Emissions from a simple 10-cm (4-in.) diameter tube combustor burning a premixed, gaseous propane/air mixture were measured. Inlet conditions included a temperature of 590 K (600 F), pressure of 5.5 atm, and reference velocity of 23 m/s (75 ft/s) for a range of equivalence ratios from the lean limit to slightly richer than stoichiometric. A nitric oxide emission index of 1 g NO2/kg fuel was measured for an equivalence ratio of 0.57

Anderson, D. N.

English

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EFFECT OF PREMIXING ON NITRIC OXIDE FORMATION
by David N. Anderson
Lewis Research Center
Cleveland, Ohio 44135
TECHNICAL PAPER proposed for presentation at the 75th National
Meeting of the American Institute of Chemical Engineers
Detroit, Michigan, June 3-6, 1973
https://ntrs.nasa.gov/search.jsp?R=19730013135 2020-03-23T02:00:49+00:00Z
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ABSTRACT
Emissions from a simple 10-cm (4-in.) diameter tube combustor
burning a premixed, gaseous propane/air mixture were measured.
Inlet conditions included a temperature of 590 K (600 F), pressure of
5. 5 atm., and reference velocity of 23 m/s (75 ft/s) for a range of
equivalence ratios from the lean limit to slightly richer than stoichio-
metric. A nitric oxide emission index of 1 g NO^/kg fuel was measured
for an equivalence ratio of 0.57.
EFFECT OF PREMIXING ON NITRIC OXIDE FORMATION
by David N. Anderson
Lewis Research Center
SUMMARY
The effect on nitric oxide emissions of using a premixed, prevapor-
ized fuel/air system in turbojet combustors was investigated. Emissions
from a simple 10-cm (4-in.) diameter tube combustor burning a premixed
gaseous propane/air mixture were measured. Inlet conditions included a
temperature of 590 K (600 F), pressure of 5. 5 atm., and reference ve-
locity of 23 m/s (75 ft/s). Concentrations of nitric oxide, carbon monox-
ide, and unburned hydrocarbons were measured at a point 46 cm (18 in.)
downstream of the flameholder for a span of fuel-air ratios ranging from
the lean blowout limit to slightly richer than stoichiometric.
With combustion efficiencies greater than 99 percent for the entire
range of stoichiometries tested, nitric oxide concentration was 10 g
NO2/kg fuel for an equivalence ratio of 0. 7 and 1 g NO«/kg fuel for
(p = 0.57.
Combustion instability occurred for equivalence ratios less than
0, 54, and the lean blowout was at <p = 0. 5.
INTRODUCTION
This study employed a simulation of a turbojet combustor primary
zone to determine the effect of premixing fuel and air on nitric oxide
emissions.
In the conventional turbojet combustor, liquid fuel is sprayed directly
into the primary zone to give a mean fuel-air ratio near stoichiometric.
Combustion is initiated and stabilized in this zone; some reactions can
continue in the secondary zone where dilution air is admitted to bring the
combustion temperature down to the level required at the turbine inlet.
The inj ection of fuel in a spray results in a wide variation of local
fuel-air ratio with a spectrum of locally lean to locally rich values.
Because nitric oxide formation is exponentially dependent on local flame
temperature, this distribution of stoichiometries results in large quanti-
ties of NO being produced in some regions and virtually none in others
even for fuel-air ratios far from stoichiometric. Thus, nitric oxide
concentration cannot easily be predicted from the average reaction zone
fuel-air ratio in a conventional combustor with a nonuniform mixture.
For a premixed, prevaporized fuel system, on the other hand, the
fuel-air ratio may be made uniform throughout the primary zone; the
result is a very strong dependence of nitric oxide concentration on
stoichiometry.
Heywood and associates (refs. 1 to 3) have studied the effect of pri-
mary zone uniformity on nitric oxide production. Their work was based
on a statistical evaluation of the equivalence ratio distribution in the
primary zone. A crossplot of some of the typical analytical results
from Fletcher and Heywood (ref. 1) is shown in figure 1 where the
strong effect of uniformity on NO concentration can be seen. It is appar-
ent from figure 1 that for a lean, uniformly mixed primary zone, sig-
nificant reductions in nitric oxide concentration could be realized com-
pared with a nonuniform conventional combustor operating near stoichi-
ometric conditions.
