In situ visualization of polycyclic aromatic hydrocarbon hydrothermal decomposition process utilizing planar induced fluorescence: A mechanistic analysis

Hydrothermal decomposition of coal or biomass for hydrogen production is a clean and efficient technology. Comprehending the hydrothermal decomposition mechanism of Polycyclic Aromatic Hydrocarbons (PAHs) is pivotal for advancing biomass or coal hydrothermal reaction technology. Traditional non-in situ measurement methods present challenges in thoroughly exploring the intricacies of PAHs’ hydrothermal reaction mechanisms. To address this issue, this study establishes an in situ visualization platform leveraging planar laser-induced fluorescence (PLIF) technology. Fluorescence signals emanating from naphthalene are meticulously measured in situ across diverse temperature conditions (450 °C-650 °C). The decomposition rate characteristics of PAH in hydrothermal process are thereby elucidated. A proportional increase in the reaction rate of naphthalene with elevated temperatures, concomitant with an augmented risk of coking. Moreover, the generation of hydroxyl radicals (OH) during the hydrothermal decomposition process was observed in the in situ visualization platform. Afterwards, to promote the generation of OH radicals and improve the conversion of PAHs, small molecule additives (formic acid, acetic acid, methanol, and ethanol) were added into the reactions. The experimental results indicate that the addition of small molecules increases the conversion rate of PAH and significantly reduces coking. These radicals effectively occupy reactive sites on naphthalene, thereby suppressing the coking proclivity of PAHs. This work provides a novel method to investigate the mechanism of hydrothermal process and offers a potential way to improve the conversion efficiency.