Synthetic fuels and combustion

Prog. Eneroy Combust. Sci., Vol. 3, pp. 127-138, 1977. Pergamon Press. Printed in Great Britain

SYNTHETIC FUELS AND COMBUSTION J. P. LONGWELL Corporate Research Laboratories, Exxon Research and Engineering Company Linden, New Jersey 07036, USA Abstract--As the supply of hydrocarbons for transportation fuels includes an increasing proportion of low hydrogen-to-carbon ratio sources, such as coal, the cost and waste of energy of converting these materials to the high hydrogen-to-carbon ratio fuels now required by land and air propulsion systems will increase. In the extreme, where coal is the major source of liquid fuel, elimination of restrictions on aromatics content (H/C ratio) could reduce refining energy cost by as much as 20% of the heat of combustion of the syncrude being processed. Refining costs are approximately proportional to refining energy consumption, and an energy saving of this magnitude would reduce the total cost of refined products by one-third. For a syncrude product cost of 30 $/bbl, this would be a cost saving of 25 e/gal, of product. Such a large conservation and economic driving force provides a powerful incentive for choice of power plants capable of burning fuels of low hydrogen-to-carbon ratio in a clean and environmentally acceptable manner. The main combustion problem is the increasing difficulty of avoiding the emission of soot, and the relative ability of power plants to completely burn out the soot formed in the early stages of combustion will be an important selection criterion. In automotive systems, the combustion problems appear much more easily solved for the Stirling cycle and the gas turbine because of the steady flow conditions and the potentially longer time that can be provided for soot burnout. The liquid injection Diesel and stratified charge engines are at a disadvantage in this regard and may not be able to compete successfully with the Otto cycle engine, for which aromatics offer an improvement in efficiency because of their high octane number. Improvement in the ability of aircraft engines to burn highly aromatic, wide boiling range fuels offers the possibility of advances in economics and fuel conservation in air transportation. Fortunately, there is every indication that combustion research and development can be expected to eliminate the need for high levels of hydrogenation and boiling range conversion in fuels manufacture. Much more research is needed in the chemistry of soot formation and burnout, and the mechanics of reactive flows involving high molecular weight liquids and vapors and soot, for the complex systems of practical interest. In development programs, highly aromatic fuels should be used even though the economic and conservation driving force for fuel specification changes might appear well into the future. While the examples and numbers used in this discussion are based on an extreme that is indeed well into the next century, the trend toward lower hydrogen-to-carbon ratio feed stocks is already underway, and it is timely to begin moving toward less energy intensive fuels manufacture in addition to working on more thermodynamically efficient propulsion machinery.

The term "synthetic fuels" is used in the energy industry to describe fossil fuels manufactured from sources other than petroleum. The term correctly implies that major changes in chemical composition are involved in converting from, for example, coal to gasoline. Since major changes in chemical composition are involved, it can be expected that the number of options for fuel composition will increase; and since the performance of combustion equipment, especially in the transportation sector, is sensitive to fuel composition, the most efficient and economical overall system may call for changes and advances in the technology of combustion as well as in the refining of fossil fuels. In this paper an attempt is made to relate efficiency of synthetic fuel manufacture to opportunity in combustion research. Emphasis is placed on energy supply in the United States, since it is in the unique position of combining very high energy consumption, increasingly inadequate petroleum resources, and extraordinarily

large coal and oil shale resources as illustrated below: TABLE1. Approximate US remaining resources. 10~Skj

Approx. net H/C ratio

Petroleum 700 2.0 Tar sands 100 1.75 Oil shale 20,000 1.9 Coal 90,000 0.75 (Current liquid fuel consumption ~ 40 x 1015 k J/year)

While the amount of petroleum remaining worldwide is estimated to be much larger (10,000 x 1015 kJ), it is predicted x that production will reach a m a x i m u m in the late 1980s or in the 1990s. With world demand for energy expected to continue its growth, the need to turn to other resources is clear. Shale oil is, for the US, an attractive alternative, since, after denitrogenation, it produces an oil very 127

128

J.P. LONGWELL

H20 COAL OR CHAR

~

PARTIAL CO+H2 COMBUSTION[ 13600K [+H2S+NH3

H20~ BOILER H AIR

~ ASH

I H2S Nil3

H20

1

FIG. 1.

Hydrogenmanufacture.

similar to conventional petroleum and since it is projected that it can be produced at a lower cost than liquids from coal. The concentration of rich deposits in a relatively small area has led to the belief that oil shale will be limited in production rate and that concurrent production of liquids from coal will be required---even though more expensive. Although projections of the build-up of liquid fuel production from oil shale and from coal are uncertain, the large coal resources and their widely distributed occurrence throughout the world indicates that coal liquefaction will, in time, be the major source of liquid fuels. It is also probable that when this time comes, the transportation sector will be the major user of liquid fuels, since other major users such as power generation and industrial have the option of direct use of solid fuels or nuclear-generated heat and electricity. For these reasons the emphasis in this paper is placed on the conversion of coal to liquid fuels for transportation use. CONVERSION OF COAL TO LIQUID FUELS

The low hydrogen-to-carbon ratio of coal, shown in Table 1, is striking, and, since conventional liquid fuels for transportation uses average around 1.9 for the hydrogen-to-carbon ratio and since combustion characteristics are a strong function of the hydrogen-tocarbon ratio, it is clear that the manufacturing of synthetic fuels from coal revolves around increasing this ratio. The quantity of hydrogen required can be very large. For example, to convert coal to conventional refined products requires around 10,000ft 3 of hydrogen to produce one barrel (42 gal.) of liquid products. Since in the 1980s the cost of producing hydrogen from coal will be around $2.00/1000ft 3, hydrogen cost alone would be $20 for a barrel of product. While this is an extreme example, it illustrates why liquids from coal will probably cost more than twice the current cost of imported crude. Superficially, it might seem that hydrogen could be manufactured efficiently and economically by the reaction: 1.45C + 0.450 2 + 2H2Ot0 ~ 1.45CO 2 + 2H 2.

