The invention relates to a method for reducing nitrogen oxide emissions in a bubbling fluidized bed boiler . BACKGROUND OF THE INVENTION

In the near future, new, significantly strict ^¬ er nitrogen oxide (NO _x ) emission limits depending on the fuel power of the boiler are about to enter into force in the member states of the European Union (Industrial Emissions Directive, IED) . There is thus the need for developing better ways to reduce nitrogen oxide emissions. In fluidized bed combustion, nitrogen oxides are mainly the result of the oxidation of organic nitrogen in the fuel. The quantity of nitrogen oxides can be re- duced e.g. by methods of combustion technology, by in ^¬ jecting urea or ammonia into the furnace and by cata ^¬ lytic purification of the flue gases.

Methods of combustion technology are used to prevent formation of nitrogen oxides or to convert into other compounds those nitrogen oxides, which are pro ^¬ duced in earlier stages of combustion. Widely used post-combustion NOx-control technologies include Selec ^¬ tive Catalytic Reduction (SCR) process and the Selec ^¬ tive Non-catalytic Reduction (SNCR) process. Both pro- cesses reduce NO _x to 2 and ¾0 with ammonia and urea based reagents. The operating temperatures of these two processes differ from each other. The SNCR process typ ^¬ ically takes place at a temperature between 950 and 1,100 °C . On the contrary, the SCR process takes place in a much lower temperature range, i.e. at a tempera- ture between 160 and 350 °C . Also, the investment and operating costs of these two processes are different.

For a power plant of a fuel power of above 300 MW, the new upper limit for nitrogen oxide emissions will be 200 mg/Nm ³ . The new limit may be achieved either by Selective Catalytic Reduction process or by combin ^¬ ing advanced combustion technology with Selective Non- Catalytic Reduction process. The investment costs for Selective Catalytic Reduction are approximately 10 MEur whereas the investment costs for the combination of ad ^¬ vanced combustion technology and Selective Non- Catalytic Reduction are much lower, approximately 1 to 2 MEur. In addition, bubbling fluidized bed boilers typically utilize large quantities of biofuels, which have the tendency of rapidly deactivating the catalytic material, thus increasing the operating costs of the power plant when catalytic methods are used.

In Selective Non-Catalytic Reduction of nitro ^¬ gen oxides, nitrogen oxide reductant is injected into the boiler furnace. Both urea and ammonium water can be used for SNCR process, each having its own advantages and disadvantages. The following overall post- combustion reactions take place: NH ₂ CONH ₂ + 2 NO + ½ 0 ₂ → 2 N ₂ + C0 ₂ + 2 H ₂ 0 (for urea)

4 NH ₃ + 4 NO + 0 ₂ → 4 N ₂ + 6 H ₂ 0 (for ammonia)

As a result of these reactions molecular nitrogen (N ₂ ) , water vapor (H ₂ 0) and with urea carbon dioxide (C0 ₂ ) are formed .

The temperature window for achieving an adequate nitrogen oxide reduction with a minimum N¾ slip is narrow. The optimum temperature range for urea and ammonia is between about 950 and 1100 °C . When the tem ^¬ perature is above this range, nitrogen oxides are formed. When the temperature is below this range, the reaction rate is slowed down causing ammonia slip. In the incorrect temperature range, nitrous oxide ₂ O is formed. The temperature range depends on the flue gas composition .

Optimum nitrogen oxide reduction is achieved by evenly distributing and mixing the reagent in the flue gas within the appropriate temperature window. Of ^¬ ten, the temperature in the furnace is very high, espe ^¬ cially when full boiler load is used. It may be that injecting nitrogen oxide reductant into the furnace is not possible due to the high temperature. Alternative ^¬ ly, nitrogen oxide reductant may be supplied in the up ^¬ per part of the furnace, e.g. at the height of the fur ^¬ nace nose.

Mixing nitrogen oxide reductant with flue gas ^¬ es is challenging, especially when the nitrogen oxide reductant is injected at the level of furnace nose where it is not possible to place the injection nozzles on two opposite walls. The superheaters in the upper part of the furnace also pose a mechanical barrier in ^¬ hibiting efficient distribution of the nitrogen oxide reductant over the entire cross-section of the injec ^¬ tion level. In addition, injecting the nitrogen oxide reductant in the upper part of the furnace results in corrosion of superheaters. The dwell time of the nitro ^¬ gen oxide reductant in the boiler furnace is also rela ^¬ tively short when the nitrogen oxide reductant is in ^¬ jected in the upper part of the furnace. Therefore, the overall nitrogen oxide reduction is not satisfying.

In addition to new nitrogen oxide emission limit, new ammonia and nitrous oxide emission limits enter into force in the member states of the European Union in the near future, making it even more important to achieve the correct temperature range in the boiler furnace when using SNCR process to reduce nitrogen ox ^¬ ide emissions. In order to achieve efficient nitrogen oxide reduction meeting the new nitrogen oxide emission limits, more efficient ways of reducing nitrogen oxide concentration in the flue gas are needed, including combustion and post-combustion processes. An even distribution of the nitrogen oxide reductant in the flue gas at the right temperature has to be achieved.

PURPOSE OF THE INVENTION

The purpose of the invention is to provide an efficient method for reducing nitrogen oxide emissions in a bubbling fluidized bed boiler. The method combines combustion technology and SNCR technology and results in efficient nitrogen oxide reduction. Further, the purpose of the invention is to provide a bubbling flu ^¬ idized bed boiler and use of nitrogen oxide reductant in the bubbling fluidized bed boiler according to the invention .

SUMMARY

The present invention relates to a method for reducing nitrogen oxide emissions in flue gas in a bub ^¬ bling fluidized bed boiler burning fuel. The bubbling fluidized bed boiler comprises a fluidized bed includ ^¬ ing bed material and a boiler furnace comprising a first combustion zone (I) . Air needed for burning the fuel in the fluidized bed is supplied in stages into the boiler furnace for causing substoichiometric com- bustion in the first combustion zone (I) . The method comprises :

- supplying primary air into the first combus ^¬ tion zone (I) from under the fluidized bed for fluidiz- ing the bed material; and

- supplying nitrogen oxide reductant for re ^¬ ducing nitrogen oxides in the boiler furnace at a cer- tain injection level where the temperature in the fur ^¬ nace is such as to enable the nitrogen oxide reductant to reduce nitrogen oxide concentration in the flue gas.

Combustion air for volatile matter is supplied into the first combustion zone (I) along with the fuel supply such that part of the combustion air for vola ^¬ tile matter is supplied as a first combustion air sup ^¬ ply mixed with the fuel supply and part of the combus ^¬ tion air for volatile matter is supplied as a second combustion air supply surrounding at least part of the fuel supply.

The velocity at which the combustion air for volatile matter is supplied in both the first and the second combustion air supplies is 10 to 25 m/s .

