Method of combustion of pulverized coal by pulverized coal burner

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United States Patent 19)

Izuha et al.

54 method of combustion of

Pulverized coal by pulverized

Coal burner

(75) Inventors: Akira Izuha, Koganei; Tomoyuki Shinamo, Yokohama; Yuho

Shibayama, Warabi; Chikatoshi

Kurata, Akashi; Kenichi Fujii, Kobe, all of Japan (73) Assignee: Kawasaki Jukogyo Kabushiki Kaisha, Kobe, Japan

52 U.S. C. ...... ... 110/347; 110/263

3,868,211 2/1975 La Haye et al. ...................... 431/10

4,246,853 1/1981 Mehta ................................. 110/347 4,294,178 10/1981 Borio ................................... 1.10/347 4,308,810 l/1982 Taylor................................. 110/347 Primary Examiner- Edwad G. Favors

Attorney, Agent, or Firm-Jordan and Hamburg

A method of combustion of pulverized coal in two stages of combustion to reduce the amount of NOx produced by combustion. In a first stage, exhaust gas is added to air for combustion in a proportion of 35-60% of the air necessary for carrying out combustion. In a second stage, air for combustion is supplied in a propor tion of 25-35% of the air necessary for carrying out combustion. In this method of combustion, the pulver ized coal is combusted slowly at low temperature with the formation of a flame of low brightness to produce in large amounts activated intermediate components re sponsible for denitration reaction to promote reduction of produced NOx, and the concentration of CO pro duced can be reduced by abstaining from using the minimum air necessary for combustion.

2 Claims, 1 Drawing Figure

Residual

A2 + residual v

Drawings

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ized coal can be markedly reduced by causing the pull

METHOD OF COMBUSTION OF PULVERIZED Verized coal to burn slowly at low temperature with COAL BY PULVERIZED COAL BURNER low O2, and that the non-combusted components, in cluding hydrocarbons which are activated intermediate

BACKGROUND OF THE INVENTION 5 products responsible for denitration reaction, NH3, This invention relates to improvements in or relating HCN and CO are produced in large amounts and exist to the method of two-stage combustion of pulverized for a prolonged period of time in non-combusted condi coal by means of a pulverized coal burner capable of tion to enable decomposition of the produced NOx to reducing the amount of NOx produced by combustion. take place vigorously.

When pulverized coal is combusted by a pulverized 10 The present invention is based on the aforesaid dis coal burner, NOx of an amount in the range between covery and provides method of combustion of pulver 500 and 600 ppm is produced as a result of complete ized coal capable of positively reducing the amount of combustion of the pulverized coal. Release into the the produced NOx to a value below 100 ppm by effec atmosphere of the exhaust gas containing a large tively carrying out combustion of the pulverized coal at amount of NOx should be avoided from the point of 15 low speed and at low temperature with low O2, regard view of avoiding air pollution. To this end, it has been less of whether the pulverized coal is of high quality or earnestly desired that the amount of NOx produced by low quality.

the combustion of pulverized coal be reduced as much The aforesaid object can be accomplished according as possible. to the invention by a method of combustion of pulver In our experience, two-stage combustion and exhaust 20 ized coal comprising the steps of burning the pulverized gas recycling have effect in reducing the amount of coal by adding exhaust gas to air for combustion in a NOx produced by the combustion of pulverized coal. In proportion which is in the range between 35 and 60% of a two-stage combustion method of the prior art, it has hitherto been customary to burn pulverized coal in two the air required for carrying out combustion in a first stage of combustion, and carrying out combustion by steps by limiting the proportion of air for combustion in 25 supplying the second stage to 20% or less at most of the air re tion whichairis for on second stage combustion in a propor the range between 25 and 35% of the quired for combustion and by mixing the exhaust gas in air necessary for combustion in a second stage of com a proportion of 20% or less or most. The reason for bustion.

adopting these proportions is that if the proportion of air for combustion in the second stage is increased 30 BRIEF DESCRIPTION OF THE DRAWING above the aforesaid limit, non-combusted air would

The single drawing is a sectional view showing the increase in amount and if the exhaust gas is increased condition above the aforesaid limit, the combustion as a whole of combustion of pulverized coal by a pulver would become unstable. In the prior art, the amount of ized coal burner.

the NOx produced by the combustion of pulverized 35 DESCRIPTION OF THE PREFERRED coal has only been reduced to the range between 200 EMBODEMENT and 300 ppm. With exhaust gas being added in a propor

