An Autoignition Study of iso-Butanol: Experiments and Modeling

The autoignition delays of iso-butanol, oxygen, and nitrogen mixtures have been measured in a heated rapid compression machine (RCM). At compressed pressures of 15 and 30 bar, over the temperature range 800-950 K, and for equivalence ratio of $\phi$ = 0.5 in air, no evidence of an NTC region of overall ignition delay is found. By comparing the data from this study taken at $\phi$ = 0.5 to previous data collected at $\phi$ = 1.0 (Weber et al. 2013), it was found that the $\phi$ = 0.5 mixture was less reactive (as measured by the inverse of the ignition delay) than the $\phi$ = 1.0 mixture for the same compressed pressure. Furthermore, a recent chemical kinetic model of iso-butanol combustion was updated using the automated software Reaction Mechanism Generator (RMG) to include low- temperature chain branching pathways. Comparison of the ignition delays with the updated model showed reasonable agreement for most of the experimental conditions. Nevertheless, further work is needed to fully understand the low temperature pathways that control iso-butanol autoignition in the RCM.Comment: 6 pages, 4 figures, 8th US National Combustion Meetin

Combustion chemistry of alcohols and their application to advanced engines

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 237-266).A major challenge in energy is the identification of viable liquid fuels as alternatives to petroleum-based fuels. There are a wide variety of candidate fuels to select from and assessing each new fuel is far from trivial. Small variations in chemical structure can cause large changes in a fuel's performance. Simultaneously, engine designs are also changing rapidly. Accurately predicting how new fuels will perform in future engines are in many ways more valuable than knowing which fuels perform well in today's engines. Predictive theoretical modeling is required to efficiently screen candidates. The selection of a good candidate fuel requires the development of detailed kinetic models capable of accurately predicting fuel behavior over the entire range of engine operating conditions. Despite the fact that most literature models succeed to accurately predict primary combustion products and high temperature ignition delay, two areas require further scientific understanding: peroxy chemistry and polycyclic aromatic hydrocarbon (PAH) formation. The first section of this thesis describes significant contributions to both these areas. Peroxy chemistry is important for accurately predicting ignition in future engine designs based on the concept of low temperature combustion (LTC). This thesis provides a clear explanation of how peroxy chemistry affects low temperature ignition behavior. Simple analytical expressions are provided for the time constant for radical growth and first-stage ignition delay. To improve the understanding of PAH formation, abintio calculations to indene and naphthalene from cyclopentadiene and cyclopentadienyl radical were performed. The calculated gas phase rate constants and thermochemistry were used to develop the first elementary micro-kinetic model for the formation of indene and naphthalene from cyclopentadiene. The model is validated against cyclopentadiene pyrolysis data in flow reactors. The second section of this thesis presents a combined computational-experimental approach to rapidly construct accurate combustion chemistry simulations for alcohol fuels. In this approach experiments and quantum chemical calculations are carried out in parallel, informing an evolving chemical kinetic model. This approach was used to understand and predictively model the combustion chemistry of iso-butanol and pentanol isomers. Detailed kinetic models for iso-butanol and pentanol isomers are presented which are validated against a large number of datasets spanning the entire range of operating conditions seen during real engine operation. We see that for many performance parameters, the model predictions are as accurate as experiment and help provide mechanistic insight into differing reactivity of a fuel's isomers. Lastly, we show how detailed kinetic model can be applied in multi-dimensional CFD simulations of a new type of engine, the reactivity controlled compression ignition engine (RCCI), in order to make predictions of how iso-butanol will affect the engine efficiency and emissions. This thesis covers the entire process of predictively accessing a fuel by taking a new fuel molecule, developing a detailed model, and evaluating it in a new engine design in order to make informed decisions.by Shamel Sarfaraz Merchant.Ph. D

