Laminar flame characteristics and kinetic modeling study of ETBE

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Laminar flame characteristics and kinetic modeling study of ETBE compared with MTBE, ethanol, iso-octane and gasoline Jinfeng Ku, Erjiang Hu, Geyuan Yin, Chanchan Li, Xin Lu, and Zuohua Huang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03636 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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Energy & Fuels

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Laminar flame characteristics and kinetic modeling study of ETBE

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compared with MTBE, ethanol, iso-octane and gasoline

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Jinfeng Ku1, Erjiang Hu*1, Geyuan Yin1, Chanchan Li2, Xin Lu1, Zuohua Huang1

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1

5

University, Xi’an, People’s Republic of China

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2

7

Republic of China

State key Laboratory of Muliphase Flow in Power Engineering, Xi’an Jiaotong Chinesisch-Deutsches Hochschulkolleg, Tongji University, Shanghai, People’s

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ABSTRACT

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Laminar flame speeds of ethyl tertiary butyl ether (ETBE) were measured in a

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constant volume bomb at different initial temperatures (298 K, 373 K, 453 K) and

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pressures (1 atm, 3 atm, 5 atm). The laminar flame experiments were also conducted

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for methyl tertiary butyl ether (MTBE), ethanol, iso-octane and gasoline for the

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comparison of laminar flame speeds and Markstein lengths. Experimental results

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show that laminar flame speeds peak at the equivalence ratio of 1.1 for all tested fuels.

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Ethanol has the fastest laminar flame speed and the other fuels have similar flame

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speeds, indicating replacing MTBE with ETBE in gasoline will not influence the

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laminar flame speed of present gasoline. CRECK mechanism and Curran mechanism

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were validated by experimental results of ETBE and neither could predict laminar

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flame speeds well. Curran mechanism was optimized by updating the underlying

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mechanism, and the Modified Curran mechanism has better prediction performance

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on the laminar flame speed. Sensitivity analyses were also provided to interpret the

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differences of laminar flame speeds and the major reason of better prediction

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performance for Modified Curran mechanism. The result of Markstein length shows

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that gasoline has the smallest Markstein lengths and its flame front is the most

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unstable. The Markstein lengths of ETBE and iso-octane differ little and are the

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largest under 1.2.

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1. INTRODUCTION

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Nowadays，alternative fuels have attracted increasingly considerable attention due

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to their importance in reducing the reliance on fossil fuels，achieving lower emission

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and limiting the greenhouse gases in the atmosphere. Many types of oxygenated

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alternative fuels, such as alcohols, acyclic ethers, cyclic ethers and esters, can be

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obtained through a wide range of processes involving fermentation or catalytic

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reactions.1 To achieve higher thermal efficiency in spark ignition engines, the engine

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downsizing with a turbocharger is getting common, which causes high pressure

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combustion. Under such conditions, the addition of a good octane number enhancer is

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required in order to avoid the occurrence of engine knock. Before 1970, teraethyllead

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was added into gasoline to improve the octane number, thereby allowing higher

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compression ratio so as to improve the thermal efficiency. But from 1970 teraethylead

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began to be forbidden because of concerns over air and soil lead levels and the

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accumulative neurotoxicity of lead. In 1979, MTBE (methyl tertiary butyl ether,

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RON=118) is widely used as an oxygenate additive to enhance the octane number and

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oxygen content, improve combustion and reduce engine knock. However, MTBE can

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pollute the groundwater due to its high water solubility and make people sick,

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vomiting and dizziness after contacting with it.2, 3 Since 2004, the United States and

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some European countries began to reduce or ban the use of MTBE. But, in some Asia

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countries, including China, there are still no restrictions about MTBE as a fuel

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additive. The test results show that there are about 4-10% MTBE (%Wt) in gasoline

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of China. Similarly with MTBE, ETBE (ethyl tertiary butyl ether, RON=117) also has

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high octane number and good performance as gasoline additive but without the

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drawbacks of MTBE.

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Besides the method of liquid phase synthesis from ethanol and isobutene, ETBE

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can also be synthesized from tert-butyl alcohol (TBA) and ethanol.4 ETBE can also be

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considered as biofuel, since it can be produced from 47% bioethanol and 53%

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isobutene. After many years of scientific and technological research, Kabuskiki

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Kaisha IBF developed a competitive bioETBE mixture manufacturing technology,

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which can produce ETBE from biomass completely. Table 1 gives the properties of

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ETBE, MTBE, ethanol, iso-octane and gasoline.1, 5 ETBE, MTBE and ethanol have

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higher octane number. The lower heating value (LHV) of ETBE (26.93 MJ/L) and

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MTBE (26.04 MJ/L) is much closer to gasoline than that of ethanol (21.2 MJ/L), so

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the power reduction of ETBE and MTBE is much less than that of ethanol when they

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are added into gasoline. Reid vapor pressure (RVP) has an important impact on the

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evaporative emissions of hydrocarbons in the gasoline supply system, so regulations

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around the world have imposed strict restrictions on it.6 Despite the fact that ethanol

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has the lowest RVP because of the strong intermolecular hydrogen boding interactions

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between ethanol molecules, these additives (ETBE, MTBE, ethanol) and gasoline

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mixtures’ RVP presents a completely opposite result. According to the research of


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Energy & Fuels

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Silva3 and Rodrí guez-Antón6, the oxygenates (ETBE, ethanol), with exception of

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ETBE, increase the RVP of gasoline. The positive deviation from Raoult’s Law with

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the addition of ethanol occurs because the intermolecular interactions between ethanol

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and hydrocarbon molecules are weaker than they are in two pure liquids and also due

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to the formation of azeotrope that reduces the boiling point temperature and increases

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the vapor pressure of the mixture.6 Thus, ETBE has the advantages of mixtures RVP

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compared to MTBE and ethanol when they are added into gasoline. Additionally,

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ethanol will absorb moisture from the atmosphere, thus causing the phase separation

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of gasoline. In summary, with the advantages of low water solubility, being

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biodegradable, producing from biomass, low harmfulness to people’s health, high

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octane number, higher LHV, low mixture RVP and water absorbability, ETBE is a

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better gasoline additive compared to MTBE and ethanol.

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Generally, ETBE is blended with other fuels to combustion, so the relevant study of

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ETBE is particularly important. At present, the research of ETBE is mainly focused

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on production methods and processes, engine performance and chemical kinetics. In

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2006, Malça and Freire7 found that the use of bioethanol as raw materials can increase

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the yield of ETBE and bioETBE. Menezes et al.8 studied the optimization of the

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ETBE production process and found that azeotropic mixtures of ETBE and ethanol

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during the production process have a good prospect of application due to its higher

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octane number, lower volatility and lower production cost. Besides, in the process of

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synthesis of ETBE, an equivalent molar ratio of raw material (ethanol/isobutene)

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should be used to reduce the production of azeotropic mixtures and increase ETBE

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yield. However, under such conditions, the by-products of TBA and SBA (sec-butyl

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alcohol) will be produced and the activity of catalysts will be reduced greatly. In

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addition to being synthesized from chemical products (ethanol, isobutene, TBA)

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completely or partly, ETBE can also be produced from biomass completely according

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to the research results of Kabuskiki Kaisha IBF. With the development of Production

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methods and processes, the cost of ETBE decreases and yield increases gradually,

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which makes the use of ETBE as gasoline additive increasingly wide over the

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developed countries. Despite the fact that the main gasoline additive is MTBE in

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China due to the higher cost and lower yield of ETBE, ETBE will also be used as

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additive widely with the upgrade of domestic production processes and manufacturing

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equipment.

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For engine performance research, ETBE is mainly used to study its reduction


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effects on engine emissions. According to the research of Croezen et al.,9 replacing

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MTBE with ETBE in fuel will bring a reduction of greenhouse gas emissions by

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1.94kg CO2/GJ fuels. And compared with other oxygenated additives, ETBE does not

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cause photochemical smog.10 In 2010, Westphal et al.11 studied the toxicity and BTEX

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(Benzene, Toluene, Ethylbenzene, Xylene) emissions of gasoline engine with the

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addition of ETBE. The result indicated that the addition of ETBE to gasoline can

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improve combustion and decrease emission of BTEX. As for diesel engine, Górski et

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al.12 discovered that the diesel-ETBE blends can reduce emission of particulates and

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soot by 36% and 70%, respectively. Besides the advantages of decreasing emission,

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ETBE can also improve the spray characteristics and keep the change of physical and

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chemical properties of diesel in acceptable range at the meanwhile. Also, compared

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with ethanol, ETBE is more suitable to be added into diesel in terms of solubility and

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stability. From the research above, we can learn that the addition of ETBE into

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common fuels can reduce the emission of CO2, HC, NOX, aromatics, PM and soot and

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improve spray characteristics, solubility and stability, which provides guidance and

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basis for its wide application in actual fuels.

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For chemical kinetics research of ETBE, many efforts have also been made by

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some researchers over the last decades. In 1968, Daly and Wentrup13 found that ETBE

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will decompose into ethanol and isobutene at the temperature range of 706-757 K and

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put forward the four-center unimolecular decomposition mechanism. Dunphy and

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Simmie14 measured the ignition delay times of ETBE in a shock tube over the high

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temperature range from 1160 K to 1830 K and the pressure of 3.5 bar. They came to a

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conclusion that the isobutene controls the ignition process and ETBE and MTBE have

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similar ignition behaviors. A few years later, Cathonnet et al.15 carried out ETBE

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oxidation experiments on a jet-stirred reactor (JSR) between 800 and 1150 K, at 10

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bar and a large range of equivalence ratio (0.5-2). Meanwhile, El Kadi and Baronnet16

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utilized a static reactor to study the oxidation of ETBE at 550-800 K and concluded

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that the formation of isobutene could make an explanation for its antiknock effect.

