The present invention relates to a method of decreasing an amount of nitrogen oxides from flue gases of a boiler, which nitrogen oxides are generated in the combustion of fuels and air or other oxygen-containing gas. The invention also relates to a steam- generating boiler.

Flue gases of steam-generating boilers, such as a recovery boiler of a chemical pulp mill, are led from the furnace into contact with various heat exchangers, superheaters, boiler bank and water preheaters of the boiler, whereby the heat contained in the gases is recovered in the water, steam or mixture thereof flowing in the heat exchangers. The boiler bank refers to a heat exchanger comprising heat exchange elements, inside which the boiler water to be heated flows. The economizer (preheater) of the boiler refers to a heat exchanger comprising heat exchange elements, inside which the boiler feed water to be heated flows. Free space for flue gas flow remains in the boiler bank and the economizer between the heat exchanger elements. As the flue gas passes by the heat exchanger elements, heat is transferred into the feed water or boiler water flowing inside the elements. From the economizer the flue gases of the boiler are led in a way known per se via a flue gas discharge conduit to gas purification following the boiler, such as an electrostatic precipitator.

Figure 1 illustrates the construction of a chemical recovery boiler having a furnace 1 defined by water tube walls: front wall 2, side walls 3 and rear wall 4, as well as a bottom 5 formed of water tubes. Combustion air is fed into the furnace from several different levels as primary, secondary and tertiary air. There may be also other air levels. Waste liquid, such as black liquor, is led via nozzles 6 located between the secondary and tertiary air zones. During combustion, the waste liquid forms a smelt bed on the bottom 5 of the furnace, wherefrom the smelt is discharged via a smelt spout 7 adapted in the lower part of the furnace.

Above the furnace, heat recovery surfaces, i.e. superheaters 8 are provided, and the heat exchangers, a boiler bank 9 and economizers 10, follow the superheaters located above the furnace and are positioned on the side of the rear wall 4. The heat generated in the furnace is recovered in said boiler bank and economizers. On the boiler banks 9, water in saturated temperature is boiled partly into steam and in feed water preheaters 10 the water is heated by means of flue gas prior to leading the water into the steam-generating part 9 and the superheating parts 8 of the boiler. In the superheaters, the saturated steam is heated to generate steam at a higher temperature. The so-called bullnose is marked with reference numeral 14.

The water/steam circulation of the boiler is arranged via natural circulation, whereby the water/steam mixture formed in the water tubes of the walls and bottom of the furnace rises upwards via collection tubes into a steam drum 11 that is located crosswise in relation to the boiler, i.e. parallel to the front wall 2. Hot water flows from the steam drum via downcomers 12 into a manifold 13 on the bottom, wherefrom the water is distributed into the bottom water tubes and further into the water tube walls.

A waste liquor recovery boiler is conventionally formed of the following main parts, which are illustrated schematically in Figure 1 :

- A lower part 16 of the furnace, where combustion of waste liquor mainly takes place. - A middle part 17 of the furnace, where final combustion of gaseous combustible substances mainly takes place.

- An upper part 18 of the furnace

- A superheater zone 8, wherein the saturated steam exiting the steam drum 11 is transformed into (superheated) steam having a higher temperature. In the superheater zone or upstream thereof there is often provided a so-called screen tube system 15 that usually boils water.

- A boiler bank 9, i.e. water vaporizer, wherein water at a saturated temperature is partly boiled into steam.

- Feed water preheaters, i.e. so-called economizers 10, wherein the feed water flowing in the heat transfer elements is preheated by means of flue gases prior to leading the water into the drum 11 and the steam-generating parts 9 and superheating parts 8 of the boiler.

- A drum (or steam drum) 11 with water in the lower part and saturated steam in the upper part. Some boilers have two drums: a steam drum (upper drum) and a water drum (lower drum), wherebetween a heat transfer element, so-called boiler tubes for boiling the water are provided.

