A comprehensive experimental and modeling study of iso-pentanol combustion

Biofuels are considered as potentially attractive alternative fuels that can reduce greenhouse gas and pollutant emissions. iso-Pentanol is one of several next-generation biofuels that can be used as an alternative fuel in combustion engines. In the present study, new experimental data for iso-pentanol in shock tube, rapid compression machine, jet stirred reactor, and counterflow diffusion flame are presented. Shock tube ignition delay times were measured for iso-pentanol/air mixtures at three equivalence ratios, temperatures ranging from 819 to 1252 K, and at nominal pressures near 40 and 60 bar. Jet stirred reactor experiments are reported at 5 atm and five equivalence ratios. Rapid compression machine ignition delay data was obtained near 40 bar, for three equivalence ratios, and temperatures below 800 K. Laminar flame speed data and non-premixed extinction strain rates were obtained using the counterflow configuration. A detailed chemical kinetic model for iso-pentanol oxidation was developed including high- and low-temperature chemistry for a better understanding of the combustion characteristics of higher alcohols. First, bond dissociation energies were calculated using ab initio methods, and the proposed rate constants were based on a previously presented model for butanol isomers and n-pentanol. The model was validated against new and existing experimental data at pressures of 1–60 atm, temperatures of 650–1500 K, equivalence ratios of 0.25–4.0, and covering both premixed and non-premixed environments. The method of direct relation graph (DRG) with expert knowledge (DRGX) was employed to eliminate unimportant species and reactions in the detailed mechanism, and the resulting skeletal mechanism was used to predict non-premixed flames. In addition, reaction path and temperature A-factor sensitivity analyses were conducted for identifying key reactions at various combustion conditions.     

A shock tube and chemical kinetic modeling study of the pyrolysis and oxidation of butanols

The pyrolysis and oxidation of all four butanols (n-, sec-, iso- and tert-) have been studied at pressures from 1 to 4 atm and temperatures of 1000–1800 K behind reflected shock waves. Gas chromatographic sampling at different reaction times varying from 1.5 to 3.1 ms was used to measure reactant, intermediate and product species profiles in a single-pulse shock tube. In addition, ignition delays were determined at an average reflected shock pressure of 3.5 atm at temperatures from 1250 to 1800 K. A detailed chemical kinetic model consisting of 1892 reactions involving 284 species was constructed and tested against species profiles and ignition delays. The little-known chemistry of enols is included in this work to explain the temperature dependence of acetaldehyde produced in the thermal decomposition of isobutanol.     

Pyrolysis and oxidation of decalin at elevated pressures: A shock-tube study

Ignition delay times and ethylene concentration time-histories were measured behind reflected shock waves during decalin oxidation and pyrolysis. Ignition delay measurements were conducted for gas-phase decalin/air mixtures over temperatures of 769–1202 K, pressures of 11.7–51.2 atm, and equivalence ratios of 0.5, 1.0, and 2.0. Negative-temperature-coefficient (NTC) behavior of decalin autoignition was observed, for the first time, at temperatures below 920 K. Current ignition delay data are in good agreement with past shock tube data in terms of pressure dependence but not equivalence ratio dependence. Ethylene mole fraction and fuel absorbance time-histories were acquired using laser absorption at 10.6 and 3.39 μm during decalin pyrolysis for mixtures of 2200–3586 ppm decalin/argon at pressures of 18.2–20.2 atm and temperatures of 1197–1511 K. Detailed comparisons of these ignition delay and species time-history data with predictions based on currently available decalin reaction mechanisms are presented, and preliminary suggestions for the adjustment of some key rate parameters are made.     

