Method and system for controlling nitrogen flows in a pulp mill

Abstract

A method for controlling nitrogen flows in a pulp mill which comprises a recovery boiler, a lime kiln, a power boiler and a concentrated non-condensable gases (CNCG) incinerator and / or a wet gas sulfuric acid (WSA) plant is disclosed. Further is disclosed a system for controlling nitrogen flows in a pulp mill.

Description

[0001] METHOD AND SYSTEM FOR CONTROLLING NITROGEN FLOWS IN A PULP MILL

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a method for controlling nitrogen flows in a pulp mil l . The present disclosure further relates to a system for controlling nitrogen flows in a pulp mill .

[0004] BACKGROUND

[0005] Nitrogen oxide emissions in a pulp mill are mainly produced by a recovery boiler, a power boiler, a lime kiln, a concentrated non-condensable gases (CNCG) incinerator, and a wet gas sulfuric acid (WSA) plant . Nitrogen oxide emissions are a concern due to their negative impacts on air quality and the environment . As environmental regulations restricting nitrogen oxides tend to become stricter, there is a need to control nitrogen flows in a pulp mill . The inventors have therefore recogni zed the need for an efficient method for controlling nitrogen flows in a pulp mill .

[0006] SUMMARY

[0007] A method for controlling nitrogen flows in a pulp mill which comprises a recovery boiler, a lime kiln, a power boiler, and a concentrated non-condensable gases (CNCG) incinerator and / or a wet gas sulfuric acid (WSA) plant , wherein the WSA plant comprises a non- condensable gas (NCG) burner, a waste heat boiler, a SO2 converter and a condenser, is disclosed, wherein the method comprises ,

[0008] - passing flue gas from the recovery boiler through at least one electrostatic precipitator (ESP) to remove dust particles from the flue gas from the recovery boiler, and through a first fabric filter to remove further dust particles from the flue gas from the recovery boiler, and through a f irst selective catalytic reduction ( SCR) catalyst phase to reduce NOx emissions of the flue gas from the recovery boiler, and

[0009] - passing flue gas from the lime kiln through a second ESP to remove dust particles from the flue gas from the lime kiln, and through a second fabric filter or a ceramic candle filter to remove further dust particles from the flue gas from the lime kiln, and through a second SCR catalyst phase to reduce NOx emissions of the flue gas from the lime kiln, and

[0010] - passing flue gas from the power boiler through a third ESP to remove dust particles from the flue gas from the power boiler and / or through a third fabric filter to remove dust particles from the flue gas from the power boiler, and passing flue gas from the power boiler through a third SCR catalyst phase to reduce NOx emiss ions of the flue gas from the power boiler, and

[0011] - passing flue gas from the CNCG incinerator through a fourth SCR catalyst phase to reduce NOx emissions of the flue gas from the CNCG incinerator, and / or passing flue gas from the NCG burner through a fifth SCR catalyst phase to reduce NOx emissions of the flue gas from the NCG burner .

[0012] A system for controlling nitrogen flows in a pulp mill is disclosed . The system comprises a pulp mil l comprising a recovery boiler, a lime kiln, a power boiler, and a concentrated non-condensable gases (CNCG) incinerator and / or a wet gas sulfuric acid (WSA) plant, wherein the WSA plant comprises a non-condensable gas (NCG) burner, a waste heat boiler , a SO2 converter and a condenser, wherein the system comprises ,

[0013] - a first electrostatic precipitator (ESP) unit for removing dust particles from a flue gas from the recovery boiler, a first fabric filter for further removing dust particles from the flue gas from the recovery boiler, and a first selective catalytic reduction ( SCR) catalyst unit for reducing NOx emissions of the flue gas from the recovery boiler, and

[0014] - a second ESP unit for removing dust particles from a flue gas from the lime kiln, a second fabric filter or a ceramic candle filter for removing further dust particles from the flue gas from the lime kiln, and a second SCR catalyst unit for reducing NOx emis sions of the f lue gas from the lime kiln, and

[0015] - a third ESP unit for removing dust particles from a flue gas from the power boiler and / or a third fabric filter for removing dust particles from the flue gas from the power boiler, and a third SCR catalyst unit for reducing NOx emiss ions of the flue gas from the power boiler, and

[0016] - a fourth SCR catalyst unit for reducing NOx emi ssions of the flue gas from the CNCG incinerator, and / or a fifth SCR catalyst unit for reducing NOx emissions of the flue gas from the NCG burner .

[0017] BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The accompanying drawings , which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description help to explain the principles of the invention . In the drawings :

[0019] Figure 1 is a schematic diagram of processes of an exemplary recovery boiler in accordance with at least some embodiments of the present invention .

[0020] Figure 2 is a schematic diagram of processes of an exemplary lime kiln in accordance with at least some embodiments of the present invention . Figure 3 is a schematic diagram of processes of an exemplary power boiler in accordance with at least some embodiments of the present invention .

[0021] Figure 4 is a schematic diagram of processes of an exemplary CNCG incinerator in accordance with at least some embodiments of the present invention .

[0022] Figure 5 is a schematic diagram of processes of an exemplary WSA plant in accordance with at least some embodiments of the present invention .

[0023] Figure 6 is a schematic diagram of a system for controlling nitrogen flows in a pulp mill in accordance with at least some embodiments of the present invention .

[0024] Figure 7 is a schematic diagram of a system for controlling nitrogen flows in a pulp mill in accordance with at least some embodiments of the present invention .

[0025] DETAILED DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 illustrates a schematic diagram of processes of an exemplary recovery boiler 2 with peripheral components . The flue gas from the recovery boiler contains dust particles and the most of them are filtered within at least one electrostatic precipitator (ESP) 7a . The flue gas is then led from the first ES P 7a to a first fabric filter 8a for filtering remaining particles . The ESP may efficiently filter out particles which could otherwise create too compact particle layers on the fabric f ilter . The flue gas from the recovery boiler is then passed through a first catalytic SCR phase for reducing NOx emiss ions of the flue gas from the recovery boiler, which may take place either within the first fabric filter or within a first SCR unit 9a . Residual heat energy of the flue gas may be then recovered by at least one heat recovery unit I la . Sorbent 13a may be inj ected into f lue gas from the recovery boiler before the ESP 7a or at least before the first fabric fi lter 8a for reacting with Sox components of the flue gas . Additional or supplemental ammonia 12a may be inj ected into the flue gas from the recovery boiler before the first fabric filter 8a or at least before the first SCR unit 9a .

[0027] Figure 2 illustrates a schematic diagram of processes of an exemplary lime kiln 3 with peripheral components . The flue gas from the lime kiln contains dust particles and the most of them are filtered within at least one electrostatic precipitator (ESP) 7b . The flue gas from the lime kiln may then be led from the second ESP 7b to a second fabric filter 8b for filtering remaining particles . The ESP may efficiently filter out particles which could otherwise create too compact particle layers on the fabric filter . The flue gas from the lime kiln is then passed through a second catalytic SCR phase for reducing NOx emiss ions of the flue gas from the lime kiln, which may take place either within the second fabric filter or within a second SCR unit 9b . Residual heat energy of the flue gas may be then recovered by at least one heat recovery unit 11b . Sorbent 13b may be inj ected into flue gas from the lime kiln before the second ESP 7b or at least before the second fabric filter 8b for reacting with Sox components of the flue gas . Additional or supplemental ammonia 12b may be inj ected into the flue gas from the lime ki ln before the second fabric filter 8b or at least before the second SCR unit 9b .

