JPH0221107A - Low NOx combustion method for pulverized coal - Google Patents

Abstract

PURPOSE:To reduce NOx generated during the combustion of dust coal by turning volatile nitrogen oxides contained in the dust coal into reducing nitrogen oxides, such as NH3, and HCN in an air shortage region formed by the combustion of a main burner, and reducing NOx generated in an air excess region formed by the combustion of a sub burner. CONSTITUTION:Combustion flames formed by a main burner 52 are arranged to have an air ratio of 0.9 to 1.0 and get subjected to rather imperfect combustion, produce reducing substances generation atmosphere, and form perfect combustion state in a region where the combustion flames generated by a sub burner 53 have an excess air ratio of 1.1 and over. In other words, NOx generated by the sub burner 53 is reduced by reducing nitrogen oxides, such as NH3, and HCN so as to reduce NOx.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for burning pulverized coal, and is particularly suitable for reducing nitrogen oxides (hereinafter referred to as NOx) generated during combustion of pulverized coal. Regarding combustion methods.

[Prior art]

Fossil fuels contain N in addition to fuel components such as C and H. In particular, in the case of coal, the amount of N content is larger than that of gaseous fuel or liquid fuel. Therefore.

NOx generated during combustion of coal is greater than NOx generated during combustion of gas and liquid fuels, and it is desired to reduce this as much as possible.

NOx generated during the combustion of various fuels is thermal NOx.
It is classified into x and fuel NOx.

Thermal NOx is generated when nitrogen in the combustion air is oxidized by oxygen, and fuel NOx is
It is produced by the oxidation of N in fuel. Conventional combustion methods to suppress the generation of NOx include a multistage combustion method in which combustion air is divided into multiple stages and supplied, and an exhaust gas recirculation method in which combustion exhaust gas with a low oxygen concentration is mixed into the combustion area. be.

The common principle of these low NOx combustion methods is to lower the temperature of the combustion flame through low oxygen combustion.

Its purpose is to suppress the reaction between nitrogen and oxygen. By the way, of the two types of NOx mentioned above, thermal NOx can be suppressed from being generated by lowering the combustion temperature, and the generation of fuel NOx is less dependent on the combustion temperature. Therefore, conventional combustion methods aimed at lowering the flame temperature are effective in reducing NOx from fuel with low N content, but compared to pulverized coal combustion, where nearly 80% of the NOx generated is fuel NOx. The effect is small.

[Purpose of the invention]

The purpose of the present invention is to reduce the fuel NO during combustion peculiar to pulverized coal.
It is an object of the present invention to provide a combustion method that is effective in reducing NOx generated during pulverized coal combustion without increasing the size of a combustion device, taking into account the generation of x.

[Summary of the invention]

Combustible components in coal can be roughly divided into volatile components and solid components. According to the unique properties of coal, the combustion mechanism of pulverized coal consists of the thermal decomposition process of pulverized coal in which volatile components are released, the combustion process of the released volatile components, and the combustion process of combustible solid components after thermal decomposition (
It consists of the combustion process of char (hereinafter referred to as char). The burning rate of volatile components is much faster than that of solid components, and volatile components are burned in the initial process of combustion. Also, in the pyrolysis process.

Like other combustible components, the N contained in coal is divided into those that are volatilized and released and those that remain in the char. Therefore, fuel NOx generated during combustion of pulverized coal is divided into NOx from volatile N and NOx from N in char. Of these two types of fuel NOx, fuel NOx from char is only generated when char is burned, so NOx continues to be generated until the latter half of combustion, and countermeasures against this are an important key.

It is known that volatile N becomes compounds such as HNa and HCN in the initial process of combustion and in the oxygen-deficient combustion region. These nitrogen compounds, in addition to reacting with oxygen to become NOx, can also serve as reducing agents that react with generated NOx and decompose NOx into nitrogen. NO due to this nitrogen compound
The x reduction reaction proceeds in a system in which NOx coexists, and in a reaction system in which NOx coexists, most nitrogen compounds are oxidized to NOx. Further, the generation of this reducing substance progresses more easily as the atmosphere becomes lower in oxygen concentration.

As described above, an effective method for reducing NOx during combustion of pulverized coal is a combustion method in which a volatile nitrogen compound having reducing properties and NOx coexist and the nitrogen compound reduces NOx to nitrogen. That is, a combustion method that uses a reducing nitrogen compound such as NHa, which is a precursor of NOx, to reduce NOx, thereby eliminating the generated NOx and the NOx precursor is effective for reducing NOx.

