WO1999047745A1

WO1999047745A1 - Method for reducing emissions of detrimental nitrogen compounds of pulp mill

Info

Publication number: WO1999047745A1
Application number: PCT/FI1999/000216
Authority: WO
Inventors: Kauko Janka
Original assignee: Kvaerner Pulping Oy
Priority date: 1998-03-19
Filing date: 1999-03-19
Publication date: 1999-09-23
Also published as: SE520308C2; SE0003336L; FI980618A; SE0003336D0; FI980618A0; FI105704B

Abstract

A method for reducing detrimental nitrogen compounds of a pulp mill. In the method, the ammonia contained in weak odor gases is combusted into nitrogen oxide by supplying the weak odor gases to the combustion zone for strong odor gases in the odor gas combustor, whereby the nitrogen oxide formed reacts under reducing conditions with ammonia separated from black liquor, forming molecular nitrogen and water.

Description

METHOD FOR REDUCING EMISSIONS OF DETRIMENTAL NITROGEN COMPOUNDS OF PULP MILL

The invention relates to a method for reducing emissions of detrimental nitrogen compounds of a pulp mill, in which method ammonia and nitrogen oxide are reacted with each other in the furnace of a recovery boiler to form molecular nitrogen and water.

When black liquor or some other fuel is combusted in recovery or other boilers of a pulp mill, the combustion usually produces nitrogen oxides. Released in the atmosphere, the nitrogen oxides are considered detrimental compounds. To reduce or eliminate NO_x formation, various combustion techniques have been used, the air supply has been stepped and different reagents have been supplied to the furnace of the boilers to prevent nitrogen formation or to convert the nitrogen oxide formed into molecular nitrogen.

Various chemical compounds detrimental to the environment and the atmosphere are produced in a sulphate pulp mill. Examples of such compounds are various oxides of sulphur and nitrogen, and for example ammonia, which is formed in certain steps of the chemical cycle. Attempts have been made to reduce the emissions of nitrogen compounds, mainly nitrogen oxides, from recovery boilers and power boilers by the use of so- called stepped combustion, in which air is supplied to a boiler in several successive steps so that the combustion takes place mainly in under- stoichiometric conditions. In this case, the formation of so-called thermal NO_x can be reduced considerably. Such a solution is disclosed, for example, in Swedish Published Specification No. 468 171. There have also been attempts to reduce the NO_x contents of flue gases by supplying various reagents to the flue gases to prevent the formation of NO_x compounds or to convert them into a form in which they can be removed as easily as possible. Such an additive may be ammonia or urea purchased outside the mill, in which case the NO present in the flue gas reacts with the ammonia, forming gaseous nitrogen, which can be discharged to the atmosphere. It is also possible to use various solid or liquid ammonium salts as the reagent in this so-called SNCR method known per se. The problem in this technique is that the reagent purchased outside the mill is expensive, and that the amount of ammonia supplied to the flue gases is difficult to adjust suitably so that all ammonia would be consumed in the reaction and no 2 ammonia residues would remain in the flue gases that will be discharged to the atmosphere.

It is also known to supply hydrocarbons, such as natural gas or the like, to the flue gases of the boiler. In this case, the reduction of NO_x compounds is based on the fact that the hydrocarbon radicals speed up the reactions of the nitrogen compounds. The drawback of such methods is that they are expensive, requiring heavy investments and having high operating costs, since the additives must be purchased outside the mill, and in addition, equipment is required for storing, batching, adjusting and feeding the additives.

Finnish Patent Application No. 931 055 discloses a method in which oxygenous hydrocarbon, such as methanol, obtained in the pulp cooking process is supplied to the flue gases of a recovery boiler. In this method, the methanol and any aqueous steam are supplied to the upper part of the recovery boiler and mixed there with flue gases, whereafter the flue gases are washed with white liquor or with an aqueous solution containing ammonia- based and/or alkali-based compounds. The method is based on the fact that the nitrogen oxide NO contained in the flue gases is partly oxidated into nitrogen dioxide NO₂, which can be removed by an alkali scrubber. The drawback of the method is that it has an effect only on the reduction of the oxides of the already formed nitrogen, and the only reagent that can be used is methanol or a corresponding hydrocarbon derivative. In addition, the method requires a flue gas scrubber suitable for removing N0₂, and the treatment of any nitrogen compounds remaining in the washing liquid is still a problem.

