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Fuel

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Full Length Article

Effects of fuel composition and initial pressure on laminar flame speed ofH2/CO/CH4 bio-syngas

Quan Zhoua, C.S. Cheunga,⁎, C.W. Leunga, Xiaotian Lib, Xiaojie Lib, Zuohua Huangb

a Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, Chinab State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China

A R T I C L E I N F O

Keywords:Bio-syngasLaminar flame speedElevated pressureChemical kinetic structure

A B S T R A C T

Biomass-derived syngas composition varies considerably depending on different feedstocks and processingtechniques and thereby complicates the combustion control. A study on the effects of variations in the fuelcomposition and initial pressure on the characteristics of premixed H2/CO/CH4 flames was conducted using thespherical expanding flame method and CHEMIKIN package. Experimental measurements and numerical simu-lations were performed at an initial temperature of 303 K, equivalence ratios of 0.6–1.5 and pressures of0.1–0.5MPa with a wide range of H2/CO/CH4 compositions. The thermal and chemical kinetic analyses are alsopresented. The measured laminar flame speed was compared with simulations using the Li mechanism. Theexperimental data show a reasonable agreement with the calculated values, especially at fuel-lean and lowpressure conditions. With the increase of H2 fraction in the fuel, the laminar flame speed increases significantly,but for the CH4 enrichment flame, the behavior is quite the contrary that it has the lowest laminar flame speed.With the increase of CO fraction in the fuel, the laminar flame speed does not change much. The thermal andchemical kinetic analyses indicate that the CO addition has more effect on the adiabatic flame temperature butonly plays a small role in the chemical effect compared to that of the H2 addition. On the other hand, at elevatedpressures, the Li mechanism gives slight overestimations for lean mixtures but underestimations for rich mix-tures. The laminar flame speed decreases with the increase of initial pressure under tested equivalence ratiowhich is mainly due to the increasing unburned mixture density and decreasing H, OH radicals concentrations.

1. Introduction

Biomass, as one of the important renewable energy resources formaintaining sustainable energy development and reducing air pollu-tion, has attracted global attention in the past decades. In previousyears, biomass was usually used in direct combustion. But recently,gasification of biomass is regarded as the more promising utilizationmethod [1]. The synthesis gas obtained from biomass can be used notonly in the gas turbine for power generation but also as raw material forthe production of other chemical products. For the majority of gasifiedbiomass, it is a mixture of hydrogen (H2), carbon monoxide (CO),methane (CH4), together with nitrogen (N2), carbon dioxide (CO2) andother minor species such as C2 hydrocarbons [2–5]. The diverse bio-mass feedstock and processing techniques lead to considerable varia-tion in the composition of bio-syngas. Fig. 1 shows the statistics of re-sults of various compositions of biomass-derived syngas (bio-syngas)reported in the literature [2–10]. It can be noticed that bio-syngasmostly has high concentrations of H2 and CO but relatively low

concentration of CH4. The volume fraction of H2 in the main speciesH2/CO/CH4 can vary from 20% to 80%. H2/CO ratio of bio-syngasvaries from 1:5 to 5:1. Generally, the other species such as CO2 and N2

in bio-syngas are about half of the total composition, however, they canbe removed by using pressure swing adsorption, amine scrubbing andmembrane reactors [11]. In the present study, we only investigate themain species H2/CO/CH4 in bio-syngas as well.

Laminar flame speed is critical to understand the combustioncharacteristics of a combustible fuel mixture, since it determines theflame burning rate and flame stability. Many previous investigations onlaminar flame speed concentrated on the H2-air mixtures [12–14], CH4-air mixtures [15–18], H2-CH4-air mixtures [19–23], and H2-CO-airmixtures [24–27]. So far, the laminar flame speed of H2/CO/CH4/airmixture has been rarely reported. The H2/CO/CH4 fuel blend, which isthe main species in bio-syngas, is more complex than a single compo-nent fuel or binary fuel in terms of fuel composition and combustionperformance. Lee et al. investigated the combustion instability of H2/CO/CH4 syngas in a partially premixed gas turbine. They found that the

https://doi.org/10.1016/j.fuel.2018.10.106Received 1 August 2018; Received in revised form 15 October 2018; Accepted 19 October 2018

⁎ Corresponding author.E-mail address: mmcsc@polyu.edu.hk (C.S. Cheung).

Fuel 238 (2019) 149–158

0016-2361/ © 2018 Elsevier Ltd. All rights reserved.

T

combustion instability frequency appeared to be linearly proportionalto the adiabatic flame temperature and laminar flame speed for all testconditions [28]. In addition, many practical combustors, such as gasturbine and internal combustion engine work at high pressure condi-tions, therefore, it is necessary to carry out the fundamental research ofbio-syngas combustion and understand its combustion characteristics atdifferent fuel compositions and initial pressures.

Some researchers numerically studied the physicochemical proper-ties and laminar flame speed of bio-syngas (H2/CO/CH4) combustion[29–31] using GRI Mech 3.0 [32] and San Diego Mech [33]. The resultsindicated that the different chemical kinetic mechanisms gave reason-able agreement with each other and with experimental data. In addi-tion, Monteiro et al. [34] experimentally determined the laminar flamespeeds and Markstein numbers of three syngas produced from woodgasification at normal temperature and pressure. They found that themaximum syngas-air laminar flame speed was observed at the stoi-chiometric equivalence ratio. The Markstein numbers showed that thethree syngas-air flames were generally unstable. Vu et al. [35] alsostudied the flame characteristics of three biomass derived gases. Theyreported that the experimental results and predictions using GRI Mech3.0 of unstretched laminar burning velocity agreed well at lean andstoichiometric flames, and the peak burning velocity of the three bio-mass derived gases-air premixed flames were found at equivalence ratioof 1.4. Yan et al. [36] used the heat flux method to measure the laminarflame speeds of four biomass-derived gases at atmospheric pressure andfound that the presence of H2 and CO in the gasified biomass fuel could

contribute to a higher burning velocity than that of the bio-methanefuel. Cheng et al. [11] experimentally and numerically studied thecharacteristics of atmospheric premixed stoichiometric H2/CO/CH4/airopposed-jet flames. Their results showed that the laminar flame speedincreased with increasing H2 or CO addition to the CH4-air flame, andthe increase of the laminar flame speed with H2 addition was mostlikely due to an increase in the active radicals (chemical effect), ratherthan from changes in the adiabatic flame temperature (thermal effect).

