Reaction Mechanisms Underlying Unfunctionalized Alkyl Nitrate Hydrolysis in Aqueous Aerosols

Alkyl nitrates (ANs) are both sinks and sources of nitrogen oxide radicals (NOx = NO + NO2) in the atmosphere. Their reactions affect both the nitrogen cycle and ozone formation and therefore air quality and climate. ANs can be emitted to the atmosphere or produced in the gas phase. In either case, they can partition into aqueous aerosols, where they might undergo hydrolysis, producing highly soluble nitrate products, and act as a permanent sink for NOx. The kinetics of AN hydrolysis partly determines the extent of AN contribution to the nitrogen cycle. However, kinetics of many ANs in various aerosols is unknown, and there are conflicting arguments about the effect of acidity and basicity on the hydrolysis process. Using computational methods, this study proposes a mechanism for the reactions of methyl, ethyl, propyl, and butyl nitrates with OH- (hydroxyl ion; basic hydrolysis), water (neutral hydrolysis), and H3O+ (hydronium ion; acidic hydrolysis). Using quantum chemical data and transition state theory, we follow the effect of pH on the contribution of the basic, neutral, and acidic hydrolysis channels, and the rate coefficients of AN hydrolysis over a wide range of pH. Our results show that basic hydrolysis (i.e., AN reaction with OH-) is the most kinetically and thermodynamically favorable reaction among our evaluated reaction schemes. Furthermore, comparison of our kinetics results with experimental data suggests that there is an as yet unknown acidic mechanism responsible for acidic catalysis of AN hydrolysis.Peer reviewe

Elementary, pressure dependent model for combustion of C1, C2 and nitrogen containing hydrocarbons : operation of a pilot scale incinerator and model comparison

A 140000 BTU/hr pilot scale incinerator has been constructed, tested and run; and an online sampling train capable of taking in situ data has been established. The continuous on-line analytical instruments include a CO analyzer, an NO/NOx analyzer and an 02 analyzer. In addition, two gas chromatographs with flame ionization detector are used to determine CH4, C2H2 + C2H4 and total hydrocarbon concentrations. Typical operating conditions are at an average 02 concentration of 6 - 8 %. The NO concentration ranged from 100 - 200 ppm. Approximately I ppm of CH.4 is also present at steady state operations. The kinetic model for the combustion process in the pilot scale incinerator consists of elementary reaction kinetics for oxidation of the model fuel species: CH4, CH3OH, C2H2, C2H4, C2H6 and CH3NH2. Thermodynamic properties for these species are determine by ab initio methods and density functional theory. High-pressure limit rate constants are determine by either canonical transition state theory or variational transition states theory. In some cases, estimation techniques based on Evans-Polvani relationships are used. Pressure and temperature dependent mechanism is constructed utilizing QRRK for k(E) with either master equation or modified strong collision analysis for fall-off The mechanism is constructed over the pressure range of 0. 00 1 - 100 atm and over a temperature range of 300 - 2500 K. A reactor configuration of an isothermal perfectly stirred reactor (PSR) followed by a plugged flow reactor with heat transfer loss (PFRI), followed by a second plugged flow reactor with a different heat transfer loss (PFR2) is used to model the pilot scale incinerator. Concentration profiles are determined from the detailed kinetic model based on the reactor configuration. Results show that02, is consumed and CO2 and NO are formed mainly in the PSR. The concentration of these three components do not change throughout PFRI and PFR2. Comparison of the NO and NOx experimental data with the model shows the data are in the same range, varying from 100 - 200 ppm, with less than 50 pprn difference. The average NO:NOx ratio for experimental data is 0.97, and the average NO\u27NOx ratio from the model results is 0.98

