FI105704B - Procedure for reducing the emission of harmful nitrogen compounds by a cellulose factory - Google Patents

Description

105704

A method for reducing emissions of harmful nitrogen compounds from a pulp mill

The present invention relates to a process for reducing emissions of harmful nitrogen compounds in a pulp mill, by reacting ammonia and nitric oxide in a furnace furnace furnace to produce molecular nitrogen and water.

In the soda and other boilers of a pulp mill, the combustion of black liquor or similar fuel usually results in nitrogen oxides being considered as harmful compounds released into the atmosphere. Attempts have been made to reduce and eliminate NOx formation by various combustion techniques, by phasing the air supply, and by feeding various reagents to the furnace furnace to prevent the formation of nitrogen or to convert the formed nitric oxide to moiety.

15 The sulphate pulp mill produces various chemical compounds that are harmful to the environment and the atmosphere for environmental reasons. These include the various sulfur and nitrogen oxides and, inter alia, ammonia, which is formed at certain stages of the chemical cycle. Emissions of nitrogen compounds from soda and power boilers, mainly nitrogen oxides, have been reduced by the use of so-called. step combustion, in which air is supplied to the boiler in several successive stages such that the majority of the combustion occurs under sub-kinematic conditions. In this case, the so-called. the formation of thermal NOx can be significantly reduced. Such a solution is known e.g. See SE Bulletin 468 • · ______: 171.

25 Efforts have also been made to reduce NOx concentrations in flue gases by introducing various reagents into the flue gases either to prevent the formation of NOx compounds or to convert them to a form where they can be removed as easily as possible. As such an additive can be. . ; uses ammonia or urea purchased externally, whereby NO in the flue gas reacts with ammonia to form molecular nitrogen, which can be discharged into the open air. Various solid or solutionic ammonium salts may also be used as a reagent for this so-called SNCR process, which is generally known as such. The problem with this technique is that the reagent purchased externally is expensive, and that the amount of ammonia fed to the flue gases is difficult to fit so that it will all be consumed by the reaction and will not leave any residues of ammonia in the flue gases.

It is also known to feed hydrocarbons, such as natural gas, etc. into the flue gases of the boiler, whereby the reduction of NOx compounds is based on hydrocarbon radicals accelerate the reaction of nitrogen compounds. The disadvantages of such methods are that they are expensive in terms of investment and operating costs because the additives are purchased from outside and, in addition, they require equipment for storing, dispensing, adjusting and feeding the additives.

Further, F1 patent application 931055 discloses a process in which the flue gases of a recovery boiler are fed with an oxygenated hydrocarbon obtained in the process of cooking cellulose, such as methanol. In this method, methanol and any water vapor are fed to the top of the recovery boiler among flue gases, followed by washing with white liquor or an aqueous solution containing ammonia and / or alkali-based compounds. This method is based on the partial oxidation of nitrogen monoxide NO in the flue gases to nitrogen dioxide NO 2 which can be removed by an alkaline scrubber. A disadvantage of this process is that it affects only the reduction of oxides of nitrogen already formed and no reagent other than methanol or a corresponding hydrocarbon derivative can be used. Further, the process requires a flue gas scrubber suitable for removal of NO 2 and the treatment of nitrogen compounds remaining in the scrubbing solution remains a problem.

Ammonia emissions from the chemical cycle of the pulp mill include nitrogen from wood in connection with the black liquor combustion process. The salt melt resulting from the combustion of the liquor contains nitrogen compounds which are converted to ammonia during and after the melt leaching process. The resulting ammonia is still liberated e.g. leaching gas from the leaching tank. On the other hand, odor nuisances of carbon monoxide gases are environmentally problematic and normally require the treatment of carbon monoxide gases. It is known to purify the leaching gases of the leaching tank by means of water washing to reduce the ammonia emissions to the environment, or they have been led with other odorous gases from the pulp mill to be incinerated separately. This, in turn, causes additional costs, while the flue gases of the odorous boiler still have to be washed in the scrubber and the nitrogen compounds bound to the scrubber present a treatment problem.

It is an object of the present invention to provide a method for substantially reducing nitrogen oxide emissions in a recovery boiler while simply reducing other process emissions. The process according to the invention is characterized in that the odorous gas burner for the combustion of concentrated odorous gases is installed in the recovery boiler in such a position that the furnace has reducing conditions at the odorous gas burner with ammonia to form molecular nitrogen and water.

The essential idea of the invention is that the effervescent gases, i.e. dilute odor gases, are introduced into the lower part of the recovery boiler in its reducing zone. in the combustion zone of the odor gas burner mounted above the zone, either as combustion air or mixed with it, whereupon they burn in the flame of the odor gas burner to form nitric oxide. A further essential idea of the invention is that the formed nitric oxide reacts with ammonia released from the black liquor in the reducing zone to form molecular nitrogen and water, which results in a corresponding reduction in nitrogen oxide emissions from the recovery boiler. According to a preferred embodiment of the invention, the effluent gases, i.e. dilute odor gases, are reduced by the amount of water contained in the si-20 in a condensing scrubber, thereby reducing the amount of water unnecessarily entering the recovery boiler. An advantage of the invention is that it simultaneously eliminates ammonia emissions from carbon monoxide gases and reduces NOx emissions from the recovery boiler without the use of external reagents or complex processes.