The above review shows that the variabilities of fuel composition inbio-syngas lead to different flame performances, and thereby imposeconsiderable technological challenge in practical systems. Most of theprevious investigations on combustion characteristics of H2/CO/CH4

fuel blends are limited by the variabilities of fuel composition or ele-vated pressure conditions. Thus, in the present study we aim to in-vestigate the effects of fuel composition and initial pressure on the la-minar burning characteristics of H2/CO/CH4 bio-syngas fuels, with thefollowing specific objectives. The first is to measure and predict thelaminar flame speed of H2/CO/CH4 blended fuels under various oper-ating conditions, including a wide range of fuel compositions and dif-ferent initial pressures. A constant volume combustion chamber com-bined with a high-speed schileren system was utilized to developexpanding spherical flames. Secondly, the thermal effect includingadiabatic flame temperature and thermal diffusivity are examined, andselected chemical kinetic structures are used to investigate the influ-ences of fuel composition and initial pressures. Sensitivity analysis anda consumption pathway analysis are conducted to further identify the

0 20 40 60 80 100

0

20

40

60

80

100 0

20

40

60

80

100

CO(%

)CH4

(%)

H2 (%)

(a)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

2

4

6

8

10

12

Cou

nts

H2 / (H2+CO+CH4)

(b)

0 1 2 3 4 50

2

4

6

8

10

12

14

Cou

nts

H2 / CO ratio

(c)

0 10 20 30 40 50 600

2

4

6

8

10

12

14

Cou

nts

Other species / Total components (%)

(d)

Fig. 1. The statistics results of bio-syngas composition for various feedstock and processing techniques [2–10].

Q. Zhou et al. Fuel 238 (2019) 149–158

150

chemical effects of fuel composition and initial pressure on the laminarflame speed.

2. Experimental and numerical method

2.1. Experimental setup

In the present study, a constant volume combustion bomb systemwas adopted to measure the laminar flame speed of H2/CO/CH4/airmixtures under various conditions. As shown in Fig. 2, the systemconsisted of a constant volume combustion chamber, ignition system,gas control system, high-speed schlieren photography system and dataacquisition system. The combustion chamber was in a cylindrical shapewith an inner diameter and length of 180mm and 210mm, respec-tively. Two quartz windows with a diameter of 80mm each were lo-cated at the two sides of the vessel for optical access. The detailedstructure of the combustion chamber can be found in a previous pub-lication [37]. A K-type thermocouple was mounted in the combustionchamber to measure the temperature with an uncertainty of± 3 K.Pressure transmitter, pressure transducer, inlet and outlet valves weremounted on the chamber body. The centrally located electrodes wereutilized to ignite the combustible mixtures. Artificially prepared bio-syngas/air mixtures were introduced into the chamber through the inletpipe and quantified by pre-calculated partial pressure settings using apressure gauge (Rosemount 3051 TG1 with an accuracy of 0.075%).The schlieren images of the flame propagation were recorded by a high-speed camera (Phantom V611) operating at 10,000 frames per second.

In the experiments, the initial temperature was set as 303 ± 3 K,and the initial pressure was set as 0.1, 0.3 and 0.5MPa, respectively.The equivalence ratio ranged from 0.6 to 1.5. The H2/CO/CH4 fuelblends and air were prepared by mixing the appropriate amount of pureH2 (99.99%), CO (99.99%), CH4 (99.99%), O2 (99.995%) and N2

(99.995%). For the measurement of laminar flame speed at elevatedpressure conditions (0.3 and 0.5MPa), helium (He) was utilized as asubstitution for N2 to suppress the cellular structures in order to acquirethe accurate experimental data. The dilution ratio of helium and oxygenwas fixed at 7:1 which is widely used in previous investigations [38–40]to produce the comparable adiabatic flame temperature to that of theflames combusted in air (O2/N2).

The volumetric percentage of a specific fuel component in a H2/CO/CH4 fuel blend, α(i), is defined as,

=+ +

×α i XX X X

( ) 100%i

H CO CH2 4 (1)

where Xi is the volume fraction of the specific fuel component. A basiccondition, defined as αBasis, was set as a reference for comparison.When the proportion of one of the three fuel components was changed,the mole ratio between the other two components was kept constant toexamine the effect of fuel composition variation on flame character-istics. Besides the basic condition, six other H2/CO/CH4 fuel blendswere also studied, namely, αH2-60, αH2-80, αCO-60, αCO-80, αCH4-60and αCH4-80. The detailed composition of the fuel blends are listed inTable 1. Each test was repeated at least three times to inspect the re-peatability of the experimental results, and the average data are used inthe analysis.

2.2. Extraction of laminar flame speed

For an outwardly propagating spherical flame, the stretched flamespeed Sb is calculated by [41,42],

=S dr dt/b f (2)

where rf represents the radius of the spherical flame. The unstrechedflame speed Sb

0, which is the flame speed at zero stretch rate, can beextracted using the nonlinear expression of Frankel and Sivashinsky[43],

= − ×S S S L r2/b b b b f0 0 (3)

where Lb is the Markstein length. This extrapolation method was alsoadopted by Chen [44]. Finally, the laminar flame speed SL can be de-termined through the simplified continuity equation across the flamefront,

=ρ S ρ Su L b b0 (4)

where ρu and ρb are the densities of unburned and burned mixtures,respectively.

Chen [45] stated that various factors can affect the uncertainty ofthe measurement of laminar flame speed using outwardly propagatingspherical flames, such as the effect of mixture preparation, ignition,chamber confinement, buoyancy, instability, radiation, extrapolationmethod etc. At the early stage of spherical expanding flame develop-ment, the influence of ignition energy is inevitable. According to pre-vious investigations [21,41,46], the spark-ignition disturbance can beavoided when the flame radius is larger than 5mm. Moreover, bytaking into account the influence of pressure rise in the combustionchamber, an upper limit of the flame radius is no more than 25mm sothat the flame propagation process can be regarded as a constant-pressure one. Therefore, the flame images with flame radius rangingfrom 10mm to 20mm are used in the present study to eliminate theinfluence of ignition, pressure change and chamber confinement. Inaddition, the images of the flame surface with full-fledged cellularstructure were abandoned to guarantee the accuracy of the laminarflame speed determined.

The total experimental uncertainty of laminar flame speed in the

Fig. 2. Experimental apparatus.

Table 1The fuel blends investigated.

