The invention relates to a method for reducing emissions of detrimental nitrogen compounds from the chemical cycle of a pulp mill, said method comprising supplying black liquor produced in the chemical cycle to a recovery boiler for combustion, whereby smelt is formed on the bottom of the recovery boiler as a result of the combustion, said smelt being supplied to a dissolving tank to be dissolved in a liquid containing mainly water. 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 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 for preventing the formation of NO _x compounds or for converting them to 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 problems with this technique are that it is expensive to purchase the reagent outside the mill, and that it is difficult to adjust the amount of ammonia supplied to flue gases suitably so that all ammonia is consumed in the reaction and no ammonia residues remain in the flue gases to be discharged to the atmosphere.

It is also known to supply hydrocarbons, such as natural gas or the like, to the flue gases of a boiler. In this case, the reduction of NO _x compounds is based on the fact that the hydrocarbon radicals speed up the reactions of

nitrogen compounds. The drawback of such methods is that they are expensive, since they require heavy investments and the operating costs are high: 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. 931055 discloses a method in which oxygenous hydrocarbon obtained in the pulp cooking process, such as methanol, 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 a recovery boiler to be mixed with flue gases, whereafter the flue gases are washed with white liquor or with an aqueous solution containing ammonia- 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 to a nitrogen dioxide N0 ₂ , which can be removed by an alkali scrubber. The drawback of this method is that it has an effect only on the reduction of the oxides of 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 N0 ₂ , and the treatment of the nitrogen compounds remaining in the washing liquid is still problematic. In the chemical cycle of a pulp mill, ammonia emissions are caused by nitrogen derived from wood, for example, during the combustion process of black liquor. The salt smelt obtained as a result of the combustion of 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 a 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 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 reducing emissions of detrimental nitrogen compounds from the chemical

cycle of a pulp mill by which the drawbacks of the prior art solutions can be avoided.

The method of the invention is characterized by supplying ammonia-containing vent gases separated from the smelt during the dissolution and/or in process steps following the dissolution to the furnace of a boiler of the pulp mill at a point where the fuel supplied to the boiler has mainly burnt, whereby the ammonia contained in the vent gases reacts with the nitrogen oxide present in the furnace of the boiler to form molecular nitrogen. The essential feature of the invention is that a detrimental nitrogen compound discharged from the chemical cycle of a pulp mill as an emission is used for removing another detrimental nitrogen compound discharged from the chemical cycle of the pulp mill so that the result is normal molecular nitrogen that can be freely discharged to the atmosphere. When implemented in this manner, the invention has the advantage that it allows separate treatment of two detrimental compounds to be at least partly avoided, when no reagents purchased outside the mill or equipment for treating them are required for this purpose.

In the following, the invention will be described in greater detail with reference to the accompanying drawing, which is a schematic illustration of the part of the equipment of a pulp mill required for applying the invention.

The figure is a schematic view of a recovery boiler 1 , to which black liquor is supplied from an evaporator through a liquor supply channel 2. As a result of black liquor combustion in the recovery boiler, a smelt bed 3 is formed in the lower part of the recovery boiler. From the smelt bed 3, the smelt flows through a smelt chute 4 to a dissolving tank 5, where it is dissolved in a liquid containing mainly water, i.e. white liquor, NaOH, water or a mixture of these. The flue gases which are formed in the recovery boiler during the combustion and which contain nitrogen oxides flow to the upper part of the recovery boiler 1 and are discharged through a flue gas duct 6. During the combustion of black liquor, such nitrogen compounds are formed in the recovery boiler 1 that once said compounds have been supplied to the dissolving tank 5 with the smelt, they are converted into ammonia. The ammonia is released from the solution and discharged with vent gases from the dissolving tank through a vent gas duct 7. The vent gases of the dissolving tank also contain aqueous steam. The steam is preferably removed before the vent gases are supplied to the boiler. For this purpose it is

possible to use a condensating scrubber 8, from which the vent gases are supplied through a duct 8a to the recovery boiler at a point where the combustion of black liquor is at least mainly completed. In this case, the ammonia present in the vent gases reacts with the nitrogen oxide contained in the flue gases of the recovery boiler in accordance with the following formula:

6NO + 4NH ₃ => 5N ₂ + 6H ₂ O.

The optimum temperature for the reaction is 900°C, but if desired the temperature window of the recovery boiler can be expanded by supplying suitable additives, such as methanol or ethanol, to the furnace. These additives can be supplied to the recovery boiler either with the ammonia- containing vent gases discharged from the dissolving tank 5 or separately from them. In view of the application of the invention, an optimal point at which the additives can be supplied to the recovery boiler 1 is below the beak 9 of the boiler, where the temperature is the most suitable for the supply of ammonia- containing vent gases. If the boiler has a so-called upper-tertiary air register, the vent gases can be supplied to the boiler 1 therethrough. Alternatively, separate feed openings can be provided for the supply of vent gases. In some cases, a tertiary air register may also be suitable for the supply of vent gases to achieve a desired effect.

In addition to a recovery boiler, the figure illustrates a power boiler 10 of a pulp mill, to which fuel is supplied through a fuel channel 11. Since nitrogen oxides are also formed in a conventional power boiler, the invention can also be applied by supplying ammonia-containing vent gases of a dissolving tank 5 to the power boiler 10 as shown in the figure, i.e. through channel 8a, according to the principle disclosed in connection with the recovery boiler 1. In this case, the ammonia-containing vent gases are supplied to a temperature of about 900°C at a point where the fuel supplied to the power boiler 10 has mainly burnt, and the nitrogen oxide is reacted with the ammonia in accordance with the formula disclosed above. The flue gases of the power boiler are supplied further through a flue gas duct 12.

A significant advantage of the invention is that it allows emissions of nitrogen oxides of a boiler to be substantially reduced by utilizing another detrimental nitric compound formed in a pulp mill, i.e. ammonia. The nitrogen oxide and ammonia are reacted with each other to form molecular nitrogen,

which is harmless and can thus be freely discharged to the atmosphere. The use of reagents that have to be purchased and investments in equipment can be avoided, since the necessary equipment already exists in a pulp mill. Furthermore, the invention allows considerable savings to be achieved in the treatment of flue gases and vent gases, since these gases can be treated with fewer reagents and simpler equipment.

In the specification above and in the drawing, the invention has been described merely by way of example, and it is by no means limited to this example. If desired, ammonia-containing vent gases can be supplied from the dissolving tank 5 past the scrubber 8 directly to the recovery boiler or power boiler through duct 7', indicated with broken lines, and duct 8a. Instead of the dissolving tank, or in addition to it, it is also possible to supply ammonia- containing gases from the following process steps, such as a causticizing department. The temperature range of the supply point may naturally vary to a certain extent, the preferred temperature range being 800°C to 1000°C. The essential thing is that the ammonia separated in a dissolving tank or in the following process steps from the smelt formed in a recovery boiler of a pulp mill is supplied to the recovery boiler or a power boiler in such a manner that, when reacting in the furnace of the boiler with the nitrogen oxide contained in the flue gases, it forms molecular nitrogen and thus reduces nitrogen oxide emissions from the boiler. It will be obvious that, if necessary, the flue gases of both a recovery boiler and a power boiler can be further purified in a suitable manner to remove any remaining impurities.