This reaction is thermally neutral (the heating value of the hydrogen formed is equal to the heating value of the starting carbon). Unfortunately limitations of reaction rate and equilibrium plus the energy required for separating nitrogen from oxygen and separating carbon dioxide from hydrogen results in a complex energy intensive process as illustrated in Fig. 1. It is seen that the temperature of the process system goes up and down several times with attendant degradation of heat. The overall result is that the heating value of the hydrogen produced is in the range of 55-60% of the heating value of the starting coal; therefore, hydrogen production can be considered quite wasteful of energy. A frequently considered alternative is electrochemical production of hydrogen from nuclear or ocean thermal power. Cost of hydrogen from these sources is currently rather poorly defined but appears to be in the range of 1.25-1.5 times the cost of production from coal. The reaction of hydrogen with low hydrogen content liquids also results in substantial energy losses. This can be illustrated by the model compound reactions shown in Table 2. In all of these reactions heat is liberated, resulting in the products having a lower heat of combustion than the starting material. The result of hydrogenating an aromatic is the loss of 29% of the heat of combustion of the hydrogen used, and the combination of hydrogenation and conversion to a lower molecular weight is a 21% loss of the hydrogen heat of combustion. The combination of hydrogen production at, say, 55% efficiency, and subsequent use for aromatics hydrogenation and hydrocracking of 79% results in an overall efficiency of 43% for the hydrogen used. While the above losses are rather basic to increasing the hydrogen content of fossil fuels, in actual synthetic fuels and refining processes a considerable effort is made to conserve energy by heat exchange between the entering and leaving streams. Additional energy is also required for separation and purification, handling and cleanup of input streams such as coal and water and of output streams such as ash, flue gas and water, so that the overall energy balance is a very complex sum of savings and losses.

Synthetic fuels and combustion

129

TABLE2. Heat of H2-hydrocarbon reactions. MJ/k mol. hydrocarbon

~)~

+3H2--. ~

210 exothermic

C~,H~,

C~,HI2

Benzene

Cyclohexane 36

+2H2_~ 2C3H s C~,HI2 Cyclohexane

~

~o Loss in heat of combustion of H2 used 29

7.5

Propane 246

[ +5H2-~2C3H s

21

C~Ho Benzene

The following table summarizes estimates of the overall thermal efficiency of several synthetic fuel processes. Shale oil is produced by simply heating the stone to around 550°C. An oil of high hydrogen-to-carbon ratio is distilled off and then hydro-treated to remove sulfur and nitrogen and to reduce the boiling range. The equipment required is relatively simple.

Methanol and paraffinic fuels by Fischer-Tropsch synthesis are manufactured by first con~eerting char or coal to a mixture of carbon monoxide and hydrogen. The efficiency of this conversion is 62~o for the Koppers Totzek process. 7 Catalytic reaction of the CO/H 2 mixture to form methanol or liquid hydrocarbons is extremely exothermic, resulting in a substantial loss in heating value of the products

TABLE3. Thermal efficiencyof conversion processes. Conversion Oil shale--shale oil Coal Hydroconversion Char --* Methanol Char --*CH 2 via Fischer-Tropsch synthesis

Liq. product H/C

Name

Reference

~ 1.9 1.27 1.53 2 2

Paraho H-Coal H-Coal ---

(5) (4) (5) (20)

Use of hydrogen in coal liquefaction gives high yields of liquid. The H-coal process, which is used as an example here, produces 35-50 wt.~o on coal while a pyrolysis process such as COED produces 15-25 wt.~o of coal 6 (a high quality oil shale produces 12 wt.~o). In Table 3 two efficiencies are shown for coal liquefaction. The first corresponds to a liquid of H/C = 1.27 produced by direct distillation of the product from the coal hydrogenation reactor and consuming 0.85m 3 of hydrogen/kg of product hydrogen per barrel of producL The second example with an H/C of 1.53 is produced by additional cracking and hydrogenation of the distilled product and consumes an additional 0.75 m3/kg, bringing the total to 1.6 ma/kg. This latter hydrocracking step can be logically considered part of the refining operation. The products of pyrolysis processes, while lower in yield, have a generally similar composition.

Thermal eft. 80 65-75 55-65 (Est.) 40-50 40-50

compared with the CO/H 2 starting material ; however, much of this heat can be usefully employed in the process itself, so the overall efficiency starting with coal appears to fall in the 40-50~o range. If a Fischer-Tropsch or methanol process is combined with methane manufacture, considerable savings in energy can be realized by first producing methane from coal pyrolysis and producing the necessary CO/H 2 mixture from the remaining char. 5 Here the overall efficiency is estimated to be 65~. Since an alternate disposition for the char is combustion for boiler or industrial fuel, the ~ 50~o efficiency for the char-toliquids conversion should be used. Since further refining of coal syncrude has a thermal efficiency of ~ 80Vo, the overall efficiency to specification fuel is also around 50~. It can be said, therefore, that the overall efficiency of producing methanol or Fischer-Tropsch liquids is comparable to the efficiency of producing paraffinic

