I
CLARENCE JACKLIN, DONALD R. ANDERSON, and HARRIS THOMPSON National Aluminate Corp., Chicago, 111.
Fireside Deposits in Oil-Fired Boilers Deposit Location US. Chemical Composition I N RECENT years improved processes in oil refining have increased the yield of high value products and decreased the amount of residual oil obtained from a barrel of crude. Since most of the ash-forming materials in the original crude concentrate in the residual oil, the trend toward lower yields of residual results in higher ash concentrations in the fuel going to the consumer. This increase in ash content, along with other factors, has caused increased slag and corrosion problems in boilers burning residual fuel. These deposits and slags reduce heat absorption and the area of gas passages, cause higher draft losses, and often reduce total boiler capacity. Higher sulfur in the fuel usually results in more sulfuric acid formation in the cooler sections of the boiler and more acid corrosion of economizers and air heaters. Corrosion products build up a n insulating layer of deposit and reduce the efficiency in these sections of the boiler. These problems often become so troublesome that a boiler must be shut down and the deposits removed to restore it to normal efficiency. Severe corrosion losses also cause premature failure of air heaters and economizers. This all adds up to a real problem to boiler operators. Considerable study and research have been devoted to finding methods for control and prevention of fireside deposits during the past few years. Some methods approach the problem from the mechanical point of view, varying from minor modifications in tube cleaning equipment to major changes in boiler design. Other methods utilize the chemical approach, which relies upon the ability of added chemicals, introduced as fuel treatments or directly into the furnace gases, to reduce the adherence of deposits to boiler surfaces by altering their chemical and physical structure. Although notable progress has been made on both approaches, no completely successful answer has been developed as yet, I While this article i s not directly related to the subject matter of the symposium, it i s included because it contains information of direct interest to all manufacturing organizations who have problems with chemical deposits from residual fuel oils in boiler operation.
possibly because of the numerous factors influencing deposit formation. One of the most important pieces of information needed for developing effective fuel treatments for preventing fireside deposits is the knowledge of the chemical composition of these deposits. I n addition to the regular chemical analysis, a n integrated pattern showing the relationship between the different chemical compounds and their location in the boiler is helpful. Clarke (2) has reported the results of a detailed and careful study of the fireside deposits formed while firing highvanadium fuel oil in a pair of high-pressure stationary boilers. He noted the transition of physical properties and chemical composition which occurs in different locations in the gas path and suggested several possible mechanisms of deposit formation. The work reported here was undertaken to develop the pattern of deposit composition throughout an average residual oil-fired boiler, and relate the general methods of deposit formation to this pattern.
Experimental Program The authors’ company has conducted research on fireside deposits for a number of years. During this time several hundred samples of deposits were removed from industrial and utility boilers in plants throughout the United States. The exact sampling location was recorded for each boiler along with the general operating conditions. Most of the samples were selected to represent the bulk deposits rather than any specific portion. Conventional analytical procedures provided quantitative data for the various elements present. Seventy-five of these deposit analyses, representing more than 50 different boilers, were grouped according to their location in the boilerLe., radiant, superheater, convection, economizer, and air heater. The average deposit composition was then calculated for each location.
