i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 3 2 9 2 e1 3 2 9 9
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Characterization of biogas-hydrogen premixed
flames using Bunsen burner
H.S. Zhen a,*, C.W. Leung a, C.S. Cheung a, Z.H. Huang b
a
Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong,
China
b
State Key Laboratory of Multiphase Flows in Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China
article info
abstract
Article history:
Experimental study is conducted to clarify the effects of hydrogen addition to biogas and
Received 27 May 2014
hydrogen fraction in the biogas-H2 mixture on the stability, thermal and emission char-
Received in revised form
acteristics of biogas-H2-air premixed flames using a 9 mm-ID-tube Bunsen burner. Varia-
20 June 2014
tion in biogas composition is allowed to range from BG60 (60%CH4e40%CO2), down to BG50
Accepted 23 June 2014
(50%CH4e50%CO2) and to BG40 (40%CH4e60%CO2). For each biogas, the fraction of hydrogen
Available online 17 July 2014
in the biogas-H2 mixture is varied from 10% to 50%. The results show that upon hydrogen
addition and increasing hydrogen fraction in the fuel mixture, there are corresponding
Keywords:
changes in flame stability, laminar burning velocity, flame tip temperature and CO
Biogas-H2 fuel
pollutant emission.
Flame stabiltiy
Under the tested conditions of 400 Re 800 and 0.8 Ф 1.2, the otherwise
Laminar burnig velocity
incombustible biogas-air mixture becomes flammable upon hydrogen addition and the
CO emission
threshold value of hydrogen fraction for occurrence of stable flames increases with the
increase in the CO2 concentration of biogas. Overall, the flame stability is best at rich
equivalence ratio of Ф ¼ 1.2. The laminar burning velocity of the biogas-H2-air mixture is
found to be higher than pure biogas due to addition of hydrogen, and it monotonically
increases at higher hydrogen fraction. Further, a shift in equivalence ratio for peak laminar
burning velocity from stoichiometric Ф ¼ 1.0 to Ф ¼ 1.2 is observed. The results also show a
monotonic increase in flame temperature and a monotonic decrease in CO emission at
higher hydrogen fraction. The more efficient oxidation of CO into CO2 reveals that more
complete combustion is induced by hydrogen addition.
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
Introduction
Increasing concern over fossil fuel shortage and air pollutant
emission pushes more and more research efforts towards
the utilization of biogas as an alternative fuel around the
world. The reduction of reliance on the fossil fuel might
also be met by increasing the utilization of biogas. For
example, biogas (BG) has been used as an important
energy source in the rural areas of China and the propensity
for biogas usage will grow even greater by the end of
2020 [1].
* Corresponding author. Fax: þ852 23654703.
E-mail address: [email protected] (H.S. Zhen).
http://dx.doi.org/10.1016/j.ijhydene.2014.06.126
0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 3 2 9 2 e1 3 2 9 9
Nomenclature
Re
rmix
Vexit
d
Ф
Vfuel
Vair
a
mmix
mi
Yi
Mi
LBV
U0
q
EICO
Reynolds number of fuel/air mixture jet
density of fuel/air mixture, kg/m3
flow velocity at burner exit, m/s
inner diameter of the nozzle, m
mixture equivalence ratio
volume flow rate of fuel, m3/s
volume flow rate of air, m3/s
mole fraction of hydrogen in the fuel mixture, %
dynamic viscosity of fuel/air mixture, N s/m2
dynamic viscosity of component i, N s/m2
mole fraction of component i, %
molecular weight of component i, kg/mol
laminar burning velocity, m/s
bulk velocity of unburned mixture, m/s
half cone angle of the flame, degrees
emission index of CO, g/kg
Biogas is derived from landfills, agricultural wastes and
other sources of biomass. Thus, it is an environmentallyfriendly renewable fuel. Biogas is mainly composed of
methane (CH4) and carbon dioxide (CO2), with smaller
amounts of oxygen, nitrogen and volatile organic compounds.
Depending on the source of biogas, the fraction of CO2 present
in biogas ranges from 30% to 60% by volume. Due to the high
CO2 content, the combustion characteristics of biogas are
inferior to those of natural gas, which is almost all methane,
because the presence of CO2 results in a narrow range of flame
stability, low burning speed and reduced flame temperature
[2e4]. Therefore, the application of pure biogas in industrial
burners is restricted by its potential instability problem and
low flame temperature, as biogas has narrow flammability
range and low burning rate in nature.
