Investigation of low concentration methane combustion in a fluidized bed with Pd/Al2O3 as catalytic particles

Article abstract of DOI:10.1039/c4ra08534e

The catalytic combustion characteristics of low concentration (0.15-3 vol%) methane combustion in a lab-scale fluidized bed with 0.5 wt% Pd/Al₂O₃ as catalytic particles were studied experimentally. A mathematical model was developed according to the flow and reaction characteristics of the fluidized bed. The effects of bed temperature and inlet methane concentration on combustion were investigated and the kinetic characteristics were analyzed. The results show that methane conversion increases with increasing bed temperature, while it decreases slightly as the inlet methane concentration increases. The reaction order was evaluated as 0.608, and the activation energy was determined to be 96 185 J mol^-1 during low-concentration methane catalytic combustion in the fluidized bed. It was also found that the reaction in the fluidized bed was controlled by kinetics when the temperature was below 450°C. When the temperature exceeded 450°C, the reaction kinetic constant increased with increasing temperature, and the reaction was eventually controlled by kinetics, mass transfer and diffusion. A comparison of the results showed that the values calculated by the mathematical model agreed well with the experimental data.

Full text of DOI:10.1039/c4ra08534e

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Investigation of low concentration methane
combustion in a fluidized bed with Pd/Al O as
2
3
Cite this: RSC Adv., 2014, 4, 59418
catalytic particles
a
a
a
ab
a
Zhongqing Yang,* Peng Yang, Li Zhang, Mingnv Guo and Yunfei Yan
The catalytic combustion characteristics of low concentration (0.15–3 vol%) methane combustion in a lab-
2 3
scale fluidized bed with 0.5 wt% Pd/Al O as catalytic particles were studied experimentally. A mathematical
model was developed according to the flow and reaction characteristics of the fluidized bed. The effects of
bed temperature and inlet methane concentration on combustion were investigated and the kinetic
characteristics were analyzed. The results show that methane conversion increases with increasing bed
temperature, while it decreases slightly as the inlet methane concentration increases. The reaction order
ꢀ1
was evaluated as 0.608, and the activation energy was determined to be 96 185 J mol during low-
concentration methane catalytic combustion in the fluidized bed. It was also found that the reaction in
ꢁ
the fluidized bed was controlled by kinetics when the temperature was below 450 C. When the
Received 12th August 2014
Accepted 28th October 2014
ꢁ
temperature exceeded 450 C, the reaction kinetic constant increased with increasing temperature, and
the reaction was eventually controlled by kinetics, mass transfer and diffusion. A comparison of the
DOI: 10.1039/c4ra08534e
results showed that the values calculated by the mathematical model agreed well with the experimental
data.
www.rsc.org/advances
catalytic combustion can be an effective way to utilize low
concentration methane.
1
. Introduction
Low concentration methane exists extensively in coal bed
methane in coal mines and exhaust gases from chemical combustion involves multi-step surface reactions, the global
Though the kinetic mechanism of methane catalytic
1,2
production processes. Its volumetric concentration changes combustion reaction can be approximately expressed as follows:
under different operating conditions, which makes it difficult to
0
CH
4
+ 2O
2
/ CO
2
+ 2H
2
O DH298 ¼ ꢀ802 kJ molꢀ1
(1)
utilize. In some cases, low concentration methane emits to the
atmosphere directly without treatment. It is a waste of non-
renewable energy and severely affects global warming, as the
greenhouse effect of methane is 21–23 times greater than that of
There are different types of combustion catalysts, but noble
metals such as platinum, palladium, and ruthenium are
generally accepted as the most active catalysts for low-
Trimm and Lam studied the
characteristics of methane combustion on platinum-alumina
3
carbon dioxide. Therefore, it is important to nd appropriate
8,24
temperature combustion.
approaches to utilize low concentration methane resources.
Compared to conventional ame combustion, catalytic
combustion has several advantages in burning low concentra-
tion methane, such as low light-off temperature, low energy
consumption, wide scope of application, high efficiency, low
secondary pollution, and high oxidizing speed. Moreover, full
oxidation of methane can be achieved with suitable catalysts at
9

ber catalysts. Lee and Trimm summarized the change rules
for activation energies and reaction orders by varying the
10
experimental working conditions. The variation of activation
energy depends on catalyst and temperature, and the reaction
order for methane was found to be in the range of 0.5–1.
Chaouki et al. studied the characteristics of methane cata-
lytic combustion in a cyclic catalytic reactor with reversed ow.
