EP4638857A1

EP4638857A1 - Method for lowering emissions of a recovery boiler

Info

Publication number: EP4638857A1
Application number: EP23828416.0A
Authority: EP
Inventors: Hamilton BRANDÃO; Naveen CHENNA; Juho HILTUNEN; Jokke JANTUNEN; Jukka RÖPPÄNEN
Original assignee: Andritz Oy
Current assignee: Andritz Oy
Priority date: 2022-12-21
Filing date: 2023-12-12
Publication date: 2025-10-29
Also published as: JP2025541784A; PY23104852A; CN118224883A; FI20226146A1; WO2024134017A1; UY40580A

Abstract

The present invention relates to method for lowering emissions of a recovery boiler comprising the steps of spraying black liquor into a boiler furnace, combusting the sprayed black liquor within the boiler furnace generating steam by cooling flue gases of the combusted black liquor by passing the flue gases through heat exchangers, removing particles from the cooled flue gases with at least one electrostatic precipitator, passing the flue gases through at least one baghouse filter, removing further particles from the flue gases with bags in the at least one baghouse filter, and lowering emissions of the flue gases in an SCR catalyst phase.

Description

METHOD FOR LOWERING EMISSIONS OF A RECOVERY BOILER

DESCRIPTION

Scope of the invention

The invention relates to a method for lowering emissions of a recovery boiler. In particular, the invention relates to a method of lowering emissions from flue gases of a recovery boiler, especially a kraft recovery boiler of a pulp production plant.

Background of the invention

Lowering emissions, for example NOx emissions, of power and/or steam generating boilers is required according to official restrictions. Low NOx boiler designs and combustion air controls are not adequate as future restrictions will gradually demand lower emission limits. The emission levels required from kraft recovery boilers have been less stringent than from power boilers because of different chemistry and other properties of flue gases. Since better emission control technologies will also be developed for recovery boilers, they will be adopted to comply with official emission limitations even though they can cause additional investment and operation costs. Achieving additional reductions of other airborne emission of harmful particles and gases is also desired.

“Best Available Techniques (BAT) Reference Document for the Production of Pulp, Paper and Board” published by European Commission (2015) describes available technologies for controlling emissions of recovery boilers of pulp mills. Publication of Valmet Oy, “Recovery Boiler SCR - A Challenge and an Opportunity in Retrofit Cases” describes further solutions for NOx emission reduction methods for recovery boilers.

US8211391 , US8480984 and US8808652 describe SCR (selective catalytic reduction) and SNCR (selective non-catalytic reduction) NOx reduction methods for lowering NOx emissions of power boilers. Publications US5585081 , US20190118126 and US10071340 disclose integrated baghouse filtering and SCR phases.

Summary of the invention

The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

According to a first aspect of the present invention, there is provided a method for lowering emissions of a recovery boiler comprising the steps of spraying black liquor into a boiler furnace, combusting the sprayed black liquor within the boiler furnace, generating steam by cooling flue gases of the combusted black liquor by passing the flue gases through heat exchangers, removing particles from the cooled flue gases with at least one electrostatic precipitator. After electrostatic precipitation, the method further comprises passing the flue gases through at least one baghouse filter, removing particles from the flue gases in the at least one baghouse filter, and lowering emissions of the flue gases having a temperature of 180 °C or greater flown through bags of the at least one baghouse filter within a selective catalytic reduction (SCR) catalytic phase.

Brief Description of the drawings

Fig. 1 illustrates a schematic diagram of processes of an exemplary recovery boiler in accordance with at least some embodiments of the present invention.

Detailed Description of the Drawings

Fig. 1 illustrates a schematic diagram of processes of an exemplary recovery boiler 1 with peripheral components. An evaporator 2 processes black liquor discharged from pulp production process of a pulp mill and dries the liquor to desired dry solids content. Dried black liquor is sprayed into furnace of the recovery boiler 1 for combustion and recovery of circulated chemicals. Economizers 3 are heat exchanges which will heat feedwater and cool flue gases to exit temperature of the recovery boiler. The flue gases contain ash particles and the most of them are filtered within at least one electrostatic precipitator (ESP) 4. The flue gases are then led from the ESP 4 to a baghouse 5 for filtering remaining particles. The ESP 4 is most efficient in filtering out smallest particles which can create too compact particle layers on filtering bags of the baghouse 5. NOx components of the cleaned flue gases that are passed through filtering bags of the baghouse 5 are then reduced within a catalytic SCR phase, which will take place either within the baghouse or within a separate SCR unit 6. Residual heat energy of the flue gases will be recovered by at least one heat exchanger 7 before exiting to chimney 8.

