Fuel 95 (2012) 206–213

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Fuel

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Investigation of laminar flame speeds of typical syngas using laser basedBunsen method and kinetic simulation

Yong He a, Zhihua Wang a,⇑, Li Yang a, Ronald Whiddon b, Zhongshan Li b, Junhu Zhou a, Kefa Cen a

a State Key Laboratory of Clean Energy Utilization, Zhejiang University, 310027 Hangzhou, Chinab Division of Combustion Physics, Lund University, P.O. Box 118, S-22100 Lund, Sweden

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 June 2011Received in revised form 27 September2011Accepted 29 September 2011Available online 19 October 2011

Keywords:SyngasLaminar flame speedOH PLIFBunsen flameKinetic simulation

0016-2361/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.fuel.2011.09.056

⇑ Corresponding author. Tel.: +86 571 87953162; faE-mail address: wangzh@zju.edu.cn (Z. Wang).

Synthetic gas (syngas) fuels are promising energy sources in the future. In the current work, laminarflame speeds of typical syngas with different H2 contents were studied using both experimental measure-ments and kinetic simulations. Measurements were carried out using the Bunsen method with the flamearea derived from the OH planar laser-induced fluorescence (OH-PLIF) images; while kinetic simulationswere made using CHEMKIN with two mechanisms: GRI-Mech 3.0 and USC-Mech II. The OH-PLIF basedBunsen method was validated with previous results. Both the experimental and simulated results indi-cated that the flame speed of syngas increased with H2 concentration, which, based on the simulation,is attributed to the rapid production of highly reactive radicals and the acceleration of chain-branchingreactions by these radicals. In general, predictions with both mechanisms agreed well with measure-ments, especially for fuel-lean conditions; simulations with USC-Mech II gave better agreement withexperimental results at U = 0.8 and 0.9 (discrepancy <5%).

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

With more concern about the energy and environment, technol-ogies such as Integrated Gasification Combined Cycle (IGCC) be-come increasingly attractive for the utilization of solid fuels,especially coal. In IGCC systems, syngas is produced from the gas-ification process and then burnt in gas turbines typically in fuellean conditions. Due to the high energy conversion ratio and loweremissions, syngas seems to be a promising energy source in the fu-ture. Syngas fuels consist mainly of hydrogen (H2) and carbonmonoxide (CO), and may also contain methane (CH4), nitrogen(N2), carbon dioxide (CO2), water vapor (H2O) and other hydrocar-bons [1]. Table 1 presents the mole fraction of typical compositionof syngas fuels derived from coal gasification in air or air/steamconditions [2]. It can be seen that there could be a considerablevariation in the composition of syngas dependent on various fuelsources and gasification conditions. This variability of the compo-sition brings many problems to the use of syngas. For example, itis difficult to design the burners and combustion chambers withboth high efficiency and low emission to suit these changes. To uti-lize these fuels efficiently and cleanly, it is important to understandthe effects of composition changes on their fundamentalcombustion characteristics.

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x: +86 571 87951616.

Laminar flame speed is a fundamental characteristic to the com-bustion process. Many important flame properties such as stability,extinction and flashback are all related to the flame speed. In theuse of syngas fuels, due to their low calorific values, flame stabilityis one of the major problems [2]. In previous studies, severalresearchers have investigated the laminar flame speed of binaryH2/CO mixtures [3–7]. In recent years, there has been more andmore interest in the actual syngas compositions. The laminar flamespeed of typical biomass derived syngas was studied by Monteiroet al. [8], Liu et al. [9] and Ouimette and Seers [10] through eitherexperiments or simulations. Dam et al. measured the laminarflame speed of typical coal derived syngas at a series of equivalenceratios [11]. However, systematic investigation of the effects ofcomposition changes on the laminar flame speed of typical syngashas been seldom reported.

Several methods have been used in the past to measure the lam-inar flame speed, which have been reviewed by Dam et al. [11] andDas et al. [12]. The Bunsen flame method is a relatively easy tech-nique, in which laminar flame speeds are calculated from dividingthe volumetric flow rate of gas mixtures by the reaction zone area.In previous studies, the reaction zone area was mostly decided fromthe image of flame spontaneous emission, such as the measure-ments by Natarajan et al. [4] and Dong et al. [7]. But it may be diffi-cult to precisely determine the edge of the reaction zone because thesignal of spontaneous emission is weak and the flame round tipcould cause systematic errors, which might be a problem in deter-mining the flame speed [4]. However, laser-based diagnostic

Table 1Typical composition of coal derived syngas [2].

Type of coal Gasifier Gasifying medium Composition of syngas fuels (in mole fraction, %) Lower heating value

H2 CO CH4 N2 CO2 MJ/Nm3 kcal/Nm3

Brown coal Lurgi Air/steam 25 16 5 40 14 6.28 1500.29Bituminous Lurgi Air/steam 24.8 17.2 4.1 42.7 11 6.13 1464.46Lignite Winkler Air/steam 12 22 1 55 10 4.13 986.66Coke Wellman-Galusha Air/steam 15 29 3 50 3 6.08 1452.51Subbituminous Wyoming Air 17.3 14.7 3.3 51.6 12.4 5.11 1220.78

Y. He et al. / Fuel 95 (2012) 206–213 207

techniques, such as planar laser-induced fluorescence (PLIF), whichhas been widely used in combustion diagnostics [13–16], may pro-vide a good solution to these problems due to the short exposuretime (such as 50 ns) and high signal-to-noise ratio.

