FI20245211A1

FI20245211A1 - METHOD AND SYSTEM FOR CONTROLLING NITROGEN FLOWS IN A PULP MILL

Info

Publication number: FI20245211A1
Application number: FI20245211A
Authority: FI
Inventors: Juho Hiltunen; Naveen Chenna
Original assignee: Andritz Oy
Priority date: 2024-02-21
Filing date: 2024-02-21
Publication date: 2025-08-22
Also published as: WO2025176940A1

Abstract

A method 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/or a wet gas sulfuric acid (WSA) plant (6) is disclosed. Further is disclosed a system for controlling nitrogen flows in a pulp mill (1).

Description

METHOD AND SYSTEM FOR CONTROLLING NITROGEN FLOWS IN A

PULP MILL

TECHNICAL FIELD

The present disclosure relates to a method for controlling nitrogen flows in a pulp mill. The present disclosure further relates to a system for controlling nitrogen flows in a pulp mill.

BACKGROUND

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 there- fore recognized the need for an efficient method for controlling nitrogen flows in a pulp mill.

SUMMARY

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

N gases (CNCG) incinerator and/or a wet gas sulfuric acid

N (WSA) plant, wherein the WSA plant comprises a non-

S condensable gas (NCG) burner, a waste heat boiler, a SO:

N 30 converter and a condenser, is disclosed, wherein the = method comprises, ” - passing flue gas from the recovery boiler through = at least one electrostatic precipitator (ESP) to 3 remove dust particles from the flue gas from the x 35 recovery boiler, and through a first fabric filter to remove further dust particles from the flue gas from the recovery boiler, and through a first se- lective 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 sec- ond 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 cat- alyst phase to reduce NOx emissions 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 a fifth SCR catalyst phase to reduce NOx emissions of the flue gas from the NCG burner.

A system for controlling nitrogen flows in a

N pulp mill is disclosed. The system comprises a pulp mill

N comprising a recovery boiler, a lime kiln, a power

S 30 boiler, and a concentrated non-condensable gases (CNCG)

N 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 SO, converter and = a condenser, wherein the system comprises, 3 35 - a first electrostatic precipitator (ESP) unit for x 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 - 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 fur- ther dust particles from the flue gas from the lime kiln, and a second SCR catalyst unit for reducing

NOx emissions of the flue gas from the lime kiln, and - 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 emissions of the flue gas from the power boiler, and - a fourth SCR catalyst unit for reducing NOx emis- sions 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.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and < constitute a part of this specification, illustrate em-

S bodiments of the invention and together with the de-

AI scription help to explain the principles of the inven- ? 30 tion. In the drawings:

N Figure 1 is a schematic diagram of processes = of an exemplary recovery boiler in accordance with at — least some embodiments of the present invention. 3 Figure 2 is a schematic diagram of processes

N 35 of an exemplary lime kiln in accordance with at least

N 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.

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

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

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.

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.

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a schematic diagram of processes of an exemplary recovery boiler 2 with pe- ripheral 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 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

N on the fabric filter. The flue gas from the recovery

N boiler is then passed through a first catalytic SCR

S 30 phase for reducing NOx emissions of the flue gas from

N 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 recov- = ered by at least one heat recovery unit lla. Sorbent 13a 3 35 may be injected into flue gas from the recovery boiler before the ESP 7a or at least before the first fabric filter 8a for reacting with Sox components of the flue gas. Additional or supplemental ammonia 12a may be in- jected into the flue gas from the recovery boiler before the first fabric filter 8a or at least before the first

SCR unit 9a. 5 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 par- ticle 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 filter or within a second SCR unit %.

Residual heat energy of the flue gas may be then recov- ered by at least one heat recovery unit 11b. Sorbent 13b may be injected 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 in- jected into the flue gas from the lime kiln before the second fabric filter 8b or at least before the second

SCR unit 9b.

N Figure 3 illustrates a schematic diagram of

N processes of an exemplary power boiler 3 with peripheral

S 30 components. The flue gas from power boiler contains dust

N 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 precipita- = tor (ESP) 7c. The flue gas may then be led from the 3 35 third ESP 7c to a third fabric filter 8c for filtering x 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 cata- lytic SCR phase for reducing NOx emissions of the flue gas from the power boiler, which may take place either within the fabric filter or within a third SCR unit Sc.