Pd/Al (0.5%; mass fraction) was employed as catalytic
4,5
low temperatures. Furthermore, it has been determined that
11
air–fuel mixtures can be burned effectively by catalytic
2 3
O
6,7
combustion without regard to ammability limits. Thus,
particles in the uidized bed. It was found that the use of a
catalytic bed material contributes to lowering of the NOx
emissions and that there would be no NO production from
Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing methane catalytic combustion at a temperature of 650 C, if an
University), Ministry of Education of China, Shapingba District, Chongqing, 400030, appropriate choice of operating parameters was used. Gosiew-
x
ꢁ
a
China. E-mail: zqyang@cqu.edu.cn; Fax: +86-23-65111832; Tel: +86-23-65103114
ski et al. studied the combustion characteristics of mine venti-
b
College of Mechanical and Power Engineering, Chongqing University of Science and
Technology, Shapingba District, Chongqing 401331, China
12,13
lation air in a catalytic ow reversal reactor.
They showed
59418 | RSC Adv., 2014, 4, 59418–59426
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
that the reactor can work only under the condition that the combustion in uidized beds should be further studied. For
methane concentration remains steady in the mine ventilation this study, low concentrations of methane were produced by
air. At higher temperatures, homogeneous combustion premixing methane and air, and the catalytic combustion
occurred in the combustor, causing the formation of local characteristics were studied experimentally in a lab-scale
2 3
hotspots and deactivation of the catalyst, which made the device uidized bed reactor with 0.5 wt% Pd/Al O as catalytic parti-
very unstable. Several factors could account for the deactivation cles. A mathematical model of the low concentration methane
of the catalyst, but the most critical reason was overheating. catalytic reactions in the uidized bed was established accord-
Once heat was generated without its effective removal from the ing to the ow and reaction features, and the kinetic charac-
catalyst, overheating and thermal deactivation would take place teristics of the reactions were analyzed.
14
successively.
Iamarino et al. studied the characteristics of uidized-bed
combustion using a premixed lean methane–air mixture over
2
. Experimental section
15
a copper-based catalyst. It was found that heat extraction from The experimental arrangement is shown schematically in Fig. 1.
the catalytic combustion system could be accomplished A bubbling uidized bed reactor was used and it was composed
conveniently in a uidized-bed reactor. Moreover the uidized- mainly of a pre-heater (1), a gas distributor plate (2), a furnace
bed reactor ran isothermally under different operating condi- (3), a sampling tube (4), a cyclone separator (5), and a cooling
16
tions without the occurrence of local hotspots. Another water pipe (6). The removable gas distributor plate was made of
advantage of uidized bed combustion is its adaptability to a stainless steel and installed between the pre-heater and furnace.
variety of fuels; thus, it has been used widely for low-grade solid It had 34 drilled holes with diameters of 1.2 mm, which were
fuel combustion applications, such as inferior coal, coal regularly distributed on a hexagonal grid. The furnace consisted
17–19
gangue, sludge, and biomass.
a premixed uidized-bed reactor under lean conditions, with height of 0.66 m and an inner diameter of 0.1 m, and an
.15–3% inlet methane concentrations and using Cu/g-Al as expanded freeboard with a height of 0.15 m and an inner
The authors also investigated of two main parts: a stainless-steel uidized-bed reactor with a
0
2 3
O
catalyst particles. It was found that methane could be diameter of 0.37 m. The purpose of the expanded freeboard was
completely converted when the bed temperature was below to recycle the bed material particles when the uidized bed ran
ꢁ
14
7
00 C. Zukowski et al. investigated catalytic combustion of under the bubbling condition. The height between the reactor
gaseous fuel in a bubbling uidized bed with manganese oxides and the expanded freeboard was 0.14 m. The two zones of the
20
as the bed material. Heterogeneous reactions on the surfaces furnace were heated by electrical heaters that could be
of the catalyst particles and homogeneous reactions in the inter- controlled independently. During the experiment, three ther-
particle spaces were both observed when the bed temperature mocouples, shown as T , T , and T in Fig. 1, were used to
1
2
3
ꢁ
exceeded 750 C. Besides heterogeneous methane oxidation measure temperatures; and three pressure measuring points,
taken place on the catalytic surface, and homogenous reactions shown as of P , P , and P , were installed. T and P were
1
2
3
1
1
could be neglected. Hayhurst et al. studied catalytic combustion basically located in a horizontal line, which was located 90 mm
of propane–air mixtures in a uidized bed of hot sand, with Pt/ above the distributor plate; T2 and P2 were located 280 mm
21
Al O as catalyst pellets. Interestingly, sand inhibited the above the distributor plate. The data measured by the thermo-
2
3
combustion, whereas platinized alumina particles catalyzed the couples and pressure sensors were collected by a data sampling
combustion during the experiments. Foka and Sotudeh- system with a sampling frequency of 10 Hz. Gas was sampled
Gharebagh investigated the combustion characteristics of pre- from the outlet of the furnace through a stainless-steel tube 6.3
2
2,23
mixed natural gas in a turbulent uidized bed.