130

J.P. LONGWELL

specification fuels from the aromatics compounds in coal, but that there are substantial energy, investment and manufacturing cost savings potentially available if it were possible to make direct use of the low hydrogen-to-carbon ratio liquids produced from coal. Since production of liquids from coal offers the possibility of energy and monetary economy by choice of fuel composition, it is of interest to consider the process of coal liquefaction and the nature of its products in more detail. Figure 2 shows a block diagram of a generalized coal liquefaction process.

purposes. Hydrogen consumption of these processes varies over a wide range. Simple pyrolysis requires no hy,drogen for liquid production ; however, liquid production is limited, and the hydrogen-to-carbon ratio is low while the high liquid production processes, including product hydrogenation, may consume as much as 8000 cubic feet of hydrogen per barrel of product, so that a high price in hydrogen requirement is paid for the increase in liquid yield and quality. If the logistic and institutional problems involved could be solved, it would, however, appear advantageous to skim off the DI STILLABLE I LIQUIDS AND GAS

(PRODUCT )

H/C-1.27 HIck3

I

H 2 TO REACTOR

II-

HYDROGEN HYDROGEN REACTION

COAL H/C=.75

SEPARATION

PRODUCTION CO 2

HEAT

HEAT

SOLIDS SOLVENT

SOLVENT

I

P(~4ER

_ AIR N2 < ~ SEPARATIONI

lAIR FIG. 2. Generalized coal liquefaction process. Coal is heated in the presence of hydrogen and, in some cases, a solvent. At this stage a varying fraction of the organic matter in coal can be liquefied. If the solids are filtered out and pyrolysis kept to a minimum, a very high boiling tar is produced as in the solventrefined coal process. For synthetic crude production, the liquid products are distilled out, and in this process carbonization of part of the organic matter occurs. The hydrogen, pressure, temperature and time employed, plus the effect of catalysts and solvents that might be used, determines the yield of distillable liquids which, as previously mentioned, vary over the range of 15 wt.~o for low pressure pyrolysis to around 50 wt.~o for a process operating with high hydrogenation intensity such as Synthoil or H-coal. Gas, consisting mostly of methane, which has a very high hydrogen content is also produced. The undistillable heavy tars or coke can be used to produce the hydrogen required for this process. If these products are in surplus as in the case of less intense pyrolysis processes, they can be used as boiler fuel for power generation or other

liquids by pyrolysis from all coal used and to use the remaining char as boiler fuel. In Fig. 3 the boiling range of examples of products produced from the H-coal process are compared with a good quality petroleum crude oil (Arab light). The petroleum is fairly evenly distributed in boiling point from 40 to 550°C. About 18~o boils above 570°C and is considered undistillable. In the "synthetic crude" the undistillable residue has been left behind in the process to be further converted to hydrogen or coke and distillable liquids. The low H/C (1.27) syncrude has a distribution somewhat similar, while higher boiling components in the more highly refined syncrude (H/C = 1.64) have been eliminated to produce a fuel containing mostly material in the gasoline and diesel fuel boiling range. Fuels used for mobile equipment are characterized by their boiling ranges. As shown in Table 4 the gasoline boiling range varies from slightly above room temperature to around 220°C. This range is primarily set by the requirement for adequate evaporation in the

TABLE4. Petroleum product yields Fuel Gasoline Jet A Diesel oil

Boiling temp. °C

Yield, vol. ~o Arab light

~ 50-220 ~ 180 290 ~ 180 340

42 14 30

H-coal product H/C 1.27 H/C 1.67 30 30 45

50 45 60

Synthetic fuels and combustion

131

500 ARAB LIGHT CRUDE

~AW H-COAL PRODUCT H/C-1.27

400

300

H=COAL SYNCRUDE

B/c=t.64

200

100

[

I

I

I

I

I

I

I

I

I

I

0

10

20

30

40

50

60

70

80

90

100

VOLU/~ PERCENT DISTILLED

FIG. 3. Distillation curves for petroleum and syncrude. carburetor and intake manifold, the light components being required for ease of starting in cold weather and the maximum boiling point by the ability to essentially complete evaporation before ignition. In aviation and diesel fuels the maximum boiling point is primarily set by the requirement that the fuel does not freeze in cold weather, while in the case of jet fuel the initial boiling point is set by the safety requirement that the volatility be sufficiently low that the vapors formed above the liquid not be within the limits of inflammability under normal storage and operating conditions. Petroleum is seen to be an excellent source of hydrocarbons in the gasoline boiling range. In fact, until recently there has been a worldwide surplus of light hydrocarbons--partly due to the composition of petroleum and natural gas liquid and partly due to the large use of high boiling components for nontransportation uses such as industrial heating and power generation. While the synthetic crudes would appear to be an excellent source of Jet A and other distillate fuels, it must be remembered that they are higher in aromatic content (40-80~o), and that current types of engines would not tolerate such fuels. Modern petroleum refining technology is, however, capable of adjusting the boiling range of liquid hydrocarbons by means of refining processes such as hydrocracking and catalytic cracking, while aromatic content can be reduced by hydrogenation. These processes inevitably produce gas and/or coke as byproducts and consume hydrogen to reduce aromatic content and to form lower molecular weight products. The energy consumed in refining varies over a wide range, as shown in Table 5. If only distillation, moderate octane improvement for gasoline and partial desulfurization are required to convert crude petroleum to the required distribution

of products, energy consumption approaching a minimum of 3~o is possible. While such refineries match the needs in some parts of the world, refineries in the United States, because of high gasoline requirement relative to fuel oil, must carry out a substantially greater degree of boiling range conversion and octane number improvement with the result that energy consumption is in the 8-127/o range. The increased cost of fuel for refinery energy has stimulated a concerted effort to reduce energy consumption, and it is expected that a greater than 15~o reduction in energy loss will be TABLE5. Refinery energy consumption.

