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Proceedings of the Combustion Institute 34 (2013) 2279–2287
www.elsevier.com/locate/proci
Hetero-/homogeneous combustion of
ethane/air mixtures over platinum at pressures up to
14 bar
Xin Zheng, John Mantzaras ⇑, Rolf Bombach
Paul Scherrer Institute, Combustion Research, CH-5232 Villigen PSI, Switzerland
Available online 14 June 2012
Abstract
The hetero-/homogeneous combustion of fuel-lean ethane/air mixtures over platinum was investigated
experimentally and numerically at pressures of 1–14 bar, equivalence ratios of 0.1–0.5, and surface temperatures ranging from 700 to 1300 K. Experiments were carried out in an optically accessible channel-flow
reactor and included in situ 1-D Raman measurements of major gas phase species concentrations across
the channel boundary layer for determining the catalytic reactivity, and planar laser induced fluorescence
(LIF) of the OH radical for assessing homogeneous ignition. Numerical simulations were performed with a
2-D CFD code with detailed hetero-/homogeneous C2 kinetic mechanisms and transport. An appropriately
amended heterogeneous reaction scheme has been proposed, which captured the increase of ethane catalytic reactivity with rising pressure. This scheme, when coupled to a gas-phase reaction mechanism, reproduced the combustion processes over the reactor extent whereby both heterogeneous and homogeneous
reactions were significant and moreover, provided good agreement to the measured homogeneous ignition
locations. The validated hetero-/homogeneous kinetic schemes were suitable for modeling the catalytic
combustion of ethane at elevated pressures and temperatures relevant to either microreactors or large-scale
gas turbine reactors in power generation systems. It was further shown that the pressure dependence of the
ethane catalytic reactivity was substantially stronger compared to that of methane, at temperatures up to
1000 K. Implications for high-pressure catalytic combustion of natural gas were finally drawn.
Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Keywords: Ethane heterogeneous and homogeneous combustion; Catalytic reactivity on platinum; Homogeneous
ignition; In situ Raman and OH-LIF
1. Introduction
Combined hetero-/homogeneous combustion
has demonstrated the potential of ultra-low NOx
emissions in power generation systems ranging
from large-scale gas turbine burners to microreac-
tors for portable power generation. Commercial
utilization of such combustion technologies
requires knowledge of the heterogeneous and
moderate-temperature homogeneous kinetics of
conventional fuels (e.g. natural gas) under realistic
operating conditions, so as to facilitate reactor
⇑ Corresponding author. Fax: +41 56 3102199.
E-mail address: [email protected] (J. Mantzaras).
1540-7489/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.proci.2012.05.028
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X. Zheng et al. / Proceedings of the Combustion Institute 34 (2013) 2279–2287
design and delineate operational envelops. In the
past decade, development and validation of heterogeneous methane kinetics have been accomplished over a wide range of operating conditions
[1–11]. For ethane, however, which is another
important natural gas component (comprising up
to 16% per volume, depending on gas provenance),
studies were mainly limited to partial oxidation
catalytic kinetics due to the industrial use of ethane
as the primary feedstock for ethylene production
in short-contact-time catalytic reactors.
The first detailed ethane heterogeneous kinetic
scheme, consisting of 82 elementary reactions
among 19 surface species over Pt, was reported
by Zerkle et al. [12] with reaction rates based on
literature data and appropriate fitting to measurements; their model predictions yielded good
agreement to earlier atmospheric-pressure measurements [13]. Subsequently, Donsi et al. [14]
constructed a scheme for ethane partial oxidation
over Pt and Pt/Sn catalysts, by combining detailed
hydrogen and carbon monoxide kinetics with
lumped steps of ethane heterogeneous reactions.
