The kinetics of the reaction CO + HO2* --> CO2 + *OH was studied using a combination of ab initio electronic structure theory, transition state theory, and master equation modeling. The potential energy surface was examined with the CCSD(T) and CASPT2 methods. The classical energy barriers were found to be about 18 and 19 kcal/mol for CO + HO2* addition following the trans and cis paths, respectively. For the cis path, rate constant calculations were carried out with canonical transition state theory. For the trans path, master equation modeling was also employed to examine the pressure dependence. Special attention was paid to the hindered internal rotations of the HOOC*O adduct and transition states. The theoretical analysis shows that the overall rate coefficient is independent of pressure up to 500 atm for temperature ranging from 300 to 2500 K. On the basis of this analysis, we recommend the following rate expression for reaction R1 k(cm(3)/mol x s) = 1.57 x 10(5) T(2.18)e(-9030/T) for 300 < or = T < or = 2500 K with the uncertainty factor equal to 8, 2, and 1.7 at temperatures of 300, 1000, and 2000 K, respectively.

No abstract

The influence of the route via the NCN radical on NO formation in flames was examined from a thermochemistry and reaction kinetics perspective. A detailed analysis of available experimental and theoretical thermochemical data combined with an Active Thermochemical Tables analysis suggests a heat of formation of 457.8 ± 2.0 kJ/mol for NCN, consistent with carefully executed theoretical work of Harding et al. (2008) [5]. This value is significantly different from other previously reported experimental and theoretical values. A combination of an extensively validated comprehensive hydrocarbon oxidation model extended by the GDFkin3.0_NCN-NO x sub-mechanism reproduced NCN and NO mole fraction profiles in a recently characterized fuel-rich methane flame only when heat of formation values in the range of 445-453 kJ/mol are applied. Sensitivity analysis revealed that the sensitivities of contributing steps to NO and NCN formation are strongly dependent on the absolute value of the heat of formation of NCN being used. In all flames under study the applied NCN thermochemistry highly influences simulated NO and NCN mole fractions. The results of this work illustrate the thermochemistry constraints in the context of NCN chemistry which have to be taken into account for improving model predictions of NO concentrations in flames.

The primary products of the bimolecular reactions of ortho-benzyne, o-C6H4 (1,2-dehydrobenzene), with ethene, propene, and acetylene have been detected by molecular beam mass spectrometry at a combustion relevant temperature of T = 1475 K. o-Benzyne was produced by flash pyrolysis of phthalic anhydride in the absence and presence of the respective reactant. Potential reaction pathways of the addition reactions were investigated by quantum chemical calculations. Channels with biradical intermediates were found to be energetically more favorable than alternative quasi-concerted [2+1] cycloaddition and concerted H-transfer pathways. Bicyclic benzocyclobutene and benzocyclobutadiene were identified as the main products of the reactions with C2H4 and C2H2, respectively. At combustion temperatures, however, these cyclic products are likely to undergo sequential ring opening. In the case of propene, the presence of an allylic H atom initiates a favorable ene-type reaction sequence yielding the open-chain product allylbenzene. Overall, hydrocarbon reactivity was found to increase in the order C2H2, C2H4 to 3H8. The range of the estimated bimolecular rate constants is comparable to the rate constants of the corresponding phenyl radical reactions and hence point out a potentially important role of o-C6H4 reactions in flame and soot formation chemistry.

We present the adaptation of the sectional model approach to the pyrolysis of cellulose, (C 6 H 10 O 5) n. Cellulose is the major component of lignocellulosic biomass. Due to its longitudinal structure, cellulose is characterized by one-dimensional chains composed of a varying number of cellobiose molecules, C 12 H 22 O 11. Fragments of those chains with similar mass are grouped into size classes (BINs) determined by characteristic numbers of cellobiose monomers. During the pyrolysis of cellulose, reaction temperatures of more than approximately 500 K (depending on the heating rate) initiate bond dissociation within the chains, i.e. between cellobiose units, resulting in smaller chain fragments. We have developed a new reaction scheme for the pyrolysis of cellulose based on existing models from literature. However, we present the sectional approach as new concept for modelling the degradation (depolymerisation) step. Our model comprises the degradation of solid cellulose, its devolatilisation to either glucose or to tars (e.g. levoglucosan) including primary gaseous products (e.g. CO and CO 2), the formation of char and water from various pathways, as well as secondary gas-phase reactions. For the cellulose degradation, we propose a kinetic data set of A = 2.2•10 13 s-1 and E A = 225.9 kJ/mol for the dissociation of a single glycosidic bond. The model was tested via reactor simulations. Cellulose is pyrolysed at a constant heating rate (1, 10, 15, and 150 K/min) from 323 K up to a preset final temperature of 1073 K. Our simulations show good agreement with two different experimental data sets from literature for all heating rates.