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; II I II I !OR M II III
A COMPARISON OF THE INFLUENCE OF FUEL/AIR UNMIXEDNESS ON
NO EMISSIONS IN LEAN PREMIXED, NON-CATALYTIC AND
CATALYTICALLY STABILIZED COMBUSTION
A. Schlegel, M. Streichsbier, R. Mongia, R. Dibble
Combustion Analysis Laboratory
Department of Mechanical Engineering, University of California - Berkeley
ABSTRACT
Experimental results on the influence of temporal unmixedness
INTRODUCTION
Lean premixed combustion is one of the most successful
on NO, emissions are presented for both non-catalytic and
catalytically stabilized, lean premixed combustion. The test rig used
techniques to reduce NO. emissions in gas turbines. However, the
fuel/air mixing quality, that is achieved prior to combustion, has a
for the experiments consists of a fuel/air mixing section which allows
great influence on NO. emissions. Poor mixing quality means that
variation of the degree of temporal unmixedness while maintaining a
uniform "average over time" concentration profile over the cross
leaner and richer pockets around the mean fuel concentration are
burnt in the combustion process. Although NO, formation rate in
section at the inlet to the oanbustion chamber. The unmixedness is
measured as "rms fluctuations in fuel concentration" by an optical
leaner pockets is smaller due to lower temperatures, the mean NO :
formation rate is increased, since the formation rate in richer pockets
probe using laser absorption at 3.39um over a 9mm gap. "Average
over time" measurements are taken with "conventional" suction
is much higher. This is due to the non-linear, highly temperature
dependent NO, formation in the overall lean regime.
probe analyzers. The combustion chamber is an insulated, tubular
Unmixedness of fuel and air occurs in two forms which are
reactor (i.d. 26.4mm). At the inlet to the combustion chamber a
honeycomb monolith section is inserted. This monolith is either
referred to as spatial and temporal unmixedness. These two forms of
catalytically active or inactive for catalytically stabilized or non-
unmixedness do not differ in principle, since temporal fluctuations
are nothing but spatial unmixedness in the direction of flow.
catalytic combustion respectively. For both modes, the exact same
inlet conditions are applied. In catalytically stabilized combustion a
fraction of the fuel is consumed within the catalyst and the remaining
detected with conventional suction probe analyzers. Conventional
analyzers have a rather high response time greater than 1 Hz and are
fuel is burnt in the subsequent homogeneous combustion zone.
It is shown that catalytically stabilized combustion yields lower
therefore not capable of resolving temporal fuel fluctuations at higher
frequencies.
The problem with temporal unmixedness is, that it cannot be
NO emissions and, more important, that the effect of temporal
As has been shown recently. NO. emissions in lean premixed
fuel/air unmixedness on NO. emissions is much smaller than with
combustion are strongly affected by temporal unmixedness alone
(Fric, 1992, Mongia et al., 1996). These investigations emphasize
non-catalytic combustion under identical inlet conditions.
Experimental evidence leads to the conclusion, that the catalyst is
that, although spatial uniformity in time-mean fuel concentration may
capable of reducing temporal fluctuation in fuel concentration and/or
temperature in the combustion process, thereby preventing excess
be achieved in the premixing section, "hidden" temporal
NO. formation. As a result, the requirements on mixing quality are
less stringent when using catalytically stabilized combustion instead
of conventional, non-catalytic combustion.
concentration fluctuations will contribute significantly to higher NO.
emissions. "Hidden" in this context means that these fluctuations
could not be resolved by the suction probes used. Since it is very
difficult to achieve perfect temporal mixing in technical applications,
Presented at the International Gas Turbine & Aeroengine Congress & Exhibition
Orlando, Florida — June 2–June 5,1997
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EXPERIMENTAL
the lowest theoretically possible NO: emissions for perfectly
premixed, lean combustion are rarely being reached.
