FI98842C - Procedure for purifying the flue gases from a boiler - Google Patents

Description

Method for cleaning the flue gases of a recovery boiler 98842

The present invention relates to a method 5 according to the preamble of claim 1 for purifying the flue gases of a recovery boiler.

According to such a method, the flue gases are contacted with a scrubbing solution in a flue gas scrubber to absorb the sulfur and nitrogen oxides contained in the flue gases and, if desired, a scrubbing solution containing the absorbed sulfur and nitrogen compounds is further processed to recover these compounds.

The invention also relates to a method according to the preamble of claim 12 for reducing the amount of oxygen compounds contained in the dirty condensates of the pulp process.

15

Primary and secondary methods are known for the removal of nitrogen oxides from flue gases. Primary methods aim to reduce the amount of nitrogen oxides generated in the combustion process through combustion engineering changes. Secondary methods are used to remove nitrogen oxides from the flue gases. The most commonly used secondary methods are SCR (selective catalytic reduction) and SNCR (selective non-catalytic reduction).

In addition to the above, so-called simultaneous methods have been developed in which nitrogen and sulfur oxides are removed simultaneously. Simultaneous methods can be divided into dry and wet-25 methods. Dry methods are based on catalyst or adsorption. Wet processes are based on the simultaneous removal of nitrogen and sulfur oxides by scrubber technology. The scrubber method is commonly used, for example, to remove sulfur oxides from the flue gases of a recovery boiler in the pulp industry.

30 The simultaneous removal of nitrogen oxides in an existing desulphurisation scrubber requires the oxidation of nitric oxide to nitrogen dioxide or the addition of additives to enhance the absorption of nitric oxide to the scrubbing solution used in the absorber.

2,98842

In pulp production, the use of ammonia and urea injections to reduce nitrogen monoxide to nitrogen in a recovery boiler at a temperature of 900 to 1000 ° C has been tried.

However, these solutions have not been developed to an industrially applicable level.

It is an object of the present invention to provide a new process for the simultaneous removal of nitrogen and sulfur oxides from the flue gases of a recovery boiler, which process is based on the oxidation of nitrogen monoxide to nitrogen dioxide in the recovery boiler.

The solution according to the invention is based on the idea that in order to oxidize nitrogen monoxide, an oxygen-containing hydrocarbon is fed to the recovery boiler. According to the invention, methanol, ethanol, acetone or a similar compound is then used as the oxygen-containing hydrocarbon, which is then derived from a pulping process. To separate the nitrogen dioxide formed from the flue gases, these are washed with a scrubbing solution which, on reaction with the sulfur dioxide in the flue gases, forms sulphite and possibly bisulphite-15 compounds which react with the nitrogen dioxide.

More specifically, the method according to the invention for cleaning the flue gases of a recovery boiler is mainly characterized by what is set forth in the characterizing part of claim 1.

20

The process for reducing the amount of oxygen-containing hydrocarbons in the condensates and for rendering the compounds harmless, in turn, is characterized by what is stated in the characterizing part of claim 12.

25 For the purposes of this application, "recovery boiler" means a boiler or oven in which concentrated waste liquor from pulp cooking is incinerated. The purpose is to recover the energy contained in the waste liquor, for example by producing water vapor. The effluent fed to the soda boiler typically contains sulfide compounds which form sulfur dioxide during combustion. In addition, nitrogen oxides (NOJ) are formed during combustion, some of which are in the form of nitrogen monoxide-30 din.

According to the invention, the oxidation of NO with methanol or similar oxygen-containing hydrocarbons is carried out by injecting liquid or evaporated methanol with a suitable carrier into a boiler at a temperature of about 500 to 900 ° C in the flue gases. Methanol produces H02 radicals on decomposition, which oxidize NO to NO2 according to Equation I.

5 NO + H02 NO2 + OH (I)

The reaction can proceed in both directions and has an optimum residence time depending on the temperature.

10

According to the invention, the methanol (and other oxygen-containing compounds) used to oxidize nitric oxide is taken from the pulp process.