The reactions responsible for the formation of nitric oxide are rela-
tively slow compared with the other reactions which define the combus-
tion process. Therefore, the concentration of nitric oxide is also
strongly dependent on the residence time of the hot gases in the combus-
tor. Since a premixed configuration will burn more intensely, and with
a shorter flame zone, than will a conventional combustor, it should be
possible to decrease the hot-gas residence time by decreasing the com-
bustor length for a premixed system.
Although a lean premixed primary zone will result in lower concen-
trations of nitric oxide compared with those from a conventional combus-
tor, combustion stability can be expected to suffer. The lean flamma-
bility limit for most hydrocarbon fuels is about 50 percent of stoichio-
metric. In a nonuniform system, however, the mean fuel-air ratio can
be considerably less than the lean limit and locally rich zones will main-
tain combustion. For the uniform system which results from premixing
this advantage is lost and blowout will occur near the lean flammability
limit.
The present program was conceived to determine the effect of pre-
mixing on nitric oxide formation for conditions applicable to turbojet
combustors. A 10-cm (4-in,) diameter combustor burning gaseous pro-
pane fuel was used. The flame was stabilized with a drilled-plate which
provided a total pressure drop of 4 percent of the upstream total pres-
sure, a value typical of that in a turbojet combustor0 Inlet conditions
were maintained at a temperature of 590 K (600° F) and 5. 5 atm. pres-
sure; the reference velocity was 23 m/s (75 ft/s). Concentrations of
nitric oxide, carbon monoxide, carbon dioxide and unburned hydrocar-
bons were measured for a range of fuel-air ratios from lean blowout to
slightly rich.
APPARATUS AND INSTRUMENTATION
Details of the apparatus are given in figure 2. "Air entering the test
section was indirectly preheated to 590 K (600 F) for all tests. Gaseous
propane fuel was introduced through a fuel tube located about 2 m (7 ft)
upstream of the test combustor to allow sufficient time for thorough
mixing of fuel and air. A small swirler was located on the end of the
fuel tube to give some initial mixing.
The flameholder was made by drilling 61 holes of 0.6 cm (0. 25 in.)
diameter in a 0. 6 cm (0. 25 in.) thick stainless steel plate. The hole
pattern is shown in figure 3. This arrangement resulted in an open area
of 25 percent of the inlet duct cross-sectional area. At 23 m/s (75 ft/s)
reference velocity and 590 K (600° F) inlet temperature, the total pres-
sure drop across the plate was 4 percent of the upstream total pressure.
The combustor was 10 cm (4 in.) in diameter, the same as the inlet
duct, and 60 cm (24 in.) long (see fig, 2). Water cooling was used. At
the downstream end quench water was sprayed into the gas stream to
cool the exhaust to about 370 K (200° F), This mixture of combustion
products and water passed through a remotely operated back-pressure
valve for control of rig pressure
Rakes for inlet instrumentation were avoided to eliminate any possi-
ble flameholders in the inlet duct. Instead, static pressure was mea-
sured at the wall and a single Chromel-Alumel thermocouple was in-
serted to a depth of about 2 cm (3/4 in.) from the duct wall. Downstream
static pressure was measured at a tap in the spark igniter boss, about
5 cm (2 in.) downstream of the flameholder. ASME standard orifices
were used to measure air and fuel flow.
Details of the water-cooled stainless-steel gas-sampling probe are
also shown in figure 2. The probe was 1.3 cm (0, 5 in.) outside diameter
with a center sampling tube of 0. 6 cm (0. 25 in.) diameter. Samples
were drawn only from a single position on the combustor center line at a
point 46 cm (18 in.) downstream from the flameholder.
It is well known that metal probes can cause a catalytic reduction
of the nitric oxide by carbon monoxide. However, Halstead and Munro
(ref, 4) found that for equivalence ratios less than unity, there was vir-
tually no reduction in nitric oxide concentration by this mechanism. For
fuel-rich conditions they found a reduction in concentration greater than
an order of magnitude.
