Aircraft Turbine Fuel Properties Affecting Combustor Carbon 13. P. B-iRNARD ~ N D LAMOXT ELTIKGE Research Department, Standard Oil Co. (Indiana), Whiting, Ind.
111 high speed flight, maximum power output per unit of engine size and weight is essential. Jet fuel must burn in such a small space and so fast that compromises with clean combustion are inescapable. Excess air passes through the jet engine, but zones of oxygen deficiency lead to carbon formation. The same fuel properties influence combustion as in any other furnace-volatility and hydrogen to carbon ratio. Smoke points determined in a standard test lamp are a practical measure of carbon-forming tendencies. rcmft, gas turbine, or jet engine, is attractive because it delivers much more power than any other engine that, can be crowded into the same space. The need for abundant porn-er in a small, light package is all-import,a,nt in high speed military aircraft. Ir. must always be borne in mind when the problems of aircraft power plants and fuels are considered. The jet engine's high specific output (power per unit of weight or size) is essential to the attainment of sonic and higher speeds because the speed of really fast aircraft is a function of the power available per pound of weight and per square foot of frontal area. The engine of the Douglas Skyray, ivhich last fall set a speed record of 753 miles per hour, develops more than 20,000 hp. a t 750 miles per hour; yet it is about the same m i g h t and size as t.he latest and biggest aircraft piston engine, which develops only one sixth that much power. Jet engines of maximum specific output are frequently troubled by the deposition of carbon at critical locations. Carbon deposits can materially aff ect, engine performance and durability and necessitate early and frequent engine maintenance. Carbon deposition is affected by both engine design and fuel. Engine design involves compromises that are usually decided in favor of specific output ra,ther than deposition resistance. Attention must, therefore, he turned to fuel quality if carbon deposits are to be avoitid,
The combust,ors must be kept small to lrcep the engine small and light-that is, to maximize specific power. The volume available for burning fuel to release heat must t'hcrefore he minimized; only a very short t'ime can be made available for combustion. I n ot,her words, the specific heat release must be inasimized. Jet engine combustors have higher specific heat release than any other burners-I0 t'o 100 times greater than industrial furnaces and as much as 1000 times greater than home furnaccs. If home furnaces attained equally high heat releaie, the fire box would only conta.in 16 cubic inches-about the volume o€ a baseball. If sufficient combustor space were provided to complete the mixing, complete the combustion, and finally introduce the secondary cooling air, the engine would be most tolerant of fuel characteristics and the combustor carbon problem would not exist. However, such space is not available in jet engines; evaporation, mixing, combustion, and cooling intermingle and cause carbon dcposits. Therefore, the jet is not as omnivorous as was supposed vhen it was removed from security rcstrict'ions after Korld K a r 11. Some land-borne and sea-borne gas turbines of loivci specific out,put have been able to burn residual fuel successfully. EIomver, jets have never been operated sat fuels. CARBON DEPOSITS
JET ESGINE
A typical jet eiigirie is shown in Figurc 1. Air enters, is coinpressed, and passes to the combustor. Here, about 20% of it mixes with the fuel and burns it. The combustion products may reach temperatures above 3500" F. Because t,he best available materials cannot survive sust,ained temperatures above 2000" F., early cooling of the burning gases is imperative, even though some extinguishing of combust,ion may result. The remaining 807, of the air is used for this cooling. The mixture passes through the turbine, which is just large enough to drive the compressor and accessories. After leaving the turbine, t'he expand and accelerate as they pass out through the exhaust nozzle. Sewton's "equal and opposite" reaction t'o this acceleration drives the aircraft ahead. The combustor section of the cngine is detailed in Figure 2. At the head end, air enters through small holes or swirl vanes. The fuel is introduced through a spray nozzle. A spark across the electrodes of the igniter fires the mixture; once started, the combustion is self-sustaining. hlore air enters through large holes lorateti downst,reani and cools the burning gases.
