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An experimental and modeling study of n-octanol combustion
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Proceedings of the Combustion Institute xxx (2014) xxx–xxx
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An experimental and modeling study of n-octanol combustion Liming Cai a,⇑, Yasar Uygun b, Casimir Togbe´ c, Heinz Pitsch a, Herbert Olivier b, Philippe Dagaut c, S. Mani Sarathy d a
Institute for Combustion Technology, RWTH Aachen University, 52056 Aachen, Germany b Shock Wave Laboratory, RWTH Aachen University, 52074 Aachen, Germany c CNRS-INSIS, 1C, Avenue de la recherche scientifique, 45071 Orle´ans cedex 2, France d Clean Combustion Research Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia
Abstract This study presents the first investigation on the combustion chemistry of n-octanol, a long chain alcohol. Ignition delay times were determined experimentally in a high-pressure shock tube, and stable species concentration profiles were obtained in a jet stirred reactor for a range of initial conditions. A detailed kinetic model was developed to describe the oxidation of n-octanol at both low and high temperatures, and the model shows good agreement with the present dataset. The fuel’s combustion characteristics are compared to those of n-alkanes and to short chain alcohols to illustrate the effects of the hydroxyl moiety and the carbon chain length on important combustion properties. Finally, the results are discussed in detail. Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: n-Octanol; Chemical mechanism; High pressure shock tube; Jet stirred reactor
1. Introduction Alcohols are regarded as alternative fuels and blending fuel components for internal combustion engines. There is a huge amount of experimental and numerical studies in the literature addressing short chain alcohols, e.g. methanol [1,2] and ⇑ Corresponding author. Address: Institute for Combustion Technology, RWTH Aachen University, Templergraben 64, 52056 Aachen, Germany. Fax: +49 241 80 92923. E-mail address: [email protected] (L. Cai).
ethanol [3,4], as well as for medium chain alcohols, such as the isomers of butanol [5,6] and pentanol [7,8]. Recently, a longer chain alcohol, noctanol, has been identified as a biofuel candidate that is obtained from biomass-derived platform chemicals [9]. While the shorter chain ethanol is well known as alternative fuel for spark-ignition engines [10,11], n-octanol can be applied in diesel engines due to its increased cetane number and because it is proven to allow for almost soot and NOx free combustion over a wide range of engine loads [12]. For a better understanding of its oxidation behavior, knowing its fundamental combustion characteristics is necessary. In addition, the
http://dx.doi.org/10.1016/j.proci.2014.05.088 1540-7489/Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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chemical reaction model of n-octanol is required in computational fluid dynamics (CFD) calculations to simulate its combustion performance in research and in engine development [13–15]. Nevertheless, no study on reaction kinetics of n-octanol is found in the literature. The main goal of this study is the first investigation of the combustion characteristics of n-octanol and to explore the features of the longer alcoholic fuels compared to n-alkanes and the shorter alcohols. A detailed kinetic model for n-octanol including both low- and high-temperature kinetic schemes is developed based on established reaction classes and rate rules. The ignition delay times sign of stoichiometric n-octanol/air mixtures are measured spanning a wide range of temperatures at high pressures of 20 and 40 bar in a high pressure shock tube (HPST). Concentration profiles of stable intermediates and products are determined for n-octanol oxidation in a jet stirred reactor (JSR) at 10 atm for a variety of equivalence ratios and temperatures. The agreement between measured and computed results meets expectations, and the model is further used to elucidate the oxidation pathways of n-octanol under representative conditions. The effects of chain length in alcoholic fuels and the differences between linear alkanes and alcohols are discussed in detail. 2. Kinetic model development In this section, the formulation of a comprehensive chemical kinetic model for n-octanol using current knowledge of combustion chemistry for structurally similar molecules, such as alcohols and alkanes, is described. 2.1. Naming of species n-octanol consists of a hydroxyl functional group with a normal octyl chain and is referred to as C8H17OH in the model (see Fig. 1 for its molecular structure). The carbon sites are labeled numerically (i.e., 1, 2, 3) starting with the location of the carbon bonded to the hydroxyl group. In this way, the n-octanol radical at the a site (i.e., carbon bonded to the oxygen site) is denoted by C8H16OH-1. In enol species, the location of a double bond is indicated by a hyphen followed by the
Fig. 1. Structure of n-octanol (C8H17OH).
number of the first carbon in the double bond, for instance C8H14OH-1. 2.2. General model features The proposed n-octanol kinetic model includes both low-temperature and high-temperature kinetics and is based on the hierarchical nature of chemical mechanisms for n-butanol [5] and n-pentanol [7]. The high-temperature fuel chemistry is required to simulate the high-temperature combustion phenomena, such as flame speed, flame extinction, and flame structure. The present work is concerned with simulating ignition delay times and JSR species profiles at low and intermediate temperatures. Under these conditions, low temperature reactions are of particular importance. The model was developed by going through the various reaction classes and selecting the corresponding rate rules. The sub-mechanism for C6–C8 alkanes and alkenes was extracted from Westbrook and Dryer [17] and integrated into the present model to describe the oxidation of the intermediate species formed during n-octanol combustion. The thermodynamic data and the transport properties for n-octanol and the related radicals were calculated using the RMG program [16]. The entire model consists of 1280 species and 5537 reactions. These input files for numerical simulation are available as supplementary material. Reaction classes and rate rules have been discussed extensively in former studies (e.g. [5,17– 19]). 30 reaction classes of elementary reactions and their rate rules that have been considered for butanol [5] and pentanol isomers [7,8] were adopted for this study with n-octanol. The major classes include unimolecular fuel decomposition, hydrogen abstraction from the fuel, fuel radical decomposition, and the reaction steps to complete the low temperature chain-branching channel. For a detailed discussion, see [5]. The most significant model developments in this study are as follows: As demonstrated in Refs. [5,7], C-H bond scission energies at a; b and c sites in alcohols are typically lower than those for alkanes. In this study, for n-octanol, the a; b, and c carbon sites were treated as alcohol specific, and thus the rate rules were adopted from analogies with n-butanol [5] and n-pentanol [7]. The remaining five C-H sites were treated like analogous sites in normal alkanes, since the effect of the hydroxyl group on these sites is insignificant. Their rate rules were taken from the previous studies on alkanes [17,18] and 2-methylalkanes [20]. The rate constants for the reaction class describing O2 with a-hydroxyoctyl radicals to directly form aldehyde +HO2 and the reaction class for O2 addition to a-hydroxyoctylhydrop-
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eroxide (QOOH) radicals were modified within their uncertainty bounds to predict ignition delay times at low temperatures more accurately. These treatments are consistent with the previous studies on n-butanol [5] and npentanol [7]. The high-pressure limit of the rate constants was decreased by a factor of 2.5, while the rates at low pressures were multiplied by 2.5. Recently, Rosado-Reyes and Tsang [21] experimentally determined the rate expressions for each channel contributing to n-butanol decomposition. The same reaction rates were applied to n-octanol decomposition for the analogous reactions. Several reaction pathways were added to the model based on recent experimental and theoretical studies by Welz et al. [22,23], who state that the pathways involving hydrogen transfer from the OH group are important for the cand d-hydroxyalkylperoxy (RO2) radicals via 7- and 8-membered transition state rings, similar to the Waddington reaction pathway [24,25]. Moreover, Welz et al. [23] also present an unconventional water elimination pathway from hydroxy-alkylhydroperoxide radicals, where the hydroperoxide is on the c site and the radical is on the a site. These reactions were included in the present model and their rate constants were selected based on the recent study for iso-pentanol oxidation [8].