Their model reproduced experiments in a catalytic
monolith at pressures up to 10 bar. More recently,
Vincent et al. [15] extended the work of Zerkle
et al. [12] to a more detailed scheme comprising
283 elementary reactions among 35 surface species, with kinetic parameters determined by Density Function Theory (DFT) and the UBI-QEP
method. Furthermore, kinetics of ethane catalytic
partial oxidation has also been studied over other
catalysts, e.g. metal oxides which are low-cost
substitutes for noble metals [16]. In contrast to
partial oxidation, fewer studies are reported for
ethane total oxidation relevant to large- or
micro-scale power generation. Early work [17]
proposed a global step having first order kinetics
with respect to ethane and zeroth order with
respect to oxygen concentration, for various catalysts (Ni, Pd and Pt) on the basis of subatmospheric-pressure, fuel-lean experiments in a
recirculating batch reactor. Deshmukh et al. [18]
suggested a three-step mechanism with kinetic
parameters estimated by using the bond-order
conservation theory and their kinetic predictions
agreed well with experimental literature data at
atmospheric pressure. The aforementioned kinetics was only tested for partial oxidation or total
oxidation at low pressures. Variation in operating
pressure may break down their validity under microreactor (up to 6 bar) [19] or gas turbine
(16 bar) [20] operating conditions. Therefore,
extension of the available heterogeneous kinetic
schemes to the high pressure and temperature
total oxidation conditions necessitates further
investigation.
The present work continues previous kinetic
studies of H2 [21], syngas [22], methane [10] and
propane [23] by experimentally and numerically
investigating heterogeneous and homogeneous
combustion of ethane over platinum in a channel-flow reactor. The main objective is to establish
suitable heterogeneous and homogeneous kinetic
schemes for ethane, which are capable of reproducing key catalytic and gas-phase combustion
processes at elevated pressures. Particular interests are to improve fundamental understanding
regarding the impact of pressure on the heterogeneous kinetics and on the hetero-/homogeneous
chemistry coupling, and thus advance the development of natural-gas fired catalytic combustion
systems.
2. Experimental methodology
2.1. Test rig and channel flow reactor
Experiments were performed in an optically
accessible channel flow reactor (Fig. 1 and details
in [10]), consisting of two horizontal, platinumcoated Si[SiC] ceramic plates (300 mm long,
9 mm thick, and placed 7 mm apart) and two vertical quartz windows (300 mm long and 3 mm
thick). The inner Si[SiC] surfaces were coated via
plasma vapor deposition with a 1.5 lm thick
non-porous Al2O3 layer, followed by a 2.2 lm
thick Pt layer. Measurements of the total and
active catalyst surface areas with BET (Kr-physisorption) and CO-chemisorption, respectively,
have verified the absence of surface porosity,
while post-combustion X-ray photoelectron spectroscopy (XPS) attested the presence of only Pt
and the lack of bulk Si or Al at the catalyst surface
[10]. A water-cooled metal block was attached to
the reactor entry (Fig. 1) to control the surface
temperatures and thus to allow for kineticallycontrolled catalytic ethane conversion, away from
the mass-transport-limit. Surface temperatures
Fig. 1. High-pressure test rig and optical layout of
Raman and OH-LIF (all distances in mm).
X. Zheng et al. / Proceedings of the Combustion Institute 34 (2013) 2279–2287
2281
Table 1
Experimental conditions.a
Case
p (bar)
u
C2H6 (%vol)
TIN (K)
UIN (m/s)
ReIN
1
2
3
4
5
6
7
8
9
10
1
2
4
10
12
1
4
6
8
14
0.30
0.31
0.41
0.22
0.11
0.49
0.44
0.42
0.40
0.34
1.75
2.70
2.39
1.35
1.93
2.83
2.52
2.41
2.30
1.95
460
460
454
455
461
449
515
487
505
652
3.47
1.89
0.90
0.46
0.39
1.75
0.75
0.52
0.51
0.63
1310
1430
1390
1770
1760
689
936
1070
1320
1850
a
Pressure, equivalence ratio, % vol. ethane, inlet temperature, velocity and Reynolds number.
along the x–y symmetry plane were monitored by
S-type thermocouples (12 for each plate) embedded 0.9 mm beneath the catalyst surface through
holes eroded from the outer uncoated Si[SiC]
surfaces.