Experimental and modeling study of the oxidation of n-butylbenzene

New experimental results for the oxidation of n-butylbenzene, a component of diesel fuel, have been obtained using three different devices. A rapid compression machine has been used to measure autoignition delay times after compression at temperatures in the range 640–960 K, at pressures from 13 to 23 bar, and at equivalence ratios from 0.3 to 0.5. Results show low-temperature behavior, with the appearance of cool flames and a negative temperature coefficient (NTC) region for the richest mixtures. To investigate this reaction at higher temperatures, a shock tube has been used. The shock tube study was performed over a wide range of experimental temperatures, pressures, and equivalence ratios, with air used as the fuel diluent. The ignition temperatures were recorded over the range 980–1740 K, at reflected shock pressures of 1, 10, and 30 atm. Mixtures were investigated at equivalence ratios of 0.3, 0.5, 1.0 and 2.0 in order to determine the effects of fuel concentration on reactivity over the entire temperature range. Using a jet-stirred reactor, the formation of numerous reaction products has been followed at temperatures from 550 to 1100 K, at atmospheric pressure, and at equivalence ratios of 0.25, 1.0, and 2.0. Slight low-temperature reactivity (below 750 K) with a NTC region has been observed, especially for the leanest mixtures. A detailed chemical kinetic model has been written based on rules similar to those considered for alkanes by the system EXGAS developed at Nancy. Simulations using this model have been compared to the experimental results presented in this study, but also to results in the literature obtained in a jet-stirred reactor at 10 bar, in the same rapid compression machine for stoichiometric mixtures, in a plug flow reactor at 1069 K and atmospheric pressure, and in a low-pressure (0.066 bar) laminar premixed methane flame doped with n-butylbenzene. The observed agreement is globally better than that obtained with models from the literature. Flow rate and sensitivity analyses have revealed a preponderant role played by the addition to molecular oxygen of resonantly stabilized, 4-phenylbut-4-yl radicals.     

Determination of particle temperatures in a silica-generating counterflow flame via flame emission measurements

We propose a simple technique to measure particle temperatures in a particle generating counterflow flame. The silica particle temperature was derived from flame light emission measurements. This technique allows the non-intrusive measurement of particle temperatures over 2000 K. In addition, the OH concentration distribution in the hydrogen–oxygen flame was estimated from flame emission spectra in the ultraviolet region. A numerical model of the combustion processes, which included the reactions of SiCl₄ leading to the formation of silica particles, verified that the measured particle temperatures and OH concentration were close to the theoretical values.     

An experimental and kinetic modelling study of n-butyl formate combustion

The oxidation of n-butyl formate, a potential biofuel candidate, is studied using three different experimental approaches. Ignition delay times have been measured for stoichiometric mixtures of fuel and air for pressures of about 20 and 90 bar at temperatures from 846 up to 1205 K in a high-pressure shock tube. A rapid compression machine has been used to determine the low-temperature ignition delay times for stoichiometric mixtures at pressures close to 20 bar over the temperature range from 646 K up to 861 K. Laminar burning velocities have been determined for stoichiometric ratios ranging from 0.8 to 1.2 using the high-pressure chamber method combined with an optical Schlieren cinematography setup in order to acquire experimental data at elevated pressures of about 10 bar and a temperature of 373 K. A detailed kinetic model has been constructed including high-temperature and low-temperature reaction pathways. The enthalpies of formation, entropies, and specific heats at constant pressure for the fuel, its primary radicals, and several combustion intermediates have been computed with the CBS-QB3 methods and included in the mechanism. This model was validated successfully against the presented data and used to elucidate the combustion of this interesting ester. The importance of accurate inclusion of the low-temperature peroxy chemistry has been highlighted through sensitivity and reaction path analysis. This study presents the first combustion study of n-butyl formate and leads to an improved understanding of the chemical kinetics of alkyl ester oxidation.     

Shock tube study of methanol,methyl formate pyrolysis: CH3OH and CO time-history measurements