Impurities in Residual Fuel Oil By the time residual fuel oil reaches the consumer’s plant, it contains the original impurities in the crude oil plus varying amounts of additional contam-
inants. The usual sources of fuel oil ash and sulfur are: (1) oil-soluble compounds present in the crude; (2) watersoIubIe compounds in the water portion of the crude; (3) corrosion products resulting from the attack of acidic material in the oil on metal surfaces such as tanks; and (4) contaminants such as salt water, scale, and petroleum catalyst. Table I gives a summary of the forms in which the more common ash-forming elements are thought to occur in crude petroleum. The concentration range for impurities, expressed as parts per million of the oxide of the element present in residual fuel oils, is as follows: Nan0
vzo5
CaO NiO MgO Fez03
Si02 AIzOs PZOK
Sulfate as SOa Sulfur
to 320 to 550 to 250 t o 300 0 to 30 2 to 350 0 t o 275 0 to 50 0 to 15 0 to 450 0 . 5 to 3 . 0 % 2 2 0 0
Thermal Characteristics of Deposit Areas Temperature is the most important single factor affecting fireside deposits. It determines the kind of deposits and where they will form. A thorough understanding of the temperature contour lines in a boiler is extremely helpful for interpreting the formation and composition of fireside deposits. Since boilers are built in a great variety of sizes, shapes, and arrangements, any attempt to classify the particular areas where fireside deposits form should be based on general patterns rather than any specific design. Figure 1 illustrates the five principal areas of a boiler where fireside deposits are found. Individual boilers vary in their arrangement. One or more of the areas shown may be absent, but this representative sketch illustrates the typical areas. When describing these areas there will be, in many boilers, unavoidable overlapping from one area to the next. In general, there is a more or less smooth transition of fireside conditions from the burners to the stack of a boiler. The areas illustrated have been selected as representative rather than as sharply defined zones. Starting at the high-temperature end VOL. 48, NO. 10
OCTOBER 1956
193 1
/
R. S. C. E. A.
C
Radiant Superheater Convection Economizer Air heater
tures exists for the gas and metal conditions in the five different sections. The temperature range in each section would be smaller for a specific boiler than those shown. For example, the temperatures found in a large boiler (1500 pounds per square inch) might be near the upper limits shown, while those in a small boiler (450 pounds per square inch) might be near the lower limits shown. Each boiler has its own particular temperature pattern. This description has been given only to define clearly the location and temperature conditions existing in different parts of the boiler.
Analytical Method
uw/
Figure 1 .
Fireside deposit areas
of the boiler, the radiant section, R in Figure 1, is roughly defined as all areas which are exposed to direct radiation from the flame. The exposed surfaces in the radiant section may be mostly tube metal-typical in newer high-pressure, high-capaci ty boilers- or they may have a relatively large proportion of exposed refractory-typical of older four-drum bent-tube boilers. Temperature differences in the radiant area are very large. Flame or gas temperatures may be as high as 3000' F., while tube surfaces in protected areas are near the saturated steam temperature, usually between 450' and 650" F. for the boilers which experience fireside deposits. When slag forms on steam generating tubes, temperature differences of 1500' F. may be present in a 1-inch layer of deposit. The tube metal may be near 500 O F. while the outer surface of the slag approaches 2000" F. Slag attached to refractory surfaces shows lower temperature differences between the inner layer attached to the refractory and the outer layer exposed to furnace radiation. The superheater section, S, includes primary and secondary superheaters and reheaters. Temperature differences in this section are somewhat less because the gases are cooler and the metal temperatures are higher than in the radiant section. Gas temperatures are in the range from 1000" to 2000' F., while tube metal temperatures are in the range from 600' to 1200' F. A few exposed metal parts such as support brackets may operate a t higher temperatures.
1932
\
Practically all exposed metal in this section is heat-resistant alloy composition. The convection section, C, includes all steam generating and downcomer tubes exposed to combustion gases but not direct furnace radiation. Gas temperatures are lower, 500' to 1500' F., and tube metal temperatures are slightly above saturation tempeature, 450 O to 650" F. The economizer section, E, includes feedwater heating equipment in the combustion gas stream. Gas temperatures range from 300' to 800' F., with metal temperatures below steam saturation temperature, 200' to 600' F. The air heater section, A , which is illustrated as a tubular design, is intended to include both tubular and regenerative types. Gas temperatures range from 300" to 600" F.,while metal temperatures range from 200 ' to 500 ' F. Thus, a rather wide range of temperaTable I. Element Aluminum Calcium Iron Magnesium Nickel Silicon Sodium Vanadium a (1).
