US2412809A - Corrosion reduction in heat exchangers - Google Patents

Description

Dec. 17,1946.

w. F. HARLOW 2,412,809

CORROSION REDUCTION IN HEAT EXCHANGERS Fil ed Aug. 8, 1945 A a/vc/WALJ f/c Rear/N6 FLU/0 Marl/2.5.

. i t I l Q-JTEAM on Am.

An; HEATER.

Q \SUPERHEA ER.

mmvrm Walter]? Harlan 7 Patented Dec. 17, 1946 CORROSION REDUCTION IN HEAT EXCHANGERS Walter F. Harlow, Quarndon, England, assignor to Combustion Engineering Company, Inc., New

' York, N. Y.

Application August 8, 1945; Serial No. 609,605 In Great Britain June 21, 1944 1 The invention relates to reducing corrosion and clogging in heat exchange apparatus particularly a in economizers and/or air heaters provided in steam boiler plants for recovering heat from flue gases after they have left the boilerproper.

. In such plants, more especially those operating with a high final steam temperature say of 800 F. upwards and fired by mechanical stokers, serious difficulty is frequently encountered with choking of the gas passages of the economizer and/or air heater which greatly hinders the oper ation of the plant and often results in corrosion and destruction of the heating surfaces.

Much research has been carried out on this problem and it is fairly well known that this choking and corrosion is largely due to acidic condensate forming on the heat absorbing surfaces of economizers or air preheaters although these surfaces are at a temperature above that at which condensation should normally occur; that is to say, it is a result of the existence of an abnormally high dew point temperature of the flue registered on certain plants whereas in other cases the more normal figure of 1.30 F. was obtained. It was also shown that in the plants where the high dew point temperatures were recorded there was a greater amount of sulphur trioxide in the flue gases than in plants where the dew point temperature was lower. It was concluded Claims. Cl. 257-1) acid or direct combination with the iron of the heat absorbing surface results in the corrosion and depositsdescribed. I have moreover discovered that the sulphur trioxide is formed as a result of the passage of the flue gases containing $02 over the surfaces of heat absorbing elements in advance of the economizer or air preheater such as the superheater and/or boiler tubes,

Y the amount produced depending on the nature that this factor accounted for the difference since the sulphur trioxide'would combine with the water vapor which is also present, forming sulphuric acid vapor; and sulphuric acid having a higher boiling point, depending on its concentration, than water would consequently condense at higher temperatures.- The reason for the existence in some cases of a greater amount of sulphur trioxide, however, was not determined.

The object of the invention is to minimize such corrosion and clogging by removing the causes from which this trouble arises.

The single figure of. the accompanying drawing is a diagrammatic elevational view of an illustrative steam generating boiler in which the present invention is practiced.

The present applicant has studied the problem over a period of fifteen years and has confirmed by a great many experiments on operating plants that the difficulties referred to are caused by relatively excessive amounts of' sulphur trioxide in the fiue gases which by deposition of sulphuric and temperature of these surfaces ahead of the heat exchanger. I have found by laboratory investigations and confirmed by observations taken on plants that this is principally due to the catalytic effect of a film of ferric oxide (F8203) and to a lesser extent, of the refractory-like scale which frequently forms on iron or steel surfaces of heat absorbing elements such as superheaters; also that the'extent of this action while comparatively slight at surface temperatures of say up to 600 F. afterwards increases and in the case of ferric oxide becomes very much greater,

between 800 F. and 1200" F.

One method by which I eliminate this catalytic action is to coat these surfaces of iron with the black magnetic oxide (F8304) such as is obtained by heating the iron to a temperature of say 1600 in air or flue gas or to 1200" F.'in an atmosphere of steam, and then the heated iron does not produce the catalytic effect referred to.

Another method is to coat the surfaces of iron or of refractory or of scale which are catalytic with certain fine dusts (say of the fineness of 10 micron or less) if only to a thickness of say one thousandth of an inch; the surfaces then lose their catalytic properties, providing of course the dust particles adhere to the surfaces. For in- I stance if the surfaces, preferably when heated, are

sprayed with or even merely dipped in an aqueous suspension of very fine pulverized fuel ash containing say 10% dust or a suspension of" lime hydrate containing as little as 3% lime the catalytic action is almost completely eliminated.

Applicant has also found that if the catalytic surfaces referred to above are sprayed with, or even dipped (preferably when in a heated condition) in a concentrated aqueous solution of sodium carbonate (commercial soda ash is quite suitable) the catalytic effect is also practically eliminated. In practice I have found that by coating s'uperheater tubes with an adherent film of lime before erection in the boiler a considerable advantage has been gained.

