FI105703B - Method for reducing the NOx emission from the smoke gases of a soda pan - Google Patents

Description

, 105703

A method for reducing NOx emissions from the flue gases of a recovery boiler

The invention relates to a method for reducing NOx emissions in the flue gases of a recovery boiler by supplying to the furnace of the recovery boiler a pulp mill containing concentrated sulfur-containing odorous gases.

When burning liquor in the recovery boiler, different nitrogen compounds are always formed, which are harmful when released into the environment. Efforts have been made to reduce the formation of nitrogen oxides by various combustion methods, such as phasing of combustion air or by feeding various reagents and catalytically active substances to the furnace furnace furnace or flue gases. Such methods are known, for example, from SE 504 056 and EP 705 363. The disadvantage of many methods is that the reagents are expensive and not only the use of the reagent and the catalyst are sufficient, but further purification and treatment of the flue gases is required.

In known processes, the basic starting point has generally been either the decomposition of the formed nitrogen oxides to give molecular nitrogen or the prevention of the formation of nitrogen oxides. This is done by feeding the reagent at one point into the furnace of the recovery boiler, either in combination with the combustion air or with a separate nozzle.

However, the known methods have not achieved the desired reduction of the emission i i, but even more efficient and better methods and equipment are needed. In addition, investment and operating costs are high.

A: It has also been found that several known methods are required to achieve the desired emission level simultaneously.

: Y: 25 The present invention is intended to provide a more effective and efficient method. The process of the invention is characterized in that concentrated ammonia-rich concentrated odor gases are fed to the lower part of the recovery boiler below the liquefaction feed point and correspondingly low ammonia-containing concentrated odorous gases are fed to the upper part of the recovery boiler.

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The essential idea of the invention is that the odorous gases of different ammonia content are fed into the furnace furnace furnace so that the ammonia-rich odorous gases are fed into the lower part of the furnace furnace furnace and burned with almost stoichiometric air either directly to the top of the recovery boiler or to a separate burner so that it burns clearly with a subchiometric air ratio.

An advantage of the invention is that without the need for any complicated equipment, by simply selecting the source of the odorous gases, the odorous gases containing ammonia and other hydrocarbon derivatives can be fed into two or even more different locations so that they are fed to the most suitable locations in the furnace. At the same time, ammonia and sulfur emissions are also eliminated, as the reactions will decompose the ammonia and bind the sulfur to the dusty form 10.

The invention will be explained in more detail in the accompanying drawings, in which Figure 1 schematically shows a process diagram of a pulp mill and Figure 2 schematically shows the supply of odorous gases to a recovery boiler in accordance with the invention.

Figure 1 schematically illustrates a pulp mill process generally known to one of ordinary skill in the art.

The wood is brought to the cooking plant 1, from where the pulp is led to the washing 2 and then removed from the process. From the wash 2, the weak black liquor is led to the evaporator 3 and from there to further concentration 4. The evaporator 3 has 20 several evaporation stages which can be both in succession and in parallel. Black liquor flows through various evaporation stages to remove water vapor and volatile gases. Evaporator 3 is also associated with so-called. stripping to remove impurities from the dirty condensate. in stripper 3a. From the stripper gases obtained therefrom, methanol is further separated in a methanol liquefier: Y: 25b, whereupon the non-liquefied gases are conveyed as concentrated odor gases. The methanol from methanol liquefaction 3b is usually quite ammonia-rich, whereupon methanol is quite well suited for use as a support or auxiliary fuel in the bottom burner of the recovery boiler. The figure schematically illustrates how ammonia-rich odorous gases 30 are conveyed through ducts 8 to the collection tank 9 and thereafter through ducts · · · '; Low ammonia-containing concentrated odor gases from further concentration are passed through a conduit 11 to a recovery boiler 5 at the top for feeding as shown in Figure 2. The operation and construction of the various evaporators, as well as the stripper 35 and the liquefier, are well known to those skilled in the art and are therefore not described in detail herein.

In the event of further concentration, the 4 black liquors are further evaporated to remove odor gases. The black liquor is then fed to a recovery boiler 5 where it is incinerated to recover chemicals. The salt melt generated in the soda boiler is dissolved in water in the leaching tank 6 and the green liquor thus formed is led to causticization 7 where it is formed into white liquor and the white liquor is fed to the cooking plant 1 as a cooking solution. At different stages of the cellulose process, odorous gases with different concentrations of ammonia and sulfur are produced. of the concentrated or high sulfur compounds 10, some are high in ammonia and low in ammonia respectively. In accordance with the invention, these odorous gases generated in the various stages are introduced into the furnace furnace furnace to reduce NOx emissions, such that ammonia-rich gases from, for example, a boiler or evaporator Typically, the odorous gases are introduced into the furnace via a separate burner, the solution shown in Figure 2. In this case, the combustion air is supplied separately to the burner so that the air coefficient is almost stoichiometric, in practice at least 0.7. Similarly, ammonia-poor odor gases are fed into the upper part of the recovery boiler above the liquefaction feed point either

