WO1994020676A1

WO1994020676A1 - Process for purifying the combustion gases of a recovery boiler

Info

Publication number: WO1994020676A1
Application number: PCT/FI1994/000084
Authority: WO
Inventors: Pauli Dernjatin; Harri Kannela; Pia Salokoski; Hanne Siikavirta
Original assignee: Imatran Voima Oy
Priority date: 1993-03-10
Filing date: 1994-03-09
Publication date: 1994-09-15
Also published as: FI98842C; FI931055A0; FI98842B; AU6208894A; FI931055A

Abstract

The invention relates to a method for cleaning the flue gases of a recovery furnace. According to the method, the flue gases are contacted with a scrubbing liquid in a flue gas scrubber (3) in order to absorb the sulfur and nitrogen compounds contained in the flue gases, and when desired, subjecting the scrubbing liquid containing the absorbed sulfur and nitrogen compounds to further processing in order to recover said compounds. According to the invention, to the recovery furnace (1) is fed an oxygen-containing hydrocarbon derivative such as methanol obtained from the pulp cooking process to the end of achieving at least a partial oxidation of nitric oxide contained in the flue gases to nitrogen dioxide, and the flue gas scrubber (3) is then run using such a scrubbing liquid in which by virtue of the absorption of sulfur oxide are formed sulfite compounds capable of reacting with nitrogen dioxide.

Description

IProcess for purifying the combustion gases of a recovery boiler .

The present invention relates to a method according to the preamble of claim 1 for cleaning the flue gases of a recovery furnace.

According to the present method, flue gases are contacted with a scrubbing liquid in a flue gas scrubber to the end of absorbing the sulfur and nitrogen oxides contained in the flue gases, and when desirable, subjecting the spent scrubbing liquid containing absorbed sulfur and nitrogen compounds to further processing to the end of recovering said compounds.

The invention also concerns a method as defined in the preamble of claim 13 for reducing the quantity of oxygen-containing compounds in the contaminated con¬ densates of a cellulose manufacturing process.

Conventionally, the removal of nitrogen oxides from flue gases is implemented using primary and secondary methods. Using improved combustion techniques, primary methods aim at reducing the amount of nitrogen oxides generated in a combustion process. Secondary methods aim at removing the nitrogen oxides from flue gases. The most frequently applied secondary methods are SCR (selective catalytic reduction) and SNCR (selective noncatalytic reduction).

In addition to those described above, so-called simultaneous methods have been developed in which nitrogen and sulfur oxides are removed concurrently. Simultaneous methods are categorized as dry and wet methods. Dry methods are based on the use of a catalyst or adsorption. Wet methods are based on simultaneous removal of nitrogen and sulfur oxides by means of scrubber techniques. The scrubber technique is conventionally used for, e.g., removal of sulfur oxides from the flue gases of a recovery furnace in the pulp industry. Simultaneous removal of nitrogen oxides in a scrubber designed for sulfur removal presumes the oxidation of nitric oxide to nitrogen dioxide, or alternatively, the dosing of nitric oxide absorption promoting additives into the scrubbing liquid.

Use of ammonia and urea injections has been attempted in pulp production in order to reduce nitric oxide to nitrogen in the recovery furnace at a temperature of 900 - 1000 °C. However, these methods have not been developed to a practical level at the industrial scale.

It is an object of the present invention to provide a novel method for simultaneous removal of nitrogen and sulfur oxides from the flue gases of a recovery furnace, said method being based on the oxidation of nitric oxide to nitrogen dioxide in the recovery furnace.

The invention is based on the concept of injecting an oxygen-containing hydro¬ carbon derivative to the recovery furnace to the end of oxidizing the nitric oxide. According to the invention, the oxygen-containing hydrocarbon derivative used herein can be methanol, ethanol, acetone or similar compound obtained from the pulping process. To remove the nitrogen dioxide thus formed from the flue gases, the gases are scrubbed with a scrubbing liquid which reacts with the sulfur dioxide contained in the flue gases so as to form sulfite compounds and possibly also bisulfite compounds which then react with the nitrogen dioxide.

