This invention relates to a method in accordance with the preamble of claim 1 for burning pulverized fuel in a tangentially fired boiler and for the production of reducing conditions to reduce nitrogen oxides.

The invention also concerns an apparatus for implementing the method.

Today reducing harmful emissions of power plant's exhaust gases has become one of the main objectives when developing modern combustion technologies and apparatures. Emissions of, for example, sulphur oxides and solids can be controlled to a great extent with modern technologies, but emissions of nitrogen oxides are still a problem that has not been entirely solved. It is well known that NO _x formed in the combustion process is one of the main causes of air pollution; thus, certain basic improvements in burners or improvements in the whole combustion system have been made. A particular problem raised in the combustion of pulverized fuel is that the organically bound nitrogen (nitrogen content is for example 1- 2 wt % in coal and peat) is released to gas phase during the combustion process, generating a lot of NOx emissions.

During the first stage of coal combustion, the pyrolysis, most of the fuel-N release to gas phase and gaseous compounds such as HCN and NH ₃ are formed. After the pyrolysis smaller volumes of fuel-N are retained in the after pyrolysis to remaining solid particles, called "char-N". If, during this process, there is a lot of oxygen, most of NH ₃ and HCN is oxidized to NO-.. If oxygen concentration is low enough, these compounds tend to reduce to molecular N ₂ . It is also known that HCN and NH ₃ can reduce the already formed NO _x to molecular N ₂ under the conditions of low oxygen content and high temperature.

Another fact is that some chemical radicals, especially CH _; radicals, which are

intermediate combustion products, can reduce NO _x , and as a consequence, NH ₃ and HCN are again formed, which can further reduce NO _x . The higher the temperature and the lower the oxygen content, the more liable those reducing reactions are to proceed. Accordingly, in order to suppress the generation of NO _x during combustion of coal or other pulverized fuels, the technical problem is how to create such an atmosphere that has a low oxygen concentration and high temperature.

In general, a combustion process referred to as two-stage combustion applies to this low-oxygen region for reducing the emissions of NO _x . In this process, an air-deficient zone is formed in the burner zone of a combustion furnace, and an amount of air corresponding to the above deficient amount of air is supplied through the so-called after-air port provided downstream of burners to effect complete combustion, whereby combustion over the whole of the combustion furnace is improved thereby reducing the amount of NO _x discharged. However, in case of a two-stage combustion like this, half-burned coal particles (char) are formed in the air-deficient zone of the burner, and a large free space is required in the furnace for complete combustion of the char with after-air. Thus, although the above combustion process (two-stage combustion) is rather efficient in lowering NO _x emissions of the combustion, it has still certain limitations, such as like unburned carbon and unstable flame conditions.

Thus, a new type low-NO _x burner has been constructed so that the air-deficient zone is formed very close to the tip end of the burner, and the two-stage combustion is carried out by means of a single-burner. This single burner staging technique combined with staging in the whole furnace (OFA, Over Fire Air technique) is very efficient in lowering NO _x emissions. The United States patent No. 4,545,307 describes this kind of a low-NO _x burner. The burner described in the US 4,545,307 is designed to be mounted perpendicularly in the wall of the furnace. These burners are equipped with a flame holder at the open end of the fuel feeding pipe, which promotes rapid ignition of the pulverized fuel; hence it is possible to allow a high temperature reducing zone to form near the burner. The flame holder is efficient

also in reducing the amount of unburnt carbon, in addition to reducing NO _x emissions. In these burners, pulverized fuel is fed with carrier air, amounting to 20 to 30% of the total combustion air passed through a coal pipe and injected through an injection port and flame holder into a combustion furnace. At the outer peripheral part of the burner, a stream of secondary air having a swirling motion imparted by air vanes is passed through a secondary air register. Further in the outer peripheral part, a stream of tertiary air is passed through a tertiary air register, and it has a swirling motion imparted by a radial swirler. In order to achieve a low NO _x concentration, it is required that the primary combustion zone is separated from the secondary and tertiary air streams near the burner throat in order to form a good reducing atmosphere and, on the other hand, to enhance postflame mixing between unburnt carbon and tertiary air.