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v or through a separate burner so that the burner burn zone

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v: air ratio not greater than 0.5. This produces the formation of nitrogen oxides

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v,: to reduce or inhibit, and to re-establish the nitrogen oxides formed to the molecular nitrogen form. At the same time, it avoids excess ammonia concentration: 25 which would otherwise be necessary to prevent ammonia emissions. ---------------- just because they are high or low in ammonia Typically, in a normal pulp mill process, the ammonia content of the odor gases that are • separated from further hardening is typically 1000 ppm, which means Ammonium-30 low-odor odor gas and for example from a cooker, evaporator, etc.

* I · '; * * concentrated strong odor gases typically contain about 10,000 ppm of ammonia: ···' nia, meaning ammonia-rich odor gases. One limit between ammonia-rich and ammonia-poor odors may be considered to be: 5000 ppm ammonia, _____ 35 In the combustion of ammonia-rich odorous gases with a high production factor, the reaction results mainly in nitrogen monoxide NO. Similarly, in the reductive lower portion of the recovery boiler, the nitrogen compounds in the black liquor are mainly released as NH3 ammonia, whereby the ammonia and NO can react in accordance with the following reaction: 6NO + 4NH3 → 5N2 + 6H2O.

The reaction thus results in water and molecular nitrogen, which in turn are harmless to the environment and the atmosphere.

On the other hand, when feeding low odor ammonia-containing odor gases to the top of the recovery boiler, the situation, based on the normal combustion process of the boiler, is such that the ammonia released from the black liquor is mainly converted into NO and NO2. The low odor ammonia-containing concentrated odor gases supplied in this connection act in a so-called. reburning as waste fuel, resulting in the conversion of nitric oxide into molecular nitrogen.

Figure 2 is a schematic representation of a recovery boiler depicting four different air supply levels ^-14 by way of example. In this embodiment, the lye supply L occurs clearly above the second air supply plane l2. The concentrated odor gases from the boiler and evaporator 10 are precisely supplied to the recovery boiler via a separate burner 12, whereupon they are supplied with combustion air P1 and possibly support fuel P. . In this embodiment, the total air coefficient of the burner 12 is at least 0.7. In this situation, the results of combustion of sulfur and ammonia-containing odor gases burned in the burner react in the furnace furnace furnace in such a way as to prevent the formation of nitrogen oxides and result mainly in molecular nitrogen. The low odor ammonia-containing concentrated odor gases from the additional concentration 11 are in turn fed directly to the soda recovery furnace above the third air supply level 13. If the odorous gases with low ammonia content are supplied through a separate burner, the air coefficient of up to 0.5 is used in the burner so that much of the combustion occurs outside the burner combustion zone. 35 Final air supply occurs from the last air supply level l4, whereby the combustion is as complete as possible.

The invention is set forth by way of example only in the specification and claims and is not limited thereto. It is essential that the concentrated odorous gases are led to different locations in the soo-5 dacetter according to the ammonia content so that the ammonia-rich odorous gases are fed to the lower part of the furnace via a separate burner, through the odor gas burner such that the air coefficient of the burner is so low that a substantial amount of ammonia does not burn in the combustion zone. The process step from where the ammonia-containing concentrated odorous gas is led to where depends on the amount of ammonia in those odorous gases in the process of the plant in question.

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Claims (5)

A process for reducing NOx emissions in the flue gases from a soda boiler (5) by supplying to the fire hearth of the soda boiler (5) strong sulfur-containing odorous gases separated from a process in the cellulose plant, characterized by strong odorous gases containing ammonia ammonia the lower part of the soda boiler (5) below the lutin feed point (L) and similarly, strong odor gases containing small amounts of ammonia are measured in the upper part of the soda boiler (5) above the lutin feed point (L). {10 Method according to claim 1, characterized in that the strong odorous gases containing ammonia ooh as measured in tili | the lower part of the soda boiler is burnt with a separate burner (12) so that the air factor is at least 0.7 in the combustion zone of the burner (12). 3. A process according to claim 1 or 2, characterized in that the strong odorous gases containing small amounts of ammonia are measured directly to the hearth of the boiler (5). 4. A method according to claim 1 or 2, characterized in that the strong odorous gases containing small amounts of ammonia are measured in the fire hearth of the soda boiler (5) via a separate odor gas burner such that the air factor is at most 0.5 in the combustion zone of the burner. 5. A method according to any of the preceding claims, characterized; t e c k n a t that the ammonia content of the strong odorous gases containing:.: 'plenty of ammonia is at least 5,000 ppm. • • • - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - (*) • · · «» »