Torrefaction of

biomass Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...

, e.g., wood or grain, is a mild form of

pyrolysis Pyrolysis is a process involving the Bond cleavage, separation of covalent bonds in organic matter by thermal decomposition within an Chemically inert, inert environment without oxygen. Etymology The word ''pyrolysis'' is coined from the Gree ...

at temperatures typically between 200 and 320 °C. Torrefaction changes biomass properties to provide a better fuel quality for combustion and

gasification Gasification is a process that converts biomass- or fossil fuel-based carbonaceous materials into gases, including as the largest fractions: nitrogen (N2), carbon monoxide (CO), hydrogen (H2), and carbon dioxide (). This is achieved by reacting ...

applications. Torrefaction produces a relatively dry product, which reduces or eliminates its potential for organic

decomposition Decomposition is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is ess ...

. Torrefaction combined with densification creates an energy-dense fuel carrier of 20 to 21 GJ/ton lower heating value (LHV). Torrefaction causes the material to undergo Maillard reactions. Torrefied biomass can be used as an energy carrier or as a feedstock used in the production of bio-based fuels and chemicals. Biomass can be an important energy source. However, there exists a large diversity of potential biomass sources, each with its own unique characteristics. To create efficient biomass-to-energy chains, torrefaction of biomass, combined with densification ( pelletisation or briquetting), is a promising step towards overcoming the logistical challenges in developing large-scale

sustainable energy Energy system, Energy is sustainability, sustainable if it "meets the needs of the present without compromising the ability of future generations to meet their own needs." Definitions of sustainable energy usually look at its effects on the e ...

solutions, by making it easier to transport and store. Pellets or briquettes have higher density, contain less moisture, and are more stable in storage than the biomass they are derived from.

Process

Torrefaction is a thermochemical treatment of biomass at . It is carried out under atmospheric pressure and in the absence of

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

. During the torrefaction process, the water contained in the biomass as well as superfluous volatiles are released, and the biopolymers (

cellulose Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...

hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all embryophyte, terrestrial plant cell walls. Cellulose is crystalline, strong, an ...

and

lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidit ...

) partly decompose, giving off various types of volatiles. The final product is the remaining solid, dry, blackened material that is referred to as ''torrefied biomass'' or ''bio-coal''. During the process, the biomass typically loses 20% of its mass (bone dry basis) and 10% of its heating value, with no appreciable change in volume. This energy (the volatiles) can be used as a heating fuel for the torrefaction process. After the biomass is torrefied it can be densified, usually into briquettes or pellets using conventional densification equipment, to increase its mass and energy density and to improve its hydrophobic properties. The final product may repel water and thus can be stored in moist air or rain without appreciable change in moisture content or heating value, unlike the original biomass. The history of torrefaction dates to the beginning of the 19th century, and gasifiers were used on a large scale during the Second World War.

Added value of torrefied biomass

Torrefied and densified biomass has several advantages in different markets, which makes it a competitive option compared to conventional biomass

wood pellets Pellet fuels (or pellets) are a type of solid fuel made from compressed organic material. Pellets can be made from any one of five general categories of Biomass (energy), biomass: industrial waste and co-products, food waste, Crop residue, agricul ...

Higher energy density

An energy density of 18–20 GJ/m³ – compared to the 19-24 GJ/m³ heat content of natural

anthracite coal Anthracite, also known as hard coal and black coal, is a hard, compact variety of coal that has a submetallic lustre. It has the highest carbon content, the fewest impurities, and the highest energy density of all types of coal and is the highe ...

– can be achieved when combined with densification (pelletizing or briquetting) compared to values of 10–11 GJ/m³ for raw biomass, driving a 40–50% reduction in transportation costs. Importantly, pelletizing or briquetting primarily increases energy density. Torrefaction alone typically decreases energy density, though it makes the material easier to make into pellets or briquettes.

More homogeneous composition

Torrefied biomass can be produced from a wide variety of raw biomass feedstocks that yield similar product properties. Most woody and herbaceous biomass consists of three main polymeric structures:

and

. Together these are called

lignocellulose Lignocellulose refers to plant dry matter (biomass), so called lignocellulosic biomass. It is the most abundantly available raw material on the Earth for the production of Biofuel, biofuels. It is composed of two kinds of carbohydrate polymers, c ...

. Torrefaction primarily drives moisture and oxygen-rich and hydrogen-rich functional groups from these structures, producing similar char-like structures in all three cases. Therefore, most biomass fuels, regardless of origin, produce torrefied products with similar propertieswith the exception of ash properties, which largely reflect the original fuel ash content and composition.

Hydrophobic behavior

Torrefied biomass has

hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...

properties, i.e., repels water, and when combined with densification make bulk storage in open air feasible.

Elimination of biological activity

All biological activity is stopped, reducing the risk of fire and stopping biological decomposition like rotting.

Improved grindability

Torrefaction of biomass leads to improved grindability of biomass. This leads to more efficient co-firing in existing

coal-fired power station A coal-fired power station or coal power plant is a thermal power station which burns coal to generate electricity. Worldwide there are about 2,500 coal-fired power stations, on average capable of generating a gigawatt each. They generate ...

s or entrained-flow gasification for the production of chemicals and transportation fuels.

Markets for torrefied biomass

Torrefied biomass has added value for different markets. Biomass in general provides a low-cost, low-risk route to lower CO₂-emissions. When high volumes are needed, torrefaction can make biomass from distant sources price competitive because the denser material is easier to store and transport. Wood powder fuel: * Torrefied wood powder can be ground into a fine powder and when compressed, mimics

liquefied petroleum gas Liquefied petroleum gas, also referred to as liquid petroleum gas (LPG or LP gas), is a fuel gas which contains a flammable mixture of hydrocarbon gases, specifically propane, Butane, ''n''-butane and isobutane. It can also contain some ...

(LPG). Large-scale co-firing in coal-fired power plants: * Torrefied biomass results in lower handling costs; * Torrefied biomass enables higher co-firing rates; * Product can be delivered in a range of LHVs (20–25 GJ/ton) and sizes (briquette, pellet). * Co-firing torrefied biomass with coal leads to reduction in net power plant emissions. Steel production: * Fibrous biomass is very difficult to deploy in furnaces; * To replace injection coal, biomass product needs to have LHV of more than 25 GJ/ton. Residential/decentralized heating: * Relatively high percentage of transport on wheels in the supply chain makes biomass expensive. Increasing volumetric energy density does decrease costs; * Limited storage space increases need for increased volumetric density; * Moisture content important as moisture leads to smoke and smell. Biomass-to-Liquids: * Torrefied biomass results in lower handling costs. * Torrefied biomass serves as a 'clean' feedstock for production of transportation fuels (

Fischer–Tropsch process The Fischer–Tropsch process (FT) is a collection of chemical reactions that converts a mixture of carbon monoxide and hydrogen, known as syngas, into liquid hydrocarbons. These reactions occur in the presence of metal catalysts, typically at te ...

), which saves on production costs. Miscellaneous uses: * Several guitar builders have used torrefaction to obtain more dimensionally stable wood for guitar parts than traditional kiln-drying or air-drying provides, including Yamaha, Martin, Gibson, and luthier Dana Bourgeois.