During the mm in length. To keep particles from entering into the tube, a
ꢁ
experiments, the temperature varied over the range 400–600 C small plug of quartz wool was used. Aer drying, the samples
and the inlet mixture contained 4 vol% methane. It was found were collected into sampling bags made of Teon. The
that the turbulent ow regime was more suitable for the composition of the samples was determined by using a gas
combustion of natural gas than the bubbling regime. Xu, et al. chromatograph (Shimadzu GC-8A).
studied the methane catalytic combustion over noble metal
The bed materials employed were inert particles (limestone)
24,25
catalysts and Mn1ꢀxCe
x
O2ꢂy catalysts.
They found that the and a commercial catalyst (G-74D), with a mass ratio of 40 : 1.
reaction order with respect to oxygen decreased with tempera- During the experiments, the reaction temperature varied within
ꢁ
ture while the order with respect to methane remained constant the range of 450–700 C, which is below the decomposition
when lean-burn methane combustion were investigated in a low temperature of limestone; consequently, limestone can be
temperature range of 473–673 K, and 3.3 wt% Pd/Al
catalytic particles. And, Pd oxide affected the adsorption and combustion. Particles of catalyst and limestone were in the
activation of reactants.
same size range of 180–325 mm and were evenly mixed before
2 3
O acted as considered as inert particles for low concentration methane
3
However, current research is focused mainly on natural gas, being loaded into the uidized bed. The total weight of the bed
and the catalytic combustion of low concentration methane material was 2.8 kg and the height of the static bed was about
with concentrations below 3 vol% has been reported rarely. The 0.2 m. The active component of the commercial catalyst G-74D
effects of bed temperature and inlet methane concentration on was 0.5 wt% palladium on an alumina carrier, and the mass of
the combustion characteristics, mathematical models, and the catalyst was 70 g. During the experiments, the low concen-
kinetic analysis of low concentration methane catalytic tration methane fed into the reactor was obtained by premixing
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 59418–59426 | 59419

View Article Online
RSC Advances
Paper
Fig. 1 Schematic drawing of the experimental system.
methane (99% purity, Praxair) and air (extra dry, Praxair) so the [the variation of methane in the bubble phase] ¼ [the amount of
varying uidized velocity or gas mixture ratio could be methane reacted in the bubble phase] + [the amount of methane
controlled by regulating the ow of air and methane. To ensure
the accuracy of the experimental data, the experiments were
repeated three times under the same conditions.
transferred from the bubble phase to the emulsion phase]
[the variation of methane in the emulsion phase] ¼ [the amount of
The concentration of CO produced was less than 10 ppm methane reacted in the emulsion phase] + [the amount of methane
during the experiments, which was quite low compared to other
products. The methane conversion fraction (XCH4) was dened
as follows:
transferred from the emulsion phase to the bubble phase]
Therefore, the mathematical expressions for the bubble and
(2) emulsion phases are given by:
C
CH ðinÞ ꢀCCH ðoutÞ
4
4
X
CH4
¼
C
4
CH ðinÞ
dC
b
6kbed
ꢀ
3
3
ꢀ u
b
¼ dR
b
þ
ðC
b
ꢀ C
e
Þ
(3)
where CCH (in) is the inlet methane concentration (mol m ),
4
dz
d
b
ꢀ
and CCH (out) is the outlet methane concentration (mol m ).
4
dC
e
6kbed
ꢀ
u
mf
¼ð1ꢀ dÞR
e
ꢀ
ðC
b
ꢀ C Þ
e
(4)
dz
d
b
3. Mathematical model
A mathematical model of the relevant reaction and kinetics was
Studies have shown that heterogeneous reactions dominate
established on the basis of the characteristics of the uidized the catalytic combustion of methane, as the activity of the
bed. The bed materials employed were limestone and catalyst could inhibit the occurrence of homogeneous reac-
commercial catalyst (Geldart B) and the supercial velocity tions. Thus, it was reasonable to ignore homogeneous reactions
ꢀ
1
varied from 0.1–0.25 m s in a bubbling uidized bed during in the mathematical model.
the experiments. It was assumed that bubble and emulsion
phases exist in the uidized bed, the gas in the emulsion phase can be expressed by:
runs through the bed at the minimum uidization velocity and
the excess gas runs through the bed in the form of bubbles.
