Method of Removing Nitrogen Oxides From Recovery Boiler Flue Gases

The present invention relates to a method of removing nitrogen oxides from flue gases of a recovery boiler of a cellulose pulp mill in a cost effective and environmentally friendly manner.

The waste liquor produced during the production of chemical cellulose pulp in the pulp and paper industry, i.e. black liquor, is usually burned in a recovery boiler to recover chemicals and heat. In a conventional recovery boiler, the process chemical is recovered by spraying the black liquor into a furnace. When the liquor enters the boiler it dries quickly and is burned under reducing conditions, producing smelt which is removed from the boiler. The combustion air is usually introduced into the boiler at three different levels so as to provide reducing conditions at first and oxidizing conditions thereafter. Primary air is introduced into the lower section of the furnace, secondary air above the level of the primary air but below the liquor nozzles, and, in order to secure complete combustion, tertiary air above the liquor nozzles. These three air levels are typically the basic air levels in a modern recovery boiler, but the boiler may also have other air levels. A recovery boiler with multiple air introduction levels is shown in U.S. Pat. No. 5,701,829 and "Pulp and Paper Manufacture," Third Edition, Volume 5, .COPYRGT.1989, by Grace et al, FIG. 482, page 533 20 (the disclosures of which are hereby incorporated by reference herein).

In the combustion of black liquor, a large volume of flue gases is produced, which gases contain various impurities, such as nitrogen oxides (NOXs). The NO.sub.x compounds originate either from the thermal oxidation of the nitrogen in the combustion air, or from the release of the nitrogen bound in the fuel and the following oxidation. In the reducing conditions prevailing in the furnace of the recovery boiler, the nitrogen fed into the furnace is converted during the combustion processes into ammonia and nitrogen compounds ending up in the chemical smelt. During conventional combustion, or in conventional combustion effected with staged oxidation (i.e. air is introduced in several different stages as described above) the ammonia forms low NO.sub.x combustion molecular nitrogen and nitrogen oxides that are harmful to the environment. Typically, half of the ammonia identified above is converted into nitrogen oxides and the other half to nitrogen gas. By the low NO.sub.x combustion (with staged air feed when the understoichiometric conditions turn to overstoichiometric conditions in the final combustion), the conversion of the ammonia to nitrogen may be reduced, typically by about 20%.

Methods, which try to reduce the NO.sub.x content of recovery boilers burning black liquor by changing the ratios and the location of the introduction points of combustion air in the direction of the combustion process, have been disclosed for example in U.S. Pat. No. 5,454,908. According to the method therein, the retention time of flue gases in the furnace under understoichiometric conditions has been prolonged so that the last introduction of combustion air takes place at least ten meters above the liquor feed level. This method requires the boiler to have a certain height in order to maintain the reducing conditions for a sufficiently long time.

Some methods of reducing the NO.sub.x emissions from boilers are based on the use of ammonia or urea. A known method of reducing the amount of nitrogen oxides produced during combustion is based on introduction of ammonia into the process in the upper part of the furnace, or after the furnace, within a precise temperature window so that a selective reaction between ammonia, oxygen and nitrogen oxide takes place, producing nitrogen gas and steam. The use of this method is limited to a temperature window of below 1050.degree. C., which is very difficult to control, and it requires introduction of ammonia (or some other nitrogen-based chemicals), which are difficult to treat, into the flue gases.

Finnish patent application no 951690 discloses a method in which additional fuel is introduced into the black liquor recovery boiler above the liquor level. The material to be added comes from the pulp manufacturing process and may be, for example, a malodorous gas or soap. These additional fuels contain hydrocarbons or hydrocarbon compounds which, when burning, generate hydrocarbon radicals, intensifying the reactions of NO.sub.x compounds, and thus reducing the amount of NO.sub.x compounds.

Also the use of other chemicals to remove nitrogen oxides from the flue gases of a recovery boiler has been suggested. According to the method of U.S. Pat. No. 5,639,434, chlorine dioxide or ozone is introduced into the flue gases (immediately before the flue gas scrubber subsequent to the recovery boiler) in order to oxidize nitrogen oxide to nitrogen dioxide. After this the flue gases are scrubbed in a gas scrubber in order to reduce nitrogen oxide to nitrogen for example with green liquor, which has been oxidized or sulphited. This method requires increasing the use of chlorine chemicals in the pulp mill, which is undesirable environmentally and to facilitate process effectiveness. On the other hand, ozone is an expensive chemical. Further, green liquor must be treated before it is used in gas scrubbing.