More specifically, the method according to the invention for cleaning the flue gases of a recovery furnace is principally characterized by what is stated in the characterizing part of claim 1.

Furthermore, the method for reducing the quantity of oxygen-containing hydro¬ carbon derivatives contained in condensates and converting said compounds to a harmless form is characterized by what is stated in the characterizing part of claim 13. In the context of this patent application, the term "recovery furnace" refers to such a recovery furnace or boiler in which the concentrated waste liquor resulting from pulp cooking is combusted. One purpose of such a combustion process is energy recovery from waste liquor by producing, e.g., steam. The waste liquor combusted in the recovery furnace typically contains sulfide compounds which in the combustion process are converted to sulfur dioxide. Additionally, the combustion process generates nitrogen oxides (NO_χ), part of which occur as nitric oxide.

According to the invention, the oxidation of NO with the help of methanol or similar oxygen-containing hydrocarbon derivatives is implemented through the injection of liquid or vaporized methanol along with a suitable carrier to the furnace at a point where the flue gas temperature is approx. 500 - 900 °C. The decomposition of methanol produces HO radicals which perform the oxidation of NO to NO₂ in accordance with Eq. I:

NO + HO₂ <-» NO₂ + OH (I)

The above reaction can take place in both directions and it has an optimum retention time which is dependent on the reaction temperature.

According to the invention, the methanol (and other oxygen-containing com¬ pounds) used in the oxidation of nitric oxide is obtained from the pulping process.

The pulping process generates significant amounts of contaminated condensate streams ("dirty condensates") containing oxygen-rich hydrocarbon derivatives originating from the cooking process, digester blow and digester relief or from black liquor deaeration and the evaporation plant. Methanol and similar oxygen- containing hydrocarbon derivatives become stripped and accumulated in the collected condensate. Methanol is principally formed through alkaline hydrolysis according to Eq. II: L-OCH3 + OH- → L-O^" + CH3OH (II)

A contaminated condensate containing methanol, foul-smelling sulfur compounds and other organic compounds (e.g., ethanol and acetone) can be purified by steam distillation, air stripping or biological treatment. Distillation by steam stripping is a conventional method particularly favored in the Nordic countries. According to the present method, the methanol separated by steam stripping is advantageously condensed and purified prior to its use for oxidation of nitric oxide. This is because the methanol gas obtained from the steam stripping step also contains sulfur compounds. However, as their boiling point is lower than that of methanol, methanol is condensed first in the condensation step thus being recovered in pure form, whereby no smell problems are incurred by the sulfur compounds. Further¬ more, handling of liquid methanol is easier. Conventionally, a separate column is required for the condensation step.

The amount of methanol formed in the pulp cooking process is approx. 6 - 13 kg per ton of pulp, and it causes the major portion of the BOD content imposed by condensates, namely, approx. 60 - 80 % of the total BOD. In addition to methanol, also ethanol is formed in amounts of approx. 1 kg/tn pulp and acetone approx. 0.15 kg/tn pulp.

Methanol can be injected to the recovery furnace in liquid or gas phase. The methanol obtained from the steam stripping and subsequently condensed and puri¬ fied can be fed to the furnace as an aqueous solution of suitable concentration. Alternatively, the condensed methanol can be vaporized and fed to the recovery furnace along with a suitable carrier medium.

Effective mixing of the methanol in the flue gases is implemented by means of jet nozzles. The concentration of methanol in its aqueous solution or gas phase is dependent on the dimensions of the recovery furnace, required number of jet nozzles and their location. When using vapor-phase injection of methanol, safety precaution must be taken against potential explosion hazards. The injection rate of the methanol-water or methanol-carrier mixture is dimensioned so that the methanol jets cover the entire cross section of the flue gas duct at the injection point and provide as effective mixing as possible.

To avoid explosion hazard, methanol concentration in the injection feedstock should be either below 5 %, or advantageously, greater than 25 %. Most advanta¬ geously the methanol concentration is approx. 30 - 60 %, typically approx. 40 - 50 %.

As noted above, the methanol injection can be complemented with or replaced by injection of other hydrocarbon derivatives such as ethanol or acetone, both of which are obtainable from the contaminated condensate streams.