Type of refinery

Energy consumption-~o of heating value of oil refined

Petroleum low conversion Petroleum US high conversion Synthetic coal liquid-high conversion (H/C = 1.25) (H/C = 1.5) Shale oil--hydrogenated (H/C = 1.9)

3+ 8-12 28-31 18-21 11-15

attained by improved design and conservation practices. Further improvements are possible in use of low level heat and a higher degree of heat integration; however, these will require substantial capital investment and will come slowly and must be justified by the economics of fuel savings. Refining energy requirements starting with hydrogenated syncrude (H/C ~ 1.52 for H-coal and 1.9 for shale oil) were calculated from a refining study reported in reference (5). Some additional detail for the efficiencies reported in Table 5 are shown in Table 6.

132

J.P. LONGWELL TABLE6. Refiningenergy requirements Feed

Gasoline

Products--vol. % Distillates

Other

Energy loss % of HV of feed

63 93 66 87

32 -27 --

5 7 7 13

18 21 I1 15

H-Coal liquids H/C = 1.52 Shale oil H/C = 1.9

The energy requirement is a function of the degree of conversion to gasoline; however, the coal syncrude requires about 7% more energy than shale oil for a similar product distribution. The coal syncrude used in this study had been previously hydrogenated from H/C = 1.27 to 1.52 which would require roughly 10% additional energy, bringing the total refining energy cost to the 28-31 range. This is approximately 20% higher than for a refinery starting with Arab light crude and producing a similar set of products, and the total loss of around 30% is large enough to deserve serious consideration of ways of avoiding the hydrogen-consuming conversion processes. It seems necessary, however, to remove sulfur and fuel nitrogen. This might be done for around 10% energy loss, so that the remaining 20% loss can be attributed to the requirement for reduction of boiling range and aromatics content. This 20% represents an extreme number, since some reduction could be expected by blending products from shale and petroleum and since complete relaxation of boiling range and aromatic content requirements is probably unrealistic; however, it does give a measure of the magnitude of energy loss that might be avoided by modification of fuel composition in future transportation systems. TRANSPORTATION FUEL REQUIREMENTS

Table 7 shows the ways in which petroleum was consumed in 1974 in the United States. TABLE7. Liquid hydrocarbon consumption (petroleum) Use Power generation Industrial Commercial and residential Transportation Automotive Aircraft Other Petrochemical and other Total

l0 6 bbl/day 1.5 1.8 2.4 8.9

% 9 11 15 54

(6.5) (1.0) (1.4) 1.9

11

16.5

100

When petroleum has become sufficiently scarce and expensive so that a major fraction of liquid hydrocarbons come from coal and oil shale, it is expected that a substantial reduction in power generation and industrial use will result, since direct use of coal and nuclear heat will offer viable options. In commercial and home heating, use of electricity plus heat pumps and solar heating or synthetic gas offer options which should in time substantially reduce the need for liquid

fuels. Petrochemical use of paraffinic hydrocarbons to produce plastics, rubber and a great variety of other products is expected to grow relative to other uses and will place increasing stress on the high hydrogen-tocarbon ratio components which are the starting point for the bulk of these products. Transportation is expected, however, to continue to depend primarily on liquid fuels for the time period of interest here. As time goes on, it will become an increasing fraction of the total with use as petrochemical feed stock also growing and requiring an increasing portion of the high quality fractions of the available hydrocarbons. Within the transportation sector, automotive use currently accounts for about 73% of the total. Aircraft fuels account for 12%, and the remainder are consumed by truck, rail, etc. Projections of the growth of ground and air transportation fuel requirements generally show a continuation of the higher growth rate for air transportation. While both growth rates are expected to be lower in the future, it is predicted that air transportation growth will exceed the rate of ground transportation fuel consumption growth. Projections vary from aircraft consuming 15% of transportation fuels in the 1990s to as much as 30% by the year 2000. 9 Because of the growth potential for aircraft fuels and because the problems involved in their production and use typify many of the problems and opportunities in the supply of future fuels, it is of interest to examine aircraft fuels in more detail. The major jet fuel specifications from a manufacturing point of view are shown in Table 8. TABLE8. Aviation turbine fuel specifications.

Flash point (°C) Ried vapor pressure (kPa) Distillation 10~o 50% 90% Final BP Freezing point (°C) Vol. % aromatics

Jet A(,)

j.p-4 ~b~

Min. 40 --

14-20

Max. Max. Max. Max. Max. Max.

200 230 280 300 - 40 20

-190 240 - 58 25

t"IASTM D 1655-71. tb~WAF MIL-T-56244.

The Jet A specification is typical of kerosene-type jet fuel used throughout the world. The flash point of > 40°C has been shown to result in some improvement in crash and ground handling safety ~° over the more volatile J.P.-4, which has been primarily used in military aircraft and to some extent in commercial