[0028] Figure 3 illustrates a schematic diagram of processes of an exemplary power boiler 3 with peripheral components . The flue gas from power boiler contains dust particles , which may be fi ltered using an ESP and / or a fabric filter . The flue gas from the power boiler may be filtered using at least one electrostatic precipitator (ESP) 7c . The flue gas may then be led from the third ESP 7c to a third fabric filter 8c for filtering remaining particles . The ESP may efficiently filter out particles which could otherwise create too compact particle layers on the fabric filter . The flue gas from the power boiler is then passed through a third catalytic SCR phase for reducing NOx emiss ions of the f lue gas from the power boi ler, which may take place either within the fabric filter or within a third SCR unit 9c . Residual heat energy of the flue gas may be then recovered by at least one heat recovery unit 11c . Sorbent 13c may be inj ected to flue gas from the power boiler before the third ESP 7c or at least before the third fabric filter 8c for reacting with Sox components of the flue gas . Additional or supplemental ammonia 12c may be inj ected to the flue gas from the power boiler before the third fabric filter 8c or at least before the third SCR unit 9c .

[0029] Figure 4 illustrates a schematic diagram of processes of an exemplary CNCG incinerator 4 with peripheral components . The flue gas from the CNCG incinerator is passed through a fourth SCR catalyst unit 9d for reducing NOx emissions of the flue gas from the CNCG incinerator . Additional or supplemental ammonia 12d may be inj ected to the flue gas from the CNCG before the SCR unit 9d . Residual heat energy of the flue gas may be then recovered by at least one heat recovery unit l id .

[0030] Figure 5 illustrates a schematic diagram of processes of an exemplary wet gas sulfuric acid (WSA) plant 5 with peripheral components . The flue gas from the NCG burner 6a is passed through a waste heat boiler 6b to cool the flue gas . Then the flue gas from the NCG burner 6a is passed through a SCR catalyst unit 9e for reducing NOx emissions of the flue gas from the NCG burner . Additional or supplemental ammonia 12e may be inj ected to the f lue gas from the NCG burner after the waste heat boiler 6b and before the fifth SCR unit 9e . The WSA plant further comprises a SO2 converter 6c, wherein SO2 is converted to SO3 us ing a catalyst . Then , the temperature of the flue gas is lowered using a condenser 6d and most of SO3 is recovered as sulfuric acid . After this , remaining SO2 may be removed from the flue gas by using an SO2 scrubber 6e . Residual heat energy of the flue gas may be then recovered by at least one heat recovery unit l ie .

[0031] Figure 6 is a schematic illustration of a system for controlling nitrogen flows in a pulp mill 1 , which comprises a recovery boiler 2 , a lime kiln 3 , a power boiler 4 , and a concentrated non-condensable gases (CNCG) incinerator 5 , and a wet gas sulfuric acid (WSA) plant 6 in accordance with at least some embodiments of the present invention . The flue gas from the recovery boiler contains dust particles and the most of them are filtered within at least one electrostatic precipitator (ESP) 7a . The flue gas is then led from the first ES P 7 a to a first fabric filter 8 a for fi ltering remaining particles . The flue gas from the recovery boiler is then passed through a first catalytic SCR phase for reducing NOx emissions of the flue gas from the recovery boiler, which may take place either within the first fabric filter 8a or within a first SCR unit 9a . Res idual heat energy of the flue gas may be then recovered by at least one heat recovery unit I la . Sorbent 13a may be inj ected into flue gas from the recovery boiler before the first ESP 7a or at least before the first fabric filter 8b for reacting with Sox components of the flue gas . Additional or supplemental ammonia 12a may be inj ected into the flue gas from the recovery boiler before the first fabric filter 8a or at least before the first SCR unit 9a . The flue gas from the lime kiln contains dust particles and the most of them are filtered within at least one electrostatic precipitator (ES P) 7b . The flue gas from the lime kiln is then led from the second ESP 7b to a second fabric filter 8b for filtering remaining particles . The ESP may efficiently filter out particles which could otherwise create too compact particle layers on the fabric filter . The flue gas from the lime kiln is then passed through a second catalytic SCR phase for reducing NOx emissions of the flue gas from the lime kiln, which may take place either within the second fabric f ilter or within a second SCR unit 9b . Residual heat energy of the flue gas may be then recovered by at least one heat recovery unit 11b . Sorbent 13b may be inj ected into flue gas from the lime kiln before the second ESP 7b or at least before the second fabric filter 8b for reacting with Sox components of the flue gas . Additional or supplemental ammonia 12b may be inj ected into the f lue gas from the l ime kiln before the second fabric filter 8b or at least before the second SCR unit 9b . The flue gas from power boiler contains dust particles , which may be filtered using an ESP and / or a fabric filter . The f lue gas from the power boiler may be filtered using at least one electrostatic precipitator (ESP) 7c . The flue gas is then led from the third ES P 7c to a third fabric filter 8c for filtering remaining particles . The ESP may efficiently filter out particles which could otherwise create too compact particle layers on the fabric filter . The flue gas from the power boiler is then pas sed through a third catalytic SCR phase for reducing NOx emissions of the flue gas from the power boiler, which may take place either within the third fabric filter or within a third SCR unit 9c . Residual heat energy of the f lue gases may be then recovered by at least one heat recovery unit 11c . Sorbent 13c may be inj ected to flue gas from the power boiler before the third ESP 7c or at least before the third fabric filter 8c for reacting with Sox components of the flue gas . Additional or supplemental ammonia 12c may be inj ected to the flue gas from the power boiler before the third fabric filter 8 c or at least before the third SCR unit 9c . The flue gas from the CNCG incinerator is passed through a fourth SCR catalyst unit 9d for reducing NOx emissions of the flue gas from the CNCG incinerator . Additional or supplemental ammonia 12d may be inj ected to the flue gas from the CNCG before the fourth SCR unit 9d . The flue gas from the NCG burner 6a is passed through a waste heat boiler 6b to cool the flue gas . Then the flue gas from the NCG burner 6a is passed through a fifth SCR catalyst unit 9e for reducing NOx emissions of the flue gas from the NCG burner . Additional or supplemental ammonia 12e may be inj ected to the flue gas from the NCG burner after the waste heat boiler 6b and before the fifth SCR unit 9e . The WSA plant further comprises a SO2 converter 6c, wherein SO2 is converted to SO3 using a catalyst . Then, the temperature of the flue gas is lowered using a condenser 6d and most of SO3 is recovered as sulfuric acid . After this , remaining SO2 may be removed from the flue gas by using SO2 scrubber 6e . Residual heat energy of the flue gas may be then recovered by at least one heat recovery unit l ie .