Stainless steel tubing of 0. 6 cm (0. 25 in.) diameter connected the
gas sample probe with the exhaust gas analyzers. To prevent condensa-
tion of unburned hydrocarbons this sample line tubing was heated by
electrical tracing to maintain the sample gas temperature between
410 K (280° F) and 450 K (350° F). The sample line was approximately
18 m (60 ft) long.
Gas analysis equipment included a Beckman Flame lonization De-
tector for measuring unburned hydrocarbons, Beckman nondispersive
infrared analyzers for measuring concentrations of carbon monoxide
and carbon dioxide, and a Thermo-Electron Chemiluminescent instru-
ment for nitric oxide concentration.
Commercial grade (92 percent pure) gaseous propane was used for
all tests (see table I). Propane has a heating value near that of jet air-
craft fuels so that combustion characteristics, especially NO production,
are similar. By using a gaseous fuel, problems associated with atom-
ization were avoided and it was possible to simulate a prevaporized as
well as a premixed system.
The propane was supplied and stored as a liquid, then superheated
to 370 K (200° F) at 10 atm. to insure complete vaporization before
metering the flow.
After each change in conditions a delay of 3 to 5 minutes was allowed
for stabilization before taking data, All the millivolt signals from pres-
sure transducers, thermocouples, and gas analysis equipment for each
test point could be recorded in less than 3 seconds by the automatic data
system used.
RESULTS AND DISCUSSION
Sample Validity
It was important to determine if the emissions measured from the
gas sample were representative of the average concentrations in the
exhaust. For this purpose an equivalence ratio based on the measured
carbon concentrations was compared with that from fuel and air flow
measurements. The agreement should be within ±15 percent to insure
good sample validity (ref. 5). The comparison is shown in figure 4 for
the test data reported here. Agreement between the two equivalence
ratios was generally within 10 percent showing that the single center-
line probe was able to provide a good sample for analysis.
Data will be reported as a function of the equivalence ratio computed
from the emissions measurements.
Combustor Heat Losses
Because the combustor was water cooled, quenching of the combus-
tion could have occurred at the walls. Heat loss through cooling was
measured to be consistently about 10 percent. However by sampling
gases only at the combustor centerline it should be possible to assume
that the sample is the same as would result from an adiabatic combustor.
Combustion Stability
Combustion was stable for the present test conditions at an equiva-
lence ratio as low as 0. 54. For leaner mixtures the combustion became
unstable until blowout occurred at <p = 0. 5. The fuel-rich combustion
limit was not investigated.
Nitric Oxide Concentration
In this study, the only nitrogen oxide concentration measured was
that of nitric oxide. The only other one of importance to turbojet com-
bustors is nitrogen dioxide. Other studies (ref. 6) have shown that total
NOV (i. e., NO + NO0) concentration can be on the order of 10 percent
X Lt
more than the NO concentration for the range of conditions studied here.
The measured concentrations of nitric oxide are given in figure 5.
By convention, these are reported in terms of the emission index of
NO£ (g NOo/kg fuel); this value "is obtained by multiplying the nitric
oxide emission index (g NO/kg fuel) by the ratio of molecular weights
of NOg to NO. The equilibrium concentration of NO is shown for refer-
ence in figure 5.
Also given are the concentrations predicted at (p = 1 by a well-
stirred-reactor and well-stirred-reactor-plus-plug-flow analytical
solutions. The former used the model of Edelman and Economos
3 3(ref. 7) with a reactor volume of 344 cm (21 in. ); this is equivalent
to assuming that the recirculation zone extends about 4. 2 cm (1. 7 in.)
downstream from the flameholder. The results of this analysis were
used as input to a plug-flow program to determine the effect of continued
reaction for a distance of 40 cm (16 in.); that is, the distance to the gas
sampling probe in the test combustor.
The experimental data show the strong dependence on equivalence
ratio that can be expected from premixed, prevaporized operation. For
the inlet temperature used for the present tests (590 K (600° F)), it was
possible to obtain emission indexes of less than 1 g NO«/kg fuel for
equivalence ratios less than 0.57. This compares with a minimum
emission index of about 2 g NO2/kg fuel from an experimental combustor
designed for low NO (ref. 6), operating at about the same inlet condi-
X
tions, but with the advantage of a quick quench after a very short pri-
mary zone. Values for more conventional combustors are in the range
of 3 to 5 g NOg/kg fuel for these inlet conditions.