3. Experimental facility 3.1. High pressure shock tube (HPST) The experiments determining ignition delay times were performed in a high pressure shock tube with an inner diameter of 140 mm and a total length of 15.5 m. The material of the shock tube is stainless steel and resists pressures up to 1000 bar in a heated state of 473 K. The 11 m long low pressure test section is separated from the 4 m driver section by a double diaphragm chamber. The HPST is therefore highly suitable for investigating low vapor pressure fuels as well as undiluted fuels at very high pressures and for a wide range of temperatures. A detailed explanation of this HPST is given in Ref. [26]. The preparation of the stoichiometric fuel/air (synthetic air: 20.5% O2 and 79.5% N2) mixture was performed directly in the evacuated low pressure section of the shock tube. After one hour of homogenization, the experiments were conducted. Due to the very low vapor pressure of n-octanol and the high filling pressure up to 4.3 bar, the shock tube was resistively (with heating wires) heated up to a temperature of about 400 K. The
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temperatures of possible condensation areas, such as the filling pipes as well as the fuel partial pressure, were controlled precisely to ensure the evaporation of the injected fuel. To exclude the influence of the high heating process, several experiments at the similar post-shock conditions with different initial temperatures were performed; no anomalies in the ignition delay time curves could be found. For the determination of the filling pressures, a high temperature resistive pressure gauge was used to ensure the complete transition of the fuel into the gas phase. For capturing the pressures during the experiment and hence the shock velocities, 8 Kistler 603B sensors were mounted at several positions of the test section. The positions began from the endwall up to a distance of 2510 mm. A photomultiplier (Hamamatsu, RR212UH) with a narrow band pass filter (LOT 430FS10, FWHM 10 2 nm) for the CH* emission detection was located 10 mm in front of the endwall. The gas temperature behind the reflected shock was calculated from the initial conditions and the shock velocities considering real gas effects using the in-house code KASIMIR [27]. The ignition delay time is determined as the time between the rise of pressure caused by the reflected shock and the steepest rise of the pressure signal due to ignition. The largest rate of increase of the emission signal (PMT) was also checked for comparison with the pressure signals, for which a deviation in ignition delay time less than 2.5% was found. The ignition delay times presented in this paper were obtained from the pressure sensor located at the 10 mm position. The uncertainties in the experimental temperatures are close to 10 K. Due to the large inner diameter of the shock tube, shock attenuation is <1%/ms and does not affect the measurements. The error of ignition delay measurements is determined to be within 20%. 3.2. Jet stirred reactor (JSR) The JSR experimental setup used here has been described earlier in [28,29]. The JSR consists of a 4 cm diameter fused silica sphere (42 cm3) equipped with four nozzles of 1 mm inner diameter. Before the injectors, the reactants were diluted with nitrogen (<100 ppm H2O, <50 ppm O2, <1000 ppm Ar, <5 ppm H2 from Air Liquide). A high degree of dilution (1000 ppm of fuel) was used, reducing temperature gradients and heat release in the JSR. The reactants were high-purity oxygen (99.995% pure form Air Liquide) and high-purity n-octanol, which was sonically degassed before use. The reactants were preheated before injection to minimize temperature gradients inside the reactor. A HPLC pump (Shimadzu LC10 AD VP) with an on-line degasser (Shimadzu DGU-20) was used to deliver n-octanol to an
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in-house atomizer-vaporizer assembly that was maintained at 210 °C. Good thermal homogeneity along the vertical axis of the reactor (gradients of <1 K/cm) was measured by a thermocouple (0.1 mm Pt-Pt/Rh-10%, located inside a thin-wall fused silica tube). The reacting mixtures were sampled using a movable fused silica low-pressure sonic probe. The samples were directed to analyzers via a Teflon heated line maintained at 200 °C. Analyses were performed online with a FTIR spectrometer (10 m path length, resolution of 0.5 cm1, 200 mbar in the cell) as well as off-line after collection and storage in 1 L Pyrex bulbs. Off-line analyses were performed with gas chromatographs (GC) which were equipped with capillary columns (60 m and 0.32 mm i.d.: DB-624; 50 m and 0.32 mm i.d.: CP-Al2O3-KCl; 25 m and 0.53 mm i.d.: Carboplot-P7), a TCD (thermal conductivity detector), and an FID (flame ionization detector). A GCMS (Shimadzu QP2010SE) operating with electron ionization (70 eV) was used for product identification. Steady state experiments were performed in this study at a constant mean residence time (s = 0.7 s) and pressure of 10 atm, with equivalence ratios / = 0.5, 1.0 and 2. The reactants flowed continually in the reactor and the temperature of the gases inside the JSR was increased stepwise. Good repeatability of the measurements and an atomic balance of carbon, oxygen, and hydrogen at the level of 100 10% were observed. The maximum uncertainty in the GC analysis was estimated to be 10%. 4. Results and discussion The following section presents the comparisons between experimental measurements and numerical calculations performed with the proposed model. In order to gain further insights into the impacts of the hydroxyl moiety on ignition of hydrocarbon species, kinetic modeling of ignition delay times for fuels that are structurally similar to n-octanol are performed to compare with those of n-octanol. In addition, reaction pathway analyses are presented to elucidate the oxidation of n-octanol under the conditions of interest.