Additive A is injected to flue gases before the ESP 4 or at least before the baghouse 5 for reacting with SOx components of the flue gases. The additive A may comprise for example calcium hydroxide or more preferably sodium bicarbonate. Those additives and resulting reaction products of them will be the same chemicals which are circulated within the pulp mill. The sodium bicarbonate of additive A may be made at the pulp mill from available Sodium carbonate, carbon dioxide and water. The amount of injected additive can be more than needed without extra cost as the all injected additive can be recycled within the pulp mill. That improves the economy of the SOx emission reduction. The surplus additive will ensure that unstable combustion occasions can also be tolerated. Additionally, the injected additive contributes to cleaning the baghouse filter since it has a bigger or a different particle size distribution than the recovery boiler ash. The reaction products are returned to the evaporator 2 and thus will be mixed with the sprayed black liquor. When SOx content of flue gases are reduced to an adequate low level, the ash dust accumulating on filtering bags of the baghouse 5 will not comprise a remarkable amount of sticky components. That will enable continuous NOx emission reduction by the SCR phase. The baghouse will also ensure that particle bursts due to occasional malfunction of the ESP 4 will always be filtered out and cannot contaminate the catalytic surfaces of the SCR phase. Additional or supplemental ammonia and/or urea may be injected to the flue gas stream before the baghouse 5 or at least before the separate SCR unit 6. Embodiments

It is an aim of embodiments to overcome at least some of the problems described above and provide a method for lowering emissions of a recovery boiler. The method involves combusting sprayed black liquor within the boiler furnace and generating steam by cooling flue gases from the combusted black liquor flowing through heat exchangers and removing particles from the cooled flue gases with at least one electrostatic precipitator. It is a further aim of embodiments to achieve reliable emission reductions of flue gases of a kraft recovery boiler without compromising efficiency of power generation. It has surprisingly been found that emissions of a recovery boiler are lowered and energy efficiency of a recovery boiler are improved in a method which comprises the steps of spraying black liquor into a boiler furnace, combusting the sprayed black liquor within the boiler furnace, generating steam by cooling flue gases of the combusted black liquor by passing the flue gases through heat exchangers, removing particles from the cooled flue gases with at least one electrostatic precipitator, passing the flue gases through at least one baghouse filter, removing further particles from the flue gases with bags in the at least one baghouse filter, and lowering emissions of the flue gases in an SCR catalyst phase.

Detailed Description

Embodiments of the present invention describe a method of lowering emissions of a recovery boiler comprising the steps of spraying black liquor into a boiler furnace, combusting the sprayed black liquor within the boiler furnace, generating steam by cooling flue gases of the combusted black liquor by passing the flue gases through heat exchangers, removing particles from the cooled flue gases with at least one electrostatic precipitator, passing the flue gases through at least one baghouse filter, removing further particles from the flue gases with bags in the at least one baghouse filter, and lowering emissions of the flue gases in a selective catalytic reduction (SCR) catalyst phase.

Power boilers have utilized baghouse filters downstream of electrostatic precipitators (ESP) for minimizing particle emissions. Baghouse filters have not been earlier used in kraft recovery boilers, mainly because of distinct dust properties. Kraft recovery boiler ash is extremely fine and has a very narrow size distribution. Typically 95% of the particles by mass have a diameter of 1.0 pm ±0.5 pm. On average, particulates exiting the boiler have a diameter of one micrometer (1.0 pm). Dust removal in baghouse filters is based on a dust cake formation on bags and the small uniform particle size makes the cake too dense. A dense cake on baghouse filters may increase pressure drop too much, for example pressure through the filter fabric may be increased too much due to the density of the cake and/ or the pressure drop in flue gases due to drag caused by clean filters and cake layers may increase too much. Additionally, the dense cake being formed from small, uniform-sized particles is liable to break easily, resulting in dust that is difficult to remove from the system. SOx emissions of the flue gases have also limited the use of baghouse filters due to sticky compounds in the dust that could glue dust particles to each other and onto the bags. The sticky particle layers grow too thick and restrict throughflow and the layers do not reliably fall when attempting reverse flow cleaning cycles. Therefore, ESPs have been used instead of baghouses in recovery boilers (RB).