Compressed air was electrically preheated,
mixed with ethane in two sequential static mixers
and the resultant flow was driven into a rectangular steel duct (200 mm long, 104 mm wide and
7 mm high) equipped with cross-flow grids to produce a uniform flow at the reactor entry. Flows of
air and ethane were controlled by two mass flow
meters while the incoming gas temperature was
monitored by a sheathed K-type thermocouple,
positioned at the channel inlet (x = 0 mm). The
reactor assembly was mounted inside a high-pressure stainless steel tank. Two quartz windows
(350 mm long, 50 mm high and 35 mm thick) on
the tank provided optical access through both
reactor sides (Fig. 1). Two additional quartz windows, one at the reactor exhaust and the other at
the rear flange of the tank, offered an additional
optical access in the streamwise direction, which
was used for the LIF excitation beam.
The experimental conditions are presented in
Table 1, with pressure and equivalence ratios ranging from 1 to 14 bar and 0.11 to 0.49, respectively.
The provided Reynolds numbers (ReIN) based on
the uniform inlet properties and the channel
hydraulic diameter (=13.1 mm) manifested laminar flow conditions. It is emphasized that laminar
conditions were guaranteed even at ReIN 5000
due to the strong flow laminarization induced by
the heat transfer from the hot catalytic plates [24].
2.2. Laser diagnostics
The Raman and LIF set-up (Fig. 1) is similar to
that employed in recent H2/air studies [25]. Planar
OH-LIF was used to detect gas-phase combustion.
Excitation (k = 285 nm) was achieved by a frequency-doubled Nd:YAG pulsed laser (Quantel
YG981E20-CL), which pumped a tunable dye
laser (Quantel TDL90 NBP2UVT3). A cylindrical
lens telescope and a 1 mm slit mask transformed
the 285 nm beam into a light sheet propagating
counterflow, along the x–y symmetry plane of
the reactor. Fluorescence from both (1–1) and
(0–0) OH transitions at 308 and 314 nm, respectively, were collected at 90 with an ICCD camera
(LaVision Imager Compact HiRes IRO). Each
recorded LIF image corresponded to a 100 7 mm2 channel area and the camera was traversed
axially to map the entire 300 mm reactor length.
Given the steady operating conditions, 400 images
were averaged at each measuring location.
For the Raman measurements, a pulsed
Nd:YLF laser (Quantronix Darwin Duo,
k = 586.5 nm) was operated at 1.5–2 kHz with
pulse duration and energy of 130 ns and 37–
43 mJ, respectively. The k = 586.5 nm beam was
focused through the side windows into a vertical
line (0.3 mm thick) by an f = 150 mm cylindrical
lens. The focal line spanned the entire 7 mm channel height and was offset laterally (z = 15 mm) to
increase the collection angle and minimize thermal
beam steering, as in [21–23]. Two f = 300 mm
lenses collected the scattered light at 50° angle
with respect to the sending optical path and
focused it on a 25 cm imaging spectrograph
(Chromex-250i) equipped with an ICCD camera
(Princeton Instruments PI-MAX1024GIII). Signal of 200,000–400,000 pulses was integrated on
the detector chip. Effective Raman cross sections,
which included transmission efficiencies, were
evaluated by recording the signals of pure ethane,
air, and completely burnt gases of known composition. Raman data were acquired at different
positions by traversing axially a table supporting
the sending and collecting optics, and also the
Nd:YLF laser (Fig. 1). Measurement accuracies
were ±5% for species molar fractions down to
1%, while lower concentrations entailed larger
uncertainties. Finally, data points closer than
0.7 mm to either wall were discarded due to
low Raman signal-to-noise ratios.