Residual heat energy of the flue gas may be then recov- ered by at least one heat recovery unit llc. Sorbent 13c may be injected 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 in- jected to the flue gas from the power boiler before the third fabric filter 8c or at least before the third SCR unit 9c.

Figure 4 illustrates a schematic diagram of processes of an exemplary CNCG incinerator 4 with pe- ripheral components. The flue gas from the CNCG incin- erator 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 injected 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 11d.

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

N 6b to cool the flue gas. Then the flue gas from the NCG

N burner 6a is passed through a SCR catalyst unit 9e for

S 30 reducing NOx emissions of the flue gas from the NCG

N burner. Additional or supplemental ammonia 12e may be = injected 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 SO, converter 6c, 3 35 wherein 50, is converted to SOs using a catalyst. Then,

R the temperature of the flue gas is lowered using a con- denser 6d and most of 503 is recovered as sulfuric acid.

After this, remaining SO; may be removed from the flue gas by using an SO, scrubber 6e. Residual heat energy of the flue gas may be then recovered by at least one heat recovery unit lle.

Figure 6 is a schematic illustration of a sys- tem 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 ESP la to a first fabric filter 8a for filtering 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. Residual heat energy of the flue gas may be then recovered by at least one heat recovery unit lla. Sorbent 13a may be injected 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 injected into the flue gas from the recovery boiler before the first fab-

N ric filter 8a or at least before the first SCR unit 9a.

N The flue gas from the lime kiln contains dust particles

S 30 and the most of them are filtered within at least one

N 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 parti- = cles. The ESP may efficiently filter out particles 3 35 which could otherwise create too compact particle layers x 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 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 1lb. Sorbent 13b may be injected 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 injected 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 parti- cles, which may be filtered using an ESP and/or a fabric filter. The flue gas from the power boiler may be fil- tered using at least one electrostatic precipitator (ESP) 7c. The flue gas is then 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 lavers on the fabric filter. The flue gas from the power boiler is then passed 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 flue gases may be then recovered by at least one heat recovery unit 11c. Sorbent 13c may be injected to flue gas from the power boiler before the

N third ESP 7c or at least before the third fabric filter

N 8c for reacting with Sox components of the flue gas.

S 30 Additional or supplemental ammonia 12c may be injected

N to the flue gas from the power boiler before the third = fabric filter 8c 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 3 35 emissions of the flue gas from the CNCG incinerator. x Additional or supplemental ammonia 12d may be injected to the flue gas from the CNCG before the fourth SCR unit ad. 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. Addi- tional or supplemental ammonia 12e may be injected 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 SO, converter 6c, wherein SO: is converted to S03; using a catalyst. Then, the temper- ature of the flue gas is lowered using a condenser 6d and most of 503 is recovered as sulfuric acid. After this, remaining SO; may be removed from the flue gas by using SO, scrubber 6e. Residual heat energy of the flue gas may be then recovered by at least one heat recovery unit lle.

Figure 7 is a schematic illustration of a sys- tem 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 b? and chips b3. Nitrogen bound to bark b? and chips b3 is in organic form. The nitrogen bound to bark b? is combusted in the power boiler 4 with sludge bl7. Then the chips b3 and white liquor bl? are

N directed to cooking A2. The nitrogen in white liquor bl2

N is mostly in the form of ammonia and cyanate. During

S 30 cooking A2, some non-condensable gases (NCGs) b4 are

N generated, which may contain a small amount of ammonia.

E After washing and screening A3, the produced pulp bb is free of nitrogen, while the nitrogen of the weak black = liquor b6 is primarily in an organic form, although 3 35 there may be some presence of ammonia. In the evapora- x tion 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 recaustization Ab, a por- tion of nitrogen is separated from the white liquor bl2 as ammonia in non-condensable gases (NCGs) bl3. In the effluent treatment A6, the nitrogen content in the ef- fluents bl4 should be minimal. The primary nitrogen load arises from nutrients bl5, either in the form of urea or ammonia. Through biological activity, the nitrogen is converted into organic nitrogen and eventually trans- formed into sludge bl7.