In this paper, the OH-PLIF based Bunsen method was employedto study the effects of H2 content on the laminar flame speeds oftypical syngas, which simulated the actual composition of typicalbituminous coal derived syngas, at both fuel-lean and stoichiome-tric conditions. In addition, kinetic simulations were carried outusing CHEMKIN with two mechanisms, GRI-Mech 3.0 and USC-Mech II, to explore the flame speed enhancement mechanism withvarious H2 contents.

2. OH-PLIF based Bunsen method

The laminar flame speed is defined as the velocity that a planarflame front travels toward the unburned gas in a direction normalto the flame surface [17]. Among the methods of flame speed mea-surements, the Bunsen flame approach has been widely used forvarious gas mixtures [4,18,19]. Although the Bunsen flame has aconical shape instead of a one-dimensional surface, according tothe discussion by Natarajan et al. [4], the flame speed calculatedby the reaction zone area can be very close to the one-dimensionalreaction zone flame speed. Based on this assumption, the one-dimensional reaction zone flame speed can be shown as:

S0b � Sb ¼

_mqbAb

; ð1Þ

where the subscript b denotes the reaction zone, the superscript 0denotes the one-dimensional flame, _m is the mass flow rate, q is

Fig. 1. Flame reaction zone determination: (a) the OH-PLIF image of Bunsen flame, (b)detection program, (d) polynomial fit to the reaction zone.

the density, and A is the flame area. For the one-dimensional flame,the mass balance can be expressed as:

qbS0b ¼ quS0

u; ð2Þ

where the subscript u means the unburned zone. From Eqs. (1) and(2), the one-dimensional unburned zone flame speed, namely thelaminar flame speed can be calculated as:

S0u ¼

qbS0b

qu¼

_mquAb

¼_Q

Ab; ð3Þ

where _Q is the volumetric flow rate of the unburned gas mixture.In this study, the reaction zone area was calculated from the

OH-PLIF images of Bunsen flames. OH radicals are one of the mostimportant intermediate reactants during the combustion of fuels.The distribution of OH can be used to determine the reaction zone[20]. Images of OH also represent the boundary of hot productswith cold reactants, and the flame front can be extracted fromOH images through processing the signal gradient [21]. An exam-ple of the measured OH-PLIF image is shown in Fig. 1a. The flamehas an axial symmetry about the burner center, and the OH-PLIFimage was cut in half along the burner center (see Fig. 1b). To cal-culate the reaction zone area, an edge detection program in MAT-LAB was employed to find the reaction zone by locating themaximum derivative of the intensity of OH-PLIF signals along theradius of the flame (see Fig. 1c). Then a polynomial line was usedto fit the points in the reaction zone (see Fig. 1d). After that, thereaction zone area was calculated through a spatial integral basedon the axially symmetric assumption. Finally, the laminar flamespeed was calculated from Eq. (3).

the half of the OH-PLIF image, (c) points in the reaction zone decided by the edge

Fig. 2. Schematic of the experimental setup (MFC = mass flow controller). The mixing was achieved by the long transport time in the tubes.

Table 2Composition of syngas for each H2 content condition.

No. Composition of syngas fuels (in mole fraction, %) Lower heating value

H2 CO CH4 N2 CO2 MJ/Nm3 kcal/Nm3

1 0 37.4 8.9 42.7 11 7.91 1889.212 5 33.4 7.9 42.7 11 7.58 1811.923 10 29.3 7.0 42.7 11 7.28 1740.164 15 25.3 6.0 42.7 11 6.96 1662.865 20 21.2 5.1 42.7 11 6.66 1591.106 25 17.2 4.1 42.7 11 6.34 1513.80

208 Y. He et al. / Fuel 95 (2012) 206–213

3. Experimental measurement

3.1. Experimental setup

Fig. 2 shows a schematic of the experimental setup, which con-sists of a premixed Bunsen burner, a gas supply system and a PLIFsystem. The Bunsen burner was made of a straight cylindricalstainless steel tube. Three tubes were used in different conditions,with the inner diameter (D) 7.8 mm, 11.6 mm, and 15.2 mm,respectively. For each experimental condition, a proper diameterwas chosen and the length of the tube was designed to be at least50D [4] to ensure a laminar and stable flame. A coaxial coolingwater tube was also fixed outside the Bunsen tube to keep the ini-tial temperature of gas mixtures constant. All the measurementswere conducted at the pressure of 1 atm and the initial tempera-ture of 298 K.