[0032] Figure 7 is a schematic illustration of a system for controlling nitrogen flows in a pulp mill 1 , which comprises a recovery boiler 2 , a lime kiln 3 , a power boiler 4 , and a concentrated non-condensable gases (CNCG) incinerator 5 , and a wet gas sulfuric acid (WSA) plant 6 in accordance with at least some embodiments of the present invention . In the wood handling Al , wood bl is converted into bark b2 and chips b3 . Nitrogen bound to bark b2 and chips b3 is in organic form . The nitrogen bound to bark b2 is combusted in the power boiler 4 with sludge bl 7 . Then the chips b3 and white liquor bl2 are directed to cooking A2 . The nitrogen in white liquor bl2 is mostly in the form of ammonia and cyanate . During cooking A2 , some non-condensable gases (NCGs ) b4 are generated, which may contain a small amount of ammonia . After washing and screening A3 , the produced pulp b5 is free of nitrogen , while the nitrogen of the weak black liquor b6 is primarily in an organic form, although there may be some presence of ammonia . In the evaporation A4 , all ammonia is separated from weak black liquor b6 and converted into NCGs b8 and condensates . The final strong black liquor b7 contains only organic nitrogen . After the recovery boiler 2 , the nitrogen in smelt b9 is in the form of cyanate . In recausti zation A5 , a portion of nitrogen is separated from the white liquor bl2 as ammonia in non-condensable gases (NCGs ) bl 3 . In the ef fluent treatment A6 , the nitrogen content in the effluents bl 4 should be minimal . The primary nitrogen load arises from nutrients bl 5 , either in the form of urea or ammonia . Through biological activity, the nitrogen is converted into organic nitrogen and eventually transformed into sludge bl 7 .

[0033] The flue gas from the recovery boiler contains dust particles and the most of them are filtered within at least one electrostatic precipitator (ESP) 7a . The flue gas is then led from the first ESP 7a to a first fabric filter 8a for filtering remaining particles . The ESP may efficiently filter out particles which could otherwise create too compact particle layers on the fabric filter . The flue gas from the recovery boiler is then passed through a first catalytic SCR phase for reducing NOx emissions of the flue gas from the recovery boiler, which may take place either within the first fabric filter 8a or within a first SCR unit 9a . Residual heat energy of the flue gas may be then recovered by at least one heat recovery unit I la . Sorbent 13a may be inj ected into flue gas from the recovery boiler before the first ESP 7a or at least before the first fabric fi lter 8b for reacting with Sox components of the flue gas . Additional or supplemental ammonia 12a may be inj ected into the flue gas from the recovery boiler before the first fabric filter 8a or at least before the first SCR unit 9a . The flue gas from the lime kiln contains dust particles and the most of them are filtered within at least one electrostatic precipitator (ESP) 7b . The flue gas from the lime kiln is then led from the second ESP 7b to a second fabric filter 8b for filtering remaining particles . The ES P may efficiently filter out particles which could otherwise create too compact particle layers on the fabric filter . The flue gas from the lime kiln is then passed through a second catalytic SCR phase for reducing NOx emiss ions of the flue gas from the lime kiln, which may take place either within the second fabric filter 8b or within a second SCR unit 9b . Residual heat energy of the flue gas may be then recovered by at least one heat recovery unit 11b . Sorbent 13b may be inj ected into flue gas from the lime kiln before the ESP 7b or at least before the fabric f ilter 8b for reacting with Sox components of the flue gas . Additional or supplemental ammonia 12b may be inj ected into the flue gas from the lime kiln before the second fabric filter 8b or at least before the second SCR unit 9b . The flue gas from power boiler contains dust particles , which may be filtered using an ESP and / or a fabric filter . The flue gas from the power boiler may be filtered using at least one electrostatic precipitator (ESP) 7c . The flue gas may then be led from the third ESP 7c to a third fabric filter 8c for filtering remaining particles . The ESP may efficiently filter out particles which could otherwise create too compact particle layers on the fabric filter . The flue gas from the power boiler is then passed through a catalytic SCR phase for reducing NOx emissions of the flue gas from the power boiler, which may take place either within the third fabric filter 8c or within a third SCR unit 9c . Residual heat energy of the flue gases may be then recovered by at least one heat recovery unit 11c . Sorbent 13c may be inj ected to flue gas from the power boiler before the third ESP 7c or at least before the third fabric filter 8c for reacting with Sox components of the flue gas . Additional or supplemental ammonia 12c may be inj ected to the flue gas from the power boiler before the third fabric filter 8 c or at least before the third SCR unit 9c . The flue gas from the CNCG incinerator is passed through a fourth SCR catalyst unit 9d for reducing NOx emissions of the flue gas from the CNCG incinerator . Additional or supplemental ammonia 12d may be inj ected to the flue gas from the CNCG before the fourth SCR unit 9d . The flue gas from the NCG burner 6a is passed through a waste heat boi ler 6b to cool the flue gas . Then the flue gas from the NCG burner 6a is passed through a fifth SCR catalyst unit 9e for reducing NOx emissions of the flue gas from the NCG burner . Additional or supplemental ammonia 12e may be inj ected to the flue gas from the NCG burner after the waste heat boi ler 6b and before the fifth SCR unit 9e . The WSA plant further comprises a SO2 converter 6c, wherein SO2 is converted to SO3 using a catalyst . Then, the temperature of the flue gas is lowered using a condenser 6d . After thi s , SO2 may be removed from the flue gas by using SO2 scrubber 6e . Residual heat energy of the flue gas may be then recovered by at least one heat recovery unit l ie .

[0034] DETAILED DESCRIPTION

[0035] A method for controlling nitrogen flows in a pulp mill which comprises a recovery boiler, a lime kiln, a power boiler, and a concentrated non-condensable gases (CNCG) incinerator, and / or a wet gas sulfuric acid (WSA) plant , wherein the WSA plant comprises a non- condensable gas (NCG) burner, a SO2 converter and a condenser is disclosed, wherein the method comprises ,

[0036] - passing flue gas from the recovery boiler through at least one electrostatic precipitator (ESP) to remove dust particles from the flue gas from the recovery boiler, and through a first fabric filter to remove further dust particles from the flue gas from the recovery boiler, and through a f irst selective catalytic reduction ( SCR) catalyst phase to reduce NOx emissions of the flue gas from the recovery boiler, and

[0037] - passing flue gas from the lime kiln through a second ESP to remove dust particles from the flue gas from the lime kiln, and through a second fabric filter or a ceramic candle filter to remove further dust particles from the flue gas from the lime kiln, and through a second SCR catalyst phase to reduce NOx emissions of the flue gas from the lime kiln, and

[0038] - passing flue gas from the power boiler through a third ESP to remove dust particles from the flue gas from the power boiler and / or through a third fabric filter to remove dust particles from the flue gas from the power boiler, and passing flue gas from the power boiler through a third SCR catalyst phase to reduce NOx emiss ions of the flue gas from the power boiler, and

[0039] - passing flue gas from the CNCG incinerator through a fourth SCR catalyst phase to reduce NOx emissions of the flue gas from the CNCG incinerator, and / or passing flue gas from the NCG burnerthrough a fifth SCR catalyst phase to reduce NOx emi ssions of the flue gas from the NCG burner .