Catalytically stabilized combustion has been proposed as a way
to improve lean stability and to achieve low NO ' emissions (Pfefferle,
1975). Due to temperature limitations of materials and the steady
increase in gas turbine inlet temperatures, "hybrid" designs have to
be applied, where the catalyst is kept at a lower temperature than the
overall flame temperature. This can be achieved, if only a certain
fraction of the fuel is converted within the catalyst while combustion
is completed in a homogeneous combustion zone downstream of the
catalyst (Furuya et al., 1987, Scherer et al., 1993, DaIla Rena et al.,
1994).
Recent investigations have shown that NO : emissions can be
reduced when using catalytically stabilized combustion instead of
conventional, non-catalytic combustion under identical inlet
conditions (Schlegel et al., 1994, 1996). The reduction of NO :
emissions depends on the fraction of fuel converted within the
catalyst, i.e. the higher this fraction, the lower the NO % emissions.
These results were obtained under perfectly premixed inlet conditions
only. It is therefore not yet clear how temporal unmixedness
influences NO: emissions from catalytically stabilized combustion.
Although spatially uniform fuel concentration is important in order to
avoid "hot spots" in the catalyst, temporal fluctuations may be
"absorbed" to a certain degree by the catalysts thermal inertia.
In this paper, experimental data on the influence of temporal
urunixedness on NO% emissions are presented for both non-catalytic
and catalytically stabilized, lean premixed combustion. In a first
approach, propane is used as fuel and all experiments are performed
at atmospheric pressure. According to literature, temporal
fluctuations in fuel concentration are usually quantified by the
"unmixedness" parameter U (Danckwerts, 1952):
Apparatus
Figure 1 shows schematically the test rig consisting of a mixing
section and a combustion chamber. For the purpose of this
investigation, the mixing section has to supply the following inlet
conditions to the combustor (1) a uniform in-time fuel concentration
profile as well as plug flow profiles in terms of temperature and
velocity distribution over the cross section; (2) superimposed
temporal fluctuations in fuel concentration variable in magnitude and
frequency. The above requirements can be met by a mixing section
setup as shown in Fig. 1:
x COrtm
41•■•
suctbn
probe
combustion
chamber
honeycomb
facitve Or
h-adiVe)
wire mesh
Th
distance L
healing
taw
let
_______-mbeng tube
ri
alr
(preheated) —..
soiencect
Antve
_ t fuel A _art.
UC
•(
1- c)
(1)
Fig. 1: Schematic of the test rig with its mixing section and
combustion chamber
where I is the mean over time fuel mole fraction and c„,„
. is the "root
mean square" of the fuel mole fraction fluctuation. For a perfectly
mixed stream, U = 0 and for a completely unmixed stream U = 1. In
this study, a more descriptive parameter to quantify temporal
unmixedness is used instead:
Air is electrically preheated and fed Into a lm mixing tube (id.
26.4mm) to act as hot callow. The tube is wrapped with heating tape
maintained at the inlet temperature to the combustion chamber, Tin,
to prevent the development of radial temperature profiles. At the exit
of the mixing section (the inlet to the combustion chamber) a fine
wire mesh in several layers guarantees a constant velocity profile over
the cross section. Additionally, upstream radiation from the
combustion chamber is absorbed by the mesh and uniformly
transferred to the fuel/air mixture, therefore allowing very accurate
measurement of the true inlet temperature T ei to the combustion
chamber.
Propane and a small part of the total air are injected via a center
jet tube (id. 1.4rom). The additional air enhances turbulent mixing
with the coflow. All flows are measured and controlled with mass
(2)
where F is the "rms fluctuations in fuel concentration" around the
mean over time fuel mole fraction.