Flame Fuel mixture (vol %) Fuel mixturedensity (g/L)

Stoichiometric air/fuelratio

XH2 XCO XCH4

αBasis 40 40 20 0.604 3.808αH2-60 60 26.7 13.3 0.429 3.332αH2-80 80 13.3 6.7 0.255 2.856αCO-60 26.7 60 13.3 0.773 3.332αCO-80 13.3 80 6.7 0.943 2.856αCH4-60 20 20 60 0.621 6.664αCH4-80 10 10 80 0.629 8.092

Q. Zhou et al. Fuel 238 (2019) 149–158

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present study can be evaluated by the method of Moffat [47],

⎜ ⎟= + ⎛⎝

⎞⎠

δ Bt S

M( )S S

M S2 1.952

L LL

(5)

where SSL is the standard deviation of SL; M represents the repeat timesfor each experimental condition; tM-1.95 is the value at the confidenceinterval of 95%; BSL is the total systematic uncertainty of the mea-surement method, and can be calculated by,

∑ ⎜ ⎟= ⎛⎝

∂∂

⎞⎠=

B S xx

u( )S

nL i

ii

i 1

2

L(6)

where xi is the factor affecting the uncertainty of laminar flame speedand ui represents the fixed error of each factor xi. Moreover, there existsa correlation between laminar flame speed and equivalence ratio, initialtemperature and pressure [48]. The uncertainty of ϕ is mainly causedby the measurements of the partial pressure of H2, CO, CH4, O2, and N2,and it can be estimated to be± (3–1.2)%. The accuracy of initialpressure and temperature are± 3 KPa and±3 K, respectively. Finally,the total experimental uncertainty was estimated to be± 0.8–4.6 cm/sby using Eq. (5).

2.3. Numerical method

A one-dimensional freely propagating plane premixed flame wasadopted to calculate the laminar flame speed of premixed H2/CO/CH4/air flames for comparison with the measured values using the PREMIXcode [49] in the CHEMKIN package [50]. All the calculations in thepresent paper were carried out with the Li mechanism [51], which wasupdated from the previous mechanism for hydrogen-oxygen [52] oncarbon monoxide, formaldehyde, and methanol kinetics. It has beenvalidated against a wide range of experimental results. The detailedreaction mechanism consists of 84 reversible reactions and 21 species.For the present calculation, the Soret effects and multi-componenttransport model were taken into consideration in the simulation. GRADand CURV values were set at 0.02 to ensure the independence of gridsolution, and final grid numbers were above 900 to assure a fullyconverged flame speed prediction.

3. Results and discussion

3.1. Effect of fuel composition

The experimentally determined and computed laminar flame speedof the premixed H2/CO/CH4/air flames under various fuel compositions(as shown in Table 1), and equivalence ratios from 0.6 to 1.5 are givenin Fig. 3. A comparison of the experimental and the calculated resultsshow that the Li mechanism can have a good prediction of the laminarflame speed of premixed H2/CO/CH4/air flames especially under fuellean conditions. For the fuel-rich conditions, there is a small differencebetween the predicted and the experimental results. This is consistentwith the conclusion drawn by Das et al. [24] that the Li mechanism isbetter in predicting laminar flame speed at fuel-lean conditions. Inaddition, a comparison among different fuel compositions yields thefollowing trends. First, the results clearly show that the SL for the H2-enriched flames increases significantly compared to that of the αBasisflame. With the increase of H2 content in the fuel, the peak value of SLshifts toward fuel-rich condition as a result of the high diffusivity ofhydrogen [22]. Second, it can be seen that with the increase of CH4

concentration in the fuel, the laminar flame speed decreases evidentlycompared with the αBasis condition, and the SL for the CH4-enrichedflames are lower than that of the CO-enriched flames. This observationis consistent with the results presented by Wu et al. [53] that the ad-dition of an appropriate amount of CO to CH4-air mixtures could in-crease the flame propagation of premixed CH4/CO/air flames. Third, itis worthy to note that the difference in laminar flame speed between the

αCO-60 and the αBasis flame is small, while for the αCO-80 flame, theSL slightly decreased. These trends can be explained from the thermaland chemical aspects in the following Sections 3.1.1–3.1.3.

3.1.1. Thermal diffusivity and adiabatic flame temperatureFig. 4 gives the thermal diffusivities and adiabatic flame tempera-

tures of the fuel mixtures used in the present study, which were cal-culated by using STANJAN [54] and EQUIL [55], respectively. Thethermochemical and transport properties are provided by the CHEMKINthermodynamic database [56]. According to the laminar flame theory[57], SL ∝ (αRR)1/2, laminar flame speed is proportional to the thermaldiffusivity (α) and the adiabatic flame temperature (Tad), which has apositive correlation with the reaction rate (RR). As shown in Fig. 4, theadiabatic flame temperature reaches its peak value at around ϕ=1.0for all the tested flames, while the thermal diffusivity monotonicallyincreases with equivalence ratio from ϕ=0.6 to ϕ=1.5 for all testedfuel mixtures. More importantly, with the increase of H2 fraction in thefuel mixture, thermal diffusivity increases significantly with increase ofequivalence ratio and this leads to the right shift of peak flame speed ofH2-enriched flame. On the contrary, with the increase of CO fraction orCH4 fraction in the fuel mixture, thermal diffusivity increases moremodestly or has changed little with increase of equivalence ratio, re-spectively. Consequently, for the CO-enriched flames and the CH4-en-riched flames, the peak flame speeds shift left compared to the H2-en-riched flames. Compared with the αBasis condition, both thermaldiffusivity and adiabatic flame temperature increase for the H2-en-riched condition but decrease for the CH4-enriched condition. This isconsistent with the results in Fig. 3 that the laminar flame speed in-crease with the increase of H2 fraction but decrease with the increase ofCH4 fraction in the fuel mixture. Although the thermal diffusivity of theCO-enriched fuel mixtures decrease compared to that of the αBasiscondition, there is an increase of adiabatic flame temperature with theincrease of CO fraction in the fuel mixture, and the adiabatic flametemperature of the αCO-80 flame is even higher than that of the αH2-60and αH2-80 flames. This is different from the laminar flame speed re-sults that the SL for the CO-enriched flames are lower than that of theH2-enriched and αBasis flames. Xie et al. [58] also found that theadiabatic flame temperature increases with the increase of CO/H2 ratiosin the fuel mixture. This suggests that the addition of CO to the H2/CO/CH4 mixture has more effect on the adiabatic flame temperature thanthat of the H2 addition.

3.1.2. Sensitivity analysisTo find out the most important elementary reactions that affect the

laminar flame speed at different fuel compositions, the normalized

0.6 0.8 1.0 1.2 1.4 1.60

50

100

150

200αBasis Li ModelαH2-60 αH2-80αCO-60 αCO-80αCH4-60 αCH4-80

S L (cm

/s)

φ

P=0.1MPaT=303K

Fig. 3. Laminar flame speed of H2/CO/CH4/air mixtures at different fuelcompositions.

