# Effect of Hydrogen Content and Dilution on  Laminar Burning Velocity and Stability Characteristics of Producer Gas-Air Mixtures

Authors: V. Ratna Kishore; M. R. Ravi; Anjan Ray
Journal: International Journal of Reacting Systems
 (2008)

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## Abstract

Producer gas is one of the promising alternative fuels with typical constituents of H2, CO, CH4, N2, and CO2. The laminar burning velocity of producer gas was computed for a wide range of operating conditions. Flame stability due to preferential diffusional effects was also investigated. Computations were carried out for spherical outwardly propagating flames and planar flames. Different reaction mechanisms were assessed for the prediction of laminar burning velocities of CH4, H2, H2-CO, and CO-CH4 and results showed that the Warnatz reaction mechanism with C1 chemistry was the smallest among the tested mechanisms with reasonably accurate predictions for all fuels at 1 bar, 300 K. To study the effect of variation in the producer gas composition, each of the fuel constituents in ternary CH4-H2-CO mixtures was varied between 0 to 48%, while keeping diluents fixed at 10% CO2 and 42% N2 by volume. Peak burning velocity shifted from ϕ=1.6 to 1.1 as the combined volumetric percentage of hydrogen and CO varied from 48% to 0%. Unstable flames due to preferential diffusion effects were observed for lean mixtures of fuel with high hydrogen content. The present results indicate that H2 has a strong influence on the combustion of producer gas.


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## Body

## 1. Introduction

Synthetic gas, often called syngas or producer gas,
can be generated from natural gas,
petroleum, gasification of coal, biomass, or even organic wastes. The
availability and flexibility of feedstock for production of producer gas make
it a potential future fuel. Producer gas mainly consists of combustible gases, namely,
hydrogen, carbon monoxide, and traces of methane and diluents, namely, carbon
dioxide and nitrogen. Producer gas is used to generate hydrogen for various
applications and as a source of carbon monoxide, which is used for
carboxylation reactions [1]. Recently, producer gas has also been directly used
as a gas turbine fuel in integrated coal-gasification combined cycle (IGCC) power
plants, which provide cost-effective and environmentally sound options for
meeting future coal-utilizing power generation needs [2]. Compositions and
calorific values of producer gas vary widely depending on raw materials and
gasifier types. Gaseous fuel, produced in various gasifiers, has a calorific
value of 4–13 MJ/m3 at STP conditions.Laminar burning velocity, which is a fundamental property of any combustible mixture,
is useful in the determination of its combustion rates and hence is a key
parameter in the design of combustion systems. The stability of these flames with
respect to preferential diffusion is often characterized by their Markstein
lengths. Experimental data for laminar burning velocity values generated by
various research groups almost until the turn of this millennium were not consistent
with each other, primarily because the effect of flame stretch rate was not
taken into account. Hence, it is essential to always quote a stretch-free value
of the laminar burning velocity. Any flame configuration chosen for the
measurement of burning velocity should subject the flame to a known stretch
rate that is uniform over the flame surface. This would help in determining the
stretch-free burning velocity by extrapolation. Spherical flames propagating at constant pressure
fulfil these requirements well and hence were used in the present investigation.Mishra et al. [3] developed a code for
spherical inwardly and outwardly propagating premixed flames to determine the
effect of stretch. They showed that a single step mechanism is not good for accurate
computation of burning velocities. Bradley et al. [4] developed a code for
spherical laminar flame propagation in three modes for methane-air mixtures:
explosion (outwardly propagating), implosion (inwardy propagating), and
stationary spherical flames to determine burning velocities, the effect of
stretch, and hence the unstretched burning velocity. Sun et al. [5] presented computational results for spherical inwardly
and outwardly propagating flames, planar flames, and counter flow premixed flames
using Sandia National Laboratories' PREMIX and OPPDIF codes [6] for the dynamics of weakly stretched flames and
also for quantitative description of the nonequidiffusion effect on flame stretch.Vagelopoulos
and Egolfopoulos [7] studied the
influence of hydrogen and methane addition on the propagation and extinction of
CO-air flames using the counterflow premixed flame configuration. Vagelopoulos
and Egolfopoulos [8] have reported the direct experimental determination of unstretched
laminar burning velocity with the flame impinging onto a plate at various
distances from the burner exit. They measured burning velocities for methane,
ethane, and propane at ultra-low strain rates.Faeth et al. [9–13] measured
laminar burning velocity along with stretch correction and Markstein numbers to
understand stability using a schlieren apparatus and high-speed photography for
constant pressure spherical outwardly propagating flames of methane, hydrogen,
and hydrogen/carbon monoxide with air. They also computed the laminar burning
velocity using RUN1DL [14] and PREMIX [6] and discussed
the stretch/preferential diffusion interactions based on
the computed flame structures.The present work involves the
computational determination of laminar burning velocity of producer gas-air mixtures
using the RUN1DL computational code developed by Rogg and Wang [14] and Sandia National
Laboratory’s PREMIX [6]. The prime objective is to study the effect of hydrogen content and
diluent content in the producer gas fuel mixture on the laminar burning
velocity and flame stability. Different reaction mechanisms, namely, the C1 and
C2 reaction mechanisms of Warnatz et al. [15] and the GRI Mech 3.0 reaction mechanism
[16] have been evaluated for computing laminar burning velocities of pure
methane, pure hydrogen, H2-CO and CO-CH4 mixtures burning
in air, by comparing the results against published experimental data. The purpose of such an exercise was to choose
a relatively small mechanism which can be suitably used for computation for
such mixtures properly and for which computational effort is reasonable. Hence,
other large mechanisms like Konnov release 0.5 [17] and the San Diego
mechanism [18] were not considered.
Computations for producer gas, which is represented as ternary mixtures of CO-CH4-H2 with diluents N2 and CO2 with different compositions,
have also been carried out. A systematic study of the variations in burning
velocity and equivalence ratio at peak burning velocity was carried out. The stability
of the flames was studied using both a Markstein number and an effective Lewis
number. The latter is found to predict the occurrence of instabilities for
different mixtures.


