US 3343908 A
Description (OCR text may contain errors)
Sept. 26, 1967 50 ox/o/zfp 70 50 200 300 700 560 600 700 800 900 /O00 //00 lUO m 5 19/9. WEE C K URT FERDINAND INVENTOR W/CKE/F T ATTORNEYS United States Patent 3,343,908 METHOD OF REMOVENG SULFUR TRIOXIDE FROM COMBUSTHGN GASES TO REDUCE THE CGRROSIVE EFFECTS THEREGF Kurt Ferdinand Wickert, Berlin-Siemensstadt, Germany, assignor to Deutsche Goldund Silber-Scheideanstalt vormals Roessler, Frankfurt am Main, Germany Filed Feb. 6, 1964, Ser. No. 342,953 Claims priority, application Germany, Feb. 13, 1963, 1) 40,856 4 Claims. (Cl. 232) ABSTRACT OF THE DISCLOSURE Process for reducing dewpoint acid corrosion and fouling of surfaces contacted at temperatures between below about 400 C. and the ambient atmospheric temperature with combustion gases containing sulfur trioxide and water vapor comprising incoporating in a fine state of distribution a mixture of (1) finely divided basic material selected from the group consisting of calcium oxide, magnesium oxide, magnesium carbonate, basic magnesium carbonate and mixtures thereof and (2) finely divided acidic oxide material selected from the group consisting of silicon dioxide, aluminum oxide and mixtures thereof, said finely divided basic material and acidic oxide material having a surface area of at least 30 m. /g., into such combustion gases when they have cooled to a temperature of about 400 C., the molar proportion of said acidic material being at least 0.5 mol per mol of basic material calculated as oxide.
The present invention relates to an improved process for hindering acid dewpoint corrosion, particularly those caused by sulfuric acid, and fouling of equipment subjected to combustion gases in the lower temperature region.
It is known that liquid and solid fuels, such as oil and coal, contain sulfur which in the combustion gases is partially oxidized to sulfur trioxide. Such sulfur trioxide forms sulfuric acid vapor at temperatures around 250 C. with the water vapor which is always also present in the combustion gases and such sulfuric acid vapors, depending upon their concentration in the combustion gases, deposit at 180 C. and lower upon the available surfaces of the equipment in contact with such combustion gases. The strong corrosive effects of the deposition of the sulfuric acid under some circumstances can even be noticeable in the chimneys provided for exhausting the cooled combustion gases to the atmosphere. In addition, the sulfuric acid which may be carried along with the exhausted combustion gases under unfavorable weather conditions can cause considerable pollution of the atmosphere which can be disturbing to health, as well as plant growth, in the surrounding territory. Attempts have already been made to bind the sulfur trioxide contained in the exhaust gases with magnesium oxide or magnesium compounds such as dolomite and thereby prevent any occurrence of an acid dewpoint. However, only unsatisfactory results have been attained thereby as it was only possible to shift the acid dewpoint to somewhat lower temperatures. Previously, attempts to eliminate the more or less strong fouling caused by deposits of solids have not been successful to any degree worth mentioning. To the contrary. the deposits of the substances added to reduce the dewpoint which have partially been converted to sulfates produce hard firmly adhering crusts, for example, in the low temperature region of the boiler tubes and the plates of the air preheaters, which cannot be removed with the usual means without interrupting the operation.
3,343,908 Patented Sept. 26, 196? According to the invention it was found that the occurrence of an acid dewpoint and the deposit of firmly adherent encrustations on the surfaces contacted with combustion gases in the lower temperature region could be practically completely avoided if a mixture of highly disperse finely divided basic and acidic oxides is uniformly distributed in the combustion gases when they have cooled down to about 400 C. Instead of basic oxides, carbonates or basic carbonates of elements forming such oxides can be used. Highly disperse silicon dioxide or aluminum oxide or mixtures thereof have proved especially suited as the oxides of acidic character which are used according to the invention and magnesium oxide or calcium oxide or mixtures thereof are especially suited as the basic oxides. Magnesium carbonate or basic magnesium carbonate may also be used with advantage in place of the magnesium oxide or calcium oxide. It is essential for the success of the process according to the invention that the highly disperse acidic and basic materials employed are of sufficient fine grain that their surface area measured by the BET method is at least In. /g. Preferably, the surface area of the acidic oxides is larger, for example, about 100 m. g. The primary particle size of the acidic oxide, especially of the silicon dioxide, should be under 150 mu and preferably be under 100 m Under these conditions, practically all of the sulfur trioxide is bound in a very short period of time to magnesium sulfate or calcium sulfate, for example, in less than 1 second. Magnesium oxide is especially suited as the basic component of the oxide mixture as it practically does not react with sulfur dioxide.
The quantities of the oxides employed insofar as the basic component is concerned depends upon the quantity of sulfur trioxide present in the combustion gases to be treated. The quantity of such basic oxide should be stoichiometric with respect to sulfate formation. In order to insure prevention of the formation of encrustations and solid deposits at least 0.5 mol of acidic oxide should be employed per mol of alkaline oxide.
The addition of the oxide mixture to the combustion gases as indicated above should be at temperatures not substantially above 400 C. as further formation of S0 by the catalytic oxidation of S0 practically does not occur under this temperature.
The accompanying drawing is a graph showing the equilibrium curves 0n the catalytic oxidiation of S0 to S0 plotted against the temperature.