In the pulp cooking process, dirty condensates ("dirty condensates") containing significant amounts of oxygen-containing hydrocarbons are formed in the digester, buttocks and digester gasification or in the black liquor expansion and evaporator. Methanol and other oxygen-containing hydrocarbons distill and condense into the condensate. Methanol is formed mainly by alkaline hydrolysis according to Equation II: 20 L-OCH3 + OH '- L-O + CH3OH (II)

Methanol, odorous sulfur compounds, and other organic compounds (e.g., ethanol and acetone) distill in the pulp cooking process and condense into a condensate commonly referred to as dirty condensate. The condensate can be purified by steam distillation, air stripping or biological purification. Steam distillation is common, especially in the Nordic countries.

The methanol separated by steam distillation can be liquefied and purified.

Methanol is formed in the pulp cooking process at about 6 to 13 kg / tonne of pulp and causes the major part, i.e. about 60 to 80%, of the BHK load of condensate. In addition to methanol, 30 ethanol is formed about 1 kg / ton of pulp and acetone about 0.15 kg / ton of pulp.

According to a preferred embodiment of the invention, the methanol is fed to the recovery boiler in the form of a water vapor / methanol mixture from a steam stripping condensate used to purify its dirty condensate having a methanol content of at least about 10%, preferably about 30-60%, typically about 40-50%. As stated above, in addition to or instead of methanol, other oxygen-containing hydrocarbons, such as ethanol and acetone, derived from dirty condensates can be fed to the recovery boiler.

According to the invention, in addition to the oxygen-containing hydrocarbons obtained from the pulp process, corresponding pure chemicals can also be introduced into the recovery boiler, if necessary. The use of these has been discussed e.g. U.S. Patent 4,350,669 and GB Patent 1,547,531.

The water vapor / methanol mixture from the steam stripping can be fed to the recovery boiler in the vapor phase or in the liquid phase. According to a preferred embodiment of the invention, the methanol injection is carried out by introducing the methanol-steam mixture into the boiler by means of suitable auxiliary devices. Pressure nozzles, for example, operating under pressure can be used as auxiliary devices.

15 If necessary, a carrier, eg steam, can be added. This embodiment is particularly suitable for use in plants in which dirty condensates are treated by steam stripping and in which a methanol-steam mixture of a suitable composition of about 10 to 90%, preferably at least 30% and particularly preferably about 30 to about 60% methanol is available.

20

In plants with biological purification (i.e., methanol is not separated from dirty condensate), methanol can be separated from the condensate by a separate separation unit, such as a steam distillation (steam stripping) or distillation unit.

Oxygen-containing hydrocarbons are introduced into the recovery boiler at a point where the flue gas temperature is about 500 to 900 ° C. This zone is typically located at the boiler jacket tubes and superheater. The residence time is usually about 0.05 to 5 s, preferably about 0.1 to 1.5 s. When the hydrocarbons are fed to a temperature zone having a temperature of e.g.

600 to 650 ° C, the residence time can be selected to be, for example, about 0.5 to 1 second.

30

According to the invention, more methanol is preferably fed than there are nitrogen oxides in the flue gases. Thus, the molar ratio of feed methanol to flue gas nitrogen oxides 5,98842 is maintained at a value greater than 1, preferably about 1.2-5. High molar ratios promote the oxidation of nitric oxide, but at the same time increase the formation of carbon monoxide.

5 The flue gas scrubber of a soda boiler uses scrubbing solutions whose compounds react with sulfur dioxide to form sulphite and possibly bisulphite compounds. These compounds are collectively referred to in this application as "sulfite compounds". Washing solutions, which are usually aqueous, may contain ammonium compounds such as ammonium sulfide, ammonium hydroxide or ammonium carbonate, or they may contain alkali metal compounds such as sodium sulfide, sodium hydroxide and / or sodium carbonate. Washing solutions may also contain mixtures of the above compounds. In addition to pure chemicals, e.g. white liquor can also be used. Since mainly sodium and to some extent ammonium-based broths are used in pulp cooking processes, the use of washing liquids containing these cations is particularly advantageous for the present invention.

As a result of the absorption of SO 2 into the sodium-based washing solution, sodium sulfite (Na 2 SO 3) is formed, e.g. according to formula III.

