The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
Laminar burning velocities of C2H5OH + O2 + CO2 flames
Jenny D. Naucler, Moah Christensen, Elna J.K. Nilsson, Alexander A. Konnov
Division of Combustion Physics, Department of Physics, Lund University,
Lund, Sweden
jenny.naucler@forbrf.lth.se
ABSTRACT
First measurements of the adiabatic laminar burning velocities of ethanol + oxygen +
carbon dioxide flames are presented. The oxygen content O2/(O2 + CO2) in the artificial
air was 35%. The initial temperature of the gas mixture was 298 K and 318 K. Nonstretched flames were stabilized on a perforated plate burner at atmospheric pressure.
The heat flux method was used to determine burning velocities under conditions when the
net heat loss of the flame is zero. The measurements were found in qualitative agreement
with the modelling performed using recent version of the Konnov mechanism.
Keywords: Liquid Fuel, Heat Flux, Ethanol, Oxy-Fuel, Laminar burning velocity
1. INTRODUCTION
Biofuels are considered as economically promising and environmentally friendly fuels.
Among the biofuels, ethanol is popular as an alternative to traditional fuels [1]. As
biofuels differ in composition from traditional fuels, the chemistry behind the combustion
require further research. Detailed knowledge is needed to increase combustion efficiency
and minimize pollutant formation.
Oxy-fuel combustion is considered to be of strategic importance, since it offers a pathway
for CO2 capture. In oxy-fuel combustion, air is replaced by an oxidizing mixture of O2
and CO2. Oxy-fuel combustion has been shown to have the additional benefit of reducing
the formation of NOx-pollutants in the emission [2, 3]. Implementation of this approach
to combustion of solid fossil and gaseous fuels has motivated a significant number of
recent investigations [4]. However, combustion of liquid fuels under oxy-fuel conditions
was only considered for specific cases, like Diesel engines for underwater applications
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[5]. No attempts to investigate flames of liquid biofuels in the oxidizing mixtures of O2
in CO2 have been reported so far, to the best of the authors’ knowledge. To this end, the
goal of the present work was to measure the adiabatic laminar burning velocity of ethanol
under oxy-fuel conditions. The adiabatic laminar burning velocity is a fundamental
parameter of each combustible mixture, which depends on the stoichiometric ratio,
pressure and temperature. The laminar burning velocity at standard conditions, i.e.
atmospheric pressure and initial temperature of 298 K is invaluable for the
characterization of combustion properties of the given fuel, for understanding of the
underlying chemistry, validation of models, etc.
In the present work the heat flux method was used to determine burning velocities under
conditions when the net heat loss of the flame was zero. The measurements were
compared with the modelling performed using recent version of the Konnov mechanism.
In the following sections experimental setup and modelling details are outlined. First
measurements of the adiabatic laminar burning velocities of ethanol + oxygen + carbon
dioxide flames are then presented and discussed.
2. EXPERIMENTAL DETAILS
The heat flux method for the stabilization of adiabatic premixed laminar flames on a flat
flame burner was proposed by de Goey et al [6], and further developed by van Maaren et
al [7]. The method is extensively used for measuring laminar burning velocities of
gaseous fuels, e.g. [8, 9]. In Fig. 1 a schematic overview of the burner is shown. A 2 mm
thick burner plate perforated with small holes (0.5 mm in diameter) is attached to the
burner outlet.
The burner head has a heating jacked supplied with water from a
thermostatic water bath, to keep the temperature of the burner plate constant. During the
experiments the temperature of the heating jacket was fixed at 368 K. In the plenum
chamber the initial temperature of the fresh gas mixture is adjusted using a separate
thermostatic water bath, supplying water at temperatures in the range 298 K to 358 K.
This fixes the initial temperature of the burning mixture. The heating jacket keeps the
burner plate edges at a certain temperature higher than the initial gas temperature, thus
warming up the (unburned) gases flowing through. Conductive heat transfer of the flame
to the burner plate cools the gas flow on its turn. If the flame is stabilized under sub-2-
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adiabatic conditions, the gas velocity is lower than the adiabatic flame burning velocity
and the sum of the heat loss and heat gain is higher than zero. Then, the center of the
burner plate is hotter than the heating jacket. If the unburned gas velocity is higher than
the adiabatic burning velocity (super-adiabatic conditions), the net heat flux is lower than
zero and the center of the burner plate is cooler than the heating jacket. By changing the
flow rate of the gas mixture an appropriate value of the gas velocity can be found to
nullify the net heat flux. In this case the radial temperature distribution in the burner
plate is uniform and equal to the temperature of the heating jacket. The temperature
distribution of the burner plate is measured using the series of thermocouples shown in
Fig. 1. The flow rate at which the net heat flux is zero is the adiabatic flame burning
velocity [6, 10].
Figure 1: A schematic overview of the burner.
