DE4027404A1 - Nitric oxide oxidn. in gas stream by scrubbing with nitric acid - contg. nitrous acid by desorbing of acid cpds. into gas and replacing nitrous acid - Google Patents

Abstract

In the oxidn. of NO in a gas stream by scrubbing with HNO3, the new features are that (a) dil. HNO3 contg. HNO2 is used: (b) HNO2 and HNO3 are desorbed into the gas stream in scrubbing, due to the higher partial pressures in the liq; and (c) the desorbed HNO2 in the scrubbing liquor is replaced. Also claimed is the appts. Pref. oxidn involves the following reactions. HNO2 + HNO3 + H2O (I), NO + NO2 + H2O (2) 2 HNO2 + 2 HNO3 =4NO2 + 2 H2O (3). Scrubbing is carried out in 1 or 2 stages. In 2-stage operation, HNO2 and HNO3 are vapourised from the scrubbing liquor in stage 1; and pt. of the NO and NO2 in the gas stream are removed (equation 2) in stage 2, to form HNO2, and the liquor is recycled to stage 1. A dwell . stage may be used between stages 1 and 2 for oxidn. (equation 1). In the dwell stage, the surface area for reaction is increased, pref by using exchange inserts. USE/ADVANTAGE- The process is useful for oxidising NO to NO2 in process gas or flue gas from combustion. The HNO2 is replaced continuously and H2O2 consumption is reduced.

Description

The invention relates to a process for the oxidation of nitrogen monoxide NO to nitrogen dioxide NO ₂ in a process gas or in a flue gas from a combustion.

The industrial application of the method PCT / DE88 / 00 412 to the Removal of nitrogen oxides NOx from flue gases has shown that for the oxidation of the vaporous nitrogen monoxide NO with HNO₃ at the procedural low levels of stick dioxide NO₂ in the flue gas long residence times required are.

In the known document DE 26 54 234 - A1 is a Verfah described for the removal of nitrogen oxides from flue gases, in the first stage for the oxidation very high nitric acid Concentrations needed, and thus always a consumption of expensive, highly concentrated nitric acid is present.

The first method has the disadvantage that for the realization of longer residence times, the installation of a large-volume and expensive wash tower is required.

The second method of Ugine Kuhlman has the disadvantage that The used nitric acid in dilute form as Abschläm accumulated and concentrated before re-task must become.

It is the object of the present invention to provide these two To avoid disadvantages and the oxidation state so too operate that

• 1. the residence time of the gas stream for the oxidation of the NO to NO ₂ becomes shorter,
• 2. a separate concentration of nitric acid from the Oxidation level is not required.

According to the invention, the first solution to this problem To consider the sum equation of the oxidation:

NO + 2 HNO₃ → 3 NO₂ + H₂O; (1)

This equation is in the gas phase. It is anticipated set that above a liquid nitric acid a drerei Partial pressure to carry out the oxidation in the Gas phase is present.

In the presence of water vapor are still the other Reactions with nitrogen oxides possible:

NO + NO₂ + H₂O → 2HNO₂ (2)

HNO₂ + HNO₃ → 2NO₂ + H₂O (3)

From the sum of equations (2) and (3), equation (1) is obtained. Equation (3) describes the absoption process in the production of nitric acid and then proceeds from right to left. The literature (Ullmann Vol. 15, p. 24, 1964) describes that the NO ₂ concentration in the gas space determines the rate of the reaction.

Is in the gas stream only a little NO₂ present and predominates in Equation (3) the left side, so determine ENT₂ and ENT₃ the Speed of the reaction and it is formed from ENT  and ENT₃ according to equation (3) nitrogen dioxide and water vapor.

According to the invention, therefore, according to claim 1 of the present invention, the NO-containing gas stream for the oxidation of NO in a scrubber is treated with a scrubbing liquid containing HNO ₂ and HNO ₃ and the beginning of the oxidation initiated with a reaction according to equation ( 3 ).

According to PCT / DE88 / 00 412, the gas stream is preheated after prewash over the dew point temperature or via the heat exchanger according to claim 9 of the present invention, the scrubber can be supplied via a liquid preheating heat. In the has the same effect that when a contact of washing liquid and gas stream by evaporation of the Waschflüs fluid saturation of the gas stream occurs. According to the invention, this saturation also includes the components HNO ₂ and HNO ₃ .