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 fab- ric 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 lla. Sorbent 13a may be injected into flue gas from the recovery boiler before the first ESP 7a or at least before the first fabric

N filter 8b for reacting with Sox components of the flue

N gas. Additional or supplemental ammonia 12a may be in-

S 30 jected into the flue gas from the recovery boiler before

N the first fabric filter 8a or at least before the first = SCR unit %a. 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 3 35 flue gas from the lime kiln is then led from the second

R ESP 7b to a second fabric filter 8b for filtering re- maining particles. The ESP may efficiently filter out particles which could otherwise create too compact par- ticle 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 filter 8b or within a second SCR unit %.

Residual heat energy of the flue gas may be then recov- ered by at least one heat recovery unit 11b. Sorbent 13b may be injected into flue gas from the lime kiln before the ESP 7b or at least before the fabric filter 8b for reacting with Sox components of the flue gas. Additional or supplemental ammonia 12b may be injected 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 fil- ter. 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 parti- cles. 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 reduc- ing 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

N energy of the flue gases may be then recovered by at

N least one heat recovery unit lic. Sorbent 13c may be

S 30 injected to flue gas from the power boiler before the

N 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 injected = to the flue gas from the power boiler before the third 3 35 fabric filter 8c or at least before the third SCR unit x 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 injected 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 1s passed through a fifth SCR catalyst unit 9e for reducing NOx emissions of the flue gas from the NCG burner. Additional or sup- plemental ammonia 12e may be injected 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 $S0, converter 6c, wherein SO, is converted to SO; using a catalyst. Then, the temperature of the flue gas is lowered using a condenser 6d. After this,

SO, may be removed from the flue gas by using SO, scrubber be. Residual heat energy of the flue gas may be then recovered by at least one heat recovery unit lle.

DETAILED DESCRIPTION

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 SO, converter and a con-

N denser is disclosed, wherein the method comprises, a - passing flue gas from the recovery boiler through ? at least one electrostatic precipitator (ESP) to

N 30 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

N from the recovery boiler, and through a first se-

N lective catalytic reduction (SCR) catalyst phase

N to reduce NOx emissions of the flue gas from the recovery boiler, and - passing flue gas from the lime kiln through a sec- ond 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 cat- alyst phase to reduce NOx emissions 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 burnerthrough a fifth

SCR catalyst phase to reduce NOx emissions of the flue gas from the NCG burner .

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

N power boiler 4, and a concentrated non-condensable gases

N (CNCG) incinerator 5 and/or a wet gas sulfuric acid

S 30 (WSA) plant 6, wherein the WSA plant comprises a NCG

N burner 6a, a waste heat boiler 6b, a SO, converter 6c = and a condenser 6d, wherein the system comprises, ” - a first electrostatic precipitator (ESP) unit 7a 5 for removing dust particles from a flue gas from a 35 the recovery boiler, a first fabric filter 8a for

R further removing dust particles from a flue gas from the recovery boiler, and a first selective catalytic reduction (SCR) catalyst unit 9a for re- ducing NOx emissions of the flue gas from the re- covery 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 remov- ing 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 - 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 emis- sions 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 CNCG incinerator and/or a fifth SCR catalyst unit 9e for reducing

NOx emissions of the flue gas from the NCG burner.

Nitrogen oxide emissions are a concern due to their < negative impacts on air quality and the environment.

S In a pulp mill, nitrogen can exist in various

N forms, including organic, ammonia, cyanate (OCN), NOx,

ES 30 N2, or other types of nitrogen (such as salts and dis-

E solved nitrogen). Initially, the majority of nitrogen - may arrive at the pulp mill bound to wood in the form

N of organic nitrogen, approximately 1.5 to 3.0 kgN/Adt,

N depending on the wood type. In the wood handling pro-

N 35 cess, 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 boiler, where the fuel nitrogen can undergo transformation into either NOx or N,. Typi- cally, 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.

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 atmos- pheric nitrogen with hydrocarbon radicals. Compared to the total NOx generated from combustion, prompt NOx typ- ically constitutes a relatively small portion. However, as environmental regulations restricting nitrogen ox- ides tend to become stricter, even the contribution of prompt NOx becomes more important.

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 + (H-O) and nitrogen (N.) by using a catalyst and a reduc-

S 25 ing agent. SCR may effectively reduce NOx emissions from

N cooler flue gases compared to SNCR (Selective Non Cat-

ES alytic 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

N 30 catalytic elements of SCR are very vulnerable to con-

N taminations that may cover the catalyst elements and

N block the active porous catalytic surfaces.

