SE1750990A1 - METHOD FOR NOx REDUCTION IN A CIRCULATING FLUIDIZED BED BOILER, A CIRCULATING FLUIDIZED BED BOILER AND USE THEREOF - Google Patents

Abstract

The current invention relates to a method for reducing NOx emissions in a circulating fluidized bed boiler (CFB boiler), the CFB boiler comprising a furnace, the furnace comprising a bottom and fluidizable bed material. The method comprises feeding primary air from below through the bottom and the bed material into the furnace for fluidizing the bed material and entrapping at least a part of the bed material for circulating it 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 for combusting the fuel. The method iscomprising feeding over-fire air (OFA) into the furnace at a height above the secondary air feeding, for super-stoichiometric combustion of the fuel.(FIG. 1)

Description

lO METHOD FOR NOX REDUCTION IN A CIRCULATING FLUIDIZED BEDBOILER, A CIRCULATING FLUIDIZED BED BOILER AND USETHEREOF FIELD OF THE INVENTION The present disclosure relates to a method ofNOX reduction in a circulating fluidized bed boiler. Itfurther relates to a circulating fluidized bed boiler and to use thereof.

BACKGROUND OF THE INVENTION Fluidized bed boilers are used for producingheat 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-the fuels used in fluidized bed boil- ers have relatively high water content and can be con- fuels. Typically, 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 boilertypes, the fuel material is combusted in a furnace us-ing inert particulate bed material through which air isblown to fluidize it. This increases the rate of heattransfer in the furnace and stabilizes the combustionWhile in a BFB mains in the lower part the bed material re-in a CFB fur- process. furnace,of the furnace,nace, a part of the bed material circulates within thefurnace. Some of it is carried to the upper part of thefurnace and even out of it with flue gas. Therefore, toprevent loss of the bed material, and the uncombusted fuel material possibly carried out with it, a separa- l0 tion cyclone is used to separate flue gas from solidsand the solids are returned to the furnace.

The CFB boilers are more flexible than the BFBboilers in terms of the fuel used. Different fuels canbe used simultaneously, shifting from one fuel to an-other can be done directly and a wider range of fuelscan be used. Thus they are the most promising alterna- tive for nmlti-fuel combustion, which is advantageousas 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 thusfluidizing it. The primary air velocity is at least 3-5 m s* 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 bediJ1 a CFB furnace result material in efficient ndxing and heat transfer of the fuel and bed material, as wellas in an even temperature distribution within the fur-nace. The combustion temperature of the fuel in a CFB°C and 900 °C. Thefuel retention time during combustion is longer for CFB adding to the effi- furnace is typically' between 800furnaces than for the BFB furnaces,ciency of the combustion. The particles carried out ofthe furnace with the gas flow are separated in a cy-clone, from which they are fed back into the lower partof the furnace through a return pipe.

The main source for the combustion reactionsis the so-called secondary air, which is fed into thefurnace some meters above the primary air feeding lev-el. Also the fuel is added to the furnace in the lowerpart of the furnace, above the level at which the bedmaterial is fluidized. Either a dedicated feed pipe orthe return pipe from the cyclone can be used for fuel feeding. l0 Like all combustion the harmful industrial combustion processes, reactions in CFB boilers produceinto the environment has to include NOX (NO whose releaseThe harmfulderived from. the nitrogen containedthe substances,be restricted. substancesand NOQfuel. Their boiler and. on the in the relative amounts depend on type of fuel used. CFB furnaces generally produce low amounts of NOX (currently in the range of300-500 mg mfn), but the tightening environmental regu-lations pose a challenge for all combustion-based ener-of reduction. of 'NOX emissions gy' production. in terms 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-stoichiometricin the fuel. Thiswhich species inevitably formed during com- relative to the combustible carbonresults in the formation of hydrocarbon radicals,scavenge the NOXbustion and reduce them to molecular nitrogen. This is effective in reducing the NOX emissions of the combus- tion. However, to ascertain the complete combustion ofthe fuel, the oxygen-to-fuel stoichiometry' has to beincreased to slightly above l, i.e. to a super- stoichiometric range.reduction is used to At the moment, the Also post-combustion NOXfurther reduce the NOX emissions.most cost-effective method for the post-combustion NOX reduction is the so-called selective non-catalytic re- duction (SNCR). In SNCR, NOX are reduced into molecularnitrogen (N2) using nitrogen-containing reductants, typ-ically water solutions of either ammonia or urea. The general reactions for nitrogen monoxide areNH2CONH2 + 2 NO + % O2 Q 2 N2 + CO2 + 2 H20 and4 NH3 + 4 NO + O2 Q 4 N2 + 6 H20 for urea and ammonia, respectively. 900-1,100 In temperatures below 900 The temperature window for SNCR is°C, preferably 950-1,050 °C.

O , the NOX reduction takes place too slowly to be effec-tively applied and the reductants can escape from thefurnace turning into unwanted emissions themselves.This poses a challenge for utilizing SNCR in CFB fur-naces, as the temperatures in them usually remain belowor at the threshold value of 900 °C, the CFB boiler does not function at full capacity. especially when Patent document CN 103721552 discloses a meth-od for implementing SNCR denitration during low loadingof a CFB boiler. a reducer spray gun for implementing SNCR, The method comprises secondary air andwherein thesecondary air is arranged on an upper layer and a lowerlayer. The distance between the two layers of secondaryair is 3-5 m and the secondary air on the upper layerThe spray gun isThe method solves the problem that a denitration effect is accounts for 20-40 % of the total air. arranged in the secondary air on the upper layer. poorer when the smoke temperature at an inlet of a cy-clone separator is under 800 °C at low loading condi- tions of the boiler.

Thereduce the NOX solutions presented. in prior art do notemissions below the limits required bythe tightening' future environmental protection stand-ards. The inventors have therefore recognized the need to further reduce the NOX emissions in a CFB furnace.

PURPOSE OF THE INVENTIONThe purpose of the present disclosure is toproblems related to solve, or* at least to alleviate, prior art solutions. Especially, it is the purpose ofthe current disclosure to present a method and an appa- ratus for reducing the NOX emissions of CFB furnaces. lO SUMARY The method according to the present disclosureis characterized by what is presented in claim l.

The CFB boiler according to the present dis-closure is characterized by what is presented in claim22.

The use according to the present disclosure is characterized by what is presented in claim 31.

