating time is 5ms and the first preset temperature is 700 ℃.

Optionally, ammonia gas is mixed into the pulverized coal before the secondary preheating at a second position, the second preset temperature is 700-800 ℃, and ammonia gas is mixed into the pulverized coal after the secondary preheating at the second position, and the second preset temperature is 600-700 ℃.

Because ammonia is mixed into the pulverized coal before secondary preheating at the second position, when the pulverized coal is secondarily preheated, the ammonia absorbs part of heat, the primary preheating efficiency of the pulverized coal is affected, and the second preheating temperature can be set to be higher, such as 750 ℃. If ammonia gas is mixed into the pulverized coal after the secondary preheating, only the pulverized coal is preheated when the secondary preheating is carried out at the second position, the pulverized coal has higher heat absorption efficiency, and the second preheating temperature can be set lower, such as 650 ℃.

In some embodiments, the time for the three stages of preheating ranges from 100ms to 1000ms, and the third preset temperature ranges from 900 ℃ to 1100 ℃. For example, the three-stage preheating time is 600ms, and the first preset temperature is 1000 ℃.

Optionally, ammonia gas is mixed into the pulverized coal before the third-stage preheating at a third position, the third preset temperature is 1000-1100 ℃, ammonia gas is mixed into the pulverized coal after the third-stage preheating at the third position, and the third preset temperature is 900-1000 ℃.

Because ammonia is mixed into the pulverized coal before the tertiary preheating at the third position, when the tertiary preheating is carried out on the pulverized coal, the ammonia absorbs part of heat, the primary preheating efficiency of the pulverized coal is influenced, and the third preheating temperature can be set to be higher, such as 1150 ℃. If ammonia gas is mixed into the pulverized coal after the three-stage preheating, only the pulverized coal is preheated when the three-stage preheating is carried out at the third position, the heat absorption efficiency of the pulverized coal is higher, and the third preheating temperature can be set to be lower, such as 950 ℃.

Optionally, the wind speed of the primary wind is 15m/s-25 m/s. For example, the wind speed of the primary wind is 20 m/s.

Optionally, the secondary wind has a wind speed of 15m/s to 40 m/s. For example, the wind speed of the secondary wind is 30 m/s.

Optionally, ammonia gas is blended into the pulverized coal before the primary preheating at the first position, wherein the ammonia blending ratio is 0-80%. For example, the blending ratio of the ammonia gas and the coal powder is 40%, and the specific blending ratio of the ammonia gas and the coal powder can be adjusted according to the actual use requirement.

Optionally, ammonia gas is mixed into the pulverized coal before the secondary preheating at the second position, and the ammonia mixing ratio is 0-70%. For example, the blending ratio of the ammonia gas and the pulverized coal is 35%, and the specific blending ratio of the ammonia gas and the pulverized coal can be adjusted according to the actual use requirement.

Optionally, ammonia gas is mixed into the pulverized coal before the three-stage preheating at the third position, wherein the ammonia mixing ratio is 0% -50%. For example, the blending ratio of the ammonia gas and the pulverized coal is 25%, and the specific blending ratio of the ammonia gas and the pulverized coal can be adjusted according to the actual use requirement.

Optionally, ammonia gas is mixed into the pulverized coal in the fourth position, and the ammonia mixing ratio is 0% -20%. For example, the blending ratio of the ammonia gas and the pulverized coal is 10%, and the specific blending ratio of the ammonia gas and the pulverized coal can be adjusted according to the actual use requirement.

Optionally, the pulverized coal has a particle size of 75 μm or less. For example, the pulverized coal has a particle size of 40 μm.

In the description of the present invention, it is to be understood that the terms "central," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," "circumferential," and the like are used in the orientations and positional relationships indicated in the drawings for convenience in describing the invention and to simplify the description, and are not intended to indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and are therefore not to be considered limiting of the invention.

Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one such feature. In the description of the present invention, "a plurality" means at least two, e.g., two, three, etc., unless specifically limited otherwise.

In the present invention, unless otherwise expressly stated or limited, the terms "mounted," "connected," "secured," and the like are to be construed broadly and can, for example, be fixedly connected, detachably connected, or integrally formed; may be mechanically coupled, may be electrically coupled or may be in communication with each other; they may be directly connected or indirectly connected through intervening media, or they may be connected internally or in any other suitable relationship, unless expressly stated otherwise. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.

In the present invention, unless otherwise expressly stated or limited, the first feature "on" or "under" the second feature may be directly contacting the first and second features or indirectly contacting the first and second features through an intermediate. Also, a first feature "on," "over," and "above" a second feature may be directly or diagonally above the second feature, or may simply indicate that the first feature is at a higher level than the second feature. A first feature being "under," "below," and "beneath" a second feature may be directly under or obliquely under the first feature, or may simply mean that the first feature is at a lesser elevation than the second feature.