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Experiments covering the low and high temperature oxidation regimes were

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conducted in a JSR by Dagaut et al.17 to demonstrate the inhibitory effects of ETBE

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on n-heptane oxidation. Their findings indicated that ETBE strongly influences the

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n-heptane oxidation rate only below 800 K and the presence of ETBE reduces the

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emission of butadiene but increases the emission of formaldehyde and acetaldehyde

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moderately. Experimental data attained by Goldaniga et al.18 in a JSR at a residence


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Energy & Fuels

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time of 0.5 s and pressure of 10 bar were used to validate a semi-detailed oxidation

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mechanism of ether oxidation and combustion, and the good agreement was observed

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between model prediction and experimental data. In 2000, Glaude et al.19 developed a

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kinetic mechanism automatically generated by EXGAS and modeled the JSR

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experimental results of Dagaut et al.17 The discrepancy between modeling and the

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experimental values was very small. Over the next few years, little work had been

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done on chemical reaction kinetics of ETBE until 2007. In this year, Ogura et al.20

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constructed an ETBE sub-mechanism from 2, 2-dimethyl-pentane and its derivatives

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by replacing the CH2 group for an O atom in those species by KUCRS.21 Yasunaga et

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al.22 from Currran’s research group studied the high temperature pyrolysis of ETBE

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behind reflected shock waves coupled with single-pulse method and UV absorption

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spectroscopy in the temperature range of 1000-1500 K and pressure of 1-9 bar and put

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forward a 170-reaction mechanism. Almost at the same year, Yahyaoui et al.23

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measured a series of ignition delay times of ETBE over a wide range of temperature

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(1280-1750 K), pressure (2-10 bar) and equivalence ratio (0.25-1.5) and laminar

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flame speeds of ETBE at room temperature and atmospheric pressure. In this study,

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Yahyaoui et al.23 also compared the experimental ignition delay time and laminar

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flame speed with the computed values calculated by a detailed chemical kinetic

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reaction mechanism and analyzed the main pathways and sensitivity. Then Gong24

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measured the laminar flame speeds of the blending of ETBE, TBA and ethanol at

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different temperature (373 K, 423 K, 473 K) and different pressure (1 bar, 2.5 bar, 5

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bar) and analyzed the flame instability of the blending. A high temperature chemical

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kinetic mechanism of ETBE, MTBE, EME and DEE consisting of 215 species and

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1051 reactions constructed by Yasunaga25 was validated by a series of shock tube

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experimental data. Recently, Liu26 determined the oxidation characteristic and

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products of ETBE using ARC (accelerating rate calorimetry) and considered that the

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oxidation reaction process of ETBE with oxygen occurred through absorption of

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oxygen by ETBE, followed by thermal decomposition and oxidation reaction. Liu

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also found that ETBE is easy to absorb oxygen in the air and organic peroxides will

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accelerate the oxidation of ETBE once generated. More recently, extinction limit

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measurements for ethers were carried out by Hashimoto et al.27 using a counterflow

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burner. According to the experimental results, ETBE, DIPE and TAME almost have

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the same extinction strain rate and the extinction limit of ethers decreases with the

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increase of carbon atoms. As has been said, many studies of ETBE available in the


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literature about chemical kinetics are mainly focus on the ignition delay times, mole

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fractions of radicals and the developments of chemical kinetic reaction mechanism.

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However, laminar flame speeds of ETBE are rare except the only experimental data23

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at room temperature and atmosphere pressure. As we know, laminar flame speed is

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also an important parameter of hydrocarbons and can be used to validate the chemical

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kinetic mechanism, develop gasoline surrogate mechanism and provide the guidance

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for combustor design of engine and numerical simulation of turbulent combustion.

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Due to various advantages mentioned above of ETBE and the importance of

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laminar flame speeds of ETBE for mechanism validation and development and

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engineering application, propagating spherical flame method was applied to attain the

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laminar flame speeds of ETBE at different temperatures and pressures in a constant

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volume bomb in this study. The laminar flame speeds of MTBE, ethanol, iso-octane

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and gasoline were also measured at certain conditions for the purpose of comparison

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with ETBE. Additionally, two different chemical kinetic mechanisms of ETBE were

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validated with the experimental results. Finally, the discrepancy of laminar flame

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speeds of the tested fuels was explained by sensitivity analysis and radical

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concentration analysis.

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2. EXPERIMENTAL SETUP AND DATA PROCESSING

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2.1. Experimental Setup

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In this study, the laminar flame speeds were measured using the spherically

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propagating flame in a constant volume vessel. The detailed description of the

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apparatus has been introduced in literatures28, 29 and here only brief introduction is

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provided. The combustion chamber is cylindrical with inner diameter of 180 mm and

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volume of 5.8 L. Two glass windows for optical purpose with visible diameter of

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80mm are mounted at both ends of the cylinder. The heating-tape surrounding outside

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of the cylinder was used to heat the chamber and the temperature in the chamber was

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monitored by a K-type thermocouple with the accuracy of ±2 K. High precision

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pressure transmitter with relative pressure deviation of ±0.075% was used to prepare

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the mixtures. The liquid fuel was injected into the chamber by micro syringes. The

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mixtures are kept at least 3 minutes to guarantee full evaporation and high mixing

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homogeneity before the mixtures was ignited with the electrodes located in the

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chamber center. The spherical expanding flame was recorded by a high-speed camera

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Phantom V611 at 10000 fps.

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ETBE, MTBE, ethanol and iso-octane have the high purity of over 99%. The tests


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Energy & Fuels

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of ETBE were performed with initial temperatures of 298 K, 373 K and 453 K and

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initial pressure of 1atm, 3atm and 5atm. Under each temperature and pressure, the

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equivalence ratios cover the range of 0.7-1.6. For comparison, the tests of MTBE,

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ethanol, iso-octane and gasoline were conducted at certain conditions shown in Table

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2.

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2.2. Data Processing

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The flame radius history Rf = Rf (t) could be acquired from schlieren pictures

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recorded by the camera. Then the stretched flame propagation speed can be deduced

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from the equation: Sb 

dR f

(1)

dt

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after getting the Sb , the unstreched flame propagation speed was extrapolated

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employing the nonlinear expression of Frankel and Chen30, 31: Sb  Sb0 

2Sb0 Lb Rf

(2)

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where Lb is Markstein length of the burned mixture. According to the mass

217

conservation across the flame front, the laminar burning velocities could be calculated

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from the following equation: S u0  S b0

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b u

(3)

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

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Various factors, such as mixture preparation, ignition, buoyancy, instability,

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confinement, radiation, nonlinear stretch behavior, and extrapolation, will undermine

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the accuracy of the laminar flame speeds obtained from outwardly spherical

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propagating flames.32 In order to avoid the effect of ignition,33 space confinement34

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and pressure increase during late flame propagation,35 the flame radius ranging from

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9mm to 25mm were picked for the image processing. The uncertainty caused by

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radiation could be modified with an expression of Yu et al.36 : S u0, RCFS  S u0, Exp  0.82S u ,Exp (

S u0,Exp S0

) 1.14 (

Tu pu 0.3 )( ) T0 p0

(4)

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where S0=1 cm/s, T0=298 K, and p0=0.1 MPa. S0u,RCFS and S0u,Exp are the

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radiation-corrected laminar flame speed and the experimental values. In 1988, Moffat

229

et al.37 put forward a method of describing the uncertainties in an engineering

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experiment considering both system errors and random errors, which is widely used in


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evaluating uncertainty of laminar flame speed. The detail expression of the method is:

 S  ( BS ) 2  (t M 1.95   S ) 2 0 u

0 u

(5)

0 u

232

where tM-1.95 is the student’s t distribution at 95% confidence interval and the degrees

233

of M-1 and M is the experimental repetition times;  S 0 means the standard deviation u

234

of S u0 ; BS 0 represents the total uncertainty of the measurement method, which can u

235

be acquired from:

BS 0  u

n

 (ui i 1

Su0 ( xi ) 2 ) xi

(6)

236

where xi and ui are the influence factors of S u0 and the deviation of every factor xi,

237

respectively. Thus, the experimental uncertainty of this study was at the range of

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about 1-4cm/s according to the above analysis.

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2.3. Computational Methods

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Laminar flame speed calculations were carried out using PREMIX code38 of

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CHEMKIN-PRO package39. In present work, CRECK mechanism40 and Curran

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mechanism25, 41 were used to simulate the laminar flame speeds of ETBE. CRECK

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mechanism (Version 1412, PRF+PAH+Alcohols+Ethers) consists of 225 species and

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7645 reactions, of which the ethers sub-mechanism is derived from Goldaniga18.

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Curran mechanism is made up of 215 species and 1051 reactions. Besides, the

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underlying mechanism of Curran mechanism was updated with AramcoMech2.042-48

247

and a modified mechanism, called Modified Curran mechanism, was developed. The

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Modified Curran mechanism is given in the Supporting Information. For comparative

249

purpose, the ethanol mechanism of Leplat et al.,49 consisting of 36 species and 252

250

reactions which has been comprehensively validated, was used to calculate the

251

experimental results of ethanol. Similarly, a mechanism of iso-octane of Chaos et al.50

252

involving 107 species and 723 reactions which has been validated against a set of

253

experimental data, was used to simulate the experimental results of iso-octane. During

254

the simulation, Soret effect and mixture-averaged transport were considered.

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3. RESULTS AND DIUCUSSION

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3.1. Apparatus Validation

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Figure 1 shows the laminar flame speed of ETBE at 1 atm and 298K, compared

258

with experimental data from Yahyaoui23 under the same experimental condition. The

259

two sets of data show good agreement with each other, which prove the good

260

performance of the experimental apparatus. Similarly, Figure 2 compares the laminar


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flame speed of ethanol at 1 atm and 373 K published by different research groups51-54.

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It can be seen that the present data are very close to the data of Konnov,51 Broustail52

263

and Bradley,53 which can also verify the accuracy of the experimental data in this

264

study. All the measured laminar flame speed values and corresponding errors at

265

various temperatures and pressures are given in the Appendix.

