Gas turbine engine emissions—Problems, progress and future

Pergamon Presb Lid I qT~,

Printed ID Great Britai~

GAS TURBINE ENGINE EMISSIONS--PROBLEMS. PROGRESS AND FUTURE ROBERT E. JONES Head. Experimental Comhustor Section, National Aeronmttics and Space Administrati(m. I_z,wis Research Center. Cleveland, Ohio 44135. U.S.A.

exhaust pollutants was begun by the United States Environmental Protection Agency, EPA. Numerous studies of ambient air quality in and around various airports were conducted. 8-~ Models of the atmosphere, pollutant sources, and mixing were developed. ~2-1 s Studies of the state of aircraft combustor technology were undertaken in attempts to estimate what reduced levels of emissions might be achieved b~ employment of various technological approaches. 16.1 In 1972 the first proposed EPA Aircraft Emission Standards were published. 18 These standards called for strict control of the levels of unburned hydrocarbons, carbon monoxide, oxides of nitrogen and smoke. These regulations did not become effective immediately. The studies had indicated that advanced combustor technology was required. Time was going to be allowed for the development of this technolog). Time also had to be allowed to certify that the changes to the aircraft combustion system would not adversely impact the overall performance, reliability and safety of the aircraft engine. In 1973 the Aircraft Engine Standards were modified slightly by allowing a twofold increase in the levels of carbon monoxide and unburned hydrocarbon emissions.~9 There has been no further change in the standards since that time. The Clean Air Act provided that during the interim period between the establishment of standards i19731 and the enforcement date (1979t, there would be public reviews to evaluate the progress being made in controlling emissions. A public hearing was conducted in January 1976, for the purpose of giving testimony to the progress of emission control technology efforts. Presentations were made by aircraft engine manufacturers, airline and aircraft associations and by NASA. 2°'2~ At the present time this information is being considered by the EPA. Some changes are contemplated either in the standards or in the implementation date or both. This paper will review in some detail the present emissions problem area and potential future areas of concern. The presently promulgated EPA standards and their implications for aircraft gas turbines will be discussed. The progress and status of emissions reduction technology programs and other NASA efforts which have emphasized advanced combustor technology will be reviewed in detail. Also included arc those efforts underway to determine the emission:s floor and incorporate those techniques into practical combustors of the future.

INTRODUCTION

The concern over emissions from aircraft gas turbines has been the motivating factor for a vast amount of combustion research conducted during the last 6 years. Since the problems of emission control and minimization are by no means solved, a significantly large research effort will continue into the future. The research efforts conducted have been worldwide in extent and have included engine manufacturers, universities, private and governmental research agencies. ~ h is the purpose of this paper to review the problem of gas turbine emissions as they affect combustor design, illustrate and discuss progress made in controlling emissions by advanced combustor technology, and point out potential problem areas that may arise in the future. It is, however, beyond the scope of this paper to include and discuss all of the pollution reduction combustion programs being conducted. This paper will be limited to the work being done by or under the sponsorship of the NASA-Lewis Research Center. It would have been preferable to include results and discussion of the many aircraft engine manufacturer's inhouse pollution reduction programs. Unfortunately most of such material is considered company proprietary and little useful information is available in the open literature. In 1970 the Congress of the United States passed the Clean Air Act. This act empowered the newly formed Environmental Protection Agency to establish regulations that controlled the levels of emissions from aircraft gas turbine engines. This legislation was not the first attempt to control levels of pollutants emitted into the atmosphere. Concern over ambient air quality had been increasing throughout the previous decade and in the State of California strict regulations affecting industry and automobiles were enacted, z'3 There was also increasing concern over the levels of aircraft engine exhaust smoke.'* The growing commercial use of JT8D, JT3D and other "'smoky" engines made the public visibly aware of engine emissions. The military had been concerned with levels of emitted smoke for some time. 5-~ Military aircraft flying low altitudes could be detected by ground observers before the aircraft itself could be seen. Therefore. prior to enactment of the Clean Air Act considerable effort was underway by the engine manufacturers to reduce smoke levels below the visible level." The effort to establish allowable levels of engine 73

74

ROBERT E. JONES

EMISSION REGULATION There are two main areas of concern relative to emissions regulations. They are the local or urban area and the high altitude region, principally the stratosphere.

The Local Problem The potential problem which is encountered locally involves all of the pollutants from gas turbine engines. Industrial plants, homes and automobiles have been shown to be the major contributors to urban pollution and active programs are underway to limit and minimize pollution from these sources. Even though aircraft contribute only slightly to the total urban pollution problem their contribution is large in and around airports. Up to 50% of the measured pollution at airports can be attributed to aircraft operation, the remammg 50/0 being caused by automobile and truck traffic. As emission control of automobiles became more effective, uncontrolled emissions from increased aircraft traffic would be a major source of pollution,a-t The damaging effects of pollutants have been extensively reported elsewhere, t'22 It is sufficient to realize that sustained exposure to carbon monoxide and oxides of nitrogen can adversely affect human health. Hydrocarbons combine with oxides of nitrogen in the presence of sunlight to produce photochemical smog. Unburned or partially oxidized hydrocarbons are principal causes of exhaust odors prevalent around airports. Particulate and smoke emissions contribute to a general haziness of the air and deposit across the countryside. In addition, concern has been expressed that carcinogens may be present in such smokes or aerosols) Recognition of all these facts has led to the establishment of standards to limit emissions into the urban environment. •

"

O/

The Stratosphere Problem The situation with regard to the stratosphere is different from the urban problem. In the stratosphere the primary concern has been over emitted levels of oxides of nitrogen, N O , , and oxides of sulfur SOx. 23-26 Levels of SOx emissions from aircraft are presently very low as strict controls exist as to the amount of sulfur allowable in the fuel. The pollutant of concern then is N O x and its reaction with ozone in the stratosphere. The reaction mechanism is illustrated below. NO + 0 3 --+ N O 2 + 0 2 N O 2 + O --, N O + O 2 In this reaction scheme ozone, 03, is destroyed and the nitric oxide, NO, is reconstituted to react again with ozone molecules. The result of extensive ozone destruction would be an increase in the levels of ultraviolet radiation on the earth's surface. Such an increase could have serious biological impact. An increase in skin cancer among humans has been shown to be one likely result.26.2 7

It was originally felt that the Supersonic Transport would be the major source of NO~ emissions into the stratosphere. This was a valid concern for it seemed initially that large SST fleets might become an early reality. Since SST's would fly at altitudes of 16--23km where ozone concentrations are high, NO~ emissions would be expected to cause a significant change in the ozone concentration. It is now apparent that large fleets of SST's are still some years in the future. Considerable research efforts are required to develop practical combustors having the extremely low levels NO x emissions that are deemed necessary. Thus, the concern has shifted in recent years to the effect of NOx emissions from subsonic transports flying the lower levels of the stratosphere. There are today large numbers of subsonic transports flying in the lower levels of the stratosphere. Both the numbers of aircraft and their flight altitudes will surely increase with time. Uncontrolled NOx emissions from these aircraft could have an environmental impact long before large SST fleets are a reality. The stratospheric problem was studied extensively in the Climatic Impact Assessment Program, CIAP, conducted by the United States Department of Transportation. 26 The entire question of the impact of engine emissions injected into the stratosphere is, however, far from resolved. The results of recent modeling efforts indicate that the effect of such pollutants may have been considerably overestimated in the past.~ 21 The High Altitude Pollution Program (HAPP) being conducted by the U.S. Department of Transportation is continuing to provide new information on the effects of stratospheric pollution upon the earth's climate and population. ~25 Until the effect of pollutants is known with greater accuracy it is virtually impossible to establish standards for emitted levels of pollutants. The possibility does exist though, that such standards may be issued sometime in the future. In anticipation of such standards. NASA has begun combustion programs to determine how to achieve minimal NOx emissions in practical combustion systems. These efforts and early results are described in detail later in this paper.

AIRCRAFT ENGINE REGULATIONS In an attempt to reasonably classify aircraft engines for regulation, the EPA established the various engine classes shown in Table 1. The smaller general aviation turbofan engines are in the T1 class. The large commercial transport engines are in the T2 class. The separation between the T1 and T2 engines classes has been selected to be 80001bs of thrust, 35.6kN. The exception to this is that the JT3D and JT8D engines were put into separate classes. T3 and T4. respectively• This was done as smokeless combustor retrofit programs were underway by Pratt & Whitney for those engines. By the end of 1973 virtually all JT8D engines were retrofitted with the low smoke combustor. Low smoke combustors for the JT3D engine are presently

G~s lurbine engine emissions- problems, progress and future

-~

7~,lu v I EPA aircraft engine classes.

ENGI HE

DEFINITION

APPLICATION

CLASS

TI

T2

TURBOFAN/TURBOJET <8000

LB THRUST

<35.6

kN

GENERAL

TURBOFAN/TURBOJET

AVIATION

WIDE BODY TRANSPORTS

>8000 LB THRUST > 35.6

kN

T3

JT3D ENGINES

B707,

DC-8

T4

JTSD ENGINES

B727,

B737,

T5

SUPERSONIC

SUPERSONIC

PI

PISTON ENGINES

GENERAL AVIATION

P2

TURBOPROP

GENERAL

CRUISE ENGINES

ENGINES

undergoing field service evaluation. The T5 class applies to engines used for the supersonic transport.-" s At present only the Olympus 593 engine used in the Concorde transport is in this class. The P1 class applies to piston engines and the P2 class to turboprop engines. The level of emissions is obtained by integrating the engine emissions over the specified landing-takeoff cycle. This cycle is illustrated in Fig. 1 for T2 class engines. Note that only operations below an altitude of 3000 ft (914m)are regulated.* This altitude was chosen as it is the typical altitude of temperature inversions over cities. The cycle consists of five segments: taxi-idle outbound, takeoff, climb to 3000f t, approach from 3000 ft to the ground and taxi-idle inbound. The reason for the two taxi-idle legs is that engine emissions are usually higher on outbound taxi-idle when the engine may be colder than on inbound taxi-idle where the engines are quite warm. Table 2 lists the EPA specified conditions of time in mode. TIM, and engine power setting for the various engine classes. The power setting at taxi-idle is usually set by the engine manufacturers. There are minor variations in the times in mode for the * The author apologizes f~r the use of the English system of units, but the emission regulations are promulgated in those units. An arbitrary change to S.I. units would add only confusion rather than clarification. Elsewhere, S.I. units have been used. The exception being that pressures are reported as atmospheres rather than kilo or mega Pascals.

DC-9

TRANSPORTS

AVIATION

various engine classes which reflects the general operation of aircraft using engines of those classes. An exception is the T5 class where the approach leg occurs in two steps. This has been done in an attempt to minimize noise levels near airports. II

3OOO

,-:_ .2000

CLIMBOUT-. / ~

I

g iooo

TAXI/IDLE IOUT)

I I

I

TAKEOFF ",, I

\~F.~ APPROACH \

~\ TAXIIIDI E

___l___l__..J,_.b;l/L Ik-~N)--l---'] 5 10 15 20-- 25 30 TIME, MIN c.~-79oa~ FIG. 1. The EPA landing-takeoff cycle.

The integrated value of pollutant levels over the landing-takeoffcycle has come to be known as the EPA Parameter or EPAP. The parameter has the units of: pounds of pollutant per thousand pounds thrust (h.p.) per hour per cycle. To compute the value of this parameter one must know the engine pollutant level, fuel flow rate and thrust {h.p.) at each state point in the landing-takeoff cycle. The engine emission level is usually specified at state points by the emission index. as pounds of pollutant per 1000 pounds of fuel burned.

76

ROBERT E. JONES

TABLE 2. EPA landing-takeoffcycle conditions.

OPERATING CONDITION

TIME

IN M O D E ,

ENGINE

MINUTES

POWER,

%

TI,P2

T2,T3,T4

T5

TI,P2

T2,T3,T4

T5

19.0

19.0

19.0

(a)

(a)

(a)

TAKEOFF

0.5

0.7

1.2

i00

i00

i00

CLIMB

2.5

2.2

2.0

90

85

65

DESCENT

N/A

N/A

1.2

N/A

N/A

15

APPROACH

4,5

4.0

2.3

30

30

34

TAXI-IDLE, IN

7.O

7.O

7,0

(a)

TAXf-IDLE, OUT

(a)

(a)

a Manufacturers recommended power setting. The EPAP is computed by: 5

Z (EI)(WI) (EPAP) = 5~ (Fu)(TIM) I

where: E1 Wf F.v TIM

emission index, Ib/lO00 Ib fuel fuel flow rate, lb/h engine thrust, lb force time in mode, rain

The EPA standards for the various engine classes are listed in Table 3. For comparison, engine emission EPAP numbers are given for present engines in each class. A comparison of present engine levels versus the standards shows that reductions in emissions by factors of two to ten or more are required. In general, all emissions in any engine class must be simultaneously reduced. The one exception to this is P2 class, turboprop engines. For these engines the required NOx EPAP is 12.9, while most engines are presently operating below that level. This affords the combustion engineer the relatively rare opportunity to trade-off emissions performance. A large decrease in CO and THC emissions can be achieved at the expense of an increase in NOx. If the NOx increase is small then the EPAP will increase only slightly and the standard value will not be exceeded. Table 3 lists both the 1979 and the 1981 standards} °'2s The 1979 standards apply to all engines manufactured after that date. Many engines currently in production would therefore have to incorporate emission reduction technology. The 1981 standards apply to new engines certified after 1981. Note that these standards are stricter than the 1979 standards in

that the levels of hydrocarbons and carbon monoxide are reduced. The levels of smoke emissions in Table 3 are taken from the curve of smoke level as a function of engine thrust and reproduced in Fig. 2 for turbojet and turbofan engines. A similar curve exists for P2 class engines. The smoke number is determined by the technique of ARP 1179.29 The values shown in Fig. 2 represent the demarkation between visible and nonvisible exhaust smoke. 50

lot

0

I

I0

I

2o

I

3o.

I

4o

!

50

ENGINERATEDPOWER(I000 Ib OFTHRUST)

FIG. 2. The EPA smoke number relation for turbojet turbofan aircraft engines. The EPA has set emissions standards for gas turbine engines in applications other than aircraft engines. The standards for a ground power and automotive applications are listed in Table 4. 3°'31 For gas turbines used for ground power, only oxides of nitrogen and oxides of sulfur emissions are regulated. The reason is that gas turbine engines in these applications spend only the minimum time necessary at idle or low power where CO and THC emissions are high. At high power CO and THC emissions are minimal but NO.~ emissions are at

Gas turbine engine emissions--problems, progress and future

7-

TABLE3. EPA aircraft engine emission standards. 1979 Standards ENGINE

THC ~

NO a x

CO a

SMOKE

CLASS PRES

STD

PRES

Ti

4-16

1.6

15-60

9.4

T2,T3,T4

2-21

0.8

7-20

5.3

P2

6-12

4.9

20-30

26.8

PRES

STD

STD

PRES

STD

3.7

~0

32

3-1o

3.0

15-30

25

5-10

12.9

55

50

.01 0

20

2.5-4.5

1981 Standards

IT ,T3,T

I

7- 01 3O I 3-10 I 3O 1980 Standards

i

Pounds of pollutant/1000 pounds thrust-hours/cycle or pounds of pollutant/1000 horsepower-hours'cycle.

a maximum. The proposed NO~ standard is 75 ppm as measured in the exhaust stack. The NOx value is adjusted by correcting the measured oxygen in the exhaust to 15°/o. Additional adjustments are allowed due to the size or firing rate of the engine. Oxides of sulfur emissions are set at a value of 100ppm and would be controlled by limiting the amount of sulfur in the fuel. Smoke is not regulated by the EPA. in part due to the difficulty of maintaining transmissometer type instruments on exhaust stacks. However. many local communities, counties and states do have regulations as to allowable smoke levels, a2.a3 The level of emissions of NOx, 75 ppm, will require that an effective NO~ control technique be employed. A "'dry" N O.,. fix is presently not available for en~nes in service today. The EPA has assumed that users will have to resort to either water or steam injection techniques to minimize NO x emissions. The effectiveness of water injection in controlling NOx emissions has been well documented in the past, particularly for aircraft engines.a4-3 ~Users would prefer not to use water due to the expense of maintaining large quantities of demineralized water at the engine site. The EPA has planned for possible exemptions to this requirement but on a case-by-case basis. The automotive gas turbine emission standards are the same as those for the automotive piston engine. The emission levels are obtained bv summing the emissions over the EPA Urban Driving Cycle. For the ~as turbine engine to successfully compete with the

piston engine, the emission levels must be met at comparable or better fuel economy. At present the fuel economy of automotive gas turbine engines is not comparable to piston engines. However. in the larger engine sizes approaching 600 h.p., 44.7 kW, gas turbine engines are actively being studied as replacements for truck diesel engines. NASA and the U.S. Energy Research and Development Administration (ERDA/are conducting programs to develop gas turbine engines for automotive and truck transportation. CAUSES OF GAS TURBINE ENGINE EMISSIONS

Aircraft engine exhaust emissions can be broadly divided into five principal groups of constituents as shown in Table 5. The exhaust concentration values are given for a typical aircraft gas turbine engine. The first two groups of constituents are those associated with air and the normal products of combustion of a hydrocarbon fuel. The constituents of the three remaining groups have been termed pollutants and are those resulting from inefficient combustion (CO and THCt. the high temperature occurring during the combustion process (NOx)and impurities contained in the fuel ISO2 and SO3 or SO.,). The emissions caused by inefficient combustion and high temperatures can be substantially reduced by combustor desigr.. Fuel related pollutants can be removed by improving the quality of the fuel. In practice, the levels of sulfur in aviation grade kerosines are usually welt belo~ the allowable level. 38

78

R O B E R T t7 . J O N E S

TABLE 4. Gas turbine engine emissions standards.