[0040] A system for controlling nitrogen flows in a pulp mill is disclosed . The system comprises a pulp mill 1 , comprising a recovery boiler 2 , a lime kiln 3 , a power boiler 4 , and a concentrated non-condensable gases (CNCG) incinerator 5 and / or a wet gas sulfuric acid (WSA) plant 6 , wherein the WSA plant comprises a NCG burner 6a, a waste heat boiler 6b, a SO2 converter 6c and a condenser 6d, wherein the system comprises ,

[0041] - a first electrostatic precipitator (ESP) unit 7a for removing dust particles from a flue gas from the recovery boiler, a first fabric fi lter 8a for further removing dust particles from a flue gas from the recovery boiler, and a first selective catalytic reduction ( SCR) catalyst unit 9a for reducing NOx emis sions of the f lue gas from the recovery boiler, and

[0042] - a second ESP unit 7b for removing dust particles from a flue gas from the lime kiln, a second fabric filter 8b or a ceramic candle filter 10 for removing further dust particles from a flue gas from the lime kiln, and a second SCR catalyst unit 9b for reducing NOx emissions of the flue gas from the lime kiln, and

[0043] - a third ESP unit 7c for removing dust particles from a flue gas from the power boiler and / or a third fabric filter 8c for removing dust particles from the flue gas from the power boiler, and a third SCR catalyst unit 9c for reducing NOx emi ssions of the flue gas from the power boiler, and

[0044] - a fourth SCR catalyst unit 9d for reducing NOx emissions of the flue gas from the CNCG incinerator and / or a fifth SCR catalyst unit 9e for reducing NOx emissions of the flue gas from the NCG burner .

[0045] Nitrogen oxide emissions in a pulp mill are mainly produced by a recovery boiler, a power boiler, a lime kiln, a concentrated non-condensable gases (CNCG) incinerator, and a wet gas sulfuric acid (WSA) plant . Nitrogen oxide emissions are a concern due to their negative impacts on air quality and the environment .

[0046] In a pulp mi ll , nitrogen can exi st in various forms , including organic, ammonia, cyanate (OCN) , NOx, N2 , or other types of nitrogen ( such as salts and dis solved nitrogen) . Initially, the maj ority of nitrogen may arrive at the pulp mill bound to wood in the form of organic nitrogen, approximately 1 . 5 to 3 . 0 kgN / Adt , depending on the wood type . In the wood handling process , wood may be transformed into both bark and wood chips , with approximately 10% to 30 % of the organic nitrogen transferring to the bark . The bark may then be combusted in the power boi ler , where the fuel nitrogen can undergo transformation into either NOx or N2 . Typically, 30 % of fuel nitrogen may transformed into NOx . Nitrogen oxides are mainly generated from nitrogen bound to different incinerated flows , such as black liquor, wood residues , sludge , support fuels and ammonia in CNCGs .

[0047] Nitrogen oxide emissions in a pulp mill are primarily formed through two main mechanisms : thermal NOx (nitrogen in air converts into NOx) and fuel NOx . Beside these , also prompt formation of NOx is possible . Prompt NOx may form from the rapid reaction of atmospheric nitrogen with hydrocarbon radicals . Compared to the total NOx generated from combustion, prompt NOx typically constitutes a relatively small portion . However, as environmental regulations restricting nitrogen oxides tend to become stricter, even the contribution of prompt NOx becomes more important .

[0048] Selective catalytic reduction ( SCR) is used to describe a chemical reaction in which harmful nitrogen oxides (NOx) in a flue gas are converted into water (H2O) and nitrogen (N2 ) by using a catalyst and a reducing agent . SCR may effectively reduce NOx emissions from cooler flue gases compared to SNCR ( Selective Non Catalytic Reduction) . In SCR, the flue gas temperature may range from 150 to 500 ° C, while in SNCR the flue gas temperature may range from 800 to 1100 ° C . However, the catalytic elements of SCR are very vulnerable to contaminations that may cover the catalyst elements and block the active porous catalytic surfaces . The dust from the recovery boiler contains huge amounts of sodium and the dust from the lime ki ln contains calcium . Both of these elements can be considered detrimental to the catalyst material , acting as potential contaminants and poison . Consequently, the integration of SCR technology into recovery boilers and lime kilns has not been feasible due to the presence huge and variable amount of dust in their flue gases . However, the inventors surprisingly found out that the dust could be further removed from the flue gases by using fabric filters and then SCR could be integrated into recovery boiler and lime kiln .

[0049] The expression "fabric filter" should be understood in this specification, unless otherwise stated as a fi lter, that util i zes fabric f iltration to remove dust particles from the flue gas by depositing the dust particles on fabric material . The fabric filter may also sometimes be referred to as a baghouse filter . The use of the fabric filter not only extends the lifespan of SCR but also offers additional advantages . Specifically, employing the fabric filter allows SCR to operate with a reduced catalyst pitch, resulting in lower catalyst consumption and lower overall catalyst expenses .

[0050] An electrostatic precipitator (ESP) may be used for removing dust particles from the flue gases . However, ESPs are not always rel iable for continuous particle removal due to occasional performance issues , resulting in uneven dust particle removal . These performance issues may increase the dust particle amount in the flue gases after ESP , caus ing plugging of the SCR catalyst bed . The inventors surprisingly found out that dust particles could be further removed from the flue gases by using fabric filters and thus making the dust particle removal more reliable .

[0051] After passing through the ESP the dust particle level of the flue gas may decrease to 30 mg / Nm3, and further, after fabric filter, it may reduce to 5 mg / Nm3.

[0052] In one embodiment , after the ESP the dust particle level is in the range of 10 - 30 mg / Nm3, or 10 - 50 mg / Nm3, or 10 - 100 mg / Nm3. In one embodiment , after the fabric filter the dust particle level is in the range of 1 - 5 mg / Nm3, or 1 - 10 mg / Nm3, or 1 - 15 mg / Nm3. In one embodiment , after the ceramic candle filter the dust particle level is in the range of 1 - 5 mg / Nm3, or 1 - 10 mg / Nm3, or 1 - 15 mg / Nm3. The dust particle level may be measured according to standard SFS-EN 13284 - 1 .

[0053] As mentioned, the catalytic elements of SCR are very vulnerable to contaminations that may cover the catalyst elements and block the active porous catalytic surfaces . The f lue gases of the power boiler, CNCG incinerator, lime kiln and recovery boiler contain SO2 which, when combined with NH3 may form ammonia salt that may block the active porous catalytic surfaces . The ammonia salt formation, in which SO2 and NH3 convert to mostly ammonium sulfate on the surface of the catalyst, especially in low temperatures , can be avoided either with flue gas temperature control or with sulphur removal .

[0054] In one embodiment , the WSA plant comprises the following devices in the following order : a NCG burner 6a, a waste heat boiler 6b, a SO2 converter 6c and a condenser 6d . In one embodiment , the WSA plant comprises the following devices in the following order : a NCG burner 6a, a waste heat boiler 6b, a SO2 converter 6c, a condenser 6d and a SO2 scrubber 6e . In one embodiment , the fifth SCR unit 9e is between a waste heat boiler 6b and a SO2 converter 6b .