The syngas studied here contains H2, CO, CH4, N2, and CO2,deliberately composed for simulating the actual composition oftypical bituminous coal derived syngas. To define the equivalenceratio in experiments, syngas and air are assumed to combine asfollows:

FðXFH2H2 þ XFCO COþ XFCH4

CH4 þ XFN2N2 þ XFCO2

CO2Þ

þ AðXAO2O2 þ XAN2

N2Þ; ð4Þ

where F and A are the number of moles of fuel and air respectively,and XF,i and XA,i are the mole fractions of the different species in fueland air respectively (e.g. XAO2

¼ 0:21). Assuming that the productsof combustion are CO2 and H2O, the equivalence ratio is given by:

/ ¼ F=AðF=AÞstoich

¼ FA

XFH2þ XFCO þ 4XFCH4

2XAO2

ð5Þ

To study the effects of H2 concentration on laminar flame speeds,the H2 content in syngas changed from 0% to 25% in volume. Consid-ering in the actual air gasification process, both CO and CH4 wereconverted from the carbon in coal; by assuming the same conver-sion ratio, the ratio of CO/CH4 was then kept the same in syngascompositions. To eliminate the effects of dilution by N2 or CO2, thecontents of these two compositions were kept constant. The de-tailed mole fraction of syngas composition for each experimentalcondition is shown in Table 2. In experiments, each gas was accu-rately controlled by a mass flow controller with the accuracy of±(0.8% of reading + 0.2% of full scale). All the compositions were

mixed thoroughly in the tubes before burned at the exit of the Bun-sen burner. To measure the OH distribution in the Bunsen flames, aPLIF system was employed. The laser system contains a QuantelYG981-E Nd:YAG laser pumped Quantel TDL-90 dye laser withRhodamine 590, providing a 567.106 nm laser beam. The dye laserwas then doubled to produce an UV laser at 283.553 nm, whichwas used to excite the Q1(8) transition of the A2P+ X2P (1, 0)band of OH radicals. The laser pulse energy used in experimentswas around 7 mJ per pulse. A cylindrical lens with the focal lengthf = �100 mm and a spherical lens with f = 500 mm were used tomake a laser sheet with the height of 37 mm. The fluorescence sig-nal was captured with a PI MAX2 ICCD camera (512 � 512 pixels,produced by Princeton Instruments), to record the OH distributionin the flame. The dimensions of the region imaged by the camerawere 40 mm � 40 mm, which determined the resolution of theCCD system was about 13 pixels/mm. With a short exposure time(50 ns) and an interference filter (band pass 310 ± 5 nm), the scat-tering laser light and flame spontaneous emission were blockedeffectively.

Fig. 3 shows some OH-PLIF images of Bunsen flames for typicalsyngas with different H2 content at the equivalence ratio U = 0.9.From these OH-PLIF images, the reaction zone and the flame areacan be determined.

3.2. Uncertainty estimate

The uncertainty of the measured flame speed was estimatedfrom two main sources: the uncertainty of the total flow rate ofthe unburned gas mixture ðU _Q Þ, and the uncertainty of the calcu-lated flame area ðUAb

Þ. U _Q came from the mass flow controlleruncertainty, which was estimated to be �2%. And UAb

derived from

Fig. 3. OH-PLIF images of Bunsen flames for syngas/air mixture at U = 0.9, with the diameter of the tube 11.6 mm: (a) H2 = 0%, Q = 4.436 SL/min, (b) H2 = 5%, Q = 5.704 SL/min,(c) H2 = 10%, Q = 6.972 SL/min, (d) H2 = 15%, Q = 8.873 SL/min.

0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.45

10

15

20

25

30

35

40

45

Flam

e sp

eed

(cm

/s)

Equivalence ratio, φ

Present Data Lowry, 2011 Tahtouh, 2009 Chen, 2007 Bosschaart, 2004 Rozenchan, 2002 Gu, 2000 Vagelopoulos, 1998 VanMaaren, 1994 GRI-Mech 3.0 USC-Mech II

Fig. 4. Comparison of laminar flame speeds for CH4/air mixtures.

Y. He et al. / Fuel 95 (2012) 206–213 209

the camera resolution, which was estimated to be �4%. Then theoverall uncertainty was estimated from

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiU2

Abþ U2

_Q

q: �5% for all

the measured flame speeds.The uncertainties in equivalence ratio and H2 content were also

estimated respectively for each experimental condition, whichwere calculated from the mass flow controller uncertainty. Theuncertainty in equivalence ratio was found to be within 0.03, andin H2 content was found to be within 0.8 for all the conditions.

4. Laminar flame speed simulation

Kinetic simulations were carried out with the PREMIX moduleof CHEMKIN 3.7 to compare with measurements. The laminarflame speed was calculated using the one dimensional freely prop-agating laminar flame model. For simulation, the initial tempera-ture of gas mixtures was fixed at 298 K, and the pressure was at1 atm. The computations used multispecies and thermal diffusionalong with GRAD 0.05-0.1 and CURV 0.1-0.2 leading to grids con-sisting of about 180 points on average, which showed fine enoughto make the simulation grid-independent. The calculated profilesof H and OH radicals as well as the rates of their productions werediscussed for a better understanding of the mechanism of the lam-inar flame speed enhancement with increased H2 content.

For the flame speed modeling, two mechanisms were employed:GRI-Mech 3.0 [22] and USC-Mech II [23]. The GRI-Mech 3.0, whichwas originally developed for CH4 combustion, consists of 53 speciesand 325 reactions. This mechanism has also been employed for theCH4/H2/air flame simulation [24] and for the H2/CO/air flame simu-lation [4]. The GRI-Mech 3.0 has been the most popular model forsimulating the CH4 flame, which stimulated interest to explore thecapability to simulate the flame of actual syngas compositions.The second mechanism USC-Mech II consists of 111 species and784 reactions. This mechanism was especially developed forH2/CO/C1–C4 hydrocarbon combustion, and has been developedthrough a series of studies over the last decades [25–28]. Comparedwith GRI-Mech 3.0, USC-Mech II specially includes the high-temper-ature chemistry of H2/CO/C1–C4 compounds and covers the elemen-tary reactions of C5 and C6 compounds, which may be more preciseto describe the combustion process of actual syngas compositions.

