DE3735391A1 - Process for removing NOx and, where required, SO2 from exhaust gases - Google Patents

Abstract

In the known process, in order to remove NOx and, where required, SO2, ozone is fed in in at least stoichiometric amount, based on NOx, and the exhaust gas is then brought into contact with an aqueous recirculating solution to bind the oxidized NOx and, where required, the SO2. It is questionable whether, in the known process, the removal of oxidized N2O5 is achieved, since this is stable only up to a temperature of 47@C. The object of the novel process is to provide for removal of NOx even at temperatures above 47@C and, at the same time, to result in a higher degree of separation. The addition of the ozone is carried out at a temperature above 47@C, and the circulating solution used is a weakly acidic to alkaline solution containing at least one alkaline earth and/or alkali and an oxidant, the oxidant content resulting in superstoichiometry, based on the NOx in the exhaust gas.

Description

The invention relates to a method for removing NO _x and possibly SO₂ from exhaust gases, in which, for the oxidation of NO _x, an ozone based on NO _x in at least a stoichiometric amount of gas is supplied to the exhaust gas at temperatures between ambient temperature and a temperature which is higher than this. and then the exhaust gas is contacted with at least one aqueous circulation solution for incorporating the oxidized NO _x and optionally the SO₂.

Such a method is known from DE-PS 24 59 913, in which the gas containing ozone is supplied to the exhaust gas at temperatures between ambient temperature and 100 ° C. for setting the value "x" of NO _x between 1.2 and 2.5 , and the exhaust gas treated in this way is contacted in a first zone with sulfuric acid, nitric acid and an iron compound as catalyst containing a strongly acidic solution, the sulfuric acid concentration being between 0.5 and 10% by weight. Air or oxygen is added to the solution thus obtained in a second zone, and the solution is then returned as a circulating solution. It is preferably provided that part of the circulating solution is drawn off and reacted with lime in order to convert the sulfuric acid contained in the aqueous solution into gypsum, whereupon the mother liquor obtained, which remains after the gypsum formed has been separated off, can be neutralized with sodium hydroxide to convert the contained nitric acid into sodium nitrate.

According to column 3, lines 37-45, when the ozone-containing gas is added to the exhaust gas, the NO _{x is} subject to oxidation as follows from the following reaction equations:

NO + O₃ → NO₂ + O₂ (1)

2 NO₂ + O₃ → N₂O₅ + O₂. (2)

It is therefore suggested that the ozone dosage for the To carry out oxidation in the gas phase so that the Integration in the aqueous circulation solution according to the following equation can continue:

N₂O₅ + H₂O → 2 HNO₃. (3)

It is questionable whether at the specified temperature range from approx. 30-100 ° C this procedure is possible at all is because according to the book "Handbook of Chemistry and Physics" (1981), CRC Press Inc., Boca Raton, Florida, USA, N₂O₅ only is stable up to a temperature of 47 ° C.

It is therefore the object of the present invention to provide a method of the type mentioned above in which the removal of NO _x also takes place at temperatures above 47 ° C. and at the same time with a higher degree of separation.

This object is achieved in that the addition of the ozone to the exhaust gas takes place at a temperature above 47 ° C., and in that a solution which contains at least one alkaline earth and / or alkali and is slightly acidic to alkaline and contains an oxidizing agent is used as the circulating solution, the content of oxidizing agent leads to an over-stoichiometry based on the NO _x in the exhaust gas.

It therefore occurs in the process management according to the invention no longer for the formation of N₂O₅ in the gas phase reaction (1).

The implementation of NO₂ in the aqueous phase then takes place according to the following equation:

2 NO₂ + H₂O → HNO₃ + HNO₂. (4)

The fact that the circulating solution based on NO _x in the exhaust gas has an over-stoichiometric content of an oxidizing agent, which is preferably done by adding a gas to the exhaust gas that contains ozone in an appropriate amount, is after the almost stoichiometric reaction in the gas phase (1 ) an excess of O₃ is present, which is dissolved in the liquid phase, so that the nitrite can be broken down there according to the following equation:

HNO₂ + O₃ → HNO₃ + O₂. (5)

This means a shift in the Reaction equilibrium in (4) to the right and yields thus the decisive increase in the deposition rate.

