The Advanced Low Emissions Catalytic Combustors Program (ALECC) is being undertaken to evaluate the feasibility of employing catalytic combustion technology in aircraft gas turbine engines as a means to control emission of oxides of nitrogen during subsonic stratospheric cruise operation. The ALECC Program is being conducted in three phases. The first phase, which was completed in November, 1978, consisted of a design study to identify catalytic combustor designs having the greatest potential to meet the emissions and performance goals specified. The primary emissions goal of this program was to obtain cruise NO emissions of less than 1g/kg (compared with levels of 15 to 20 g/x obtained with current designs)/ However, good overall performance and feasibility for engine development were heavily weighted in the evaluation of combustor designs

Dodds, W. J.

English

NASA Technical Reports Server

PHASE 1 - DESIGI4 STUDY 
NASA PROGRAM MANAGER - A, J'. SZANISZLO 
GE PROGRAM MANAGER - A s  L. MEYER 
TECHNICAL PROGRAM FIANAGER - D, W, BAHR 
PRINCIPAL INVESTIGATOR - C a  C B  GLEASOM 
I 
COMBUSTION ENGINEER - w # '  J DODDS 
https://ntrs.nasa.gov/search.jsp?R=19790016840 2020-03-21T22:17:24+00:00Z
INTRODUCTION 
The Advanced Low Emissions Catalytic Combustor Program (ALECC) is 
being undertaken to evaluate the feasibility of employing catalytic 
combustion technology in aircraft gas turbine engines as a means to 
control emission of oxides of nitrogen during subsonic stratospheric 
cruise operation. The ALECC Program is being conducted in three 
phases, as illustrated in Table I. The first phase, which was com- 
pleted in November, 1978, consisted of a design study to identify 
catalytic c~mbustor designs having the greatest potential to meet 
the emissions and performance goals specified in Table 11. The 
primary emissions goal of this program was to obtain cruise NOx emis- 
sions of less than lg/kg (compared with levels of 15 to 20 g/kg obtained 
with current designs). However, good overall performance and feasi- 
bility for engine development were heavily weighted in the evaluation 
of combustor designs. The General Electric design effort was supported 
by a subcontract with Engelhard Industries, specialists in the catalytic 
combustion field. 
Catalytic Combustor Design Considerations 
Reference Engine Operating requirements are compared with projected 
catalyst performance in Table 111, Performance projections in this 
table were based on Engelhard Industries estimates of catalyst develop- 
ment over a 5 to 10 year period. 
It is apparent that the catalyst cannot cover the entire range of opera- 
tion. Specifically, idle inlet temperature is not high enough for 
catalyst ignition, and exit temperatures at the idle, approach, and 
minimum cruise conditions are too low to obtain high combustion 
efficiency. 
Catalyst combustion efficiency characteristics as a function of fuel/ 
air ratio(at constant inlet conditions) are shown in Figure 1, In 
order to obtain efficiency above 99.9%, the fuel/air ratio must be high 
enough to assure operation in the catalytically supported homogeneous 
combustion mode. Also shown in Figure 1 is the maximum fuel/air ratio 
corresponding to the catalyst maximum use temperature. In order to 
obtain high efficiency and avoid exceeding the catalyst maximum use 
temperature at the conditions shown, the mixture entering the catalyst 
must be between fuel/air ratios of about 24 and 35 g/kg. This provides 
for about +20% spacial variation in mixture uniformity if average fuel/ 
air ratio is exactly 29.5 g/kg. However, in practice, mixture uni- 
formity within about +lo% will be required to allow some margin for 
fuel injector deterioration and control system inaccuracy. 
Obtaining a uniform, fully evaporated fuel/air mixture is complicated 
by autoignition considerations, Autoignition delay times predicted 
based on References 1-3 are between 9.6 and 16.1 ms at the maximum 
cruise conditions, decreasing to between 2.2 and 3.1 ms at hot day 
takeoff. Within this period fuel must be injected, evaporated and 
thoroughly nixed with the air stream. 
