US3306235A - Corrosion reducing method and material for furnaces - Google Patents

Description

Feb. 28, 1967 E. c. LEWIS ETAL 3,306,235

CORROSION REDUCING METHOD AND MATERIAL FOR FURNACES Filed Oct. 26, 1964 2 Sheets-Sheet l Rae arr Fa a W475? Z/ INVENTORS EVERETT c- LEWIS JOHN T. REESE Feb. 28, 1967 c, w TAL 3,305,235

CORROSION REDUCING METHOD AND MATERIAL FOR FURNACES Filed Oct. 26, 1954 2 Sheets-Sheet 2 FIG-.2

flflfl/f/Vf l3 FT Q /Z m I Q 9 \J E 8 Q r b 7 W x 6 Q g; 5 4 f 2 W 0 Z 6 8 l0 l2 l4 /6 /8 Z0 am/ixpy 0 -/0//Z0) //v Ffffl- Z INVENTORS EVERETT C... LEWIS JOHN "F. REESE ATTORNEY United States Patent Ofifice 3,395,235 Patented Feb. 28, 1967 3,306,235 CORROSION REDUCING METHOD AND MATERIAL FOR FURNACES Everett C. Lewis, Avon, and John T. Reese, Simsbury, Conn., assignors to Combustion Engineering, Inc.,

Windsor, Conn., a corporation of Delaware Filed Oct. 26, 1964, Ser. No. 406,500

5 Claims. (Cl. 110-1) This invention relates to additive materials for use in reducing the corrosion of heat transfer surfaces in large furnaces due to the action of combustion products and more particularly to a material for and method of obtaining better adhesion of the additive materials to the heat transfer surfaces.

A problem of increasing importance in modern furnaces and high capacity steam generators in power plants is corrosion and fouling of tube surfaces as a result of the combined action of corrosive combustion products in the gases and the existing high temperatures. Such corrosion weakens the tubes and leaves scaled deposits which have a deleterious effect on the heat transfer capacity of the furnace.

It has been recognized in the past that an additive might be sprayed or blown onto the heat transfer surfaces to reduce or eliminate corrosion and scale formation. It has been discovered, for instance, that sodium or calcium oxides, or materials which will decompose to give these oxides, when sprayed into the high temperature superheater regions of a furnace ahead of the low temperature sections such as the heat economizers and air heaters, will reduce corrosion in these low temperature sections (see United States patent to Harlow, No. 2,412,- 809). This method, however, is not satisfactory for the purposes of reducing the corrosion of the high temperature surfaces such as occurs in the superheater and reheater. As set forth in the co-pending application of Richard C. Ulmer, Serial No. 190,614, filed on April 27, 1962, now abandoned, corrosion and fouling of the high temperature superheater and reheater surfaces in solid fuel fired furnaces may be reduced by the addition of magnesium oxide or magnesium salts which will decompose in situ to form magnesium oxide. A great many additional materials have been employed as additives with varying degrees of success and a few will be mentioned hereinafter.

A major problem with a large number of additive materials which are otherwise very desirable has been to get them to adhere to the high temperature tube surfaces. To be effective as a corrosion retardant, the additives must deposit on, and adhere to, the metal surface. Very poor adhesion is realized if the materials are added by themselves in the dry form. This is particularly true of coal fired units as contrasted with oil fired units since the vanadium ash deposits produced in oil fired units provides a molten, sticky surface to which the additives will inherently adhere. This type of a sticky deposit is not present in coal fired furnaces so that the invention is particularly useful with coal fired units although not limited thereto.

One method of achieving increased adhesion is to mix the additives with water and to spray the resultant slurry onto the tubes through the soot-blowers which are common to most large furnaces. The water slurry in some manner tends to increase the adhesion to some degree. However, this slurry method is not entirely satisfactory since greater adhesion is desired than can be achieved thereby, and the spraying of the water on the high temperature tubes produces thermal shock and early failure of the tubes.

Accordingly, it is an object of the present invention to provide a method of reducing furnace corrosion and scale deposit.

It is a further object to provide a method of increasing the amount of furnace additives which adhere to the desired heat transfer surfaces.

A particular object of the invention is to provide a furnace additive mixture incorporating a material which will increase the adherence of the additives.

According to the present invention these and other objects are achieved by mixing with the conventional furnace additives a quantity of sodium tetraborate and then introducing the resulting mixture into the steam generating unit. Such a procedure and technique will significantly increase the amount of additives that will adhere to the tubes without impairing the beneficial effect of the additive itself.