In the present disclosure, the terms "one embodiment," "some embodiments," "an example," "a specific example," or "some examples" and the like mean that a specific feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present disclosure. In this specification, the schematic representations of the terms used above are not necessarily intended to refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, various embodiments or examples and features of different embodiments or examples described in this specification can be combined and combined by one skilled in the art without contradiction.

Although the above embodiments have been shown and described, it should be understood that they are exemplary and should not be construed as limiting the present invention, and that many changes, modifications, substitutions and alterations to the above embodiments may be made by those of ordinary skill in the art without departing from the scope of the present invention.

Claims

1. The multistage preheating combustion method for the ammonia coal is characterized by comprising the following steps:

conveying pulverized coal to a first position through primary air;

primary preheating is carried out on the primary air and the pulverized coal at the first position, and the primary air and the pulverized coal are preheated to a first preset temperature;

conveying the primary air and the pulverized coal which are preheated at the first stage to a second position;

performing secondary preheating on the primary air and the pulverized coal at the second position, and preheating to a second preset temperature to enable the pulverized coal to be combusted to generate primary flame, exhaust oxygen in the primary air, and generate pyrolysis reaction and generate pyrolysis gas when the pulverized coal is subjected to secondary preheating, wherein the second preset temperature is higher than the first preset temperature;

conveying the pyrolysis gas and the pulverized coal subjected to secondary preheating to a third position;

carrying out three-stage preheating on the pulverized coal and the pyrolysis gas at the third position, and preheating to a third preset temperature, wherein the third preset temperature is higher than the second preset temperature;

conveying the pulverized coal and the pyrolysis gas subjected to the three-stage preheating to a fourth position through secondary air, and combusting at the fourth position;

2. The ammonia coal multi-stage preheating combustion method according to claim 1, wherein the preheating time of the first stage is 20ms-100ms, and the first preheating temperature is 150 ℃ -200 ℃.

3. The ammonia-coal multistage preheating combustion method according to claim 2, characterized in that ammonia gas is blended into the pulverized coal before the primary preheating at the first position, the first preheating temperature is 175 ℃ to 200 ℃, ammonia gas is blended into the pulverized coal after the primary preheating, and the first preheating temperature is 150 ℃ to 175 ℃.

4. The ammonia coal multi-stage preheating combustion method according to claim 1, wherein the secondary preheating time is 1ms to 10ms, and the second preset temperature is 600 ℃ to 800 ℃.

5. The ammonia coal multistage preheating combustion method according to claim 4, characterized in that ammonia gas is blended into the pulverized coal before the secondary preheating at the second position, the second preset temperature is 700 ℃ to 800 ℃, and ammonia gas is blended into the pulverized coal after the secondary preheating at the second position, and the second preset temperature is 600 ℃ to 700 ℃.

6. The ammonia coal multistage preheating combustion method according to claim 1, wherein the time of the three-stage preheating is 100ms to 1000ms, and the third preset temperature is 900 ℃ to 1100 ℃.

7. The ammonia coal multistage preheating combustion method according to claim 5, characterized in that ammonia gas is blended into the pulverized coal before the tertiary preheating at the third position, the third preset temperature is 1000 ℃ to 1100 ℃, ammonia gas is blended into the pulverized coal after the tertiary preheating at the third position, and the third preset temperature is 900 ℃ to 1000 ℃.

8. The ammonia coal multistage preheating combustion method according to claim 1, wherein the wind speed of the primary wind is 15m/s-25 m/s; and/or the wind speed of the secondary wind is 15m/s-40 m/s.

9. The ammonia-coal multistage preheating combustion method according to claim 1, wherein ammonia gas is mixed into the pulverized coal before the first-stage preheating at the first position, and the ammonia mixing ratio is 0% -80%.

10. The ammonia coal multi-stage preheating combustion method according to claim 1, wherein ammonia gas is mixed into the pulverized coal before the secondary preheating at the second position, and the ammonia mixing ratio is 0% -70%.

11. The ammonia coal multistage preheating combustion method according to claim 1, wherein ammonia gas is mixed into the pulverized coal before the tertiary preheating at the third position, and the ammonia mixing ratio is 0% -50%.

12. The ammonia coal multistage preheating combustion method according to claim 1, wherein ammonia gas is blended into the pulverized coal in the fourth position, and the ammonia blending ratio is 0% -20%.

13. The ammonia-coal multistage preheat combustion method according to any one of claims 1 to 12, wherein the pulverized coal particle size is 75 μm or less.