the maximum rate of rise in temperature and pressure measured in the present HPST is negligible. The results obtained from the proposed model, compared to the measurements of n-octanol/air mixtures for / ¼ 1:0 at 20 and 40 bar, are shown in Fig. 2. It can be seen immediately that n-octanol shows a clear negative temperature coefficient (NTC) behavior, which is not observed for the shorter chain alcohols, such as n-butanol [5] and n-pentanol [35]. Using the present model, ignition delay times of stoichiometric n-octanol/air mixtures are predicted fairly well over the entire range experimentally investigated. More specifically, while the ignition delay times in the high temperature regime at 20 bar are slightly overpredicted, the computed ignition delay times appear very satisfactory for the intermediate to low temperatures, especially at the pressure of 40 bar. The observed deviations between simulations and experiments are within the reported experimental uncertainties. In general, the kinetic model presented in this paper is capable of predicting the experiments accurately. 4.2. Kinetic modeling simulations of JSR data The measurements in the JSR have been conducted at 10 atm and at equivalence ratios of 0.5, 1.0, and 2.0 for the temperature range of 500–1200 K with a constant mean residence time of 0.7 s. The initial fuel mole fraction is 1000 ppm. For the simulation, an end time of 20 s is specified to be sure that a steady state has been accomplished. As revealed in previous studies [33,34] and found also in this one, a comparison between experiment and simulation at / = 1.0 is representative for other equivalence ratios. Thus, it suffices to show only the experimental and numerical results for stoichiometric n-octanol/oxidizer mixtures (Fig. 3). The results for the equivalence ratios 0.5 and 2.0 are available in the supplementary material.
4.1. Kinetic modeling simulations of shock tube ignition delay times The numerical calculations presented in this study are performed using the appropriate reactor modules in the Chemkin PRO code [31] and the open source FlameMaster code [32]. Both codes yield identical numerical results. The computed ignition delay time is defined as the time necessary to reach the maximum rate of rise in temperature. As described in earlier publications [26,30], the difference between the times necessary to reach
Fig. 2. Ignition delay times of stoichiometric n-octanol/ air mixtures at 20 and 40 bar. Symbols denote the experimental data and solid lines show numerical results.
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Fig. 3. n-octanol oxidation in a JSR at 10 atm, t = 0.7 s, and / = 1.0. The initial fuel mole fraction is 1000 ppm. Symbols denote the experimental data and solid lines show numerical results.
Consistent with the experimental observations in the HPST, both experiments and simulations for the JSR configuration show a clear NTC behavior of n-octanol. The model gives satisfactory results for the major species, i.e., carbon monoxide (CO), carbon dioxide (CO2), water (H2O) and oxygen (O2) under all conditions. For the intermediate formaldehyde CH2O, the model slightly overpredicts its mole fraction in the high temperature regime, while it does a decent job at lower temperatures, especially in case of the minimum mole fraction in the NTC regime. As demonstrated in Fig. 3, the fuel reactivity in the cool flame regime is underpredicted by a factor of 2 using the proposed model. More importantly, however, the model correctly reflects the NTC behavior of n-octanol. The results reveal that the shapes of the model profiles for the minor species methane (CH4), ethylene (C2H4), ethane (C2H6), propene (C3H6), 1-butene (C4H8-1), 1-pentene (C5H10-1) and 1-hexene (C6H12-1) closely match the measured ones, but their maximum concentrations are slightly underpredicted. While the mole fraction profile of heptanal is very well reproduced in the simulations, the maximum concentrations of octanal and hexanal are overpredicted by more than a factor of 2, which could be attributed to the missing low-temperature consumption pathways of the large aldehydes or the overestimation of their primary formation. Recently, Veloo et al. [36] conducted an experimental and modeling study on the oxidation of n- and iso-butanal and revealed the importance of their low-temperature consumption reactions in accurately predicting the JSR data. In the presented mechanism, the simplified reaction schemes of the large aldehydes are included to describe the H-abstraction from aldehydes and further decomposition of aldehyde radicals, similar to the earlier
studies on butanol and pentanol isomers [5,7,8]. As found in these studies on alcohols, the global combustion characteristics, e.g. ignition delay times, are highly sensitive to the production of the large aldehydes. Nevertheless, their consumption channels are proven to be irrelevant for the ignition delays. These simplified schemes of the aldehyde consumption are generally adopted in a variety of modeling works [18,37,39]. As the model size can grow rapidly by including the detailed consumption channels of the large aldehydes and their theoretical and experimental kinetic knowledge are strongly limited, the aforementioned simplifications are further applied in this study for n-octanol. Generally speaking, the presented model has demonstrated the capability to predict the present data with satisfactory accuracy. 4.3. Kinetic modeling simulations of ignition delay times In order to gain further insights into the ignition behavior of n-octanol, numerical simulations with the proposed model were performed for the shock tube configuration and then compared with the results for species having a similar molecular structure, e.g. n-octane, n-butanol and n-pentanol. This improves the knowledge of the effects of the alcohol functional group and the chain length on auto-ignition. Figure 4 depicts the calculated ignition delay times for stoichiometric n-butanol/air [5], n-pentanol/air [7] and n-octanol/air mixtures at a pressure of 20 bar. Compared with n-butanol and n-pentanol, n-octanol exhibits a clear NTC behavior and a significantly stronger reactivity at low temperature. While the differences of ignition delays between three linear alcohols are comparably small in the high temperature range, the increased
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reduced reactivity in the low temperature regime. The reasons will be explored in the next section. At high temperatures, n-octanol shows an improved ignition propensity compared with noctane. This can be attributed to the different Habstraction rates by HO2 radicals in the n-octane [20] and n-octanol models, which are rate-determining in the high temperature range. The slower rates in the n-octane model [20] lead to its inhibited high temperature reactivity. 4.4. Reaction path analysis Fig. 4. Ignition delay times of n-butanol/air, n-pentanol and n-octanol/air mixtures at 20 bar and / = 1.0. Solid lines show numerical results.
chain length obviously contributes to an improved fuel reactivity at intermediate to low temperatures. The reason is that the chain branching pathways initialized by isomerization of peroxy radicals are promoted. One could expect that the low temperature and NTC ignition reactivity of long alcohols would not increase further after a critical length of carbon chain is reached, as demonstrated for n-alkanes by Westbrook and Dryer [17] and for 2-methylalkanes by Sarathy et al. [20]. The computed and experimental ignition delays for n-octanol are compared to those for n-octane [20] in Fig. 5. It should be mentioned here that the computed ignition delay times for n-octane are larger than the experimental observations reported in Ref. [20] due to the absence of rate constant tuning and the uncertainties in both the selection of reaction pathways and the estimation of reaction rate coefficients of the n-octane mechanism. Over the entire range of the investigated temperatures, n-octanol exhibits a similar ignition performance as n-octane besides the
Fig. 5. Ignition delay times of n-octane and n-octanol/ air mixtures at 20 bar and / = 1.0. Symbols denote the experimental data for n-octanol from this study and for n-octane from Ref. [20] and solid lines show numerical results.