A bullnose, where the boiler narrows and which is a common boundary area between the furnace and the heat recovery surfaces, is located at the upper part of the furnace on the rear wall of the boiler. The bullnose is formed of a recess in the rear wall of the boiler, which recess is directed towards the front wall of the boiler. Thus, the bullnose comprises a lower wall part that is typically directed diagonally from the rear wall towards the front wall of the boiler, an upper wall part that is directed from the front wall of the boiler diagonally towards the rear wall, and a bullnose arch or tip that combines these. The purpose of the bullnose area is to protect the superheater part against direct heat radiation from the furnace and to assist the upwards flowing flue gas in turning around the corner towards the flue gas discharge conduit of the boiler so that the gases flow evenly by the heat recovery surfaces. The so-called depth of the bullnose, which plays an important part in guiding the flue gas flow into the upper part of the furnace, is e.g. in single drum boilers typically 40 -50 % of the total depth of the furnace, which means the horizontal length of the side wall of the furnace.

Many recovery boilers are additionally provided with screen tubes upstream of the superheaters in the gas flow direction typically horizontally at the deepest part of the bullnose. Typically, a saturated mixture of water and steam flows in the screen tubes, which is connected to the water circulation of the boiler. The purpose of the screen is to cool the flue gases to some extent before they enter the superheater zone, to prevent heat radiation from the furnace to the superheater tubes and to retain a part of so- called carry-over particles escaping from the furnace.

An abundant amount of flue gases containing various impurities, such as nitrogen oxides, are generated in the combustion of various fuels, such as black liquor. During combustion, nitrogen oxide is generated from a part of nitrogen entrained in air and fuel, while the rest of the nitrogen exits as molecular nitrogen (N ₂ ) and as small amounts of hazardous compounds such as dinitrogen oxide (N ₂ O), ammonia (NH ₃ ) and hydrogen cyanide (HCN). Nitrogen oxides are formed via several various routes, depending on the conditions and fuels.

The purpose of methods for removing nitrogen oxides is to minimize polluting nitro- gen oxide emissions and thus to maximize the portion of harmless molecular nitrogen N ₂ , simultaneously keeping the emissions of all other hazardous compounds at a low level. Typical nitrogen oxide removal methods include fuel staging, air staging and selective non-catalytic reduction, SNCR. Selective non-catalytic reduction is reduction of nitrogen oxide generated in combustion by addition of a reagent, such as ammonia. The efficiency of the method is influenced by operation conditions, the composition of the fuel and the reagent pre- sent. Thus, this technique has provided known embodiments, comprising a fuel-lean process using ammonia, [US-patent 3,900,554], a fuel-rich process using ammonia [US-patent 4,325,924], and a fuel-rich process using urea [US-patent 4,335,084].

SNCR variations comprise addition of a reducing agent via various flows, e.g. with reburning fuel, with air or alone. The operation of each variation is limited to precisely determined conditions. In the absence of carbon monoxide (CO), fuel-lean SNCR operates in ranges 1100-1400 K (827-1127°C), while fuel-rich SNCR operates at higher temperatures. However, carbon monoxide is present in almost all processes utilizing the SNCR-method, and the detrimental result thereof is shifting and narrowing of temperature windows. Optimal conditions for SNCR are hard to create in several combustion apparatuses.

US-patent 5820838 describes a circulating fluidized bed boiler, where heat transfer pipes, such as omega-pipes, are installed in the flue gas flow. In the solution, means for injecting an agent that reacts with nitrogen oxides (e.g. ammonia or urea) are integrated in the omega-pipes. The aim is to obtain adequate cooling of the reducing agent to a low temperature, e.g. 100-600 °C, while injecting so that the reducing agent does not decompose. However, in this patent no attention has been paid to creating a suitable temperature window between nitrogen oxide and the reducing agent.

Decreasing of NOx-contents in recovery boilers has already been applied by methods based on staging or SNCR-technique using i) "quaternary air" in the upper part of the recovery boiler at a high level, in one embodiment of which ammonia is added entrained in said air (WO 97/21869), ii) "vertical air staging" [Fl 101420 B], where air jets are fed into the furnace of the recovery boiler by means of nozzles located at several vertical elevations, iii) "Mitsubishi Advanced Combustion Technology" (MACT) [Arakawa Y., lchinose T., Okamoto A., Baba Y, Sakai T., in Proc. of the Int. Chemical Recovery Conf., Whistler, British Columbia, Jun. 11-14, 257-260, 2001], where a reducing agent (urea) can be added after air staging, and iv) black liquor staging [Fl Patent 103905], where black liquor is fed from at least two levels into a furnace having vertical air staging according to (ii). By means of these techniques, a NOx-reduction of 30-50% has been reached, but in practice they require adjustments that are not optimal for a recovery boiler. Often these techniques require oversized furnaces for keeping the temperature after the furnace adequately low and/or more expensive material solutions for preventing corrosion. In practice staged combustion or SNCR-technique in recovery boilers requires temperatures even as low as 850- 1000°C, which are reached only in such recovery boilers that are bigger and thus more expensive than conventional boilers.