Based on this principle, a combustion method in which fuel is dividedly supplied in two stages for generating NOx and for generating a reducing agent is already known. This system uses two stages of fuel supply within the furnace. This combustion method is a process in which main combustion is performed by bringing the flame from the main burner to an air ratio of 1 or more, and then fuel is supplied from the second stage burner to reduce the NOx generated in this process, and the air ratio is 1 or less. This process consists of a step of forming a reduction region, and a step of supplying air from the third stage burner to burn excess fuel in the reduction region. It is well known that this combustion method can reduce NOx, but in order to apply this method to coal combustion and increase the NOx reduction effect, it is necessary to reduce the distance between the -, second, and third stage burners. Since it is necessary to increase the size of the chicks and clearly distinguish each combustion area, the combustion furnace becomes large, which is economically disadvantageous in practical use.

Furthermore, when applied to an actual boiler, for example, in a large boiler for electric power, the cross-sectional area of the combustion furnace is large, with a width of 22 m and a depth of 15.5 m. It is difficult to completely mix the fuel and air ejected from the third stage burner. Therefore, the non-uniform distribution of fuel and air within the cross section of the combustion furnace and the associated N
Non-uniform distribution of Ox concentration occurs, and the same NOx reduction effect as obtained in a small test reactor cannot be obtained. In particular, when the air from the third stage burner is poorly mixed, the amount of unburned matter released increases, resulting in a decrease in combustion efficiency.

Furthermore, a cell burner combining a plurality of burners was developed to carry out two-stage fuel supply combustion.
No. 6-906, JP-A-56-149517.

A burner disclosed in Japanese Patent Application Laid-Open No. 57-1673 is known.

According to this known technique, it is effective to prevent the combustion furnace from increasing in size, and it also prevents the mixture of reaction products from the main combustion zone, which is the fire combustion zone, and the reduction combustion zone, which is the zone containing secondary combustion. This is an improvement over the combustion method typified by Special Publication No. 55-21922. However, in order to ideally realize the generation of the reducing agent and NOx and the mixing of the two, it is necessary to
Eliminate mutual interference between generation areas. That is, it is necessary to mix the products from each reaction zone after the reaction in each zone is completed, and it is necessary to reduce the amount of mixing of each zone during the reaction.

In the above-mentioned known example, in order to further increase the effect of reducing NOx,
As previously mentioned, there is a need to further improve the reaction promotion of the air starved combustion zone and the mixing of reaction products from the air starved zone with reaction products from the air rich zone.

Generally, when attempting to reduce NOx, the amount of unburned substances tends to increase, and it is difficult to simultaneously achieve low NOx and low unburned substances using conventional combustion methods. Therefore, when pulverized coal is burned, the release of unburned matter depends on coarse particles of 50 mesh or more, so a means is provided to separate the pulverized coal into fine powder and coarse powder, and the coarse powder and coarse powder are placed in separate burners. A pulverized coal combustion device that achieves low NOx and low unburned emissions by supplying fine powder and increasing the air ratio during coarse powder combustion and lowering the air ratio during fine powder combustion was published in Japanese Patent Application Publication No. No. 51-106241. This combustion device is more effective in reducing NOx and reducing the release of unburned substances than conventional combustion methods, but like the publicly known Japanese Patent Publication No. 55-21922, it is a pulverized combustion device that burns at a low air ratio. The problem is how to mix the unburned material from the flame with the air used to burn it.

The present invention is characterized by a combustion method in which fuel coal is divided and supplied in multiple stages, and includes a primary fuel nozzle provided in the center of a cylindrical burner that blows out fuel coal and primary combustion air that also serves as fuel coal conveyance. , a secondary fuel nozzle equipped with a swirling flow generating means provided on the outer periphery of the primary fuel nozzle so as to be concentric with the secondary fuel nozzle, which blows out secondary combustion air that also serves as fuel coal conveyance, and tertiary combustion. A low NOx combustion burner consisting of a secondary fuel nozzle that blows out air and a tertiary air nozzle equipped with a swirling flow generation means provided on the outer periphery of the main burner and a sub-burner is arranged in two stages. is installed upstream of the exhaust gas flow, and a sub-burner is installed downstream of the main burner, and the nitrogen oxides in the combustion exhaust gas of the main burner are combusted at an air ratio of 1.1 or more. The purpose is to perform gas phase reduction and complete combustion using the combustion gas from the auxiliary burner.

The present invention is capable of converting volatile nitrogen compounds in pulverized coal into NII3. Reducing nitrogen compounds such as HCN are used to reduce NOx generated in the air excess area formed by combustion in the auxiliary burner.
to reduce.

