A method and a system for recovering chemicals in a kraft pulp mill

Abstract

The invention relates to new solutions for effective recovery of chemicals in a Kraft pulp mill A method for recovering chemicals in a Kraft pulp mill is provided. The method comprises producing green liquor in a recovery boiler, conveying green liquor from the recovery boiler to a green liquor stripper, stripping the green liquor with a gas stream comprising or consisting of CO2 so as to produce a stripper off-gas, feeding the stripper off-gas to a sulfuric acid plant, and producing sulfuric acid from the stripper off-gas. Further, a system as well as use of the same for recovering chemicals in a Kraft pulp mill are provided.

Description

[0001] A method and a system for recovering chemicals in a Kraft pulp mill

[0002] Technical field

[0003] The disclosure relates to a method for recovering chemicals in a Kraft pulp mill. The disclosure also relates to a system for recovering chemicals in a Kraft pulp mill as well as to use of the same for recovering chemicals in a Kraft pulp mill.

[0004] Background

[0005] The most important chemical balance of the Kraft pulp mill is the Na-S balance. Na-S balance simply tells sodium and sulfur mass flows into and out of the pulp mill. Superiority of the Kraft cooking is based on the high pulp quality and yield, but also on the efficient chemical recovery. The active chemicals used in the cooking, sodium hydroxide and sodium sulfide, are recovered in recovery loop. In an ideal case chemical recovery would be totally closed, but in real world some of the chemicals are leaking from the loop and must be replaced with make-up chemicals.

[0006] Pulp mills are using high amounts of sulfuric acid. If sulfuric acid is imported to mill and spent acid is dumped to the recovery cycle, sulfur will start to accumulate. To reach continuity, all the imported sulfur must be dumped out of the pulp mill. In a modem pulp mill with tall oil production and chlorine dioxide production sulfur dumping related to sulfuric acid import can be 10 kgS / ADt. Lignin extraction is also consuming sulfuric acid up to 10 kgS / ADt. In a medium size pulp mill with 1 000 000 ADt / a production sulfuric acid importing leads to 57 tS / d dumping. If this is calculated to Na2SO4 which is mainly the form of dumping, it means 254 tNa2SO4 / d. New environmental permits will be difficult to get for the mills which are dumping more than 100 tons of salt per day to nearest lake. Also, for the mills at seaside, we have already seen tightening of environmental permit limits related to salt load.

[0007] The primary purpose of the pulp mill’s recovery boiler to discharge Na2SO4, primarily through the disposal of recovery boiler ESP ash, is to eliminate the harmful elements chlorine and potassium, which can cause severe corrosion and plugging in the recovery boiler. As a byproduct, sulfur and sodium are also removed.

[0008] Modern recovery boiler ash treatment systems aim to minimize the discharge of sodium and sulfur by selectively removing chlorine and potassium from the ash. This allows the treated ash, now with reduced chlorine and potassium content, to be recycled back into the system without the detrimental effects of potassium and chlorine. This will decrease the total solid waste the pulp mill produces and thereby lower the environmental impact of the mill. However, this process can lead to an imbalance of sodium and sulfur within the pulp mill, as these elements are no longer being removed from the process to the same extent through ash disposal.

[0009] To get rid of the imported sulfuric acid and dumping of salt related to that, sulfuric acid plants which are producing sulfuric acid from concentrated noncondensable gases (CNCG) have been developed. However, availability of CNCG as the sulfur source is limited. Typically only 3-5 kgS / ADt can be extracted to sulfuric acid plant with CNCG as the sulfur source.

[0010] Green liquor stripping and black liquor heat treatment have been introduced as supplementary sources of sulfur extraction for sulfuric acid production. Presently, green liquor stripping is based on stripping green liquor with carbon dioxide containing flue gas. Such a method however requires a complicated system with rather high investment and operational costs. Further, the contaminated flue gas streams must be properly handled after their use in stripping. Black liquor heat treatment, for one, relies on the usage of middle pressure steam, which is not optimal from electricity production point of view. Hence, new more (cost-)efficient processes are needed for boosting the target of closing the pulp mill chemical recovery.

