JPH05240410A - Method of burning pulverized coal and burner for combustion - Google Patents

Abstract

PURPOSE:To enhance an effect of suppressing the generation of NOx by jetting a fuel-air mixed gas containing pulverized coal from a burner, jetting secondary air and tertiary air concentrically from the outer periphery of the burner and forming a primary combustion region and a secondary combustion region to reduce NOx. CONSTITUTION:The subject burner for combustion of pulverized coal comprises pulverized coal nozzles 12, 13 adapted to feed a fuel-air mixed gas composed of pulverized coal and primary air, and secondary and tertiary air nozzles 17, 18. A primary combustion region 21 where combustion is performed with the fuel-air mixed gas and the secondary air in the amount of air for combustion less than a theoretical amount of air is formed in the vicinity of the forward end of the burner, and a secondary combustion region 22 where further combustion of the combustion products from the primary combustion region 21 by tertiary air is formed in the wake of the primary combustion region 21. After ammonia compounds and cyanic compounds have been produced from the N content in coal in addition to NOx in the primary combustion region 21, these nitrified compounds are allowed to react with the combustion gas in the secondary combustion region on reduce NOx. The pulverized coal nozzles 13 are arranged in the outer periphery of the tertiary air nozzles 18.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulverized coal combustion method and a burner for combustion, and more particularly to a combustion method and burner suitable for reducing nitrogen oxides (hereinafter referred to as NOx).

[0002]

2. Description of the Related Art NOx produced during combustion of fossil fuel is
It is classified into fuel NOx and thermal NOx. Fuel NOx is generated by oxidizing a nitrogen component (hereinafter referred to as N component) contained in fuel, and thermal NOx is generated by oxidizing nitrogen in the air. Coal has a high N content, and nearly 80% of NOx generated during combustion is fuel NOx. On the other hand, the combustion technology that has been developed in the past, as represented by the two-stage combustion method and the exhaust gas recirculation method,
The mainstream is effective in reducing thermal NOx by suppressing the oxidation of nitrogen in the air by lowering the combustion temperature.

Fuel NOx generated during combustion of pulverized coal
The generation route of is described as follows according to the combustion mechanism. Pulverized coal combustion consists of the processes of ignition, pyrolysis, gas combustion, and solid combustion. The initial region of combustion is the region where ignition and thermal decomposition proceed, and the N content present in coal is divided into those that volatilize as gas and those that remain in solid. In the combustion region following pyrolysis, gas combustion in which volatile matter in coal burns and solid combustion in which solids that released volatile matter burn progresses, and N content released as gas and N content in solids also burn. Partly converted to NOx and part to nitrogen in the region.

N released as a gas during the thermal decomposition of coal
Some of the components are hydrogen cyanide (HCN) and ammonia (NH ₃ ), and these nitrogen compounds are oxidized to NOx in a high temperature and high oxygen concentration atmosphere. It has the property of selectively reducing NOx in the coexistence to form nitrogen (N ₂ ). By utilizing this property, it is possible to improve the conventionally developed two-stage combustion and reduce NOx in the pulverized coal combustion, and a burner based on the two-stage combustion principle has been developed. The two-stage combustion is a reducing region with a low air ratio in the first stage, and it is a method of reducing N content in coal to N ₂ as much as possible. To further reduce NOx, generation of NOx in the combustion flame It is necessary to clearly distinguish and control the region, the reducing agent generation region for NOx reduction, and the NOx reduction region.

[0005]

The object of the present invention is NO.
(EN) It is intended to provide a pulverized coal combustion method and a burner for combustion, which have a great effect of suppressing generation of x and reduce emission of unburned components.

[0006]

According to the present invention, a fuel mixture consisting of pulverized coal and primary air is ejected from a burner, secondary air is ejected from the outer periphery of the fuel mixture, and 3 from the outer periphery of the secondary air.
By injecting secondary air, a primary combustion region is formed near the burner tip where the fuel mixture and secondary air burn at a combustion air amount less than the theoretical air amount, and NOx, ammonia and cyanide are generated from the nitrogen content in the pulverized coal. The combustion is performed to generate a system compound, and a secondary combustion region is formed by the tertiary air and the combustion exhaust from the primary combustion region in the wake of the primary combustion region to generate NO.
A pulverized coal combustion method characterized by performing combustion for reducing x.

Further, a first nozzle for burning a fuel mixture composed of pulverized coal and primary air, a second nozzle provided on the outer periphery of the first nozzle for ejecting secondary air, and a second nozzle The present invention provides a burner for pulverized coal combustion, comprising a third nozzle that is provided on the outer circumference of the nozzle and that ejects tertiary air.