The dust from the recovery boiler contains huge amounts of sodium and the dust from the lime kiln con- tains calcium. Both of these elements can be considered detrimental to the catalyst material, acting as poten- tial contaminants and poison. Consequently, the inte- gration 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.

The expression “fabric filter” should be understood in this specification, unless otherwise stated as a filter, that utilizes fabric filtration 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 s expenses.

S 25 An electrostatic precipitator (ESP) may be used

N for removing dust particles from the flue gases. How-

ES ever, ESPs are not always reliable for continuous par- = ticle removal due to occasional performance issues, re- - sulting in uneven dust particle removal. These perfor-

N 30 mance issues may increase the dust particle amount in

N the flue gases after ESP, causing plugging of the SCR

N 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.

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/Nm?.

In one embodiment, after the ESP the dust particle level is in the range of 10 - 30 mg/Nm?, or 10 — 50 mg/Nm?®, or 10 — 100 mg/Nm*. In one embodiment, after the fabric filter the dust particle level is in the range of 1 - 5 mg/Nm®, or 1 — 10 mg/Nm, or 1 — 15 mg/Nm*. In one embodiment, after the ceramic candle fil- ter the dust particle level is in the range of 1 - 5 mg/Nm*, or 1 — 10 mg/Nm®, or 1 - 15 mg/Nm'. The dust particle level may be measured according to standard

SFS-EN 13284-1.

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 flue gases of the power boiler, CNCG in- cinerator, lime kiln and recovery boiler contain SO; which, when combined with NH; may form ammonia salt that may block the active porous catalytic surfaces. The am- + monia salt formation, in which SO; and NH; convert to

S 25 mostly ammonium sulfate on the surface of the catalyst,

N especially in low temperatures, can be avoided either

ES with flue gas temperature control or with sulphur re-

E moval. - In one embodiment, the WSA plant comprises the

N 30 following devices in the following order: a NCG burner

N ca, a waste heat boiler 6b, a SO, converter 6c and a

N 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 SO, converter oc, a condenser 6d and a SO; scrubber ce. In one embodiment, the fifth SCR unit 9e is between a waste heat boiler 6b and a 50, converter 6b.

In one embodiment, ammonia is injected into the flue gas before the first, second, third, and/or fourth

SCR catalyst phase(s). In one embodiment, ammonia is injected into the flue gas before the first, second, and/or third fabric filter(s). In one embodiment, ammo- nia is injected 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 sys- tem comprises an inlet 12a configured for receiving am- + monia before the first fabric filter 8a, an inlet 12b

S 25 configured for receiving ammonia before the second fab-

N ric filter 8b, and/or an inlet configured for receiving

ES ammonia before the third fabric filter 8c. In one em- = bodiment, the inlet configured for receiving ammonia is - a flange or a welded connection at flue gas duct or at

N 30 separate chamber.

N In one embodiment, the ammonia is an ammonia

N 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 emission to into water (H,O0) and nitrogen (N,) at the surface of the catalyst.

The method and system as disclosed in the current spec- ification have the added utility of enabling the inter- nal utilization of ammonia in the pulp mill.

Ammonia water may be sprayed into the duct us- ing a pump, nozzle, and, if necessary, a mechanical mixer. Alternatively, the ammonia water solution can be vaporized in a separate chamber and then mixed with the flue gases.

When using anhydrous ammonia, nozzles may be used as the anhydrous ammonia is already in high pres- sure and gas format.

Even though most of SOx emissions can be re- moved from the flue gases before injecting ammonia, still ammonium bisulphate (ABS) may slowly accumulate on the catalytic surfaces. The accumulated ABS can be vaporized 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 flue gas led to the SCR phase may

O 25 be activated when ammonia slip increases over a prede-

S termined value. The temperature is lowered back to nor-

N mal operating temperature when ammonia slip decreases

E below a smaller predetermined value. The other ways to — ensure the catalytic reactions are to wash, change or,

N 30 in the case of catalyst poisoning, regenerate the cat-

X alytic elements. The SCR phase may be bypassed via a bypass conduit, if necessary for the washing or changing operations.

The expression “ammonia slip” should be under- stood in this specification, unless otherwise stated, as an amount of ammonia passing through the SCR unre- acted. This may occur when ammonia is injected in ex- cess, 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 pur- poses of demonstration, ammonia slip is limited to a maximum of 5 mg/Nm® with reference 02 of 6 % by increasing the temperature when ammonia slip reaches e.g. 4 or 4.5 mg/Nm* or even exceeds 5mg/Nm'. If the ammonia slip is too high, e.g. above the maximum acceptable level of 5mg/Nm®, less ammonia is injected into the flue gases, which also reduces the level of ammonia slip. Addition- ally, 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.