According to the invention, the injection of the oxygen-containing hydrocarbon derivative compounds obtained from the cooking process to the recovery furnace can be complemented with a feedstock of corresponding pure chemicals when desired. The use of such feedstock is disclosed in, i.a., US patent publication 4,350,669, GB patent publication 1,547,531 and JP patent application 54-55602 (ABIPC Vol. 51, No. 2 (1980), Abstract No. 2356). However, these publications do not teach the use of cooking-process-based methanol as the reactive feedstock. The use of the cooking-process-based methanol achieves significant savings in process operating costs and thus also contributes to the alleviation of the contaminated condensates handling problem.

In plants employing biological treatment (whereby methanol is not separated from the contaminated condensates), methanol can be recovered from the condensates by means of a purpose-designed separating unit such as a steam distillation (steam stripping) or distillation plant. Oxygen-containing hydrocarbon derivative compounds are injected to the recovery furnace at a point where the flue gas temperature is approx. 500 - 900 °C. This zone is typically at the level of the furnace tube wall and the superheater tubes. The retention time is typically in the order of 0.05 - 5 s, advantageously approx. 0.1 - 1.5 s. If the hydrocarbon derivative compounds are fed to the temperature zone of, e.g., approx. 600 - 650 °C, the retention time can be designed for, e.g., approx. 0.5 - 1 s.

According to the invention, methanol is advantageously injected in excess relative to the content of nitrogen oxides in the flue gases. Accordingly, the mole ratio in which methanol is mixed with the nitrogen oxides of flue gases is maintained at a value greater than 1, advantageously in the range of approx. 1.2 - 5. High values of the mole ratio are advantageous to the oxidation of nitrogen oxides, whilst the formation of carbon monoxide is simultaneously increased.

The flue gas scrubbers following the recovery furnace are wetted with scrubbing liquids containing compounds which react with sulfur dioxide forming sulfite compounds and possibly also bisulfite compounds. These compounds are in the context of the present patent application generally referred to as "sulfite compounds". Scrubbing liquors which typically are in the form of aqueous solutions may contain ammonium compounds such as ammonium sulfide, ammonium hydroxide or ammonium carbonate, or alternatively, they may contain alkali metal compounds such as sodium sulfide, sodium hydroxide and/or sodium carbonate. The scrubbing liquids may also be comprised of the mixtures of the above-listed compounds. In the preparation of the scrubbing liquid, pure chemicals may be complemented with, e.g., white liquor. As pulp cooking processes chiefly employ sodium-based cooking liquors, and also to some extent, ammonium-based cooking liquors, cooking techniques based on such cationic liquors are particularly advantageously compatible with the present invention. As a result of the absoφtion of SO₂ in the sodium-based scrubbing liquid, sodium sulfite (Na₂SO₃) is formed according to, e.g., Eq. Ill given below:

2 NaOH + SO₂ → Na₂SO₃ + H₂O (HI)

Also sodium bisulfite (NaHSO₃) is formed in the spent scrubbing liquor. Correspondingly, ammonium sulfite and bisulfite are formed in ammonium-based scrubbing liquors. Effective absorption of NO₂ in aqueous solutions of the above mentioned compounds has been verified.

Obviously, the above-mentioned scrubbing liquid compositions can be partially replaced by compositions initially containing sulfite compounds (e.g., using solutions of ammonium or alkali metal sulfites).

The spent scrubbing liquor obtained from the scrubber contains, among other things, sulfite, bisulfite, nitrite and nitrate ions. To separate and recover the sulfur and nitrogen containing compounds, the spend liquid can be taken to further processing.

However, according to a preferred embodiment of the invention, these compounds are not separated, but rather, the further processing of the spent scrubbing liquor comprises returning the liquor as such to the pulp cooking process. Accordingly, the spent scrubbing liquor is taken to the evaporation plant, or alternatively, it can be taken to the dissolving tank of smelt received from the recovery furnace. In the latter case, the NO₂ ^~ and NO₃ ^~ ions contained in the scrubbing liquor are directly routed to the pulp cooking process along with the cooking chemicals. Correspond¬ ingly, if the spend scrubbing liquor is taken to evaporation plant, the nitrogen compounds contained therein remain in the liquor solids which are then combusted in the recovery furnace. Though the exact path of the nitrogen compounds in the combustion conditions of the recovery furnace is not known exactly, it is plausible that at least a portion of the compounds become entrapped in the smelt and circulated through that path back to the pulp cooking process. The nitrogen compounds also partially end up in the gas phase, where they under the reducing conditions are reduced to nitrogen.