Automated Reaction Mechanism Generation Including Nitrogen as a Heteroatom

The open source rate-based Reaction Mechanism Generator (RMG) software and its thermochemical and kinetics databases were extended to include nitrogen as a heteroatom. Specific changes to RMG and the mining of thermochemistry and reaction kinetics data are discussed. This new version of RMG has been tested by generating a detailed pyrolysis and oxidation model for ethylamine (EA, CH3CH2NH2) at ∼1400 K and ∼2 bar, and comparing it to recent shock tube studies. Validation of the reaction network with recent experimental data showed that the generated model successfully reproduced the observed species as well as ignition delay measurements. During pyrolysis, EA initially decomposes via a CC bond scission, and the CH2NH2 product subsequently produces the first H radicals in this system via β-scission. As the concentration of H increases, the major EA consuming reaction becomes H abstraction at the α-site by H radicals, leading to a chain reaction since its product generates more H radicals. During oxidation, the dominant N2-producing route is mediated by NO and N2O. The observables were found to be relatively sensitive to the CC and CN EA bond scission reactions as well as to the thermodynamic values of EA; thermodynamic data for EA were computed at the CBS-QB3 level and reported herein. This work demonstrates the ability of RMG to construct adequate kinetic models for nitrogenous species and discusses the pyrolysis and oxidation mechanisms of EA.Swiss National Science Foundation (Grant PBEZB2-140081)US Department of Energy, Office of Basic Energy Sciences (Award DE-SC0014901

A Coordinated Investigation Of The Combustion Chemistry Of Diisopropyl Ketone, A Prototype For Biofuels Produced By Endophytic Fungi

Several classes of endophytic fungi have been recently identified that convert cellulosic biomass to a range of ketones and other oxygenated molecules, which are potentially viable as biofuels, but whose oxidation chemistry is not yet well understood. In this work, we present a predictive kinetics model describing the pyrolysis and oxidation of diisopropyl ketone (DIPK) that was generated automatically using the Reaction Mechanism Generator (RMG) software package. The model predictions are evaluated against three experiments that cover a range of temperatures, pressures, and oxygen concentrations: (1) Synchrotron photoionization mass spectrometry (PIMS) measurements of pyrolysis in the range 800-1340. K at 30. Torr and 760. Torr; (2) Synchrotron PIMS measurements of laser photolytic Cl-initiated oxidation from 550. K to 700. K at 8. Torr; and (3) Rapid-compression machine measurements of ignition delay between 591. K and 720. K near 10. bar. Improvements made to the model parameters, particularly in the areas of hydrogen abstraction from the initial DIPK molecule and low-temperature peroxy chemistry, are discussed. Our ability to automatically generate this model and systematically improve its parameters without fitting to the experimental results demonstrates the usefulness of the predictive chemical kinetics paradigm. © 2013 The Combustion Institute

Kinetics and Products of Vinyl + 1,3-Butadiene, a Potential Route to Benzene

The reaction between vinyl radical, C[subscript 2]H[subscript 3], and 1,3-butadiene, 1,3-C[subscript 4]H[subscript 6], has long been recognized as a potential route to benzene, particularly in 1,3-butadiene flames, but the lack of reliable rate coefficients has hindered assessments of its true contribution. Using laser flash photolysis and visible laser absorbance (λ = 423.2 nm), we measured the overall rate coefficient for C[subscript 2]H[subscript 3] + 1,3-C[subscript 4]H[subscript 6], k[subscript 1], at 297 K ≤ T ≤ 494 K and 4 ≤ P ≤ 100 Torr. k[subscript 1] was in the high-pressure limit in this range and could be fit by the simple Arrhenius expression k[subscript 1] = (1.1 ± 0.2) × 10[superscript –12] cm[superscript 3] molecule[superscript –1] s[superscript –1] exp(−9.9 ± 0.6 kJ mol[superscript –1]/RT). Using photoionization time-of-flight mass spectrometry, we also investigated the products formed. At T ≤ 494 K and P = 25 Torr, we found only C[subscript 6]H[subscript 9] adduct species, while at 494 K ≤ T ≤ 700 K and P = 4 Torr, we observed ≤∼10% branching to cyclohexadiene in addition to C[subscript 6]H[subscript 9]. Quantum chemistry master-equation calculations using the modified strong collision model indicate that n-C[subscript 6]H[subscript 9] is the dominant product at low temperature, consistent with our experimental results, and predict the rate coefficient and branching ratios at higher T where chemically activated channels become important. Predictions of k[subscript 1] are in close agreement with our experimental results, allowing us to recommend the following modified Arrhenius expression in the high-pressure limit from 300 to 2000 K: k[subscript 1] = 6.5 × 10[superscript –20] cm[superscript 3] molecule[superscript –1] s[superscript –1] T[superscript 2.40] exp(−1.76 kJ mol[superscript –1]/RT).United States. Dept. of Energy. Office of Basic Energy Sciences (Energy Frontier Research Center for Combustion Science. Grant No. DE-SC0001198