Figure 6 illustrates the reaction path analysis for stoichiometric n-octanol/air combustion at 20 bar and temperatures of 769 K and 1000 K. The reaction fluxes are given at 30% fuel consumption. The oxidation of n-octanol is initiated via hydrogen atom abstraction from the fuel by molecular oxygen. Due to its merely slight contribution to the overall consumption of n-octanol, this pathway is not shown here. As demonstrated, n-octanol is mainly consumed by the build-up of OH. Through the abstraction of an H atom, eight primary radicals are produced. The H abstraction from the OH site is very minor at both considered conditions because of the strong OH bond dissociation energy [5,38]. It is therefore not presented in the pathway diagram. Similar to short alcoholic fuels [5,7], the production of the a radical is important at both temperatures. In contrast to the typical alkane pathways, the a hydroxyalkyl radicals react with oxygen mainly to form aldehydes and HO2 radicals instead of hydroxyalkyl peroxy radicals at low to intermediate temperatures, which prevents the degenerate chain branching pathway and retards the ignition of alcohols. Therefore, the short alcohols, n-butanol and n-pentanol, show a lower reactivity at low temperatures compared to their corresponding alkanes, i.e., n-butane and n-pentane as presented in Refs. [5,7]. However, this inhibitory effect decreases as the chain length grows in n-octanol. Since more carbon sites can be found in the molecule, the branching ratio producing a fuel radicals is comparatively decreased, even though this pathway remains favorable. Moreover, the increased chain length provides also the possibility to enhance the low temperature reactivity of the fuel, because the number of favorable 6-membered ring RO2 and peroxy-hydroxyalkylhydroperoxide (O2QOOH) radical isomerizations is increased. Overall, the effect of the hydroxyl functional group decreases for the larger alcohols, while its presence plays a major role in the short chains. Therefore, n-octanol shows a very similar ignition behavior as n-octane in Fig. 5. As shown in Fig. 6, except a radicals, the consecutive pathways of the other fuel radicals are almost identical to these of alkyl radicals. While the decomposition of the fuel radicals takes place
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Fig. 6. Reaction path analysis for n-octanol/air mixture at 769 K and 1000 K and at 30% fuel consumption, 20 bar, and / = 1.0.
at high temperatures, these radicals are oxidized by oxygen addition to yield peroxy radicals in the low temperature range. After the following isomerization and the second O2 addition, the peroxy-hydroxyoctylhydroperoxide radical is produced, which isomerizes rapidly and then decomposes to give an OH radical and a ketohydroperoxide. The important intermediate species, ketohydroperoxide, can decompose further to form the second OH radical, which completes the low temperature chain branching process. The isomerization of peroxy radicals competes with the production of enols and HO2 radicals. For b RO2 radicals, the Waddington reaction [24] can also take place and returns an OH radical, which results in a chain propagation pathway. However, the effect of this chain-propagating pathway is minor, as only a small number of b peroxy radicals are produced at the relevant conditions. To better understand the n-octanol kinetics, a sensitivity analysis for ignition delay times was performed at the same conditions. The results are presented in the supplementary material, and are consistent with those from the path analysis. The ignition delay times are found to be very sensitive towards the fuel H-abstraction by OH radicals and the isomerization of peroxy radicals.
5. Concluding remarks This paper presents the first chemical kinetic investigation on the oxidation of n-octanol. A detailed kinetic model for n-octanol combustion was developed by applying methods recently developed for n-butanol and n-pentanol combustion models. Ignition delay times and stable species concentration profiles were experimentally obtained for n-octanol oxidation in a shock tube and a jet stirred reactor, respectively. Good agreement between simulations and experiments was presented over a wide range of initial conditions. The major differences between linear alcohols and alkanes with the same carbon chain length are that the presence of the hydroxyl moiety enhances H atom abstraction from the a carbon site, and the subsequent a hydroxyalkyl radical’s reaction with O2 yields an aldehyde. This reaction sequence inhibits low to intermediate temperature chain branching in alcohol fuels. However, this inhibitory effect of the hydroxyl moiety on the ignition propensity is reduced for the longer chain n-octanol, because the greater number of carbon sites in the molecule decreases the branching fraction of the a fuel radical production. The present work demonstrates the ability to develop adequate chemical kinetic models for high
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molecular weight alcohols in the absence of fundamental experimental and theoretical studies on reaction rates. Further measurements and quantum chemistry calculations are required to verify the model’s predictive capability and to minimize predictive uncertainties. Hydrogen abstractions by OH and HO2 radicals are important reactions for ignition delay times. Their theoretical or experimental studies could lead to an improved prediction of fuel auto-ignition. Reactions, such as hydroxyalkylperoxy radical isomerization, formation of epoxy alcohols, and concerted elimination warrant also further investigation. In addition, the detailed oxidation chemistry of large aldehydes and enols is required to improve the prediction of JSR data of high molecular weight alcohol fuels.
Acknowledgments This work was performed as part of the Cluster of Excellence “Tailor-Made Fuels from Biomass” which is funded by the Excellence Initiative by the German federal and state governments to promote science and research at German universities and as part of the collaborative research center (SFB) 1029 which is funded by the German Research Foundation (DFG). The work at KAUST was funded by the Clean Combustion Research Center. Co-author S.M.S. acknowledges funding from the TMFB Visiting Fellowship program. At CNRS, the research leading to these results has received funding from the European Research Council under the European Community’s Seventh Framework Programme (FP7/ 2007-2013)/ERC Grant Agreement No. 2910492G-CSafe. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.proci.2014.05.088. References [1] C.K. Westbrook, F.L. Dryer, Combust. Flame 37 (1980) 171–192. [2] W. Ren, E. Dames, D. Hyland, D.F. Davidson, R.K. Hanson, Combust. Flame 160 (2013) 2669– 2679. [3] N. Leplat, P. Dagaut, C. Togbe´, J. Vandooren, Combust. Flame 158 (2011) 705–725. [4] P.S. Veloo, Y.L. Wang, F.N. Egolfopoulos, C.K. Westbrook, Combust. Flame 157 (2010) 1989–2004. [5] S.M. Sarathy, S. Vranckx, K. Yasunaga, M. Mehl, P. Osswald, W.K. Metcalfe, C.K. Westbrook, W.J. Pitz, K. Kohse-Ho¨inghaus, R.X. Fernandes, H.J. Curran, Combust. Flame 159 (2012) 2028–2055.