2
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temperature to the combustion chamber (T io) was set at 300 and
flow controllers (MFC). The fuel flow can be pulsed in adjustable
frequencies by a solenoid valve (on/off mode). In this work, three
400°C respectively. All measurements are taken at atmospheric
pressure.
different frequencies of 10, 20 and 30Hz are used. The magnitude of
the fuel fluctuations is varied by varying the mixing distance L, which
is defined as the distance between the center jet exit and the
T,,
combustion chamber inlet plane. This set of frequencies and mixing
IC]
cot /ow air
PM
jet air
(SUN]
jet C3H8
(SLM)
distances were found to produce a wide range of "ms fluctuations in
fuel concentration" suitable for this investigation (compare Fig. 3). In
A
300
96
4
2.10
practical applications, higher frequencies can be observed, but this
B
400
96
4
2.10
experimental range was purposely chosen, since lower frequencies
are expected to have a greater impact on the catalyst performance.
Tab. 1: Flow conditions and inlet temperatures. The fuel flow
is either constant for perfectly premixed inlet
conditions (OHz) or pulsed with frequencies of 10, 20
or 30Hz for temporal fluctuations in fuel concentration.
The combustion chamber is an insulated, tubular reactor (i.d.
26.4tam). Honeycomb monoliths of 20mm length and 1.5mm square
cells were placed at the inlet of the combustion chamber. These
monoliths (cordierite) are either inactive for non-catalytic combustion
or coated with Pt on a A1203 washcoat for catalytically stabilized
Figure 2 shows radial fuel concentration profiles at the inlet to
the combustion chamber (x = Omm) in dependence of the mixing
distance L for case A and the fuel flow pulsed at 20Hz. These
combustion. In all cases, monoliths of the same length and cell size
are used.
In order to stabilize the homogeneous combustion at a fixed
measurements are taken with the "conventional" suction probe
analyzer revealing "average over time" fuel concentrations. The
location and therefore to achieve the same residence time in all cases,
a thin, electrically heated SIC filament is placed immediately
profiles flatten quickly with increasing mixing distance L. At mixing
distances greater than L = 28.5 cm the spatial fuel concentration
downstream of the monolith. The additional energy input into the gas
stream by this flame stabilizer is small compared to the chemical heat
distribution is flat in time.
released by the fuel oxidation (< 1%), thereby minimizing the effect
1.4
on NO emissions. However, the electric power supplied is monitored
-4.-L =95mm
- 4-L=115mm
- 4-L=135mm
and kept constant in all cases.
- 4-L =160mm
- 4-L =285mm
Piaanostics
Temporal fluctuations in fuel concentration are measured by
using absorption of laser light at 3.39sun over a path length of 9mm.
This method is described in more detail in Yoshiyama et al.. (1996)
.a
and Motigia et al., (1996). Sapphire rods (o.d. lmm) transmit the
infrared light from the He-Ne laser via the probe gap to the detector.
A
2
The probe is placed perpendicular to the flow at the inlet plane to the
combustion chamber (x = Omm). The fluctuation measurements are
0.9
0.8
conducted without combustion, but at exact same inlet conditions as
with combustion (see below).
-15
•10
-5
•
0
5
10
15
radial position /mm
Species measurements such as CO, CO2. 02. UHC, NO/NOz are
made with "conventional" suction probe analyzers (Horiba). A water-
Fig. 2: Development of radial "average over time" fuel
concentration profiles in function of mixing distance L.
measured with a "conventional" suction probe analyzer
at the inlet to the combustion chamber (x = Otom).
Case A. fuel pulse frequency 20Hz.
cooled, stainless steel probe (ad. 3mm) is placed at the exit of the
combustion chamber (x = 400mm). Condensation of water vapor is
prevented by using probe cooling water at 60 °C and heated lines to
the analyzers. The water is removed from the sample gas by a porous
dryer purged with N2 gas; therefore, all concentration measurements
are on dry basis.