The method and device according to the presentdisclosure may offer at least one of the following ad-vantages: The formation of NOX species is reduced duringcombustion and their concentration in the flue gas isreduced. Another advantage of the method and device ac-cording to the present disclosure is that if post-combustion removal of NOX is performed by selective-non-(SNCR), can be further reduced. the NOX concentration in theThis catalytic reductionflue gas is due to themore amenable temperature range for SNCR both at highand low boiler loading conditions. therange and the ensuing increased efficiency of SNCR, the Furthermore, more optimal temperature re-risk of reductants into unwanted duces turning emissions. Additionally the tightening future NOX emis-sion standards can be reached with the cost-effectiveSNCR 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 thefuel can be combusted more efficiently. If the fuel ismixed with the secondary air before feeding into thethe combustion furnace, increased efficiency' of fuel can be more pronounced. lO BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are includedto provide a further understanding of the invention andconstitute 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 l is a schematic presentation of an em-bodiment of a method and a CFB furnace according to thepresent 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 (CFB) combustion. device meant for combusting' many' types of A circulating fluidized bed boiler is a 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 NO and N03 in the formation. of nitrogenoxides,as NOX. which are collectively referred toThey are considered pollutants and their concen-tration in the flue gas released from. a CFB boilershould be minimized.

In one aspect, a method for reducing NOX emis-(CFB boil- the furnace sions in a circulating fluidized bed boilerer), the CFB boiler comprising a furnace,comprising' a bottonl and fluidizable bed Inaterial, isdisclosed. The method comprises simultaneously' or insequence in any order - feeding primary air from below through thebotton1 of the furnace and. the bed 1naterial into the furnace for fluidizing the bed material and entrapping lO at least a part of the bed material for circulating thebed material in the furnace; - feeding secondary air into the furnace at aheight above the air for sub- primary feeding, stoichiometric combustion of fuel; and - feeding fuel into the furnace at a heightabove the primary air feeding height for combusting thefuel. The næthod is further comprises characterized. in that the method (OFA) nace at a height above the secondary air feeding, - feeding over-fire air into the fur-forsuper-stoichiometric combustion of the fuel.

The CFB boiler comprises a furnace, in whichThe in height the combustion reactions take place. furnace is usually' between 20 and 50 meters and its cross section is a rectangle. As is known in the art, aCFB boiler comprises a range of additional instrumenta-tion for regulating the combustion conditions and forcapturing the energy released from the combustion. Forthe combustion. to function efficientlyy a CFB boilercontains fluidizable bed nmterial within the furnace.The bed material has a high thermal capacity, it is in-ert and durable enough to withstand the conditions inIt has also the furnace. suitable density' and grain size for fluidization. For example quartz sand is usedas the bed material.

When fuel is combusted in a CFB boiler, air isblown from the bottom of the furnace to fluidize thebed 1naterial and. to circulate it within. the furnace.The air blown through the bed material is called prima-approximately 3-5 In s_Ü The to be kept high ry air. Its velocity is mass flow of the primary' air needs enough to fluidize the bed material.At a height above the primary air feeding, sec- ondary air is fed into the furnace. fed through the bottom. of the Since the primaryall relative to the primary' air feeding' are the air is furnace, heights lO same relative 11) the bottom. However, ii: is possible for the furnace bottom to have variable shape andtherefore 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 ofprimary air feeding.

In a typical situation, the height of secondaryair feeding is 2-10 meters above the primary air feed-for exam- ing. The secondary air feeding height can be, ple, 4-8 m above the height of primary air feeding. Forexample, the secondary air can be fed into the furnaceabove the height of the fuel theheight of the fuel feeding. Itthe Thus, feeding or belowis possible to dividesecondary air feeding in a vertical direction. for example, some secondary air can be fed intothe furnace 2-6 m above the fuel feeding, while somesecondary air is fed at the same height as the fuel orbelow the height some meters, for example 2-6 meters, of fuel feeding. The main purpose of the secondary airis to provide oxygen for the combustion reactions. How-ever, the air current created by the secondary air alsocontributes to the material flow within the furnace.The direction of the secondary air fed into the furnaceis substantially horizontal or downwards from horizon-tal.nace can be,tal. to the furnace needs to be high enough for the The angle of the secondary air fed into the fur-for example 20-30° downwards from horizon-The velocity at which the secondary air is fed in-second-ary air to thoroughly mix with the fuel and bed materi-al. Both the primary air and the secondary air provideoxygen for the combustion reactions in the furnace. Bythe velocity of the air being fed into the furnace isherein meant the velocity of the air at the end pointof the channel leading to the furnace.

The stoichiometry of the combustion reactions(SR) of If the air coefficient is l, is generally described by the air coefficient the combustion reaction. lO the combustion is stoichiometric. I.e. the theoreticalinto the furnace would. be If the air amount of oxygen providedjust enough to oxidize the fuel completely. coefficient is below l, there is less oxygen than oxi- dizable fuel. In this case, the combustion is sub-stoichiometric. At air coefficient values above l, thecombustion is super-stoichiometric. In this case, there is excess oxygen present in relation to the combustiblefuel. of combustible components contained in them.

It should be noted that fuels vary in the amountTherefore,the amount of air needed for completely combusting aunit 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 (SR¿) and for secondary air (SRfi.However, as both contribute to the combustion reac-tions, usually their sum (SR¿+SRfi is used to describethe combustion stoichiometry in a CFB furnace. In the method according to the present disclosure, the combus-tion reactions depending on the primary and secondaryair have a sub-stoichiometric air-to-fuel ratio. This means, that SR¿+SR2 is less than l. 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. aircoefficient of the primary(SR¿+SR2) is 0.5-0.95, preferably 0.8-0.9.

Without limiting the current disclosure to any air and secondary air specific theory, sub-stoichiometric combustion can [beadvantageous as it increases the rate at which the fuel“diluted” This improves the heat generation and conse- is combusted, since the fuel is not with ex- cess air.quently' the combustion. efficiency' of the CFB boiler.Further, the sub-stoichiometric combustion may increasethe concentration of hydrocarbon radicals produced dur-ing the combustion. As radicals the hydrocarbon are very reactive, they may function as NOX scavengers inthe furnace thus reducing the amount of NOX present inthe flue gas.

Since the air coefficient is observed for as-certaining efficient combustion reactions in the fur-nace, the amount of air needs to be regulated in re-sponse to the boiler load. Boiler load refers to theproportion of the full boiler combustion capacity beingutilized at a given time. The amount of air needed forachieving the targeted air coefficient decreases withboiler load. At the same time, there is a lower limitfor the mass flow of primary air needed to fluidize thebed Inaterial and. to circulate it within. the furnace.Therefore, with some boiler loads and/or some fuels, itmight be beneficial to feed recirculated flue gas intothe 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 ofprimary air can be adjusted. Mass flow of primary aircan thus be increased without corresponding increase in the amount of oxygen, affecting' the air coefficient.