[0008]

As described above, the N content in coal becomes compounds such as NOx, nitrogen (N ₂ ), ammonia (NH ₃ ), hydrogen cyanide (HCN) in the thermal decomposition process. Among these compounds, NH ₃ has a property of selectively reducing NOx even in the coexistence of oxygen, and it is a well-known fact in the field of flue gas denitration technology that the effect of reducing NOx is high. Therefore, in order to effectively reduce NOx in the combustion gas containing oxygen by using coal, a large amount of NH ₃ is generated from coal and this is converted into N 2.
It is sufficient to react with Ox, and NOx reduction during coal combustion is
NH ₃ generation method and NH ₃ and NOx mixing method are major technical problems. However, the type of nitrogen compound in which N content in coal is converted during pyrolysis depends on the pyrolysis conditions of coal,
In order to generate a large amount of the target NH ₃ , it is necessary to set the optimum thermal decomposition conditions for NH ₃ generation. As a result of intensive studies by the inventors, in the temperature range close to the coal combustion temperature, the oxygen concentration in the coal pyrolysis atmosphere has a great influence on the NH ₃ conversion rate of N in the coal, and the optimum NH ₃ conversion rate is maximized. It was confirmed by the thermal decomposition experiment of pulverized coal that there existed various oxygen concentrations.

The point of the present invention is to utilize a thermal decomposition product from coal for the reduction of NOx. Therefore, a portion of the coal for combustion is burned in an oxygen concentration atmosphere in which the NH ₃ conversion rate of N content in the coal is maximized. Pyrolysis and mixing this with NOx-containing combustion gas.

Further, in the burner of the present invention, the pulverized coal combustion flame is divided into three areas of the primary, secondary, and third time combustion areas, and the NOx generation area and NOx are formed in the combustion flame.
The pulverized coal thermal decomposition product generation region for reduction and the NOx reduction reaction region are clarified. The primary combustion region is a complete combustion region in which pulverized coal is completely combusted with air having a theoretical air amount or more, and most of the fuel is combusted therein. In this region, since the air ratio is burned at 1 or more, the unburned fuel remaining in the combustion ash becomes very 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, in which combustion with an air ratio of less than 1, that is, thermal decomposition of pulverized coal proceeds. Since this region is an oxygen-deficient reducing region, the proportion of N in the pulverized coal that is oxidized to NOx is very small, and ammonia (NH ₃ ), which is an intermediate product of N in the combustion process, Hydrogen cyanide (HCN) etc. are generated. The third combustion region is a region where the thermal decomposition product of pulverized coal generated in the secondary combustion region reacts with NOx generated in the primary combustion region and surplus oxygen in the primary combustion region. Reduction reactions and oxidation reactions of hydrocarbons, carbon monoxide, hydrogen, unburned solids, etc. proceed.

By making the amount of pulverized coal that is completely burned in the primary combustion region larger than that of the pulverized coal that is thermally decomposed in the secondary combustion region, the emission of unburned fuel can be reduced, and further, the high temperature in the primary combustion region can be achieved. Since the thermal decomposition products generated in the secondary combustion region are oxidized by the excess oxygen heated to the above, the chemical reaction in the tertiary combustion region can be efficiently promoted.

In one embodiment of the present invention, the pulverized coal combustion exhaust gas is used for adjusting the oxygen concentration, and the mixing ratio of the combustion exhaust gas and the combustion air is controlled, whereby the oxygen concentration in the thermal decomposition atmosphere is controlled. Is adjusted.

Further, in one embodiment of the present invention, in order to facilitate adjustment of the oxygen concentration in the pyrolysis zone, the above mixed gas is used to convey pulverized coal and is jetted to the pyrolysis zone of the pulverized coal in the combustion furnace. It is used to

Further, in order to more effectively bring out the effect of the present invention, the pulverized coal gas ejected by pyrolysis in the secondary combustion region,
That is, the oxygen concentration of the mixture of combustion exhaust gas and air is preferably 3 to 5% by volume. That is, NO even in the presence of oxygen
In order to efficiently generate NH ₃ capable of selectively reducing x in the thermal decomposition reaction of pulverized coal, it is necessary to select thermal decomposition conditions. As a result of intensive studies, the inventors have found that the oxygen concentration in the thermal decomposition atmosphere is high. It was discovered that NH ₃ is most abundant from pulverized coal when 3 to 5%. Therefore, the oxygen concentration in the secondary combustion region is set to 3 to 5% by volume, that is, the oxygen concentration of the air-fuel mixture is adjusted to 3 to 5% by volume, and the mixing ratio of the air and the combustion exhaust gas is adjusted so that the tertiary combustion is performed. It is possible to effectively promote the NOx reduction reaction that proceeds in the region.

