Formation of polycyclic aromatic hydrocarbons and their growth to soot—a review of chemical reaction pathways

Kinetic modeling of hydrocarbon/nitric oxide interactions in a flow reactor

Detailed chemical kinetic models for the combustion of hydrocarbon fuels

https://pepiot.mae.cornell.edu/pdf/Blanquart_CF_2009.pdf

Chemical mechanism for high temperature combustion of engine relevant fuels with emphasis on soot precursors

This review of the role of reaction kinetics in combustion chemistry traces the historical evolution and present state of qualitative and quantitative understanding of a number of reaction systems. Starting from the H2–O2 system, in particular from the reaction between H and O2, mechanisms and key reactions for soot formation, for the appearance of NOx, and for processes of peroxy radicals in hydrocarbon oxidation are illustrated. The struggle for precise rate constants on the experimental and theoretical side is demonstrated for the example of the reaction H + O2 → OH + O. The intrinsic complexity of complex-forming bimolecular reactions, such as observed even in this reaction, also dominates most other key reactions of the systems considered and can be unravelled only with the help of quantum-chemical methods. The multi-channel character of these reactions often also requires the combination with master equation codes. Although kinetics provides an already impressive database for quantitative modelling of simple combustion systems, considerable effort is still required to quantitatively account for the complexities of more complicated fuel oxidation processes.

Unravelling combustion mechanisms through a quantitative understanding of elementary reactions

Shock-tube experiments on the high-temperature pyrolysis of phenol were performed in the temperature range from 1450 to 1650 K using atomic and molecular resonance absorption spectroscopy as probing tools. From the measurements, it was concluded that the dominating initiation step of the phenol pyrolysis is the molecular channel R1: C 6 H 5 OH-ix}C 5 H 6 +CO For the rate of reaction, an Arrhenius expression k 1 =1.0×10 12 exp(−30,600/ T )s −1 was deduced. The experimental data can be satisfactorily modeled if the coefficient for a secondary radical channel R1a: C 6 H 5 OH-ix}C 6 H 5 O+H does not exceed the value of 0.15 k 1 . At temperatures below 1510 K, there exists very good agreement between measured and computed results over the whole observation time: at higher temperatures, the calculated H concentrations exceeded the experimental ones at 800 μs by up to 50%. This might be due to uncertainties of the rate constants of the propargyl radical. The reaction of phenol with H atoms using ethyl iodide as precursor for H atoms has also been investigated. The experimental data compare very well with the sum of the rate coefficients for R2: C 6 H 5 OH+H-ix}C 6 H 6 +OH and R3: C 6 H 5 OH+H-ix}C 6 H 5 O+H 2 given by He et al. [5]. A reaction mechanism consisting of 18 reactions to reproduce our experimental data is presented.

Shock-tube study on the high-temperature pyrolysis of phenol

The shock tube technique has been used to investigate high-temperature reactions of C 5 -species, which appear to play an important role in polycyclic aromatic hydrocarbon (PAH) and soot formation. The unimolecular decomposition of cyclopentadiene (C 5 H 6 ) has been studied behind reflected shock waves. The temperature ranged from 1260 to 1600 K at pressures between 0.7 and 5.6 bar. Initial cyclopentadiene concentrations ranged from 0.5 to 120 ppm, diluted in argon. Atomic Resonance Absorption Spectrometry (ARAS) was used to record the temporal concentration profiles of H-atoms during the pyrolysis of cyclopentadiene under very low concentration conditions. Absorption spectrometry and optical multichannel analyzer for the detection of acetylene during C 5 H 6 pyrolysis were applied. For the main channel of cyclopentadiene decomposition C 5 H 6 →C 5 H-c+H (R1) a revised rate expression of k 1 =4.0×10 14 ×exp(−38760/ T ) s −1 was deduced, after reevaluation of the previous experiments with an improved experimental calibration curve. For evaluating the decay rate of the cyclopentadienyl radical C 5 H 5 -c→C 2 H 2 +C 3 H 3 (R3) PUMP2 level calculations were performed. The results were validated by means of the measured C 2 H 2 -and H-absorption profiles. Theory and experiments presented in this work verify quantitatively the decomposition process of the C 5 H 5 -c radical.