3. Chemistry and flow simulations
Chemistry was modeled with detailed heterogeneous and homogeneous reaction mechanisms. An
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Table 2
Heterogeneous reaction mechanism for ethane oxidation on platinum.
Reactions
S0/A
n
Ea
Reference
Adsorption and desorption reactions
R1
C2H6 + 2Pt(s) ! C2H6(s)
R2
C2H6(s) ! C2H6 + 2Pt(s)
R3
CH4 + 2Pt(s) ! CH3(s) + H(s)
R4
O2 + 2Pt(s) ! O(s) + O(s)
R5
O2 + 2Pt(s) ! O(s) + O(s)
R6
2O(s) ! O2 + 2Pt(s)
R7
CO + Pt(s) ! CO(s)
R8
CO(s) ! CO + Pt(s)
R9
OH + Pt(s) ! OH(s)
R10
OH(s) ! OH + Pt(s)
R11
H2O + Pt(s) ! H2O(s)
R12
H2O(s) ! H2O + Pt(s)
R13
CO2(s) ! Pt(s) + CO2
3.0E3
1.0E+13
1.0E2
2.3E2
1.8E+21
3.2E+21
1.6E+20
1.0E+13
1.0
1.0E+13
7.5E1
1.0E+13
1.0E+13
0.0
0.0
0.0
0.0
0.5
0.0
0.5
0.0
0.0
0.0
0.0
0.0
0.0
0.0
20,900
0.0
0.0
0.0
224,7106000h0
0.0
125,500
0.0
192,800
0.0
40,300
20,500
[14]ab
[12]
[4]ab
[4]a
[4]
[4]
[4]a
[4]
[4]a
[4]
[4]a
[4]
[4]
Surface reactions
R14
C2H6(s) ! C2H5(s) + H(s)
R15
C2H6(s) + O(s) ! C2H5(s) + OH(s) + Pt(s)
R16
C2H5(s) + 6Pt(s) ! 2C(s) + 5H(s)
R17
CH3(s) + Pt(s) ! CH2(s) + H(s)
R18
CH2(s) + Pt(s) ! CH(s) + H(s)
R19
CH(s) + Pt(s) ! C(s) + H(s)
R20
H(s) + O(s) M OH(s) + Pt(s)
R21
H(s) + OH(s) M H2O(s) + Pt(s)
R22
OH(s) + OH(s) M H2O(s) + O(s)
R23
C(s) + O(s) ! CO(s) + Pt(s)
R24
CO(s) + O(s) ! CO2(s) + PT(s)
1.0E+13
1.0E+22
5.0E+21
3.7E+21
3.7E+21
3.7E+21
3.7E+21
3.7E+21
3.7E+21
3.7E+21
3.7E+21
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
57,700
25,100
20,000
20,000
20,000
20,000
11,500
17,400
48,200
62,800
105,000
[26]
[12]
[14]b
[4]
[4]
[4]
[4]
[4]
[4]
[4]
[4]
a
Reaction rates are in terms of sticking coefficients (S0).
Reaction order with respect to Pt(s) is 1.1, 2.3 and 1.0 for R1, R3 and R16, respectively. Units: S0 [–], A [cm, s, K],
Ea [J/mol], coverage h [–].
b
ethane surface reaction mechanism has been constructed based on reactions of ethane and its derivatives, collected by compiling literature data
[12,14,26] and further including CH4/CO/H2 reactions on Pt [4]. The added ethane reactions had
only been tested for atmospheric-pressure partial
oxidation conditions and thus their applicability
has not been previously assessed for deep oxidation at elevated pressures. In order to reproduce
the measurements, the reaction order of ethane
adsorption with respect to Pt was increased to
1.1 from its original value of 1.0; this change was
supported by detailed sensitivity analysis as will
be elaborated in Section 4.1. While the reaction
order modification had a negligible impact at
p 6 2 bar, it had a progressively larger effect with
rising pressure above 2 bar. It is further emphasized that the order of hydrocarbon adsorption
with respect to Pt is crucial in capturing the correct
pressure dependence of the catalytic reactivity.