In the case of a boiler in which the fouling of the air heater was such that its availability was only 384, hours, this was extended to 1402 hours auaeoa the coal even if the normal amount of sulphur trioxide only (i. e., neglecting the sulphur dioxide ordinarily produced) is taken into account, showing that the process really prevents the acid from 1 forming and does not merely neutralize it after formation: in fact none of the methods of treatment disclosed herein are intended to chemically neutralize disadvantageous constituents of flue gases. 7

I cover the superheater tubes or other heating surfaces commonly used ahead of the air preheater or economizer with a film of non-catalytic materialby spraying them with or otherwise applying a liquid having an adherent suspension of lime, sodium carbonate, finely pulverized coal ash I or other suitable material, preferably when the surfaces are in a heated condition. The spray may be produced by delivering the liquid suspension through a fine nozzle from a container under pressure or by introducing the suspension into an air or steam pipe so that the coating material is dispersed, and carried onto the heating heater, as shown solely by way of illustration in the drawing. It is proposed to coat the surfaces from time to time as may be necessary whilst the boiler is inoperation, and/or to apply the coat-'.

ing before use is made of the boiler.

It has been found that the deposits of siliceous material which sometimes form on the heating surfaces of boilers and superheaters are also absorbing elements that 4 s are located in advance of said heat exchanger in the direction of gas flow which method comprises: applying an adherent liquid solution containing sodium carbonate to the surfaces'of said heat absorbing elements.

3. The method of minimizing corrosion by action of sulphuric acid on the metallic surfaces of a heat exchanger under circumstances wherein combustion gases containing 80: are subjected to catalytic action in passing over hot iron or steel heat absorbing elements that are located in advance of said heat exchanger in the direction of gas flow. which method comprises: periodically spraying the surfaces of said heat absorbing lements with an aqueous solution containing sodium carbonate equivalent to approximately one pound per'hundred square feet of surface to form a non-catalytic coating thereon. 4. The method of minimizing corrosion by action of sulphuric acids on the metallic surfaces of a heat exchanger under circumstances wherein combustion gases containing are subjected to catalyticaction in passing over iron or steel superheater elements that are located in advance of said heat exchanger in the direction of gas flow and heated to a temperature in the range 800 to 1200 E, which method comprises: periodically spraying the surfaces of said superheater elements during operation thereof with an aqueous solution containing sodium carbonate to form a non-catalytic surface thereon.

5.' The'method of minimizing corrosion by action of sulphuric acid on "the metallic surfaces Y in advance of said heat exchanger in the direccatalytic but to a lesser extent although they become effective when the encrustation is substantial. The methods described may be effectively employed for avoiding this undesirable condition.

This application is a continuation in part of that died in my name on October 11,1944, under Serial No. 558,291.

What I claim is:

1. The method of minimizing corrosionby action of sulphuric acid on the metallic surfaces of a heat exchanger under circumstances. wherein combustion gases containing, SO: are subjected to catalytic action in passing over hot iron or steel heat absorbing elements that are located in advance of said heat exchanger in the direction of gas flow, which method comprises: treating the gas contacted surfaces of said heat absorbing elements with a material which is non-catalytic in the presence'of the sulphur dioxide in the furnace gases to thereby avoid its conversion to sulphur trioxide and thesubsequent formation of a heat exchanger under circumstances wherein combust on gases containing 802 are subjected to catalytic action in passing over iron or st el heat tion of sulphuric acid on the metallic surfaces tion of gas flow and heated to a temperature in the range 860 to 1200". R, which method comprises: periodically spraying the surfaces of said superheater elements with an aqueous solution containing sodium hydrate to form a non-catalytic surface thereon.

6. The method of minimizing corrosion by action of sulphuric acid on the metallic surfaces of a heat exchanger under circumstances wherein combustion gases containing SO: are subjected to catalytic action in passing over hot iron or.

steel heat absorbing elements that are located in advance of said heat exchanger in the direction of gas flow, which method comprises: pe-

riodically spraying the surfaces of said heat absorbing elements during operation thereof with an aqueous solution containing sodium'hydrate to form anon-catalytic surface thereon.

7. The method of minimizing corrosion by acof a heat exchanger under circumstances wherein combustion gases containing are subjected to catalytic action in passing over iron or steel heat absorbing elements that are located in advance of said heat exchanger in the direction of gas flow and heated to a temperature in the range 8009 to 1200 E, which method comprises: periodically spraying the surfaces of said heat absorbing elements with an aqueous suspension containing lime hydrate to form a non-catalytic coating thereon.

8. The method of minimizing corrosion by action of sulphuric acids on the metallic surfaces of a heat exchanger under circumstances wherein combustion gases containing 80: are subjected to catalytic action in passing over iron or steel superheater elements that are located in advance" of said heat exchanger in the direction of gas flow and heated to a temperature in the range 800 to 1200 E, which method comprises: periodically spraying the surfaces of said superheater elements during operation thereof with an aqueous suspension vcontaining lime hydrate to form a non-catalytic coating thereon.

9. The method of minimizing corrosion by action of sulphuric acid on the metallic surfaces of a heat exchanger under circumstances wherein combustion gases containing 80: are subjected to catalytic action in passing over hot iron or steel heat absorbing elements that are locatedin advance of said heat exchanger in the direction of gas flow, which method comprises: periodically spraying the surfaces of said heat absorbing eleof said heat exchanger in the direction of gas how and heated to a temperature in the range 800 to 1200 F., which method comprises: periodically spraying the surfaces of said. superheater elements during operation thereof with an aqueous solution containing very fine puiverized fuel ash. I

WALTER F. HARIDW.

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