Conversely, with a given mass flow, less oxygen can befed air into the furnace. In one embodiment, the primarycomprises recirculated flue gas for adjusting theair coefficient of the combustion. By recirculated flueis herein meant gas conveyed from any position af-The for exam- gaster the furnace to the primary air feeding system.recirculated flue gas can be collected from,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 andto 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 lO ll or other additives can be incorporated into the fuel ator before feeding the fuel into the furnace. Typically,the height of fuel feeding is l-5 m above the height ofprimary air feeding. The height of fuel feeding can bethe same as the height of secondary air feeding. Alter-natively, the secondary air can be fed into the furnaceThe fuel can be fed into the furnace from one or more dedicated l-6 meters above or below the fuel feeding. inlets. Alternatively, the same pipe can be used forreturning 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 heightabove the secondary air feeding. This air, termed over-fire air (OFA), has the main purpose of increasing the in the furnace to a valuethe OFA (SRtot)can be total air coefficient above l. A SR value calculated for(SRMÄ). By SRUÉ is herein meant the sum of air coeffi-ciencies of all the fed theSRUm=SR¿+SR¿+SRM¶. Thus, feeding OFA results in a super- stoichiometric combustion air into furnace, i.e. in the furnace. However, itis possible that the OFA has also other functions, suchas participating in the mass flow and/or in generatingbeneficial heat exchange conditions in the furnace.Although sub-stoichiometric combustion has thebenefits of faster reaction rates and decreased releaseof NOX, 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 idealconditions and thus remains incomplete even if SRuü isl. 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. 12 By raising the a value above feeding OFA, SRDW, to 1 bythe complete combustion of the fuel is as-certained. As much of the fuel energy as possible isthus captured and the flue gases contain only minimalamounts of uncombusted material. By material is hereinmeant all substances, irrespective of their presence insolid or liquid form, present in the furnace.(SRtot) Of achieved. by gaseous,In one embodiment, a total air coefficient1.1-1.4, the OFA feeding. preferably' approximately' 1.2 is Taken together, the method according to thepresent disclosure combines the benefits of both sub-stoichiometric and super-stoichiometric combustion pro-ducing as little NOX as possible but at the same time combusting the fuel substantially completely.

The fed the The remaining heat of the flue gas after en- air into furnace can be pre-heated.trapping most of it through superheater(s) and econo-mizer(s)air and/or OFA by, gas-gas heaters.temperature of the the and/or OFA is typically 150-250 °C. can be used for heating primary air, secondaryThe air for example, primary air, secondary 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- in the furnace betweenand the OFA feeding stoichiometric combustion zonethe primary air height; feeding heightand feeding OFA results in the formation of athe OFAIn such an embodiment of the method ac-the OFA feeding is super-stoichiometric combustion zone above feeding height. cording to the present disclosure, positioned. so that two combustion zones form. in the furnace in a vertical direction. Since the nitrogen re- maining in the organic compounds forms NOX when super- lO l3 stoichiometric combustion takes place, ii: is advanta-geous to keep the combustion sub-stoichiometric as longas possible. However, the combustion needs to be super-stoichiometric for a sufficient period to attain com-plete enough combustion before the nmterial exits thefurnace.

The lower combustion zone extends between theprimary air feeding and the OFA feeding. In the lower combustion zone, the combustion is sub-stoichiometric.The oxygen used for combustion in this combustion zonecomes from primary air and secondary air and the aircoefficient is SR¿+SR2. The second combustion tends above the height of OFA feeding. zone ex-In this combus-tion the combustion is the zone, super-stoichiometric andThe - together with some of the bed material and fuel is substantially completely combusted. flue gases 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 betweenthe two combustion zones is not clear-cut. The materialin the furnace is in continuous motion and therefore,some sort of combustion a gradient of 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 14 might be utilized more efficiently as the fuel is mixedwith 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. the fuel For these reasons,ignites earlier, as it both heats up fasterand 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 besuper-stoichiometric. While the fuel is igniting, theremight momentarilyf be more air available for the com-busting nmterial.fuel,part of the furnace, fron1 the fuel The better' early' combustion. of theagain, leads to higher temperature in the lowerwhich leads to earlier release ofnitrogen compounds Thus, into the gas phase.the lower part of the furnace might contain lo-calized super-stoichiometric volumes in which NOX for-mation might actually be increased. However, as the ma-jority of the furnace volume below OFA feeding remainssub-stoichiometric, the formed NOX are likely to be re-duced to nwlecular nitrogen before they reach the su-per-stoichiometric height (above OFA feeding level).

Often, the OFA. feeding takes place substan-tially higher in the furnace than the secondary air and the OFA feeding heightmore preferably 28-32 m Without limit- fuel feeding. In one embodiment,is 20-40 m, above the secondary air feeding height. preferably 25-35 m,ing the current disclosure to any specific theory, whenthere is vertical space in the range of tens of metersbetween the secondary air and OFA feeding, the condi-tions might be amenable for the formation of the two combustion zones detailed above.

The residence time of fuel in a furnace can beused to næasure the furnace throughput. The residencetime can be to some degree proportional to the effi-ciency' of combustion. As a parameter describing the furnace function, residence time is independent of thespecific measures of the furnace. In most CFB furnaces, a residence time of 3 seconds can be considered. theminimum 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. the CFB boiler further com- prises an exit chute and the OFA is fed into the fur- In one embodiment,nace 2-8 m below the exit chute of the furnace. By anexit chute is herein meant an arrangement of at leastone opening in the CFB furnace wall through which theflue gas leaves the furnace. The exit chute can com-prise one opening or two or more openings. cal for CFB boilers, As is typi-the exit chute leads into a sepa-ration cyclone, in which flue gas is At least a part of the solids separated from 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 theknowledge of the skilled person. A distance of 2-8 me-ters between. OFA. feeding' and. flue gas exit from. thefurnace can be considered sufficient to allow the fuel combustion to be complete. 16 The and secondary air combined with OFA feeding has a sur- sub-stoichiometric combustion by primaryprising effect on the temperature of the CFB furnace.The temperature in the furnace might increase by tensof degrees Celsius compared. with known CFB furnaces. above 900 Even temperatures above 1,000 °C might be achieved.With- Furnace temperatures°C are achievable.out limiting the current disclosure to the any specific theory, sub-stoichiometric combustion by primaryand secondary air might accelerate the initial combus-tion in the furnace below the level of OFA feeding. Inaddition to this, the height at which the fuel is com-pletely combusted might be raised compared to prior artfurnaces. This could allow the combustion energy to be more equally' distributed. in the furnace, which againcan 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,950-1,050 °C There may be areas of the furnace in which the tempera- preferably ofin the majority' of the furnace volume.in the method ac- ture remains below 900 °C. However, cording' to the present disclosure, the proportion ofthe furnace, in which the temperature is above 900 °Cis large enough to affect the combustion processes inthe furnace. at least 50 %,least 80 % of the furnace volume can have a temperatureabove 900 °C. at least 50 %,least 80 % of the furnace volume can have a temperature of 950-1,050 °C.