These modern low-NO _x burners are intended to be mounted perpendicularly in the wall of a furnace (wall-fired boilers), and the flames are directed perpendicularly towards the center of the furnace. In wall-fired boilers there are sevaral individual burners mounted next to each other, and all burners have their own separate flames.

All these flames are stabilized and staged separately using burners with high-swirling motion for the combustion air. In the burners of the prior art (e.g. that of US 4 545 307), staged combustion in air-deficient zones occurs in each flame separately, and the reducing zone is formed near the burner, and the amounts of nitrogen oxides and unburnt carbon are reduced.

In the tangentially fired boilers, burners are located perpendicularly in each comer, and the flames and the combustion air are directed to the opposite comer to form a vortex in the center of the furnace. In case of the tangentially fired boilers, fuel and combustion air are injected axially into the boiler, and the final mixing occurs in the central vortex (fireball). Central vortex compensates the lack of swirl of the combustion air and takes care of the flame stabilization. A tangential jet burner of the prior art typically comprises a fuel pipe, secondary air channel and sometimes intermediate air channel for cooling materials between fuel pipe and secondary air

channel. Normally, using jet burners, the distance between the ignition point and the throat of the bumer is 2 - 3 meters, and the burning of the fuel occurs mainly in the central vortex. Before the ignition point the parallel streams of fuel and combustion air have been mixed together causing combustion in oxidizing atmosphere and forming NO _x emissions. In case of two-stage combustion an air- deficient reduction zone does not form until the central vortex, and no staging occurs in the fuel stream between the throat of the burner and the central vortex. The staging concerns only the central vortex flame, and as deep staging as in the modem wall low-NO _x burners can not be achieved by using jet burners.

The NO _x emissions of existing tangentially fired boilers can be reduced by modifying the boiler and burners and installing an over fire air system (OF A), instead of installation of totally new low-NO _x burners. Normally this means that the combustion is delayed, and, as a consequence, the amount of unburnt carbon increases, and only a moderate NO _x reduction can be achieved. The United States patent 5 020 454 describes a firing system for tangentially fired boilers. This system inculdes a windbox and a first cluster of fuel nozzles mounted in the windbox operative for injecting clustered fuel into the furnace so as to create a first fuel-rich zone therewith, a second cluster of fuel nozzles mounted in the windbox operative for injecting clustered fuel into the furnace so as to create a second fuel-rich zone therewith, and an offset air nozzle mounted in the windbox operative for injecting offset air into the furnace and towards the walls of the furnace. The system also includes two sets of overfire air nozzles. With this system, fuel-rich zones are formed in the furnace and staged combustion occurs over the whole furnace. Emissions of nitrogen oxides are thereby reduced, but the system has several drawbacks. It is very complicated, and the furnace has requires quite extensive modifications. No deep staging can not be achieved, since the combustion air mixes rapidly with the fuel, and it is therefore difficult to maintain reducing conditions in the flame zone. In these modified boilers, staging occurs in the main vortex, instead of the primary combustion zone, because ignition is delayed.

As the requirements for controlling emmissions of nitrogen oxides in tangentially fired boilers are also growing, there is a need to improve better combustion methods and burners which can be fitted in existing tangentially fired boilers, too.

The object of this invention is to provide a totally new type of bumer and a combustion method for reducing the emissions of nitrogen oxides in tangentially fired boilers.

Another object of this invention is to introduce a new method for reducing slagging problems of tangentially fired boilers, to reduce the amount of unburnt carbon and to improve flame stability.

The invention is based on controlling the air and fuel flows in burners of a tangentially fired boiler, whereby an air-deficient mixture of primary air and fuel is fed through a flame holder into the combustion chamber, and at least one stream of combustion air is routed around the stream of primary fuel into the central vortex so that the combustion air is not essentially mixed with the fuel until in the central vortex, and an air-deficient reducing zone is formed in the vicinity of the bumer outlet.

According to one embodiment of the invention, a stream of secondary combustion air is passed around the flame formed of the fuel to provide a separating blanket of air around the flame, and a stream of tertiary combustion air is directed towards the water walls and horizontally away from the flame.

