Formation of Polycyclic Aromatic Hydrocarbons (PAHs) in Thermal Systems: A Comprehensive Mechanistic Review

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are formed invariably through oxidative and pyrolytic degradation of organic materials and fuels. Understanding the highly complex reaction mechanisms that dictate their synthesis in thermal systems has been given a great deal of focus. Such interest stems from two broad perspectives, namely, enhancing the efficiency of the combustion system, and energy recovery from fuels and protecting the environment. Health and environmental effects widely vary among PAHs where certain compounds exhibit carcinogenic tendencies. This critical review mainly aims to provide a general mechanistic view of the commonly discussed formation pathways of PAHs. The attained mechanistic knowledge often incorporates experimental measurements and kinetic modelings, as well as pathways mapped out by quantum chemical calculations. A chemical sampling of species is typically conducted via the molecular beam (MB)− mass spectroscopy (MS) technique connected to the reactor (flow reactor, jet-stirred reactor, or a shock tube). Generally, PAH precursors mainly encompass four categories of species (radicals and molecules): acetylenic compounds, alkyl radicals, phenyl radicals, and resonance-stabilized cyclic radicals. Overall, the relevance of a mechanism depends on the consistency between the proposed kinetic model that incorporates mechanism pathways, and the experimental profiles of products at the investigated conditions (i.e., temperatures, pressures, and distance from the burners). The effect of seeding common PAH precursors, with other precursors, has been explored by surveying pertinent experimental studies. Growth of higher PAHs, including the toxic pyrene, most likely involves a hydrogen abstraction acetylene addition (HACA)-like mechanism starting from anthracene and bimolecular reactions that involve benzyl radicals and indene molecules. A synergistic collaboration between different mechanisms is often suggested to account for the observed fast growth rate of PAHs. As experimentally shown, sequential mass growth by 15 u, 24 u, 26 u, and 74 u, indicate formation routes by the MAC, HACA, HAVA*, and PAC routes, respectively. Whether a physical or a chemical process is the initial step in the conversion of PAHs into soot (the inception step) is still debated in the literature. Recent experimental evidence underscores that the soot inception is likely to commence by creating PAH dimers through physical clustering under real flame conditions (i.e., 400− 1200 K). Formation of PAHs from degradation of synthetic and natural polymers ensues from condensation of their fragments as well as from structural arrangements of the polymeric structural entities, prior to their fragmentation. The surveyed and presented knowledge in this review will be useful for readers who aim to comprehend the highly complex chemistry underlying the synthesis of PAHs in thermal systems.