20 2 NaOH + SO2 - Na2SO3 + H2O (III)

Sodium bisulfite (NaHSO 3) is also formed in the solution. Ammonium-based solutions form ammonium sulfite and bisulfite, respectively. NO2 has been found to be efficiently absorbed by this. aqueous solutions of the compounds.

25

It is clear that the above-mentioned washing solutions can be partially replaced by solutions which already contain sulphite compounds as such (e.g. ammonium or alkali metal sulphite solutions).

30 A used washing solution is obtained from the scrubber, which contains e.g. sulfite, bisulfite, nitrite and nitrate ions. It can be subjected to further processing to separate and recover sulfur- and nitrogen-containing compounds.

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However, according to a preferred embodiment of the invention, these compounds are not separated, but in the further treatment of the spent washing solution, the solution is returned to the pulp process. Thus, the solution can be led to an evaporator or it can be led to a melting tank from the recovery boiler. In the latter case, the NO 2 'and NO 3'-5 ions in the washing solution enter the pulp cooking process directly with the cooking chemical. If the spent washing solution is in turn passed to the evaporator, the nitrogen compounds remain in the solid, which is burned in a recovery boiler. Although the behavior of the compounds under the combustion conditions of the recovery boiler is not yet precisely known, it seems obvious that at least some of the compounds are melted and thus returned to the process. The compounds are also partially subjected to the gas-10 phase, whereby they are reduced to nitrogen under the reducing conditions of the recovery boiler.

The invention provides considerable advantages. Thus, in the sulphate pulp industry, the methanol formed in the process can be used to oxidize NO. Traditionally, methanol and sulfur compounds have been considered to be the worst source of odor in the plant, and hitherto rich methanol (40-50%) steam has been exported directly for combustion. Instead, the present invention provides a completely new method for reducing and rendering harmless the amount of oxygen-containing compounds contained in these dirty condensates, which method can at the same time reduce nitrogen oxide emissions from the recovery boiler. As can be seen from the above, this method comprises feeding a mixture of water vapor and oxygen-containing compounds from steam stripping to a recovery boiler, where the oxygen-containing compounds are reacted in the gas phase with nitric oxide to form nitrogen dioxide. The nitrogen dioxide-containing flue gases from the soda boiler are washed in a flue gas scrubber with a scrubbing solution containing at least some sulfite ions, the scrubbing solution is recycled in the scrubber and the spent scrubbing solution is passed to an evaporator or a melt tank.

According to the invention, it is thus possible to simultaneously reduce the odor nuisances caused by methanol and substantially reduce the nitrogen oxide emissions of the recovery boiler. Since the used washing solution is recycled to the process, no wastewater is generated in the scrubber, which as a result of NOx-ab-30 sorption would contain nitrite and nitrate ions requiring wastewater treatment.

The invention will now be examined in more detail with reference to the accompanying drawings and

II

7 98842 with a few application examples.

Figure 1 shows a process diagram of the method according to the invention.

Figure 2 shows a schematic diagram of the test apparatus used in Example 1.

Figure 3 shows the proportion of NOx absorbed in the scrubbing solution as a function of NO2 / NO ratio.

Figure 4 shows the analysis results of the washing solution in an experiment in which all NO 2 was NO 2.

The apparatus according to the invention comprises a recovery boiler 1, an electric filter 2 and a scrubber 3.

In the figure, the different zones of the recovery boiler are provided with reference numerals, where number 4 means the firebox of the recovery boiler, number 5 curtain tubes, number 6 superheater, number 7 boiler tubes and number 8 water preheater. The temperature of the soda boiler varies depending on the zone, being above 1000 ° C for the furnace 4 (usually about 1200-15150000 C), about 500-900 ° C for the curtain tubes 5 and superheater 6 and about 250-900 ° C for the boiler tubes 7 and the water preheater 8. 400 ° C. The soda boiler 1 further has an inlet pipe 9 and a methanol supply nozzle 10 from the methanol mixture obtained from the pulp process, as well as an exhaust flue gas outlet pipe 11 to which an electric filter 2 is connected. The electric filter communicates with a flue gas scrubber 3 with a first zone 20 12, which is the actual scrubbing zone, and a second zone 13, where heat recovery takes place. The washer is equipped with a chimney 14.