The invention offers significant benefits. Namely, methanol formed in the pulp cooking process of sulfate pulp industry can be used for oxidation of NO. Conven¬ tionally, methanol and sulfur compounds have been considered to form the most problematic source of foul-smelling emissions from a pulp mill, and consequently, methanol-rich steam (with a methanol content of 40 - 50 %) has directly been taken to combustion in the prior art. By contrast, the present invention provides an entirely novel method for reducing the quantity of such oxygen-containing com¬ pounds incorporated in contaminated condensates and converting them to non- hazardous compounds, whereby said method simultaneously is capable of reducing nitrogen oxide emissions from the recovery furnace. As is evident from the above- given description, the present invention comprises feeding the mixture of steam from steam stripping and the oxygen-containing compounds to the recovery fur¬ nace where the oxygen-containing compounds are reacted in gas phase with nitric oxide to form nitrogen dioxide. The NO_χ-containing flues gases of the recovery furnace are scrubbed in a flue gas scrubber with a scrubbing liquid containing at least a certain amount of sulfite ions, the scrubbing liquid is circulated in the scrubber and the spent scrubbing liquid is taken to the evaporation plant or the dissolving tank of the smelt obtained from the recovery furnace.

According to the invention, it is thus possible to simultaneously reduce the odor nuisance caused by methanol emissions and to essentially reduce the NO_χ effluents from the recovery furnace. Because the spend scrubbing solution is recycled back to the process, the scrubber consequently produces no waste water which due to the NO_χ absoφtion would contain nitrite and nitrate ions requiring waste water treatment. In the following, the invention will be examined in more detail with the help of a few exemplifying embodiments illustrated in the attached drawings, in which:

Figure 1 is a process flow diagram of the method according to the invention; Figure 2 is a diagrammatic view of the bench-scale test setup used in Example 1 ;

Figure 3 is a graph showing the effect of a fixed bed in the reactor on the oxidation of NO (that is, NO conversion vs. temperature);

Figure 4 is a diagrammatic view of the bench-scale test setup used in Example 2; Figure 5 is a graph showing the proportion of NO_x absorbed in the scrubbing liquid as a function of the NO₂:NO ratio; and

Figure 6 is a graph showing the analysis results of the scrubbing liquid in a test where all NO_χ was NO₂.

With reference to the drawings, the apparatus according to the invention comprises a recovery furnace 1, an electrostatic precipitator 2 and a scrubber 3. In the diagram the different zones of the recovery furnace are marked with reference numerals so that the furnace of the soda recovery boiler is indicated by reference numeral 4, the boiler tube wall is indicated by reference number 5, the superheater by reference numeral 6, the boiler tubes by reference numeral 7 and the water preheater by reference numeral 8. The internal temperature of the recovery furnace varies according to the above-mentioned zones so that the temperature at furnace 4 is greater than 1000 °C (usually approx. 1200 - 1400 °C), at the furnace tube wall 5 and the superheater 6 approx. 500 - 900 °C and at the boiler tubes 7 and the water preheater 8 approx. 250 - 400 °C. The recovery furnace 1 is further provided with an inlet pipe 9 of the methanol mixture obtained from the pulp cooking process, methanol injection nozzles 10 and flue gas duct 11 to which an electrostatic precipitator 2 is connected. The electrostatic precipitator is followed by the flue gas scrubber 3 having a first zone 12, which is the actual scrubbing zone, and a second zone 13, where heat recovery takes place. The scrubber is provides with a stack 14. The method according to the invention is applied to, e.g., a conventional sulfate pulping process. Accordingly, a recovery furnace 1 is fed with black liquor comprising spent cooking liquor which is separated and recovered at maximum concentration. When necessary, the recovered black liquor is further concentrated by evaporation prior to taking it to combustion in the recovery furnace 1 to the end of recovering the cooking chemicals and the energy contained in the black liquor. The smelt of chemical salts obtained from the recovery furnace 1 is regenerated to form fresh white liquor.

