CA1137704A - Air heater corrosion prevention - Google Patents

Abstract

Abstract of the Disclosure An additive, e.g., MgD.Mg silicate, is injected into the flue gas stream of a coal-or oil-fired furnace, at 2300-1700 deg. F., thereby reducing accumulation of sulfuric acid on the air heater inlet and outlet.

Description

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~ he present invention is dIrected to reduction of corrosion and fouliny ln the alr heater of an industrial furnace or utility boiier burniny coal or oil. This is accornplished by injecting an adrJitive into the flue gas stream where the stream has a temperature of about 2300 dey.
F. to 1700 deg. F., at a rate of 0.05 to 10.0 pounds of additive per short ton of coal burned. The additive reduces the concentration of free sulphur trioxic~e in the -flue gas.
This in turn reduces the dew-point oF sulfuric acid and rate of acid build-up at the air heater inlet and outlet to a level which does not promote air heater corrosion and fouling. The additive required is a compound or mixture of , two compounds which are capable of forming refractory materials under these conditions. This includes metal oxides themselves, e.g., magnesium oxide, calcium oxide, or sllicon dioxide, and also includes compounds which will break down to metal refractory materials a-t this temperature (2300 - 1700 deg. F.), e.g. magnesium silicate, calcium silicate, iron silicate, or the like, including mixtures of any of the foregoing. Preferably the additive is a mixture of magnesia i,~
and magnesium silicate (suitably in a weight ratio of 3-0.25:1 and preferably 1.5:1) and pre-ferably the additive is added at a zone of the furnace where the temperature is about 2200 deg. F. and at a feed rate of 0.50 to 1.5 pounds per ton , of coal or other fuel burned. Preferably the addi-tive lS
i ground or finely divided, e.g., typically 5-7 microns. The additive should exclude any substantial amount of free alumina.
The pre-ferred magnesium silica-te is talc. Other useful magnesium silicates include: magnesium trisilicate serpentine, steatite, soapstone, enstatite, and diopside.

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As is well known in the furnace art, metal ternperatures in air heaters are often below the sulfuric acid dew-point of the flue gases, and hence such metal surfaces face the continual problem of sulfuric acicl condensation leading to ; fouling and corrosion. Use of the present invention not only prevents acid condensation but also keeps heat transfer surfaces in the air heater clean.

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Prior Art Industrial or utility boilers burning sulFur containing fossil fuels experience not only the problern oF emitting hazardous pollutant in the atrnosphere but also pluggage and corrosion in air heaters. Inorganic constituents of the fly ash catalyze the oxidation of sulfur dioxide to sulfur trioxide. Sulfur trioxide combines readily with water vapor in lower temperature zones oF -the boiler forrning corrosive sulfuric acid. Condensed sulFuric acid initiates corrosion, which subsequently leads to fouling and pluggage. Of`ten, acid smut fallout is also associated with unburned carbon and condensed sulfuric acid.
Technical litera-ture oF recent years (1,5) (references at the end oF this section) shows considerable interest in the area of additive application for preventing cold end acid corrosion and fouling. Basically, published approaches can be grouped into three different categories. It should be noted that classification is strictly based on the method oF
SO3 removal.
1) Physical absorption on the finely dispersed phase in the gaseous media, such as silica and alumina. (3,4)

2) PreFerential reaction of atomic oxygen -to preven-t 503 formation. (5)

3) Introduction oF materials in the back end of the boiler which selectively combine with sulFur trioxide to form rion-corrosive end products. (2,5) Based on the-published data in the llterature, the mos-t effective approach in reducing corrosion and fouling of the air heater is in-troducing a neutralizing agent which selectively cornbines with sul~uric acid. (5) However, ~L~3'77~9~

