An experimental program demonstrated that fuel-rich catalytic combustion can be accomplished soot free as long as the combustion temperature is less than the temperature at the rich limit of combustion. Although soot was not measured directly, three pieces of data strongly suggest that it was not present: (1) the product gases were completely transparent and produced no radiation characteristic of soot, (2) measured reaction temperatures followed closely those calculated for equilibrium with no soot present, and (3) over 99 percent of the carbon was accounted for in the measured reaction products. Data for two catalyst configurations were taken along with gas samples at two locations downstream of the catalyst bed

Brabbs, T. A.

Olson, S. L.

English

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r10
NASA Technical Memorandum 87042
,
4
uel Rich Catalytic Combustion the First
Stage of a Two-Stage Combustor
(NASA -TM-d7042)
	 FUEL 6ICN CJTALYTIC
	 HE5-2ESF3
COAU^)TIUN: THE FIIASI 5IA(;B GP A TWC-:;TA B
COMBUSTO6 ( NASA)	 12 p HC AO[/Mf A01
CCLc1B	 Ur.cldf
G3/25 21536
Theodore A. Brabbs and Sandra L. Olson:
Lewis Research Center
Cleveland, Ohio
Prepared for the
Fall Meeting of the Eastern Section of the Combustion Instigate
Clearwater Beach, Florida, December 3--5, 1984
A^
NASA
0
Mme' IDVA
`^	 Mx
FUEL-RICH CATALYTIC COMBUSTION - THE FIRST STAGE OF A TWO-STAGE COMBUSTOR
Theodore A.
	
Brabbs and Sandra L. Olson
National Aeronautics and Space Administration
Lewis Research Center
Cleveland,	 Ohio 44135
ABSTRACT
An experimental	 program demonstrated that fsel-rich ( gyp = 4.8 to 7.8) {
catalytic combustion can be accomplished soot free as	 long as the combustion
temperature is less than the temperature at the rich limit of combustion.
Although soot was not measured directly, three pieces of data strongly suggest
that it was not present: 	 (1)	 the product gases were completely transparent and
produced no radiation characteristic of soot, 	 (2) measured reaction tempera-
tures
	
followed closely those calculated for equilibrium with no soot present, d
and (3) over 99 percent of the carbon was accounted for in the measured reac-
tion products.	 Data for two catalyst configurations were taken along with gas
samples at two locations downstream of the catalyst bed.
INTRODUCTION
{11
Soot formation in gas turbine combustors is a problem that is expected to
get worse as the quality of fuel deteriorates. 	 This increased soot loading has
adverse effects on liner temperature and fails to meet environmental	 standards. kil
The mechanism of soot formation has received a great deal
	
of attention recently; }^
a number of studies have been undertaken to relate combustion parameters and
fuel	 properties to the mechanism. 	 Another approach to the problem of soot is {f
a two-stage combustion system in which the first stage is a fuel-rich catalytic I
combustion fuel processor. 	 Catalytic combustion can be soot free at equivalence }
ratios beyond the rich limit of combustion and can produce a fuel-rich gaseous I	 j
mixture containing mostly small hydrocarbon molecules.	 This approach was
prompted by the research of Street and Thomas (ref.
	
