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Flammability Limit and NO
XFormation of Fuel Enriched Lean Premixed
Flames
Hongsheng Guo, Gregory J. Smallwood
Institute for Chemical Process and Environmental Technology National Research Council, Ottawa, Ontario, Canada K1A 0R6
Email: Hongsheng.guo@nrc-cnrc.gc.ca
Abstract
Lean premixed combustion has been of great interest to combustion scientists and engineers because of its potential to reduce fuel consumption and pollutant formation. However, lean premixed combustion has some intrinsic weaknesses. A strategy to overcome these weaknesses
is to adopt fuel enrichment. This paper investigated the extinction and NOX formation
characteristics in reformate gas enriched CH4/air lean premixed flames. Detailed chemistry and
complex thermal and transport properties were used. The composition of reformate gas was
assumed to be the products of partially oxidation of CH4. The results indicate that reformate gas
enrichment can significantly enlarge the flammable region and lower the lean flammability limit
of CH4/air premixed flame. As a result, the reformate gas enrichment technology allows a
combustor to operate under leaner condition, and therefore significantly reduces the formation of
NOX.
1. Introduction
Lean premixed combustion is a promising concept for substantial reduction in fuel consumption and emissions of greenhouse gases and pollutants. It involves operation with excess air-fuel ratios to substantially reduce the flame temperature. At these low temperatures and excess air-fuel ratios, NO formation from thermal and prompt routes can be effectively suppressed. Emission of soot, the predominant source of particulate matter and a major global warming contributor, can also be essentially eliminated in these flames.
Unfortunately lean premixed combustion has some intrinsic weaknesses. One of the weaknesses is that at excessive air-fuel ratios, the lean-flammability limit is approached and the flame becomes less stable. A strategy to overcome this weakness is to adopt fuel enrichment, i.e. adding a small amount of other component to the fuel to enhance the flame stability, while maintaining the advantages of lean combustion. Previous studies [1-4] have shown that hydrogen enrichment can significantly extend the lean flammability limit and reduce NO formation in
counterflow CH4/air premixed flames by allowing a combustor to operate under leaner condition.
One way to obtain hydrogen is the reforming of hydrocarbon fuels. However, the reformate gas contains not only hydrogen, but also carbon monoxide and some minor components. Instead of hydrogen, if the reformate gas of a hydrocarbon fuel can be directly used as the enrichment component, the fuel enrichment combustion technology will be more practical and economical.
Therefore it is of practical interest to understand the combustion and pollutant formation characteristics of reformate gas enriched lean premixed flames.
The purpose of this paper is to investigate the extinction and NOX formation characteristics
in reformate gas enriched lean CH4/air premixed flames by numerical simulation. The reason to
choose methane is that the chemical kinetics of its combustion are less uncertain than those of other hydrocarbon fuels. The reformate gases were assumed to be the products of partial
oxidation of methane. The effect of reformate gas addition on the flammability limit and NOX
formation will be discussed.
2. Numerical Model
The flame configuration studied is an axisymmetric laminar counterflow premixed flame, stabilized near the stagnation plane of two opposed-jets.
The simulations assumed the stagnation point flow approximation. The governing equations can be found elsewhere [5]. The potential boundary conditions were used. Upwind and center difference schemes were respectively employed for the convective and diffusion terms in all the governing equations. Adaptive refinement of meshes was done. The arc-length continuation method [6] was adopted to obtain the extinction limits. The pressure and the fresh mixture temperature were 1 atm and 300 K, respectively.
A previous investigation [7], using an improved radiation model, in which the radiation reabsorption was taken into account, showed that reabsorption has little influence on the
extinction limits and temperatures of lean counterflow CH4/air premixed flames. To simplify the
calculation, an optically thin radiation model [1] was used in this paper.
The chemical reaction mechanism used is GRI-Mech 3.0 [8]. The thermal and transport properties were obtained by using the database of GRI-Mech 3.0 and the algorithms given in [9,10].
3. Results and Discussion
Except in the discussion of extinction limits, a stretch rate of 30 1/s was used for all the flames. This is an arbitrary choice.