The mixing panel shown in Fig. 2 was used to provide controlled flow of the vaporized
fuel and air. The liquid fuel is controlled and evaporated with Cori-Flow liquid massflow controller (MFC 77) with Controlled Evaporator Mixer (MFC 75), both from
Bronkhorst B.V. N2 is used as carrier gas for the fuel from the reservoir to MFC 77 and
MFC 75. The fuel is evaporated at 110 °C. The artificial air supply is divided in two
flows. One air flow is controlled by MFC-74 and mixed with the liquid fuel in the
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evaporator and the other air flow is controlled by MFC-76 and mixed downstream with
the main flow to provide the required mixture composition.
Figure 2: A schematic overview of the mixing panel and burner for the heat flux setup. Figure
created by J.P.J. van Lipzig and used with permission.
3. MODELLING DETAILS
One dimensional premixed flames of C2H5OH + O2 + CO2 were modelled using the
CHEMKIN - II collection of codes [11-13], including transport properties [14], from
Sandia National Laboratories. Multi-component diffusion and thermal diffusion options
were taken into account. Adaptive mesh parameters were GRAD = 0.1 and CURV = 0.5.
Relative and absolute error criteria were RTOL=1.0*10-9 and ATOL =1.0*10-9,
respectively. The flames were modelled using the Konnov detailed C/H/N/O reaction
mechanism for combustion of small hydrocarbons [15]. The current version of the
Konnov mechanism (Release 0.6) consists of 1230 reactions among 129 species. It
differs from the previous Release 0.5 only in the part of the prompt-NO sub-mechanism.
Therefore, it retains predicting capabilities and performance of the previous version in
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terms of laminar flame modelling. Validation of Release 0.5 of the Konnov mechanism
was summarized recently [15].
4. Results and discussion
First sets of measurements of the adiabatic laminar burning velocities of ethanol +
oxygen + carbon dioxide flames have been performed. The oxygen content O2/(O2 +
CO2) in the artificial air was 35%. This concentration was chosen to be compatible with
the earlier studies in methane (ethane, propane) + oxygen + carbon dioxide mixtures [16].
The oxidizing mixture was prepared at the plant with the accuracy of 0.5 % of O2. The
initial temperatures of the gas mixture was 298 K and 318 K. The flames were stabilized
on a perforated plate burner at atmospheric pressure. The equivalence ratio was varied in
the range 0.5-0.7. The maximum equivalence ratio that can be investigated using the
present method at room temperature is 0.7, at higher equivalence ratios partial
vaporisation pressure of ethanol would lead to condensation of the fuel [17, 18].
The measured laminar burning velocities are presented in Fig 3. Error bars show an
experimental uncertainty of the results mainly resulting from accuracy of the liquid flow
from the cori-flow (MFC 77 in Fig 2), the accuracy of the gas-flow MFCs and
temperature measurements. This uncertainty was estimated to be about ±1 cms-1, [18] for
fastest flames, in slowly-burning flames both the uncertainty of the burning velocity and
of the equivalence ratio increases since the set-points of the MFCs approached the
recommended low-limit of 10 % from the total scale. Also shown is the modelling
results performed using recent version of the Konnov mechanism. The measurements are
apparently in qualitative agreement with the modelling considering existing experimental
uncertainty and rather narrow range of the measurements, but the model slightly
underpredicts the experimental results. To extend the range of accessible equivalence
ratios the initial temperature of the mixture was increased up to 318 K. Figure 4 shows
adiabatic burning velocity for ethanol burned under oxy-fuel conditions at atmospheric
pressure and initial temperature of 318 K. The maximum equivalence ratio accessible at
this temperature is limited by the full range of the liquid MFC. Similar to the results at
298 K the modelling systematically under-predicts the experimental values. Further
analysis will focus on the model behaviour at oxy-fuel conditions. Also the composition
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of the oxidizer (O2 + CO2) could be modified and thus the range of accessible
equivalence ratios and initial temperatures will be extended, which is the objective of the
authors.
50
Burning velocity, cm/s
40
30
20
10
0
0.4
0.6
0.8
1.0
1.2
1.4
Equivalence ratio
Figure 3: Adiabatic burning velocity vs. equivalence ratio for ethanol burned under oxy-fuel
conditions at atmospheric pressure and initial temperature of 298 K.
50
Burning velocity, cm/s
40
30
20
10
0
0.4
0.6
0.8
1.0
1.2
Equivalence ratio
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1.4
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Figure 4: Adiabatic burning velocity vs. equivalence ratio for ethanol burned under oxy-fuel
conditions at atmospheric pressure and initial temperature of 318 K.
5. CONCLUSIONS AND PERSPECTIVES
The feasibility of the measuring the adiabatic laminar burning velocities of alternative
liquid biofuels at oxy-fuel conditions has been demonstrated.
First two sets of
measurements in ethanol + oxygen + carbon dioxide flames has been presented. The
experimental results were found in satisfactory agreement with the modelling performed
using recent version of the Konnov mechanism. Further research will be focused on
improvement of the experimental accuracy and on extension of the range of initial
temperatures of the flames. This would also allow extension of the mixture composition
towards richer flames.
6. ACKNOWLEDGEMENTS
The authors would like to thank the Swedish Energy Agency (Energimyndigheten) and
the Foundation for Strategic Research (SSF) for financial support through the Center of
Combustion Science and Technology (CECOST).
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Research Foundation – IFRF
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