A bumer according to this invention is called a NR-JET burner in the following.

More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

Furthermore, the apparatus according to the invention is characterized by what is

stated in the characterizing part of claim 10.

The invention provides essential benefits.

The main object and advantage of this invention is a substantial reduction of emissions of nitrogen oxides in flue gases. By means of this invention, NO _x emissions of tangentially fired boilers can be reduced at least to the same level as emissions of the modem wall fired boilers. The staging occurs both in a separate primary combustion zone in front of the bumer and further in the main vortex with the overfire-air. With this new combustion method, a much deeper staging of the combustion can be achieved than in the conventional tangentially fired boilers.

The slagging problem of tangentially fired boilers is avoided by directing air to the waterwalls and thereby providing an oxidizing atmosphere near the walls. The amount of unburnt carbon is reduced because of rapid ignition of the fuel, and, at the same time, flame stability is improved. The construction of the NR-JET bumer is relatively simple. Accoring to the invention the main application of the NR-JET burner is retrofitting old tangentially fired boilers. When an old boiler is retrofitted with these burners, NO _x emissions are reduced remarkably, and combustion efficiency is improved, too.

This invention provides a totally new type of low-NO _x bumer for the tangentially fired boilers, NR-JET bumer, applying some of the above mentioned principles used in wall fired low-NO _x burners. In a boiler which has been fitted with NR-JET burners, staging occurs both in the primary combustion zone in front of the bumer and in the main vortex with OFA. In the NR-JET bumer, pulverized fuel is injected by carrier air to the amount of about 20 to 30 % of the total combustion air into the combustion furnace. Around the fuel pipe there is concentrically a secondary air channel, for injecting the secondary air into the furnace. In uppermost and lowermost parts of the bumer there are tertiary air channels and representative injection ports. The fuel stream is separated from the tertiary air streams with

spacers in order to form a good reducing atmosphere in the primary combustion zone. In addition to the spacers, both tertiary air injection ports are equipped with outwardly directed quide sleeves, which direct the tertiary air streams vertically away from the primary combustion zone. Furthermore, the tertiary air can be directed horizontally away from the center of the furnace and towards the waterwalls. This way oxygen is kept on the waterwalls, and the lower furnace heat absorption is improved. This also prevents the tendency of high volumes of over-fire air to slag the lower furnace and to increase the furnace outlet temperature.

Below, the invention is explained in detail with references to the enclosed drawings.

Fig. 1 is a schematic front and cross sectional view of the conventional Jet-bu er for tangentially fired boilers.

Fig. 2 is a schematic front and cross sectional view of one embodiment of the invention.

Fig. 3 is a schematic front and cross sectional view of the second embodiment of the invention.

Fig. 4 is a schematic front and cross sectional view of the third embodiment of the invention.

Fig. 5 is a schematic view of the fourth embodiment of the invention.

Fig. 6 is a schematic view of the principle of directing the jet of the tertiary air horizontally to the direction of the water wall. In the figure the angles are just an illustrating example.

Basically, there are three versions of the burner, namely NR-JET 1, NR-JET 2 and NR-JET 3. All these NR-JET burners have basically the same functional idea, but

the need for a different construction is based on the lack of space in the existing boilers/bumer throats. NR-JET 3 bumer has the best combustion performance and the lowest NO _x emissions, but the bumer throat diameter is the largest, limiting the applicability of this burner.

The preferred structure of the burner used for implementation of the invention is illustrated in figures 2 - 5. Figures 1 - 4 also show the flame shape and the various combustion zones illustrating the combustion process.

In the figures, _ _< ■ is the volatilization zone, I the primary recirculation zone, II the reducing zone, HI the vigorous turbulent combustion zone, IV the tertiary recirculation zone, V the stagnation zone, VI the secondary recirculation zone and VII the main vortex.

A conventional jet-bumer consists of rectangular pulverized coal pipe 1 and injection port 2 of the coal pipe. Around fuel pipe 1 there is upper secondary air channel 3 with upper secondary air injection port 4 and lower secondary air channel 5 with lower secondary air injection port 6. As it can be seen in the figure 1, reducing zone π is very small.

