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Torrefaction: Roasting agricultural, forest and animal residues to produce “green coal” with improved properties and value

What is torrefaction?

Torrefaction is a thermochemical treatment process for biomass upgrading that occurs at a temperature range of 390 to 570 degrees F at near atmospheric pressure, in the absence of oxygen, and at a reactor residence time of 10 to 30 minutes, or longer. During torrefaction, water and volatile organic compounds (VOCs) are removed, and hemicellulose fractions of the biomass are degraded leaving cellulose and lignin in the biomass. The final product looks similar to charcoal.

 

How to torrefy biomass?

The first step in biomass torrefaction involves feeding raw biomass into the 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 degrees F, which is maintained for up to 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, producing heat that can be used to preheat the incoming raw biomass for torrefaction. The torrefied biomass can be pelletized to obtain high density, uniform biomass pellets (Figure 1).

 

Why torrefy biomass?

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 — Torrefaction drastically reduces the moisture content of the biomass, resulting in a relatively hydrophobic product that absorbs and retains less water during storage, reducing biomass deterioration.  The low moisture content also decreases transportation cost.
  • Increased brittleness — Torrefaction destroys the biomass cell structure, which increases the brittleness of the material. Biomass usually needs to be reduced in size for use, such as fuels, chemicals, or other materials. Increased brittleness of torrefied biomass decreases the energy required for grinding by 80-90%.
  • Higher energy content per unit weight — The calorific value of the biomass typically increases from 5,000-7,000 BTUs per pound to 8,500-10,500 BTUs per pound. This makes it an ideal candidate to be used in thermochemical plants as well as household burners.
  • Homogeneous solid with less smoke — Raw biomass have a wide range of moisture content 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 odor and pathogenic microorganisms — Torrefaction involves heat treatment, which can eliminate odors and pathogenic microorganisms that arise from decomposition of biomass during storage.

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 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 to 53 pounds per cubic foot) compared to raw biomass pellets (34 to 44 pounds per cubic foot).

Ultimately, torrefaction of biomass and densification reduces dry matter losses and enhances handling and transportation logistics.

 

Applications of torrefied biomass

Torrefied biomass/pellets can be used for different applications, such as:

  • An alternative to coal in conventional coal fired plants — Torrefied biomass are renewable, and have calorific value (8,500-10,500 BTU per pound) comparable to coal (~11,000 BTU per pound), and lower ash and mineral contents.
  • Feedstock for biofuel production — Torrefied biomass primarily contains cellulose, which can be converted to biofuel, 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.

 

How much does it cost to torrefy biomass?

Torrefaction of biomass requires a reactor that provides inert reaction conditions without oxygen. Such a facility, at commercial scale, requires a large capital investment. Torrefaction operational 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. Torrefaction costs of between $17 and $42 per ton of torrefied biomass have been reported.

Dr. Ajay Shah is an Assistant Professor in the Department of Food, Agricultural and Biological Engineering of The Ohio State University.  He can be reached at shah.971@osu.edu and (330) 263-3858.  This column is provided by the OSU Department of Food, Agricultural and Biological Engineering, OSU Extension, Ohio Agricultural Research & Development Center, and the College of Food, Agricultural and Environmental Sciences

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