For a more detailed understanding of the present invention, reference may be had to the accompanying drawings in which:

FIG. 1 schematically illustrates a steam generator to which the present method may be applied; 7

FIG. 2 is an elevational view of a device for admitting the additives of the present invention to the steam generator; and I FIG. 3 is a graph illustrating the effect of theadditive constituent of the present inventio Referring to FIG. 1, there is shown a typical boiler furnace or steam generator having a combustion chamber 10 in which a solid fuel, such as pulverized coal, is burned. The fuel is introduced into the-furnace by means of burners 12, while the combustion air enters through wind boxes 14. The combustion gases generated pass upwardly through the chamber 10, through the horizontal gas pass 16, down through the vertical gas pass 18, and out the lower end thereof to the stack (not shown).

Feedwater is supplied to the economizer 20 by means of header 21, where the water is heated to a certain extent. This heated water then flows to the header 22, through the tubes 23 to the outlet header 24, and from there to the steam and water drum 26.

Water from the drum 26 flows through a downcomer' 28 to lower headers 30, which supply water to the tubes 32 which completely line the walls of the furnace 10. Most of the steam generation occurs in the tubes 32. Leaving the top of the tubes 32 is a mixture of steam and Water which is returned to drum 26 'byan upper header 33.

The steam separates from the water and flows on to distribution header 34. The steam passes fromthe header 34 down through the tubes 36 to the supply headers 38 and then to the primary superheaters 40 and 42. The superheated steam then flows to the final superheater section 44 by way of the header 45, and from there to a turbine (not shown).

Many modern turbines are designed with a plurality of stages. In order to prevent condensation inthe lower pressure stages, and further to obtain the highest possible thermal efliciency, partially expanded steam is withdrawn from the high pressure stages and returned to the furnace to be reheated. The reheated steam is used to drive the lower pressure stages of the turbine. In the furnace 3 shown in FIG. 1, reheater sections 46 and 47 in the gas pass 16 are provided for this purpose.

Modern steam generators of the type illustrated in FIG. 1 are characterized by large physical dimensions. Thus, one unit having a capacity of 1,200,000 pounds of steam per hour extends upwardly about 170 feet from its foundation with the furnace measuring 28 feet from front to rear and being 40 feet wide. The invention is, of course, also applicable to furnaces of other capacities, dimensions and types. Combustion gases from the furnace entering into and passing over the superheater 44 will typically be at a temperature within the range from about 1500 F. to about 2500 F. Gas-side corrosion and fouling is most severe on the high temperature tubes of the superheater 44 and the reheaters 46 and 47. Steam outlet temperatures from these tubes may be in the range from 1000 F. to 1200 F. and the tube metal temperatures may be as high as 1300 F.

The present invention is carried out by introducing the additive mixture into the steam generator at locations which will give adequate and relatively uniform coverage of the superheater and reheater tubes. The exact locations and the required number of locations will, of course, vary from furnace to furnace. A plurality of additive supply tubes 50 have been illustrated in FIG. 1. These tubes terminate in nozzles which protrude through suitable access openings in the furnace wall and which are located so as to give a distribution of the additive material to adequately cover superheater 44 and reheaters 46 and 47.

Apparatus for feeding a dry additive mixture prepared in accordance with the present invention is illustrated in FIG. 2. The mixture is stored in a conical feeder 52 and fed from the bottom thereof into the inclined chute means 54. Both the conical feeder and the chute means are vibrated by means 56 and 58 respectively to keep the dry material flowing properly from the cone and the chute. These vibrating means 56 and 58 are operated from the control means 60. Any conventional vibrating apparatus may be used for this purpose.

The additive mixture drops from the chute means 54 into the funnel 62. The funnel is attached to and part of a simple aspirator 64 into which air is fed from line 66. The air will aspirate the additive mixture into the aspirator 64 and carry the mixture into the furnace through the line or lines 50.

The present invention as previously stated involves the mixing of sodium tetraborate with conventional additive materials to improve the adhesion of the additives to the desired tube surfaces. Sodium tetraborate is particularly desirable since it not only performs this function satisfactorily but it is also a comparatively inexpensive material. The usual commercial deca-hydrate form of sodium tetraborate, borax (Na B O .10H O), is preferred for the practice of the invention. The anhydrous or the penta-hydrate forms could also be used but the problem of caking due to water absorption wouldthen have to be contendedwith. The deca-hydrate when introduced into the furnace will melt (at about 167 F.) and then lose its water of hydration. The anhydride will then melt again at about 1366 F. and will deposit on the tubes in this molten, sticky condition and provide a bonding agent to bond the corrosion retarding additive material to the tubes.