GROUND

POWER APPLICATION

NO X

SO

x

SMOKE

AUTOMOTIVE

- 75 PPM

(15% O X Y G E N A N D F I R I N G

- I00 PPM

- OPACITY

VALUE

BY L O C A L

RULE

APPLICATION

THC

- 0.42 g/mi

CO

- 3.4 g/mi

NO

RATE A D J U S T M E N T S )

- 0.4 g/mi x

AVERAGE

O V E R EPA U R B A N

DRIVING

CYCLE

TABLE 5. Engine exhaust constituents.

ESTIMATED

SOURCE

CONSTITUENTS

CONCENTRATION

N2

AIR

77% (VOL)

02 A

AIR

16.6% (VOL)

AIR

0.9%

(VOL)

H20

EFF COMBUSTION

2.7%

(VOL)

C02

EFF C0b~USTION

2.8%

(VOL)

CO

!NEFF COMBUSTION

10-50 PPM

UNBURNED HC

!NEFF COMBUSTION]

PARTIALLY OXIDIZED HC

INEFF COMBUST!ONJ

H2 SMOKE (PARTICULATES)

INEFF COMBUSTION INEFF COMBUSTION

5-50 PPM 0.4-50 PPM (MASS)

NO, NO 2

HEATING OF AIR

50-400 PPM

S02, SO 3 TRACE METALS

FUEL FUEL

i-i0 PPM 5-20 PPB

5-25 PPMC

Gas turbine engine emissions--problems, progress and future The level of the gaseous emission pollutants varies with the engine operating condition. For most conventional combustors the pollutants vary as illustrated in Fig. 3. The emission indices (g of pollutant/kg of fuel burned) of CO and T H C are highest at low power (engine idlei and decrease rapidly as engine power increases. On the other hand NO~ emissions are low at idle and increase rapidly as power increases. At engine idle it is difficult to achieve 100% combustion efficiency as pressure levels are low, 2-3.5atm, and inlet-air

7o

80--

70D

60 m

50-O

70 m

x" 6O

40-

Z

o t~

5O

f x"

40

29--

Z

30

,

X

10-2O

Ol

1600

lO

0

20

40

60

80

100

FIG. 3. Emission characteristics of advanced high pressure ratio gas turbine engines. temperatures are low, 394-450 K. Also at idle, the fuel is often poorly atomized and distributed. Prior to concern for emissions, an engine idle combustion efficiency of 90" ° o was considered to be quite satisfactory. However, to meet the 1979 EPA standards for C O and T H C a combustion efficiency of 99+°.o is required at idle. As en~ne power increases the combustor pressure and inlet-air temperatures increase rapidly until at takeoff the pressure and temperature are such that virtually 100",, combustion efficiency is achieved. Some NO.,. emissions are obtained during engine idle operation. Maldistribution of fuel. resulting in hot pockets is responsible for these NOx emissions levels. As engine power increases the fuel nozzles spray the fuel very uniformly, but any gains here are more than compensated for by the increased pressure and inlet-air temperature acting to cause an increase in the flame temperature. The rate of NO~ production is known to be a strong function of the flame temperature as well as the residence time that the air and combustion gases remain at high temperature. The exponential dependence of N O , emissions on flame temperature is shown in Fig. 4. These results were calculated for premixed combustion at an inlet-air temperature of 800 K. a pressure of 5.5 arm and a residence time of 2ms. Conventional combustors have average flame temperatures of the order of 2300-2500K in their 4 ~--B

2000

I

2200

2400

2600

FLAMETEMPERATURE. K

ENGINEPOWER,

J.P.E(.5

1800

FIG. 4. Effect of flame temperature on NO~ emission index for an ideal premixing-prevaporizing combustor: combustor inlet temperature 800 K ; pressure 5.5 atm "and residence time 2ms. primary zone. Because a conventional combustor operates with a nonhomogeneous diffusion flame, the effect of average primary-zone flame temperature on NO~ formation is not as strong as that shown in Fig. 4. The effect of residence time on NO~ emissions is shown in Fig. 5. The formation of NOx is relatively slow as illustrated. Typical residence times in the primary zone of a combustor are of 2 - 4 m s . thus there is usually insufficient time to achieve equilibrium NO.~ ermssion levels. The relationship between engine operating con329--

x"

24O _~_

EQUILIBRIUM

Zt.k.

oN~-

o z

160

80

2 4 RESIDENCE TIME, m s e c

6 cs-;;:~I

FIG. 5. Effect of residence time on NO~ emissions tor an ideal premixing-prevaporizing combustor: combustor inlet temperature 800 K: pressure 5.5 atm: equivalence ratio 1.0.

80

ROBERTE. JONES

ditions, the causes of polutant formation and the cures that can be effected are shown in Fig. 6. At low power the quenching of combustion reactions, poor fuel atomization etc. result in low combustion efficiency with CO and THC the principal pollutants. At high power, the high flame temperature contributes to NO~ emissions. An examination of the "cures" to reduce these emissions poses a dilemma at the two operating extremes. Those "cures" which can reduce CO and THC run counter to those required to reduce NO:,. The single exception is improved fuel distribution. The challenge posed by the present Emissions Regulations is to develop combustor technology that can effectively utilize these "cures" to reduce pollutants at one operating condition without increasing production of other pollutants at the other operating condition. EMISSION CONTROL APPROACHES The idle emission control approaches shown in Fig. 6 are: (1) increase primary zone residence time: (2) retard mixing of hot gases with diluent air; (3) optimize the primary-zone equivalence ratio: (4) improve fuel distribution and atomization. All of the approaches will reduce CO and THC and, in general, the more approaches used, the lower the emission levels will be. Increasing the residence time of hot combustion gases in the primary zone can be done by lengthening the primary zone, increasing zone height or by reducing the airflow. Dimensional changes to the combustor are generally not possible once the combustor is in production. Dilution air injection can be delayed by moving dilution holes downstream. This has the effect of increasing residence time as well as retarding mixing of diluent air. Major adjustments to combustor airflow are usually not possible though minor variations can be made. Minor airflow variations can help to optimize the equivalence ratio in the primary zone. If the primary zone is fuel-rich, excess CO and

THC will exist. The concentrations of CO and THC will be reduced somewhat by mixing with dilution air but complete consumption is often stopped as the dilution air quenches further combustion. If the primary zone is too fuel-lean, often there is insufficient temperature to cause the combustion to be completed within the primary zone. Additional air just reduces the temperature further and CO and THC levels are unchanged. A possible combustor design solution is the staged combustor consisting of pilot and a main zone. The pilot zone would be designed specifically to minimize CO and THC emissions. At all other operating conditions the pilot zone serves as a hot gas source to stabilize efficient combustion in the main stage. The pilot zone of a two stage combustor can be designed to effectively utilize the approaches just described. At idle a more optimum fuel/air ratio in the primary zone can often be achieved by use of engine bleed or fuel scheduling. 39 When compressor bleed is used the airflow rate through the combustor is reduced and a higher local fuel/air ratio exists. The resulting increase in temperature can be very effective in reducing CO and THC. The use of excessive bleed should be avoided as this can reduce the engine speed, resulting in reduced compressor exit pressure and temperature. These effects tend to increase CO and THC levels. Locally high temperatures can be obtained by fuel scheduling. In fuel scheduling, fuel would be supplied to only a portion of the fuel nozzles. With the total idle fuel flow passed through fewer fuel injectors, locally high fuel/air ratios are produced and fuel distribution and atomization are improved. The difficulty with this approach is that a circumferentially nonuniform temperature pattern is imposed upon the turbine stator and rotor. This could reduce engine life or reduce cycle efficiency by requiring greater clearances within the turbine. Improved fuel atomization is one approach that usually reduces idle emissions substantially. At low power the fuel nozzle injection pressure is usually low. RESULT

COMBUSTION INEFFICIENCY CARBONMONOXIDE UNBURNEDHYDROCARBONS CAUSES

LOW:

Tin p~ -]

EFFECTS

7 QUENCHING / POORCOMBUSTION LOW POWER IDLE / STABILITY / POORFUELATOMIZATION AND DISTRIBUTION

/

CURE INCREASE RESIDENCETIME REDUCEFLOW VELOCITY RETARD MIXING (QUENCHING) INCREASEEQUIVALENCERATIOTOI IMPROVEFUELATOMIZATIONAND DISTRIBUTION

- ,-~___POLLUTANTS

EXCESS RESIDENCE ~i~ "J HIGH POWERTAKEOFF " TEMP TIMEHIGHFLAME POORLOCAL

Tin

FUELDISTRIBUTION

REDUCERESIDENCETIME INCREASE FLOWVELOCITY ENHANCEMIXING IQUENCHING) REDUCEEQUIVALENCERATIO TO 0.5-0.7 IMPROVE LOCALFUELDISTRIBUTION

OXIDES OF NITROGENSMOKE FIG. 6. Combustor emission considerations.

Gas turbine engine emissions--probiems, progress and future This can result in large fuel drops, poor spray distribution or even a collapsed fuel spray. Increasing the injection pressure will help remedy this. but nozzle size must be reduced or fuel scheduling used. Several alternative schemes are the use of air-assist fuel injection or air-blast fuel nozzles. 4 ° ~ Air-assisted fuel injection uses small quantities of high pressure air passing through the fuel nozzle. This air serves to finely atomize the fuel. In tests of air-assisted fuel nozzles both CO and THC have been significantly reduced. 42 Air-blast fuel nozzles use the dynamic pressure of the air to finely atomize the fuel. At idle conditions, however, the dynamic pressure is low and the fuel may not be atomized as finely as required for best emission reduction. These nozzles though, do give an improved fuel distribution within the primary zone and that feature alone helps to reduce emissions. The approaches to reduce high power NO~ emissions are: (1) reduce residence time; (2) enhanced mixing; 13t lean combustion;and (4) improved fuel distribution. High flame temperatures, which are the cause of high NOx emissions, can be reduced by water injection. This is a very effective technique, but requires water flow rates that could be as large as the fuel flow rate. 43'~4 In addition, the water must be demineralized to prevent deposits on and corrosion of the turbine. A more functional approach is to achieve low NOx emissions by combustor design technique. The effect of residence time on NO~ emissions was shown in Fig. 5. Increasing the velocity within the combustor will generally enhance mixing as well. This enhanced mixing of combustion gases with diluent air serves to quench the NOx forming reactions by cooling the combustion gases to a level where NOx formation is less, Fig. 4. If lean combustion could be stabilized and made efficient in a high velocity zone, then NOx emissions would be substantially reduced. Fuel distribution not, becomes very important as local fuel-rich regions will have higher flame temperatures and high NOx levels. In practice it has proven to be very difficult to achieve simultaneous reductions in all pollutants by applying these techniques to single stage combustors. The techniques employed to minimize pollutants at one condition, usually lead to enhanced pollutants at another condition. One way to effectively use these emission control approaches is in staged combustors having separate lot, power and high power zones. In this way the best techniques at each operating condition can be employed without the control techniques of one stage affecting the performance of the other stage. APPLICATION AND RESULTS OF EMISSION CONTROL APPROACHES

This report discusses the results of research and development programs being sponsored, directed and/ or conducted by NASA.4s-48 Work supported by other government agencies, such as the Department of Defense iDOD), the Federal Aviation Administration tFAAI and the Environmental Protection Agency ~EPA) and industry have also provided much data on

81

advanced technology lot' emission gas turbine combustors. 39-123'~2'~The results of the two major NASA technology development programs, the Experimental Clean Combustor Program (ECCP) and the Pollution Reduction Technology Program {PRTPI will be discussed in detail.

Experimental Clean Comhustor Pro¢lram The Experimental Clean Combustor Program (ECCP}, was initiated in December 1972, with the objective to develop and demonstrate, in a full-scale engine, advanced technology combustors that are capable of reducing pollutant emissions in the large high bypass ratio engines tEPA Class T2. thrust over 80001bs, 35.6kN1. The original emission level goals were established from NASA studies and were subsequently adjusted to be consistent with the EPA Standards published in mid 1973. The two contractors that were selected are Pratt & Whitney Aircraft (JT9D-7 engine) and the General Electric Company (CF6-50 enginel. The program is a three-phased effort with each contractor, scheduled to culminate in engine demonstration tests in 1976-77. Phase I of the program consisted of screening a variety of low emission combustor concepts. References 49 and 50 cover the results obtained in detail. The Phase II effort consisted of refinement of two combustor concepts prior to engine adaptation. Detailed results of the Phase II programs are given in Refs. 51-54. The Phase III effort consisted of full-scale engine tests of the most "'engine-ready'" combustor from the Phase II program. The entire threephase program will be discussed for each contractor separately and the major program results summarized at the end.

General Electric Company Phase I: comhustor screening. The Phase I program was an 18-month effort to screen a variety of low emission combustor designs. 49 The effort involved the definition of four advanced combustor designs, detailed aeromechanical design of each to fit within the CF6-50 engine envelope, fabrication of full/annular test combustors and performance testing. The combustors that were studied were a very lean primary zone version of the production CF-50 combustor, a swirl-can combustor, a radial/axial staged combustor and a double/ annular combustor. The swirl-can module combustor has been extensively investigated at NASA and had shown potential for much lower NO x emissions) ~-6° Most of the testing in this Phase was conducted at simulated takeoff and idle operating conditions. The best emission results are summarized in Table 6. The most promising results were obtained with the radial' axial staged and double/annular combustor designs. These favorable results are attributed to the use of two discrete zones within the combustors, wherein the combustion process can be optimized for each zone. Based on the Phase I results these two combustor concepts were selected for further development in Phase II. Two additional efforts were conducted during Phase I, an Advanced Supersonic Technology IASTi and a

82

ROBERT E, JONES

TABLE6. Summary of the Phase I screening tests---General Electric.

CO~USTOR

CONFIGURATIONS

CONCEPT

TESTED

LOWEST EMISSION

INDICES

CO b

THC b

NO a x SINGLE

(EI), g/kg FUEL

4

28

~

48

17

28

15

6T

RADIAL/AXIAL

7

14

2

38

DOUBLE

6

17

i0

42

ANNULAR SWIRL-CAN

ANNULAR a NO~ at

takeoff conditions, extrapolated. b THC and CO at idle conditions. Combustion Noise Program. The objective of the AST effort was to evolve combustor designs based on the Phase I combustor concepts, that would have reduced NO~ emissions when operating at supersonic cruise conditions typical of future AST engines. The purpose of the Combustion Noise Program was to obtain experimental data on the acoustic characteristics of low pollution combustors. Results of the AST program are presented in Ref. 49 and the Combustion Noise Program in Ref. 61. Phase II: concept refinement. The two advanced technology CF6-50 engine combustor concepts that were evaluated in Phase II are shown along with the conventional CF6-50 engine combustor in Fig. 7. Both designs utilize a form of fuel scheduling {staged combustion) for reducing pollutant emissions over the entire engine operating range. The pilot stages of both the radial/axial staged and the double/annular are optimized for high efficiency (low CO and THC emissions) at engine low power (idle). The main stages are optimized for lean combustion (low NO~) at ,a~l power (takeoff). Various combinations of fuel staging can be used for off-design operation such as approach power settings. The radial/axial staged configuration utilizes a premixed fuel/air technique in the main stage whereas the double/annular configuration uses an airblast fuel nozzle and airflow control in the main stage. These two concepts employ four of the previously discussed control techniques :(1) fuel scheduling: (2) airblast fuel nozzles; 13) lean mixture combustion; and (4) premixing. Based on theresults of Phase I1 testing, the double/annular concept was chosen for the Phase III engine demonstration tests. All of the testing in Phases 1 and II was performed in a full/annular combustor test rig which closely duplicates the flow path of the CF6-50 engine. All engine inlet and exit operating conditions were simulated except for combustor inlet pressure which was limited to a maximum of 10 atm. Further details are ~ven in Ref. 51.