[0055] In one embodiment , ammonia is inj ected into the flue gas before the first , second, third, and / or fourth SCR catalyst phase ( s ) . In one embodiment , ammonia is inj ected into the flue gas before the first , second, and / or third fabric filter ( s ) . In one embodiment , ammonia is inj ected into the flue gas from the NCG burner after the waste heat boiler and before the fifth SCR catalyst phase . In one embodiment , the system comprises an inlet 12a configured for receiving ammonia before the first SCR catalyst unit 9a, an inlet 12b configured for receiving ammonia before the second SCR catalyst unit 9b, an inlet 12c configured for receiving ammonia before the third SCR catalyst unit 9c, an inlet 12d configured for receiving ammonia before the fourth SCR catalyst unit 9d and / or an inlet 12e configured for receiving ammonia after the waste heat boiler 6b and before the fifth SCR catalyst unit 9e . In one embodiment , the system comprises an inlet 12a configured for receiving ammonia before the first fabric filter 8a, an inlet 12b configured for receiving ammonia before the second fabric filter 8b, and / or an inlet configured for receiving ammonia before the third fabric filter 8c . In one embodiment , the inlet configured for receiving ammonia is a flange or a welded connection at flue gas duct or at separate chamber .

[0056] In one embodiment , the ammonia is an ammonia gas , a pure ammonia gas , an anhydrous ammonia, an ammonia water solution, or urea . Ammonia may be used as a reducing agent to convert NOx emi ssion to into water (H2O) and nitrogen (N2 ) at the surface of the catalyst . The method and system as disclosed in the current specification have the added utility of enabling the internal utili zation of ammonia in the pulp mill .

[0057] Ammonia water may be sprayed into the duct using a pump, noz zle , and, if necessary, a mechanical mixer . Alternatively, the ammonia water solution can be vapori zed in a separate chamber and then mixed with the flue gases .

[0058] When using anhydrous ammonia, noz zles may be used as the anhydrous ammonia is already in high pres sure and gas format .

[0059] Even though most of SOx emissions can be removed from the flue gases before inj ecting ammonia, still ammonium bisulphate (ABS ) may slowly accumulate on the catalytic surfaces . The accumulated ABS can be vapori zed by occasional increases of the temperature of the flue gases . The temperature may be at least 250 °C . In practice , the temperature may be raised to over 300 °C and more preferably to over 350 °C in order to clean the catalytic surfaces sufficiently fast . The raised temperature phase of f lue gas led to the SCR phase may be activated when ammonia slip increases over a predetermined value . The temperature is lowered back to normal operating temperature when ammonia slip decreases below a smaller predetermined value . The other ways to ensure the catalytic reactions are to wash, change or, in the case of catalyst poi soning, regenerate the catalytic elements . The SCR phase may be bypassed via a bypass conduit, if necessary for the washing or changing operations .

[0060] The expression "ammonia slip" should be understood in this specification, unless otherwise stated, as an amount of ammonia passing through the SCR unreacted. This may occur when ammonia is injected in excess, operating temperatures may be too low for ammonia to react, or the catalyst has been poisoned. The ammonia slip provides an indication how much ammonia may be injected into the flue gas. For example, for the purposes of demonstration, ammonia slip is limited to a maximum of 5 mg / Nm3with reference O2 of 6 % by increasing the temperature when ammonia slip reaches e.g. 4 or 4.5 mg / Nm3or even exceeds 5mg / Nm3. If the 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.

[0061] In one embodiment, the operating temperatures of the first and / or third SCR catalyst phases are in the range of 180 °C - 250 °C, or 190 °C - 240 °C, or 200 °C

[0062] - 230 °C, or 220 °C - 230 °C. In one embodiment, the operation temperature of the second SCR catalyst phase is in the range of 260 - 350 °C or 280 - 320 °C when using the ceramic candle filter or the operation temperature of the second SCR catalyst phase is in the range of 180 °C - 250 °C, or 190 °C - 240 °C, or 200 °C

[0063] - 230 °C, or 220 °C - 230 °C when using the second fabric filter. In one embodiment, first SCR catalyst unit 9a, second SCR catalyst unit 9b, third SCR catalyst unit 9c are configured to operate at a temperature in the range of 180 °C - 250 °C, or 190 °C - 240 °C, or 200 °C - 230 °C, or 220 °C - 230 °C. In one embodiment, the second SCR catalyst unit 9b is configurated to operate at a temperature in the range of 260 - 350 °C or 280 - 320 °C when the ceramic candle filter 10 is used or the second SCR catalyst unit 9b is configured to operate at a temperature in the range of 180 °C - 250 °C, or 190 °C - 240 °C, or 200 °C - 230 °C, or 220 °C - 230 °C when the second fabric filter 8b is used.

[0064] The operating temperature range of 220 °C - 230 °C can be considered energy efficient for the first, second and third SCR phase. This temperature range allows for optimal energy utilization while maintaining effective performance. In one embodiment, the operating temperatures of the fourth SCR catalyst phase is in the range of 210 °C - 500 °C, or 300°C - 450 °C, or 350 °C

[0065] - 400 °C, or 400 °C - 420 °C. In one embodiment, the fourth SCR catalyst unit 9d is configured to operate at a temperature in the range of 210 °C - 500 °C, or 300°C

[0066] - 450 °C, or 350 °C - 400 °C, or 400 °C - 420 °C. In one embodiment, the operating temperature of the fifth SCR catalyst phase is in the range of 400 - 420 °C. In one embodiment, the fifth SCR catalyst unit 9e is configured to operate at a temperature in the range of 400

[0067] - 420 °C.

[0068] The operating temperature of SCR phase may 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 may be above the dew point of sodium bisulphate (SBS) to avoid accumulation of SBS on the catalytic surface . Accumulated ABS and / or SBS may cover active surfaces of the catalysts and thus may inhibit the reduction of NOx by the catalyst . The operating temperatures may be , in the range of 200 ° C to 250 °C, preferably 230 ° C or 220 ° C to avoid accumulation of ABS and / or SBS .

[0069] In one embodiment , the flue gas (es ) is / are cooled within at least one heat recovery phase after the first , second, third, and / or fourth SCR catalyst phase ( s ) . Heat recovered in the heat recovery phase may be used in power generation, for example in the generation of electricity by reducing the usage of process steam . In one embodiment , the system comprises at least one heat recovery unit I la, 11b, 11c, l id configured to cool the flue gas after the first SCR catalyst unit 9a, after the second SCR catalyst unit 9b, after the third SCR catalyst unit 9c, after the fourth SCR catalyst unit 9d . In one embodiment , the f lue gas from the WSA plant is cooled within at least one heat recovery phase after the WSA plant . In one embodiment , the system comprises at least one heat recovery unit l ie configured to cool the flue gas after the WSA plant 6 .

[0070] In one embodiment , the method further comprises passing the flue gas from the NCG burner through the waste heat boiler to cool the flue gas from the NCG burner before the fifth SCR phase . In one embodiment , the waste heat boiler 6b is configured to cool the flue gas from the NCG burner before the fifth SCR catalyst unit . Typically, the temperature of the flue gas from the NCG burner may be in the range of 900 - 1300 ° C . In one embodiment , the waste heat boiler 6b i s conf igured to cool the flue gas from the NCG burner to temperature range of 400 ° C - 420 ° C .

[0071] In one embodiment, the heat recovery unit is a heat exchanger or a scrubber . In scrubber, water or chemical solvents are sprayed into the flue gas and heat is absorbed into the small droplets . Then, the heat from the sprayed liquid can be recovered with a separate heat exchanger, transferring the heat to water .