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I n any burner, carbon results when hydrocarbon fuels are burned without sufficient air. Such rich combustion occurs in the burning zone of jet combustors bccause the air present is not uniformly mixed with t'he fuel. Drops of fuel or pockets of fuel vapor are surrounded by air and burning gases, and pockets of air and burning gases occur in masses of fuel vapor. If the vapor is heated enough by adjacent comhust'ion, pyrolysis taBcs place and carbon results. This carbon may deposit on the liner, dome, spray nozzle, or igniter. $1~0, fuel droplets map impinge on the liner; if so, either combustion of overrich mixtures or liquid-phase cracking causes deposition. The incidence of localized overrich mixtures depends on coinhustor design-primarily the location and size of the air entry holes. It, also depends on nozzle design and spray pattern and on engine operation. Small changes in these variables can change deposition significantly; deposit tests under apparently identical conditions give results t'hat differ by 20%. Liner surface conditions, too, have a marked effect on deposition; differences in conditioning or cleaning may change deposits as much as 30%. 4 s shown in Figure 3, deposits can cause trouble by disrupting
INDUSTRIAL AND ENGINEERING CHEMISTRY
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-Turbine FUEL FACTORS
the mixing of the fuel and air, preventing ignition of the mixture, disrupting the even cooling of the hot gases, arid causing local thermal stresses. Deposits on the fuel nozzle distort the spray pattern; those on the dome distort the flow of combustion air. In either case, they disrupt the good mixing of fuel and air and lead to poor engine performance. Deposits on the igniter short-
Carbon deposit'ion depends on fuel quality as well as on combustor design and operating conditions. Fuels affect deposition in two ways. One is by influencing the formation of locally over-rich regions where carbon is formed. The other is by differing in resistance to carbon formation under the prevailing high t,emperatures and low oxygen concentrations. The formation of overrich regions is affected by volatility and viscosity. Resistance t o coking is a function of hydrocarbon type. The effect of volatility is shown in Figure 4 (8). Relative deposits are plotted against average boiling point,. Two curves are shown, one for fuels of about 10% aromatic content, and the other for fuels containing 20% aromatics. Both curves indicate an increase in deposits with average boiling point. The separat'ion of the t,wo lines indicat'es that composition, too, has an effect on deposition. The effect of aromat,ics content,, the most important composition variable, is shown more directly in Figure 5 ($, 8, 11). This plot shows how deposits increase with aromatics content. Other worlr has shown that dicyclic aromatics cause much more deposition than monocyclic ones, whereas olefins and naphthenes cause little more deposit'ion than paraffins. The structure of paraffins affects carbon formation in bench tests ( 5 ) , but' in jet combustors the effect has not been observed. Although all tests indicate t'he same trends of increases in deposits with average boiling point and aromatics content, different engines do not necessarily rate all fuels in the same order. Further, a single combustor operated a t two different conditions can rat,e a series of fuels in a different order a t each condition. Today's engines may differ slightly from one another in fuel appetites, but they all require high quality fuels. Some burn kerosine. Others burn fuels meeting JP-3 or JP-4 specifications; JP fuels have a wider distillation range than lrerosines but a similar high burning qualky.
E X H A U S NOZZLE
Figure 1.
Jet,Engine
circuit the Lipark and prevent ignition of the fuel-air mixture. Carbon that covers the liner holes upsets the mixing of cooling sir with the burning gases and causes hot streaks in the exhaust. If fuel flow is reduced to cool these streaks below limits imposed by the material, t.he engine gives less power and is less efficient. Deposits can also cause uneven heat flow t'o the liner. Warping results, and air flow is distorted. Deposits that break loose and lodge in the turbine later burn ou) and overheat the nozzles. Several other factors influence combustor design and intensify the deposition problem. Jets operate over a wide range of pressures. Ambient pressure ranges from 1 atmosphere at sea level to less than 1/8 atmosphere at 50,000 feet. The burner must afford complete combustion at high altitude and yet not) form deposits at' sea level. Making the openings in the dome smaller and sending less air through the combustion zone favors high altitude combustion. These design changes also favor deposition, so a compromise must be reached. If both high altitude comhust,ion and deposition resistance are better than necessary, the combustor should be reduced in size so aircraft performance can he improved. The jet burner must be kept light and simple. ,4loft, there is no high pressure air or steam for atomizing fuel or blowing away soot. Such equipment would add intoleralily to engine size 2nd weight and ~rouldimpose big penalties on performance.
COMBUSTION ZONE
FUEL RATINGS
Knowing the effects of individual fuel variables on deposition not enough. A laboratory method for rating the deposition tendencies of commercial fuels is needed for defining fuel quality in specifications and for refinery control. Although any of several suggested procedures mill serve, the deposition problem is so important that every reasonable effort must be made to select or devise the best one. The practical problem in fuel rating is primarily one of choosing a method for appraising volatilityaromaticity relationships in a manner that best reflects behavior in scrvice equipment. Because fuel ratings change with combustor design and operation, a single laboratory test cannot be expected to agree perfectly with all deposit experience. To provide the best protection again3t deposition, the laboratory test should be selected on the basis of correlation with average i5
COOLIHG Z O N E
C O M W S T I O N ZONE
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Fuels-
COOLING Z O N E
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--%PUEL
Figure 2.
