FI122982B - Method for reducing nitrogen oxide emissions from a recovery boiler and a recovery boiler - Google Patents

Description

Method for reducing nitrogen oxide emissions from a recovery boiler and a recovery boiler

Field of the Invention 5

The invention relates to a method for reducing nitrogen oxide emissions from a recovery boiler according to the preamble of claim 1. The invention also relates to a recovery boiler according to the preamble of claim 14.

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Background of the Invention

The sludge boiler burns the waste liquor from the manufacture of sulphate pulp, i.e. black liquor. The purpose of the recovery boiler is not only to recover the energy contained in the 15 black liquors, but also to recover the chemicals it contains, which can be recycled back to the pulp. The combustion air required for the combustion of the black liquor is usually introduced into three different levels into the furnace: primary air from the bottom of the furnace, secondary air between the primary air plane and the liquor nozzles, and tertiary air 20 above the liquor nozzles. Burning of black liquor results in flue gases containing nitrogen oxides. These oxides consist of both nitrogenous materials contained in the lye and gaseous nitrogen in the combustion air.

About two-thirds of the nitrogen in the black liquor is released during the pyrolysis step following drying of the lye drop with pyrolysis ™ gases. About half of the nitrogen transferred to the volatile portion is immediately converted to the molecular nitrogen, N2, and the remainder remains in reactive form as ammonia, NH3. When the pyrolysis gases burn, this ammonia is easily oxidized to nitric oxides. Most of the nitrogen oxide emissions from the flue gases of the recovery boiler are formed along this route.

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§ A variety of techniques are used to reduce the nitrogen oxide content of flue gases from a boiler. One of these is the phasing of the combustion air. This is based on the conversion of a substantial portion of the ammoniacal nitrogen to a molecular nitrogen by suitable pyrolysis gas combustion. Usually, during the phasing of the combustion air, the combustion air is fed to the boiler from 3 to 5 different air levels. The purpose is to burn the liquor in the boiler so as to provide reducing furnace reducing conditions up to the final air supply step 5, whereby the ammonia in the flue gases is reduced to a molecular nitrogen according to the following reaction equation: NH3- + oh-> NHg-> NH-ίί! - »N + NO> Ng (1) 10

Pyrolysis also produces hydrogen cyanide, HCN. HCN is also reduced to the molecular nitrogen by phased air supply as follows: HCN + ^ -> NCO—> NH ——> N + NO> N2 (2)

Some of the nitrogen from the fuel is reduced to molecular nitrogen 20 and some is oxidized to nitrogen oxides, which are further reduced to molecular nitrogen by the hydrocarbon radicals formed in pyrolysis to contribute to the reduction of nitrogen oxides. An example of such a reaction is described in Reaction Equation (3) wherein the hydrocarbon radical is -CH 1.

25 g N (excipient) + O 2 -> NO + CH 2> HCN + ° '+ OH »H 3 NCO + H> NH 1 + NO> N 2

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9 (3) w Sub stoichiometric conditions for oxygen are maintained in the furnace to the upper air supply level, maximizing the dwell time required for reactions (1) 05 30 and (2) and minimizing the amount of NH 3 and HCN.

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The air required to complete the pyrolysis gases is supplied to the furnace in the final air supply step, which creates ilma free conditions.

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Another possible method for reducing the nitrogen oxide content of the flue gases from the recovery boiler is Selective Non-Catalytic Reduction (SNCR). In this process, nitrogen oxides produced by the combustion of the liquor are reduced by feeding nitrogen oxide reducing compounds to the furnace. The most commonly used compounds are ammonia and urea.

The third method used to reduce the nitrogen oxide content of the flue gases from the recovery boiler is the step of refining the fuel feed. The process is based on the ability of the fuel-derived radicals to reduce nitrogen oxides to the molecular nitrogen as shown in Reaction Equation (3) above. In the method, the lye is fed to the furnace from a plurality of feed levels with respect to the height of the furnace. The liquor fed to the lower part of the furnace is mainly burned under the reducing conditions of the lower part of the furnace. The combustion of the pyrolysis gases released from the liquor is carried out in an oxygen-containing, i.e., free, zone formed below the liquor nozzles. Combustion with excess air produces some nitrogen oxides. These nitric oxides are sought to be removed by reduction to a molecular nitrogen. This is done by re-feeding the furnace from a higher level of liquor, whereby the combustion of the liquor results in the formation of radicals which reduce the nitrogen oxides formed in the flue gases. At the top of the furnace, there is an over-charge burner, which is used for post-combustion. As discussed above, the combustion of the lye produces hydrogen-25 cyanide, HCN. It has been found to be extremely harmful to the furnace process ^ as it is a highly corrosive substance to the wall surfaces of the boiler.

