E. Therssen

A rapid compression machine investigation of oxidation and auto-ignition of n-heptane: measurements and modeling

n-Heptane oxidation and auto-ignition in a rapid compression machine is studied in the low and intermediate temperature regimes at high pressures. Experimental ignition delay times and some phenomenological aspects related to knock in engines are presented, providing additional information at lower temperatures on previously published delays from shock tube experiments. The products of oxidation are identified and time profiles are measured during a two-stage ignition process. Eight C7 heterocycles, heptenes, lower 1-alkenes, aldehydes, and carbon monoxide are the main species. Their origin is discussed in relation to the isomerization and decomposition of heptylperoxy radicals. The high selectivity observed in the formation of lower 1-alkenes is explained by the scission of the β CC bond of the β-hydroperoxyheptyl radicals weakened by the presence of oxygen atoms. Numerical simulation of the experiments with Warnatzs comprehensive chemical mechanism gives satisfactory results for cool flame and total ignition delays, but fails to reproduce the detailed chemistry before auto-ignition.

Comparison of oxidation and autoignition of the two primary reference fuels by rapid compression

New experimental data on autoignition delays and product distributions during two-stage autoignitions for the two primary reference fuels n -heptane and iso -octane (2,2,4-trimethylpentane) have been obtained by rapid compression in the low and intermediate range of temperature for enginelike conditions of stoichiometry and dilution. The lower reactivity of iso -octane has been compensated by a four times increase in pressure. A good correlation between our data and that published is obtained when the compressed charge density of the core gas is considered. Both fuels show many common features in this temperature range: a marked negative temperature coefficient region that shifts to higher temperatures as the pressure is increased and a similarity in the nature of the intermediate species. However, the importance of the cool flame zone is greater for n -heptane, and the negative temperature coefficient region extends toward higher temperatures. The evolution of the main intermediate products formed during the two-stage autoignition is presented and discussed according to a common generic mechanism that takes into account the various isomerizations of alkylperoxy radicals and scissions of the hydroperoxyalkyl radicals. Cyclic ethers are important intermediates. For both hydrocarbons, tetrahydrofurans are the major O heterocycles formed in cool flames, especially in the case of iso -octane. The observed high selectivity in the lower alkenes demonstrates the importance of β carbon-carbon scission of the hydroperoxyalkyl radicals that leads to terminal alkenes in the case of n -heptane and to methylpropene and substituted pentenes in the case of iso -octane. These channels have to be taken into account in the improvement of detailed mechanisms for good predictions of pollutants.

The chemistry of pre-ignition of n-pentane and 1-pentene

The pre-autoignition chemistry of n-pentane and 1-pentene was studied by rapid compression in the low temperature range (600–900 K). The pressure traces, light emissions, intensities of cool flames, autoignition delays, and hydrocarbon conversions before final ignition indicate that there are similarities of behavior, but a lower reactivity of 1-pentene over the whole temperature range. Chemical analysis of the stable intermediate species after the cool flame, but before final ignition, shows marked differences in selectivities for O-heterocycles and aldehydes. Relatively high amounts of propyloxirane and butanal in the oxidation of 1-pentene suggest additions of oxidizing radicals to the double bond. The classical low temperature peroxidation scheme of alkanes can be applied, not only to n-pentane, but also to 1-pentene, if the higher reactivity of the allylic hydrogens and direct addition of OH and HO2 radicals are taken into account. Some peroxy radicals are common to both fuels and are responsible for their similar features of pre-autoignition chemistry. However, oxidation of 1-pentene is still deeply marked by the presence of an olefinic bond.

Cavity ring-down measurements of OH radical in atmospheric premixed and diffusion flames.: A comparison with laser-induced fluorescence and direct laser absorption

Abstract Cavity ring-down spectroscopy (CRDS) is tested in two atmospheric burners: a premixed flat flame burner and a Wolfhard–Parker burner. The quantitative nature and the spatial resolution of CRDS are compared with those of laser-absorption and laser-induced fluorescence by recording OH concentration profiles. Losses per pass due to the abundant OH sample in the CRD cavity need to be carefully controlled to obtain an exponential ring-down decay. Index refraction gradients can be responsible for important random off-resonance losses which perturb CRDS measurements. In contrast, line-of-sight CRD measurements performed along the axis of the gradients are found to be very accurate.

Devolatilization of coal particles in a flat flame : experimental and modeling study

Abstract Pulverized coals have been tested under the conditions of industrial flames, with high heating rate and high temperatures. Coal particles were injected (5 g/h) into a flat, air-propane flame at 1400°C. The chars were collected after different pyrolysis times. The thermal history of the particles, as monitored by two color pyrometry, shows that particles undergo very large heating rates (6 × 106 K/s) and reach a peak temperature of 1100°C. For eight coals (volatile matter 1–57% d.a.f.), the devolatilized fraction of coal has been measured, as well as those of carbon, hydrogen and nitrogen. In every case, the devolatilized fraction of coal was greater than the predicted A.S.T.M. value, but proportional to it. The devolatilized fraction of hydrogen seems to be a more sensitive parameter than the coals weight loss. During pyrolysis, the evolution of the texture of the grains has been studied by measurement of their microporous surface area, which undergoes a large increase, depending on coal rank. The composition of the volatiles, as deduced from the ultimate and proximate analysis of chars, showed high volatile bituminous coals to essentially produce tars with an aromatic structure. In that case, the surface area passed through a maximum at the beginning of devolatilization. The subsequent decrease could be due to recondensation of heavy molecular weight volatile matter at the chars surface. Low and medium volatile bituminous coals produced light hydrocarbons on devolatilization and the chars surface area continued increasing slowly during the whole of devolatilization, according to the slow increase of the fraction of hydrogen devolatilized. The chars reactivity with oxygen was followed by measurements of Active Surface Area (A.S.A.). It was shown that the A.S.A. continuously decreases during devolatilization. This may be due to the departure of prompt volatiles made of aliphatic groups, which are potential active sites for oxygen. Five models of devolatilization in the literature were tested and compared to the experimental results, assuming first-order reactions with respect to the remaining volatile matter. Badziochs model correctly fitted the experimental results and values of the rate constant obtained by computer trial and error adjustment were higher for lower ranks of the four bituminous coals. Anthonys model also fits the measurements, provided an adjustment of the preexponential factor and activation energy (k0, E0), for which it is shown that an infinite number of such pairs is suitable. If the model is run isothermally at the flames peak temperature, it also correctly fits the experimental results.