[0011] Summary

[0012] This disclosure provides new solutions for effective recovery of chemicals in a Kraft pulp mill. The method and the system disclosed herein enhance extraction of sulfur from the pulp mill liquor cycle by providing a new sulfur cycle for the Kraft pulp mill. New solutions for replacing flue gas as a green liquor stripping agent are presented. Lower capital investment when compared to the traditional flue gasbased system is required for the system according to this disclosure.

[0013] According to an embodiment, a method for recovering chemicals in a Kraft pulp mill is provided. The method comprises producing green liquor in a recovery boiler, conveying green liquor from the recovery boiler to a green liquor stripper, stripping the green liquor with a gas stream comprising or consisting of CO2 so as to produce a stripper off-gas, feeding the stripper off-gas to a sulfuric acid plant, and producing sulfuric acid from the stripper off-gas.

[0014] According to another embodiment, a system for recovering chemicals in a Kraft pulp mill is provided. The system comprises a recovery boiler for producing green liquor, a green liquor stripper for producing stripper off-gas, a conduit for conveying green liquor from the recovery boiler to the green liquor stripper, a conduit for conveying a gas stream comprising or consisting of CO2 to the green liquor stripper, a sulfuric acid plant for producing sulfuric acid, and a conduit for conveying the stripper off-gas to the sulfuric acid plant.

[0015] Further, use of the system as disclosed herein for recovering chemicals in a Kraft pulp mill is provided.

[0016] Brief description of the drawings

[0017] Fig. 1 illustrates, by way of an example, a general overview of the Kraft pulp mill chemical recovery cycle with the novel sulfur cycle according to this disclosure,

[0018] Fig. 2 illustrates, by way of an example, a general overview of the system according to this disclosure as well as its interconnection to the pulp mill chemical recovery cycle, and

[0019] Fig. 3 illustrates, by way of an example, a green liquor stripper according to this disclosure.

[0020] The figures are schematic. The figures are not on any particular scale. Detailed description

[0021] The solution is described in the following in more detail with reference to some embodiments, which shall not be regarded as limiting.

[0022] The features recited in the embodiments and examples of the description and in the claims are mutually freely combinable unless otherwise explicitly stated.

[0023] Unit of temperature expressed as degrees C corresponds to °C. The following reference numbers and denotations are used in this application:

[0024] A cooking and fiberline

[0025] B evaporation plant

[0026] C recovery boiler

[0027] D recausticizing plant

[0028] 201 , 301 green liquor stripper

[0029] 202 sulfuric acid plant

[0030] 203 mini causticizer

[0031] 204 chlorine dioxide plant

[0032] 211 , 311 conduit for conveying green liquor from recovery boiler to green liquor stripper

[0033] 212, 312 conduit for conveying a gas stream comprising or consisting of

[0034] CO2 to the green liquor stripper

[0035] 213, 313 conduit for conveying stripper off-gas to the sulfuric acid plant

[0036] 214, 314 conduit for conveying carbonate solution from the green liquor stripper to the mini causticizer

[0037] 215 conduit for conveying sodium hydroxide from mini causticizer to cooking and fiberline

[0038] 216 conduit for conveying sulfuric acid from sulfuric acid plant to chlorine dioxide plant

[0039] 217 conduit for conveying chlorine dioxide from the chlorine dioxide plant to cooking and fiberline

[0040] 218 conduit for conveying sulfate from chlorine dioxide plant to evaporation plant

[0041] 219 conduit for conveying CNCG from evaporation plant to sulfuric acid plant

[0042] 220 recovery boiler ash treatment bleed The Kraft pulp mill chemical recovery cycle is a crucial process in the Kraft pulping method, which is used to produce cellulose pulp from wood. The cycle is designed to recover and reuse chemicals used in the pulping process, thereby reducing environmental impact and improving economic efficiency. The Kraft pulp mill chemical recovery cycle with the novel sulfur cycle according to this disclosure is illustrated in Fig. 1.