The present invention further relates to a bubbling fluidized bed boiler comprising a fluidized bed including bed material, a boiler furnace comprising furnace walls and a first combustion zone (I),

primary air nozzles under the fluidized bed for supplying primary air into the first combustion zone (I) for fluidizing the bed material;

at least one fuel feed pipe on at least one furnace wall in the first combustion zone (I) for sup ^¬ plying fuel into the fluidized bed, the at least one fuel feed pipe comprising a first outlet, and

a first inlet on at least one furnace wall for supplying nitrogen oxide reductant for reducing nitrogen oxides in the boiler furnace at a certain injection level where the temperature in the furnace is such as to enable the nitrogen oxide reductant to reduce nitro ^¬ gen oxide concentration in the flue gas.

The boiler comprises a second inlet for sup ^¬ plying combustion air for volatile matter into the first combustion zone (I) along with the fuel supply, and

at least one air feed channel around at least part of the length of the fuel feed pipe and surround- ing at least part of the fuel feed pipe, the at least one air feed channel comprising a second outlet.

Part of the combustion air for volatile matter is arranged to be supplied into the boiler furnace through the fuel feed pipe as a first combustion air supply mixed with the fuel supply and part of the com ^¬ bustion air for volatile matter is arranged to be sup ^¬ plied into the furnace through the at least one air feed channel as a second combustion air supply sur- rounding at least part of the fuel supply. The cross- sectional area of the fuel feed pipe at the first out ^¬ let and of the at least one air feed channel at the second outlet is arranged to be such that the velocity at which the combustion air for volatile matter is sup- plied in the first combustion air supply and in the second combustion air supply is 10 to 25 m/s .

The present invention further relates to a bubbling fluidized bed boiler comprising a fluidized bed including bed material, a boiler furnace comprising furnace walls and a first combustion zone (I),

primary air nozzles under the fluidized bed for supplying primary air into the first combustion zone (I) for fluidizing the bed material;

at least one fuel feed pipe on at least one furnace wall in the first combustion zone (I) for sup ^¬ plying fuel into the fluidized bed, and

a first inlet on at least one furnace wall for supplying nitrogen oxide reductant for reducing nitrogen oxides in the boiler furnace at a certain injection level where the temperature in the furnace is such as to enable the nitrogen oxide reductant to reduce nitro ^¬ gen oxide concentration in the flue gas.

The boiler comprises a second inlet for sup ^¬ plying combustion air for volatile matter into the first combustion zone (I) along with the fuel supply, and at least one air feed channel around at least part of the length of the fuel feed pipe and surround ^¬ ing at least part of the fuel feed pipe.

Part of the combustion air for volatile matter is arranged to be supplied into the boiler furnace through the fuel feed pipe as a first combustion air supply mixed with the fuel supply and part of the com ^¬ bustion air for volatile matter is arranged to be sup ^¬ plied into the furnace through the at least one air feed channel as a second combustion air supply sur ^¬ rounding at least part of the fuel supply.

The present invention further relates to a use of nitrogen oxide reductant for reducing nitrogen oxide and ammonia emissions in the bubbling fluidized bed boiler according to the invention.

The inventors surprisingly found out that when nitrogen oxide reductant is supplied in the furnace of a bubbling fluidized bed boiler and combustion air for volatile matter is supplied into the first combustion zone (I) along with the fuel supply in a manner de ^¬ scribed, efficient nitrogen oxide reduction can be achieved. Nitrogen oxide emissions are reduced by a combination of combustion technology and post- combustion technology. The nitrogen oxide emissions are reduced to the level of 250 to 300 mg/Nm ³ by using the combustion technology according to the invention. The nitrogen oxide emissions are further reduced to below 200 mg/Nm ³ by utilizing direct injection of nitrogen oxide reductant into the furnace comprising hot flue gas- es. Efficient nitrogen oxide reduction by the nitrogen oxide reductant is achieved by ensuring a good tempera ^¬ ture range with the aid of the combustion technology according to the invention. The overall nitrogen oxide reduction is 30 - 50 % as compared to conventional staged combustion.

The post-combustion technology includes sup ^¬ plying nitrogen oxide reductant in the boiler furnace at a certain injection level where the temperature in the furnace is such as to enable the nitrogen oxide re- ductant to reduce nitrogen oxide concentration in the flue gas. The efficiency of nitrogen oxide reductants depends on the temperature of the boiler furnace at the injection level and the efficiency of mixing of the re ^¬ agent with the flue gases. The desired operating tem ^¬ perature range for typically used nitrogen oxide re ^¬ ductants, i.e. urea or ammonia water, is approximately 950 - 1, 100 °C. When the temperature of the flue gases at the injection level is outside the desired operating temperature, the efficiency of nitrogen oxide reduction is reduced. Especially when the boiler load is high, the desired temperature range is difficult to achieve.

As a result of supplying combustion air for volatile matter into the first combustion zone (I) along with the fuel supply in the manner described, the temperature of the flue gas at the furnace exit (Fur ^¬ nace Exit Gas Temperature, FEGT) is reduced. In bub- bling fluidized bed boilers of furnace loads of 144 MW/m ³ and 120 MW/m ³ , temperature of the flue gas at the furnace exit (FEGT) has been reduced by 50 to 100 °C as compared to conventional staged combustion. Consequent ^¬ ly, the described combustion method enables supplying nitrogen oxide reductant into the furnace at a lower height as compared to conventional staged combustion. When the temperature in the furnace is lowered, it is possible to supply the nitrogen oxide reductant below the furnace nose, where the nitrogen oxide reductant can be supplied from two opposite furnace walls, which improves mixing of the nitrogen oxide reductant with the flue gas. The lower height of the injection level also ensures longer dwell time of the nitrogen oxide reductant in the furnace, thereby allowing the nitrogen oxide reductant more time to react.

When distribution and mixing of the nitrogen oxide reductant in the flue gas stream is improved, formation of undesired ammonia N¾ and nitrous oxide ₂ O is minimized. Thus, ammonia and nitrous oxide emissions are also reduced, thereby reducing the need for expen ^¬ sive systems for removing ammonia and nitrous oxide. In bubbling fluidized bed boilers of furnace loads of 144 MW/m ³ and 120 MW/m ³ , ammonia emissions in the flue gas have been reduced to 5 mg/Nm ³ by the current invention. The above result is achieved when SR _VOL in the first combustion zone is approximately 0.95 and the refracto- ry lining in the lower part of the furnace extends to a height of 1.8 meters from the surface of the fluidized bed .

In addition to post-combustion technology, nitrogen oxide emissions are reduced by improved combus- tion technology according to the current invention. The combustion technology reduces nitrogen oxide emissions by supplying combustion air for volatile matter into the first combustion zone (I), thus improving combus ^¬ tion of volatile matter released from the fuel in the pyrolysis reaction and reduction of nitrogen oxides in the furnace .