For fuel-rich equivalence ratios (values greater than 1) the data
approach the equilibrium concentrations. This result suggests that
probe catalytic effects are absent from the present experiments.
The strong effect of residence time on nitric oxide concentration
can be seen by comparing the well-stirred-reactor and well-stirred-
reactor-plus-plug-flow analytical solutions. At y = 1, these results
predict that an order of magnitude increase in nitric oxide concentration
results during the time the hot combustion products flow from the end of
the reaction zone to the probe (about 4 msec). It is likely that the nitric
oxide concentrations near the flameholder are significantly less than
those reported here, even for regions where the combustion is essenti-
ally complete. In a practical combustor rapid quenching of combustion
reactions by dilution air introduced at the end of the recirculation region
could prevent concentrations from growing to the levels reported here.
For this reason the emissions from a practical premixed combustor
could be lower than the values reported here and might approach the
concentrations predicted by the well-stirred-reactor solution.
Carbon Monoxide, Unburned Hydrocarbons, and
Combustion Efficiency
These reported concentrations of nitric oxide are only meaningful
if the combustion reactions have gone to completion so that essentially
100 percent combustion efficiency is realized. For this reason, con-
centrations of carbon monoxide and unburned hydrocarbons were mea-
sured.
From figure 6 it can be seen that the measured carbon monoxide
concentrations were essentially at the equilibrium level for the entire
8range of equivalence ratio covered.
The measurements of unburned hydrocarbons showed concentrations
less than 0.4 gHC/kg fuel over the entire span of stoichiometries inves-
tigated. Such values are negligible.
Combustion efficiency can be determined from the difference be-
tween the measured and equilibrium concentrations of carbon monoxide
and unburned hydrocarbons. Since both emissions were virtually at the
equilibrium level, the computed combustion efficiency for these tests
was greater than 99 percent for all test points.
SUMMARY OF RESULTS
Emission levels from a simple 10-cm (4-in.) diameter tube com-
bustor burning a premixed gaseous propane/air mixture were measured.
Inlet conditions included a temperature of 590 K (600° F), pressure of
5. 5 atm., and reference velocity of 23 m/s (75 ft/s). Concentrations
of nitric oxide, carbon monoxide and unburned hydrocarbons were mea-
sured at a point 46 cm (18 in.) downstream of the flameholder for a span
of fuel-air ratios ranging from the lean blowout limit to slightly richer
than stoichiometric. These were the results:
1. Nitric oxide was strongly dependent on equivalence ratio with an
emission index of 10 for an equivalence ratio of 0.7 and 1 for tp = 0.57.
2. Measured levels of carbon monoxide corresponded with equilib-
rium concentrations, unburned hydrocarbons were negligible, and com-
bustion efficiencies were greater than 99 percent for the entire range of
fuel-air ratios.
3. Combustion instability occurred for equivalence ratios less than
0. 54; the lean blowout limit was q> - 0. 5.
REFERENCES
1. Fletcher, R. S., and Heywood, J. B., "A Model for Nitric Oxide
Emissions from Aircraft Gas Turbine Engines," AIAA Paper
71-123 (1971).
2. Heywood, «!„ B., "Gas Turbine Combustor Modelling for Calculating
Nitric Oxide Emissions," AIAA Paper 71-712 (1971).
3. Pompei, F., and Heywood, J. B., "The Role of Mixing in Burner
Generated Carbon Monoxide and Nitric Oxide, " Comb, and Flame,
19, 407 (1972).
4. Halstead, C. J., and Munro, A. J. E., "The Sampling, Analysis,
and Study of the Nitrogen Oxides Formed in Natural Gas/Air
Flames, " Presented at the Conference on Natural Gas Research
and Technology, Chicago, 111., Feb. 28 - Mar. 3, 1971.
5. Anon., "Procedure for the Continuous Sampling and Measurement
of Gaseous Emissions from Aircraft Turbine Engines, " SAE ARP
1256 (Oct. 1971),
6. Niedzwiecki, R. W,, and Jones, R. E., "Pollution Measurements
of a Swirl-Can Combustor, " NASA TM X-68160 (1972).
