Comparative Chemical Kinetic Analysis and Skeletal Mechanism Generation for Syngas Combustion with NO_x Chemistry

Abstract

Emission of nitrogen oxides (NOx) are one of the major environmental concerns arising from the combustion of syngas. Strategies to reduce emission and improve the efficiency of syngas combustion can be developed using computational fluid dynamic simulations to design cleaner and more efficient combustors. Toward this end, an accurate and efficient chemical kinetic mechanism that can describe the combustion chemistry of syngas with NOx under engine-relevant conditions is critical. In this work, a comprehensive survey of detailed mechanisms available in the literature for syngas/NOx combustion reaction system is first conducted. A systematic and comparative chemical kinetic analysis of five detailed mechanisms is performed based on reaction pathway and sensitivity analyses to identify the key reactions of the nitrogen species for a wide range of mixtures including the formation of NOx during syngas combustion, ignition of NH3, H2/N2O, and H2/NO2 mixtures.Comparisons of the reaction pathways from different detailed mechanisms indicate that the detailed 2 chemistry is controlled by a small set of reactions and species. Recent high-level theoretical studies on HONO and HNO2 chemistry including previously neglected important reactions are updated. The rate constants for HNO + O2 = NO + HȮ2 are calculated using ab initio calculations in this work. An efficient high-fidelity skeletal mechanism consisting of 27 species and 130 reactions is developed based on a combination of the directed relation graph with error propagation (DRGEP) method and the simplified iterative screening and structure analysis (ISSA) method. Compared to the detailed mechanisms, the skeletal mechanism retains the major species and reactions for the syngas/NOx system and is validated against typical experimental data resulting in a very good performance.

Reaction of the N Atom with Electronically Excited O2 Revisited: A Theoretical Study

“…Note that these composite techniques were chosen for comparative analysis because they had previously been proved to be effective in determining energy barriers on the excited PESs. 94,108,109,153,154 One can see that the use of modern higher-level methods in general leads to some decrease in the entrance energy barriers on the 2 A′ and 4 A′ PESs compared with early data by Walch and Jaffe. 17 with CASPT2 calculations 37 giving the lowest E a + (a) value [aside from the zero value obtained by Starik et al 44 during the plain CASSCF calculations with apparently insufficient (9,7) active space].…”

“…The calculations exhibited that along pathway (d) the quartet 4 A″ PES, correlating with the N + O 2 (a 1 Δ g ) system, crosses the sextet 6 A′ one, correlating with N + O 2 (X 3 Σ g − ) reactants, very close to the 4 TS 1 structure: let us denote this intersystem crossing point as IC 1 . This means that the N + O 2 (a 1 Δ g ) system just before passing through the 4 TS 1 critical point has a probability of leaving path (d), leading to NO(X 2 Π) + O( 3 P) bimolecular products, and of descending toward the ground-state reactants.…”

“…Nevertheless, since the reverse reaction has no substantial energy barrier (there is a fairly deep intermediate complex 6 IM 0 on the product side of the 6 TS 0 transition state), the reaction channel backward to (c) may be of interest for kinetics of electronically excited NO states, which is important for describing the nonequilibrium radiation from heated air. 155 As for the N + O 2 (a 1 Δ g ) reacting system, by virtue of the twofold orbital degeneracy of molecular oxygen in the singlet delta state, 76,156 the interaction of N with O 2 (a 1 Δ g ) can occur along two different PESs, resulting in the formation of the ground-state NO(X 2 Π) + O( 3 P) bimolecular products via the transition states 4 TS 1 [pathway (d)] and 4 TS 2 [pathway (e)] of different energies. The obtained values of energy barriers for pathways (d) and (e) are 0.619 and 1.964 eV, respectively.…”

“…As for the second term of the N + O 2 (a 1 Δ g ) system, Figure 2 shows that along pathway (e) the quartet 4 A′ PES crosses three terms before passing through the 4 TS 2 structure: two doublet terms of the high-lying N( 2 D) + O 2 (X 3 Σ g − ) system and the sextet term of N( 2 S) + O 2 (X 3 Σ g − ). In principle, the presence of these PES intersections can affect the kinetics of reaction by lowering the activation energy; however, due to the very high energy barrier for this pathway, at least, with respect to pathway (d) (the heights of these barriers differ more than three times; see Table 6), we will not dwell on these intersystem crossings in detail.…”

“…The reaction of the ground-state atomic nitrogen ( 4 S) with normal oxygen molecules is one of two principal elementary channels involved in the Zeldovich mechanism for high-temperature conversion of air N 2 to nitrogen oxides (NO x ), − while this mechanism, also known as the “thermal NO” one, underlies the complex chemical kinetic models describing the NO x pollutant emission from combustion or high-temperature industrial processes − and the nitrogen oxidation in re-entry flows. − The kinetics of the R1 process has been extensively studied for a long time, both experimentally (see for the low-, − elevated-, , and high-temperature measurements , ) and theoretically. ,− In addition, there are some available experimental − and theoretical data ,,− ,,,,,, on the nascent distribution of vibrationally excited NO­( v ) formed in the R1 reaction.…”