™ Furthermore, if it is not completely reduced to molecular nitrogen, it is also a hazardous substance when released into the air.

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£ 30 in Fl patent application 20040763 (corresponding WO application

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05/118113) discloses a method for reducing the amount of nitrogen oxides produced by combustion, wherein the fuel is fed to the furnace

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g delta from different input tray. According to the publication, the liquor fed from the second fuel supply level in the higher furnace is to be incinerated at such a temperature and under reducing conditions so as to obtain as much hydrogen cyanide as possible. Hydrogen cyanide is converted to the molecular nitrogen by the introduction of excess air into the top of the furnace. The problem with this method is that the piping and other equipment to be built to meet the needs of the two liquor feed planes are complex and expensive.

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Description of the Invention Ivhvt

It is an object of the present invention to provide a novel fuel-phasing solution for reducing nitrous oxide-10 emissions from a recovery boiler.

To accomplish this purpose, the method according to the invention is essentially characterized in what is set forth in the characterizing part of independent claim 1.

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The recovery boiler according to the invention, in turn, is mainly characterized by what is disclosed in the characterizing part of independent claim 14.

Other dependent claims disclose some preferred embodiments of the invention.

The invention is based on the idea that black liquor is fed into the furnace furnace furnace from one level so as to provide two zones of combustion with respect to different heights of the fire chamber. All black liquor is fed to the furnace at substantially the same level. The black liquor ™ is fed with substantially 9 different types of liquor nozzles mounted on the same level. The droplet size of the oily liquor to be fed from the first liquor nozzles is close to one lye droplet size generally used in a soda recovery boiler. The droplet size of the liquor fed from the other liquor nozzles is substantially smaller §! than the droplet size of the liquor fed from the first liquor nozzles.

§ The liquor fed from the first and second liquor nozzles is ™ from the same liquor reservoir.

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The droplets from the first nozzles are larger in size than the droplets from the second nozzles. The size of the droplets, and thus the weight, affect where they burn. Larger droplets from the first nozzles fly through the air levels below them through various stages of combustion. The droplets from the second nozzles are substantially smaller in size, and thus also in weight, than the droplets produced by the first nozzles, and dry and burn substantially at the same level as or slightly above the second nozzles.

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By means of the invention, the nitrogen oxides produced from fuel fed from the first nozzles can be reduced by the reducing conditions provided by the fuel fed from the second nozzles.

15 The lye droplets fed from the first liquor nozzles fall down, dry on the way, and burn as low as possible in the furnace. The reductive zone in the lower part of the furnace is intended to maintain sub-cytometric conditions to minimize nitrogen oxide formation. The NH3 formed during combustion is also sought to be completely reduced to the molecular nitrogen. Above this combustion zone, just prior to the liquor spray, is a combustion zone having an air factor greater than 1. This zone thus has excess air conditions and is intended to ensure complete combustion of the liquor fed to the lower part of the furnace and the resulting pyrolysis gases. By-product of combustion also generates some nitrogen oxides, e.g. NO.

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o Liquid droplets fed from other liquor nozzles form a '' drop cloud '' in the center of the boiler, at or slightly above the level of the liquor nozzles. However, the droplet size is adjusted to such an extent that the droplets also penetrate into the center of the cross section of the boiler in the plane of the nozzles. Because of the small size of the droplets, their drying, pyrolysis, and combustion (LT) g occur almost immediately after the liquor is fed. The amount of liquor to be fed from the other nozzles is considerably small, which results in a low temperature in the afterburner zone above the nozzles with reducing conditions, about 950 to 1500 ° C, most preferably about 1050 to 1400 ° C. Due to the low temperature, combustion of the liquor fed from the other lye nozzles in the furnace produces more hydrocarbon radicals, which react with nitrogen oxides contained in the flue gases from the lower furnace and reduce at least some of them to hydrogen cyanide (HCN). The liquor supplied from the other nozzles is burned down in the burn-out zone by the supply of air to the upper part of the furnace, which is fed so much that over-free conditions are created. The temperature in this zone is from about 950 ° C to about 1200 ° C, preferably from about 950 ° C to about 1050 ° C, and the hydrogen cyanide formed is converted to the molecular nitrogen (N 2).

