Mechanism Generation with Integrated Pressure Dependence:  A New Model for Methane Pyrolysis

Abstract

Autocatalytic, lower-temperature (≤1100 K) methane pyrolysis has defied mechanistic explanation for almost three decades. The most recent attempt (by Dean in 1990) invoked the chemically activated addition of an allyl radical to acetylene, leading to a cyclopentadiene/cyclopentadienyl chain-branching system that prompted the observed autocatalysis. However, newer, more accurate thermochemical data for the cyclopentadienyl radical render that explanation untenable. A new model for methane pyrolysis is constructed here, using a novel mechanism generation approach that automatically computes any needed rate constants k(T,P) for chemically or thermally activated pressure-dependent reactions. The computer-generated mechanism accurately predicts the observed autocatalysis and concentration profiles without any adjustable parameters. Radical-forming reverse disproportionation reactionswhich involve propyne, allene, and fulveneaccount for at least half of the experimentally observed autocatalytic effect. Many of these reverse disproportionations were neglected in previous studies. The cyclopentadienyl radical is also important, but it is formed primarily by the chemically activated reaction of propargyl with acetylene. New rate estimates for unimolecular ring-closure reactions of unsaturated radicals are also presented. This approach is the first to incorporate pressure-dependent reactions generally and systematically during computerized mechanism construction. It successfully identifies complex but critical chemical-reaction pathways and autocatalytic loops missed by experienced kineticists.

Modeling and simulation of materials synthesis: Chemical vapor deposition and infiltration of pyrolytic carbon

“…Furthermore, in the present work methane is used as carbon precursor and the mechanism of methane pyrolysis is considerably less understood in the temperature regime of CVD/CVI than those of C 2 -pyrolysis [11][12][13][14][15][16], which were mainly applied in the validation of the above described mechanisms of elementary reactions [8,9]. Thus lumped gas-phase and surface reaction mechanism were developed to calculate carbon deposition from those gas-phase species that dominate CVI of pyrolytic carbon from methane.…”

Hydrocarbon Bond Dissociation Enthalpies: From Substituted Aromatics to Large Polyaromatics

“…[11][12][13][14][15][16] They can arise from incomplete combustion of organic matter and as side-products in thermal cracking units used in the petroleum industry for the production of light olefines such as ethylene and propylene. For each of these processes the homolytic bond dissociation of the organic structures of coal into smaller molecules is an important reaction step and an understanding of the thermochemistry and reactivity of specific bonds may lead to advances in coal processing.…”

Thermochemical properties from G3MP2B3 calculations, Set‐2: Free radicals with special consideration of CH2CHC•CH2, cyclo−•C5H5, •CH2OOH, HO•CO, and HC(O)O•

“…• ), helped to identify an important new class of reactions hitherto not widely used in combustion modelling [2]. The standard heat of formation of c-C 5 H 5…”