JPH0259361B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for combustion of pulverized coal, and particularly to a method suitable for reducing nitrogen oxides (hereinafter referred to as NOx).

NOx generated during the combustion of fossil fuels is classified into fuel NOx and thermal NOx. Fuel NOx is the nitrogen content (hereinafter referred to as N) contained in fuel.
) is oxidized and generated, resulting in thermal NOx
is generated when nitrogen in the air is oxidized. Coal is N
80% of NOx generated during combustion
Fuel NOx is nearby. In contrast, combustion technologies that have been developed in the past include thermal NOx, which suppresses the oxidation of nitrogen in the air by lowering the combustion temperature, as typified by the two-stage combustion method and the exhaust gas recirculation method. The most effective measures are the ones that are effective.

The generation route of fuel NOx generated during pulverized coal combustion can be explained as follows in conjunction with the combustion mechanism. Pulverized coal combustion involves ignition, thermal decomposition, gaseous combustion,
It consists of the process of solid combustion. The initial region of combustion is a region where ignition and thermal decomposition proceed, and the N present in the coal is divided into those that volatilize as a gas and those that remain in solid form. In the combustion region following pyrolysis, gaseous combustion in which the volatile matter in the coal is combusted, and solid combustion in which the solids that have released the volatile matter are combusted, progress.
The N released as a gas and the N in the solid are also partially converted into NOx and partially into nitrogen in the respective combustion zones.

Some of the nitrogen released as a gas during thermal decomposition of coal becomes hydrogen cyanide (HCN) and ammonia (NH ₃ ), and these nitrogen compounds are oxidized to NOx in an atmosphere with high temperature and high oxygen concentration. If an appropriate reaction temperature is set, NOx can be selectively reduced to nitrogen (N ₂ ) in the presence of oxygen. By utilizing this property, it is possible to improve the conventionally developed two-stage combustion and reduce NOx in pulverized coal combustion, and burners based on the principle of two-stage combustion have been developed. Two-stage combustion is a method in which the N content of the coal is reduced to _N2 as much as possible in the low air ratio reducing region of the first stage.In order to further reduce NOx, NOx generation within the combustion flame is region,
It is necessary to clearly distinguish and control the generation region of the reducing agent for NOx reduction and the NOx reduction region.

In addition, a pulverized coal combustion burner has already been developed, which is an improved version of the two-stage combustion originally developed as a measure against thermal NOx, for use as a measure against fuel NOx. An example is shown in FIG. The burner includes a pulverized coal nozzle 01 that supplies a fuel mixture consisting of pulverized coal and primary air;
It is composed of a secondary air nozzle 02 and a tertiary air nozzle 03. A primary combustion region is formed near the burner tip by the fuel mixture and secondary air, where combustion occurs with an amount of combustion air less than the stoichiometric air amount. A combustion zone is formed. In the primary combustion region, N in the coal
In addition to NOx, ammonia and cyanide compounds are generated from the minute, and these nitrogen compounds react in the secondary combustion area to reduce some of the NOx, resulting in primary and secondary combustion.
This occurs more than in a burner that burns pulverized coal in one combustion zone without dividing the next combustion zone and combustion zone.
NOx decreases.

The burner shown in Figure 1 has NOx emissions due to the above reasons.
However, the amount of combustion air in the primary combustion region is sensitive to NOx concentration and unburned fuel emissions.
In order to reduce unburned fuel emissions and suppress NOx generation, it is necessary to strictly control the amount of combustion air. In addition, to further improve the NOx suppression effect,
The primary combustion zone that generates NOx reducing compounds and 2
Clarify the distinction between secondary combustion areas, and
It is necessary to improve the structure so that it is easy to set the optimal reaction conditions for the NOx reducing reaction.

An object of the present invention is to provide a pulverized coal combustion method that is highly effective in suppressing NOx generation and reduces emissions of unburned matter.

The present invention provides a method for burning pulverized coal using a burner having two pulverized coal supply nozzles, in which a mixture of pulverized coal and air for conveying it is divided into two systems and fed to each nozzle, and one Pulverized coal ejected from one nozzle is combusted at an air ratio of 1 or more, pulverized coal ejected from the other nozzle is combusted at an air ratio of less than 1, and a flame from the pulverized coal ejected from the one nozzle and a flame from the other nozzle are generated. This is a pulverized coal combustion method characterized by merging the ejected pulverized coal with a flame in the trailing region of the pulverized coal. Here, the air ratio is a ratio obtained by dividing the amount of air to be supplied by the theoretical amount of air required to completely burn the fuel.