For example, preferably at For example, preferably at It is beneficial to mix the OFA fed into thefurnace with the nmterial 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 theOFA is fed into the furnace. The minimum velocity giv- ing a sufficient mixing in most CFB boiler applications 17 can be considered to be 50 m s*.

However, the entry ve-locity of OFA cannot be increased indefinitely due toincreased energy consumption and the abrasive effectsof the bed material and fuel particles on the internalcomponents of the furnace. In one embodiment, the ve-locity of the OFA feeding is 50-100 m s_É60-90 m s”.

The direction of the OFA entry into the fur- preferably nace is another parameter having an effect on the mix-ing of the OFA. zontal direction or a direction downwards from horizon- The inventors have found out that hori- tal may' have a beneficial effect on the OFA Inixing.Without limiting the current disclosure to any specificit might be that such a direction promotes theIt is theory,formation of two combustion zones in the furnace.also possible, that when fed in a horizontal directionor downwards from horizontal, the OFA is not capturedtoo early into the upward currents present in the fur-the OFA is fed into the fur- nace. In one embodiment, nace in a horizontal direction, or in a direction at anangle of 20-40°, ward from horizontal. preferably of approximately 30° down- Yet another parameter affecting the mixing ofthe OFA with other material present in the furnace isthe geometry of the OFA feeding into the furnace. Inone 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 artfor feeding air into a CFB furnace. Selecting a suita-ble alternative is within the knowledge of the skilledperson. By a nozzle is herein meant an inlet shaped tocreate a jet stream of gas or liquid towards the fur-nace interior. The opening in the nozzle can be of anyThe nozzle can be directed. shape. strean1 of gas or liquid. emitted. fronl the 18 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 otherwalls. This the case, if the might be for example, walls of the furnace are of variable width. Practicalreasons might lead into one or more wall not having anynozzles. The nozzles can be placed at regular intervalsor their distance from each other can vary. For exam-that eight or ten nozzles and the two walls adjacent to the ple, it is possible one furnace wall has six, said wall have two, three or four nozzles each. Anotheralternative would be that two walls opposite to eachother have several nozzles, for example eight nozzles,each.

Without limiting the current disclosure to anyspecific theory, it might be beneficial for the nozzleson 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 possiblethat the nozzles in a given wall are arranged in two ormore rows. It might also be possible that the nozzleson a given furnace wall are staggered on two verticallevels. all nozzles can be on the same height.

Alternatively, In one embodiment of the næthod according tothe present disclosure,the catalytic reduction nitrogen reductant is fed into furnace for reducing NOX selective(SNCR).herein meant any substance containing nitrogen and be-(N2)- BY SNCR is herein meant selective non-catalytic reduction. through non- By nitrogen reductant ising able to reduce NOX into molecular nitrogenIt is a temperature-dependent process for reducing NOXinto Ng. For SNCR to be of practical relevance in reduc-ing NOX levels in the flue gas, a sufficient temperature is required. The temperature should be at least 900 °C 19 under all boiler loading conditions. However, the tem- perature should not exceed 1,100 °C since the nitrogenreductant will start reacting with oxygen at about thistemperature, possibly forming NOX. Preferably, the tem-should. be maximally' 1,050 °C. utilization of SNCR allows the scavenging of NOX so that perature The effectivethe NOX concentration in the flue gas can be further re-duced. The utilization SNCR in combination with the OFAfeeding produces a beneficial cumulative effect in theNOX reduction. This is due to the favorable temperaturerange achievable with the OFA feeding according to thepresent disclosure.

In one embodiment, the nitrogen reductant iswater solution of ammonia, or water solution of urea.Without limiting the current disclosure to any specifictheory, both ammonia and urea are able to reduce NOX species into nwlecular nitrogen. The selection of thereductant depends on the process parameters in each CFBboiler. The selection of a suitable reductant for ac-complishing SNCR in each case is within the knowledgeof the skilled person. The process parameters need tobe further optimized in each application in terms of,for example, ammonia or urea concentration, possibleSuch adjustment is within the competence of the skilled per- pre-heating of the ammonia or urea solution etc. son.

Since OFA feeding is connected to the changeof the combustion stoichiometry in the furnace to su-per-stoichiometric, also the production of NOX species might be enhanced at this height and above it. The fur-nace may comprise dedicated inlets for feeding the ni-the same inlets used for the OFA feeding can be used for trogen reductant into the furnace. Alternatively, feeding nitrogen reductant into the furnace. It is pos-sible to feed all the nitrogen reductant into the fur-used. for OFA. part of the nitrogen reductant may be fed into the fur- nace through. the inlets Alternatively, lO nace through the inlets used for OFA.ble to utilize all of the OFA inlets for feeding nitro-the some of the OFA inlets can be used for feeding the ni- It is also possi- gen reductant into furnace. Alternatively, only trogen reductant into the furnace. The nitrogen reduct-ant can be mixed with the OFA before the entry into thefurnace. Both the OFA and the nitrogen reductant can beheated before feeding in to the furnace. The optimalfeeding configuration in terms of velocity of feeding,the proportion of common inlets is to be determined foreach application.

In embodiments where the OFA is sprayed intothe In one embodiment, thealong with it. furnace, nitrogen reductant may be sprayedat least part of thenitrogen reductant is with the OFA. sprayed. into the furnace alongWithout limiting the current disclosureto any specific theory, such a feeding method might im-prove the dispersion. and. penetration. of the nitrogenFurther, reductant in the furnace. the droplet size of the nitrogen reductant can be adjustedr Depending onthe 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 withinthe knowledge of the skilled. person. to optimize thedroplet size of the nitrogen reductant according to therelevant parameters.

It is possible to monitor the furnace tempera-The can be utilized in the optimization of nitrogen reduct- ture on-line. on-line measurement of temperature ant feeding. The nitrogen reductant can be fed into thefurnace 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 individuallyresponsive to temperature changes. The on-line regula- tion of nitrogen reductant feeding can be implemented 21 both in cases where there are dedicated inlets the fornitrogen reductant and in ant is fed through the OFA inlets. cases where nitrogen reduct- 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 somesecondary 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 ndxed 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_Ü example 15 to 20 m s_Ü 25 to 40 m s*second secondary air may enter the furnace at a veloci- ty of 30 m sfl.