In the method according to the invention for the tion of about 20%, the air necessary for combustion has combustion a demand for a relatively high rate of O2 and conse supplied together of pulverized coal, pulverized coal 1 is quently diffusion of O2 to the combustion zone occurs at with primary air A1 through a com high speed, so that the pulverized coal is essentially bustion air outlet 1 of a pulverized coal burner and combusted at high temperature and at high speed with caused by a swirler 2 to be injected into the furnace high O2. The result of this. is that the pulverized coal while flowing slowly in vortical form. Secondary air burns in non-homogeneous diffusion combustion, so A2 is injected into the furnace with exhaust gas through that the flame has a high temperature and the amount of 45 an inner annular outlet 3 surrounding the combustion NOx produced increases due to rapid combustion of the air outlet 1, the secondary air either flowing slowly in pulverized coal with high O2. Rapid combustion of the vortical form or not flowing in vortical form as the case pulverized coal also produces non-combusted compo may be. Tertiary air A3 is injected into the furnace with nents, such as hydrocarbons which are activated inter exhaust gas through an outer annular outlet 4 surround mediate products. responsible for denitration reaction, 50 ing the inner annular outlet 3 while flowing in vortical NH3, HCN and CO. These non-combusted components form. Pulverized coal supplied to the furnace together are combusted and disappear as soon as they are pro with primary air is combusted to form a primary flame duced, so that they do not help in decomposition of 5. The primary flame 5 is formed by slow combustion of NOx and the amount of the latter does not shown a the pulverized coal at low temperature with low O2 and reduction. 55 is low in brightness, because the primary air A1 is about

Summary of the invention

20-30% in amount of the air necessary for combusting all the pulverized coal supplied therewith to the fur

This invention has as its object the provision of a nance and mixing of secondary and tertiary air there two-stage combustion method for a pulverized coal with is prohibited. Combustion of a volatile component burner for reducing the amount of NOx produced by V of the pulverized coal is mainly responsible for for the combustion of pulverized coal. mation of the primary flame 5, so that the pulverized In particular, the invention has been developed for coal is combusted slowly at low temperature with a the purpose of providing a combustion method for flame of low brightness. In this type of combustion, burning pulverized coal in two stages which is capable production of NOx is greatly produced and the non of reducing the amount of NOx produced by the com 65 combusted components, such as hydrocarbons which bustion of pulverized coal to a value below 100 ppm. are activated intermediate products responsible for The results of research conducted by us show that the denitration reaction, NH3 HCN and CO, are produced amount of NOx produced by the combustion of pulver in large amounts and exist for a prolonged period of 4 time in non-combusted condition. Thus these non-com the reduction of NOx which are produced are too small busted components react with NOx, to ultimately re to permit the concentration of NOx to be reduced to a duce the NOx to N2. Char which is produced in large valve below 100 ppm. When the proportion is above amounts as a non-combusted component of the primary 35%, the amounts of the non-combusted components flame 5 is combusted in the secondary flame 6. The which are produced are excessive, so that although the residual volatile component is combusted mainly by the amount of NOx is reduced, completion of the combus secondary air A2 ejected through the inner annular tion of the non-combusted components is delayed in outlets 3 to form a secondary flame 6. Most of char is spite of the incorporation of the air for combustion in combusted by the secondary air A2 and the tertiary air the second stage. Thus the range between 25 and 35% is A3 to form a tertiary flame range 7. The secondary 10 preferred. When the valve is within this range, it is flame 6 and the tertiary flame 7 are formed by the com possible for the method of combustion according to the bustion of relatively low speed and low temperature invention to cope with combustion of pulverized coal of with low O2, because the secondary and tertiary air is both high and low quality.

about 55-80% in amount of the air necessary for the To enable the effects achieved by the method of combustion of all the pulverized coal and the air con 15 combustion of pulverized coal according to the inven tains exhaust gas in 35-60%. tion to be better understood, the examples of the inven The secondary flame 6 and the tertiary flame 7 are tion will now be described in comparison with the prior thus in condition of air deficiency as a whole, but burns art, slowly so that O2 exists therein although it is low in concentration. The reduced substances (activated inter 20 PRIOR ART mediate products), such as hydrocarbons, NH3, HCN In combusting coal mined in Japan (Sorachi coal) in and CO, produced gradually from the char, react in the pulverized form by a 3 T/H burner, combustion of the presence of O2 with NOx produced on the upstream pulverized coal was carried out in two stages. In the side, to reduce NOx to N2. In the tertiary flange range first stage, exhaust gas was mixed with air for combus 7 reductive denitration is promoted more than NOx 25 tion in a proportion of 20% of the air necessary for production under the reductive circumstances. There carrying out combustion. In the second stage, air for fore, the reduction of NOx is performed in this range. combustion was in a proportion of 20% of the air neces Combustion of the non-combusted components (re sary for carrying out combustion.

sidual char) is completed in a rear portion of the tertiary flame region 7 when two stage combustion air A4 is 30 EXAMPLE 1 (THE INVENTION) supplied, in an amount which is 25-35% of the air nec In combusting coal mined in Japan (Sorachi coal) in essary for combusting all the pulverized coal, to the pulverized form by a 3 T/H burner, combustion of the furnace in which combustion is taking place with the pulverized coal was carried out in two stages. In the formation of a flame of low brightness by the combus first stage, exhaust gas was mixed with air for combus tion of the pulverized coal at low speed and at low 35 tion in a proportion of 35% of the air necessary for temperature with noncatalytic denitration.