Thus, the variations of methane in the bubble and emulsion
phases can be expressed by:
Therefore, the reaction rate of methane in the bubble phase
m
R
b
¼ g
b
k
r
C
b
(5)
59420 | RSC Adv., 2014, 4, 59418–59426
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Moreover, the reaction rate of methane in the emulsion
phase can be expressed by:
The above eqn (14)–(16) can be solved given the following
boundary conditions:
m
R
e
¼ (1 ꢀ 3mf)k
r
C
e
(6)
when x ¼ 0, Q
b
¼ Q
e
¼ 1
(17)
It is worth pointing out that catalytic reactions on the
surfaces of particles are continuous and stable for low concen-
tration methane in the emulsion phase. That is to say, the mass
of reacted species transferred from the gas phase to particle
surfaces is equal to that consumed by catalytic reactions in unit
time. So it can be described by the differential equation:
4
. Results and discussion
The effects of bed temperature and inlet methane concentration
on low concentration methane combustion in a uidized bed
reactor were studied experimentally. Numerical calculations
were carried out with the mathematical model and the results
were compared and discussed.
dn
¼
k
g
S
s
ðC
e
ꢀ C
s
Þ¼ðꢀR
A
ÞV
s
¼ hV
s
k
c
f ðC
s
Þ
(7)
dt
4.1 Effects of bed temperature on methane conversion
The efficiency factor of catalysts is a function of the Thiele
number, and is given by:
Fig. 2 depicts the effects of bed temperature in the range of 450–
ꢀ
ꢁ
ꢁ
700 C on low concentration methane combustion in a uidized
1
1
1
h ¼
ꢀ
(8)
bed. It can be seen that methane conversion rises quickly with
increasing bed temperature, particularly when it is below
4
tanhð34Þ 34
where, 4 is the Thiele number for a spherical catalyst, which can
be expressed by:
ꢁ
ꢁ
5
50 C. When the bed temperature was 450 C, the methane
conversion fraction was 61.4% at an inlet methane concentra-
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢀ1
tion of 2 vol% and a uidization velocity of 0.1 m s . When it
d
c
mk
c
mꢀ1
ꢁ
4
¼
C
(9) increased to 550 C, the methane conversion fraction increased
6
D
e
to 96.3%, and full conversion was achieved at a bed temperature
ꢁ
of 650 C.
e
where, D is regarded as the effective diffusion coefficient of the
The bed materials employed during the experiments were
the catalyst and inert particles. It was reasonable to ignore the
homogeneous combustion reactions and to consider that only
(10) heterogeneous reactions took place during the process of
methane reacting with oxygen on the surfaces of solid catalyst
during low concentration methane combustion in the uidized
gas.
Introduce dimensionless factors:
u
b
u
mf
C
b
C
e
z
u
b
¼
; u
e
¼
; Q
b
¼ C ; Q
e
¼ C ; x ¼
u
0
u
0
in
in
H
20
The index of mass transfer between the bubble and emulsion bed. The processes of chemical adsorption, chemical reac-
phases was estimated as:
tions, and chemical desorption occurring on the surfaces of
particles were regarded as chemical kinetics processes for
heterogeneous reactions, while the others were regarded as
diffusion process. The reactions of low concentration methane
catalytic combustion were controlled by not only catalytic
kinetics and diffusion, but also affected by mass transfer
between the bubble and emulsion phases. According to the
kinetics of methane catalytic reactions, the kinetic constant and
reaction rate increased with rising temperature, and simulta-
neously diffusion was enhanced.
6kbedH
Mt ¼
(11)
(12)
(13)
b 0
d u
The Stanton number of the mass transfer was given by:
À
Á
6
k
g
ð1ꢀ dÞ 1ꢀ 3mf
H
St
m
¼
d
c
u
0
The Damkohler number was expressed by:
mꢀ1
d
c
k
c
C
in
4.2 Effects of inlet methane concentration on methane
conversion
Da ¼
6k
g
Low concentration methane is produced as a by-product of the
coal mining process, and methane emissions from coal mines
vary in concentration based on different mining operations.
