Chemical kinetic modelling of the effect of NO on the oxidation of CH4 under fluidized bed combustor conditions

Loeffler, G.; Wargadalam, Verina J.; Winter, Franz; Hofbauer, Hermann

doi:10.1016/s0016-2361(02)00002-9

Cited by 18 publications

(18 citation statements)

References 15 publications

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“…The incorporation of NO leads to additional C 3 H 8 reactions that are up to two or more orders of magnitude higher than the number of NO molecules added, depending on residence times and alkane pressures used, which suggests that NO acts as a catalyst instead of getting stoichiometrically consumed in the reaction. This catalytic role of NO x in hydrocarbon conversion has been proposed and probed mechanistically in many previous studies. − Such large enhancements lead to homogeneous reactions rates comparable to those on solid catalysts, as shown in the Supporting Information via comparisons of productivity of NO x -mediated reactions with several previously reported catalytic reactions (Table S2), which may make it practical to run these reactions at large scales. The C 3 H 8 conversion increases with increasing NO pressure and is greater for the larger reactor, suggesting that reactions proceed in the gas-phase via homogeneous pathways enabled by NO or its oxidation products.…”

Section: C3h8 Activation Rates and Selectivities In C3h8–o2–no Reactionssupporting

confidence: 53%

Selective C–H Bond Activation via NO_x-Mediated Generation of Strong H-Abstractors

2019

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Mechanistic details and product distributions for C3H8–O2 reactions catalyzed by gas-phase NO x species are compared to reactions on solid V2O5 catalysts. C3H8 conversions are greatly enhanced by addition of small concentrations of NO to C3H8–O2 mixtures without solid catalysts because homogeneous catalytic redox cycles involving oxidation of NO to NO2, reduction by H-addition to form HONO and release of OH radicals facilitate abstraction of H-atoms from strong C–H bonds in C3H8. NO x -mediated conversions exhibit C3H6 selectivity values among the highest reported at similar oxidative conditions because OH radicals are strong abstractors that activate C–H bonds via early transition states that do not exhibit significant bond elongation, which dampens bond-strength sensitivity and the preference to activate weak allylic C–H bonds in C3H6 products over strong bonds in C3H8. C3H8 conversion rates increase with residence time and NO, O2, and H2O pressures and exhibit supra-linear dependence on C3H8 pressure with trends analogous to homogeneous systems involving H-abstraction by OH radicals but significantly different from V2O5 catalysts that exhibit linear C3H8 pressure dependence and weak sensitivity to the other parameters. Temperature dependencies of rate-constant-ratios representing activation energy differences between primary and secondary C3H8 and allylic C3H6 C–H bonds in NO x -catalyzed routes are much weaker than V2O5, consistent with dampened bond-strength sensitivity that is also observed in density functional theory estimates of these differences for OH radicals. NO x mediation enables efficient alkane activation providing high productivity and yields at moderate temperatures, which is important for chemical transformations requiring rate-limiting C–H activation.

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Section: C3h8 Activation Rates and Selectivities In C3h8–o2–no Reactionssupporting

confidence: 53%

Selective C–H Bond Activation via NO_x-Mediated Generation of Strong H-Abstractors

2019

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“…Addition of NO and NO 2 has been found to reduce the temperature for the onset of methane oxidation in a number of experimental and/or chemical kinetics modelling studies [e.g. 6, [15][16][17][18][19][20][21][22]. The present study extends these findings by showing that the addition of small amounts of ammonia is also capable of enhancing the oxidation of methane, leading to a lowering of the auto-ignition temperature.…”

Section: Discussionsupporting

confidence: 76%

A numerical study of the influence of ammonia addition on the auto-ignition limits of methane/air mixtures

Schoor¹,

Norman²,

Vandebroek³

et al. 2009

Journal of Hazardous Materials

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“…H-addition to CH 3 O from H 2 or CH 4 forms more than 90% of all CH 3 OH molecules. The sequential nature of the reactions in eq 1 has been proposed for CH 4 -O 2 and CH 4 -O 2 -NO x reactants, 27,29,36,39 but detailed contributions and the relative kinetic relevance of the various elementary steps involved remain unclear. Figures 4 and 5 show rate-of-formation data for CH 4 and HCHO, respectively, including steps that form or convert >2% of all CH 4 and HCHO.…”

Section: Pathways For Hcho Synthesis and Consumption With And Without Nomentioning

confidence: 99%

NO_x-Mediated Homogeneous Pathways for the Synthesis of Formaldehyde from CH₄−O₂ Mixtures

Zalc

Green

Iglesia

2006

Ind. Eng. Chem. Res.