Research efforts have been made to convert low-quality
pure/raw biogas into higher quality fuel through blending it
with a higher grade fuel [3,5,6]. Lee and Hwang [3,5] proposed
raising the heating value of biogas through addition of a
higher grade fuel, LPG (liquefied petroleum gas). They
observed higher heating value and higher burning speed of
the biogas-LPG mixed fuel in comparison with raw biogas.
They thus proposed that the mixed fuel can be used as an
interchangeable gas of liquefied natural gas. Cardona and
Amell [6] blended biogas with propane and hydrogen, finding
that the laminar burning velocity of the mixture is higher than
that of pure biogas due to addition of propane and hydrogen.
Research efforts have also been devoted to enhancing the
stability of biogas flames. Lee and Hwang [3] conducted a
comparison of the flame stability of biogas, biogas with
addition of LPG (liquefied petroleum gas), and LNG (liquefied
natural gas). They obtained rather good interchangeability
between the biogas-LPG mixed fuel and LNG. Leung and
Wierzba [7] investigated the stability of a diffusion flame
burning biogas blended with a small amount of hydrogen. In
their study, two compositions of pure biogas, 60%CH4e40%
CO2 and 50%CH4e50%CO2 were considered and the authors
found a significantly wider range for stable flames when a
small amount of hydrogen was introduced into the biogas.
13293
Examining a biogas diffusion flame with the biogas composition of 60%CH4e40%CO2, Zhen et al. [8] observed a significant
improvement in the flame stability through adding a trace
amount of hydrogen into the biogas. Hydrogen has low ignition energy and high burning rate, thereby the biogas-hydrogen
mixture is easier to ignite and the relative slow reaction rate of
the biogas fuel accelerates, which improve flame stability
[7,8]. In a later investigation by the same authors [9], similar
improvement in stability was reported for a premixed flame
burning biogas blending with hydrogen as hydrogen added
extends the lean stability limit and allows leaner combustion.
Like biogas, hydrogen is also a renewable, alternative fuel,
attracting many studies conducted on hydrogen-air flame, for
example by Toro et al. [10]. However, not many efforts were
given to hydrogen-biogas-air flame, and the literature review
shows that interest in upgrading biogas by blending it with
hydrogen has expanded in recent years. In comparison to
blending biogas with any hydrocarbon fuel, the biogashydrogen mixture has greater potential to be a cleaner fuel,
as hydrogen burned has no emission of CO/CO2. In Refs. [7e9],
the biogas considered to be mixed with hydrogen is of relatively high quality, with the volume fraction of CH4 in the
biogas as high as 60%, while lower quality biogas with the
fraction of CH4 ranging from 50% down to 40% was not
considered. Therefore, it becomes the objective of this study
to make a full picture of the stability improvement effect for a
Bunsen-type premixed flame burning low-quality biogas with
different levels of hydrogen addition. Variation in biogas
composition is allowed in this study as it helps understand the
detrimental effect of the component CO2 on the flame stability. The flame stability testing will be conducted in a wide
range of Reynolds number and equivalence ratio, and the
experimental measurements of flame height, flame temperature and CO emission are also performed to gain a better
understanding of the changed thermal and emission characteristics of the biogas flame caused by hydrogen addition. The
findings from this investigation will provide guidance for
better utilization of biogas in premixed burner applications.
Experimental setup and method
A schematic layout of the experimental facilities used in this
study is shown in Fig. 1. It comprised the supplies of fuel
gases, fuel metering and blending devices, and temperature
registering instrumentation as well as the rig for measuring
gaseous CO emission. In the current study, biogas was prepared by blending pure CH4 and CO2 gases at proper volume
ratios to simulate the biogas supplies typically encountered in
practice. The CH4 fraction in the CH4eCO2 mixture was varied
from 60% down to 50% and 40%, respectively with the symbols
BG60, BG50 and BG40 to designate each biogas composition,
where the subscripts represent the fraction of CH4 in the
biogas. For each biogas prepared, pure hydrogen was incrementally introduced for mixing at a volumetric ratio from
10% to 50%. Namely, the level of hydrogen addition, a, is
defined as:
a¼
VðH2 Þ
100%
VðCH4 Þ þ VðCO2 Þ
(1)
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 3 2 9 2 e1 3 2 9 9
Fig. 1 e Layout of experimental facilities.