Earlier methane studies [10] showed that the good
performance of the therein tested catalytic scheme
[4] at elevated pressures cardinally depended on
the 2.3 order of methane adsorption with
respect to Pt. The resulting ethane heterogeneous
scheme comprised 24 reactions among 14 surface
species (see Table 2), with surface site density
C = 2.7 109 mol/cm2. Methane reactions in
Table 2 were included for completeness, as this
species was part of the ethane gas-phase mechanism; however, the ensuing ethane hetero-/homogeneous combustion simulations were practically
unaffected by these reactions.
The gas phase mechanism, involving 192 reactions among 35 species, was extracted from a
detailed C3 combustion mechanism [27] by removing C3 reactions as they were largely irrelevant
under the present very lean conditions. Species
thermodynamic and transport data were also
taken from [27]. Surface and gas-phase reaction
rates were evaluated with CHEMKIN [28,29].
Finally, a mixture-average diffusion model was
used for species transport.
A steady elliptic 2-D CFD code (details given
in [21,22]) simulated the 300 7 mm2 (x–y) channel domain. Uniform gas temperature, velocity,
and species composition were applied at the inlet
(x = 0) and prescribed temperatures at the gaswall interfaces (y = 0 and 7 mm), constructed by
polynomial curve fits to the measured 12 surface
temperatures per plate. Given the particularly
large gas-phase and catalytic mechanisms, it was
imperative to first use the solution of a fast parabolic code [5] as initial guess to the elliptic code. In
the presence of strong gaseous combustion, the
parabolic code always yielded homogeneous
X. Zheng et al. / Proceedings of the Combustion Institute 34 (2013) 2279–2287
2283
ignition distances longer than those of the elliptic
code, by up to 2 cm (comparisons between elliptic and parabolic homogeneous ignition predictions have been elaborated in [5]). Despite the
good initial guess, maximum 15 CPU days were
still necessary for convergence of the elliptic code
in each examined case, in a Xeon 3.0 MHz
processor.
4. Results and discussion
Experimental and numerical results are compared against each other. For Cases 6-10 in Table
1, flames were anchored inside the channel reactor
as manifested by the OH-LIF measurements, thus
allowing validation of homogeneous kinetics. On
the other hand, the absence of flames for the
lower-wall temperature Cases 1-5 allowed evaluation of heterogeneous kinetics.
4.1. Pressure effect on catalytic reactivity
Computed catalytic (C) and gaseous (G)
hydrogen consumption rates (the latter integrated
over the 7 mm channel-height) as well as measured surface temperatures are presented in
Fig. 2 for four selected cases without appreciable
gas-phase chemistry contribution. For the same
cases, predicted and measured transverse molefraction profiles for C2H6 and the major product
H2O are shown in Fig. 3 at three selected streamwise positions x 6 80 mm, whereby the gas-phase
contribution on ethane conversion was negligible.
The magnitudes of the catalytic conversions (C) in
Fig. 2 bear the combined effects of pressure,
equivalence ratio, gas inlet temperature and velocity, and surface temperatures. The pressure effect,
of particular interest herein, will be numerically
delineated in the next section by fixing all other
parameters. The vertical arrows marked xag in
Fig. 2 denote the onset of appreciable gas-phase
ethane conversion (therein G amounts to 5% of
the C conversion). By limiting the heterogeneous
reactivity investigation to x < xag, potential falsification of the heterogeneous kinetics by gaseous
chemistry is eliminated. It is clarified that the
onset of appreciable gaseous conversion does
not imply the onset of vigorous exothermic gaseous reactions: it was shown [10,23] that the
incomplete oxidation of hydrocarbons to CO
can still yield significant gaseous fuel consumption
without appreciable exothermicity and hence
without the establishment of a flame.