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 x = 230 °C, or 220 °C - 230 °C. In one embodiment, the & 25 operation temperature of the second SCR catalyst phase

S is in the range of 260 - 350 °C or 280 — 320 °C when

N using the ceramic candle filter or the operation tem-

E perature of the second SCR catalyst phase is in the — range of 180 °C — 250 °C, or 190 °C — 240 °C, or 200 °C

N 30 — 230 °C, or 220 °C — 230 °C when using the second fabric

N 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.

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 al- lows 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 — 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 — 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 con-

S 25 figured to operate at a temperature in the range of 400

S - 420 °C.

N The operating temperature of SCR phase may be z above the dew point of ammonium bisulphate (ABS) in - order to avoid accumulation of ABS on the catalytic

N 30 surfaces. Preferably, the operating temperature of the

N SCR phase may be above the dew point of sodium bisulphate

N (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 re- duction of NOx by the catalyst. The operating tempera- tures may be, in the range of 200 °C to 250 °C, prefer- ably 230 °C or 220 °C to avoid accumulation of ABS and/or

SBS.

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 gener- ation of electricity by reducing the usage of process steam. In one embodiment, the system comprises at least one heat recovery unit lla, 1lb, 11c, 11d 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 flue 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 lle configured to cool the flue gas after the WSA plant 6.

In one embodiment, the method further comprises s passing the flue gas from the NCG burner through the

S 25 waste heat boiler to cool the flue gas from the NCG

N burner before the fifth SCR phase. In one embodiment,

ES 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

N 30 the NCG burner may be in the range of 900 - 1300 °C. In

X one embodiment, the waste heat boiler 6b is configured to cool the flue gas from the NCG burner to temperature range of 400 °C - 420 °C.

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.

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 1s 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 cool- ers, the heat can be recovered by transferring it to water, while in the case of LUVO, it is directly recov- + ered and transferred to the combustion air.In one em-

S 25 bodiment, a sorbent is injected into the flue gas from

N the recovery boiler before the first fabric filter, the

ES 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 cat-

N 30 alyst may be also installed between an economizer and

N an air preheater, which are typical heat exchanger sur-

N faces within the power boiler.

In one embodiment, the system comprises an in- let 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 injected 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 re- ceiving sorbent before the first ESP unit 7a, an inlet 13b configured for receiving sorbent before the second

ESP unit 7b, and/or an inlet 13c configured for receiv- ing sorbent before the third ESP unit 7. In one embod- iment, the inlet configured for receiving sorbent is a flange or a welded connection at flue gas duct.

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 vac- uum. The vacuum will naturally suck the sorbent into the flue gas. In one embodiment, the sorbent is sodium hy- droxide, calcium carbonate, sodium carbonate, sodium + bicarbonate or sodium sesguicarbonate.

S 25 In one embodiment, the sorbent is continuously

N injected into the flue gas(es) from the recovery boiler,

ES 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

N 30 prevent formation of ABS, SBS, and layers of sticky

N particles on the surface of the fabric filter. The

N sorbent has the added utility of cleaning the fabric filter surfaces. Due to the larger size of the sorbent particles compared to the dust particles of the flue gas after ESP, the dust particle size is reduced to approx- imately 1 pm after ESP, the sorbent particles adhere to the dust particles, effectively cleaning the fabric fil- ter.

In one embodiment, the size of the sorbent par- ticles is 10 - 150 pm, or 10 - 100 pm, or 10 - 90 um, or 10 — 80 um, or 10 — 70 pm or 10 — 60 um, 10 — 50 um, or 10 — 40 um, or 10 - 30 pm, or 10 — 20 pm.

In one embodiment, the first, second, third, fourth, and/or fifth SCR catalyst phase is a SCR cata- lyst bed. In one embodiment, the first SCR catalyst unit 9a, the second SCR catalyst unit 9b, the third SCR cat- alyst 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.

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 filter. In one embodiment, the + first SCR catalyst unit 9a is configured to the take

S 25 place within the first fabric filter 8a, the second SCR

N catalyst unit 9b is configured to take place within the

ES second fabric filter 8b, and/or the third SCR catalyst = unit 9c is configured to take place within the third - fabric filter 8c.