As biogas was assumed to be the primary fuel and
hydrogen as the secondary additive fuel, so the hydrogen
fraction in the mixed fuel was no larger than 50%. Pure gases
of CH4, CO2 and H2 were taken from high-pressure cylinders
located in separate feeding lines. All gases were of commercial
grade with over 99% purity to avoid any change of the mixture
composition during the experiments. Air was provided by an
air compressor. All gaseous flows were controlled by regulating needle valves, with the flow rate metered by calibrated
flow meters, and then were mixed together in a cylindrical
mixing chamber full of stainless steel beads, prior to entering
the burner.
The burner used in this study is of Bunsen-type, consisting of a single copper tube, which is sufficiently long for
establishing fully developed flow at the tube exit. The tube
has 9 mm inner diameter and 11 mm outer diameter. For
each biogas-air or biogas-H2-air mixture, the flame jet
Reynolds number is evaluated based on cold fuel-air
mixture as:
Re ¼
rmix Vexit d
mmix
(2)
where mmix is calculated according to Ikoku [11] as:
pffiffiffiffiffiffi
P
mi Yi Mi
mmix ¼ P pffiffiffiffiffiffi
Yi Mi
(3)
Regarding each biogas or mixed biogas-H2 as the fuel, the
equivalence ratio of the fuel-air mixture, Ф, is:
F¼
Vfuel Vair
fuel to air ratio
¼
ðfuel to air ratioÞst
Vfuel Vair st
(4)
where Vfuel and Vair represent volume flow rates of fuel and
air, respectively, and St stands for stoichiometric conditions.
For the experiments conducted, the values of Re and Ф are
varied from 400 to 800 and from 0.8 to 1.2, respectively.
A high-resolution CCD camera with a shutter speed of
1/60 s was used to record luminous flame images with a dark
background. Flame height of the premixed flame, defined as
the vertical distance from the tip of the inner conical flame
front to the tube exit plane, was measured with a ruler with
0.5 mm precision. Flame temperature was measured by an
uncoated type-B thermocouple with a wire diameter of
0.25 mm and bead diameter of 0.5 mm. The registered temperatures were then corrected for radiation and conduction
losses.
The emission of gaseous pollutant CO was measured by
probe sampling followed by in-situ analysis of its concentration. The sampling probe, mounted axially in the post-flame
region, is a tapered quartz probe of 1 mm inner diameter
and 2 mm outer diameter. The quartz probe was connected to
a 1 m long stainless steel pipe, which was then connected to
the CO/CO2 analyzer (California Instruments Corp., Model 300,
NDIR). Emission index of CO, defined as grams of pollutant
emitted per kilogram of fuel burned, was evaluated based on
the CO concentrations measured along the flame axis from
70 mm to 100 mm above the tube exit plane, and the EICO
values obtained varied less than 1.5%. Hence, emission data at
100 mm above the burner exit were treated as the average
representative values reported in this study.
All the measurements were repeated three times and the
averaged data were used for an uncertainty analysis [12].
Using a 95% confidence level, the uncertainties are 1.6% in
flame temperature, 0.3% in flame height, and 4.9% in EICO.
Results and discussions
Improved flame stability
For a Bunsen-type laminar premixed flame, the flame would
be stabilized and attached to the burner rim when the laminar
burning velocity and flow speed of the flame are matched with
each other. In addition, the flow speed component of the
flame perpendicular to the flame front is equal to the local
burning velocity at all points of the flame front [13]. At both
extremely high and sufficiently low flow speed, instable
flames occur. The flame would blow off the burner rim if the
flow speed is extremely higher than the laminar burning velocity and would flash-back when the flow speed becomes
sufficiently lower than the burning velocity [14].
The flame stability testing was conducted by firstly
providing the burner with pure biogas of BG40, BG50 and BG60,
respectively, at a certain fixed biogas-air Reynolds number
(Re ¼ 400, 600 and 800) and equivalence ratio (F ¼ 0.8, 1.0 and
1.2). Next, in each case of the pure biogas, hydrogen was added
and the fraction of hydrogen introduced into the biogas-H2
mixture was gradually increased from 10% to 50%, at an interval of 10%, while maintaining constant Re and F. Then, the
igniter was placed near the burner rim to check whether the
flame can be stably operating or not. The stability map is
shown in Fig. 2.