The fuel used is methane. Two reformate gases were examined. One is the product of partial oxidation of methane by oxygen, and the other is the product of partial oxidation of methane by air. The two reformate gases differ in the content of nitrogen. The former (reformate gas 1) does not contain nitrogen, while the latter (reformate gas 2) does. The partial oxidation was assumed
to follow the reaction: 2CH4 + O2 + γN2 = 2CO + 4H2 + γN2, where γ equals 0 for the first case
and 3.76 for the second. In the following discussion, αrg is the volume fraction of reformate gas
in the fuel mixture, i.e.αrg =Vrg/(Vrg+VCH4), where Vrg and VCH4 are the volume flow rates of
reformate gas and methane, respectively. The variable φ represents the equivalence ratio. For the
sake of comparison, the performances of pure hydrogen enrichment will also be presented. Since the objective of this paper is to study the effect of fuel enrichment in lean premixed flames, the maximum equivalence ratio investigated was limited to 0.9.
3.1. Extinction Limits
A counterflow premixed flame can be extinguished by an extra high or low stretch rate [1]. At an extra high stretch rate, a flame is extinguished due to the shortened residence time. This extinction limit is named as stretch extinction limit. Figure 1 shows the effect of fuel enrichment on the stretch extinction limit for the flames of equivalence ratio 0.6. For comparison, the effect of hydrogen enrichment is also shown. The flames of other equivalence ratios have the similar phenomenon. It is demonstrated that similar to the addition of hydrogen [1], the addition of a reformate gas monotonically increases the stretch extinction limit. However, the effect of a reformate gas addition is not as effective as that of the pure hydrogen addition in increasing the stretch extinction limit. This is because hydrogen is one of the main components of a reformate gas, but a reformate gas also contains carbon monoxide. The addition of carbon monoxide
enhances the combustion intensity of a CH4/air premixed flame due to the reaction CO + OH = H
+ CO2. However, the effect of its addition is not as effective as that of the hydrogen addition. In
addition to hydrogen and carbon monoxide, reformate gas 2 contains nitrogen that is basically an inert species for primary combustion reactions. These lead to that the addition of either hydrogen
or a reformate gas increases the stretch extinction limit of a CH4/air premixed flame. The effect
of hydrogen addition is most significant, while that of reformate gas 2 addition is least.
At a lower equivalence ratio, a counterflow premixed flame also can be extinguished because of radiation heat loss. This extinction limit is called radiation extinction limit. Figure 2 displays the variation of radiation extinction limit with the addition of a reformate gas or pure hydrogen for the flame of equivalence ratio 0.43. Similarly, it is noted that the addition of a reformate gas or pure hydrogen causes a monotonous decrease of radiation extinction limit for
CH4/air premixed flames.
φ = 0.6
Fraction of reformate gas, αrg
0.0 0.2 0.4 0.6 0.8 1.0 St ret c h exti nct ion lim it , 1 /s 100 1000 10000 reformate gas 1 reformate gas 2 H2
Fig. 1 Effect of fuel enrichment on stretch extinction limit.
φ = 0.43
Fraction of reformate gas, αrg
0.0 0.2 0.4 0.6 0.8 1.0 Radiation ex tinc ti on limit, 1/s 0.0 0.5 1.0 1.5 2.0 2.5 Reformate gas 1 Reformate gas 2 H2
Fig. 2 Effect of fuel enrichment on radiation extinction limit.
If all the stretch and radiation extinction limits and the corresponding equivalence ratios are plotted in a stretch rate – equivalence ratio plane, a C-shaped curve is formed for a constant reformate gas enrichment fraction [1]. The upper branch of the curve is the stretch extinction limit branch, and the lower branch is the radiation extinction limit branch. The region bounded by the two branches is the flammable region of counterflow lean premixed flames. At a critical equivalence ratio, the two branches merge at one point. This critical equivalence ratio is the flammability limit of counterflow lean premixed flame. As an example, the C-shaped curves for
pure CH4/air flame and those flames with 40% reformate gas addition are shown in Fig. 3. It is
limits lead to the wider flammable region and lower lean flammability limit when a reformate gas or hydrogen is added, with the addition of hydrogen being most effective and that of
reformate gas 2 being least. The lean flammability limit of pure CH4/air premixed flame is 0.420,
while this value is extended to 0.335, 0.353 and 0.380, respectively, when 40% hydrogen, reformate gas 1 or 2 is added.
Equivalence ratio 0.3 0.4 0.5 0.6 0.7 0.8 0.9 E x tinc tion limit, 1/s 0.1 1 10 100 1000 10000 Pure CH4 40% H2 40% Reformate gas 1 40% Reformate gas 2 φ = 0.65 φ = 0.68
Fig. 3 C-shaped curves of flames without and with 40% reformate gas enrichment.