As seen in Fig. 3, predictions of the herein-constructed heterogeneous scheme are in good agreement with the Raman measurements of C2H6 and
H2O mole fractions at pressures up to 12 bar;
however, simulations in Fig. 3 without the corrected C2H6 adsorption (with the 1.1 platinum
order), overpredict substantially the catalytic reac-
Fig. 2. Measured wall temperatures (upper-wall:
squares and fitted solid lines, lower-wall: circles and
fitted dashed lines), and computed catalytic (C) and gasphase (G) ethane conversions for four flameless cases in
Table 1.
Fig. 3. Measured (symbols) and predicted (dashed-lines
and solid-lines for the original and the herein proposed
reaction scheme, respectively) transverse profiles of
C2H6 (triangles) and H2O (circles) mole fractions, at
three axial locations for Cases 2–5. For Case 2, the solid
and dashed-line predictions practically overlap.
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X. Zheng et al. / Proceedings of the Combustion Institute 34 (2013) 2279–2287
tivity at p > 2 bar. Such in situ, highly-resolved
measurements of gas-phase species concentrations
across the catalyst boundary layer have been provided for the first time, allowing for direct evaluation of the ethane heterogeneous kinetics. It is
noted that mass-transport-limited conversion
was not approached for pressures up to 12 bar
and wall temperatures up to 1100 K, as manifested by the appreciable C2H6 concentrations
near both walls (Fig. 3). The attained kinetically-controlled catalytic conversion was also
facilitated by the larger than unity Lewis number
of ethane (for fuel-lean C2H6/air stoichiometries,
LeC2H6 1.8), which in turn led to strongly underadiabatic surface temperatures at the upstream
parts of the reactor. This effect, present during
fuel-lean catalytic combustion of higher hydrocarbons, was also discussed in earlier propane studies
[23].
The sensitivity analysis (SA) in Fig. 4(a) identified ethane adsorption (R1) as the most sensitive
catalytic reaction controlling the consumption of
ethane at all pressures; this SA was constructed
using the surface perfectly stirred reactor (SPSR)
code of CHEMKIN [30] at u = 0.3, constant wall
temperature of 900 K, and a residence time of
0.1 s. The limiting reaction in ethane catalytic oxidation is thus its adsorption:
s_ ad ¼ k ad ½C2 H6 mw ½ChPt n ;
ð1Þ
Fig. 4. (a) Sensitivity analysis, SPSR computations for
a = 0.3 ethane/air mixture (reaction numbering as in
Table 2), (b) SPSR-computed temperatures for CH4/
C2H6 mixtures versus inlet temperature, for three % vol.
C2H6 contents in the total fuel at 15 bar, and (c) SPSRcomputed fuel conversions for CH4/air, C2H6/air and
C3H8/air mixtures (u = 0.3) versus pressure.
where kad is the adsorption rate constant, [C2H6]w
the ethane concentration at the gas-wall interface,
U the total surface site density (2.7 109 mol/
cm2), hPt the Pt coverage, while m = 1.0 and
n = 1.1 are the reaction orders regarding ethane
and Pt, respectively. As previous catalytic reactivity studies of lean propane/air [23] and methane/
air mixtures [10] have indicated, the rise in oxygen
partial pressure at elevated pressures increases the
O(s) surface coverage at the expense of Pt(s). This
in turn reduces the catalytic reaction rate due to
the drop in available free platinum surface concentration ([UhPt] in Eq. (1)). On the other hand,
a rise in pressure increases proportionally the concentration of the limiting reactant at the wall
([C2H6]w in Eq. (1)), thus promoting ethane
adsorption. These two competing factors determine the overall dependence of the catalytic reactivity on pressure, as also shown for methane [10]
and propane [23].