The method and the fluidized bed boiler ac ^¬ cording to the present invention lead to improved re ^¬ duction in nitrogen oxide emissions. Better nitrogen oxide reduction is achieved when combustion air for volatile matter is supplied both mixed with the fuel supply and surrounding at least part of the fuel supply together, as compared to supplying combustion air for volatile matter either mixed with the fuel supply or surrounding the fuel supply solely or together but with a large velocity difference between the two air sup ^¬ plies. Similar results have been achieved for boilers of both high and low furnace load. In bubbling fluid ^¬ ized bed boilers of furnace loads of 144 MW/m ³ and 120 MW/m ³ , nitrogen oxide emissions have been reduced by 100 - 150 mg/Nm ³ , thereby reducing overall nitrogen oxide emissions by 20 - 30 % as compared to conventional staged combustion. The above results are achieved when S RVOL in the first combustion zone is approximately 0.95 and the refractory lining in the lower part of the furnace extends to a height of 1.8 meters from the surface of the fluidized bed.

The current invention reduces the tempera ^¬ ture of the flue gas at the furnace exit (Furnace Exit Gas Temperature, FEGT) . Low FEGT makes it possible to supply nitrogen oxide reductant lower in the furnace, thereby improving the efficiency of nitrogen oxide re ^¬ duction caused by post-combustion technology. Volatile matter released from the fuel is burnt as low in the furnace as possible. As a result, most of the volatile matter can be burnt before the second combustion zone. Also, the fuel particles are forced to the fluidized bed and therefore do not escape to the upper parts of the furnace. Thus the temperature in the upper part of the furnace and of flue gases at the nose of the fur ^¬ nace is not excessively risen. Low FEGT also improves the efficiency of the boiler. Similar results have been achieved for boilers of both high and low furnace load. In bubbling fluidized bed boilers of furnace loads of 144 MW/m ³ and 120 MW/m ³ , temperature of the flue gas at the furnace exit (FEGT) has been reduced by 50 to 100 °C as compared to conventional staged combustion. The above results are achieved when S R _V OL in the first com ^¬ bustion zone is approximately 0.95 and the refractory lining in the lower part of the furnace extends to a height of 1.8 meters from the surface of the fluidized bed.

When part of the combustion air for volatile matter is supplied as mixed with the fuel supply and part of the combustion air for volatile matter is sup ^¬ plied as surrounding at least part of the fuel supply in a manner where the velocities of the two air sup ^¬ plies are controlled, fluid dynamics at the outlet of the fuel feed pipe are improved. According to the invention, combustion air for volatile matter is supplied as divided into first and second combustion air supplies. The first combustion air supply is mixed with the fuel supply. The second combustion air supply surrounds at least part of the fuel supply. The fuel is more efficiently migrated into the fluidized bed when it is mixed with combustion air for volatile matter, causing efficient combustion of the fuel. In addition, combustion air for volatile mat- ter forms a curtain of combustion air around the fuel stream or part of the fuel stream, thereby directing the fuel stream, including fine fuel particles, into the fluidized bed and preventing escape of fuel parti ^¬ cles to the upper parts of the furnace. The air curtain also prevents fuel particles from ending up on the boiler walls

The velocity at which the combustion air for volatile matter is supplied in both the first and the second combustion air supplies is 10 to 25 m/s . The ve- locity at which the combustion air for volatile matter is supplied means the velocity of the combustion air for volatile matter at the outlet of the air feed pipe or the air feed channel. The velocity of the air supply depends on the cross-section of the pipe or channel in which the supply flows, i.e. the smaller the cross- section, the faster the flow. Good fluid dynamics at the outlet of the fuel feed pipe are achieved when the velocity at which the combustion air for volatile mat ^¬ ter is supplied in both supplies is 10 to 25 m/s. Ac- cording to CFD (Computational Fluid Dynamics) calcula ^¬ tions, the overall fluid dynamics in fluidized bed boiler furnace relative to nitrogen oxide emissions are improved when output velocities of both combustion air supplies are in this velocity range. Because the com- bustion air for volatile matter is directed to the fur ^¬ nace both mixed with the fuel supply and surrounding at least part of the fuel supply, the total additional air flow needed to increase S R _V OL near value 1 is divided into larger cross-section of the pipe, and the velocity of the air is decreased. As a consequence, nitrogen ox ^¬ ide emissions are decreased and combustion is enhanced. Too high velocities end up in increasing nitrogen oxide emissions .

The first combustion zone (I) begins from the height level of the primary air nozzles and extends up to below the height level of secondary air nozzles. For a bubbling fluidized bed boiler, the length of the first combustion zone (I) may be optimized by optimiz ^¬ ing the height of secondary air nozzles by the method described in EP 2574841 A2, the contents of which is incorporated herein by reference.

By nitrogen reductant is herein meant any sub ^¬ stance containing nitrogen and being able to reduce NO _x into molecular nitrogen (N ₂ ) . By injection level, the height level in the boiler furnace is meant at which the nitrogen oxide reductant is supplied. In one embod- iment, the temperature at the injection level is such as to enable the nitrogen oxide reductant to reduce ni ^¬ trogen oxide concentration in the flue gas by at least 30 %. In one embodiment, nitrogen oxide reduction effi ^¬ ciency is at least 30 %. The nitrogen oxide reduction efficiency describes the percentage of nitrogen oxide reduction as compared to the original concentration. In one embodiment, the temperature at the injection level is such as to enable the nitrogen oxide reductant to reduce nitrogen oxide concentration in the flue gas by at least 25 %. In one embodiment, nitrogen oxide reduc ^¬ tion efficiency is at least 25 %. In one embodiment, the temperature at the injection level is 900 - 1, 100 °C. In one embodiment, the temperature at the injection level is 950 - 1,050 °C.

In one embodiment, the method and the bubbling fluidized bed boiler reduce ammonia emissions in flue gas . Part of the combustion air for volatile mat ^¬ ter is supplied as a first combustion air supply mixed with the fuel supply. A small amount of carrier air may also be supplied with the fuel supply.

Part of the combustion air for volatile mat ^¬ ter is supplied as a second combustion air supply sur ^¬ rounding at least part of the fuel supply. In one em ^¬ bodiment, the second combustion air supply fully sur ^¬ rounds the fuel supply. In one embodiment, the second combustion air supply surrounds the fuel supply on three sides but not on side below the fuel supply. In one embodiment, the at least one air feed channel sur ^¬ rounds the fuel feed pipe on all sides.

In one embodiment, the at least one air feed channel surrounds the fuel feed pipe on three sides but not on side below the fuel feed pipe. In one embodi ^¬ ment, the at least one air feed channel surrounds the fuel feed pipe on all sides. The at least one air feed channel may be one air feed channel or several separate air feed channels surrounding at least part of the fuel feed pipe.

In one embodiment, at least part of the length of the fuel feed pipe is surrounded by at least one air feed channel. In one embodiment, at least part of the circumference or perimeter of the cross section of the fuel feed pipe is surrounded by at least one air feed channel. The inlet for supplying combustion air for volatile matter or the at least one air feed channel may contain means for directing the air flow, such as guide vanes.