In such graph, curve a is the normal equilibrium curve, whereas curves [2 and 0 give the amounts of the S0 oxidiation with deposited oil ash b and with deposited coal fly ash c as the catalyst. Curve d corresponds to the conversion of S0 to S0 in a quartz apparatus. As can be seen from these curves, if the addition of the oxides to the combustion gases is made when the latter still have a temperature of 600 C., further S0 can be formed after the neutralization of the S0 present at such temperature with the stoichiometric quantity of the basic oxide such as magnesium oxide and as a consequence the further cooled gases will still con tain S0 as the basic oxide was all constuned in the neutralization of the S0 present at 600 C.
Evidently the effect of the acidic oxide in the oxide mixture employed according to the invention is that the finely divided oxide with large surface area prevents the formation of firmly adhering encrustations which would have been produced from the magnesium sulfate by the action of the water vapor contained in the combustion gases through interstratification.
The introduction of the oxide mixture into the combustion gases can be effected at a suitable location at which the combustion gases have cooled to about 400 C. by blowing such mixture in solid form as a dust. However, according to an advantageous embodiment of the invention, such oxide mixture is introduced into the combustion gases in the form of an aqueous dispersion. For this purpose the disperse oxide mixture, is first dispersed in water and such dispersion blown into the combustion gases with the aid of an injector or other suitable device. The aqueous dispersing agent vaporizes quickly at the 400 C. temperature, leaving a uniformly distributed oxide smoke which effectively binds the su lfur trioxide chemically and through the presence of the acidic oxide effectively prevents the formation of encrustations.
In view of the fine particle size and large surface area of the oxides employed according to the invention, they are easily converted to stable aqueous dispersion. In the system of magnesium oxide and water, a solids concentration of about 8% by weight should be selected whereas in the system of silicon dioxide and water it can even be about 20% by weight. A 1:1 mixture of silicon dioxide and magnesium oxide can still be processed to a suitable dispersion at a 10% by weight solids content.
Example A boiler of a power station needs 10 t./h. of fuel oil which a below heating value of 9,600 kcaL/kg. The excess air is 10%. The oil has a sulfur content of 3.0% in the average.
1 kg. of fuel oil generates at burning with 10% of air excess a smoke gas volume of 12.4 Nmfi. 10 t of fuel oil generate under the same conditions a waste gas amount of 124,000 Nm. This waste gas contains 600 kg. of S if the Whole sulfur would be burned to S0 The waste gas examination showed that the thintieth part of this S0 amount consists of S0 that means that 20 kg. of S0 were transformed into 25 kg. of S0 These 25 kg. of S0 will be neutralized by blowing in of MgO. Theoretically 12.5 kg. of MgO will be needed for this. It has been practically Worked with an MgO excess of On a spot where the smoke gas stream had a temperature of about 400, a mixture of 14.0 kg. of MgO and kg. of SiO was blown in per hour. By the treatment of the waste gas according to the invention the content of kg. of S0 in 124,000 Nm. of Waste gas (corresponding to 20 kg. of 80;) was reduced to 0.37 kg. of 50;; or 0.0003 g. SO /Nm. of waste gas. This S0 concentration will no more effect a raising of the water vapor 'dewpoint, that means an acid dewpoint is not anymore demonstrable.
The Water vapor dewpoint of a Waste gas containing i 10 vol-percent water vapor is at a temperature of 40- 45 C. It raises up to 150180 C., if there is a small content of S0 in such gas-mixture.
SiO was obtained by the hydrolysis in the gas phase in the pyrogenic manner, the primary particle size was about 0005-0025 11, the specific surface according to the BET method 190 m. /g. The MgO had a specific surface of about -m. g. and consisted of scaly agglomerates up to 3 1 size.
Analogical results will be obtained by replacing of silicon dioxide in the whole or partly by titanium oxide and magnesium oxide in the Whole or pantly by calcium oxide or basic or neutral magnesium carbonate.
1. A process for reducing dewpoint acid corrosion and fouling of surfaces contacted at temperatures between below about 400 C. and the ambient atmospheric temperature with combustion gases containing sulfur trioxide and Water vapor comprising incorporating in a fine state of distribution a mixture of (1) finely divided basic material selected from the group consisting of calcium oxide, magnesium oxide, magnesium carbonate, basic magnesium carbonate and mixtures thereof and (2) finely divided acidic oxide material selected from the group consisting of silicon dioxide, aluminum oxide and mixtures thereof, said finely divided basic and acidic oxide materials having a surface area of at least 30 In. /g., into such combustion gases when they have cooled to a temperature of about 400 C., the molar proportion of said acidic material being at least 0.5 mol per mol of basic material calculated as oxide.
2. The process of claim 1 in which said basic material is finely divided magnesium oxide having a surface area of about 30 mF/g. and said acidic oxide is silicon dioxide having a surface area of about m. g.
3. The process of claim 1 in lVVhlCh. the quantity of basic material incorporated in the combustion gases is substantially stoichiometric for the formation of the corresponding sulfate with the sulfur trioxide contained in such combustion gases.
4. The process of claim 1 in which said mixture of finely divided basic material and finely divided acidic oxide material is supplied to the combustion gases in the form of an aqueous dispersion.
References Cited UNITED STATES PATENTS 2,718,453 9/1955 Beckman EARL C. THOMAS, Primary Examiner.