To facilitate the forthcoming discussion, the
negative pressure dependence of hPt is represented
as [hPt] pb, where b is a positive number that is
not necessary constant, but function of local
conditions. According to Eq. (1), the adsorption
reaction becomes a product of the positive
pressure dependent gas-phase concentration,
½C2 H6 mw pm , and the negative pressure dependent
free-site coverage, [hPt]n pnb. Thus, the catalytic
reactivity assumes an overall pressure dependence
pmnb with m > nb, such that a total positive pressure dependence is guaranteed. This pressure effect
is well-reproduced by the proposed kinetic scheme,
as the comparisons in Fig. 3 indicate. Crucial for
this performance is the adopted 1.1 reaction order
with respect to Pt(s) for C2H6 adsorption (R1 in
Table 2).
Having established appropriate ethane highpressure kinetics, comparisons of heterogeneous
reactivities for C2H6 and CH4 and C3H8 (main
components of natural gas) were performed using
SPSR [30] simulations; the equivalence ratio for
all cases was u = 0.3, the surface-to-volume ratio
was 2.86 cm1 (equal to that of the channel in
Fig. 1), the reactor temperature was constant at
900 or 1000 K, pressure varied from 1 to 15 bar,
and residence time increased proportionally with
rising pressure (s = 10 ms at 1 bar) to maintain a
constant reactor mass throughput. The CH4 and
C3H8 high-pressure heterogeneous schemes were
taken from [4,23], respectively. Reactivity is
shown in terms of fuel conversion versus pressure
in Fig. 4(c). It is evident that reactivity drops in
the order of C3H8, C2H6 and CH4 and, most
importantly, relative differences between the fuel
reactivities increase with rising pressure. For the
examined temperatures, methane conversion
reaches a nearly constant value above a certain
pressure, while C2H6 and C3H8 conversions
strongly increase with rising pressure up to
15 bar. The continuous rise of C2H6 and C3H8
X. Zheng et al. / Proceedings of the Combustion Institute 34 (2013) 2279–2287
reactivities with pressure indicates that these fuel
components could facilitate the natural gas lightoff at elevated pressures relevant to many power
generation systems. This is further illustrated by
additional SPSR simulations at a fixed pressure
of 15 bar and inlet temperatures varying from
700 to 900 K. Computed reactor (or outlet) temperature is plotted in Fig. 4(b) as a function of
inlet temperature, demonstrating a reduction of
the light-off temperature by about 20 and 50 K
when replacing volumetrically CH4 with 5% or
16% C2H6 (contents typical to various natural
gas compositions). It is clarified, however, that
using the scheme in Table 2 for natural gas would
also entail further validation experiments with
CH4/C2H6 mixtures. Nonetheless, the results
indicate that the presence of ethane is highly beneficial for the light-off of natural-gas-fueled high
pressure systems.
4.2. Homogeneous ignition
Predicted and LIF-measured 2-D OH distributions are presented in Fig. 5 for Cases 6–11,
wherein flames are anchored inside the reactor;
vertical arrows indicate locations of homogenous
ignition (xig), defined as the far-upstream positions whereby OH rises to 5% of its maximum
value inside the reactor. Differences between
measured and predicted xig are less than 8%, apart
from Case 8 (16%). Moreover, simulations
mildly underpredict the ignition distance at p <
6 bar and overpredict it at higher pressures.
Comparisons of flame sweep angles, which are
linked to laminar flame speeds, are also in good
agreement. The predicted flames are thinner than
the measured ones, particularly at higher pressures, but this is largely attributed to the fact that
OH-LIF images have not been calibrated with
adsorption measurements to have quantitative
traits.
Fig. 5. (a) LIF-measured and (b) predicted OH distributions for Cases 6–10 in Table 1. The color bars
provide predicted OH in ppmv.