field observation and data appears to inclicate that this approach is only partially effect:ive; the reason beiny that the temperature zone where the neutraliziny additive is injected is generally about 600 deg. F. I-t is probable that a gas to solid phase reaction for sulfur trioxide rernoval, does not occur. Instead, sulfur trioxide reacts with water vapor forming sulfuric acid, which subsequently condenses on relatively cool heat transFer surfaces. This acid is then neutralized by contact with additive at the surFace aFter condensation.
This method of application, there-fore presents certain problems:
1) The neutralizing additive only neutralizes sulfur trioxide which is condensing as sulfuric acid.
Measurements of sulfur trioxide before and aFter the air heater, with and without additive, will show very little change in the amount of S0 removed in the air heater with this process.
2) Neutralization occurs on the cold air heater surfaces by a physical contact process.
Tllerefore the additive rnust be dispersed over all cold air heater surfaces to insure that all condensing acid is neutralized efFectively in stopping cold end corrosion. If there are cold surfaces in the unit which do not receive adequate quan-tities oF neutralizing additive, corrosion will still occur on -them.
3) It is very difFicult to measure the extent of corrosion protection achieved in the air heater wilh the appllcatlon oF neutralizing additive.

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This can only be done by either visual lnspection of the unit or by placiny corrosion strips in different areas o~` the air heater in an atternpt to measure the effect.
; 4) One of the problems associated with sulfuric acid condensation in air heaters is a build Up of cor-rosion debris and fly ash causin0 pluygage. This can create conditions which force the unit off line for cleaning, leading to decreased availabil-ity and costly rnaintenance. Since the low temperature application of neutraliziny aclditive works as a physical contact process in the air i heater, sticky deposits can still occur within the unit causing pluggage and necessitating shut down.
; 5) The problem of acid smut ernissions is created by condensation of sulfuric acid at some point in the , flue gas stream after the air heater. The priMary surface available for condensation at that point ` is the fly ash itself. Condensation in this manner creates the sticky acidic fly ash particu-;~ late in the stacl< known as acid smut. If after ~ leaving the stack this rmaterial -falls on cars, ., boats and buildings, corrosive damage is caused due to -the presence of sulfuric acid on the surface of the particle. Since the cold end application of a neutralizing additive only removes SO~ condensillg as sulfuric acid in the air heater, residual 503 can still be available to condense at a later stage in the flue gas stream, creating the acid smut emission.

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If 503 levels present in the unit, under norrnal conditlons, are so low that condensation of sul-furic acid does not normally occur in the air heater, the concentration of S03 will no-t be significantly affected by the cold end application of a neutralizing additive. Thus, an acid srnut problem will not be alleviated under these conditions.
This invention reduces 50`3 levels in the flue gas before it reaches the air heater. The concentration of 503 in the flue gas can be reduced well below -the point at which condensation will occur. The product of this reaction is a dry powdery sulfate salt. Utilizing this process minimizes the problems mentioned above for the followlng reasons:
3 1) Removal of 503 from the flue gas stream prior to the air heater allows rneasurements of 503 ahead of the air heater (in a temperature zone where no condensation can have occurred), thus allowiny direct measuremen-t and comparison of ` corrosion potential with and without applica-;~ tion of additive. This means that the dosage of the additive can be accurately set and '~ main-tained to provide protection for -the unitwith minimal quantities o-f neutralizing material.
We have found -that -this has enabled utilization of much lower quan-tities of the additive than in the cold end application.
i ~ 2) Since 503 is effectively removed, protection is ,~ .
afforded equally to all the cold surfaces of an ` air hea-ter. Thus, the unit can be completely pro-tected with a relatively small dosage of the _ 7 _ `
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adciitive.
3) Since the product oF the reaction of S03 with the additive is a dry powdery ma-l,erial, no pluggage occurs in the air heater.

4) Efficient removal of S03 from the flue gas to a point where no significant condensation of the sulfuric acid can occur in the unit will also minimize the potential For creating acid smut since this is caused by sulfuric acid condensation at some point after the air heater.
In addition to the utilization of the neutralizing materials described in high temperature zones of the boiler, we have -found that the introduction of natural minerals in the additive results in activation of the minerals for gas phase reactions. This actlvation may be caused by the disruption of the original crystal structure, resulting in increased porocity of the dispersed solid phase. This increased surface area with activated adsorption sites increases the efficiency of sulfur trioxide removal from the flue gas. The neutralization and physical adsorption process desc'ribed above effectively prevents cold end corrosion ancl fouling.
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: , 7~91 REFERENCES