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3.2. Laminar Flame Speeds of ETBE

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Generally, there is a close relationship between turbulent flame speed and laminar

268

flame speed. The turbulent flame speed can provide guidance for the combustion

269

modeling and engine design, so the laminar flame speed is very important in practical

270

application. The empirical formula of laminar flame speed by fitting the existing

271

experimental data is obviously vital for application. Because the laminar flame speed

272

varies with equivalence ratio, temperature and pressure, the following equation is

273

used to fit:

Su0  Su0,ref (

Tu  pu  ) ( ) Tu ,ref pu ,ref

(7)

274

where the Tu ,ref =298 K, pu ,ref =1 atm and the reference laminar flame speed Su0,ref ,

275

temperature dependence exponents β and pressure dependence exponents θ are regard

276

as a third order polynomial of equivalence ratio as following:

Su0,ref   0  1   2 2   3 3

(8)

  0  1   2 2  3 3

(9)

   0  1   2 2  3 3

(10)

277

During the fitting, the unconstrained minimization function fminunc in MATLAB

278

was employed to solve the twelve coefficients above and the detail results were

279

shown in Table 3.

280

Figure 3 shows the measured and fitting laminar flame speed of ETBE at different

281

temperatures and different pressures. The maximum value of laminar flame speed

282

appears near the equivalence ratio of 1.1 for each experimental condition. As is shown

283

in Figure 3(a), the laminar flame speed increases monotonically with the increase of

284

temperature, which is mainly due to the increase of adiabatic flame temperature with

285

initial temperature increasing. Additionally, the equivalence ratio of maximum

286

laminar flame speed slightly shifts to the direction of rich mixture with the increasing

287

temperature. And in Figure 3(b), we can see opposite tendency as the pressure


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288

increases compared with the result of various temperatures. Main reason behind this

289

phenomenon is the competition between the chain branching reactions and chain

290

termination reactions. As for the lines in Figure 3, they are the fitted results with Eq.(7)

291

using the fminunc function of MATLAB. The fitted results have good agreement with

292

the experimental results.

293

3.3.

Mechanism Validation and Optimization

294

In this part, the laminar flame speeds of ETBE were simulated using the latest

295

ethers mechanisms (Curran mechanism and CRECK mechanism). Figure 4 gives the

296

comparison between the measured and simulated laminar flame speeds at different

297

temperatures and pressures. From this figure, we can see that both Curran and

298

CRECK mechanism do not give satisfactory prediction overall and even all the peak

299

values of Curran mechanism appear at the equivalence ratio of 1.2. Only when the

300

equivalence ratio is larger than 1.3, the simulated results deviate from experimental

301

results relatively slightly. When the equivalence ratio is smaller than 1.3, the

302

simulated results of Curran mechanism are larger than the measured results and the

303

simulated results of CRECK mechanism are smaller than the corresponding measured

304

results. Additionally, with the increase of temperature and pressure, the deviation

305

between simulated results and experimental data also increase. Therefore, the

306

prediction performance of Curran and CRECK mechanisms does not meet the

307

requirements of understanding the chemical kinetics deeply.

308

To improve the prediction performance of Curran mechanism and understand the

309

chemical kinetics of ETBE, some refinements were done for the Curran mechanism.

310

As mentioned in section 2.3, the underlying mechanism of Curran mechanism was

311

updated and a mechanism called Modified Curran mechanism was developed. The

312

Modified Curran mechanism was employed to simulate the experimental results, as

313

shown in Figure 5. Compared to the Curran mechanism, the Modified Curran

314

mechanism gives good prediction for such experimental conditions globally. Firstly,

315

the maximum laminar flame speed lies at the equivalence ratio of 1.1, which is

316

consistent with the experimental results. Secondly, the prediction performance

317

improves greatly for lean mixtures and stoichiometric mixture. Finally, the deviation

318

from experimental data becomes smaller than that of Curran mechanism with the

319

increase of temperature and pressure.

320

The ignition delay time was also validated for the Modified Curran mechanism.

321

Figure 6 shows the comparison of measured and simulated ignition delay time of


Page 10 of 40

Page 11 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

322

ETBE. The agreement between Modified Curran mechanism and measured data is

323

more satisfactory than that of Curran mechanism, especially for the conditions of high

324

temperature. But when the temperature is lower than 1400K, Modified Curran

325

mechanism still over-predicts the experimental data slightly, which means there are

326

still some works to do on the Modified Curran mechanism.

327

To clearly understand why the Modified Curran mechanism behaves better on

328

predicting laminar flame speed than Curran and CRECK mechanisms, the normalized

329

sensitivity analysis for ETBE was carried out using these three mechanisms

330

respectively, as shown in Figure 7. We can see that the three mechanisms’ sensitivity

331

analysis differs greatly from each other. Firstly, the three most important reactions

332

promoting laminar flame speed are: R1, O2+HO+OH, R2, CO+OHCO2+H,

333

and R3, HCO+MH+CO+M for the three mechanisms and the two most important

334

reactions inhibiting the laminar flame speed are: R4, H+OH+MH2O+M and R5,

335

H+O2(+M)HO2(+M). But the reaction rate constants of the five reactions are

336

different from each other, as the underlying mechanism was different, which may

337

account for the differences of laminar flame speed. The Curran and CRECK

338

mechanism were developed in 2011 and 2014, respectively and nobody updated them

339

until now with the quick development of chemical reaction kinetics. So the authors

340

update

341

AramcoMech2.0 mechanism which includes the sub-mechanisms of C1-C4 based

342

hydrocarbon and oxygenated fuels. Secondly, the reactions involving IC4H8, IC4H7

343

and IC4H9 appear to be sensitive for Curran mechanism, but such phenomenon does

344

not happen to the other two mechanisms. We can infer that the production of IC4H8

345

dominates the reaction of the system partly for Curran mechanism, but the system

346

reaction is not restricted by IC4H8 broadly for Modified Curran and CRECK

347

mechanisms, which probably means the three mechanisms may differ in the IC4H8

348

sub-mechanism thus causing different prediction performance. Thus the explanation

349

will be developed around the above two main differences with reaction pathway

350

analysis, comparison of reaction rates and laminar flame speed calculations.

the

Curran

mechanism’s

underlying

mechanism

with

the

latest

351

First of all, the reaction pathway analysis of IC4H8 coupling with reaction rates was

352

conducted to illustrate the differences. As shown in Figure 8, the reaction pathway of

353

IC4H8 differs greatly from each mechanism. In order to explain the differences, the

354

comparison of mole fraction of OH, H and O radicals (Figure 9) and rate constants for

355

different consumption pathways of IC4H8 (Figure 10) between Curran, Modified


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

356

Page 12 of 40

Curran and CRECK mechanism was carried out.

357

There are six kinds of main consumption pathways of IC4H8 in the three

358

mechanisms. And with the exception of reaction pathway of IC4H8+HIC4H7+H2,

359

the other five pathways differs greatly among the three mechanisms. From the Figure

360

R10(a), we can find the rate constant of IC4H8+HIC4H9 in Modified Curran

361

mechanism is far less than that in Curran and CRECK mechanism and thus there

362

almost no IC4H8 reacts with H radical to produce IC4H9 in Modified Curran

363

mechanism. As the AramcoMech2.0 mechanism includes the latest detailed

364

sub-mechanism of IC4H8, so the IC4H8 sun-mechanism is more accurate and

365

comprehensive in Modified Curran mechanism than the other two mechanisms. Thus

366

we may conclude that the rate constants of IC4H8+HIC4H9 are too high in Curran

367

and

368

IC4H8+OHIC4H7+H2O, the consumption branching ratio in Modified Curran

369

mechanism is higher than that in the other two mechanism. Because of the similarities

370

in OH radical mole fractions and rate constants in three mechanisms, we can’t explain

371

the branching ratio difference by rate constants. But as some IC4H8 is consumed by

372

the reaction of IC4H8+HIC4H9 in Curran and CRECK mechanisms, the

373

consumption percentages of IC4H8+OHIC4H7+H2O are lower naturally. Not like

374

Curran and Modified Curran mechanisms, the pathway of IC4H8+HC3H6+CH3 is

375

absence in CRECK mechanism, which means the sub-mechanism of IC4H8 in

376

CRECK is incomplete. From Figure 9, we can find mole fractions of O radical are

377

close in the three mechanisms, so the consumption percentages of IC4H8 are

378

determined by the rate constants of IC4H8+OIC4H7+OH. Since the rate constants

379

of IC4H8+OIC4H7+OH in Curran mechanism is far less than that in Modified

380

Curran and CRECK mechanism, IC4H8 is hardly consumed by O radicals in Curran

381

mechanism, which means there are some problems in the rate constants of

382

IC4H8+OIC4H7+OH in Curran mechanism. Generally the H atoms of IC4H8 at two

383

locations can be removed by OH radical, but there exits only one kind of

384

H-abstraction reaction by OH radical in Curran and CRECK mechanisms, which

385

means the sub-mechanisms of IC4H8 of Curran and CRECK mechanisms are

386

incomplete.

CRECK

mechanisms.

As

for

the

IC4H8

consumption

pathway

of

387

Like IC4H8, the comparison of rate constants for different consumption pathways

388

of IC4H7 (Figure 11) between Curran, Modified Curran and CRECK mechanism was

389

also carried out to explain the reaction pathway differences of IC4H7. From Figure 8,


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Energy & Fuels

390

we can find that IC4H7 is mainly consumed by reaction of IC4H7AC3H4+CH3

391

(59.53%) and the other IC4H7 is consumed by reaction of IC4H7+HIC4H8 in

392

Modified Curran mechanism (34.34%). On the contrary, IC4H7 is mainly consumed

393

by reaction of IC4H7+HIC4H8 (64.59%) in Curran mechanism. It can be find that

394

the rate constants of IC4H7+HIC4H8 in Curran and Modified Curran mechanisms

395

are close to each other during the whole reaction temperature range from Figure 11(a),

396

which can’t account for the higher branching ratio of IC4H7+HIC4H8 in Curran

397

mechanism. Also we can find the rate constants of IC4H7AC3H4+CH3 in Modified

398

Curran mechanism is at least 3 times higher than that in Curran mechanism, which

399

can account for the higher branching ratio of IC4H7+AC3H4+CH3 in Modified

400

Curran mechanism. After replacing the rate constants in Curran mechanism with that

401

of

402

IC4H7+HIC4H8 and IC4H7AC3H4+CH3 are changed to 34.94% and 62.03%

403

respectively, which is very close to that of Modified Curran mechanism. Thereby, we

404

can draw a conclusion that the rate constant of IC4H7AC3H4+CH3 in Curran

405

mechanism is too low. As for CRECK mechanism, another reaction pathway of

406

IC4H7+OCH2CCH3+CH3 appears and its branching ratio is the highest among the

407

three reaction pathways. But there doesn’t include such reaction pathway of

408

IC4H7+OCH2CCH3+CH3 in Modified Curran mechanism, which perhaps mean

409

this pathway isn’t reasonable in CRECK mechanism.