The testing results show that for pure biogas flames
burning raw BG40, BG50 and BG60, no flame exists. As a certain
amount of hydrogen is added into the raw biogas, the mixed
biogas-H2 gases can be flammable and a burner-stabilized
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 3 2 9 2 e1 3 2 9 9
Fig. 2 e Flame stability map for various biogases with
hydrogen addition.
flame is established. This reveals that addition of hydrogen
improves the ignitability of the fuel and enhances the flame
stability. Hydrogen has very low ignition energy and very wide
range between lower and upper flammability limits. Furthermore,
addition of hydrogen to biogas enhances the laminar flame
burning velocity [7e9]. Therefore, both the ignition and stability of biogas flames are improved upon hydrogen addition.
For an example, the flame burning raw BG60 blows off at
Re ¼ 400 and F ¼ 1.0, and becomes stable, attached to the
burner rim as 10% hydrogen is introduced into the flame.
Additionally, stable flames are observed as the hydrogen
fraction in the biogas-H2 mixture increases from 10% to 40%.
At a ¼ 50%, the flame flashes back due to the sufficiently high
laminar burning velocity. It can be seen that the threshold
value of hydrogen fraction in the mixture for occurrence of
stable flames for the flames burning BG60 is 10%. Similarly, the
flames burning BG50 and BG40 under Re ¼ 400 and F ¼ 1.0,
stable flames appear from a ¼ 20% to a ¼ 40% for the former
case and from a ¼ 40% to a ¼ 50% for the latter. It seemingly
shows a trend that the threshold value of hydrogen fraction
over which the biogas-H2-air flame becomes burner-stabilized
elevates from 10% for BG60 to 20% for BG50 and further to 40%
for BG40, following the increase in the CO2 concentration of the
biogas. Or in other words, more amount of hydrogen is
required to be added to the raw biogas to achieve stable flames
if the biogas has more CO2 in its composition. This observation
identifies the detrimental effect of CO2 on the flame stability
as CO2 acts as a combustion-inhibiting gas by lowering the
concentration of combustible gases and species, reducing the
flame temperature and decreasing laminar burning velocity
[2e4].
As Re is increased from 400 to 600, the flames burning BG60
are stable at a 10% and the flames burning BG50 and BG40 are
stable at a 30% and a 40%, respectively. It confirms the
trend that the critical value of hydrogen fraction in the fuel
mixture for occurrence of stable flames increases with the
increase in the CO2 content of the biogas. On the other hand,
the flames burning either BG60 or BG50 at a ¼ 50% do not flashback at Re ¼ 600, while they do at Re ¼ 400, indicating that the
flow speed at Re ¼ 600 which is higher than that at Re ¼ 400 can
well match the laminar burning velocity in the high Reynolds
number case. At further higher Re of 800, the threshold fraction of hydrogen for stable flames is 20%, 30% and 40% for
13295
BG60, BG50 and BG40, respectively, with no flash-back observed.
The testing results also show that at any particular Re, there
are more stable flames/conditions associating with the biogas
with lower CO2 content. For instance, the BG60 flame has lower
CO2 content than the BG50 and BG40 flames, and has four stable
conditions at Re ¼ 400, while there are three and two stable
conditions for the BG50 and BG40 flames, respectively. Similar
phenomena are observable at Re ¼ 600 and 800.