As mentioned above, the N content in coal is converted into NOx, nitrogen (N ₂ ), ammonia (NH ₃ ),
It becomes compounds such as hydrogen cyanide (HCN). In particular, among these compounds, NH ₃ is
It is already a well-known fact in the field of flue gas denitrification technology that it has the property of selectively reducing NOx and has a high NOx reducing effect. Therefore, in order to effectively reduce NOx in combustion gas containing oxygen using coal,
All you have to do is generate a large amount of NH ₃ from coal and react with NOx, and reduce NOx during coal combustion.
The main technical issues are how to generate NH ₃ and how to mix NH ₃ and NOx. However, the type of nitrogen compounds that the N content of coal converts to during pyrolysis depends on the pyrolysis conditions of the coal, and in order to generate a large amount of the desired NH ₃ , it is necessary to select the optimal pyrolysis conditions for NH ₃ production. Must be set. As a result of intensive studies, the inventors found that in the temperature range close to the combustion temperature, the oxygen concentration in the coal pyrolysis atmosphere has a large effect on the NH ₃ conversion rate of the N component in the coal, and found that the optimum method for maximizing the NH ₃ conversion rate was found. The presence of oxygen concentration was confirmed by pyrolysis experiments of pulverized coal.

The gist of the invention is to remove pyrolysis products from coal.
N content in coal is used to reduce NOx.
The purpose is to thermally decompose a portion of the combustion coal in an oxygen-concentrated atmosphere that maximizes the NH ₃ conversion rate, and mix it with NOx-containing combustion gas.

In addition, in the burner of the present invention, the pulverized coal combustion flame is divided into three regions: primary, secondary, and tertiary combustion regions, and within the combustion flame, there is a NOx generation region and a pulverized coal for NOx reduction. Pyrolysis product generation area,
The NOx reduction reaction region is defined. The first combustion region is a complete combustion region in which pulverized coal is completely combusted with air that is greater than or equal to the theoretical air amount, and most of the fuel is combusted here. In this region, combustion is performed at an air ratio of 1 or more, so the amount of unburned fuel remaining in the combustion ash is extremely small, and at the same time, a large amount of NOx is generated. The secondary combustion region is a combustion region of pulverized coal ejected with a mixed gas of combustion exhaust gas and air, and here combustion at an air ratio of 1 or less, that is, thermal decomposition of the pulverized coal proceeds. Since this region is a reducing region lacking oxygen, the rate at which N in pulverized coal is oxidized to NOx is extremely small, and the intermediate products of N in the combustion process, ammonia (NH ₃ ), Hydrogen cyanide (HCN) etc. are generated. The tertiary combustion zone is a region where the thermal decomposition products of pulverized coal generated in the secondary combustion zone react with NOx generated in the primary combustion zone and surplus oxygen in the primary combustion zone. Reduction reactions and oxidation reactions of hydrocarbons, carbon monoxide, hydrogen, unburned matter in solids, etc. proceed.

The pulverized coal that is completely combusted in the primary combustion area is
By using more pulverized coal than the pulverized coal that is thermally decomposed in the secondary combustion area, it is possible to reduce the emissions of unburned fuel, and furthermore, the surplus oxygen heated to a high temperature in the primary combustion area can be used to
Since the thermal decomposition products generated in the secondary combustion zone are oxidized, the chemical reaction in the tertiary combustion zone can be efficiently promoted. Furthermore, if the two pulverized coal nozzles are provided apart or if air is ejected from between the two pulverized coal nozzles, the primary combustion area and the secondary combustion area can be clearly formed.

In one embodiment of the present invention, pulverized coal combustion exhaust gas is used to adjust the oxygen concentration, and the oxygen concentration in the pyrolysis atmosphere is adjusted by controlling the mixing ratio of the combustion exhaust gas and combustion air. Ru.

Further, in one embodiment of the present invention, in order to facilitate the adjustment of oxygen concentration in the pyrolysis region, the above-mentioned mixed gas is used for transporting pulverized coal and injected into the pyrolysis region of pulverized coal for a combustion furnace. used.