[0072] In one embodiment , the first and second heat recovery phase is a flue gas cooler . In one embodiment , the first and the second heat recovery unit is a flue gas cooler . In one embodiment , the third heat recovery phase is an air preheater . In one embodiment , the third heat recovery unit is an air preheater . In one embodi ment , the fourth heat recovery unit is a waste heat boiler . In one embodiment , the fourth heat recovery phase is a waste heat boiler . In one embodiment , the fifth heat recovery phase is a scrubber . Cooling may be achieved using flue gas coolers at recovery boilers and lime kilns . In power boilers , heat can be recovered using an air preheater ( such as LUVO) . In flue gas coolers , the heat can be recovered by transferring it to water, while in the case of LUVO, it is directly recovered and transferred to the combustion air . In one embodiment , a sorbent is inj ected into the flue gas from the recovery boiler before the first fabric filter, the flue gas from the lime kiln before the second fabric filter, and / or the flue gas from the power boiler before the third fabric filter . In a power boiler, an SCR catalyst may be also installed between an economi zer and an air preheater, which are typical heat exchanger surfaces within the power boiler . In one embodiment , the system comprises an inlet 13a configured for receiving sorbent before the first fabric filter 8a, an inlet 13b configured for receiving sorbent before the second fabric filter 8b, and / or an inlet 13c configured for receiving sorbent before the third fabric filter 8c . In one embodiment , a sorbent is inj ected into the flue gas from the recovery boiler before the first ESP, the flue gas from the lime kiln before the second ESP, and / or the flue gas from the power boiler before the third ESP . In one embodiment , the system comprises an inlet 13a configured for receiving sorbent before the first ES P unit 7a , an inlet 13b configured for receiving sorbent before the second ESP unit 7b, and / or an inlet 13c configured for receiving sorbent before the third ESP unit 7c . In one embodiment , the inlet configured for receiving sorbent is a flange or a welded connection at flue gas duct .

[0073] The sorbent may be dosed using a dosing screw . The sorbent is in a powder form, and it is typically dosed at the location, where the flue gases are in vacuum . The vacuum will naturally suck the sorbent into the flue gas . In one embodiment , the sorbent is sodium hydroxide , calcium carbonate , sodium carbonate , sodium bicarbonate or sodium sesquicarbonate .

[0074] In one embodiment , the sorbent is continuously inj ected into the flue gas (es ) from the recovery boiler, power boiler, and / or the lime kiln . Continuous feeding of the sorbent to the flue gas has the added utility of maintaining low Sox levels . Eliminating Sox emissions prevent formation of ABS , SBS , and layers of sticky particles on the surface of the fabric filter . The sorbent has the added utility of cleaning the fabric filter surfaces . Due to the larger si ze of the sorbent particles compared to the dust particles of the flue gas after ESP, the dust particle si ze is reduced to approximately 1 pm after ESP, the sorbent particles adhere to the dust particles , effectively cleaning the fabric filter .

[0075] In one embodiment , the si ze of the sorbent particles is 10 - 150 pm, or 10 - 100 pm, or 10 - 90 pm, or 10 - 80 pm, or 10 - 70 pm or 10 - 60 pm, 10 - 50 pm, or 10 - 40 pm, or 10 - 30 pm, or 10 - 20 pm .

[0076] In one embodiment , the first , second, third, fourth, and / or fifth SCR catalyst phase is a SCR catalyst bed . In one embodiment , the first SCR catalyst unit 9a, the second SCR catalyst unit 9b, the third SCR catalyst unit 9c, the fourth SCR catalyst unit 9d, and / or the fifth catalyst unit is a SCR catalyst bed . In one embodiment , one or more of the SCR catalyst phases is a SCR catalyst bed .

[0077] In one embodiment , the first SCR catalyst phase takes place within the first fabric filter , the second SCR catalyst phase takes place within the second fabric filter, and / or the third SCR catalyst phase takes place within the third fabric f ilter . In one embodiment , the first SCR catalyst unit 9a is configured to the take place within the first fabric filter 8a, the second SCR catalyst unit 9b is configured to take place within the second fabric filter 8b , and / or the third SCR catalyst unit 9c is configured to take place within the third fabric filter 8c .

[0078] When the SCR catalyst phase takes place within the fabric filter it has the added utility of reducing the installation space . The fabric filter may be embedded with the catalysts and coated with dust protective filter either with two separate filters or as integrated as one filter .

[0079] In one embodiment , the first SCR catalyst phase takes place within the first fabric filter and in the SCR catalyst bed, the second SCR catalyst phase takes place within the second fabric filter and in the SCR catalyst bed, and / or the third SCR catalyst phase takes place within the third fabric filter and in the SCR catalyst bed . In one embodiment , the first SCR catalyst unit 9a is configured to the take place within the first fabric filter 8a and in the SCR catalyst bed, the second SCR catalyst unit 9b is configured to the take place within the second fabric filter 8b and in the SCR catalyst bed, and / or the third SCR catalyst unit 9c is configured to the take place within the third fabric filter 8c and in the SCR catalyst bed . When SCR catalyst phase takes place within the fabric filter and in the SCR catalyst bed this enables better control of NOx during unexpected highloads and reduces risk of ammonia slip .

[0080] The method and system as disclosed in the current specification has the added utility of allowing flexibility in combustion design in the recovery boiler, the lime ki ln, and the power boiler . A low NOx boi ler , 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 , power boilers and lime kilns . The method and system as disclosed in the current specification also provides flexibility in non- condensable gas (NCG) handling, because NOx emissions can be eliminated with the method as disclosed in the current specification and thus NOx control poses no additional design concerns .

[0081] The method and system as disclosed in the current specification has the added utility of reducing NOx emissions up to 80 % in the pulp mill .

[0082] In addition to reducing NOx emissions , the method and system as di sclosed in the current specifi cation have the added utility of simultaneously reducing dust and SOx emissions . Both of these emissions may cause problems in flue gas heat recovery due to formation of acidic liquids at low temperatures . When dust and SOx are reduced to minimal levels in flue gases , then the only limiting factor for flue gas heat recovery is the natural dew point of water vapor, which typically ranges from 60 to 70 ° C . For example , for the currently used recovery boilers , exit flue gas temperature is limited to 120 - 130 ° C and cooling water inlet temperature to 100 ° C due dissolving dust particles . With the method as disclosed in the current specification, the exit flue gas temperatures of 100 ° C and incoming cooling water temperature of 80 ° C are possible , which will lead to better heat utili zation, higher recovery boiler energy efficiency and increased electricity production at the pulp mill .

[0083] The method and system as disclosed in the current specification have the added utility of increasing energy recovery from flue gases in recovery boiler, lime kiln and power boiler by lowering flue gas exit temperature by minimi zing the generation of acidic compounds from dust and SO2. The method and system as disclosed in the current specification have the added utility of lowering impacts of nutrient feeding to effluent treatment plant by destroying NOx generated from sludge incineration in power boiler .

[0084] Nutrient (NH3 ) is transformed to organic nitrogen bound to sludge . The resulting sludge is then incinerated in the power boiler . By using the method as disclosed in the current specification, the NOx emissions resulting from the use of NH3 as a nutrient can be reduced . Thus , the method as disclosed in the current specification has the added utility of minimi zing environmental impacts of nitrogen from nutrient feeding to effluent treatment plant .