Quantitative Features and Sensitivity of Cavity Ring-Down Measurements of Species Concentrations in Flames

Absolute concentrations of minor species can be measured by cavity ring-down spectroscopy (CRDS) by analysing the exponential time decay of the CRDS signal. This paper shows that quantitative concentrations can be measured by CRDS using a moderately narrowband multimode dye laser, even though the ring-down decays exhibit a multi-exponential behavior (nonlinear variation of the losses with the absorbance). A model based on Fabry-Perot theory has been developed to fit the multi-exponential decays by taking into account the convolution of the laser lineshape and of the absorption line. From this model, true absorbances, corrected for nonlinear effects, can be obtained, leading to quantitative measurements of concentrations. Using the model, the dynamic range of CRD measurements is increased by a factor of ten. The sensitivity of the technique is shown to be reduced in the region of the thermal gradient, which induces an important increase of the off-resonance losses/pass. The best fractional absorption/pass we could obtain was estimated to be 10 ppm in the flame front and 5 ppm in the burnt gases of a low-pressure premixed flame of methane and air. The sensitivity is greater when the laser is coupled to the TEM00 mode of the cavity. CRD measurements of (CH) performed in two different spectral ranges in the C-X and B-X bands are compared.

Parametric study of polycyclic aromatic hydrocarbon laser desorption

We present the use of a combined laser desorption/multi-photon ionization/time-of-flight mass spectrometry technique for the analysis of polycyclic aromatic hydrocarbon (PAH) solid samples. A thorough characterization of the first step (laser desorption) of this experimental technique has been performed. By varying the energy of the laser pulse, a specific response of each PAH has been evidenced for pure and mixed PAH sample desorption. This behaviour has also been studied with respect to the fragmentation processes. Similar studies on PAHs adsorbed on graphite evidenced the possibility of desorbing molecules from the adsorbed phase only, i.e. without a contribution from the graphite substrate. These findings represent important preliminary steps towards the final goal of setting up a completely characterized analytical method for the investigation of the adsorbed phase of soot particles generated in combustion processes.

CCl, CH, and NO LIF measurements in methane-air flames seeded with chlorinated species: Influence of CH3Cl and CH2Cl2 on CCl and NO formation

In this work, the laser-induced fluorescence (LIF) technique is used to detect minor species (CCl, NO, and CH) in premixed stoichiometric methane-air flames seeded with monochloromethane or dichloromethane. Quenching data are extracted from time-resolved fluorescence lifetime measurements for all the excited species. First quenching measurements of CCl under flame conditions are reported. It is shown that LIF measurements are strongly perturbed by the presence of background emissions issued from the radiative relaxation of photolytic fragments (HCl*, CCl*, CH*, and C2*) formed upon laser excitation. The parent molecules that are partly responsible for these emissions are C2H3Cl (for HCl*, CH*) and CHCl2 (for CCl*). Profiles of both photolytic fragments and species directly measured by LIF are used to study the influence of CH3Cl and CH2Cl2 addition on CCl and NO formation in methane-air flames. CCl radical is found to be formed in the reaction zone of the flames. The reaction path leading to CCl appears to be dependent on the nature of the chlorinated hydrocarbon (CHC) seeded in the flame. The suggested reaction paths may preferentially involve the contribution of CHCl2 in case of CH2Cl2 degradation and CH2Cl in case of CH3Cl degradation. An important increase of NO in presence of CHC is pointed out for the first time. The NO formation in flames containing CHC appears to occur in the reaction zone of the flames, and [NO] is found to be constant in the burned gases: This suggests a predominance of the prompt-NO mechanism in this kind of flame as confirmed experimentally by the observed [CH] increase. Reaction paths involving the degradation of CHCs, particularly CHCl2, should largely contribute to the formation of CH in flames seeded with CHCs.

Etude expérimentale de la dévolatilisation rapide de charbons pulvérisés

Abstract Experimental study of the rapid devolatilization of pulverized coals. Rapid devolatilization of various pulverized coals have been studied in a laboratory bench constituted by a flat flame burner of propane which reproduces thermal conditions of an industrial flame. The particles, which undergo a heating rate of 6.10 6 K −1 .s −1 with a peak temperature of 1 100 °C, are completely devolatilized within 24 ms. Fifteen coals, included in a wide range (anthracite to subbituminous coal) have been tested. The coal weight loss is globally proportional to the normalized volatile matter content with some exceptions which confirm the advantage of this laboratory bench. The formation of tars or hydrocarbons has been related to the coal weight loss. Carbon, hydrogen and nitrogen devolatilized fraction have been followed with total mass weight loss. In spite of the fact that hydrogen and carbon devolatilized fractions present a good correlation with the total mass weight loss, the nitrogen devolatilized fraction have an anarchic evolution. For high heating rates, the devolatilization of tars observed for the bituminous coals seems to explain this unpredictable phenomenon. These results will be valorized in comparison with those obtained in industrial flame conditions.