DK141493B - Process for removing hydrogen chloride from flue gas. - Google Patents

Description

(11) PUBLICATION 141493 DENMARK (61) lnt.C, .3 B 01 O 53/34 § (21) Application No 4850/77 (22) Filed on 31. October 1977 (23) Race day 31- Oct. 1977 (44) The application presented and the petition published on 31 · m & T · 1 980 DIRECTORATE FOR #eA. __ PATENT AND TRADEMARK BASIS (3 °) Priority requested from (71) A / s NIRO ATOMIZER, Gladsaxevej 305, 2860 Søborg, DK.

R2) Inventor: Karsten Fele vang, Sortemosen 14, 3450 Allerød, DK: Martin, Andreae Grønlund Scholten, Vejlesøvej 13, 2840 Holte, DK.

(74) Plenipotentiary in the proceedings:

International Patent Office.

(64) Methods for removing hydrogen chloride from flue gas. *

The invention relates to a process for removing hydrogen chloride from flue gas resulting from the incineration of waste by absorption of the hydrogen chloride by an alkaline agent.

When incinerated waste is incinerated, a flue gas with a hydrogen chloride content is produced, which is mainly due to the waste content of chlorine containing substances, in particular polyvinyl chloride. The HCl content of the flue gas will be of the order of 1¾¾ (calculated by weight) in the incineration of typical waste disposal waste, but this amount is increased proportionally with the increased use of chlorine-containing plastics for packaging. In case the waste that is incinerated does not consist solely of household waste, but also waste from companies that process chlorine-containing plastics, the HC1 content of the flue gas will of course be much greater.

It is a widespread desire to remove hydrogen chloride from the combustion plant's flue gas, but despite the well-known high reactivity of hydrogen chloride, no method has been developed in the past, with which it is possible to achieve as far HCl removal as desired. , without using any measures that cause significant disadvantages.

Thus, of course, it is possible to achieve effective removal of the hydrogen chloride by washing the flue gas with an aqueous alkaline liquid, e.g. in a washing tower, obtaining a chloride-containing sludge which can be returned to the washing process.

However, this method can only be used in conjunction with lowering the flue gas temperature to such a low value that it cannot immediately be passed to electrophilters and upon release at the chimney opening will give rise to an undesirable visible tab.

Therefore, so far only such washing has been applied to a partial flow of the flue gas and combined with hot non-washed gas prior to application to the electro filters. However, this solution is not satisfactory as it does not allow a sufficient reduction of the total amount of hydrogen chloride in the combustion flue gas.

If the total amount of flue gas is treated by the described washing process, it is necessary to reheat the washed flue gas before it is passed to the electric filter and chimney, which reheating results in a considerable energy consumption.

Methods are also known for reducing the hydrogen chloride content of flue gases of the kind in question, whereby the flue gas is contacted with solid alkaline materials such as calcium or magnesium oxide. These processes do not require such far-reaching cooling of the flue gases that re-heating is necessary, but they do have other drawbacks.

Since the reaction takes place in the solid phase, it is difficult at the same time to obtain a sufficiently small concentration of HCl in the outgoing gas and a satisfactory conversion rate of the solid absorption material.

In order to achieve an effective contact between the solid absorption material and the gas stream, the former is fluidized therein and a considerable thickness of the fluid absorption material layer is required, which results in a large pressure drop over the treatment zone, which increases the energy consumption of the process. Such methods using solid absorbent are e.g. described 3 141493 in the descriptions of French usage patterns Nos. 2,189,101 and 2,205,354.

According to the disclosure of U.S. Patent No. 3,966,418, which discloses a process for removing sulfur dioxide from flue gas resulting from the combustion of sulfur-containing fuel, it is also possible to remove hydrogen chloride by spraying in the flue gas an aqueous sodium carbonate solution under such conditions, the bulk of the water content of the solution evaporates to form a moist powder which is withdrawn by means of a auger located at the bottom of the treatment zone. Such a method is not particularly suitable in the case where primarily hydrogen chloride is desired to be removed since the moist powder is not convenient to handle and the exhaust gas will be so close to the dew point that it cannot be passed directly to the electrofilter.