Synthetic luels and combustion airlines outside the United States. A major advantage of the J.P.-4 type fuel lies in the greater fraction of natural petroleum that can be included which increases potential supply--an important military consideration. In the past it has also been cheaper due to a general oversupply of the low boiling naphthas which are included in J.P.-4 but excluded from Jet A. In the future, the supply advantage of including volatile components is expected to diminish, since tars, coal liquids and shale oil contain relatively little of these components. Another advantage of including light components is the greater ease of meeting the freezing point specifications which limit the inclusion of high boiling components. Since future fuel supplies tend to be rich in the higher boiling components and since the need for stringent freezing point control can, in principle, be technologically eliminated by tank insulation and heating, relaxation of this requirement in the long term appears logical from a cost supply and refining efficiency point of view. If such a change occurs, the gas turbine combustion systems would have to burn higher molecular weight fuels than they are currently designed for. The restriction of 207o aromatics is of particular interest, since liquids produced from coal are very high in aromatics. As shown in Table 3, the raw coal liquids have a hydrogen-to-carbon ratio of 1.27-1.53 which corresponds to 50-80~o aromatics. Clearly, large amounts of hydrogen must be added to meet a 20~o aromatics specification. The percent aromatics specification was set to avoid a series of combustion-related problems that were encountered with fuels containing higher than the specification amounts. These problems are: 1. Overheating of the combustion chamber from flame radiation. 2. Smoke formation under high power conditions. 3. Formation of solid carbonaceous deposits which caused fuel spray distortion and turbine damage when deposits become detached. These effects have correlated quite well with hydrogen-to-carbon ratio. Recent work at NASA x' reports results with a variety of fuels seen in a single JT8D combustor. Figure 4 shows the correlation of smoke number with hydrogen-to-carbon ratio. Smoke 50 %0

40

\.

3020-

I0o

I 1.4

I 1.6

I 1.8

I 2.0

2.2

H/C RATIO FIG. 4. S m o k e formation in a J T 8 D combustor.

133

number is determined by drawing a given volume of exhaust gas through a filter paper. A dark disk is formed which is assigned a number according to the darkness (SAE/ARP 1179). The pronounced rise in smoke number with reduced hydrogen content is striking. A similar correlation of combustion liner temperature is shown in Fig. 5 from the same series of tests. The pronounced rise in liner temperature results from increased flame radiation resulting from the formation of larger numbers of carbon particles as hydroben content is decreased. o

i

1200

ii00

•

1ooo

;

.

900

800

I

I

I

I

I

1.4

1.6

1.8

2.0

2.2

HYDROGEN/CARBON RATIO

FIG. 5. Liner temperature in a JT8D combustor (cruise conditionsl. These correlations are, of course, a function of the particular combustion system design which must be empirically determined, since adequate predictive techniques for the complex set of phenomena involved do not, at present, exist. From the point of view of relating fuel composition to performance, the success of hydrogen-to-carbon ratio in correlating results is a significant simplification. It is, of course, related to fuel composition. Jet fuels can be considered to be made up of a mixture of a more limited number of basic classes of compounds such as those shown in Table 9. It is seen that on the basis of hydrogen-to-carbon ratio one would predict that naphthalenes would be significantly more difficult to burn, and indeed this is found in laboratory tests of pure compounds. It can be expected that differences among the structural variations possible within these compound types will occur that are not accounted for by simple hydrogento-carbon ratio; however, hydrogen-to-carbon ratio appears to be a good first order measure of burning quality as well as of energy efficiency and cost in manufacturing fuels from a material such as coal. The aviation gas turbine combustor and its fuel requirements can be considered as an example of the type of system that would be required for ground transportation gas turbines and for the combustion fuel system needed for a Stirling cycle engine in that it represents a continuous combustion system that must operate over a wide range of heat output and atmospheric conditions. With the possible exception of more stringent emissions control and problems due to more frequent changes of output, the requirements for ground transportation appear less demanding in that

134

J.P. LONGWELL TABLE9. Major hydrocarbon types. Type

Typical structure C C I I C--C--C--C--C--C--C--C--C~

Paraffin Mono cyclo paraffin

C C C

Dicyclo paraffin

C I C

\ / C

C

CnH2n+2

2.1

C~H 2n

2.0

Cn H2n - 2

1.83

C.H2._ ~

1.5

C, H 2. - J2

1.0

C

c d'\ %/ C

c I C--C--C--C--C II I C C

C C / / \ / % C C C-~C--C C

H/C for n = 12

C--C--C~C--C

C C / \ / \ C C C--C--C I I I C C C \ / \ / C C

Alkyl benzene

Alkyl naphthalene

C

Empirical formula

C % / \ / / c c

C

the range of atmospheric pressure and temperature is more limited and the penalties for flame-out or inability to relight are less severe. These new types of ground transportation systems seem to offer an excellent opportunity to design the combustors to operate satisfactorily over a wide range of fuel composition and volatility. Automotive fuel consumption will require an increasing fraction of total liquid fuels as nontransportation uses diminish with time. Efficiency and fuel requirements of engines for these systems are receiving considerable study, since higher thermal efficiency alternates to the conventional spark ignition engine could be developed. The following table of projected fuel economy for 1400 kg (compact) cars of equivalent performance 12 is illustrative. The performance of these vehicles has been estimated at the CIT Jet Propulsion Laboratory for TABLE 10. VEHICLE FUEL ECONOMY.