One of the purposes of the present invention is to provide a method for controlling the emissions of detrimental nitrogen compounds, especially nitrogen oxides, entering from combustion processes, in a way that is more efficient and more economical than the methods described in the above. Especially a purpose of the present inven- tion is to provide a method and an apparatus for arranging a suitable temperature window for a method of removing nitrogen oxides based on SNCR-technique. The present invention can be applied especially in a chemical recovery boiler, but also in other steam-generating boilers, where creating a temperature window required by the SNCR-technique is needed.

For reaching these objectives the present invention relates to a method of decreasing the amount of nitrogen oxides from flue gases of a boiler, which nitrogen oxides are generated in combustion of fuels and air, said boiler having a water circulation system comprising superheaters and a furnace for combusting fuel and for generat- ing flue gases containing nitrogen oxides, which flue gases flow mainly upwards in the furnace and further to a superheater zone and via other heat recovery surfaces of the boiler out of the boiler, and an agent for reducing nitrogen oxides is introduced in said flue gases. It is characteristic for the invention that the nitrogen oxides reducing agent is introduced into the flue gases prior to the superheater zone, before which the temperature of the flue gases is decreased by means of at least one heat exchanger that is located in the flue gas flow upstream of the introduction of the reducing agent, for obtaining a suitable temperature window in the flue gas flow in order to reduce nitrogen oxides. Also, the invention relates to a steam-generating boiler having a water circulation system comprising heat recovery surfaces, such as superheaters, and a furnace for combusting fuel and for generating flue gases, which flue gases flow mainly upwards in the furnace and further to a superheater zone and via other heat recovery surfaces out of the boiler, and feeding means for introducing an agent for reducing nitrogen oxides in the flue gases. It is characteristic of the invention that one or more heat exchangers are located in the flue gas flow in the furnace for decreasing the temperature of the flue gases and for creating a suitable temperature window in the flue gas flow for reducing nitrogen oxides, and that the feeding means for the reducing agent are located in the flue gas flow direction after one or more heat exchangers and prior to the superheater zone.

In this connection, a heat exchanger refers to an apparatus, wherein heat is recovered from flue gas indirectly into a medium. Typically the apparatus comprises pipes, inside which the medium receiving heat from the flue gases flows.

According to a preferred embodiment of the invention, heat is recovered from flue gases in said heat exchanger or heat surface into the water circulation system of the boiler for superheating steam and/or for boiling boiler water and/or for preheating feed water. Heat can be recovered also for heating combustion air of the boiler and/or for heating another medium by means of the heat exchanger. An essential feature of the invention is that heat is recovered from flue gases into a heat exchanger mounted in the upper part of the furnace, the number of said heat exchangers being at least one, and thus the temperature of the flue gas is decreased to be suitable for decreasing the ni- trogen oxide amount by means of a reducing agent, such as ammonia.

It is essential that the heat exchanger or heat exchangers are located in such a point that an adequate volume is formed for feeding a reagent and for reacting at a proper temperature prior to the superheater zone that typically is located above the bullnose.

As earlier described, a boiler, especially a chemical recovery boiler, can be provided with screen tubes upstream of the superheaters in the gas flow direction, typically horizontally at the deepest point of the bullnose. In the arrangement according to the present invention said heat exchanger can be a screen tube system located in accor- dance with one preferred embodiment in the vertical direction of the furnace at such an elevation that a space favorable to injecting a SNCR-reagent is formed screen tube system in the flue gas flow upstream of the superheaters located above the bullnose.