Modified

Curran

mechanism,

we

can

find

the

branching

ratio

of

410

Secondly, the rate constants comparison of the three most important reactions

411

promoting laminar flame speed (R1-R3) and two most important reactions inhibiting

412

laminar flame speed (R4-R5) was also conducted in order to explain the differences of

413

Curran, Modified Curran and CRECK mechanisms. As the hydrogen oxidation

414

mechanism is a central part of many combustion systems, the precision of the rate

415

constants are of great importance (R1, R4, R5). The correlative reactions of CO are

416

also of great importance for hydrocarbon fuels (R2, R3). Figure 12 gives out the

417

detailed comparison of rate constants of R1-R5. Rate constants of many gas-phase

418

elementary reactions’, which have been measured by several groups and at a given

419

temperature, typically have an uncertainty of 10-30%, even for the best know

420

reactions.55 The chain-branching reaction R1: H+O2OH+H is the best known

421

elementary reaction and even considering an uncertainty of 10% on its rate constants,

422

it can cause an accuracy of 30% on the prediction of mechanisms56. From Figure

423

12(a), we can find the difference of rate constants of R1 is about 10% at temperature


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

424

above 1000K between Curran and Modified Curran mechanisms. The corresponding

425

difference between CRECK and Modified Curran mechanisms is about 20-50%,

426

which means the rate constants of R1 in CRECK mechanism are too high. As for R2:

427

CO+OHCO2+H, we can find there has 3 times difference of rate constants

428

between CRECK and modified mechanisms at temperature about 1000 K and the rate

429

constants are very similar among the three mechanisms at temperature above 1600 K.

430

The difference of rate constants of R3 between Curran and Modified Curran

431

mechanisms is 20%, but the difference between CRECK and Modified Curran

432

mechanisms is about 3 times at temperature over 2000K. Compared to R1-R3, the

433

differences of rate constants of R4 and R5 are relatively small, which are no more

434

than 30% and 15% for R4 and R5, respectively. In order to evaluate such differences’

435

effects on the prediction performance of mechanisms, the rate constants of R1-R5 in

436

Curran and CRECK mechanisms were replaced with that of Modified mechanism

437

respectively to calculate laminar flame speed of ETBE at temperature of 373 K and

438

pressure of 1 atm, as shown in Figure 13.

439

As it can be seen from Figure 13(a), replacing rate constants of R2, R3 and R5

440

has little effects on the modeling result of Curran mechanism. And after replacing rate

441

constants of R1 and R4, there have lower and higher modeling results for R1 and R4

442

respectively. However, all the replacing of rate constants doesn’t give better

443

prediction performance and the phenomenon of equivalence ratio deviation still exists.

444

Thus we may come to a conclusion that the crux of Curran mechanism lies on IC4H8

445

sub-mechanism instead of R1-R5. Unlike Curran mechanism, the replacing brings a

446

different outcome for CRECK mechanism. Except R2 and R5, changing rate

447

constants of the other three sensitive reactions creates larger effects on laminar flame

448

speed calculation results. Changing rate constants of R3 and R4 makes the modeling

449

results further deviate from experimental results. Only when the rate constant of R1 is

450

replaced, CRECK mechanism presents a better prediction performance than original

451

CRECK mechanism. Consequently, we may consider the difference of between

452

CRECK and Modified mechanisms not only lies on IC4H8 sub-mechanism, but also

453

some central elementary reactions, such as R1-R5.

454

3.4. Comparison of Laminar Flame Speed

455

Before the ETBE was widely used as gasoline antiknock additive, MTBE played

456

the role of antiknock additive in some Europe and American countries. And the flame

457

speed is vital in spark ignition engine design. So, in order to clarify if replacing


Page 14 of 40

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Energy & Fuels

458

MTBE with ETBE as additive will have an effect on the flame speed of gasoline, the

459

comparison of laminar flame speeds between ETBE and MTBE was conducted.

460

Figure 14 illustrates the laminar speed of ETBE and MTBE at atmospheric pressure.

461

It can be seen that the two ethers’ laminar flame speeds are very close to each other

462

and the biggest discrepancy is less than 3 cm/s at the whole range of equivalence

463

ratio.

464

The laminar flame speed similarity of the two ethers indicates a corresponding

465

similarity with the flame structure and processes in the flame. Ji et al.57 stated that

466

since the initial fuel reacts to form fuel fragments relatively fast, it is the fuel fragment

467

products that enter the active oxidation zone and their subsequent reactions, that

468

control the heat release and the flame propagation. Figure 15 shows the temperature

469

and heat release profiles for the two ethers calculated with Modified Curran

470

mechanism. It is seen that the temperature and heat release profiles for the two ethers

471

are almost identical. Generally speaking, these two parameters are the most important

472

factors influencing the chemical reactions and flame propagation. The congruence of

473

their profiles is consistent with the result that the flame speeds are so similar.

474

Besides the similarities of temperature and heat release profiles for the two ethers,

475

we can find that their key reactions and corresponding sensitivity coefficients are also

476

basically identical from Figure 16. Table 4 gives the ETBE and MTBE’s main

477

consumption pathways at 373 K, 1atm and . We can see that both ETBE and

478

MTBE are predominantly consumed by H-abstraction reactions instead of

479

decomposition reactions and the consumption pathways are very similar. Additionally,

480

we can know that ETBE and MTBE consume nearly at the same rate along the whole

481

reaction zone from the Figure 17 of the fuel fractions of ETBE and MTBE. According

482

the analyses above, we can know that all of the flame structure, sensitivity analysis,

483

main consumption pathways and fuel fraction of ETBE and MTBE are similar, which

484

perhaps give supports to the observed similarity in the laminar flame speed. In the

485

research of Dunphy and Simmie14, we can learn that ETBE and MTBE almost have

486

the same ignition delay times and their oxidation rates are very close based on the

487

shock tube experiment results. The ignition delay times and laminar flame speeds of

488

ETBE and MTBE show similarities, which means replacing MTBE with ETBE as

489

additive is absolutely feasible and reasonable on the fundamental combustion

490

characteristics.

491

Besides MTBE and ETBE, ethanol is also added into gasoline as gasoline additive.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

492

Iso-octane is commonly used as the surrogate or representative species for gasoline

493

fuels. Thus, the laminar flame speed of ETBE is also compared with that of ethanol,

494

iso-octane, as well as gasoline. Figure 18 shows the laminar flame speed of ETBE,

495

ethanol, iso-octane and gasoline at 373 K and 453 K and corresponding simulated

496

results of ethanol and iso-octane. It is seen that the peak of laminar flame speed for

497

these four fuels appears at the equivalence ratio around 1.1 at both 373 K and 453 K.

498

Among the four fuels, ethanol has the largest laminar flame speed. For the other three

499

fuels, the differences among their laminar flame speeds are relatively small, especially

500

for the iso-octane and gasoline. Overall, in the lean mixture conditions, laminar flame

501

speed between ETBE, iso-octane and gasoline differs little and the difference is no

502

more than 3 cm/s. While in rich mixture conditions, the maximum gap is less than 6

503

cm/s and ETBE has the largest flame speed among the three fuels. Therefore mixing

504

ETBE with gasoline as additive will not influence the flame speed of gasoline greatly.

505

To explain the reasons for this discrepancy of laminar flame speed of ETBE,

506

ethanol and iso-octane, the thermodynamic analysis and chemical kinetic analysis

507

were conducted. Figure 19 depicts the adiabatic flame temperature of ETBE, ethanol

508

and iso-octane at 373 K and 453 K. It can be seen that ethanol has the lowest adiabatic

509

flame temperature; ETBE and iso-octane have almost the same adiabatic flame speed

510

and the largest difference is less 6 K.

511

Combining Figure 18 and Figure 19, the laminar flame speed and adiabatic flame

512

temperature of ethanol, ETBE and iso-octane presents the opposite tendency, which

513

means chemical kinetic plays a more important role in determining the laminar flame

514

speed than thermodynamic factor. In order to further interpret the laminar flame speed

515

differences, the sensitivity analyses of ethanol, ETBE and iso-octane were also

516

provided at the condition of T=373K, p=1atm, =1.0, as shown in Figure 14. The

517

reaction of O2+HO+OH has the largest positive coefficient among the 20 most

518

sensitive reactions for the three fuels.

519

Figure 20(a) gives the 20 most sensitive reactions of ETBE. There are four

520

reactions involving species of IC4H8, which generates IC4H7 by dehydrogenation

521

reaction. IC4H8 and IC4H7 will consume large amount of H and OH radicals and are

522

inactive. So they will inhibit the combustion and lower the globle reactivity of ETBE.

523

Similarly, among the sensitive reaction of iso-octane as shown in Figure 20(b),

524

there are also some reactions involing IC4H8 and IC4H7, as well as C3H6. C3H6 will

525

consume OH radical and produce propenyl C3H5-A. Like IC4H8 and IC4H7, C3H6 and


Page 16 of 40

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Energy & Fuels

526

C3H5-A will also consume active radicals and are less active. So These radicals will

527

reduce the reactivity of iso-octane.