N 30 When the SCR catalyst phase takes place within

N the fabric filter it has the added utility of reducing

N the installation space. The fabric filter may be embedded with the catalysts and coated with dust pro- tective filter either with two separate filters or as integrated as one filter.

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 cat- alyst 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.

The method and system as disclosed in the cur- rent specification has the added utility of allowing + flexibility in combustion design in the recovery boiler,

S 25 the lime kiln, and the power boiler. A low NOx boiler,

N where the reduction of NOx emissions is achieved with

ES combustion measures, is remarkable more expensive to = construct than a conventional boiler. An effective NOx - reduction from flue gases will enable remarkable cheaper

N 30 designs of new recovery boilers, power boilers and lime

N kilns. The method and system as disclosed in the current

N 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 ad- ditional design concerns.

The method and system as disclosed in the current specification has the added utility of reduc- ing NOx emissions up to 80 % in the pulp mill.

In addition to reducing NOx emissions, the method and system as disclosed 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 for- mation of acidic liguids 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 lim- ited 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

S better heat utilization, higher recovery boiler energy

N 25 efficiency and increased electricity production at the

S pulp mill.

N r The method and system as disclosed in the cur-

E rent specification have the added utility of increasing = energy recovery from flue gases in recovery boiler, lime 3 30 kiln and power boiler by lowering flue gas exit temper-

N ature by minimizing the generation of acidic compounds from dust and 502.

The method and system as disclosed in the cur- rent 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.

Nutrient (NH3) is transformed to organic ni- trogen 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 emis- sions resulting from the use of NH; as a nutrient can be reduced. Thus, the method as disclosed in the current specification has the added utility of minimizing envi- ronmental impacts of nitrogen from nutrient feeding to effluent treatment plant.

EXAMPLES

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

The description below discloses some embodi- ments in such a detail that a person skilled in the art is able to utilize the method based on the disclosure.

Not all steps of the embodiments are discussed in de- tail, as many of the steps will be apparent for the < person skilled in the art based on this specification.

S 25

N

S Example 1 - Controlling nitrogen flows in a pulp mill

N In this example, it is illustrated how nitrogen

E is controlled in a pulp mill by means of at least some

T= embodiments of the invention. The first column of Table

O 30 1 indicates the flows under consideration, the second

O 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 accord- ing to at least some embodiments of the invention.

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. NH; may be measured by various means as de- scribed e.g. on page 28 chapter 5 of Päästömittausten

Käsikirja Osa 1 (Handbook of emission measurements part 1) published by VTT in June 2007. In the present example,

NH; is not an emission the amount of which is lowered, but to a small extent increases due to the injection of

NH: into flue gases. Its presence may be measured e.g. by chemiluminescence. Typically, in a pulp mill, meas- urements 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

Päästömittausten Käsikirja Osa 1 (Handbook of emission measurements part 1), published by VIT in June 2007 and x in Päästömittausten Käsikirja Osa 2 (Handbook of emis- & 25 sion measurements part 2), published by VTT in April

S 2004.

N

= a Table 1. Nitrogen flows in a pulp mill 5 Input (N content | Output (N content 3 [3 from wood's|[% from wood's & Flows NJ) NJ)

oe k 1 mm k 1 ams [pe sam | [pr m 1 1 k [1 >=

Table 2. Comparative example — Nitrogen flows in a pulp mill

Input (N content | Output (N content [% from wood’'s | [% from wood's

Flow N]) NJ) oe tk 1 mme [1 sm | Pk am | or m [1 = <t

N s er

N

E 5 As shown in Table 1 above, both NOx emissions — and ammonia in blow gases were significantly reduced

N compared to the comparative example in Table 2. At table 3 2, blow gases were untreated, while at table 1 the gases

O

N 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.

