FIELD OF THE INVENTION

The present disclosure relates to a method of NO _x reduction in a circulating fluidized bed boiler. It further relates to a circulating fluidized bed boiler and to use thereof.

BACKGROUND OF THE INVENTION

Fluidized bed boilers are used for producing heat and electricity by combusting solid fuels, such as coal, peat, wood chips, wood processing by-products and community waste, as well as from a range of other bio- fuels. Typically, the fuels used in fluidized bed boil ^¬ ers have relatively high water content and can be con ^¬ sidered difficult to combust. Further, the size distri- bution of the fuel particles, as well as heat value be ^¬ tween fuels, may vary.

There are two types of fluidized bed boilers, namely bubbling fluidized bed (BFB) boilers and circu ^¬ lating fluidized bed (CFB) boilers. In both boiler types, the fuel material is combusted in a furnace us ^¬ ing inert particulate bed material through which air is blown to fluidize it. This increases the rate of heat transfer in the furnace and stabilizes the combustion process. While in a BFB furnace, the bed material re- mains in the lower part of the furnace, in a CFB fur ^¬ nace, a part of the bed material circulates within the furnace. Some of it is carried to the upper part of the furnace and even out of it with flue gas. Therefore, to prevent loss of the bed material, and the uncombusted fuel material possibly carried out with it, a separa- tion cyclone is used to separate flue gas from solids and the solids are returned to the furnace.

The CFB boilers are more flexible than the BFB boilers in terms of the fuel used. Different fuels can be used simultaneously, shifting from one fuel to an ^¬ other can be done directly and a wider range of fuels can be used. Thus they are the most promising alterna ^¬ tive for multi-fuel combustion, which is advantageous as the price and availability of different fuels varies in many locations.

Current CFB furnaces comprise primary air in ^¬ lets at the bottom of the furnace, through which prima ^¬ ry air is blown into the bed material from below thus fluidizing it. The primary air velocity is at least 3-5 m s ^-1 in a CFB furnace, which is sufficient to distrib ^¬ ute the bed material throughout the furnace. The rela ^¬ tively even distribution and high velocities of the bed material in a CFB furnace result in efficient mixing and heat transfer of the fuel and bed material, as well as in an even temperature distribution within the furnace. The combustion temperature of the fuel in a CFB furnace is typically between 800 °C and 900 °C. The fuel retention time during combustion is longer for CFB furnaces than for the BFB furnaces, adding to the effi ^¬ ciency of the combustion. The particles carried out of the furnace with the gas flow are separated in a cy ^¬ clone, from which they are fed back into the lower part of the furnace through a return pipe.

The main source for the combustion reactions is the so-called secondary air, which is fed into the furnace some meters above the primary air feeding lev ^¬ el. Also the fuel is added to the furnace in the lower part of the furnace, above the level at which the bed material is fluidized. Either a dedicated feed pipe or the return pipe from the cyclone can be used for fuel feeding . Like all industrial combustion processes, the combustion reactions in CFB boilers produce harmful substances, whose release into the environment has to be restricted. The harmful substances include NO _x (NO and NO ₂ ) derived from the nitrogen contained in the fuel. Their relative amounts depend on the type of boiler and on the fuel used. CFB furnaces generally produce low amounts of NO _x (currently in the range of 300-500 mg m ^~3 n) , but the tightening environmental regu- lations pose a challenge for all combustion-based ener ^¬ gy production in terms of reduction of NO _x emissions from current levels.

In the early stages of the combustion process, it is advantageous to keep the amount of oxygen availa ^¬ ble in the air fed into the furnace sub-stoichiometric relative to the combustible carbon in the fuel. This results in the formation of hydrocarbon radicals, which scavenge the NO _x species inevitably formed during com- bustion and reduce them to molecular nitrogen. This is effective in reducing the NO _x emissions of the combus ^¬ tion. However, to ascertain the complete combustion of the fuel, the oxygen-to-fuel stoichiometry has to be increased to slightly above 1, i.e. to a super- stoichiometric range.

Also post-combustion NO _x reduction is used to further reduce the NO _x emissions. At the moment, the most cost-effective method for the post-combustion NO _x reduction is the so-called selective non-catalytic re- duction (SNCR) . In SNCR, NO _x are reduced into molecular nitrogen (N ₂ ) using nitrogen-containing reductants, typ ^¬ ically water solutions of either ammonia or urea. The general reactions for nitrogen monoxide are

NH2CONH2 + 2 NO + ½ 0 ₂ → 2 N ₂ + C0 ₂ + 2 H ₂ 0 and 4 NH ₃ + 4 NO + 0 ₂ → 4 N ₂ + 6 H ₂ 0

for urea and ammonia, respectively. The temperature window for SNCR is 900-1,100 °C, preferably 950-1,050 °C. In temperatures below 900 °, the NO _x reduction takes place too slowly to be effec ^¬ tively applied and the reductants can escape from the furnace turning into unwanted emissions themselves. This poses a challenge for utilizing SNCR in CFB fur ^¬ naces, as the temperatures in them usually remain below or at the threshold value of 900 °C, especially when the CFB boiler does not function at full capacity.

Patent document CN 103721552 discloses a meth ^¬ od for implementing SNCR denitration during low loading of a CFB boiler. The method comprises secondary air and a reducer spray gun for implementing SNCR, wherein the secondary air is arranged on an upper layer and a lower layer. The distance between the two layers of secondary air is 3-5 m and the secondary air on the upper layer accounts for 20-40 % of the total air. The spray gun is arranged in the secondary air on the upper layer. The method solves the problem that a denitration effect is poorer when the smoke temperature at an inlet of a cy ^¬ clone separator is under 800 °C at low loading conditions of the boiler. The solutions presented in prior art do not reduce the NO _x emissions below the limits required by the tightening future environmental protection stand ^¬ ards. The inventors have therefore recognized the need to further reduce the NO _x emissions in a CFB furnace.

PURPOSE OF THE INVENTION

The purpose of the present disclosure is to solve, or at least to alleviate, problems related to prior art solutions. Especially, it is the purpose of the current disclosure to present a method and an appa ^¬ ratus for reducing the NO _x emissions of CFB furnaces. SUMMARY

The method according to the present disclosure is characterized by what is presented in claim 1.

The CFB boiler according to the present dis- closure is characterized by what is presented in claim 22.

The use according to the present disclosure is characterized by what is presented in claim 31. The method and device according to the present disclosure may offer at least one of the following ad ^¬ vantages: The formation of NO _x species is reduced during combustion and their concentration in the flue gas is reduced. Another advantage of the method and device ac- cording to the present disclosure is that if post- combustion removal of NO _x is performed by selective-non- catalytic reduction (SNCR) , the NO _x concentration in the flue gas can be further reduced. This is due to the more amenable temperature range for SNCR both at high and low boiler loading conditions.