The invention will be explained in more detail in the accompanying drawing, which schematically illustrates the application of the process of the invention in a recovery boiler.

The figure shows a recovery boiler in which the furnace 1 burns. There are a plurality of air supply levels 2a to 2c in the elevation direction of the recovery boiler, from which the combustion air required for the combustion of the black liquor is supplied so that it is carried out under sub-isometric conditions in the main furnace 1. The figure further shows the lye syringes 3 from which the black liquor is fed to the recovery boiler for combustion. In the lower part of the sump boiler furnace 1, i.e. in the area where the combustion takes place sub-cytometrically, the soda gas-35 burner 4 is still installed in the soda boiler, to which the concentrated odor gases are supplied e.g. evaporator and $ 4,105,704 in the final so-called evaporator. super-enrichment step, and in addition support fuel, which reliably and safely enables combustion of concentrated odor gases. Further, combustion air is supplied to the combustion air duct of the odorous gas burner, which is mixed with carbon monoxide gases, i.e. non-concentrated odorous gases. These carbon monoxide gases, or dilute odor gases, are odorous gases which are high in ammonia but otherwise of low calorific value. Typically, such odor gases are derived, for example, from a molten leaching tank, a green liquor tank, a green liquor clarifier, a lime slurry / sorter, a causticization tank, a white liquor filter 10 and / or a lime sludge filter. Thus, for the purposes of this patent application and the claims, the terms "carbon monoxide gas" and "dilute odor gas" are specifically intended to mean such odor gases. Ammonia in the flue gases fed with the combustion air burns in the flame in the combustion zone of the odor gas burner to form nitric oxide NO. Similarly, ammonia NH3 is separated from the black liquor at the bottom of the firebox 15 of the recovery boiler under reducing conditions, whereby nitric oxide and ammonia react with each other according to the following formula; 6NO + 4NH3 => 5N2 + 6H2O 20 resulting in molecular nitrogen and water. As a result, there are compounds that are harmless to the atmosphere and can be freely released into the atmosphere. · Diluted odorous gases may also be fed directly through a separate conduit into the combustion zone of the odorous burner such that at least a substantial portion of the ammonia is oxidized to nitrogen monoxide.

It is essential to the invention that the odorous gas burner is located in the furnace of the recovery boiler in such a position that the combustion conditions at the odorous gas burner are reduced, i.e., the combustion takes place by sub-chiometry. 30 In this case, the nitrous oxide formed by combustion of ammonia in the odor gas burner is capable of reacting with the separated ammonia to give molecular nitrogen and water.

The invention has been described above by way of example only and is not limited thereto. It is essential 35 that the carbon monoxide gases, or dilute odor gases, are introduced into the combustion zone of the odor gas burner 580704 as combustion air or mixed so that the ammonia therein returns to nitrogen monoxide. Further, the odorous gas burner is mounted in the lower part of the recovery boiler in an area where there are reducing conditions, i.e. the liquor combustion has occurred by sub-chiometry. The odor gas burner may be located 5 below, for example, the lye sprays or elsewhere before the final air supply step. The trace gases can either be mixed with the combustion air of the odor gas burner or, in some circumstances, replace the whole combustion air. As a fuel for the odor gas burner, it is possible to use some combustible waste from the pulp mill, such as methanol, soy or turpentine, whereby the ammonia possibly containing it also burns into nitric oxide and consequently a greater amount of ammonia separated from the black liquor is converted into molecular nitrogen and water.

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Claims (6)

1. A process for reducing emissions of harmful nitrogen compounds from a cellulose plant, in which process ammonia and nitric oxide are reacted to react their semen in a fireplace hearth in the manner that molecular nitrogen and water are formed, characterized by a strong combustion gas burning the intended odor gas burner is mounted in the soda boiler at such a location that the fireplaces at the site of the odor gas burner exhibit reducing properties and that weak odor gases are measured in the combustion zone of the odor gas burner, whereby the ammonia burned in from black liquor and form molecular nitrogen and water. Process according to claim 1, characterized in that the odor gas burner is placed under the liquor nozzle in such a way that the nitrogen oxide formed in the odor gas burner (4) reacts with ammonia separated from black liquor substantially completely before the liquor nozzles (3). Method according to claim 1 or 2, characterized in that the weak odor gases are measured in the combustion zone of the odor gas combustor as combustion air. 20 4. A process according to claim 3, characterized in that the weak odor gases are measured at the odor gas combustor as its only combustion air. 5. A method according to claim 3, characterized in that the weak odor gases are measured in the combustion gas combustor mixed with other combustion air. Method according to claim 1 or 2, characterized in that the weak odor gases are measured in the combustion zone of the odor gas combustor along a separate duct. _________________ 1 ------