Therefore, the effect of inlet methane concentration on
(14) conversion was studied with its volumetric fraction varying
from 0.15 to 3%, and the results were shown in Fig. 3. It is clear
that the methane conversion fraction decreased at higher inlet
Combining the above expressions, eqn (3), (4) and (7) can be
simplied as follows:
dQ
b
ꢀ
u
b
¼ MtðQ
b
ꢀ Q Þ
e
dx
dQ
e
ꢀ
1
ꢀ
u
e
¼ St
m
ðQ
e
ꢀ Q
s
ÞꢀMtðQ
b
ꢀ Q
e
Þ
(15)
methane concentrations. At a supercial velocity of 0.1 m s
dx
ꢁ
and a bed temperature of 500 C, the methane conversion
fraction decreased from 97% to 88% as the inlet methane
concentration increased from 0.15 to 3 vol% for Fig. 3(a). While
m
s
(
Q ꢀ Q ) ¼ hDaQ
(16)
e
s
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 59418–59426 | 59421

View Article Online
RSC Advances
Paper
ꢀ1
¼ 0.1 m sꢀ1 (b) aCH4 ¼ 2%, u
¼
Fig. 2 Effects of temperature on methane conversion at a fluidization velocity of 0.1 m s (a) aCH4 ¼ 3%, u
0
0
ꢀ1
ꢀ1
ꢀ1
0
.1 m s (c) aCH4 ¼ 0.15%, u
0
¼ 0.1 m s (d) aCH4 ¼ 0.3%, u
0
¼ 0.1 m s
.
it displayed similar trends that the methane conversion fraction
The CO concentration was very low (less than 10 ppm)
decreased with increasing inlet methane concentration in compared with the other products. Therefore, it was reasonable
Fig. 3(b)–(d). And, the principal analysis of Fig. 3(a)–(d) are as to ignore the component of CO, and the methane conversion
follows.
fraction can be approximately expressed as the ratio between
¼ 0.1 m sꢀ , T ¼ 500 C (b) u
1
ꢁ
¼ 0.1 m sꢀ1
,
Fig. 3 Effects of inlet methane concentration on methane conversion under different conditions (a) u
0
0
ꢁ
ꢀ1
ꢁ
ꢀ1
ꢁ
T ¼ 550 C (c) u
0
¼ 0.15 m s , T ¼ 550 C (d) u ¼ 0.15 m s , T ¼ 600 C.
0
59422 | RSC Adv., 2014, 4, 59418–59426
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
0
the molar volume of methane reacted and the molar volume of tting of the experimental data, the pre-exponential factor, k ,
1
0
3 ꢀ1
inlet methane (eqn (18)).
was evaluated to be 1.35
ꢃ
10
mol (s mcatalyst )
ꢀ
3 ꢀ0.608
m
(mol m
96 185 J mol
)
and the activation energy, E, was evaluated to be
R
ðCH4Þ
s
krCCH4
s
ꢀ1
X
CH4 ¼ C
¼ C
(18)
.
CH ðinÞ
4
CH ðinÞ
4
During low concentration methane catalytic combustion, the
where, R(CH4) represents the methane reaction rate, s is the reaction is controlled by intrinsic kinetics when the temperature is
contact reaction time; k
r
is the reaction kinetic constant, and m relatively low. As the temperature increases, the reaction gradually
is the reaction order.
becomes controlled by kinetics, mass transfer, and diffusion
Methane conversion can be regarded as an approximate together. Fig. 5 shows the dimensionless parameters Da and hDa
function of methane concentration, which can be seen from the varying with temperature; h represents the catalyst efficiency
above equation. Based on the selection of catalysts for the factor, of which the value is less than or equal to 1. The Damkohler
methane catalytic combustion reaction, the methane combus- number Da is the ratio of the chemical reaction rate to the diffu-
tion reaction order is in the range between 0.5 and 1. For the sion rate. It can be seen from Fig. 5 that the Da curve is generally in
catalyst (0.5 wt% Pd/Al
2
O
3
) employed in these experiments, the good agreement with the hDa curve and h is approximately equal
ꢁ
reaction order m was 0.608. The details of the reaction order to 1 when temperature is below 450 C. It should be noted that
ꢁ

tting will be shown in the next section. It can be seen from eqn when the temperature exceeds 450 C, the value of h decreases
(18) that methane conversion decreases with increasing inlet with increasing temperature, and the deviation value between the
methane concentration, as m is less than 1.
Da and hDa curves gradually increases. The reaction in the uid-
ized bed is controlled by kinetics when the temperature is no more
ꢁ
than 450 C; while at higher temperatures, the reaction kinetic
4
.3 Kinetic analysis of low Concentration methane catalytic
constant increases, and the reaction is gradually controlled by
kinetics, mass transfer and diffusion.