528

In Figure 20(c), all the sensitive reactions are the elementary reactions involving

529

only C1-C2 species for ethanol. Thus the ethanol generates small species quickly and

530

has higher chemical reactivity. Additionally, the small species diffuse faster than

531

larger species generally. Thereby, ethanol has the larger laminar flame speed

532

compared to ETBE and iso-octane.

533

According to the analysis above, ETBE and iso-octane have lower laminar flame

534

speed than ethanol, though ethanol has lower adiabatic flame temperature the ETBE

535

and iso-octane. As for who has higher laminar flame speed between ETBE and

536

iso-octane, more experiments and deep thermodynamic, diffusion and kinetic analysis

537

should be done to come to a clear conclusion.

538

To support the analysis from the sensitivity analysis, the mole fractions of some

539

active species (H, OH, O) and inactive species (IC4H8, IC4H7, C3H6, C3H5-A) are

540

shown in Figure 21. The mole fractions of H, OH and O of ethanol are higher than

541

that of ETBE and iso-octane, especially for the H radical, so the reactivity of ethanol

542

is the highest. And the differences between ETBE and iso-octane are very small in

543

terms of mole fractions of active species. Unlike ETBE and iso-octane, ethanol

544

generates very little IC4H8, IC4H7, C3H6, C3H5-A, which consume large amount of

545

active radicals and inhibit the overall reactivity during the combustion process, as

546

shown in Figure 21(b). Meanwhile, mole fractions of some inactive species between

547

ETBE and iso-octane differ little. Therefore, the results of radical analysis are

548

consistent with the results of sensitivity analysis, which proves the validity of the

549

laminar flame speed results.

550

3.5. Markstein length

551

Markstein length represents the flame sensitivity to stretch and characterize the

552

flame instability. Positive Markstein length means stable flame front and negative

553

Markstein length means unstable flame front. And Bradley58 put forward that the

554

flame keeps stable until the critical flame radius is reached when the Markstein length

555

is larger than 1.5. Although the Markstein lengths of different fuels vary from each

556

other, the change rule of Markstein length respect to the equivalence ratio is very clear.

557

In order to reduce the uncertainty, the Markstein length is the average of three times

558

experimental results. Figure 22 shows the Markstein length of ETBE at different

559

temperatures and different pressures.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

560

At each temperatures and pressures, the Markstein length of ETBE decreases with

561

the increase of equivalence ratio, so the flame stability decreases gradually. The

562

reasons are that ETBE is heavier than air and the heat diffusion and mass diffusion are

563

non-equal. From Figure 22(a), we can see that temperature has little significant effect

564

on Markstein length. The differences between the Markstein lengths under different

565

temperatures are almost less than 0.4mm. The effects of pressure on Markstein length

566

are shown in Figure 22(b). As the pressure increases, the Markstein length decreases

567

and the flame becomes more and more unstable. The same conclusion can be drawn

568

based on the experimental schilieren images under different pressures. The higher the

569

pressure, the earlier the cellularity appears.

570

The measured Markstein lengths of ETBE, ethanol, iso-octane and gasoline were

571

also compared with each other, as shown in Figure 23. With the increase of

572

equivalence ratio, the Markstein lengths of all tested fuels decrease at both

573

temperatures. Among the tested fuels, gasoline has the smallest Markstein lengths at

574

both temperatures, which means its flame front is the most unstable at the tested

575

conditions. Additionally, the differences of Markstein lengths between ETBE and

576

iso-octane are very small at the whole range of equivalence ratio. When 1.2, the result is just contrary. Therefore, under lean conditions, the flame fronts of

579

ETBE and iso-octane might be more stable and under rich conditions, the flame front

580

of ethanol might be the most stable.

581

4. CONCLUSIONS

582

In this work, the laminar burning characteristics of ETBE were conducted at

583

different temperatures (298K, 373K, 453K) and pressures (1atm, 3atm, 5atm) using a

584

constant volume bomb. Besides, the laminar burning characteristics of MTBE,

585

ethanol, iso-octane and gasoline were compared to that of ETBE. The CRECK

586

mechanism, Curran mechanism and modified Curran mechanism were used to model

587

the laminar flame speeds of ETBE. Sensitivity analyses of the tested fuels were made

588

to interpret the differences between the Curran and modified Curran mechanism and

589

the differences of laminar flame speed between the tested fuels. The main conclusions

590

are as follows:

591

(1) Laminar flame speeds of all the tested fuels reach the peak value at the

592

equivalence ratio of 1.1. As the ignition delay times, the laminar flame speeds of

593

ETBE and MTBE are very close with each other, which indicates that replacing


Page 18 of 40

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Energy & Fuels

594

MTBE with ETBE in gasoline is absolutely feasible and reasonable. Ethanol

595

shows the fastest laminar flame speed and the other three fuels (ETBE,

596

iso-octane, gasoline) have similar laminar flame speeds. Sensitivity analysis of

597

ethanol indicates that the sensitive reactions of ethanol only involve species

598

whose number of C atoms is below 3. However, the sensitive reactions of ETBE

599

and iso-octane consist of several large species, such as IC4H8, IC4H7, C3H6 and

600

C3H5-A. Thus the laminar flame speed of ethanol is faster than that of ETBE and

601

iso-octane.

602

(2) Both the CRECK mechanism and Curran mechanism can’t well predict the

603

laminar flame speed of ETBE. The modified Curran mechanism has a better

604

prediction performance over the tested conditions. The sensitivity analysis and

605

pathway analysis of ETBE using Curran, Modified Curran and CRECK

606

mechanisms shows that the update of reaction rate constants for critical sensitive

607

reactions accounts for the better prediction performance.

608

(3) The results of Markstein length show that temperature has little effects on

609

Markstein lengths of ETBE and the Markstein lengths decrease as the pressure

610

increase. Among the tested fuels, gasoline has the lowest Markstein length and

611

its flame is the most unstable. Under lean conditions, the flame front of ETBE

612

and iso-octane might be more stable and under rich conditions, the flame front of

613

ethanol might be the most stable one.

614 615

ASSOCIATED CONTENT

616 617

Supporting Information

618

The measured laminar flame speeds and errors at various temperatures and pressures.

619

The Modified Curran mechanism (mechanism, thermodynamic data, transport data).

620

AUTHOR INFORMATION

621

Corresponding Authors *

623 624

E-mail: [email protected] (Erjiang Hu) State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, People’s Republic of China

625

ORCID

626

Erjiang Hu: 0000-0002-0762-9018

622


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

627 628 629

Notes The authors declare no competing financial interest.

630 631

ACKNOWLEDGEMENTS

632

This study is supported by the National Natural Science Foundation of China (Grants

633

91641124 and 91441118). The support from the Fundamental Research Funds for the

634

Central Universities is also appreciated.

635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668

REFERENCES (1) Tran, L. S.; Sirjean, B.; Glaude, P. A.; Fournet, R. PROGRESS IN DETAILED KINETIC MODELING OF THE COMBUSTION OF OXYGENATED COMPONENTS OF BIOFUELS. Energy 2012, 43 (1), 4-18. (2) Squillace, P. J.; Zogorski, J. S.; And, W. G. W.; Price, C. V. Preliminary Assessment of the Occurrence and Possible Sources of MTBE in Groundwater in the United States, 1993−1994. Rev.fac.nac.salud Pública 1996, 30 (5), 39-42. (3) Silva, R. D.; Cataluña, R.; Menezes, E. W. D.; Samios, D.; Piatnicki, C. M. S. Effect of additives on the antiknock properties and Reid vapor pressure of gasoline. Fuel 2005, 84 (7–8), 951-959. (4) Yang, B. L.; Yang, S. B.; Yao, R. Q. Synthesis of ethyl tert -butyl ether from tert -butyl alcohol and ethanol on strong acid cation-exchange resins. Reactive & Functional Polymers 2000, 44 (2), 167-175. (5) Wu, X.; Li, Q.; Jin, F.; Tang, C.; Huang, Z.; Daniel, R.; Tian, G.; Xu, H. Laminar burning characteristics of 2,5-dimethylfuran and iso- octane blend at elevated temperatures and pressures. Fuel 2012, 95 (1), 234-240. (6) Rodríguez-Antón, L. M.; Hernández-Campos, M.; Sanz-Pérez, F. Experimental determination of some physical properties of gasoline, ethanol and ETBE blends. Fuel 2013, 112 (10), 178-184. (7) Malça, J.; Freire, F. Renewability and life-cycle energy efficiency of bioethanol and bio-ethyl tertiary butyl ether (bioETBE): Assessing the implications of allocation. Energy 2006, 31 (15), 3362-3380. (8) Menezes, E. W. D.; Cataluña, R. Optimization of the ETBE (ethyl tert -butyl ether) production process. Fuel Processing Technology 2008, 89 (11), 1148-1152. (9) Croezen, H.; Kampman, B. The impact of ethanol and ETBE blending on refinery operations and GHG-emissions. Energy Policy 2009, 37 (12), 5226-5238. (10) Matsumoto, N.; Sano, D.; Elder, M. Biofuel initiatives in Japan: Strategies, policies, and future potential. Applied Energy 2009, 86 (11), S69-S76. (11) Westphal, G. A.; Krahl, J.; Brüning, T.; Hallier, E.; Bünger, J. Ether oxygenate additives in gasoline reduce toxicity of exhausts. Toxicology 2010, 268 (3), 198-203. (12) Górski, K.; Sen, A. K.; Lotko, W.; Swat, M. Effects of ethyl-tert-butyl ether (ETBE) addition on the physicochemical properties of diesel oil and particulate matter and smoke emissions from diesel engines. Fuel 2013, 103 (1), 1138-1143. (13) Daly, N. J.; Wentrup, C. The thermal decomposition of t-butyl ethyl ether. Australian Journal of Chemistry 1968, 21 (6), 1535-&. (14) Dunphy, M. P.; Simmie, J. M. Preliminary observations on the high temperature oxidation of ethyl tert‐ butyl ether. International Journal of Chemical Kinetics 1991, 23 (6), 553-558. (15) Dagaut, P.; Reuillon, M.; Cathonnet, M.; Presvots, D. Gas chromatography and mass spectrometry identification of cyclic ethers formed from reference fuels combustion. Chromatographia 1995, 40 (3-4), 147-154.