Measurements of temporal fluctuations in fuel concentration
taken with the optical probe at the same location (x = Omm) are
shown in Fig. 3, again for case A but for different fuel pulse
frequencies. The magnitude of fuel concentration fluctuations is
gxaerlmental conditions
The flow conditions applied in this investigation are summarized
decreasing linearly with increasing mixing distance. Note that all
measurements are taken at mixing distances greater than 28.5cm,
in Table 1. Throughout the measurements, all flows are kept constant
yielding an overall equivalence ratio equal to 0.5. The inlet
3
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▪
45
when the spatial average over time fuel concentration is already
uniform over the cross section. In order to achieve perfectly premixed
40
inlet conditions, the injector nozzle is placed at the maximum mixing
distance (L = lm approx.) and the fuel flow is kept constant with no
pulsation, i.e. the fuel concentration is not only spatially but also
35
temporarily uniform. This case is referred to as "OHS' case.
40
e
35 .
g 30 -
•
• 10 Hz
• 20 Hz
• % Hz
•
5•
8 25 '
76
•
200
s
•
15 -
g
t 10 5-
400
800
600
SOO
1000
1200
hum
Fig. 4: Measured NO, emissions at the combustion chamber
exit (x = 400mm) versus mixing distance L for case B.
both non-catalytic and catalytically stabilized combustion. Fuel flow pulse frequency = 30Hz, T z• = 400°C,
= 0.5. NO: emissions for perfectly premixed inlet
conditions are shown as well (OHz).
•
case A:
300°C
200
600
mixing distance L
•
0
0
400
1000
mixing distance L Imm
In Figures 5 and 6, the "mixing distance" is replaced with
Fig. 3: Temporal fluctuations in fuel concentration versus
mixing distance L, measured with the optical probe at
the inlet to the combustion chamber (x = Omm).
Case A. fuel pulsed with 10,20 and 30Hz.
measured "rms fluctuations in fuel concentration" which are a
function of mixing distance and fuel pulse frequency (see Fig. 3).
Again, NOx emissions in lean, catalytically stabilized combustion are
far less influenced by temporal munixedness than lean, non-catalytic
combustion. Note, that all measurements are taken at inlet conditions
RESULTS AND DISCUSSION
where only temporal unmixedness is present. The spatial, average
Figure 4 shows measured NO: emissions versus mixing distance
over time fuel distribution is perfectly flat over the cross section
for case B, both non-catalytic and catalytically stabilized combustion.
(compare Fig. 2).
The fuel flow is pulsed with a frequency of 30Hz (on/off mode). In
addition, data for perfectly premixed (OHz) inlet conditions are shown
as well, i.e. these measurements are taken with no temporal
fluctuations in fuel concentration. Under these conditions, NO x
emissions can be reduced by about 60% when using catalytically
stabilized combustion instead of non-catalytic combustion. These
measurements are in close agreement with data presented by Schlegel
et al., (1994). It has been shown that for lean, perfectly premixed
inlet conditions, NO: emissions can be reduced with catalytically
stabilized combustion in comparison to non-catalytic combustion. The
reduction is dependent on the amount of fuel converted within the
catalyst. In the present study, the amount of fuel converted within the
catalyst cannot be measured, Nevertheless, according to that data, the
amount of fuel converted within the catalyst can be estimated to be
0
roughly 70 to 80%.
If temporal fluctuations in fuel concentration occur at the inlet to
5
10
15
20
25
30
rms fluctuations in fuel concentration F
the combustion chamber, the differences in NO x emissions are even
Fig. 5: Measured NO• emissions at the combustion chamber
exit (x = 400mm) versus "rms fluctuations in fuel
concentration F' for case A, both non-catalytic and
catalytically stabilized combustion. Fuel flow pulse
frequency = 20Hz, Ti n = 300°C, = 0.5.
more significant (Fig. 4). With decreasing mixing distance, NO x
emissions increase strongly for non-catalytic combustion whereas for
catalytically stabilized combustion, NOx emissions increase only
slightly.