Thus, in the scrubber, HNO ₂ and HNO _{3 are} spontaneously transferred to the vapor phase of the NO ₂ lean gas stream by a mass transfer process and form NO ₂ and H ₂ O vapor according to equation (3). Due to the NO ₂ formation in the gas stream, there is sufficient NO ₂ present for the further oxidation of NO to NO ₂ that the formation of NO ₂ can proceed via the normal formation path via equation (2) and (3).

In practical scrubbing operation, less than 0.3 g of HNO ₂ / l is dissolved in the scrubbing liquid in an acidic wash liquor and at a NO x concentration of 400 ppm NO and NO ₂ in the scrubbing liquid. After the amount of liquid stored in the washing liquid circuit is small compared to the hourly Durchge set gas flow to 20 m3 / h come 1 liter of liquid, the present in the wash liquid at the beginning nitrous acid in the scrubber quickly desorbed and not nachgebil det sufficiently.

According to a further feature of claim 1, the nitrous Acid in the washing liquid continuously added.

One possibility for the reproduction of nitrous acid is the Washing liquid continuously near the boiling point to heat up and / or the washing liquid longer on elevated Temperature level to store the formation of HNO₂ promote.

Another possibility is the reduction of a part of the dissolved nitric acid according to claim 12 to nitrous acid. For this purpose, a buffer is in the circulation of the washing liquid built-in liquid container. In the buffer tank is a device for dipping a reducing agent z. B. in mattal form as steel balls or sheets. As with the industrial pickling of metals is formed in the liquid is nitrous acid. Another reducing agent are liquid or gaseous hydrocarbons. these can  as pentane, butane, alcohol or methane simply in the above Buffer be dosed.

In all cases, the nitric acid is oxidizing and is partially reduced to HNO ₂ .

This type of process control is technically easy to carry out, but has the disadvantage that the entire newly formed HNO ₂ in the downstream absorption process for Auswas monitoring of nitrous gases z. B. is to be re-oxidized by H ₂ O ₂ .

It is a further object of the invention to avoid this additional requirement for H ₂ O ₂ .

According to the dependent claim 3 of the invention, this object is achieved in that the gas stream is washed in a further 2 th washing stage after the oxidation with the washing liquid, which comes from the previously described washing step (now referred to as washing stage 1 ), in countercurrent.

After the oxidation, the gas stream contains nitrous gases with a higher proportion of NO ₂ and a corresponding proportion of vaporous nitrous acid HNO ₂ . This nitrous acid is NEN back Won ₂ lean washing liquid from the washing step 1 in the washing step 2 through a countercurrent washing with HNO.

With regard to the mass transfer processes of HNO ₂ , the washing stage 1 works as a stripper for the volatile nitrous acid, while the washing stage 2 works as an absorber for HNO ₂ from the gas stream.

In the following the invention with reference to a drawing described.

According to Fig. 1, the gas stream to be treated via the line 10 of the washing stage 1 is supplied. At this stage, the nitrous acid is stripped from the dilute nitric acid into the gas stream to be treated.

The gas stream can be from an industrial process z. B. for  Production of nitric acid or an artificial fertilizer, in which also nitrous gases are released. The procedure ren is suitable in the illustrated form for the Treatment of flue gases.

The washing stage 1 is shown as a countercurrent scrubber with a housing 3 , a packing 4 , a liquid distributor. 5 The washing liquid to be treated as a carrier for HNO₃ and HNO ₂ is fed via line 11 in countercurrent at the point 6 in the washing stage 1 .

In the treatment of flue gases, the temperature in line 10 is generally higher than the dew point temperature of the gas stream by a preheat, not shown. Under the countercurrent conditions in washing stage 1 , the partial Ver evaporation of HNO ₃ and H ₂ O from the washing liquid and the desired evaporation or stripping the HNO ₂ from the liquid in the vapor phase of the gas stream.

At the outlet of the gas stream from the packing 4 , the components of the washing liquid namely HNO ₂ , HNO ₃ , and H ₂ O are in equilibrium with their vaporous components in the treated gas stream with respect to the pressure. Here, therefore, the physical conditions are fulfilled in the gas stream, that the inventive formation of NO ₂ according to equation (3) can proceed spontaneously.

