Detailed chemical kinetic mechanism for the oxidation of biodiesel fuels blend surrogate

Detailed chemical kinetic mechanisms were developed and used to study the oxidation of two large unsaturated esters: methyl-5-decenoate and methyl-9-decenoate. These models were built from a previous methyl decanoate mechanism and were compared with rapeseed oil methyl esters oxidation experiments in a jet-stirred reactor. A comparative study of the reactivity of these three oxygenated compounds was performed and the differences in the distribution of the products of the reaction were highlighted showing the influence of the presence and the position of a double bond in the chain. Blend surrogates, containing methyl decanoate, methyl-5-decenoate, methyl-9-decenoate and n-alkanes, were tested against rapeseed oil methyl esters and methyl palmitate/n-decane experiments. These surrogate models are realistic kinetic tools allowing the study of the combustion of biodiesel fuels in diesel and homogeneous charge compression ignition engines.     

Detailed chemical kinetic oxidation mechanism for a biodiesel surrogate

A detailed chemical kinetic mechanism has been developed and used to study the oxidation of methyl decanoate, a surrogate for biodiesel fuels. This model has been built by following the rules established by Curran and co-workers for the oxidation of n-heptane and it includes all the reactions known to be pertinent to both low and high temperatures. Computed results have been compared with methyl decanoate experiments in an engine and oxidation of rapeseed oil methyl esters in a jet-stirred reactor. An important feature of this mechanism is its ability to reproduce the early formation of carbon dioxide that is unique to biofuels and due to the presence of the ester group in the reactant. The model also predicts ignition delay times and OH profiles very close to observed values in shock tube experiments fueled by n-decane. These model capabilities indicate that large n-alkanes can be good surrogates for large methyl esters and biodiesel fuels to predict overall reactivity, but some kinetic details, including early CO₂ production from biodiesel fuels, can be predicted only by a detailed kinetic mechanism for a true methyl ester fuel. The present methyl decanoate mechanism provides a realistic kinetic tool for simulation of biodiesel fuels.     

Modeling of the oxidation of methyl esters—Validation for methyl hexanoate, methyl heptanoate, and methyl decanoate in a jet-stirred reactor

The modeling of the oxidation of methyl esters was investigated and the specific chemistry, which is due to the presence of the ester group in this class of molecules, is described. New reactions and rate parameters were defined and included in the software EXGAS for the automatic generation of kinetic mechanisms. Models generated with EXGAS were successfully validated against data from the literature (oxidation of methyl hexanoate and methyl heptanoate in a jet-stirred reactor) and a new set of experimental results for methyl decanoate. The oxidation of this last species was investigated in a jet-stirred reactor at temperatures from 500 to 1100 K, including the negative temperature coefficient region, under stoichiometric conditions, at a pressure of 1.06 bar and for a residence time of 1.5 s: more than 30 reaction products, including olefins, unsaturated esters, and cyclic ethers, were quantified and successfully simulated. Flow rate analysis showed that reactions pathways for the oxidation of methyl esters in the low-temperature range are similar to that of alkanes.     

Experimental study of the oxidation of methyl oleate in a jet-stirred reactor

The experimental study of the oxidation of a blend containing n-decane and a large unsaturated ester, methyl oleate, was performed in a jet-stirred reactor over a wide range of temperature covering both low and high temperature regions (550-1100 K), at a residence time of 1.5 s, at quasi atmospheric pressure with high dilution in helium (n-decane and methyl oleate inlet mole fractions of 1.48 × 10⁻³ and 5.2 × 10⁻⁴) and under stoichiometric conditions.The formation of numerous reaction products was observed. At low and intermediate temperatures, the oxidation of the blend led to the formation of species containing oxygen atoms like cyclic ethers, aldehydes and ketones deriving from n-decane and methyl oleate. At higher temperature, these species were not formed anymore and the presence of unsaturated species was observed. Because of the presence of the double bond in the middle of the alkyl chain of methyl oleate, the formation of some specific products was observed. These species are dienes and esters with two double bonds produced from the decomposition paths of methyl oleate and some species obtained from the addition of H-atoms, OH and HO₂ radicals to the double bond.Experimental results were compared with former results of the oxidation of a blend of n-decane and methyl palmitate performed under similar conditions. This comparison allowed highlighting the similarities and the differences in the reactivity and in the distribution of the reaction products for the oxidation of large saturated and unsaturated esters.     