Urban driving cycle (Compact car--mature technology) Engine type Miles/gal-gasoline equivalent Otto--compression ratio 8.0 (compression ratio 12.0) Diesel--compression ratio 15.0 Gas turbine Stirling

18.3 (21) 20.7 20.8 26.3

"mature" technology which makes use of projected advances in technology, but stops short of, for example, ceramic high temperature components which correspond to their "advanced" technology case. Resuits for the Urban Federal Driving Cycle are shown. Comparative mileage is on a heating value basis, since gasoline is assumed for fuel in all cases. The standard Otto cycle engine with a compression ratio of 8.0 is distinctly lowest in system efficiency; however, use of high octane fuels with a high compression ratio could improve efficiency by approximately 15~. This higher performance of the Otto engine matches or exceeds the Diesel (or stratified charge engines) in spite of their somewhat higher thermodynamic efficiency because of the lower engine weight for the same performance. The comparative Diesel performance is also reduced below that popularly anticipated by demanding the same acceleration performance for all engines and omitting the normal ~ 10~o volumetric heating value advantage of Diesel fuel versus gasoline. The "mature" gas turbine is projected to have the same, or higher, mileage than the Diesel engine although this has yet to be demonstrated in production systems. The Stirling engine is projected to have a very high system efficiency. There is some disagreement over this projection; however, the projections range from matching Diesel performance to the very high performance shown. Since differences in refining energy losses between hydrocarbon fuels can range up to

Synthetic fuels and combustion

135

expected to be a problem with these stratified charge engines. On the other hand, burnout of carbon is less of a problem in continuous flow engines such as the gas turbine and the Stirling cycle engine, thus potentially reducing their requirements for highly hydrogenated fuel and increasing their potential advantage over intermittent combustion engines. For the spark ignition engine, the higher boiling range of future fuels will tend to require increased boiling range conversion to supply conventional gasoline. In this case, however, the higher aromatic content of these fuels will tend to increase the octane number of the gasoline produced. Since octane number is a measure of the compression ratio at which knock occurs, the higher the octane number, the higher the compression ratio and therefore the higher the cycle efficiency; for a compression ratio of 12, an octane number of ~ 105 is required. Selected octane number data 21 are shown below.

20%, it is apparent that, when these losses are taken into account, the relative overall efficiency of these engines could very well change, because they have quite different fuel requirements. A number of studies have been published 5'13'~4 in which the effect of refining efficiency along with projected engine efficiency is taken into account for refining situations where the use of high quality petroleum feed stocks is continued. In such a refining system, production of distillate (Diesel)fuels requires mainly distillation and relatively little boiling range conversion; whereas gasoline production requires boiling range conversion plus reforming of some of the components to increase octane number by converting naphthenes and paraffins to aromatics. The overall result is that refining efficiency is improved by increasing the proportion of "distillate" fuels produced. Since all the engines described in Table 10, with the exception of the spark ignition engine, can use "distillate"

TABLEll. Typical octane number of gasoline components.

Component Current clear gasoline Light petroleum naphtha Aromatics Toluene p-Xylene n-Butyl Benzene l, 4 Di-ethyl Benzene 1 Methyl Naphthalene Methanol Methanol + 10% water

Motor octane number

Research octane number

Boiling range, °C

83 65

91 65

50-220 65-150

112 124 116 138 114 87.4 92.8

124 145 114 151 123 110 114

110 138 183 184 240 64

fuels, it is pointed out that this improvement in refining efficiency augments the inherently higher efficiency of the engines to widen the efficiency gap. The situation is not necessarily the same, however, when refining coal liquids is considered; matching the properties of current paraffinic distillates requires, as in the case of jet fuels, substantial consumption of hydrogen to increase the hydrogen-to-carbon ratio by aromatic hydrogenation and to reduce the boiling range by hydrocracking. On the other hand, if lower hydrogento-carbon fuels could be accepted, the advantage of high efficiency could, at least partially, be retained. In the case of the compressed ignition automotive Diesel engine, the fuel must be highly paraffinic, since aromatics ignite poorly in this type of system and greatly increase the tendency toward smoking. For this reason, the Diesel engine would seem to be at an increasing disadvantage in the future if it is assumed that these combustion problems cannot be solved. Combustion of hydrocarbons in the form of liquid drops will generally produce carbon particulates. Since reduction of hydrogen-to-carbon ratio greatly increases this problem and since carbon burnout is difficult in the intermittent combustion of reciprocating engines, formation of carbon particulates is also

The pure hydrocarbon compound data are the blending octane number in a 40/60 n-heptaneisooctane base stock. Aromatic compounds are clearly very high octane number components; however, current boiling range limitations exclude the fused ring aromatics such as the naphthalenes. The possibility seems to exist for developing aromatic high octane fuels; however, a special fuel vaporization system would be required to handle the higher boiling fuels. Methanol-water blends offer a significant performance advantage by combining high compression ratio operation and low NO x emissions due to reduced flame temperature, 22 and it appears that a spark ignition engine optimized around this fuel could compete with the Diesel engine in efficiency. Methanol is, however, a high hydrogen-to-carbon ~atio fuel, and its manufacture results in energy losses such that there appears to be little or no net gain in overall thermal efficiency over manufacture of gasoline when coal is the common starting material. In summary, the relative fuel/engine system efficiency for both air and ground transportation is expected to be strongly influenced by the ability of an engine to burn low hydrogen-to-carbon ratio aromatic

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fuels without formation of carbon particulates, engine deposits, or maintenance problems. For ground transportation, the gas turbine and Stirling cycle engines seem particularly favored with Diesel and injected stratified charge engines at an increasing disadvantage unless the problem of increasing particulates with decreasing hydrogen-to-carbon ratio can be overcome. Since none of these systems are presently developed for such fuels and since basic or even empirical understanding of the requirements for clean burning is lacking, there is a major opportunity for contribution by combustion research and development to resource conservation.

laminar flat flame burner, and the lower curve from an intensely back-mixed jet-stirred reactor. For toluene, the stoichiometric O/C ratio is 2.57 ; and it is seen that for a vaporized toluene/air mixture, it is possible to go as low as O/C = 1.72 before carbon is formed, and that below that point the amount of carbon produced increases rapidly. Intense mixing offers a wider range of carbon-free combustion than does a laminar flame. C.