Furthermore, the more optimal temperature range and the ensuing increased efficiency of SNCR, re ^¬ duces the risk of reductants turning into unwanted emissions. Additionally the tightening future NO _x emis- sion standards can be reached with the cost-effective SNCR method without the need for more expensive cata ^¬ lytic methods under all boiler loading conditions.

A further advantage of the method and the de ^¬ vice according to the present disclosure is that the fuel can be combusted more efficiently. If the fuel is mixed with the secondary air before feeding into the furnace, the increased efficiency of fuel combustion can be more pronounced. 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:

Figure 1 is a schematic presentation of an em ^¬ bodiment of a method and a CFB furnace according to the present disclosure.

Figure 2 is a schematic presentation of sec ^¬ ondary air and a fuel feeding into the furnace accord ^¬ ing to the present disclosure. DETAILED DESCRIPTION

A circulating fluidized bed (CFB) boiler is a combustion device meant for combusting many types of solid fuels, such as coal, peat, wood chips, wood pro ^¬ cessing by-products, as well as community waste, sewag- es sludge and oil-based fuels. The fuels can be used as different mixtures with variable proportions. Combus ^¬ tion inevitably results in the formation of nitrogen oxides, NO and O ₂ , which are collectively referred to as NO _x . They are considered pollutants and their concen- tration in the flue gas released from a CFB boiler should be minimized.

In one aspect, a method for reducing NO _x emis ^¬ sions in a circulating fluidized bed boiler (CFB boil ^¬ er) , the CFB boiler comprising a furnace, the furnace comprising a bottom and fluidizable bed material, is disclosed. The method comprises simultaneously or in sequence in any order

feeding primary air from below through the bottom of the furnace and the bed material into the furnace for fluidizing the bed material and entrapping at least a part of the bed material for circulating the bed material in the furnace;

feeding secondary air into the furnace at a height above the primary air feeding, for sub- stoichiometric combustion of fuel; and

feeding fuel into the furnace at a height above the primary air feeding height for combusting the fuel. The method is characterized in that the method further comprises

- feeding over-fire air (OFA) into the fur ^¬ nace at a height above the secondary air feeding, for super-stoichiometric combustion of the fuel.

The CFB boiler comprises a furnace, in which the combustion reactions take place. The furnace is usually between 20 and 50 meters in height and its cross section is a rectangle. As is known in the art, a CFB boiler comprises a range of additional instrumenta ^¬ tion for regulating the combustion conditions and for capturing the energy released from the combustion. For the combustion to function efficiently, a CFB boiler contains fluidizable bed material within the furnace. The bed material has a high thermal capacity, it is in ^¬ ert and durable enough to withstand the conditions in the furnace. It has also suitable density and grain size for fluidization . For example quartz sand is used as the bed material.

When fuel is combusted in a CFB boiler, air is blown from the bottom of the furnace to fluidize the bed material and to circulate it within the furnace. The air blown through the bed material is called prima ^¬ ry air. Its velocity is approximately 3-5 m s ^-1 . The mass flow of the primary air needs to be kept high enough to fluidize the bed material.

At a height above the primary air feeding, sec- ondary air is fed into the furnace. Since the primary air is fed through the bottom of the furnace, all heights relative to the primary air feeding are the same relative to the bottom. However, it is possible for the furnace bottom to have variable shape and therefore the level of primary air feeding is decisive. Further, if primary air is fed from two or more levels, the lowermost level is to be considered the height of primary air feeding.

In a typical situation, the height of secondary air feeding is 2-10 meters above the primary air feed ^¬ ing. The secondary air feeding height can be, for exam- pie, 4-8 m above the height of primary air feeding. For example, the secondary air can be fed into the furnace above the height of the fuel feeding or below the height of the fuel feeding. It is possible to divide the secondary air feeding in a vertical direction. Thus, for example, some secondary air can be fed into the furnace 2-6 m above the fuel feeding, while some secondary air is fed at the same height as the fuel or some meters, for example 2-6 meters, below the height of fuel feeding. The main purpose of the secondary air is to provide oxygen for the combustion reactions. How ^¬ ever, the air current created by the secondary air also contributes to the material flow within the furnace. The direction of the secondary air fed into the furnace is substantially horizontal or downwards from horizon- tal . The angle of the secondary air fed into the fur ^¬ nace can be, for example 20-30° downwards from horizon ^¬ tal. The velocity at which the secondary air is fed in ^¬ to the furnace needs to be high enough for the second ^¬ ary air to thoroughly mix with the fuel and bed materi- al . Both the primary air and the secondary air provide oxygen for the combustion reactions in the furnace. By the velocity of the air being fed into the furnace is herein meant the velocity of the air at the end point of the channel leading to the furnace.

The stoichiometry of the combustion reactions is generally described by the air coefficient (SR) of the combustion reaction. If the air coefficient is 1, the combustion is stoichiometric. I.e. the theoretical amount of oxygen provided into the furnace would be just enough to oxidize the fuel completely. If the air coefficient is below 1, there is less oxygen than oxi- dizable fuel. In this case, the combustion is sub- stoichiometric . At air coefficient values above 1, the combustion is super-stoichiometric . In this case, there is excess oxygen present in relation to the combustible fuel. It should be noted that fuels vary in the amount of combustible components contained in them. Therefore, the amount of air needed for completely combusting a unit of given fuel, has to be determined empirically, as is known to the skilled person. The air coefficient can be calculated separate ^¬ ly for primary air (SRi) and for secondary air (SR ₂ ) . However, as both contribute to the combustion reac ^¬ tions, usually their sum (SR ₁ +SR ₂ ) is used to describe the combustion stoichiometry in a CFB furnace. In the method according to the present disclosure, the combus ^¬ tion reactions depending on the primary and secondary air have a sub-stoichiometric air-to-fuel ratio. This means, that SR ₁ +SR ₂ is less than 1. In other words, there is less oxygen available from primary and second- ary air than would be required for the fuel to be com ^¬ busted completely. In one embodiment, a combined air coefficient of the primary air and secondary air (SR ₁ +SR ₂ ) is 0.5-0.95, preferably 0.8-0.9.

Without limiting the current disclosure to any specific theory, sub-stoichiometric combustion can be advantageous as it increases the rate at which the fuel is combusted, since the fuel is not "diluted" with ex ^¬ cess air. This improves the heat generation and conse ^¬ quently the combustion efficiency of the CFB boiler. Further, the sub-stoichiometric combustion may increase the concentration of hydrocarbon radicals produced during the combustion. As the hydrocarbon radicals are very reactive, they may function as NO _x scavengers in the furnace thus reducing the amount of NO _x present in the flue gas .

