Modeling of particulate formation in combustion and pyrolysis

We use a detailed chemical kinetic mechanism to explore the effects of C/O ratio, temperature and pressure on the formation of high molecular weight aromatic species in premixed flames and shock tubes in;I wide range of operating conditions. Key sequences of reactions in the formation of aromatics are the addition of acetylene to smaller aromatics, activated by H-atom abstraction and the combination of resonantly stabilized radicals, including cyclopentadienyl radical combination, propargyl addition to benzyl radicals and the sequential addition of propargyl radicals to aromatic rings. The modeling results are compared with those obtained by considering only the H-abstraction acetylene addition (HACA) route for aromatic formation to identify the controlling steps in different combustion regimes and to assess the viability of the HACA and resonantly stabilized radical (RSR) pathways to model aromatic growth. The full model is able to predict the concentration and formation rate of total organic carbon collected in slightly sooting rich flames at different temperatures and pressures. On the contrary,he HACA model predicts concentrations and formation rates more than one order of magnitude lower than those measured in experiments. This result indicates that a combination of resonantly stabilized radicals are the controlling steps in the formation of aromatics in flames. The oxidative environment is pivotal in the formation of aromatics to activate the pathways involving the resonantly stabilized radicals in the aromatic growth process. Since the model simulates only the formation of two- and three-ring aromatics, it can be hypothesized that total organic material collected in flames, i.e. a mass quantity much larger than the chromatographable polycyclic aromatic hydrocarbons, is the result of a fast reactive coagulation of small aromatics, forming structures of high molecular mass. Results indicate that reactions leading to the formation of two- and three-ring aromatics are rate-limiting, and combination reactions involving these aromatics control soot formation in oxidative or slightly sooting regimes. In very fuel-rich and pure pyrolysis conditions, the two models predict the same amount and formation rate of aromatics; this indicates that the HACA route controls formation in these conditions since the RSR pathways are activated only in oxidative environments. (C) 1999 Elsevier Science Ltd. All rights reserved.