Figure 2 shows NR-JET 1 burner according to the invention. The NR-JET comprises rectangular pipe 1 for pulverized fuel and injection port 2 in the outlet end of the fuel pipe. Around the fuel pipe there is concentrically arranged rectangular secondary air channel 7, forming a secondary air passageway around the outer periphery of pulverized fuel pipe 1, and injection port 8 for channel 7. NR- JET 1 bumer is also equipped with flame holder 9, which comprises ring 9a inside coal pipe 1 and guide sleeve 9b in secondary air channel 7. Ring 9a has the same rectangular form as the cross section of injection port 2 of fuel pipe 1, and it extends perpendicularly towards the central axis of fuel pipe 1. The cross section of ring 9a may be a continous ring, but in this construction ring 9a is provided with teeth, that extend into fuel pipe 1. Secondary air channel 7 surrounds the end part of coal pipe

1 and outward secondary air guide sleeve 9b of flame holder 9 extends into channel 7. In addition, the outer part of secondary air channel 7 of NR-JET 1 bumer is provided with angled guide sleeve 10. The vertical outward angle of angled guide sleeve 0 ₂ is typically between 5-40 ° in relation to the central axis of the bumer.

Flame holder 9 is a ring that surrounds the inner wall of fuel pipe 1, and it is made of, or coated by, a wear and heat-resistant material such as ceramics or heat- resistant steel. In this construction, flame holder 9 is a rectangular or cylindrical bluff body having a hole through which the pulverized coal stream is passed in the central part thereof, and it is arranged in the opening end of fuel pipe 1. The inner side of the flame holder, ring 9a, extends nearly perpendicularly to the axial direction of fuel pipe 1, and the secondary air guide sleeve 9b thereof being formed either in parallel to the axial direction of the pulverized coal pipe toward the com¬ bustion furnace or at such an angle that the diameter of the guide sleeve is enlarged to the radial direction of secondary air channel 7. Furthermore, in order to enhance ignitability at the exit of the injection port of fuel pipe 1, and to generate the high temperature reducing flame at the exit end with certainty, ring 9a forms a toothed apron protruding at the inner peripheral surface of the fuel pipe 1 at the exit of injection port 2 thereof towards the center of fuel pipe 1 to ensure efficiency of the present invention. The apron may be a continuous ring, but in this embodiment it is serrated, i.e. provided with cut-away parts in it. The inner diameter or dimension d _t of ring 9a of flame holder 9 and inner diameter d ₂ of fuel pipe 1 are preferably determined to satisfy a relation of 0.7 < < 0.98, and most preferably determined so as to give a dj/d ₂ of about 0.9. The ratio of d-/d ₂ is not limited to the above range, but if the ratio of d,/d ₂ is too low, the flame holder protmdes too much into fuel pipe 1, increasing the flow rate of the pulverized coal stream passing through the injection port, and hence increasing the pressure drop inside the fuel feeding pipe.

Angle 0 _ι formed between angled secondary air guide sleeve 9b and central axis of the fuel pipe is typically between 15 - 25 ° in order to give enough flame

maintenance effect and to separate well the central reducing flame from the oxidizing main flame and the combustion air.

NR-JET 2 bumer comprises rectangular fuel pipe 1 having injection port 2. Around the fuel pipe there is concentrically arranged rectangular secondary air channel 7 forming a secondary air passageway around the outer periphery of fuel pipe 1, and injection port 8 of channel 7. In the uppermost and lowermost parts of the bumer there is upper tertiary air channel 11 and lower tertiary air channel 13, and corresponding injection ports 12 and 14. Between tertiary air channel 11 and secondary air channel 7 there is upper spacer 16 and between lower tertiary air channel 13 and secondary air channel 7 there is lower spacer 15. The primary function of the spacers is to separate the secondary and the tertiary air streams in order to protect the formation of reduction zone II in front of the bumer. The height (d ₃ ) of spacers 15, 16 varies normally between 30 and 350 mm. Flame holder 9 is similar to that in NR-JET 1 bumer.