As mentioned earlier, the bullnose of the boiler forms a recess in the rear wall of the boiler, which recess is directed towards the front wall of the boiler. Thus, the bullnose comprises a lower wall part that is typically directed diagonally from the rear wall towards the front wall of the boiler, an upper wall part that is directed from the front wall of the boiler diagonally towards the rear wall, and a bullnose arch or tip that can also be a mainly upright wall part (the vertical part of the rear wall of the boiler). According to an embodiment of the invention, said at least one heat exchanger, such as screen, is located in the elevational direction of the boiler below the bullnose. Depending on the shape of the bullnose, at least one heat exchanger, such as a screen, can according to an embodiment of the invention also be located in the area of the bullnose. In that case the tip of the bullnose is preferably formed of a vertical wall part combining the inclined lower and upper walls, whereby the bullnose area in the vertical direction is adequately long for locating the heat exchanger or heat exchangers.

In the solution according to the invention, the heat exchanger or heat exchangers are to be located at such a distance from the superheaters thereabove that between the heat exchanger and the superheater an agent for reducing nitrogen oxides can be fed in an advantageous way so that said reducing agent has enough time to react with the nitrogen oxides for removing them from the flue gas to the largest possible extent prior to the superheater zone. The required distance is influenced by retention time, the efficiency of mixing of the reducing agent with the flue gas and the tem- perature of the flue gas.

An advantage of the invention is that the agent reacting with NOx (e.g. ammonia or urea) can be injected in the proper temperature window in large volume, whereby an adequate retention time is obtained. The agent can be introduced e.g. entrained in air jets above a heat exchanger, such as screen, by evaporating the ammonia into the air, whereby efficient mixing is obtained simultaneously. An additional advantage worth mentioning is that the location of the screen in accordance with the invention decreases the escape of liquor particles, i.e. so-called carry over up onto the superheater surfaces. In the solution of the invention preferably at least one heat exchanger that is located in the flue gas flow direction upstream of the injection of a SNCR-reagent acts as a superheater. In other words, at least part of the screen transfers heat from the flue gas into the superheated steam. Thus, the size of the boiler or the volume of the su- perheating surface does not grow, because the screen tubes form a part of the superheating surface capacity required in the boiler.

The heat exchanger or heat exchangers located upstream of the injection of the SNCR-reagent are dimensioned such that the flue gas temperature decreased ade- quately for obtaining the desired temperature window. So, in accordance with the invention, a number of heat exchangers with adequate capacity for decreasing the flue gas temperature for a suitable temperature window is located in the flue gas flow upstream of the reaction of the reagent and the nitrogen oxides in the flue gas.

The solution according to the invention allows lowering the height of the superheaters that typically are located above the bullnose and thus also lowering the total height of the boiler.

Obtaining the desired temperature window in the furnace of a boiler where heat is transferred mainly into the walls of the furnace only, would make the furnace of the boiler, and thus whole boiler and the boiler plant very high.

The invention allows utilizing the SNCR-technique especially in a chemical recovery boiler or other steam boiler where mixing of injected ammonia or urea is difficult, at a required temperature.

By installing in accordance with the invention at least one heat exchanger in the furnace upstream of the injection of an agent (e.g. ammonia) reacting with NOx, a lower temperature is obtained, which allows introducing the reducing agent in a proper temperature window in the furnace, whereby nitrogen oxides form nitrogen and water. This has been problematic especially in a chemical recovery boiler of a chemical pulp mill where the temperatures in the furnace are typically too high for applying the SNCR-method. Additionally, passing of a reducing agent, such as ammonia or urea, on the superheater surfaces is undesired, because feeding of substances at a later stage would be disadvantageous due to superheater corrosion. In bubbling fluidized bed (BFB) boilers the temperatures are typically lower than in a recovery boiler, but the present invention can be applied in connection with them as well, if needed.

The present invention allows e.g. feeding the injected reducing agent, such as urea and/or ammonia together with a medium, e.g. air or circulated flue gas, effectively in the furnace upstream of the superheaters, which thus will be better protected against possible corrosive effect of the SNCR-agent. Feeding of the reducing agent together with combustion air of the boiler is advantageous, because then there is no need to provide the boiler with additional openings for feeding said agent. The carrier gas for the reducing agent can originate from the boiler's combustion air system or a separate dedicated gas source. The flue gas used as carrier gas can originate from a boiler wherein the invention is applied or from another boiler at the mill.

The feeding can be effected with ammonia gas also pressurized together with steam. Ammonia can also be sucked from a container by means of a steam ejector and injected into the boiler together with steam. The ammonia can also be liquefied, mixed into water and sprayed into the boiler.

In the feed of the reducing agent, the medium can also be e.g. a combination of the above mentioned media, e.g. air and flue gas.