5. Results and discussion

The primary objective of this work was to study the effects of H2

content on the laminar flame speed of typical syngas. In the

beginning, laminar flame speeds of CH4 and 5%H2–95%CO mixtureswere measured and compared with previous results to make a val-idation for the present method. Then, experimental measurementsand kinetic simulations were carried out for typical syngas at bothfuel-lean and stoichiometric conditions with H2 content rangingfrom 0% to 25%. The calculated radical profiles were also discussedto help understand the flame speed enhancement mechanism. Inthe end, USC-Mech II was employed to calculate the laminar flamespeeds over a wide range of stoichiometry for different H2 content.

5.1. Experimental validation

In order to validate the Bunsen flame method based on the OH-PLIF images, experiments were conducted at atmospheric pressurefor two fuels: CH4 and 5%H2–95%CO, respectively, to compare withprevious published results.

Fig. 4 shows the measured laminar flame speeds of CH4 at dif-ferent equivalence ratios. Values from previous works [29–36]and kinetic simulations with GRI-Mech 3.0 and USC-Mech II arealso given in this figure for comparison. In these experiments,the burner diameter was chosen to be 15.2 mm for the equivalenceratio 0.7, and 11.6 mm for the equivalence ratios from 0.75 to 1.0.As seen in Fig. 4, measurements with the present method were in

24

28

32

36

40

44

Φ=0.9

Φ=1.0

Flam

e sp

eed

(cm

/s)

GRI-Mech 3.0 USC-Mech II

Φ=0.8

210 Y. He et al. / Fuel 95 (2012) 206–213

good agreement with both previous results and kineticsimulations.

Similar comparisons were carried out for 5%H2–95%CO fuelmixtures, which were previously measured with the Bunsen flameapproach based on the flame spontaneous emission [4] and thespherically expanding flame method [5,37]. The results are shownin Fig. 5. Kinetic simulations were conducted for a wide range ofequivalence ratios. For current experiments, the burner diameterwas chosen to be 15.2 mm for the equivalence ratios 0.7 and0.75, and 11.6 mm for the equivalence ratios from 0.8 to 1.0. Asseen in Fig. 5, the current measurements also agree well with pre-vious values and model results.

0 5 10 15 20 25

20

H2 content in syngas fuel (%)

Fig. 6. Laminar flame speeds of typical syngas versus H2 content at fuel-lean andstoichiometric conditions; measurements based on OH-PLIF images of Bunsenflames (symbols) and kinetic simulations with GRI-Mech 3.0 and USC-Mech II(lines).

5.2. Effects of H2 content

Fig. 6 shows the experimental results for the laminar flamespeed of typical syngas as a function of H2 content. Kinetic simula-tions with GRI-Mech 3.0 and USC-Mech II are also directly com-pared with experiments. Generally, flame speeds calculated withGRI-Mech 3.0 were slightly larger than both experimental resultsand simulation results with USC-Mech II. For the more interestingfuel-lean conditions, predictions with USC-Mech II showed a betteragreement with measurements, with differences within 5% overthe whole range of investigation at both U = 0.8 and 0.9. In thisstudy, original thermodynamic and kinetic data was used in GRI-Mech 3.0. Recent studies have reported that the heat of formationfor OH can be updated to a more accurate value 8.9 kcal/mol[38,39] instead of the default 9.4 kcal/mol from database of Gur-vich et al. [40]. But, updating the heat of formation for OH to thenew value will lead to the formation of larger amounts of OH,increasing the radical pool and thus the flame speed, which willfurther deteriorate the agreement seen in Fig. 6. Therefore, simula-tions were performed using USC-Mech II in the followingdiscussions.

From both experimental and modeling results, it is apparentthat the laminar flame speed increased significantly with H2 con-tent, which was in accordance with previous results reported inRefs. [4,7] although the composition of syngas was different fromeach other. Moreover, this increase was almost linear. The similarlylinear behavior was also observed by Yu et al. [41] and Tang et al.[42] when H2 was added to hydrocarbon–air mixtures. This can beexplained probably by the fact that the H2 content is relatively lowin the combined syngas–air mixtures. For example, even for 25% H2

at U = 1.0, the H2 mole fraction in the total reactant is only about10%. It should be noticed that the maximum flame speed of

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.010

20

30

40

50

60

70

80

Flam

e sp

eed

(cm

/s)

Equivalence ratio, φ

Present Data Natarajan , 2007 Sun, 2007 Hassan, 1997 GRI-Mech 3.0 USC-Mech II

0.6 0.7 0.8 0.9 1.0 1.110

20

30

40

50

Fig. 5. Comparison of laminar flame speeds for 5%H2–95%CO fuel mixtures.