Principal catalytic combusto? design considerations and possible 
design solutions are summarized in Table IV, 
Combustor Conceptual Design 
The six catalytic combustor conceptual designs are shown in 
Figures 2 - 7 .  APk of these concepts incorporate (1) a conventional 
pilot stage designed specifically for relight and low idle emissions, 
and (2) a lean premixed catalytic stage sized specifically for ultralsw 
NO, emissions at cruise. 
Concept 1(Figure 2) is a basic series staged combustor design, At 
power levels up to about 25% of rated thrust, only the pilot stage 
is fueled, and the catalyst is used as a cleanup device, A& power 
levels above 25%,  the pilot stage is cut back and fuel is injected 
through multiple point injectors located in the 90 main stage mixing 
chutes. This fuel is atomized by, and mixed with approximately 40% 
of combustor airflow which also passes through the chutes, During 
intermediate power operation, sector fueling is used to control catalyst 
inlet fuel/air ratio. Combustor pressure drop with this combustor is 
between 5 and 6% at all operating conditions. 
The cross-sectional area of this combustor is reduced at the plane of 
the mixing chutes to accelerate the pilot stream, improving the velocity 
profile at the fuel injection plane and increasing the fuel/air mixing 
length, which is limited by autoignition requirements. Immediately 
upstream of the catalyst, the flow is rapidly diffused to the velocity 
required to obtain acceptable conversion and pressure drop. 
This series staged design provides good emissions reduction potential 
because all fuel is reacted in the catalyst at all operating conditions, 
However, a major problem with this design is obtaining uniform fuel/air 
mixtures and avoiding autoignition with the main stage fuel injector 
system. This design also suffers because of increased system length 
and the difficulty of cooling the fuel injector chutes. 
Concept 2(Figure 3)is a series staged combustor which incorporates (1) 
variable geometry, (2) a folded pilot burner, (3) external fuel/air 
nixing chutes, and (4) a third combustion stage downstream of the 
catalyst. At low power operating conditions, the variable geometry 
vanes are closed and all fuel is burned in the pilot stage. At inter- 
mediate and high power conditions, the variable vanes are opened, and 
fuel is injected through multiple point injectors located in the mixing 
ducts, At takeoff conditions and during transients, the third fuel 
injector stage may be fueled to avoid catalyst over temperature. As 
in the basic series staged design, circumferential fuel staging is 
utilized for catalyst fuel/air ratio control during intermediate power 
operation. 
The use of variable geometry in this concept allows catalyst pressure 
drop to be increased relative to the basic series staged design, and 
a lso  increases the air flow admitted through the fuel injection 
chutes to about 70% of combustor air flow at cruise conditionsg The 
use of external fuel injection chutes eliminates the chute cooling 
problem encountered with Concept 9, and the reverse flow pilot stage 
provides some length reduct.ion relative to the basic series staged 
eombustor. In the analysis of this design, it was determined that 
the takeoff stage shown in Figure 3 would not be required if combustos 
aft section film cooling flow was eliminated and used instead as 
catalyst air flow. Since the takeoff stage positioned at the catalyst 
exit was considered a high risk design feature, a revised design in 
which the takeoff stage was removed and the aft section was convectively 
cooled using turbine cooling air was considered in the final eombustor 
evaluation. 
The increased fuel injection chute air flow in this concept tends to 
decrease fuellair mixing requirements relative to the basic series 
staged design. However, obtaining uniform catalyst inlet %uel/air mix- 
tures without encountering autoignition still presents a difficult 
problem, Other problem areas with this design are increased idle pres- 
sure drop (10% vs. 5% at cruise), and additional mechanical design and 
operational complexity. 