Experimental and chemical kinetic modeling study of small methyl esters oxidation: Methyl (E)-2-butenoate and methyl butanoate

This study examines the effect of unsaturation on the combustion of fatty acid methyl esters (FAME). New experimental results were obtained for the oxidation of methyl (E)-2-butenoate (MC, unsaturated C₄ FAME) and methyl butanoate (MB, saturated C₄ FAME) in a jet-stirred reactor (JSR) at atmospheric pressure under dilute conditions over the temperature range 850-1400 K, and two equivalence ratios (Φ=0.375,0.75) with a residence time of 0.07 s. The results consist of concentration profiles of the reactants, stable intermediates, and final products, measured by probe sampling followed by on-line and off-line gas chromatography analyses. The oxidation of MC and MB in the JSR and under counterflow diffusion flame conditions was modeled using a new detailed chemical kinetic reaction mechanism (301 species and 1516 reactions) derived from previous schemes proposed in the literature. The laminar counterflow flame and JSR (for ?=1.13) experimental results used were from a previous study on the comparison of the combustion of both compounds. Sensitivity analyses and reaction path analyses, based on rates of reaction, were used to interpret the results. The data and the model show that MC has reaction pathways analogous to that of MB under the present conditions. The model of MC oxidation provides a better understanding of the effect of the ester function on combustion, and the effect of unsaturation on the combustion of fatty acid methyl ester compounds typically found in biodiesel.     

Studies of C4 and C10 methyl ester flames

The oxidation of three model biodiesel fuels, namely methyl butanoate (C₅H₁₀O₂, CAS No. 623-42-7), methyl crotonate (C₅H₈O₂, CAS No. 623-43-8), and methyl decanoate (C₁₁H₂₂O₂, CAS No. 110-42-9) was investigated in laminar premixed and non-premixed flames. The experiments were conducted in the counterflow configuration at atmospheric pressure, for a wide range of equivalence or inert-dilution ratios, and elevated reactant temperatures. Laminar flame speeds and local extinction strain rates were determined by measuring the flow velocities using digital particle image velocimetry. The experimental data were compared against those derived for flames of n-alkanes of similar carbon number, in order to assess the effects of saturation, the length of carbon chain, and the presence of the ester group. Several recent chemical kinetic models were tested against the experimental data, and major differences were identified and assessed. The accuracy of the Lennard–Jones potential parameters assigned to the methyl esters in the transport databases of the different models was evaluated and new values were estimated. Insight was provided into the high-temperature kinetic pathways of methyl esters in flame environments. Additionally, the reduced sooting propensity of methyl ester flames compared to n-alkane flames was investigated computationally.     

A shock tube study of methyl decanoate autoignition at elevated pressures

A shock tube study of the autoignition of methyl decanoate, a candidate surrogate for biodiesel fuels containing large methyl esters, has been carried out. Ignition delay times were measured in reflected-shock-heated gases by monitoring electronically-excited OH chemiluminescence and pressure. Methyl decanoate/air mixtures were studied at equivalence ratios of 0.5, 1.0, and 1.5, at temperatures from 653 to 1336 K, and for pressures around 15–16 atm. The experimental results illustrate negative-temperature-coefficient behavior characteristic of alkanes, with ignition delay times very similar at high temperatures and somewhat longer at low temperatures than those for n-decane. Experimental results are compared to the kinetic modeling predictions of Herbinet et al. [Combust. Flame 154 (2008) 507–528] with remarkable agreement. Both reaction flux analysis and the comparison of experimental methyl decanoate and n-decane ignition delay times illustrate the importance of the long alkyl chain in controlling methyl decanoate overall reactivity and the subtle role the methyl ester group has on inhibiting low-temperature reactivity.     