Q. Zhou et al. Fuel 238 (2019) 149–158

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sensitivity coefficients are calculated using the PREMIX code inCHEMKIN. Fig. 5 shows the normalized sensitivity coefficients to thelaminar flame speed of stoichiometric H2/CO/CH4/air flames withdifferent fuel compositions. Generally speaking, the major reaction

steps of H2/CO/CH4/air mixtures mainly involve the H2/O2 reactions,the oxidation of CO and CH4, and the production of intermediate CO. Asshown in Fig. 5, the reactions: H+O2↔O+OH (R1), CO+OH↔CO2+H (R29) and CH3+O↔ CH2O+H (R48) are the three mostimportant elementary reactions for the H2/CO/CH4/air mixtures. Thereaction (R1) is the dominant chain branching reaction which producesa lot of OH and O radicals. The oxidation of CO (R29) and consumptionof CH3 (R48) contribute to building up the H radial pool. These reac-tions increase the concentration of active radicals (H, OH, and O), andthereby have a positive impact on the laminar flame speed.

When the fraction of H2 in the fuel increases to 80%, besides re-actions (R1), (R29) and (R48), the reactions H2+OH↔H2O+H (R3),H2+O↔H+OH (R2), and HO2+H↔OH+OH (R15) play an in-creasing role in promoting the combustion. For the αCO-80 flame, thereaction (R1) has the lowest positive sensitivity coefficient than that ofother flames, while the reaction (R29) is of critical importance in theoverall reaction process. For the αCH4-80 flame, the reaction (R1) hasthe highest positive sensitivity coefficient compared with other flamesand the reaction HCO+M↔H+CO+M (R30) becomes important.On the other hand, the reaction CH3+H+(M) ↔ CH4+(M) (R53) hasthe highest negative sensitivity coefficient compared with other flames,combined with reactions H+OH+M↔H2O+M (R12),H+O2+ (M)↔HO2+(M) (R13), HCO+H↔ CO+H2 (R32), andCH4+H↔ CH3+H2 (R54), they result in an effective termination ofthe radical chain process.

3.1.3. Chemical kinetic structuresDetailed chemical kinetic structures of four selected stoichiometric

flames, namely αBasis, αH2-80, αCO-80 and αCH4-80, were in-vestigated via numerical simulation using the Li mechanism. The spe-cies mole fraction, production rate, and net reaction rate of the majorelementary steps at various fuel compositions are plotted in Figs. 6–8,respectively. First, for the αBasis flame (40% H2, 40% CO, 20% CH4), itcan be seen in Fig. 6(a) that the decrease of H2 mole fraction occursearlier than those of CH4 and CO and the consumption of CO starts atthe latest due to a positive production rate of CO found in Fig. 7(a).Fig. 8(a) shows that the chain cycle (R1)–(R3) in the H2-O2 reactionscheme plays an important role in the αBasis flame. With abundant H,OH and O radicals produced, the dehydrogenation of CH4 is achievedthrough reactions: CH4+H↔ CH3+H2 (R54), CH4+O↔ CH3+OH(R55), and CH4+OH↔ CH3+H2O (R56). As noticed from Fig. 7(a),the oxidation of CO starts when the CH4 has been consumed to a largeextent, by the reaction: CO+OH↔ CO2+H (R29). The production ofintermediate CO is mainly from the oxidation of CH4 through the re-actions: CH3+O↔ CH2O+H (R48), CH2O+H↔HCO+H2 (R42)and HCO+M↔H+CO+M (R30). On the other hand, some CH3

radicals also react with H radicals through the reaction:CH3+H+ (M)↔ CH4+ (M) (R53) to terminate the radical chainprocess.

For the αH2-80 flame (80% H2-13.3% CO-6.7% CH4), it can benoticed from Fig. 6(b) that the mole fraction of CH3 radical decreasesevidently compared with the αBasis flame due to the fraction of CH4 inthe fuel is small, moreover, the consumption of CO still does not occurduring the oxidation of CH4 stage as observed by the production ofintermediate CO in Fig. 7(b). As found in Fig. 8(b), the chain cycle(R1)–(R3) in the H2-O2 reaction scheme plays a dominant role in theoverall reaction process, and the reaction rates of these reactions aremuch higher than those of the αBasis flame. In addition, formation andconsumption of HO2 are noticed through the reactions:H+O2+ (M)↔HO2+ (M) (R13) and HO2+H↔OH+OH (R15),which also generate the active radicals to speed up the overall reactionprocess and contribute to a higher laminar flame speed than that of theαBasis flame.

For the αCO-80 flame (13.3% H2-80% CO-6.7% CH4), it can benoticed from Fig. 6(c) that as the concentration of CO in the fuel mix-ture is increased to 80%, the mole fraction of CH3 radical decreases

0.6 0.8 1.0 1.2 1.4 1.6

2.5x10-5

3.0x10-5

3.5x10-5

4.0x10-5

4.5x10-5

0.6 0.8 1.0 1.2 1.4 1.61600

1700

1800

1900

2000

2100

2200

2300

2400

(a)Th

erm

al d

iffus

ivity

(m2 /s

)

φ

P=0.1MPa T=303KαH2-60 αH2-80αCO-60 αCO-80αCH4-60 αCH4-80αBasis

Adia

batic

flam

e te

mpe

ratu

re (K

)

φ

P=0.1 MPa T=303KαBasis αH2-60 αH2-80αCO-60 αCO-80αCH4-60 αCH4-80

(b)

Fig. 4. Thermal diffusivity and adiabatic flame temperature at different fuelcompositions.

Fig. 5. Sensitivity analysis for various H2/CO/CH4/air mixtures.

Q. Zhou et al. Fuel 238 (2019) 149–158

153

further as compared to the αBasis and αH2-80 flames. Unlike the formertwo flames, the dominant chemistry of this flame shifts to the CO ki-netics as revealed by Fig. 8(c). The rate of reaction (R29) for CO oxi-dation increases significantly and even exceeds those of the reactions(R1)–(R3). However, it can be found in Fig. 8 that the net reaction ratesfor the αCO-80 flame are lower than those of the αBasis and αH2-80flames. Recall that the results of laminar flame speed in Fig. 3 and thecalculated adiabatic flame temperatures in Fig. 4(b), the αCO-80 flamehas a lower laminar flame speed than that of the αBasis and H2-en-riched flames which implies that although the addition of CO has moreeffect on the adiabatic flame temperature than that of the H2 addition,the chemical effect of CO addition is not comparable to that of the H2

addition. This conclusion is consistent with Vagelopoulos and Egolfo-poulos [59] that the added CO in the fuel mixture will not react untilmost of the hydrocarbon species have been consumed, and its effect isthermal in nature.

Finally, for the αCH4-80 flame (10% H2-10% CO-80% CH4), it canbe seen from Fig. 6(d) that the mole fraction of CH3 radical increasesobviously, and there is an increasing amount of CO produced mainly bythe reactions (R48), (R42), (R44) and (R30) as shown in Fig. 7(d). FromFig. 8(d) we know that the most significant reaction for the αCH4-80flame is the chain branching reaction (R1), followed by the oxidation ofCH4 (R48). The dehydrogenation of CH4 plays an increasing role in theoverall reaction process mainly through the reactions (R56), (R54), and(R55). In these three reactions, the active H, OH and O radicals areexchanged for the less active radical CH3, leading to the lowest laminar

flame speed compared with the other flames, as shown in Fig. 3. On theother hand, it is noteworthy that with the abundance of CH3 radicals inthe reaction pool, some CH3 radicals can react by itself:CH3+CH3+ (M)↔ C2H6+ (M) (R52). The reaction (R52) is a radicaltermination step, which inhibits ignition and this explains why it is hardfor the CH4-enrichment flames to be ignited for rich mixture conditions.