The steam stripping of contaminated condensate streams provides a condensate rich in methanol and possibly other oxygen-containing hydrocarbon derivatives that is injected to the upper part of the recovery furnace 1 via a methanol inlet pipe 9. As shown in Fig. 1 , the methanol-steam mixture is injected to the recovery furnace at a point (zones 5 - 6) where the flue gas temperature is approx. 500 - 900 °C. The injection is implemented with the help of high-pressure jet nozzles

10. When required, the jet nozzles can also be used for introducing an additional amount of a carrier medium such as steam to the recovery furnace.

The methanol-steam mixture is injected to the flue gas flow in either liquid or vapor phase, whereby the nitric oxide contained in the flue gases reacts with the methanol forming nitrogen dioxide.

The flue gases are cooled in the heat recovery zones 5 - 8 of the recovery furnace prior to taking them via the electrostatic precipitator 2 to the scrubber 3. The electrostatic precipitator performs the separation of fly ash and other particulates from the flue gases. The flue gases are scrubbed in a contacting zone 12 of the scrubber by an alkaline scrubbing liquor which is introduced via atomizing spray nozzles 12' to the scrubber 3. The alkaline scrubbing liquid employed can be an aqueous solution of, e.g., sodium hydroxide or white liquor. In the contacting zone the nitrogen dioxide and sulfur dioxide are bound with the circulating scrubbing liquor forming nitrite, nitrate, sulfite and bisulfite compounds in the liquor. The spent scrubbing liquor obtained from the scrubber is taken either to the dissolving tank of the recovery furnace plant, or alternatively, to the evaporation plant. The unreacted flue gases are discharged from the scrubber via a stack 14.

With reference to Fig. 2, the diagrammatic configuration of the bench-scale test setup used in Example 1 is shown. The test setup includes a reactor 15 housing a fixed bed 16. The reactor is placed in a furnace 17. Via a nozzle at the top of the reactor, the reactor is fed from gas containers 19 - 21 via a flow rate regulator 18 with a gas mixture containing NO, air (O₂) and N . The zone above the fixed bed 16 is fed via an inner pipe 22 with a mixture of CH₃OH and N₂. Said mixture is prepared by taking a nitrogen flow from a gas container 23 via a rotameter 24 to a methanol container 25, wherein methanol is evaporated into the nitrogen carrier flow. The internal temperature of the furnace 17 is controlled by means of temperature controller unit (not shown), which for the test of Example 1 was programmed to first elevate the furnace temperature to a desired initial temperature, after which the furnace temperature was ramped up to the end temperature, and correspondingly, lowered back down to the initial temperature at a rate of 4 °C/min.

After the fixed bed 16, the reacted gas mixture passes via an inner pipe 26 to analyzers 27 - 30. The gas mixture was analyzed for NO, NO₂ and NO_χ concen¬ trations using a continuous-function chemiluminescence-based NO_χ analyzer 27 designated as Monitor Labs type 8840. The CO concentration was measured using an IR-absoφtion based analyzer 28 designated as Siemens Ultramat 22 and the O₂ concentration was measured using a paramagnetic O₂ analyzer 29 designated as

M&C PMA 25. Measurement data was collected by means of a recorder 30.

With reference to Fig. 4, the diagrammatic configuration of the bench-scale test setup used in Example 2 is shown. The test setup includes a gas scrubbing bottle 31, which is equipped with a thermostat and a magnetic stirrer (not shown). The apparatus is provided with a sample inlet nozzle 32, which is connected to a tubing pump 33. Reference numeral 34 designates a receiving vessel for the liquid sample pumped from the scrubbing liquid. A gas feed pipe 35 is further adapted to the gas scrubbing bottle 31 with the pipe end lowered so deep that it remains below the surface of the liquid fed into the bottle. Gases can then be passed to the feed pipe 35 via a three-way valve 36, or alternatively, the gas scrubbing bottle can be bypassed. Reference numerals 37 - 41 designate flow-regulating valves. Gas samples can be passed either directly from the gas sources or the gas scrubbing bottle 31 via the three-way valve to the NO_χ analyzer 42 and the SO₂ analyzer 43, respectively. The analyzers 42, 43 are connected to a data retrieval unit 44. Diluting air is passed to the SO₂ analyzer to the end of diluting the concentration of the sample to be measured in the sample measuring chamber.

