DE1260667B - Process for the prevention of acid dew point corrosion and surface contamination in the low temperature area of incineration plants - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY GERMAN #re PATENT OFFICE

EDITORIAL

Int. Cl .:

F23j

German class: 24 g-6/80

Number:
File number:
Registration date:
Display day:

1260 667
D40866IVc / 24g
February 13, 1963
February 8, 1968

It is known that liquid and solid fuels contain sulfur, which in the combustion gases is partially oxidized to form sulfur trioxide. Depending on the fuel, the quality of the combustion and the type of boiler, the combustion gases contain from 0.0010 to 0.010 volume percent SO ₃ . The SO ₃ is created by catalytic SO ₂ oxidation in the temperature range: combustion chamber temperature down to about 400 ° C. Below 400 ° C, no SO ₂ molecules are oxidized _{to SO 3.} The implementation depends on the dwell

₃ y

time of the combustion gases in this temperature range, the oxygen content and the activity of the dusty catalysts (V ₂ O ₅ , Fe ₂ O ₃ ). This sulfur trioxide forms sulfuric acid vapor at around 250 ° C and less with the water vapor that is always present in flue gases, from which, depending on the content of the combustion gases, the sulfuric acid is deposited on the accessible surfaces of the system at temperatures of 180 ° C and below. In this way, strong signs of corrosion can occur, which under certain circumstances also make themselves noticeable in the chimneys of such systems. In addition, the sulfuric acid that gets into the outside air creates severe air pollution in unfavorable weather conditions, which can be the cause of health disorders and disturbances to the growth of plants in the area.

Attempts have already been made to reduce the sulfur trioxide in the exhaust gases with high-melting metal oxides and / or carbonates, such as. B. magnesium oxide or dolomite, which is a double salt of MgCO ₃ and CaCO ₃ , to bind and to eliminate the occurrence of an acid dew point at all. However, so far only partially satisfactory results have been achieved in this direction, since it has only been possible either to shift the acid dew point to somewhat lower temperatures or to remove the sulfur trioxide from the exhaust gas, but with the precipitation of its reaction products with the additive in Form of firmly adhering layers in the plants could not be prevented. A removal of the more or less severe soiling has not yet been achieved to any significant extent under these conditions. Rather, the deposits of the substances added to lower the dew point, some of which are converted into sulphates, form firmly adhering hard crusts on the pipes in the low-temperature area of the boiler and on the plates of the air preheater, which cannot be removed with the means available during operation.

Procedure to prevent
Acid dew point corrosion and surface contamination in the low temperature area
of incinerators

Applicant:

German gold and silver refinery

formerly Roessler,

6000 Frankfurt, Weißfrauenstr. 9

Named as inventor:

Dr. Kurt Wickert, 1000 Berlin

In a method for preventing acid dew point corrosion and surface contamination in the low temperature range of incineration plants by adding finely divided, high-melting metal oxides and / or carbonates to the combustion gases, according to the invention, a mixture of alkaline oxides with a specific surface area of about 30 measured by the BET method m ² / g and / or their carbonates or basic carbonates with acidic oxides with a specific surface area of about 100 m ² / g to the combustion gases at a temperature not exceeding 400 ° C, with 1 part of basic oxide and / or carbonate at least 0.5 parts of acidic oxide are used.

Highly disperse silicon dioxide or aluminum oxide and mixtures thereof have proven particularly useful as oxides with an acidic character, and disperse magnesium oxide or calcium oxide and mixtures thereof as the basic component. Instead of magnesium oxide or calcium oxide, magnesium carbonate or basic magnesium carbonate can also be used with advantage. The primary particle size of the acidic oxide, in particular of silicon dioxide, should be below 150, preferably below 100 ταμ .

The combined effect: Removal of the sulfur trioxide formed up to 400 ° C from the fuel gases and prevention of crust formation in the furnace, it is enough on its own not off, as in the previously known processes, the first step of removing the sulfur trioxide from the combustion gases only with fine dust

809 507/272

Γ 260 667

ground basic oxides or carbonates, e.g. B. dolomite to make, but it is this one highly disperse and thus highly active oxide, e.g. B. MgO or CaO necessary because the residence time the gases in the temperature range between 400 and about 100 ° C (end of air preheater) no longer than 0.5 seconds and the reaction temperatures are relatively low.