The velocity may be, for Also higher velocities, such as are possible. As an example, fuel andThe velocity allows the mixed fuel andsecondary air to fly towards the center of the furnaceand mix efficiently with the bed material and primaryair.

When fuel enters the furnace, it is entrappedin the upwards-flowing air current. In a CFB furnace,the upwards flow velocities are high enough to entrapfuel soon after it enters the furnace. This means thata substantial portion of the fuel might fly upwards inthe vicinity of the furnace walls. When the sulphur-containing and chlorine-containing substances possiblycontained fuelsuch as H25 and HCl,This might cause corrosion of the furnace wall compo- Thus, in the are vaporized, corrosive com- pounds, respectively are released. nents. advantageously, fuel flows upwards and iscombusted at a distance from the side walls of the fur-If fuelproportion of fuel might be carried away from the walls the of the nace. is mixed. with secondary' air, a larger towards center furnace before being en- trapped in the upwards-flowing current. This might be due to the Velocity of air and fuel entering the fur- lO 22 nace. In other words, of the side walls, less fuel remains in the vicinityif fuel is mixed with at least somesecondary air. The larger the proportion of fuel resid-ing at a distance from the furnace walls, the less con-tact the potentially corrosive compounds have with theThus, side walls of the furnace. 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 atleast two meters before the entry point into the fur-nace. There can be more than one site of feeding fuelinto the furnace. The secondary air can be mixed withfuel 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 andfed the with the fuel through said fuel pipe. There may be more than one fuel into furnace simultaneouslypipe and the first secondary air can be mixed with thefuel in one or more of these fuel pipes. If secondaryair is fed into the furnace both mixed with fuel andwithout fuel mixing, a part of the secondary air notmixed with the fuel can be fed into the furnace abovethe level of fuel feeding. Some secondary air can befed 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 limitingthe current disclosure to any specific theory, in suchan arrangement, the fuel might start to be heated bythe secondary air already before it enters the furnace.This can be nmnifested in the earlier evaporation ofwater from the fuel and earlier volatilization of thefuel. This leads to the acceleration of the combustion process, in volatilizable components contained. in theeffect possibly' meaning more heat thus released. per unit time, increasing the temperature in the furnace. lO 23 It might further* be possible that the mixing' of thesecondary air with the fuel early on, makes oxygen morereadily available for the combustible components in the fuel.

It is further possible to divide the secondaryair.fuel into air into first secondary air and second secondaryThe while the second secondary air is fed separately first secondary' air can be mixed. with the the furnace. The second secondary air may be fed into the furnace as an air stream at least partially sur-air. that rounding the mixture of fuel and first secondaryThe inventors of the current disclosure found outsuch a configuration in a CFB furnace can lead to sur-prising advantages. The fuel dispersion in the furnacemay become more favorable leading to a decrease in theslagging of furnace walls as well as to further reduc-tion of NOX 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 NOX production. In one embodiment, part of the secondary air is fed as first secondary' air mixed. with. fuel and. part of thesecondary air is fed as second secondary air, thestream. of second secondary air surrounding at least part of the fuel and the first secondary air fed intothe furnace.

In one embodiment, the second secondary air isfed into the furnace through an air feed channel ar-ranged around at least part of the length of the fuelpipe 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 ofthe fuel and the first secondary air fed into the fur- nâCG . 24 For the benefits of the method. according tothe present disclosure might be most prominent if morethan half of the secondary air is fed as the first sec-ondary air.

In one embodiment, the first secondary air O comprises 60-70 6 of the secondary air, and the second O secondary air comprises 30-40 6 of the secondary air.

Since during' the operation. of a CFB boiler,the bed material is in a constant movement due to theprimary air blown into it, the penetration of the fuelinto the furnace needs to be secured. By penetration isherein meant the ability of the fuel to get dispersedThe secondary air, throughout the cross-sectional area of the furnace. same concept can be applied for thesince it depends on the actual distribution of oxygenin the furnace whether the targeted air coefficient isThisthe furnace actually performs as expected. at which the fuel, actually' achieved. in turn partly' determines if Therefore, the velocities 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_É preferably 15-20 m s_É and the velocity of thesecond secondary air is 15-40 m s_É 4 preferably 20-30 m S With the method according to the present dis- closure, low NOX levels can be achieved. The NOX concen- tration can be given as mass (in milligrams) per nor- malized gas volume (cubic meters), denoted as mg mfn(6%, 02the volume of gas næasured in conditions as known in the art. dry). By normalized gas volume is herein meant In one embodiment, the flue gas comprises lessthan 200 mg mfn NOX species, preferably less than 150mg mfn NOX species, more preferably less than 100 mg m- 3n NOX species. lO 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 recircu-lating solids escaping from the furnace; - at least one primary air inlet at the bot-tonl of the furnace for feeding' primary' air into thefurnace from below; - at least air inlet at a one secondary 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-The CFB boiler ischaracterized in that it further comprises (OFA) inlet for feeding OFA ally secondary air, into the furnace. - at least one over-fire air inlet at aheight above the secondary airinto the furnace.

The furnace according to the present disclosure comprises a bottom, through which primary air can beefed into the furnace. Therefore,let(s) known in the art of CFB furnaces, the primary air in-are located at the bottom of the furnace. As isa solids recircula-tion system is used to return the escaped bed materialand the possible uncombusted fuel particles back to theThe solids furnace. recirculation. systen1 comprises an exit chute, a separation cyclone, flue gas exit piping and a solids return pipe. The flue gas and the solidsare carried from the furnace to the separation cyclonethrough the exit chute. The solids and the flue gas areseparated from each other in the separation cyclone andthe solids returned to the furnace through the returnpipe. The flue recovery systems, gas continues through possible heat-scrubbers and/or dust collectors out of the boiler arrangement. lO 26 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 canalternatively be below both. The solids return pipe can or the returnable solidsIf the fuel the returnable solids end in an inlet on its own,can be combined with the fuel. is mixedwith at least some secondary air,can be mixed with the fuel either before or after thesecondary air is mixed with the fuel.

The secondary air and fuel can be fed into thefurnace through dedicated inlets for secondary air andfuel, have at least one common inlet, respectively. Alternatively, it is through which fuel and possible to 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 onecommon inlet for fuel and secondary air and additionalsecondary air inlets. It is equally possible that therefuel on opposite walls of the furnace. are two common inlets for and secondary air, placed, for example,In addition to the two common inlets for fuel and sec-ondary air there can be additional secondary air inletsand/or fuel inlets. All said inlets can be on the sameheight or on different heights.