Since the air coefficient is observed for as- certaining efficient combustion reactions in the furnace, the amount of air needs to be regulated in re ^¬ sponse to the boiler load. Boiler load refers to the proportion of the full boiler combustion capacity being utilized at a given time. The amount of air needed for achieving the targeted air coefficient decreases with boiler load. At the same time, there is a lower limit for the mass flow of primary air needed to fluidize the bed material and to circulate it within the furnace. Therefore, with some boiler loads and/or some fuels, it might be beneficial to feed recirculated flue gas into the primary air. Since the recirculated flue gas con ^¬ tains less oxygen than ambient air, the amount of oxy ^¬ gen brought into the furnace by a given mass flow of primary air can be adjusted. Mass flow of primary air can thus be increased without corresponding increase in the amount of oxygen, affecting the air coefficient. Conversely, with a given mass flow, less oxygen can be fed into the furnace. In one embodiment, the primary air comprises recirculated flue gas for adjusting the air coefficient of the combustion. By recirculated flue gas is herein meant gas conveyed from any position af ^¬ ter the furnace to the primary air feeding system. The recirculated flue gas can be collected from, for exam ^¬ ple exit chute, cyclone or flue gas outward piping af- ter the cyclone. If the flue gas is to be recirculated, it is within the knowledge of the skilled person to se ^¬ lect an appropriate position for the recirculation and to design the necessary equipment for the recircula ^¬ tion.

Fuel is fed into the furnace at a height above the primary air feeding. As is known in the art, lime or other additives can be incorporated into the fuel at or before feeding the fuel into the furnace. Typically, the height of fuel feeding is 1-5 m above the height of primary air feeding. The height of fuel feeding can be the same as the height of secondary air feeding. Alternatively, the secondary air can be fed into the furnace 1-6 meters above or below the fuel feeding. The fuel can be fed into the furnace from one or more dedicated inlets. Alternatively, the same pipe can be used for returning the solids separated from the flue gas in the cyclone and for feeding the fuel into the furnace.

In the method according to the present disclo ^¬ sure, air is fed into the furnace also at a height above the secondary air feeding. This air, termed over- fire air (OFA) , has the main purpose of increasing the total air coefficient ( SR _to t ) in the furnace to a value above 1. A SR value can be calculated for the OFA (SR _0FA ) · By SRtot is herein meant the sum of air coeffi- ciencies of all the air fed into the furnace, i.e. SR _t ot= SRi +SR2+ SRoFA- Thus, feeding OFA results in a super- stoichiometric combustion in the furnace. However, it is possible that the OFA has also other functions, such as participating in the mass flow and/or in generating beneficial heat exchange conditions in the furnace.

Although sub-stoichiometric combustion has the benefits of faster reaction rates and decreased release of ΝΟχ, the total combustion in a CFB furnace should be super-stoichiometric . The reason is that in practice, combustion in a furnace does not take place under ideal conditions and thus remains incomplete even if SR ot is 1. The products of incomplete combustion, such as car ^¬ bon monoxide, contain energy and thus decrease the ef ^¬ ficiency of the furnace. They also present a hazard as pollutants and as potentially flammable components within the gas flow system of the CFB boiler. By raising the SR _to t to a value above 1 by feeding OFA, the complete combustion of the fuel is as ^¬ certained. As much of the fuel energy as possible is thus captured and the flue gases contain only minimal amounts of uncombusted material. By material is herein meant all substances, irrespective of their presence in gaseous, solid or liquid form, present in the furnace. In one embodiment, a total air coefficient ( SRtot ) of 1.1-1.4, preferably approximately 1.2 is achieved by the OFA feeding.

Taken together, the method according to the present disclosure combines the benefits of both sub- stoichiometric and super-stoichiometric combustion pro ^¬ ducing as little NO _x as possible but at the same time combusting the fuel substantially completely.

The air fed into the furnace can be pre ^¬ heated. The remaining heat of the flue gas after en ^¬ trapping most of it through superheater ( s ) and econo- mizer(s) can be used for heating primary air, secondary air and/or OFA by, for example, gas-gas heaters. The temperature of the primary air, the secondary air and/or OFA is typically 150-250 °C. In some applica ^¬ tions, however, values of 300 °C or above, such as 350 °C can be envisaged.

In one embodiment, feeding primary air and sec ^¬ ondary air results in the formation of a sub- stoichiometric combustion zone in the furnace between the primary air feeding height and the OFA feeding height; and feeding OFA results in the formation of a super-stoichiometric combustion zone above the OFA feeding height. In such an embodiment of the method ac ^¬ cording to the present disclosure, the OFA feeding is positioned so that two combustion zones form in the furnace in a vertical direction. Since the nitrogen re ^¬ maining in the organic compounds forms NO _x when super- stoichiometric combustion takes place, it is advanta ^¬ geous to keep the combustion sub-stoichiometric as long as possible. However, the combustion needs to be super- stoichiometric for a sufficient period to attain com- plete enough combustion before the material exits the furnace .

The lower combustion zone extends between the primary air feeding and the OFA feeding. In the lower combustion zone, the combustion is sub-stoichiometric. The oxygen used for combustion in this combustion zone comes from primary air and secondary air and the air coefficient is SR ₁ +SR ₂ . The second combustion zone ex ^¬ tends above the height of OFA feeding. In this combus ^¬ tion zone, the combustion is super-stoichiometric and the fuel is substantially completely combusted. The flue gases - together with some of the bed material and other solids - exit the furnace at the top end of this second combustion zone. This means that to exit the furnace, the fuel has to cross the super-stoichiometric combustion zone. This again increases the likelihood of complete combustion of even the more resilient fuel components .

It should be understood that the border between the two combustion zones is not clear-cut. The material in the furnace is in continuous motion and therefore, some sort of a gradient of combustion stoichiometry and/or oxygen concentration might be observed in the furnace. Further, the location of the border may depend on many parameters, such as boiler load, the velocity, location and direction of the OFA entering the furnace, as well as the general material streams in the furnace.

Despite the sub-stoichiometric overall combus ^¬ tion stoichiometry at heights below the OFA feeding, increased furnace temperatures can surprisingly be achieved in the lower part of the furnace. Without lim ^¬ iting the current disclosure to any specific theory, the oxygen available in the lower part of the furnace might be utilized more efficiently as the fuel is mixed with secondary air, and thus spread more evenly across the horizontal area of the furnace. Thus, not only the secondary air, but also the primary air is more effi- ciently used for early combustion. For these reasons, the fuel ignites earlier, as it both heats up faster and oxygen is available a larger portion of the fuel (the fuel is not clumped) .

Therefore, according to the current disclosure, although the total combustion remains sub- stoichiometric below the OFA feeding height, the air and fuel mixing might lead to the formation of local ^¬ ized volumes in which the combustion might actually be super-stoichiometric . While the fuel is igniting, there might momentarily be more air available for the com ^¬ busting material. The better early combustion of the fuel, again, leads to higher temperature in the lower part of the furnace, which leads to earlier release of nitrogen compounds from the fuel into the gas phase. Thus, the lower part of the furnace might contain lo ^¬ calized super-stoichiometric volumes in which NO _x for ^¬ mation might actually be increased. However, as the ma ^¬ jority of the furnace volume below OFA feeding remains sub-stoichiometric, the formed NO _x are likely to be re- duced to molecular nitrogen before they reach the su ^¬ per-stoichiometric height (above OFA feeding level) .