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669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712

Energy & Fuels

(16) Kadi, B. E.; Baronnet, F. Study of the oxidation of unsymmetrical ethers (ETBE, TAME) and tentative interpretation of their high octane numbers. Journal de Chimie Physique et de Physico-Chimie Biologique 1995, 92, 706-725. (17) Dagaur, P.; Koch, R.; Cathonnet, M. The Oxidation of N-Heptane in the Presence of Oxygenated Octane Improvers: MTBE and ETBE. Combustion Science & Technology 1997, 122 (1-6), 345-361. (18) Goldaniga, A.; Faravelli, T.; Ranzi, E.; Dagaut, P.; Cathonnet, M. Oxidation of oxygenated octane improvers: MTBE, ETBE, DIPE, and TAME. Symposium on Combustion 1998, 27 (1), 353-360. (19)Glaude, P. A.; Battin-Leclerc, F.; Judenherc, B.; Warth, V.; Fournet, R.; Côme, G. M.; Scacchi, G.; Dagaut, P.; Cathonnet, M. Experimental and modeling study of the gas-phase oxidation of methyl and ethyl tertiary butyl ethers. Combustion & Flame 2000, 121 (1–2), 345-355. (20) Ogura, T.; Sakai, Y.; Miyoshi, A.; Koshi, M.; Dagaut, P. Modeling of the Oxidation of Primary Reference Fuel in the Presence of Oxygenated Octane Improvers: Ethyl Tert-Butyl Ether and Ethanol. Energy Fuels 2007, 21 (6), 3233-3239. (21) Miyoshi, A. In OS3-1 KUCRS - Detailed Kinetic Mechanism Generator for Versatile Fuel Components and Mixtures(OS3 Application of chemical kinetics to combustion modeling,Organized Session Papers), The ... international symposium on diagnostics and modeling of combustion in internal combustion engines, 2017; 2017; pp 116-121. (22) Yasunaga, K.; Kuraguchi, Y.; Hidaka, Y.; Takahashi, O.; Yamada, H.; Koike, T. Kinetic and modeling studies on ETBE pyrolysis behind reflected shock waves. Chemical Physics Letters 2008, 451 (4–6), 192-197. (23) Yahyaoui, M.; Djebailichaumeix, N.; Dagaut, P.; Paillard, C. E. Ethyl Tertiary Butyl Ether Ignition and Combustion Using a Shock Tube and Spherical Bomb. Energy & Fuels 2008, 22 (6), 3701-3708. (24) Gong, J.; Jin, C.; Huang, Z.; Wu, X. Study on Laminar Burning Characteristics of Premixed High-Octane Fuel−Air Mixtures at Elevated Pressures and Temperatures. Energy & Fuels 2010, 24 (24), 965-972. (25) Yasunaga, K.; Simmie, J. M.; Curran, H. J.; Koike, T.; Takahashi, O.; Kuraguchi, Y.; Hidaka, Y. Detailed chemical kinetic mechanisms of ethyl methyl, methyl tert -butyl and ethyl tert -butyl ethers: The importance of uni-molecular elimination reactions. Combustion & Flame 2011, 158 (6), 1032-1036. (26)Liu, X.; Ito, S.; Wada, Y. Oxidation characteristic and products of ETBE (ethyl tert-butyl ether). Energy 2015, 82, 184-192. (27) Hashimoto, J.; Hosono, J.; Shimizu, K.; Urakawa, R.; Tanoue, K. Extinction limits and flame structures of ETBE, DIPE and TAME non-premixed flames. Proceedings of the Combustion Institute 2017, 36 (1), 1439-1446. (28) Hu, E. J.; Huang, Z. H.; He, J. J.; Jin, C.; Miao, H. Y.; Wang, X. B. Measurements of laminar burning velocities and flame stability analysis for hydrogen-air-diluent mixtures. Chinese Science Bulletin 2009, 54 (5), 846-857. (29) Tang, C.; Huang, Z.; Wang, J.; Zheng, J. Effects of hydrogen addition on cellular instabilities of the spherically expanding propane flames. International Journal of Hydrogen Energy 2009, 34 (5), 2483-2487. (30) FRANKEL, M. L.; SIVASHINSKY, G. I. On Effects Due To Thermal Expansion and Lewis Number in Spherical Flame Propagation. Combustion Science & Technology 1983, 31 (3-4), 131-138. (31) Chen, Z. On the extraction of laminar flame speed and Markstein length from outwardly propagating spherical flames. Combustion & Flame 2011, 158 (2), 291-300. (32) Chen, Z. On the accuracy of laminar flame speeds measured from outwardly propagating spherical flames: Methane/air at normal temperature and pressure. Combustion & Flame 2015, 162 (6), 2442-2453. (33) Bradley, D.; Gaskell, P. H.; Gu, X. J. Burning velocities, markstein lengths, and flame quenching for spherical methane-air flames: A computational study. Combustion & Flame 1996, 104 (1–2), 176-198. (34) Burke, M. P.; Chen, Z.; Ju, Y.; Dryer, F. L. Effect of cylindrical confinement on the determination of laminar


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713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756

flame speeds using outwardly propagating flames. Combustion & Flame 2009, 156 (4), 771-779. (35) Zhang, Z.; Huang, Z.; Wang, X.; Xiang, J.; Wang, X.; Miao, H. Measurements of laminar burning velocities and Markstein lengths for methanol–air–nitrogen mixtures at elevated pressures and temperatures. Combustion & Flame 2008, 155 (3), 358-368. (36) Yu, H.; Han, W.; Santner, J.; Gou, X.; Sohn, C. H.; Ju, Y.; Chen, Z. Radiation-induced uncertainty in laminar flame speed measured from propagating spherical flames. Combustion & Flame 2014, 161 (11), 2815-2824. (37) Moffat, R. J. Describing uncertainties in experimental results. Exp Therm Fluid Sci J 1:3-7. Experimental Thermal & Fluid Science 1988, 1 (1), 3-17. (38) Kee, R. J.; Grcar, J. F.; Smooke, M. D.; Miller, J. A.; Meeks, E. PREMIX. A Fortran Program for Modeling Steady Laminar One-dimensional Premixed Flames. 1985, 143 (5), 65. (39) Reaction Design: San Diego, C.-P., CHEMKIN-PRO 15112, Reaction Design: San Diego, 2011. 2011. (40) Ranzi, E.; Corbetta, M.; Manenti, F.; Pierucci, S. Kinetic modeling of the thermal degradation and combustion of biomass. Chemical Engineering Science 2014, 110 (7), 2-12. (41) Yasunaga, K.; Gillespie, F.; Simmie, J. M.; Curran, H. J.; Kuraguchi, Y.; Hoshikawa, H.; Yamane, M.; Hidaka, Y. A multiple shock tube and chemical kinetic modeling study of diethyl ether pyrolysis and oxidation. Journal of Physical Chemistry A 2010, 114 (34), 9098-9109. (42) Li, Y.; Zhou, C. W.; Somers, K. P.; Zhang, K.; Curran, H. J. The oxidation of 2-butene: A high pressure ignition delay, kinetic modeling study and reactivity comparison with isobutene and 1-butene. Proceedings of the Combustion Institute 2017, 36(1), 403–411. (43) Zhou, C. W.; Li, Y.; O'Connor, E.; Somers, K. P.; Thion, S.; Keesee, C.; Mathieu, O.; Petersen, E. L.; Deverter, T. A.; Oehlschlaeger, M. A. A comprehensive experimental and modeling study of isobutene oxidation. Combustion & Flame 2016, 167, 353-379. (44) Burke, U.; Metcalfe, W. K.; Burke, S. M.; Heufer, K. A.; Dagaut, P.; Curran, H. J. A detailed chemical kinetic modeling, ignition delay time and jet-stirred reactor study of methanol oxidation. Combustion & Flame 2016, 165 (5), 125-136. (45) Burke, S. M.; Burke, U.; Mc Donagh, R.; Mathieu, O.; Osorio, I.; Keesee, C.; Morones, A.; Petersen, E. L.; Wang, W.; DeVerter, T. A.; Oehlschlaeger, M. A.; Rhodes, B.; Hanson, R. K.; Davidson, D. F.; Weber, B. W.; Sung, C.-J.; Santner, J.; Ju, Y.; Haas, F. M.; Dryer, F. L.; Volkov, E. N.; Nilsson, E. J. K.; Konnov, A. A.; Alrefae, M.; Khaled, F.; Farooq, A.; Dirrenberger, P.; Glaude, P.-A.; Battin-Leclerc, F.; Curran, H. J. An experimental and modeling study of propene oxidation. Part 2: Ignition delay time and flame speed measurements. Combustion and Flame 2015, 162 (2), 296-314. (46) Burke, S.; Metcalfe, W.; Herbinet, O.; Battinleclerc, F.; Haas, F.; Santner, J.; Dryer, F.; Curran, H. J. In An experimental and modeling study of propene oxidation. Part 1: Speciation measurements in jetstirred and flow reactors, International Symposium on Web and Wireless Geographical Information Systems, 2014; 2014; pp 171-186. (47) Metcalfe, W. K.; Burke, S. M.; Ahmed, S. S.; Curran, H. J. A Hierarchical and Comparative Kinetic Modeling Study of C 1 − C 2 Hydrocarbon and Oxygenated Fuels. International Journal of Chemical Kinetics 2013, 45 (10), 638–675. (48) Kéromnès, A.; Metcalfe, W. K.; Heufer, K. A.; Donohoe, N.; Das, A. K.; Sung, C. J.; Herzler, J.; Naumann, C.; Griebel, P.; Mathieu, O. An experimental and detailed chemical kinetic modeling study of hydrogen and syngas mixture oxidation at elevated pressures. Combustion & Flame 2013, 160 (6), 995-1011. (49) Leplat, N.; Dagaut, P.; Togbé, C.; Vandooren, J. Numerical and experimental study of ethanol combustion and oxidation in laminar premixed flames and in jet-stirred reactor. Combustion & Flame 2011, 158 (4), 705-725. (50) Chaos, M.; Kazakov, A.; Zhao, Z.; Dryer, F. L. A high‐temperature chemical kinetic model for primary