3.2. Effect of initial pressure

Fig. 9 shows the effect of initial pressure (0.1MPa, 0.3 MPa and0.5MPa) and equivalence ratio on the laminar flame speed of the H2/CO/CH4/He mixtures at αBasis condition (40%H2, 40%CO and20%CH4). A comparison between the experimental and the calculatedresults shows that the Li mechanism slightly overestimates the SL of H2/CO/CH4/He mixture at fuel-lean conditions but underestimates the SLat fuel-rich conditions for each pressure condition. It is seen that the SLdecreases with the increase of initial pressure under the testedequivalence ratios and the peak SL shifts closer to ϕ=1.0 at higherinitial pressure due to the decrease of thermal diffusivity of the fuelmixture with the increase of initial pressure [60]. More analyses wereconducted as shown in the following Sections 3.2.1–3.2.4.

3.2.1. Laminar burning fluxAccording to the research of the Law and Sung [42], laminar

burning flux, =f ρ Su L0 , is a fundamental parameter for flame propa-

gation. It can manifest reactivity, diffusivity, and exothermicity of a

0.0 0.1 0.2 0.3 0.41E-4

1E-3

0.01

0.1

0.0 0.1 0.2 0.3 0.41E-4

1E-3

0.01

0.1

0.0 0.1 0.2 0.3 0.41E-4

1E-3

0.01

0.1

0.0 0.1 0.2 0.3 0.41E-4

1E-3

0.01

0.1

αBasis

Spe

cies

mol

e fr

actio

n

Distance (cm)

(a)

H2OO

2 CO

H2

CH3

HOOH

CH4

CO2

(b)

αH2-80

Spe

cies

mol

e fr

actio

n

Distance (cm)

H2O

H2

O2

CO

CH4

CH3

O H OH

CO2

(c)

αCO-80

Spe

cies

mol

e fr

actio

n

Distance (cm)

CO2

CO

O2

H2

CH4

H2O

H

OH

O

CH3 (d)

αCH4-80

Spe

cies

mol

e fr

actio

n

Distance (cm)

O2 H

2O

CH4

H2

CO

CO2

O H

OH

CH3

Fig. 6. Computed species mole fraction of premixed stoichiometric H2/CO/CH4 flames at T= 303 K, P= 0.1MPa: (a) αBasis; (b) αH2-80; (c) αCO-80; (d) αCH4-80.

Q. Zhou et al. Fuel 238 (2019) 149–158

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combustible fuel mixture. Fig. 10 gives the laminar burning flux of theH2/CO/CH4/He mixtures at different initial pressures and equivalenceratio for the αBasis condition. It is seen that although the SL decreaseswith the increase of initial pressure as shown in Fig. 9, f 0 increases withthe increase of initial pressure. This is consistent with the results of Law[61]. He indicated that the decreasing trend of laminar flame speedwith increasing initial pressure could be caused by the increasingdensity. According to the work of Law et al. [42], we have

∼S λ c w ρ[( / ) ] /b p b b b0 1/2 , which shows that laminar flame responses de-

pend on the flame kinetic through the characteristic reaction rate wb,and on the transport processes through the density-weighted transportcoefficient λ c( / )p b. It is important for us to take density into accountbecause the density weighting intrinsically affects the interpretation ofthe role of diffusive transport as well as that of bulk mass flow rate [61].

3.2.2. Sensitivity analysisTo find out the most important elementary reactions that affect the

laminar flame speed at different initial pressures, Fig. 11 gives thenormalized sensitivity coefficients to the laminar flame speed of stoi-chiometric H2/CO/CH4/He flames at different initial pressures for theαBasis condition. The results indicate that as initial pressure increases,the collision between molecules and free radicals becomes more fre-quent and reactions are facilitated, and thereby the positive sensitivitycoefficients of reaction (R1), (R29), and (R48) increase with the in-crease of initial pressure. This finding can be also demonstrated by theincreasing trend of f 0 in Fig. 10 that the chemical reactivity increaseswith the increase of initial pressure [61]. Moreover, it should be noticedthat with the increase of initial pressure, the three-body, pressure-

sensitive, chain termination reactions H+OH+M↔H2O+M (R12),H+O2+ (M)↔HO2+ (M) (R13), and CH3+H+ (M)↔ CH4+ (M)(R53) also become more important as shown in the negative coeffi-cients side. It has been suggested in the previous literature [62] thatwhen the pressure is increased, the termination reactions become im-portant, removing key radicals, such as H and OH, from the radical pooland producing the relatively inactive radical HO2 or stable moleculeCH4. A retarding effect is therefore imposed on the overall reactionprocess. These termination reactions could replace the dominating roleof the branching reactions particularly when they are three-body as itsrate can be substantially increased with increasing pressure.

3.2.3. Chemical kinetic analysisPrevious studies [58,63] have shown a strong positive correlation

between the laminar flame speed and the concentrations of free radicalsH, OH and O in the reaction zone. Therefore, Fig. 12 gives the calcu-lated mole fractions of these free radicals at different initial pressuresunder the stoichiometric αBasis condition. It can be seen that themaximum concentrations of H, OH and O radicals decrease with theincrease of initial pressure, respectively. This indicates that for the samefuel composition, with the increase of initial pressure, the decreasing oflaminar flame speed is indeed related to the reduction of concentrationsof H, OH and O key free radicals.

3.2.4. Consumption pathway analysisTo further analyze the initial pressure effect on the chemical kinetic

in flames, Fig. 13 presents the results of species consumption pathwayanalysis for P=0.1MPa and P=0.5MPa. The results show that the

0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24

-0.02

-0.01

0.00

0.01

0.02

0.03

0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24

-0.010

-0.005

0.000

0.005

0.010

0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

αBasis

Pro

duct

ion

rate

(mol

e/cm

3 -s)

Distance (cm)

H2O

CO2

H

O2

CH4 CH

3

CO

H

OOH

(a) (b)

αH2-80

Pro

duct

ion

rate

(mol

e/cm

3 -s)

Distance (cm)

H2O

H

CH3CO

2

CH4

H2

O2

CO

OHO

(c)

αCO-80

Pro

duct

ion

rate

(mol

e/cm

3 -s)

Distance (cm)

CO2

H2OHO OHCH

3

O2

CH4 H

2

CO(d)

αCH4-80

Pro

duct

ion

rate

(mol

e/cm

3 -s)

Distance (cm)

H2O

CO

CO2

OH

H2

O2

CH4

CH3 O

H

Fig. 7. Computed production rate of premixed stoichiometric H2/CO/CH4 flames at T=303 K, P=0.1MPa: (a) αBasis; (b) αH2-80; (c) αCO-80; (d) αCH4-80.