Methanol and methyl formate pyrolysis were studied by measuring CH₃OH and CO concentration time-histories behind reflected shock waves. In the study of methanol pyrolysis, experimental conditions covered temperatures of 1266–1707 K, pressures of 1.1–2.5 atm, and initial fuel concentrations of 1% and 0.2% with argon as the bath gas. Detailed comparisons of CH₃OH and CO concentration profiles with the predictions of the detailed kinetic mechanism of Li et al. (2007) [8] were made. Such comparisons combined with sensitivity analysis identified the need to include an additional methanol decomposition channel, CH₃OH ? CH₂(S) + H₂O, into the mechanism. Pathway and sensitivity analyses for methanol decomposition were performed, leading to rate constant recommendations both for CH₃OH unimolecular decomposition and H-abstraction reactions with improved model performance. In the study of methyl formate pyrolysis, methanol concentration time-histories were measured at temperatures over the range of 1261–1524 K, pressures near 1.5 atm, and initial fuel concentrations of 1% with argon as the bath gas. Our current work, and CO time-histories from previous work, indicates that the Dooley et al. (2010) [3] model is able to accurately simulate most species concentrations in shock tube experiments at early times. However, model improvement is still needed to match the CH₃OH and CO time-histories at later times. Incorporation of the modified rate constants in the methanol sub-mechanism leads to good predictions of the full methanol time-histories at all temperatures. The kinetic implications of some aspects of the CO time-histories and suggestions for further improving the predictive capabilities of these mechanisms are discussed. The current results are the first quantitative measurements of CH₃OH time-histories in shock tube experiments, and hence are a critical step toward understanding of the chemical kinetics of oxygenates.     

Negative pressure dependence of mass burning rates of H2/CO/O2/diluent flames at low flame temperatures

Experimental measurements of burning rates, analysis of the key reactions and kinetic pathways, and modeling studies were performed for H₂/CO/O₂/diluent flames spanning a wide range of conditions: equivalence ratios from 0.85 to 2.5, flame temperatures from 1500 to 1800 K, pressures from 1 to 25 atm, CO fuel fractions from 0 to 0.9, and dilution concentrations of He up to 0.8, Ar up to 0.6, and CO₂ up to 0.4. The experimental data show negative pressure dependence of burning rate at high pressure, low flame temperature conditions for all equivalence ratios and CO fractions as high as 0.5. Dilution with CO₂ was observed to strengthen the pressure and temperature dependence compared to Ar-diluted flames of the same flame temperature. Simulations were performed to extend the experimentally studied conditions to conditions typical of gas turbine combustion in Integrated Gasification Combined Cycle processes, including preheated mixtures and other diluents such as N₂ and H₂O.Substantial differences are observed between literature model predictions and the experimental data as well as among model predictions themselves – up to a factor of three at high pressures. The present findings suggest the need for several rate constant modifications of reactions in the current hydrogen models and raise questions about the sufficiency of the set of hydrogen reactions in most recent hydrogen models to predict high pressure flame conditions relevant to controlling NO_x emissions in gas turbine combustion. For example, the reaction O + OH + M = HO₂ + M is not included in most hydrogen models but is demonstrated here to significantly impact predictions of lean high pressure flames using rates within its uncertainty limits. Further studies are required to reduce uncertainties in third body collision efficiencies for and fall-off behavior of H + O₂(+M) = HO₂(+M) in both pure and mixed bath gases, in rate constants for HO₂ reactions with other radical species at higher temperatures, and in rate constants for reactions such as O + OH + M that become important under the present conditions in order to properly characterize the kinetics and predict global behavior of high-pressure H₂ or H₂/CO flames.     

High pressure study of 1,3,5-trimethylbenzene oxidation

The high pressure and high temperature kinetics of the 1,3,5-trimethylbenzene oxidation were investigated in the High Pressure Single Pulse Shock Tube at University of Illinois at Chicago. Experiments were performed at nominal reflected shock pressures of 20 and 50 atm, for three different equivalence ratios (Ф = 0.51, 0.95 and 1.86) and for a temperature range of 1017–1645 K. A variety of stable species ranging from aliphatic hydrocarbons to single ring and polycyclic aromatic hydrocarbons were sampled from the shock tube and analyzed using standard gas chromatographic techniques.A detailed chemical kinetic model was developed to simulate the fuel and oxygen decay and the stable intermediate species profiles as obtained from the high pressure oxidation experiments. The model shows satisfactory predictions for the formation and consumption of most of the major intermediates.     