Low Temperature Kinetic Studies using a Pulsed Laval Nozzle Apparatus

Laboratory kinetic studies of reactions relevant to interstellar environments have been performed using a pulsed Laval nozzle apparatus coupled with pulsed laser photolysis-laser induced fluorescence spectroscopy in the temperature range 54-148 K. Rate coefficients for the reactions of the hydroxyl radical with several oxygenated organic molecules are reported in Chapters 3 and 4. At low temperatures, the rate coefficients for these reactions are found to be significantly enhanced despite barriers to hydrogen abstraction. A common mechanism has been identified involving the initial formation of a weakly bound complex (~ 15-30 kJ mol-1), which has an extended lifetime at lower temperatures. The extended lifetime of the complex facilitates two competing channels: collisional stabilisation into the pre-barrier well, or quantum mechanical tunnelling through the hydrogen abstraction barrier. The role of these channels is assessed through studies of pressure dependence. The mechanism is also found to be operative for even very weakly bound complexes, such as in the reaction of OH with ammonia as reported in Chapter 5. Pressure dependence and product detection studies enable the low temperature yield of NH2 radicals from this reaction to be quantified. The potential interstellar implications of these reactions in light of the rate coefficients obtained in this work are reported. In Chapter 6, the reaction of the postulated products from the OH + methanol and ethanol reactions, methoxy and ethoxy radicals, with NO are studied for the first time at low temperatures. The role of pressure stabilization of the complex, RONO, versus bimolecular product formation is investigated through pressure dependence studies and detection of the NO + methoxy radical product, HCHO, via laser induced fluorescence spectroscopy. The low temperature high pressure limiting rate coefficients of the OH + oVOC and OH + NH3 reactions are explored in Chapter 7 using the proxy method of Jaffer, Smith, Quack and Troe. Rate coefficients for the reactions are obtained at different quanta of vibrational excitation of OH, and the validity of the proxy method for weakly bound complexes at low temperatures is considered with regards to efficient intramolecular vibrational relaxation

Entwicklung von Reaktionsmechanismen für Systeme bei der Polygeneration

Polygeneration bezeichnet die flexible Umwandlung zwischen thermischen, chemischen und mechanischen Energieformen. Bei Verbrennungsprozessen ist die Implementierung der Polygeneration eine interessante Alternative zur Erzeugung von Wärme und Arbeit sowie wertvoller Chemikalien aus der Oxidation von Kohlenwasserstoffen. Die Model-lierung extrem brennstoffreicher Bedingungen, die für Polygenerationsprozesse relevant sind, erfordert Reaktionsmechanismen, die speziell für die Beschreibung der Reaktions-kinetik solcher Gemische entwickelt wurden. In Rahmen dieser Arbeit wurde ein neuer detaillierter Elementarreaktionsmechanismus zur Prognose und Beschreibung der Kinetik von Methan/Additive-Gasgemischen bei derartig unkonventionellen Reaktionsbedingun-gen entwickelt. Der hier entwickelte Polygenerationsmechanismus (PolyMech) wurde anhand von Daten aus unterschiedlichen experimentellen Versuchsaufbauten validiert. Hierbei wurden Simulationsergebnisse der Zündverzugszeiten und der Speziesverläufe mit experimentellen Daten in einem breiten Bereich von Druck, Temperatur und Äquiva-lenzverhältnis verglichen. Ergebnisse haben gezeigt, dass der Mechanismus die Experi-mente zuverlässig beschreiben kann. Parallel zur Weiterentwicklung und Validierung des PolyMech erfolgte die Entwicklung einer effizienten numerischen Methode zur automa-tischen Vereinfachung von hochdimensionalen Reaktionsmechanismen. Das in dieser Ar-beit verwendete Reduktionsmodell basiert auf charakteristischen Zeitskalen- und Entro-pieproduktionsanalysen. Dadurch werden Informationen aus komplex reagierenden Sys-temen automatisch erhalten, die eine Zerlegung der Dynamik des Systems in niedrigdi-mensionale Systeme ermöglichen. Das Reduktionsmodell kann durch die Erzeugung von Skelettmechanismen oder die direkte Integration des resultierenden Differentialglei-chungssystems verwendet werden. Die Anwendbarkeit des in dieser Arbeit implemen-tierten Reduktionsmodells bei hochdimensionalen Reaktionsmechanismen wurde über-prüft, indem Simulationsergebnisse des reduzierten Modells mit Vorhersagen des detail-lierten Mechanismus verglichen wurden. Hierbei zeigen die Ergebnisse aus beiden Mo-dellen eine gute Übereinstimmung. Eine Reduktion um ca. 60% der Anzahl unabhängi-ger, für die Modellierung zu lösender Variablen des detaillierten Mechanismus, wird hier-bei ermöglicht

Summaries of FY 1997 Research in the Chemical Sciences

The objective of this program is to expand, through support of basic research, knowledge of various areas of chemistry, physics and chemical engineering with a goal of contributing to new or improved processes for developing and using domestic energy resources in an efficient and environmentally sound manner. Each team of the Division of Chemical Sciences, Fundamental Interactions and Molecular Processes, is divided into programs that cover the various disciplines. Disciplinary areas where research is supported include atomic, molecular, and optical physics; physical, inorganic, and organic chemistry; chemical energy, chemical physics; photochemistry; radiation chemistry; analytical chemistry; separations science; heavy element chemistry; chemical engineering sciences; and advanced battery research. However, traditional disciplinary boundaries should not be considered barriers, and multi-disciplinary efforts are encouraged. In addition, the program supports several major scientific user facilities. The following summaries describe the programs

