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

Highlights

• Production of aromatic hydrocarbons from bench-scale catalytic pyrolysis of lignin.

• Combined in-situ and ex-situ pyrolysis configurations.

• In-situ catalysts of bentonite, olivine and spent FCC; ex-situ catalyst of HZSM-5.

• Maximum selectivity to aromatic hydrocarbons using bentonite catalyst.

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.

Introduction

Increased concerns for greenhouse gases emission, which is a serious threat to the environment and is caused by the extensive consumption of fossil fuels, necessitates the supply of an alternative energy resource (Kim et al., 2018, Kim et al., 2018, Adhikari et al., 2014, Ohra-aho and Linnekoski, 2015). Lignocellulosic biomass, which consists of hemicellulose, cellulose, and lignin, is an important renewable energy source for the production of sustainable fuels and chemicals (Adhikari et al., 2014). Fast pyrolysis is an efficient and simple thermochemical method for the conversion of solid biomass into liquid fuel (bio-oil) in the absence of O₂. Among the three major components of lignocellulosic biomass (hemicellulose, lignin, and cellulose), lignin is a promising source for aromatic chemicals (Ohra-aho and Linnekoski, 2015, Li et al., 2012) and is produced in vast quantities as the waste by-product of the pulp-paper, food, wood, and ethanol production industries (Li et al., 2012, Zhu et al., 2017, Zakzeski et al., 2010). Lignin has a heterogeneous polymeric structure consisting of hydroxycinnamyl alcohols (coniferyl, p-coumaryl, and sinapyl alcohol building blocks) linked with C‒O‒C and C‒C bonds (Adhikari et al., 2014, Ohra-aho and Linnekoski, 2015, Kim et al., 2018, Kim et al., 2018, Shafaghat et al., 2017). The high phenolic structure of lignin leads to the formation of monomeric phenols, such as alkylphenols and methoxyphenols (particularly methoxyphenols), through fast pyrolysis process. Although the production of alkylphenols, which are used as a building block in the manufacture of pharmaceuticals, detergents, stabilizers, wood-adhesives, resins, and polymers (de Wild et al., 2012) from lignin pyrolysis is interesting, the potential of lignin for the production of aromatic hydrocarbons as fuel components/additives or chemical intermediates is commercially valuable. Indeed, catalytic pyrolysis makes the conversion of lignin-derived phenolics to aromatic hydrocarbons possible. Overall, there are two configurations, in-situ and ex-situ, for catalytic pyrolysis. With in-situ pyrolysis, the catalyst is mixed with the feedstock (biomass/lignin) and upgrading occurs in the pyrolyzer, whereas with ex-situ pyrolysis, the pyrolysis vapors are upgraded by passing through a catalyst bed placed remotely from the pyrolyzer. Among various types of catalysts, HZSM-5 has been vastly used in ex-situ and in-situ catalytic pyrolysis (Wang et al., 2014a, Iisa et al., 2016, Luo and Resende, 2016, Yildiz et al., 2013). It was reported that the deoxygenation of lignin-derived pyrolyzates to aromatic hydrocarbons was enhanced by the in-situ/ex-situ catalytic pyrolysis of biomass using HZSM-5 (Yildiz et al., 2013). Catalytic upgrading of lignin-derived pyrolyzates over Y-zeolite and HZSM-5 catalysts in an analytical micropyrolyzer was studied by Ohra-aho and Linnekoski (2015), in which an enhanced formation of aromatic hydrocarbons was achieved by using the acidic zeolite catalysts compared to the non-catalytic pyrolysis. It was reported by Adhikari et al. (2014) that the acidity of catalyst plays a key role in the conversion of lignin pyrolyzates into aromatic hydrocarbons. On the other hand, catalyst deactivation caused by coke formation is a main challenge in the catalytic pyrolysis of biomass. Phenolic compounds, such as guaiacols produced from the thermal degradation of lignin, are the main cause of coke formation in the catalytic pyrolysis process. Therefore, finding catalysts with high activity in the conversion of lignin-derived vapors to aromatic hydrocarbons and low capability in coke production is a great point of interest in the catalytic pyrolysis of biomass. Lee et al. (2016) carried out the lignin pyrolysis in a two stage fixed bed reactor using HZSM-5 and natural zeolite as ex-situ and in-situ catalysts, respectively. They mentioned that combining the ex-situ pyrolysis with in-situ pyrolysis decreased the coke formation on the HZSM-5 and enhanced the production of BTXs (benzene, toluene, and xylenes) and alkylphenols.

In this study, the sequential in-situ and ex-situ catalytic fast pyrolysis of Kraft lignin was carried out using a bench-scale fixed bed reactor at 500 °C. The objective of this research was to examine the effects of various additives as an in-situ catalyst on the pyrolysis of Kraft lignin, and to determine the best additive that can promote the subsequent ex-situ upgrading efficiency. Bentonite, olivine, and spent FCC were used as in-situ catalysts, whereas HZSM-5 was used as ex-situ catalyst. Although the zeolite type catalysts have been vastly used in lignin pyrolysis, the application of natural clay catalysts of bentonite and olivine which are inexpensive and easily available is a newly-focused research. The effectiveness of the additives was evaluated based on the selectivity to aromatic hydrocarbons (mono- and polycyclic aromatic hydrocarbons) in the lignin pyrolysis oil. The potential of the in-situ catalysts for decreasing the levels of char and coke formation during the lignin pyrolysis was also examined. In addition, the influence of in-situ catalysts of bentonite, olivine, and spent FCC on the reusability of ex-situ HZSM-5 (with and without regeneration) for the production of aromatic hydrocarbons was examined.