Experimental Study of a Diluted Methane-Air Mixture Combustion
under Filtration in a Packed Bed
VALERI I. BUBNOVICH, MARIO TOLEDO
Department of Chemical Engineering
Universidad de Santiago
B. O´Higgins 3363, Casilla 10233, Santiago
CHILI
Abstract: - The present work studies in experimental form the ignition and the combustion of the lean methane –
air mixtures in inert porous media formed by small balls of alumina T162 of 4.8 mm of diameter in a quartz tube
of 30 cm of height and 5.2 cm of inner diameter. With the objective to reduce the time that the combustion take
in being developed in stable form within the PIM, the inferior layers of porous media are preheated by the free
flame until reaching the temperature of ignition. After the warm-up period of approximately 2-3 min., the fuel
equivalence ratio is changed to the desired value,  = 0.6664, and, as to result, the flame enter into packed bed
and migrated downstream in porous medium. For the three speeds of filtration of the mixture, 1.03, 1.54 and
2.06 m/s, this paper studies the properties of combustion waves that are: the maxima temperature and the speed
of displacement of the combustion front and the emissions of CO, CO2, SO2 and NOx when coming out of the
burner.
Key-Words: - Porous burners, porous media, combustion, emissions
1 Introduction
Porous media combustion offers exceptional
advantages compared with techniques involving free
flame burners. Porous media burners are
characterized by higher burning rates, increased
flame stability, and lower combustion zone
temperatures, which lead to a reduction in NOx
formation [1]. In addition, they show a very high
turndown ratio, low emissions of CO, and are of
very small size [2]. The combustion inside the
porous medium is very intense and the reaction zone
in the porous matrix has an elongated form with a
length of several centimeters in the stream wise
direction when using premixed natural gas with air
under atmospheric pressure [3].
Flame propagation and heat transfer through
porous media has already been investigated by
several scientists. The time and the distance
characteristic of cooling of gases of combustion
within a PIM were studied in [4] demonstrating that
the cooling of combustion products is practically
finished in the same section where the chemical
reactions disappear. The different mechanisms from
heat transport (thermal radiation, conduction,
convection, diffusion and dispersion) in PIM were
studied in [5,6], the excess of entalpía in the
combustion front was analyzed in [7,8].
Nowadays, exist different methods from
stabilization of the flame within the PIM, among
them counts on a) the criterion of the critical Peclet
number [9], b) stabilization of the flame in the
radiant mode to near the downstream interface of the
porous medium [10], c) evacuation of the heat from
the combustion zone by means of embedded coolant
tubes [11], d) combustion zone stabilization by
periodically reversed of the gas flow direction (RFB)
[12]. In literature exists much information with
respect to the principles of construction of porous
burners [1,13], the numerical [14] and the analytical
solutions [15] of the combustion in PIM, the
mathematical simulation of the kinetic formation of
CO and NOx [16], the heat exchange between the
solid and gaseous phase [17] and the different
applications from this technology of combustion in
modern industry [2].
In order to inflame a mixture and to reach a
self-sustaining combustion within a PIM, exists a
method simple to obtain it presented in the work
[13]. The mixture with the equivalent ratio of 0.75 is
ignited at the exit of the burner using a methane
lighter. Once ignited, the mixture briefly stabilized
on the top of the porous plug and, after a period of
approximately 5 min., the flame migrated down to
the section of the mixture entrance.
In the present work, to reduce the development
time of the ignition in a self-sustaining combustion
within the PIM and, in addition, to study the
characteristics of the wave of combustion in its
displacement downstream and against the gravity, a
method is propoused for ignition of the mixture and
preheating of the PIM with the free flame that is
applied to the MPI from down, as it is seen in figure
1. For the three different speeds of filtration of the
mixture, 1.03, 1.54 and 2.06 m/s, with the same
value of the fuel equivalence ratio  = 0.6664, the
characteristics of combustion waves that are: the
maximum temperature and the speed of
displacement of the combustion front and the
emissions of CO, CO2 and NOx at the exit of the
Burner, are studied.
Flame
Figure 1. Alumina heating with free flame
2 Experimental Study
Gas Analyzer
Termocouple
Figure 2. Experimental prototype
The experimental prototype of generation and
registry of waves of combustion in porous media
formed by small balls of alumina is presented in
figure 2 and consists of a) a system of provision and
mixing of air and methane, b) a porous burner, c) a
system of thermocuples and d) the emissions
measuring equipment. Each gas flow (methane and
air) is controlled by means of fluxmeters, model
GFC 47 (for methane) and GFC 67 (for air), marks
Alborg, with exactitude 1.5%. The flows are joined
by means of union in "Y" of 45° and are lead by
means of flexible hose for gas to the ignition camera
where they become inflamed by means of an
electrical arc produced by a coil of automovil of 12
volts. The air is provided under the pressure of 20
psi by a free oil compressor and the compressed
methane of 99.99% of purity is blown from a
cylinder of high pressure.