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L. Cai et al. / Proceedings of the Combustion Institute xxx (2014) xxx–xxx [30] S. Vranckx, J. Beeckmann, W.A. Kopp, C. Lee, L. Cai, H.K. Chakravarty, H. Olivier, K. Leonhard, H. Pitsch, R.X. Fernandes, Combust. Flame 160 (2013) 2680–2692. [31] Reaction Design, Inc. CHEMKIN-PRO Release 15101; Reaction Design, Inc.: San Diego, CA, 2010. [32] H. Pitsch. FlameMaster: A C++ Computer Program for 0D Combustion and 1D Laminar Flame Calculations, 1993. [33] G. Dayma, S. Gail, P. Dagaut, Energy Fuels 22 (2008) 1469–1479. [34] S. Gail, M. Thomson, S. Sarathy, S. Syed, P. Dagaut, P. Dievart, A. Marchese, F. Dryer, Proc. Combust. Inst. 31 (2007) 305–311.
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[35] C. Togbe´, F. Halter, F. Foucher, C. MounaimRousselle, P. Dagaut, Proc. Combust. Inst. 33 (2011) 367–374. [36] P.S. Veloo, P. Dagaut, C. Togbe´, G. Dayma, S.M. Sarathy, C.K. Westbrook, F.N. Egolfopoulos, Combust. Flame 160 (2013) 1609–1626. [37] C.K. Westbrook, W.J. Pitz, O. Herbinet, H.J. Curran, E.J. Silke, Combust. Flame 156 (2009) 181–199. [38] C. W Zhou, J.M. Simmie, H.J. Curran, Int. J. Chem. Kinet. 44 (2012) 155–164. [39] L. Cai, A. Sudholt, D.J. Lee, F.N. Egolfopoulos, H. Pitsch, C.K. Westbrook, S.M. Sarathy, Combust. Flame 161 (2014) 798–809.
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An experimental and modeling study of n-octanol combustion Liming Cai a,⇑, Yasar Uygun b, Casimir Togbe´ c, Heinz Pitsch a, Herbert Olivier b, Philippe Dagaut c, S. Mani Sarathy d a
Institute for Combustion Technology, RWTH Aachen University, 52056 Aachen, Germany b Shock Wave Laboratory, RWTH Aachen University, 52074 Aachen, Germany c CNRS-INSIS, 1C, Avenue de la recherche scientifique, 45071 Orle´ans cedex 2, France d Clean Combustion Research Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia
Abstract This study presents the first investigation on the combustion chemistry of n-octanol, a long chain alcohol. Ignition delay times were determined experimentally in a high-pressure shock tube, and stable species concentration profiles were obtained in a jet stirred reactor for a range of initial conditions. A detailed kinetic model was developed to describe the oxidation of n-octanol at both low and high temperatures, and the model shows good agreement with the present dataset. The fuel’s combustion characteristics are compared to those of n-alkanes and to short chain alcohols to illustrate the effects of the hydroxyl moiety and the carbon chain length on important combustion properties. Finally, the results are discussed in detail. Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: n-Octanol; Chemical mechanism; High pressure shock tube; Jet stirred reactor
1. Introduction Alcohols are regarded as alternative fuels and blending fuel components for internal combustion engines. There is a huge amount of experimental and numerical studies in the literature addressing short chain alcohols, e.g. methanol [1,2] and ⇑ Corresponding author. Address: Institute for Combustion Technology, RWTH Aachen University, Templergraben 64, 52056 Aachen, Germany. Fax: +49 241 80 92923. E-mail address: [email protected] (L. Cai).
ethanol [3,4], as well as for medium chain alcohols, such as the isomers of butanol [5,6] and pentanol [7,8]. Recently, a longer chain alcohol, noctanol, has been identified as a biofuel candidate that is obtained from biomass-derived platform chemicals [9]. While the shorter chain ethanol is well known as alternative fuel for spark-ignition engines [10,11], n-octanol can be applied in diesel engines due to its increased cetane number and because it is proven to allow for almost soot and NOx free combustion over a wide range of engine loads [12]. For a better understanding of its oxidation behavior, knowing its fundamental combustion characteristics is necessary. In addition, the
http://dx.doi.org/10.1016/j.proci.2014.05.088 1540-7489/Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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chemical reaction model of n-octanol is required in computational fluid dynamics (CFD) calculations to simulate its combustion performance in research and in engine development [13–15]. Nevertheless, no study on reaction kinetics of n-octanol is found in the literature. The main goal of this study is the first investigation of the combustion characteristics of n-octanol and to explore the features of the longer alcoholic fuels compared to n-alkanes and the shorter alcohols. A detailed kinetic model for n-octanol including both low- and high-temperature kinetic schemes is developed based on established reaction classes and rate rules. The ignition delay times sign of stoichiometric n-octanol/air mixtures are measured spanning a wide range of temperatures at high pressures of 20 and 40 bar in a high pressure shock tube (HPST). Concentration profiles of stable intermediates and products are determined for n-octanol oxidation in a jet stirred reactor (JSR) at 10 atm for a variety of equivalence ratios and temperatures. The agreement between measured and computed results meets expectations, and the model is further used to elucidate the oxidation pathways of n-octanol under representative conditions. The effects of chain length in alcoholic fuels and the differences between linear alkanes and alcohols are discussed in detail. 2. Kinetic model development In this section, the formulation of a comprehensive chemical kinetic model for n-octanol using current knowledge of combustion chemistry for structurally similar molecules, such as alcohols and alkanes, is described. 2.1. Naming of species n-octanol consists of a hydroxyl functional group with a normal octyl chain and is referred to as C8H17OH in the model (see Fig. 1 for its molecular structure). The carbon sites are labeled numerically (i.e., 1, 2, 3) starting with the location of the carbon bonded to the hydroxyl group. In this way, the n-octanol radical at the a site (i.e., carbon bonded to the oxygen site) is denoted by C8H16OH-1. In enol species, the location of a double bond is indicated by a hyphen followed by the
Fig. 1. Structure of n-octanol (C8H17OH).