Often, the OFA feeding takes place substantially higher in the furnace than the secondary air and fuel feeding. In one embodiment, the OFA feeding height is 20-40 m, preferably 25-35 m, more preferably 28-32 m above the secondary air feeding height. Without limit ^¬ ing the current disclosure to any specific theory, when there is vertical space in the range of tens of meters between the secondary air and OFA feeding, the conditions might be amenable for the formation of the two combustion zones detailed above. The residence time of fuel in a furnace can be used to measure the furnace throughput. The residence time can be to some degree proportional to the effi ^¬ ciency of combustion. As a parameter describing the furnace function, residence time is independent of the specific measures of the furnace. In most CFB furnaces, a residence time of 3 seconds can be considered the minimum time needed for the sub-stoichiometric combus ^¬ tion processes to be completed. However, the exact res- idence time depends on many aspects of the combustion process and is determined empirically. In one embodi ^¬ ment, the OFA is fed into the furnace at a height, which gives a fuel residence time of at least 3 sec ^¬ onds, preferably 4-5 seconds, for reaching the OFA feeding height.

In one embodiment, the CFB boiler further comprises an exit chute and the OFA is fed into the fur ^¬ nace 2-8 m below the exit chute of the furnace. By an exit chute is herein meant an arrangement of at least one opening in the CFB furnace wall through which the flue gas leaves the furnace. The exit chute can com ^¬ prise one opening or two or more openings. As is typi ^¬ cal for CFB boilers, the exit chute leads into a sepa- ration cyclone, in which flue gas is separated from solids. At least a part of the solids is returned to the furnace by a return pipe. By solids is herein meant all the solid particles in the furnace. The flue gas can comprise some solid particles, typically at the small end of the particle size range present in the furnace. Such particles may be removed at the later stages of flue gas processing. The design of an appro ^¬ priate exit chute for a given application is within the knowledge of the skilled person. A distance of 2-8 me- ters between OFA feeding and flue gas exit from the furnace can be considered sufficient to allow the fuel combustion to be complete. The sub-stoichiometric combustion by primary and secondary air combined with OFA feeding has a surprising effect on the temperature of the CFB furnace. The temperature in the furnace might increase by tens of degrees Celsius compared with known CFB furnaces. Furnace temperatures above 900 °C might be achieved. Even temperatures above 1,000 °C are achievable. With ^¬ out limiting the current disclosure to any specific theory, the sub-stoichiometric combustion by primary and secondary air might accelerate the initial combus ^¬ tion in the furnace below the level of OFA feeding. In addition to this, the height at which the fuel is com ^¬ pletely combusted might be raised compared to prior art furnaces. This could allow the combustion energy to be more equally distributed in the furnace, which again can be manifested as an increase in the furnace temper ^¬ ature. In one embodiment, feeding OFA results in a com ^¬ bustion temperature of at least 900 °C, preferably of 950-1,050 °C in the majority of the furnace volume. There may be areas of the furnace in which the tempera ^¬ ture remains below 900 °C. However, in the method ac ^¬ cording to the present disclosure, the proportion of the furnace, in which the temperature is above 900 °C is large enough to affect the combustion processes in the furnace. For example, at least 50 %, preferably at least 80 % of the furnace volume can have a temperature above 900 °C. For example, at least 50 %, preferably at least 80 % of the furnace volume can have a temperature of 950-1,050 °C.

It is beneficial to mix the OFA fed into the furnace with the material in the furnace as fast and efficiently as possible. One of the parameters influ ^¬ encing the efficient mixing of the OFA with the materi- al present in the furnace is the velocity at which the OFA is fed into the furnace. The minimum velocity giv ^¬ ing a sufficient mixing in most CFB boiler applications can be considered to be 50 m s ^~ . However, the entry ve ^¬ locity of OFA cannot be increased indefinitely due to increased energy consumption and the abrasive effects of the bed material and fuel particles on the internal components of the furnace. In one embodiment, the ve ^¬ locity of the OFA feeding is 50-100 m s ^-1 , preferably 60-90 m s ^"1 .

The direction of the OFA entry into the furnace is another parameter having an effect on the mix- ing of the OFA. The inventors have found out that hori ^¬ zontal direction or a direction downwards from horizontal may have a beneficial effect on the OFA mixing. Without limiting the current disclosure to any specific theory, it might be that such a direction promotes the formation of two combustion zones in the furnace. It is also possible, that when fed in a horizontal direction or downwards from horizontal, the OFA is not captured too early into the upward currents present in the fur ^¬ nace. In one embodiment, the OFA is fed into the fur- nace in a horizontal direction, or in a direction at an angle of 20-40°, preferably of approximately 30° down ^¬ ward from horizontal.

Yet another parameter affecting the mixing of the OFA with other material present in the furnace is the geometry of the OFA feeding into the furnace. In one embodiment, the OFA is fed into the furnace through nozzles located at least at two walls, preferably at least at three walls of the furnace, each said wall comprising at least one, preferably at least two noz- zles. There are many types of nozzles known in the art for feeding air into a CFB furnace. Selecting a suitable alternative is within the knowledge of the skilled person. By a nozzle is herein meant an inlet shaped to create a jet stream of gas or liquid towards the fur- nace interior. The opening in the nozzle can be of any shape. The stream of gas or liquid emitted from the nozzle can be directed. It is possible to feed OFA through nozzles that are placed on all walls of the furnace. It is pos ^¬ sible that some walls have more nozzles than some other walls. This might be the case, for example, if the walls of the furnace are of variable width. Practical reasons might lead into one or more wall not having any nozzles. The nozzles can be placed at regular intervals or their distance from each other can vary. For example, it is possible that one furnace wall has six, eight or ten nozzles and the two walls adjacent to the said wall have two, three or four nozzles each. Another alternative would be that two walls opposite to each other have several nozzles, for example eight nozzles, each .

Without limiting the current disclosure to any specific theory, it might be beneficial for the nozzles on each wall to be approximately at the same height. For example, all nozzles could be positioned within two meters, preferably one meter, along the furnace wall in the vertical direction. Further, it might be possible that the nozzles in a given wall are arranged in two or more rows. It might also be possible that the nozzles on a given furnace wall are staggered on two vertical levels. Alternatively, all nozzles can be on the same height.