[Embodiments of the invention]

FIG. 1 shows a burner according to an embodiment of the present invention. This burner is composed of two fuel coal injection nozzles 11 and 13, a tertiary air nozzle 14, and a combustion furnace preheating fuel injection nozzle 17. Thermal coal is divided into primary fuel and secondary fuel.
each primary fuel nozzle 11. It was ejected from the secondary fuel nozzle 13. Combustion air is primary air used to convey primary fuel.

The secondary air used for conveying the secondary fuel is further separated into tertiary air, and each is connected to a secondary fuel nozzle 11. Secondary fuel nozzle 1
3. It was ejected from the tertiary air nozzle 14. A cylindrical combustion furnace preheating fuel injection nozzle 17 is installed in the center of the burner,
When the combustion furnace was preheated during startup, gaseous fuel was ejected from this and combusted using primary air, secondary air, and tertiary air. When the temperature inside the combustion furnace reached a predetermined temperature, the injection of gaseous fuel was stopped, and pulverized coal, which was the primary fuel and secondary fuel, was ejected from the respective injection nozzles.

The secondary fuel nozzle 11 is a fuel injection nozzle 17 for preheating the combustion furnace.
The secondary fuel nozzle 13 is a cylindrical nozzle arranged in a concentric circle on the outer periphery of the cylinder, and has a double cylindrical structure with the combustion furnace preheating fuel injection nozzle 17 as the inner pipe and the primary fuel nozzle 11 as the outer pipe. is an annular nozzle that is installed on the outer periphery of the primary fuel nozzle 11 and is concentric with the primary fuel nozzle 11. Further, an axial swirling flow generator 15 was installed at the ejection port of the secondary fuel nozzle 13 to give a swirling flow to the mixed flow consisting of the secondary fuel and secondary air and eject it. The tertiary air nozzle 14 was installed concentrically with the outer circumference of the secondary fuel nozzle 13. The tertiary air nozzle 14 was composed of eight circular tubes, and the eight circular tubes were installed at equal intervals on a pitch circle.

In addition, the eight circular pipes were installed so that the jet ports were inclined in the tangential direction of the pitch circle so that the tertiary air jet was a swirling flow.

FIG. 2 is a view taken along the -n arrow in FIG. 1, and the direction in which the tertiary air is ejected is indicated by an arrow.

FIG. 3 shows the amount of NOx generated when pulverized coal is burned by the burner shown in FIGS. 1 and 2. The horizontal axis in FIG. 3 indicates the air ratio λ, which is the sum of the primary, secondary, and tertiary air amounts divided by the theoretical air amount required to completely burn the primary fuel and the secondary fuel. The vertical axis indicates the NOx concentration in the combustion exhaust gas. The coal used was Taiheiyo Coal, which was pulverized to a particle size that allowed about 80% of the particles to pass through a 200-mesh sieve.

The amount of pulverized coal supplied is 30 kg/h, and the size of the combustion furnace is 700 nwa in inner diameter and 2 m in length.
This was obtained under experimental conditions in which the fuel flow rate was 5 kg/h and the air ratio λ1 of the inner flame formed by the primary fuel was set to 0.4. Further, the overall air ratio λ was changed by adjusting the tertiary air amount. From FIG. 3, it can be seen that NOx generated during pulverized coal combustion increases as the air ratio λ increases. In order to clarify the effects of the present invention, Fig. 3 shows experimental results 32 using the burner shown in Fig. 1, using all of the supplied coal 30 kg/h as primary fuel, and setting the secondary fuel to zero. was also shown. As is clear from comparing experimental results 31 and 32 in FIG. 3, it can be seen that the two-stage fuel supply method allows for a greater reduction in NOx at the same air ratio.

FIG. 4 shows the experimental results under the same conditions as the experimental results 31 shown in FIG. 3, that is, when pulverized coal was supplied at a rate of 15 kg/h for both the primary and secondary fuels. However, the overall air ratio λ was kept constant at about 1.3, and the air ratio λl of the inner flame formed by the primary fuel and primary air (hereinafter referred to as the primary air ratio) was varied. Furthermore, in order to keep the overall air ratio λ constant, the tertiary air amount was changed as the primary air ratio λl changed. The horizontal axis in Figure 4 is the primary air ratio λ1
, and the vertical axis indicates the NOx concentration in the combustion exhaust gas.

From Figure 4, there is an optimal value for the primary air ratio λl, and NO
It can be seen that there exists a primary air ratio where x is minimum.