## 2. Computational Codes

Unsteady numerical simulations of outwardly propagating spherical
laminar flames were performed using the computational code RUN-1DL by Rogg and Wang [14]. RUN1DL employs the Euler
extrapolation scheme. In order to tackle the stiffness of the governing
equations, which are introduced through the chemical source terms and profiles
of quantities involved in combustion processes which typically exhibit steep
gradients and strong curvature, the use of adaptive methods is resorted to, so
that control of both temporal and spatial discretization errors is possible. The
calculations for unstretched (planar) flames were carried out using the steady,
one-dimensional laminar premixed flame code PREMIX [6].
The latter computations were performed for the sake of comparison of the
unstretched burning velocities obtained by extrapolation of the results of
RUN1DL simulations to zero stretch limit.


## 3. Calculation of Stretch Rate

The flame stretch rate,κ, at a point on a flame surface is the time rate of change of an infinitesimally
small element of area A surrounding the point, normalized by the area. For the
spherically symmetric outwardly propagating flames under consideration, the
total stretch rate was shown to be [4](1)κ=2Snru, where Sn is flame speed and  ru is flame radius.
The flame position was obtained by tracking a
particular isotherm and then flame speed and burning velocity were calculated.
Unstretched burning velocity (SL∞) was obtained by linear extrapolation of
stretched burning velocity to zero stretch using (2)SL=SL∞−Lκ. Here, L is the Markstein length. Two dimensionless numbers were
defined to conveniently characterise the effect of stretch: the Karlovitz
number (Ka=κδD/SL) and the Markstein number (Ma=L/δD) [9]. The Karlovitz number represents the ratio
of characteristic flame residence time to the characteristic flame stretch
time, κ−1.
The Markstein number represents the sensitivity of laminar burning velocities
to flame stretch and can be either positive (stable flames) or negative
(unstable flames) [19]. For the present work, the characteristic flame
thickness δD(=Du/SL) was based on a characteristic mass
diffusivity (Du) and the laminar burning velocity of the
stretched flame, in accordance with the approach followed in [9]. Putting the
above relations into (2) and rearranging will yield(3)SL∞SL=1+MaKa.


## 4. Chemical Reaction Mechanism

Two detailed chemical
reaction mechanisms due to Warnatz et al. [15], one consists of only C1 chemistry consists of 16 reactive species and 97 chemical
reactions (referred to in the present paper as C1
mechanism) and another mechanism consists of C1-C2 chemistry, consists of 23 reactive species and 140 chemical reactions
(referred to as C2 mechanism), were used in the present work
in the RUN1DL code. These mechanisms have smaller sets of reactions and are
widely used at 1 bar pressure. GRI-Mech 3.0 [16] reaction mechanism with 53
species and 325 reactions is an optimised reaction mechanism available for
methane combustion—it is used with the
PREMIX code. Computation
of the spherical outwardly propagating flame for the stoichiometric methane-air
mixture with GRI-Mech 3.0 using RUN1DL even up to a small flame radius of 4 cm required
a computation time up to 50 hours on an Intel P4-based 3 GHz, 32-bit computer.
Thus, in the computation of spherically expanding flames, this mechanism was
not used. Also, other large mechanisms like San Diego
mechanism [18] and Konnov release 0.5
[17] were not considered when using RUN1DL. On PREMIX, the computational time
using this mechanism is relatively shorter, and hence it was decided to perform
the PREMIX simulations using GRI Mech 3.0, while the RUN1DL calculations were
confined to the Warnatz C1 and C2 mechanisms. It has however been verified for
specific cases that the predicted unstretched burning velocities from both
codes using the same mechanism are nearly identical.


