Synergistic cracking-oxidation of polycyclic aromatic hydrocarbons: Mechanistic insights, application in environmental matrices, and energy efficiency

Polycyclic aromatic hydrocarbons (PAHs) are toxic environmental contaminants presented in most of environmental matrices. The mullite and Fe 2 O 3 were identified as the reactive species of coal fly ash for the cracking-oxidation of PAHs (3∼5 benzene rings, e.g., anthracene, pyrene, benzo(a)pyrene). The conversion of PAHs reached over 99 % at 250–300 ℃ with the non-methane total hydrocarbons (NMHC) in the off-gas being below 50 mg/m 3 . The in-situ DRIFTS and TGA/GC–MS revealed the synergy of Brønsted acid sites from mullite and oxidation active sites from Fe 2 O 3 for the pyrene decomposition. The reaction process was initiated by self-condensation of a minor fraction of pyrene that provide hydrogen radicals to convert remaining pyrene to dihydropyrene. Then dihydropyrene underwent a series of Brønsted acid-catalyzed β-scission and alkyl transfer reactions to form light hydrocarbons that were then converted to CO 2 and H 2 O over Fe 2 O 3 species. The cracking-oxidation of PAHs were furtherly verified in terrestrial/aqueous systems which is known as the most common environment matrices of PAHs. A demonstrative equipment comprised of a riser-downer reactor was used for such cracking-oxidation process to ensure particle fluidization, mass transfer and circulation efficiency. PAHs can be almost completely removed from the environment matrices with the NMHC concentration of 46.3–49.6 mg/m 3 in the off-gas and COD of 0.6–4.1 mg/L in the condensate water. The energy released from the exothermic cracking-oxidation reaction was re-utilized to heat the feed air, so the energy consumption and related operation cost outperformed those of traditional methods.