Finally, the flame stability testing was conducted by operating the flames at different equivalence ratios of F ¼ 0.8, 1.0
and 1.2. Similar findings are obtained and it can be concluded
that the threshold fraction of hydrogen addition beyond
which nozzle-stabilized flames appear would increase with
the increase of the CO2 content of the biogas, i.e. the higher
CO2 fraction in the biogas, the higher H2 fraction in the biogasH2 mixture in order to stabilize the flame. Besides, the results
show that the biogas with lower CO2 content tends to have
wider stable operational conditions. For the present experiments, the BG60 flame has the lowest CO2 content and is
associated with the widest range of stable operational conditions, followed by the BG50 flame and then the BG40 flame. The
results also indicate a wider range of stable operation for the
biogas-H2 flames at higher equivalence ratio, i.e. fuel-rich
condition of F ¼ 1.2. Moreover, flash-back is more prone to
occur for the biogas-H2 flame with lower CO2 content. The
reason is that when maintaining the flames burning BG40,
BG50 and BG60 at constant Re, F and a, the flow speed is the
lowest in the case of the flame burning BG60, followed by BG50
and then BG40. Namely, the flow speed is lower for the biogasH2-air mixture with lower CO2 content. On the other hand, the
laminar burning velocity tends to be higher for the bigas-H2air mixture with lower CO2 content. Therefore, the combined
effect is a higher tendency for occurrence of flash-back for
the biogas-H2-air flame with lower CO2 content (more CH4
content).
Shortened flame height
The structure of the biogas flames as a result of increasing
hydrogen concentration in the mixed fuel was investigated in
this section. Throughout the tests, as hydrogen was added,
both the Reynolds number and equivalence ratio of the flames
were kept constant.
Within the range of 400 Re 800 and 0.8 F 1.2, visual
inspection of the flame heights revealed that for each biogas
composition of BG40, BG50 or BG60, the flame shortens upon
monotonically at higher hydrogen concentration in the
biogas-H2 mixture. For each flame, hydrogen addition induces
a higher volumetric fuel/air flow rate and a higher flow speed
as the Reynolds number and equivalence ratio are fixed constant. Thus, if the laminar burning velocity of the flame is
assumed constant, the conical flame front must increase in
height to match the increased flow speed. However, the fact is
that a shorter flame is observed at higher flow speed, meaning
that the laminar burning velocity of the biogas-H2-air mixture
is increased at higher hydrogen fraction. Taking the BG60
flames under Re ¼ 800 and F ¼ 1.0 for an example, as shown in
Fig. 3 (middle row), when the level of hydrogen addition increases from a ¼ 20%e50%, the flame monotonically shortens
from 27 mm to 19 mm, while the volumetric fuel/air flow rate
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 3 2 9 2 e1 3 2 9 9
Fig. 3 e Flames heights against hydrogen fraction at
Re ¼ 800, Ф ¼ 0.8 (upper row), Ф ¼ 1.0 (middle row) and
Ф ¼ 1.2 (lower row).
higher flow speed tends to push the flame to develop upwards,
thus increasing the flame height. The second reason is that as
the BG40 flame has the largest CO2 fraction among the three
flames, therefore the flame temperature would be the lowest
due to the much amount of diluting CO2. Both the diluting
effect of CO2 and the resultant low flame temperature would
lower the laminar burning velocity [2e4,18]. Consequently,
the combined effect is that the BG40 flame has the longest
flame height among the three flames which are under identical values of Re, F and a.
Fig. 3 also presents the images of the BG60 flames at F ¼ 0.8
(upper row) and F ¼ 1.2 (lower row). At F ¼ 0.8, due to instable
operation, no flame exists from a ¼ 10%e40%. There is only
one flame with a ¼ 50% is shown in Fig. 3 (upper row). At
F ¼ 1.2, five flames are shown in Fig. 3 (lower row) as the BG60
flames are stable from a ¼ 10%e50%. Another finding is that
the flame height monotonically reduces as the fraction of
hydrogen added into the biogas-H2 mixture is increased from
a ¼ 10%e50% at F ¼ 1.2, indicative of enhanced laminar
burning velocity caused by hydrogen addition. On the other
hand, in both cases of F ¼ 0.8 and F ¼ 1.2, comparison of the
BG60 flames with the BG50 and BG40 flames under the same
level of hydrogen addition, i.e. a ¼ 50%, reveals similar finding
to the case of F ¼ 1.0. That is, the BG60 flame is the shortest
while the BG40 flame is the longest, indicating that the flame
height is larger for the biogas with higher CO2 fraction.
Enhanced laminar burning velocity
increases from 9.7 105 m3/s to 10.12 105 m3/s (Table 1),
indicative of a higher laminar burning velocity at higher a. The
result is consistent with Refs. [7e9], which similarly reported
an increased laminar burning velocity of biogas flames upon
hydrogen addition. The behavior in flame height of the biogasH2-air flames in this study is similar with the studies of the
CH4eH2-air flames [15e17]. All the studies reported reduced
height of the reaction cone upon hydrogen addition, and
that the laminar burning velocity increases as a function of
hydrogen fraction in the fuel mixture.