The ESP is not a reliable enough particle removal method. ESPs cannot guarantee continuous removal of particles within the RB environment, but will occasionally have lower performance, meaning that not all particles are continuously removed. The reason is for example due to sudden combustion process changes, incomplete combustion or operational problems. These lower performance moments will substantially increase the dust particle amount in the flue gases after ESP, causing plugging of the SCR catalyst bed. A recovery boiler is normally continuously operated at full capacity, so it is not a simple task to wash SCR catalyst beds or replace catalysts during operation. Servicing operations are much easier with power boilers which often operate at variable power levels or discontinuously.

In one embodiment the temperature of the flue gases led to the SCR catalyst phase is 180 °C or higher, preferably 250 °C or higher, particularly 300 °C or higher when ammonia slip increases over a predetermined value and the temperature is lowered when ammonia slip decreases below a smaller predetermined value. For example, for the purposes of demonstration, ammonia slip is limited to a maximum of 5 mg/Nm³ with reference O2 of 6 % by increasing the temperature when ammonia slip reaches e.g. 4 or 4.5 mg/Nm³ or even exceeds 5mg/Nm³. The amount of ammonia slip provides an indication how much ammonia may be injected into the flue gases. If the amount of ammonia slip is too high, e.g. above the maximum acceptable level of 5mg/Nm³, less ammonia is injected into the flue gases, which also reduces the level of ammonia slip. Additionally, if NOx reduction must be limited due to the amount of ammonia slip, this indicates that the activity of the catalyst has been reduced from that of a new catalyst. The operating temperature of SCR phase should be above the dew point of ammonium bisulphate (ABS) in order to avoid accumulation of ABS on the catalytic surfaces. Preferably the operating temperature of the SCR phase should be above the dew point of sodium bisulphate (SBS) to avoid accumulation of SBS on the catalytic surface. Accumulated ABS and/or SBS covers active surfaces of the catalysts and thus inhibits the reduction of NOx by the catalyst. The operating temperature should, preferably, be in the range of 200 °C to 250 °C, preferably 230 °C or 220 °C to avoid accumulation of ABS and/or SBS. For optimum efficiency of electric power and steam generation, the temperature of flue gases after economizer heat exchangers of a recovery boiler should be close to 200 °C in line with current best practice. Combustion air preheater or other heating needs of the pulp mill can effectively utilize the still hot flue gases from the SCR phase. The temperature of flue gases led to the SCR catalyst phase is advantageously kept over 180 °C for at least 95 % of the operating time of the recovery boiler. The normal temperature range for a conventional recovery boiler provides for the maintenance of full steam power generation efficiency and lowest carbon dioxide emissions per produced unit of energy. The temperature range is the normal flue gas temperature of the flue gases after economizers. The temperature can be kept so low by ensuring that particles are finely filtered out and SOx emissions are sufficiently reduced from the flue gases. The flue gases should be cooled down to exit temperature within at least one heat exchanging phase after the SCR catalyst phase for full heat recovery to the heating of combustion air or other processes of the pulp mill.

In a further embodiment ammonia or urea is injected to the flow of flue gases before the SCR catalyst phase. For NOx reduction, injection of ammonia or urea to the flow of flue gases should take place before the SCR catalyst phase. Thus, in an embodiment additional or supplemental, ammonia, urea or a mixture thereof may be injected into the flow. The ammonia and/or urea injection for the SCR phase is preferably done downstream of the SOx reduction phase. In one embodiment, ammonia is injected in the form of ammonia gas, e.g. pure ammonia gas or anhydrous ammonia. In a further embodiment the ammonia is injected in the form of an ammonia water solution (ammonia water). The amount of ammonia in the ammonia water solution or ammonia water is not particularly limited. The amount of ammonia present may be up to and including 50 wt % of the ammonia water, e.g. 19 wt % or 29 wt %, preferably 25 wt%, most preferably 24 wt %.