## 5. Assessment of Suitability of Reaction  Mechanisms for Present Simulations

The two Warnatz [15] reaction mechanisms have been evaluated
for their suitability in accurately determining the laminar burning velocity of
various mixture combinations using experimental data in the literature. This was
done by comparing the computationally obtained burning velocities for outwardly
propagating spherical flames using these mechanisms in RUN1DL with published
data from the literature as well as results obtained for planar flames using
PREMIX.The adaptive grid parameters (GRAD and CURV) which place more grid
points at steep gradients and sharp curvatures, respectively, are so chosen that
the number of grid points in the reaction zone was more than 50 [4] to ensure
grid independent results. The effects of these grid parameters on the predicted
value of laminar burning velocity of
stoichiometric methane-air mixture
are shown in Table 1. It is clear that the effect of changing GRAD from 0.1 to
0.01 on burning velocity is very little. Hence, a GRAD value of 0.01 and a CURV value
of 1.5 were chosen for use in all the computations presented in this paper.Table 1
Effect of grid parameters on burning velocity for stoichiometric methane-air mixture at 1 bar
and 300 K.
Adaptive grid parametersNo. ofBurning velocityGRADCURVgrid points(cm/s)0.51.57036.70.11.512034.90.011.519034.8For all computations,
the cold boundary was taken at 12 cm radius. The burnt gas temperature obtained
from the above computations was 2240 K for stoichiometric
methane-air mixture at 1 bar and 300 K, which is equal to the adiabatic equilibrium temperature, showing that
the extent chosen for the computational domain was sufficient in this case.The suitability of various reaction mechanisms used in the present
work was assessed by comparing the laminar burning velocities predicted using
these mechanisms against experimental data available in the literature. As a
first step, computations were carried out for pure gaseous fuels, such as methane
and hydrogen burning in air at 1 bar with unburnt gas temperature 300 K, since chemical
kinetics and experimental data on laminar burning velocity are well established
for these mixtures.For methane-air, agreement of
computational results with Warnatz C1 chemistry with the available experimental
data for aϕ range of 0.7–1.2 was better
than those with Warnatz C2 chemistry and GRI-Mech 3.0. For very rich mixtures (ϕ=1.3), however, C1 chemistry predicts burning
velocity lower than experiments because for rich mixtures the CH3 radical would combine to give C2 radicals. Thus, C2 chemistry is
expected to work better for such mixtures than C1 chemistry. For hydrogen-air
mixture predictions using the C1 mechanism in RUN1DL up to an equivalence
ratio of 3.0 are in good agreement with experimental data, as well as
predictions using GRI-Mech 3.0 mechanism in PREMIX.The Warnatz C1 mechanism produces reasonably
accurate results for methane-air, and also hydrogen-air mixtures at 1 bar, 300 K.
In order to evaluate the accuracy of predictions using this mechanism when CO
is also present in the combustible mixture, as well as when more than one
combustible constituent is present in the mixture, simulation results at the
same initial pressure, and temperature for binary fuel mixtures are compared
with the experimental results available in the literature. Computations were
performed for the combustion of binary mixtures of CO-H2 and CH4-CO in air.Only one published work (Vagelopoulos and Egolfopoulos [7], using the counter-flow
premixed flame technique) is available for unstretched burning velocities of CH4-CO
mixtures burning in air at 1 bar pressure with unburnt gas temperature 300 K. Simulations
were performed for the compositions tested in [7]. The results are shown in Figure 1. The format of presentation follows that of [7], where the abscissa is
the mole fraction of CH4 in the mixture, and curves are plotted for
different mole fractions of the fuel (CH4-CO) in the mixture. The predictions are within the experimental
uncertainties mentioned in [7].Figure 1
Unstretched  burning
velocities of CH4-CO/air
mixtures at 1 bar pressure, 300 K.Computations were done at 1 bar pressure and unburnt
gas temperature 300 K for fuel mixtures of composition 5%H2-95%CO
and 50%H2-50%CO. The thermal diffusion effect was included in these computations
[14]. Predictions of unstretched laminar burning velocities for these mixtures
with RUN1DL using the Warnatz C1 mechanism for spherically expanding flames are
plotted in Figure 2 and compared with spherically expanding flame measurements by
Hassan et al. [10], McLean et al. [20],
and Sun et al. [21] and those obtained with PREMIX using GRI-MECH 3.0 for freely
propagating planar flames. The comparisons with experiments are quite good and
RUN1DL predicts peak burning velocities at the same equivalence ratios as in
the experiments.Figure 2
Unstretched  burning velocities ofH2-CO/air
mixtures at 1 bar pressure, 300 K. Red lines indicate predictions
without inclusion of thermal diffusion effect.In Figure2, results of computations with and without thermal diffusion
using RUN1DL and PREMIX for H2 (5%)-CO (95%) is shown. Bongers and de
Goey [22] have shown that for light species like hydrogen, taking the Soret
effect or thermal diffusion into account makes a difference of about 10% in the
predicted burning velocity. Figure 2 shows that the inclusion of thermal
diffusion caused burning velocity to reduce by 5% for RUN1DL simulations and
8% for PREMIX simulations for 5%H2-95% CO mixture. Hence, for correct
simulation of flame propagation in a fuel containing hydrogen the inclusion of
thermal diffusion is essential.The results presented in the
foregoing sections indicate clearly that the C1 mechanism of Warnatz et al. [15] is
able to predict burning velocities of single fuels and fuel mixtures containing
CO, H2, and CH4 burning in air at atmospheric pressure
within the experimental uncertainties in almost all the cases presented. The
percentage difference between experimental data and simulation results using
Warnatz C1 mechanism was less than 5% for methane-air and hydrogen-air flames.
The percentage difference was less than about 10% for binary fuel mixtures.
Therefore, this mechanism was used for all subsequent computations presented in
this paper. Thermal diffusion effects have been included in the results
presented hereafter.