If the gas stream in the line 10 is not preheated the washing stage 1 , it is possible alternatively to supply the necessary amount of heat through the heat exchanger 9 in the liquid line 11 of the washing stage 1 .

The spent washing liquid is withdrawn via line 13 from the washing stage 1 . The associated fluid circuit is described below.

The gas stream to be treated from the scrubbing stage 1 in the line 12 thus has the composition that according to equation (3) to next N0 ₂ is formed and then via the equations (2) and (3) the oxidation of NO to NO ₂ can proceed.

The oxidation depends, inter alia, on the residence time and proceeds more quickly in the presence of moisture. A simple gas flow in the line 12 is sufficient for the course of the reaction. In Fig. 1, however, an alternative to Darge provides by the gas stream is passed through a moist exchange body in a further washing stage 14 , to improve the reaction conditions.

To this washing stage 14 , a liquid circuit with the pipe 17 is formed, in which a liquid tank 15 and a pump 16 are used.

After the residence time section 12 or the described washing stage 14 , the nitrous gases are oxidized in the gas stream. For the further treatment of the gas flow, a ratio of NO 2 to NO of 1 to 1 is expedient.

At a content of NO and NO ₂ of 400 ppm in the gas stream, a proportion of about 60 ppm of vaporous HNO ₂ in the gas stream has been set to equilibrium at 320 K by normal operating temperatures. A portion of this vaporous HNO ₂ is recovered by absorption in the washing stage 2 from the gas stream in countercurrent and fed again according to the idea of the invention the gas stream in the washing stage 1 . In the example in FIG. 1, the feed takes place directly without intermediate buffering via the line 11 into the washing stage 1 .

The washing stage 2 is comparable in construction and in shape to the previously described steps 1 and 14 . It is therefore possible to accommodate the 3 stages 1 , 14 and 2 in a common container and run the stage 14 as a pure residence time with the possibility of a Flüssigkeitsseindüsung.

The washing liquid from the washing stage 1 in the line 13 passes through a buffer 18 and the feed pump 19 in the washing stage. 2 In the conduit 13 , a further heat exchanger 20 is installed. This heat exchanger 20 is operated as a cooler ben and is not required for the operation of the method.

By regulating the cooling capacity in the heat exchanger results in a simple way to regulate the ENT ₃ -ge stop in the washing liquid according to claim 9. If the Waschflüs fluid is further cooled, the gas flow in the washing stage 2 is further cooled below the dew point and it condenses more water vapor, so that the acid concentration in the washing liquid decreases.

The liquid analysis takes place via the sensor 21 , is processed via the regulator / transmitter 22 and given as a signal to the control valve 22 in the coolant line.

The cycle of the washing liquid closes according to FIG. 1 via the line 11 into the washing stage 1 . In the liquid circuit, a supply 24 for the dilute nitric acid and possibly fresh water and a line 25 is provided for the withdrawal of a slurry.

For the safe operation of the washing stage 2 as an absorber, the task of an HNO _2- free washing liquid is low. Therefore, the washing stage 1 is just applied with a quantity of liquid so large that a possible HNO ₂ -free washing liquid is generated by the stripping.

The regulation of the amount of liquid in the line 13 and 11 can therefore also via a liquid analysis z. B. by means of electrical conductivity at the point 21 . The output of the regulator / transmitter 22 then goes to the motor of the pump 19 or to a throttle valve (not shown) in the line 13 .

By the above invention, it is possible in the Oxida tion of NO ₂ -my nitrous gases shorten the very slow way on the equations (2) and (3) by first desorbing into the gas stream HNO ₃ and HNO ₂ .

According to a further aspect of the invention, it is possible to recover the nitrous acid from the nitrous gases and thus to avoid an additional need for oxidizing agents such as H ₂ O ₂ in a later absorption.

The embodiment of the method described with reference to FIG. 1 thus comprises

• with the washing stage 1 a desorption for HNO ₃ , HNO ₂ from the washing liquid,
• a middle washing stage 14 for producing HNO ₂ and NO ₂ and
• - With the washing stage 2, an absorption for HNO ₃ , HNO ₂ from the gas in the washing liquid.

With respect to nitrous acid, HNO ₂ as the important reaction partner according to equation (3) in step 2 with a HNO ₂ low-scrubbing liquid according to the invention produced NO ₂ , and with the existing NO according to equation (2) and the generated HNO ₂ recovered from the treated gas. In the liquid then the HNO _{2 is} buffered. In the washing stage 1 , the HNO _{2 is} desorbed in the untreated and HNO ₂ poor gas.