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reference fuels. International Journal of Chemical Kinetics 2007, 39 (7), 399-414. (51) Konnov, A. A.; Meuwissen, R. J.; Goey, L. P. H. D. The temperature dependence of the laminar burning velocity of ethanol flames. Proceedings of the Combustion Institute 2011, 33 (1), 1011-1019. (52) Broustail, G.; Seers, P.; Halter, F.; Moréac, G.; Mounaim-Rousselle, C. Experimental determination of laminar burning velocity for butanol and ethanol iso-octane blends. Fuel 2011, 90 (1), 1-6. (53) Bradley, D.; Lawes, M.; Mansour, M. S. Explosion bomb measurements of ethanol–air laminar gaseous flame characteristics at pressures up to 1.4 MPa. Combustion & Flame 2009, 156 (7), 1462-1470. (54) Egolfopoulos, F. N.; Du, D. X.; Law, C. K. A study on ethanol oxidation kinetics in laminar premixed flames, flow reactors, and shock tubes. Symposium on Combustion 1992, 24 (1), 833-841. (55) Turányi, T.; Nagy, T.; Zsély, I. G.; Cserháti, M.; Varga, T.; Szabó, B. T.; Sedyó, I.; Kiss, P. T.; Zempléni, A.; Curran, H. J. Determination of rate parameters based on both direct and indirect measurements. International Journal of Chemical Kinetics 2012, 44 (5), 284–302. (56) Miller, J. A.; Pilling, M. J.; Troe, J. Unravelling combustion mechanisms through a quantitative understanding of elementary reactions. Proceedings of the Combustion Institute 2005, 30 (1), 43-88. (57) Ji, C.; Dames, E.; Wang, Y. L.; Wang, H.; Egolfopoulos, F. N. Propagation and extinction of premixed C 5 – C 12 n -alkane flames. Combustion & Flame 2010, 157 (2), 277-287. (58) Bradley, D.; Lawes, M.; Liu, K.; Verhelst, S.; Woolley, R. Laminar burning velocities of lean hydrogen–air mixtures at pressures up to 1.0 MPa. Combustion & Flame 2007, 149 (1), 162-172.

775 776


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

List of Tables and Figures

777 778

Table 1. Properties of tested fuels.1, 5

779

Table 2. Experimental conditions of tested fuels.

780

Table 3. Fitting coefficients of reference laminar flame speed, temperature and

781

pressure.

782

Table 4. Comparison of the main consumption pathways between ETBE and MTBE.

783

Figure 1. Comparison of laminar flame speed of ETBE at 1 atm and 298 K.

784

Figure 2. Comparison of laminar flame speed of ethanol at 1 atm and 373 K.

785

Figure 3. Laminar flame speeds of ETBE at different temperatures and pressures.

786

Figure 4. Comparison of measured (symbols) and simulated laminar flame speeds

787

(Dot line: Curran mechanism, dash line: CRECK mechanism) of ETBE.

788


789

(Dot line: Curran mechanism, solid line: Modified Curran mechanism) of ETBE.

790

Figure 6. Comparison of measured (symbols) and simulated ignition delay time (Dot

791

line: Curran mechanism, solid line: Modified Curran mechanism) of ETBE.

792

Figure 7. Comparison of sensitivity analyses of ETBE between Curran, Modified

793

Curran and CRECK mechanism.

794

Figure 8. Comparison of reaction pathway analyses of IC4H8 between Curran,

795

Modified Curran and CRECK mechanism.

796

Figure 9. Comparison of mole fractions of OH, H and O radicals between Curran,

797


798

Figure 10. Rate constants comparison for different consumption pathways of IC4H8

799

between Curran, Modified Curran and CRECK mechanism.

800


801


802

Figure 12. Comparison of rate constants of R1-R5 between Curran, modified Curran

803

and CRECK mechanisms.

804

Figure 13. Effects of replacing rate constants of R1-R5 on the prediction of laminar

805

flame speed.

806

Figure 14. Comparison of laminar flame speed between ETBE and MTBE.

807

Figure 15. Temperature and heat release profiles for ETBE and MTBE flames

808

calculated with Modified Curran mechanism, =1.0, T=373 K, p=1 atm.

809

Figure 16. Sensitivity analysis comparison of ETBE and MTBE, =1.0, T=373 K, p=1

810

atm.


Page 24 of 40

Page 25 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

811

Figure 17. Fuel fraction of ETBE and MTBE at 373 K, 1 atm and =1.0.

812

Figure 18. Laminar flame speed of ETBE, ethanol, iso-octane and gasoline at 373K

813

and 453K.

814

Figure 19. Adiabatic flame temperature of ETBE, ethanol and iso-octane at 373K and

815

453K.

816

Figure 20. Sensitivity analysis of ethanol, ETBE and iso-octane at =1.0, T=373K,

817

p=1atm.

818

Figure 21. Mole fractions of some main species of ethanol, ETBE, iso-octane flames

819

at T=373K, p=1atm, =1.0.

820

Figure 22. Measured Markstein length of ETBE at different temperatures and

821

pressures.

822

Figure 23. Measured Markstein length of ETBE, ethanol, iso-octane and gasoline at

823

373K and 453K.

824


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

825

Page 26 of 40

Table 1. Properties of tested fuels.1, 5 ETBE

MTBE

Ethanol

Iso-octane

Gasoline

C4H9OC2H5

C4H9OCH3

C2H5OH

(CH3)2CHCH2C(CH3)₃

C4-C14

102.17

88.14

46.07

114.23

≈102

Oxygen content (Wt%)

15.7

18.2

34.8

0

≈1.8

RVP (kPa)

30.6

55

15.6

-

45-85

Density (kg/m3)

750

740

790

691.9

0.74

Boiling point (K)

346.0

328.5

351.6

372.0

303-473

LHV (MJ/kg)

35.9

35.2

26.8

44.3

42.7

LHV (MJ/L)

26.93

26.04

21.2

30.7

31.60

RON/MON

117/101

118/101

109/90

100/100

92-98/82-88

12.16

11.74

9.00

15.13

14.18

Chemical formula MW (g/mol)

Stoichiometric A/F ratio

826 827

Table 2. Experimental conditions of tested fuels.

Initial temperature T/K Initial pressure p/atm Equivalence ratio 

ETBE

MTBE

Ethanol

Iso-octane

Gasoline

298、373、453

298、373

373、453

373、453

373、453

1、3、5

1

1

1

1

0.7-1.6

0.7-1.6

0.7-1.6

0.7-1.6

0.7-1.6

828 829

Table 3. Fitting coefficients of reference laminar flame speed, temperature and

830

pressure.

Su0,ref

β

θ

α0

-137.51

β0

-0.57

θ0

0.12

α1

377.34

β1

5.15

θ1

-1.99

α2

-255.62

β2

-4.81

θ2

2.48

α3

48.24

β3

1.82

θ3

-0.91

831 832 833 834 835


Page 27 of 40

836

Table 4. Comparison of the main consumption pathways between ETBE and MTBE. Reactions

Consumption Pathway/ %

ETBE+HTC4H9OC2H4S+H2 ETBE+HC2H5OC4H8I+H2 ETBE(+M)IC4H8+C2H5OH(+M) ETBE+OHC2H5OC4H8I+H2O ETBE+HTC4H9OC2H4P+H2 ETBE+OHTC4H9OC2H4P+H2O ETBE+OC2H5OC4H8I+OH ETBE+OTC4H9OC2H4S+OH ETBE(+M)C2H4+TC4H9OH(+M) ETBE(+M)C2H5O+TC4H9(+M)

33.74 22.16 12.49 10.45 7.39 3.48 3.45 1.52 1.21 1.04

Reactions

Consumption Pathway/ %

MTBE+HTC4H9OCH2+H2 MTBE+HCH3OC4H8I+H2 MTBE+OHCH3OC4H8I+H=O MTBE(+M)IC4H8+CH3OH(+M) MTBE+OCH3OC4H8I+OH MTBE+OTC4H9OCH2+OH MTBE+OHTC4H9OCH2+H2O MTBE+HO2CH3OC4H8I+H2O2 MTBE(+M)CH3O+TC4H9(+M) MTBE(+M)CH3+TC4H9O(+M)

837

40

ETBE T=298K p=1atm

0

Su (cm/s)

30

20

Present work Yahyaoui [24]

10 0.6

0.8

1.0

838 839

1.2

1.4

1.6



Figure 1. Comparison of laminar flame speed of ETBE at 1 atm and 298 K.

70 60

ethanol T=373K p=1atm

50 Present work Konnov [52] Broustail [53] Bradley [54] Egolfopoulos [55]

40

0

Su (cm/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

30 20 10 0.6

0.8

1.0

1.2

1.4

1.6



840 841

Figure 2. Comparison of laminar flame speed of ethanol at 1 atm and 373 K.


71.41 10.65 8.32 4.91 1.69 1.05 0.58 0.50 0.20 0.18

Energy & Fuels

70 60

(a)

ETBE p=1atm

0

Su (m/s)

50 40 30

T=298K T=373K T=453K Line:fitting line

20 10 0.6

0.8

1.0

1.2

1.4

1.6



842

50

(b)

ETBE T=373K

30

p=1atm p=3atm p=5atm Line:fitting line

0

Su (m/s)

40

20 10 0.6

0.8

1.0

843 844

1.2

1.4

1.6



Figure 3. Laminar flame speeds of ETBE at different temperatures and pressures.