Premixed ignition behavior of C9 fatty acid esters: A motored engine study

An experimental study on the premixed ignition behavior of C₉ fatty acid esters has been conducted in a motored CFR engine. For each test fuel, the engine compression ratio was gradually increased from the lowest point (4.43) to the point where significant high temperature heat release (HTHR) was observed. The engine exhaust was sampled and analyzed through GC-FID/TCD and GC-MS. Combustion analysis showed that the four C₉ fatty acid esters tested in this study exhibited evidently different ignition behavior. The magnitude of low temperature heat release (LTHR) follows the order, ethyl nonanoate > methyl nonanoate ? methyl 2-nonenoate > methyl 3-nonenoate. The lower oxidation reactivity for the unsaturated fatty acid esters in the low temperature regime can be explained by the reduced amount of six- or seven-membered transition state rings formed during the oxidation of the unsaturated esters due to the presence of a double bond in the aliphatic chain of the esters. The inhibition effect of the double bond on the low temperature oxidation reactivity of fatty acid esters becomes more pronounced as the double bond moves toward the central position of the aliphatic chain. GC-MS analysis of exhaust condensate collected under the engine conditions where only LTHR occurred showed that the alkyl chain of the saturated fatty acid esters participated in typical paraffin-like low temperature oxidation sequences. In contrast, for unsaturated fatty acid esters, the autoignition can undergo olefin ignition pathways. For all test compounds, the ester functional group remains largely intact during the early stage of oxidation.     

Experimental study of the oxidation of large surrogates for diesel and biodiesel fuels

The experimental study of the oxidation of two blend surrogates for diesel and biodiesel fuels, n-decane/n-hexadecane and n-decane/methyl palmitate (74/26 mol/mol), has been performed in a jet-stirred reactor over a wide range of temperatures covering both low, and high-temperature regions (550-1100 K), at a residence time of 1.5 s, at quasi atmospheric pressure with high dilution in helium (hydrocarbon inlet mole fraction of 0.002) and at stoichiometric conditions.Numerous reaction products have been identified and quantified. At low and intermediate temperatures (less than 1000 K), the formation of oxygenated species such as cyclic ethers, aldehydes and ketones has been observed for n-decane, n-hexadecane, and methyl palmitate. At higher temperature, the formation of these species was not observed any more, and small amounts of unsaturated species (olefins and unsaturated methyl esters) have been detected.Results obtained with methyl palmitate and n-hexadecane have been compared in order to highlight similarities and differences between large n-alkanes and methyl esters.     

Hierarchical and comparative kinetic modeling of laminar flame speeds of hydrocarbon and oxygenated fuels

The primary objective of the present endeavor is to collect, consolidate, and review the vast amount of experimental data on the laminar flame speeds of hydrocarbon and oxygenated fuels that have been reported in recent years, analyze them by using a detailed kinetic mechanism for the pyrolysis and combustion of a large variety of fuels at high temperature conditions, and thereby identify aspects of the mechanism that require further revision. The review and assessment was hierarchically conducted, in the sequence of the foundational C₀–C₄ species; the reference fuels of alkanes (n-heptane, iso-octane, n-decane, n-dodecane), cyclo-alkanes (cyclohexane and methyl-cyclo-hexane) and the aromatics (benzene, toluene, xylene and ethylbenzene); and the oxygenated fuels of alcohols, C₃H₆O isomers, ethers (dimethyl ether and ethyl tertiary butyl ether), and methyl esters up to methyl decanoate. Mixtures of some of these fuels, including those with hydrogen, were also considered. The comprehensive nature of the present mechanism and effort is emphasized.     

