FI126253B

FI126253B - Method for reducing nitrogen oxide emissions in a bubbling fluidized bed boiler and bubbling fluidized bed boiler

Info

Publication number: FI126253B
Application number: FI20155088A
Authority: FI
Inventors: Pauli Dernjatin; Jouko Heikkilä
Original assignee: Fortum Oyj
Priority date: 2015-02-09
Filing date: 2015-02-09
Publication date: 2016-08-31
Also published as: EP3256780A1; WO2016128616A1; FI20155088A

Description

METHOD FOR REDUCING NITROGEN OXIDE EMISSIONS IN A BUBBLING FLUIDIZED BED BOILER AND BUBBLING FLUIDIZED BED BOILER

FIELD OF THE INVENTION

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 stricter nitrogen oxide (N0X) 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 guantity of nitrogen oxides can be reduced e.g. by methods of combustion technology, by injecting urea or ammonia into the furnace and by catalytic 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 produced in earlier stages of combustion. Widely used post-combustion NOx-control technologies include Selective Catalytic Reduction (SCR) process and the Selective Non-catalytic Reduction (SNCR) process. Both processes reduce N0X to N2 and H2O with ammonia and urea based reagents. The operating temperatures of these two processes differ from each other. The SNCR process typically 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 temperature between 160 and 350 °C. Also, the in vestment 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/Nm3. The new limit may be achieved either by Selective Catalytic Reduction process or by combining 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 advanced 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 nitrogen 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:

(for urea)

(for ammonia)

As a result of these reactions molecular nitrogen (N2) , water vapor (H2O) and with urea carbon dioxide (CO2) are formed.

The temperature window for achieving an adequate nitrogen oxide reduction with a minimum NH3 slip is narrow. The optimum temperature range for urea and ammonia is between about 950 and 1100 °C. When the temperature 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 N2O 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. Often, the temperature in the furnace is very high, especially 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. Alternatively, nitrogen oxide reductant may be supplied in the upper part of the furnace, e.g. at the height of the furnace nose.

Mixing nitrogen oxide reductant with flue gases 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 inhibiting efficient distribution of the nitrogen oxide reductant over the entire cross-section of the injection 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 nitrogen oxide reductant in the boiler furnace is also relatively short when the nitrogen oxide reductant is injected 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 oxide 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 fluidized 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 bubbling fluidized bed boiler burning fuel. The bubbling fluidized bed boiler comprises a fluidized bed including 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 combustion in the first combustion zone (I). The method comprises: - supplying primary air into the first combustion zone (I) from under the fluidized bed for fluidizing the bed material; and - 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 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 volatile matter is supplied as a first combustion air supply mixed with the fuel supply and part of the combustion 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 supplying 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 nitrogen oxide concentration in the flue gas.

The boiler comprises a second inlet for supplying 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 surrounding 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 combustion air for volatile matter is arranged to be supplied into the furnace through the at least one air feed channel as a second combustion air supply surrounding 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 described, efficient nitrogen oxide reduction can be achieved. Nitrogen oxide emissions are reduced by a combination of combustion technology and postcombustion technology. The nitrogen oxide emissions are reduced to the level of 250 to 300 mg/Nm3 by using the combustion technology according to the invention. The nitrogen oxide emissions are further reduced to below 200 mg/Nm3 by utilizing direct injection of nitrogen oxide reductant into the furnace comprising hot flue gases. Efficient nitrogen oxide reduction by the nitrogen oxide reductant is achieved by ensuring a good temperature 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 supplying 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 reductant to reduce nitrogen oxide concentration in the flue gas. The efficiency of nitrogen oxide reduct-ants depends on the temperature of the boiler furnace at the injection level and the efficiency of mixing of the reagent with the flue gases. The desired operating temperature range for typically used nitrogen oxide reductants, 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 (Furnace Exit Gas Temperature, FEGT) is reduced. In bubbling fluidized bed boilers of furnace loads of 144 MW/m3 and 120 MW/m3, temperature of the flue gas at the furnace exit (FEGT) has been reduced by 50 to 100 °C as compared to conventional staged combustion. Consequently, 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 NH3 and nitrous oxide N2O is minimized. Thus, ammonia and nitrous oxide emissions are also reduced, thereby reducing the need for expensive systems for removing ammonia and nitrous oxide. In bubbling fluidized bed boilers of furnace loads of 144 MW/m3 and 120 MW/m3, ammonia emissions in the flue gas have been reduced to 5 mg/Nm3 by the current invention. The above result is achieved when SRVol 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.