1), who studied the forma-
tion of carbon (soot)	 in premixed flames.	 They observed that carbon was not
formed in flames when the gas mixture was being burnt near the rich limit of {Il
combustion.	 This same observation was made by Burgoyne and Neal 	 (ref.	 2).
Street and Thomas suggested that the reason for this behavior was that rate-
controlling step in the formation of soot has a high activation energy and thus
carbon would form very slowly at these temperatures. {;
Recently an experimental program was undertaken to demonstrate that soot-
free combustion of very fuel-rich gaseous mixtures is possible as long as com-
bustion occurs at temperatures below the rich limit of combustion. 	 This paper
describes preliminary results of the catalytic combustion of iso-octane at
equivalent ratios
	 in	 the range	 (p = 4.5 to 8.0.
EXPERIMENTAL APPARATUS
A 5.1 cm insulated flow reactor (fig. 1) was built to experimentally
demonstrate that soot-free fuel-rich catalytic combustion is feasible in a
practical combustion system. Platinum-Palladium on a honeycomb substrate was
selected as the catalyst because it is an excellent oxidation catalyst and the
I
toN
w
substrate has only a small pressure loss. Iso-octane was used as the base fuel
because it is a pure single component liquid which was relatively inexpensive.
A multltube conical fuel infector (ref. 3) produced a uniform gaseous mixture
of fuel and air entering the catalyst bed. A simple gas sampling system
(fig. 2) withdrew samples from two locations downstream of the catalyst bed.
These were stored in 1 L stainless steel bottles and later analyzed on a gas
chromatograph with a thermal conductivity detector. Thermocouple ports were
located along the reactor for monitoring the temperature and an optical port
was located 7.6 cm downstream of the catalyst for visual observations. The
reactor was designed always to operate in the fuel-rich mode so that the cata-
lyst was not destroyed by the temperature of a stoichiometric burn.
The catalyst was a 10 percent coating of Platinum-Palladium on 2.54 cm
thick honeycomb substrates. Table I gives a description of the catalyst sub-
strate and the four configurations used in the testing program. Results for
the 1290 and 1900 cm 2 catalyst configurations will be presented in this
report.
RESULTS AND DISCUSSION
In order to have uniform reaction occurring in the catalyst it is neces-
sary to have a near homogeneous fuel-air gas mixture entering the bed. Temper-
ature profiles measured diametr :ally across and reactor upstream of the
catalyst demonstrated that good mixing and vaporization had been obtained.
Figure 3 shows a comparison between the measured temperature and the theoreti-
cal one calculated for an air inlet temperature of 800 K and liquid Iso-octane
at 298 K. The agreement is excellent. Combustion temperature profiles across
the outlet of the catalyst bed were much flatter than those for the vaporized
fuel-air mixture upstream of the catalyst.
The first thing to be shown is that the reactions taking place in the
catalyst bed produces a soot free product. Although direct measurements of
soot were not taken, other measurements and observations strongly suggest that
the gaseous products are soot free. '
 These are (1) the reaction temperature,
(2) visual observations of the gaseous products, and (3) the carbon atom bal-
ance across the catalyst bed.
The reaction temperature is the temperature measured 3.7 cm downstream of
the catalyst bed. Figure 4 is a plot of the temperature as a function of
equivalence ratio for two theoretical equilibrium calculations. The solid line
shows the calculated temperatures when solid carbon is not a product while the
dashed line shows the temperatures when it is a product. The difference in the
temperature for the two calculations is about 150 K. The measured temperature
agrees very well with the no soot calculations and strongly suggests the gases
are soot free.
Visual observations of the hot reaction products 7.6 cm downstream of the
catalyst bed show a completely transparent gas quite different from the lumin-
ous gas of the soot producing flame of a Bunsen burner. A small amount of
light was observed through the window but this was found to emanate from the
hot substrate of the catalyst bed. This is the second bit of information which
tends to justify the no soot assumption.
The final piece of evidence is the carbon balance across the catalyst bed
calculated from the measured reaction products. Figure 5 shows the ratio of
the carbon out to the carbon in expressed as a percent plotted versus the
measured reaction temperature for the data found in table II. The dashed line
is the 100 percent line drawn across the plot. The fit of the data to the line
is excellent and the average value of the ratio is 99.6 percent.
Having established that combustion was soot free, the effect on the prod-
uct distribution of changing parameters such as catalyst surface areas and gas
F;iase dwell time was investigated. Table III compares the temperature and
product distribution at the same equivalent" ratio for catalyst configurations
having 1290 and 'E900 cm 2 of surface area. As can be seen only small changes
were observed for most of the products formed with the exception of hydrogen
and water, which changed by more than a factor of two. For the smaller cata-
lyst configuration 0.068 cm3 of water per liter of product gas was collected
(about 8.5 percent water as a product) and 5.35 mole percent hydrogen. The
larger area produced only about 3.5 percent water but 12.11 mole percent hydro-
gen. Thus increasing the catalyst surface area increased the mole percent of
hydrogen and decreased the fraction of water produced. This yields a more
energy rich and clean burning product gas which is one of the objectives of
this work. At present there is not enough data to determine what the mechanism
might be for this conversion, but reformation of the hydrocarbons by water
appears to be a good candidate.
Another difference in the data was the decrease in the temperature when
the surface area was increased. At first glance this was unexpected and dif-
ficult to understand until it was noted that there was a significant difference
in the volume of water collected for the two samples. The analysis of the
reaction products have consistently shown that reactions involving oxygen lead
ultimately to the formation of CO, CO2 , andH 2O. The heats of formation
of these compounds at 1200 K are -27.2, -94.4, and -59.5 kcal/mole, respec-
tively. Since the formation of CO is exothermic by only 27 kcal while water
is exothermic by 59.5 kcal, its clear that the catalyst configuration producing
the most water would have a higher temperature.
Table II compares the reaction products for the 1900 cm 2 catalyst con-
figuration as a function of temperature (equivalence ratio) for samples taken
at two locations along the drift tube. The first location (about 16 msec of
gas phase reaction time) describes the product distribution resulting from
	