An experimental and modeling study of the low- and high-temperature oxidation of cyclohexane

The experimental study of the oxidation of cyclohexane has been performed in a jet-stirred reactor at temperatures ranging from 500 to 1100 K (low- and intermediate temperature zones including the negative temperature-coefficient area), at a residence time of 2 s and for dilute mixtures with equivalence ratios of 0.5, 1, and 2. Experiments were carried out at quasi-atmospheric pressure (1.07 bar). The fuel and reaction product mole fractions were measured using online gas chromatography. A total of 34 reaction products have been detected and quantified in this study. Typical reaction products formed in the low-temperature oxidation of cyclohexane include cyclic ethers (1,2-epoxycyclohexane and 1,4-epoxycyclohexane), 5-hexenal (formed from the rapid decomposition of 1,3-epoxycyclohexane), cyclohexanone, and cyclohexene, as well as benzene and phenol. Cyclohexane displays high low-temperature reactivity with well-marked negative temperature-coefficient (NTC) behavior at equivalence ratios 0.5 and 1. The fuel-rich system (? = 2) is much less reactive in the same region and exhibits no NTC. To the best of our knowledge, this is the first jet-stirred reactor study to report NTC in cyclohexane oxidation. Laminar burning velocities were also measured by the heated burner method at initial gas temperatures of 298, 358, and 398 K and at 1 atm. The laminar burning velocity values peak at ? = 1.1 and are measured as 40 and 63.1 cm/s for T_i = 298 and 398 K, respectively. An updated detailed chemical kinetic model including low-temperature pathways was used to simulate the present (jet-stirred reactor and laminar burning velocity) and literature experimental (laminar burning velocity, rapid compression machine, and shock tube ignition delay times) data. Reasonable agreement is observed with most of the products observed in our reactor, as well as the literature experimental data considered in this paper.     

The thermal cracking of canola and soybean methyl esters: Improvement of cold flow properties

A study was performed to evaluate the use of thermal cracking to overcome cold flow and stability limitations of current biodiesel. Experiments were conducted in a batch cracking reactor system using soy methyl ester and canola methyl ester feedstocks. The amount of high-MW C₁₆–C₂₄ FAMEs was reduced from nearly 100% in the original feedstock by an order of magnitude. Yields of desirable cracking product ranged from 70 to 85% while cloud and pour points decreased around 20 °C and 15 °C, respectively. The stability of the fuel was improved by converting all of the unsaturated esters into lower-MW saturated esters. This method may lead to an attractive process to produce an improved biodiesel that is more conductive to cold temperature utilization and more stable during storage.     

Decomposition and hydrocarbon growth processes for hexadienes in nonpremixed flames

Alkadienes are formed during the decomposition of alkanes and play a key role in the formation of aromatics due to their degree of unsaturation. The experiments in this paper examined the decomposition and hydrocarbon growth mechanisms of a wide range of hexadiene isomers in soot-forming nonpremixed flames. Specifically, C3 to C12 hydrocarbon concentrations were measured on the centerlines of atmospheric-pressure methane/air coflowing nonpremixed flames doped with 2000 ppm of 1,3-, 1,4-, 1,5-, and 2,4-hexadiene and 2-methyl-1,3-, 3-methyl-1,3-, 2-methyl-1,4-, 3-methyl-1,4-pentadiene, and 2,3-dimethyl-1,3-butadiene. The hexadiene decomposition rates and hydrocarbon product concentrations showed that the primary decomposition mechanism was unimolecular fission of CC single bonds, whose fission produced allyl and other resonantly stabilized products. The one isomer that does not contain any of these bonds, 2,4-hexadiene, isomerized by a six-center mechanism to 1,3-hexadiene. These decomposition pathways differ from those that have been observed previously for propadiene and 1,3-butadiene, and these differences affect aromatic hydrocarbon formation. 1,5-Hexadiene and 2,3-dimethyl-1,3-butadiene produced significantly more C₃H₄ and C₄H₄ than the other isomers, but less benzene, which suggests that benzene formation pathways other than the conventional C3 + C3 and C4 + C2 pathways were important in most of the hexadiene-doped flames. The most likely additional mechanism is cyclization of highly unsaturated C5 decomposition products, followed by methyl addition to cyclopentadienyl radicals.     