In one embodiment, the fuel is biofuel, peat or oil-based waste. In one embodiment, biofuel compris ^¬ es wood and industrial sewage sludge. In one embodi ^¬ ment, oil-based waste comprises plastics waste. In one embodiment, the fluidized bed boiler is a circulating fluidized bed boiler and the fuel comprises coal. In one embodiment, several fuel feed pipes are located on two opposite furnace walls. The fuel feed pipe comprises a first outlet for directing fuel into the furnace. The outlets of the fuel feed pipes are usually located side by side at the same height. The cross-section of the fuel feed pipe may be of any shape. In one embodiment, the cross-section of the fuel feed pipe is perpendicular. In one embodiment, the cross-section of the fuel fee pipe is round.

The bed material is fluidized by supplying fluidizing gas from under the fluidized bed. In one embodiment, fluidizing gas is supplied trough primary air nozzles. The fluidizing gas may consist solely of pri ^¬ mary air or it may be a mixture of primary air and an inert gas, such as flue gas. The fluidizing gas is set to flow with such a velocity that the particles in the fluidized bed are in continuous motion and the bed ef ^¬ ficiently mixes together the bed material and the fuel supplied into it. The fluidizing gas velocity is set such that the particles will not escape along with the gas flow into the upper part of the boiler but will form a fluidized bed in the lower part of the boiler.

EP 2574841 A2 discloses ways to adjust certain boiler parameters, which may be used together with the current invention for improving nitrogen oxide reduction. The content of EP 2574841 A2 is disclosed herein by reference. The distance of the fuel supply openings from the surface of the bubbling fluidized bed de ^¬ scribed in EP 2574841 A2 may be used to improve nitro- gen oxide reduction. Also, the vertical supply angle and horizontal supply angle of the fuel chutes, the ar ^¬ rangement of secondary and tertiary air nozzles in rows including nozzles blowing a small, medium and large air jet, and the side air nozzles placed between the outer- most fuel supply chutes and the side wall described in EP 2574841 A2 may be used to improve nitrogen oxide re ^¬ duction. In one embodiment the method is carried out in a bubbling fluidized bed boiler, the fluidized bed has a top surface and the boiler furnace has a lower part which is equipped with a refractory lining extending to a height of 1.8 - 2.4 meters from the top surface of the fluidized bed. In one embodiment the fluidized bed has a top surface and the boiler furnace has a lower part which is equipped with a refractory lining extending to a height of 1.8 - 2.4 meters from the top sur ^¬ face of the fluidized bed.

In one embodiment, the refractory lining ex ^¬ tends to a height of 1.8 - 2.0 meters from the top sur ^¬ face of the fluidized bed. One way of lowering the flu ^¬ idized bed temperature in a bubbling fluidized bed boiler is by reducing the boiler' s refractory lining surface in the region between the primary air level and the secondary air level. In typical bubbling fluidized bed boilers of today, the refractory lining surface in the lower part of the furnace is made by laying bricks to a height of about 2.5 - 5 meters from the surface of the bubbling fluidized bed. The purpose of the refrac ^¬ tory lining is to protect the boiler' s water pipes against corrosion and contamination, but at the same time it also increases the temperatures in the part above the fluidized bed, because the refractory lining prevents radiation heat transfer to the water pipes lining the furnace. When the refractory lining is lowered, the generated heat is more efficiently trans ^¬ ferred into the water pipes. Consequently, the tempera ^¬ ture of the flue gas in the upper part of the furnace is decreased, thereby enabling injection of nitrogen oxide reductant lower in the furnace.

In one embodiment the boiler furnace comprises a second combustion zone (II) located above the first combustion zone (I) and a third combustion zone (III) located above the second combustion zone (II), and sec ^¬ ondary air is supplied into the second combustion zone (II) through secondary air nozzles and tertiary air is supplied into the third combustion zone (III) through tertiary air nozzles. The second combustion zone (II) begins from the height level of the secondary air noz ^¬ zles and extends up to below the height level of ter- tiary air nozzles. The third combustion zone (III) be ^¬ gins from the height level of the tertiary air nozzles. The first and second combustion zones (1,11) are sub- stoichiometric . By substoichiometric combustion it is meant that the total air coefficient S R _T OT is kept sub- stoichiometric. The total air coefficient S R _T OT is kept superstoichiometric in the third combustion zone (III), in which the combustion is completed.

In one embodiment the bubbling fluidized bed boiler comprises a second combustion zone (II) located above the first combustion zone (I) and a third combus ^¬ tion zone (III) located above the second combustion zone (II), and secondary air nozzles on at least one furnace wall above the first combustion zone (I) for supplying secondary air into the second combustion zone (II) and tertiary air nozzles on at least one furnace wall above the second combustion zone (II) for supply ^¬ ing tertiary air into the third combustion zone (III) .

In one embodiment, nitrogen oxide reductant is supplied into the second combustion zone (II) along with the secondary air through the secondary air nozzles. In one embodiment, the nitrogen oxide reductant is supplied into the third combustion zone (III) along with the tertiary air through the tertiary air nozzles. In one embodiment the tertiary air nozzles are placed 2 - 4 meters below the furnace nose. In one embodiment the nitrogen oxide reductant is supplied into the third combustion zone (III) above the tertiary air nozzles. In one embodiment the nitrogen oxide reductant is sup ^¬ plied into the third combustion zone (III) above the tertiary air nozzles through nitrogen oxide reductant nozzles. The nozzles may be air or pressure atomizing nozzles. In one embodiment nitrogen oxide reductant is supplied simultaneously into the second combustion zone (II) along with the secondary air through the secondary air nozzles and into the third combustion zone (III) along with the tertiary air through the tertiary air nozzles. In one embodiment nitrogen oxide reductant is supplied into the third combustion zone (III) simulta ^¬ neously along with the tertiary air through the tertiary air nozzles and above the tertiary air nozzles.

In one embodiment the first inlet is at least one of the secondary air nozzles for supplying the nitrogen oxide reductant into the second combustion zone (II) along with the secondary air. In one embodiment the first inlet is at least one of the tertiary air nozzles for supplying the nitrogen oxide reductant into the third combustion zone (III) along with the tertiary air. In one embodiment the first inlet is located on at least one of the furnace walls above the tertiary air nozzles for supplying the nitrogen oxide reductant into the third combustion zone (III) above the tertiary air nozzles.

The nitrogen oxide reductant may be injected into the boiler furnace through separate nozzles in ^¬ stalled on boiler walls, the nozzles typically being air or pressure atomizing nozzles. It is, however, challenging to achieve adequate penetration and distribution of the nitrogen oxide reductant into the boiler furnace with these kinds of nozzles. This is because of the small amount and penetration capacity of the steam or air used as a driving medium. Also, undesired ammo- nia N¾ and nitrous oxide ₂ O are formed due to ineffi ^¬ cient distribution and mixing of the reductant in the flue gas stream.