Finally, in the washing stage 14 , the gas to be treated with the washing liquid is in constant mass transfer. According to Eq. (3) arises in the gas phase NO ₂ and H ₂ O from HNO ₂ and HNO ₃ . ENT ₂ is modeled according to equation (2) by the resulting NO ₂ continuously. HNO ₃ as the reactant comes as vapor with the incoming gas in the stage 14 and can be replaced by post-evaporation from the washing liquid of the liquid circulation stage 14 .

To enhance the re-evaporation of HNO ₃ , the container 15 as the container 18 can be equipped with a similar acid and acid discharge line. The discharged from the circulation acid can be disposed of in the other circuit on the loading container 18 .

Fig. 1 is thus a plant for the invention imple out of the method.

The proposed solution according to Fig. 1 for carrying out the method has the disadvantage that it is expensive in terms of apparatus. In practice, a simpler solution is more pleasant.

It is the further object of the invention to carry out the method of FIG. 1 in a single apparatus / reactor.

This object is achieved in that

• 1. The amount of liquid is chosen to be so small that, upon exiting the liquid from the reactor of FIG. 2, the liquid is in equilibrium with the incoming gas stream with respect to the partial pressures of the components and the HNO ₂ content in the liquid approaches zero.
• 2. The amount of liquid that is fed into the reactor at the gas outlet of Fig. 2, just so large that the HNO ₂ content in the liquid is as high as possible and as much, the important for the process sequence component HNO ₂ , from the gas is recovered;
• 3. The liquid, if it is outside the reactor, if possible HNO ₂ -free and if it is within the reactor, serves as a buffer for HNO ₂ , to carry out the reactions according to the equi conditions (2) and (3);
• 4. as a reactor, a design with internals is selected, which the operation with small amounts of liquid and the Training of several separation stages to carry out the substance exchange operations, as here 1-3. described, guaranteed guaranteed.

In the method according to the invention, the surface of exchange bodies is formed, which are wetted with liquid. While the known washing system are operated in the flue gas cleaning with a liquid amount of 10-20 ltr / Nm ³ flue gas, so the required amount of liquid according to the new method is on the order of 1% of the usual liquid load z. B. at 0.1 ltr./Nm ³ .

For this case, the reactor of Fig. 2 must be carried out either with column bottoms or it is the installation of an orderly packing necessary.

The saturation temperature of the flue gases, especially from the United combustion of garbage can also higher z. B. above 60 ° C. In this case, care must be taken that the solubility of HNO ₂ in the water is lower and therefore the amount of liquid must be increased.

A reactor for 100,000 Nm ³ / h of exhaust gas has the following dimensions:

Diameter | 4.5 m packing height 4,0 m Pack (Sulzer) Type 350 Y - Mellapak

With reference to Fig. 2, the invention will be described again.

Fig. 2 shows a reactor with a cycle for Waschflüs fluid. The circuit is formed by the components, container 18 , pump 14 , pipes 13 and the reactor 30 . The heat exchanger 20 can be installed additionally. The Bauprin zip of the reactor 30 is already described in connection with the stage 1 in Fig. 1.

The fluid circuit has according to claim 14 different Liche concentrations of HNO ₂ . At the outlet 35 from the reactor 30 , the HNO ₂ concentration is as low as possible and remains constant until re-application of the liquid at the point 36 in the reactor. In the reactor, the liquid passes in countercurrent to the rising gas through the zones 32-34 . Zone 31 , which is included here as part of Zone 32 , and the supply of liquid at point 27 will be discussed below in connection with mercury removal.

In zone 32 the liquid absorbs by equilibrium with the gas according to equation (2) HNO ₂ . The total amount of uptake HNO ₂ increases as the amount of fluid increases.

In zone 33 , HNO ₂ is buffered in the liquid and oxidation of NO can be performed according to equations (2) and (3).

In zone 34 , the liquid HNO₂ by desorption into the incoming gas from again. There is the formation of NO ₂ according to the equation (3).

The raw gas enters the reactor 30 at 7 and is discharged after the oxidation via the line 26 from the reactor.