## 6. Results and Discussion

### 6.1. Burning Velocities and Markstein Numbers of Producer Gas-Air Flames

The following sections present computational results for ternary gaseous fuel
mixtures of CO,H2, and CH4 with diluents CO2 and N2. Table 2 gives three compositions of producer gas [23], obtained
from different feed stocks using different kinds of biomass gasification technology.
A brief perusal of Table 2 indicates that PG2 has nearly the same relative
proportions of fuel constituents as PG1. The diluents in PG2, which again are
in the same relative proportions of CO2 and N2 as in PG1,
are nearly 50% more than those in PG1. PG3 has nearly the same amount of total
diluents as PG1, while the hydrogen content in PG3 is about 40% more than that
in PG1. Thus, in the following sections, a comparison of PG1 and PG2 could be
expected to bring out the effect of diluents, whereas a comparison of PG1 and PG3
could be expected to illustrate the effect of hydrogen content.Table 2
Gas compositions of producer gas from different feed stocks.
Feed stockGasifier usedGas composition (by % volume)H2COCH4CO2N2Producer gas 1 (PG1) [23]Wood chipsDowndraft222241042Producer gas 2 (PG2) [23]Rice HuskDowndraft101221561Producer gas 3 (PG3) [24]Wood chipsTwo-stage311811535Computational investigations of outwardly
propagating spherical flames were carried out for these producer gas mixtures
at 1 bar and 300 K using Warnatz C1 mechanism. Two features were intended to be
studied: (i) the effect of composition on unstretched burning velocity and (ii)
the sensitivity of the laminar burning velocity to stretch.Figure3 shows the unstretched
burning velocity as a function of equivalence ratio for the different producer
gas compositions, obtained using linear extrapolation of stretched burning
velocity data to zero stretch. Clearly, the laminar burning velocity increases
with increase in hydrogen content in the fuel at any equivalence ratio. For PG2,
the burning velocity peak occurs at an equivalence ratio of 1.1. With higher
hydrogen content, the occurrence of peak shifts to higher values of equivalence
ratio. For PG1, it occurs at an equivalence ratio of 1.3, whereas for PG3 (31% hydrogen),
it occurs at an equivalence ratio of about 1.4. It can be concluded from these
observations that laminar burning velocity of producer gas is strongly
influenced by the hydrogen content in the mixture.Figure 3
Unstretched burning velocity as a function of  equivalence ratio for producer gas-air
mixtures at 1 bar and 300 K.Figure4 shows predicted Markstein numbers over a range of equivalence
ratios for different producer gas compositions. As outlined in Section 3,
Markstein length (L) was obtained in the process of linear
extrapolation of stretched burning velocity data to zero stretch for the
simulations of spherically expanding flames considered here. For the calculation of Markstein number (=L/δD), the characteristic flame thickness, δD(=Du/SL), was based on the diffusivity (Du) of hydrogen into nitrogen. It can be seen
from Figure 4 that for lean and moderately rich mixtures of all three producer
gases, the Markstein number is negative.Figure 4
Predicted Markstein numbers for producer gas as function of equivalence ratio.IfMa<0, the flame is in the preferential diffusion
instability regime, and if Ma>0, it is in the stable regime [25]. If Ma=0, the flame is neutrally stable and SL=SL∞ at all values of stretch rate. The flame can be seen to be
neutrally stable at ϕ≈0.9 for PG1, and at ϕ≈1.15 for PG2 and PG3. Comparing the stability
behaviour of PG1 and PG3, we find that PG1
exhibits stability with respect to preferential diffusion for a wider range of
equivalence ratio compared to PG3. This could be attributed to the higher
hydrogen content in PG3. This behaviour was also observed by Hassan et al. [10];
with an increase of H2 fraction (by volume) from 3% to 50% in a
binary fuel mixture of H2-CO, the neutral stability shifts from ϕ≈1.1 to 1.6, showing the effect of hydrogen
content in fuel on preferential diffusion stability.A comparison of the
preferential diffusion stability characteristics of PG1 and PG2 shows that PG2
is more unstable, and this can be attributed to the larger amount of diluents
in PG2. Kwon and Faeth [13] have shown a similar result for H2-O2-Ar
mixtures: at ϕ=0.6, with variation of O2/(O2+Ar)
from 0.36 to 0.21, H2/O2/Ar mixtures also showed a  transition from stable to unstable condition.
Also Qiao et al. [26] have shown that for H2-air mixtures at ϕ=1 an increase in dilution with Ar, N2,
or CO2 from 0%–40% results in
transition from stable to unstable condition. Taylor
[19] found that with dilution of H2-air
mixture with nitrogen from 0–60%, Markstein number
changed from positive to negative values, signifying transition.