An advantage of the invention is that it enhances the reduction of nitrogen oxides in the combustion process of the recovery boiler. Further, the equipment required to carry out the invention is simple and inexpensive to implement. The apparatus according to the invention is also easy to implement in connection with boiler renewals and repairs.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be explained in more detail with reference to the accompanying figures, in which Figure 1 schematically shows a side view of a furnace for a recovery boiler,

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Fig. 2 shows an embodiment for positioning the first and second liquor nozzles according to the invention on the walls of the furnace c3 of the soda ash, and

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Fig. 3 shows another embodiment for positioning the first and second lye nozzles according to the invention on the walls of a soda ash boiler.

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DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows a recovery boiler 1 according to the invention with a furnace 2 with a fuse box 3 at its bottom. Fuse assembly 3 is formed when black-5 liquor is fed from droplet syringe syringes 6a into droplet furnace 2. Upon entering the furnace, the liquor dries; As a result of the coke combustion, a melt is formed which collects at the bottom of the fire chamber and is then led to the melt leaching and chemical recovery plant.

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In order to burn the liquor so that the resulting flue gases contain as little nitrogen oxides as possible, the combustion air is supplied to the furnace from nozzles located at several different levels with respect to the height of the furnace. The primary air nozzles 4 are disposed closest to the bottom of the furnace, at a distance therefrom 15, but the secondary air nozzles 5 are disposed above the primary air nozzles 4, but still below the lye nozzles 6a and 6b which follow. As can be seen from the figure, the secondary air 5 is divided into two sections which are fed to the furnace 2 at different heights. Part of the 20 secondary air is fed into the furnace for 2 so-called. through the secondary secondary air nozzles 5a and the rest is fed to the furnace 2 via the upper secondary secondary air nozzles 5b. Above the secondary air nozzles 5 are provided liquor nozzles 6a and 6b for supplying the liquor to the furnace. Above the liquor nozzles are tertiary air nozzles 7 which supply 25 tertiary air to the furnace. Above the tertiary air nozzles are the quaternary air nozzles 8, through which the

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By supplying the combustion air from different levels to the furnace 2, different combustion zones are formed in the furnace 30, to which the amount of combustion air to be fed can be adjusted. At level 4 of the primary air nozzles is O) §! reductive zone A with undercutometric, reducing conditions.

§ Air is supplied to this zone from primary air nozzles 4.

S This is where the lye coke burns. About 35 on or slightly above the secondary air nozzles 5, however, below the liquor nozzles 6a and 6b is a combustion zone B, with an air factor of 8 slightly above 1. The purpose of this zone is to ensure complete combustion of the liquor. Air is supplied to the combustion zone B from the secondary air nozzles 5.

Approximately at or just above the tertiary air nozzles 7 is a post-combustion zone C having undercutometric, reducing conditions. The afterburn zone C is supplied with combustion air from a tertiary air nozzle 7. The highest of all furnaces 2 has a burn-out zone D located approximately at or above the level of the quaternary air nozzles. The burn-up zone is supplied with combustion air from the quaternary air nozzles 8. Zone D has an air coefficient well above 1 and performs post-combustion of the pyrolysis gases still present in the furnace.

15 The black liquor is fed into the furnace from one level so as to provide two zones of combustion. Between the secondary air nozzles 5 and the tertiary air nozzles 7 are provided first and second liquor nozzles 6a and 6b. The liquor nozzles 6a and 6b are disposed in substantially the same plane with respect to the height of the furnace. There are 20 different types of liquor nozzles that produce different sized droplets of liquor. Larger droplets of liquor fed from the first liquor nozzles 6a fly downward and the liquor and the pyrolysis gases formed therefrom are burned in the reducing zone A and the combustion zone B below the liquor nozzles 6a and 6b.

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The second nozzles 6b are such that the droplets contained in the resulting liquor jet ™ are substantially smaller than the droplets generated by the first nozzles 6a. These lye droplets and the pyrolysis gases formed therefrom by the second lye nozzles 6b are burned by the lye at the level of the nozzles 30 and upwards in the post-combustion zone C and the burn-up zone D.

