Experimental and Kinetic Modeling Study of C₂H₂ Oxidation at High Pressure

Abstract

A detailed chemical kinetic model for oxidation of C 2 H 4 in the intermediate temperature range and high pressure has been developed and validated experimentally. New ab initio calculations and RRKM analysis of the important C 2 H 3 + O 2 reaction was was used to obtain rate coefficients over a wide range of conditions (0.003-100 bar, 200-3000 K). The results indicate that at 60 bar vinyl peroxide, rather than CH 2 O and HCO, is the dominant product.The experiments, involving C 2 H 4 /O 2 mixtures diluted in N 2 , were carried out in a high pressure flow reactor at 600-900 K and 60 bar, varying the reaction stoichiometry from very lean to fuel-rich conditions. Model predictions are generally satisfactory. The governing reaction mechanisms are outlined based on calculations with the kinetic model. Under the investigated conditions the oxidation pathways for C 2 H 4 are more complex than those prevailing at higher temperatures and lower pressures. The major differences are the importance of the hydroxyethyl (CH 2 CH 2 OH) and 2-hydroperoxyethyl 1 (CH 2 CH 2 OOH) radicals, formed from addition of OH and HO 2 to C 2 H 4 , and vinyl peroxide, formed from C 2 H 3 + O 2 . Hydroxyethyl is oxidized through the peroxide HOCH 2 CH 2 OO (lean conditions) or through ethenol (low O 2 concentration), while 2-hydroperoxyethyl is converted through oxirane. [2][3][4][5][6][7], shock tubes [8][9][10][11][12] and premixed laminar flames [13][14][15][16][17], covering a wide range of stoichiometries and temperatures. Most of the reported work, however, have been carried out at near atmospheric pressure. A few results are available from flow reactor studies at 5-10 bar [6], but despite their relevance for the chemistry in engines, gas turbines, and gas-to-liquid processes, data at high pressures are limited.The objective of the present study is to obtain experimental results for the oxidation of C 2 H 4 at high pressure (60 bar) as functions of temperature (600-900 K) and stoichiometry (lean to fuel-rich) and analyze them in terms of a detailed chemical kinetic model. The oxidation pathways for C 2 H 4 under these conditions are different from those prevailing at higher temperatures and lower pressures and the results of the current work help to extend the validation range for chemical kinetic modeling of C 2 H 4 oxidation. This paper is part of a series investigating the high-pressure, medium temperature oxidation of simple fuels: previously work has been reported for CO/H 2 , CH 4 , and CH 4 /C 2 H 6 mixtures [18,19]. The present kinetic model draws on this work, as well as recent results in tropospheric chemistry. Furthermore, the important reaction of C 2 H 3 with O 2 was characterized from ab initio calculations over a wide range of pressure and temperature. 3 ExperimentalThe experimental setup is a laboratory-scale high pressure laminar flow reactor designed to approximate plug-flow. The setup is described in detail elsewhere [18] and only a brief description is provided here. The system enables well-defined investigations of...

The C2H2 + NO2 reaction: Implications for high pressure oxidation of C2H2/NOx mixtures

“…The values obtained this way are in good agreement with those reported by Gimenez-Lopez et al [32] calculated from kinetic modeling of the inlet system under similar conditions. Figure 3 , which compares data obtained in the present work with those of Gimenez-Lopez et al [9] obtained under similar conditions, shows the effect of adding NO x on acetylene oxidation at different stoichiometries. In the absence of nitrogen oxides, the temperature for onset of reaction is about 725 K, independent of stoichiometry.…”

“…In the absence of nitrogen oxides, the temperature for onset of reaction is about 725 K, independent of stoichiometry. Above this temperature, acetylene is rapidly oxidized, as predicted by the model [9] . Addition of NO x shifts the onset temperature to values below 600 K. However, upon onset of reaction the oxidation rate in the presence of nitrogen oxides is lower than in the absence of NO x ; this is most pronounced for reducing conditions.…”

“…The reaction mechanism and corresponding thermodynamic properties were mostly drawn from recent work on high-pressure acetylene oxidation [9] and on nitrogen chemistry [33] . Reactions added in the present work for the C 2 H 2 /NO x subset are listed in Table 2 ; the full mechanism is available as supplementary material.…”

“…Con-sequently, we have estimated the rate constant for CHCHO + NO (R6) to be roughly similar to CH 2 + NO. A subset for the 3 CHCHO radical was included in the mechanism with estimated rate constants [9] .…”

“…Published by Elsevier Inc. All rights reserved. [2][3][4][5] , and few experiments have been conducted at increased pressure [6][7][8][9] . In particular, there is little knowledge about the interaction of acetylene with nitrogen oxides.…”