The emission results obtained to date for a selected "best" configuration of each of the advanced technology combustor concepts shown in Fig. 7 are summarized and compared to the baseline engine combustor in

(a) ENGINECONVENTIONAL(BASELINE)COMBUSTOR.

,,i,

"--~'~'I STAGE ~ _ ~ .

___._---~..-/~--

,//

../

.__-

~)) RADIAL/AXIALSTAGEDCONCEPT.

. ' - ~ . ~ i PILOT

(c) DOUBLEANNULARCONCEPT. FIG. 7. Combustors for the CF6-50 engine, Phase II of ECCP--cornbustor refinement.

Gas turbine engine emissions--problems, progress and future

83

TAmm 7. Summary of emissions performance of advanced technology combustors for the CF6-50 engine. ECCP. Phase II. (Values in E.I.) CONFIGURATION

EMISSIONS = CO

CF6-50 ENGINE

IDLE

BASELINE COMBUSTOR

APPROACH

73

THC 3O

NOX

SMOKE

2.5 10.0

L.3

29.5

CLIMBOUT SLTO

35.5

EPAP c

10.8

4.3

19.3

2.2

7.7

13

CRUISE DOUBLE/ANNULAR CONCEPT

IDLE

CONFIGURATION D/A-13

APPROACH b

3.0

12.~

3.1

CLIMBOUT

13.3

SLTO

16.9

EFAP

3.0

0.3

4.25

CRUISE

~.8

0.17

8.0

RADIAL/AXIAL CONCEPT

IDLE

53.8

6.1

3.1

CONFIGURATION R/A-2

APPROACHb

1.3

0.2

9.2

CL!MBOL~

10.9

0.2

14.2

SLTO

8.5

0.i

16.1

EPAP

9.56

o.88

~.3o

4.3

0.8

3.O

CRUISE 1979 EPA STANDARDS

EPAP

19

a All values computed at actual entnne operating conditions {standard day I. b Pilot stage only fueled at approach. c EPAP units are pounds of pollutant/lO00 pounds thrust-hours/cycle.

Table 7. All of the emission index (Ell values listed in Table 7 are computed to be those that would occur at combustor operating conditions consistent with actual engine operation. To perform this computation, combustor test rig data, which was limited to 6--10atm pressure, were extrapolated to engine related pressure and correlated for appropriate values of reference velocity, fuel/air ratio, and inlet temperature using the procedure described in Ref, 51. The computation of the listed Environmental Protection Agency Parameter (EPAPI values is consistent with the recommended procedure from Ref. 19. Details of the EPAP calculations are given in Refs. 21 and 51. Table 7 presents the emissions in terms of El's at the various discrete operating conditions that are used in the landing-takeoff (LTOt cycle computation and the resultant EPAP values. Also included are the representative EI's obtained at simulated altitude cruise conditions (10.Tkm, Mach 0.8). The selected "best" concept configurations, double/annular, D'A-13. and radial/axial, R/A-2, Isee Ref. 51 for definitions), were chosen from all of the configurations tested in Phase It based on the following factors: (1t the lowest corn-

bined emission levels obtained at all of the engine operating conditions: (2/ acceptable performance in terms of pressure drop, combustion efficiency, and exit temperature pattern factor: and (3) acceptable staging characteristics at the approach condition. Development potential in terms of durability, operational stability, altitude relight capability, and overall engine/control integration were also considered. Many of the double/annular and the radial/axial configurations achieved emission levels nearly equal to and in some instances less than those shown in Table 7 and a complete listing of all of the Phase II test results is given in Ref. 51. Nevertheless, D/A-13 and R/A-2 were judged to be the best overall based on the selection factors used. Comparison of the EPAP values achieved with D/'A-13 and R/A-2 and the baseline engine combustor shows that significant reductions in all emission levels w'ere achieved. Smoke levels are not listed because the values for all the combustors were well below the requirements established in the EPA standards. The D/A-13 configuration was capable of reducing the CO and THC emissions to levels equal to or less than those

83

ROBERT E. JONES

required to comply with the EPA standards. A NOx EPAP value of 4.25 was obtained with the D/A-13 compared to the 3.0 required by the standards. Although not meeting the EPA standards, the 4.25 value represents a 45o/0 reduction from the current baseline value. The reduction in N O , El at high power (takeoff) conditions was greater than 50~°/oof the baseline combustor value. That the percentage reduction in EPAP was less than 50~ can be explained by comparing the EI's at approach. An increase in the NOx EI value as approach was obtained using the D/A-13 configuration as compared to the baseline combustor. This increase is caused by higher flame temperatures produced by higher fuel/air ratios needed when operating the pilot only during approach. The R/A-2 configuration also made significant reductions but was not capable of reducing any of the gaseous emissions to the levels required by the EPA standards. From an emissions and performance viewpoint, the double/annular and the radial/axial concepts represent major steps for reducing engine emissions without compromising performance. The results obtained from Phases I and II of this program indicate that the combustor technology exemplified by the double/ annular and radial/axial designs have only very limited potential for further NO x reduction. The results presented in this paper are the best levels that were • achieved. Minor design changes generally result in trade-offs of emission levels and generally poorer combustor performance. Substantially lower NOx emission levels by additional combustor refinement do not appear possible with these combustor design approaches. Additional efforts were also conducted during Phase II. The Combustion Noise program was continued to obtain additional acoustic data and to relate this data to combustor design and operating parameters. Results of this program are presented in Ref. 62. An Alternate Fuels Program was also conducted to investigate the effect of alternate fuels on the emissions and performance of advanced technology combustors. 63 Results of this program are discussed later in this paper. Phase III: engine demonstration. The double/annular combustor installed for engine tests is shown in Fig. 8. A comparison of this design with the double/annular combustor used in the rig tests, Fig. 7, indicates several changes incorporated to increase combustor durability. While the changes are logical and appear to

-

FIG. 8. Double/annular combustor configuration for Phase III of ECCP--engine demonstration.

be minor they resulted in a large degradation in idle emissions performance. A development effort was required to improve the emissions performance of the pilot stage used at idle. Once suitable improvements were determined, the combustor was modified and installed in the engine for testing. Table 8 lists the airflow distribution for this combustor. Twice as much air flows through the main zone (inner annulus) as through the pilot zone (outer annulus). The reference velocity in the pilot zone is 10 ms and 29 ms in the main zone. Average reference velocity for the combustor is 23ms. The greatest changes in airflow distribution from the best Phase II configuration are in the amounts of liner cooling air. All cooling airflows were increased somewhat, but the biggest change was a 60Vo increase in the pilot zone cooling flow. The cooling air increases were obtained by reducing the amount of air available for trim dilution and centerbody cooling. The emissions test program consisted of obtaining emissions data at all specified EPA cycle points as well as some intermediate power settings. Emissions were sampled using a cruciform rake as specified by the EPA. In addition, another cruciform rake was used, rotated 45 ° from the first rake which doubled the number of sampling ports. The entire assembly, of the two rakes. was then rotated and data were taken at each 5° . These three sampling techniques gave results that varied generally within plus or minus 5°0. Table 9 presents the results of these tests and compares the engine EPAP values to those estimated from the Phase II effort. As indicated the double/annular combustor in engine test was only capable of achieving the hydrocarbon standard. The levels of CO and NO~ emissions were considerably reduced below the level of the production combustor but could not achieve the EPA standard levels. Surprisingly the exhaust smoke level was high. Rig tests at 6--8atm pressure had not indicated that smoke levels would be as high as in the production combustor. In general the combustor did not achieve the emission levels expected from Phase II tests results. It is now apparent that the changes incorporated into the engine combustor design had an even greater than expected impact on emissions performance. Also the fuel injectors used in the engine appeared to be improperly sized for this application and hence poor fuel spray quality and distribution may have been responsible for the higher than anticipated levels of CO, NO x and smoke. The engine test results presented in o/ Table 9 were obtained at an idle power setting of 3.3/0, the usual engine idle level. As the idle power setting was increased, all the calculated emission parameter values decreased. An idle power setting of approximately 6.5% would produce CO and THC levels below the EPA standards. 64 It is unfortunate that the engine combustor could not duplicate the excellent emissions performance obtained with the double/annular combustor during Phase II. These results obtained by rig and engine test, verify the potential of the double.' annular combustor concept to achieve low emissions.

~5

Gas turbine engine emissions--problems, progress and future TABLE 8. Airflow distribution for the double/annular combustor.

COMBUSTOR

PERCENT OF TOTAL

FEATURE

COMBUSTOR AIRFLOW

OUTER ANNULUS SWiRLERS

12.6

DILUTION

4.5

COOLING

7.2

INNER ANNULUS SWIRLERS

33.0

DILUTION

10.6

COOLING

5.4

CENTERBODY

3.1

INNER LINER TRIM DILUTION

2.0

20.2

LINER COOLING

1.4

AFT SEAL

TABLE 9. Summary of emissions of the CF~50 en~ne with the double/annular combustor, ECCP, Phase III.

CO

THC

NO

4.3

0.8

3.O

20

PRODUCTION COMBUSTOR

10.8

4.3

7.7

13

D/A IN CF6-50 ENGINE

6.3

0.3

5.6

25

D/A IN TEST RIG

3.0

0.3

4.25

NIL

1979 EPA STANDARDS

These results also illustrate how sensitive the combustor design is to minor changes and the large impact on emissions performance. Should the double/annuiar concept be developed further, there is every reason to believe that engine performance can and will match test rig performance. In all other combustor performance aspects, the double/annular combustor performed at least as well as the production combustor. G r o u n d starting, engine

X

SMOKE

acceleration and fuel staging posed no problem at all. Combustor durability was excellent. There were no areas damaged by local overheating and there were no soot deposits around the fuel injectors, Complete engine test results will be found in Ref. 64.

Pratt & Whitney Phase I: comhustor screenin#. The Phase I program at Pratt & Whitney was'similar in duration and ob-

86

-

ROBERT E. JONES

TABLE t0. Summary of Phase l screening tests--Pratt & Whitney. COMBUSTOR

CONFIGURATIONS

CONCEPT

TESTED

LOWEST EMISSION INDICES (El), g/kg FUEL NO a

THC b

CO b

13

13.6

58.5

78.5

9

20.6

1

9

l0

12.4

4.5

X

SWIRL-CAN STAGED PREMIX VORBIX

29

a Extrapolated to SLTO conditions. b Ground idle conditions. jective to that at General Electric. 50 Three advanced technology combustor concepts were investigated in this effort. They were a swirl-can combustor, a staged premix combustor and a vorbix (vortex burning and mixing) combustor. The best emission results with these combustor concepts are summarized in Table 10. As in the General Electric tests the best emissions performance was obtained with those combustor designs that use a separate stage for low power and high power operation. The best idle performance was obtained with the staged premix combustor. At simulated takeoff none of the combustors met the NOx goal but best results were obtained with the swirl-can and vorbix concepts. The N O x reductions of the stagedpremix combustor were accompanied by a fall-off in combustion efficiency. All of the testing in Phases I and II was conducted in a 90 ° sector test rig which closely duplicated the JT9D-7 engine flow path. All operating conditions except inlet pressure, which was limited to approximately 6 atm, were simulated. Complete details of the test housings and facility are given in Refs. 50 and 52. As in the General Electric program an Advanced Supersonic Technology lAST) and Combustion Noise effort were conducted during Phase I with Pratt & Whitney. Results of these efforts are found in Refs. 50 and 66 respectively. Phase I1: comhustor refinement. The two advanced technology JT9D-7 engine combustor concepts that were evaluated in Phase II are shown schematically along with the conventional JT9D-7 engine combustor in Fig. 9. 52 As with the CF6-50 engine concepts, both designs use fuel scheduling (staging) as the principal approach to controlling overall pollutant emissions. The hybrid concept utilizes a parallel (radial) fuel staging approach which includes premixing in the pilot stage and a variation of the swirl-can concept in the main stage. This configuration is an attempt to mate the lowest CO and THC emission design (premix pilot stage) with the lowest NO.~ emission design (swirl-canmodule stagel that were tested in Phase I. see Ref. 50. The vorbix configuration utilizes a series-type (axial) fuel staging approach with standard pressure atomizing

fuel nozzles in both the pilot and main stages. High intensity swirlers are located immediately downstream of the main stage fuel injection point to promote very intense, rapid mixing of the fuel and air in the flame zone. The combination of the intense mixing and hot gases exiting from the pilot stage allows lean combustion in the main stage and also reduces residence time due to quick quenching of the hot gases. These concepts also employ four control techniques:(1) fuel

la) ENGINECONVENTla~AL(BASEUNE)COMBUS'K)R.

~) HYBRID CONCEPT.

(c) VORBIXCONEPT

FtG. 9. Combustors for the JT9D-7 engine, Phase II of ECCP--combustor refinement.

Gas turbine engine emissions--problems, progress and future scheduling: (2j lean mixture combustion: (3) premixing: and (4) quick quenching. Table 11 presents the emission level results obtained for the two selected concept configurations, vorbix S-25. and hybrid. H-6 (see Ref. 52 for definition). The factors used in selecting these two configurations were the same as those previously discussed in relation to the CF6-50 engine configurations. Comparison of results with baseline engine emissions shows that considerable reductions in all emission levels were obtained in terms of both EPAP and El at specific operating points. As with the CF6-50 configurations, smoke levels were acceptably below the EPA standards and are therefore not listed. The S-25 combustor configuration achieved THC and NO x emission levels below the EPA standards but did not achieve the required level for CO emissions. Two engine idle operating conditions were tested; indicated in Table 11 as Idles 1 and 2. At present the EPA specifications are inexact as to how the engine should be operated at idle. Idle operation usually entails the use of some engine bleed and ,or power extraction. For the JT9D-7 engine bleed and power extraction reduce engine speed, which results in lower compressor exit pressure and temperature. Emissions obtained during such tests are labeled Idle 1. Emissions representative of the engine operation with no bleed are labeled Idle 2. The two EPAP values then represent the changes due to the large increase in idle emissions for the case where engine bleed was used. At the unbled idle condition, CO was reduced below that of the baseline engine combustor but was still approximately 50°<~, higher than the EPA standard. The NO x EPAP value of 2.2 is well below the EPA standard value of 3.0. This represents a significant accomplishment in obtaining NO.~ emission reductions. The H-6 configuration was able to achieve substantial reduction in all emissions though only meeting the EPA standard for CO and THC emissions at the unbled conditions. However, CO, THC and NO.~ levels are very low and only small improvements would be required to meet all the EPA standards. Based on the testing performed in Phase II, the vorbix combustor was selected for the Phase III engine demonstration tests. During Phase II, the Combustion Noise effort was continued along with Alternate Fuels effort. The Combustion Noise results are presented in Ref. 66 and the fuels effort is discussed later. 67 Phase III: enqfne demortstration. The Phase II combustor rig results showed that the vorbix combustor could meet all the EPA standards except CO. In an attempt to reduce CO emissions for the engine test, the pilot stage was lengthened to increase its volume. A sketch of the vorbix combustor used in the engine tests is shown in Fig. 10, The vorbix combustor concept incorporates two burning zones separated axially by a high velocity throat section. The pilot zone is a conventional swirl-stabilized, direct-injection combustor employing thirty fuel injectors. It is sized to provide the required heat release rate for idle operation at high efficiency. Emissions of carbon monoxide and un-

~7

burned hydrocarbons are minimized at idle operating conditions primarily by maintaining a sufficiently high pilot zone equivalence ratio to allow complete burning of the fuel. ,- PILOT ZONE

/ SWIRLER FUEL INJECTOR-

:

'

" ''~'

-

IGNITER"~: ':'-i'~""~ MAIN ZONE FUEL INJECTOR

FIG. 10. Vorbix combustor configuration for Phase Ill of ECCP-- engine demonstration.

