Biomass is a versatile energy resource that could be used as a sustainable energy resource in solid, liquid and gaseous form of energy sources. Torrefaction is an emerging thermal biomass pretreatment method that has an ability to reduce the major limitations of biomass such as heterogeneity, lower bulk density, lower energy density, hygroscopic behavior, and fibrous nature. Torrefaction, aiming to produce high quality solid biomass products, is carried out at 200-300 °C in an inert environment at an atmospheric pressure. The removal of volatiles through different decomposition reactions is the basic principle behind the torrefaction process. Torrefaction upgrades biomass quality and alters the combustion behavior, which can be efficiently used in the co-firing power plant. This paper presents a comprehensive review on torrefaction of biomass and their characteristics. Despite of the number of advantages, torrefaction is motivated mainly for thermochemical conversion process because of its ability to increase hydrophobicity, grindability and energy density of biomass. In addition to this, torrefied biomass could be used to replace coal in the metallurgical process, and promoted as an alternative of charcoal.

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Many correlations are available in the literature to predict the higher heating value (HHV) of raw biomass using the proximate and ultimate analyses. Studies on biomass torrefaction are growing tremendously, which suggest that the fuel characteristics, such as HHV, proximate analysis and ultimate analysis, have changed significantly after torrefaction. Such changes may cause high estimation errors if the existing HHV correlations were to be used in predicting the HHV of torrefied biomass. No study has been carried out so far to verify this. Therefore, this study seeks answers to the question: “Can the existing correlations be used to determine the HHV of the torrefied biomass”? To answer this, the existing HHV predicting correlations were tested using torrefied biomass data points. Estimation errors were found to be significantly high for the existing HHV correlations, and thus, they are not suitable for predicting the HHV of the torrefied biomass. New correlations were then developed using data points of torrefied biomass. The ranges of reported data for HHV, volatile matter (VM), fixed carbon (FC), ash (ASH), carbon (C), hydrogen (H) and oxygen (O) contents were 14.90 MJ/kg–33.30 MJ/kg, 13.30%–88.57%, 11.25%–82.74%, 0.08%–47.62%, 35.08%–86.28%, 0.53%–7.46% and 4.31%–44.70%, respectively. Correlations with the minimum mean absolute errors and having all components of proximate and ultimate analyses were selected for future use. The selected new correlations have a good accuracy of prediction when they are validated using another set of data (26 samples). Thus, these new and more accurate correlations can be useful in modeling different thermochemical processes, including combustion, pyrolysis and gasification processes of torrefied biomass.

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A two-stage, inclined continuous rotary torrefier with novel flights has been developed in the Biomass Conversion Laboratory at Dalhousie University for improving biomass torrefaction processes. Experimental work on torrefaction of small poplar wood particles (0.5–1.0 mm) in the torrefier was undertaken for a deeper understanding of the working of such torrefiers where the volatile gas released was used as the torrefaction medium instead of nitrogen. The rotary torrefier is operated under different operating conditions by varying its rotational speed, tilt angle and temperature. Measured chemical and physical properties of the torrefied products included ultimate and proximate analysis, structural analysis, energy density, mass yield, energy yield, and bulk density. A novel probe was developed to collect samples of biomass and measure temperature at different interior points along the length of the rotary torrefaction reactor while the biomass was being progressively torrefied in it. Axial temperature distribution of the rotary torrefier showed a parabolic profile but the fixed carbon content, volatile, and energy density of biomass undergoing torrefaction varied linearly along the length of the torrefier. For torrefaction at 300 °C and 5 rpm and 1° of tilt angle the change in heating value was 40%, while the mass yield and energy yield of torrefied biomass were 34% and 48%, respectively. Results showed that temperature is the most important parameter in this torrefaction process.

Torrefaction, a mild roasting process in inert atmosphere, is an emerging thermo-chemical pretreatment process that can eliminate many of the shortcomings of raw biomass, but the supply of an inert gas like nitrogen in large industrial units may not be cost-effective. This paper examines the use of air as a potential substitute for the expensive nitrogen gas through a simple innovative means. It proposes to use a mildly pressurized batch reactor instead of an open continuous reactor continuously fed by nitrogen. Torrefaction of poplar wood was conducted in a 25.4 mm diameter × 304.8 mm long batch reactor under different operating parameters (Gauge Pressures, 0, 200, 400, and 600 kPa, temperatures, 220, 260, and 300 °C, and residence times, 15, 25, and 35 min) in air and nitrogen. Results show that torrefaction in pressurized air has higher energy density, higher fuel ratio, and similar energy yield but reduced mass yield compared to those in pressurized nitrogen. While reactor pressure was increased from 200 to 600 kPa, fuel ratio, energy density enhancement factor, fixed carbon increased but mass yield decreased in both air and nitrogen medium. Data obtained further showed that torrefaction temperature is the most important operating parameter influencing the process. Using Response Surface Methodology, this work also developed correlations to predict mass loss for given values of temperature, pressure, and time in air and nitrogen media. Correlations to estimate torrefied product properties like energy density enhancement and fuel ratio for known mass loss during torrefaction were then established. This also offers a quantitative characterization of different modes of torrefaction that could be used for design selection.

Biomass is seen as a viable alternative energy source for replacing fossil fuel. However, the inherent properties of raw biomass, including high moisture content, low energy density, hydrophilic behavior, and fibrous nature, limit its use. Thus, pretreatment becomes an important step in the path of biomass to energy conversion routes. Torrefaction is one such emerging pretreatment process, which makes biomass hydrophobic, more energy dense, and easier to grind. This paper investigates the torrefaction process in a continuous two-stage, indirectly heated rotary torrefier under the volatile gases medium. The rotary reactor is operated at different angular speeds, inclinations, and temperatures, and the produced torrefied solid product is characterized for physical and chemical compositions. The observed characteristics of the torrefied biomass produced under the volatile gases medium indicate that nitrogen supply can be avoided. Experimental results showed that torrefaction temperature has a more significant effect than angular speed and inclination. At 320 °C, 4 rpm, and 1°, fixed carbon in torrefied poplar is double that in raw poplar, with a solid mass yield of 80.2% and an increase in volumetric energy density of 8.9%. The energy balance showed the net system efficiency to be above 88%.