Q. Zhou et al. Fuel 238 (2019) 149–158

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consumption of H2 is through two reactions: H2+O↔H+OH (R2)and H2+OH↔H2O+H (R3); the consumption of CO is mainlythough the reaction: CO+OH↔ CO2+H (R29); and the consumptionof CH4 is through three reactions: CH4+H↔ CH3+H2 (R54),CH4+O↔ CH3+OH (R55), and CH4+OH↔ CH3+H2O (R56).With the increase of initial pressure, the production of OH radical isincreased through the reaction: O+H2O↔OH+OH (R4), marked byan increase in the consumption percentage of O radical from 18% to

54%. However, the consumption percentage of HO2 by H (R15) togenerate OH radical decreases substantially from 55% to 33%, andanother elementary reaction: HO2+OH↔H2O+O2 (R17) changesgreatly from 24% to 50%, indicating increased consumption of OHradicals. On the other hand, as the initial pressure is increased, theproduction of H radical by the reaction: CH3+O↔ CH2O+H (R48)decreases from 27% to 9%. Moreover, more H radicals are consumed bythe three-body reaction: H+O2+ (M)↔HO2+ (M) (R13) to formHO2, and thereby less H radicals are consumed through the reaction:

0.12 0.14 0.16 0.18 0.20 0.22

0.0

5.0x10-3

1.0x10-2

1.5x10-2

2.0x10-2

0.12 0.14 0.16 0.18 0.20

0.0

1.0x10-2

2.0x10-2

3.0x10-2

4.0x10-2

5.0x10-2

6.0x10-2

0.12 0.14 0.16 0.18 0.20 0.22-2.0x10-3

0.0

2.0x10-3

4.0x10-3

6.0x10-3

8.0x10-3

1.0x10-2

1.2x10-2

0.12 0.14 0.16 0.18 0.20 0.22-2.0x10-3

0.0

2.0x10-3

4.0x10-3

6.0x10-3

8.0x10-3

1.0x10-2

1.2x10-2

αBasisN

et re

actio

n ra

te (m

ole/

cm3 -s

)

Distance (cm)

R1R3

R48

R42

R29R54

R53

R56R30

R2

R55

(a) αH2-80(b)

Net

reac

tion

rate

(mol

e/cm

3 -s)

Distance (cm)

R3

R1

R13

R15

R48R53

R2

R42R29

R54

R56

αCO-80

Net

reac

tion

rate

(mol

e/cm

3 -s)

Distance (cm)

R29R1

R3R48

R15

R13

R56

R42

R55

R2

R54

(c) αCH4-80(d)

Net

reac

tion

rate

(mol

e/cm

3 -s)

Distance (cm)

R1

R48 R3

R30R42

R44

R55

R56

R53

R29R2

R54

Fig. 8. Computed net reaction rate of premixed stoichiometric H2/CO/CH4 flames at T= 303 K, P= 0.1MPa: (a) αBasis; (b) αH2-80; (c) αCO-80; (d) αCH4-80.

0.6 0.8 1.0 1.2 1.4 1.620

40

60

80

100

120

140

S L (cm

/s)

φ

αBasis P=0.1 MPa P=0.3 MPa P=0.5 MPa

Li Model

Fig. 9. Laminar flame speed of H2/CO/CH4/He mixtures at different initialpressures.

0.6 0.8 1.0 1.2 1.4 1.60.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

f 0 (g/c

m2 -s

)

φ

αBasis P=0.1 MPa P=0.3 MPa P=0.5 MPa

Fig. 10. Laminar burning flux of H2/CO/CH4/He mixtures at different initialpressures.

Q. Zhou et al. Fuel 238 (2019) 149–158

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H+O2↔O+OH (R1). Consequently, the chain-branching cycle (R1)to (R3) is broken, leading to a fall in total concentration of H and OHradicals. In conclusion, with the increase of initial pressure, a poorer

radical pool is presented quantitatively, which restrains the overallreaction activity. Recall that the result of laminar burning flux indicatesthat it is also important to allow for density weighting in the inter-pretation of combustion phenomena due to the pressure variation [61].The initial pressure has influence on the chemical kinetic and density,while both of them contribute to the reduction of laminar flame speedwith increasing pressure.

4. Conclusions

The effects of fuel composition and initial pressures on the laminarflame speed of bio-syngas (H2/CO/CH4) were investigated over a widerange of fuel compositions, 0.1–0.5MPa initial pressures, and 0.6–1.5equivalence ratios. The expanding spherical flame method coupled witha numerical method was used in the present study. The main results aresummarized as follows:

1. For the effect of fuel composition, the experimental data and themodel prediction using the Li mechanism at different fuel compo-sitions show good agreement with each other, especially at fuel-leanconditions. A comparison among the laminar burning characteristicsof the H2/CO/CH4/air mixtures at different fuel compositions showsthat with the increase of H2 fraction in the fuel, the overall reactionactivity was strongly promoted, and thereby the SL of the H2-en-riched flames are much higher than that of the αBasis flame. Withthe increase of CO fraction in the fuel, the αCO-60 flame showsnearly the same SL with the αBasis flame, while the SL of the αCO-80flame slightly decreased. The results indicated that the CO additionplays a small role in chemical kinetic effect, but has more impact onadiabatic flame temperature than that of the H2 addition. With theincrease of CH4 fraction in the fuel, the SL decreases evidentlycompared with the αBasis flame, and the SL of the CH4-enrichedflames are lower than that of the CO-enriched flames. This is causedby the combination of thermal and chemical kinetic effects of theCH4 addition.

2. For the effect of initial pressure, the Li mechanism gives slightoverestimations for fuel lean mixtures but underestimations for fuelrich mixtures at elevated pressures. The SL decreases with the in-crease of initial pressure under tested equivalence ratios. As pressureis increased, the growing importance of the three-body chain ter-mination reactions (R12), (R13), and (R53) is noticed. Moreover,the analyses show that with the increase of initial pressure, thedecrease of SL is mainly caused by the increase of ρu and decrease ofthe concentrations of H, OH and O radicals.