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sj- LT) g The first lye nozzles can be commonly used spoon nozzles, which give a droplet size of a few 35 millimeters. It is intended that the droplets formed fall 9 downward, dry on the way and burn as low as possible in the fire chamber.

The second lye nozzles are nozzles capable of producing substantially smaller droplets of lye than the first lye nozzles. The size of the liquor droplets is a few hundred microns. The droplet size is adjusted so that the liquor fed from the other nozzles forms a '' liquor cloud '' in the center of the boiler, at or just above the level of the liquor nozzles. However, the droplet size is adjusted to such an extent that the droplets 10 migrate to the center of the cross-sectional area of the boiler. Carrier gas can be used to assist in feeding the liquor. The second lye nozzles may be, for example, nozzles provided with a carrier gas channel which surrounds the lye channel in the nozzle, thereby forming a gas curtain around the lye. The carrier gas improves the penetration of the liquor into the central parts of the furnace 15 and prevents the formation of a liquor cloud near the furnace walls.

The liquor fed from the other nozzles may be heated or treated as necessary to facilitate the formation of small droplets. However, the liquor fed from the nozzles is the same as the liquor fed from the first nozzles.

The first and second liquor nozzles may be arranged in different ways on the walls 9 of the fire chamber. Figure 2 shows an embodiment for positioning them. The first nozzles 6a are disposed symmetrically such that each wall has three nozzles. The second nozzles 6b ^ are located at the corners of the furnace 2. In the embodiment of Figure 3, the first 9 nozzles 6a have two pieces on each wall 9. The second nozzles 6b are disposed on each wall 9 in the center of the first nozzles 6a g 30, substantially in the middle of the furnace wall. Naturally, the number and placement of the first and second liquor nozzles with respect to the furnace walls may vary from the example shown in Figures 2 and 3. ..

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The invention is not intended to be limited to the exemplary embodiments set forth above, but is intended to be extensively applied within the scope of the inventive idea defined in the following claims.

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Claims (26)

