The role and potential of attapulgite in catalytic pyrolysis of refinery waste activated sludge

Abstract

Pyrolysis is a promising technology for the treatment of refinery waste activated sludge (rWAS). In this study, attapulgite as a natural clay was used to enhance the pyrolysis of rWAS. The yields, characteristics of pyrolytic products, pyrolytic kinetics and mechanisms were investigated. The attapulgite improved the conversion of rWAS into non-condensable gases and pyrolytic liquids. The bio-oil quality improved and the biochar yield reduced. The average activation energy of Stage I (230–400 °C) and Stage II (400–500 °C) decreased by 36.5% and 49.7%, respectively, compared to rWAS alone. Al2O3 and Fe2O3 in attapulgite enhanced the dealkylation reaction and cracking of C–C bonds. The content of the gasoline (<C13) fraction of bio-oil doubled relative to rWAS alone. Attapulgite promoted the deoxygenation, dehydroxylation and dehydrogenation reactions of O-containing compounds, and the content of CO and CO2 in non-condensable gases increased. Addition of attapulgite (rWAS:attapulgite ratio of 1:1) decreased the O mobility from 14.6% to 12.8% relative to rWAS alone. Also, the content of saturates in bio-oil increased from 38.5 wt% to 47.2 wt% and the lower heating value (LHV) increased from 6.8 kcal/kg to 8.4 kcal/kg. The heavy metals originally in rWAS were fixed into the pyrolytic residue and the environmental risks are low. This study demonstrates the role and potential of attapulgite in catalytic pyrolysis of rWAS with an added advantage of increased cost-effectiveness.