A homogeneous distribution of the nitrogen oxide reductant in the boiler furnace is difficult to ob- tain as flue gases are very viscous and it is therefore challenging to mix different gases. According to CFD modelling performed by the inventors, considerably bet- ter penetration and distribution is achieved when nitrogen oxide reductant is injected into the boiler fur ^¬ nace directly through secondary or tertiary air nozzles. This may be achieved by installing nitrogen oxide reductant lances in the existing secondary or tertiary air openings. The lances may be equipped with air or pressure atomizing nozzles. In this case the nitrogen oxide reductant is efficiently distributed and mixed with the aid of combustion air into the whole cross- section of the boiler.

However, when nitrogen oxide reductant is injected through tertiary air nozzles with full boiler load there is a risk that the temperature of the flue gas at the injection level is above the allowable tem- perature of 1,100 °C and nitrogen oxide reduction is not sufficient. When conventional combustion technolo ^¬ gies are used, the temperature at the height level of tertiary air nozzles is typically too high for injec ^¬ tion of nitrogen oxide reductant. With the aid of the combustion technology according to the invention the temperature of the flue gas at the height level of the tertiary air nozzles is decreased by 50 to 100 °C. The invention thus enables injection of the nitrogen oxide reductant at the height level of tertiary air nozzles, even with full boiler load. Injection of the nitrogen oxide reductant efficiently through tertiary air noz ^¬ zles is thus enabled. Efficient nitrogen oxide reduc ^¬ tion is achieved while keeping the undesired ammonia emissions below the allowable limits. In addition, the height level of tertiary air nozzles can be used for nitrogen oxide reductant injection at considerably wid ^¬ er power range of boilers.

The temperature in the furnace depends on the boiler load. When the boiler load is small, it is pos- sible to inject the nitrogen oxide reductant at the height level of secondary air nozzles. With higher boiler load, the nitrogen oxide reductant can be in- jected at the height level of tertiary air nozzles. With maximum boiler load, the nitrogen oxide reductant can be injected at the height level above the tertiary air nozzles. The lower the nitrogen oxide reductant is supplied in the furnace, the longer is the dwell time of the reductant in the furnace, thereby improving mix ^¬ ing of the reductant with the flue gas and improving the nitrogen oxide reduction. The temperature in the furnace also depends on the furnace load. With low fur- nace load, it is possible to inject the nitrogen oxide reductant at the height level of secondary air nozzles even with high boiler load.

In one embodiment, the bubbling fluidized bed boiler comprises temperature sensors on at least one furnace wall at the height level of secondary air noz ^¬ zles. In one embodiment, the bubbling fluidized bed boiler comprises temperature sensors on at least one furnace wall at the height level of tertiary air noz ^¬ zles. In one embodiment, the bubbling fluidized bed boiler comprises temperature sensors on at least one furnace wall at the height level above the height level of tertiary air nozzles. The temperature in the boiler furnace is continuously measured by the temperature sensors during the operation of the power plant. De- pending on the measured temperature at different height levels, a computer-assisted control system allows in ^¬ jection of nitrogen oxide reductant at such a height level, in which the temperature is within a suitable range for the nitrogen oxide reductant. The nitrogen oxide reductant may be injected at different injection levels at the same time. The computer-assisted control system may be e.g. Acoustic Gas Temperature Measurement System (AGAM) commonly used with SNCR injection systems. In one embodiment, the bubbling fluidized bed boiler comprises a computer-assisted temperature con ^¬ trol system for adjusting the injection level of nitrogen oxide reductant. In one embodiment, the boiler load is 30 to 50 %, and the nitrogen oxide reductant is supplied into the second combustion zone (II) along with the second ^¬ ary air through the secondary air nozzles. In one em- bodiment, the boiler load is 60 to 90 %, and the nitro ^¬ gen oxide reductant is supplied into the third combus ^¬ tion zone (III) along with the tertiary air through the tertiary air nozzles. In one embodiment, the boiler load is 100 % and the nitrogen oxide reductant is sup- plied into the third combustion zone (III) above the tertiary air nozzles. In this case, nitrogen oxide re ^¬ ductant may be supplied at the height level of e.g. 3 to 4 meters above the height level of tertiary air noz ^¬ zles. Boiler load refers to the proportion of the full furnace combustion capacity being utilized at a given time .

When nitrogen oxide reductant is injected in ^¬ to the furnace through separate SNCR injection nozzles, additional holes have to be made to the furnace walls. This requires stopping the operation of the power plant for installing the SNCR nozzles, thereby causing costs. When nitrogen oxide reductant is supplied through ex ^¬ isting tertiary or secondary air nozzles, installing SNCR lances is easier and faster.

In one embodiment the nitrogen oxide reductant comprises a water solution of ammonia, a water solution of urea or gaseous ammonia. In one embodiment the ni ^¬ trogen oxide reductant consists of a water solution of ammonia, a water solution of urea or gaseous ammonia.

In addition to primary air, the first combustion zone (I) is supplied with combustion air for volatile matter in order to enhance nitrogen oxide reduc ^¬ tion. The amount of primary air supplied into the first combustion zone (I) as a fluidizing gas or as a part of it does not change as compared to conventional fluid- ized bed combustion. The total amount of combustion air in the first combustion zone (I) is thus increased by adding combustion air for volatile matter. In one embodiment, the air coefficient in relation to volatile matter S R _V OL in the first combustion zone (I) is in the substoichiometric area. That is, S R _V OL is below 1.

The air coefficient or the stoichiometric ra ^¬ tio SR tells how much air must be used for the combus ^¬ tion in comparison with the theoretical (stoichio ^¬ metric) volume of air needed for complete combustion of the fuel. In substoichiometric combustion, the air co- efficient SR is under 1, and in superstoichiometric combustion the air coefficient SR is over 1.

Since S R _V OL is in substoichiometric area, com ^¬ bustion of volatile matter released from the fuel in pyrolysis takes place in substoichiometric conditions in relation to volatile matter. In one embodiment, the air coefficient in relation to volatile matter is below 1, but as high as possible in order to enhance combus ^¬ tion of volatile matter in the first combustion zone (I) . The higher the air coefficient S R _V OL in relation to volatile matter, the more quickly the volatile matter will burn, at the same time causing a high local temperature and forming a maximum quantity of hydrocarbon radicals, which are needed for the reduction of nitro ^¬ gen oxides formed from the fuel. Most of the volatile matter can be burnt in the first combustion zone (I) before the supply of secondary air. In one embodiment, the total air coefficient S R _T OT in the first combustion zone is substoichiometric.

In one embodiment the air coefficient in rela- tion to volatile matter S R _V OL in the first combustion zone (I) is 0.9 - 1.0. When the air coefficient in re ^¬ lation to volatile matter S R _V OL in the first combustion zone (I) is 0.9 - 1.0, nitrogen oxides are efficiently reduced, whereby a major part of the fuel's volatile matter will burn already in the first combustion zone (I) . In one embodiment the air coefficient in relation to volatile matter S R _V OL in the first combustion zone (I) is 0.95 - 1.0. The optimum air coefficient in rela ^¬ tion to volatile matter S R _V OL in the first combustion zone (I) depends on the fuel, because different fuels have different contents of volatile matter.

