Physicochemical characterisation of torrefied biomass

Abstract

Torrefaction is a mild pyrolysis treatment under inert atmosphere that has been found to improve the physical and chemical properties of biomass as a fuel. In this study, several wood fuels: willow, eucalyptus, a mixture of hardwoods (oak and birch) and a mixture of softwoods (pine, larch and spruce) were torrefied at 270 and 290 °C with two residence times (30 and 60 min). Physicochemical properties of the torrefied materials, such as grindability, density, hydrophobicity and surface area were investigated. Furthermore, information was also obtained from microscopic and spectrometric studies, namely transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), to gain an insight into any changes in morphology and chemical composition due to the treatments. Temperature plays a major role during torrefaction and choosing an optimum condition is fundamental to attaining a satisfactory energy yield. Results show the lower temperature and shorter residence time was the best treatment to achieve good physical properties with a relatively high energy yield. When treated at these conditions, the softwood mixture had the highest energy (95%), followed by the hardwood mixture (80%), then willow (79%), and finally eucalyptus (75%). Increasing the severity of the torrefaction conditions greatly improved the physical characteristics of the torrefied biomass, in terms of grindability properties and hydrophobicity. While little difference in porosity and surface area were detected by the methods used, the XPS and FTIR studies showed that torrefaction results in a decrease in the ROH groups and an increase in CO groups within the fuels. This results in a decrease in the affinity of the fuels to absorb water, and therefore improves their hydrophobicity. For all the fuels, there was a critical temperature for torrefaction, above which abrupt changes in many properties occurred. These include hydrophobicity, energy yield, and grindability. The critical temperatures were fuel dependent and therefore the results indicate that careful optimisation is required for all fuel types to maximise the benefits of torrefaction whilst maintaining a good energy yield.

Introduction

The mitigation of climate change, together with the gradual decline and insecurity of non-renewable resources, such as coal, oil and gas, have become the main drivers for the urgent need to make a transition to renewable resources for energy in order to attain a sustainable environment [1]. The European communities (EU) proposed a directive in 2008 for the utilisation of such resources [2]. The agreement aims to establish an overall binding target to achieve reductions in EU greenhouse gases (GHG) emissions of 20% by 2020. The UK has signed up to this EU target and committed to produce 15% of its energy from renewable resources by 2020 [2], [3]. Thus, there followed the establishment of the latest Renewable Energy Strategy in 2012, where targets are focused on three energy sectors: electricity, heat and transport [3]. Of all renewable resources, biomass is considered to be the largest that is utilised worldwide, which includes underdeveloped, developing and developed countries [4]. It has a higher availability than most renewables such as wind and hydropower [5]. Moreover, well-managed and regulated biomass is a sustainable source of energy, offering significant life-cycle GHG savings compared to fossil fuels. The carbon dioxide produced from burning biomass is captured during plant growth and thus it is perceived as carbon-neutral and able to reduce the net carbon dioxide emissions when displacing fossil fuel use [6]. However, GHG emissions from cultivation, harvesting, drying, storage, transportation and conversion (for example, pelletisation and co-firing) need careful assessment in any fuel procurement strategy.

Biomass absorbs moisture easily which leads to degradation upon long storage and results in a low heating value. This characteristic, coupled with low energy density, makes it more expensive for transportation [7], [8]. Furthermore, biomass is fibrous in nature and tenacious to grind to desired particle sizes and this requires high energy input. It is difficult to accomplish a perfect combustion, and in entrained fuel boilers or gasifiers, finely powdered biomass is required. Therefore, the making of wood pellets is becoming necessary especially in the co-firing industry, where biomass is used with coal in large scale boilers [9]. However, the grinding of wood to pellets is very costly and usually requires high capital investment and maintenance [10]. Prior to grinding, the biomass requires drying as moisture is an important factor during milling. Mani et al. [11] stated that “the higher the moisture content, the higher the specific energy consumption is needed to mill such biomass”.

The chemical and physical properties of a biomass can be upgraded by implementing torrefaction. Torrefaction is a developing area that is believed to become a leading technology [12]. It is a mild pyrolysis pre-treatment, which involves a moderate temperature of above 200 °C in the absence of oxygen. Several studies have been conducted using different temperatures within the range of 200–300 °C and residence times, mainly 30–60 min [4], [13], [14], [15], [16], [17], [18]. A few studies extended the reaction time to three to five hours [9], [19]. In general, all results showed that the more severe the torrefaction conditions are, the easier is the grinding and the greater the amount of energy that can be saved during this process [20]. However, the mass loss of the solid torrefied product must be kept as low as possible to attain a high energy yield [20]. Therefore, choosing an optimum operating condition is crucial, as different types of biomass give different outcomes. What is required and acceptable is that this approach is able to retain approximately 70% of the initial biomass dry weight, and about 80–90% of the biomass's original energy content [7]. Biomass has a higher O/C ratio than coal, which explains its lower calorific value. Torrefaction has the ability to increase this value. Furthermore, this thermal pre-treatment produces a more hydrophobic and grindable solid product compared to the raw material.

In torrefaction, hemicellulose, as one of the main components that make up biomass, is considered to be the most reactive [7], [17]. Chen et al. carried out torrefaction processes that focussed on the three lignocellulosic materials using a thermogravimetry and recorded the weight losses with increasing temperature [4]. The results showed that hemicellulose degraded effectively by torrefaction even at a temperature as low as 230 °C [4]. This can be explained by the decomposition of various saccharides and branches in the hemicellulose, which are easy to remove from the main backbone [24]. This effect is followed by the decomposition of cellulose which can eventually take place at higher temperatures [4]. Cellulose is made up of a polymer of glucose with no branches, hence, it is thermally more stable [24]. Lignin does not seem to show any significant impact on torrefaction [4]. It decomposes to three products: a char solid, tars and volatiles/gases, the latter two contributing to the significant loss of mass upon torrefaction at higher temperatures.

With regard to the solid torrefied biomass, there has been a great deal of research considering the standard fuel analysis, mass yield and energy yield [4], [13], [17], [19], [21], [22], [23]. A few studies report on the improvement of their grindability properties [4], [9]. However, very little research has given a thorough look into the structure and physicochemical properties of the solid product [4]. The present study focuses on the investigation of not only the morphology and composition of the solid torrefied biomass (several woody biomass including eucalyptus and short rotation willow coppice (SRC)) but also, on their physical and chemical characteristics. A range of characterisation methods are used, including: transmission electron microscopy (TEM) and scanning electron microscopy (SEM) for morphology examination, X-ray photoelectron spectroscopy (XPS), which is used to study the changes in the O/C ratio and components in the biomass, and Fourier transform infrared spectroscopy (FTIR), to follow changes in chemical structure. The surface area and pore size distribution are also investigated using the Brunauer–Emmett–Teller (BET) method. Furthermore, the density, hydrophobicity and grindability of the torrefied products are also studied.