The effect of torrefaction on the chlorine content and heating value of eight woody biomass samples

Highlights

• Eight woody biomass samples were torrefied at 260 °C.

• The chemical and fuel properties of different wood species were analyzed.

• Torrefaction reduced the biomass chlorine content.

• Torrefaction increased the biomass HHV at maximum by a factor of 1.11.

• Torrefaction decreased the biomass elementary O to C-ratio.

Abstract

This study examined and compared the effect of torrefaction on the heating value, elementary composition, and chlorine content of eight woody biomasses. The biomass samples were torrefied in a specially constructed batch reactor at 260 °C for 30, 60, and 90 min. The original biomasses as well as the solid, liquid, and gaseous torrefaction reaction products were analyzed separately. The higher heating values (HHV) of dry samples increased from 19.5–21.0 MJ  kg⁻¹ to 21.2–23.2 MJ  kg⁻¹ during 60 min of torrefaction. In all samples, the HHV increased 9 % on average. Furthermore, the effect of torrefaction time on the biomass HHV was studied. Measurements showed that after a certain point, increasing the torrefaction time had no effect on the samples' HHV. This optimal torrefaction time varied considerably between the samples. For more reactive biomasses, i.e., birch and aspen, the optimal torrefaction time was close 30 min whereas the HHV of less reactive biomasses, e.g., stumps, increased markedly even after a 60-min torrefaction. Another significant observation was that torrefaction reduced the chlorine content of the biomass samples. The chlorine concentration of the solid product dropped in most samples from the original by half or even as much as 90 %. The highest relative chlorine decrease was observed in the Eucalyptus dunnii sample, which also had the highest chlorine content of all the studied biomasses. The relative carbon content of the biomass samples increased during torrefaction as the average elementary composition changed from CH_0.123O_0.827 to CH_0.105O_0.674 after a 60-min torrefaction.

Introduction

The growing world population and accelerating industrialization keep increasing the energy demand. The concurrent global warming and concerns about the depletion of fossil fuel reserves necessitate the development of sustainable ways to produce energy. Because biomass is considered a carbon-neutral source of energy, partial replacement of coal with biofuels in commercial combustion units lowers the carbon dioxide emissions [1]. However, biomass properties, such as heterogeneous and tenacious structure, hydrophilic nature, and high moisture content are posing challenges to using biomass for energy production.

Torrefaction, i.e., thermal treatment at temperatures ranging from 200 to 300 °C in the absence of oxygen, transforms biomass properties close to those of fossil coal [2], [3]. Torrefaction increases biomass bulk density and improves its storage and handling properties [4]. Furthermore, torrefaction reduces the biomass moisture content in two ways. First, increasing temperature evaporates the free water in biomass, and at above 200 °C releases the physically bound water [5]. Moreover, biomass loses partly its hydrophilic property as the hydroxyl groups decompose [1]. Torrefaction decreases the biomass oxygen content and increases the relative proportion of carbon, thus improving biomass fuel properties [2]. The vaporization of water and stripping of carbon dioxide (both with zero heating value) increase the biomass heating value. Even a 20-% increase in the biomass heating value during torrefaction has been observed [6]. Torrefaction has also shown to improve the grindability of biomass in terms of lowered energy demand and more spherical particles produced [7], [8], [9].

Arias et al. [10] have studied the effect of torrefaction on the reactivity and combustion properties of woody biomass and found out that torrefaction affects only to the most reactive hemicellulose components. Because of the low volatile content of torrefied biomass, the activation energy of the first stage of combustion increases [10]. Generally, hardwoods show better reactivity during torrefaction than softwoods because of their higher content of the most reactive hemicellulose component, i.e., glucuronoxylan, or xylan [6]. Compared to coal, the crucial problem in torrefied biomass use is its explosibility and higher flame speed referring to the ignition sensitivity of combustible dust and air mixture and the higher burning velocity of this powder, respectively [11].

This study focused on comparing the behavior of eight woody biomasses during torrefaction. Elementary analyses were conducted on the samples to better understand the changes in biomass during torrefaction. The effect of torrefaction on the biomass chlorine content was examined because fuel derived chlorine compounds may heavily corrode boilers [12], [13], [14], [15] and in flue gas mitigate to the environment. Hydrogen chlorine (HCl) cause acidification [16] and dioxins are a risk to the human health because of their persistence, toxicity, and bio-accumulation resulted from their lipophilicity [17], [18]. The effect of torrefaction on biomass chlorine content has not been studied commonly; however, methyl chloride has been detected in the volatile torrefaction products [19]. The torrefaction device in this study is a batch reactor with a relatively large sample particle size and sample volume together with slow torrefaction. Kim et al. [20] and Na et al. [21] have reported similar experimental set-ups.