In one embodiment of the method according to the present disclosure, nitrogen reductant is fed into the furnace for reducing NO _x through selective non- catalytic reduction (SNCR) . By nitrogen reductant is herein meant any substance containing nitrogen and being able to reduce NO _x into molecular nitrogen (N ₂ ) . By SNCR is herein meant selective non-catalytic reduction. It is a temperature-dependent process for reducing NO _x into N ₂ . For SNCR to be of practical relevance in reduc ^¬ ing ΝΟχ levels in the flue gas, a sufficient temperature is required. The temperature should be at least 900 °C under all boiler loading conditions. However, the temperature should not exceed 1,100 °C since the nitrogen reductant will start reacting with oxygen at about this temperature, possibly forming NO _x . Preferably, the tem- perature should be maximally 1,050 °C. The effective utilization of SNCR allows the scavenging of NO _x so that the Οχ concentration in the flue gas can be further reduced. The utilization SNCR in combination with the OFA feeding produces a beneficial cumulative effect in the ΝΟχ reduction. This is due to the favorable temperature range achievable with the OFA feeding according to the present disclosure.

In one embodiment, the nitrogen reductant is water solution of ammonia, or water solution of urea. Without limiting the current disclosure to any specific theory, both ammonia and urea are able to reduce NO _x species into molecular nitrogen. The selection of the reductant depends on the process parameters in each CFB boiler. The selection of a suitable reductant for ac- complishing SNCR in each case is within the knowledge of the skilled person. The process parameters need to be further optimized in each application in terms of, for example, ammonia or urea concentration, possible pre-heating of the ammonia or urea solution etc. Such adjustment is within the competence of the skilled per ^¬ son .

Since OFA feeding is connected to the change of the combustion stoichiometry in the furnace to su- per-stoichiometric, also the production of NO _x species might be enhanced at this height and above it. The fur ^¬ nace may comprise dedicated inlets for feeding the ni ^¬ trogen reductant into the furnace. Alternatively, the same inlets used for the OFA feeding can be used for feeding nitrogen reductant into the furnace. It is pos- sible to feed all the nitrogen reductant into the fur ^¬ nace through the inlets used for OFA. Alternatively, part of the nitrogen reductant may be fed into the fur- nace through the inlets used for OFA. It is also possi ^¬ ble to utilize all of the OFA inlets for feeding nitro ^¬ gen reductant into the furnace. Alternatively, only some of the OFA inlets can be used for feeding the ni- trogen reductant into the furnace. The nitrogen reduct ^¬ ant can be mixed with the OFA before the entry into the furnace. Both the OFA and the nitrogen reductant can be heated before feeding in to the furnace. The optimal feeding configuration in terms of velocity of feeding, the proportion of common inlets is to be determined for each application.

In embodiments where the OFA is sprayed into the furnace, the nitrogen reductant may be sprayed along with it. In one embodiment, at least part of the nitrogen reductant is sprayed into the furnace along with the OFA. Without limiting the current disclosure to any specific theory, such a feeding method might improve the dispersion and penetration of the nitrogen reductant in the furnace. Further, the droplet size of the nitrogen reductant can be adjusted. Depending on the nitrogen reductant, as well as on the furnace com ^¬ bustion and material flow parameters, a smaller or larger droplet size might be preferable. It is within the knowledge of the skilled person to optimize the droplet size of the nitrogen reductant according to the relevant parameters.

It is possible to monitor the furnace tempera ^¬ ture on-line. The on-line measurement of temperature can be utilized in the optimization of nitrogen reduct- ant feeding. The nitrogen reductant can be fed into the furnace only in locations where the temperature is suf ^¬ ficiently high to allow efficient SNCR. Nitrogen re ^¬ ductant can be fed only through those inlets around which the temperature is sufficiently high for SNCR. The inlets can be computer-controlled and individually responsive to temperature changes. The on-line regula ^¬ tion of nitrogen reductant feeding can be implemented both in cases where there are dedicated inlets the for nitrogen reductant and in cases where nitrogen reduct- ant is fed through the OFA inlets. It might bring further benefits for the com ^¬ bustion process, if the method according to the present disclosure comprises mixing the fuel with at least some secondary air before feeding the fuel into the furnace. In one embodiment, fuel is mixed with at least some secondary air before feeding the fuel into the furnace. When fuel is mixed with at least some secondary air, the mixed fuel and air are fed into the furnace with a velocity of at least 12 m s ^-1 . The velocity may be, for example 15 to 20 m s ^-1 . Also higher velocities, such as 25 to 40 m s ^-1 are possible. As an example, fuel and second secondary air may enter the furnace at a veloci ^¬ ty of 30 m s ^-1 . The velocity allows the mixed fuel and secondary air to fly towards the center of the furnace and mix efficiently with the bed material and primary air.

When fuel enters the furnace, it is entrapped in the upwards-flowing air current. In a CFB furnace, the upwards flow velocities are high enough to entrap fuel soon after it enters the furnace. This means that a substantial portion of the fuel might fly upwards in the vicinity of the furnace walls. When the sulphur- containing and chlorine-containing substances possibly contained in the fuel are vaporized, corrosive com ^¬ pounds, such as ¾S and HC1, respectively are released. This might cause corrosion of the furnace wall compo ^¬ nents. Thus, advantageously, fuel flows upwards and is combusted at a distance from the side walls of the fur ^¬ nace. If fuel is mixed with secondary air, a larger proportion of fuel might be carried away from the walls towards the center of the furnace before being en ^¬ trapped in the upwards-flowing current. This might be due to the velocity of air and fuel entering the fur- nace . In other words, less fuel remains in the vicinity of the side walls, if fuel is mixed with at least some secondary air. The larger the proportion of fuel residing at a distance from the furnace walls, the less con- tact the potentially corrosive compounds have with the side walls of the furnace. Thus, if fuel is mixed with secondary air, furnace corrosion might be reduced.

In a typical situation, the air is fed into the furnace through a fuel pipe. The air is fed into the fuel pipe, for example one meter, preferably at least two meters before the entry point into the fur ^¬ nace. There can be more than one site of feeding fuel into the furnace. The secondary air can be mixed with fuel in one or more of these sites. In one embodiment, the fuel is fed into the furnace through a fuel pipe, and the first secondary air is mixed with the fuel and fed into the furnace simultaneously with the fuel through said fuel pipe. There may be more than one fuel pipe and the first secondary air can be mixed with the fuel in one or more of these fuel pipes. If secondary air is fed into the furnace both mixed with fuel and without fuel mixing, a part of the secondary air not mixed with the fuel can be fed into the furnace above the level of fuel feeding. Some secondary air can be fed into the furnace 2-6 m above the fuel feeding.

It is possible to heat the secondary air al ^¬ ready before it is fed into the fuel. Without limiting the current disclosure to any specific theory, in such an arrangement, the fuel might start to be heated by the secondary air already before it enters the furnace. This can be manifested in the earlier evaporation of water from the fuel and earlier volatilization of the volatilizable components contained in the fuel. This leads to the acceleration of the combustion process, in effect possibly meaning more heat released per unit time, thus increasing the temperature in the furnace. It might further be possible that the mixing of the secondary air with the fuel early on, makes oxygen more readily available for the combustible components in the fuel .