80 70

ETBE p=1atm

(a)

60 50

0

Su (cm/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

40 30 20

T=298K T=373K Dot line： Curran T=453K Dash line：CRECK

10 0 0.6

845

0.8

1.0

1.2

1.4

1.6




Page 28 of 40

Page 29 of 40

ETBE T=373K

50

(b)

0

Su (cm/s)

40 30 20

p=1atm p=3atm Dot line： Curran p=5atm Dash line：CRECK

10 0 0.6

0.8

1.0

1.2

1.4

1.6



846 847


848

(Dot line: Curran mechanism, dash line: CRECK mechanism) of ETBE.

849 80 70

ETBE p=1atm

(a)

60

0

Su (cm/s)

50 40 30 20 10 0 0.6

T=298K T=373K Dot line： Curran T=453K Solid line：Modified Curran 0.8 1.0 1.2 1.4 1.6 

850 50

(b)

ETBE T=373K

40

0

Su (cm/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

30 20

p=1atm p=3atm Dot line： Curran p=5atm Solid line：Modified Curran

10 0 0.6

851

0.8

1.0

1.2

1.4

1.6



852


853

(Dot line: Curran mechanism, solid line: Modified Curran mechanism) of ETBE.


Energy & Fuels

10000

Ignition delay time (s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

p5=3.5bar 1000

100 Yahyaoui [24] Dunphy and Simmie [15] Curran Modified Curran

10

1 6.0

854

0.3% ETBE + 4.5% O2 + 95.2% Ar

6.4

6.8

7.2 4

7.6

8.0

-1

10 /T (K )

855

Figure 6. Comparison of measured (symbols) and simulated ignition delay time (Dot

856

line: Curran mechanism, solid line: Modified Curran mechanism) of ETBE.

857

858 859

Figure 7. Comparison of sensitivity analyses of ETBE between Curran, Modified

860

Curran and CRECK mechanism.

861


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Energy & Fuels

862 863

Figure 8. Comparison of reaction pathway analyses of IC4H8 between Curran,

864


865

866 867

Figure 9. Comparison of mole fractions of OH, H and O radicals between Curran,

868


869


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a) IC4H8+HIC4H9

Page 32 of 40

(b) IC4H8+OIC4H7+OH

870

Figure 10. Rate constants comparison for different consumption pathway of IC4H8

871


872

(a) IC4H7+HIC4H8

(b) IC4H7AC3H4+CH3

873


874


875


Page 33 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(a) R1: H+O2O+OH

(b) R2: CO+OHCO2+H

(c) R3: HCO+MH+CO+M

(d) R4: H+OH+MH2O+M

(e) R5: H+O2(+M)HO2(+M) 876 877

Figure 12. Comparison of rate constants of R1-R5 between Curran, modified Curran

878

and CRECK mechanisms.

879


Energy & Fuels

(a) Curran mechanism

(b) CRECK mechanism

880

Figure 13. Effects of replacing rate constants of R1-R5 on the prediction of laminar

881

flame speed.

882 60

p=1atm

T=373K

50 40

0

Su (cm/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 40

T=298K

30 20 ETBE MTBE

10 0 0.6

883 884

0.8

1.0

1.2

1.4

1.6



Figure 14. Comparison of laminar flame speed between ETBE and MTBE.

885


Page 35 of 40

×10

10 2400

Temperature

2100 1800

6

1500

4

ETBE MTBE

1200 900 600

2 Heat release 0 0.10

0.15

Temperature (K)

8

Heat release rate (erg·cm-3·s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

300 0 0.25

0.20

Distance (cm)

886 887

Figure 15. Temperature and heat release profiles for ETBE and MTBE flames

888

calculated with Modified Curran mechanism, =1.0, T=373 K, p=1 atm.

889 H+OH+MH 2O+M H+O2(+M)HO 2(+M) CH3+H(+M)CH=(+M) HCO+HCO+H 2 HO2+HH2+O 2 C2H3+HC2H2+H2 IC4H8+OHIC4H7+H2O HCO+O2CO+HO 2 H2+OHH+H 2O

MTBE ETBE

CH3+OHCH 2OH+H H2+OH+OH

T=373K p=1atm =1

CH3+HO2CH3O+OH HO2+H2OH HCO+MH+CO+M CO+OHCO 2+H O2+HO+OH

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

Normalized sensitivity coefficients

890 891

Figure 16. Sensitivity analysis comparison of ETBE and MTBE, =1.0, T=373 K,

892

p=1 atm.

893


Energy & Fuels

ETBE MTBE

1.0

T=373K p=1atm 

Fuel fraction

0.8 0.6 0.4 0.2 0.0 0.12

0.13

895

0.14

0.15

0.16

0.17

0.18

Distance (cm)

894

Figure 17. Fuel fraction of ETBE and MTBE at 373 K, 1 atm and =1.0.

70 60

T=373K p=1atm

(a)

0

Su (m/s)

50 40

ETBE ethanol isooctane gasoline

30 20

Leplat-ethanol Chaos-isooctane

10 0.6

0.8

1.0

1.2

1.4

1.6



896 90 80

T=453K p=1atm

(b)

70 60

0

Su (m/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50 40 30

Leplat-ethanol Chaos-isooctane

20 10 0.6

0.8

1.0

1.2

1.4

1.6



897 898

Figure 18. Laminar flame speed of ETBE, ethanol, iso-octane and gasoline at 373K

899

and 453K.


Page 36 of 40

Page 37 of 40

2400

p=1atm Adiabatic flame temperature (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2300 2200 ETBE-373K ethanol-373K isooctane-373K ETBE-453K ethanol-453K isooctane-453K

2100 2000 1900 0.6

0.8

1.0

1.2

1.4

1.6



900 901

Figure 19. Adiabatic flame temperature of ETBE, ethanol and iso-octane at 373K and

902

453K.

903 H+OH+MH2O+M H+O2(+M)HO2(+M) CH3+H(+M)CH4(+M) HCO+HCO+H2 HO2+HH2+O2 C2H3+HC2H2+H2 IC4H8+OHIC4H7+H2O HCO+O2CO+HO2 IC4H8+HIC4H7+H2 IC4H8IC4H7+H C2H4+H(+M)C2H5(+M) IC4H8+OHIC4H7-I1+H2O H2+OHH+H2O CH3+OHCH2OH+H H2+OH+OH CH3+HO2CH3O+OH HO2+H2OH HCO+MH+CO+M CO+OHCO2+H O2+HO+OH

-0.2 904

-0.1

(a)

Modified Curran ETBE T=373K p=1atm =1

0.0 0.1 0.2 0.3 Normalized sensitivity coefficients


0.4

Energy & Fuels

H+OH+MH2O+M H+CH3(+M)CH4(+M) IC4H8IC4H7+H C3H6+OHC3H5-A+H2O H+HCOH2+CO H+O2(+M)HO2(+M) HCO+O2HO2+CO HO2+OHH2O+O2 OH+HCOH2O+CO O+CH3H+CH2O

O+CH3=>H+H2+CO O+H2H+OH HO2+CH3OH+CH3O IC4H7C3H4-A+CH3 CH2+O2=>2H+CO2 HCO+H2OH+CO+H2O HCO+MH+CO+M OH+CH3CH2(S)+H2O OH+COH+CO2 H+O2O+OH

-0.2 905

(b)

Chaos iso-octane T=373K p=1atm =1

-0.1 0.0 0.1 0.2 Normalized sensitivity coefficients

0.3

906 H+O2+MHO2+M H+HO2H2+O2 CH3+H(+M)CH4(+M) OH+HO2H2O+O2 HCO+O2CO+HO2 H+OH+MH2O+M HCO+HCO+H2 2CH3(+M)C2H6(+M) CH3CHOH+O2CH3HCO+HO2 O+H2OH+H OH+H2H+H2O CH2+O2CO2+2H CH3CHOH+HO2CH3HCO+2OH CH3+HO2CH3O+OH HCO+MH+CO+M CH3+OHCH2(S)+H2O HCO+H2OH+CO+H2O H+HO22OH CO+OHCO2+H O2+HO+OH

(c)

A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 40

-0.2 907

Leplat ethanol T=373K p=1atm =1

-0.1 0.0 0.1 0.2 Normalized sensitivity coefficients

0.3

908

Figure 20. Sensitivity analysis of ethanol, ETBE and iso-octane at T=373K, p=1atm,

909

=1.


Page 39 of 40

Species mole fraction

0.012

T=373K p=1atm =1.0

Pink line: ethanol Red line: ETBE Blue line: isooctane

(a)

Solid line: H Dash line:OH Dot line:O

0.008

0.004

0.000 0.0

0.1

0.2

0.3

0.4

0.5

Distance (cm)

910

0.004

Species mole fraction

T=373K p=1atm =1.0

Red line: ETBE Blue line: isooctane Solid line:

0.000 0.00

(b)

IC4H8

Dash line:

IC4H7

Dot line:

C3H6

Dash dot line: C3H5-A

0.002

0.05

0.10

0.15

0.20

Distance (cm)

911 912

Figure 21. Mole fractions of some main species of ethanol, ETBE, iso-octane flames

913

at T=373K, p=1atm, =1.0.

914 4 (a) 3 2

Lb (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1 0 -1 -2 0.6

915

ETBE p=1atm T=298K T=373K T=453K 0.8

1.0

1.2

1.4

1.6




Energy & Fuels

3

(b)

Lb (mm)

2 1

ETBE T=373K

0

p=1atm p=3atm p=5atm

-1 -2 0.6

0.8

1.0

1.2

1.4

1.6



916 917

Figure 22. Measured Markstein length of ETBE at different temperatures and

918

pressures. 4 (a) 3

Lb (mm)

2 1 0 -1


T=373K p=1atm

-2 0.6

0.8

1.0

1.2

1.4

1.6



919

2.0

(b)

1.5 1.0

Lb (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.5 0.0 -0.5 -1.0


T=453K p=1atm

-1.5 0.6

0.8

1.0

1.2

1.4

1.6



920 921

Figure 23. Measured Markstein length of ETBE, ethanol, iso-octane and gasoline at

922

373K and 453K.


Page 40 of 40

Laminar flame characteristics and kinetic modeling study of ETBE

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