The graph of 'FIG. 3 illustrates the effect of sodium tetraborate on the ability of the additive materials to adhere to the tube surfaces. The graph shows the relationship between the percent borax in the additive mixture fed to the test furnace and the percentage of the material impinging upon the tubes which adheres thereto. The impinging material is calculated by multiplying the total material introduced into the furnace by the ratio of the projected tube area to the total gas pass area. By projected tube area is meant the cross sectional area of the gas pass which is filled with tubes. The graph therefore indicates the percentage of the material which actually adheres relative to that which theoretically strikes the tubes and has an opportunity to adhere. Curve A shows the percentage of the total impinging material which will adhere while curve B shows the percentage of the impinging additive, in this case magnesium oxide, which adheres. It can be seen that when no borax is present in the additive mixture very little magnesium oxide adheres to the tubes while the addition of the borax produces a sharp and significant increase in the amount of material adhering to the tubes. It has been found that the borax and the magnesium oxide will deposit in approximately equal proportions by weight and thus a 50-50 mixture of the two materials by weight is a desirable mixture although the invention is not limited thereto since the addition of any amount of borax has the desirable effect.

The amount of additive required per unit period of time for a particular size furnace depends primarily upon the tube metal temperature and the corrosiveness of the combustion products and thus the amounts may vary widely. Accordingly, it is not feasible to specify in advance the amount of any particular additive mixture required for a particular furnace. The amounts must be determined under the operating conditions and adjusted to effectively reduce the corrosion.

A preferred additive for use in the present invention with the sodium tetraborate is magnesium oxide which was used in the test illustrated in the graph of FIG. 3. This material is commercially available and ulitized as an additive in either the -200 or 325 mesh size range. A typical utilization rate for such an additive by itself without sodium tetraborate might be on the order of 720 pounds per 24 hours in a steam generator with a capacity of 1,200,000 pounds of steam per hour although, as previously stated, this can vary widely. This could be introduced continuously throughout the 24-hour period, at various intervals or all at one time. This latter procedure, however, would not be the most desirable since a large single application would tend to build up an excessively thick coating on the tubes and thus reduce the heat transfer rate to an unacceptably low level. The preferred embodiment of the present invention would, therefore, involve the mixing of magnesium oxide of the stated commercial grade with an equal portion of borax by weight and introducing the resulting mixture into the steam generator at the required rates. The rate of application would, of course, be far less with the borax added than without.

The present invention may, however, be applied to a variety of additive materials. One group of such materials is the alkaline earth oxides which will react with 50;; or acid salts produced in coal ash deposits. The most important member of this group from a practical standpoint, besides the above-mentioned magnesium oxide, is calcium oxide. Barium oxide would be rather expensive. Salts of these alkaline earth metals, such as magnesium carbonate, may also be used, since they will decompose when introduced into the furnace to give the desired oxides.

The present invention is intended to be applicable not only to the additive materials specifically mentioned 'but to all additives which exhibit the same adhering problems and to which the invention would be beneficial. It will therefore be understood that the examples given above are for illustrative purposes only and that additions thereto or alterations thereof may be made without departing from the s irit of this invention as defined by the following claims.

We claim:

1. A method of reducing corrosion of heat transfer surfaces due to the action of combustion products in a steam generator comprising applying a mixture of a corrosion retardant material and sodium tetraborate to the surfaces subject to said corrosion.

2. A method of reducing corrosion of heat transfer surfaces due to the action of combustion products in a steam generator comprising introducing into said steam generator a mixture of a corrosion retardant material and sodium tetraborate so that said mixture will impinge upon and deposit on said heat transfer surfaces.

3. The method of claim 2 wherein said corrosion retardant material and said sodium tetraborate are present 5 6 References Cited by the Examiner UNITED STATES PATENTS 2,412,809 12/1946 Harlow 165-1 2,859,146 11/1958 Prust 10614 X 3,234,580 2/1966 Keck ll0l FOREIGN PATENTS 795,919 1/ 1936 France.

10 DONLEY J. STOCKING, Primary Examiner.

ROBERT A. DUA, Examiner.

Claims (1)

1. A METHOD OF REDUCING CORROSION OF HEAT TRANSFER SURFACES DUE TO THE ACTION OF COMBUSTION PRODUCTS IN A STEAM GENERATOR COMPRISING APPLYING A MIXTURE OF A CORROSION RETARDANT MATERIAL AND SODIUM TETRABORATE TO THE SURFACES SUBJECT TO SAID CORROSION.

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