Concept 3 (Figure 4) is an annular, parallel-staged combustor, In this 
design, approximately 40% of the combustor air flow is used for pilot 
dome combustion and liner cooling air. The remaining 60% is used as 
catalyst air flow. Only the pilot stage is operated up to about 25% 
thrust. Above this level, pilot stage fuel flow is minimized and a 
majority of fuel flow is routed to the catalyst stage, At intermediate 
power levels, sector burning is utilized to control catalyst inlet fuel/ 
air ratio. At higher power levels and during transients catalyst stage 
fuel flow is limited by catalyst maximum use temperature, and excess 
fuel is injected into the pilot stage. At the normal cruise condition, 
approximately 25% of the fuel is burned in the pilot stage. 
Catalyst stage fuel flow is injected from orifices located in the central 
splitter vane of the inlet diffuser, A nominal flow velocity of 61 m/s 
is used in the prenixing duct to provide adequate mixing length while 
meeting the 2 ms autoignition delay time requirement, Immediately up- 
stream of the catalyst, the duct area is rapidly increased. The duct 
walls in this region are contoured to simulate the streamlines which 
would be observed in unconfined flow approaching the catalyst blockage, 
Concept $(Figure 5) is a similar parallel staged design except that 
a cannular catalyst stage consisting of 30 cylindrical catalytic re- 
actors is used. This catalyst stage has been relocated outboard of the 
pilot stage, and a reverse-flow configuration has been used to decrease 
combustor length, This cannular design provides advantages in %uel/air 
mixing duct velocity profile control and catalytic reactor access. Very 
uniform combustor exit temperature profiles are anticipated with ehis 
design because of improved mixing between the pilot and catalyst stages, 
Catalyst stage emissions and performance are also expected to bc 
markedly improved during sector burning because individual reactors caw 
be fueled, and the lean "fringe" area between fueled and eawfueled 
annular sectors is avoided. 
A problem common to both Concepts 3 and 4 is the inability to meet 
cruise NO, goals because a relatively large proportion of fuel (about 
25%) must be burned in the pilot stage to avoid catalyst over tempera- 
Lure during cruise operation, In Concept 5(Figure 6), catalyst air 
$%ow at cruise condition is increased from 60 to about 80% by the 
use of variable geometry, thereby enabling approximately 95% of com- 
bustor fuel flow to be reacted in the catalyst. Within this concept, 
low power operations are conducted with the variable vanes closed, 
Under these conditions flow splits are similar to Concepts 3 and 4, 
but combustor pressure drop is increased to about 15%. Above the 25% 
thrust level, the variable geometry vanes are opened to increase 
catalyst flow to about 80% and reduce pressure drop to the 5% design 
level, As with Concept 2, the takeoff stage shown in this design was 
eliminated for the final evaluation. 
Although NOx emissions reduction potential is improved with this design, 
the use of variable geometry results in a significant increase in 
mechanical complexity and control requirements. Of concern is de- 
creased compressor stall margin due to increased pressure drop during 
idle operation, which leads to an increased risk of stall during tran- 
sient operation. 
Concept 6CFiguse 7) is essentially two complete combustors in parallel. 
A11 operations within the landing/takeoff cycle are conducted with the 
outer combustor, which is a piloted premixing design based on the radial/ 
axial configuration investigated in the NASAJGE Experimental Clean Com- 
bustor Program (Reference 4). When this combustor is in operation, the 
catalytic combustor vanes are closed, and only about 10% leakage flow 
passes through the catalyst, At cruise conditions, the variable geome- 
try vanes are rotated to direct as much as 90% of combustor air flow 
to the catalytic combustor located in the inner annulus. Instead of 
sector burning in this design, catalyst inlet fuel/air ratio is controlle 
by opening the main stage control vanes to bypass air flow around the 
catalytic combustor, 
This design approach takes maximum advantage of conventional combustor 
design technology since the outboard mounted combustor used for landing/ 
takeoff maneuvers can be of conventional design. The required range 
of operation of the catalytic combustor is thus limited to cruise range 
conditions, which results in less severe operating constraints for the 
catalytic reactor and fuel/air carburetion system, On the other hand, 
this system does not take advantage of the catalytic combustor emis- 
sions reduction potential during landing/takeoff maneuvers. Transition 
from main combustion to catalytic combustor operation also presents a 
major control challenge, and overall system length, weight, and com- 
plexity are increased with this design. 