It is further possible to divide the secondary air into first secondary air and second secondary air. The first secondary air can be mixed with the fuel while the second secondary air is fed separately into the furnace. The second secondary air may be fed into the furnace as an air stream at least partially sur ^¬ rounding the mixture of fuel and first secondary air. The inventors of the current disclosure found out that such a configuration in a CFB furnace can lead to sur- prising advantages. The fuel dispersion in the furnace may become more favorable leading to a decrease in the slagging of furnace walls as well as to further reduc ^¬ tion of NO _x emissions. Reduced slagging can lead to re ^¬ duced operating costs and better boiler availability due to extended maintenance intervals. Without limiting the current disclosure to any specific theory, the im ^¬ proved fuel dispersion might positively affect the com ^¬ bustion reactions, thus reducing the NO _x production. In one embodiment, part of the secondary air is fed as first secondary air mixed with fuel and part of the secondary air is fed as second secondary air, the stream of second secondary air surrounding at least part of the fuel and the first secondary air fed into the furnace .

In one embodiment, the second secondary air is fed into the furnace through an air feed channel ar ^¬ ranged around at least part of the length of the fuel pipe and surrounding at least part of the fuel pipe. Through this arrangement, the stream of second second- ary air can be directed to surround at least part of the fuel and the first secondary air fed into the fur ^¬ nace . For the benefits of the method according to the present disclosure might be most prominent if more than half of the secondary air is fed as the first sec ^¬ ondary air. In one embodiment, the first secondary air comprises 60-70 % of the secondary air, and the second secondary air comprises 30-40 % of the secondary air.

Since during the operation of a CFB boiler, the bed material is in a constant movement due to the primary air blown into it, the penetration of the fuel into the furnace needs to be secured. By penetration is herein meant the ability of the fuel to get dispersed throughout the cross-sectional area of the furnace. The same concept can be applied for the secondary air, since it depends on the actual distribution of oxygen in the furnace whether the targeted air coefficient is actually achieved. This in turn partly determines if the furnace actually performs as expected. Therefore, the velocities at which the fuel, and when secondary air is mixed with the fuel, the secondary air are fed into the furnace, need to be adjusted. In one embodi ^¬ ment, the velocity of the first secondary air is 12-25 m s ^-1 , preferably 15-20 m s ^-1 ; and the velocity of the second secondary air is 15-40 m s ^-1 , preferably 20-30 m s ^"1 .

With the method according to the present dis ^¬ closure, low NO _x levels can be achieved. The NO _x concen ^¬ tration can be given as mass (in milligrams) per nor- malized gas volume (cubic meters) , denoted as mg m ^~3 n (6%, O ₂ dry) . By normalized gas volume is herein meant the volume of gas measured in conditions as known in the art. In one embodiment, the flue gas comprises less than 200 mg m ^~3 n NO _x species, preferably less than 150 mg m ^~3 n NO _x species, more preferably less than 100 mg m ^{~ 3} n Οχ species. In another aspect, a CFB boiler is disclosed. The CFB boiler according to the present disclosure com ^¬ prises

a furnace comprising a bottom;

- a solids recirculation system for recirculating solids escaping from the furnace;

at least one primary air inlet at the bot ^¬ tom of the furnace for feeding primary air into the furnace from below;

- at least one secondary air inlet at a height above the primary air inlet (s) for feeding sec ^¬ ondary air into the furnace; and

at least one fuel inlet, at a height above the primary air inlet (s) for feeding fuel, and option- ally secondary air, into the furnace. The CFB boiler is characterized in that it further comprises

at least one over-fire air (OFA) inlet at a height above the secondary air inlet for feeding OFA into the furnace.

The furnace according to the present disclosure comprises a bottom, through which primary air can beefed into the furnace. Therefore, the primary air in ^¬ let (s) are located at the bottom of the furnace. As is known in the art of CFB furnaces, a solids recircula- tion system is used to return the escaped bed material and the possible uncombusted fuel particles back to the furnace. The solids recirculation system comprises an exit chute, a separation cyclone, flue gas exit piping and a solids return pipe. The flue gas and the solids are carried from the furnace to the separation cyclone through the exit chute. The solids and the flue gas are separated from each other in the separation cyclone and the solids returned to the furnace through the return pipe. The flue gas continues through possible heat- recovery systems, scrubbers and/or dust collectors out of the boiler arrangement. The solids return pipe leads to the lower part of the furnace. Typically, the level at which the re ^¬ turnable solids are fed back to the furnace is approxi ^¬ mately at the level of fuel or secondary air feeding, for example within four meters of one of them. It can alternatively be below both. The solids return pipe can end in an inlet on its own, or the returnable solids can be combined with the fuel. If the fuel is mixed with at least some secondary air, the returnable solids can be mixed with the fuel either before or after the secondary air is mixed with the fuel.

The secondary air and fuel can be fed into the furnace through dedicated inlets for secondary air and fuel, respectively. Alternatively, it is possible to have at least one common inlet, through which fuel and secondary air are fed into the furnace. It is possible to combine the inlet configurations in many different ways. For example, it is possible that there is one common inlet for fuel and secondary air and additional secondary air inlets. It is equally possible that there are two common inlets for fuel and secondary air, placed, for example, on opposite walls of the furnace. In addition to the two common inlets for fuel and sec ^¬ ondary air there can be additional secondary air inlets and/or fuel inlets. All said inlets can be on the same height or on different heights.

The CFB boiler according to the present disclo ^¬ sure comprises one or more over-fire air (OFA) inlets. They are located at a height above the secondary air and fuel. In one embodiment, the at least one OFA inlet is at a height of 20-40 m, preferably 25-35 m, more preferably 28-32 m above the secondary air feeding height. It is alternatively possible to position the location of the OFA inlet in respect to the exit chute. In one embodiment, the CFB boiler further comprises an exit chute and the OFA inlet is 2-8 m, preferably 3-5 m below the exit chute of the furnace. As described above, the residence time of the fuel in the furnace can be used to observe the combustion reactions in the furnace. In one embodiment, the at least one OFA inlet is at a height which gives a fuel residence time of at least 3 seconds, preferably 4-5 seconds, for reaching the OFA inlet (s) .

The OFA inlet can be configured to be a nozzle. In one embodiment, the at least one OFA inlet is a noz ^¬ zle .

In one embodiment, the CFB boiler according to the current disclosure further comprises means for feeding nitrogen reductant into the furnace. The nitro ^¬ gen reductant is fed into the furnace for accomplishing SNCR in order to reduce the NO _x emissions from the fur- nace . Means for feeding nitrogen reductant into the furnace can be nozzles. As is known to the skilled per ^¬ son, the means for feeding nitrogen reductant into the furnace are connected to an appropriate arrangement of storing, delivering and regulating the amount of the nitrogen reductant. The set-up of such an arrangement is to be determined for each application and reductant.