number of the first carbon in the double bond, for instance C8H14OH-1. 2.2. General model features The proposed n-octanol kinetic model includes both low-temperature and high-temperature kinetics and is based on the hierarchical nature of chemical mechanisms for n-butanol [5] and n-pentanol [7]. The high-temperature fuel chemistry is required to simulate the high-temperature combustion phenomena, such as flame speed, flame extinction, and flame structure. The present work is concerned with simulating ignition delay times and JSR species profiles at low and intermediate temperatures. Under these conditions, low temperature reactions are of particular importance. The model was developed by going through the various reaction classes and selecting the corresponding rate rules. The sub-mechanism for C6–C8 alkanes and alkenes was extracted from Westbrook and Dryer [17] and integrated into the present model to describe the oxidation of the intermediate species formed during n-octanol combustion. The thermodynamic data and the transport properties for n-octanol and the related radicals were calculated using the RMG program [16]. The entire model consists of 1280 species and 5537 reactions. These input files for numerical simulation are available as supplementary material. Reaction classes and rate rules have been discussed extensively in former studies (e.g. [5,17– 19]). 30 reaction classes of elementary reactions and their rate rules that have been considered for butanol [5] and pentanol isomers [7,8] were adopted for this study with n-octanol. The major classes include unimolecular fuel decomposition, hydrogen abstraction from the fuel, fuel radical decomposition, and the reaction steps to complete the low temperature chain-branching channel. For a detailed discussion, see [5]. The most significant model developments in this study are as follows: As demonstrated in Refs. [5,7], C-H bond scission energies at a; b and c sites in alcohols are typically lower than those for alkanes. In this study, for n-octanol, the a; b, and c carbon sites were treated as alcohol specific, and thus the rate rules were adopted from analogies with n-butanol [5] and n-pentanol [7]. The remaining five C-H sites were treated like analogous sites in normal alkanes, since the effect of the hydroxyl group on these sites is insignificant. Their rate rules were taken from the previous studies on alkanes [17,18] and 2-methylalkanes [20]. The rate constants for the reaction class describing O2 with a-hydroxyoctyl radicals to directly form aldehyde +HO2 and the reaction class for O2 addition to a-hydroxyoctylhydrop-
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eroxide (QOOH) radicals were modified within their uncertainty bounds to predict ignition delay times at low temperatures more accurately. These treatments are consistent with the previous studies on n-butanol [5] and npentanol [7]. The high-pressure limit of the rate constants was decreased by a factor of 2.5, while the rates at low pressures were multiplied by 2.5. Recently, Rosado-Reyes and Tsang [21] experimentally determined the rate expressions for each channel contributing to n-butanol decomposition. The same reaction rates were applied to n-octanol decomposition for the analogous reactions. Several reaction pathways were added to the model based on recent experimental and theoretical studies by Welz et al. [22,23], who state that the pathways involving hydrogen transfer from the OH group are important for the cand d-hydroxyalkylperoxy (RO2) radicals via 7- and 8-membered transition state rings, similar to the Waddington reaction pathway [24,25]. Moreover, Welz et al. [23] also present an unconventional water elimination pathway from hydroxy-alkylhydroperoxide radicals, where the hydroperoxide is on the c site and the radical is on the a site. These reactions were included in the present model and their rate constants were selected based on the recent study for iso-pentanol oxidation [8].
3. Experimental facility 3.1. High pressure shock tube (HPST) The experiments determining ignition delay times were performed in a high pressure shock tube with an inner diameter of 140 mm and a total length of 15.5 m. The material of the shock tube is stainless steel and resists pressures up to 1000 bar in a heated state of 473 K. The 11 m long low pressure test section is separated from the 4 m driver section by a double diaphragm chamber. The HPST is therefore highly suitable for investigating low vapor pressure fuels as well as undiluted fuels at very high pressures and for a wide range of temperatures. A detailed explanation of this HPST is given in Ref. [26]. The preparation of the stoichiometric fuel/air (synthetic air: 20.5% O2 and 79.5% N2) mixture was performed directly in the evacuated low pressure section of the shock tube. After one hour of homogenization, the experiments were conducted. Due to the very low vapor pressure of n-octanol and the high filling pressure up to 4.3 bar, the shock tube was resistively (with heating wires) heated up to a temperature of about 400 K. The
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temperatures of possible condensation areas, such as the filling pipes as well as the fuel partial pressure, were controlled precisely to ensure the evaporation of the injected fuel. To exclude the influence of the high heating process, several experiments at the similar post-shock conditions with different initial temperatures were performed; no anomalies in the ignition delay time curves could be found. For the determination of the filling pressures, a high temperature resistive pressure gauge was used to ensure the complete transition of the fuel into the gas phase. For capturing the pressures during the experiment and hence the shock velocities, 8 Kistler 603B sensors were mounted at several positions of the test section. The positions began from the endwall up to a distance of 2510 mm. A photomultiplier (Hamamatsu, RR212UH) with a narrow band pass filter (LOT 430FS10, FWHM 10 2 nm) for the CH* emission detection was located 10 mm in front of the endwall. The gas temperature behind the reflected shock was calculated from the initial conditions and the shock velocities considering real gas effects using the in-house code KASIMIR [27]. The ignition delay time is determined as the time between the rise of pressure caused by the reflected shock and the steepest rise of the pressure signal due to ignition. The largest rate of increase of the emission signal (PMT) was also checked for comparison with the pressure signals, for which a deviation in ignition delay time less than 2.5% was found. The ignition delay times presented in this paper were obtained from the pressure sensor located at the 10 mm position. The uncertainties in the experimental temperatures are close to 10 K. Due to the large inner diameter of the shock tube, shock attenuation is <1%/ms and does not affect the measurements. The error of ignition delay measurements is determined to be within 20%. 3.2. Jet stirred reactor (JSR) The JSR experimental setup used here has been described earlier in [28,29]. The JSR consists of a 4 cm diameter fused silica sphere (42 cm3) equipped with four nozzles of 1 mm inner diameter. Before the injectors, the reactants were diluted with nitrogen (<100 ppm H2O, <50 ppm O2, <1000 ppm Ar, <5 ppm H2 from Air Liquide). A high degree of dilution (1000 ppm of fuel) was used, reducing temperature gradients and heat release in the JSR. The reactants were high-purity oxygen (99.995% pure form Air Liquide) and high-purity n-octanol, which was sonically degassed before use. The reactants were preheated before injection to minimize temperature gradients inside the reactor. A HPLC pump (Shimadzu LC10 AD VP) with an on-line degasser (Shimadzu DGU-20) was used to deliver n-octanol to an
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in-house atomizer-vaporizer assembly that was maintained at 210 °C. Good thermal homogeneity along the vertical axis of the reactor (gradients of <1 K/cm) was measured by a thermocouple (0.1 mm Pt-Pt/Rh-10%, located inside a thin-wall fused silica tube). The reacting mixtures were sampled using a movable fused silica low-pressure sonic probe. The samples were directed to analyzers via a Teflon heated line maintained at 200 °C. Analyses were performed online with a FTIR spectrometer (10 m path length, resolution of 0.5 cm1, 200 mbar in the cell) as well as off-line after collection and storage in 1 L Pyrex bulbs. Off-line analyses were performed with gas chromatographs (GC) which were equipped with capillary columns (60 m and 0.32 mm i.d.: DB-624; 50 m and 0.32 mm i.d.: CP-Al2O3-KCl; 25 m and 0.53 mm i.d.: Carboplot-P7), a TCD (thermal conductivity detector), and an FID (flame ionization detector). A GCMS (Shimadzu QP2010SE) operating with electron ionization (70 eV) was used for product identification. Steady state experiments were performed in this study at a constant mean residence time (s = 0.7 s) and pressure of 10 atm, with equivalence ratios / = 0.5, 1.0 and 2. The reactants flowed continually in the reactor and the temperature of the gases inside the JSR was increased stepwise. Good repeatability of the measurements and an atomic balance of carbon, oxygen, and hydrogen at the level of 100 10% were observed. The maximum uncertainty in the GC analysis was estimated to be 10%. 4. Results and discussion The following section presents the comparisons between experimental measurements and numerical calculations performed with the proposed model. In order to gain further insights into the impacts of the hydroxyl moiety on ignition of hydrocarbon species, kinetic modeling of ignition delay times for fuels that are structurally similar to n-octanol are performed to compare with those of n-octanol. In addition, reaction pathway analyses are presented to elucidate the oxidation of n-octanol under the conditions of interest.