In one embodiment the total air coefficient

S RTOT in the second combustion zone (II) is 0.75 - 0.85. In one embodiment the total air coefficient S R _T OT in the second combustion zone (II) is 0.8. Substoichiometric conditions are thus maintained above the first combus- tion zone ( I ) .

In one embodiment the combustion air for vola ^¬ tile matter comprises secondary air. In one embodiment the combustion air for volatile matter consists of sec ^¬ ondary air. The amount of secondary air provided above the first combustion zone (I) is decreased correspond ^¬ ingly. In one embodiment, part of the secondary air is supplied as combustion air for volatile matter into the first combustion zone (I) . In one embodiment, the tem ^¬ perature of the combustion air for volatile matter is 150 to 250 °C. Secondary air is typically preheated in order to enhance combustion in the furnace, the temperature of secondary air in a fluidized bed boiler typi ^¬ cally being in a range of 150 - 250 °C. As a result of mixing hot combustion air for volatile matter with the fuel supply, drying of the fuel particles and subse ^¬ quent pyrolysis already begin inside the fuel feed pipe. Consequently, combustion of the fuel begins ear ^¬ lier and is enhanced in the lower part of the furnace, thereby increasing the temperature in the lower part of the furnace. The time available for combustion is in ^¬ creased. When hot combustion air for volatile matter is used, the result of combustion is better. Most of the volatile matter released from the fuel in pyrolysis is burnt in the first combustion zone before supply of secondary air. In addition, mixing of the combustion air for volatile matter with the fuel is improved by the high temperature. In one embodiment the velocity at which the combustion air for volatile matter is supplied in both the first and the second combustion air supplies is 12 to 20 m/s . Good fluid dynamics are achieved when the velocity is in this range. In one embodiment the veloc ^¬ ity at which the combustion air for volatile matter is supplied in both the first and the second combustion air supplies is 15 to 20 m/s. In one embodiment, the velocity at which the combustion air for volatile mat- ter is supplied in the first combustion air supply is the same as the velocity at which the combustion air for volatile matter is supplied in the second combus ^¬ tion air supply. This way, flux flow is achieved di ^¬ recting the fuel into the fluidized bed.

In one embodiment, the velocity at which the combustion air for volatile matter is supplied in the first combustion air supply is 12 to 20 m/s. In one em ^¬ bodiment, the velocity at which the combustion air for volatile matter is supplied in the first combustion air supply is 15 to 20 m/s. In one embodiment, the velocity at which the combustion air for volatile matter is supplied in the first combustion air supply is 15 m/s.

In one embodiment the velocity at which the combustion air for volatile matter is supplied in the second combustion air supply is 12 to 20 m/s. In one embodiment the velocity at which the combustion air for volatile matter is supplied in the second combustion air supply is 15 to 20 m/s. In one embodiment the ve ^¬ locity at which the combustion air for volatile matter is supplied in the second combustion air supply is 15 m/s .

In one embodiment the first combustion air supply comprises 60 to 70 % of the combustion air for volatile matter and the second combustion air supply comprises 30 to 40 % of the combustion air for volatile matter. According to CFD calculations and confirmed by experiments in fluidized bed boilers, fluid dynamics in the furnace in relation to reduction of nitrogen oxide emissions are improved when 60 to 70 % of combustion air for volatile matter is supplied as mixed with the fuel supply and from 30 to 40 % of combustion air for volatile matter is supplied as surrounding the fuel supply. The fuel particles end up into the fluidized bed, thereby preventing fouling of the heat transfer surfaces of the boiler.

In one embodiment the fuel is supplied into the first combustion zone (I) through a fuel feed pipe, and the first combustion air supply is mixed with the fuel supply and supplied into the boiler furnace simul ^¬ taneously with the fuel supply through said fuel feed pipe. In one embodiment, the fuel supply and first com- bustion air supply are mixed in the fuel feed pipe.

In one embodiment the second combustion air supply is supplied into the first combustion zone (I) through at least one air feed channel arranged around at least part of the length of the fuel feed pipe and surrounding at least part of the fuel feed pipe.

In one embodiment the first combustion air supply and the second combustion air supply are simul ^¬ taneously supplied into the first combustion zone (I) .

In one embodiment the second inlet is connect- ed to the secondary air for supplying secondary air at least as part of the combustion air for volatile mat ^¬ ter. In one embodiment, the second inlet is connected to the secondary air for supplying secondary air as the combustion air for volatile matter. In one embodiment, the temperature of the combustion air for volatile mat ^¬ ter is arranged to be 150 to 250°C.

In one embodiment the fuel feed pipe comprises a first opening for directing part of the combustion air for volatile matter as a first combustion air sup- ply from the second inlet into the fuel feed pipe, and the at least one air feed channel comprises a second opening for directing part of the combustion air for volatile matter as a second combustion air supply from the second inlet into the at least one air feed chan ^¬ nel. The openings may be on any side of the fuel feed pipe or the air feed channel.

In one embodiment at least one of the first and second openings comprises at least one control damper for directing 60 to 70 % of the combustion air for volatile matter from the second inlet into the fuel feed pipe and 30 to 40 % of the combustion air for vol- atile matter from the second inlet into the at least one air feed channel. In one embodiment, a first con ^¬ trol damper directs 60% of the combustion air for vola ^¬ tile matter from the second inlet into the fuel feed pipe. In one embodiment, a second control damper di- rects 40% of the combustion air for volatile matter from the second inlet into the at least one air feed channel .

In one embodiment the fuel feed pipe comprises a first outlet and the at least one air feed channel comprises a second outlet and the cross-sectional area of the fuel feed pipe at the first outlet and of the at least one air feed channel at the second outlet is ar ^¬ ranged to be such that the velocity at which the com ^¬ bustion air for volatile matter is supplied in the first combustion air supply and in the second combus ^¬ tion air supply is 10 to 25 m/s. In one embodiment, the velocity at which the combustion air for volatile mat ^¬ ter is supplied in the first combustion air supply and in the second combustion air supply is arranged to be 12 to 25 m/s. In one embodiment, the velocity at which the combustion air for volatile matter is supplied in the first combustion air supply and in the second com ^¬ bustion air supply is arranged to be 15 to 25 m/s. In one embodiment, the velocity at which the combustion air for volatile matter is supplied in the first com ^¬ bustion air supply and in the second combustion air supply is arranged to be 15 m/s. In one embodiment, the velocity at which the combustion air for volatile mat ^¬ ter is supplied in the first combustion air supply is arranged to be the same as in the second combustion air supply .

The amount of combustion air for volatile mat ^¬ ter is determined by the amount of primary air supplied into the first combustion zone (I) so that the value of SR _VOL in the first combustion zone (I) is in the correct area. The velocity of the combustion air for volatile matter in the fuel feed pipe is affected by the mass flow of the air and the cross-section of the fuel feed pipe. Similarly, the velocity of the combustion air for volatile matter in the at least one air feed channel is affected by the mass flow of air and the cross-section of the at least one air feed channel. The velocity of the combustion air for volatile matter in the fuel feed pipe is also affected by the mass flow of fuel.