But it may also be possible to generate the hyperstoichiometric content of oxidizing agent Circulating solution is an oxidizing agent in gas or liquid Form is added. Is z. B. H₂O₂ added so runs The reaction to decompose the nitrite is as follows:

HNO₂ + H₂O₂ → HNO₃ + H₂O. (6)

Other oxidizing agents can e.g. B. be O₃, NaClO, NaClO₂.  

The nitric acid is in the circulating solution contained alkaline earth and / or alkali parts z. B. according dismantled the following equations:

2 HNO₃ + CaCO₃ → Ca (NO₃) ₂ + H₂O + CO₂ (7)

when using CaCO₃ and

HNO₃ + NaOH → NaNO₃ + H₂O (8)

with the addition of NaOH.

If, in addition to NO _x , the exhaust gases also contain SO₂, the simultaneous separation of SO₂ and NO _x in one washing stage can possibly use up the excess content of oxidizing agent in the circulating solution for the following competitive reactions:

SO₂ + H₂O → H₂SO₃ (9)

and

H₂SO₃ + O₃ → H₂SO₄ + O₂ (10)

in the case of excess ozone.

Therefore, it can be advantageous that the SO₂ is first removed in an upstream wash before the addition of the ozone-containing gas, in order to possibly achieve even better ozone utilization. In many cases, despite the possibly competing reactions (9) and (10), simultaneous separation of SO₂ and NO _{x will be} sufficient.

The gas containing ozone is preferably added to the exhaust gas a temperature of 60-250 ° C, further preferred 90-200 ° C and more preferably 110-200 ° C fed.

Furthermore, it is expedient to use the circulating solution  Adjust pH in the range of 4 to 8.

The invention will now be described with reference to the accompanying figures are explained in more detail. It shows

Fig. 1 shows the NO _x -Abscheidegrad as a function of ozone dose sole, in the exhaust gas with the use of NaOH as absorbents

Fig. 2 shows the NO _x -Abscheidegrad as a function of ozone dose solely in the exhaust gas with the use of CaCO₃ as an absorbent,

Fig. 3 SO₂- and NO _x removal efficiencies as a function of the pH of the circulating solution,

Fig. 4 is a block diagram for simultaneous SO₂ and NO _x removal and

Fig. 5 is a schematic circuit diagram for a desulfurization before the NO _x removal.

With the aid of the attached FIG. 1, the degree of NO _x separation is shown as a function of the ozone metering in NaOH as an alkaline component of the circulating solution. The ozone is dosed at a temperature of 170 ° C of the exhaust gas. The pH value is in the range from 4 to 8 and the L / G value in the range from 3 to 12. Here L is the amount of circulated water in l / h and G is the gas volume flow in m³ / h in the operating state. The L / G value is usually referred to as the specific amount of wash suspension. From Fig. 1 it can be clearly read that the degree of NO _x separation increases surprisingly with overstoichiometric addition of ozone, the stoichiometric factor α _{O x being} defined as

and the degree of separation or washout is defined as

Fig. 2 shows the conditions when using CaCO₃ as an absorbent. Here, too, a surprising increase in the degree of NO _x removal can be found in the region of α _O3 in the superstoichiometric range. Although the NO _x separation using CaCO₃ shows a somewhat flatter course than when using NaOH, can be achieved with double stoichiometric ozone metering ( α _O3 = 2) in both cases NO _x separation levels of over 90% without other chemicals of the circulating washing solution would have to be added to the oxidizing agent.

From Figs. 1 and 2, is further read that it is advantageous if, based on the NO _x exhaust ozone and the circulating solution is supplied to a further oxidizing agent such in the exhaust gas, that a superstoichiometry of at most 2.5, preferably 1, 5-2.5 is reached.