[0085] EXAMPLES

[0086] Reference will now be made in detail to the described embodiments .

[0087] The description below discloses some embodiments in such a detail that a person skilled in the art is able to uti li ze the method based on the di sclosure . Not all steps of the embodiments are discussed in detail , as many of the steps will be apparent for the person skilled in the art based on this specification .

[0088] Example 1 - Controlling nitrogen flows in a pulp mill

[0089] In this example , it is illustrated how nitrogen is control led in a pulp mil l by means of at least some embodiments of the invention . The first column of Table 1 indicates the flows under consideration, the second column of Table 1 , labeled Input (N content [ % from wood' s N] ) , indicates the nitrogen input as a percentage from wood's nitrogen content, and the third column, labeled Output (N content [% from wood's N] ) , indicates the nitrogen output as a percentage from wood's nitrogen content. In addition, a comparative example (Table 2) was made, where the flue gases were not treated according to at least some embodiments of the invention.

[0090] The measurements in Table 1 and 2 were made using FT-IR. Reductions in NOx emissions were detected using chemiluminescence and measured according to SFS- EN 14792. NH3 may be measured by various means as described e.g. on page 28 chapter 5 of Paas tbmittaus ten Kasikirja Osa 1 (Handbook of emission measurements part 1) published by VTT in June 2007. In the present example, NH3 is not an emission the amount of which is lowered, but to a small extent increases due to the injection of NH3 into flue gases. Its 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 Paastbmittausten Kasikirja Osa 1 (Handbook of emission measurements part 1) , published by VTT in June 2007 and in Paastbmittausten Kasikirja Osa 2 (Handbook of emission measurements part 2) , published by VTT in April 2004.

[0091] Table 1. Nitrogen flows in a pulp mill

[0092] Table 2 . Comparative example - Nitrogen flows in a pulp mill As shown in Table 1 above , both NOx emissions and ammonia in blow gases were significantly reduced compared to the comparative example in Table 2 . At table 2 , blow gases were untreated, while at table 1 the gases were incinerated at NCG boiler as with the integration as presented in figure 4 . A person skilled in the art recognises that with the advancement of technology, the basic idea of the invention may be implemented in various ways . The invention and its embodiments are thus not limited to the examples described above , instead they may vary within the scope of the claims .

[0093] The embodiments described hereinbefore may be used in any combination with each other . Several of the embodiments may be combined together to form a further embodiment . A system and method herein, may comprise at least one of the embodiments described hereinbefore . It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments . The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages . It wi ll further be understood that reference to "an" item refers to one or more of those items . The term "compri sing" is used in this specification to mean including the feature ( s ) or act ( s ) followed thereafter, without excluding the presence of one or more additional features or acts .

[0094] REFERENCE SYMBOLS

[0095] 1 pulp mill

[0096] 2 recovery boiler

[0097] 3 lime kiln

[0098] 4 power boiler

[0099] 5 concentrated non-condensable gases (CNCG) incinerator

[0100] 6 wet gas sulfuric acid (WSA) plant

[0101] 6a NCG burner

[0102] 6b waste heat boiler

[0103] 6c SO2 converter

[0104] 6d Condenser

[0105] 6e SO2 scrubber

[0106] 7 electrostatic precipitator (ESP) unit

[0107] 8 fabric filter

[0108] 9 selective catalytic reduction ( SCR) catalyst unit

[0109] 10 ceramic candle filter

[0110] 11 heat recovery unit

[0111] 12 inlet configured for receiving ammonia

[0112] 13 inlet configured for receiving sorbent

[0113] Al wood handling

[0114] A2 cooking

[0115] A3 washing

[0116] A4 evaporation

[0117] A5 recausti zation

[0118] A6 effluent treatment bl wood b2 bark b3 chips b4 CNCG b5 pulp b6 weak black liquor b7 strong black liquor b8 CNCG b9 smelt blO CaO bll CaCOs bl2 white liquor bl3 CNCG bl4 effluents bl5 nutrients bl6 clean effluents bl7 sludge

[0119] ABS ammonium bisulphate

[0120] CNCG concentrated non-condensable gases

[0121] ESP electrostatic precipitator

[0122] NCG non-condensable gas

[0123] SBS sodium bisulphate

[0124] SCR selective catalytic reduction

Claims

CLAIMS1 . A method for controlling nitrogen flows in a pulp mi ll comprising a recovery boi ler , a lime kiln, a power boiler, and a concentrated non-condensable gases (CNCG) incinerator and / or a wet gas sulfuric acid (WSA) plant , wherein the WSA plant comprises a non-condensable gas (NCG) burner, a waste heat boiler, a SO2 converter and a condenser, wherein the method comprises ,- passing flue gas from the recovery boiler through at least one electrostatic precipitator (ESP) to remove dust particles from the flue gas from the recovery boiler, and through a first fabric filter to remove further dust particles from the flue gas from the recovery boiler, and through a f irst selective catalytic reduction ( SCR) catalyst phase to reduce NOx emissions of the flue gas from the recovery boiler, and- passing flue gas from the lime kiln through a second ESP to remove dust particles from the flue gas from the lime kiln, and through a second fabric filter or a ceramic candle filter to remove further dust particles from the flue gas from the lime kiln, and through a second SCR catalyst phase to reduce NOx emissions of the flue gas from the lime kiln, and- passing flue gas from the power boiler through a third ESP to remove dust particles from the flue gas from the power boiler and / or through a third fabric filter to remove dust particles from the flue gas from the power boiler, and passing flue gas from the power boiler through a third SCR catalyst phase to reduce NOx emiss ions of the flue gas from the power boiler, and- passing flue gas from the CNCG incinerator through a fourth SCR catalyst phase to reduce NOx emissions of the flue gas from the CNCG incinerator and / or passing flue gas from the NCG burner through afifth SCR catalyst phase to reduce NOx emissions of the flue gas from the NCG burner.

2. The method according to any of the preceding claims, wherein the method further comprises passing the flue gas from the NCG burner through the waste heat boiler to cool the flue gas from the NCG burner before the fifth SCR phase.

3. The method according to any of the preceding claims, wherein the operating temperatures of the first and / or third SCR catalyst phase (s) is / are in the range of 180 °C - 250 °C, or 190 °C - 240 °C, or 200 °C - 230°C, or 220 °C - 230°C.

4. The method according to any of the preceding claims, wherein the operation temperature of the second SCR catalyst phase is in the range of 260 - 350 °C or 280 - 320 °C when using the ceramic candle filter or the operation temperature of the second SCR catalyst phase is in the range of 180 °C - 250 °C, or 190 °C - 240 °C, or 200 °C - 230 °C, or 220 °C - 230 °C when using the second fabric filter.

5. The method according to any of the preceding claims, wherein the operating temperature of the fourth SCR catalyst phase is in the range of 210 °C - 500 °C, or 300°C - 450 °C, or 350 °C - 400 °C, or 400 °C - 420 °C.

6. The method according to any of the preceding claims, wherein the operating temperature of the fifth SCR catalyst phase is in the range of 400 °C - 420 °C.

7. The method according to any of the preceding claims, wherein the flue gas (es) is / are cooled within at least one heat recovery phase after the first, second, third, and / or fourth SCR catalyst phase (s) .