H2-air mixtures occurred at U = 1.8 [43], in which case, the H2

mole fraction in the combined fuel–air mixtures is about 43%.Therefore, although the burning velocity of H2 is nearly an orderof magnitude faster than those for CH4 and CO, its effect is some-what suppressed because of the relatively low content, resultingin a linear behavior of the flame speed with increasing H2.

As other fuel–oxidizer mixtures, the reactivity of syngas–airmixture is driven by the generation and chain propagation of freelyactive radicals. A deep discussion about the syngas chemical kinet-ics has been made by Chaos et al. [44,45]. As discussed in these twoworks, radicals will be generated and propagated through muchfaster hydrogen-related reactions when hydrogen is involved inthe reactants. Therefore, the flame speed increasing with H2 con-tent should be attributed to the rapid production of the hydro-gen-related radicals. In order to further explain these results,temperature and species distribution along the flame height werecalculated through the one dimensional freely propagating laminarflame model in CHEMKIN. Considering the better agreement withexperiments, simulation results of USC-Mech II were chosen herefor further discussion. Fig. 7 shows the profiles of two importantradicals related to the chain-branching reactions: H and OH, aswell as the temperature profile for two H2 contents 0% and 25%at U = 0.8 and U = 1.0 respectively. Due to the decrease of heatingvalue of syngas with increased H2, as shown in Table 2, tempera-ture in post flame region for 25% H2 is a little lower compared with0% H2. However, the increase of the H2 molecule in reactants re-sulted in a much faster growth of highly reactive radicals H andOH in the low-temperature region (T < 1200 K). At the early stageof combustion, concentrations of H and OH radicals were evendoubled when H2 content changed from 0% to 25%. As discussedby Das et al. [12] and Natarajan et al. [46], the flame speedenhancement was dominated by radical related chain-branchingreactions over thermal effects. Compared with U = 0.8, the increaseof H and OH radicals at U = 1.0 was more significant with increas-ing H2 content. For example, when H2 content changed from 0% to25%, the peak concentration of H radicals increased by 34.96% atU = 1.0 and by 23.94% at U = 0.8; that of OH radicals increasedby 20.85% at U = 1.0 and by 14.24% at U = 0.8. The higher temper-ature at U = 1.0 versus U = 0.8 resulted in a larger growth of H andOH radicals with increased H2 content, and thus a more significantincrease of flame speeds as shown in Fig. 6.

Fig. 8 further gives the comparison for rate of production of Hradicals between 0% H2 and 25% H2 at U = 0.8. For each condition,

0.00 0.05 0.10 0.15 0.20 0.25 0.30

400

600

800

1000

1200

1400

1600

1800H2 content 0%

Temperature H concentration OH concentration

H2 content 25% Temperature H concentration OH concentration

Distance (cm)

Tem

pera

ture

(K)

0

1000

2000

3000

4000

5000

6000

Mol

e fra

ctio

n (p

pm)

T

OH

H

(a) Φ=0.8

0.00 0.05 0.10 0.15 0.20 0.25 0.30

400

600

800

1000

1200

1400

1600

1800

2000H2 content 0%

Temperature H concentration OH concentration

H2 content 25% Temperature H concentration OH concentration

Distance (cm)

Tem

pera

ture

(K)

T

H

OH

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Mol

e fra

ctio

n (p

pm)

(b) Φ=1.0

Fig. 7. Distributions of temperature, H and OH radicals with different H2 content.

0.00 0.05 0.10 0.15 0.20 0.25 0.30

-0.0025

-0.0020

-0.0015

-0.0010

-0.0005

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

Rat

e of

pro

duct

ion

H (m

ol/c

m3 -s

)

Distance (cm)

Total R1 R2 R3 R12 R16

R31 R82 R89 R123 R39

(a) 0% H2

0.00 0.05 0.10 0.15 0.20 0.25 0.30

-0.0025

-0.0020

-0.0015

-0.0010

-0.0005

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

Rat

e of

pro

duct

ion

H (m

ol/c

m3 -s

)

Distance (cm)

Total R1 R2 R3 R12 R16

R31 R82 R89 R123 R88

(b) 25% H2

Fig. 8. Rate of production of H radicals for different H2 content at U = 0.8.

Table 3Reaction equations plotted in Figs. 8 and 9.

Legendlabel

Reaction equation

R1 H + O2 = O + OHR2 O + H2 = H + OHR3 OH + H2 = H + H2OR4 OH + OH = O + H2OR9 H + OH + M = H2O + MR12 H + O2(+M) = HO2(+M)R16 HO2 + H = OH + OHR31 CO + OH = CO2 + HR39 HCO + M = CO + H + MR82 CH2O + H = HCO + H2

R84 CH2O + OH = HCO + H2OR88 CH3 + H(+M) = CH4(+M)

Y. He et al. / Fuel 95 (2012) 206–213 211

the top 10 most sensitive elementary reactions are shown in thisfigure. The detailed reaction equations related to the legend labelare shown in Table 3. As can be seen, all reactions shifted slightlyupstream as H2 content increased. For the 0% H2 condition, the ma-jor contribution to production of H radicals was the reaction (R31)CO + OH = CO2 + H. As H2 content increased to 25%, the reaction(R3) OH + H2 = H + H2O became the dominant and the peak pro-duction rate was much larger, which resulted in the peak level oftotal production rate doubled compared with the 0% H2 condition.For consumption of H radicals, the dominant reaction was (R1)H + O2 = O + OH at both conditions, but the peak consumption rateincreased from 1.7E-3 mol/(cm3-s) to 2.4E-3 mol/(cm3-s) as H2

content changed from 0% to 25%. Similarly, the production ratesof OH radicals are shown in Fig. 9. The peak levels of OH productionand consumption rates were also found to increase with H2 con-tent. These observations indicated that the increase of H2 contentresulted in a higher production rate of H and OH radicals, andhence accelerated the whole chain-branching combustion process.

