Impacts of co-feeding alcohols on pyrolysis of cellulose

Highlights

• Co-feeding of alcohol enhances evolution of heavy organics in cellulose pyrolysis.

• Glycol or its derivatives react with bio-char, increasing char yield.

• Glycol makes bio-char more oxygen-rich, while ethanol makes it more carbon-rich.

• Ethanol enhances thermal stability of the bio-char, while glycol does not.

• Impact of the alcohols on cellulose pyrolysis is governed by pyrolysis temperature.

Abstract

Reactive atmosphere significantly affects pyrolysis of biomass, as the gases such as NH₃ involves in the reaction network in pyrolysis. In this study, instead of feeding reactive gas, the impacts of co-feeding ethanol or glycol on the pyrolysis of cellulose were investigated at varied pyrolysis temperatures (400 and 600 °C). The results showed that the co-feeding of the alcohols affected yields of bio-char and tar, evolution of light organics and heavy organics, elemental composition, formation of fused ring structures, crystallinity, thermal stability, distribution of functionalities in the bio-char. The co-feeding of both glycol and ethanol enhanced the formation of the heavy components with the π-conjugated ring structures and affected the production of the light organics such as furfural and glycolaldehyde. Glycol or its derivatives cross-polymerized with the organics in bio-char, increasing bio-char yield and made the bio-char more oxygen-rich. Ethanol, however, showed the converse effects. Co-feeding of the alcohols promoted crystallinity but showed distinct effects on thermal stability of the bio-char at the varied pyrolysis temperatures. Both pyrolysis temperature and structures of the alcohols determined their extents of involvement in the pyrolysis of cellulose.

Introduction

Biomass has been considered as a renewable source of energy, which can be used directly via combustion for supplying heat or energy. Biomass also can be converted into liquid fuel via pyrolysis, a thermo-chemical process to break down the macro-polymer structure in biomass to form the condensable liquid named as bio-oil, the solid products named as bio-char and the gaseous products [[1], [2], [3]]. The gases formed from the pyrolysis are combustible and can be used to provide the energy required for the endothermic pyrolysis process [4]. Pyrolysis of biomass involves a very complicated reaction network as both the primary decomposition of the macro structure of biomass and the secondary decomposition/condensation of the primary products took place [5,6]. Understanding the mechanism of pyrolysis of biomass has been the focus of many studies [[7], [8], [9], [10], [11], [12], [13]].

The reaction network in the pyrolysis of process is affected by many factors such as atmosphere of the pyrolysis, the heating temperature, the heat transfer efficiency, the mass transfer (the particle size of feedstock), the reactor type, etc [1]. The reaction atmosphere of pyrolysis can remarkably change the chemistry of the pyrolysis reactions, as indicated in previous studies [[14], [15], [16], [17], [18]]. Generally, nitrogen, argon and helium were regarded as the inert atmospheres for biomass pyrolysis as they generally do not react with the reaction intermediates generated in pyrolysis [[19], [20], [21]]. Other atmospheres such as carbon dioxide, methane, hydrogen and steam were also used as the reaction atmosphere of pyrolysis [[14], [15], [16], [17], [18]]. These gases were generally regarded as reactive, which could interfere with primary and secondary pyrolysis reactions and hence modified the distribution and properties of the products [[14], [15], [16], [17], [18]]. Thus, several research groups have investigated the use of the reactive atmospheres to study the possibility of controlling the occurrences of the secondary reactions to obtain the bio-oil with the improved properties [22]. For example, Ragucci et al. investigated the influence of the steam atmosphere in the slow pyrolysis of cellulose [22]. Their results showed that steam could positively improve the gas yield and carbon monoxide content. In addition, bio-char properties such as surface area were also improved [22]. Antal et al. developed the kinetics for the pyrolysis of the cellulose in nitrogen and steam [23]. Their results indicated that the presence of the steam in the pyrolysis medium did not have a considerable influence on pyrolysis kinetics of cellulose [23]. Shen et al. investigated the thermal decomposition of cellulose at different atmospheres by using TG-MS analyser [24]. They selected helium and oxidative (helium with 7% O₂, 20% O₂ and 60% O₂) atmospheres in their study. Their results confirmed that the evolution reactions of the volatiles such as furfural, acetone, 2,5-hydromethyl furfural, formaldehyde, CO and CO₂ were considerably affected by increasing the oxygen content in the atmosphere [24].

The reactive atmosphere clearly affected either the primary pyrolysis or secondary conversion of the volatiles via decomposition or condensation reactions that leads to the formation of gases, light and heavy compounds in bio-oil and also bio-char [25,26]. Tailoring the primary and secondary reactions via adjusting the reaction atmosphere could thus modify the pyrolysis process [25,26]. In addition to the gases and steam, organic compounds could also be potentially used to interfere with the primary pyrolysis or secondary conversion of the volatiles. It has been demonstrated that the volatiles (the organic compounds) generated from the primary pyrolysis are reactive at the high temperatures [[27], [28], [29], [30], [31]]. Thus, the externally added organics could also potentially affect the pyrolysis of biomass, tailoring the distribution of the main products.

In this study, the effects of externally added ethanol and glycol on the pyrolysis of cellulose were investigated. The rationale for using ethanol is because that ethanol could be produced from biomass via fermentation [32], while glycol could be produced from biomass via hydrolysis and hydrogenation [33]. These alcohols contain the hydroxyl group. At the typical pyrolysis temperature (i.e. 400-600 °C), they might decompose or degrade to form the oxygen-containing reaction intermediates that might further react with the volatiles generated from the pyrolysis and thus modify the distribution of the products. Cellulose was selected as the reaction substrate as it is one of the most abundant components in biomass [34]. It has to note that the cost of such a process with co-feeding the alcohols was not considered, as this study was a proof of concept for demonstrating the effects of alcohols on the pyrolysis behaviours of cellulose. The results indicated that the presence of the alcohols in the atmosphere of the pyrolysis remarkably affected both the primary pyrolysis and secondary conversion of the volatiles via decomposition or condensation reactions, resulting in the formation of the bio-char with the distinct properties and the distribution of the products in the bio-oil and the gases generated.