4
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0.4
3
1
2
0
5
10
15
rrns fluctuations in fuel concentration
20
0
F
10
20
30
tins fluctuations in fuel concentration
40
F
Fig. 7: Measured NO: emissions at the combustion chamber
exit (x = 400mm) versus "rms fluctuations in fuel
concentration F' for case A in catalytically stabilized
combustion. Fuel flow pulse frequency = 10, 20 and
3011z, Ts = 300°C, = 0.5.
Fig 6: Measured NO, emissions at the combustion chamber
exit (x = 400mm) versus "rms fluctuations in fuel
concentration F' for case B, both non-catalytic and
catalytically stabilized combustion. Fuel flow pulse
frequency = 30Hz, Ti c = 400°C, = 03.
Figures 7 and 8 show measured NO, emissions for catalytically
stabilized combustion versus "rms fluctuations in fuel concentration"
for cases A and B. The data is collected with different fuel flow pulse
frequencies (10, 20 and 30Hz). Note, that a certain magnitude of
"rms fluctuations in fuel concentration" can be achieved with
different fuel pulse frequencies (compare Fig. 3). Figures 7 and 8
reveal that NO1 emissions of catalytically stabilized combustion do
not depend on the frequency of the fluctuations, but on their
magnitude.
These findings strongly emphasize that the catalyst equalizes
temporal fluctuations in fuel concentration. Richer pockets passing
the catalyst experience higher fuel conversion rates than leaner
pockets. Therefore, concentration differences between richer and
0
leaner pockets decrease along the catalyst. The extra beat released by
5
10
15
20
25
30
rrns fluctuations in fuel concentration F 1%
a rich pocket is absorbed by the substrate and transferred to the
following lean, colder pocket. A rough estimation shows that the
thermal inertia and conductivity of a typical ceramic catalyst substrate
Fig. 8: Measured NO, emissions at the combustion chamber
exit (x = 400mm) versus "rms fluctuations in fuel
concentration F' for case B in catalytically stabilized
combustion. Fuel flow pulse frequency = 10, 20 and
30Hz, Tin = 400°C, $05.
are large enough to store heat fluctuations in the order of up to 50%
at 10Hz without changing the substrate temperature significantly.
Therefore, the catalyst also prevents the development of large
temperature differences which result in non-catalytic combustion.
when burning rich and . lean pockets alternately. It is these
temperature differences which cause the overall higher NO,
formation rate at a given, lean equivalence ratio. As a consequence,
remaining fuel fluctuations entering the homogeneous combustion
It should be mentioned as well, that non-catalytic combustion
yields strong acoustic vibrations in the same frequencies as the fuel
pulsation. When using catalytically stabilized combustion under exact
zone downstream of the catalyst are much smaller in magnitude than
same conditions, the vibrations are far smaller, i.e. the combustion
without a catalyst. Smaller fluctuations in fuel concentration
process is much more stable and less noisy. Though for the present
subsequently lead to smaller temperature differences which in turn
study, this observation cannot be bolstered with measured data.
produce less overall NO. emissions.
5
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REFERENCES
RA Della Betta, IC. Schlatter, S.G. Nickolas, D.K. Yee,
T.Shoji, 1994, "New Catalytic Combustion Technology for very Low
Emissions Gas Turbines", ASME Paper No. 94-GT-260.
P. V. Danckwerts, 1952, -The Definition and Measurement of
some Characteristics of Mixtures", Applied Scientific Research, Sec.
A, Vol 3, pp. 279-296.
T. F. Fric, 1992, "Effect of Fuel-Air Unmixedness on NO:
Emissions", AIAA paper 92-3345.
T. Furuya, S. Yamanaka, T. Hayata, J. Koezuka, T. Yoshine, A.
Ohkoshi, 1987, "Hybrid Catalytic Combustion for Stationary Gas
Turbine-Concept and Small Scale Test Results", ASME Paper 87GT-99.