Both upper and lower tertiary air channels 11 and 13 are also equipped with guide sleeves 17 and 18 having vertical angle 0 ₃ . Normally 0 ₃ is between 5 and 40 °. In order to achieve sufficiently good funnel like effect for tertiary air in the tertiary air injection ports 12, 14, the length of the sleeves should be designed so that the relation between the length of sleeve 1 and the height of the tertiary air passage h _j is l/h, >2 (figure 3.). It is possible to shorten the length of the sleeves by using intermediate guide sleeves 17a and 18a without losing the funnel like effect, but then the sleeves should be designed so that the relation between length 1 of the sleeves and height h ₂ of the channel formed between the intermediate guide sleeve and the wall of the air channel is l/h ₂ >2.

Tangential jet bumer NR-JET 3 is basically similar to NR-JET 2 bumer, with the exception of air vanes 19 being provided in the passageway of secondary air channel 7. These axial vanes 19 give the secondary air stream a tangential velocity component improving the turbulent combustion near the bumer throat. Typically the

number of air vanes 19 is 8 - 15, and the vanes are angled to the axial direction in an angle of 40 - 50° so that swirl number is between 0.5 and 1.0. Another difference between NR-JET 2 and NR-JET 3 is the shape of the fuel pipe and the air channels. Fuel pipe 1, fuel injection port 2, secondary air channel 7 and secondary air injection port 8 have a cylindrical shape and, as earlier, they are equipped with flame holder 9, which comprises angled secondary air guide sleeve 9b, and toothed ring 9a. Flame holder 9, spacers 15, 16, tertiary air channels 11, 13 and tertiary air injection ports 12, 14 are cylindrically shaped.

The amount of primary air depends basically on the mill conditions, being typically between 20 and 30 %. Favourable velocity of the primary air is 15 - 25 m/s. According to the invention, one object of the secondary air is to prevent spreading of the coal/primary air stream. The secondary air is passed around the reducing flame π with a great velocity, and it forms a separating blanket reducing the amount of coal particles that are driven to the furnace walls, and the slagging behavior of the boiler is reduced. In addition, the amount of primary and secondary air should enable the burning of the volatile material of the fuel. Thus, the precentage of volatiles in the coal or other fuels determines the amount of the secondary air, being normally less than 30%. In order to achieve a sufficient enveloping effect and proper mixing of the secondary air and the primary air/fuel mixture, the velocity of the secondary air has to be sufficiently high, about 30 - 80 m/s. The rest of the combustion air is injected through the tertiary air injection ports, and the mass flow ratio between secondary and tertiary air is 1:2-1:5. The velocity of the tertiary air at the tertiary air injection port is 30-80 m/s. If the content of volatiles in the fuel is low, the amount of primary air may be sufficient for combusting these volatiles in the reducing flame. In such case, mixing of secondary air with the reducing flame must be prevented. In this embodiment, the secondary air flow is similar to the tertiary air flow, and no separate secondary air streams exists unlike in NR-JET 2 and 3 burners. In this case, the combustion air channel may surround the primary air/fuel channel, or it may be arranged in two separate channels above and below the fuel pipe.

Another very important fact concerns flame stabilization and mixing: in case of swirling burners, tertiary air has high swirl number that gives good postflame mixing and stabilization. In case of tangential NR-JET bumer, tertiary air has only axial momentum, but in this case central vortex compensates the lack of the swirl and takes care of mixing and flame stabilization

In case of the conventional jet burner (axial flows, no swirl, fig. 1), ignition point is far from fuel pipe, volatilization I„ zone is large, flame is unstable and there will be no reducing zone II, or it is very small, which results in high NOx emissions. Actual flame stabilization in tangentially fired boilers using the conventional jet burners occurs in main vortex zone VII. Turbulent/oxidizing combustion zone HI occurs on the outer boundary layer of the primary air flow and in main vortex.

NR-JET 1 burner is equipped with flame holder 9, which enhances the formation of primary recirculation zone I improving ignition and flame stability. The secondary air is passed around the primary air and fuel with a great velocity, and this prevents spreading of the fuel stream. The passage for secondary air 8 is shaped to direct a part of the secondary air away (ring 9a + sleeve 9b, θ^ from the primary air and fuel. As a result, reducing zone II is larger and nearer the bumer throat than in the conventional jet burner.