R89 CH3 + O = CH2O + HR92 CH3 + OH = CH�2 + H2OR123 CH4 + H = CH3 + H2

R124 CH4 + O = CH3 + OHR125 CH4 + OH = CH3 + H2O

5.3. Effects of stoichiometry for different H2 content

In order to compare results from the present study to values forother fuels, whether oxygenated or not, the oxygen equivalenceratio, which has been proposed to represent the stoichiometry ofmixtures being or containing oxygenated species in recent years[47,48], were employed here to show the stoichiometry effectsfor typical syngas. For oxygenated hydrocarbon fuels which con-tain C, H and O atoms, the oxygen equivalence ratio is defined as

the amount of oxygen atoms required to convert all C and H atomsin the fuel/oxidizer mixture to CO2 and H2O divided by the amountof oxygen atoms present in the mixture. For the syngas/air mix-tures as shown in Eq. (4), the oxygen equivalence ratio is given by:

0.00 0.05 0.10 0.15 0.20 0.25 0.30

-0.0030

-0.0025

-0.0020

-0.0015

-0.0010

-0.0005

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

Rat

e of

pro

duct

ion

OH

(mol

/cm

3 -s)

Distance (cm)

Total R1 R2 R3 R4 R16

R31 R84 R124 R125 R92

(a) 0% H2

0.00 0.05 0.10 0.15 0.20 0.25 0.30

-0.0030

-0.0025

-0.0020

-0.0015

-0.0010

-0.0005

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

Rat

e of

pro

duct

ion

OH

(mol

/cm

3 -s)

Distance (cm)

Total R1 R2 R3 R4 R16

R31 R84 R124 R125 R9

(b) 25% H2

Fig. 9. Rate of production of OH radicals for different H2 content at U = 0.8.

0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4

5

10

15

20

25

30

35

40

45

Flam

e sp

eed

(cm

/s)

Oxygen equivalence ratio, φΩ

Exp. 25% H2 Exp. 15% H2 Exp. 0% H2 Sim. 25% H2 Sim. 15% H2 Sim. 0% H2

Fig. 10. Laminar flame speeds versus oxygen equivalence ratios for different H2

content. The dash line indicates the oxygen equivalence ratio at maximum flamespeed.

212 Y. He et al. / Fuel 95 (2012) 206–213

/X ¼FðXFH2

þ 2XFCO þ 4XFCH4Þ

FXFCO þ 2AXAO2

ð6Þ

The calculated laminar flame speeds of typical syngas with USC-Mech II as a function of oxygen equivalence ratio are shown inFig. 10. The H2 contents were chosen to be 0%, 15%, and 25% respec-tively. Experimental results in terms of oxygen equivalence ratio atboth fuel-lean and stoichiometric conditions are also shown in thisfigure. As can been seen, for fuel-rich conditions, the laminar flamespeed also increased with H2 content, which agrees with the re-sults from Monteiro et al. [8] and Dam et al. [11].

For each H2 content, the laminar flame speed first increasedwith oxygen equivalence ratio until the peak flame speed and thenstarted declining with further increase of oxygen equivalenceratios. As shown in Fig. 10, the oxygen equivalence ratio relatedto the maximum flame speed is around 1.15. According to previousstudies, the peak flame speed of pure methane occurs at UX = 1.05[10]; that of pure hydrogen occurs at UX = 1.8 [43]; for carbonmonoxide, it occurs at UX = 1.23 [7]. Comparing the peak pointof coal syngas with that of pure CH4, H2 and CO, it seems thatCH4 and CO played a significant role on the peak flame speed posi-tion. Based on the pure simulation results, the oxygen equivalenceratio related to the peak flame speed seemed to increase slightlytowards the peak position of pure H2 when H2 content increasedfrom 0% to 25%. It must be noticed that the calculated increasetrend was not obvious enough yet, even within the margin of error

for the simulations, which indicated that more experimental workwas needed in the future.

6. Conclusion

In this paper, effects of H2 content on the laminar flame speedsof typical syngas were studied based on OH-PLIF images of Bunsenflames and kinetic simulations with GRI-Mech 3.0 and USC-MechII. The main results are summarized as follows:

1). The OH-PLIF based Bunsen method employed in this studyhas been validated to deliver reasonably accurate measure-ments of the laminar flame speed of different gas mixtures.

2). The laminar flame speed of typical syngas was found toincrease with H2 content, which was attributed to the rapidproduction of highly reactive radicals such as H and OH atthe early stage of combustion and the acceleration ofchain-branching reactions by these radicals.

3). Kinetic simulations with both GRI-Mech 3.0 and USC-MechII agreed well with experimental results. For the more inter-esting fuel-lean conditions, simulations with USC-Mech IIgave better agreement with experiments with discrepancywithin 5%.