Also shown in Fig. 3 (middle row) is that under Re ¼ 800,
F ¼ 1.0 and a ¼ 50%, the flame height is the shortest at 19 mm
for the BG60 flame, followed by the BG50 flame and then the
BG40 flame, which are 21 mm and 23 mm high, respectively.
There are two reasons for the larger heights of the BG50 and
BG40 flames. One is that under the same Re, F and a, the BG40
flame has the highest flow speed at the burner exit, while the
exit flow speed is the lowest in the case of the BG60 flame. The
The laminar burning velocity for the BG60 flames at Re ¼ 800 is
calculated based on its relationship with the cone angle of the
flame front and the mean flow speed, by taking into account
that the perpendicular component of the flow speed equals to
the local burning velocity over a great trajectory of the flame
front [13]. In this case where straight-sided flame cones occur,
the laminar burning velocity can be calculated according to:
LBV ¼ U0 sin q
(5)
Where U0 is the bulk velocity of the unburned mixture at the
nozzle exit and q is the half cone angle of the flame.
By referring to Fig. 3, it is known that the BG60 flame under
Re ¼ 800 and Ф ¼ 0.8 is stable only at a ¼ 50%, and thus Fig. 4
shows that its corresponding laminar flame burning velocity
is 0.25 m/s. As for the stoichiometric and fuel-rich conditions
of Ф ¼ 1.0 and Ф ¼ 1.2, the number of stable flames, as shown
in Fig. 3, is four and five, respectively, corresponding to the
same number of laminar burning velocities given in Fig. 4. It is
Table 1 e Operational conditions for BG60 flames at Re ¼ 800, Ф ¼ 1.0.
a
0%
10%
20%
30%
40%
50%
Air volume
flow rate
(105 m3/s)
CO2 volume
flow rate
(106 m3/s)
CH4 volume
flow rate
(106 m3/s)
H2 volume
flow rate
(106 m3/s)
Fuel/air mixture
volume flow rate
(105 m3/s)
Mean flow
velocity (m/s)
8.066
8.081
8.094
8.105
8.113
8.118
5.65
5.41
5.13
4.82
4.45
4.01
8.48
8.12
7.70
7.23
6.67
6.02
0.00
1.50
3.21
5.16
7.41
10.03
9.48
9.58
9.70
9.83
9.97
10.12
1.49
1.51
1.52
1.54
1.57
1.59
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 3 2 9 2 e1 3 2 9 9
Fig. 4 e Calculated laminar burning velocity for the BG60
flames versus hydrogen fraction.
seen from Fig. 4 that as the fraction of hydrogen in the biogasH2 mixture is increased, the laminar flame burning velocity
increases monotonically, with the maximum occurring at
a ¼ 50% in either case of Ф ¼ 1.0 or Ф ¼ 1.2. The result is in
agreement with the studies [3,5e9], which reported a higher
laminar burning velocity of biogas-H2 mixture than biogas
alone. The monotonic increase in the burning velocity
of biogas-H2 premixed flames is similar to the case of
methane-H2 premixed flames [19e21]. All the investigators
observed a monotonic increase in the burning velocity of
the methane-H2-air mixture at higher fraction of hydrogen
because the burning velocity of hydrogen is higher than that
of methane.
Fig. 4 illustrates that the maximum laminar burning velocity for the flames under Ф ¼ 1.0 occurs at a ¼ 50%, i.e.