The CFB boiler according to the present disclo-(OFA)They are located at a height above the secondary air the at least one OFA inlet sure comprises one or more over-fire air inlets. and fuel. In one embodiment,is at a height of 20-40 m,the preferably 25-35 nu more preferably 28-32 m above air height. It is secondary feedingalternatively' possible to [position thelocation of the OFA inlet in respect to the exit chute.In one embodiment,exit chute and the OFA inlet is 2-8 m, the of the the CFB boiler further comprises anpreferably 3-5 m below exit chute furnace. As described lO 27 above, the residence time of the fuel in the furnacecan be used to observe the combustion reactions in thefurnace. In one embodiment, the at least one OFA inletis at a height which gives a fuel residence time of atleast 3 seconds, the OFA inlet(s). preferably 4-5 seconds, for reachingThe 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, further the CFB boiler according to the current disclosure comprises means for feeding nitrogen reductant into the furnace. The nitro-gen reductant is fed into the furnace for accomplishingSNCR in order to reduce the NOX emissions from the fur-Means for reductant into the nace. feeding nitrogen furnace can be nozzles. As is known to the skilled per-son, the means for feeding nitrogen reductant into thefurnace are connected to an appropriate arrangement ofstoring, delivering' and. regulating' the amount of thenitrogen reductant. The set-up of such an arrangementis to be determined for each application and reductant.

In one embodiment, the CFB boiler according tothe current disclosure further comprises a fuel pipeand means for mixing fuel with at least some secondaryair in the fuel pipe before feeding the fuel into thefurnace 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 secondaryair 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 secondaryair surrounding' at least part of the fuel and firstsecondary air fed into the furnace.

In one embodiment, the CFB boiler according to the current disclosure further comprises a flue gas re- 28 circulation system for recirculating flue gas. The re-circulation of flue gas can be used for adjusting theair 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 isdisclosed. The CFB boiler according to the current dis-closure can be used for reducing the NOX emissions from combusting a range of different fuels.

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

Figure J. depicts a schematic presentation ofan embodiment of a method and a CFB boiler according tothe present disclosure. The features of the embodimentof Fig. 1 are not drawn to scale and components of theCFB boiler not necessary for describing the method andthe CFB boiler according to the present disclosure areomitted for clarity.

The furnace 1 is the place where the combus-is delimited tion reactions take place. The furnace 1 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 2is depicted as black dots dispersed throughout the fur-nace 1 and the solids recirculation system 17. Primaryair 3 is blown into the furnace 1 through primary airinlets 18 located at the bottom 16 of the furnace 1.Although in Fig. 1, picted. substantially' throughout the botton1 16 of the the primary air inlets 18 are de- furnace 1, this needs not be the case. Primary air 3causes the fluidization of the bed material 2 and its dispersion throughout the furnace 1. 29 Secondary air 4 is blown into the furnace 1 ata height above the primary air 3 entry into the furnace1. The fed thethrough air feed channels 15 and secondary air inlets19. In Fig. 1, 1 from two opposite directions. secondary air 4 is into furnace 1 secondary air 4 is fed into the furnaceIt would be possible tofrom four feed the secondary air 4 also, for example, directions, each corresponding to a wall 11 of the fur- nace.

Fuel 5 is fed. into the furnace 1 through. afuel pipe 14 and. a fuel inlet 20. Although. only' onefuel 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 canbe, for example two or four fuel inlets 20 on a furnace1 wall 11. 20 and the secondary air inlets 19 can be coordinated so that, The placing of the fuel inlet(s)forexample, there is one or more secondary air inlet 19under each fuel inlet 20.

A solids recirculation system 17 is also de-Although the details of the solids recirculation system 17 are omitted from the figure for picted in Fig. 1.clarity, it comprises a cyclone 10 for separating fluegas from solids and piping 24 for returning the solids1, the solids are returned into the furnace 1. In Fig. into the furnace 1 at the approximate height of fuel feeding. In practice, the height at which the solidsare 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 fur-nace 1, it would be possible that the solids would berecirculated into the furnace 1 through the fuel inlet20. There are many options for the skilled person torealize the optional connection between the fuel feed-recirculation systems. Similarly, ing and solids sec- ondary air 4 piping to the desired number and positionof secondary air inlets 20 can be arranged in a numberof ways as is known by the skilled person.

The feeding of secondary air 4 and fuel 5 inaddition to 3 bring about the stoichiometric combustion of the fuel 5. the primary air sub-In the method according to the present disclo-(OFA) 6 is fed into the furnace 1for super-stoichiometric combustion of the fuel 5. TheOFA 6 is fed into the furnace 1 through OFA inlets 21, sure, over-fire air of which four are shown in Fig. 1. The air feed channel15 leading to the OFA inlet 21 can be a part of thesame air feeding system as the air feed channel 15 usedfor secondary air 4. One of the OFA inlets 21 in Fig. 1is presented as a side view. There could be several,for example eight OFA inlets 21 altogether, in the samedirection. Three OFA inlets 21 are shown as end-view asthey are located at the furnace 1 wall 11 farthest awayfrom the viewing direction of Fig. 1. There could bein the wall 11 closest to the wall 11 is three additional ones viewing' direction, but this not shown inFig. 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 the19, 21. there could be two or more air sources. flow of air into the different air inlets 18,Alternatively,This alternative would allow the regulation of the airwhich in turn might have advantages in the regulation of the composition. of different sources independently, 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- 31 stoichiometric combustion zone 8 above the OFA 6 feed-ing height.

The majority of the combustion products leavesthe 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-of the solids so some uncombusted fuel 5 might be carried outfurnace 1 through the exit chute 9. Most of theare returned to the furnace 1 through the solids recir-culation system 17. parts of the CFB boiler.

The flue gas continues into furtherThese parts are not depictedin Fig. 1.

However, in some situations, it might be ad-vantageous to return also some of the flue gas 12 intothe 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 material2 sufficiently. On the other hand, the combined air co-efficient and (SR1+SR2) of the primary air 3 secondary' air 4 needs to be kept below 1. As flue gas 12 con-tains less oxygen than ambient air, the mass flow ofprimary air 3 increases faster relative to the amountof oxygen introduced into the furnace 1. when primaryair 3 is supplemented with flue gas 12. The flue gas 12depicted. with. dashed. out- recirculation systen1 22 is line, 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 furnace1. In panels a)-e) of Fig. 2, part of the secondary air4 is fed as first secondary air 4a mixed with fuel 5and part of the secondary air 4 is fed as second sec-ondary air 4b, the stream of second secondary air 4bsurrounding at least part of the fuel 5 and the first secondary air 4a fed into the furnace 1. 32 In Fig. 2, the secondary air inlet 19 and the fuel inlet 20 are depicted as seen from the inside ofThe two inlets 19, the furnace 1. 20 can be of any shape. In panels a)-c) and e), the fuel inlet 20 isrectangular. In panel d) the fuel inlet 20 is round.However, for example elliptical or asymmetric shapes can be envisaged. The first fed through the fuel inlet 20 into the furnace 1. secondary' air 4a is Fuel 5 and first secondary air 4a are fed intothe 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 beone continuous secondary air inlet 19 as in panels a),b) and d), ings 19 on different sides of the fuel inlet 20, or be formed of separate secondary air open-as inpanels c) and e). Typically the secondary air inlet 19approximately follows the outer contour of the fuel in-let 20. with the fuel inlet 20, The secondary air inlet 19 can be in contactas in panels a), b) and d), or at a distance from it, as in panel c). Also it is pos-sible that the secondary airtacts the fuel inlet 20,and d), 19 surrounds the fuel inlet 20 on all sides, inlet 19 partially con-as in panel e).