-0.2 0.0 0.2 0.4

R54: CH4+H = CH3+H2

R53: CH3+H(+M) = CH4(+M)

R48: CH3+O = CH2O+H

R30: HCO+M = H+CO+M

R3: H2+OH = H2O+H

R29: CO+OH = CO2+H

R2: O+H2 = H+OH

R15: HO2+H = OH+OH

R13: H+O2(+M) = HO2(+M)

R12: H+OH+M = H2O+M

R1: H+O2 = O+OH

αBasis T=303 K ; φ=1.0

P=0.1 MPa P=0.3 MPa P=0.5 MPa

Normalized sensitivity coefficients

Fig. 11. Sensitivity analysis for different initial pressures.

0.1 0.2 0.3 0.4 0.5 0.6

0.000

0.002

0.004

0.006

0.008

0.010

Mol

e fra

ctio

n

Distance (cm)

αBasis T=303K φ=1.0

P=0.1 MPa P=0.3 MPa P=0.5 MPa

H

OH

O

Fig. 12. The calculated mole fraction of H, OH and O radicals at different initialpressures.

Fig. 13. Integrated species consumption pathways of H2/CO/CH4/air mixtures at different initial pressures.

Q. Zhou et al. Fuel 238 (2019) 149–158

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Acknowledgements

The authors thank the Hong Kong Polytechnic University for fi-nancial support (Project code: RUSZ) and thank the State KeyLaboratory of Multiphase Flow in Power Engineering, School of Energyand Power Eng., Xi’an Jiaotong University for experiment and technicalsupport.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuel.2018.10.106.

References

[1] Demirbas A. Recent advances in biomass conversion technologies. Energy Educ SciTechnol 2000;6:19–41.

[2] Chaudhari S, Bej S, Bakhshi N, Dalai A. Steam gasification of biomass-derived charfor the production of carbon monoxide-rich synthesis gas. Energy Fuels2001;15(3):736–42.

[3] Lv P, Yuan Z, Wu C, Ma L, Chen Y, Tsubaki N. Bio-syngas production from biomasscatalytic gasification. Energy Convers Manage 2007;48(4):1132–9.

[4] Panigrahi S, Dalai A, Chaudhari S, Bakhshi N. Synthesis gas production from steamgasification of biomass-derived oil. Energy Fuels 2003;17(3):637–42.

[5] Wang T, Chang J, Lv P. Synthesis gas production via biomass catalytic gasificationwith addition of biogas. Energy Fuels 2005;19(2):637–44.

[6] Chaudhari S, Dalai A, Bakhshi N. Production of hydrogen and/or syngas (H2+ CO)via steam gasification of biomass-derived chars. Energy Fuels 2003;17(4):1062–7.

[7] Hanaoka T, Inoue S, Uno S, Ogi T, Minowa T. Effect of woody biomass componentson air-steam gasification. Biomass Bioenergy 2005;28(1):69–76.

[8] Lv P, Yuan Z, Ma L, Wu C, Chen Y, Zhu J. Hydrogen-rich gas production frombiomass air and oxygen/steam gasification in a downdraft gasifier. RenewableEnergy 2007;32(13):2173–85.

[9] Sutton D, Kelleher B, Ross JRH. Review of literature on catalysts for biomass ga-sification. Fuel Process Technol 2001;73(3):155–73.

[10] Tijmensen MJ, Faaij AP, Hamelinck CN, van Hardeveld MR. Exploration of thepossibilities for production of Fischer Tropsch liquids and power via biomass ga-sification. Biomass Bioenergy 2002;23(2):129–52.

[11] Cheng TS, Chang YC, Chao YC, Chen GB, Li YH, Wu CY. An experimental andnumerical study on characteristics of laminar premixed H2/CO/CH4/air flames. IntJ Hydrogen Energy 2011;36(20):13207–17.

[12] Kwon O, Faeth G. Flame/stretch interactions of premixed hydrogen-fueled flames:measurements and predictions. Combust Flame 2001;124(4):590–610.

[13] Hermanns R, Konnov AA, Bastiaans R, De Goey L. Laminar burning velocities ofdiluted hydrogen− oxygen− nitrogen mixtures. Energy Fuels 2007;21(4):1977–81.

[14] Hu E, Huang Z, He J, Miao H. Experimental and numerical study on laminarburning velocities and flame instabilities of hydrogen–air mixtures at elevatedpressures and temperatures. Int J Hydrogen Energy 2009;34(20):8741–55.

[15] 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.

[16] Liao S, Jiang D, Cheng Q. Determination of laminar burning velocities for naturalgas. Fuel 2004;83(9):1247–50.

[17] Liao S, Jiang D, Gao J, Huang Z. Measurements of Markstein numbers and laminarburning velocities for natural gas− air mixtures. Energy Fuels 2004;18(2):316–26.

[18] Hu E, Li X, Meng X, Chen Y, Cheng Y, Xie Y, et al. Laminar flame speeds and ignitiondelay times of methane–air mixtures at elevated temperatures and pressures. Fuel2015;158:1–10.

[19] Yu G, Law C, Wu C. Laminar flame speeds of hydrocarbon+ air mixtures withhydrogen addition. Combust Flame 1986;63(3):339–47.

[20] Halter F, Chauveau C, Djebaili-Chaumeix N, Gökalp I. Characterization of the ef-fects of pressure and hydrogen concentration on laminar burning velocities ofmethane–hydrogen–air mixtures. Proc Combust Inst 2005;30(1):201–8.

[21] Huang Z, Zhang Y, Zeng K, Liu B, Wang Q, Jiang D. Measurements of laminarburning velocities for natural gas–hydrogen–air mixtures. Combust Flame2006;146(1):302–11.

[22] Law CK, Kwon OC. Effects of hydrocarbon substitution on atmospheric hydro-gen–air flame propagation. Int J Hydrogen Energy 2004;29(8):867–79.

[23] Li Y, Bi M, Li B, Zhou Y, Gao W. Effects of hydrogen and initial pressure on flamecharacteristics and explosion pressure of methane/hydrogen fuels. Fuel2018;233:269–82.

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

[25] Varghese RJ, Kolekar H, Hariharan V, Kumar S. Effect of CO content on laminarburning velocities of syngas-air premixed flames at elevated temperatures. Fuel2018;214:144–53.

[26] Zhang W, Gou X, Kong W, Chen Z. Laminar flame speeds of lean high-hydrogensyngas at normal and elevated pressures. Fuel 2016;181:958–63.

[27] Dong C, Zhou Q, Zhao Q, Zhang Y, Xu T, Hui S. Experimental study on the laminarflame speed of hydrogen/carbon monoxide/air mixtures. Fuel2009;88(10):1858–63.

[28] Lee MC, Yoon J, Joo S, Kim J, Hwang J, Yoon Y. Investigation into the cause of highmulti-mode combustion instability of H2/CO/CH4 syngas in a partially premixedgas turbine model combustor. Proc Combust Inst 2015;35(3):3263–71.