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A detailed kinetic network for homogeneous CH 4 -O 2 -NO x reactions is used to estimate maximum attainable formaldehyde (and methanol) yields and to identify elementary steps that lead to the observed enhancement effects of NO x on CH 4 oxidation rates, to HCHO yield limits, and to NO x losses to unreactive N-compounds. NO x was shown previously to increase CH 4 oxidation rates and HCHO yields in CH 4 -O 2 reactions, but maximum yields were low (<10%) and intrinsic kinetic limits were not rigorously examined. We show here that the CH 4 oxidation rate increases because NO 2 reacts with CH 4 during an initial induction period. NO and NO 2 lead to similar effects, except that residence times required for a given yield are higher for NO feeds because NO-NO 2 interconversion must first occur. CH 4 leads to supra-equilibrium NO 2 concentrations because HO 2 formed during HCHO oxidation reacts with NO to form OH and NO 2 faster than NO 2 can decompose to NO. Oxygenate selectivities decrease with increasing CH 4 conversion, because weaker C-H bonds in HCHO and CH 3 OH relative to CH 4 lead to their fast sequential oxidation to CO and CO 2 . Rate-of-formation analyses show that NO x molecules introduce more effective elementary steps for the formation of CH 3 O intermediates and for its conversion to HCHO, but H-abstraction from CH 4 and HCHO remains the predominant step in controlling rates and selectivities in the presence or absence of NO x . Without NO x , OH radicals account for all H-abstraction reactions from CH 4 , while HCHO reacts with OH but also with less reactive H and HO 2 radicals. NO x increases HCHO yields by converting these less reactive H and HO 2 radicals to OH radicals, which become the predominant H-abstractor for both CH 4 and HCHO and which react less selectively with HCHO than do H and HO 2 . Kinetic selectivity, based on C-H bond energy differences between CH 4 and HCHO, becomes weaker with increasing radical reactivity and increasing reaction temperature. Maximum HCHO yields of 37% are theoretically possible for radicals that abstract H from CH 4 and HCHO at equal rates, but radical species prevalent during CH 4 -O 2 -NO x reactions lead to maximum HCHO yields below 10% under all conditions. Higher yields appear unlikely with more reactive radicals, because their reactivity would lead to cascade reactions that form species with greater kinetic sensitivity to C-H bond energies. Maximum C 1 -oxygenate yields increase with increasing O 2 pressure, suggesting that the O 2 distribution along a reactor will not improve HCHO yields but may prove useful to inhibit NO x losses to less reactive N-compounds.

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Chemical kinetic modelling of the effect of NO on the oxidation of CH4 under fluidized bed combustor conditions

Cited by 18 publications

References 15 publications

Selective C–H Bond Activation via NO_x-Mediated Generation of Strong H-Abstractors

Selective C–H Bond Activation via NO_x-Mediated Generation of Strong H-Abstractors

A numerical study of the influence of ammonia addition on the auto-ignition limits of methane/air mixtures

NO_x-Mediated Homogeneous Pathways for the Synthesis of Formaldehyde from CH₄−O₂ Mixtures

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Chemical kinetic modelling of the effect of NO on the oxidation of CH4 under fluidized bed combustor conditions

Cited by 18 publications

References 15 publications

Selective C–H Bond Activation via NOx-Mediated Generation of Strong H-Abstractors

Selective C–H Bond Activation via NOx-Mediated Generation of Strong H-Abstractors

A numerical study of the influence of ammonia addition on the auto-ignition limits of methane/air mixtures

NOx-Mediated Homogeneous Pathways for the Synthesis of Formaldehyde from CH4−O2 Mixtures

Contact Info

Product

Resources

About

Selective C–H Bond Activation via NO_x-Mediated Generation of Strong H-Abstractors

Selective C–H Bond Activation via NO_x-Mediated Generation of Strong H-Abstractors

NO_x-Mediated Homogeneous Pathways for the Synthesis of Formaldehyde from CH₄−O₂ Mixtures