In one embodiment, the CFB boiler according to the current disclosure further comprises a fuel pipe and means for mixing fuel with at least some secondary air in the fuel pipe before feeding the fuel into the furnace through the at least one fuel inlet.

In one embodiment, at least one secondary air inlet is configured to at least partly surround at least one fuel inlet for feeding part of the secondary air as first secondary air mixed with the fuel through the at least one fuel inlet into the furnace, and for feeding part of the secondary air as second secondary air surrounding at least part of the fuel and first secondary air fed into the furnace.

In one embodiment, the CFB boiler according to the current disclosure further comprises a flue gas re- circulation system for recirculating flue gas. The recirculation of flue gas can be used for adjusting the air coefficients in the furnace. The flue gas recircu ^¬ lation system may comprise heating and/or heat collec- tion arrangements.

In another aspect, the use of a CFB boiler is disclosed. The CFB boiler according to the current dis ^¬ closure can be used for reducing the NOx emissions from combusting a range of different fuels.

EXAMPLES

Reference will now be made in detail to the embodiments of the present invention, an example of which is illustrated in the accompanying drawings.

Figure 1 depicts a schematic presentation of an embodiment of a method and a CFB boiler according to the present disclosure. The features of the embodiment of Fig. 1 are not drawn to scale and components of the CFB boiler not necessary for describing the method and the CFB boiler according to the present disclosure are omitted for clarity.

The furnace 1 is the place where the combus ^¬ tion reactions take place. The furnace 1 is delimited by furnace 1 walls 11, which are constructed as known in the art. In Fig. 1, the cross section of the furnace 1 is not shown, but it is a rectangle. Bed material 2 is depicted as black dots dispersed throughout the fur ^¬ nace 1 and the solids recirculation system 17. Primary air 3 is blown into the furnace 1 through primary air inlets 18 located at the bottom 16 of the furnace 1. Although in Fig. 1, the primary air inlets 18 are depicted substantially throughout the bottom 16 of the furnace 1, this needs not be the case. Primary air 3 causes the fluidization of the bed material 2 and its dispersion throughout the furnace 1. Secondary air 4 is blown into the furnace 1 at a height above the primary air 3 entry into the furnace 1. The secondary air 4 is fed into the furnace 1 through air feed channels 15 and secondary air inlets 19. In Fig. 1, secondary air 4 is fed into the furnace 1 from two opposite directions. It would be possible to feed the secondary air 4 also, for example, from four directions, each corresponding to a wall 11 of the fur ^¬ nace .

Fuel 5 is fed into the furnace 1 through a fuel pipe 14 and a fuel inlet 20. Although only one fuel inlet 20 is depicted in Fig. 1, it is possible that there are several of them located next to each other on the same wall 11 of the furnace 1. There can be, for example two or four fuel inlets 20 on a furnace 1 wall 11. The placing of the fuel inlet (s) 20 and the secondary air inlets 19 can be coordinated so that, for example, there is one or more secondary air inlet 19 under each fuel inlet 20.

A solids recirculation system 17 is also depicted in Fig. 1. Although the details of the solids recirculation system 17 are omitted from the figure for clarity, it comprises a cyclone 10 for separating flue gas from solids and piping 24 for returning the solids into the furnace 1. In Fig. 1, the solids are returned into the furnace 1 at the approximate height of fuel feeding. In practice, the height at which the solids are returned into the furnace 1 can vary.

The air and fuel conducting and regulation equipment, such as fans and valves, is omitted from the figure for clarity. Although in Fig. 1, the solids re ^¬ circulation system 17 has its own opening into the furnace 1, it would be possible that the solids would be recirculated into the furnace 1 through the fuel inlet 20. There are many options for the skilled person to realize the optional connection between the fuel feed ^¬ ing and solids recirculation systems. Similarly, sec- ondary air 4 piping to the desired number and position of secondary air inlets 20 can be arranged in a number of ways as is known by the skilled person.

The feeding of secondary air 4 and fuel 5 in addition to the primary air 3 bring about the sub- stoichiometric combustion of the fuel 5.

In the method according to the present disclo ^¬ sure, over-fire air (OFA) 6 is fed into the furnace 1 for super-stoichiometric combustion of the fuel 5. The OFA 6 is fed into the furnace 1 through OFA inlets 21, of which four are shown in Fig. 1. The air feed channel 15 leading to the OFA inlet 21 can be a part of the same air feeding system as the air feed channel 15 used for secondary air 4. One of the OFA inlets 21 in Fig. 1 is presented as a side view. There could be several, for example eight OFA inlets 21 altogether, in the same direction. Three OFA inlets 21 are shown as end-view as they are located at the furnace 1 wall 11 farthest away from the viewing direction of Fig. 1. There could be three additional ones in the wall 11 closest to the viewing direction, but this wall 11 is not shown in Fig. 1.

On a practical level, the source of the prima ^¬ ry air 3. secondary air 4 and OFA 6 can be the same. There are many systems known in the art to regulate the flow of air into the different air inlets 18, 19, 21. Alternatively, there could be two or more air sources. This alternative would allow the regulation of the air composition of different sources independently, which in turn might have advantages in the regulation of the combustion process.

The combustion in the furnace 1 below the OFA 6 feeding level is sub-stoichiometric . The feeding of OFA 6 turns the combustion into super-stoichiometric. In the embodiment of Fig. 1, the furnace 1 can be ver ^¬ tically divided into a sub-stoichiometric combustion zone 7 below the OFA 6 feeding height and into a super- stoichiometric combustion zone 8 above the OFA 6 feed ^¬ ing height.

The majority of the combustion products leaves the furnace 1 as flue gas 12 through an exit chute 9. Some solid particles, mainly comprising bed material 2, escape from the furnace 1 through the exit chute 9. Al ^¬ so some uncombusted fuel 5 might be carried out of the furnace 1 through the exit chute 9. Most of the solids are returned to the furnace 1 through the solids recir- culation system 17. The flue gas continues into further parts of the CFB boiler. These parts are not depicted in Fig . 1.

However, in some situations, it might be ad ^¬ vantageous to return also some of the flue gas 12 into the furnace 1 as a part of the primary air 3. As de ^¬ tailed above, the primary air 3 needs to retain a suf ^¬ ficient mass flow in order to fluidize the bed material 2 sufficiently. On the other hand, the combined air co ^¬ efficient of the primary air 3 and secondary air 4 (SR ₁ +SR ₂ ) needs to be kept below 1. As flue gas 12 con ^¬ tains less oxygen than ambient air, the mass flow of primary air 3 increases faster relative to the amount of oxygen introduced into the furnace 1 when primary air 3 is supplemented with flue gas 12. The flue gas 12 recirculation system 22 is depicted with dashed outline, since it is an optional element of the CFB boiler according to the present disclosure.