The individual zones in the reactor 30 can be formed from a homogeneous packing or from several identical trays. In Fig. 2, the zones are drawn separately only for optical reasons.

In the use of the method for the treatment of flue gases of waste incineration is, according to the investigations of Braun, Metzger and Vogg, Karlsruhe, especially in systems with vor switched wet wash still metallic mercury Hg as Hg (O) designated, available. The authors say that 20% of the originally contained in the garbage Hg in this form with the gas escape to the environment. Hg is considered a dangerous pollutant and is assigned to pollutant class I. In the future it is too Prevent this pollutant with the exhaust gases in the Atmosphere arrives.

It is known that metallic mercury Hg (O) can be washed out under the oxidizing action of HNO ₃ and by the addition of an oxidizing agent z. B. H ₂ O _2, the Auswa tion can be improved.

These conditions are given in the method of scrubbing NO _x with H ₂ O ₂ according to PCT / DE88 / 00 412 in the nitrous gas absorption stage. The deposition together with the absorption has the disadvantage that in this stage a valuable material namely HNO _{3 is} generated, which would be contaminated simultaneously with Hg.

It is therefore a further object of the invention to simultaneously wash out metallic mercury Hg (O) in conjunction with the oxidation of NO to NO ₂ and to strip off the heavy metals such as Hg together with the effluent according to the method of PCT / DE88 / 00412 the upstream flue gas cleaning can be returned.

This object is achieved in that in the washing liquid, which is free of HNO ₂ , H ₂ O _{2 is placed} at the point 27 via the line 28 and in the reactor 30 an additional zone 31 inform of package height is installed.

In this zone 31 takes place simultaneously from the nitrous gases, the formation of HNO ₃ in the liquid and the desired Oxida tion and washing of the metallic mercury Hg (O) from the gases - both using the oxidizing effect of H ₂ O ₂ .

In addition, this idea of the invention can also be represented with the aid of FIG. 2:

In the zone 31 of the reactor, the HNO ₂ -free liquid absorbs NO _x from the gases and forms from the reaction with H ₂ O ₂ nitric acid HNO ₃ . At the same time, the oxidizing effect enhances the oxidation and precipitation of metallic mercury Hg (O) and other heavy metals into the wash liquor.

After passage of the liquid through the zone 31 , the H ₂ O ₂ is consumed in the liquid and the oxidation process for nitrous gases according to claim 14 can proceed as described. With respect to H ₂ O ₂ there is in the liquid circuit is also a region which contains H ₂ O _2, and that the feed point of A 27 to the end of the theoretical zone 31st The dosage of H ₂ O ₂ in the liquid takes place continuously. To Vergeichmäßigung the H ₂ O ₂ concentration in the liquid, the simultaneous task of the necessary for the operation of Kreislau fes water via line 28 is useful.

In this case, no HNO ₃ must be supplied, since the consumed for the oxidation HNO ₃ amount is replicated via the oxidation with the H ₂ O ₂ .

The present invention thus enables the oxidation of NO _x and the simultaneous deposition of metallic mercury silver Hg (O). Both tasks can be performed in an apparatus, wherein the HNO ₂ - concentration in the liquid circulation is locally constant, but within the reactor 30 but different Lich, outside the reactor 30 is uniformly deep.

Claims (18)

1. A process for the oxidation of nitrous gases (NO) in a gas stream, characterized by a scrubbing of the gas stream with nitric acid HNO _3, characterized in that

• - a dilute nitric acid is used which contains proportions of nitrous acid HNO ₂ ,
• during washing, due to the higher partial pressures in the liquid, nitrous acid HNO ₂ and nitric acid HNO _{3 are} desorbed from the washing liquid into the gas stream,
• - And the desorbed from the washing liquid, nitrous acid HNO _{2 is added} in the washing liquid.