INDUSTRIAL AND ENGINEERING CHEMISTRY
The method used by the authors' company in analyzing slag deposits resembles, in general, the standard procedure for the analysis of silicates. Precautions, however, are taken to prevent any interference of certain elements, such as boron, vanadium, and the like, Lvhich are not generally found in silicate mixtures. The loss on ignition is determined by the weight loss which takes place when a dried sample is heated to constant weight at 1600" to 1700" F. in an oxidizing atmosphere, The decrease in weight of the sample is an indication of the amount of volatile compounds present in the slag. The residue obtained from this preliminary step is then fused, and the amounts of calcium, magnesium, barium, copper, nickel, iron, aluminum, chromium, phosphorus, silicon, vanadium, and sulfur are determined by conventional analytical procedures. Manganese, sodium, chloride, carbonate, and sulfate (as SOs) present in the slag before ignition are determined from the original dried sample. The amount of sulfate found in the sample before ignition rcpresents the total sulfate in the deposit, and the difference between total sulfate and sulfate present after ignition corresponds to the amount which is volatile at or below 1600' to 1700" F. The percentages of the various elements found in the deposit are reported as oxides, although this is not necessarily the form in which they occur in the deposit.
Chemical Form of Ash-Forming Elements in Crude Oil"
Solzlbilit~ in Oil Insoluble Insoluble Soluble Insoluble
Probable Chemical Composition Complex aluminum-silicates in suspension Calcium salts in suspension or dissolved in emulsified water Possibly porphyrin complexes Magnesium salts dissolved in emulsified water or in suspension in microcrystalline state Soluble Probably porphyrin complexes Insoluble Complex silicates and sand in suspension Insoluble Largely sodium chloride dissolved in emulsified water or in suspension in microcrystalline state Soluble Probably porphyrin complexes
A D D I T I V E S I N FUELS Table II.
Element Sodium
Methods b y Which Different Elements Deposit on Boiler Surfaces Boiler Section Economizer and Air Heater Radiant Superheater Convection Gas Temp., F. 600 to $00 800 to $00 $000 to 2000 a000 to 1000 1600 to 600 T u b e Temp., F. 660 to 460 1900 to 600 660 to 460 600 to 900 600 to 900 I, A or B. Condensation or desublima- Same a s radiant I, B. Desublimation of NaCl vap- 111. Entrapment of sodium salts
tion of NaCl vapors (converts to section NazSOd 11. Stickmg of molten particles of NaCl (converts to NazSO4) Vanadium I, A or B. Condensation or desublima- Same as radiant tion of vanadium oxide vapors section 11. Sticking of molten particles of metal vanadates Nickel I, A or B. Condensation or desublima- Same as radiant tion of nickel oxide vapors (probably section forms sulfate) Reaction of SO2 and so3 vapor with Fe, Same a s radiant Sulfur Na, Ni, Ca, and Mg section Calcium 111. Entrapment of calcium oxide and/ Same as radiant or sulfate section Magnesium III. Entrapment of magnesium oxide Same as radiant and/or sulfate section Silicon
I, B.
111. Entrapment of iron oxide
I n addition to the above analyses, the total acidity as sulfuric acid and the p H value of a 1% aqueous slurry of the deposit are also determined. The former represents the theoretical amount of sulfuric acid which is equivalent to the acidity of a n aqueous slurry of the deposit. Both free sulfuric acid and acid salts such as iron sulfate contribute to the acid value. Standard sodium hydroxide
Figure 2. Distribution o f elements in fireside deposits in different sections of boiler
Same as radiant section
111. Entrapment of vanadium oxide and salts
Same as radiant section
111. Entrapment of nickel salts
Same a s radiant section
I, A.