[0043] During the Kraft pulping process, wood chips are cooked in a solution comprising sodium hydroxide (NaOH) and sodium sulfide (Na2S), known as white liquor (step of “Cooking” in Fig. 1 ). This process breaks down the lignin that binds the cellulose fibers in the wood. The spent cooking liquor, now called black liquor, contains dissolved lignin, hemicellulose, and the inorganic chemicals used in the cooking process.

[0044] In the evaporation step, the black liquor is concentrated in multiple-effect evaporators of an evaporation plant to increase its solids content. The concentrated black liquor is burned in a recovery boiler. This combustion process serves two purposes: it generates steam and electricity for the mill, and it recovers the inorganic chemicals. The organic material in the black liquor burns to produce heat, while the inorganic chemicals form a molten smelt at the bottom of the boiler. The molten smelt, which contains sodium carbonate (Na2COs) and sodium sulfide (Na2S), is dissolved in water to form green liquor.

[0045] The green liquor is then reacted with lime (calcium oxide, CaO) in a recausticizing process to convert sodium carbonate into sodium hydroxide, regenerating the white liquor. The reaction produces calcium carbonate (CaCOs, also called lime mud) as a byproduct. The calcium carbonate is separated from the white liquor and washed. It is then calcined in a lime kiln to regenerate lime (CaO), which is reused in the re-causticizing process. The regenerated white liquor, now containing sodium hydroxide and sodium sulfide, is recycled back to the digester for use in the cooking.

[0046] As mentioned, pulp mills are using high amounts of sulfuric acid. When sulfuric acid is imported to mill the spent acid cannot just be dumped to the chemical recovery cycle described above, as sulfur will start to accumulate. Hence, all the imported sulfur must be dumped out of the pulp mill, typically in the form of Na2SO4. The dumping usually takes place by dumping recovery boiler electrostatic precipitator (ESP) ash. However, import of the sulfuric acid and dumping of the excess sulfur can be avoided by producing sulfuric acid internally.

[0047] This disclosure provides new solutions for effective recovery of chemicals in a Kraft pulp mill. The method disclosed herein enhances extraction of sulfur from the pulp mill liquor cycle by providing a new sulfur cycle for the Kraft pulp mill.

[0048] The new sulfur cycle disclosed herein is also illustrated in Fig. 1. Green liquor produced in a recovery boiler is conveyed from the recovery boiler to a green liquor stripper. As is evident based on the above discussed, not all but a fraction of the green liquor produced in the recovery boiler is conveyed to the green liquor stripper for stripping. In an example, from 5 to 10 vol-% of the green liquor produced in the recovery boiler is conveyed to green liquor stripper. The green liquor is stripped in the green liquor stripper with a gas stream comprising or consisting of carbon dioxide (CO2). As outcome of stripping, a stripper off-gas comprising hydrogen sulfide (H2S) is produced.

[0049] When green liquor is contacted with a gas stream comprising or consisting of CO2, Na2COs and Na2S contained by the green liquor react with CO2 thereby producing sodium hydrosulfide (NaHS) and sodium bicarbonate (NaHCOs) according to reactions 1 and 2.

[0050] NaHS and NaHCOs react in the stripping column to produce hydrogen sulfide and sodium carbonate, according to reaction 3.

[0051] NaHS + NaHCOz H2S + Na2CO3(3)

[0052] The stripper off-gas is fed to a sulfuric acid plant. In the sulfuric acid plant, sulfuric acid is produced from the stripper off-gas and the CNCG produced and collected within the pulp mill. The produced sulfuric acid is consumed within the pulp mill, and the sulfur contained by the sulfuric acid eventually ends up back to the chemical recovery cycle of the pulp mill. The gas stream comprising CO2 or consisting of CO2 and used for stripping of the green liquor may originate from various origins. As already mentioned, traditionally, green liquor stripping has been performed with flue gas containing CO2. However, solutions replacing the flue gas are provided herein. Hence, within context of this disclosure, the gas stream comprising CO2 or consisting of CO2 does not refer to flue gas stream as such. In other words, the gas stream comprising CO2 or consisting of CO2 is a non-flue gas stream.