In panels a) the secondary air inletthus feed-ing second secondary air 4b from all sides of the fuel5 and first secondary air 4a entering the furnace. Inone continuous inlet 19 sur- panel b), secondary air rounds the fuel inlet 20 on all sides except from one side, thereby feeding second secondary air 4b fromthree sides of the fuel inlet 20, but not from. oneside. For example, the one side from which the second- ary air inlet 19 does not surround the fuel inlet 20 isbelow. this side could be above the fuel inlet 20.

Alternatively,Panel c) depicts an embodiment similar to theone in in panel b), except that the secondary air inlet 19 comprises three separate openings. 33 of the inlet 20 on In panel e), four separate openingssecondary air inlet 19 surround the fuelall sides, all sides of the fuel inlet 20. thereby feeding second secondary air 4b from ExampleA. time-averaged computational fluid dynamics(CFD) to simulate hydrodynamics, modelling approach for fluidization was appliedcombustion and NOX formationin a CFB furnace using the method according to the pre-sent disclosure. The furnace geometry used in simula-The furnacethe sectional profile of the furnace was rectangular. tions was determined from public sources. in the simulation was 44 m in height, cross- Forthe cross sectional areaThe bottom of the majority of the furnace,of the furnace was 21 m times 10 meters.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 fromeight fuel inlets,The fuel the vertical four on each long wall of the fur- nace. inlets were rectangular in cross of the SEC- tion, sides inlets being longerthan the horizontal sides.all walls of the furnace so that each long wall had 14 Each short wall Secondary air was fed from secondary air inlets in two rows of 7.of the furnace had six secondary air inlets in two rowsof three. All the fuel inlets and the secondary air in-lets of the longer walls were located along the part ofthe furnace wall that was oblique due to the narrowingthe bottom. of the furnace towards The secondary air was fed at the same level or above the level of fuel feeding. Primary air was fed upwards through the bottomof the furnace.

Three different simulations were run. One ofthen1 was a control case with. no OFA. feeding; It was (OFA) The lower of these compared with feeding over-fire air into the fur- nace at two alternative heights. 34 heights, termed “OFA level A” was 16 meters above theheight of primary The height, termed “OFA level B” was 26 meters above the height of air feeding. higherprimary air feeding.

OFA was simulated to be fed into the furnacethrough. nozzles, eight of which. were located. on onelong 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 ofprimary air were held fixed in all cases. Primary air When OFA was \ always comprised 60 (à of the total air. \ 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 partiallybetween 800 and 900 °C. However,in which the temperatures exceeded 900 °Cwith both OFA feeding level A and OFA feeding level B the volume of furnace increased in comparison with the control case with no OFA feed-ing. The increase in the furnace temperature was morepronounced with OFA feeding at level B. in thethe temperature at the top half of thewhereas with OFA feed- the temperatures in this region exceeded 900 °C.

Further,control case,furnace remained below 890 °C,ing,This surprising effect allows the efficient use of SNCRin CFB furnace, which would not have functioned ade-quately in CFB boilers known in the art.

Fuel nwisture evaporation takes place in thebottom part leading to locally cooler regions especial-ly close to the fuel inlets. On the side of the furnaceat which solids are returned to the furnace, moistureevaporates inside the return legs and ignition occursearlier in the furnace supported by the char and solidsThe furnace temperature level was predicted to rise in the recirculated from the solids separation system. l0 cases where OFA was fed into the furnace.were the highest with the OFA level B, Temperatureswhere the veloc-ity profile in the lower part was the smoothest and thechanneling effect the weakest.

When OFA. was fed the deficient regions were formed in the areas below OFA into furnace, oxygen- feeding as expected. Consequently carbon monoxide con-centrations increased - in case of OFA level B notablyalso in the upper part of the furnace. Peak values re-thus the corrosion risk was estimated Rather mained at 2 vol-%to stay low in the temperature zone considered.similar carbon monoxide burnout with exit values of500-700 ppm was achieved in all cases due to efficientOFA mixing. Oxygen was able to come into contact withthe carbon nwnoxide before the exit chutes. The exitflow of 'uncombusted. carbon. was predicted. to decreasewhen OFA feeding is used. Particle residence times inthe furnace increased when OFA feeding was introduced.Based on the results, combustion efficiency and fly ashuncombusted carbon are not expected to weaken due to OFA feeding.

According to the model, the NOX emission was remarkably decreased in cases where OFA feeding was in-troduced compared to the control case, as reductive re- actions gain in comparison to oxidative ones in air- deficient conditions. Initial NOX 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,This might reflect the longer fuel residence time before the O and reduction up to 60 6 in case of OFA level B. final combustion zone in the latter case.

It is obvious to a person skilled in the artthat 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 36 to the examples described above, instead they may vary within the scope of the claims.

Claims (29)

1. l. A method for reducing NOX emissions in a circu-lating fluidized bed boiler (CFB boiler), the CFB boil-having a rectangular cross(16) comprising er comprising a furnace (l) section, the furnace (l) comprising a bottom andfluidizable (2), the method simultaneously or in sequence in any order bed material- feeding primary air (3) from below,bottom. (l6)(2), into the furnace (l)rial (2) and entrapping at least a part of the bed ma-terial (2) (2) in the through theof the furnace (l) and. the bed 1naterial for fluidizing the bed mate- for circulating the bed material(l);- feeding secondary air (4) furnace into the furnace (l) at a height above the primary air (3) feeding, for sub-stoichiometric combustion of fuel (5); and- feeding fuel (5) into the furnace (l) at a height above the primary air (3)the fuel (5); c h a r a ct:e r i z e d feeding for combusting in that the næthodfurther comprises - mixing' fuel (5) with. at least some secondary air (4) before feeding the fuel (5) into the furnace(l); and - feeding over-fire air (OFA) (6) into the fur-nace (l) at a height above the secondary air (4) feed- ing height,fuel (5).