7. Edelman, R., and Economos, C., "A Mathematical Model for Jet
Engine Combustor Pollutant Emissions, " AIAA Paper 71-714
(1971).
10
TABLE I. - PROPERTIES OF COMMERCIAL GRADE
PROPANE USED IN THIS STUDY
Analysis: C«Hg 82 percent
C2H4 1-2
C3H6 4-6
C4H10 1-2
Volatile S 4-6 grains/100 ft3
Dew point: 244 K (-20° F)
Heat content: 11 900 cal/g (21 400 Btu/lbm)
Vapor pressure: 11.9 atm. at 310 K (175 psi at 100° F)
Specific gravity: 0.508
CONCENTRATION,
ARBITRARY
UNITS
WELL MIXED
PARTIALLY MIXED
NONUNIFORM
.6 .8 1.0 1.2
EQUIVALENCE RATIO, if
1.4
Figure 1. - Effect of equivalence ratio distribution on
nitric oxide concentration (crossplot of analytical
results presented in figure 4 of Fletcher and
Heywood Iref. II.)
GAS SAMPLE PROBE
-COOLING WATEK
1.3 0.63|
UPSTREAM PRESSURE TAPJ— '
AND INLET TEMPERATURE7
FUEL
QUENCH
WATER
COOLING\ COOLING 1
WATER OUT \WATERIN
J l r
\ 46
/ k \ 60
PERFORATED \
FLAMEHOLDER \
.; COMBUSTION
' PRODUCTS
DOWNSTREAM
PRESSURE TAP
FUEL TUBE
Figure 2. - Rig schematic. (Dimensions are in cm.)
o°Po0
OO o
-9.7 DIAM
-0.635 DRILL
61 HOLES
Figure 3. - Flameholder. (0.64 cm plate and 25 percent open <jred.
EQUIVALENCE
RATIO
DETERMINED
FROM
EMISSIONS
MEASUREMENT
1.4
1.2
1.0
.6
+15%
-15%
o PREMIXED DATA
.2 .4 .6 .8 1.0 1.?
EQUIVALENCE RATIO BASED ON FLOW MEASUREMENTS
Figure 4. - Gas sample validity.
1.4
100
EMISSION
INDEX,
gN02/kgFUEL
10
1.0
EQUILIBRIUM
,-\VELL-STIRRED-REACTOR-
PLUS-PLUG-FLOW PRE-
. DICTION
WELL-STIRRED-
REACTOR
PREDICTION
o PREMIXED DATA
.2 .4 .6 .8 1.0
EQUIVALENCE RATIO, <p
1.2 1.4
Figure 5. - Nitric oxide concentration.
23m/s.)
590 K; pj = 5. 5 atm ; V re(
-vr
L6
L4
L 2
EQUIVALENCE '
RATIO
DETERMINED
FROM •8
EMISSIONS
MEASUREMENT
.6
.2
15*
-15%
OPREMIXEO DATA
.2 .4 .6 .8 1.0 1.2
EQUIVALENCE RATIO BASED ON FLOW MEASUREMENTS
Figure 4. - Gas sample validity.
1.4
100
EMISSION
INDEX,
gN02/kgFUEL
10
1.0
EQUILIBRIUM
rWELL-STIRRED-REACTOR-
PLUS-PLUG-FLOW PRE-
DICTION
WELL-STIRRED-
REACTOR
PREDICTION
oPREMIXED DATA
.2 .4 .6 .8 1.0
EQUIVALENCE RATIO, <p
1.2 1.4
Figure 5. - Nitric oxide concentration. IT^ = 590 K; pj = 5.5 atm; V re j
23m/s. )
1000 r-
100
EMISSION
INDEX,
10
° PREMlXED DATA
I
.4 .6 .8 1.0 1.2 1.4
EQUIVALENCE RATIO, if
Figure 6. - Carbon monoxide concentration. iT-j • 590 K
(600° F); p3 = 5. 5 atm; Vref - 23 m/s. I


Effect of premixing on nitric oxide formation

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