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

[30] Liu J, Zhang X, Wang T, Hou X, Zhang J, Zheng S. Numerical study of the chemical,thermal and diffusion effects of H2 and CO addition on the laminar flame speeds ofmethane–air mixture. Int J Hydrogen Energy 2015;40(26):8475–83.

[31] Zhang K, Jiang X. An investigation of fuel variability effect on bio-syngas com-bustion using uncertainty quantification. Fuel 2018;220:283–95.

[32] Smith GP, Golden DM, Frenklach M, Moriarty NW, Eiteneer B, Goldenberg M, et al.GRI 3.0, http://www.me.berkeley.edu/gri_mech/.

[33] Chemical Kinetic Mechanism for Combustion Applications Center for EnergyResearch (Combustion Division), University of California at San Diego http://maewebucsdedu/combustion/.

[34] Monteiro E, Bellenoue M, Sotton J, Moreira NA, Malheiro S. Laminar burning ve-locities and Markstein numbers of syngas–air mixtures. Fuel 2010;89(8):1985–91.

[35] Vu TM, Song WS, Park J, Bae DS, You HS. Measurements of propagation speeds andflame instabilities in biomass derived gas–air premixed flames. Int J HydrogenEnergy 2011;36(18):12058–67.

[36] Yan B, Wu Y, Liu C, Yu J, Li B, Li Z, et al. Experimental and modeling study oflaminar burning velocity of biomass derived gases/air mixtures. Int J HydrogenEnergy 2011;36(5):3769–77.

[37] Gu X, Huang Z, Li Q, Tang C. Measurements of laminar burning velocities andmarkstein lengths of n-butanol−air premixed mixtures at elevated temperaturesand pressures. Energy Fuels 2009;23(10):4900–7.

[38] Tse SD, Zhu DL, Law CK. Morphology and burning rates of expanding sphericalflames in H2/O2/inert mixtures up to 60 atmospheres. Proc Combust Inst2000;28(2):1793–800.

[39] Kéromnès A, Metcalfe WK, Heufer KA, Donohoe N, Das AK, Sung C-J, et al. Anexperimental and detailed chemical kinetic modeling study of hydrogen and syngasmixture oxidation at elevated pressures. Combust Flame 2013;160(6):995–1011.

[40] Lu X, Hu E, Li X, Ku J, Huang Z. Non-monotonic behaviors of laminar burningvelocities of H2/O2/He mixtures at elevated pressures and temperatures. Int JHydrogen Energy 2017;42(34):22036–45.

[41] Bradley D, Gaskell P, Gu X. Burning velocities, Markstein lengths, and flamequenching for spherical methane-air flames: a computational study. Combust Flame1996;104(1–2):176–98.

[42] Law CK, Sung CJ. Structure, aerodynamics, and geometry of premixed flamelets.Prog Energy Combust Sci 2000;26(4):459–505.

[43] Frankel M, Sivashinsky G. On effects due to thermal expansion and Lewis number inspherical flame propagation. Combust Sci Technol 1983;31(3–4):131–8.

[44] Chen Z. On the extraction of laminar flame speed and Markstein length from out-wardly propagating spherical flames. Combust Flame 2011;158(2):291–300.

[45] Chen Z. On the accuracy of laminar flame speeds measured from outwardly pro-pagating spherical flames: methane/air at normal temperature and pressure.Combust Flame 2015;162(6):2442–53.

[46] Lamoureux N, Djebaili-Chaumeix N. Paillard CE. Laminar flame velocity determi-nation for H2–air–He–CO2 mixtures using the spherical bomb method. Exp ThermFluid Sci 2003;27(4):385–93.

[47] Moffat RJ. Describing the uncertainties in experimental results. Exp Therm Fluid Sci1988;1(1):3–17.

[48] Li Q, Hu E, Cheng Y, Huang Z. Measurements of laminar flame speeds and flameinstability analysis of 2-methyl-1-butanol–air mixtures. Fuel 2013;112:263–71.

[49] Kee RJ, Grcar JF, Smooke MD, Miller J, Meeks E. PREMIX: a Fortran program formodeling steady laminar one-dimensional premixed flames. Sandia NationalLaboratories Report 1985(SAND85-8249).

[50] Kee RJ, Rupley FM, Meeks E, Miller JA. CHEMKIN-III: A FORTRAN chemical ki-netics package for the analysis of gas-phase chemical and plasma kinetics. Sandianational laboratories report SAND96-8216 1996.

[51] Li J, Zhao Z, Kazakov A, Chaos M, Dryer FL, Scire JJ. A comprehensive kineticmechanism for CO, CH2O, and CH3OH combustion. Int J Chem Kinet2007;39(3):109–36.

[52] Li J, Zhao Z, Kazakov A, Dryer FL. An updated comprehensive kinetic model ofhydrogen combustion. Int J Chem Kinet 2004;36(10):566–75.

[53] Wu CY, Chao YC, Cheng TS, Chen CP, Ho CT. Effects of CO addition on the char-acteristics of laminar premixed CH4/air opposed-jet flames. Combust Flame2009;156(2):362–73.

[54] STANJAN transport properties calculator Available at: http://navierengrcolostateedu/~dandy/code/code-2/indexhtml.

[55] Kee RJ, Dixon-Lewis G, Warnatz J, Coltrin ME, Miller JA. A Fortran computer codepackage for the evaluation of gas-phase multicomponent transport properties.Sandia National Laboratories Report SAND86-8246 1986;13:80401-1887.

[56] Kee R, Rupley F, Miller J. CHEMKIN: The Chemkin Thermodynamic Database. Rep.SAND87-8215. Sandia National Laboratories, Livermore (CA), USA 1987.

[57] Kuo K. Principles of Combustion. New York: John Wiley & Sons; 1986. Ch 1.[58] Xie Y, Wang J, Xu N, Yu S, Huang Z. Comparative study on the effect of CO2 and

H2O dilution on laminar burning characteristics of CO/H2/air mixtures. Int JHydrogen Energy 2014;39(7):3450–8.

[59] Vagelopoulos C, Egolfopoulos F, Law C. Further considerations on the determina-tion of laminar flame speeds with the counterflow twin-flame technique.Symposium (international) on combustion. Elsevier; 1994. p. 1341–7.

[60] Law CK. Combustion physics. Cambridge University Press; 2010.[61] Law C. Propagation, structure, and limit phenomena of laminar flames at elevated

pressures. Combust Sci Technol 2006;178(1–3):335–60.[62] Egolfopoulos FN, Law CK. Chain mechanisms in the overall reaction orders in la-

minar flame propagation. Combust Flame 1990;80(1):7–16.[63] Singh D, Nishiie T, Tanvir S, Qiao L. An experimental and kinetic study of syngas/

air combustion at elevated temperatures and the effect of water addition. Fuel2012;94:448–56.

Q. Zhou et al. Fuel 238 (2019) 149–158

158