In the chemical cycle of a pulp mill, ammonia is emitted from wood- derived nitrogen, for example, during the combustion process of black liquor. The salt smelt obtained as a result of the combustion of the liquor contains nitrogen compounds which are converted into ammonia during the dissolution of the smelt and in the following process steps. The ammonia produced is further released to the vent gases of the dissolving tank, for example. The odour of the vent gases is problematic in view of the environment, and therefore the vent gases must normally be treated. It is known to wash the vent gases of the dissolving tank with water in order to reduce ammonia emissions to the environment or to supply them with other odour gases of the pulp mill to an odour gas boiler, where they are burnt separately. This causes 3 extra costs, and the flue gases of the odour gas boiler must still be washed in a scrubber, whereby the nitrogen compounds accumulated in the scrubber present a treatment problem.

The object of the present invention is to provide a method for essentially reducing emissions of nitrogen oxides in a recovery boiler simultaneously as the other process emissions are reduced in a simple manner. The method of the invention is characterized in that an odor gas combustor for combusting strong odor gases is arranged such in the recovery boiler that reducing conditions prevail at the odor gas combustor in the furnace, and that weak odor gases are supplied to the combustion zone of the odor gas combustor, whereby the ammonia contained in these gases burns in the odor gas combustor, turning into nitrogen oxide, and reacts at the bottom of the recovery boiler with ammonia separated from the black liquor, thereby forming molecular nitrogen and water. The essential idea of the invention is that the vent gases, i.e. weak odor gases, are supplied to the combustion zone of the odor gas combustor arranged in the reducing zone at the bottom of the recovery boiler, at the reducing zone, either as combustion air or mixed with the combustion air, whereby they burn in the flames of the odor gas combustor, forming nitrogen oxide. Another essential idea of the invention is that the nitrogen oxide formed reacts with the ammonia released from black liquor in the reducing zone, forming molecular nitrogen and water, as a result of which the nitrogen oxide emissions from the recovery boiler are reduced accordingly. In a preferred embodiment of the invention, water contained in the vent gases, i.e. weak odor gases, is reduced from the gases in a condensing scrubber, which reduces the amount of unnecessary water in the recovery boiler. The advantage of the invention is that the ammonia emissions of the vent gases can be eliminated simultaneously as the NO_x emissions of the recovery boiler are reduced without the use of any external reagents or complicated processes. The invention is described in greater detail in the attached drawing, which is a schematic presentation showing how a method of the invention can be applied in a recovery boiler.

The figure shows a recovery boiler with a furnace 1 , where the combustion takes place. There are several air supply levels 2a to 2c in the vertical direction of the recovery boiler, the combustion air needed to combust the black liquor being supplied from the air supply levels such that in the main 4 part of the furnace 1 the combustion takes place in under-stoichiometric conditions. The figure also shows liquor sprayers 3, from which the black liquor is supplied to the recovery boiler for combustion. At the bottom of the furnace 1 of the recovery boiler, i.e. in the area where under-stoichiometric combustion takes place, the recovery boiler is also provided with an odor gas combustor 4, to which are supplied strong odor gases from, for example, an evaporator, optionally from the last 'super-concentration' step of the evaporator, and to which is also supplied auxiliary fuel to effect reliable and complete combustion of the strong odor gases. Further, combustion air, mixed with vent gases, i.e. weak odor gases, i.e. other than strong odor gases, is supplied to the combustion air channel of the odor gas combustor. The vent gases, i.e. weak odor gases, are odor gases that contain a large amount of ammonia but otherwise have a negligible combustion value. Odor gases like this are usually derived, for example, from a smelt dissolving tank, a green liquor tank, a green liquor clahfier, a caustic sludge extinguisher/sorter, a causticizing tank, a white liquor filter and/or a caustic sludge filter. The terms 'vent gas' and 'weak odor gas' in the present application and claims thereby particularly mean odor gases like this. The ammonia contained in the vent gases and supplied with the combustion air is burnt by the flames of the combustion zone of the odor gas combustor, whereby nitrogen oxide NO is formed. On the other hand, ammonia NH₃ is separated from the black liquor at the bottom of the furnace 1 of the recovery boiler under reducing conditions, whereby the nitrogen oxide and the ammonia react with each other according to the formula