In addition to post-combustion technology, nitrogen oxide emissions are reduced by improved combustion 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 combustion 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 according to the present invention lead to improved reduction 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 supplies. 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/m3 and 120 MW/m3, nitrogen oxide emissions have been re duced by 100 - 150 mg/Nm3, thereby reducing overall nitrogen oxide emissions by 20 - 30 % as compared to conventional staged combustion. The above results are achieved when SRVol 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 temperature 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 reduction 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 furnace 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/m3 and 120 MW/m3, 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 SRVol 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.

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 supplied 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 matter 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 particles 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 velocity 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 matter is supplied in both supplies is 10 to 25 m/s. According to CFD (Computational Fluid Dynamics) calculations, 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 combustion air for volatile matter is directed to the furnace both mixed with the fuel supply and surrounding at least part of the fuel supply, the total additional air flow needed to increase SRvol near value 1 is divided into larger cross-section of the pipe, and the velocity of the air is decreased. As a consequence, nitrogen oxide 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 optimizing 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 substance containing nitrogen and being able to reduce N0X into molecular nitrogen (N2). By injection level, the height level in the boiler furnace is meant at which the nitrogen oxide reductant is supplied. 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 30 %. In one embodiment, nitrogen oxide reduction efficiency 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 reduction 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 matter 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 matter is supplied as a second combustion air supply surrounding at least part of the fuel supply. In one embodiment, the second combustion air supply fully surrounds 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 surrounds 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 embodiment, 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 comprises wood and industrial sewage sludge. In one embodiment, 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 primary 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 efficiently 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 described in EP 2574841 A2 may be used to improve nitrogen oxide reduction. Also, the vertical supply angle and horizontal supply angle of the fuel chutes, the arrangement of secondary and tertiary air nozzles in rows including nozzles blowing a small, me dium and large air jet, and the side air nozzles placed between the outermost fuel supply chutes and the side wall described in EP 2574841 A2 may be used to improve nitrogen oxide reduction. 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 surface of the fluidized bed.

In one embodiment, the refractory lining extends to a height of 1.8 - 2.0 meters from the top surface of the fluidized bed. One way of lowering the fluidized 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 refractory 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 transferred into the water pipes. Consequently, the temperature 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 secondary 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 nozzles and extends up to below the height level of tertiary air nozzles. The third combustion zone (III) begins from the height level of the tertiary air nozzles. The first and second combustion zones (1,11) are substoichiometric. By substoichio-metric combustion it is meant that the total air coefficient SRtot is kept substoichiometric. The total air coefficient SRTot 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 combustion 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 supplying tertiary air into the third combustion zone (HI) .

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 supplied 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) simultaneously 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 installed 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 ammonia NH3 and nitrous oxide N2O are formed due to inefficient distribution and mixing of the re-ductant in the flue gas stream.

A homogeneous distribution of the nitrogen oxide reductant in the boiler furnace is difficult to obtain as flue gases are very viscous and it is therefore challenging to mix different gases. According to CFD modelling performed by the inventors, considerably better penetration and distribution is achieved when nitrogen oxide reductant is injected into the boiler furnace 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 temperature of 1,100 °C and nitrogen oxide reduction is not sufficient. When conventional combustion technologies are used, the temperature at the height level of tertiary air nozzles is typically too high for injection 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 nozzles is thus enabled. Efficient nitrogen oxide reduction 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 wider power range of boilers.

The temperature in the furnace depends on the boiler load. When the boiler load is small, it is possible to inject the nitrogen oxide reductant at the height level of secondary air nozzles. With higher boiler load, the nitrogen oxide reductant can be injected 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 mixing 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 furnace 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 nozzles. In one embodiment, the bubbling fluidized bed boiler comprises temperature sensors on at least one furnace wall at the height level of tertiary air nozzles. 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. Depending on the measured temperature at different height levels, a computer-assisted control system allows injection 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 control 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 secondary air through the secondary air nozzles. In one embodiment, the boiler load is 60 to 90 %, and 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 boiler load is 100 % and the nitrogen oxide reductant is supplied into the third combustion zone (III) above the tertiary air nozzles. In this case, nitrogen oxide reductant may be supplied at the height level of e.g. 3 to 4 meters above the height level of tertiary air nozzles. Boiler load refers to the proportion of the full furnace combustion capacity being utilized at a given time.