The following table of figures shows the extent to which the removal of SO ₃ from the combustion gases of a coal-fired boiler by blowing in highly disperse, basic, highly active oxides and of finely divided oxides such as. B. dust-fine dolomite, at about 400 ° C combustion temperature takes place:

additive Volume
percent SO ₃
in the exhaust behind
Air preheater No addition
Highly dispersed MgO
Highly dispersed CaO
Magnesite, powdery
Dolomite, dusty
Dolomite hydrate
Hydrated lime 0.0022
0.0000
0.00010
0.0021
0.0023
0.0016
0.0019

From the number table it can be seen that only highly dispersed MgO completely _{removes the SO 3} from the gas stream. Highly dispersed CaO is somewhat less effective because under the given conditions it also _{reacts with CO 2} and SO ₂ , which is not the case with MgO. The powdery but less active products magnesite, dolomite, dolomite hydrate and hydrated lime remove practically no SO ₃ from the combustion gases under the given conditions.

The SO ₃ removal from the combustion gases leads to the formation of MgSO ₄ (or CaSO ₄ ) which, together with the fuel ash and the water vapor that is always present, forms the dust-like deposits on the heating surfaces. By adding a further substance, namely a highly dispersed, active, acidic oxide, according to the invention, as a second step, a transfer of these dust-like substances into firmly adhering substances is prevented. Both steps can be combined in such a way that the addition takes place in the form of a mixture of MgO and, in particular, highly dispersed SiO ₂ .

To demonstrate the effect achieved according to the invention, two diagrams are shown, the curves of which _{illustrate the behavior of the reaction products of metal oxide or silicon dioxide plus metal oxide during SO 3} removal. Fig. 2 shows the adhesion values of NaSO ₄ as a function of the temperature in the presence of combustion gases (curve I) and with the addition of highly dispersed silicon dioxide (curve Π). The values 1 to 6 plotted on the ordinate mean in this order: no adhesion, trace adhesion, distinct adhesion, strong adhesion, complete adhesion, melt. 3 shows the encrustation due to MgSO ₄ deposits in the moist flue gas or air stream in the absence of highly dispersed SiO ₂ (curve I) and with the addition of highly dispersed SiO ₂ (curve II). The values 1 to 5 shown on the ordinate mean in this order: no incrustation, incipient incrustation, partial incrustation, further incrustation, solid crusts.

The presence of alkali sulfates in the dusty deposit, in the presence of small amounts of SO ₃ in the gas, leads, according to curve I (FIG. 2), to strong adhesion to the heating surfaces. According to curve Π, there is no ίο adhesion _{when SiO 2 is simultaneously present.} At point A in curve I, NaHSO _{4 has} formed, at point B this has changed to Na ₂ S ₂ O ₇ . Both substances are strong adhesion promoters. Above 600 ° C, the Na ₂ S ₂ O ₇ breaks down into Na ₂ SO ₄ and SO ₃ . Therefore, no adhesion takes place between 650 ° C and the melting point of Na ₂ SO _4. In the presence of disperse, highly active silica, no bi- and disulfates of the alkalis are formed, since they react immediately with the SiO _{2 in the state of formation to form Na 2} SiO _3. After curves, therefore, there is no sticking in the presence of SiO ₂ . Similarly, SiO ₂ prevents oil ash dust from sticking (see VGB communications, issue 84 [1963], pp. 6 to 8).

At very low temperatures z. B. MgSO ₄ by absorption of water hard crusts that adhere firmly to the heating surfaces. The absorbed water comes from the flue gases or from the humidity in the case of boiler shutdowns. According to FIG. 3, curves, this encrustation of the heating surfaces does not take place either when disperse SiO ₂ is deposited together with MgSO ₄ (CaSO ₄ ) dust. In the presence of CaSO ₄ , SiO ₂ also prevents encrustation. This process is based on the ability of the dispersed SiO ₂ to envelop the foreign particles. The method according to the invention makes it possible _{to bind the low SO 3} contents in the combustion gases to the added dusty particles by chemical reactions in the gas stream and to prevent encrustation.