At high power conditions, the pilot exhaust equivalence ratio is reduced as low as 0.3 (including pilot dilution air) to minimize formation of oxides of nitrogen. The minimum equivalence ratio for the pilot zone is determined by the overall lean blowout limits. combustion efficiency, and the need to maintain sufficient pilot zone temperature to vaporize and ignite the main zone fuel. Main zone fuel is introduced through fuel injectors located at the outer wall of the liner downstream of the pilot zone discharge location. Sixty fuel injectors are used. Main zone combustion and dilution air is introduced through sixty swirlers positioned on each side of the combustor. Table 12 itemizes the flow area distribution of the final version of the vorbix combustor. The greatest changes between this version and the S-25 configuration are a slight increase in the liner cooling flow and a slight reduction in the main dilution airflow. Considerable effort was spent to design the fuel manifold system and control which could handle a two-stage combustor. Flexibility was required so that the pilot-tomain fuel flow ratio could be varied at discrete operating conditions. The fuel system was never intended to be flight worthy, but rather an experimental setup to expedite the emissions testing. The emissions test program and sampling procedures were the same for the Pratt & Whitney JT9D-7 engine as for the CF6-50 engine described previously. The Phase III results are tabulated in Table 13 and compared with the baseline combustor and the test rig results from Phase I1.68 The vorbix combustor met all of the EPA 1979 gaseous emission standards, failing only to meet the smoke standard. The increase in volume of the Phase l l I combustor pilot zone was very successful in reducing idle CO emissions. Emissions at various operating conditions were very sensitive to the pilot-to-main fuel flow split. Emissions of THC and CO were substantially reduced at the approach power condition (30°'; power) by maintaining the pilot fuel/air ratio equal to 0.007 (overall basis). A decrease in the

ROaER'r E. JONES

88

TABLE I 1. Summary of emissions performance of advanced technology combustors for the JT9D-7 engine. ECCP. Phase II. tValues in E.I.I. CONFIGURATION

OPERATING

EMISSIONS c

CONDITION CO

NO

x

SMOKE

IDLE #i a

JT9D-7 ENGINE BASELINE

THC

COMBUSTOR

IDLE #2 b APPROACH

77 9.6

29.8

3.3

1.0

8.4

CLIMB

22.9

SLTO EPAP #2 e

31.5 14.29

5.3 a

4.90

2.9

l0

CRUISE VORBIX CONCEPT

CONFIGURATION

S-25

IDLE #I a

40.5

0.5

IDLE #2 b

18.5

.5

3.2

APPROACH

30.9

1.9

4.3

CLIMB

8.8

.9

8.5

SLT0

4.5

.4

10.8

EPAP #i

10.4

.3

2.2

EPAP #2

6.5

.3

2.2

10.8

4.0

4.5

IDLE #1 a

9.6

4.2

3.6

IDLE #2 b

3.5

2.8

2.8

.2

15.2

21.6

1.5

11.6

SLTO

8.9

i.i

16.4

EPAP Hl

4.32

.94

3.5b

EPAP #2

3.29

.71

3.45

CRUISE HYBRID CONCEPT CONFIGURATION

H-6

APPROACH d CLIMB

CRUISE EPA 1979 STANDARDS

EPAP

34.6 4.3

6.02

7-5

0.3

3.0

19

Idle # l--bled. bldle #2--unbled. c All values computed at actual engine operating conditions (standard day). d Pilot stage only fueled at approach. c EPAP units are the same as in Table 7.

pilot fuel/air ratio from 0.007 to 0.004 would triple the emission index values for THC and CO. At climb and takeoff conditions the level of NOx emissions could be reduced by reducing the pilot fuel/air ratio. Minimum NOx emissions were obtained at a pilot fuel/ air ratio of 0.0045. The smoke number value of 30 is above the EPA standard. Further development of the main stage fuel injectors would probably reduce the smoke level. Some additional air admitted around the fuel nozzles might reduce the smoke number and would also serve to shield the injector tips from soot deposits. Some soot buildups were noticed during the tests. The engine met the requirements for acceleration and deceleration. Some initial difficulty was encountered with the fuel system at the staging points. But these problems were quickly rectified and the test

program proceeded very smoothly thereafter. The engine turbine stator vanes were instrumented with thermocouples in the early phases of testing to measure the combustor exit temperature pattern factor. Measurements indicated that the vorbix combustor had a slightly better value of pattern factor than the baseline combustor. Some minor liner durability problems were observed when the combustor was removed from the engine. Several film-cooling overhangs showed signs of excessive temperature. Those film-cooling slots near the combustor throat are particularly in need of additional cooling air. A test at engine pressure levels is often required to find those liner areas where cooling is deficient. The fuel manifolds and control are two more areas where further work is necessary. Although they

Gas turbine engine emissions- problems, progress and future

TABLE 12. Airflow distribution for vorbix combustor.

COMBUSTOR

PERCENT OF TOTAL

FEATURE

COMBUSTOR FLOW

PILOT SWIRLER

12.0

PILOT I.D.

DILUTION

4.1

PILOT O.D.

DILUTION

7.0

BULKHEAD

3.O

MAIN SWIRLER,

I.D.

6.8

MAIN SWIRLER,

O.D.

27.2

MAIN DILUTION,

Z0.9

O.D.

.8

MAIN FUEL NOZZLE

7.9 17.~ 2.8

I.D. LINER COOLING 0.D. LINER COOLING TURBINE VANE P L A T F O R M

were satisfactory for these tests they would not be for a flight engine. Many improvements, simplifications and reliability' must be added to the system before it would be flight worthy. A complete description of the Phase 1II program and results is given in Ref. 68. ECCP summary

There is no doubt that the Experimental Clean Combustor Program is an outstanding success. Pratt & Whitney. with the vorbix combustor in the JT9D-7 engine, can meet all of the EPA 1979 gaseous

emissions standards, failing only the smoke standard. General Electric, with the double/annular combustor in the CF6-50 engine, though only able to achieve the hydrocarbon standard in this test, believe that with additional work all standards except the NO x standard can be achieved. Both engines were capable of meeting the acceleration and deceleration requirements. These results are particularly remarkable in view ofthe limited state of development of these combustors and their significant departure from conventional combustor designs. These results illustrate that the 1979 EPA

TABLE 13. Summary of emissions of the JT9D-7 engine with the vorbix combustor, ECCP. Phase III.

SMOKE

CO

THC

NO

4.3

O.8

3.O

19

10.4

4.8

6.5

4

VORBIX IN JT9D ENGINE

3.2

0.2

2.7

3o

VORBIX IN TEST RIG

6.5

O.3

2.2

NIL

1979 EPA STANDARDS PRODUCTION COMBUSTOR

x

90

ROBERT E. JONES

standards can be met by employment of advanced technology. Furthermore. these were real combustors, evaluated in an engine, and having to meet all the requirements demanded of less complicated, more highly developed combustors. These combustors, of course, are not flight worthy. Many problems have to be solved before they would be committed to a flight certification program. Fuel manifold and fuel control problems will require a significant effort. Combustor durability improvements will have to be incorporated over an extended time as these problems arise. However, none of these problems seems to be prohibitively difficult to solve. The high smoke levels of JT9D-7 engines should be easily solved by minor changes to the main stage of the vorbix combustor. Further reduction of NOx emissions with the double/ annular combustor will not be easily obtained. The higher pressure ratio of the CF6-50 engine (29.8:1) compared to that of the JT9D-7 (21.7:1) means that significantly more NOx is produced. Emissions of NOx have been shown to be proportional to the square root of engine pressure ratios multiplied by an exponential function of the combustor inlet-air temperature. 69-72 Thus, as engine pressure ratio increases so will the temperature of the inlet-air increase, resulting in a significant rise in the level of NO~ emissions. It is not surprising that the advanced combustors used in the CF6-50 fail to meet the NOx emissions. The present EPA emission standards do not account for tile effect of engine pressure ratio on NOx emissions. Many low pressure ratio gas turbine engines easily meet the NOx standards, while none of the present high pressure ratio engines do. This is more than a question of relative combustor technology. If the NOx emission indices from the vorbix combustor in the JT9D-7 are scaled to the pressures and temperatures of the CF6-50 engine, the resulting values are virtually equal to the NOx emission indices for the double/annular combustor. In the future, engines with pressure ratios of 40 or more may be built. Meeting the NO.~ emission standards for these engines will be very difficult. Some recognition of engine pressure ratio effects should be made a part of the emission regulations. Another important result of this program is the good agreement found between the extrapolated test rig emissions obtained at low pressure and the actual engine emissions. The agreement was far better than anticipated and serves to validate the relationships used. This validation is important as engine demonstration test programs are very expensive, but tests in combustor rigs at low pressure can substitute for such demonstrations if the data can be validly extrapolated. The results of ECCP show that this is the case. Another result of this effort is the agreement in emissions data obtained by three techniques. In the past. the EPA recommended cruciform gas sampling rake has been criticized for not obtaining a representative gas sample. Engine data from ECCP show that values obtained with a fixed cruciform rake were within _,*5 % of values obtained by a rotational traverse of the engine exhaust nozzle.

The combustors tested in Phase III represented the most "engine ready" combustors of all those concepts studied. Many combustors were tested during Phases I and II that have the potential for still further reduction in emissions. A development of some of those concepts may be needed in the future. Pollution Reduction Technology Pro qram The second major program that has been implemented to evaluate the application potential of emission control techniques is the Pollution Reduction Technology Program (P RTP). The PRTP was initiated in mid-1974 as an effort to develop advanced technology combustors to reduce pollutant emissions for the T1. T4 and P2 classes of engines. The contractors and the respective engines selected for the PRTP were Garrett AiResearch, TFE 731-2 engine (EPA Class T 1); Pratt & Whitney Aircraft, JT8D-17 engine (EPA Class T4); and the Detroit-Diesel Allison, 501-D22A engine (EPA Class P2). The combustor evaluations were conducted in a multiphase approach similar to the

rFUEL I

~'

,,

•

~

OEEL

A - MODIFICATIONS TO ENGINE CONVENTIONAL (BASELINE) COMBUSTOR.

~.~-F~L

B - PILOTEDAIRBLAST CONCEPT. PREMIX TUBES"7 ~

~

F f/AIN

L

I E PILOT

L

C - PREMIX/PREVAPORI ZATION CONCEPT.

FIG. 11. Pollution Reduction Technology Program (PRTP), Phase I advanced technology concepts for the TFE 731-2 engine (EPA Class TI ).

Gas turbine engine emissions--problems, progress and future

9i

FUEL j +

f4i-

= ~;

MAIN STAGE

" ~ - ~ • ~'-~ ;,,,

FUEL ""

"~

I.

--~,~ - IGNITOR

A - ENGINECONVENTIONAL(BASELINE) COMBUSTOR.

A - BASELINE COMBUSTOR.

FUEL

PILOT STAGE-7 f

B - REVERSEFLOWCONCEPT. FUEL's"

,"

~ 7

IGNITOR-"

m~

/-SECONDARY SWIRLERS

B

-

FUEL

VORBIX COMBUSTOR.

PRIMARY PREM X PASSAGE 7 --

j

2, --

,

~ S

.'.~,-<~',~

\

C - PRECHAMBERCONCEPT.

PILOT - -

MAIN

i

STAGE

STAGE

i

FUEL

% MAIN

r~ /,

IGNITOR--

,f

~ SECONDARY

INLETS D - STAGEDFUELCONCEPT. C - STAGED PREMIX COMBUSTOR.

FIG. 12. Pollution Reduction Technology Program. Phase I combustor concepts for the JT8D-17 engine (EPA Class T4L

ECCP. Program goals were established to be consistent with published 1979 EPA standards. The combustor concepts selected for the Phase I screening tests are shown in Figs. 11-13 for the TFE 731-2, JT8D-17, and 501-D22A engines, respectively. The advanced technology combustor concepts selected were based on a trade-off between the estimated degree of emission reduction potential and development risk. In all cases, the configurations having the least development risk (A or B) have the least likelihood of achieving all of the pollutant emission goals. The B, C or D configurations represent increasingly higher development risk. However. they provide a better potential for achieving or exceeding the pollutant emission reduction goals. The Phase I concepts for the TFE 731-2 engine used increased engine bleed, air-assist fuel nozzles, air-blast fuel nozzles, fuel scheduling (two sta~e combustion). lean combustion and premixing of fuel and air. Many of these same techniques are employed in the test programs on the JT8D-17 combustor and the 501-D22A combustor. Although all the combustor programs use similar control techniques, the applications of these techniques to the individual engines vary. Other differences of note are that the JT8D-17 and 501-D22A are can/annular combustors whereas the combustor for

FIG. 13. Pollution Reduction Technology Program. Phase I combustor concepts for the 501-D22A engine (EPA Class P2).

the T F E 731-2 engine is a full/annular reverse-flou' design. The essential point is that the methods for applying these pollution reduction techniques to actual engineconstrained designs must be varied as the engine configuration dictates. This is true even though the pollutant control techniques are similar for all of these advanced technology combustors. The wide variation in techniques and applications being evaluated in ECCP and PRTP are providing a large experience data bank on low emission combustors. Phase I results The emissions and performance results obtained from each of the advanced technology combustor concepts are summarized in Tables 14-16. As before, all values listed in the tables are corrected to actual engine operating conditions. No extrapolation of test rig data was necessary for the JT8D-17 and 501-D22A engine combustors because the test rigs could duplicate engine levels of pressure, inlet temperature, and reference velocity. For the TFE 731-2 engine combustors, the test rig data were extrapolated using procedures similar to those described in Ref. 52. TFE-731-2 engine. Most of the data obtained with TFE 731-2 combustors were taken at the extreme

ROBERT E. JONES

92

TABLE 14. Summary of the emissions performance of advanced technology combustors for the TFE-731-2 engine. PRTP, Phase I. ^

COSFIGURATION

OPERATING CONDITION CO

THC

?:0

S~'4CKE

X

TFE-731-2 BASELINE

IDLE

COMBUSTOH

APPROACH

57.9

17.3

3.1

EPAP e

17.5

5.3

b5.3

IDLE c

31.0

1.4

3.9

CLIMB 18.8

SLTO

A, MODIFIED BASELINE COMBUSTOR:

AIR-

ASSIST FUEL NOZZLE

APPROACH CLIMB 16.4

SLT0

B. PILOTED-AIRBLASE CONCEPT

EPAP 2

10.6

.4

IDLE c

CONCEPT

31.8

1.5

6.2

.I

5.7

CLIMB

9.8

.2

10.6

2.8

7.5

.2

10.5

EPAP e

I0.i

.4

b3. 9

IDLE c'd

30.7

1.5

2. ~ 6.C

APPROACH d CLIMB SLTO EPAP e

1979 EPA STANDARDS

-L0

b~..l

APPROACH SLTO

C. PREMIXED STAGED

40

8.1

.2

IS.2

1.2

3.2

6.7

.i

4.1

!0.2

.5

b2.5

9.~

1.6

3.7

-36

a All values computed at actual engine operating conditions. b Not corrected for humidity, 12% reduction estimated. c 5?,;bleed simulated. d Air-assist fuel injection in pilot stage. e Units of EPAP are pounds of pollutant/1000 pounds thrust-hours/cycle.

operating conditions of idle and takeoff. 73 Enough data at other conditions has been obtained to estimate with reasonable accuracy values of the EPA parameter. Table 14 presents the emission levels obtained during the Phase I combustor rig test program. The modifications to the baseline combustor, Fig. 1I(A), that were evaluated included: (1) Reducing the number of active fuel nozzles at ground idle by one half; 6 versus the usual 12 nozzles. (2) Increased fuel/air swirler airflow rate. (3) Replacement ofexisting fuel nozzles with a piloted air-blast fuel injector and the use of high pressure air-assist at idle. (4) Modified primary zone recirculation pattern. (5) Simulated primary zone variable geometry. The use of the air-blast fuel nozzle produced significant reductions in CO and THC emissions at idle when high pressure air (air-assist) was used, as shown in Table