1) Reid, W. T.
"External Corrosion and Deposit"
American Elsevier, New York, New York, 1971 2) Bennett, R. P.
"Chemical Recluction of Su].fur Trioxide and Part.i:culates From l-leavy Oil~
171st Symposiurn on l-leavy Fuel Oil Addi.tives New York, New York, Apri:l, 1976 3) Libutti., B. L.
"E-fficient Cold End Additives~
Symposiurn on Heavy Fuel Oil Additives New York, New York, April, 1976 .
4) U. S. Patent No. 3,886,261, May 27, 1975 '

5) Rendle, L. K.
"The Prevention of Acid Condensation in Oil-Fired Boilers"
Journal of the Institute oTf Fuel, 372 September, 1956 The following examples illustrate without limiting the invention. Examples I and II are offered as recommended : : procedures; they have not been carried out in plan-t practice. Example III has been carried out in a commercial :; furnace.
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EXAMPLE I
A 120 megawatt designed capacity, cyclone fired boiler, burning Eastern bituminous c coal, was experiencing some corrosion and fouling in the Lungstrurn air heater. To lower the sulfuric acid dew point and rate of acid build up, a powdered mixture containing magnesia and magnesium silicate (50:50) is injected into the high ternperature zones (2300 to 1900 deg. F.) of the test boiler. Acid dew point of tne treated gas at the air heater inlet is reduced, thereby reducing corrosion and fouliny.

EXAMPLE II
A 330 megawatt rating, cyclone fired boiler, burning Eastern bituminous c coal, was equipped with preheat steam coils on the forced draft fan to minimize tubular air heater corrosion and fouling! To lower sulfuric acid dew point and rate of sulfuric acid build up, a powdered mixture of magnesia and magnesium silicate is injected into the high temperature zones (2300 - 1900 deg. F.) of the test boiler.
he additive is ef-Fective in achieving these objectives.
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EXAMPLE III
A 215 217 MWs capacity boiler burning fuel oil No. 6, was experiencing corrosion and fouling of the air heater due to sulfuric acid condensation. Samples of deposit collected from the air hea-ter showed the presence of szomolnokite (FeS04 H20) and low pH of 2 in a one percent water slurry. The su:Lfur con-tent of the fuel oil was in the range o~ 0.9 to l.0 percen-t. The corrosion and fouling of the air heater was occurriny in spite of -the fact that the ~3~i'70~

air heater was receiving neutralizing additive at the air - heater inlet. A 50:50 powdered mixture of maynesia and magnesium silicate (talc) was injected a-t the rate of 30-31 lb. 1 hour in the high temperature zone of the boiler (2,300 deg. to 1,700 deg. F.).
; In this run, as regards performance in the flue gas, the highest sulfuric acid dew-point obtained was 270 deg. F. and the highest S03 levels obtained using moclified EPA method were approximately 6 ppm by volume with a flue gas moisture content of approximately 17%. With additive injection the gaseous 503 levels dropped to 0.5 ppm and the highest acid dew point obtained was 260 deg. F. As regards the air heater, there was no acid build-up. The ~lue gas ternperature in the air heater inlet was 570-580 deg. F. The acid dew point ranged from 240 dey. F. to 260 deg. F. No corrosion or fouling was noted. As regards air heater inlet conditions using the additive, particulate S03 measured 0.77 ppm on the south side and 0.37 ppm on the north side. Gaseous S03 measured 0.73 ppm on the south side and 1.28 ppm on the north side. The tempera-ture of -the air heater inlet was 555 deg.
, F. on the south side and 576.7 deg. F. on the north side.
; The air heater outlet was in two sections. In section E
particulate S03 measured 0.09 pprn and in Section F 0.02 ppm. Gaseous 503 measured 1.76 ppm in Sec-tion E and 0.96 ppm in Section F. The temperature in Section E was 250 deg.
F. and in Section F was 265 deg. F.
The above data contrasts with performance when the additive is omitted, to wit: the rate of acid build~up averaged lOO microamps per minute, as determined by electroconductivity methods. This indica-ted a high rate of deposition with consequent corrosion and fouling.