When nitrogen oxide reductant is injected into 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 existing 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 nitrogen oxide reductant consists of a water solu- tion 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 reduction. 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 fluidized 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 SRVol in the first combustion zone (I) is in the substoichiometric area. That is, SRVol is below 1.

The air coefficient or the stoichiometric ratio SR tells how much air must be used for the combustion in comparison with the theoretical (stoichiometric) volume of air needed for complete combustion of the fuel. In substoichiometric combustion, the air coefficient SR is under 1, and in superstoichiometric combustion the air coefficient SR is over 1.

Since SRVol is in substoichiometric area, combustion 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 combustion of volatile matter in the first combustion zone (I) . The higher the air coefficient SRVol 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 nitrogen 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 SRTot in the first combustion zone is substoichiometric.

In one embodiment the air coefficient in relation to volatile matter SRVol in the first combustion zone (I) is 0.9 - 1.0. When the air coefficient in relation to volatile matter SRVol 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 SRVol in the first combustion zone (I) is 0.95 - 1.0. The optimum air coefficient in relation to volatile matter SRVol 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 SRtot in the second combustion zone (II) is 0.75 -0.85. In one embodiment the total air coefficient SRtot in the second combustion zone (II) is 0.8. Substoichiometric conditions are thus maintained above the first combustion zone (I).

In one embodiment the combustion air for volatile matter comprises secondary air. In one embodiment the combustion air for volatile matter consists of secondary air. The amount of secondary air provided above the first combustion zone (I) is decreased correspondingly. 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 temperature 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 typically 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 subsequent pyrolysis already begin inside the fuel feed pipe. Consequently, combustion of the fuel begins earlier 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 increased. 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 velocity 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 matter 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 combustion air supply. This way, flux flow is achieved directing 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 embodiment, 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 velocity 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 simultaneously with the fuel supply through said fuel feed pipe. In one embodiment, the fuel supply and first combustion 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 simultaneously supplied into the first combustion zone (I).

In one embodiment the second inlet is connected to the secondary air for supplying secondary air at least as part of the combustion air for volatile matter. 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 matter 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 supply 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 channel. 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 volatile matter from the second inlet into the at least one air feed channel. In one embodiment, a first control damper directs 60% of the combustion air for volatile matter from the second inlet into the fuel feed pipe. In one embodiment, a second control damper directs 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 arranged to be such that the velocity at which the combustion air for volatile matter is supplied in the first combustion air supply and in the second combustion air supply is 10 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 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 combustion 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 combustion 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 matter 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 matter is determined by the amount of primary air supplied into the first combustion zone (I) so that the value of SRVol 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 combustion air for volatile matter into the first combustion zone along with the fuel supply in the manner described, the temperature of the flue gas at the fur nace 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 NH3 and nitrous oxide N2O emissions are minimized.

The invention makes it possible to inject nitrogen 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 bubbling fluidized bed boiler with a fuel power of above 300 MW can be reduced to below 200 mg/Nm3 and ammonia emissions can be reduced to 5 mg/Nm3, 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 together 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 embodiments of the invention and together with the description help to explain the principles of the invention. 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 reduct-ant 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 distribution 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 oxide 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 reductant 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 tertiary 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 furnace 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 located 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 fig ure 5. Supplying 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 material and extends from the top surface of a fluidized bed to a height of 1.8 - 2.4 meters. The height of the refractory lining 13 may be different from the fuel supply height. The height of the refractory lining is reduced as compared to typical height of refractory linings. 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 level of tertiary air nozzles 16 without the risk of too high temperature at the injection level 17. The nitrogen 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 tertiary air nozzles 16.

Three combustion zones are formed: sub-stoichiometric 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 combustion 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 furnace 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 SRVol 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 SRTot is in a range of 0.75 - 0.85. In the third combustion zone (III), the total air coefficient SRtot 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. rectangular or round. The air feed channel 4 may be one continuous air feed channel 4 or separate air feed channels 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 combustion air supply around three sides of the fuel supply, 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 combustion air supply around three sides of the fuel supply, 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 according to one embodiment of the invention. The fuel feeding 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 directed 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 into 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 combustion 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 matter 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 examples presented in Tables 1 and 2. The tables show stage by stage the total air coefficients SRTot and air coefficient in relation to volatile matter SRVol in a bubbling 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 coefficient SRtot 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 fluidizing 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 effect on the total air coefficient SRtot of the first combustion zone (I).