2. The method according to claim 1, characterized in that the oxidation of NO by the formation of NO ₂ according to the following equation HNO₂ + NHO₃ → 2 NO₂ + H₂Oeingeleitet and then the oxidation of the nitrous gases according to the following 2 equations proceeds: NO + NO₂ + H₂O → 2 HNO₂HNO₂ + 2 NHO₃ → 4 NO₂ + 2 H₂O 3. The method according to claims 1-2, characterized in that the washing is performed in 2 steps, a washing stage 1 for the evaporation of HNO ₂ and HNO ₃ from the washing liquid in the gas flow and a downstream washing step 2 in which the HNO ₂ - Poor washing liquid from the washing stage 1, a portion of the resulting in the gas stream nitrous gases after the sum equation NO + NO₂ + H₂O → 2 HNO₂ausweaschen, characterized in the washing liquid again HNO _{2 is} formed and rich in nitrous acid wash liquid again in the washing stage 1 is abandoned. 4. The method according to claim 1-3, characterized in that between the washing stage 1 and 2, a residence time for the execution of the oxidation reactions according to claim 2 is installed. 5. The method according to claim 1-4, characterized in that to increase the reaction surface in the residence time dilute nitric acid. 6. The method according to claim 1-5, characterized in that be installed in the residence time exchange body. 7. The method according to claim 1-6, characterized in that the heat for the desorption of nitrous acid and salve tersäure and for evaporation of the water content from the washing liquid in a heat exchanger of the washing liquid is supplied before task in the washing stage 1 . 8. The method according to claim 7, characterized in that the evaporated amount of liquid through the heat in the Heat exchanger is regulated. 9. The method according to claim 1-8, characterized in that the washing liquid is cooled before the task in the washing stage 2 in a heat exchanger and the acid concentration in the washing liquid cycle is controlled by the cooling capacity in the heat exchanger. 10. The method according to claim 1-9, characterized in that the washing stages 1 and 2 are operated in countercurrent and the discontinued washing liquid amount is less than 1 l / Nm ³ gas. 11. The method according to claim 1-10, characterized in that the nitrous acid HNO _{2 is} separated by Desoprtion in the washing stage 1 from the washing liquid and this HNO ₂ -free washing liquid characterized as the ideal liquid for Absorp tion of HNO ₂ from the gas phase is used according to claim 3 in the washing stage 2 . 12. The method according to claim 1-2, characterized in that the laundry is carried out in a washing stage and the from the Washing liquid desorbed nitrous acid by means of a Reduction stage in the cycle of the washing liquid from the Nitric acid is added. 13. The method according to claim 12, characterized in that oxidized in the reduction stage, the nitric acid, an inorganic substance such as metals or an organic substance and thereby nitrous acid HNO _{2 is formed} in the washing liquid speed. 14. The method according to claim 1-7, characterized in that the reactions are carried out according to claims 1-6 in a reactor over which a liquid circuit is maintained in countercurrent, wherein the amount of liquid is adjusted so that the liquid outside the reactor ENT ₂ - is poor and the liquid in the reactor is rich in HNO ₂ . 15. The method according to claim 14, characterized in that 3 exchange zones are installed to carry out the mass transfer with respect to HNO ₂ between liquid and gas stream and vice versa in the reactor by the installation of packages or trays and that
the zone 32 at the reactor head for HNO ₂ absorption from the gas,
the zone 33 in the middle for the oxidation of NO _x with simultaneous buffering of HNO ₂ in the washing liquid
the zone 34 at the reactor foot to HNO ₂ desorption from the liquid into the gas. 16. The method according to claim 15, characterized in that the reactor head, a further zone 31 is installed and by the additional task of H ₂ O ₂ with the washing liquid in this zone heavy metals and especially metallic mercury Hg (O) are washed out. 17. The method according to claim 16, characterized in that in the washing liquid just as much H ₂ O _{2 is} abandoned, that also a part of the nitrous gases washed out and oxidized to HNO ₃ , wherein the H ₂ O ₂ content in the liquid at the outlet is consumed from the zone 31 . 18. Apparatus for carrying out the method according to claim 14-17, characterized in that

• - In a reactor, which is designed as a washing capacitor, in the form of a tray column or an ordered package as Aus exchange body at least three zones are provided with a plurality of separation stages,
• - A liquid circuit is installed in countercurrent to the gas stream to be treated above this reactor, wherein the liquid load is set about 99% below the usual load of 10-20 ltr / Nm ³ in the Rauchgasreini,
• - By the course of the reaction in the reactor, the liquid in the circulation, outside the reactor is largely free of HNO ₂ , within the reactor in the liquid task initially HNO ₂ receives, when flowing through the reactor HNO ₂ buffers and releases in the lower zone at the outlet HNO ₂ again .
• and in the reactor by the formation of vaporous HNO ₂ over a dilute nitric acid, the oxidation of nitrous gases can be accelerated.