[0053] Using CO2 from lignin separation process

[0054] In an embodiment, the gas stream comprising CO2 originates from lignin separation process. Within context of this disclosure, the lignin separation process refers to a process wherein black liquor is treated so as to precipitate and separate lignin therefrom to produce purified lignin. The lignin separation process uses CO2 for precipitating lignin. Aim of using CO2 is to lower the pH of the black liquor, thereby reducing solubility of lignin.

[0055] Black liquor is complicated alkaline solution. It contains an organic part with carbohydrates and lignin and an inorganic part with different salts. Most important salts are carbonates and sulfides with sodium as a cation. These both have an acidic nature meaning that both have an effect to pH and are acting as pH buffer in the solution.

[0056] Carbonate buffer reaction:

[0057] Sulfide buffer reaction:

[0058] U H2S(g H++ HS~ 2H++ S2~ (5)

[0059] When CO2 (carbonic acid) or any other acid or base is dissolved to black liquor, those buffering elements tend to counteract the change by shifting the equilibrium to opposite direction illustrated in reaction equations (4) and (5). This counteracting phenomenon is related to Le Chatelier’s principle.

[0060] First, when pH is lowered S2’ reacts to HS’ and soon after that COs2’ starts to react to HCOs’. When pH is lowered even more base HS’ is neutralized even further to H2S. H2S is charge wise a neutral component and it will escape from liquid phase when its concentration exceeds the solubility limit.

[0061] In the first (the alkaline) stage of the lignin separation process, black liquor is treated with the CO2 and pH is lowered to 10-11. This means that both carbonates are mainly in form of -CO3 and -HCO3 and sulfides are mainly in form of -HS. In the second stage of the lignin separation process, filtered lignin cakes are re-slurried into sulfuric acid solution for purification. pH is 2.5-4 which prevents lignin from dissolving. Since pH is so low virtually all the sulfide and carbonate are in H2S and CO2 form. This means that equilibriums of reactions (4) and (5) have shifted to the left end of the equilibrium reaction path. CO2 and H2S are mainly released to gas phase and only a dissolved amount defined by solubility limits are staying in the solution.

[0062] High amounts of CO2 and sulfuric acid are consumed by the lignin separation process, as roughly 300 kgCO2 and the same 300 kgH2SO4 are typically consumed per ton of lignin produced. Since most of the spent CO2 and H2S is released from lignin separation process it means that there is a potential to reuse that same carbon dioxide in the green liquor stripping. To further enhance CO2 and H2S release from lignin separation process acidic and alkaline filtrates can be mixed in a separate reactor.

[0063] Thus, in this embodiment, the method disclosed herein further comprises treating black liquor with CO2 so as to precipitate lignin in a lignin separation process, collecting a gas stream comprising CO2 from lignin separation process, conveying the collected gas stream to a green liquor stripper, and using the collected gas stream for stripping green liquor. As described above, the gas stream collected from the lignin separation process also comprises H2S, thereby further boosting the eventual sulfuric acid production.

[0064] The gas stream comprising CO2 may be collected from an alkaline slurry tank (first stage of the lignin separation process), an acidification tank (second stage of the lignin separation process), or both of them. In an example, to further enhance the CC and H2S release from lignin separation process acidic and alkaline filtrates can be mixed in a separate reactor before collecting the gas stream comprising CO2. It should be noted that the oxygen content of the off-gas originating from the lignin separation process (i.e., lignin separation off-gas) must be minimized to avoid sulfide oxidation to sulfate and to prevent gas mixture to enter explosion zone.

[0065] Lignin separation off-gas containing CO2 can be used in a green liquor stripping system which is simpler than the one needed for the conventional method utilizing flue gas. The main advantage compared to existing flue gasbased system is lower investment cost and elimination of difficult contaminated flue gas streams which must be taken to recovery boiler or to scrubber. Also, a conventional scrubber for scrubbing the off-gas from the lignin separation process can be removed which eliminates sodium hydroxide consumption, thereby further closing the mill chemical balance. Furthermore, operational costs are lowered if concentrated CC can be used instead of flue gas because lower volume flows are needed for stripping. Flue gas originating from a lime kiln or a recover boiler of a pulp mill is a diluted source of CO2, majority of it being nitrogen from combustion air, and typically less than 20 vol-% of the flue gas being CO2. Low CO2 concentration also translates into low partial pressure. As solubility of CO2 in water solution is proportional to partial pressure of CO2 in the gas phase, and concentration of CO2 in the flue gas is low, high amounts of flue gas must be used in the conventional green liquor stripping.