Several advantages are achieved by using the current invention. As a consequence of supplying com- bustion air for volatile matter into the first combus ^¬ tion zone along with the fuel supply in the manner de ^¬ scribed, the temperature of the flue gas at the furnace exit decreases. When the temperature in the furnace is lowered, it is possible to supply nitrogen oxide lower in the furnace, thereby improving distribution and mixing of the reductant with flue gases. Undesired ammonia N¾ and nitrous oxide ₂ O emissions are minimized.

The invention makes it possible to inject ni ^¬ trogen oxide reductant through tertiary or secondary air nozzles, which further improves penetration and mixing of the nitrogen oxide reductant with flue gases . The height level of tertiary air nozzles can be used for nitrogen oxide reductant injection at considerably wider power range of boilers.

With the aid of the described combination of combustion and post-combustion technology for nitrogen oxide reduction, the nitrogen oxide emissions in a bub- bling fluidized bed boiler with a fuel power of above 300 MW can be reduced to below 200 mg/Nm ³ and ammonia emissions can be reduced to 5 mg/Nm ³ , thereby meeting the new nitrogen oxide emission limit entering into force in the member states of the European Union in the near future.

The embodiments of the invention described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined to- gether to form a further embodiment of the invention. A product, a method or a use, to which the invention is related, may comprise at least one of the embodiments of the invention described hereinbefore. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate em ^¬ bodiments of the invention and together with the de- scription help to explain the principles of the inven ^¬ tion. In the drawings:

Fig. 1 is a schematic sectional side view of the bubbling fluidized bed boiler according to one embodiment of the invention illustrating the distribution of the nitrogen oxide reductant when the reductant is supplied through tertiary air nozzles.

Fig. 2 is a schematic sectional side view of tertiary air nozzle comprising SNCR lance according to one embodiment of the invention.

Fig. 3 is schematic sectional front view of the furnace of a bubbling fluidized bed boiler,

Figs. 4a-4e show the fuel feed pipe and the air feed channel from the inside the furnace according to a first, second, third, fourth and fifth embodiment of the present invention, Fig. 5 is a schematic sectional view of the fuel feed pipe and the air feed channel according to one embodiment of the invention.

DETAILED DESCRIPTION

Figure 1 shows a schematic sectional side view of the bubbling fluidized bed boiler according to one embodiment of the invention illustrating the distribu- tion of the nitrogen oxide reductant when the reductant is supplied into the boiler furnace 2 through tertiary air nozzles 16. The distribution of the nitrogen oxide reductant has been modelled using CFD modelling. The boiler furnace 2 comprises furnace walls 5. Tertiary air nozzles 16 are located on two opposite furnace walls 5. The injection level 17 is the height level of the tertiary air nozzles 16. The injection level 17 is under the furnace nose 22. The first inlet 20 is at least one of the tertiary air nozzles 16. Nitrogen ox- ide reductant is supplied into the boiler furnace along with the tertiary air through the tertiary air nozzles 16. As seen from the figure, efficient distribution of the nitrogen oxide reductant is achieved when the re ^¬ ductant is supplied through tertiary air nozzles 16. The penetration depth of the nitrogen oxide into the boiler furnace 2 is good. Moreover, the nitrogen oxide reductant does not collide with the superheaters 23, and corrosion of superheaters is thus diminished.

Figure 2 is a schematic sectional side view of a tertiary air nozzle 16 comprising an SNCR lance 21 according to one embodiment of the invention. The ter ^¬ tiary air nozzle 16 is located at a certain height on furnace wall 5. The tertiary air nozzle 16 is the first inlet 20 for supplying nitrogen oxide reductant. The tertiary air nozzle 16 comprises an SNCR lance 21 for supplying nitrogen oxide reductant into the boiler fur- nace along with the tertiary air. Similar arrangement may be used for secondary air nozzles. When nitrogen oxide reductant is supplied through an SNCR lance 21 located in the tertiary air channel, the nitrogen oxide reductant is efficiently distributed into the boiler furnace and mixed with the flue gas . Efficient nitrogen oxide reduction is thus achieved.

Figure 3 shows a schematic sectional front view of the furnace of a bubbling fluidized bed boiler. The figure is a basic view of the boiler and it is not intended to present the fluidized bed boiler on its correct scale. The fluidized bed 1 is in the lower part 12 of the furnace 2. The fluidized bed 1 consists of fluidized bed material, into which fluidizing gas is supplied through primary air nozzles 6 arranged in the bottom of the furnace 2, which primary air makes the fluidized bed material float and bubble.

Fuel is supplied above the fluidized bed 1 surface through one or several fuel feed pipes 3 locat- ed on two opposite furnace walls 5. Combustion air for volatile matter is supplied into the first combustion zone (I) along with the fuel supply. The fuel feeding means 18 are presented in more detail in figure 5. Sup ^¬ plying combustion air for volatile matter along with the fuel according to the invention reduces the temperature at the height level of tertiary air nozzles 16.

In figure 3, secondary air is supplied into the furnace 2 from secondary air nozzles 7, and tertiary air is supplied into the furnace 2 from tertiary air nozzles 16 located above the fuel supply level on two opposite furnace walls 5, the reference number of which walls are not presented in figure 3. The tertiary air nozzles 16 are usually placed 2 - 4 meters below the furnace nose.

The lower part 12 of the furnace 2 comprises a refractory lining 13 which protects the walls of the furnace 2 from erosion caused by fluidizing bed materi- al and extends from the top surface of a fluidized bed to a height of 1.8 - 2.4 meters. The height of the re ^¬ fractory lining 13 may be different from the fuel sup ^¬ ply height. The height of the refractory lining is re- duced as compared to typical height of refractory lin ^¬ ings. This way, heat transfer in the lower part of the furnace 2 is enhanced and the temperature in the upper part of the furnace 2 is reduced.

The temperature decrease allows the nitrogen oxide reductant to be supplied into the furnace 2 at an injection level 17 which corresponds to the height lev ^¬ el of tertiary air nozzles 16 without the risk of too high temperature at the injection level 17. The nitro ^¬ gen oxide reductant is supplied along with tertiary air through tertiary air nozzles 16. The first inlet 20 is thus at least one tertiary air nozzle 17. Similarly, the first inlet 20 can be at least one secondary air nozzle 7. It is also possible to supply nitrogen oxide reductant simultaneously through secondary 7 and ter- tiary air nozzles 16.