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Combustor Design
Figure 3.
Combustor Deposits
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ratings in current service engines operated under the most critical conditions. The folloxing five tests and factors have been suggested: Specific gravity Watson characterization factor Sational Advisory Committee on Aeronautics K factor Per cent aromatics boiling over 400" F. Smoke point
300
400
The second modification, smoke volatility index, is t,he smoke point plus 0.42 times the per cent dist'illed at' 2100" F. It' is embodied in tmhespecifications for JP-3 and J P J jet' fuels to control deposition tendency. It wae selected rather than smoke point on the basis of slightly bet,ter correlation with the results of deposition tests on JP-4 fuels. I n the few cases where fuels of widely different volatility were tested, smoke volatility index did not predict t,he results as well as smoke point. Therefore, smoke volatility index should be used caut,iously to evaluate fuels that differ significantly from JP-4 in volatility. Theoretical analyses of the pyrolysis of fuel to carbon deposits in jet combustors have not been reported. Hosever, some work has been done on the formation oi smoke in diffusion flames. These results are rrlevant because combustion in the jet, combustor and t'he lamp is similar. Both depend to some ext,ent on diffusional mixing, and, while thio mixing occurs, the fuel is subjected to high temperatures that mag cause carbon formation.
Mo
AVERAGE BOILING POINT-OF.
Figure 4. Effect of Average Boiling Point on Deposition
Bririah workers ( 2 , 8) showed that specific gravity correlated with deposition; other investigat,ors found this correlation fair but not sufficiently accurate. The T a t s o n characterization factor, which is based on the average boiling point and specific gravity, gives fair correlation. The National Advisor? Committee for A4eronauticsIi factor, which is based on hydrogen to carbon ratio and average boiling point, gives good correlation; however, accurate determination of hydrogen to carbon ratio has proved difficult,. The use of per cent aromatics boiling above 400" F. as a correlating parameter is based on combustor tests showing that dicyclic aromatics greatly increase depohition. This factor does not recognize the influence of front-end volat'ility or the differences between the deposition tendencies of aromatic compounds that boil over 400' F.; therefore, good correlation cannot be expected. Smoke point corI I 1 relates with dcposition bett,er t,han any other factor. Smoke point is the 2 height of a small difX fusion flame a t the point of incipient smoking in t,he conE ventional test lamp 4 0 5 10 15 20 25 illustrated in Figure AROMATICS C O N T E N T - % G . I n testing a fuel, Figure 5. Effect of .4romatics the flame height is Content on Deposition raised to the Doint of incipient smoking, and t,he height of the flame is observed on the scale. Fuels of good burning quality permit a high flame before smoking occurs. Two modifications of smoke point have been introduced. Smoking tendency is the reciprocal of smoke point multiplied by a factor (usually 320). Smoking tendency is used because it gives large characterization numbers for fuels that cause large amounts of deposits. It is a useful illustrative device but does not change the relationships.
SCALE
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T
FLAME HEIGHT
\-COMBUSTION CHAMBER
A i f i AND GUIDE
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'AIR
FLOW
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CANDLE
Figure 6.
S m o k e Lamp
Some invedgators have indicated that carbon particles are i'ornied in flames by dehj-drogenation of the fuel and formation of large condensed cyclic molecules ( 1 , 4, 6, 7 ) . Ahotherhas suggested tha,t' large molecules form and condense into droplets where the carbonization takes place (6). Still others have suggested that fuel molecules dehydrogenate to carbonaceous particles by other mechanisms (3, 9, I O ) . h-one of these explanations is widely accepted as complete and comprehensive enough to account for the smoking of even the simple compounds. However, a continuation of this tvpe of work and the inclusion of more complex compounds, and t)he mixtures of compounds called jet fuels, should aid underst'anding of the formation of ca,rbon deposits in jet combustors. FUTURE TRENDS
One might ask Thether high quality jet fuels will still be needed in the future, or whether combustor design can be improved t o permit the use of poorer fuels without sacrificing engine performance, Design can be improved, and it has been. The engine industry has made great strides. However, it' is always necessary to choose between reduced fuel quality and increased engine output and efficiency. Comparison of an 1-16 engine built in 1945 and a 5-33 built in 1952 illustrates the steps that have been taken in 7 gears. The 5-33 gives more than three times as much heat release per cubic
INDUSTRIAL AND ENGINEERING CHEMISTRY
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-Turbine foot of combustor as the 1-16 and has less deposition trouble with a given fuel, If it had been designed for it, the 5-33 might have burned poor fuel without encountering deposition. However, design compromises favored specific output rather than deposition resistance, so high quality fuels are still required. Similarly, the continuing need for increased specific output will probably absorb the benefit of most future improvements in combustor design, Hence, the need for high quality jet fuels will probably continue. LITERATURE CITED
(1) Arthur, J. R., Kapur, P. K., and Yapier, D. H., Nature, 169,
372 (1953).