[0066] Example calculation

[0067] Considering a situation wherein 200 t / d lignin is produced by the lignin separation process as described herein and assuming that half of the carbon dioxide consumed in the lignin separation process can be reutilized, 1250 kg / h carbon dioxide can be recovered. With this amount of carbon dioxide assuming stochiometric consumption and 1 ,5-time margin for bicarbonate production, stripped sulfur amount in the green liquor stripping is 700 kgS / h. From this sulfur flow 49 500 kg / d sulfuric acid can be produced. Another assumption is that direct H2S release molar-wise is half of the CO2 release from lignin separation process meaning of 425 kgS / h flow. From this sulfur flow 30 000 kg / d sulfuric acid can be produced. Hence, altogether sulfuric acid production would be 79500 kg / d (for the green liquor stripper off-gas produced by stripping with a gas stream originating from lignin separation process). Using gas stream consisting of CO2

[0068] In an alternative embodiment, the gas stream used for green liquor stripping consists of CO2. The gas stream consisting of CO2 refers to pure CO2.

[0069] When green liquor stripping is performed with the pure CO2, it greatly reduces the amount of eguipment needed for the process, when compared to the existing flue gas-based system. Pure CO2 has higher partial pressure and it can be fed to green liquor with higher pressure. This increases the solubility of CO2 and thus carbonation of the liguid. When CC can be used more efficiently and there isn’t inert nitrogen load like with normal flue gas, the whole gas flow can be taken from green liquor stripping to sulfuric acid plant.

[0070] In an embodiment, the gas stream consisting of CO2 and used for green liquor stripping is produced by a carbon capture technology from a flue gas produced by the Kraft pulp mill. The flue gas produced by the Kraft pulp mill may refer for example to flue gas from a lime kiln or from a recovery boiler. In a carbon capture technology, a chemical solvent or a porous solid material may be used for separating the CO2 from other components of the flue gas stream produced by the mill. The carbon capture technology may comprise passing the flue gas stream through an amine solvent, wherein the amine solvent is capable of binding the CO2 molecule, and heating the thus produced CO2-rich solvent so as to release the CO2 from the solvent. The released CO2 stream may be conditioned to remove impurities and used in the method and system as disclosed herein. Porous solid materials suitable for carbon capture technologies include for example metal-organic frameworks and other solid sorbents, such as zeolites, silicas, activated carbon, alumina and metal oxides.

[0071] Boosted chemical recovery

[0072] As described above, utilizing the green liquor stripping off-gas in sulfuric acid production within the Kraft pulp mill enhances extraction of sulfur from the pulp mill liquor cycle by providing a new sulfur cycle for the Kraft pulp mill.

[0073] The sulfuric acid produced in the sulfuric acid plant may be used in any location of the pulp mill utilizing sulfuric acid. Typically, chlorine dioxide plant, bleaching plant, lignin separation, and / or tall oil plant of the pulp mill are the main consumers of sulfuric acid. Hence, the method disclosed herein may further comprise conveying the sulfuric acid produced from the stripper off-gas to chlorine dioxide plant, bleaching plant, lignin separation, and / or tall oil plant of the Kraft pulp mill.

[0074] The green liquor stripping produces, besides the green liquor off-gas, a carbonate solution. The carbonate solution can be collected and causticized by a normal causticizing process so as to produce sodium hydroxide. This internally produced sodium hydroxide solution is substantially sulfide-free, which differentiates it from white liquor. The sodium hydroxide produced may be conveyed to and used in any process within the pulp mill requiring sodium hydroxide. Examples of pulp mill processes using sodium hydroxide include for example bleaching, tall oil cooking, oxygen delignification, waste water effluent neutralization, scrubbing, and use as a make-up chemical. After the usage, the sodium contained by the sodium hydroxide can be recycled back to the chemical recovery loop.