### 6.2. Extraction of Effective Lewis Number of Mixture

Preferential
diffusion stability characteristics can be explained based on the effective Lewis
number of the mixture. However, the calculation of effective Lewis number for a
multicomponent fuel mixture is not as straightforward as for a pure fuel-air
mixture such as CH4-air. Chung and Law [27] presented an expression
for computing the deviation in burnt gas temperature from the stretch-free adiabatic
flame temperature due to the simultaneous presence of preferential diffusion and
stretch. Based on this expression, the effective Lewis number can be written in
terms of the actual burnt gas temperature (Tb) and stretch-free burnt gas temperature (Tb0) as(4)TbTb0=1+Ka(1Leeff−1)⇒Leeff=1(1/Ka)(Tb/Tb0−1)+1.At every time step in the simulation the burnt gas temperature (Tb) is obtained. The
value of Tb0, the adiabatic flame temperature under equilibrium assumption, was
determined using STANJAN [28]. After extensive study for various mixtures, it was
found that Tb/Tb0 varies linearly with Ka for Ka<0.05 and the
intercept is very close to unity. Effective Lewis number (Leeff) values
obtained using (4) for PG3 at different equivalence ratios are tabulated in Table 3. It has to be mentioned here that in the absence of an expression for an
effective Lewis number in terms of individual reactant Lewis numbers for the
multicomponent mixtures studied in this work, the above semiempirical approach
has been used to arrive at an effective Lewis number from the observed linear
variation of Tb with stretch. The effective Lewis number that
obtained is independent of stretch.Table 3
Effective Lewis number (Leeff) of PG3 mixture at different equivalence ratios.
ϕKaLeeffMaStability0.80.10.69−0.72Unstable1.10.050.82−0.57Unstable1.20.031.020.20Stable1.80.11.141.86StableAtϕ=0.8 and ϕ=1.1, where Leeff is less than
unity, we observe that the Markstein number (Ma) is negative indicating
unstable flames. At ϕ=1.2 and ϕ=1.8, where Leeff is greater than
unity, the Markstein number is positive indicating stable flames. Here, it can
be clearly seen that Leeff can be used as the parameter based on
which one can characterise the stability of the flame for the mixture. However,
the computation of either of the parameters indicating flame stability, the
effective Lewis number, and the Markstein number, needs the results of
computations of the flame propagation, that is, these parameters cannot be
calculated from the known values of unburnt gas properties and composition.


### 6.3. Parametric Study on the Effect of Composition on Burning Velocity and Flame Stability

Although a comparison of PG1, PG2, and PG3 demonstrated the effects of diluent content and
hydrogen content in the fuel mixture on the burning velocity and flame
stability, a more systematic study is necessary to be able to attribute the
behaviour of the flame to changes in the fuel composition. Hence, a parametric
study was carried out to quantify the effect of fuel composition on the burning
velocity and flame stability characteristics of producer gas-air flames. The
simulations were carried out using RUN1DL with variation in volumetric
percentage of fuel components, namely, CH4, H2, and CO in
the producer gas, keeping N2 at 42% and CO2 at 10%. Amongst
fuel components, CH4 was kept at 0 and 12%, and H2 and
CO were varied in steps of 12 such that total fuel composition was 48%. Each
gas mixture was simulated for spherical flame propagation over a range of
equivalence ratios at a constant pressure of 1 bar and unburnt gas temperature
of 300 K. The results of this exercise bring out clearly the effect of each fuel
constituent on the combustion behaviour of the mixture.In Figure5 burning
velocities of mixtures with 0% CH4 are plotted as a function of
equivalence ratio to demonstrate the effect of relative proportions of CO and H2.
In these mixtures, H2 and CO were varied between 12 to 48% by volume,
and the equivalence ratio was varied in the range 0.8–2.0. The value of
the peak burning velocity increases by as much as 200% with an increase in
hydrogen content from 12% to 48%. This behaviour was also shown in Figure 3,
where PG3 was shown to have a much higher burning velocity than PG1. From the
consistency between the two cases, it can be concluded that the higher burning
velocity of PG3 was clearly owing to its higher hydrogen content.Figure 5
Unstretched burning velocity with 0% CH4 as
function of equivalence ratio at 1 bar pressure and 300 K.It can also be observed that the peak burning
velocity shifts fromϕ=1.6 for 12%H2 to ϕ=1.4 for 48%H2.  With increase in CO content, H2-CO
mixtures [10, 21] show peak burning velocities at equivalence ratios greater
than 2, while pure hydrogen shows a peak at ϕ=1.8 [5, 13]. Thus, an increase in CO and a decrease
in H2 content in the mixture result in the shift of the burning
velocity peak toward a higher value of ϕ.Predicted Markstein numbers for different
fuel compositions are shown in Figure6. With the increase in H2 percentage
by volume in the fuel from 12% to 36%, the equivalence ratio at which stable to
unstable transition occurs shifts from ϕ=0.9 to ϕ=1.05. This shows that the increase of
hydrogen in fuel causes a shift in the threshold equivalence ratio for
stability of laminar flames to higher values. This finding is in consonance
with stability characteristics observed for producer gas mixtures 1 and 3 as
discussed before.Figure 6
Predicted Markstein numbers with 0% CH4 as function of
equivalence ratio at 1 bar pressure and 300 K.In Figure7, burning velocities of mixtures with
12% CH4 by volume and the remaining 36% comprising H2 and
CO are plotted over a range of equivalence ratios. The peak occurs at ϕ=1.2 for 0%H2 and shifts to 1.1
for 24%H2. It can be noticed that the burning velocity values for
all compositions for rich mixtures vary within a narrow range. The steep
decrease in burning velocity from 23 cm/s (12%H2 and 24% CO mixture)
at ϕ=1.4 to 3 cm/s at ϕ=1.6 can be seen in
sharp contrast with Figure 5. The principal
difference between Figures 5 and 7 is the presence of 12% CH4 in the
latter. It is interesting to note that the presence of even a small quantity of
CH4 restricts the range of equivalence ratios over which the mixture
can burn. Also, a comparison of the values of burning velocity for the 12% and
24%H2 cases in the two figures shows that replacing CO with CH4 results in a sharp decrease in burning velocity values in the rich side,
especially at equivalence ratios exceeding 1.2.Figure 7
Unstretched burning velocity with 12% CH4 as function of equivalence ratio
at 1 bar pressure and 300 K.