Combustion generated fine carbonaceous particles

Soot is of importance for its contribution to atmospheric particles with their adverse health impacts and for its contributions to heat transfer in furnaces and combustors, to luminosity from candles, and to smoke that hinders escape from buildings during fires and that impacts global warming or cooling. The different chapters of the book adress comprehensively the different aspects from fundamental approaches to applications in technical combustion devices

Ignition and Heat Release Behaviour of iso-Butanol and Gasoline Blended Fuels: An Experimental and Kinetic Modelling Study

The decarbonisation of transport and the introduction of further renewable energy sources are required to minimise the impacts of climate change, while meeting the energy needs of a developing global population. Introducing alternative fuels into existing and developing spark ignition (SI) engine technologies requires the thorough characterisation of the fuel’s combustion behaviour. The propensity of fuel to autoignite is a key property which limits SI engine performance through the development of engine knock. Autoignitive behaviour can be characterised by ignition delay time (IDT) measurements in rapid compression machines (RCM) or by measuring knocking behaviour within practical engines. RCMs provide an opportunity to study a fuels ignition behaviour at the fundamental level, the measurements of which often serve as a prediction for behaviour in more complex systems, such as SI engines. Through the application of both techniques, this work investigates the influence of iso-butanol blending on the combustion behaviour of gasoline (with particular focus given to the anti-knock properties of fuel blends), as well as assessing the validity of applying fundamental studies to predict practical engine level combustion behaviour. Accurate computational modelling provides an opportunity for the prediction of combustion behaviour quickly and cheaply when compared to experiments, facilitating the rapid optimisation of engine and fuel blend designs. To enable the computational modelling of gasoline, surrogate fuels are required which replicate the target behaviours of the reference fuel, while minimising molecular complexity. The ability of a newly developed five component surrogate (5-C) to reproduce the autoignition behaviour of a research grade gasoline (RON 95 MON 86.6) is investigated within an RCM, at temperatures of 675-870 K, a pressure of 20 bar and equivalence ratios of 0.5 and 1.0, producing an excellent representation at stoichiometric conditions but displaying much lower reactivity than gasoline at lean conditions. When blended with iso-butanol (at 10, 30, 50 and 70% iso-butanol by volume), the representation of gasoline by 5-C continues to be generally good but at low temperatures (<770 K) and high iso-butanol concentrations (iB50/70), 5-C blends are considerably less reactive than gasoline blends. Upon investigation within a motored, skip-firing SI research engine, the 5-C continued to provide an accurate representation of gasoline’s normal and knocking combustion behaviour at spark advance timings of 2-10 CA° bTDC. Under blending with iso-butanol the surrogate continued to perform well but blends were observably less reactive at spark advance timings <8 CA° bTDC. Blends of 20-50% iso-butanol were found to be optimal for use in SI engines, providing considerable anti-knock benefits and comparable indicated power to gasoline. Correlations between RCM and engine measurements display the proficiency of fundamental measurements in predicting combustion behaviour within an engine at similar thermodynamic conditions. Changes in the autoignition behaviour of 5-C due to blending with iso-butanol (5-70% iso-butanol) were studied experimentally within the RCM and computationally via chemical kinetic modelling. At low temperatures, iso-butanol generally reduces reactivity, suppressing the intensity of LTHR. As temperatures are increased, iso-butanol appears to suppress NTC behaviour and a cross-over in IDT measurements is observed between blends of 5 and 10% iso-butanol, wherein the 10% blend becomes the most reactive at intermediate to high temperatures. Modelling results largely failed to replicate complex blending behaviour and largely underpredicted IDTs in the NTC region. It is proposed that the model’s misrepresentation of LTHR behaviour is a cause for such global model failures, as evidenced by local OH, brute force enthalpy of formation and reaction A-factor sensitivity analyses which highlighted the importance of reactions and species of significance to first stage ignition and low temperature oxidation processes, in the determination of IDTs and characteristic LTHR properties. Minimising uncertainties in the thermodynamic properties of complex oxygenated species typical of low temperature oxidation would produce more accurate model predictions, as these uncertainties are currently large for many important species. The influence of these uncertainties on the parameters investigated in this study is substantial. Current computer models therefore cannot be effectively applied in the prediction of the combustion behaviour for gasoline/iso-butanol blends until these issues are resolved. Further studies of the species and key reactions identified in this research would help to improve current kinetic mechanisms