Further, a reaction temperature of 900 ° C. or higher is suitable for effectively reacting NH ₃ and NOx in the presence of oxygen, and therefore the pulverized coal pyrolysis product and the NOx-containing combustion gas are burned. It is preferable to mix them in a furnace to accelerate the NOx reduction reaction.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a pulverized coal combustion method and a burner for combustion according to the present invention will be described below with reference to the drawings.

In FIG. 1, a pulverized coal combustion burner comprises a pulverized coal nozzle 01, a secondary air nozzle 02 and a tertiary air nozzle 0 for supplying a fuel mixture composed of pulverized coal and primary air.
3 and 3. Combustion of fuel mixture and secondary air near the burner tip with a combustion air amount less than the theoretical air amount 1
A secondary combustion zone is formed, and a secondary combustion zone is formed in the subsequent flow by the tertiary air and the combustion exhaust from the primary combustion zone. In the primary combustion region, ammonia and cyan compounds are produced in addition to NOx from the N content in the coal, and in the secondary combustion region, these nitrogen compounds react to reduce NOx.

In FIG. 2, the pulverized coal combustion burner comprises two pulverized coal nozzles 12, 13, a secondary air nozzle 17, a tertiary air nozzle 18, and an igniter 16. Most of the fuel pulverized coal is ejected from the pulverized coal nozzle 12 which ejects the fuel mixture composed of the pulverized coal and the primary air for carrying and ejecting the pulverized coal, which is ejected from the secondary air nozzle 17 and the tertiary air nozzle 18. The 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 the tertiary air. Further, in the present embodiment, the secondary air nozzle 17 and the tertiary air nozzle 18 are provided on the outer periphery of the pulverized coal nozzle 12 in order to shorten the flame in the primary combustion region 21 and to make it independent of other combustion regions. , And a secondary air swirl vane 14 and a tertiary air swirl vane 15 for imparting a swirl flow to the combustion air are installed in the respective nozzles. The pulverized coal nozzle 13 is installed on the outer periphery of the tertiary air nozzle 18,
From the nozzle 13, a fuel mixture composed of a combustion exhaust gas and air whose oxygen concentration is adjusted to 3 to 5% by volume and a pulverized coal is ejected. The fuel mixture injected from the nozzle 13 forms a secondary combustion region 22 on the outer periphery of the primary combustion region 21, and the heat and the mixture transmitted from the primary combustion region 21 are included in this region in the secondary combustion region 22. The thermal decomposition reaction of pulverized coal proceeds with oxygen, and a reducing thermal decomposition product is generated.

Oxidation of the reducing thermal decomposition products generated in the secondary combustion zone 22 and NO generated in the primary combustion zone 21
A third combustion zone 23 in which a reduction reaction of x takes place is formed in the wake of the first and second combustion zones.

Example 1 Japanese domestic charcoal and char produced by heating Japanese domestic charcoal in an inert atmosphere at 1000 ° C. (combustible solid remaining during pyrolysis of coal) were packed in a platinum holder and heated to 1000 ° C. The NH ₃ conversion rate of N content in coal and N content in char was measured by quantifying NH ₃ generated at the time. Figure 3 shows the oxygen concentration in the pyrolysis atmosphere at 1 to 7% by volume.
It is the result of studying the influence of the atmospheric oxygen concentration on the NH ₃ conversion rate by changing the temperature in the range. The horizontal axis shows the oxygen concentration during thermal decomposition, and the vertical axis shows the NH ₃ conversion rate of N content in coal and char. As is clear from FIG. 3, the NH ₃ conversion rate is greatly affected by the oxygen concentration, and there is an optimum oxygen concentration that maximizes the NH ₃ conversion rate. NH ₃ conversion in coal N content is maximized at an oxygen concentration of about 3 vol%, N content in the char is NH ₃ conversion is maximized at an oxygen concentration of about 5% by volume.

Example 2 FIG. 4 shows the same apparatus as the experimental apparatus for which the experimental results of Example 1 were obtained, and using the same apparatus as in Example 1, NH ₃ conversion of N content in coal having oxygen concentration in the thermal decomposition atmosphere was carried out. This is the effect on the rate. In this example, in order to examine the effect of coal type, an experiment of thermal decomposition reaction of Australian coal and Chinese coal was tried. The thermal decomposition temperature is 1000 ° C. as in Example 1. As is clear from FIG. 4, although there is a slight difference in the optimum oxygen concentration range depending on the type of coal, the oxygen concentration in the thermal decomposition atmosphere was about 3 for both Australian and Chinese coals, similar to the Japanese domestic coals of Example 1.
The NH ₃ conversion rate of N content in coal is maximized near volume%.