Furthermore, in order to more effectively exhibit the effects of the present invention,
The oxygen concentration of the ejected gas of the pulverized coal to be thermally decomposed in the secondary combustion region, that is, the mixture of combustion exhaust gas and air, is preferably 3 to 5% by volume. In other words, NH ₃ , which can selectively reduce NOx even in the presence of oxygen,
In order to efficiently generate pulverized coal through the thermal decomposition reaction,
It is necessary to select the pyrolysis conditions, and as a result of intensive study, the inventors found that the pyrolysis atmosphere has an oxygen concentration of 3 to 5.
%, it was discovered that NH ₃ was generated most from pulverized coal. Therefore, by setting the oxygen concentration in the secondary combustion region to 3 to 5% by volume, that is, by adjusting the mixture ratio of air and combustion exhaust gas to the oxygen concentration of the air-fuel mixture to 3 to 5% by volume, the tertiary It can effectively promote the reduction reaction of NOx in the combustion region.

In addition, a reaction temperature of 900°C or higher is suitable for effectively reacting NH ₃ and NOx in the coexistence of oxygen, so the pulverized coal pyrolysis products and NOx-containing combustion gas are separated in the combustion furnace. It is preferable to mix them and advance the NOx reduction reaction.

An embodiment of the burner of the present invention will be described below with reference to the drawings. In FIG. 2, the pulverized coal combustion burner includes two pulverized coal nozzles 12, 13, a secondary air nozzle 17, a tertiary air nozzle 18, and an igniter 16.
It consists of Most of the fuel pulverized coal is ejected from the pulverized coal nozzle 12 that ejects a fuel mixture consisting of pulverized coal and primary air for transporting and ejecting the pulverized coal, and the secondary air nozzle 17 and the tertiary air nozzle 1
A primary combustion region 21 having an air ratio of 1 or more is formed at the tip of the burner by the secondary air and tertiary air ejected from the burner. Furthermore, in this embodiment, in order to shorten the flame in the primary combustion region 21 and make it independent from other combustion regions, the secondary air nozzle 1 is
7 and a tertiary air nozzle 18 are installed on the outer periphery of the pulverized coal nozzle 12, and a secondary air swirl vane 14 and a tertiary air swirl vane 15 are installed in each nozzle for giving a swirling flow to the combustion air. There is. The pulverized coal nozzle 13 is installed on the outer periphery of the tertiary air nozzle, and from the nozzle 13, a fuel mixture consisting of pulverized coal and a mixture of combustion exhaust gas and air whose oxygen concentration is adjusted to 3 to 5% by volume is ejected. do. A secondary combustion area 22 is formed on the outer periphery of the primary combustion area 21 by the fuel mixture injected from the nozzle 13,
In this region, the thermal decomposition reaction of the pulverized coal progresses due to the heat transmitted from the primary combustion region 21 and the oxygen contained in the mixture, and reducing thermal decomposition products are generated.

Oxidation of reducing pyrolysis products generated in the secondary combustion zone 22 and generated in the primary combustion zone 21
The tertiary combustion zone 23 where the NOx reduction reaction occurs is
It is formed downstream of the primary and secondary combustion areas.

Example 1 This occurred when Japanese domestic coal and char produced by heating Japanese domestic coal in an inert atmosphere at 1000°C (flammable solids remaining during coal pyrolysis) were packed in a platinum holder and heated to 1000°C. By quantifying the amount of _NH3 in the coal, the N content in the coal and the N in the
The _NH3 conversion rate in minutes was measured. FIG. 3 shows the results of examining the effect of atmospheric oxygen concentration on NH ₃ conversion rate by varying the oxygen concentration in the pyrolysis atmosphere in the range of 1 to 7% by volume. The horizontal axis is the oxygen concentration during pyrolysis,
The vertical axis shows the NH ₃ conversion rate of N in coal and coal. As is clear from FIG. 3, the NH ₃ conversion rate is greatly influenced by the oxygen concentration, and there is an optimum oxygen concentration that maximizes the NH ₃ conversion rate. The _NH3 conversion rate for N in coal reaches its maximum at an oxygen concentration of approximately 3% by volume, and the N content in coal reaches a maximum at approximately 5% by volume.
The _NH3 conversion rate is maximized.