## 6.1. Burning Velocities and Markstein Numbers of Producer Gas-Air Flames

The following sections present computational results for ternary gaseous fuel
mixtures of CO,H2, and CH4 with diluents CO2 and N2. Table 2 gives three compositions of producer gas [23], obtained
from different feed stocks using different kinds of biomass gasification technology.
A brief perusal of Table 2 indicates that PG2 has nearly the same relative
proportions of fuel constituents as PG1. The diluents in PG2, which again are
in the same relative proportions of CO2 and N2 as in PG1,
are nearly 50% more than those in PG1. PG3 has nearly the same amount of total
diluents as PG1, while the hydrogen content in PG3 is about 40% more than that
in PG1. Thus, in the following sections, a comparison of PG1 and PG2 could be
expected to bring out the effect of diluents, whereas a comparison of PG1 and PG3
could be expected to illustrate the effect of hydrogen content.Table 2
Gas compositions of producer gas from different feed stocks.
Feed stockGasifier usedGas composition (by % volume)H2COCH4CO2N2Producer gas 1 (PG1) [23]Wood chipsDowndraft222241042Producer gas 2 (PG2) [23]Rice HuskDowndraft101221561Producer gas 3 (PG3) [24]Wood chipsTwo-stage311811535Computational investigations of outwardly
propagating spherical flames were carried out for these producer gas mixtures
at 1 bar and 300 K using Warnatz C1 mechanism. Two features were intended to be
studied: (i) the effect of composition on unstretched burning velocity and (ii)
the sensitivity of the laminar burning velocity to stretch.Figure3 shows the unstretched
burning velocity as a function of equivalence ratio for the different producer
gas compositions, obtained using linear extrapolation of stretched burning
velocity data to zero stretch. Clearly, the laminar burning velocity increases
with increase in hydrogen content in the fuel at any equivalence ratio. For PG2,
the burning velocity peak occurs at an equivalence ratio of 1.1. With higher
hydrogen content, the occurrence of peak shifts to higher values of equivalence
ratio. For PG1, it occurs at an equivalence ratio of 1.3, whereas for PG3 (31% hydrogen),
it occurs at an equivalence ratio of about 1.4. It can be concluded from these
observations that laminar burning velocity of producer gas is strongly
influenced by the hydrogen content in the mixture.Figure 3
Unstretched burning velocity as a function of  equivalence ratio for producer gas-air
mixtures at 1 bar and 300 K.Figure4 shows predicted Markstein numbers over a range of equivalence
ratios for different producer gas compositions. As outlined in Section 3,
Markstein length (L) was obtained in the process of linear
extrapolation of stretched burning velocity data to zero stretch for the
simulations of spherically expanding flames considered here. For the calculation of Markstein number (=L/δD), the characteristic flame thickness, δD(=Du/SL), was based on the diffusivity (Du) of hydrogen into nitrogen. It can be seen
from Figure 4 that for lean and moderately rich mixtures of all three producer
gases, the Markstein number is negative.Figure 4
Predicted Markstein numbers for producer gas as function of equivalence ratio.IfMa<0, the flame is in the preferential diffusion
instability regime, and if Ma>0, it is in the stable regime [25]. If Ma=0, the flame is neutrally stable and SL=SL∞ at all values of stretch rate. The flame can be seen to be
neutrally stable at ϕ≈0.9 for PG1, and at ϕ≈1.15 for PG2 and PG3. Comparing the stability
behaviour of PG1 and PG3, we find that PG1
exhibits stability with respect to preferential diffusion for a wider range of
equivalence ratio compared to PG3. This could be attributed to the higher
hydrogen content in PG3. This behaviour was also observed by Hassan et al. [10];
with an increase of H2 fraction (by volume) from 3% to 50% in a
binary fuel mixture of H2-CO, the neutral stability shifts from ϕ≈1.1 to 1.6, showing the effect of hydrogen
content in fuel on preferential diffusion stability.A comparison of the
preferential diffusion stability characteristics of PG1 and PG2 shows that PG2
is more unstable, and this can be attributed to the larger amount of diluents
in PG2. Kwon and Faeth [13] have shown a similar result for H2-O2-Ar
mixtures: at ϕ=0.6, with variation of O2/(O2+Ar)
from 0.36 to 0.21, H2/O2/Ar mixtures also showed a  transition from stable to unstable condition.
Also Qiao et al. [26] have shown that for H2-air mixtures at ϕ=1 an increase in dilution with Ar, N2,
or CO2 from 0%–40% results in
transition from stable to unstable condition. Taylor
[19] found that with dilution of H2-air
mixture with nitrogen from 0–60%, Markstein number
changed from positive to negative values, signifying transition.


