Selective production of aromatics from waste plastic pyrolysis by using sewage sludge derived char catalyst

Abstract

Sewage sludge char was used as the catalyst to realize the selective production of aromatics in the pyrolysis of waste mixed plastics. The effect of catalytic temperature, residence time, and plastic compositions on the product yield and distribution were investigated. The catalytic behavior of main ash components was evaluated and discussed. It was found that the selectivity of sludge char to monocyclic aromatics was up to 75.3% when catalytic temperature was 600 °C with a residence time of 1 s. The content of styrene and xylene reached 29.1% and 12.5% in oil, respectively. The selectivity of sludge char to bicyclic aromatics was up to 64.4% when the temperature was 800 °C with the same residence time of 1 s. Naphthalenes accounted for 47.5% of the pyrolysis oil. The interaction among polyethylene, polypropylene, and polystyrene improved the selectivity of bicyclic aromatics from 46.8% to 53.7%. It can be concluded that principal ash components in sludge char could increase the aromatization degree of chain hydrocarbons and reduce the condensation degree of already formed aromatics. Acid sites (mainly aluminum phosphate) and dehydrogenation active sites (mainly -PO, sulfides, etc.) could promote the formation of aromatics by catalyzing direct dehydrogenation, hydrogen transfer reaction, and Diels-Alder reaction; while ring-opening active sites (mainly calcium oxide and iron species) could prevent the formation of excessively heavy aromatics by hydrogenation and ring-opening process.