The invention is next described in detail with the help of following examples:

Example 1

Oxidation:

The test setup shown in Fig. 2, a gas mixture was passed at a rate of 3 1/min containing 150 ppm NO, 4.9 vol-% O₂ and 300 ppm CH₃OH. The gas mixture was analyzed for NO, NO₂, O₂ and CO. Reaction temperature varied during the test in the range of 500 - 900 °C. The reaction time was 0.3 s.

The test setup was equipped with a fixed bed in the reactor vessel. The goal was to assess the effect of a high content of or the properties of the fly ash contained in flue gases of a recovery furnace on the conversion of NO to NO₂.

As is evident from Fig. 3, the maximum conversion under the reaction time used occurs at a temperature of 550 - 650 °C. When the reactor is empty, more than 95 % of the NO is oxidized to NO₂. The fixed bed lowers the conversion by approx. 20 - 30 %. For instance, a fixed bed containing approx. 10 wt.-% of Na.--.SO₄, that is, the main component in the fly ash discharge of a recovery furnace lowers NO conversion to a level of approx. 80 %. It must be noted, however, that the solids in this test setup fixed bed is greater by more than 1000-fold (in proportion to gas flow rates) relative to the fly ash content of flue gas discharge from a conventional recovery furnace.

Example 2

A 1 1 aliquot of the scrubbing liquid to be investigated was prepared in a gas scrubbing bottle shown in Fig. 4. The scrubbing liquid was analyzed for pH and the ion concentrations of NO ^~ and NO₃ ^~ by ion chromatography.

This test was made to investigate both the effect of the NO₂:NO mole ratio on the removal efficiency of NO_χ and the change of the ion concentration of NO₃ ^~ and NO₂ ^~ in the scrubbing liquid as a result of the NO_x removal by absoφtion.

The scrubbing liquid was prepared by dissolving 6.30 g Na₂SO₃ and 4.75 g Na-,S₂O₅ to 1 1 distilled water, whereby the pH of the liquid was approx. 6.5. Into the scrubbing liquid was passed a gas mixture having an NO_χ concentration of 250 ppm, an SO₂ concentration of approx. 700 ppm and an O₂ concentration of approx. 5 vol-%. The ratio of NO₂ to NO was varied so that the proportion of NO₂ of total NO_x was 50, 70 and 100 %. With reference to Fig. 5, the proportion of NO_χ absorbed in the tested scrubbing liquid is plotted as a function of the NO₂:NO mole ratio. With reference to Fig. 6, the analysis results of the scrubbing liquid are plotted for a test in which all NO_χ was NO₂. As is evident from Figs. 5 and 6, the removal efficiency of NO_x is a linear function of the proportion of NO₂ and the absorbed NO is seen to form mostly NO₂ ^~ ions, but also to some extent NO₃ ^_ ions in the scrubbing liquid. Example 3

This example describes a plant-scale implementation of the process illustrated in Fig. 1.

The flue gas discharge rate from the recovery furnace is 230 m³(NTP)/s (moist). The composition of the flue gas is 5.3 % O₂, 14.2 % CO₂, 22 vol-% H₂O, 500 ppm SO₂, 100 ppm NO (moist) and 10 ppm CO. Methanol is injected in the recovery furnace at a mole ratio of 3. Then, the methanol consumption is approx. 100 g/s. Methanol is injected in the flue gas flow in the form of an aqueous solution to a temperature zone of 680 °C.

The composition of the flue gas prior to the scrubber is 5.3 % O₂, 14.2 % CO₂, 22 vol-% H₂O, 500 ppm SO₂, 40 ppm NO (moist), 60 ppm NO₂ and 150 ppm CO.