According to the present invention there is provided a method by which the nitrogen oxides of the flue gases from the recovery boiler of a pulp mill may be removed in an economical, simple, and environmentally friendly manner.

The method of the invention basically provides introduction of a peroxide solution (e.g. it is sprayed) into recovery boiler flue gases having a temperature of about 300-800.degree. C., and typically flowing in the heat recovery section of a recovery boiler, in order to oxidize nitrogen oxides .

A central aspect of the present invention is that, in the removal of nitrogen oxides from flue gases, a chemical (i.e. peroxide) is used which may also be used for other purposes in the pulp mill, for example in the bleaching of pulp. Usually, hydrogen peroxide is used in the practice of the invention, but other conventional peroxides are acceptable too. An advantage provided by peroxide is that it reacts quickly with nitrogen monoxide, producing nitrogen dioxide, according to the reaction NO+H.sub.2 O.sub.2 .fwdarw.NO.sub.2 +H.sub.2 O. The retention time in the reaction zone need only be between about 0.5-5 seconds, preferably about 1-2 seconds. The use of peroxide does not cause undesirable chemical residues in the process waters of the mill.

The flue gases of the recovery boiler are transported from the furnace to contact different heat transfer equipment, superheaters, the boiler bank, and economizers, thus recovering the heat contained in the gases into the water, steam, or mixtures thereof flowing in the heat transfer equipment. The boiler bank of the boiler and the economizer comprise heat transfer members formed by heat transfer elements inside of which the boiler feed water to be heated flows. Typically, there is free or open space for the flue gas flow in the boiler bank and in the economizer between the heat transfer elements. While the flue gas flows past the heat transfer elements heat is transferred to the feed water flowing inside the elements. In order to practice the method of the invention, peroxide solution is sprayed into the flue gases when they have a temperature of between about 300-800.degree. C., a conventional temperature when they flow through the boiler bank and the economizer. The peroxide is preferably added in the conventional flue gas duct between the boiler bank and the economizer where the most preferred temperature range (between about 400-600.degree. C.) prevails. Peroxide solution may be added at more than one point in the region of the boiler bank and the economizer.

The peroxide solution utilized typically comprises about 1-35% hydrogen peroxide in water, e.g. between about 5-20%. The amount of peroxide used is at least the stoichiometric amount, typically between about 1.1-2 times the stoichiometric amount. The introduction of the peroxide results in the conversion of the majority of NOXs in the flue gases to N.sub.2 O.sub.2, typically 60-90%, but possibly even more than 90%. For example if the No.sub.x content of the flue gases is about 100-150 ppm, the invention reduces it to about 30-40 ppm.

The treated flue gases from the recovery boiler are preferably transported from the economizer (in a manner known per se) via a flue gas outlet to a wet scrubber downstream of the recovery boiler. In the scrubber, the flue gases are scrubbed, according to the invention, with an alkaline solution containing for example sodium hydroxide or potassium hydroxide, in order to bind nitrogen dioxide, thus producing nitrate, for example sodium nitrate or potassium nitrate. The scrubbing solution preferably is an alkaline solution used in the mill, such as white liquor, or oxidized white liquor, or alkaline condensate from the pulp mill evaporation plant. The alkaline liquid preferably has a pH of greater than 9, e.g. between about 10-11.5, and converts more than half of the NO.sub.2 to nitrate (e.g. more than 90%). Also according to the invention, the nitrate-containing scrubbing solution may be transported to the biological effluent treatment plant of the pulp mill where the nitrate is used as a nutrient.

An advantage provided by the invention is that it is simple to practice. No remarkable new apparatus is needed to carry out the invention. When peroxide and alkaline solutions are used, no harmful chemical residues are produced, such as are produced when chlorine dioxide is used.

In the above description all narrower ranges within a broad range are also specifically included. For example 400-600.degree. C. means 410-510, 450-600, 500-598, and all other narrower ranges within the broad range.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.