combustion
The reaction order can be obtained by measuring the methane
Fig. 6 shows the Stanton number St and the mass transfer
conversion as it varies with inlet concentration at a given bed index of the Mt variation with temperature in the bubble and
temperature. Fig. 4 shows the effects of inlet methane concen- emulsion phases. The Stanton number represents the mass
ꢁ
tration on conversion at a bed temperature of 450 C. It can be transfer between catalyst particle surfaces and external gas. It
seen that conversion is a power function of inlet methane can be seen that the value of St is an order magnitude larger
concentration. It was assumed that the reaction order of oxygen than that of Mt, that is to say, the effects of mass transfer
was zero for the one-step reaction between methane and oxygen between the outer surfaces of the catalyst particles and the
under the condition that an excess of oxygen was always present external gas were obviously signicant, and were substantially
during the experiments; moreover, the reaction order of greater than that of the mass transfer between the bubble and
methane was evaluated to be 0.608 by least-squares tting of the emulsion phases during low concentration methane combus-
experimental data.
tion in a uidized bed. Consequently, it is reasonable to
Similarly, the pre-exponential factor and activation energy surmise that the gas concentration on the outer surfaces of the
can be evaluated by measuring methane conversion as it varies catalyst particles was approximately equal to that in the emul-
with bed temperature at a given inlet concentration, then the sion phase of the uidized bed.
reaction kinetic constant can also be obtained. By least-squares
ꢁ
Fig. 4 Effects of inlet methane concentration on conversion at 450 C. Fig.
5
Variation of dimensionless parameters Da and hDa with
temperature.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 59418–59426 | 59423

View Article Online
RSC Advances
Paper
calculated data are shown in Fig. 7. It can be seen the model
calculated values agreed well with the experimental data and
the maximum deviation was less than 15%.
The major cause of the errors was the strong gas–solid
turbulent ow in the uidized bed. The turbulence enhanced
the effects of mass transfer between the gas and catalyst parti-
cles. Simultaneously, a small amount of catalyst particles was
splashed into the freeboard by the effect of turbulence. In the
mathematical model, considering that most of the methane was
consumed in the dense zone, the small number of heteroge-
neous reactions in the freeboard was ignored. Besides, it was
assumed for the simplied model that gas ran through the
emulsion phase at the minimum uidization velocity and the
rest of the gas ran through the bed in the form of bubbles.
However, the bubbles that formed in the bubbling uidized bed
Fig.
temperature.
6
Variation of dimensionless parameters St and Mt with were not strictly spherical and the growth of bubbles occurred
with the merging of bubbles, and there was a certain difference
between the shapes and growth of bubbles in the model and in
the actual uidized bed.
4.4 Comparisons of model predictions and experimental
data
5
. Conclusions
Numerical calculations were carried out by the mathematical
model of low concentration methane combustion in a uidized
bed under various operating conditions, which can be seen in
Fig. 2 and 3. Many more comparisons of experimental and
Low concentration methane catalytic combustion characteris-
tics were experimentally studied in a lab-scale uidized bed
reactor that used 0.5 wt% Pd/Al O as catalytic particles. A
2 3
mathematical model of low concentration methane catalytic
reactions in a uidized bed was developed, and the kinetic
characteristics were analyzed. Model predictions were generally
in good agreement with the experimental data. The major
conclusions can be summarized as follows:
(1) Methane conversion increases with increasing bed
temperature, while it decreases slightly with increasing inlet
methane concentration. The kinetic rate constant increases
rapidly with rising temperature, resulting in more methane
ꢁ
being consumed. As the bed temperature increased to 650 C,
methane conversion reached approximately 100%.
(2) The mathematical model of low concentration methane
catalytic combustion was developed according to the charac-
teristics of ow and catalytic reactions in a uidized bed. By
least-squares tting of the experimental data, several kinetic
parameters were obtained, the reaction order was evaluated to
be 0.608 and the activation energy was evaluated to be 96 185 J
ꢀ
1
mol for the reaction.
3) According to analysis of the dimensionless parameters
(
Da, St, and Mt, the reactions in the uidized bed were
ꢁ
controlled by kinetics when the temperature was 450 C. When
ꢁ
the temperature exceeded 450 C, as the reaction kinetic
constant of methane increases with increasing temperature, the
reaction gradually became controlled by kinetics, mass transfer
and diffusion together.
Nomenclature
Cb
Methane concentration in the bubble phase
ꢀ3
(mol m
)
Fig. 7 Comparisons of experimental and calculated data.
59424 | RSC Adv., 2014, 4, 59418–59426
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
C
e
Methane concentration in the emulsion phase
Acknowledgements
ꢀ3
(
mol m
Methane concentration (mol m
CCH4(in) Inlet methane concentration (mol m
)
ꢀ3
The authors would like to say thanks for the nancial support of
the Natural Science Foundation of China with Project no.