Condensation of SO3 and
HzO vapor, forming HzSO4 Same a s radiant section
Same a s radiant section
Same a s radiant section
Same as radiant section
Same a s radiant I, B. section
Desublimation of silica
11. Sticking of molten silicates 111. Entrapment of silica and silicates Iron
ors (converts to NazSO4)
Desublimation of silica
111. Entrapment of silica and sili- 111. Entrapment of silica and si& cates cates Same a s radiant Tube corrosion product Tube corrosion product section 111. Entrapment of iron oxide 111. Entrapment of iron oxide
is used to titrate the deposit solution to the phenolphthalein end point. The p H value of a 1% water slurry of the deposit is determined with a glass-electrode p H meter. General Methods of Deposit Formation
All of the impurities in the fuel oil go through the flame, and most of them undergo some form of thermal decomposition. O n cooling they usually recombine into different compounds and collect in the temperature zones where they are stable. This idealized explanation of fireside deposit formation can be compared to the thermal cracking and fractionation of crude oil. The crude oil is heated and vaporized, then passed through successively cooler zones. Each fraction is collected in a separate temperature zone which corresponds to its condensation point. I n a typical boiler the separation of fireside deposits into separate zones is not clean-cut or sharp. There is considerable overlap from one zone to the next. This might be compared to a fractionating column which gives sharp separations when it is operated a t low capacity, but which gives a mist carry-through or carry-over of high-boiling fractions into low-boiling zones when it is overloaded. Nevertheless, the general deposit pattern
clearly shows that the basic principle underlying fireside deposit formation is thermal decomposition of the oil impurities in the flame, with reformation and deposition in the various sections of the boiler according to a definite and predictable pattern. Once this basic principle is established and understood, the exceptions and variations which occur can be recognized and explained. Deposit Formation Mechanisms
Methods of deposit formation, based on the authors’ studies and experience, are outlined below as a working hypothesis. They are offered in an endeavor to untangle a knotty problem and provide a useful framework for seemingly unrelated facts. Modifications and replacements will undoubtedly be necessary as new facts and relationships are uncovered.
I. Precipitation from the vapor state by condensation and/or desublimation (to pass from vapor to solid form without becoming liquid) A . Condensation 1. I n high gas-temperature region on varied temperature surface-e.g., condensation of liquid droplets of sodium chloride or sodium sulfate. Sodium VOL. 48, NO. 10
0
OCTOBER 1956
1933
I
n
z
"* O I nI
PH OF; 1 %
SOLUTION
OF DEPOSITS
,
Equivolent H2SO,
Volatlle SO3
I
I /
7
gzlot= Stablo L SO, L Figure 3. Variation in amount and type of sulfate in different sections of boiler
chloride converts to the sulfate according to the reaction ( 3 ) : 2NaC1 SOz '/z Oz HzO + NazSOd 2HC1 2. I n high gas-temperature region on low-temperature surface-e.g., formation of trisodium iron trisulfate, Na3Fe (SO4)3 3. I n low gas-temperature region on low-temperature surface-e.g.. SO3 H20 + H2S04 H2S04 Fe FeSOd and F e d S 0 4 ) ~ H Z B. Desublimation of crystalline substances such as vanadium compounds 11. Sticking of molten or semimolten particles to hot surfaces. This would include ash that is heated only enough to melt or sinter it before it deposits 111. Physical entrapment of solid particles on molten or sticky surfaces, or on dry irregular surfaces of deposits formed by the above methods
++ +
+
-
+
+
gases through the boiler. Three general patterns are apparent. The first is the similarity between the curves for vanadium and nickel, where, in both cases, the maximum percentage occurs in the superheater deposits. The second pattern of significance is the parallel trend of the iron and sulfur. This trend is particularly significant in the convection, economizer. and air heater sections where the large increase is due to iron sulfate. much of which is formed by the action of sulfuric acid on the iron surfaces. The third group includes the remaining elements; sodium, silicon, calcium, and magnesium. The general trend is a gradual reduction in the amount of these elements retained in the colder boiler sections. A point which is often overlooked in the study of deposit formation is the tremendous rate of heat absorption from the combustion gases. It is well to remember that the bulk of these gases is cooled 2000' F. or more during a