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 a romatic species in premixed flames and shock tubes in;I wide range of opera ting conditions. Key sequences of reactions in the formation of aromatics a re the addition of acetylene to smaller aromatics, activated by H-atom abst raction and the combination of resonantly stabilized radicals, including cy clopentadienyl radical combination, propargyl addition to benzyl radicals a nd the sequential addition of propargyl radicals to aromatic rings. The mod eling results are compared with those obtained by considering only the H-ab straction acetylene addition (HACA) route for aromatic formation to identif y the controlling steps in different combustion regimes and to assess the v iability of the HACA and resonantly stabilized radical (RSR) pathways to mo del aromatic growth. The full model is able to predict the concentration an d formation rate of total organic carbon collected in slightly sooting rich flames at different temperatures and pressures. On the contrary,he HACA mo del predicts concentrations and formation rates more than one order of magn itude lower than those measured in experiments. This result indicates that a combination of resonantly stabilized radicals are the controlling steps i n the formation of aromatics in flames. The oxidative environment is pivota l in the formation of aromatics to activate the pathways involving the reso nantly stabilized radicals in the aromatic growth process. Since the model simulates only the formation of two- and three-ring aromatics, it can be hy pothesized that total organic material collected in flames, i.e. a mass qua ntity much larger than the chromatographable polycyclic aromatic hydrocarbo ns, is the result of a fast reactive coagulation of small aromatics, formin g structures of high molecular mass. Results indicate that reactions leadin g to the formation of two- and three-ring aromatics are rate-limiting, and combination reactions involving these aromatics control soot formation in o xidative or slightly sooting regimes. In very fuel-rich and pure pyrolysis conditions, the two models predict the same amount and formation rate of ar omatics; this indicates that the HACA route controls formation in these con ditions since the RSR pathways are activated only in oxidative environments . (C) 1999 Elsevier Science Ltd. All rights reserved.