Table 1. Total air coefficients SRtot and air coefficients in relation to volatile matter SRVol in a bubbling fluidized bed boiler in which no combustion air for volatile matter is supplied.

In this air distribution, the air coefficient in relation to volatile matter of the fuel in the first combustion zone (I), that is, SRvow is in a range of 0.65 - 0.75, whereby the combustion temperatures are low in the lower part of the furnace. Addition of secondary 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 SRTot·

Table 2 shows the air distribution in a bubbling 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 combustion 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 matter is supplied as surrounding at least part of the fuel supply. The air coefficient SRVol 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 SRTot and air coefficients in relation to volatile matter SRVol in a bubbling fluidized bed boiler in which combustion air for volatile matter is supplied along with the fuel.

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 dis tribution 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 supply.

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.

Claims

A method for reducing nitrous oxide emissions from a flue gas in a fluidized bed boiler which burns fuel, the fluidized bed boiler comprising a bed material containing bed material (1) and a boiler furnace (2) comprising a first combustion zone (I) (2) stepwise providing ali-stoichiometric combustion in the first combustion zone (I), the method comprising: - supplying to the first combustion zone (I) primary air from below the fluidized bed (1) to fluidize the bed material; and - supplying a nitric oxide reducing agent to reduce nitrogen oxides in the boiler furnace (2) from a specific injection level (17) at which the furnace (2) temperature is such that the nitrous oxide concentration in the flue gas can be reduced by means of feeding with a portion of the volatile combustion air as the first combustion air supply to be mixed with the fuel supply and a portion of the volatile combustion air supplying at least a portion of the fuel supply with a surrounding second combustion air supply, and the rate at which the volatile combustion air is - 25 m / s.

Method according to claim 1, characterized in that the method is carried out in a sandwich boiler, in which the fluidized bed (1) has an upper surface and the boiler furnace (2) has a lower part (12) provided with refractory masonry (13) 2.4 meters above the upper surface of the fluidised bed (1).

A method according to claim 1 or 2, characterized in that the furnace furnace (2) 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 the second combustion zone (II) is supplied with secondary air through the secondary air nozzles (7) and the third combustion zone (III) is supplied with tertiary air through the tertiary air nozzles (16).

Method according to Claim 3, characterized in that the nitric oxide reducing agent is supplied to the second combustion zone (II) together with the secondary air through the secondary air nozzles (7).

Method according to claim 3 or 4, characterized in that the nitric oxide reducing agent is supplied to the third combustion zone (III) together with the tertiary air through tertiary air nozzles (16).

Method according to one of Claims 3 to 5, characterized in that the nitric oxide reducing agent is introduced into the third combustion zone (III) above the tertiary air nozzles (16).

A process according to any one of the preceding claims, characterized in that the nitric oxide reducing agent comprises aqueous ammonia, aqueous urea or ammonia in gaseous form.

Method according to one of the preceding claims, characterized in that the air coefficient SRVol of the volatile substances in the first combustion zone (I) is between 0.9 and 1.0.

Method according to one of the preceding claims 3 to 9, characterized in that the total air coefficient SRTot is in the second combustion zone (II) from 0.75 to 0.85.

A method according to any one of the preceding claims, characterized in that the combustion air of the volatile substances comprises secondary air.

A method according to any one of the preceding claims, characterized in that the first combustion air supply comprises 60-70% of the volatile material combustion air and the second combustion air supply comprises 30-40% volatile material combustion air.

Method according to one of the preceding claims, characterized in that the fuel is supplied to the first combustion zone (I) via the fuel supply pipe (3) and the first combustion air supply is mixed with the fuel supply and fed to the boiler furnace (2) simultaneously with the fuel supply pipe (3). through.

Method according to claim 12, characterized in that the second combustion air supply is supplied to the first combustion zone (I) from at least one air supply duct (4) arranged at least a portion of the length of the fuel supply pipe (3) and surrounding at least a portion of the fuel supply pipe (3). .