0.37 m/s, being higher than that under Ф ¼ 0.8 and a ¼ 50%,
simply because the fuel/air mixture under Ф ¼ 1.0 has more
complete combustion in comparison to the flame under
Ф ¼ 0.8. Under further higher equivalence ratio, the fuel/air
mixture becomes fuel-rich, and thus incomplete combustion
appears again. However, Fig. 4 shows that at a ¼ 50%, the
laminar burning velocity under Ф ¼ 1.2 is 0.47 m/s, higher than
that under Ф ¼ 1.0. The higher laminar burning velocity for the
fuel-rich flame under Ф ¼ 1.2 can be verified by referring to
Fig. 3, which shows that at fixed a ¼ 50%, the height of the BG60
flame at Ф ¼ 1.2 is shorter than the BG60 flame at Ф ¼ 1.0, being
noted that the fuel/air flow rate at Ф ¼ 1.0 (10.36 105 m3/s) is
higher than that at Ф ¼ 1.2 (10.12 105 m3/s). Here, the authors want to note that even though the calculation of laminar
burning velocity using Equation (4) is a rather rough estimate,
the results are in acceptable range, as particular care was
taken to fabricate sharp-edged burner rim to avoid any irregularities and thus to maintain straight-sided flame cones. For
reference purpose, the maximum laminar burning speed of a
66%CH4e34%CO2 mixture was reported to be around 0.26 m/s
at Ф ¼ 1.0 by Cardona and Amell [6], which is in close proximity of 0.25 m/s in the case of the 60%CH4e40%CO2 mixture
at Ф ¼ 1.0 and a ¼ 20% reported in the present study.
13297
Therefore, the value of the calculated LBV is reasonable as the
fraction of CH4 in the biogas of BG60 is lower than that in
Ref. [6].
The higher flow speed as well as shorter flame height
associating with the flame at Ф ¼ 1.2 reveals that the laminar
burning velocity is higher at Ф ¼ 1.2. The shift in maximum
laminar burning velocity from stoichiometric equivalence
ratio to the fuel-rich side is related to the high fraction of
hydrogen added in the biogas-H2 mixture. When examining
natural gas/air premixed flame with hydrogen addition, Hu
et al. [21] found a shift in the peak value of laminar flame
burning velocity from stoichiometry to the richer side with the
increase of hydrogen fraction. They found that as the fraction
of hydrogen in the hydrogenemethane mixture exceeds 60%,
the otherwise equivalence ratio of Ф ¼ 1.0 for occurrence of
peak laminar burning velocity shifts to Ф ¼ 1.2. In this study,
for the BG60 flame with a ¼ 50%, the ratio of hydrogen to
hydrogen plus methane is 0.625, over the value of 60% stated in
Ref. [21]. Therefore, the BG60 flame with 50% hydrogen addition
has similar behavior to the natural gas/air mixture when the
fraction of hydrogen added in the fuel mixture is high. The shift
in equivalence ratio for peak laminar burning velocity was
also reported by other researchers in Refs. [22e24].
Increased flame temperature
From Section 3.1, it is known that the biogas flame operating
at larger equivalence ratio has wider range of stable operation,
so the following experiments are focused on the biogas-H2-air
flames operating at Re ¼ 800 and Ф ¼ 1.2, which are also typical
values of Re and Ф for household cooking flames.
For a Bunsen-type premixed flame, the tip of the inner
reaction cone is the hottest part of the flame [13]. Hence, the
thermocouple bead was located at the tip to measure the
maximum temperature. The tip temperatures are shown in
Fig. 5, which indicate that at fixed Re ¼ 800 and Ф ¼ 1.2, the
flame temperature monotonically increases with the increase
in the fraction of hydrogen added in the biogas-H2 mixture. In
addition, all three sets of data demonstrate that with addition
Fig. 5 e Flame tip temperatures versus hydrogen fraction at
Re ¼ 800 and Ф ¼ 1.2.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 3 2 9 2 e1 3 2 9 9
of hydrogen, the increase in the peak flame temperature
against a shows a linear trend. This is because pure hydrogen
combusting with air has a much higher temperature than the
raw biogas flame [25]. Moreover, addition of hydrogen lowers
the concentration of the diluting gas CO2 present in the fuel
mixture, thus leading to higher temperature. As a result, addition of hydrogen into biogas enhances the flame temperature.
The enhanced flame temperature caused by adding hydrogen
to biogas is coincident with previous studies [8,9].
Fig. 5 also shows that at the same level of hydrogen addition, the flame burning BG60 has higher temperature than
BG50, and in turn the BG50 flame has higher temperature than
BG40, revealing a trend that the higher CO2 concentration in
the biogas, the lower the flame temperature. This is because
CO2 is an inert gas. The presence of CO2 in the biogas dilutes
the CH4 concentration on the one hand and on the other absorbs a portion of heat released during the burning of CH4.
by accelerating the oxidization rate. Thus, the CO concentration in the flame drops. Lastly, the higher flame temperature
due to hydrogen addition provides a high-temperature environment for more efficient CO oxidization [27]. As a result of
these three reasons, EICO is reduced with hydrogen addition,
which is in line with the studies reported in the literature for
hydrogen-enriched methane-air flames [28e30].