Acknowledgements

This work is supported by the National Natural Science Founda-tion of China (Contact No. 51176169), the National Basic ResearchProgram of China (Contact No. 2012CB214906), and the Program ofIntroducing Talents of Discipline to University (Contact No.B08026).

References

[1] Klimstra J. Interchangeability of gaseous fuels – the importance of the wobbe-index. SAE Trans 1986;95(6):962–72.

[2] Chomiak J, Longwell JP, Sarofim AF. Combustion of low calorific value gases:problems and prospects. Prog Energy Combust Sci 1989;15(2):109–29.

[3] Brown MJ, McLean IC, Smith DB, Taylor SC. Markstein lengths of CO/H2/airflames, using expanding spherical flames. Symp (Int) Combust1996;26(1):875–81.

[4] Natarajan J, Lieuwen T, Seitzman J. Laminar flame speeds of H2/CO mixtures:effect of CO2 dilution, preheat temperature, and pressure. Combust Flame2007;151(1–2):104–19.

Y. He et al. / Fuel 95 (2012) 206–213 213

[5] Sun HY, Yang SI, Jomaas G, Law CK. High-pressure laminar flame speeds andkinetic modeling of carbon monoxide/hydrogen combustion. Proc Combus Inst2007;31(1):439–46.

[6] Prathap C, Ray A, Ravi MR. Investigation of nitrogen dilution effects on thelaminar burning velocity and flame stability of syngas fuel at atmosphericcondition. Combust Flame 2008;155(1-2):145–60.

[7] Dong C, Zhou QL, Zhao QX, Zhang YQ, Xu TM, Hui S. Experimental study on thelaminar flame speed of hydrogen/carbon monoxide/air mixtures. Fuel2009;88(10):1858–63.

[8] Monteiro E, Bellenoue M, Sotton J, Moreira NA, Malheiro S. Laminar burningvelocities and Markstein numbers of syngas–air mixtures. Fuel2010;89(8):1985–91.

[9] Liu C, Yan B, Chen G, Bai XS. Structures and burning velocity of biomass derivedgas flames. Int J Hydrogen Energy 2010;35(2):542–55.

[10] Ouimette P, Seers P. Numerical comparison of premixed laminar flame velocityof methane and wood syngas. Fuel 2009;88(3):528–33.

[11] Dam B, Ardha V, Choudhuri A. Laminar flame velocity of syngas fuels. J EnergyResour Technol 2010;132(4):#044501.

[12] Das AK, Kumar K, Sung CJ. Laminar flame speeds of moist syngas mixtures.Combust Flame 2011;158(2):345–53.

[13] Desgroux P, Domingues E, Cottereau MJ. Measurements of OH concentration inflames at high pressure by two-optical path laser-induced fluorescence. ApplOpt 1992;31(15):2831–8.

[14] Arnold A, Bombach R, Kappeli B, Schlegel A. Quantitative measurements of OHconcentration fields by two-dimensional laser-induced fluorescence. ApplPhys B-Lasers Opt 1997;64(5):579–83.

[15] Kiefer J, Li ZS, Zetterberg J, Bai XS, Alden M. Investigation of local flamestructures and statistics in partially premixed turbulent jet flames usingsimultaneous single-shot CH and OH planar laser-induced fluorescenceimaging. Combust Flame 2008;154(4):802–18.

[16] Wang ZH, Li B, Ehn A, Sun ZW, Li ZS, Bood J, et al. Investigation of flue-gastreatment with O3 injection using NO and NO2 planar laser-inducedfluorescence. Fuel 2010;89(9):2346–52.

[17] Law CK, Sung CJ. Structure, aerodynamics, and geometry of premixedflamelets. Prog Energy Combust Sci 2000;26(4-6):459–505.

[18] Sigfrid IR, Whiddon R, Collin R, Klingmann J. Experimental investigation oflaminar flame speeds for medium calorific gas with various amounts ofhydrogen and carbon monoxide content at gas turbine temperatures. ASMEConf Proc 2010:173–81.

[19] Vega EV, Lee KY. An experimental study on laminar CH4/O2/N2 premixedflames under an electric field. J Mech Sci Technol 2008;22(2):312–9.

[20] Dally BB, Masri AR, Barlow RS, Fiechtner GJ. Instantaneous and meancompositional structure of bluff-body stabilized nonpremixed flames.Combust Flame 1998;114(1-2):119–48.

[21] Nguyen Q-V, Paul PH. The time evolution of a vortex-flame interaction observedvia planar imaging of CH and OH. Symp (Int) Combust 1996;26(1):357–64.

[22] Smith GP, Golden DM, Frenklach M, Moriarty NW, Eiteneer B, Goldenberg Met al. GRI-Mech 3.0. <http://www.me.berkeley.edu/gri_mech/>.

[23] Wang Hai, You Xiaoqing, Joshi Ameya V, Davis Scott G, Laskin Alexander,Egolfopoulos Fokion et al. USC Mech version II. High-temperature combustionreaction model of H2/CO/C1–C4 compounds; May 2007. <http://ignis.usc.edu/USC_Mech_II.htm>.

[24] Halter F, Chauveau C, Djeballi-Chaumeix N, Gokalp I. Characterization of theeffects of pressure and hydrogen concentration on laminar burning velocitiesof methane–hydrogen–air mixtures. Proc Combust Inst 2005;30(1):201–8.