Three combustion zones are formed: substoichi- ometric first (I) and second combustion zone (II), and superstoichiometric third combustion zone (III). The first combustion zone (I) begins from the height level of the primary air nozzles 6 and extends up to below the height level of secondary air nozzles 7. The second combustion zone (II) begins form the height level of the secondary air nozzles 7 and extends up to below the height level of tertiary air nozzles 16. The third com- bustion zone begins from the height level of tertiary air nozzles 16. Air conducted into the first combustion zone (I) along with the fuel supply is taken from the secondary air register, whereby it reduces the quantity of air to be supplied into the second combustion zone (II) . A larger supply of air into the first combustion zone (I) will result in high temperatures in the first combustion zone (I) . When air is supplied into the fur- nace 2 along with the fuel supply, the fuel is made to ignite quickly and a major part of the fuel's volatile matter can be burnt before the second combustion zone (II) . The temperature of the furnace in the upper part is thus decreased.

The air coefficient S R _V OL for volatile matter in the first combustion zone (I) is in a range of 0.9 - 1.00. In the second combustion zone (II), the total air coefficient S R _T OT is in a range of 0.75 - 0.85. In the third combustion zone (III), the total air coefficient S RTOT is about 1.15.

Figures 4a-4e show the fuel feed pipe 3 and the at least one air feed channel 4 from the inside the furnace according to the first, second, third, fourth and fifth embodiment of the present invention. The fuel feed pipe 3 comprises a first outlet 14 for supplying fuel and combustion air for volatile matter into the furnace. The at least one air feed channel 4 comprises a second outlet 15 for supplying combustion air for volatile matter into the furnace. The cross-section of the fuel feed pipe 3 may be of any shape, e.g. rectan ^¬ gular or round. The air feed channel 4 may be one con ^¬ tinuous air feed channel 4 or separate air feed chan ^¬ nels 4 on different sides of the fuel feed pipe 3.

In figures 4a and 4d, one continuous air feed channel 4 surrounds the fuel feed pipe 3 on all sides, thereby forming a second combustion air supply around the whole fuel supply. In figure 4b, one continuous air feed channel 4 surrounds the fuel feed pipe 3 on all sides except from below, thereby forming a second com ^¬ bustion air supply around three sides of the fuel sup ^¬ ply, but not below it. In figure 4c, three separate air feed channels 4 surround the fuel feed pipe 3 on all sides except from below, thereby forming a second com- bustion air supply around three sides of the fuel sup ^¬ ply, but not below it. In figure 4e, four separate air feed channels 4 surround the fuel feed pipe 3 on all sides, thereby forming a second combustion air supply around the whole fuel supply.

Figure 5 shows a schematic sectional view of the fuel feed pipe 3 and the air feed channel 4 accord- ing to one embodiment of the invention. The fuel feed ^¬ ing means 18 comprise a fuel feed pipe 3 and an air feed channel 4 around part of the length of the fuel feed pipe 3 and surrounding the fuel feed pipe 3 on all sides. The fuel feeding means 18 further comprise an inlet 8 for supplying combustion air for volatile matter along with the fuel. The fuel feed pipe 3 and the air feed channel 4 end in the first outlet 14 and the second outlet 15, respectively. Through these outlets fuel and combustion air for volatile matter are di- rected into the furnace. The inlet 8 is connected to secondary air for supplying secondary air as combustion air for volatile matter. The upper side of the fuel feed pipe 3 comprises a first opening for directing combustion air for volatile matter from the inlet 8 in- to the fuel feed pipe 3. The upper side of the air feed channel 4 comprises a second opening for directing combustion air for volatile matter from the inlet 8 into the air feed channel 4. The control dampers 11 direct 60 % of the combustion air for volatile matter from the inlet into the fuel feed pipe and 40 % of the combus ^¬ tion air for volatile matter into the air feed channel 4. The guide vanes 19 direct the air flow smoothly into the fuel feed pipe 3. The cross-sectional area of the fuel feed pipe 3 at the first outlet 14 and of the air feed channel 4 at the second outlet 15 is such that the velocity at which the combustion air for volatile mat ^¬ ter is supplied in both the fuel feed pipe 3 and the air feed channel 4 is 15 to 20 m/s . In the following, the distribution of air in the boiler furnace is described by referring to exam ^¬ ples presented in Tables 1 and 2. The tables show stage by stage the total air coefficients S R _T OT and air coef ^¬ ficient in relation to volatile matter S R _V OL in a bub ^¬ bling fluidized bed boiler in which no combustion air for volatile matter is supplied (Table 1) and in a boiler in which combustion air for volatile matter is supplied along with the fuel as described above (Table 2) when using peat or wood as fuel. The total air coef ^¬ ficient S RTOT increases in the vertical direction of the furnace as more air is supplied into the furnace.

In table 1, furnace air is supplied into the first combustion zone (I) mainly together with the flu- idizing gas as fluidizing air and in connection with the fuel supply as carrier air. The small air volume used for cooling start-up burners has only a minor ef- feet on the total air coefficient S R _T OT of the first com ^¬ bustion zone ( I ) .

Table 1. Total air coefficients S R _T OT and air coefficients in relation to volatile matter S R _V OL in a bubbling fluidized bed boiler in which no combustion air for volatile matter is supplied.

1.15 1.59 1.15 1.95

In this air distribution, the air coefficient in relation to volatile matter of the fuel in the first combustion zone (I), that is, S RVOL, is in a range of 0.65 - 0.75, whereby the combustion temperatures are low in the lower part of the furnace. Addition of sec ^¬ ondary air at the beginning of the second combustion zone (II) and addition of tertiary air at the beginning of the third combustion zone (III) clearly raise the total air coefficient S RTOT -

Table 2 shows the air distribution in a bub ^¬ bling fluidized bed boiler, where additional air taken from the secondary air register and intended for the combustion of fuel's volatile matter in the first com- bustion zone (I) is supplied into the boiler furnace along with the fuel supply. Part of the combustion air for volatile matter is supplied as mixed with the fuel supply and part of the combustion air for volatile mat ^¬ ter is supplied as surrounding at least part of the fuel supply. The air coefficient S R _V OL in relation to volatile matter in the first combustion zone (I) is kept within an optimum range for the reduction of nitrogen oxides, which range is 0.9 - 1.0. Table 2. Total air coefficients S RTOT and air coefficients in relation to volatile matter S R _V OL in a bubbling fluidized bed boiler in which combustion air for volatile matter is supplied along with the fuel.

Carrier air

0.45 0.62 0.48 0.82

Combustion air for volatile

matter 0.62 0.86 0.56 0.95

Cooling air for start-up

burners 0.65 0.90 0.59 1.00

Secondary air

0.80 1.11 0.80 1.36

Cooling air for load bearing

burners 0.83 1.15 0.83 1.41

Tertiary air

1.15 1.59 1.15 1.95

In this case, the combustion air for volatile matter supplied into the first combustion zone (I) along with the fuel supply clearly raises the total air coefficient. However, after the supply of secondary air, the total air coefficient is at the same level as in Table 1. Thus, the total air volume to be supplied into the bubbling fluidized bed boiler is the same as in the case shown in Table 1, but the air distribution is different, when in the solution according to Table 2 a part of the secondary air of Table 1 is supplied into the first combustion zone (I) along with the fuel sup ^¬ ply. It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above, instead they may vary within the scope of the claims.