14. Idle NOx emissions increased slightly. The resultant EPAP values meet the EPA standards for hydrocarbons, closely approach the standard for CO but still exceed the standards for NOx and smoke. The use of air-blast nozzles did reduce takeoff NOx emissions by about one half of the required amount. The NO x EPAP value given in Table 14 represents the best estimate available as data were not obtained at approach and climb conditions. The second combustor concept shown in Fig. ll(B) also utilized piloted air-blast fuel nozzles. This combustor has been significantly redesigned when compared to the baseline combustor shown in Fig. 1I(A). The fuel injectors now are mounted through the dome end of the combustor rather than through the outer wall. The air-blast fuel injector was surrounded by an air swirler. The design intent was to make the swirler a variable geometry device such that the swirler would

Gas turbine engine emissions--problems, progress and future

93

TABLE 15. Summary of the emissions performance of advanced technology combustors for the JT8D-17 engine. PRTP. Phase I. CONFI GURATIO.U

OPERATING

Er,~rSSiONSa

CONEITION CO JT8D-17 ENGINE BASE-

IDLE

LINE COMBUSTOR

APPROACH

AIR

BLAST NOZZLE,

i2.@

3.7

.7

~-5

B. VORBIX COMBUSTOR 11-9

C. STAGED PREMIX,

111-3

.6 . . . . . . . 2~

EPAP d

16.1

CRUISE

3

IDLE

19-3

3.7

2.4

i.[

6.4

CLIMB

.~

.i

18.~

SLTO

.4

.i

25.2

EPAP

6.9

1.5

7.5

~.~

IDLE

2.7

II

3.3

i~.9

0.2

a.9

.i

5.8

CLIMB

7.8

.3

9.3

SLTO

5.5

.2

12.1

EPAP

~-9

.2

4.4

CRUISE

5.2

.I

7.2

27 8

0.6

IDLE APPROACH CLIMB SLSO EPAF

.~=

17.3

.~

7.9

7.3

.3

i0.0

CRUISE c

166 12.2 4.3

.~ 256

12

27

3.7

7.8

i~.3

CRUISE b

2S

8.2

APPROACH

EPA 1979 STANDARDS

SMOKE

.9 . . . . . . . 20

APPROACH

1-4

X

7.5

SLT0

COMBUSTOR:

NO

4~.5

CLIMB

A. MODIFIED BASELINE

THC

?..a

4.6 2.5

.3: 0.~

7.~ 3.0

<25

~'All values computed at actual engine operating conditions. bAll six main fuel injectors used. c Only three main fuel injectors used. d EPAP units as in Table 7.

vary the primary zone airflow. At idle the swirler would have a minimum flow area thus raising t'he primary zone equivalence ratio to a value near 1. At higher power settings the swirler flow area would increase, admitting more air and thus reducing the primary zone equivalence ratio to a value that would minimize NO~ emissions. In this phase of the program the variable geometry feature was simulated by adding or removing blockage to the swirler. The best combustor configuration yielded the results presented in Table 14. The EPA standard values for CO and NO~ are nearly met and the hydrocarbon standard was easily achieved. As the data were extrapolated to actual engine pressures from rig pressure levels, there is some doubt as to the

accuracy to the computed EPAP values. During Phase II of this contract program, this combustor concept will be further refined. It appears that the additional testing will result in achievement of all the EPA standards. The most advanced combustor concept to be studied in this program is shown in Fig. 11(C). This is a twostage combustor similar in design intent to the twostage combustor studied in the Experimental Clean Combustor Program. The pilot stage consisted of twenty pressure atomizing fuel nozzles located at the dome end of the combustor. The main combustion zone is located downstream of the pilot zone. The main zone is fueled at all conditions other than taxi-idle. The main fuel is supplied to forty fueDair premixing tubes located

94

ROBERT E. JONES

TABLE 16. Summary of the emissions performance of advanced technology combustors for the 501-D22A engine, PRTP, Phase I. (Values in E.I. ) OPERATING

CONFIGURATION

EMISSIO?~S a

CONDITION

A. 501-D22A E N G I N E BASELINE C O M B U S T O R

IDLE

CO

THC

NO

X

42.9

17.6

3.7

45

APPROACH

5.1

2.0

7.5

59

CLIMBOUT

2.0

.9

9.2

38

SLTO

2.0

.3

8.9

37

31.5

15.0

b.2

59

EPAP b CRUISE B. REVERSE

FLOW C O N C E P T

CONFIGURATION:

MOD IV

--

IDLE

5.1

0.2

3.9

3

APPROACH

2.6

.3

5.8

16

CLIMBOUT

I.I

.2

10.8

13

SLTO

1.1

.1

ll.0

17

EPAP

4.6

.3

7.3

CRUISE C. P R E C H A M B E R

CONCEPT

CONFIGURATION:

MOD Ill

CONFIGURATION:

MOD IV

IDLE

1.6

0.4

3-5

i

APPROACH

3.1

.2

3 6

i

CLIMBOUT

.9

.I

17.7

I

SLTO

.8

.1

19.0

1

EPAP

2.1

.4

8.5

i --

10.2

0.2

4.5

3

APPROACH

IDLE

1.9

.5

9.2

2

CLIMBOUT

2.4

,5

9.2

4

SLTO

1.7

.i

9-3

--

EPAP

8.4

.4

8.1

CRUISE

EPA 1979 S T A N D A R D S

17 --

CRUISE D. STAGED FUEL CONCEPT

SMOKE

EPAP

4 --

26.8

4.9

12.9

22

All values computed at actual engine operating conditions (standard day). bEPAP units same as in Table VII.

along the outer surface of the combustor. This fuel/air mixture is ignited by the hot gases from the pilot. During the test program variations in premixer tube length, pilot to main fuel flow split and premixer tube velocity were varied. Typical premixer tube operating parameters at simulated engine takeoff are; 24% of combustor airflow, a local equivalence ratio of 0.66, a velocity of 107m/s, and a residence time of 1.9ms. Results obtained with the best configuration are presented in Table 14. The use of fuel/air premixing has reduced the NO x emission index from 18.8 for the baseline combustor to a value of 4.1. It is estimated that all EPA standards can be met with this design. Further refinements should improve emissions performance.

For both the piloted-air-blast and premixing combustors, sdaoke levels have been essentially zero at the test pressure of 4atrn. The smoke goal of 25 or less should be easily met. A complete description of this Phase I effort is given in the contract final report. 124 JTBD-17 engine. The three combustor concepts investigated in this program are shown in Fig. 12 and emissions results obtained with the "best" configuration of each concept are presented in Table 15. Also given in Table 15 are the emission indices and EPAP values for the production JTBD-17 engine. The modification to the baseline combustor Fig. 12(A), consisted of replacing the standard fuel nozzle with air-blast nozzles of various designs. In addition

Gas turbine engine emissions--problems, pr¢~gressand future the primary zone airflow distribution was altered to richer or leaner equivalence ratios compared to the production combustor. This was accomplished by decreasing or increasing the size of the primary zone air injection holes. The best results were obtained with a lean primary zone. As shown in Table 15 this combustor achieved reduced levels of all the emissions including smoke but was not able to achieve the levels required by the EPA standards. The reductions in CO and THC EPAP values closely approach the required levels whereas only a minor reduction in the NO~ EPAP value was achieved. Smoke was substantially reduced. It is unlikely that such "minor'" type modifications to the baseline combustor would provide sufficient reductions to simultaneously achieve the EPA standards for CO, THC and NO~. Cruise NO~ emissions were not measured but would be expected to be the same as the baseline combustor since the high power NO~ EI's were not affected. The vorbix concept (Fig. 12(B)) produced substantial reductions in all of the gaseous emissions while maintaining smoke levels comparable to the baseline combustor. The vorbix combustor is virtually identical in design concept with the vorbix combustor used in the JT9D-7 engine as part of the Experimental Clean Combustor Program. In this case the vorbix concept appears as a single combustor can. The combustor consists of a swirl stabilized pilot stage, a throat section separating the pilot and main burning zones and an array of swirlers for the introduction of the main zone combustion air. In this design version, the main fuel is mixed with air at the upstream end of the combustor. swirled about the exterior of the pilot zone through two carburetor tubes, and then injected into the hot pilot gas at the throat section. Combustor design features that were varied during the testing included the throat velocity, location and airflow rate of the main zone swirlers and the amount and location of dilution air. The emission results of the best configuration are presented in Table 15. The EPA parameter value, EPAP. for THC emissions was below the EPA standard requirement but the CO and NO~ values were higher than required. Both the CO and NO~. values were reduced to approximately 50°,0 of the baseline combustor EPA P value. This percentage reduction in NO~ was the best of any of the combustors tested in these two emission reduction programs. The vorbix concept can also provide a potential 30°; reduction cruise NO~. Smoke levels were virtually the same as the producti on combustor. The most significant effect discovered was the effect of throat velocity. As throat velocity decreased emissions of CO and THC were markedly reduced with only slight increase in the level of NO~ emissions. The staged-premix combustor is shown in Fig. 12~CI. This concept represents an attempt to improve fuel/air homogeneity in the main burning zone by vaporizing the fuel prior to injection in the premix passage. The approach taken was to regeneratively heat the fuel while it is maintained above the critical pressure of approximately 22 atm. The hot liquid fuel is allowed to JP.EC'.S ~ 2 - - C

95

flash vaporize upon injection into the premix passages. The regenerative heat exchanger that lined the ~alls of the pilot stage was sized to provide fuel temperatures in the range of 500-700 K at takeoff with the pilot burning at an equivalence ratio of 0.75. Considerable difficulty was encountered with the premixed pilot stage. Repeated damage to this stage at low fuel flows necessitated replacement with the pilot stage used on the vorbix combustor. Therefore there was no improvement in idle emissions beyond that obtained with the vorbix combustor. The use of premixing in the main stage was successful in reducing the takeoff NO~ emissions. However, CO emissions were increased particularb during climb, which leads to a higher CO EPAP value than was obtained with the vorbix. Operation at cruise was very poor when fuel was supplied to all six main stage injectors. Apparently the overall fuel/air ratio in the main stage was too lean for efficient combustion in the available time. When fuel was supplied to only three main stage injectors the performance improved markedly, becoming nearly equal to the values obtained with the vorbix combustor. The NO~ emissions at cruise equal those of the vorbix indicating that the fuel preheating and premixing was not effective in reducing N O x emissions further. 501-D22A enaine. The first advanced combustor concept shown in Fig. 13(B) is called a reverse flow combustor because two louvers in the front end of the combustor are reversed to deflect air forward. This air sweeps along the wall to the front of the combustor. enhancing the recirculation zone and carrying any fuel that gets to the wall back into the combustion zone. A comparison of this combustor configuration with the baseline combustor, Fig. 13(A), shows that only the combustor dome and film cooling were altered to achieve the reverse flow design. The second concept, Fig. 13(C). is a prechamber combustor. This concept employs a variable area swirler to change the prechamber fuel/air ratio for different operating conditions. The fuel/air mixture from the prechamber is burned in a sudden expansion primary zone. The third combustor concept, Fig. 13(D), employs staged combustion. A pilot stage is used for low power operation and serves as a heat source for the main stage during high power operation. Fuel is supplied to the main stage through six equally spaced tubes. Air-blast fuel injectors are mounted at the entrance to each tube. Advanced liner cooling, consisting of a combination of film and convection cooling, was used so that more air would be available for mixing with the hot combustion gases to effectively quench NO~ producing reactions. The emission levels obtained during the Phase I combustor rig tests of the "best" 501-D22A combustor are given in Table 16. A substantial amount of test data was obtained from all three advanced concepts shown in Fig. 13 at all operating conditions. All of the "best" configurations selected were capable of controlling all of the emissions to values below the levels required by the EPA standards. ~6''' In the case of CO. THC, and smoke emissions this required substantial reduction :however. the N O.~emission level actualiy in-

96

ROBERTE. JONES

TABLE 17. Engine EPAP comparison assumed CO E.I. values.

IDLE

.

.

.

.

.

.

.

.

.

.

.

.

.

20

.

APPROACH . . . . . . . . . . . . .

CL IFflB

.

.

.

.

.

.

.

.

.

.

.

.

.

0.5

.

.

.

.

.

.

.

.

.

0.4

TAKEOFF

.

.

.

JT8D-I7

.

.

.

JT9D-7

EPA

.

.

.

.

5

.

.

.

.

.

.

.

.

.

EPAP

=

7 . 5

.

.

.

EPAP

=

3.9

EPAP

= 4.3

STANDARD . . . . . . .

creased compared to the baseline combustor. Further development of the prechamber and staged fuel concepts should provide some reduction in the NOx levels. No cruise data was obtained with these concepts. The performance and characteristics of all the configurations were acceptable and the measured exit temperature pattern factors were better than those of the baseline combustor. Program status

The Phase II combustor refinement tests of combustors for the Garrett AiResearch TFE 731-2 engine are in progress and Phase III engine demonstration tests are planned for 1978. The Phase II program efforts for the JT8D and 501-D22A engines were canceled. The JT8D program was terminated after Phase I primarily because none of the advanced combustor concepts showed much likelihood of achieving the program goals and the concepts employed were very complicated. The ability to achieve the emission goals was severely affected by the higher value of thrust specific fuel consumption (TSFC) of the JT8D engine. The JT8D engine is representative of an earlier period of gas turbine technology. Today's engines are more efficient, have lower values of specific fuel consumption and have a reasonable chance to meet the emission standards, Table 17 compares carbon monoxide EPAP values for the vorbix combustor in the JT8D and JT9D engines and reflects the importance of TSFC upon the ability to achieve EPA standards. The assumed values of CO emission index are typical for both vorbix combustors tested in these programs. These emission indices yield a CO EPAP value of 7.5 for the JT8D

engine and 3.9 for the JT9D engine as compared to the EPA standard value of 4.3. Thus, the effect of the lower TSFC of the JT9D engine resulted in achievement of the standard. For the JT8D to meet the standard, the CO emission indices, as well as those of THC and NOx, would have to be about one half of the best results achieved with the vorbix combustor. It is not likely that any combustor investigated in these programs could have achieved such low levels of emissions. In-house programs conducted by Pratt & Whitney on the JT8D combustor have demonstrated combustor designs that should achieve the CO and THC emission standards, if the NOx standard would be waived or increased. Simultaneous achievement of all standards does not appear possible for the JT8D engine with any combustor configuration studied. The program with the 501-D22A engine was terminated primarily because the emission standards were easily met with a very minor change to the combustor design. More complicated combustor technology was not required. The simple changes of concept B, Fig. 13, met the program goals. Since the 501-D22A engine has a can/annular combustor and single can fig tests were conducted at correct engine pressures and temperatures, there seemed to be little need for a complete engine evaulation of this concept. PR TP summary

All of the advanced technology combustors tested exhibited the potential for substantial reductions of exhaust emissions of the three engines selected for this program. In the case of the 501-D22A engine, the selected "best" configurations exhibited the capability

Gas turbine engine emissions--problems, progress and future of meeting all of the emission levels required by the EPA standards. This was possible only because this engine has a very low engine cycle pressure ratio and, subsequently, low baseline NO~ emissions compared to the EPA standards. Therefore, an increase in NO,. emissions was allowable. Emissions of CO and NO~ require substantial decreases to meet the EPA standards for the JT8D-17 engine. Even though substantial emission reductions were obtained with the vorbix combustor, only the THC standard was met. Further reductions in emissions might have been achieved but other combustor performance levels would probably have been affected adversely. The Phase I results of the advanced combustor concepts for the TFE-731-2 engine have been very encouraging. Emissions of the pilotedair-blast and premixing concepts have been significantly reduced and the expectations are that all EPA standards can be met with further development. Combustor exit temperature pattern factors have been excellent throughout the program with most values less than 0.2. Other factors such as altitude relight and fuel staging seem to be within developable limits. ULTIMATE EMISSION LEVELS The results of the Clean Combustor and Pollution Reduction Technology programs show that advanced technology combustors can achieve most if not all of the required EPA standards for 1979. However, the 1981 EPA standards have lower values of CO and THC. Achieving these levels, particularly CO emissions, will be difficult. The levels of cruise NO~ emissions were reduced with these eombustors but the low levels recommended by CIAP and other programs were not achieved. 26 In fact, these combustor programs clearly demonstrate that further technology advances are required if these values are to be met. It is important to consider what are the ultimate possible levels of engine emissions and what kinds of approaches must be explored to reach those levels. The problem of CO and THC emissions at idle and low power conditions is directly related to combustion efficiency. To meet the 1981 EPA standards a combustion efficiency of 99.5 ~ % is required. Programs are being conducted to explore further optimization of pilot stages and primary zones that will minimize these emissions. The most difficult emission to minimize is NO~. As shown before, NO~ emissions can be reduced by lowering the flame temperature. As equivalence ratios are reduced, NO~ emissions will be reduced due to lower flame temperatures. Eventually, combustion will become unstable and blowout will occur. However, before blowout occurs there will be a loss in combustion efficiency. Neither the loss in combustion efficiency nor operation close to blowout can be tolerated in commercial aircraft gas turbine engines. The lean combustion, so desirable for low NO~ emissions requires the fuel/air mixture be highly uniform. Unmixedness results in locally high fuel/air

97

ratios, which burn at elevated flame temperatures and produce high NOx emissions. 78 Achieving uniformity of the fuel/air mixture is very difficult as long as the fuel exists as drops. It is usually conceded that best uniformity results when the fuel is vaporized completely during the mixing process. Combustors employing lean, premixed and prevaporized fuel/air mixtures should produce the theoretical low limit of NOx emissions. Research efforts at NASA and elsewhere have explored premixed-prevaporized combustion. ~9- 82 Flame tube studies using test ducts like the one shown in Fig. 14 have been conducted. In this program, propane fuel GAS

FUEL

IOCM

i

SAMPLE-~

1

~

~ r"

210CM

',

~ -~ /

l_PERFORATED

PREHEATED

AIRFLOW

FLAMEHOLDER

FIG. 14. NASA experimental test facility for studies of premixed-prevaporized combustion.

was used to simulate kerosine. Typical results are shown in Fig. 15. Data were obtained at inlet-air temperatures of 600 and 800 K. Residence time in the combustion zone was varied by changing the gas sampling probe position downstream of the flameholder. Data are compared to a well stirred reactor prediction of NOx emissions. 83 As expected NO~ emissions decrease with decreasing equivalence ratio and emission index levels below I g/kg were obtainable. T3.

lO0F P3=5.Satm

K

VDc F = 25 & 30 mls

BOO

R ,DE CET, E- ms

,.=

Z

x" _z z u~

I 'I " /

"°

"/~/.