Fuels-
( 2 ) Bass, E. L., Lubbock, I., and Williams, C. G., Shell Aviation News, 156 (1951). (3) Comerford, F. &I.,Fuel, 32, 67 (1953). (4) IIadai, D,, Ibid,, 32, 112 (1953). cHEM., 45, 602 (1953). (5) Hunt, R. A., J ~ .rZrD, , (6) Parker, W. G., and Wolfard, H. G., J. Chem. SOC., 1950, p. 2038. ( 7 ) Schalla, R. L., and RIcDonald, G. E., IXD.ENG.CHEX., 45,
-
1497, (1953).
(8) Sharp, J. G., Aircraft Eng., 23, 2 (1951). (9) Thorp, K,, Long, R., and Garner, F. H., Fuel, 30, 266 (1951). (10) Ibid.,32,116 (1953). (11) Williams, C. G., Shell Aviation S e w s , 105, 106 (1947). RECEIVED for review April 24, 1954.
AccmrEn J u n e 26, 1954.
Corrosion and Deposit in Gas Turbines B. 0. BUCKLAND General Electric Co., Schsnectady, N . Y .
Use of residual fuels in gas turbines has been limited by the fact that the ash from these fuels is often very corrosive to high temperature turbine parts and also tends to form slag deposits that reduce turbine efficiency. Vanadium and sodium are two of the most corrosive ash constituents usually encountered, and methods of inhibiting both by chemical additives have been developed. Chromium compounds, which are needed to inhibit sodium, however, augment slag formation, and methods have been developed to remove the sodium from residual fuels by washing and centrifuging. When this process is used it is only necessary to introduce an additive that prevents vanadium corrosion and at the same time forms a nonsticking ash. A water solution of magnesium sulfate, thoroughly mixed with the oil to form an emulsion, is an economical additive which is satisfactory for this purpose. ,4s a result of tests and operating experiences, a fuel specification has been prepared that defines residual fuels suitable for use in gas turbines.
c
OhlhfERCIALLY acceptable gas turbines using natural gas
fuel are a t present available. By Dee. 1, 1953, 36 General Electric gas turbines using this fuel had accumulated 140,000 hours of satisfactory operation. Three power generation units had operated 50,000 hours, and 33 gas pipeline pumping and compressor drive units had operated 90,000 hours. The usefulness and market for these prime movers could be considerably increased if they could be made to operate satisfactorily on both coal and residual fuel oil. For this reason, the General Electric Co. has undertaken the development of the residual fuel oil burning gas turbine, and this paper deals with the problems involved in using this fuel. As of December 1, there were 13 General Electric residual fuelburning gas turbines in use which had accumulated an operating time on this fuel of 50,000 hours. This service experience has shown that two basic problems exist in burning residual fuelslag-forming substances that are present in the oil corrode the metallic parts, and these same substances deposit on the nozzles and buckets, thus producing an accumulation of material in the gas path of the turbine and causing a loss in efficiency and capacity. As much as 2 tons of ash is fed through one of these machines during operation a t full load for 1000 hours. Since the first stage nozzle, for example, cannot tolerate an accumulation of more than 1 to 2 pounds of this material without a substantial
October 1954
loss of efficiency and capacity, these deposits are a source of trouble. Shortly after the failure by corrosion of the first stage nozzle of a locomotive unit while on factory test in 1948, a program of laboratory investigations was started to learn more about the nature of oil ash corrosion and its prevention. This program has continued from that time. Two basic tools that were developed for use in the study are the so-called crucible tests and the small burner tests. The results of some of the work on the crucible. tests and the small burner tests have been reported ( 3 ) and a report on corrosion experience during operation has been presented (@. Work by the General Electric Go. on the corrosion phase of the bunker C problem, was first undertaken because it was far more important than the deposit phase and affected the operation of the turbines before any substantial interference occurred due t o the ash deposition. Furthermore, it was necessary to discover the nature of the possible cures for the corrosion before an effective attack on the deposit problem could be made. Work on the corrosion phase of the problem led to the prediction that satisfactory life could be obtained by a proper adjustment of the constituents of the ash by means of additives, together with a proper choice of alloys of construction. A fuel specification defining the ash constituent relationships, together
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