Therefore we can conclude that fuel enrichment by hydrogen or a reformate gas can improve the combustion stability and extend the lean flammability limit, although the effect of different reformate gas addition varies. This is a significant advantage of fuel enrichment technology. It allows a combustor to operate under leaner conditions, and thus reduce the emission of green house gases. For example, the stretch extinction limits of the 40% reformate gas 1 enriched flame at the equivalence ratio of 0.65 and the 40% reformate gas 2 enriched flame at the
equivalence ratio of 0.68 are the same as that of the pure CH4/air premixed flame at the
equivalence ratio of 0.80, as shown in Fig.3. This indicates that if a combustor has to run at an equivalence ratio greater than or equal to 0.80 due to the high stretch rate (turbulent intensity)
using pure CH4/air premixed flame, it is allowed to run at an equivalence ratio of as low as 0.65
or 0.68, when 40% reformate gas 1 or 2 is added.
3.2. NOX Formation
Figures 4 and 5 display the distribution of NO mole fraction in pure CH4/air and 40%
reformate gas enriched flames at two typical equivalence ratios (0.65 and 0.9). These two equivalence ratios were selected, since they represent a near-stoichiometric flame and an extra-lean flame, respectively. It is observed that the addition of reformate gas 1 increases the formation of NO for both near-stoichiometric and extra-lean flames. However, the situation of reformate gas 2 addition differs. When 40% reformate gas 2 is added, the formation of NO in the extra-lean flame is enhanced, while that in the near-stoichiometric flame is reduced. Although figures 4 and 5 only show the result for 40% reformate gas addition, the simulation indicated that the result is qualitatively similar when the fraction of added reformate gas changes, i.e. the addition of reformate gas 1 monotonically increases the formation of NO for all flames, while the addition of reformate gas 2 increases the formation in extra-lean flames and reduces that in near-stoichiometric flames.
φ = 0.65
Distance from stagnation plane, cm
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 M o le f raction of NO 0.0 1.0e-6 2.0e-6 3.0e-6 4.0e-6 5.0e-6 Pure CH4 flame 40% reformate gas 1 40% reformate gas 2
Fig. 4 NO mole fraction in flames with equivalence ratios of 0.65.
φ = 0.9
Distance from stagnation plane, cm
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Mol e f ract ion of NO 0 20x10-6 40x10-6 60x10-6 80x10-6 100x10-6 Pure CH4 flame 40% reformate gas 1 40% reformate gas 2
Fig. 5 NO mole fraction in flames with equivalence ratio of 0.9.
The reason for above result can be discussed now. Since reformate gas 1 does not contain
nitrogen, its addition increases flame temperature for all the CH4/air flames. Besides, the addition
of reformate gas 1 to a flame increases the concentrations of some radicals, such as H and O. The higher flame temperature and concentrations of radicals H and O cause the formation rates of
NO by the thermal, N2O and NNH intermediate routes to increase. Although the formation rate
of NO by the prompt route may decrease due to the reduction in the concentration of radical CH, this effect is much smaller than the rate increase of the other three routes. Therefore the addition of reformate gas 1 enhances the formation of NO at a given equivalence ratio.
As for reformate gas 2, its addition decreases the peak temperature in the flame of equivalence ratio 0.9, while increases that in the flame of equivalence ratio 0.65. The decrease in the temperature of a near stoichiometric flame is due to the lower heat release of the reformate gas 2, since part of the energy is released during the reforming process. At a lower equivalence ratio, such as φ = 0.65, the air requirement of the reformate gas 2 combustion is less than that of pure methane combustion and therefore less excess air exists when the reformate gas 2 is added, which results in higher temperatures for reformate gas enriched extra-lean premixed flames. The variation of the effect of reformate gas 2 addition on flame temperature in near-stoichiometric and extra-lean flames cause the difference in the effect of reformate gas 2 addition on NO formation. When reformate gas 2 is added to an extra-lean flame, the increased flame temperature causes the higher NO formation rate, while the addition of reformate gas 2 reduces the formation of NO in a near-stoichiometric flame because of the decreased flame temperature.