•

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.7

800 K DATA 600 K DATA

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k .9

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EQUIVALENCERATIO

FIG. 15. Nitrogen oxide emissions from premix tests using propane fuel.

98

ROBERTE. JONES

The relationship between NO~ emissions, combustion efficiency, residence time and equivalence ratio is shown in Fig. 16. These curves were obtained using the apparatus shown in Fig. 14 and are obtained from crossplots of data such as that in Fig. 15 to illustrate the effect of residence time. At an equivalence ratio of 0.4 very low NO.~ emissions can be obtained at high efficiency almost regardless of the residence time. As equivalence ratio increases, NOx emissions rise rapidly and low values, approaching 1 or less, can only be obtained at very short residence times with a potential for reduced combustion efficiency. Similar experiments were conducted at the General Applied Science Laboratories (GASL) with kerosine fuel and Fig. 17 is a sketch of their combustion tunnel, s4-s6 There was an attempt to make this experimental setup appear more like the high power stage of a combustor. Test conditions simulated the operating conditions of a primary combustor for a supersonic transport engine. Data obtained in this program are shown in Fig. 18 and are compared to NASA data with propane fuel. At these operating conditions stable combustion could be obtained at equivalence ratios a~ low as 0.3 and NO~ levels were near 0.1 g/kg of fuel. However, combustion inefficiencies were as high as 4%. The figure does show that a NOx emission index of 0.5 can be obtained at a combustion efficiency of 99.5% and an equivalence ratio near 0.5. These lean, premixed-prevaporized combustion studies have illustrated the lower theoretical limit of NO~ emissions with conventional flame technology. Recently there has been a significant interest in catalytic combustion. By using a catalyst it should be possible to maintain combustion at temperature levels where virtually no NO.~ emissions are formed. The recent development of high temperature ceramic substrates like those used in the regenerators of automotive gas turbine engines has made catalytic combustion a reality. Prior to the availability of these substrates no material or process existed that would provide the surface area required for combustion and withstand the high combustion temperatures. Today an increasing effort is being devoted to studies of catalytic combustion, sT-gs From the combustion engineers' point of view the catalytic combustor would appear to be the ultimate in control of exhaust emissions. Once the reaction is started it should be possible to consume all of the fuel

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FIG. 16. The relationship between NO,, emissions, residence time, equivalence ratio and combustion efficiency.

(no CO or THC emissions) and conduct the entire combustion process at a temperature level around 1400 K (negligible NOx emissions). Studies of catalytic reactors have been conducted by NASA and others for some time. More recent studies have concentrated upon the evaluation of various catalysts and substrates, as•9s'96 The test facility shown in Fig. 19 is being used in this work at NASA. Up to four 2.54cm thick catalyst sections can be tested at one time. For these tests gaseous propane has been used as the fuel to eliminate the complexity and operating problems associated with liquid injection. In operation the air is heated to a level where the catalyst will be effective in initiating reaction of the fuel. A premixed-prevaporized fuel/air mixture is admitted to the hot catalyst bed and the reaction begins. The temperature rises rapidly through the bed as shown in Fig, 20. The performance of catalytic combustors is highly dependent upon the reference velocity of the fuel/air mixture as shown in Fig. 21. These data are results from the test rig shown in Fig. 19. The strong dependence of combustion efficiency on reference velocity indicates that it is possible to overload a catalytic reactor having a fixed surface area. If the surface area for reaction were increased at a given reference velocity then combustion efficiency should increase• Unfortunately the reactor pressure drop will also increase with an increase in surface area, where diameter is held constant. Pressure drops for the reactors tested in this program have O/ varied from 2 to 6/°. a value comparable to con-

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Gas turbine engine emissions- problems, progress and future

VREFlm/sl

ventional gas turbine engine combustors. Emissions from these reactors or combustors have been virtually nil when combustion efficiency is near 100%. Emissions of NO~ have varied from none to a few ppm. When

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less than 100}o combustion efficiency is obtained emissions of CO and THC increase rapidly• In well designed combustors using flame combustion, the usual principal cause of inefficiency is CO. In the catalytic combustor the major source of inefficiency is THC or unburned fuel. At high velocities there is not sufficient time for all of the fuel to diffuse to the catalyst surface and react. For CO fractions, however, the temperature is usually high enough for rapid gas phase oxidation to reach completion. GAS SAMPLETO ANALYZERS COOLINGWATER, CATALYSTELEMENTS~ /

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Fro. 20. Temperature history through catalyst reactor, inlet mixture temperature 800 K: inlet pressure 3 atm. durability limit the_maximum level of exit temperature. In spite of the many problems associated with the catalytic combustor, it is the concept with the greatest potential for emissions reduction. 100--

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FIG. 19. Test arrangement for catalytic combustion studies.

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The catalytic combustor is just in its infancy. Many problems have to be solved before they will be practical. The f,,el preparation and distribution is critical: cold starts and catalyst preheat must be solved: narrow limits of operation due to critical catalyst temperature requirements mean that variable inlet and outlet flow geometry is needed; poisoning and catalyst life problems must be solved and problems of substrate

80

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Fl(;. 21. Combustion efficiency of Grace DAVEX 512 B catalyst at various reference velocities, inlet mixture temperature 800 K. pressure 3 atm.

100

ROBERTE. JONES

FUTURE PROBLEM AREAS

Stratospheric Contamination The studies conducted by the United States Departmerit of Transportation as the Climatic Impact Assessment Program (CIAP) and the Report of Findings by the National Academy of Science have indicated that NOx emissions of engines operating in the stratosphere must be reduced. 26"27The levels of reductions required vary with the numbers of aircraft that fly into the stratosphere. Various scenarios have been proposed as to the future numbers and types of aircraft. From these scenarios a level of allowable NOx emissions was estimated that would cause a minimal environmental impact. In general, a reduction in cruise NO~ emissions by a factor of six or more was deemed necessary. The anticipated level of NO~ emissions for various levels of combustor technology are shown in Fig. 22. These levels have been computed for advanced supersonic transport engines that employ variable engine cycles. The variable cycle engine is so designed that both subsonic and supersonic operation are optimized. The cycle pressure ratios of these engines are greater than those of the present SST engines and those of engines designed for use on the previous US supersonic tramport. The figure shows that NO~ emissions from these engines using conventional combustor technology are extremely high with emission index values over 35. Application of Experimental Clean Combustor Program technology will reduce NO~ emissions only to the level of that of the engines used in the Concorde SST. Emissions from combustors employing forced circulation are close to the desired levels but only concepts using prevaporizing and catalytic combustion appear to have sufficient potential to meet the challenge.

cruise NO~ emission. The techniques include the last three shown in Fig. 22, forced circulation, prevaporizedpremixed and catalytic combustors. Catalytic combustion has been discussed in detail previously and will not be covered here. The state of development of catalytic combustors is such that major research efforts in many areas must succeed before such combustors are acceptable for aircraft use. The major research efforts at this time are in forced circulation combustors, low emission duct burners and basic research efforts to establish technology for prevaporized-premixed combustors. •

Forced-circulation combustors The term "forced-circulation" is used to describe the use of strong swirling flow or impinging jets to form a powerful recirculation cell in the combustory primary zone. This recirculation cell entrains hot combustion gases into the flame region and establishes a stable zone where lean combustion can occur. When these cells are coupled with partially premixed-prevaporized fuel/air mixtures as shown in Fig. 23, the combustion process begins to approach that of a homogeneous or well stirred reactor. The two concepts shown in Fig. 23 are the Jet-Induced-Circulation (JIC) and Vortex Air-Blast (VAB) concepts and are being evaluated under NASA contract to the Solar Division of International Harvester Company. To date these concepts have been studied only in tubular configuration, but a translation to a full/annular design and test program will be conducted. References 99-101 give details of the prorFUEL

Combustor Programs Many combustion programs are now being conducted by NASA to evaluate techniques to obtain low

,.-i 40 2

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CLEAN COMBUSTOR TECHNOLOGY

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PREVAPORIZING PREMIXED

Fro. 22. NOx emissions forecast for supersonic cruise engines, Mach number 2.32 ; altitude 16 km.

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Gas turbine engine emissions--problems, progress and future

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FIG. 24. Effect of combustor temperature rise on the NO.~ emissions of SOLAR forced-circulation combustors. Jet A-1 fuel, inlet temperature 830 K; pressure 2 atm.

FIG. 25. Effect of an optimized idle equivalence-ratio on the cruise emissions of a fixed geometry version of the SOLAR VAB combustor concept. Jet A-1 fuel, inlet pressure 1.4 atm.

grams conducted to date. The best emission results obtained with each concept are shown in Fig. 24. The data were obtained at inlet and exit temperatures typical of that for the primary combustor of an advanced SST engine at supersonic cruise, but at a lower pressure level. The VAB concept achieved a NO~ level of approximately 1 gNOz/kg fuel and the JIC concept a value of 2 g N O 2 / k g fuel at the designated operating point. These values represent substantial reductions from conventional combustors operating at similar conditions. Both concepts have been optimized for lean combustion at the supersonic cruise design point. Operation at other engine conditions, including ground idle, will require substantial further development and modification. For example, when the VAB concept was reconfigured to optimize the primary zone equivalence ratio at idle, the results of Fig. 25 were obtained. For these tests, the VAB combustor had dilution air ports added in the downstream portion of the can. This was done so that the primary zone equivalence ratio could be t~ept near 1 during idle operation. Although low idle emissions were obtained, the superior performance at cruise was lost. As the combustor temperature rise was increased the primary zone equivalence ratio became greater than one, establishing a rich-burn condition which was not corrected by subsequent dilution air injection with the resulting high levels of CO and N O x emissions. These results clearly indicate the need for either staged combustion or a variable geometry scheme that will main-

tain combustion zone equivalence ratios at optimum values to minimize emissions throughout the entire operating regime. Regardless of the approach taken, the impact of off-design performance on emissions must be considered when evaluating the potential gains of advanced combustion techniques. The requirements and constraints are a significant challenge to the combustion engineer. Many comprormses and trade-otis will be required before practical, operational combustors are a reality.

r- PRECHAMBER / ' FUEL MANIFOLD f

Duct burners NASA is currently sponsoring two contracts whose purpose is to evaluate the application of low NOx combustion techniques to duct burners. Studies of advanced engines for a supersonic transport have shown that thrust augmentation by duct burning will be required. 1o2jo3 These studies show that extensive augmentation is required at takeoff and during transonic acceleration but substantially less is required during supersonic cruise. An analytical study conducted by Pratt & Whitney has identified several promising near-term and far-term combustor concepts, t°4 A near-term concept utilizes the vorbix principal employed in the Experimental Clean Combustor Program and in previous augmentor applications and is shown in Fig. 26.1°* This design features three stages of fuel injection used in prechamber, pilot and main combustion stages. The prechamber is used to obtain good altitude relight and provide for soft

AIR SWlRLER -T ,,'

r HIGH POWER COMBUSTION I ZONE ,,'

I ,

'-' PILOTSECONDARY "~ HIGHPOWER ' ZONE ~" PILOTSECONDARY FUELINJECTORS

STAGEFUEL INJECTORS

FIG. 26. Sketch of the three-stage vorbix duct-burner concept.

102

ROBERT

E.JONES

TABLE18. Ductbumeroperatingconditions mr Pratt & Whimeyvariablestream controlengine, VSCE. TEST PARAMETERS

SIMULATED FLIGHT CONDITION SLTO

CLIMB

0.3

1.3

2.~

0

ll.l

16.1

2.6

1.8

2.5

438

446

604

K

1605

1396

i000

g/kg - GOAL

I.O

1.0

1.0

99

99

99

N/A

N/A

6.5

MACH NUMBER ALTITUDE,

km

BURNER PRESSURE,

atm.

INLET-AIR TEMPERATURE,

K

DESIGN EXIT TEMPERATURE, NO x EMISSIONS,

COMBUSTION EFFICIENCY, TOTAL PRESSURE LOSS,

%; GOAL

%; GOAL

ESTIMATED PERFORMANCE NO

X

EMISSIONS,

g/kg

COMBUSTION EFFICIENCY, PRESSURE LOSS,

OF THREE-STAGE

%

VORBIX

1.78

1.22

2.75

99

99

99

%

ignition of the pilot stage. A soft ignition is characterized by a small pressure rise during ignition of the augrnentor. Ignitions that generate a large pressure pulse can induce a stall in the fan stage of these engines. The overall length of the duct burner is 1.37 m and the duct height is 33 cm. The estimated emissions and other performance features for this cycle are shown in Table 18. These emission values seem high compared to the goal values, but are estimates for this combustor design based on the best available data. A test program follows this study phase where the NO~, emissions will be minimized while maintaining all other performance features. Several far-term duct burning concepts were also identified in this study. These duct burners would all have three-stages of fuel injection but the fuel would be premixed and prevaporized. Prevaporizing the fuel at the takeoff and transonic climb conditions is difficult as the burner inlet-air temperature is only about 444 K, near the initial boiling point of the fuel. Some designs are being considered where hot air from the core engine could be used to help vaporize the fuel. Other techniques such as heat pipes and exhaust gas recirculation will also be studied. A detailed discussion of this program will be found in Ref. 104. A schematic illustration of one duct burner configuration experimentally investigated by General Electric is shown in Fig. 27.1°6 This is a staged

CRUISE

4.25

combustion concept similar to those employed in the Experimental Clean Combustor Program where the pilot stage is optimized for low CO and THC emissions at cruise conditions and the main stages are optimized for low NO.~ during takeoff and acceleration. Main stage fuel is injected with spray bars located upstream of the combustor. The main fuel flow premixes with the air bypassing the pilot zone and combustion is stabilized by main stage V-gutter flameholders. The NOx emission goal for both programs is 1 g NOz/kg fuel at supersonic cruise at a combustion efficiency of 99% or higher. MAIN FUEL -~

~

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FIG. 27. Sketch of the General Electric low-emissions duct-burner.

Gas turbine engine emissions--problems, progress and future

103

TABLE 19. General Electric duct burner operating conditions and results.

S I~,TCLATED FLIGHT CONDITION

TEST PARAMETERS

SLTO

CLIMB

CRUISE

FLIGHT MACH NUMBER

0

i.0

2.4

ALTITUDE,

0

9.45

16.7

BURNER PRESSURE, arm.