A comparison can be made of the values of EICO for the
BG60, BG50 and BG40 flames in Fig. 6. It shows that at the same
level of hydrogen addition, the BG40 flame has the highest
value of EICO due to both its highest CO2 content and thus
lowest flame temperature, which leads to inefficient oxidization of CO. Similarly, due to the lowest CO2 content and
highest flame temperature, the BG60 flame has the least CO
emission among the three biogas flames.
Conclusion
Reduced CO emission
The effect of hydrogen addition to biogas on the variation of
CO emission is investigated in this section. The post-flame
region was considered where volume concentrations of CO
were measured. Measurement shows that the CO concentration decreased exponentially with axial elevation once the
quartz probe was above the flame, and the EICO becomes
rather uniform when the probe was at moderately large axial
elevations. In this study, the gaseous CO was sampled at the
axial distance of 100 mm above the burner exit plane. The
resultant EICO is shown in Fig. 6.
Fig. 6 shows that there is a monotonic decrease in CO
emission with the increase of the hydrogen fraction. There are
three reasons ascribable for it. First, the increase in hydrogen
fraction at constant Reynolds number and equivalence ratio
results in a reduced biogas flow rate, thus a lower carbon input
to the flame, potentially reducing CO formation. Second, the
increase in hydrogen fraction prompts an increase in the
concentration of the radicals OH and H. Just as the case of
methane-air flames with hydrogen addition [26], these radicals play an important role during the oxidation of CO to CO2
Fig. 6 e EICO of the flames at Re ¼ 800 and Ф ¼ 1.2.
This paper presents an experimental study investigating the
stability, thermal and emission characteristics of Bunsen-type
laminar premixed flames burning biogas with hydrogen
addition. The main objective is to examine the enhancement
effect of hydrogen addition on the stability of the flames
burning biogas with various compositions including BG60 (60%
CH4e40%CO2), BG50 (50%CH4e50%CO2) and BG40 (40%CH4e60%
CO2). Biogas is used as the main fuel and thus the mole fraction of hydrogen in the biogas-H2 mixture is varied from 0% to
50%. The fuel/air mixture jet Reynolds number and equivalence ratio are varied from 400 to 800 and 0.8 to 1.2, respectively. The experimental results obtained indicate that with
addition of hydrogen and variation in hydrogen fraction, there
are changes in flame stability, laminar burning velocity, flame
tip temperature and CO pollutant emission.
1. Under the conditions tested, it is found that the otherwise
incombustible biogas-air mixture becomes flammable upon
hydrogen addition and the threshold value of hydrogen
fraction for occurrence of stable flames increases with the
increase in CO2 content of biogas. Overall, the flame stability
is best for fuel-rich condition of Ф ¼ 1.2.
2. With the increase in hydrogen fraction at constant Reynolds number and equivalence ratio, the height of the
inner reaction cone is observed to reduce at higher level of
hydrogen addition, indicating an increase in laminar
burning velocity.
3. The laminar burning velocity shows a monotonic increase
at higher hydrogen fraction. Further, a shift in equivalence
ratio for peak laminar burning velocity to richer side of
Ф ¼ 1.2 is observed.
4. At higher hydrogen fraction, there is a monotonic increase
in flame temperature, and a monotonic decrease in CO
emission. The more efficient oxidation of CO into CO2 reveals that more complete combustion is induced by
hydrogen addition.
5. The experimental results obtained in this study serve as
guidance for better application of biogas-hydrogen mixture
in Bunsen burners. To achieve the most stable flame with
optimal thermal and emission performance (high flame
temperature and low CO emission), the flame should be
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 3 2 9 2 e1 3 2 9 9
operating at Ф ¼ 1.2, instead of Ф ¼ 1.0 and 0.8. Furthermore, the fraction of hydrogen in the biogas-hydrogen
mixture is better at a ¼ 40% for BG40 and BG50, and
a ¼ 30% for BG60 in the range of 400 Re 800.
Acknowledgment
The Authors thank for the fully financial support from the
Research Grants Council of the Hong Kong SAR (B-Q39F) to the
present project.
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