Conce~t Evaluation 
A conceptual design evaluation summary is presented in Table V, where 
the six conceptual designs are ranked with respect to predicted emission 
and developmental risk in several areas of combustor performance. The 
overall trend observed in the evaluation of these concepts is 
increased emissions reduction potential with increasing develop- 
ment risk. The parallel staged, non variable geometry concepts 
(BP and CRP) consistently sated highest in performance, but were 
lowest rated with respect to emissnons. Therefore, %he selection 
sf &he two most p~omising designs depended largely on the relative 
weighting of emissions and performance, 
Predicted emissions for each of the combustor designs are presented 
in Table V I .  As indicated in this figure, although cruise NO 
emissions for Concepts 3 and 4 were 2 to 3 times as high as tHose 
of the other concepts, absolute levels were an order of magnitude 
lower than emissions obtained with current technology combustors, 
These two concepts were therefore, selected for further study, 
Preliminary Designs 
Preliminary designs of the selected concepts are shown in Figure 8, 
In these designs, major emphasis was placed on reducing combustor 
length and increasing catalyst air flow. Features used to increase 
catalyst air flow include the reduction of film cooling in the pilot 
dome and combustor aft section. By incorporating these features, 
predicted cruise NOx levels are decreased to about 1 . 2  to 1.4 g/kg, 
compared to about 2 g/kg in the conceptual designs, 
Conclusions 
- 
Based on ALECC Phase I studies, catalytic combustion appears to be 
s promising means for obtaining ultra low NO, emissions at aircraft 
cruise operating conditions. Levels below 2 g/kg appear to be 
obtainable without the use of variable geometry, Circumferential 
duel staging appears to be a viable means for controlling catalyst 
inlet fuellair ratio, Circumferentially non-uniform exit temperature 
patterns resulting from circumferential staging are expected to be 
acceptable because of the relatively low peak exit temperatures, 
which are catalyst limited, 
Major challenges in the application of catalytic combustion to 
practical aircraft combustors include the following: 
- Development of fuel/air carburetion systems to meet 
mixing and autoignition criteria, 
- Development of catalyst, support, and mounting 
.systems to obtain good high cycle performance 
(durabiEity, thermal shock resistence), 
- Development of advanced liner cooling techniques 
to reduce cooling flow requirements. ,A&* 
- Development of precise fuellair ratio sensing 
and control techniques, 
- Determination of the effects of circumferential 
staging on catalyst performance. 
These will be major areas of study in the ALECC Phase I1 and III 
experimental programs. 
REFERENCES 
% , Marek, CJ; Papathakos, LC; and Verbulecz, PW: qqPreEiminary 
Studies of Autoignition and Flashback in a Premixing- 
Brevaporizing Flame Tube using Jet A Fuel at Lean Equivalence 
Rati~s", NASA TM X-3526, May, 1977, 
2. Stringer,FW; Clarke, AE; and Clarke, JS: "The Spontaneous 
Ignition of Hydrocarbon Fuels in a Flowing Systemss, Paper 
No. 20 ,  Symposium on Diesel Engine Combustion, Institution 
of Mech. Engineers, London, April, 1970, Proceedings Pages 
198-221, 
3. Spadaccini, LJ; "Autoignition Characteristics of Hydrocarbon 
Fuels at Elevated Temperatures and PressuresPv, ASME 76-GT-3, 
March, 1976. 
4. Bahr, DW; and Gleason, CC: "Experimental Clean Combustor 
Program, Phase I Final Reportm, NASA-CR-134737 ,June 1975. 

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Advanced low emissions catalytic combustor program at General Electric

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