It has now been found that it is possible to reduce the HCl content to very small values (less than 50 ppm, by volume) by a method which is free of the disadvantages associated with the aforementioned known practices that require less investment costs and less labor than the known methods.

According to the invention, this method is characterized by introducing the flue gas at a temperature between 200 and 330 ° C in a chamber in which an aqueous solution of alkali metal carbonate or an aqueous suspension of calcium hydroxide or carbonate or magnesium oxide is atomized. wherein the amount of solution or suspension is adjusted such that the temperature at which the flue gas leaves the drying chamber is greater than 125 ° C and the material formed by the atomized solution or suspension drying leaves the chamber as a dry free flowing powder.

The particular advantages of the present process are that the flue gas is not cooled and humidified more than is permissible by the use of plate-type electro filters and that the HCl content can be reduced to very small values while achieving almost complete utilization. of the absorbent, the powder obtained being capable of being converted to chloride for more than 90%. Thus, no recycling of the absorbent is required. The disposal of the powder produced as a waste product, consisting mainly of alkali metal or calcium or magnesium chloride as well as fly ash, is greatly facilitated by the fact that this waste material appears as a dry free flowing powder.

An advantage is also that the method results in only a slight loss of pressure.

4 141A 9 3

It is noted that in connection with the removal of sulfur dioxide from flue gas resulting from the combustion of sulfur-containing fuel, it is known to carry out an absorption process in a manner similar to that used in the invention, in which the absorbent is also atomized in the flue gas by this known process. and leaving the treatment zone as a dry powder, see U.S. Patent No. 3,932,587.

Furthermore, in German Patent Specification No. 21 59 186 (corresponding to British Patent Specification No. 13 33 635), it is also proposed to remove other harmful gases by processes of this kind which result in a dry powder. In addition to SO 2, this patent mentions examples of harmful gases which can be removed NO, HgS, HF and F.

However, the method of the invention differs from the methods known from U.S. Patent Nos. 39,328,587 and German Patent Nos. 21,586, not only in that the component being removed is another, but in particular that the method is practiced on flue gas having a temperature which is substantially greater and is substantially farther from the dew point of the gas than is the case by the aforementioned methods.

This results in the fact that the treated gas can be fed directly to plate type electrofilters, thus eliminating an energy-intensive reheating. By the above-mentioned known methods, the temperature of the purified gas, on the other hand, is so close to its dew point that it is only suitable for purification in electro filters if a reheating is done beforehand.

In the SC 2 absorption process known from the latter US Patent, a regeneration of the absorbent material is performed, which regeneration makes it desirable that the powder coming from the treatment zone continue to contain a significant amount of unreacted sodium carbonate. In the process of the invention, on the other hand, it is essential to obtain as far as possible the reaction of the absorption material.

Surprisingly, it is possible in the process according to the invention to obtain an absorption of a very large part of the hydrogen chloride present in the smoke, while at the same time obtaining an effective utilization of the basic materials, as they are almost completely converted into chlorides , as one would expect in advance that due to the high temperature of the flue gas during treatment, very unfavorable conditions for the reaction between hydrogen chloride and the basic materials would prevail. In the reaction considered in the process, the presence of water is crucial for the rate of reaction, since the reaction in the absence of water proceeds slowly, i. because the chlorides that form on the surface of the particles inhibit the contact of the hydrogen chloride with the basic materials in the interior of the particles. The process according to the invention works at such a high temperature and so far from the dew point that the water evaporates very quickly from the atomized particles, so that for the individual particles only reaction is possible in the presence of water for a very short time. In addition, in this short time, there will be less favorable reaction conditions than in the known processes, since the very high temperature of the drying gas causes a strong flow of steam from the individual particles, which is expected to be strongly inhibitory for contact between HCl and the particles while still moist. Thus, the higher temperature not only significantly shortens the time available for the reaction under humid conditions, but also causes such conditions to exist during this very short time that the possibility of contact between the HCl present in the gas and the moist particles would be expected to be substantially degraded relative to the conditions prevailing by the known methods mentioned.