Equivalence ratio 0.5 0.6 0.7 0.8 0.9 N O e m is s ion i nde x , g -NO /J 0.0 5.0e-9 1.0e-8 1.5e-8 2.0e-8 2.5e-8 3.0e-8 Pure CH4 40% reformate gas 1 40% reformate gas 2 0.60 0.65 0.70 0.75 0.80 0.0 2.0e-9 4.0e-9 6.0e-9 8.0e-9 1.0e-8 (0.68,3.0E-09) (0.65,2.6E-09) (0.8,8.8E-09)
Figure 6 shows NO emission indices for flames with equivalence ratio varying from 0.5 to 0.9 and 40% reformate gas addition. NO emission index is defined as the ratio of total formed NO to total heat release in a flame. As indicated before, the addition of a reformate gas allows a
combustor to operate at a lower equivalence ratio than that a pure CH4/air flame has to run at. It
is seen that the decrease in equivalence ratio significantly reduces the formation of NO. Since the addition of 40% reformate gas 1 or 2 allows a combustor’s operation equivalence ratio to be lowered from 0.8 to 0.65 or 0.68, as shown in Fig. 3, NO emission index can be reduced from
8.8×10-9 to 2.6×10-9 or 3.0×10-9
g-NO/J-heat. This is the biggest advantage of fuel enrichment technology. Equivalence ratio 0.5 0.6 0.7 0.8 0.9 NO 2 em issi on index , g-N O2 /J 0.0 2.0e-11 4.0e-11 6.0e-11 8.0e-11 1.0e-10 Pure CH4 flame 40% reformate gas 1 40% reformate gas 2
Fig. 7 Emission index of NO2.
Equivalence ratio 0.5 0.6 0.7 0.8 0.9 N2 O em is si on i ndex , g-N 2 O/ J 0.0 1.0e-10 2.0e-10 3.0e-10 4.0e-10 5.0e-10 6.0e-10 7.0e-10 8.0e-10 Pure CH4 40% reformate gas 1 40% reformate gas 2
Fig. 8 Emission index of N2O.
Figure 7 shows the NO2 emission indices of flames with equivalence ratio varying from 0.5
to 0.9 and 40% reformate gas addition. It is noted that the addition of a reformate gas suppresses
the formation of NO2 at all equivalence ratios. This effect becomes more significant with the
decrease of equivalence ratio. The addition of reformate gas 1 is more effective than that of reformate gas 2 at lower equivalence ratios, but less effective at near-unity equivalence ratios. A
sensitivity analysis indicates that the main NO2 formation and destruction reactions are HO2 +
NO = NO2 + OH and NO2 + H = NO + OH, respectively. The activation energy of the former is
negative, and the latter is positive. With the addition of a reformate gas, the formation reaction rate changes slowly due to the simultaneous increase of NO concentration and flame
temperature. However, the destruction reaction rate of NO2 quickly increases when a reformate
gas is added, because of the enhanced production of radical H. Owing to the more hydrogen content, the addition of reformate gas 1 is more effective than that of reformate gas 2 at lower equivalence ratios. When equivalence ratio is close to unity, more NO is formed due to the addition of reformate gas 1, while less NO is formed when reformate gas 2 is added. Therefore,
more NO2 is formed when reformate gas 1 is added at a near-unity equivalence ratio, owing to
the reaction HO2 + NO = NO2 + OH.
The N2O emission indices of flames with equivalence ratio varying from 0.5 to 0.9 and 40%
reformate gas addition are shown in Fig. 8. It is noted that except at an extra low equivalence
ratio, the addition of a reformate gas reduces the formation of N2O at a constant equivalence
ratio. The most significant formation and destruction reactions of N2O are N2O (+M) = N2 + O
(+M) and N2O + H = N2 + OH, respectively. At a constant equivalence ratio, the addition of a
reformate gas causes the concentration of radical H to increase quickly, which leads to the higher
The formation of N2O increases with the decrease in equivalence ratio for a given fraction of
reformate gas. This is an undesirable side effect of the lean premixed combustion technology. However, this side effect is moderated by the addition of a reformate gas. This is another advantage of fuel enrichment combustion technology.
4. Conclusions
The extinction and NOX formation characteristics in reformate gas enriched CH4/air lean
premixed flames have been numerically investigated by detailed chemistry and complex thermal and transport properties. The effects of two reformate gases additions were studied.
The results indicate that although a reformate gas from partial oxidation contains other components in addition to hydrogen, its addition can still significantly enlarge the flammable
region and lower the flammability limit of lean CH4/air premixed flame. As a result, the
reformate gas enrichment technology allows a combustor to operate under leaner condition, and therefore significantly reduces the formation of NO. Besides, the addition of a reformate gas can
moderate the side effect of the relatively higher NO2 and N2O formation under leaner condition.
The effect of reformate gas 1 is more significant than that of reformate gas 2 due to its more hydrogen content.
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