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l~L3 ~ 7C~4 Sulfur trioxide in the air heater inlet is also found to be much higher when the additive is omi-tted. In the south side of the air heaters inlet particulate S03 measured 0.02 ppm, the same as on the north side. This value is actually lower than when the additive was used. However, gaseous ,:
S03 was much higher, when the adclitive was omitted, narnely 5.89 pprn, south side and 3.30 ppm north side. At other ; stages oF the air heater inlet, gaseous S03 rneasured 2.05 and 0.960 ppm. At the outlet particulate S03 measured ~l 0.06 ppm, for both the E and F sections; yaseous S03 ; measured 3.57 ppm in Section E and 1.30 ppm for Section F.
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Thus when using the additive substantially lower results are generally obtainable for gaseous 503, bo-th in the air heater inlet and the air heater outlet.

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TABLE I

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FUEL ANALYSIS AND UNIT OPERATION DATA

FOR EXAMPLE III-Iest Parameter_ Witl~ Additive Wi-thout Additive .. .
Carbon, % 82.26 86.31 Hydrogen, % 12.76 12.72 Su]fur, % 1.02 O.'J5 Nitrogen, % 0.13 0.18 Ash, % 0.08 0.0~l BTU/lb. 18,900 18,750 Fuel Firing Rate 103,000 #/hr 102,569.8 Heat Ihput1946.7 million1923.18 million BTU/Hr. BTU/Hr.
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Theoretical Gas . . ,,,, : .
~ ~ Flow, SCFMD " 307,874 306,414 .j .
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., /The data are presented to show substantial equivalence of furnace operations, as to fuel used, heat input, etc., during -~i~ runs with and without add;tive. The run without the additive ;~ was made in the same furnace on the day following the run ~; with the additive.
/Standard cubic feed per minute (dry).

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A steel tube probe, one inch in diameter, 4-6 ft.
long, was used in Example III to inject the powder into the furnace. The powdered additive is taken off a hopper with a screw feeder whlch meters the powder into an alr conveying system, which delivers the powder to the probe. The air flow 1 is sufficient to cool the probe to resist heat distortion.
' ' Preferred conditions The preferred additive is a homoyeneous ground mixture of magnesia and magnesium silicate, preferably talc, in the Q respective weight ratio of 3 to 0.25:1, and more preferably 2 to 1:1; a useful commercial ratio is 1.5:1. The MgO:
; magnesium sllicate mixes of this invention are believed to be novel compositions.
This additive is added to the furnace at a temperature in the range of 2300-1700 deg. F., more preferably 2200 deg.
F to 2000 deg. F.; and still more preferably about 2200 deg.
F.
The ratio of additive to fuel is, in the case of coal, - 0.5 to 1.5 lbs. additive/ton of coal; preferably 0.5 to 1.0 ` ~ lbs./ton; and even more preferably, about O.75 lbs/ton. For oil, the rates are about the same.

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Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Method of reducing corrosion of metal surfaces in an air heater in an industrial or utility fossil fuel furnace comprising injecting an additive into the furnace at a temperature in the range of 2300°-1700°F. at a rate of 0.05-10 lbs. additive/ton of fuel burned; the additive being essentially free of alumina and consisting essentially of a combination of magnesium oxide and magnesium silicate in a weight ratio of from 2 to 1:1 of magnesia to magnesium silicate.

2. Method according to claim 1 in which the tem-perature is 2200 to 2000 deg. F.

3. Method according to claim 2 in which the tem-perature is about 2200 deg. F.

4. Method according to claim 1 where the additive feed is at a rate of 0.5 to 1.5 lbs./ton of fuel burned.

5. Method according to claim 1 in which the mag-nesia;magnesium silicate ratio is 1.5:1.

6. Method according to claim 4 where the magnesia:-magnesium silicate is 1.5:1.

7. Method according to claim 4 where the magnesium silicate is talc.

8. Method according to claim 1 in which the ratio is 1:1 and the magnesium silicate is talc.