In situ temperature measurements in sooting methane/air flames using synchrotron x-ray fluorescence of seeded krypton atoms

Synchrotron x-ray fluorescence has been used to measure temperatures in optically dense gases where traditional methods would fail. These data provide a benchmark for stringent tests of computational fluid dynamics models for complex systems where physical and chemical processes are intimately linked. The experiments measured krypton number densities in a sooting, atmospheric pressure, nonpremixed coflow flame that is widely used in combustion research. The experiments not only form targets for the models, but the simulations also identify potential sources of uncertainties in the measurements, allowing for future improvements.

Reactive Molecular Dynamics Simulations and Quantum Chemistry Calculations To Investigate Soot-Relevant Reaction Pathways for Hexylamine Isomers

Sooting tendencies of a series of nitrogen-containing hydrocarbons (NHCs) have been recently characterized experimentally using the yield sooting index (YSI) methodology. This work aims to identify soot-relevant reaction pathways for three selected C₆H₁₅N amines, namely, dipropylamine (DPA), diisopropylamine (DIPA), and 3,3-dimethylbutylamine (DMBA) using ReaxFF molecular dynamics (MD) simulations and quantum mechanical (QM) calculations and to interpret the experimentally observed trends. ReaxFF MD simulations are performed to determine the important intermediate species and radicals involved in the fuel decomposition and soot formation processes. QM calculations are employed to extensively search for chemical reactions involving these species and radicals based on the ReaxFF MD results and also to quantitatively characterize the potential energy surfaces. Specifically, ReaxFF simulations are carried out in the NVT ensemble at 1400, 1600, and 1800 K, where soot has been identified to form in the YSI experiment. These simulations account for the interactions among test fuel molecules and pre-existing radicals and intermediate species generated from rich methane combustion, using a recently proposed simulation framework. ReaxFF simulations predict that the reactivity of the amines decrease in the order DIPA > DPA > DMBA, independent of temperature. Both QM calculations and ReaxFF simulations predict that C₂H₄, C₃H₆, and C₄H₈ are the main nonaromatic soot precursors formed during the decomposition of DPA, DIPA, and DMBA, respectively, and the associated reaction pathways are identified for each amine. Both theoretical methods predict that sooting tendency increases in the order DPA, DIPA, and DMBA, consistent with the experimentally measured trend in YSI. This work demonstrates that sooting tendencies and soot-relevant reaction pathways of fuels with unknown chemical kinetics can be identified efficiently through combined ReaxFF and QM simulations. Overall, predictions from ReaxFF simulations and QM calculations are consistent, in terms of fuel reactivity, major intermediates, and major nonaromatic soot precursors.

Sooting tendencies of oxygenated hydrocarbons in laboratory-scale flames

Sooting tendencies have been measured for 186 oxygenated and 89 regular hydrocarbons under controlled laboratory conditions. The test compounds include alcohols, ethers, aldehydes, ketones, esters, alkanes, alkenes, and cycloalkanes ranging in size from methanol to isododecane. Sooting tendency was characterized with a new method based on measuring particle concentrations in methane/air nonpremixed flames when 1000 ppm of each test compound was added to the fuel. This method offers high precision and high sensitivity to the direct chemical effects of the additive. The results provide a wide-ranging yet detailed quantitative picture of how fuel oxygen affects soot formation, which will be useful for optimizing the soot-reducing benefits of oxygenated renewable fuels. The measured sooting tendencies of 1-alcohols are similar to those of n-alkanes with the same number of carbon atoms, while those of secondary alcohols are slightly higher. Aldehydes and ketones soot the same as n-alkanes with one less carbon atom. The sooting tendencies of esters depend strongly on molecular structure and increase in this order: methyl and ethyl esters < carboxylic acids, propyl esters, and n-alkanes < butyl and pentyl esters. The high sooting tendencies of the secondary alcohols and higher esters suggest that four-center and six-center reaction pathways are important.

Decomposition and Hydrocarbon Growth Processes for Esters in Non-Premixed Flames

Biomass fuels are a promising renewable energy source, and so, the mechanisms that may produce toxic oxygenated byproducts and aromatic hydrocarbons from oxygenated hydrocarbons are of interest. Esters have the form R-(C=O)-O-R' and are components of biodiesel fuels. The five specific esters studied here are isomers of C5H10O2. The experiments were performed in atmospheric pressure coflowing methane/air non-premixed flames. A series of flames were generated by separately doping the fuel mixture with 5,000 ppm of each ester. This concentration is sufficiently large to produce measurable changes in intermediate hydrocarbon concentrations, yet small enough to not disturb the overall flame structure. Since the overall structure is not perturbed, the measured changes in the intermediate hydrocarbons can be directly attributed to the reactions of the esters. Analysis of these changes reveals that unimolecular six-centered dissociation is the primary decomposition pathway for the three esters with molecular arrangements capable of undergoing that mechanism. The remaining two esters exhibited decomposition rates and products that are consistent with simple fission as the dominant decomposition mechanism, though we do not exclude other pathways from playing a significant role in their decomposition. All of the esters produce aromatic hydrocarbons at higher rates than the undoped fuel, and the molecular arrangement of the ester isomers plays a role in the degree of aromatic formation. Isomer variations also influence the type and quantity of toxic oxygenates that are produced in the flames.

Sensitive in situ detection of chlorinated hydrocarbons in gas mixtures

We detect chlorinated hydrocarbons (CHC's) in gas mixtures by dissociating the CHC's with a 193-nm laser and measuring the subsequent concentration of the CCl fragmentation by means of laser-induced fluorescence. Sub-ppm detection, where ppm indicates parts in 10(6), is achieved for C(2)H(5)Cl with a 10-mm(3) measurement volume and integration over 50 laser shots. Every other CHC tested is also detectable, with the same or better detection limits. The CCl forms promptly during the fragmentation laser pulse through unimolecular dissociation of the parent CHC's. The technique should be a useful diagnostic for CHC incineration systems.