Production of an upgraded lignin-derived bio-oil using the clay catalysts of bentonite and olivine and the spent FCC in a bench-scale fixed bed pyrolyzer

Abstract

Lignocellulosic biomass is an abundant renewable energy source that can be converted into various liquid fuels via thermochemical processes such as pyrolysis. Pyrolysis is a thermal decomposition method, in which solid biomass are thermally depolymerized to liquid fuel called bio-oil or pyrolysis oil. However, the low quality of pyrolysis oil caused by its high oxygen content necessitates further catalytic upgrading to increase the content of oxygen-free compounds, such as aromatic hydrocarbons. Among the three different types of lignocellulosic biomass components (hemicellulose, lignin, and cellulose), lignin is the most difficult fraction to be pyrolyzed because of its highly recalcitrant structure for depolymerization, forming a char as a main product. The catalytic conversion of lignin-derived pyrolyzates is also more difficult than that of furans and levoglucosan which are the main pyrolysis products of hemicellulose and cellulose. Hence, the main purpose of this study was to develop a bench-scale catalytic pyrolysis process using a tandem catalyst (both in-situ and ex-situ catalysis mode) for an efficient pyrolysis and subsequent upgrading of lignin components. While HZSM-5 was employed as an ex-situ catalyst for its excellent aromatization efficiency, the potential of the low-cost additives of bentonite, olivine, and spent FCC as in-situ catalysts in the Kraft lignin pyrolysis at 500 °C was investigated. The effects of these in-situ catalysts on the product selectivity were studied; bentonite resulted in higher selectivity to aromatic hydrocarbons compared to olivine and spent FCC. The reusability of HZSM-5 (with and without regeneration) was examined in the pyrolysis of lignin mixed with the in-situ catalysts of bentonite, olivine, and spent FCC. In the case of using bentonite and spent FCC as in-situ catalysts, there were no obvious changes in the activity of HZSM-5 after regeneration, whereas using olivine as in-situ catalyst resulted in a remarkable decrease in the activity of HZSM-5 after regeneration.

Keywords: Bench-scale fixed bed pyrolyzer; Bentonite; HZSM-5 reusability; In-situ/Ex-situ pyrolysis; Lignin.