[25] Davis SG, Law CK, Wang H. Propyne pyrolysis in a flow reactor: anexperimental, RRKM, and detailed kinetic modeling study. J Phys Chem A1999;103(30):5889–99.

[26] Wang H. A new mechanism for initiation of free-radical chain reactions duringhigh-temperature, homogeneous oxidation of unsaturated hydrocarbons:ethylene, propyne, and allene. Int J Chem Kinet 2001;33(11):698–706.

[27] Davis SG, Joshi AV, Wang H, Egolfopoulos F. An optimized kinetic model of H2/CO combustion. Proc Combust Inst 2005;30(1):1283–92.

[28] Smallbone AJ, Liu W, Law CK, You XQ, Wang H. Experimental and modelingstudy of laminar flame speed and non-premixed counterflow ignition of n-heptane. Proc Combust Inst 2009;32(1):1245–52.

[29] Lowry W, de Vries J, Krejci M, Petersen E, Serinyel Z, Metcalfe W, et al. Laminarflame speed measurements and modeling of pure alkanes and alkane blends atelevated pressures. J Eng Gas Turbines Power 2011;133(9):091501–91509.

[30] Tahtouh T, Halter F, Mounaïm-Rousselle C. Measurement of laminar burningspeeds and Markstein lengths using a novel methodology. Combust Flame2009;156(9):1735–43.

[31] Chen Z, Qin X, Ju YG, Zhao ZW, Chaos M, Dryer FL. High temperature ignitionand combustion enhancement by dimethyl ether addition to methane–airmixtures. Proc Combust Inst 2007;31(1):1215–22.

[32] Bosschaart KJ, de Goey LPH. The laminar burning velocity of flamespropagating in mixtures of hydrocarbons and air measured with the heatflux method. Combust Flame 2004;136(3):261–9.

[33] Rozenchan G, Zhu DL, Law CK, Tse SD. Outward propagation, burningvelocities, and chemical effects of methane flames up to 60 atm. ProcCombust Inst 2002;29(2):1461–70.

[34] Gu XJ, Haq MZ, Lawes M, Woolley R. Laminar burning velocity and Marksteinlengths of methane–air mixtures. Combust Flame 2000;121(1–2):41–58.

[35] Vagelopoulos CM, Egolfopoulos FN. Direct experimental determination oflaminar flame speeds. Symp (Int) Combust 1998;27(1):513–9.

[36] Vanmaaren A, Thung DS, Degoey LPH. Measurement of flame temperature andadiabatic burning velocity of methane/air mixtures. Combust Sci Technol1994;96(4–6):327–44.

[37] Hassan MI, Aung KT, Faeth GM. Properties of laminar premixed CO/H2/airflames at various pressures. J Propul Power 1997;13(2):239–45.

[38] Herbon JT, Hanson RK, Golden DM, Bowman CT. A shock tube study of theenthalpy of formation of OH. Proc Combust Inst 2002;29(1):1201–8.

[39] Ruscic B, Wagner AF, Harding LB, Asher RL, Feller D, Dixon DA, et al. On theenthalpy of formation of hydroxyl radical and gas-phase bond dissociationenergies of water and hydroxyl. J Phys Chem A 2002;106(11):2727–47.

[40] Gurvich LV, Veyts IV, Alcock CB, editors. In: Thermodynamic properties ofindividual substances, 4th ed., vol. I. Parts 1 and 2, Hemisphere, New York;1989.

[41] Yu G, Law CK, Wu CK. Laminar flame speeds of hydrocarbon + air mixtureswith hydrogen addition. Combust Flame 1986;63(3):339–47.

[42] Tang CL, Huang ZH, Law CK. Determination, correlation, and mechanisticinterpretation of effects of hydrogen addition on laminar flame speeds ofhydrocarbon–air mixtures. Proc Combust Inst 2011;33(1):921–8.

[43] Aung KT, Hassan MI, Faeth GM. Flame stretch interactions of laminar premixedhydrogen/air flames at normal temperature and pressure. Combust Flame1997;109(1–2):1–24.

[44] Chaos M, Dryer FL. Syngas combustion kinetics and applications. Combust SciTechnol 2008;180(6):1053–96.

[45] Chaos M, Burke MP, Ju YG, Dryer FL. Syngas chemical kinetics and reactionmechanisms. In: Lieuwen T, Yang V, Yetter R, editors. Synthesis gascombustion: fundamentals and applications, Taylor and Francis, New York;2009. pp. 29–70.

[46] Natarajan J, Kochar Y, Lieuwen T, Seitzman J. Pressure and preheat dependenceof laminar flame speeds of H2/CO/CO2/O2/He mixtures. Proc Combust Inst2009;32(1):1261–8.

[47] Mueller CJ, Pitz WJ, Pickett LM, Martin GC, Siebers DL, Westbrook CK. Effects ofoxygenates on soot processes in DI diesel engines: experiments and numericalsimulations. SAE paper, No. 2003-01-1791; 2003.

[48] Mueller CJ. The quantification of mixture stoichiometry when fuel moleculescontain oxidizer elements or oxidizer molecules contain fuel elements. SAEpaper, No. 2005-01-3705; 2005.