[0075] System for recovering chemicals

[0076] The system suitable for implementing the method according to this disclosure as well as its interconnection to the pulp mill chemical recovery cycle are illustrated in Fig. 2.

[0077] The system for recovering chemicals in a Kraft pulp mill comprises a recovery boiler C for producing green liquor, a green liquor stripper 201 for producing stripper off-gas, and a conduit 211 for conveying green liquor from the recovery boiler to the green liquor stripper. The system further comprises a conduit 212 for conveying a gas stream comprising or consisting of CO2 to the green liquor stripper, a sulfuric acid plant 202 for producing sulfuric acid, and a conduit 213 for conveying the stripper off-gas to the sulfuric acid plant.

[0078] According to an embodiment, the system further comprises a lignin separation system for producing the gas stream comprising CO2. In that case, the conduit 212 is for conveying the gas stream comprising CO2 from the lignin separation system to the green liquor stripper 201 . The lignin separation system may comprise an alkaline slurry tank and an acidification tank, and the conduit may be arranged to convey the gas stream comprising CO2 from at least one of the tanks to the green liquor stripper. The lignin separation system may further comprise a reactor for mixing the acidic and alkaline filtrates of the lignin separation process, before collecting the gas stream comprising CO2 and conveying it to the green liquor stripper.

[0079] According to an alternative embodiment, the system further comprises a source for pure CO2. The system may comprise a carbon capture arrangement for producing a gas consisting of CO2. As described above, the carbon capture arrangement may for example comprise a chemical solvent or a porous solid material for separating the CO2 from other components of the flue gas stream produced by the pulp mill.

[0080] In any of the alternative embodiments described above, the system may further comprise a mini causticizer 203 for producing sodium hydroxide by causticizing a carbonate solution produced in the green liquor stripper. The system also comprises a conduit 214 for conveying the carbonate solution from the green liquor stripper to the mini causticizer.

[0081] In any of the alternative embodiments described above, the system may further comprise a conduit for conveying the sodium hydroxide from the mini causticizer to anywhere in the Kraft pulp mill, for example to the cooking and fiber line, including the bleach plant. Such a conduit 215 is illustrated in Fig. 2 with a dashed line.

[0082] In any of the alternative embodiments described above, the system may further comprise a chlorine dioxide plant, a bleaching plant, a lignin separation system, and / or tall oil plant, and a conduit for conveying the sulfuric acid from the sulfuric acid plant to the chlorine dioxide plant, a bleaching plant, a lignin separation system, and / or tall oil plant. Fig. 2 illustrates the system comprising a chlorine dioxide plant 204 and a conduit 216 for conveying the sulfuric acid from the sulfuric acid plant to the chlorine dioxide plant. Further, as illustrated in Fig. 2, the system may also comprise a conduit 217 for conveying chlorine dioxide from the chlorine dioxide plant 204 to cooking and fiberline A, particularly to the bleach plant, a conduit 218 for conveying sulfate from chlorine dioxide plant 204 to evaporation plant B, and a conduit 219 for conveying CNCG from evaporation plant B to sulfuric acid plant 202. Preferably the system further comprises a recovery boiler ash treatment system for producing recovery boiler ash treatment bleed (illustrated with line 220 in Fig. 2). The recovery boiler ash treatment system is arranged to selectively remove chlorine and potassium from the recovery boiler ash. The treated ash with reduced chlorine and potassium content may be recycled back into the system.

[0083] A simplified schematic illustration of the green liquor stripper useable in the method and system according to this disclosure is illustrated in Fig. 3. The green liquor stripper 301 comprises an inlet for a conduit 311 for conveying green liquor from the recovery boiler to the green liquor stripper. The green liquor stripper further comprises an inlet for a conduit 312 for conveying a gas stream comprising or consisting of CO2 to the green liquor stripper. The green liquor stripper further comprises an outlet for a conduit 313 for conveying the stripper off-gas to the sulfuric acid plant, as well as an outlet for a conduit 314 for conveying carbonate solution from the green liquor stripper to the mini causticizer.