The method according to the invention is applied e.g. to a conventional sulphate pulp production process. Thus, a black liquor consisting of spent cooking liquor 25, which has been separated and recovered as concentrated as possible, is introduced into the recovery boiler 1. The recovered black liquor is still, if necessary, concentrated by evaporation before it is passed for incineration in the recovery boiler 1 in order to recover the cooking chemicals and the energy contained in the black liquor. A new white liquor is regenerated from the salt melt coming from the soda boiler 1.

30

Steam distillation of dirty condensates gives a condensate containing methanol and possibly other oxygen-containing hydrocarbons, which is fed to the recovery boiler 1 via a methanol inlet pipe 9 98842 8. As shown in Figure 1, the methanol / water vapor mixture is passed to the top of the recovery boiler at a point (zone 5-6) where the flue gas temperature is about 500-900 ° C. Pressure nozzles 10 are used for the supply. If necessary, additional carrier, e.g. steam-5, can be introduced into the recovery boiler via the supply nozzles.

The methanol / water vapor mixture is injected into the flue gases either in the form of a liquid jet or as a vapor, whereby the nitrogen monoxide in the gases reacts with methanol to form nitrogen dioxide.

10

The flue gases are cooled in the heat recovery zones 5-8 of the recovery boiler before being passed through the electrostatic precipitator 2 to the scrubber 3. The electrostatic precipitator separates fly ash and other solid impurities from the flue gases. In the scrubber washing zone 12, the flue gases are washed with an alkaline scrubbing solution which is fed to the scrubber 3 through the feed nozzles 15 12 '. As the alkaline washing solution, for example, sodium hydroxide or white liquor is used. In the wash zone, nitrogen dioxide and sulfur dioxide bind to the recyclable wash solution to form nitrite, nitrate, sulfite and bisulfite compounds.

The spent washing solution from the scrubber is passed either to the recovery tank of the recovery boiler or to the evaporator. Unreacted flue gases are removed from the scrubber through the chimney 14.

20

Figure 2 shows a simplified diagram of the test apparatus according to Example 1.

It has a scrubber bottle 15 equipped with a thermostat and a magnetic stirrer (not shown). The device has a sampling connection 16 connected to a hose pump 17. Reference numeral 18 describes a container into which a liquid sample of a washing solution is pumped.

A gas supply pipe 19 is further arranged in the gas scrubbing bottle 15, the end of which is inserted so deep that it remains below the surface of the liquid fed to the bottle. Gases can be fed to the supply pipe 19 by means of a three-way valve 20, or the scrubber can be bypassed. Reference numerals 21 to 25 denote gas flow control valves. Through the three-way valve, 30 gas samples can be fed to the NOx analyzer 26 and the SO2 analyzer 27, respectively, both directly from the gas sources and from the scrubber bottle 15. The analyzers 26, 27 are connected to the data acquisition unit 28. Dilution air is required to dilute the sample to the SO2 analyzer.

li 9 98842

The following examples further illustrate the invention:

Example 1 1 1 test solution was prepared in the scrubbing bottle of Figure 2. The wash solution was analyzed for pH and NO 2 and NO 3 ion concentrations by ion chromatography.

The experiments examined the effect of the NO2 / NO2 molar ratio on the degree of NOx separation and the change in the NO3 and NO2 ion content of the scrubbing solution as a result of NOx absorption.

10

The washing solution was prepared by dissolving 6.30 g of Na 2 SO 3 and 4.75 g of Na 2 S 2 O 5 in a liter of distilled water to give a pH of about 6.5. A gas mixture with a NOx content of 250 ppm, a SO 2 content of about 700 ppm and an O 2 content of about 5% by volume was introduced into the washing solution. The ratio of NO2 to NO was changed so that NO2 accounted for 50%, 70% and 100% of the total NOx content. Figure 3 shows the proportion of NOx absorbed in the test solution as a function of the NO2 / NO ratio. Figure 4 shows the analysis results of the scrubbing solution in an experiment in which all NOx was NO2. It can be seen from Figures 3 and 4 that the degree of NOx separation is directly proportional to the proportion of NO 2 and the absorbed NO 2 in the scrubbing solution forms mainly NO 2 'ions, but also some NO 3' ions.