I1
surface and gas phase reactions. The second location (about 60 msec) gives
some insight into the types of gas phase reactions occurring. The mayor prod-
ucts (>2 percent) are H2, CO, CO 2 , H 2D, CHq, C2Hq, C 3H 6 , and Cq hydrocarbons.
Figure 6 shows the change in the mole percent of some of these compounds as the 	 j
reaction temperature changes. The lines are eyeball fits to the data and were
meant to demonstrate that the trends at the two dwell times are very similar
even though the mole percents are difficult. The most significant changes in
the two sets of data are the decrease in the mole percent of H2, C 3 H 6 , and Cq
hydrocarbons and increase in CHq with increase in gas phase reaction time.
This indicates that pyrolysis of the larger molecules is occurring followed by
consumption of the molecular hydrogen by the hydrocarbon fragments. Figure 7
shows the carbon atom balance present:d previously with data showing the amount
of carbon present in molecules having two or fewer carbon atoms. It is clear 	 L'
that as the temperature increases pyrolysis of the larger molecules result in
more of the carbon appearing in the smaller molecules, thus demonstrating the
fuel processing mechanism. 	 i
SUMMARY
Fuel-rich catalytic combustion producing a soot free pyrolyzed fuel
appears to be a viable approach to a two-stale combustion system. Such a soot
free process is feasible as long as the combustion temperature is less than the
temperature at the rich limit of combustion. Although direct measurements of
soot were not taken, three pieces of data strongly suggest that soot was not
present. These are (1)the product gases are completely transparent and pro-
duce no radiation which would be characteristic of soot being present, (2)
measured reaction temperatures follow closely those calculated for equilibrium
with no soot present, and (3) over 99 percent of the carbon was accounted for
in the measured reaction products. Analysis of gas samples for two catalyst
configurations indicated that the mole percent of water, an undesirable prod-
uct, can be greatly reduced by using more catalyst. Gas samples taken at two
locations along the drift tube showed that the pyrolysis of the larger mole-
cule: produced mainly methane and ethylene.
REFERENCES
1. Street, J.C. and Thomas, A.: Carbon Formation in Pre-mixed Flames. Fuel
34, 4 (1955).
2. Burgoyne, J.N. and Neale, R.F.:
ei, sz, iieoa).
3. Tacina, R.R.: Degree of Vaporization Using an Air
a Premixed-Prevaporized Combustor. NASA TM-78836,
1
4
ORIGINAL PAC,W 6a
OF POOR QUALITY
TABLE L - DESCRIPTION OF CATALYST, CONFIGURATION, AND HOLDER
6hIt escrlplion
Cell sufacel 290
fArveiTStacking order
2	 1
1900 413,211density' areab
31 7f8
15.5 522
7,75 334
TABLE 11. - REACTION PRODUCTS FOR CATALYST SURFACE AREA 1900 cm2
Reac tfon tomp„ K 1300 1170 11]8 11 14 109]	 - - 1'dl 4 1190 1101 1145
CqulYdlence testa 4.00 5,50 6.94 7.22 7.87 4,02 5.11 6,61 6,26
Mole's tuellmin 1.2bG 1.465 1.751 1,9]7 2.099 L2fi5 6]59 4468 1,466
Moles elrlmin 15,75 15.09 15,93 16.00 15,92 15,66 15,86 15,90 15.86
Dwell time, msee 15,9 IG,I l7J 10.5 19,5 5916 60.3 60,9 64.2
Catalyst bed pressure drop, psf4
._. ,_^^
UP Stream preswro
	