In the scrubber the flue gases are contacted with an Na-based scrubbing liquor containing SO₃ ions. The composition of the flue gas after the scrubber is 5.3 % O₂, 14.2 % CO₂, 50 ppm SO₂, 40 ppm NO (moist), 1 ppm NO₂ and 150 ppm CO.

This example achieves approx. 60 % removal efficiency of NO_χ. The spent scrubbing liquor is taken to the evaporation plant and combusted along with the black liquor in the recovery furnace. This arrangement does not increase the NO_χ of the flue gas effluent, because the nitrogen compounds of the scrubbing liquor are reduced to N₂.

Claims

Claims:

1. A method for cleaning the flue gases of a recovery furnace (1), comprising

- contacting said flue gases with a scrubbing liquid in a flue gas scrubber (3) to the end of absorbing the sulfur and nitrogen compounds contained in the flue gases, and

- when desired subjecting the spent scrubbing liquid with the thus absorbed sulfur and nitrogen compounds to a further processing suited to recover said compounds, characterized by

- feeding to said recovery furnace (1) an oxygen-containing hydrocarbon derivative obtained from the pulp cooking process to the end of achieving at least a partial oxidation of nitric oxide contained in the flue gases to nitrogen dioxide, and - using in the flue gas scrubber (3) such a scrubbing liquid in which are formed, by virtue of absoφtion of sulfur oxide, sulfite compounds capable of reacting with nitrogen dioxide.

2. A method as defined in claim 1, characterized by using as the oxygen-containing hydrocarbon derivatives such methanol which has been obtained from the steam stripping of condensates from pulp processing.

3. A method as defined in claim 2, characterized by using methanol which has been obtained by condensing the distillate from steam stripping performed in order to remove sulfur compounds from said methanol.

4. A method as defined in claim 2 or 3, characterized by feeding said methanol to the recovery furnace (1) in the form of such a water vapor-methanol mixture in which the methanol content is at least approx.25 %, advantageously approx.30 - 60 %.

5. A method as defined in any of foregoing claims 2-4, characterized by feeding the water-methanol mixture to the recovery furnace (1) in liquid phase.

6. A method as defined in any of foregoing claims 2-4, characterized by feeding the water vapor-methanol mixture to the recovery furnace (1) in vapor phase.

7. A method as defined in claim 4, characterized by feeding the water vapor-methanol mixture to the recovery furnace (1) at a temperature of 500 - 900 °C.

8. A method as defined in any foregoing claim, characterized by having the mole ratio of methanol to nitrogen oxides adjusted greater than 1, advantageously approx.1.2-5.

9. A method as defined in claim 1, characterized by using as the scrubbing liquid either white liquor or an aqueous solution containing ammonium- based compounds such as ammonium sulfide, ammonium hydroxide and/or ammonium carbonate, and/or alkali-metal-based compounds such as sodium sulfide, sodium hydroxide and/or sodium carbonate.

10. A method as defined in any foregoing claim, characterized by passing the spent scrubbing liquid from the scrubber (3) to the evaporation plant.

11. A method as defined in any of the foregoing claims 1-8, character¬ ized by passing the spent scrubbing liquid from the scrubber (3) to the dissolving tank of smelt received from the recovery furnace (1).

12. A method as defined in claim 2, characterized by complementing the methanol used as the oxygen-containing hydrocarbon derivative with ethanol and acetone obtained from the steam stripping of condensates.

13. A method for reducing the quantity of oxygen-containing compounds contained in the contaminated condensates of a pulping process, said method comprising

- steam-stripping the condensates to the end of separating the oxygen- containing compounds, c h a r a c t e r i z e d by

- feeding the mixture of steam and oxygen-containing compounds obtained from the steam-stripping step to a recovery furnace (1) where the oxygen-containing compounds are reacted in gas phase with nitric oxide to the end of forming nitrogen dioxide,

- scrubbing the nitric-oxide-containing flue gases of the recovery furnace (1) in a flue gas scrubber (3) with a scrubbing solution containing sulfite ions to at least some extent,

- circulating the scrubbing liquid in the scrubber (3), and - passing the spent scrubbing liquid to the evaporation plant, or alternatively, the dissolving tank of smelt received from the recovery furnace.