A comprehensive chemical kinetic combustion model for the four butanol isomers

Alcohols, such as butanol, are a class of molecules that have been proposed as a bio-derived alternative or blending agent for conventional petroleum derived fuels. The structural isomer in traditional “bio-butanol” fuel is 1-butanol, but newer conversion technologies produce iso-butanol and 2-butanol as fuels. Biological pathways to higher molecular weight alcohols have also been identified. In order to better understand the combustion chemistry of linear and branched alcohols, this study presents a comprehensive chemical kinetic model for all the four isomers of butanol (e.g., 1-, 2-, iso- and tert-butanol). The proposed model includes detailed high-temperature and low-temperature reaction pathways with reaction rates assigned to describe the unique oxidation features of linear and branched alcohols. Experimental validation targets for the model include low pressure premixed flat flame species profiles obtained using molecular beam mass spectrometry (MBMS), premixed laminar flame velocity, rapid compression machine and shock tube ignition delay, and jet-stirred reactor species profiles. The agreement with these various data sets spanning a wide range of temperatures and pressures is reasonably good. The validated chemical kinetic model is used to elucidate the dominant reaction pathways at the various pressures and temperatures studied. At low-temperature conditions, the reaction of 1-hydroxybutyl with O₂ was important in controlling the reactivity of the system, and for correctly predicting C₄ aldehyde profiles in low pressure premixed flames and jet-stirred reactors. Enol–keto isomerization reactions assisted by radicals and formic acid were also found to be important in converting enols to aldehydes and ketones under certain conditions. Structural features of the four different butanol isomers leading to differences in the combustion properties of each isomer are thoroughly discussed.     

Biologically derived diesel fuel and NO formation: An experimental and chemical kinetic study, Part 1

Pyrolysis and oxidation experiments have been conducted on two representative biodiesel surrogate components, methyl octanoate (C9:0) and methyl trans-2-octenoate (C9:1), using the UIC High-Pressure Shock Tube (HPST). The nominal experimental pressures ranged from 27 atm to 53 atm and temperatures varied from 900 to 1450 K with nominal reaction times of 1.65 ms. Dilute reagent mixtures of ∼100 ppm of each fuel were prepared in bulk argon and shock heated to study the stable intermediates. The experimental data have been used to develop and validate a kinetic model for the pyrolysis and oxidation of saturated and unsaturated C₈ methyl esters. The developed model has also been coupled to an existing NO mechanism to predict prompt NO formation spanning the experimental temperature regime. It has been predicted that an increased amount of NO is formed from the unsaturated methyl ester, methyl trans-2-octenoate (C9:1) compared to the saturated methyl ester, methyl octanoate (C9:0) over the intermediate temperature range of 1050–1450 K.     

A wide range kinetic modeling study of pyrolysis and oxidation of methyl butanoate and methyl decanoate – Note II: Lumped kinetic model of decomposition and combustion of methyl esters up to methyl decanoate

The aim of this work is to develop and discuss a lumped kinetic model to simulate the pyrolysis and combustion behavior of methyl decanoate. Validation of the lumped kinetic model of methyl decanoate in a very wide range of conditions, with temperature ranging from 500 to more than 2000 K, pressures up to 16 bar and equivalent ratios from lean to pyrolysis conditions, proved that, despite the drastic simplifications, the model can properly reproduce the experimental measurements in pyrolysis as well as in an oxidation environment, in both the low temperature regime and in flame conditions. This model is an extension of the lumped model of methyl butanoate developed and discussed in the first part of this work [1]. Thus, the lumped kinetic model of methyl butanoate and methyl decanoate is also quite simply applied to simulating the combustion behavior of intermediate methyl esters, by using the lever rule between the two reference components. The overall agreement with experimental measurements is very encouraging and lays the basis for the extension to the lumped kinetic scheme to soy and rapeseed biodiesel fuels.     