Method according to one of the preceding claims, characterized in that the first combustion air supply and the second combustion air supply are simultaneously supplied to the first combustion zone (I).

A fluidized bed boiler comprising a bed of fluid containing bed material (1), a firebox (2) of the boiler, comprising walls of the furnace (5) and a first combustion zone (I), primary air nozzles (6) below the fluidized bed (1). for supplying air to the first combustion zone (I) for fluidizing the bed material; at least one fuel feed pipe (3) on at least one combustion chamber wall (5) in the first combustion zone (I) for supplying fuel to the fluidized bed (1), the at least one fuel feed pipe (3) comprising a first outlet (14) and a first inlet (20) (5) for supplying a nitric oxide reducing agent to reduce nitrogen oxides in the boiler fire chamber (2) from a certain injection level (17) at which the temperature of the fire chamber (2) is such that nitrogen gas oxide reducing agent in the flue gas can be reduced; 8) for supplying combustion air of volatiles to the first combustion zone (I) together with the fuel supply, the boiler comprising at least one air supply duct (4) about at least a portion of the length of the fuel supply pipe (3) and venting at least a portion of the fuel supply line (3), the at least one air supply channel (4) comprising a second outlet (15); a portion of the volatile combustion air is provided for supply to the boiler furnace (2) through a fuel supply line (3) as a first combustion air supply to be mixed with the fuel supply and a portion of the volatile combustion air is provided to the furnace (2) through at least one air supply the supply as a surrounding second supply of combustion air; and the cross sectional area of the fuel supply pipe (3) at the first outlet (14) and the cross sectional area of the at least one air supply channel (4) at the second outlet (15) is arranged such that the rate of volatile combustion air input at the first combustion air supply and - 25 m / s.

A fluidized bed boiler according to claim 15, characterized in that the fluidized bed (1) has an upper surface and the boiler furnace (2) has a lower part (12) provided with a refractory masonry (13) extending from 1.8 to 2, 4 meters above the upper surface of the fluidised bed (1).

A fluidized bed boiler according to claim 15 or 16, characterized in that the fluidized bed boiler 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 a secondary combustion zone (II). air nozzles (7) on at least one furnace wall (5) above the first combustion zone (I) for supplying secondary air to the second combustion zone (II) and tertiary air nozzles (16) on at least one furnace wall (5) above the second combustion zone (II) for supplying air to the third combustion zone (III).

A fluidized bed boiler according to claim 17, characterized in that the first inlet (20) is at least one of the secondary air nozzles (7) for feeding the nitric oxide reducing agent to the second combustion zone (II) together with the secondary air.

A fluidized bed boiler according to claim 17 or 18, characterized in that the first inlet (20) is at least one of the tertiary air nozzles (16) for supplying the nitric oxide reducing agent to the third combustion zone (III) together with the tertiary air.

Layer fluidized bed boiler according to one of claims 17 to 19, characterized in that the first inlet (20) is located at least one of the furnace walls (5) above the tertiary air nozzles (16) for feeding the nitric oxide reducer to the third combustion zone (III) .

A fluidized bed boiler according to any one of claims 17 to 20, characterized in that the second inlet (8) is connected to the secondary air for supplying the secondary air at least as a part of the combustion air of the volatile substances.

A fluidized bed boiler according to any one of claims 15 to 21, characterized in that the fuel supply pipe (3) comprises a first opening (9) for directing volatile combustion air from the second inlet (8) to the fuel supply pipe (3) as the first combustion air supply (4) comprising a second opening (10) for controlling the combustion air of the volatiles from the second inlet (8) to the at least one air supply channel (4) as a second supply of combustion air.

A fluidized bed boiler according to claim 22, characterized in that at least one of the first and second openings (9,10) comprises at least one adjusting plate (11) for controlling 60-70% of the combustion air of the volatile substances from the second inlet (8). 3) and 30 - 40% of the volatile matter combustion air from the second inlet (8) to at least one air supply duct (4).

Use of a nitric oxide reducing agent for reducing nitrogen oxide and ammonia emissions in the bed fluidized bed state of any one of claims 15 to 23.

Use according to claim 24, characterized in that the nitric oxide reducing agent comprises aqueous ammonia, aqueous urea or ammonia in gaseous form.