2. The method according to claim l, for super-stoichiometric combustion of the wherein feed- ing primary air (3) and secondary air (4) results inthe formation of za sub-stoichiometric combustion (7) in the zonefurnace (l)and the results between the primary air CEOFA (6)in the formation of a super-above the OFA (6) feeding height and feeding OFA (6) feeding height; stoichiometric combustion zone (8) feeding height. l0 38

3. The method according to claim l or 2, whereinfeeding OFA (6) at least 900 °C, results in a combustion temperature ofpreferably of 950-l,050jority of the furnace (l) °C in the ma-volume.

4. The method according to any of the precedingwherein the OFA (6) claims, feeding height is 20-40 m, preferably 25-35 m, more preferably 28-32 m above thesecondary air (4) feeding height.l-3, wherein the CFB boiler further comprises an exit chute

5. The method according to any of claims (9) and the OFA (6) is fed into the furnace (l) 2-8 mbelow the exit chute (9) of the furnace (l).

6. The method according to any of claims l-3,wherein the OFA (6) is fed into the furnace (l) at a height, which gives a fuel residence time of at least 3 seconds, preferably 4-5 seconds, (6) feeding height. for reaching the OFA

7. The method according to any of the precedingclaims, wherein the Velocity of the OFA (6)50-100 m så preferably 60-90 m s?

8. The method according to any of the precedingwherein the OFA (6) feeding is claims, is fed into the furnace (l) in a horizontal direction, or in a direction at an an-gle of 20-40°, from horizontal. preferably of approximately 30° downward

9. The method according to any of the precedingwherein the OFA (6) is fed into the furnace (l)(ll), of the furnace claims,located. at least at two walls(ll) comprising at least one, through nozzlespreferably at least at three walls(1), (ll) erably at least two nozzles. each said wall pref-

10. l0. The method according to any of the preceding claims, wherein a combined air coefficient of the pri- mary air (3) (SRfiSR2) is 0.5- 0.95, preferably 0.8-0.9.

11. ll. The method according to any of the preceding (sRtOt) Of 1.1- and secondary air (4) claims, wherein a total air coefficient 39 1.4, preferably' approximately' 1.2 is achieved. by theOFA (6) feeding.

12. The method according to any of the precedingclaims, wherein the primary air (3) comprises recircu- (12) of the combustion. lated flue gas for adjusting the air coefficient

13. The method according to any of the preceding claims, wherein nitrogen reductant (13) is fed into thefurnace (1) for reducing NOX through selective non-catalytic reduction (SNCR).

14. The method according to claim 13, wherein at least part of the nitrogen reductant (13) is sprayedalong with the OFA (6).

15. The method according to claim 13,(13) or water solution of urea. into the furnace (1)wherein thenitrogen reductant is water solution of ammonia,

16. The method according to any of the preceding claims, wherein part of the secondary air (4) is fed asfirst secondary air (4a) mixed with fuel (5) and partof the secondary air (4) is fed as second secondary air(4b), the stream of second secondary air (4b) surround-ing at least part of the fuel (5) and the first second-ary air (4a) fed into the furnace (1).

17. The method according to claim 16, wherein the (4a) is 12-25 m såand the Velocity of the second Velocity of the first secondary air preferably 15-20 m s_Ü secondary air (4b) is 15-40 m s_É 1 preferably 20-30 m s-

18. The method according to claim 16 or 17, where- in the first secondary air (4a) comprises 60-70 % ofthe secondary' air (4), and. the second secondary air(4b) comprises 30-40 % of the secondary air (4).

19. The method according to any of claims 16-18,wherein the fuel (5) is fed into the furnace (1)through a fuel pipe (14), and the first secondary air (4a) is mixed with the fuel (5) and fed into the fur- nace (1) simultaneously with the fuel (5) through saidfuel pipe (14).

20. The method according to claim 19, wherein the (4b) through. an air feed channel is fed into the furnace (1)(15) arranged. around atleast part of the length of the fuel pipe (14)(14). second secondary air and sur-rounding at least part of the fuel pipe

21. A CFB boiler comprising- a furnace (1) having a rectangular cross (16): SEC- tion and comprising a bottom - a solids recirculation system (17) for recircu-lating solids escaping from the furnace (1);- at least one primary air inlet (18) at the bot- (16) to the furnace (1) tom of the furnace (1) for feeding primary air in-from below;(19) at a for feeding inlet(18) - at least one secondary air height above the primary air inlet(s)(1);(20), secondary air (4) into the furnace- at least one fuel inlet (18) for feeding fuel and(4), the (1); in that the CFB boiler further at a height above(5), furnace the primary air inlet(s) optionally secondary air intoc h a r a ct:e r i z e dcomprises (14)with at least some secondary air (4)(14) before feeding the fuel (5) through the at least one fuel inlet - a fuel pipe and means for mixing fuel (5)in the fuel pipeinto the furnace (1)(20):(OFA) (19) and(21) at for feeding - at least one over-fire air inleta height above the secondary air inletOFA (6)

22. The CFB boiler according to claim 21,the at least one OFA inlet (21) is at a height of 20-40 more preferably 28-32 In above into the furnace (1). wherein m, preferably 25-35 m,the secondary air (4) feeding height.

23. The CFB boiler according to claim 21, wherein the CFB boiler further comprises an exit chute (9) and 41 the OFA inletexit chute (9) (21) of the furnace (1). is 2-8 m, preferably 3-5 m below the

24. The CFB boiler according to claim 21, wherein the at least one OFA inlet (21) is at a height whichgives a fuel residence time of at least 3 seconds,preferably 4-5 seconds, for reaching the OFA (6) in-let(s) (21).

25. The CFB boiler according24, wherein the at least one OFA to any of claims 21- inlet (21) is a noz- zle.

26. The CFB boiler according to any of claims 21- 25 further comprising' means for feeding' nitrogen re- (13)

27. The CFB boiler according ductant into the furnace (1). to claim 26, whereinis configured to at(20) as first second- at least one secondary air inletleast partly surround at least one fuel inlet for feeding part of the secondary air (4) ary air (4a) mixed with the fuel (5) through the atleast one fuel inlet (20) into the furnace (1), and forfeeding part of the secondary air (4) as second second- (4b) and first secondary air ary air surrounding at least part of the fuel (5)(4a)

28. The CFB boiler according to any of claims 21- fed into the furnace (1). 27 further comprising a flue gas recirculation system(22) for recirculating flue gas.

29. The use of a CFB boiler according to any of claims 21-28.