28,77	 20,6]	 20,61	 28.79	 20,01	 28,05	 28.94	 28.61	 28.66
BOwdtlream prettura 	 20.53	 20,67	 20,5]	 28.66	 20,69	 28,69	 28,77	 28.54	 20.62
aP	 across bed	 .24,.	 T-.. ,00	 .OB ... __.17	 ,12	 .16	 .19	 .07	 .OA
Reactor lemperatur q , K
Preheat air	 815	 BU	 814	 813	 811	 820	 BIB	 815	 Bit
Yaporleetlon	 549	 532	 508	 49l	 476	 551	 543	 573	 513_	 .t
Gat sample concentrations, percent, grou p s 1 and 2
Hydrogen K.sr-12,17- 0.26 5.84 4.58 0.93 10.30 9,46
Oxygen .73 .75 .79 .07 .92 .65 ,63 .46 .69
Nitrogen 53,23 53.10 55,86 58.14 59.97 52.69 53,18 53.74 54.66
Carbon monoxide 17.41 16.32 13,28 11.49 10.33 17.43 16,64 15.41 14.5B
Carbon dioxide 1.78 2.07 2.6B 7.10 3,37 1.53 1.03 2.20 2.70
Methane 6.34 5,03 6.12 5.71 4.91 8.86 8.82 8.79 8,16
Acetylene .15 .OB .06 .D3 .04 .fit .40 .30 .14
Ethylene Am 3,21 4 . 20 3.29 2 .00 4.762 4 . 85 4.65 4.43
Ethane .42 .50 .59 .51 ,48 .21 .29 .42 .55
Group i total 96.60 93,97 91.86 B9.OD 06.56 97.57 96,94 95.71 93.81
Pro pylene 1.66 2.57 3.61 4.13 4.13 .62 1.17 1.52
2J6
Alleno ,24 .25 .22 .16 .13 .12 ,16 .20 .29
Methyl acetylene .20 .I6 .12 .10 .10 .23 .26 .29 .26
C4 hydrocarbons 1.42 2.77 4.05 5.59 6.17 .59 .26 .94 2,54
C5 hydrocarbons .06 .23 .48 .91 .95 .06 .04 .11 .31
Benzene .12 ,On .10 .09 ,09 .36 .39 .43 .35
Toluene .03 .03 .04 .21 .00 .07 .00 .11 .13
Iso-octene .00 .03 .24 .89 2.04 .00 .00 .00 .00
Total groups 1 and 2 100.42 100.10 100.74 101.26 100.26 99.53 1	 99.26 99.32 100.47
Liquid sample data
Llquld deposit c	 Il 0.0]3 0,028 0.061 0.059 D.073 0.024 0.0]] 0.028 0.0q]
Dons ty q/cmt 1,00 1.05 1.01 1.05 l.07 1.01 1.02 1.00 1.02
PH 4.5 4,5 to 5 4.5 to 5 4 lm 4.5 4 to 4.5 4.5 4.0 6.0 6.0
a
I^
}
^?	 Ii
j
.I,
!i
TABLE M. • REACTION PRODUCT
Or CAIALISI
C
to 
It Surface Freer CM2
1 1 it ,I 1em91raturt, R
tRUlwleno ratio
I attion Rrodutl4, potent
11Ydro9en
011 Ran
Nlpelan
Carbon Monoxide
Carbon dioxide
Methane
Ant lens
Cth7lare
Et hint
P mOYl me
All ant
NothYl Itet9lene
C41
CeS
Iennnt
Toluene
Ica-octane
ratioN9It fltlo Itr011
1 AND TEMPERATURE AS A IUNC110N
SURFACE AREA
1790	 11900
1115,
 