Figure 2 is a schematic presentation of embod- iments in which fuel 5 is mixed with at least some sec ^¬ ondary air 4 before feeding the fuel 5 into the furnace 1. In panels a) -e) of Fig. 2, part of the secondary air 4 is fed as first secondary air 4a mixed with fuel 5 and part of the secondary air 4 is fed as second sec- ondary air 4b, the stream of second secondary air 4b surrounding at least part of the fuel 5 and the first secondary air 4a fed into the furnace 1. In Fig. 2, the secondary air inlet 19 and the fuel inlet 20 are depicted as seen from the inside of the furnace 1. The two inlets 19, 20 can be of any shape. In panels a) -c) and e) , the fuel inlet 20 is rectangular. In panel d) the fuel inlet 20 is round. However, for example elliptical or asymmetric shapes can be envisaged. The first secondary air 4a is fed through the fuel inlet 20 into the furnace 1.

Fuel 5 and first secondary air 4a are fed into the furnace through the fuel inlet 20. The second sec ^¬ ondary air 4b is fed into the furnace through the sec ^¬ ondary air inlet 19. The secondary air inlet 19 may be one continuous secondary air inlet 19 as in panels a), b) and d) , or be formed of separate secondary air open- ings 19 on different sides of the fuel inlet 20, as in panels c) and e) . Typically the secondary air inlet 19 approximately follows the outer contour of the fuel in ^¬ let 20. The secondary air inlet 19 can be in contact with the fuel inlet 20, as in panels a) , b) and d) , or at a distance from it, as in panel c) . Also it is pos ^¬ sible that the secondary air inlet 19 partially con ^¬ tacts the fuel inlet 20, as in panel e) .

In panels a) and d) , the secondary air inlet 19 surrounds the fuel inlet 20 on all sides, thus feed- ing second secondary air 4b from all sides of the fuel 5 and first secondary air 4a entering the furnace. In panel b) , one continuous secondary air inlet 19 sur ^¬ rounds the fuel inlet 20 on all sides except from one side, thereby feeding second secondary air 4b from three sides of the fuel inlet 20, but not from one side. For example, the one side from which the second ^¬ ary air inlet 19 does not surround the fuel inlet 20 is below. Alternatively, this side could be above the fuel inlet 20.

Panel c) depicts an embodiment similar to the one in in panel b) , except that the secondary air inlet 19 comprises three separate openings. In panel e) , four separate openings of the secondary air inlet 19 surround the fuel inlet 20 on all sides, thereby feeding second secondary air 4b from all sides of the fuel inlet 20.

Example

A time-averaged computational fluid dynamics (CFD) modelling approach for fluidization was applied to simulate hydrodynamics, combustion and NO _x formation in a CFB furnace using the method according to the pre ^¬ sent disclosure. The furnace geometry used in simula ^¬ tions was determined from public sources. The furnace in the simulation was 44 m in height, the cross- sectional profile of the furnace was rectangular. For the majority of the furnace, the cross sectional area of the furnace was 21 m times 10 meters. The bottom of the furnace was smaller, as the furnace narrowed sym ^¬ metrically (as depicted in Fig. 1) . The area of the bottom was 21 meters times 6 meters. Fuel was fed from eight fuel inlets, four on each long wall of the fur ^¬ nace. The fuel inlets were rectangular in cross sec ^¬ tion, the vertical sides of the inlets being longer than the horizontal sides. Secondary air was fed from all walls of the furnace so that each long wall had 14 secondary air inlets in two rows of 7. Each short wall of the furnace had six secondary air inlets in two rows of three. All the fuel inlets and the secondary air in ^¬ lets of the longer walls were located along the part of the furnace wall that was oblique due to the narrowing of the furnace towards the bottom. The secondary air was fed at the same level or above the level of fuel feeding. Primary air was fed upwards through the bottom of the furnace.

Three different simulations were run. One of them was a control case with no OFA feeding. It was compared with feeding over-fire air (OFA) into the fur ^¬ nace at two alternative heights. The lower of these heights, termed "OFA level A" was 16 meters above the height of primary air feeding. The higher height, termed "OFA level B" was 26 meters above the height of primary air feeding.

OFA was simulated to be fed into the furnace through nozzles, eight of which were located on one long wall of the furnace and two on each short wall. The second long wall was assumed to be inaccessible be ^¬ cause of the solids separation system.

The total combustion air flow and the share of primary air were held fixed in all cases. Primary air always comprised 60 % of the total air. When OFA was fed into the furnace, it comprised 23 % of the total air and secondary air flow was reduced by the same amount.

Simulated furnace temperatures were partially between 800 and 900 °C. However, the volume of furnace in which the temperatures exceeded 900 °C increased with both OFA feeding level A and OFA feeding level B in comparison with the control case with no OFA feeding. The increase in the furnace temperature was more pronounced with OFA feeding at level B. Further, in the control case, the temperature at the top half of the furnace remained below 890 °C, whereas with OFA feed ^¬ ing, the temperatures in this region exceeded 900 °C. This surprising effect allows the efficient use of SNCR in CFB furnace, which would not have functioned ade ^¬ quately in CFB boilers known in the art.

Fuel moisture evaporation takes place in the bottom part leading to locally cooler regions especial ^¬ ly close to the fuel inlets. On the side of the furnace at which solids are returned to the furnace, moisture evaporates inside the return legs and ignition occurs earlier in the furnace supported by the char and solids recirculated from the solids separation system. The furnace temperature level was predicted to rise in the cases where OFA was fed into the furnace. Temperatures were the highest with the OFA level B, where the veloc ^¬ ity profile in the lower part was the smoothest and the channeling effect the weakest.

When OFA was fed into the furnace, oxygen- deficient regions were formed in the areas below OFA feeding as expected. Consequently carbon monoxide con ^¬ centrations increased - in case of OFA level B notably also in the upper part of the furnace. Peak values re- mained at 2 vol-% thus the corrosion risk was estimated to stay low in the temperature zone considered. Rather similar carbon monoxide burnout with exit values of 500-700 ppm was achieved in all cases due to efficient OFA mixing. Oxygen was able to come into contact with the carbon monoxide before the exit chutes. The exit flow of uncombusted carbon was predicted to decrease when OFA feeding is used. Particle residence times in the furnace increased when OFA feeding was introduced. Based on the results, combustion efficiency and fly ash uncombusted carbon are not expected to weaken due to OFA feeding.

According to the model, the NO _x emission was remarkably decreased in cases where OFA feeding was in ^¬ troduced compared to the control CcL S Θ cL S reductive re- actions gain in comparison to oxidative ones in air- deficient conditions. Initial NO _x formation in the lower part of the furnace was similar in all cases. Emission reduction of 40 % was predicted in case of OFA level A, and reduction up to 60 % in case of OFA level B. This might reflect the longer fuel residence time before the final combustion zone in the latter case.

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.