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Study on the gasification mechanism of acrylonitrile-butadiene-styrene in supercritical water using experimental and ReaxFF simulation: Products, pathways, and nitrogen migration
Summary
Scientists found a way to break down ABS plastic (a common material in electronics, toys, and car parts) using superheated, high-pressure water instead of burning it, turning the waste into clean hydrogen gas fuel. This matters because it offers a way to recycle hard-to-break-down plastics into useful energy while controlling harmful nitrogen byproducts, potentially reducing plastic waste that would otherwise pollute the environment and break into microplastics.
Supercritical water gasification (SCWG) enables the clean resource utilization of acrylonitrile-butadiene-styrene (ABS) plastic waste. The elucidation of the reaction mechanism of ABS is a prerequisite for obtaining the target gas and controlling N pollution. Combining experiments and ReaxFF molecular dynamics simulations, this study investigated the effects of reaction temperature, time, feedstock concentration and pressure on the SCWG of ABS. Results showed that at 700 °C, 23 MPa, 10 min and 6% concentration, the carbon gasification efficiency (CE) and hydrogen gasification efficiency (HE) reached 59.78% and 142.96%, respectively, with H2 yield increasing from 1.38 mol/kg at 500 °C to 15.87 mol/kg. Simulations revealed that SCW acted as both hydrogen donor and reaction medium, accelerating the cleavage of ABS molecular bonds by overcoming an energy barrier of 165 kJ/mol. Benzene rings underwent ring-opening via hydroxylation, hydrogenation and pyrolysis-induced pathways. H2 was derived from chain reactions of hydrogen radicals released by ABS cracking and dissociated from supercritical water. Nitrogen was converted to ammonia via hydrogenation and hydrolysis of nitrogen-containing compounds, or to nitrogen via direct denitrification to form hydrogen cyanide and cyano groups followed by subsequent transformation. This study may contribute to understanding the transformation mechanism of ABS in SCW from a multi-scale perspective, thereby facilitating the development of regulatory strategies.