## 6.2. Extraction of Effective Lewis Number of Mixture

Preferential
diffusion stability characteristics can be explained based on the effective Lewis
number of the mixture. However, the calculation of effective Lewis number for a
multicomponent fuel mixture is not as straightforward as for a pure fuel-air
mixture such as CH4-air. Chung and Law [27] presented an expression
for computing the deviation in burnt gas temperature from the stretch-free adiabatic
flame temperature due to the simultaneous presence of preferential diffusion and
stretch. Based on this expression, the effective Lewis number can be written in
terms of the actual burnt gas temperature (Tb) and stretch-free burnt gas temperature (Tb0) as(4)TbTb0=1+Ka(1Leeff−1)⇒Leeff=1(1/Ka)(Tb/Tb0−1)+1.At every time step in the simulation the burnt gas temperature (Tb) is obtained. The
value of Tb0, the adiabatic flame temperature under equilibrium assumption, was
determined using STANJAN [28]. After extensive study for various mixtures, it was
found that Tb/Tb0 varies linearly with Ka for Ka<0.05 and the
intercept is very close to unity. Effective Lewis number (Leeff) values
obtained using (4) for PG3 at different equivalence ratios are tabulated in Table 3. It has to be mentioned here that in the absence of an expression for an
effective Lewis number in terms of individual reactant Lewis numbers for the
multicomponent mixtures studied in this work, the above semiempirical approach
has been used to arrive at an effective Lewis number from the observed linear
variation of Tb with stretch. The effective Lewis number that
obtained is independent of stretch.Table 3
Effective Lewis number (Leeff) of PG3 mixture at different equivalence ratios.
ϕKaLeeffMaStability0.80.10.69−0.72Unstable1.10.050.82−0.57Unstable1.20.031.020.20Stable1.80.11.141.86StableAtϕ=0.8 and ϕ=1.1, where Leeff is less than
unity, we observe that the Markstein number (Ma) is negative indicating
unstable flames. At ϕ=1.2 and ϕ=1.8, where Leeff is greater than
unity, the Markstein number is positive indicating stable flames. Here, it can
be clearly seen that Leeff can be used as the parameter based on
which one can characterise the stability of the flame for the mixture. However,
the computation of either of the parameters indicating flame stability, the
effective Lewis number, and the Markstein number, needs the results of
computations of the flame propagation, that is, these parameters cannot be
calculated from the known values of unburnt gas properties and composition.