20

Example 2 This example illustrates the practical implementation of the process shown in Figure 1.

25 The amount of flue gas produced by a soda boiler is 56 m3n / s. The flue gas composition is 13,900 mg / m 3 (dry), 350 mg / m 3 SO 3, 3.6 vol% O 2, 62.4 vol% N 2, 11.2 vol% CO 2, 22.5 vol% H 2 O and 135 mg / m3n NO.

Methanol is injected into the soda boiler with a molar ratio of CH3OH / NO of 1.5, whereby 100% methanol is required at about 45 kg / h. The methanol is evaporated and injected into the boiler with a suitable carrier at a point where the flue gas temperature is between 500 and 900 ° C.

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As a result of methanol injection, about 70% of the flue gas NO is oxidized to NO 2. The flue gases are passed through the separation of the particles to a flue gas scrubber, where they are first washed with an alkaline solution of SO 3 and HSO 3 ', whereby the part of the NOx oxidized to NO 2 is absorbed in the scrubbing solution.

5

The washing solution is recycled in the scrubber. The spent washing solution is led to an evaporator or a melt tank from the boiler, i.e. no wastewater is formed in the scrubber, which as a result of NOx absorption would contain NO2 and NO3 ions requiring wastewater treatment.

Claims (11)

A process for purifying the flue gases from a boiler (1), according to which the process - the flue gases are contacted with a washing solution in a flue gas washer (3) for absorbing the sulfur and nitrogen oxides in the flue gases and - the washing solution containing the absorbed sulfur - and the nitrogen compounds, are further subjected to a further treatment for the recovery of these compounds, if desired, characterized by the 10. The boiler (1) is supplied with oxygen-containing hydrocarbon derived from steam distillation of the cellulose production condensate, for at least partial oxidation of the nitrogen oxide in the nitrogen oxide. the flue gases are added to nitrogen dioxide and - in the flue gas washer (3), a washing solution is used in which sulfite compounds are formed in connection with the absorption of the sulfur oxide, which enhance the reaction with the nitrogen dioxide. 2. A process according to claim 1, characterized in that hydrocarbon hydrocarbon is used, methanol is supplied to the soda boiler (1) in the form of a water meadow / methanol mixture which has a methanol content of at least about 10%, preferably about 30-60 20%. 3. Process according to claim 2, characterized in that the water vapor / methanol mixture is added to the boiler (1) in steam phase. 25 4. A process according to claim 2, characterized in that the water meadow / methanol mixture is supplied to the liquid boiler (1) in liquid phase. 5. Process according to claim 2, characterized in that the water meadow / methanol mixture is supplied to the boiler (1) at a temperature of 500 - 900 ° C. 30 Process according to any of the preceding claims, characterized in that the molar ratio of methanol to the nitrogen oxides is greater than 1, preferably about 1, 2 ... 5. 98842 14 7. A process according to claim 1, characterized in that, as a washing solution, white liquor or a washing 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 and / or sodium sulfide, is used. Method according to any of the preceding claims, characterized in that the detergent used (3) originating from the detergent solution is added to an evaporation system. 10 9. A method according to any of claims 1-7, characterized in that the detergent used (3) originating from the washing solution is introduced into a solvent system for the melt which comes from the soda boiler (1). 15 10. A process according to claim 2, characterized in that oxygen-containing hydrocarbons are used in addition to methanol, including ethanol and acetone, which are obtained from the steam distillation of the condensate. A process for reducing the amount of oxygen-containing compounds contained in impure condensates in the cellulose process, according to which the process - the condensate is distilled to separate the oxygen-containing compounds, characterized by - the mixture of the oxygen-containing compounds and the water stripping obtained by the steam stripping. the soda boiler (1), where the oxygen-containing compounds are allowed to react with nitrogen oxide in gas phase to form nitrogen dioxide, - the nitrogen dioxide-containing flue gases in the soda boiler (1) are purified in a flue gas washer (3) by a washing solution containing at least a small amount of sulfuric acid, is circulated in the washer (3) and - the used washing solution is led to an evaporation plant or to a solvent system for the melt from the boiler. •