2285
As the heterogeneous kinetics was already validated in the foregoing section, the comparison in
Fig. 5 allows for direct evaluation of homogeneous kinetics. The overall good agreement demonstrated that the selected kinetic scheme is
suitable for describing hetero-/homogeneous high
pressure ethane combustion. In Fig. 6, heterogeneous and homogeneous conversions rates of ethane are plotted for Cases 7, 8 and 9. Catalytic
conversion is only considerable in the first few
centimeters, and is subsequently overtaken by
gas-phase reactions well-upstream of the homogeneous ignition location. As explained previously,
the appreciable gas-phase ethane conversion
upstream of xig in Fig. 6 is due to the incomplete
reaction of C2H6 to CO. It is also evident that the
surface temperatures in Fig. 6 are 100 K higher
than those in the flameless cases in Fig. 2, a necessary condition for flame establishment.
Additional hetero-/homogeneous validations
are provided in Fig. 7 by comparing predicted
transverse profiles of ethane and water against
Raman measurements upstream of homogeneous
ignition, in regions whereby homogeneous reactions are still non-negligible. Catalytic fuel conversion is kinetically controlled in Fig. 7(a)–(d)
(manifested by the non-zero concentration of
C2H6 at both walls), while in Fig. 7(e) and (f)
the nearly zero transverse wall gradients of C2H6
(dxC2H6 =dy 0 at y = 0 and 7 mm) indicate negligible catalytic fuel conversion and predominant
gas-phase conversion (as also seen in Fig. 6 at
the corresponding streamwise positions). The
good agreement between measurements and predictions in Fig. 7, particularly in regions where
both reaction pathways contribute to fuel consumption, demonstrates that the combined heter-
Fig. 6. Measured wall temperatures (upper wall: triangles and fitted solid lines, lower wall: circles and fitted
dashed lines) and computed catalytic (C) and gas-phase
(G) ethane conversions for three cases with vigorous
homogeneous combustion. For clarity, the C rates are
multiplied by three.
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X. Zheng et al. / Proceedings of the Combustion Institute 34 (2013) 2279–2287
was suitable for ethane/air total oxidation over Pt
at elevated pressures relevant to both microreactors and gas turbines. Finally, the pressure dependence of the ethane catalytic reactivity was
considerably stronger compared to that of CH4,
exemplifying the advantages of high-pressure catalytic combustion of natural gas.
Acknowledgments
Fig. 7. Measured (symbols) and predicted (lines) transverse profiles of C2H6 (triangles) and H2O (circles) mole
fractions at selected streamwise positions for Cases 8
and 9.
ogeneous and homogeneous kinetic schemes used
herein reproduce the underlying combustion processes over the pressure and temperature ranges
of interest.
The impact of catalytically-produced major
species and radical adsorption/desorption reactions on homogeneous ignition was finally investigated. Sensitivity analyses were conducted in an
SPSR at pressures 1-14 bar and constant reactor
temperature of 1000 K. The most important surface reactions (negative sensitivity) affecting
homogeneous ignition were the adsorption of
C2H6 and O2 as they determined the catalytic fuel
consumption which in turn reduced the availability of fuel for the gaseous pathway. In terms of
radicals, their adsorption/desorption reactions
played only a minor effect, in agreement to earlier
methane studies [10].
5. Conclusions
The catalytic combustion of ethane/air mixtures over Pt was investigated in a channel-flow
reactor under fuel-lean equivalence ratios of 0.1–
0.5, pressures of 1–14 bar, and surface temperatures up to 1300 K. A heterogeneous reaction
scheme, with ethane adsorption having an order
of 1.1 with respect to Pt(s), was constructed. This
scheme reproduced the Raman-measured transverse major species profiles across the reactor
boundary layer at conditions whereby only heterogeneous reactions were important, and for all
examined pressures and temperatures. Coupling
of the catalytic scheme with an elementary homogeneous reaction scheme resulted in good predictions over the reactor zones where combined
hetero-/homogeneous reactions were important,
and also yielded homogeneous ignition distances
within 10% to those measured by planar OHLIF. The validated hetero-/homogeneous kinetics
Support was provided by Swiss-Electric Research and the Swiss Competence Center of Energy and Mobility (CCEM). We thank Mr. Rene
Kaufmann for assistance in the experiments.
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