As is clear from the above examples, in order to effectively use the thermal decomposition product of coal as a reducing agent for NOx, it is necessary to control the oxygen concentration in the thermal decomposition atmosphere of coal to an optimum value. .

[0023]

As described above, according to the pulverized coal combustion method and the combustion burner of the present invention, the oxygen concentration in the pulverized coal pyrolysis region is adjusted by mixing the combustion exhaust gas with the air, and the N in coal is adjusted. Since it is possible to improve the conversion rate of NH ₃ into NH ₃ which is effective for NOx reduction, low NOx combustion of pulverized coal becomes possible. That is, the N content in coal, which is converted to NOx in normal combustion, is converted to NH _{3 in the} thermal decomposition region, and this is converted to N 3.
Since it can be used for the reduction of Ox, the combustion of pulverized coal can be reduced to NOx. Further, according to the present invention, since the complete combustion region having an air ratio of 1 or more is formed, it is possible to reduce the emission of unburned fuel. Furthermore, according to the present invention, since the pulverized coal is divided into two systems and burned to form a flame under the condition of good ignitability with an air ratio of less than 1, even if the combustion load is low, that is, Even if the pulverized coal concentration in the mixture of the pulverized coal ejected from the other nozzle and the air carrying the same is low, the flame from the other nozzle is easily formed.

[Brief description of drawings]

FIG. 1 is a side sectional view of a burner according to the present invention.

FIG. 2 is a side sectional view of a burner according to another embodiment of the present invention.

FIG. 3 is a graph showing the relationship between the NH ₃ conversion rate and the oxygen concentration.

FIG. 4 is a graph showing the relationship between the NH ₃ conversion rate and the oxygen concentration.

[Explanation of symbols]

 01 Pulverized coal nozzle 02 Secondary air nozzle 03 Tertiary air nozzle 04 Ignition burner 05 Swivel blade 06 Swirl blade 07 Auxiliary fuel supply hole for ignition 08 Pulverized coal supply hole 09 Secondary air supply hole 10 Tertiary air supply hole 11 Fine powder Charcoal supply hole 12 Pulverized coal nozzle 13 Pulverized coal nozzle 14 Secondary air swirl vane 15 Tertiary air swirl vane 16 Igniter 17 Secondary air nozzle 18 Tertiary air nozzle 21 Primary combustion region 22 Secondary combustion region 23 Third Next combustion area

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Norio Arashi 3-1-1, Saiwaicho, Hitachi, Ibaraki Hitachi Ltd. Hitachi Research Laboratory (72) Inventor Toru Inada 3-chome, Hitachi, Hitachi, Ibaraki No. 1 in Hitachi Research Laboratory, Hitachi Ltd. (72) Inventor Kenichi Soma 3-1-1, Saiwaicho, Hitachi City, Ibaraki Hitachi Ltd. Hitachi Research Laboratory in Hitachi (72) Inventor Kazushi Otsuka Hitachi City, Ibaraki Prefecture 3-1, 1-1 Sachimachi Hitachi Research Laboratory, Hitachi, Ltd. (72) Inventor Takao Hishinuma 3-1-1 1-1 Sachimachi Hitachi City, Ibaraki Prefecture Hitachi Ltd. Hitachi Research Laboratory

Claims (3)

[Claims] 1. A fuel mixture composed of pulverized coal and primary air is ejected from a burner, secondary air is ejected from the outer periphery of the fuel mixture, and tertiary air is ejected from the outer periphery of the secondary air. Thereby forming a primary combustion region near the tip of the burner where the fuel mixture and the secondary air are burned at a combustion air amount less than the theoretical air amount to remove NOx, ammonia and cyan compounds from the nitrogen content in the pulverized coal. Combustion is generated to form a secondary combustion region in the wake of the primary combustion region by the tertiary air and the combustion exhaust from the primary combustion region to generate N.
A pulverized coal combustion method characterized by performing combustion for reducing Ox. 2. The secondary air according to claim 1, and / or
Alternatively, the pulverized coal combustion method is characterized in that the tertiary air is ejected as a swirling flow. 3. A first nozzle for burning a fuel mixture composed of pulverized coal and primary air, a second nozzle provided on the outer circumference of the first nozzle for ejecting secondary air, and the second nozzle. A burner for pulverized coal combustion, comprising: a third nozzle provided on the outer periphery of the nozzle for ejecting tertiary air.