When being generated the inflammation of the
mixture, takes place a flame that hits directly on the
inferior layers of alumina (to see figure 1), of the
porous media, until reaching the temperature of
necessary ignition. The process of preheating by the
free flame lasts approximately 2-3 minutes. Later,
the mixture becomes impoverished, the flame in the
ignition camera is extinguished and begin to be
developed the combustion without flame within
porous media. This process lasts approximately 20
minutes and is accompanied by the formation of a
luminous front of several centimeters of thickness
which moves downstream.
At all the moments of its displacement the
temperature is registered with 10 seconds of
frequency by a termocupla inserted in the center del
tube of quartz and to 13 cm respect to its base. The
termocupla of type C is formed by tugsteno renio
alloys of the company OMEGA and is connected to
the card of data acquisition IOTECH model
DAQS/56 of 10 digital channels. The termocupla is
covered by a cover of alumina of 99.8 % of purity to
protect it of the oxidation by free radicals. Its
precision is of 1% in the rank of temperatures 427 ºC
< T < 2316 ºC.
Also a gas analyzer KM9106 Deluxe marks IO
Tech is used for measure the concentrations of gases
of combustion of CO, CO2, NOx, SO2 and O2 when
coming out of the burner. The porous means are
formed by small balls T-162 of alumina of 4.8 mm
of diameter of the company ALCOA, Japon, with
99.7 % of purity and 2.16 g/cm3 of volumetric
density. The tube that contains porous means is of
quartz, of 5.2 cm of internal diameter and of 30 cm
of height, its density in 2.21 mass is g/cm, the heat
capacity is cp = 670 J/(kg K) and the thermal
conductivity is 1.4 k = W/(m K). In all the
experimental tests, the necessary temperatures of
ignition of mixtures methane - air in the inferior part
of porous means is determined on the basis of the
following formula [16]:
Ea
e R0 TW
(
1
Ea
)
R0  Tig
7
3
(
K    d P  T0 hc2  W02  R05
)(
)
u0
2   2  C P2  E a5
(1)
In figure 2 the system of combustion and data
acquisition in operation is presented.
3 Results and Conclusions
Table 1. Summary of results obtained in three experimental tests
Air (Its/min)
40
60
80
CH4 (Its/min) O2 (%)
2,8
9,5
4,2
8,17
5,6
7,8
CO (ppm)
71,6
20,83
14,66
For the taking of data, the three presented tests
were made in table 1 with the same equivalence
ratio,  = 0.6664, but with different volumetric flows
of the mixture originating in porous media gas
filtrations with the equal speeds from 1.03, 1.54 and
2.06 m/s.
Temperatura v/s Tiempo (40 2,8)
CO2 (%)
6,5
7,28
7,47
NOx (ppm)
4
7,67
9,67
SO2 (ppm) Tmax, k
u , w/s
1
1582
1,03
6,17
1634
1,54
6
1810
2,06
to optimize the emissions of CO and NOx it is
necessary to control the maximum temperatures in
the front of combustion in rank 1200<T<1400ºC.
Like the main conclusion of this work, one can
affirm that the ignition of lean mixtures methane-air
with the free flame in the inferior part of porous
media offers a significant saving in the time that
delays a combustor of this type in leaving to a
permanent regime.
1800
1600
Aire=60 lts/min CH 4=4,2 lts/min
1200
1800
1000
1600
800
1400
Temperatura
(K)
Temperature
(K)
Temperatura
Temperature
(K) (K)
1400
600
400
200
0
0
200
400
600
800
1000
1200
1400
1200
1000
800
600
400
200
Tiempo
Time(seg)
(s)
0
0
200
400
600
Figure 3. Temperature v/s time in the first test
800
1000
1200
Time (s)
Tiempo
(seg)
Figure 4. Temperature v/s time in the second test
Aire=80 lts/min CH 4=5,6 lts/min
2000
1800
1600
Temperatura(K)
(K)
Temperature
The development of the temperatures with time
in the three mentioned cases appears in the figures 35. The reached maximum temperatures, as it is also
seen of table 1, are 1582, 1634 and 1810 K
respectively. Of the combustion gas analysis, whose
results summary in table 1, it can be seen that with
the increase of the speed of filtration of the 1.03
mixture in 2.06 m/s, the CO production diminishes
of 71.6 to 14.66 PPM and the CO2 production
increases of 6.5 to 7.47 PPM. In addition, in the
same table it is seen that the concentration of O2
when coming out of the burner also diminishes from
9.5 to 7.8 %, confirming therefore the thermal
conversion of CO2 CO to greater temperature. On
the other hand, the generation of NOx and SO2
increases of 4 and 1 PPM to 9.67 and 6 PPM,
respectively. The SO2 presence, probably is
originated by the impurities that reach 100 PPM in
the provided methane. The increase of the NOx
when coming out of the burner with the increase of
the speed of filtration is in agreement with the
thermal mechanism of its formation. Consequently,
1400
1200
1000
800
600
400
200
0
0
200
400
600
800
1000
1200
1400
1600
Tiempo
Time (seg)
(s)
Figure 5. Temperature v/s time in the third test
Acknowledgments – The results reported here were
obtained during an investigation supported by the
CONICYT-Chile, project Fondecyt 1010354 and by APA –
Academia Politécnica Aeronautica FACH-Chile.
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