Assessment of H2-CH4-air mixtures oxidation kinetic models used in combustion

Because of a wide number of applications, the potential hazards of H₂-CH₄-air mixtures have to be characterised. For hazard evaluation, an important element is a reliable detailed kinetic scheme. In the present study, three modern kinetic models, those of Konnov, of Dagaut and the GRI-mech 03, have been evaluated with respect to a large set of experimental data, including species profiles obtained in jet-stirred reactor, laminar flame speed, ignition delay time and detonation cell size, for hydrogen-methane-air mixtures. For jet-stirred reactor data, the model of Dagaut provides significantly better results. For flame speed data modeling, the three models are as reliable. For ignition delay times, the model of Dagaut seems the most reliable. For detonation cell size predictions, the model of Konnov is the best. Important chemical reactions are underlined through sensitivity and reaction pathway analysis and are discussed in the frame of rate constant values recommended by Baulch et al.     

Lean methane premixed laminar flames doped by components of diesel fuel II: n-Propylcyclohexane

For a better understanding of the chemistry involved during the combustion of components of diesel fuel, the structure of a laminar lean premixed methane flame doped with n-propylcyclohexane has been investigated. The inlet gases contained 7.1% (molar) methane, 36.8% oxygen, and 0.81% n-propylcyclohexane (C₉H₁₈), corresponding to an equivalence ratio of 0.68 and a C₉H₁₈/CH₄ ratio of 11.4%. The flame has been stabilized on a burner at a pressure of 6.7 kPa (50 Torr) using argon as diluent, with a gas velocity at the burner of 49.2 cm/s at 333 K. Quantified species included the usual methane C₀–C₂ combustion products, but also 17 C₃–C₅ hydrocarbons, seven C₁–C₃ oxygenated compounds, and only four cyclic C₆₊ compounds, namely benzene, 1,3-cyclohexadiene, cyclohexene, and methylenecyclohexane. A new mechanism for the oxidation of n-propylcyclohexane has been proposed. It allows the proper simulation of profiles of most of the products measured in flames, as well as the satisfactory reproduction of experimental results obtained in a jet-stirred reactor. The main reaction pathways of consumption of n-propylcyclohexane have been derived from rate-of-production analysis.     

Numerical and experimental study of ethanol combustion and oxidation in laminar premixed flames and in jet-stirred reactor

The main objectives of this research consist in achieving both experimental and numerical studies of the combustion and oxidation of ethanol. Experimental mole fraction profiles of chemical species (stable, radical, and intermediates) were measured in three C₂H₅OH/O₂/Ar flat premixed flames stabilized at low pressure (50 mbar) and with equivalence ratios equal to 0.75, 1, and 1.25, respectively. The experimental setup used to determine the structure of one-dimensional laminar premixed flames consists of a molecular beam mass spectrometer system (MBMS) combined with electron impact ionization (EI). The oxidation of ethanol was also experimentally studied using a fused silica jet-stirred reactor (JSR). Experiments were performed in the temperature range 890–1250 K, at 1 atm, at four equivalence ratios equal to 0.25, 0.5, 1, and 2 and with an initial fuel concentration of 2000 ppm.A kinetic study was conducted in order to simulate all experimental data measured. It enabled building a kinetic mechanism by thoroughly reviewing the available literature and by taking into account specificities of the two kinds of experiments performed. Validity of the mechanism was also checked against experimental results previously published (ethanol oxidation in a JSR at 10 atm, ignition in a shock tube, combustion in premixed, partially-premixed, and non-premixed flames). This mechanism ensures a reasonably good modelling of the combustion and oxidation of ethanol over the wide range of experimental conditions investigated.