High-temperature investigations on the pyrolysis of cyclopentadiene

Two reduced reaction mechanisms were established that predict reliably for pressures up to about 20 bar the heat release for different syngas mixtures including initial concentrations of methane. The mechanisms were validated on the base of laminar flame speed data covering a wide range of preheat temperature, pressure, and fuel-air mixtures. Additionally, a global reduced mechanism for syngas, which comprises only two steps, was developed and validated, too. This global reduced and validated mechanism can be incorporated into CFD codes for modeling turbulent combustion in stationary gas turbines.

Reduced Reaction Mechanisms for Methane and Syngas Combustion in Gas Turbines

The thermal dissociation of ortho-benzyne (o-C6H4) has been studied behind reflected shock waves under very isolated conditions. In the shock tube experiments 1,2-diiodobenzene was employed as a thermal source for the o-C6H4 radical. For different series of experiments the temperature ranged from 1600 to 2400 K at pressures between 1.4 and 2 bar. Very low initial concentrations of the radical precursor, 0.5–4 ppm diluted in argon, were used. In situ atomic resonance absorption measurement of iodine atoms formed during the thermal dissociation of the radical precursor molecule provides for a precise determination of the initial 1,2-diiodobenzene concentration. ARAS (atomic resonance absorption spectroscopy) was also used to record the absorption profiles of hydrogen atoms obtained during the pyrolysis of ortho-benzyne. From the measured H atom absorption profiles and by taking into account recent results from literature, it is shown that the thermal dissociation of ortho-benzyne occurs via two pathways: besides the molecular route: (R1a) o - C 6 H 4 → C 4 H 2 + C 2 H 2 the second product channel R1b leading to direct H atom elimination had to be included (R1b) o - C 6 H 4 → c - C 6 H 3 + H Due to lack of experimental data, the rate expression for R1a was estimated by taking the activation energy of 347 kJ/mol from literature. Depending on k1a a rate expression k1b was deduced from modeling H atom formation, with a value for the activation energy of about 419 kJ/mol. The reaction rates of both channels are coupled by the branching ratio α =  k1b/(k1a +  k1b), with α = 2.36E − 04 · T − 0.3. It was found that for a wide variation of k1a (factors 0.1–20), k1b had also be adjusted with the same factors to match the H atom formation, thus the branching ratio is independent of the value of k1a in the investigated temperature range.

A shock tube investigation of H atom production from the thermal dissociation of ortho-benzyne radicals

The thermal decomposition of cyclopentadiene has been studied in the temperature range 1260–1530 K behind reflected shocks. The total pressure ranged from 1.5 to 2.3 bar. Resonance absorption was used to record the temporal concentration profiles of H atoms. This sensitive technique allowed the study of the reaction systems under favorable conditions by applying very low initial concentrations (0.5–8 ppm). For cyclopentadiene decomposition R1, C5H6 C5H3-c + H, a rate expression of k1 = 1.1 × 1015 exp(–38760/T) s−1 was deduced. In a separate series of experiments the consumption by cyclopentadiene of H atoms, which had been generated by the thermal decay of ethyl iodide, was investigated. A preliminary value of k2 = 1.4 × 1014 exp(–2739/T) cm3 mol−1 s−1 was deduced for the total rate of H-atom consumption by cyclopentadiene R2, C5H6 + H products.

Peter Frank

Papers

Shock-tube study on the high-temperature pyrolysis of phenol

High-temperature investigations on the pyrolysis of cyclopentadiene

Reduced Reaction Mechanisms for Methane and Syngas Combustion in Gas Turbines

A shock tube investigation of H atom production from the thermal dissociation of ortho-benzyne radicals

Shock tube study of high-temperature reactions of cyclopentadiene