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C O M B U S T I O N RESEARCH

The major combustion problem related to lowering the hydrogen-to-carbon ratio is the greatly increased tendency to form soot. This statement is based on the assumption that the future will call for reduced emissions of soot and that increases would not be acceptable. At present, regulation of soot formation is sketchy and the public health problems related to soot have been only partially identified. Limited studies of soot composition have shown extreme variability depending both on fuel composition and on the conditions of combustion. Of greatest interest are the extractable polynuclear aromatics, some of which are known carcinogens. The quantity of extractable materials varies from essentially zero for soots that have been severely thermally treated to a few percent for others; however, quantitative information on the variables that determine the amount of extractable material are extremely limited. It is expected that fuel composition will have a pronounced effect. Formation of soot is thought to occur through several mechanisms. The most commonly considered is initial formation of acetylene followed by condensation. Direct polymerization of polynuclear aromatic components of fuel is also known to occur and is expected to be of importance for high boiling low hydrogen-tocarbon fuels. If a mixture of vaporized fuel and air is burned in a Bunsen burner with the stoichiometric amount of air, a non-luminous flame is formed; however, as the amount of air is reduced to below stoichiometric, a point is reached where carbon particles are formed, and the flame emits the black body radiation characteristics of a luminous flame. As air is reduced further, unburned soot can be found in the combustion products. This is illustrated in Fig. 6 where soot formation is shown as a function of the oxygen/carbon ratio for toluene. The upper curve shows results from a

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FIG. 6. Soot formation in a toluene-air flame. Thermodynamic calculations show that, at these high temperatures, carbon would not be formed at equilibrium at values of O/C appreciably greater than 1.0. The observed formation of carbon is, then, a kinetic effect. Thomas 16 suggests that the combustion mechanism is such that greater than equilibrium amounts of CO2 and H 2 0 are formed early in the combustion sequence, and, since the rate of reaction of carbon and carbon precursors with CO2 and H 2 0 is relatively slow, time is available for polymerization to form soot particles. The effect of back mixing is interpreted as an indication that the polymerization process to form soot is a higher order process than the fuel oxidation process.15 Table 12 below shows further data on two additional fuels. 17 It is seen that in a laminar flame methyl naphthalene, H/C = 0.9, forms carbon with mixtures only slightly richer than stoichiometric. Intense backmixing, however, allows operation at an equivalence ratio as rich as 1.5. Toluene appears to be not much worse than the higher hydrogen-to-carbon ratio hexane. These data show that it is basically possible to burn low hydrogen-to-carbon fuels without flame luminosity or soot by proper mixture preparation and with sufficient mixing intensity in the combustion zone. With the exception of the spark ignition Otto cycle engine, none of the major liquid fuel combustion systems vaporize and pre-mix the fuel and air. Instead,

TABLE12. Stoichiometry for carbon formation.

Fuel Toluene CvHa Methyl naphthalene C 11H1o

Equivalenceratio for incipient carbon formation Well-stirred H/C O/C = 1 Flat flame reactor 1.14 0.91

2.57 2.45

1.34 1.03

1.50 1.51

Synthetic fuels and combustion the fuel is dispersed in the form of small drops, and a droplet-air mixture is burned. A luminous flame invariably results, since even an evaporated drop will leave a fuel rich zone, and if the burning zone envelops the liquid drop, a very smoky diffusion flame can result, as discussed by Sjogren. 18 Figure 7 illustrates two models of droplet combustion which are dependent on the relative velocity between fuel and air.

~

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f

1

AIRDROP

137

The Otto cycle engine which in principle vaporizes its fuel before ignition is not faced with the same problems ; however, if vaporization is incomplete and large fuel drops are present, soot is formed. This requirement for vaporization before ignition, limits the boiling range of the fuel and prevents going to very high aromatic content because of their high boiling points, thereby limiting the fuel octane number and the compression ratio of the engine. In all of these propulsion systems, fuel vaporization or very fine atomization and pre-mixing with air seems called for. Fuel vaporization is, in fact, of considerable current interest in gas turbine combustion systems, although the practical advantages of droplet combustion systems make development of improved systems capable of handling low hydrogen-to-carbon ratio fuels attractive. While a substantial literature on soot formation and burnout exists, the closely coupled trade-offs between formation of soot and NOx, combustion operating range, fuel composition and many other factors makes optimized design an exceedingly complex problem for which much of the needed basic data on the chemistry of carbon formation, carbon burnout kinetics, droplet combustion for small drops and turbulent flow are lacking, particularly for low hydrogen-to-carbon ratio fuels.

RELATIVE VELOCITY CONCLUSIONS

FIG. 7. Types of droplet combustion. When mixing of the total combustion air with fuel is completed, soot will oxidize to CO2 if sufficient time at high temperature is available. In large boilers and industrial furnaces the cooling rate of the combustion products slows, and, even with combustion of coarsely atomized, heavy aromatic tars with substantial initial soot formation, soot burnout is attainable. In transportation systems, however, the need for compactness results in very rapid cooling rates, and soot burnout is more difficult to achieve. As previously discussed, current aviation gas turbines quench the combustion gases so rapidly that it is difficult, even when aromatic content of fuel is restricted to 20%, special care must be taken to achieve low smoke combustion. 19 Similar, though less severe, problems can be expected in designing burners for automotive gas turbines. For intermittent fuel injection combustion systems like the Diesel and fuel-injected stratified charge engines, fuelrich combustion zones and very short times at high temperature are inherent in their design, so that some soot particles are emitted. One would expect that the low rpm Diesels used in ships, rail locomotives, etc., would show some improvement, since more time is allowed for burnout, and this effect shows up in their ability to successfully use less refined grades of diesel fuel. It should be noted that a longer time at high temperature, while excellent for destruction of soot, CO and unburned hydrocarbons, will generally result in increased production of nitric oxide.