The reason why the proportion of the exhaust gas carrying combustion out combustion. In the second stage, air for was in a proportion of 26% of the air neces mixed with the air for combustion is set at 35-60% of sary for carrying out combustion. the air necessary for combusting the pulverized coal in the method of combustion of pulverized coal according 40 EXAMPLE 2 (THE INVENTION) to the inventions as follows. When the proportion is In combusting coal mined in Japan (Sorachi coal) in below 35%, the air necessary for combusting the pull pulverized form by a 3 T/H burner, combustion of the verized coal has a relatively high demand for O2 and the speed of diffusion of O2 to the combustion zone is high, pulverized coal was carried out in two stages. In the so that the pulverized coal is combusted quickly at high 45 first tion stage, exhaust gas was mixed with air for combus in a proportion of 41% of the air necessary for temperature. The combustion is nonhomogeneous diffu carrying out combustion. In the second stage, air for sion combustion with a formation of bright flame, so combustion was in a proportion of 30% of the air neces that the amount of NOx produced in the combustion sary for carrying out combustion.

increases. When the proportion is above 60%, the de mand for O2 of the air necessary for combusting the 50 EXAMPLE 3 (THE INVENTION) pulverized coal becomes so low that the speed of diffu In combusting coal mined in Japan (Sorachi Coal) in sion of O2 to the combustion zone becomes too low to pulverized form by a 3 T/H burner, combustion of the prevent the phenomenon of noncombustion from taking pulverized coal was carried out in two stages. In the place or to avoid incomplete combustion of the pulver first stage, exhaust gas was mixed with air for combus ized coal. The result of this is that non-combusted con 55 tion in a proportion of 55% of the air necessary for ponents of high concentration, particularly CO, are produced. Thus the range between 35 and 60% is pre carrying combustion out combustion. In the second stage, air for was in a proportion of 34% of the air neces ferred. When the value is within this range, it is possible sary for carrying out combustion. to reduce the amount of NOx produced in the combus The concentrations of NOx and CO detected in the tion of pulverized coal to a value below 100 ppm, re exhaust gases of combustion of the pulverized coal gardless of whether coal is high or low in quality.

The reason why the proportion of air for combustion method according to the inventionprior carried out in two stages in the were art and by the measured. The in the second stage is set at 25-35% of the air necessary results are shown in Table i.

for combusting the pulverized coal in the method of TABLE combustion of pulverized coal according to the inven 65 tion is as follows. When the proportion is below 25%, Concentration of NOx (ppm)

Concentration of

CO (ppm) the amounts of the non-combusted components (hydro carbons, NH3, HCN and CO) necessary for promoting Prior Art 250 92

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TABLE 1-continued achieve the excellent result of reducing the concentra Concentration of Concentration of tionWhat of CO in the exhaust gas.

is claimed is:

NOx (ppm) CO (ppm) 1. A method of combustion of pulverized coal by a Example 1 90 65 5 pulverized coal burner, comprising the steps of:

90 combusting the pulverized coal by supplying thereto air for combustion mixed with exhaust gas in a proportion of 35-60% of the air necessary for car

As can be clearly seen in Table 1, the exhaust gases rying out combustion in a first stage of combustion; produced in Examples 1-3 of the present invention in 10 and which combustion of pulverized coal was combusted in supplying to a flame formed in the first stage of com two stages contained NOx and CO in concentrations bustion air for combustion in a proportion of lower than in the prior art in which combustion of 25-35% of the air necessary for carrying outcom pulverized gas was also carried out in two stages. This bustion.

shows that the method of combustion according to the 15 2. A method of combustion of pulverized coal by a invention has marked effects in reducing the amounts of pulverized coal burner, comprising the steps of: NOx and CO in the exhaust gases. carrying out combustion in a first stage in which a From the foregoing description, it will be appreciated primary flame is formed by mixing the pulverized that in the method of combustion of pulverized coal by coal with primary air in a proportion of 20-30% of using a pulverized coal burner according to the inven 20 the air necessary for combusting all the pulverized tion, the pulverized coal is combusted in a first stage of coal mixed with the primary air, a secondary flame combustion with air mixed with exhaust gas in a propor is formed by supplying to the residual volatile of tion of 35-60% of the air necessary for combusting the the pulverized coal mainly a mixture of secondary pulverized coal so that the combustion takes place 25 air with exhaust gas, and a tertiary flame is formed slowly at low temperature with the formation of a flame by combusting most of char with the secondary air of low brightness. Thus the NOx produced in combus and a tertiary air mixed with the exhaust gas, tion is reduced in amount and the NOx produced has its wherein said secondary and tertiary air used in the concentration reduced by being reduced to N2 by char second and tertiary flame being in a proportion of 55-80% of the air necessary for combusting all the and non-combusted components (hydrocarbons, NH3, 30 pulverized coal and said exhaust gas being in a NCN and CO) which are spread widely in the furnace proportion of 35-60% of the air necessary for car and exist for a long period of time. In a second stage of rying out combustions; and combustion, second stage combustion air is supplied in a carrying out combustion in a second stage in which proportion of 25-35% of the air necessary for carrying two stage air is supplied in a proportion of 25-35% Out combustion to burn non-combusted CO and char, to 35 of the air necessary for carrying out combustion. reduce the concentration of CO. Thus the invention can sk k k k is