The value of the primary air ratio λl that minimizes this NOx is the primary,
Although it varies depending on the distribution ratio of the secondary fuel, it is a value of 1 or less. From this experimental result, the inner flame formed by the primary fuel is in a reducing atmosphere, and the outer flame formed by the secondary fuel has an air ratio of 1.
It can be seen that setting the complete combustion range as above is effective in reducing NOx.

Therefore, secondary fuel is used as a low NOx combustion method.

A fuel split burner in which the complete combustion region with an air ratio of 1 or more formed by secondary air and tertiary air is an outer flame, and the reducing region with an air ratio of 1 or less formed by primary fuel and primary air is an inner flame. It was found that even greater NOx reduction effects could be obtained by arranging the main burner and the auxiliary burner in two stages, creating a reducing atmosphere for the combustion flame from the main burner, and creating a complete combustion region for the combustion flame from the auxiliary burner. .

Hereinafter, the present invention will be explained in more detail with reference to Examples.

Example 1) Based on the experimental results from FIGS. 3 and 4 explained above, FIG. 5 shows a system for a low NOx combustion method for pulverized coal using the burner shown in FIG. 1.

Burners 52 and 53 of the present invention are arranged in two stages in a boiler 51. The fuel coal 55 stored in the coal hopper 54 is pulverized by a pulverizer 56 so that about 80% of the coal is 74 μm or less. The fuel coal, which is pulverized coal, is transported to the main burner 52 by coal conveying air 58 from the blower 57. It is divided into two stages and supplied to the sub burners 53, respectively. Furthermore, combustion air 60 from the blower 59 is used as tertiary air in addition to the coal conveying air to the main burner 52. It is supplied to the auxiliary burner 53.

According to the above overall configuration, the combustion flame formed by the main burner 52 is set to an air ratio of 0.9 to 1.0 to cause a somewhat incomplete combustion to create a reducing substance generating atmosphere, and the combustion flame formed by the auxiliary burner 53 is The aim is to form a complete combustion state in an excessive air ratio of 1.1 or more. That is, the sub burner 5
The NOx generated by the combustion of No. 3 is reduced by reducing nitrogen compounds such as NHa and HCN generated by the combustion of the main burner 52, thereby further reducing NOx.

FIG. 6 shows the NOx generation characteristics when pulverized coal is burned in the combustion furnace shown in FIG. 5. The horizontal axis in FIG. 6 shows the air ratio λ of the main burner, and the vertical axis shows NOx in the combustion exhaust gas.

However, the overall air ratio λT.

That is, depending on the minimum amount of air required to completely burn the fuel coal of the pulverized coal supplied to the main burner and the auxiliary burner, the primary of each of the main burner and the auxiliary burner is determined.

The air ratio λT, which is the value obtained by dividing the sum of the secondary and tertiary air amounts, was kept constant at about 1.5, and the air ratio λ of the flame formed by the main burner was varied. In addition, in order to keep the overall air ratio constant, as the air ratio λ of the main burner changes,
The amount of air in the secondary burner was changed. The coal used was Pacific coal, similar to the coal used in one example. The amount of pulverized coal supplied to both the main burner and the auxiliary burner was 30 kg/kl, and the combustion furnace was a rectangular vertical furnace with an inner diameter of 1 m square and a height of 6 m.

The experimental results shown in FIG. 6 were obtained when the primary and secondary fuel supply rates were equal to 15 kg/h for both the main burner and the auxiliary burner. From Figure 6, there is an optimum value for the main burner air ratio λ, and the main burner air ratio λ at which NOx is minimized.
It can be seen that the value is between 0.9 and 1.0. From this experimental result, the flame formed by the main burner has an air ratio of 0.
It can be seen that creating a reducing atmosphere of 9 to 1.0 is effective in reducing NOx.

Similarly to FIG. 6, FIG. 7 shows the NOx generation when pulverized coal is combusted in the combustion furnace shown in FIG. The horizontal axis in FIG. 7 represents the overall air ratio λT, and the vertical axis represents the degree of NOx 'a in the combustion exhaust gas. Experimental results 75 shown in Figure 7
In this example, 15 kg/h of pulverized coal was supplied equally to both the primary and secondary fuels of the main burner and the sub burner, and the air ratio λ of the main burner was set to 0.9 to 1.0.

This was obtained under experimental conditions in which the air ratio λ of the auxiliary burner was set to 1.1 or more. Further, the overall air ratio λT was changed by adjusting the tertiary air amount of the sub-burner.