## 6.3. Parametric Study on the Effect of Composition on Burning Velocity and Flame Stability

Although a comparison of PG1, PG2, and PG3 demonstrated the effects of diluent content and
hydrogen content in the fuel mixture on the burning velocity and flame
stability, a more systematic study is necessary to be able to attribute the
behaviour of the flame to changes in the fuel composition. Hence, a parametric
study was carried out to quantify the effect of fuel composition on the burning
velocity and flame stability characteristics of producer gas-air flames. The
simulations were carried out using RUN1DL with variation in volumetric
percentage of fuel components, namely, CH4, H2, and CO in
the producer gas, keeping N2 at 42% and CO2 at 10%. Amongst
fuel components, CH4 was kept at 0 and 12%, and H2 and
CO were varied in steps of 12 such that total fuel composition was 48%. Each
gas mixture was simulated for spherical flame propagation over a range of
equivalence ratios at a constant pressure of 1 bar and unburnt gas temperature
of 300 K. The results of this exercise bring out clearly the effect of each fuel
constituent on the combustion behaviour of the mixture.In Figure5 burning
velocities of mixtures with 0% CH4 are plotted as a function of
equivalence ratio to demonstrate the effect of relative proportions of CO and H2.
In these mixtures, H2 and CO were varied between 12 to 48% by volume,
and the equivalence ratio was varied in the range 0.8–2.0. The value of
the peak burning velocity increases by as much as 200% with an increase in
hydrogen content from 12% to 48%. This behaviour was also shown in Figure 3,
where PG3 was shown to have a much higher burning velocity than PG1. From the
consistency between the two cases, it can be concluded that the higher burning
velocity of PG3 was clearly owing to its higher hydrogen content.Figure 5
Unstretched burning velocity with 0% CH4 as
function of equivalence ratio at 1 bar pressure and 300 K.It can also be observed that the peak burning
velocity shifts fromϕ=1.6 for 12%H2 to ϕ=1.4 for 48%H2.  With increase in CO content, H2-CO
mixtures [10, 21] show peak burning velocities at equivalence ratios greater
than 2, while pure hydrogen shows a peak at ϕ=1.8 [5, 13]. Thus, an increase in CO and a decrease
in H2 content in the mixture result in the shift of the burning
velocity peak toward a higher value of ϕ.Predicted Markstein numbers for different
fuel compositions are shown in Figure6. With the increase in H2 percentage
by volume in the fuel from 12% to 36%, the equivalence ratio at which stable to
unstable transition occurs shifts from ϕ=0.9 to ϕ=1.05. This shows that the increase of
hydrogen in fuel causes a shift in the threshold equivalence ratio for
stability of laminar flames to higher values. This finding is in consonance
with stability characteristics observed for producer gas mixtures 1 and 3 as
discussed before.Figure 6
Predicted Markstein numbers with 0% CH4 as function of
equivalence ratio at 1 bar pressure and 300 K.In Figure7, burning velocities of mixtures with
12% CH4 by volume and the remaining 36% comprising H2 and
CO are plotted over a range of equivalence ratios. The peak occurs at ϕ=1.2 for 0%H2 and shifts to 1.1
for 24%H2. It can be noticed that the burning velocity values for
all compositions for rich mixtures vary within a narrow range. The steep
decrease in burning velocity from 23 cm/s (12%H2 and 24% CO mixture)
at ϕ=1.4 to 3 cm/s at ϕ=1.6 can be seen in
sharp contrast with Figure 5. The principal
difference between Figures 5 and 7 is the presence of 12% CH4 in the
latter. It is interesting to note that the presence of even a small quantity of
CH4 restricts the range of equivalence ratios over which the mixture
can burn. Also, a comparison of the values of burning velocity for the 12% and
24%H2 cases in the two figures shows that replacing CO with CH4 results in a sharp decrease in burning velocity values in the rich side,
especially at equivalence ratios exceeding 1.2.Figure 7
Unstretched burning velocity with 12% CH4 as function of equivalence ratio
at 1 bar pressure and 300 K.


## 7. Conclusions

The effects of stretch and
composition on laminar burning velocities and stability of multicomponent
mixtures like producer gas were studied computationally using outwardly
propagating spherical flames. The major conclusions of the study are as follows.(1)
Three
reaction mechanisms, Warnatz C1 chemistry mechanism, C1-C2 chemistry mechanism, and GRI MECH 3.0 were
tested for prediction against experimental results of binary mixtures ofH2-CO
and CH4-CO. Warnatz C1 chemistry is the smallest mechanism considered
in this paper, which was found to predict burning velocities better than
other mechanisms for all the tested fuels at 1 bar and 300 K. This
mechanism is used for prediction of burning velocities of producer gas in
this paper.(2)
The variation in burning velocity when the
Soret effect is accounted for is at least 5% even with 5% of hydrogen (by
volume) in the binary fuel ofH2-CO. Hence, inclusion of
thermal diffusion for fuels containing light species like hydrogen is very
important.(3)
This study indicates that the effect of
hydrogen enhances the burning velocity and lowers the equivalence ratio at
the peak burning velocity. It also shows that increase in dilution
promotes onset of instability.(4)
An effective Lewis number was calculated
from change in temperature of flames due to simultaneous presence of
stretch and preferential diffusion. An effective Lewis number less than
unity indicated unstable flames. Stable flames were obtained for Lewis
numbers greater than unity in the cases studied.(5)
A parametric study of the effect of composition on burning velocity for
ternary mixtures shows the following.(a)
The increase in burning velocity is dictated by the amount of
hydrogen present. The extent of increase is limited by the presence of
methane. Methane also limits the range of values of equivalence ratio over
which the mixture burns.(b)
The equivalence ratio at which the peak burning velocity occurs
shifts toward lower values with increase in hydrogen content and decrease in CO.



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*Source: 310740-2008-09-30.xml*