A detailed experimental and kinetic modeling study of n-decane oxidation at elevated pressures

The oxidation of n-decane/oxygen/nitrogen is studied at stoichiometric conditions of 1000 ppm fuel in the Princeton variable pressure flow reactor at temperatures of 520–830 K and pressures of 8 and 12.5 atm. The overall oxidative reactivity of n-decane is observed in detail to show low temperature, negative temperature coefficient (NTC) and hot ignition regimes. Detailed temporal speciation studies are performed at reactor initial temperatures of 533 K and 740 K at 12.5 atm pressure and 830 K at 8 atm pressure. Significant amounts of large olefins are produced at 830 K, at conditions of transition from NTC to hot ignition behavior. The predictions using available chemical kinetic models for n-decane oxidation are compared against each other and the experiments. Only the kinetic models of Westbrook et al., Ranzi et al., and Biet et al. capture the NTC behavior exhibited by n-decane. However, each of these models yields varying disparities in the mechanistic predictions of major intermediate species, including ethylene and formaldehyde. Analyses of the Westbrook et al. model are compared with the new data. The predicted double-peaked species yield of ethylene, a behavior not found for the other models or in the experimental observations results from deficiencies in the C₂ chemistry. Mechanistic validation information about fuel oxidation chemistry is also provided by the measurement of various larger carbon number alkene isomers at 830 K and 8 atm. The modeling analysis suggests that in addition to n-alkyl beta-scission chemistry, alkyl peroxy radical chemistry contributes significantly to the formation of these alkenes. Specific reaction pathways and rate constants which affect the computation of these observations are discussed.     

Laminar burning speed measurement of premixed n-decane/air mixtures using spherically expanding flames at high temperatures and pressures

Normal-decane (n-C₁₀H₂₂) is regarded as a major component of possible surrogates for jet fuels and diesel fuels. The structure of spherically expanding premixed n-decane/air flames has been studied at high temperatures and pressures. The laminar burning speeds of n-decane/air mixtures have been measured for the temperatures of 350–610 K and pressures of 0.5–8 atm. The experiments were performed in lean conditions (0.7 ? ? ? 1). Laminar burning speed was measured using a thermodynamic model based on the pressure rise during the flame propagation in constant volume vessels. A cylindrical vessel equipped with a high speed CMOS camera was employed to investigate the flame structure and a spherical vessel was used for the burning speed measurements. The results are in good agreement with other experimental data available in the published literature.     

Experimental observation of the flame structure of a bimodal ammonium perchlorate composite propellant using 5 kHz PLIF

The self deflagration of a bimodal ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene (HTPB) propellant was studied using high speed (5 kHz) planar laser induced fluorescence (PLIF) for the first time. The qualitative OH concentration was characterized near the surface. In addition to OH, it was found that the larger AP particles can be imaged as they fluoresce when exposed to laser radiation centered at 283.2 nm. Single AP particle ignition delay, lifetime, and flame height are determined as a function of particle diameter over a range from 100 to 500 μm at 1 atm within the burning sample. High speed visible imaging was also completed to confirm the trends seen during PLIF experiments, although the fluoresced particles have much improved contrast. Ignition delay times and single particle burn times were compared with a model proposed by Shannon and Peterson. The measured final diffusion flame height above a regressing AP crystal was compared with an expression used by the Beckstead, Derr, and Price (BDP) model. It was found that the AP/HTPB propellant flame structure varies significantly with particle size, even at 1 atm. The models are found to adequately predict the observed trends, but do not capture the interaction of adjacent particles. The interaction of particles close to each other appear to affect behavior significantly and more detailed modeling is needed. It is shown that 5 kHz OH PLIF can be a valuable tool to characterize AP composite propellant combustion.     

Shock tube measurements of ignition delay times for the butanol isomers

Ignition delay times of the four isomers of butanol were measured behind reflected shock waves over a range of experimental conditions: 1050–1600 K, 1.5–43 atm, and equivalence ratios of 1.0 and 0.5 in mixtures containing 4% O₂ diluted in argon. Additional data were also collected at 1.0–1.5 atm in order to replicate conditions used by previous researchers. Good agreement is seen with past work for 1-butanol ignition delay times, though our measured data for the other isomers were shorter than those found in some previous studies, especially at high temperatures. At most conditions, the ignition delay time increases for each isomer in the following order: 1-butanol, 2-butanol and i-butanol nearly equal, and t-butanol. In addition, t-butanol has a higher activation energy than the other three isomers. In a separate series of high-pressure experiments, ignition delay times of 1-butanol in stoichiometric air were measured at temperatures as low as 800 K. At temperatures below 1000 K, pre-ignition pressure rises as well as significant rollover of ignition delay times were observed. Modeling of all collected data using several different chemical kinetic mechanisms shows partial agreement with the experimental data depending on the mechanism, isomer, and conditions. Only the mechanism developed by Vranckx et al. [1] partially explains the rollover and pre-ignition observed in stoichiometric experiments in air.     