Therefore, it has not been obvious to one skilled in the art to seek to effect the KCl absorption under the conditions characteristic of the process of the invention, although, as mentioned, significant advantages are thereby obtained.

The method can be practiced using a conventional spray drying system, and of these can be used both the type where the gas is introduced at the top and the types where the gas is introduced centrally in the drying chamber or the lower part thereof.

The concentration of the absorbent solution or suspension is adapted in each case to the dimensions and operating conditions of the plant, including the rotational speed of the atomizer, with a smaller concentration allowing finer particles and therefore a more extensive reaction with the hydrogen chloride, while very small concentrations, on the other hand, have the disadvantage. In order to obtain sufficient HCl absorption, they require the evaporation of a large volume of water with corresponding lowering of the temperature and the increase in humidity of the flue gas.

The concentration of solution or suspension will usually be substantially lower than that used in the known SO 2 absorption processes, in which a regeneration of the absorption material is carried out.

In the process according to the invention, neither regeneration nor recirculation of the absorbent is used, and thus to achieve a nearly complete reaction thereof, concentrations of the solution or suspension of only a few S. will often be employed.

The invention is further illustrated below with reference to the drawing / which schematically shows a plant / which can be used for carrying out the method according to the invention.

In the drawing, 1 denotes an atomizer dryer in which, by means of a rotary atomizer wheel 2, atomizes a solution or suspension of absorbent supplied from a reservoir 3. Of course, a hydrogen chloride absorption can also be obtained by using the absorption materials mentioned in the claim. alkali metal hydroxides, but due to their higher cost and the greater risk associated with their handling / such are not considered.

Through a pipeline 4, hot flue gas resulting from waste incineration is passed to the atomizer 1, where the gas comes into intimate contact with the atomized particles of solution or suspension of absorbent. Thereby, almost the total amount of the water present in the particles evaporates, and a chemical reaction takes place, whereby a very substantial part of the hydrogen chloride present in the flue gas is consumed, and particles consisting mainly of alkali metal, calcium or magnesium chloride.

A portion of these mainly chloride and fly ash particles will accumulate in the bottom of the nebuliser, where they can be removed through a sluice 5. The remaining portion, which will often be the predominant one, is entrained by the flue gas leaving the nebulizer through a pipeline 6. which leads to a plate type electro-filter 7. At this point, the flue gas will remain so far from its dew point that its introduction into the electro-filter will not cause problems.

In this, the remaining amount of the mainly chloride particles is separated off with fly ash and the resulting free flowing powder is withdrawn through the powder outlets 8 into the bottom of the electro filter.

From the electro filter 7, the purified flue gas is usually passed through a (not shown) blower to a chimney 9, and at this point it has continued to be sufficiently high to be able to disperse into the atmosphere without any visible tab.

The method according to the invention is further illustrated by the following exemplary embodiments.

Example 1

An experimental cleaning system such as the one outlined in the drawing was used with the exception that the flue gas to be purified was fed to the nebulizer through a ceiling air distributor, and a cyclone was inserted immediately after the nebulizer to collect part of the powder and fly ash. The spray dryer had a diameter of '1.2 m and a cylindrical height of' Q, 8 m.

The flue gas treated came from an industrial waste incinerator and had a HCl content of 850 ppm (calculated by volume). The temperature of introduction into the nebulizer was 322 ° C and the gas was supplied at an amount of 520 kg / h.

A 3.9 wt% aqueous sodium carbonate solution was added to the atomizer wheel.

The purified flue gas at the cyclone had a temperature of 142 ° C and the HCl concentration was measured to 60 ppm, corresponding to 93% purification. The stoichiometric ratios of the per. amounts of sodium carbonate and hydrogen chloride added in unit time were calculated