Fig. 3 shows in the case of a simultaneous separation of NO _x and SO₂ the NO _x - and SO₂ separation degree as a function of the pH value using the example of NaOH as an alkaline component of the circulating solution and with a stoichiometric factor α _O3 from 1.43 to 1 , 45.

From Fig. 3 it can be seen that with a lower SO₂ separation level, the NO _x separation is better at otherwise constant conditions, ie a smaller excess of oxidizing agent is then necessary for the same NO _x separation level. Fig. 3 also shows that it makes sense to work in the pH range from 4 to 8.

In Fig. 4, an arrangement for the simultaneous deposition of NO _x and SO₂ is shown schematically. The flue gas is fed to a scrubber ( 2 ) via a channel ( 1 ) after an ozone-air mixture generated in an ozone generator ( 3 ) has been fed to it. In the scrubber ( 2 ), a washing solution is sprayed in counterflow to the flue gas, which is led in a circulation ( 4 ) with a circulation pump ( 5 ). An alkaline earth and / or alkali-containing aqueous solution is fed to the circulation ( 4 ) at ( 6 ), while nitrates and sulfates are drawn off at ( 7 ). At ( 8 ) it is indicated that with stoichiometric or post-stoichiometric addition of ozone in the raw gas channel ( 1 ) to adjust the overstoichiometry in the circulating solution ( 4 ) an additional oxidizing agent can be supplied.

Fig. 5 shows a two-stage process in which in a first scrubber ( 9 ) an alkaline earth and / or alkali-containing washing solution is run in a circuit ( 10 ) to wash out the SO₂ from the flue gas supplied via the channel ( 11 ). At ( 12 ) the wash solution is added and at ( 13 ) sulfates are withdrawn from the wash cycle if oxidation occurs in the scrubber, or mixtures of sulfites and sulfates.

The flue gas leaves the scrubber ( 9 ) at a temperature above the temperature of 47 ° C, e.g. B. at a temperature of 67 ° C.

In the description and in FIGS. 1 to 3, the brackets [] denote concentrations.

Claims (8)

1. Process for the removal of NO _x and possibly SO₂ from exhaust gases, in which for the oxidation of NO _x an ozone based on NO _x in at least stoichiometric amount containing gas at temperatures between ambient temperature and an elevated temperature compared to the exhaust gas is supplied, and then the exhaust gas is contacted with at least one aqueous circulation solution for the incorporation of the oxidized NO _x and optionally the SO₂, characterized in that the ozone is added to the exhaust gas at a temperature above 47 ° C, and that at least one alkaline earth metal and / or Alkali-containing weakly acidic to alkaline solution containing an oxidizing agent is used, the content of oxidizing agent leading to an over-stoichiometry based on the NO _x in the exhaust gas. 2. The method according to claim 1, characterized characterized in that to generate the hyperstoichiometric content of the oxidizing agent Exhaust gas is added, the ozone in appropriate amount contains. 3. The method according to claim 1, characterized characterized in that to generate the hyperstoichiometric content of oxidizing agent  Circulating solution an oxidant in gas or liquid form is added. 4. The method according to any one of claims 1 to 3, characterized in that before Addition of the gas containing ozone first the SO₂ in an upstream laundry is removed. 5. The method according to any one of claims 1 to 3, characterized in that SO₂ and NO _x are removed together in one wash. 6. The method according to any one of claims 1 to 3 and 5, characterized in that the Gas containing ozone the exhaust gas at a temperature of 60-250 ° C, further preferably 90-200 ° C and even further preferably 110-200 ° C is supplied. 7. The method according to any one of claims 1 to 5, characterized in that the pH is set in the range of 4 to 8. 8. The method according to any one of claims 1 to 6, characterized in that based on the NO _x in the exhaust gas, the exhaust gas ozone and the circulating solution, a further oxidizing agent is fed such that an overstoichiometry of at most 2.5, preferably 1.5-2 , 5 is reached.