In one embodiment the SCR catalyst phase takes place within the bags of the baghouse filter. Remarkable reduction of installation space is achieved if the SCR phase takes place within the bags of the baghouse. The bags may be embedded with the catalysts or support structures of the bags may be provided with catalytic coatings. By reducing the number of ESP phases and installing the baghouse filter in their place, the SCR can be added to existing recovery boiler without requiring additional, maybe unavailable space around the recovery boiler.

In an embodiment the removed particles from flue gases are mixed with virgin black liquor. The removed particles, contain valuable chemicals which can be recovered after mixing with the black liquor. Otherwise the chemicals e.g. sodium sulphate would be lost to landfill. It follows, therefore that in an embodiment the environmental impact of a mill or plant comprising a recovery boiler is decreased by decreasing the amount of particles that are removed from flue gases going to landfill.

In a further embodiment the dry solids content of the sprayed black liquor is more than 75 weight percentage by weight of the black liquor. If dry solids content of the sprayed black liquor is more than 75 weight percentage, SOx level is adequately low, e.g. 5 ppm or lower, preferably lower than 5 ppm, except during operation disturbances. As the solid content of the black liquor increases, the temperature in the furnace increases whereby more of the sulphur present in the black liquor binds itself to sodium sulphate. As the solid content of black liquor decreases, the temperature in the furnace is lower and the sulphur present in the black liquor tends to form sulphur dioxide.

In a particular embodiment the flue gases are cooled within at least one heat exchanging phase after the SCR catalyst phase. Heat is recovered in the heat exchanging phase which is useful in power generation, for example in the generation of electricity. Cooling the flue gases down to exit temperature within at least one heat exchanging phase after the SCR catalyst phase allows for full heat recovery for the heating of combustion air or for other processes of the pulp mill.

Continuous feeding of sorbent to the flue gas flow ensures that the SOx levels will stay low all the time. In a typical recovery boiler SOx emission peaks are found from time to time, the continuous feed of sorbent decreases the amount of SOx to essentially zero, which means lower than 5 ppm, e.g. 0, 1 , 2, 3 or 4 ppm. Thus, for limiting the amount of SOx produced, the dry solids content is not critical in an embodiment in which sorbent is continuously fed or continuously injected into the flow of the flue gases. Thus in an embodiment the sorbent is continuously injected into the flow of the flue gases. Sorbent for reacting with at least SOx gases is preferably dry powder. It should be injected to the flow of flue gases before baghouse filter. More preferably the injection takes place before the ESP phase in order to enable more reaction time with SOx gases. The reactions leads to recyclable materials of collected particles. Eliminating SOx emissions prevent formation of ABS, SBS, and layers of sticky particles on the bags of the baghouse. Since the sorbent comprises substances which are normally circulated within chemical recovery processes of a pulp mill, reaction products can be returned to the chemical circulation and preferably mixed with black liquor.

Thus, in an embodiment the sorbent is injected to the flow of flue gases before the baghouse filter, preferably before the electrostatic precipitator for reduction of possible SOx emissions. Since the sorbent has a bigger or a different particle size distribution than the recovery boiler ash, the injected sorbent provides the additional benefit of cleaning the baghouse filter surfaces. The sorbent typically comprises an alkali metal or an alkali earth metal salt or a mixture thereof. In an embodiment the sorbent comprises sodium hydroxide and/or calcium carbonate and/or sodium carbonate and/or sodium bicarbonate and/or sodium sesquicarbonate.