Laminar flame speeds of cyclohexane and mono-alkylated cyclohexanes at elevated pressures

The propagation speeds of expanding spherical flames of cyclohexane, methylcyclohexane and ethylcyclohexane in mixtures of oxygen/inert were measured in a heated, dual-chamber vessel, with the corresponding laminar flame speeds extracted from them through nonlinear extrapolation. Measurements were conducted at atmospheric and elevated pressures up to 20 atm. Computational simulations were conducted using the JetSurF 2.0 mechanism, yielding satisfactory agreement with the present measurements at all pressures, with a slight over-prediction at 1 atm. Measurements reveal the following trend for the flame speeds: cyclohexane > n-hexane > methylcyclohexane ≈ ethylcyclohexane at all pressures, with the maximum difference being approximately 5% at 1 atm and 13% at 10 atm. Examination of the computed flame structure shows that owing to its symmetric ring structure, decomposition of cyclohexane produces more chain-branching 1,3-butadiene and less chain-terminating propene. On the contrary, a more balanced distribution of intermediates is present in the flames of methylcyclohexane and ethylcyclohexane due to substitution of the alkyl group for H.     

Effects of ambient pressure,gas temperature and combustion reaction on droplet evaporation

The effects of ambient pressure, initial gas temperature and combustion reaction on the evaporation of a single fuel droplet and multiple fuel droplets are investigated by means of three-dimensional numerical simulation. The ambient pressure, initial gas temperature and droplets’ mass loading ratio, ML, are varied in the ranges of 0.1–2.0 MPa, 1000–2000 K and 0.027–0.36, respectively, under the condition with or without combustion reaction. The results show that both for the conditions with and without combustion reaction, droplet lifetime increases with increasing the ambient pressure at low initial gas temperature of 1000 K, but decreases at high initial gas temperatures of 1500 K and 2000 K, although the droplet lifetime becomes shorter due to combustion reaction. The increase of ML and the inhomogeneity of droplet distribution due to turbulence generally make the droplet lifetime longer, since the high droplets’ mass loading ratio at local locations causes the decrease of gas temperature and the increase of the evaporated fuel mass fraction towards the vapor surface mass fraction.     

Upgrading of syngas derived from biomass gasification: A thermodynamic analysis

Hydrogen yields in the syngas produced from non-catalytic biomass gasification are generally low. The hydrogen fraction, however, can be increased by converting CO, CH₄, higher hydrocarbons, and tar in a secondary reactor downstream. This paper discusses thermodynamic limits of the synthesis gas upgrading process. The method used in this process is minimization of Gibbs free energy function. The analysis is performed for temperature ranges from 400 to 1300 K, pressure of 1–10 atm (0.1–1 MPa), and different carbon to steam ratios. The study concludes that to get optimum H₂ yields, with negligible CH₄ and coke formation, upgrading syngas is best practiced at a temperature range of 900–1100 K. At these temperatures, H₂ could be possibly increased by 43–124% of its generally observed values at the gasifier exit. The analysis revealed that increasing steam resulted in a positive effect. The study also concluded that increasing pressure from 1 to 3 atm can be applied at a temperature >1000 K to further increase H₂ yields.     