R. Mongia, E. Tomita, F. Hsu, L Talbot and R. Dibble, 1996,
"Use of an Optical Probe for Time-Resolved in Situ Measurement of
Local Air-to-Fuel Ratio and Extent of Fuel Mixing with Applications
to Low NO. Emissions in Premixed Gas Turbines", 26th Symposium
(International) on Combustion, The Combustion Institute, Pittsburgh,
in press.
W.C. Pfefferle, 1975, "Catathermal Combustion: A New Process
for Low-Emissions Fuel Conversion", ASME Paper 75-WA/Fu-1.
V. Scherer, W. Weisenstein, T. Griffin, P. Benz, A. Schlegel,
S. Buser, 1993, "Catalytically-Stabilized Combustion of Natural Gas:
Experiments under Cras Turbine Conditions", Proceedings of the
Second International Conference on Combustion Technologies for a
Clean Environment, Lisbon, Portugal, 19-22 July 1993.
A. Schlegel, S. Buser, P. Benz, H. Bockhorn, F. Mauss, 1994,
"Nat Formation in Lean Premixed Non-catalytic and Catalytically
Stabilized Combustion of Propane", 25th Symposium (International)
on Combustion, The Combustion Institute, Pittsburgh, pp. 1019-1026.
A. Schlegel, P. Benz, T. Griffin, W. Weisenstein, H. Bockhorn,
F. Mauss. 1996, "Catalytic Stabilization of Lean Premixed
Combustion: Method for Improving NO x Emissions", Combustion
and Flame, Vol. 105, pp. 332-340.
S. Yoshiyama, Y. Hamamato, E. Tomita, K. Minami, 1996.
"Measurement of Hydrocarbon Fuel Concentration by Means of
Infrared Absorption Technique with 3.39turt He-Ne Laser", .1SAE
Review, Vol. 17, pp. 339-345.
Future investigations, including modeling work, will be
performed to clarify the above mentioned assumptions and to identify
the interaction between the thermal inertia of the substrate, the
magnitude and frequency of fuel fluctuations and their effect on NO.
formation in the subsequent homogeneous combustion zone.
CONCLUSIONS
Experimental data on the influence of temporal unmixedness on
NO: emissions are presented for lean premixed, non-catalytic and
catalytically stabilized combustion. The experiments are performed
with propane as fuel and under atmospheric conditions. A special
mixing section is used which allows variation of frequency and
magnitude of temporal fluctuations while having a spatially uniform,
mean in time fuel concentration profile over the cross section at the
inlet to the combustion chamber.
It is shown that catalytically stabilized combustion yields lower
NO: emissions and, more important, that the effect of temporal
fuel/air unmixedness on NOx emissions is much smaller than with
non-catalytic combustion at the same conditions. Although mean in
time, spatial mixedness is important in order to avoid "hot spots" in
the catalyst, temporal unmixedness has little influence on the catalyst
performance and consequently far less influence on NO. emissions.
This fact has been overlooked in the past, since the common view on
catalytic combustion was that perfect premixing is no less crucial for
this technique.
The measurements emphasize that resulting NOx emissions do
not depend on the frequency of the fluctuations but only on their
magnitude. It is concluded that the catalyst is capable of decreasing
temporal fluctuations in fuel concentration prior to the homogeneous
combustion zone downstream of the catalyst. Smaller concentration
fluctuations lead to smaller temperature fluctuations in the
combustion process and therefore to lower overall NO. emissions.
Moreover, it was observed that catalytically stabilized combustion
yields less acoustic vibrations than non-catalytic combustion, i.e. the
combustion process is much more stable and less noisy.
Future investigations, including modeling work, will be
performed to clarify the above mentioned assumptions and to identify
the interaction between the thermal inertia of the substrate, the
magnitude and frequency of fuel fluctuations and their effect on NO :
formation in the subsequent homogeneous combustion zone.
ACKNOWLEDGMENT
We would like to thank Dr. R. A. Perry (Technor, USA) and Dr.
P. Benz (Paul Scherrer Institute, Switzerland) for their kind loan of
some experimental equipment used in this study.
6
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