In the same way as NR-JET 1 burner, NR-JET 2 burner is equipped with flame holder 9, which enhances the formation of primary recirculation zone I improving ignition and flame stability. The ignition and flame stability of NR-JET 2 bumer is improved compared to NR-JET 1 bumer thanks to the tertiary recirculation zone IV. This is a consequence of the underpressure zone formed between secondary and tertiary air streams, whereby hot flue gases from the main vortex are recirculated back to the combustion zone. In addition, less secondary air is mixed into the volatilization zone avoiding the dilution effect and enhancing the ignition and flame stability, compared to NR-JET 1 bumer. Because of these effects, the volatilization occurs more rapidly and the volatilization zone is smaller. In front of both spacers,

is formed stagnation zone V that prevents efficiently mixing of the tertiary air into reduction zone π, so it does not disturbe the formation of reducing conditions. The length of the spacers (d ₃ ) determines the horizontal length of stagnation zone V, so the longer the d ₃ , the better the stagnation zone and the better the NO _x reduction. Besides the spacers, tertiary air guide sleeves 17, 18 prevent mixing of tertiary air into reduction zone π, because the sleeves direct tertiary air away from the primary combustion zone. The upper tertiary air injection port is directed upwards, and the lower one accordingly downwards from the primary combustion zone to prevent mixing into the flame until the central vortex (fireball), wherein the final oxidation of the fuel occurs.

In addition to directing tertiary air injection ports 12 and 14 upwards and downwards, they are shaped to direct the tertiary air away from the center of the fumace and towards water walls 23 of the fumace (figure 6). By this means, oxygen in kept away from the centre of the fiimace and near water walls 23 so as to prevent reducing atmosphere to form there. The slagging of the lower fumace is also reduced, and the lower fumace heat absorption is increased. Angle 0 ₇ between tertiary air flow 26 and wall 23 is preferably 5-45°, and the guide sleeves in the tertiary air passages are arraged accordingly. Figure 6 also shows fuel flow 25 from the comer of the fumace to central vortex 24, where the fuel finally bums.

As a result of spacers and separated secondary and tertiary air, reducing zone π in NR-JET 2 bumer is larger than in conventional jet-bumer and the NR-JET 1 bumer.

NR-JET 3 burner is similar to NR-JET 2 burner, but fuel pipe 1, the secondary air channel 7 and secondary air injection port 8 are of round shape. Because of its shape, it is possible to equip secondary air channel 7 with an axial swirler. Swirl number is between 0.5 and 1.0. Because of the secondary air swirl, secondary air recirculation zone VI is formed between the primary air and tertiary air jets creating a hot spot, and heat transfer to the primary combustion zone is increased. This improves flame stabilization, and volatilization occurs more rapidly, and a larger

reduction zone is formed. With this configuration, it is possible to achieve both the lowest amount of unburnt carbon (rapid ignition) and the lowest NO _x emissions (large reduction zone).

In case of NR-JET 1,2 and 3 burner, it is possible to apply inside fuel pipe 1 venturi part 20 and pulverized fuel concentrator (P.F. concentrator) part 22. This kind of fuel pipe is shown in figure 5. Venturi 20 is located at a distance from the exit end of fuel pipe 1, and the concentrator extends through the throat of venturi 20. The dimensions of concentrator 22 start to enlarge at the same time as the inner diameter of fuel pipe 1 starts to enlarge after venturi 20. The dimensions of concentrator 22 start to diminish near the exit of pipe 1, and concentrator 22 ends in the vicinity of flame holder 9. With venturi 20 it is possible to achieve more uniform fuel particle distribution before P.F. concentrator 22. To improve the ignition, increasing concentration of pulverized fuel around the flame holder, the flame stabilization is the most effective. In a two-phase flow of gas and particles, if the path is expanded, unhomogenous concentration will be formed due to difference of momentum between gas and particles, so P.F. concentrator is used. The fuel concentrator is arranged on the centerline of the fuel pipe and has a bulge part forming an angle of 5 - 60° (0j) at the leading side of the fuel stream, and an angle of 5 - 30° (0 ₆ )at the exit side of the fuel stream.