51206200, the Fundamental Research Funds for the Central
Universities with Project no. CDJZR12140031, and the Visiting
Scholar Foundation of Key Laboratory of Low-grade Energy
Utilization Technologies and System, MOE of China in
CCH4
)
ꢀ
3
)
ꢀ3
CCH4(out) Outlet methane concentration (mol m
Material concentration on the outer surfaces of
)
C
s
ꢀ3
catalyst particles (mol m
)
2
ꢀ1
D
e
Effective diffusion coefficient of a gas (m s )
u
b
Dimensionless velocity of a bubble rising through the Chongqing University (LLEUTS-201301).
bed
u
e
Dimensionless minimum uidized velocity
References
ꢀ1
E
Activation energy (J mol
Macro-kinetic reaction rate [mol (s
)
ꢀ1
ꢀ3
R
A
m )]
1
S. Shi and C. Hongwei, Characteristics of coal mine
ventilation air ows, J. Environ. Manage., 2008, 86(1), 44–62.
Q. Sun, X. Guo, A. Liu, J. Dong, B. Liu, J. Zhang and G. Chen,
Experiment on the Separation of Air-Mixed Coal Bed
Methane in THF Solution by Hydrate Formation, Energy
Fuels, 2012, 26(7), 4507–4513.
2
Ss
Vs
db
External surface area of catalyst particles (m )
3
Volume of catalyst particles (m )
2
Bubble diameter (m)
k
c
Intrinsic kinetic reaction rate constant
3
ꢀ1
ꢀ3 ꢀ0.608
[mol (s mcatalyst
)
(mol m
)
]
ꢀ
1
k
k
g
Gas phase mass transfer coefficient (m s )
3
Z. Yang, J. R. Grace, C. Jim Lim and L. Zhang, Combustion of
low-concentration coal bed methane in a uidized bed
reactor, Energy Fuels, 2011, 25(3), 975–980.
ꢀ
1
ꢀ1
r
Reaction rate constant [mol s kgcatalyst
ꢀ
3 ꢀ0.608
(mol m
)
]
k
be
0
Mass-transfer coefficient between the bubble and
ꢀ1
4 M. Iamarino, P. Ammendola, R. Chirone, R. Pirone,
G. Ruoppolo and G. Russo, Nonpremixed Catalytic
Combustion of Methane in a Fluidized Bed Reactor, Ind.
Eng. Chem. Res., 2006, 45(3), 1009–1013.
emulsion phase (s
)
ꢀ
1
ꢀ1
ꢀ1
k
Pre-exponential factor [mol s kgcatalyst
ꢀ
3 ꢀ0.608
(mol m
)
]
ꢀ
1
RCH4
Rb
Methane reaction rate (mol s kgcatalyst
)
5
Z. Yang, J. Liu, L. Zhang, S. Zheng, M. Guo and Y. Yan,
Catalytic combustion of low-concentration coal bed
Methane reaction rate in the bubble phase
ꢀ
1
ꢀ1
(
mol s kgcatalyst
)
R
e
Methane reaction rate in the emulsion phase
methane over CuO/g-Al
2014, 4, 39394–39399.
2 3 2
O catalyst: effect of SO , RSC Adv.,
ꢀ
1
ꢀ1
(mol s kgcatalyst
)
F
Q
Dimensionless methane concentration
Dimensionless methane concentration in the bubble
phase
Dimensionless methane concentration in the
emulsion phase
Dimensionless axial bed height
Thiele number for a spherical catalyst
The index of mass transfer between the bubble and
emulsion phases
Stanton number of mass transfer
Damkohler number
6 M. Iamarino, R. Chirone, R. Pirone, G. Russo and P. Salatino,
Catalytic combustion of methane in a uidized bed reactor
under fuel-lean conditions, Combust. Sci. Technol., 2002,
174(11–12), 361–375.
b
Q
e
7
X. Hu, Q. Yu, N. Sun and Q. Qin, Experimental study of
ammability limits of oxy–methane mixture and
x

4
calculation based on thermal theory, Int. J. Hydrogen
Energy, 2012, 17(39), 9527–9533.
Mt
8
9
T. V. Choudhary, S. Banerjee and V. R. Choudhary, Catalysts
for combustion of methane and lower alkanes, Appl. Catal.,
A, 2002, 234(1–2), 1–23.
D. L. Trimm and C. W. Lam, The combustion of methane on
platinum-alumina ber catalysts-I: kinetics and mechanism,
Chem. Eng. Sci., 1980, 35(6), 1405–1413.
St
Da
m
h
m
Reaction order
The efficiency factor of catalysts
The mass of transmitter substance (mol)
Temperature (K)
n
T
ꢀ
1
10 J. H. Lee and D. L. Trimm, Catalytic combustion of methane,
Fuel Process. Technol., 1995, 42(2–3), 339–359.