The present invention provides a simple method of controlling the emissions of detrimental nitrogen oxide compounds from combustion processes.

The present invention is described in more detail in the following with reference to the appended figures, of which

Fig. 1 illustrates schematically a chemical recovery boiler know per se, and Fig. 2a, 2b and 2c illustrate schematically some embodiments of the invention.

Figures 2a-2c use the same reference numerals as figure 1 where applicable.

Figures 2a-2c illustrate the construction of a recovery boiler having a furnace defined by water tube walls: a front wall 2, side walls 3 and a rear wall 4, as well as a bottom 5 formed of water tubes. Superheaters 8 of the boiler are located above the furnace. - A lower part 16 of the furnace, where the combustion of waste liquor mainly takes place.

- A middle part 17 of the furnace, where the final combustion of gaseous combustible substances mainly takes place. - An upper part 18 of the furnace

- A superheater area 8, wherein the saturated steam exiting the steam drum is heated into (superheated) steam having a higher temperature. A so-called screen tube system 15 is provided in the flue gas flow direction upstream of the superheater zone above the bullnose.

Flue gas generated in the furnace flows upwards into the upper part of the furnace and further to other heat recovery parts of the boiler, such as superheaters 8. The main flow direction of the flue gas is marked with an arrow 19.

A bullnose 14, where the boiler narrows and which is a common boundary area between the furnace and the heat recovery surfaces, is located at the upper part 18 of the furnace on the rear 4 wall of the boiler. The bullnose is formed of a recess in the rear wall of the boiler, which recess is directed towards the front wall 2 of the boiler. Thus, the bullnose comprises a lower wall part 14b that is typically directed diagonally from the rear wall 4 towards the front wall 2 of the boiler, an upper wall part 14a that is directed from the front wall 2 of the boiler diagonally towards the rear wall 4, and a bullnose arch or tip 14c that combines these.

Figure 2a illustrates a heat exchanger according to the invention, in this case a screen 15, located in the upwards flowing flue gas flow 19 below the bullnose 14 of the boiler. Feeding means 20 for an agent reducing nitrogen oxides are located between the screen and the lower edge 8a of the superheater 8. The screen 15 extends from the front wall to the rear wall, whereby it covers the horizontal cross-sectional surface of the furnace, whereby the screen gets well into contact with the upwards flowing flue gas, and thus the temperature of the flue gas can be decreased to be advantageous for the reduction of nitrogen oxides. The screen acts in this embodiment advantageously at least partly as a superheating surface. As the screen acts partly as superheating surface, part of the screen acts as evaporator for water. A screen acting as a heat exchanger is dimensioned so that the temperature of flue gas decreases ade- quately in order to achieve a desired temperature window. In the embodiment of Fig. 2a, where the heat exchanger cooling the flue gas is located below the bullnose, the reducing agent is introduced e.g. with tertiary air.

The tip of the bullnose can also be a mainly vertical wall part 14c (Figs. 2b and 2c). In this case, according to an embodiment of the invention, the heat exchanger or heat exchanging surface, such as a screen, is located in the area of the bullnose tip (Fig. 2b). In that case the tip of the bullnose is preferably formed of a vertical wall part combining the inclined lower and upper walls, whereby the bullnose area in the verti- cal direction is adequately long for locating the heat exchanger 15 and the means 20 for feeding the reducing agent. The distance of the heat exchanger from the superheaters 8 has to be adequate in order to provide the nitrogen oxides and the reducing agent enough time to react prior to the superheater zone.

In the embodiment of Fig. 2c the tip of the bullnose is also a mainly upright wall part 14c. The area of the bullnose tip is provided with screens 15a and 15b located crosswise and staggered, which is advantageous in view of space utilization. In this case, means 20 for feeding a reducing agent for nitrogen oxides are provided also above the screen.

In the embodiments of Figures 2b and 2c the reducing agent, such as ammonia is preferably introduced entrained in air or by circulating flue gas or in another way described in the above.

The solution according to the present invention allows arranging a suitable temperature window in a steam-generating boiler, especially a chemical recovery boiler for a method of removing nitrogen oxides based on SNCR-technique.

Although only some preferred embodiments of the method according to the invention have been described in the above, the invention covers all such modifications and variations that are included in the scope defined in the claims.