As the supply of hydrocarbons for transportation fuels includes an increasing proportion of low hydrogen-to-carbon ratio sources, such as coal, the cost and waste of energy of converting these materials to the high hydrogen-to-carbon ratio fuels now required by land and air propulsion systems will increase. In the extreme, where coal is the major source of liquid fuel, elimination of restrictions on aromatics content (H/C ratio) could reduce refining energy cost by as much as 20% of the heat of combustion of the syncrude being processed. Refining costs are approximately proportional to refining energy consumption, and an energy saving of this magnitude would reduce the total cost of refined products by one-third. For a syncrude product cost of 30 $/bbl, this would be a cost saving of 25 ¢/gal of product. Such a large conservation and economic driving force provides a powerful incentive for choice of power plants capable of burning fuels of low hydrogen-tocarbon ratio in a clean and environmentally acceptable manner. The main combustion problem is the increasing difficulty of avoiding the emission of soot, and the relative ability of power plants to completely burn out the soot formed in the early stages of combustion will be an important selection criterion. In automotive systems, the combustion problems appear much more easily solved'for the Stirling cycle and the gas turbine because of the steady flow conditions and the potentially longer time that can be provided for soot burnout. The liquid injection Diesel

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and stratified charge engines are at a disadvantage in this regard and may not be able to compete successfully with the Otto cycle engine, for which aromatics offer an improvement in efficiency because of their high octane number. Improvement in the ability of aircraft engines to burn highly aromatic, wide boiling range fuels offers the possibility of advances in economics and fuel conservation in air transportation. Fortunately, there is every indication that combustion research and development can be expected to eliminate the need for high levels of hydrogenation and boiling range conversion in fuel manufacture. Much more research is needed in the chemistry of soot formation and burnout, and the mechanics of reactive flows involving high molecular weight liquids and vapors and soot, for the complex systems of practical interest. In development programs, highly aromatic fuels should be used even though the economic and conservation driving force for fuel specification changes might appear well into the future. While the examples and numbers used in this discussion are based on an extreme that is indeed well into the next century, the trend toward lower hydrogen-to-carbon ratio feed stocks is already under way and it is timely to begin moving toward less energy intensive fuel manufacture in addition to working on more thermodynamically efficient propulsion machinery. REFERENCES

1. MOODY,J. D. and ESSER,R. W., An estimate of the world's recoverable crude oil resources, World Petroleum Congress (1975). 2. HOTTEL,H. C. and HOWARD,J. B., New Energy Technology, MIT Press (1972). 3. KALFADELIS,C. D. and MAGEE, E. M., Evaluation of pollution control in fossil fuel conversion processesCOED process, EPA 650/2-74-009e (1974). 4. JAHNIG,C. E., Evaluation of pollution control in fossil fuel conversion processes--H coal process, EPA-650/274-0090m (1974).

5. KANT,F. H. et al., Feasibility study of alternate fuels for automotive transportation. Vol. II and III, EPA-460/374-009b and c (1974). 6. COCHRANE,NEIL P., Oil and gas from coal, Scientific" American 234, No. 3 (1976). 7. MAGEE,E. M., JAHNIG,C. E. and SHAW,H., Evaluation of pollution control in fossil fuel conversion processes-Koppers Totzek, EPA-650/2-74-009a (1974). 8. NATIONAL ENERGY OUTLOOK--1976, Federal Energy Administration 041-018-0097-6 (1976). 9. MASAY,A. C. and WILLIAMS,L. J., Air transportation energy consumption: yesterday, today and tomorrow, AIAA 75-319 (1975). 10. AVIATIONFUELSAFETY,CRC Report No. 482 (1976). 11. BUTZE,H. F. and EHLERS,R. C., Effect of fuel properties on performance of a single aircraft turbojet combustor, NASA Technical Memorandum NASA TM-71789 (1976). 12. Should we have a new engine? An automotive power systems evaluation, JPL SP 43-17 (1975). 13. BROWN,E. C., CORNER,E. S. and COMPTON,R. H,, New look at auto-fuel economy vs refining, Oil and Gas Journal 125 (1975). 14. TURNEY,W. T., JOHNSON,E. M. and CRAWFORD,H. R., Energy conservation of the vehicle-fuel refinery system, SAE-750673 (1975). 15. WRIGHT, F. J., Carbon formation under well stirred conditions. Part II, Combust. Flame 15, 217-222 (1970). 16. STREET,J. C. and THOMAS,A., Carbon formation in premixed flames, Fuel 34, 4 (1955). 17. WRIGHT, F. J., Carbon formation under well stirred conditions, Twelfth Symposium (International) on Combustion, p. 867, The Combustion Institute, Pittsburgh (1968). 18. S~OGREN, A., Soot formation by combustion of an atomized liquid fuel, 'Fourteenth Symposium (International) on Combustion, p. 919, The Combustion Institute, Pittsburgh (1972). 19. FAITANI,J. J., Smoke reduction in jet engines through burner design, SAE-680348 (1968). 20. VYOS, K. C. and BODLE, W. W., Coal and oil shale conversion looks better, Oil Gas J., 45 (1975). 21. Knocking characteristics of pure hydrocarbons, ASTM Special Technical Publication No. 225 (1958). 22. MOST,W. J. and LONGWELL,J. P:, Single-cylinder engine evaluation of methanol, SAE-750119 (1975).

M a n u s c r i p t received 10 M a y 1977