Selective catalytic reduction (SCR) can reduce sufficiently the NOx emissions from cooler flue gases. NOx scrubbers are also efficient, but the downside of them is toxic liquid emissions. Toxic liquid emissions are liquids that contain for example chlorine, chlorate, nitrate, nitrite etc. or strong acids including e.g. CIO2, HCI, HNO3 etc. The SCR method does not produce harmful byproducts. The SCR catalytic elements are very vulnerable to contaminations that will cover the elements and block the active porous catalytic surfaces. For example, the catalytic surfaces can be blocked, or plugged, with dust, ammonium sulphate, and/ or sodium salts such as sodium sulphate and sodium carbonate that are found and/or formed in a recovery boiler. Plugging prevents reactions at the catalyst surface as the active sites become blocked. Any failure of an ESP, even a failure that only lasts a few seconds results in contamination of the SCR catalytic elements and suppresses their NOx reduction capability. Contamination, or poisoning of the catalyst is caused by substances such as CaO, MgO, P, Na, etc. diffusing into the active sites of the catalyst and occupying them Such contamination demands washing the contaminated catalytic elements, e.g. with liquid solutions such as acids, which is difficult or even impossible to perform while the recovery boiler is in continuous operation. A reliable particle removal method is thus required before the SCR catalytic phase.

The method for reducing nitrogen oxide emissions of a kraft recovery boiler at a pulp mill comprises combustion of sprayed black liquor within boiler furnace, cooling of flue gases of combusted black liquor flowing through heat exchangers of the furnace, and removing particles from the cooled flue gases by at least one electrostatic precipitator. Then the flue gases from the at least one electrostatic precipitator flow to at least one baghouse filter, which additionally removes particles from the flue gases, and the NOx compounds of the flue gases flown through bags of the at least one baghouse filter are reduced within a SCR catalyst phase. The baghouse will remove particles which could contaminate the catalytic surfaces of the SCR catalyst phase.

For ensuring proper operation of the baghouse and omitting creation of ammonium bicarbonate (ABS) and sodium bisulphate (SBS), SOx level of the flue gas should be low enough before entering the baghouse. The SOx level is adjusted with the dry solids content of the sprayed black liquor. If the dry solids content of the sprayed black liquor is more than 75 weight percentage, SOx level is adequately low, except during operation disturbances.

The advantageous range of flue gas temperature entering the SCR phase will be lower than precipitation temperature of about 260 °C of SBS, preferably lower than the precipitation temperature of about 250 °C of ABS. Suitably, the temperature of the flue gas entering the SCR phase is 230 °C, typically 220 °C. Even though most of SOx emissions can be removed from the flue gases before injecting ammonia for the SCR phase, still ABS will slowly accumulate on the catalytic surfaces. The accumulated ABS can be vaporized by occasional raises of the temperature of the flue gases. The temperature should be at least 250 °C. In practice, the temperature should be raised to over 300 °C and more preferably to over 350 °C in order to clean the catalytic surfaces sufficiently fast. Still, the raised temperature cleaning occasions should take far less than 5 % of the operating hours of the recovery boiler. The raised temperature phase of flue gases led to the SCR phase is activated when ammonia slip increases over a predetermined value and the temperature is lowered to normal operating temperature when ammonia slip decreases below a smaller predetermined value. The normal operating temperature range may be possible to be used over a month or even later, depending operating conditions. The other ways to ensure the catalytic reactions are to wash, change or, in the case of catalyst poisoning, regenerate the catalytic elements. The SCR phase may be bypassed via a bypass conduit, if necessary for the washing or changing operations.

The reduction of NOx emissions of recovery boilers does not only cause installation and operation costs. A low NOx boiler, where the reduction of NOx emissions is achieved with combustion measures, is remarkable more expensive to construct than a conventional boiler. An effective NOx reduction from flue gases will enable remarkable cheaper designs of new recovery boilers.

Hence the proper operation of the SCR is guaranteed also at moderate temperatures. So, there should not be a need for constant operation of additional duct burners/heaters before the SCR phase. The flue gas temperatures can be kept energy efficiency-wise on the optimal level.

The following Table (Table 1 ) illustrates how emissions are lowered by means of at least some embodiments of the invention. The first column indicates the emission from a recovery boiler under consideration, the second column indicates the amount of the emissions from the recovery boiler when flue gases are not treated according to at least some embodiments of the invention and the third column indicates the amount of emissions from the recovery boiler when flue gases are treated by means of at least some embodiments of the invention.