Ignition delay times of low-vapor-pressure fuels measured using an aerosol shock tube

Gas-phase ignition delay times were measured behind reflected shock waves for a wide variety of low-vapor-pressure fuels. These gas-phase measurements, without the added convolution with evaporation times, were made possible by using an aerosol shock tube. The fuels studied include three large normal alkanes, n-decane, n-dodecane and n-hexadecane; one large methyl ester, methyl decanoate; and several diesel fuels, DF-2, with a range of cetane indices from 42 to 55. The reflected shock conditions of the experiments covered temperatures from 838 to 1381 K, pressures from 1.71 to 8.63 atm, oxygen concentrations from 1 to 21%, and equivalence ratios from 0.1 to 2. Ignition delay times were measured using sidewall pressure, IR laser absorption by fuel at 3.39 μm, and CH^* and OH^* emission. Measurements are compared to previous studies using heated shock tubes and current models. Model simulations show similar trends to the current measurement except in the case of n-dodecane/21% O₂/argon experiments. At higher temperatures, e.g. 1250 K, the measured ignition delay times for these mixtures are significantly longer in lean mixtures than in rich mixtures; current models predict the opposite trend. As well, the current measurements show significantly shorter ignition delay times for rich mixtures than the model predictions.     

Shock tube studies of methyl butanoate pyrolysis with relevance to biodiesel

Methyl butanoate pyrolysis and decomposition pathways were studied in detail by measuring concentration time-histories of CO, CO₂, CH₃, and C₂H₄ using shock tube/laser absorption methods. Experiments were conducted behind reflected shock waves at temperatures of 1200–1800 K and pressures near 1.5 atm using mixtures of 0.1%, 0.5%, and 1% methyl butanoate in Argon. A novel laser diagnostic was developed to measure CO in the ν₁ fundamental vibrational band near 4.56 μm using a new generation of quantum-cascade lasers. Wavelength modulation spectroscopy with second-harmonic detection (WMS-2f) was used to measure CO₂ near 2752 nm. Methyl radical was measured using UV laser absorption near 216 nm, and ethylene was monitored using IR gas laser absorption near 10.53 μm. An accurate methyl butanoate model is critical in the development of mechanisms for larger methyl esters, and the measured time-histories provide kinetic targets and strong constraints for the refinement of the methyl butanoate reaction mechanism. Measured CO mole fractions reach plateau values that are the same as the initial fuel mole fraction at temperatures higher than 1500 K over the maximum measurement time of 2 ms or less. Two recent kinetic mechanisms are compared with the measured data and the possible reasons for this 1:1 ratio between MB and CO are discussed. Based on these discussions, it is expected that similar CO/fuel and CO₂/fuel ratios for biodiesel molecules, particularly saturated components of biodiesel, should occur.     

Bio-butanol: Combustion properties and detailed chemical kinetic model

Autoignition delay time measurements were performed at equivalence ratios of 0.5, 1 and 2 for butan-1-ol at reflected shock pressures of 1, 2.6 and 8 atm at temperatures from 1100 to 1800 K. High-level ab initio calculations were used to determine enthalpies of formation and consequently bond dissociation energies for each bond in the alcohol. A detailed chemical kinetic model consisting of 1399 reactions involving 234 species was constructed and tested against the delay times and also against recent jet-stirred reactor speciation data with encouraging results. The importance of enol chemistry is highlighted.     

Structure of laminar premixed flames of methane near the auto-ignition limit

Auto-ignition and flame propagation are the two different controlling mechanisms for stabilizing the flame in secondary stage combustion in hot vitiated air environment and at elevated pressure. The present work aims at the investigation of the flame stabilization mechanism of flames developing in such an environment. In order to better understand the structure of turbulent flames at inlet temperature well above the auto-ignition temperature, the behavior of laminar flames at those conditions needs to be analyzed. As an alternative to challenging and expensive measurements at high temperature and pressure, the behavior of laminar flames at such conditions can be predicted from theory using mathematical simulation. In the present work, the laminar burning velocities and flame structures of premixed stoichiometric methane/air mixtures for inlet temperatures from 300 to 1450 K and absolute pressures from 1 to 8 bar have been calculated using a freely propagating laminar, one dimensional, planar flame model. The prediction shows that at inlet temperatures below the auto-ignition temperature, the predicted laminar burning velocity which corresponds to the unburned mixture velocity in order to create a steady laminar flame decreases with increase in pressure. When the inlet temperature of the mixture goes well beyond the auto-ignition temperature of the mixture, however, the unburned mixture velocity increases steeply at higher pressure level, because of a complete transition of the flame structure.