0
1197	 11
5.49	 5
5.75 17.11
.96 .75
59,59 57.10
M14 16.77
7.40 7.01
6.55 5.07
16 .00
4.15 7.P
57 .50
7.91 7.61
76 .25
77 .16
7.91 7.11
.70 .77
.09.
05
05
.07
OD .0]
1,700 1.747
OF POOct QW,U) `7
I
1
e	 r	 o4	 t
Uk"161tv;,L E'..";
OF POOR QUALITYVENT TO	 50•LITER
ROOF BURNER	 PRESSURIZED
FUEL TANK ^
VAPORIZATION
SECTION
CATALYST	 ;	 FUEL
DRIFT TUDE
EXHAUST
VALVE
GAS
FLOWMETER	
`-14-kW ELECTRIC HEATER
L.
0.5 m
Fig, 1,- Schematic drawing of catalytic flow tuba reactor.
r
i
i
i
:D SAMPLING LINES
SAMPLING
^—I PUMP
I	 1	 ABSOLUTE PRESSURE
ICE BATH	
PROBE I I 	 +
REACTOR
	1•LITER	 VACUUM	
_►
 VENTSAMPLE BOTTLE	 _, PUMP ^—
^^	 ti
t !i
II
iJ
I^
VENT
WET TEST METER
Fig. Z, - SamPling System.
515
550
se 525
^t
500
475
450
425
I	 ^
I	 r
v	 ^. 	 9
Fig. 3. • Comparison of measured vaporization
temperature with theoretical temperature
as function of equivalence ratio.
1250
4
C
1200
1150
z
^ uao ^^\
1050	 `.\	 I
1000
4	 5	 6	 7	 a	 9
EQUIVALENCE RATIO
Fig. 4, - Comparison of measured reaction
temperatures with calculated equilibrium
temperatures, All reaction temperatures
corrected to Inlet conditions of calculation.
A2
110
K
100 - b — - -Oro— ®— O
g 80
60
8 40 —
E5 	 _
TEMPERATURE, K
Fig, 5,- Carbon atom balance -percent of Initial
carbon In reaction products Itabie 2) as function
of temperature.
12
8	 --_—o----0-0-0-
O
G 4
0
(a) Methane,
16
LL
O
12	 O
e	 o	 ^O
O
4
6
DWELL
TIME,
msec
4—	 O	 O 16.19
^N 	 q 60-64
2
^t
0
(c) Propylene,
''^_ q
1100	 1140	 1180
	
1220	 1060	 1100	 1140	 1180	 1
TEMPERATURE, K
(b) Hydrogen.
	 (d) C4 hydrocarbons.
Fig. 6. - Change In the percentage of reaction products as function of temperature and dwell time,
d
i
f
I
l
i
11
N 110
s 100 —3-45-0-0-0-0- 00-45-0-0---o-O--®-°
r^
60
O Y'OTALCARBON
riS p	 O CARBON IN GROUP 1
I	 I	 I	 I	 I	 I
Fig, 7, • Carbon atom balance and percent of
carbon found in molecules having two or
fewer carbon atoms,
•	 1.
1. Report No. 2, Government Accession No. 0, Recipient's Catalog No.
NASA TM-47042
rTlllo and Subillie 5. Report onto
Fuel-Rich Catalytic Combustion - The First Stage
0. Porfoemina Organization Codeof a Two-Stage Combustor'
505-31-04
7, Author(e) 0. Performing Organization Report No.
Theodore A. Brabbs and Sandra L. Olson E-2601
10. Work Unit No.
0, Performing organization Noma end Address
National Aeronautics and Space Administration
1 	 Contractor Grant No.
Lewis Research Center
Cleveland,  Ohio	 44135 19, Typo of Report and Period Covered
Technical Memorandum12, Sponeoring Agency Noma and Address
National Aeronautics and Space Administration
Washington, D.C.	 20546
14. Sponsoring Agency Code
15. Supplementary Notes	 `-
Prepared for the Fall Meeting of the Eastern Section of the Combustion
Institute, Clearwater Beach, Florida, 	 December 3-5,	 1984,
10, Abstract
An experimental program demonstrated that fuel-rich (rp - 4.8 to 7.8) catalytic
combustion can be accomplished soot free as long as the combustion temperature
is less than the temperature at the rich limit of combustion.	 Although soot was
not measured directly, three pieces of data strongly suggest that it was not
present:	 (1) the product gases were completely transparent and produced no
radiation characteristic of soot, 	 (2) measured reaction temperatures followed
closely those calculated for equilibrium with no soot present,	 anal (3) over 99
percent of the carbon was accounted for in the measured reaction products. 	 Data
for two catalyst configurations were taken along with gas samples at two
locations downstream of the catalyst bed.
17. Key Words (Suggested by Authorial) 18, Distribution statement
Catalytic combustion; Fuel-rich Unclassified - unlimited
combustion; Two-stage combustion STAR Category 25
system; Soot-free combustion
19, Security Classll. (of this report)
Unclassified
20, security Clasalf. (of this page)
Unclassified
21. No, of peace 22. Prlco•
For sale by the National Technical Information Service, Springfield, Voglnle 22161
G
i
s


Fuel rich catalytic comustion: The first stage of a two-stage combustor

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