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 hereinbe- fore. It will be understood that the benefits and advantages described above may relate to one embodi- ment or may relate to several embodiments. The embod- iments 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 will further be understood that reference to "an” item refers to one or more of those items. The term "comprising” is used in this specification to mean including the feature(s) or act(s) followed thereafter, without excluding the pres- ence of one or more additional features or acts. <t

N

O

N

Q 30

N

I a a

N

< 35

N

O

N

REFERENCE SYMBOLS

1 pulp mill 2 recovery boiler 3 lime kiln 4 power boiler 5 concentrated non-condensable gases (CNCG) incin- erator 6 wet gas sulfuric acid (WSA) plant 6a NCG burner 6b waste heat boiler

Oc 50, converter 6d Condenser be SO, scrubber 7 electrostatic precipitator (ESP) unit 8 fabric filter 9 selective catalytic reduction (SCR) catalyst unit 10 ceramic candle filter 11 heat recovery unit 12 inlet configured for receiving ammonia 13 inlet configured for receiving sorbent

Al wood handling

A2 cooking

A3 washing

Ad evaporation

AD recaustization

N A6 effluent treatment

N bl wood

S 30 b2 bark

N b3 chips

E b4 CNCG b5 pulp 5 be weak black liquor

S 35 Db7 strong black liquor

N b8 CNCG bo smelt blo CaO bll CaCO; b12 white liguor bl3 CNCG

Dbl4 effluents b15 nutrients ble clean effluents b17 sludge

ABS ammonium bisulphate

CNCG concentrated non-condensable gases

ESP electrostatic precipitator

NCG non-condensable gas

SBS sodium bisulphate

SCR selective catalytic reduction <t

N

O

N

<Q

N

I a a

N

O

+

N

O

N

Claims

1. A method for controlling nitrogen flows in a pulp mill 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 SO, 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 first se- lective 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 sec- ond 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 S gas from the power boiler and/or through a third AN fabric filter to remove dust particles from the ? 30 flue gas from the power boiler, and passing flue N gas from the power boiler through a third SCR cat- = alyst phase to reduce NOx emissions of the flue gas — from the power boiler, and N - passing flue gas from the CNCG incinerator through N 35 a fourth SCR catalyst phase to reduce NOx emissions N 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.

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 N SCR catalyst phase is in the range of 400 °C - 420 °C.

N 7. The method according to any of the preceding S 30 claims, wherein the flue gas(es) is/are cooled within N at least one heat recovery phase after the first, sec- = ond, 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 3 35 within at least one heat recovery phase after the WSA R plant.

9. The method according to any of the preceding claims, wherein ammonia is injected into the flue gas before the first, second, third, and/or fourth SCR cat- alyst phase(s).

10. The method according to any of the preced- ing claims, wherein ammonia is injected 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 preced- ing 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 preced- ing claims, wherein a sorbent is injected 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 preced- ing claims, wherein the sorbent is sodium hydroxide, calcium carbonate, sodium carbonate, sodium bicarbonate or sodium sesquicarbonate.

14. The method according to any of the preced- ing claims, wherein the sorbent is continuously injected into the flue gas(es) from the recovery boiler, power boiler, and/or the lime kiln.

15. The method according to any of the preced- N ing claims, wherein the first, second, third, fourth, N and/or fifth SCR catalyst phase is a SCR catalyst bed. S 30

16. The method according to any one of claims N 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 3 35 the third fabric filter. R

17. The method according to any one of claims 1 - 14, wherein the first SCR catalyst phase takes place within 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 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 50, converter (oc) 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 N from the lime kiln, and N - a third ESP unit (7c) for removing dust particles S 30 from a flue gas from the power boiler and/or a N third fabric filter (8c) for removing dust parti- = cles 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, 3 35 and R - 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.

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 cat- alyst 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 (Sb) 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 N 18 - 22, wherein the fifth SCR catalyst unit (9e) is N configured to operate at a temperature in the range of S 30 400 °C - 420 °C. N

24. The system according to any one of claims = 18 - 23, wherein the system comprises at least one heat * recovery unit (lla, l1l1b,11c,11d) configured to cool the = flue gas after the first SCR catalyst unit (9a), after 3 35 the second SCR catalyst unit (9b), after the third SCR N 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 (lle) 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 re- ceiving ammonia before the second SCR catalyst unit (Sb), 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. N

29. The system according to any one of claims N 18 - 27, wherein the first SCR catalyst unit (9a) is S 30 configured to the take place within the first fabric N filter (8a), the second SCR catalyst unit (9b) is con- = figured to take place within the second fabric filter ” (8b), and/or the third SCR catalyst unit (9c) is con- = figured to take place within the third fabric filter 3 35 (8c). R

30. 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) 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. i N O N N <Q N I a a N O + N O N