8. The method of any one of the preceding claims, wherein the flue gas from the WSA plant is cooled within at least one heat recovery phase after the WSA plant .9 . The method according to any of the preceding claims , wherein ammonia is inj ected into the flue gas before the first , second, third, and / or fourth SCR catalyst phase ( s ) .10 . The method according to any of the preceding claims , wherein ammonia is inj ected into the flue gas from the NCG burner after the waste heat boiler and before the fifth SCR catalyst phase .11 . The method according to any of the preceding claims , wherein the ammonia is an ammonia gas , a pure ammonia gas , an anhydrous ammonia, an ammonia water solution, or urea .12 . The method according to any of the preceding claims , wherein a sorbent is inj ected into the flue gas from the recovery boiler before the first fabric filter, the flue gas from the lime kiln before the second fabric filter, and / or the flue gas from the power boiler before the third fabric filter .13 . The method according to any of the preceding claims , wherein the sorbent is sodium hydroxide , calcium carbonate, sodium carbonate, sodium bicarbonate or sodium sesquicarbonate .14 . The method according to any of the preceding claims , wherein the sorbent is continuously inj ected into the flue gas (es ) from the recovery boiler, power boiler, and / or the lime kiln .15 . The method according to any of the preceding claims , wherein the first , second, third, fourth, and / or fifth SCR catalyst phase is a SCR catalyst bed .16 . The method according to any one of claims 1 - 14 , wherein the first SCR catalyst phase takes place within the first fabric filter, the second SCR catalyst phase takes place within the second fabric filter, and / or the third SCR catalyst phase takes place within the third fabric filter .17 . The method according to any one of claims 1 - 14 , wherein the first SCR catalyst phase takes placewithin the first fabric filter and in a SCR catalyst bed, the second SCR catalyst phase takes place within the second fabric filter and in a SCR catalyst bed, and / or the third SCR catalyst phase takes place within the third fabric filter and in a SCR catalyst bed.

18. A system for controlling nitrogen flows in a pulp mill (1) comprising a recovery boiler (2) , a lime kiln (3) , a power boiler (4) , and a concentrated noncondensable gases (CNCG) incinerator (5) and / or a wet gas sulfuric acid (WSA) plant (6) , wherein the WSA plant comprises a NCG burner (6a) , a waste heat boiler (6b) , a SO2 converter (6c) and a condenser (6d) , wherein the system comprises,- a first electrostatic precipitator (ESP) unit (7a) for removing dust particles from a flue gas from the recovery boiler, a first fabric filter (8a) for further removing dust particles from the flue gas from the recovery boiler, and a first selective catalytic reduction (SCR) catalyst unit (9a) for reducing NOx emissions of the flue gas from the recovery boiler, and- a second ESP unit (7b) for removing dust particles from a flue gas from the lime kiln, a second fabric filter (8b) or a ceramic candle filter (10) for removing further dust particles from the flue gas from the lime kiln, and a second SCR catalyst unit (9b) for reducing NOx emissions of the flue gas from the lime kiln, and- a third ESP unit (7c) for removing dust particles from a flue gas from the power boiler and / or a third fabric filter (8c) for removing dust particles from the flue gas from the power boiler, and a third SCR catalyst unit (9c) for reducing NOx emissions of the flue gas from the power boiler, and- a fourth SCR catalyst unit (9d) for reducing NOx emissions of the flue gas from the CNCGincinerator, and / or a fifth SCR catalyst unit (9e) for reducing NOx emissions of the flue gas from the NCG burner.

19. The system according to claim 18, wherein the waste heat boiler (6b) is configured to cool the flue gas from the NCG burner before the fifth SCR catalyst unit ( 9e) .

20. The system according to claims 18 or 19, wherein the first SCR catalyst unit (9a) and / or third SCR catalyst unit (9c) is / are configured to operate at a temperature in the range of 180 °C - 250 °C, or 190 °C - 240 °C, or 200 °C - 230 °C, or 220 °C - 230 °C.

21. The system according to any one of claims 18 - 20, wherein the second SCR catalyst unit (9b) is configurated to operate at a temperature in the range of 260 - 350 °C or 280 - 320 °C when the ceramic candle filter (10) is used or the second SCR catalyst unit (9b) is configured to operate at a temperature in the range of 180 °C - 250 °C, or 190 °C - 240 °C, or 200 °C - 230 °C, or 220 °C - 230 °C when the second fabric filter (8b) is used.

22. The system according to any one of claims 18 - 21, wherein the fourth SCR catalyst unit (9d) is configured to operate at a temperature in the range of 210 °C - 500 °C, or 300°C - 450 °C, or 350 °C - 400 °C, or 400 °C - 420 °C.

23. The system according to any one of claims 18 - 22, wherein the fifth SCR catalyst unit (9e) is configured to operate at a temperature in the range of 400 °C - 420 °C.

24. The system according to any one of claims 18 - 23, wherein the system comprises at least one heat recovery unit ( 1 la, 1 lb, 11c, 1 Id) configured to cool the flue gas after the first SCR catalyst unit (9a) , after the second SCR catalyst unit (9b) , after the third SCR catalyst unit (9c) , and / or after the fourth SCR catalyst unit ( 9d) .

25. The system according to any one of claims 18 - 24, wherein the system comprises at least one heat recovery unit (lie) configured to cool the flue gas after the WSA plant (6) .

26. The system according to any one of claims 18 - 25, wherein the system comprises an inlet (12a) configured for receiving ammonia before the first SCR catalyst unit (9a) , an inlet (12b) configured for receiving ammonia before the second SCR catalyst unit (9b) , an inlet (12c) configured for receiving ammonia before the third SCR catalyst unit (9c) , an inlet (12d) configured for receiving ammonia before the fourth SCR catalyst unit (9d) and / or an inlet (12e) configured for receiving ammonia after the waste heat boiler (6b) and before the fifth SCR catalyst unit (9e) .

27. The system according to any one of claims 18 - 26, wherein the system comprises an inlet (13a) configured for receiving sorbent before the first fabric filter (8a) , an inlet (13b) configured for receiving sorbent before the second fabric filter (8b) , and / or an inlet (13c) configured for receiving sorbent before the third fabric filter (8c) .

28. The system according to any one of claims 18 - 27, wherein the first SCR catalyst unit (9a) , the second SCR catalyst unit (9b) , the third SCR catalyst unit (9c) , the fourth SCR catalyst unit (9d) , and / or the fifth catalyst unit (9e) is a SCR catalyst bed.

29. The system according to any one of claims 18 - 27, wherein the first SCR catalyst unit (9a) is configured to the take place within the first fabric filter (8a) , the second SCR catalyst unit (9b) is configured to take place within the second fabric filter (8b) , and / or the third SCR catalyst unit (9c) is configured to take place within the third fabric filter (8c) .

30. The system according to any one of claims 18 - 27, wherein the first SCR catalyst unit (9a) isconfigured to the take place within the first fabric filter ( 8a) and in a SCR catalyst bed, the second SCR catalyst unit ( 9b) is configured to the take place within the second fabric filter ( 8b) and in a SCR catalyst bed, and / or the third SCR catalyst unit ( 9c) is configured to the take place within the third fabric filter ( 8c) and in a SCR catalyst bed .

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