the maximum rate of rise in temperature and pressure measured in the present HPST is negligible. The results obtained from the proposed model, compared to the measurements of n-octanol/air mixtures for / ¼ 1:0 at 20 and 40 bar, are shown in Fig. 2. It can be seen immediately that n-octanol shows a clear negative temperature coefficient (NTC) behavior, which is not observed for the shorter chain alcohols, such as n-butanol [5] and n-pentanol [35]. Using the present model, ignition delay times of stoichiometric n-octanol/air mixtures are predicted fairly well over the entire range experimentally investigated. More specifically, while the ignition delay times in the high temperature regime at 20 bar are slightly overpredicted, the computed ignition delay times appear very satisfactory for the intermediate to low temperatures, especially at the pressure of 40 bar. The observed deviations between simulations and experiments are within the reported experimental uncertainties. In general, the kinetic model presented in this paper is capable of predicting the experiments accurately. 4.2. Kinetic modeling simulations of JSR data The measurements in the JSR have been conducted at 10 atm and at equivalence ratios of 0.5, 1.0, and 2.0 for the temperature range of 500–1200 K with a constant mean residence time of 0.7 s. The initial fuel mole fraction is 1000 ppm. For the simulation, an end time of 20 s is specified to be sure that a steady state has been accomplished. As revealed in previous studies [33,34] and found also in this one, a comparison between experiment and simulation at / = 1.0 is representative for other equivalence ratios. Thus, it suffices to show only the experimental and numerical results for stoichiometric n-octanol/oxidizer mixtures (Fig. 3). The results for the equivalence ratios 0.5 and 2.0 are available in the supplementary material.
4.1. Kinetic modeling simulations of shock tube ignition delay times The numerical calculations presented in this study are performed using the appropriate reactor modules in the Chemkin PRO code [31] and the open source FlameMaster code [32]. Both codes yield identical numerical results. The computed ignition delay time is defined as the time necessary to reach the maximum rate of rise in temperature. As described in earlier publications [26,30], the difference between the times necessary to reach
Fig. 2. Ignition delay times of stoichiometric n-octanol/ air mixtures at 20 and 40 bar. Symbols denote the experimental data and solid lines show numerical results.
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Fig. 3. n-octanol oxidation in a JSR at 10 atm, t = 0.7 s, and / = 1.0. The initial fuel mole fraction is 1000 ppm. Symbols denote the experimental data and solid lines show numerical results.
Consistent with the experimental observations in the HPST, both experiments and simulations for the JSR configuration show a clear NTC behavior of n-octanol. The model gives satisfactory results for the major species, i.e., carbon monoxide (CO), carbon dioxide (CO2), water (H2O) and oxygen (O2) under all conditions. For the intermediate formaldehyde CH2O, the model slightly overpredicts its mole fraction in the high temperature regime, while it does a decent job at lower temperatures, especially in case of the minimum mole fraction in the NTC regime. As demonstrated in Fig. 3, the fuel reactivity in the cool flame regime is underpredicted by a factor of 2 using the proposed model. More importantly, however, the model correctly reflects the NTC behavior of n-octanol. The results reveal that the shapes of the model profiles for the minor species methane (CH4), ethylene (C2H4), ethane (C2H6), propene (C3H6), 1-butene (C4H8-1), 1-pentene (C5H10-1) and 1-hexene (C6H12-1) closely match the measured ones, but their maximum concentrations are slightly underpredicted. While the mole fraction profile of heptanal is very well reproduced in the simulations, the maximum concentrations of octanal and hexanal are overpredicted by more than a factor of 2, which could be attributed to the missing low-temperature consumption pathways of the large aldehydes or the overestimation of their primary formation. Recently, Veloo et al. [36] conducted an experimental and modeling study on the oxidation of n- and iso-butanal and revealed the importance of their low-temperature consumption reactions in accurately predicting the JSR data. In the presented mechanism, the simplified reaction schemes of the large aldehydes are included to describe the H-abstraction from aldehydes and further decomposition of aldehyde radicals, similar to the earlier
studies on butanol and pentanol isomers [5,7,8]. As found in these studies on alcohols, the global combustion characteristics, e.g. ignition delay times, are highly sensitive to the production of the large aldehydes. Nevertheless, their consumption channels are proven to be irrelevant for the ignition delays. These simplified schemes of the aldehyde consumption are generally adopted in a variety of modeling works [18,37,39]. As the model size can grow rapidly by including the detailed consumption channels of the large aldehydes and their theoretical and experimental kinetic knowledge are strongly limited, the aforementioned simplifications are further applied in this study for n-octanol. Generally speaking, the presented model has demonstrated the capability to predict the present data with satisfactory accuracy. 4.3. Kinetic modeling simulations of ignition delay times In order to gain further insights into the ignition behavior of n-octanol, numerical simulations with the proposed model were performed for the shock tube configuration and then compared with the results for species having a similar molecular structure, e.g. n-octane, n-butanol and n-pentanol. This improves the knowledge of the effects of the alcohol functional group and the chain length on auto-ignition. Figure 4 depicts the calculated ignition delay times for stoichiometric n-butanol/air [5], n-pentanol/air [7] and n-octanol/air mixtures at a pressure of 20 bar. Compared with n-butanol and n-pentanol, n-octanol exhibits a clear NTC behavior and a significantly stronger reactivity at low temperature. While the differences of ignition delays between three linear alcohols are comparably small in the high temperature range, the increased
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reduced reactivity in the low temperature regime. The reasons will be explored in the next section. At high temperatures, n-octanol shows an improved ignition propensity compared with noctane. This can be attributed to the different Habstraction rates by HO2 radicals in the n-octane [20] and n-octanol models, which are rate-determining in the high temperature range. The slower rates in the n-octane model [20] lead to its inhibited high temperature reactivity. 4.4. Reaction path analysis Fig. 4. Ignition delay times of n-butanol/air, n-pentanol and n-octanol/air mixtures at 20 bar and / = 1.0. Solid lines show numerical results.
