Torrefaction is a thermal treatment process for biomass upgrading that occurs at a temperature range of 390 to 570 degrees Fahrenheit (200 to 300 degrees Celsius) at near atmospheric pressure, in the absence of oxygen, and at a reactor residence time of 10 to 30 minutes or longer (Medic et al., 2012). During torrefaction, water and volatile organic compounds (VOCs) are removed and hemicellulose fractions of the biomass are mainly degraded, leaving cellulose and lignin with minimal degradation in the biomass (Prins et al., 2006). For biomass torrefied at higher temperatures, the final product looks similar to charcoal.
The first step in biomass torrefaction involves feeding raw biomass into a torrefaction system. The particle size may vary depending on the feedstock source; however, a uniform size will give the best results. The biomass is then slowly heated in the absence of oxygen in the torrefaction chamber to reach a temperature of 390 to 570 F, which is maintained for 10 to 30 minutes. This partially decomposes the biomass and releases some volatile matter, including water, VOCs and hemicellulose. The remaining solid portion is the torrefied biomass, which is the main final product. The volatiles, vapors and gases produced during torrefaction can be burned in the combustion section to produce heat that can be used to preheat the incoming raw biomass before torrefaction. The torrefied biomass can be pelletized to obtain a high density and uniform particle size (Figure 1).
|Raw biomass||Torrefied biomass||Torrefied pellets|
Figure 1. Raw biomass converted to torrefied biomass and torrefied pellets. (Disclaimer: Figure is assembled from different internet sources.)
Benefits of Torrefaction
Agricultural and forest residues, energy crops, and animal litter can be used as a feedstock to generate bioenergy, such as ethanol, heat, and electricity. However, these feedstocks usually have a low bulk density and energy density. Biomass upgrading techniques, such as torrefaction, change biomass properties for improved storage, transportation, pretreatment, and conversion.
The advantages of torrefied biomass over raw biomass include:
- Reduced moisture content and improved hydrophobicity. With torrefaction, the moisture content of the biomass is drastically reduced to less than 1 percent, when measured right after the torrefaction. It produces a relatively hydrophobic product that absorbs minimal water from the atmosphere, thus it has minimal increase in moisture content during storage, reducing biomass deterioration (Commandre and Leboeuf, 2015). Also, the low moisture content decreases transportation costs.
- Increased brittleness. Torrefaction disrupts the biomass cell structure, which increases the brittleness of the material. Biomass usually needs to be reduced in size for conversion to the end products, such as fuels, chemicals, or other materials. Increased brittleness of torrefied biomass decreases the energy required for grinding by 80 to 90 percent (Phanphanich and Mani, 2011).
- Higher energy content per unit weight. After torrefaction, the calorific value of the biomass typically increases from 5,000-7,000 BTU per pound to 8,500-10,500 BTU per pound (Tumuluru et al., 2011). This makes it an ideal candidate to be used in thermochemical plants as well as for household burners.
- Homogeneous solid with less smoke. Raw biomass has a wide range of moisture contents and VOCs depending on the type, source, and harvest conditions. Torrefaction significantly reduces the moisture content and VOCs, resulting in a homogeneous solid fuel that generates less smoke during combustion.
- Elimination of odors and pathogenic microorganisms. Torrefaction at high temperature eliminates pathogenic microorganisms (if any) and most of the odorous VOCs (Isemin et al., 2017).
Torrefied biomass can be further densified into pellets. Torrefied pellets have a more uniform size and form, which results in better flowability compared to the raw biomass, and improves its handling and storage. Furthermore, torrefied biomass pellets have several benefits compared to the raw biomass pellets, including:
- Improved durability. The biomass contains a higher fraction of hot lignin immediately after torrefaction, which helps it to be compacted with strong adhesion between the biomass particles, improving pellet durability and handling properties.
- Increased bulk density. Torrefied pellets usually have a higher bulk density (47-53 lb/ft3) compared to raw biomass pellets (34-44 lb/ft3) (Bergman, 2005).
Ultimately, torrefaction of biomass and densification reduces dry matter losses and enhances its handling and transportation logistics.
Material Balance and Energy Value of Biomass During Torrefaction
Torrefaction of biomass occurs at high temperatures, which removes the moisture present in the biomass. In addition to the moisture, torrefaction also removes the VOCs and degrades the hemicellulose fraction of the biomass, which typically ranges between 10 to 35 percent of dry biomass weight, depending on the type of biomass. Thus, torrefaction reduces the biomass weight and energy content compared to the raw biomass. However, the reduction in absolute energy content is lower compared to the weight reduction, which results in higher energy density (Figure 2).
Figure 2. Effect of torrefaction on weight and energy value of biomass (adapted from HM3 Energy, 2013).
Applications of Torrefied Biomass
Torrefied biomass/pellets can be used for different applications, such as:
- Alternative to coal in conventional coal fired plants. Torrefied biomass is renewable, and has a calorific value (8,500-10,500 BTU per pound) comparable to that of coal (~11,000 BTU per pound). In addition, it has a lower ash and mineral content compared to coal. These make torrefied biomass "green coal."
- Feedstock for biofuel production. Torrefied biomass can be converted to biofuels, especially through thermochemical conversion routes.
- Soil amendment. Torrefied biomass has almost all the minerals and nutrients in the biomass and thus can be land applied as a soil amendment (Kikuchi, 2016; Ogura et al., 2016).
Cost of Torrefaction
Torrefaction of biomass requires a reactor that provides inert reaction conditions (without oxygen). Such a facility, at a commercial scale, requires a large capital investment. Torrefaction operating costs vary considerably depending on the scale of the facility, feedstock type, desired torrefaction conditions, and potential use of heat or VOCs for other purposes, such as offsetting heating costs. A wide range of torrefaction costs, varying from $17 to $65 per ton of torrefied biomass, have been reported in the literature (Shah et al., 2012; Strauss, 2014; Tiffany et al., 2013; Uslu et al., 2008).
Authors thank Dr. Sushil Adhikari, Professor, Department of Biosystems Engineering, Auburn University; Dr. Harold Keener, Professor Emeritus and Associate Chair; and Mary Wicks, Program Coordinator, Department of Food, Agricultural and Biological Engineering, The Ohio State University for technical and editorial review of this factsheet.
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