3.8

2.1

2.6

INLET AIR TEMPERATURE, K

~55

~6

595

km

DESIGN FUEL-AIR RATIO

0.0513

0.0505

0.0202

2014

19~7

1283

CO, g/kg

24.2

33.1

24.2

THC, g/kg

18.5

17.7

23.1

N0 x, g/kg

1.06

1.18

1.19

0.0314

0.0339

0.0252

97.6

97.5

97.1

DESIGN EXIT TEMPERATURE,

K RESULTS

FUEL-AIR RATIO COMBUSTION EFFICIENCY,

Test results of the "best" configuration tested by General Electric are given in Table 19. To date~ the NO~ emissions of this duct burner have been very close to the program goal. However. as indicated by the levels of CO and THC this has been achieved at the expense of some reduction in combustion efficiency. These duct burners have exhibited the tendency for NO., emissions to increase exponentially as combustion efficiency increases from 98 towards 100°/o. Combustion instability, or screech, has also been a problem. The fuel/air ratios listed at takeoff and climb are near the maximum that can be tested without causing screech. At cruise a higher than design fuel/air ratio was needed to maintain a high combustion efficiency. Further work will be required to develop this concept into a practical duct burner for a supersonic cruise engine.

in this program should materially aid in the development of low NOx combustors for supersonic mrcraft engines. The prevaporized-premixed combustion technique will be studied in great detail. The effect of fuel drop size on emissions will be examined. The emission data shown in Fig. 28 was obtained by the General Applied Science Laboratories (GASLI from the experiment

47

EQUIVALENCE

3--

0.6~

RATIO, ~P

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Z

x" z

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Stratospheric cruise emission reduction propram

o_

The objective of this program is to develop and demonstrate the combustor technology required to reduce cruise NO, emissions by a factor of 6 or more from current levels and :o meet the EPA standards for emissions over the landing-takeoff cycle.l°~l oB The program efforts are focused on combustors for highbypass ratio, high-pressure ratio engines used on current subsonic transports. The technology developed

g z

1 m

0[

I

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1

8 INLETPRESSURE,PIN' atm.

1

12

FIG, 2~. Effect of inlet pressure and equivalence ratio on NO~ emissions from GASL experiment. Inlet temperature 900 K : Jet A fuel.

104

ROBERT E. JONES

shown in Fig. 17. The NOx emission index for several lean equivalence ratios is shown versus the pressure. The expected dependency of NO~ on the square root of pressure was not obtained with a minimum occurring near 8 atm. This result may be due to the prevaporizingpremixing process, in that improved vaporization and/ or mixing are occurring as the pressure increases. A similar result was obtained with the Vortex Air-Blast (VAB) combustor described previously. The high pressure test conducted with the VAB combustor also resulted in a su.dden decrease in NOx emissions as pressure increased. TM However, the tests conducted with various NASA swirl-can combustor modules over wide pressure ranges have not indicated any unusual pressure dependency: 7.so The conclusion is that some part of the premixing-prevaporizing process itself may be responsible in some still undetermined way. A clear understanding of these effects may lead to significant NOx reductions. The success of premixing and prevaporizing techniques depends critically upon our ability to predict and control autoignition and flashback. The flame tube studies conducted at GASL and NASA experienced autoignition, sometimes with drastic consequences. The autoignition problem for lean premixed combustors is illustrated in Fig. 29. The vaporization times shown were calculated from a simplified program developed at NASA and the autoignition delay times come from Ref. 109. It is apparent that for high pressure ratio engines if the fuel is given time to vaporize, autoignition may occur. The effect of drop size is critical. If the drop size can be kept small then evaporation can be completed prior to the onset of autoignition. The planned study will investigate autoignition of "premixed" fuel/ air streams at pressures up to 30atm. Emphasis will be placed on low equivalence ratio mixtures. Later the

100

effect of hot surfaces, boundary layers, and fuel type will be studied. Flashback and the effect of engine transients on flashback will also be investigat.ed. As the results of the early studies become available the data obtained will be applied to various combustor designs. As these designs evolve, an assessment will be made of their potential for reduced emissions and practical application. The most promising concepts will be selected for extended experimental development.

Fuel Specification Changes For the past 10 or more years the quality of aircraft gas turbine fuels has remained virtually constant. Fuel related emissions are essentially nil as the amounts of sulfur and fuel-bound nitrogen are extremely low. The concentration of aromatic compounds has been held below 20~o and as a result the atomic hydrogen/carbon ratio has been nearly constant at a value of 1.92. Fuel freezing points and distillation range have been controlled and kept within the ASTM specifications. In the past several years this situation has begun to change with the increase in the percentage of aromatic compounds allowed going up to 25%. This has been necessary due to the shortening supply of domestic crude oils, the Arab oil embargo, and the general deterioration in the quality of crude oils. This situation cannot be expected to improve and we must consider the fact that the fuels used in the past will not be available in quantity in the future. While high quality fuels can be made from virtually any source, the cost of maintaining the present fuel quality may become prohibitive. Other sources of oil are available such as tar sands, shale oil and crudes made from coal. The properties of fuels from such sources can be expected to vary markedly from present aircraft fuels. Changes in fuel specifications will impact combustor performance and the aircraft fuel system.

Combustor problems F- ,

A change in the allowable concentration of aromatics in the fuel will reduce the percent of hydrogen in the fuel as shown in Fig. 30. The figure indicates the range of variability in percent hydrogen in the fuel as aromatics content increases. The present specification for ASTM Jet-A fuel is 205/0aromatics which yields an

r-AUTOIGNITION TIME

8

1

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10 20 30 ENGINEPRESSURERATIO

1

40

FIG. 29. Fuel auto-ignition and droplet evaporation times of I00 and 25 ram drops for engines of various pressure ratios.

110

10

20

30

40

.50

60

I

I

70

80

AROMATICS CONTENT, vol.% FIG. 30. Variation of hydrogen content of fuel with in-

creasing aromatic content.

Gas turbine engine emissions--problems, progress and future average value for percent hydrogen of about 13.7%. The effect of lower hydrogen/carbon ratios will be to increase the flame radiation levels and the tendency of the combustor to emit smoke. 1~o-~ ~2 Several research efforts are being conducted to evaluate the effect of fuel specification changes. At NASA-Lewis the effect of varying the fuel hydrogen/carbon ratio is being studied using a single JTSD combustor canJ ~3 Test fuels were chosen to give wide variations in chemical composition, such as paraffins, aromatics and naphthenes as well as in fuel volatility. The study has been conducted at simulated idle and cruise conditions. The combustor performance with the test fuels has been judged on the basis of combustion efficiency, pollutant emissions, smoke, flame radiation as determined by changes in combustor liner temperature and combustor blowout.

I05

again. Fuel atomization would have to be improved by the use of air atomizers or prevaporized fuel to combat the smoke trend. Lean burning primary zones ~ould help to reduce the effects of increased flame radiation. Liner temperatures could also be kept low by the use of thermal barrier coatings. ~14 Recent studies using the same JTSD combustor have shown how thermal barrier coatings can reduce liner temperatures by up to 200 K as shown in Fig. 33.~ ~~

1240-IL~3-v

0 CRUISE A TAKEOFF

11~ --

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TAK OFF

CERAMIC-COATED LINER

5C

1080-4C

Z

o

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1000

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12 13 14 HYDROGEN. %

15

16

FIG. 31. Increase in JT8D combustor smoke number with decreasing hydrogen content of the fuel at cruise conditions. The results of these tests confirmed the expected trends. At idle operating conditions a decrease in the percent of hydrogen in the fuel causes combustion efficiency to decrease slightly. This is reflected by an increase in the idle emissions, CO and THC. At simulated cruise, changes in the hydrogen content of the fuel produced the effects shown in Fig. 31 and 32. There was a dramatic increase in smoke as shown in Fig. 31 and a nearly linear increase maximum combustor liner temperature, shown in Fig. 32. The effects of reduced hydrogen content on increased smoke and flame radiation have been well documented. The results indicate that serious problems must be resolved before such fuels are used. Aircraft smoke emissions, now virtually invisible, would increase into the visible range 1200~,~

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II0C-

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1000-

800

10

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HYDROGEN, % FIG. 32. Increasein JT8D cornbustor liner temperaturewith

decreasing hydrogen content of the fuel at cruise conditions.

/

ss"

CRUISE

9"20--

iC-

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I """"'0 f/

e"

I I I I I I I I 1100 n40 ns0 lm 1260 1300 AVERAGEEXHAUST-GAS TEMPERAIIJRE,K

FIG. 33. Effect of thermal barrier ceramic coating on maximum liner temperatures of a JT8D combustor al takeoff and cruise conditions with Jet A fuel. Measured levels of flame radiation and smoke were also reduced with the thermal barrier coating. The coating was found to have a reflectance 2-3 times greater than that of the uncoated liner walls. It is believed that the intense radiation from the ceramic coated walls back into the flame was responsible for these results. The reflected radiation could have reduced the amount of soot formed initially or enhanced soot combustion. Since hot soot particles account for most of the flame radiation, any reduction in soot concentration would appear as reduced flame radiation and reduced smoke. The levels of other pollutants were not affected. A similar fuels investigation was conducted as a part of the Experimental Clean Combustor Program. 63"6~ Although the number of fuels tested did not cover as wide a range as covered in the work of Ref. 115, they were sufficiently varied to indicate the trends that varying hydrogen concentration will have on advanced technology combustors. Table 20 shows the results of this program with the vorbix and hybrid combustors, see Fig. 9, at Pratt & Whitney. 6' The fuels listed in the table are arranged in order of increasing aromatic content. In general, there was an increase in the level of CO emissions at the idle condition with an increasing aromatic content in the fuel. At simulated engine takeoff conditions the emission indices of NOx do not vary

106

ROBERTE. JONES

TABLE20. Emissions for vorbix and hybrid combustors with various fuels. EMIS-

JET A

NUMBER 2

JET A

NUMBER 2

JET A

DIESEL

PLUS

HOME

PLUS

NAPH -

HEAT

XYLENE

SIONS

THENE AROMATICS,

%

18.0

27.0

35.5

38.5

47.9

3.1

3.2

3.4

46.0

54.0

4b.0

3.6 69.o

3.7 67.0

6.3

10.6

4.2

10.2

6.9

98.2

97.5

98.4

97.2

97.6

IDLE CONDITIONS VORBIX

NO a

COMBUSTOR

CO

X

THC EFF. HYBRID

NO

COMBUSTOR

CO

4.3

4.5

4.5

4.3

4.5

10.0

21.0

15.0

18.5

12.0

4.4

2.5

4.0

3.2

4.7

EFF.

99.2

99.2

99.2

99.2

99.2

VORBIX

NO a

15.6

14.7

16.1

CO

ll.0

15.7 12.1

14.9

COMBUSTOR

ll.0

5.8

.I

.i

.1

.I

EFF.

99.7

99.7

99.7

99.9

99.8

HYBRID

NO

18.3

18.6

22.0

20.8

21.3

COMBUSTOR

CO

5.2

2.4

3.0

2.5

X

THC

SLTO CONDITIONS X

THC

X

THC EFF.

8.1 0

.4

.2

.2

.i

3.0 .2

99.8

99.9

99.9

99.9

99.9

a All emission values are emission index, g/kg and are corrected to engine design table values.

greatly and no clear trend with fuel property is obvious. Similar results were observed with regard to idle leanblowout fuel/air ratios. Variations between fuels were minor and no clear trend with fuel property was discovered. An 8 hour endurance test program was conducted with No. 2 Home Heat fuel with both the hybrid and vorbix combustors, Fig. 9. As expected there were carbon deposition and durability problems. However, the carbon deposition problem was more severe in the more conventionally designed pilot zone of the vorbix combustor than in the premixing pilot zone of the hybrid combustor, Similar results were found by comparing smoke emissions of the two combustors. Smoke levels of the vorbix increased from an SAE smoke number of 4 with Jet A fuel to a value near 20 for the Jet A Naphthalene blend. No such trend was noted for the hybrid combustor and smoke levels with all fuels were less than an SAE smoke number of 5. A similar effort was conducted at General Electric using the three combustors shown in Fig, 7, the standard production CF6-50 combustor and the two low emission combustors. The effects obtained with the conventional production CF6-50 combustor were more

pronounced than those obtained with the low emissions combustors. Increased final boiling point of the fuel resulted in increases in CO, THC and smoke emission levels but appeared to have no effect on NO x emissions or peak metal temperatures. Decreases in hydrogen content of the fuel resulted in increases in all pollutant levels and increases in liner temperature. In general the effects of fuel variation were quite moderate but the trends were as anticipated. As in the program with Pratt & Whitney, it was also noted here that the advanced technology low emission combustors were less sensitive to fuel specification changes than the more conventional combustor designs. This is illustrated in Fig. 34 where maximum liner temperatures are shown for the various fuels tested as a function of hydrogen content of the fuel. The NASA data with the JTSD combustor can is shown along with the data from the production CF6-50 combustor and the ECCP low emission combustors, JT9D vorbix and CF6-50 double/annular. As shown, the advanced technology combustors are insensitive to the hydrogen content variation of the fuel. This is because the main power stages of these combustors are

Gz~sturbine engine emissions- problems, progress and fuu}re

10T

TABLIE21. Properties of Jet A and shale oil. Jet A fuels

FUEL TYPE

JET A

SHALE JET A

SPECIFIC

GRAVITY @ 298 K

FLASH POINT, K

0.8080

0.8057

330

329

453 476 488

444

~96

508 521

43,320

43,304

14.9

17.3

1.5

0.6

<3

<3

13.72

13.56

0.07

0.03

DISTILLATION INITIAL BOILING POINT, 20~ 50 ~

K

7O% 90~, K NET HEATING VALUE, AROMATICS,

J/g

VOL %

OLEFINS, VOL % NAPHTHENES, HYDROGEN,

VOL %

WT %

SULFUR, WT % NITROGEN,

WT PPM

,- JTgD VORB IX COMBUSTOR / {SECTOR)

/

500--

"~_

,,-CF6-50FULL-ANNULUS ,' COMBUSTOR

v

400 --

._J

_z_g, z_

x

;

o~,,..

I

~

'- CF6-50DOUBLEANNULAR COMBUSTOR(FULL-ANNULUS) I I I I

_.J

< ~

7JI'BD COMBUSTOR

300--

z 100 11

r 12

13

14

15

492

813

designed for fuel-lean operation thus tending to offset the increased flame radiation caused by a decrease in hydrogen content of the fuel. The fuels used in these programs differ markedly from Jet A fuel in the percent of aromatics, the

~-" :E

474

16

HYDROGENCONENT, wt.% FiG. 34. Effectof hydrogen content of the fuel on combustor liner surface temperatures.

freezing point, and final boiling point. Fuels meeting the Jet A specifications can be produced from a variety of petroleum and nonpetroleum sources. Such a fuel was obtained from shale oi1.116 Table 21 compares the properties of this fuel with Jet A fuel. The listed properties of the two fuels are virtually the same with the exception of the high, 800 "~ppm level of fuel-bound nitrogen in the shale Jet A. Quantities of this fuel were tested by the Air Force, by General Electric in the production CF6o50combustor and by NASA in a JTSD combustor) ~m63.~14 Results obtained with the JT8D combustor are shown in Fig. 35. Smoke number and liner temperatures are essentially the same for both fuels as would be expected. The emissions of NOx are generally 2 emission index values higher with the shale Jet A due to the high fuel-bound nitrogen content. Similar results were obtained by General Electric with the CF6-50 combustor, in that no change in combustor performance or emissions was noted other than an increase in NO x emissions. A 100", conversion of the fuel-bound nitrogen would have increased the NO~ emission index by a value of 2.6gNO2/kg fuel. Measured increases in NO~ emis-

108

ROBERT E. JONES

STANDARD JET A OIL-SHALE JP-5

....