Example 2 Figure 4 shows the effect of the oxygen concentration in the pyrolysis atmosphere on the NH ₃ conversion rate of the N content in the coal in the same manner as in Example 1, using the same experimental apparatus as that used to obtain the experimental results in Example 1. This is a look at the impact. In this example, in order to examine the influence of coal types, Australian coal,
We attempted a thermal decomposition reaction experiment of Chinese coal. The thermal decomposition temperature is 1000°C as in Example 1. As is clear from Figure 4, there are some differences in the optimum oxygen concentration range depending on the type of coal, but as with the Japanese domestic coal in Example 1, the oxygen concentration in the pyrolysis atmosphere for both Australian and Chinese coals is approximately The NH ₃ conversion rate of N in coal reaches its maximum at around 3% by volume.

As is clear from the above examples, in order to effectively utilize the coal pyrolysis product as a NOx reducing agent, it is necessary to control the oxygen concentration in the coal pyrolysis atmosphere to an optimal value.

According to the pulverized coal combustion method of the present invention, the oxygen concentration in the pulverized coal pyrolysis region is adjusted by mixing the combustion exhaust gas and air, and the conversion rate of N in the coal to _NH3 , which is effective for NOx reduction, is increased. This enables low NOx combustion of pulverized coal. That is,
The N content of coal, which is converted to NOx in normal combustion, is
Since it can be converted to NH ₃ in the pyrolysis region and used to reduce NOx, the combustion of pulverized coal can be reduced.
Can be converted into NOx. Further, according to the present invention, the air ratio 1
Since the above complete combustion region is formed, it is possible to reduce emissions of unburned fuel. Furthermore, according to the present invention, by combusting pulverized coal in two systems, a flame is formed in one nozzle under conditions with good ignitability with an air ratio of less than 1, so even when the combustion load is low, i.e. Even if the concentration of pulverized coal in the mixture of the pulverized coal ejected from the other nozzle and the air conveying it is low, flames from the other nozzle are likely to form.

[Brief explanation of drawings]

Figure 1 is a side sectional view of a conventional low NOx burner, Figure 2 is a side sectional view of a burner according to the present invention, and Figures 3 and 4 are graphs showing the relationship between NH ₃ conversion rate and oxygen concentration. be. 01...Pulverized coal nozzle, 02...Secondary air nozzle, 03...Tertiary air nozzle, 04...Ignition burner, 05,06...Swirling vane, 07...Auxiliary fuel supply hole for ignition, 08... ...Pulverized coal supply hole, 09...
...Secondary air supply hole, 10...Tertiary air supply hole, 1
1...Pulverized coal supply hole, 12...Pulverized coal nozzle, 1
3...pulverized coal nozzle, 14...secondary air swirl vane, 15...tertiary air swirl vane, 16...igniter, 17...secondary air nozzle, 18...tertiary air nozzle.

Claims (1)

[Claims] 1. A method for burning pulverized coal using a burner having two pulverized coal supply nozzles, in which a mixture of pulverized coal and air for conveying the pulverized coal is divided into two systems and fed to each nozzle. , the pulverized coal ejected from one nozzle is combusted at an air ratio of 1 or more, the pulverized coal ejected from the other nozzle is combusted at an air ratio of less than 1, a flame caused by the pulverized coal ejected from the one nozzle, and the other A method of combustion of pulverized coal, characterized by combining the flame of pulverized coal ejected from a nozzle in the trailing region of the pulverized coal. 2. The pulverized coal combustion method according to claim 1, characterized in that air is ejected from between the one nozzle and the other nozzle. 3 the one nozzle and the other nozzle,
The pulverized coal combustion method according to claim 1, characterized in that the respective flames are provided separately so that they do not mix immediately. 4. In a method of burning pulverized coal using a burner having two pulverized coal supply nozzles, a mixture of pulverized coal and air carrying it is divided into two systems and fed to each nozzle, and is ejected from one nozzle. The pulverized coal is combusted at an air ratio of 1 or more, and the pulverized coal is ejected from the other nozzle by mixing combustion exhaust gas with the air for transporting the pulverized coal, and the pulverized coal is blown out to form a flame containing ammonia. The pulverized coal is burned at an air ratio of less than 1, and the flame of the pulverized coal ejected from the one nozzle and the flame of the pulverized coal ejected from the other nozzle are combined in the trailing region of the pulverized coal. Combustion method.