travel time in the boiler of 1 minute or less. Certainly these conditions are ideal for the gases to become supersaturated with vaporized ash from the oil. Table I1 summarizes the most probable methods by which different elements form deposits in the various boiler sections. Of the eight elements listed. four of them-sodium. vanadium, nickel. and sulfur-appear to undergo condensation or desublimation from the vapor state in one or more of the boiler sections. Sodium and vanadium compounds, as molten or semimolten particles, also adhere to hightemperature tubes or deposits by splash sticking. Of the four remaining elements; calcium, magnesium, and silicon are held in the deposit primarily by entrapment, while iron may be held by entrapment or reaction with sulfuric acid. There are a few unusual deposits which do not appear to follow the normal patterns of formation. Figure 2 shows relatively high vanadium content in the economizer and air heater deposit as compared to the other entrapped materials. Sulfatic forms of vanadium do exist, although their presence in boiler deposits has not been established. However, it is felt that some factor other than entrapment may be responsible for this higher vanadium content. Sodium and sulfur form an unusual compound with furnace and superheater tube surfaces. The reaction product is trisodium iron trisulfate, a thin enamellike deposit which adheres tenaciously to the tubes. It forms an excellent bond between the metal and the bulky deposits which develop later. I t has not been identified in the outer portions of high-temperature deposits. Silica, when combined with alkali metals, produces some low-melting sili-
+
Table I1 is a tabulation of the methods of deposit formation which apply to the common impurities in each of the five sections of a boiler. For simplification, the reactions which occur after the deposits are in place have not been emphasized. The notations in the table correspond to the outline of methods listed above. Results and Discussion
Figure 2 shows the average deposit analysis for each of the five boiler sections, with the sections arranged in the same order as the flow of combustion
END OF SYMPOSIUM 1 934
INDUSTRIAL AND ENGINEERING CHEMISTRY
cates which stick to existing furnace and superheater deposits. The suIfate content of a radiant, superheater, or convection section deposit is higher in the portion next to the tube than at the outer surface exposed to the hot gases. This illustrates the second influence of temperature on deposit formation. With the large temperature gradient that exists across a thick deposit, the compounds making up the deposit are normally found in separate layers corresponding to the order of their temperature stability. Figure 3 shows in graphic form the variation in amount and type of sulfate in the five boiler sections. As described previously. stable sulfate is defined as the amount retained in the sample after ignition at 1600' to 1700' F. The amount of sulfate vaporized during the ignition procedure has been termed volatile sulfate. Although this separation is somewhat arbitrary, it provides useful data on the temperature stability of the compounds in the deposit. Iron sulfates, ferrous and ferric, are primarily responsible for the high volatile sulfur trioxide values in the convection, economizer. and air heater sections. Since both compounds are also acid salts. the equivalent acidity and pH curves should and do correlate closely with the volatile sulfur trioxide curve. Free sulfuric acid in a deposit has the same volatility-acidity correlation as the iron sulfates. The important feature of Figure 3 is the change in the ratio of alkali to acid throughout the boiler as shown by the equivalent sulfuric acid and pH curves. Figures 2 and 3 show a definite correlation between the location and composition of deposits in a boiler. This information is basic to a better understanding of this complicated problem. It has been used to develop chemical treatments for fuel oil which have proved beneficial in alleviating severe fireside deposits. I t is hoped that this work will contribute to a technically sound foundation upon which improved additives may be developed and used to obtain troublefree performance with all types of fuel oils. Acknowledgment
The authors wish to thank Frank P. Manlik for his assistance in organizing the mass of data used in this paper. References (1) Bowden, -4, T., Draper, P., Rowling, H., Engineer 195, 640 (1953). ( 2 ) Clarke, F. E., J . Am. SOL.,XTaval E n g r ~ . 65, 253-70 (1953). ( 3 ) Sulzer, P. T., Schweiz. Arch. f u r ahgew. Wzss. u . Tech. 20, 8 (1934).
RECEIVED for review October 17, 1955 ACCEPTED
J ~ l 27, y 1956