14 X~,,-

z_o

13 12

z~

11 10 9

I

1100 m

I

I

./

~' S SS

=~ o= zooom

900

S SS

I

I (1 O1

]

I

O. 012 (1 014 FUEL-AIR RATIO

I

The effect new fuels may have on the elastomers in the aircraft fuel system is being studied at the Jet Propulsions Laboratories. t ~8.t 19 The extent of chemical change caused by exposure to alternate fuels is being determined by stress relaxation studies at relevant conditions of temperature and exposure time. Chemical analysis of the elastomers, calculations of solubility parameter and tests to determine physical compatibility are also being conducted. Studies such as this are necessary to insure that reliable materials are used in future aircraft fuel systems. Some preliminary results have been obtained that compare the stress relaxation of Viton B elastomer in air and in Jet A fuels at 423 K as shown in Fig. 36. The shale oil Jet A fuel produces a significantly greater degradation in the Viton B than the conventional Jet A fuel. It has been postulated that the higher aromatic and gum content of the shale oil Jet A may be responsible for these results. Tests are planned to determine the causes for the difference in behaviour with the two fuels. 0 AIR [ ] JET-A O SHALE-OILJET-A

I (1 016

1.0

.8

FIG. 35. Comparison of shale oil Jet A and Jet A fuels at cruise conditions of a JT8D eombustor. sions indicate that between 40 to 800/0of the fuel-bound nitrogen was converted to NO2, depending upon the combustor operating conditions. These results are instructive from several standpoints. First, a fuel meeting the present Jet A specifications was readily refined from a shale oil crude. Combustor performance with this fuel was the same as with conventional Jet A. This fuel had a high fuelbound nitrogen content and removal of the fuel-bound nitrogen would require hydrotreatment of the fuel, further increasing its cost. Secondly, most of the fuelbound nitrogen entered the atmosphere as an increase in combustor N O~ emissions, tending to counteract any low NOx emission characteristics obtained by advanced combustor technology. For those applications where ultra low NOx emissions are required, the fuelbound nitrogen must be removed from the fuel. Therefore low cost removal processes for the nitrogen must be found. For those applications where fuel cost predominates, NO., emissions may have to be controlled by combustor technology. Techniques to burn high nitrogen fuels with low NO~ emissions must be perfected.

Aircraft problems The fuel specification changes that have been allowed have not created any serious aircraft fuel system problems. However, any subsequent changes must be evaluated in detail. The primary areas of concern are effects new fuels may have on various elastomers used as gaskets and seals in the fuel system and on an elevation of the fuel freezing point.

.4 o'}

.2 0

I

I

5 TIME, hr

10

FtG. 36. Comparison of stress-relaxation forces of Viton B in air and in Jet A fuels at 423 K. An increase in the fuel freezing point could result from a broadening of present fuel specifications. An increase in fuel freeze point means that for long flights, the fuel could solidify or gel in the fuel tanks. This problem and its potential solution are presently being studied by Boeing under NASA contract, t2° The typical spread in freezing point of a hydrocarbon fuel blend as a function of final boiling point is shown in Fig. 37. Freezing point is somewhat of a misnomer here since a jet fuel consists of many different organic compounds, and only a pure compound solidifies at a constant and definite temperature. For jet fuels the freezing point is defined by the final disappearance of solid hydrocarbon crystals in the liquid phase. The wide spread in freezing point for a given final boiling point is due to the variations in the types and concentrations of organic compounds found in fuels refined from different crude sources. The only difference between the specifications for Jet A and Jet A-1 is the maximum allowable freezing point, which is 233 K ( - 4 0 ° F ) and

Gas turbine engine emissions--problems, progress and future

280

2 °

2~ m

450

PEC,

I

I

5O0

550

I

600

I

650

FINAL BOILINGPOINT, K FIG. 37. The variation of fuel freezing point with final boiling point. 223 K ( - 5 8 ° F ) , respectively. The freezing point for diesel No. 2 is considerably higher and varies from about 250-255 K ( - 10 to 0°F). An extreme variation in wing tank fuel temperature over a long distance flight is shown in Fig. 38. As a safety margin for avoiding fuel line plugging, the United States Federal Aviation Administration (FAA) requires that the tank fuel temperature be maintained at least 3 K (5.4°F) above the freezing point of the fuel being used. For the extreme example shown, the tank fuel temperature will fall below the safety margin for Jet A after flying about 3700km. This fgure illustrates the necessity of using Jet A-1, which has a lower freezing point for long distance flights. It is interesting to observe that the effect of initial fuel temperature on tank fuel temperature becomes negligible for long flight times.

280KINITIAL 270L_~, FUELTEMP, 278

26O

z507

230~240t-- 2 ~ . 220L , 0

I

.A FREEZEPOINT tJETA-:'I-I:I~'E'F.ZE~ 0"IN1

I I 2000 4000 6000 8000 RANGE, KILOMETERS

FIG. 38. Calculated extreme minimum fuel temperature for long range flights of 747 aircraft. Several flight operational methods may be considered for maintaining a fuel above its freezing point. In the event that the measured tank fuel temperature approaches the safety margin, the tank fuel temperature may be increased by increasing flight Mach number, reducing flight altitude, or altering course to avoid cold air masses. All of these approaches penalize fuel consumption. Switching from outboard to inboard fuel tanks, which are at a slightly higher temperature, is relatively limited in effectiveness. As shown in the previous figure, preheating fuel on the ground has a negligible effect on the fuel tank temperature for a long flight. However, preheating fuel on the ground might be necessary during the winter in some regions just to

10q

transfer a broader specification fuel such as Diesel No. 2 to the aircraft. The use of broader specification fuel such as Diesel No. 2 would require a major redesign of the airframe fuel system, Insulating the fuel tanks could provide a partial solution. However, heating the fuel in the tank during flight would probably be required to maintain the tank fuel temperature above the freezing point. Several approaches to heating the fuel during the flight could be considered. Fuel could be recirculated through the engine heat exchanger and returned to the fuel tank. This approach would probably require changes to the present design for the fuel pumps and engine heat exchanger. Another approach could be the addition of fuel tank heaters. Whatever method is used. high local fuel temperature must be avoided to prevent gumming of fuel passages due to degradation of the fuel.

Low Quality Fuels Looking to the future, liquid fuels from other sources such as tar sands, shale oils and coal may become the predominant source of aircraft fuel. Fuels derived from these sources generally have higher levels of sulfur, fuelbound nitrogen and trace elements than fuels from conventional crude oils. If these fuels are to be used for aircraft gas turbine engines they will have to be upgraded and refined to a quality level close to that of today's fuels. This applies particularly to fuel contaminants such as sulfur and the trace metals. Ways may be found to control and minimize the emissions of fuel-bound nitrogen. Programs presently exist under EPA sponsorship to investigate rich burning combustor concepts to minimize NO~ emissions resulting from higher levels of fuel-bound nitrogen. Hopefully, refinery treatment of the fuel can eliminate fuel-bound nitrogen because the combustor concepts presently being studied may not be able to control both smoke and NOx emissions simultaneously. If aircraft cruise NO~ emission control becomes a necessity, the use of fuels with fuel-bound nitrogen would substantially offset any gains made by advanced combustor technology. There is a demand for low quality fuels, regardless of the source, primarily due to cost. Aircraft-type gas turbine engines are being used in an ever widening variety of applications. Ground power generation and ship propulsion are two areas where large gas turbine engines are being used. Since the cost of power is directly related to the cost of fuel, the operators want to use the cheapest fuel practical. In many cases high quality crude oils meet that requirement. The combustor and engine problems encountered when such fuels are used are quite formidable. The increases in smoke and liner temperatures have been mentioned previously and these problems will worsen with the use of still lower quality fuels. Tests with low quality fuels have resulted in significant carbon deposition on the combustor liner. Major advances in combustor technology are required to successfully burn low quality

110

ROBERT E. JONES

fuels while meeting the required EPA standards and retaining combustor durability While it seems unlikely that aircraft will ever use such fuels, the use of such fuels for power generation or propulsion is highly probable in the not too distant future. However, aircraft fuels may degrade to a level similar to present Diesel No. 2 specifications. Considerable efforts will be required to develop combustors that can achieve high performance and low emissions with such a fuel. A major effort will also be required in the aircraft fuel system. The high freezing point and the lower thermal stability of such fuels will require new fuel system designs as well as limiting the extent to which the fuel may be used to cool engine oil or other components. THE FUTURE OF EMISSION REGULATIONS

By now it should be apparent that emission regulations are here to stay. The aircraft engine emission standards issued in 1973 are presently being revised. In March 1978 the EPA issued revised standards and regulations. The new standards differ from the previous standards in several important aspects. The implementation date for the standards has been generally pushed back, 1984 now being the implementation date for newly certified engines. Emission regulation has been removed from piston engines (PI class), small gas turbine engines (T1 class) below 27,000N thrust (60701b thrust) and turbo-prop engines (P2 class) below 2000kW (2680h.p.). Emission levels of CO and THC are generally the same as previously promulgated, but the NO x standard has been increased in value by o/ In addition a pressure ratio correction is to be 33/0. applied to the NOx standard. The correction assumes an engine with a compressor pressure ratio of 25 to be at the nominal value of the standard. Engines having pressure ratios less than 25 have a constant NO~ emission standard while those engines of pressure ratio greater than 25 can have higher NO~ emission levels. These changes to the standards reflect the input of the NASA combustor technology programs and the experience and comments of all interested and affected parties. Regulation of aircraft emissions in the stratosphere is still a possibility and will require a major effort to achieve the emissions reductions that have been proposed. A change in fuel specifications and the use of low quality fuels would seriously impact the progress that has been made in controlling emissions. It would, however, be unrealistic to ignore the fuel situation. Clearly efforts should be started to optimize combustor performance and emissions with low quality fuels. If these efforts are successful then the desired levels of emissions control will be achieved. If they are unsuccessful, then emission standards may have to be altered. It is, however, more than unrealistic to think that emission standards will be altered until thorough and extensive research shows that there is no alternative.

CONCLUDING REMARKS

The NASA in-house and sponsored research programs to meet the 1979 EPA Aircraft Emission Standards have been in the main, extremely successful. The advanced technology combustors demonstrated substantially reduced emissions as well as surprisingly good performance considering their limited state of development. These programs illustrate that practical combustors can be built that are capable of meeting stringent emission goals. The problem of achieving the NOx standard as engine pressure ratio increases is still formidable. Some modification of the NO~ standard to account for pressure ratio effects is needed and has been recognized in the latest revision to the standards. Efforts to produce ultra low levels of pollutants are only in the early developmental stages. Much further work will be required before such concepts can be translated into practical combustion systems. The problems posed by changes to aircraft fuels require immediate attention. In the past research efforts have not emphasized the effects caused by changes in the fuel. Many new efforts should be started that will emphasize the effect of the fuel character on combustor design. The advanced technology combustors described in this paper appear to be extremely complicated in design. Certainly from a purely mechanical standpoint these combustors are more complex than combustors used in the past. From a combustor modeling point of view, however, the opposite is true. Previous combustor designs, characterized by single stage fuel injection and large quantities of liner and dilution jet air flow, are virtually impossible to model successfully. By contrast, the advanced technology combustors employ staged combustion and each stage is designed and optimized for a given task. The more advanced concepts being studied for ultra low emissions employ premixing and/ or prevaporizing concepts. The modeling of these combustor concepts is much simpler than for combustors of the past. The potential now exists for mathematical models to successfully predict the effect of design changes on the performance and emissions of an advanced technology combustor.

REFERENCES

1. SAWYER, R. F., Atmospheric pollution by aircraft engines and fuels--a survey. AGARD Advisory Report No. 40 (March 1972). 2. Secretary of Health, Education and Welfare, Nature and control of aircraft engine exhaust emissions; Report to the United States Congress, Senate Document No. 91-9, Pursuant to Public Law 90-148, Air Qualit, Act of 1967, December 1968. US Government Printing Office (March 1969). 3. Los Angeles County Air Pollution Control District. Profile of air pollution control (1971). 4. Environmental Studies: Aviation effect on air quality in the Bay Region. Report. Association of Bay Area Governments. Regional Airport Systems Study, Berkeley, California [February 19711. 5. CHAMPAGNE, n . L., Parameters affecting the measurements of aero engine smoke. Air Force Systems

Gas turbine engine emissions--problems, progres~ and future

Command, Wrighl-Patterson Air Force Base, Ohio 4FAPL-TR-70-23 ( 1970/. 6. (HAMPAGNE. D. L,. Standard measurement of aircraft gas turbine exhaust smoke, ASME Paper No, 71-GT-68 ( 1971 ). .. BAHR D. W. et al., Development of low smoke emission combustor for large aircraft turbine engines, AIAA Paper No. 69-493 (19691. 8. Los Angeles County Air Pollution Control District, Study of jet aircraft emissions and air quality in the vicinity of the Los Angeles International Airport. APTD-0662 (PB-198699)(April 19711. 9. NOLAN, M., A survey of air pollution in communities around the John F. Kennedy International Airport, September-October 1964. Robert A. Taft Sanitar~ Engineering Center, Cincinnati. Ohio (PB-216662t (June 1966). 10. DONALDSON, W, F., Air pollution by et aircraft at Seattle-Tacoma Airport, US Weather Bureau. Western Region. WBTM WR 58 (1970). 11. PARKER, J.. Air pollution at Heathro~ Airport, London: April-September 1970. SAE Paper No. 710324. In: S A E ' D O T Cm!ference on Aircraft and the Environment, P-37, part 1. Society of Automotive Engineers ( 1971 ). 12, Mn_FORD. N. et al.. Dispersion modeling of airport pollution. Sa, E Paper No. 710325. In: S A E D O T Ct)l~lerem'e on Airo'qft and the Environment. P-37, part 1. Society of Automotive Engineers {1971 ). 13. HEYWOOD. J, B. et al.. Jet Aircraft pollutant production and dispersion. AIAA Paper No, 70-115 (January 19701. 14. BocclO. J. L., WEILERSTEIN, G. and EDELMAN. R. B., A mathematical model for jet engine combustor pollutant emissions. GASL-TR-781, General Applied Science Labs,. Inc.: NASA CR-121208 (1973). 15. OTERCAMP.T. J. and FAY. J. A., Dispersion and subsidence of the exhaust of a supersonic transport in the stratosphere. AIAA paper 72-650, June 1973: d. Aircr. 10. 702-728 (19731. 16. BASTg~SS.E. K. et al.. Assessment of aircraft emission control technology. Report NREC-1168-1. Northern Research and Engineering Corp. ( 1971 ). 17. United States Environment Protection Agency Aircraft emissions: Impact on air quality and feasability of control 119721. 18. Title 40. Protection of environment; Part 8 7 1 Aircraft and aircraft engines, proposed standards for control of air pollution. Fed Rectist. 37, 26488-265133 (December 19721. 19. Environmental Protection Agency. Control of air pollution for aircraft engines--emission standards and test procedures for aircraft. Fed. Reoist. 3K 1908819103 {July 1973). 20. RUDEr, R1CHARD A.. Status of technological advancements for reducing aircraft gas turbine engine pollutant emissions. NASA TM X-71846 (1975). 21. NtEDZWIECKI, RWHARD W., The Experimental Clean Combustor Program--description and status to November 1975. NASA TM X-71849 (19751. 22. BARTH. DEL~ERT S., Effects of air pollution on man and his environment. SAE Paper No. 7103'26. In: S A E ' D O T Conference on Aircraft and the Environment, P-37. part 2, 28-39. Society of Automotive Engineers 119711. ,~,~. JOHNSTON. H., Science 173, 517-522 (19711. 24. MAC'HTA. LESTER, Water vapor pollution of the upper atmosphere by aircraft. SAE Paper No. 710323. In: S,4E 'DOT Co~iference on Aircrqft amJ the Environment, P-37. part 1. Society of Automotive Engineers ¢19711. 25. NUESSI_E. V D. and HOLCOMK R. W.. Will the SST pollute the stratosphere? Science 168, 1562 (1970). 26, GROBECKER.A. J., CORONtTE, S. C. and CANNON, R. H. JR.. Report of findings. The effects of stratospheric J P I.( ~ 4 2

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32. 33.

I I1

pollution bx aircraft. Department of Transporu~tion Report DOT-TST-75-50 {19741. National Research Council. Climatic Impact ( o m mittee. Environmental impact of stratospheric flight. National Academy of Sciences, Washington. D.C. (1975l. Environmental Protection Agency. ControI of air pollution from aircraft and aircraft engines-supersonic aircraft. Fed. Re:#st. 41. 34722-34725. (August 19761. Aircraft gas turbine exhaust smoke measurement. Aerospace recommended practice 1179. Society of Automotive Engineers {1970i. Environmental Protection Agency. Standards of performance for nex~ stationary sources. Fed. Re
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35.

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38.

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40.

41.

42.

43.

44.

45. 46.

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d,7. NIEDZWIECKI, RIr'HARD W. a n d JONES. ROBERq ~.. T h e

Experimental Clean Combustor Program--description

112

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