A method for reducing nitrous oxide emissions from a recovery boiler, comprising feeding to the recovery furnace furnace furnace (2) 5 primary air nozzles (4) to a lower portion of the furnace (2), - black liquor liquor nozzles (6a, 6b), - secondary air extractor (2) 5) above the primary air nozzles (4) but below the lye nozzles (6a, 10 6b), - the tertiary air from the tertiary air nozzles (7) above the lye nozzles (6a, 6b), - the quaternary air from the quaternary air nozzles (8) above the air nozzles (7), characterized in that the black liquor is supplied to the furnace (2) from the first liquor nozzles (6a) and the second liquor nozzles (6b) arranged at substantially the same level as the height of the furnace (2). and that the droplet size of the liquor supplied from the second liquor nozzles (6b) is substantially smaller than that of the first liquor nozzles the droplet size of the lye fed from the thyme (6a). Method according to claim 1, characterized in that the liquor fed from the first liquor nozzles (6a) and the pyrolysis gases formed therefrom are burned in the reducing zone (A) and the combustion zone (B) located below the liquor nozzles (6a, 6b). C \ J δ A method according to claim 2, characterized in that combustion air is supplied to the 9 reducing zones (A) from the primary air nozzles (4) such that the reducing zone (A) has a gauge-reducing conditions. □ _ O) A method according to claim 2, characterized in that the combustion air is supplied to the combustion zone (B) by the LO g from the secondary air nozzle (5), such that the combustion zone (B) has an overexposed condition. Method according to Claim 1, characterized in that the liquor fed from the second lye nozzles (6b) and the pyrolysis gases formed therefrom are burned in the afterburning zone (C) and the burn-up zone (D) located above the second lye nozzles (6b). 5 Method according to Claim 5, characterized in that the post-combustion zone (C) is supplied with combustion air from the tertiary air nozzles (7) such that the post-combustion zone (C) has submerged catheter reducing conditions. 10 Process according to Claim 6, characterized in that the temperature in the afterburning zone (C) is about 950 to 1500 ° C, preferably about 1050 to 1400 ° C. A method according to claim 5, characterized in that the combustion air is supplied to the burn-up zone (D) from the quaternary air nozzles (8) such that excess conditions are present in the burn-up zone (D). The process according to claim 8, characterized in that the temperature in the burn-up zone (D) is about 950-1200 ° C, preferably about 950-1050 ° C. Method according to claim 1, characterized in that black liquor is supplied to the furnace (2) from the first liquor nozzles (6a) and the second liquor nozzles (6b) arranged symmetrically with respect to the walls (9) of the furnace (2), δ i c3 A method according to claim 10, characterized in that g 30 is fed to the furnace (2) from the first liquor nozzles (6a) and the second liquor nozzles (6b), the first liquor nozzles (6a) being arranged symmetrically on the walls of the furnace (2) and The nozzles (6b) are arranged at the corners of the furnace (2). o (M Method according to Claim 10, characterized in that black liquor is supplied to the furnace (2) from the first liquor nozzles (6a) and the second liquor nozzles (6b) arranged between the first liquor nozzles (6a) on the walls (9) of the furnace (2). ). Method according to Claim 1, characterized in that the black liquor is fed to the furnace (2) from the first liquor nozzles (6a) and the second liquor nozzles (6b), which is derived from the same liquor reservoir. 14. Soda boiler having - a furnace (2), - primary air nozzles (4) for supplying primary air to the lower part of the furnace (2), - lye nozzles (6a, 6b) for supplying black liquor to a furnace (2), 15. secondary air nozzles (5) ) for supplying secondary air to the furnace (2), the secondary air nozzles (5) disposed above the primary air nozzles (4) with respect to the height of the fire chamber (2), but below the lye nozzles (6a, 6b), air nozzles (7) for supplying tertiary air to the furnace 20 (2) disposed above the liquor nozzles (6a, 6b), - quaternary air nozzles (8) for supplying quaternary air to the furnace (2), the quaternary air nozzles (8) are disposed above the tertiary air nozzles (7), characterized in that the lye nozzles (6a, 6b) consist of first lye nozzles (6a) and second lye nozzles (6b) which the tiles (6a, 6b) are arranged at substantially the same level with respect to the height of the furnace (2) and that the size of the droplet of the liquor fed from the second liquor nozzles (6b) is substantially smaller than that of the first liquor. CL o> A recovery boiler according to claim 14, characterized in that the LT) g furnace (2) has a lower sounding zone (A) and a combustion zone (B) arranged beneath the liquor nozzles (6a, 6b), wherein the zones (A, B) 35 of the first liquor nozzles (6a) the supplied liquor and the pyrolysis gases formed therefrom are arranged to be incinerated. A recovery boiler according to claim 15, characterized in that the primary air nozzles (4) are arranged to supply combustion air to the reducing zone (A). 5 A recovery boiler according to claim 15, characterized in that the secondary air nozzles (5) are arranged to supply combustion air to the combustion zone (B). A recovery boiler according to claim 14, characterized in that the furnace (2) has a post-combustion zone (C) and a burn-up zone (D) arranged above the liquor nozzles (6a, 6b), wherein the liquor supplied from the second liquor nozzles (6b) is arranged for incineration. 15 A recovery boiler according to claim 18, characterized in that the tertiary air nozzles (7) are arranged to supply combustion air to the post-combustion zone (C). A recovery boiler according to claim 19, characterized in that the temperature in the afterburning zone (C) is about 950 to 1500 ° C, preferably about 1050 to 1400 ° C. A recovery boiler according to claim 18, characterized in that 25 quaternary air nozzles (8) are arranged to supply combustion air to the burn-out zone (D). (M A recovery boiler according to claim 21, characterized in that the temperature in the exhaust zone (D) is about 950 to 1200 ° C, preferably about 950 to 1050 ° C. CL o> A recovery boiler according to claim 14, characterized in that the first and second liquor nozzles (6a, 6b) LT) g are arranged symmetrically with respect to the walls (9) of the furnace (2). 35 A recovery boiler according to claim 23, characterized in that the first liquor nozzles (6a) are arranged symmetrically on the walls of the furnace (2) and the second liquor nozzles (6b) are arranged on the corners of the furnace (2). 5 A recovery boiler according to claim 23, characterized in that the second liquor nozzles (6b) are arranged between the first liquor nozzles (6a) on the walls (9) of the furnace (2). A recovery boiler according to claim 14, characterized in that the black liquor fed from the first liquor nozzles (6a) and the second liquor nozzles (6b) into the fire chamber (2) comes from the same liquor container. C \ J δ (M δ i o (M X en CL O) {M sj- m CD o o (M