Biomass torrefaction under different oxygen concentrations and its effect on the composition of the solid by-product

Abstract

Torrefaction is a thermal treatment used to improve the properties of biomass in relation to thermo-chemical processing techniques for energy generation. It is a thermo-chemical treatment method primarily characterized by an operating temperature within the 200–300 °C range. It is carried out under conditions of atmospheric pressure and in the presence of a minimum amount of oxygen in order to avoid spontaneous combustion. The aim of this study was to evaluate the combined effect of the temperature (240 and 280 °C) and oxygen concentration (2, 6, 10 and 21%) on the physical and chemical properties of large particles of Eucalyptus grandis. A statistical analysis was carried out. The different oxygen concentrations did not significantly affect the composition of the solid by-product for low temperatures. At 280 °C, the high oxygen concentration affected some of the properties studied.

Introduction

The global problems associated with the intensive use of fossil fuels have increased interest in the use of renewable fuels worldwide, mainly in countries like Brazil, where biomass is widely available at low cost. However, a pre-treatment process is required to convert biomass into a hydrophobic solid product with an increased energy density [1]. One way to achieve that goal is to apply torrefaction, which is considered as a biomass feedstock pre-treatment, particularly for thermal conversion. It is gaining attention as an important pre-processing step for improving biomass quality in terms of physical properties and chemical composition [2], [3]. Torrefaction involves the slow heating of biomass in an inert environment or reduced O₂ content to a maximum temperature of approximately 300 °C. Torrefaction can also be defined as a group of products resulting from the partially controlled and isothermal pyrolysis of biomass occurring in a temperature range of 200–280 °C [4], [5]. The yields and properties of torrefaction by-products are influenced by several parameters, including biomass composition, particle size, processing temperature and time, heating rate and the composition of the working atmosphere [6], [7]. Much work has been done to study how these operating parameters influence torrefaction [8], [9], but there still remains a need to study the influence of different oxygen concentrations on the thermal reactivity of large biomass particles during torrefaction, as well as their effect on the physical and chemical properties of torrefied biomass.

The thermal decomposition of lignocellulosic materials involves a complex series of chemical reactions and heat and mass transfer processes [10], [11], [12]. An oxidizing atmosphere can promote the oxidative degradation of the material and the subsequent oxidation of the volatile matter released during such degradation, in addition to promoting the combustion of the char residue. Studies to evaluate the thermal behaviour of biomass in both oxidizing and inert atmospheres have been carried out by several authors [13], [14]. Most of them showed that when oxygen is present, the thermal reactivity of biomass (especially cellulose) is greatly enhanced due to the acceleration of mass loss in the first stage of pyrolysis. Analysing the influence of the initial solid weight and the sample shape on solid conversion for different oxygen concentrations, Bilbao [15] proposed a simple model. He showed that the influence of oxygen concentrations on mass loss depends on the weight of the sample and the temperature at which the process occurs. More recently, Shen and his co-authors [16] used a DAEM method and global kinetic model in both inert and oxidative atmospheres to examine the mechanisms involved in the thermal decomposition of wood. The apparent activation energy of pyrolysis and combustion varied linearly with oxygen concentration [17]. However, these studies were carried out at high temperatures using small samples. For temperature lower to 250 °C, the mass losses rates of wood were similar. Thus, it is important to evaluate the effect of different oxygen concentrations in the working atmosphere using larger samples, closer to the true conditions of use, and at the low temperatures of torrefaction process.

This study investigated the thermal reactivity of Eucalyptus grandis during torrefaction under different oxygen concentrations by examining the relationship between mass loss and the physical and chemical properties of the solid by-product.