Under these circumstances, practically all of the sulfur trioxide is used as magnesium sulfate or magnesium sulfate. Calcium sulfate in a short time, e.g. B. with dwell times of only 1 second bound. There is magnesium oxide Particularly suitable as a basic component of the oxide mixture because it is practical with sulfur dioxide not reacted.

As far as the basic component is concerned, the amounts to be used depend on the amount of sulfur trioxide present in the combustion gases. The basic oxide should be added in stoichiometric amounts with respect to sulfate formation. To safely avoid encrustations and solid deposits, at least 0.5 part of the acidic oxide is used for 1 part of basic oxide, corresponding to MgO: SiO ₂ as 1: 0.5.
As mentioned, the oxide mixture should be added to the combustion gases at temperatures not above 400 ° C., since below this temperature there is no longer _{any significant formation of SO 3} by catalytic oxidation of SO _2. This goes from the Fi g. 1, which shows the equilibrium curves of the catalytic oxidation of sulfur dioxide to sulfur trioxide as a function of temperature. Curve α means the normal equilibrium curve, while curves b and c show the extent of SO ₂ oxidation with deposited oil ash (b) and hard coal fly ash (c) as the catalyst. Finally, curve d corresponds to the implementation of

SO ₂ in SO ₃ in a quartz apparatus. From these curves it can be seen that when the oxides are added to combustion gases which have a temperature of e.g. B. 600 ° C, after the neutralization of the sulfur trioxide by MgO further SO _{3 is formed} by catalytic oxidation of the oil ash. This can then no longer be bound by the stoichiometric amount of _{MgO added to the initial SO 3} content, since this amount has been used up. ίο

The influence of the acidic oxide component in the mixture is obviously that the finely divided Oxide with a large surface by intermediate storage the formation of the firmly adhering crust, which from magnesium sulphate under the action of water vapor would be prevented.

The introduction of the oxide mixture into the combustion gases can take place in such a way that the finely divided Oxide mixture at a suitable point where the combustion gases have cooled to around 400 ° C, is blown into this in solid form as dust. The introduction is particularly advantageous but in the form of an aqueous dispersion of the oxide mixture. For this purpose, the disperse Oxide mixtures dispersed and the dispersion using as an injector or other suitable device. The aqueous dispersant evaporates then at a temperature of 400 ° C in a short time, making a uniformly beneficial Oxide smoke remains, which chemically binds the sulfur trioxide and due to the presence of the acidic Oxide component is able to effectively prevent incrustations.

As a result of their fine division and their large surface area, they can be used in accordance with the invention Easily convert oxides into stable dispersions in water. One chooses magnesium oxide-water in the system about 8 percent solids by weight while in the silica-water system can even be about 20 percent by weight. A mixture of silicon dioxide and magnesium oxide in the ratio 1: 1 can contain 10 parts by weight Solids content can still be processed into a usable dispersion.

Claims (4)

Patent claims: 1. A method for preventing acid dew point corrosion and surface contamination in the low temperature area of incineration plants by adding finely divided, high-melting metal oxides and / or carbonates to the combustion gases, characterized in that a mixture of alkaline oxides with a specific surface area measured by the BET method of about 30 m ² / g and / or their carbonates or basic carbonates with acidic oxides with a specific surface area of about 100 m ² / g is added to the combustion gases at a temperature not exceeding 400 ° C., with 1 part basic oxide and / or carbonate at least 0.5 part acidic oxide are used. 2. The method according to claim 1, characterized in that the basic component of the Oxide or oxide-carbonate mixture Magnesium oxide or carbonate and as an acidic component Silica is used. 3. Process according to Claims 1 and 2, characterized in that the basic component is added to the combustion gases in amounts _{which are at least sufficient stoichiometrically for the formation of sulfate from the SO 3} fraction of the combustion gases. 4. Process according to Claims 1 to 3, characterized in that the oxide mixture is introduced into the combustion gases as an aqueous dispersion. Considered publications:
German Patent No. 721 024;
Journal of Technical Surveillance, 1 (1960), No. 8, pp. 293 to 301. 1 sheet of drawings 809 50 // 272 1.68 © Bundesdruckerei Berlin