From FIG. 7, the amount of NOx generated during pulverized coal combustion tends to increase as the overall air ratio increases. In order to clarify the effects of the present invention, the combustion furnace shown in FIG. 5 is used, the amount of pulverized coal supplied is the same, and the air ratio of both the main burner and the auxiliary burner is set to 1. Experimental results 76 when burning at a temperature of 1 or more are also shown. As is clear from comparing experimental results 75 and 76 in FIG. 7, the external flame swirl type fuel split burner shown in FIG. 0.9
~1.0, and according to the combustion method in which the air ratio of the auxiliary burner is set to 1.1 or more, it can be seen that even with the same total air ratio, it is possible to achieve a further reduction in NOx.

Next, FIG. 8 shows a comparison of the amounts of unburned matter in the combustion ash released during pulverized coal combustion. The experimental conditions were the same as the experimental results 75.76 shown in FIG. 7, and the amounts of primary and secondary fuel supplied to the main burner and auxiliary burner were the same, 15 kg/h. In FIG. 8, experimental result 81 shows the combustion under the same experimental conditions as experimental result 75, with the air ratio of the main burner being 0.9 to 1.0 and the air ratio of the auxiliary burner being 1.1 or more. The experimental result 82 was obtained under the same conditions as the experimental result 76, that is, when both the main burner and the auxiliary burner were burned at an air ratio of 1.1 or more. As is clear from the experimental results shown in Figure 8, according to the combustion method of the present invention, the amount of unburned matter released from the combustion ash can be reduced even with the same total air ratio, that is, the combustion efficiency can be improved. I can figure it out.

According to this embodiment, NOx can be reduced by arranging the burners shown in FIG. 1 in two stages. As a result, conventional multistage combustion methods have been used. There is no need to separately provide an air supply burner for complete combustion, and NOx can be reduced without increasing utilities such as NHa. Therefore, it is possible to prevent the combustion apparatus from increasing in size, as it goes without saying that economy and volume are important considerations for the combustion apparatus.

〔Effect of the invention〕

According to the present invention, by arranging burners with the same structure in two stages relative to the gas flow in the combustion furnace, the pulverized coal combustion flame is divided into the NOx generation area due to combustion in the auxiliary burner and the main area for reducing NOx. It can be clearly divided into the area where reducing substances are generated due to burner combustion.

Since the reaction products from both can be mixed and the NOx reduction reaction can be promoted, it is possible to effectively carry out two-stage fuel supply combustion, and it is possible to achieve a reduction in NOx and at the same time a reduction in the release of unburned substances.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view of a burner used in an embodiment of the present invention, Figure 2 is a view taken along the ■-■ arrow in Figure 1, and Figure 3 is the air ratio when coal is combusted with the burner in Figure 1. Figure 4 shows the relationship between primary air ratio and NOx when coal is burned in the burner shown in Figure 1.
Figure 5 is a flowchart showing the overall configuration of an embodiment of the present invention, Figure 6 is a diagram showing the relationship between NOx and the air ratio of the main burner when coal is burned, and Figure 7 is a flowchart showing the overall configuration of an embodiment of the present invention. FIG. 8 is a diagram showing the relationship between the total air ratio and NOx during combustion, and FIG. 8 is a diagram showing the relationship between the total air ratio and the unburned content in the combustion ash. 11...Primary fuel nozzle, 13...Secondary fuel nozzle. 14... Tertiary air nozzle, 15... swirl flow generator,
17... Fuel injection nozzle for preheating, 51... Combustion furnace. 52...Main burner, 53...Sub-burner, 54...
Coal storage amount 30 Air ratio ＼ 4 Figure 1) Air ratio χl 60 Air ratio N Rate 5 Figure 7 Air F input T

Claims (1)

[Claims] 1. A combustion method in which pulverized fuel coal is divided and supplied in multiple stages, and the primary combustion air is provided in the center of a cylindrical burner that blows out the fuel coal and the primary combustion air that also serves as the conveyance of the fuel coal. Swirling flow generating means provided on the outer periphery of the primary fuel nozzle so as to eject the fuel nozzle, the fuel coal, and secondary combustion air that also serves to convey the fuel coal, and to be concentric with the primary fuel nozzle. and a tertiary air nozzle that ejects tertiary combustion air and is equipped with swirl flow generating means provided on the outer periphery of the secondary fuel nozzle. The combustion exhaust gas of the main burner is arranged in two stages, the main burner is installed upstream of the exhaust gas flow, and the auxiliary burner is installed downstream of the main burner. A low NO_x combustion method for pulverized coal, which is characterized by reducing nitrogen oxides of 20% by using combustion exhaust gas from an auxiliary burner burned at an air-to-air ratio of 1.1 or more.