6NO + 4NH₃ => 5N₂+ 6H₂O

producing molecular nitrogen and water. The resultant compounds are harmless and can be freely discharged to the atmosphere. In addition to being supplied as combustion air or mixed with combustion air, weak odor gases can also be supplied along a separate channel directly to the combustion zone of the odor gas combustor so that at least an essential amount of the ammonia is oxidated into nitrogen oxide.

The essential point of the invention is that the odor gas combustor is arranged such in the furnace of the recovery boiler that the combustion conditions at the odor gas combustor are reducing ones, i.e. under- stoichiometric combustion takes place. The nitrogen oxide formed from the 5 ammonia by combustion in the odor gas combustor can thus react with the separated ammonia to produce molecular nitrogen and water.

The invention is described in the above description and in the drawings only by way of an example, and it should not be considered to be restricted thereto in any way. The essential point is that the vent gases, i.e. weak odor gases, are supplied to the combustion zone of the odor gas combustor as combustion air or mixed with the combustion air so that the ammonia contained therein will burn, forming nitrogen oxide. Further, the odor gas combustor is arranged at the lower part of the recovery boiler in an area where reducing conditions prevail, i.e. where the liquor has combusted under- stoichiometrically. The odor gas combustor can be arranged, for example, below the liquor sprayers, or at some other point before the last air supply step. The vent gases can either be mixed with the combustion air of the odor gas combustor or in some conditions they can replace the combustion air. The auxiliary fuel used in the odor gas combustor can be a combustible waste obtained from a pulp mill, such as methanol, soap or turpentine, whereby any ammonia contained in the waste will burn, turning into nitrogen oxide, and thereby a larger amount of the ammonia separated from the black liquor can be converted into molecular nitrogen and water.

Claims

1. A method for reducing emissions of detrimental nitrogen compounds of a pulp mill, in which method ammonia and nitrogen oxide are reacted with each other in the furnace of a boiler to form molecular nitrogen and water, characterized in that an odor gas combustor for combusting strong odor gases is arranged such in the recovery boiler that reducing conditions prevail at the odor gas combustor in the furnace, and that weak odor gases are supplied to the combustion zone of the odor gas combustor, whereby the ammonia contained in these gases burns in the odor gas combustor, turning into nitrogen oxide, and reacts at the bottom of the recovery boiler with ammonia separated from the black liquor, thereby forming molecular nitrogen and water.

2. A method as claimed in claim 1, characterized in that the odor gas combustor is arranged below the liquor sprayer such that the nitrogen oxide formed in the odor gas combustor (4) reacts with the ammonia separated from the black liquor substantially entirely before the liquor sprayers

(3)*

3. A method as claimed in claim 1 or 2, characterized in that the weak odor gases are supplied to the combustion zone of the odor gas combustor as combustion air.

4. A method as claimed in claim 3, characterized in that the weak odor gases are supplied to the odor gas combustor as the only combustion air.

5. A method as claimed in claim 3, characterized in that the weak odor gases are supplied to the odor gas combustor mixed with other combustion air.

6. A method as claimed in claim 1 or 2, characterized in that the weak odor gases are supplied to the combustion zone of the odor gas combustor along a separate channel.