[0084] The green liquor stripper useable herein refers to any kind of apparatus, wherein a physical separation process resulting in removal of a H2S gas stream from a liquid green liquor stream by a gaseous stream comprising or consisting of CO2 can be performed. For example, stripping in the green liquor stripper may be conducted in frayed towers (plate columns) or packed columns.

[0085] One clear advantage of using the method and the system of this disclosure compared to the original one utilizing the flue gas-based stripping is the lower capital investment. More than half of the equipment of the traditional system can be eliminated by the novel system disclosed herein. The eliminated equipment includes flue gas cooler, precarbonator, carbonators with mixers, flue gas fan, flue gas compressors and recovery heat exchanger.

Claims

Claims:

1. A method for recovering chemicals in a Kraft pulp mill, the method comprising- producing green liquor in a recovery boiler,- conveying green liquor from the recovery boiler to a green liquor stripper,- stripping the green liquor with a gas stream comprising or consisting of CO2 so as to produce a stripper off-gas,- feeding the stripper off-gas to a sulfuric acid plant, and- producing sulfuric acid from the stripper off-gas.

2. The method according to claim 1 , wherein the gas stream comprising or consisting of CO2 is a non-flue gas stream.

3. The method according to claim 1 or 2, wherein the method further comprises- treating black liquor with CO2 in a lignin separation process,- collecting a gas stream comprising CO2 from the lignin separation process,- conveying the collected gas stream to the green liquor stripper, and- using the collected gas stream for stripping green liquor.

4. The method according to any of the preceding claims, wherein the gas stream comprising CO2 is collected from an alkaline slurry tank and / or an acidification tank of a lignin separation process.

5. The method according to claim 1 or 2, wherein the gas stream consists of CO2.

6. The method according to claim 5, wherein the method further comprises producing the gas stream consisting of CO2 by a carbon capture technology from a flue gas produced by the Kraft pulp mill.

7. The method according to any of the preceding claims, wherein from 5 to 10 vol-% of the green liquor produced in the recovery boiler is conveyed to the green liquor stripper for stripping.

8. The method according to any of the preceding claims, wherein the method further comprises conveying the sulfuric acid produced from the stripper off-gas to chlorine dioxide plant, bleaching plant, lignin separation system, and / or tall oil plant of the Kraft pulp mill.

9. The method according to any of the preceding claims, wherein the method further comprises producing a carbonate solution when stripping green liquor, collecting the carbonate solution, and causticizing the carbonate solution so as to produce sodium hydroxide.

10. A system for recovering chemicals in a Kraft pulp mill, wherein the system comprises- a recovery boiler (C) for producing green liquor,- a green liquor stripper (201 , 301 ) for producing stripper off-gas,- a conduit (211 , 311 ) for conveying green liquor from the recovery boiler to the green liquor stripper,- a conduit (212, 312) for conveying a gas stream comprising or consisting of CO2 to the green liquor stripper,- a sulfuric acid plant (202) for producing sulfuric acid, and- a conduit (213, 313) for conveying the stripper off-gas to the sulfuric acid plant.11 . The system according to claim 10, further comprising- a lignin separation system for producing the gas stream comprising CO2.

12. The system according to claim 10, further comprising a source for pure CO2.

13. The system according to claim 10 or 12, further comprising a carbon capture arrangement for producing a gas consisting of CO2.

14. The system according to any of the claims 10-13, further comprising- a mini causticizer (203) for producing sodium hydroxide by causticizing a carbonate solution produced in the green liquor stripper, and- a conduit (214) for conveying the carbonate solution from the green liquor stripper to the mini causticizer.

15. The system according to any of the claims 10-14, further comprising - a chlorine dioxide plant (204), a bleaching plant, a lignin separation system, and / or a tall oil plant, and- a conduit (216) for conveying the sulfuric acid from the sulfuric acid plant to the chlorine dioxide plant, the bleaching plant, the lignin separation system, and / or the tall oil plant.

16. Use of the system according to any of the claims 10-15 for recovering chemicals in a Kraft pulp mill.

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