Table 1

The measurements in Table 1 were made using FT-IR, with the exception of dust that was measured using gravimetry. The reductions in NOx and dust after treatment according to embodiments of the invention were measured were as the reduction in SO2 is predicted. Reductions in dust, SO2, which is included in SOx, and NOx emissions, however, can be seen using e.g. gravimetry (dust) according to SFS-EN 13284-1 , UV fluorescence detection (SO2) measured according to CEN/Ts 17021 :2017, chemiluminescence detection (NOx) measured according to SFS-EN 14792. Each of the emissions that has been reduced, as well as NH3 may be measured by various means as described e.g. on page 28 chapter 5 of Paastomittausten Kasikirja Osa 1 (Handbook of emission measurements part 1 ) published by VTT in June 2007. In the present example, NHs is not an emission the amount of which is lowered, but to a small extent increases due to the injection of NHs into flue gases. It’s presence may be measured e.g. by chemiluminescence. Typically in a pulp mill, measurements are taken continuously but also periodically, usually annually, for environmental monitoring by an external consultant. Methods for both continuous and periodic measurements are described in detail in both Paastomittausten Kasikirja Osa 1 (Handbook of emission measurements part 1 ), published by VTT in June 2007 and in Paastomittausten Kasikirja Osa 2 (Handbook of emission measurements part 2), published by VTT in April 2004.

It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases 'in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

Furthermore,, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

The verbs "to comprise" and "to include" are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, a singular form, throughout this document does not exclude a plurality.

Claims

1 . A method for lowering emissions of a recovery boiler comprising the steps of:

• spraying black liquor into a boiler furnace,

• combusting the sprayed black liquor within the boiler furnace

• generating steam by cooling flue gases of the combusted black liquor by passing the flue gases through heat exchangers,

• removing particles from the cooled flue gases with at least one electrostatic precipitator,

• passing the flue gases through at least one baghouse filter,

• removing further particles from the flue gases with bags in the at least one baghouse filter, and

• lowering NOx emissions of the flue gases in an SCR catalyst phase

2. The method according to claim 1 , wherein the temperature of the flue gases led to the SCR catalyst phase is 180 °C or higher, preferably 250 °C or higher, particularly 300 °C or higher.

3. The method according to claim 1 or 2, wherein the operating temperature of the SCR phase is in the range of 200 °C to 250 °C, preferably 230 °C or 220 °C.

4. The method according to any of the preceding claims, wherein ammonia or urea, or a mixture thereof is injected to the flow of flue gases before the SCR catalyst phase.

5. The method according to any of the preceding claims, wherein ammonia is injected to the flow of flue gases before the SCR catalyst phase, said ammonia being in the form of ammonia gas, preferably pure ammonia gas or anhydrous ammonia.

6. The method according to any of claims 1 to 4, wherein ammonia is injected to the flow of flue gases before the SCR catalyst phase, said ammonia being in the form of an ammonia water solution.

7. The method according to any of the preceding claims, wherein the SCR catalyst phase takes place within a SCR catalyst bed.

8. The method of according to any of the preceding claims, wherein the SCR catalyst phase takes place within the bags of the baghouse.

9. The method of according to any of the preceding claims, wherein the SCR catalyst phase takes place in both the SCR catalyst bed and within the bags of the baghouse.

10. The method according to any of the preceding claims, wherein the removed particles from flue gases are mixed with virgin black liquor.

11. The method according to any of the preceding claims, wherein dry solids content of the sprayed black liquor is more than 75 weight percentage.

12. The method according to any of the preceding claims, wherein the flue gases are cooled within at least one heat exchanging phase after the SCR catalyst phase.

13. The method according to any of the preceding claims, wherein sorbent is injected to the flow of flue gases before the electrostatic precipitator or at least before the baghouse for reduction of SOx emissions.

14. The method according to any of the preceding claims, wherein the sorbent comprises sodium hydroxide and/or calcium carbonate and/or sodium carbonate and/or sodium bicarbonate and/or sodium sesquicarbonate.

15. The method according to any of the preceding claims, wherein the sorbent is continuously injected into the flow of flue gases.