chain length obviously contributes to an improved fuel reactivity at intermediate to low temperatures. The reason is that the chain branching pathways initialized by isomerization of peroxy radicals are promoted. One could expect that the low temperature and NTC ignition reactivity of long alcohols would not increase further after a critical length of carbon chain is reached, as demonstrated for n-alkanes by Westbrook and Dryer [17] and for 2-methylalkanes by Sarathy et al. [20]. The computed and experimental ignition delays for n-octanol are compared to those for n-octane [20] in Fig. 5. It should be mentioned here that the computed ignition delay times for n-octane are larger than the experimental observations reported in Ref. [20] due to the absence of rate constant tuning and the uncertainties in both the selection of reaction pathways and the estimation of reaction rate coefficients of the n-octane mechanism. Over the entire range of the investigated temperatures, n-octanol exhibits a similar ignition performance as n-octane besides the
Fig. 5. Ignition delay times of n-octane and n-octanol/ air mixtures at 20 bar and / = 1.0. Symbols denote the experimental data for n-octanol from this study and for n-octane from Ref. [20] and solid lines show numerical results.
Figure 6 illustrates the reaction path analysis for stoichiometric n-octanol/air combustion at 20 bar and temperatures of 769 K and 1000 K. The reaction fluxes are given at 30% fuel consumption. The oxidation of n-octanol is initiated via hydrogen atom abstraction from the fuel by molecular oxygen. Due to its merely slight contribution to the overall consumption of n-octanol, this pathway is not shown here. As demonstrated, n-octanol is mainly consumed by the build-up of OH. Through the abstraction of an H atom, eight primary radicals are produced. The H abstraction from the OH site is very minor at both considered conditions because of the strong OH bond dissociation energy [5,38]. It is therefore not presented in the pathway diagram. Similar to short alcoholic fuels [5,7], the production of the a radical is important at both temperatures. In contrast to the typical alkane pathways, the a hydroxyalkyl radicals react with oxygen mainly to form aldehydes and HO2 radicals instead of hydroxyalkyl peroxy radicals at low to intermediate temperatures, which prevents the degenerate chain branching pathway and retards the ignition of alcohols. Therefore, the short alcohols, n-butanol and n-pentanol, show a lower reactivity at low temperatures compared to their corresponding alkanes, i.e., n-butane and n-pentane as presented in Refs. [5,7]. However, this inhibitory effect decreases as the chain length grows in n-octanol. Since more carbon sites can be found in the molecule, the branching ratio producing a fuel radicals is comparatively decreased, even though this pathway remains favorable. Moreover, the increased chain length provides also the possibility to enhance the low temperature reactivity of the fuel, because the number of favorable 6-membered ring RO2 and peroxy-hydroxyalkylhydroperoxide (O2QOOH) radical isomerizations is increased. Overall, the effect of the hydroxyl functional group decreases for the larger alcohols, while its presence plays a major role in the short chains. Therefore, n-octanol shows a very similar ignition behavior as n-octane in Fig. 5. As shown in Fig. 6, except a radicals, the consecutive pathways of the other fuel radicals are almost identical to these of alkyl radicals. While the decomposition of the fuel radicals takes place
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Fig. 6. Reaction path analysis for n-octanol/air mixture at 769 K and 1000 K and at 30% fuel consumption, 20 bar, and / = 1.0.
at high temperatures, these radicals are oxidized by oxygen addition to yield peroxy radicals in the low temperature range. After the following isomerization and the second O2 addition, the peroxy-hydroxyoctylhydroperoxide radical is produced, which isomerizes rapidly and then decomposes to give an OH radical and a ketohydroperoxide. The important intermediate species, ketohydroperoxide, can decompose further to form the second OH radical, which completes the low temperature chain branching process. The isomerization of peroxy radicals competes with the production of enols and HO2 radicals. For b RO2 radicals, the Waddington reaction [24] can also take place and returns an OH radical, which results in a chain propagation pathway. However, the effect of this chain-propagating pathway is minor, as only a small number of b peroxy radicals are produced at the relevant conditions. To better understand the n-octanol kinetics, a sensitivity analysis for ignition delay times was performed at the same conditions. The results are presented in the supplementary material, and are consistent with those from the path analysis. The ignition delay times are found to be very sensitive towards the fuel H-abstraction by OH radicals and the isomerization of peroxy radicals.
5. Concluding remarks This paper presents the first chemical kinetic investigation on the oxidation of n-octanol. A detailed kinetic model for n-octanol combustion was developed by applying methods recently developed for n-butanol and n-pentanol combustion models. Ignition delay times and stable species concentration profiles were experimentally obtained for n-octanol oxidation in a shock tube and a jet stirred reactor, respectively. Good agreement between simulations and experiments was presented over a wide range of initial conditions. The major differences between linear alcohols and alkanes with the same carbon chain length are that the presence of the hydroxyl moiety enhances H atom abstraction from the a carbon site, and the subsequent a hydroxyalkyl radical’s reaction with O2 yields an aldehyde. This reaction sequence inhibits low to intermediate temperature chain branching in alcohol fuels. However, this inhibitory effect of the hydroxyl moiety on the ignition propensity is reduced for the longer chain n-octanol, because the greater number of carbon sites in the molecule decreases the branching fraction of the a fuel radical production. The present work demonstrates the ability to develop adequate chemical kinetic models for high
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molecular weight alcohols in the absence of fundamental experimental and theoretical studies on reaction rates. Further measurements and quantum chemistry calculations are required to verify the model’s predictive capability and to minimize predictive uncertainties. Hydrogen abstractions by OH and HO2 radicals are important reactions for ignition delay times. Their theoretical or experimental studies could lead to an improved prediction of fuel auto-ignition. Reactions, such as hydroxyalkylperoxy radical isomerization, formation of epoxy alcohols, and concerted elimination warrant also further investigation. In addition, the detailed oxidation chemistry of large aldehydes and enols is required to improve the prediction of JSR data of high molecular weight alcohol fuels.
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