11 J. Chaouki, C. Guy, C. Sapimdzhiev, D. Kusohorsky and
D. Klvana, Combustion of methane in a cyclic catalytic
reactor, Ind. Eng. Chem. Res., 1994, 33(12), 2957–2963.
u
0
Supercial velocity (m s )
Tbed
ub
umf
XCH4
z
Bed temperature (K)
Bubble rising velocity through the bed (m s
Minimum uidization velocity (m s )
ꢀ
1
)
ꢀ
1
Calculated methane conversion fraction (%)
Axial bed height (m)
Contact reaction time
Void fraction of solids dispersed in the bubbles
Bubble volume fraction in the bed
1
2 K. Gosiewski and K. Warmuzinski, Effect of the mode of heat
withdrawal on the asymmetry of temperature proles in
reverse-ow reactors. Catalytic combustion of methane as
a test case, Chem. Eng. Sci., 2007, 62(10), 2679–2689.
s
g
b
d
3
mf
Void fraction in the bed at minimum uidization
13 D. Klvana, J. Chaouki, C. Guy, et al. Performance of auto-
cyclic reactor in catalytic combustion of lean fuel mixtures,
Ind. Eng. Chem. Res., 2005, 44(25), 9676–9682.
14 M. Iamarino, R. Chirone, L. Lisi, R. Pirone, P. Salatino and
G. Russo, Cu/g-Al2O3 catalyst for the combustion of
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 59418–59426 | 59425

View Article Online
RSC Advances
Paper
methane in a uidized bed reactor, Catal. Today, 2002, 75(1– 21 A. N. Hayhurst, J. J. John and R. J. Wazacz, The combustion
4
), 317–324.
of propane and air as catalyzed by platinum in a uidized
bed of hot sand, Symp. (Int.) Combust., [Proc.], 1998, 27(2),
3111–3118.
1
1
1
5 M. Iamarino, P. Salatino, R. Chirone, R. Pirone and G. Russo,
Catalytic combustion of methane and propane in a uidized-
bed reactor, Proc. Combust. Inst., 2002, 29(1), 827–834.
6 R. L. Wu, J. R. Grace and C. J. Lim, Model for heat transfer in
circulating uidized beds, Chem. Eng. Sci., 1990, 45(12),
22 M. Foka, J. Chaouki, C. Guy and D. Klvana, Natural gas
combustion in a catalytic turbulent uidized bed, Chem.
Eng. Sci, 1994, 49(24A), 4269–4276.
3
389–3398.
7 C. You and X. Xu, Utilization of Ventilation Air Methane as a
Supplementary Fuel at Circulating Fluidized Bed
23 R.
Sotudeh-Gharebaagh,
J.
ChaoukiNatural
and
R. Legrosgas, Combustion in a turbulent uidized bed of
inert particles, Chem. Eng. Sci, 1999, 54(13–14), 2029–2037.
a
Combustion Boiler, Environ. Sci. Technol., 2008, 42(7), 24 J. Xu, L. Ouyang, W. Mao, X.-J. Yang, X.-C. Xu, J.-J. Su,
590–2593. T.-Z. Zhuang, H. Li and Yi-F. Han, Operando and Kinetic
Study of Low-Temperature, Lean-Burn Methane
Catalyst, ACS Catal., 2012,
2
1
1
2
8 A. M. Amin, E. Croiset, Z. Malaibari and W. Epling,
Hydrogen production by methane cracking using Ni-
supported catalysts in a uidized bed, Int. J. Hydrogen
Energy, 2012, 14(37), 10690–10701.
Combustion over a Pd/g-Al
2 3
O
2(2), 261–269.
25 J. Xu, P. Li, X. Song, C. He, J. Yu and Yi-F. Han, Operando
Raman Spectroscopy for Determining the Active Phase in
9 Z. Chen, J. R. Grace, C. Jim Lim and A. Li, Experimental
studies of pure hydrogen production in a commercialized
One-Dimensional Mn1 Ce O Nanorod Catalysts during
ꢀx
x
2ꢂy

uidized-bed membrane reactor with SMR and ATR
Methane Combustion, J. Phys. Chem. Lett., 2010, 1(10),
catalysts, Int. J. Hydrogen Energy, 2007, 13(32), 2359–2366.
0 W. Zukowski, The role of two-stage combustion in the
development of oscillations during uidized bed
combustion of gases, Fuel, 2000, 79(14), 1757–1765.
1648–1654.
59426 | RSC Adv., 2014, 4, 59418–59426
This journal is © The Royal Society of Chemistry 2014