Torrefaction of biomass, e.g., wood or grain, is a mild form of pyrolysis at temperatures typically between 200 and 320 °C. Torrefaction changes biomass properties to provide a better fuel quality for combustion and gasification applications. Torrefaction produces a relatively dry product, which reduces or eliminates its potential for organic decomposition. Torrefaction combined with densification creates an energy-dense fuel carrier of 20 to 21 GJ/ton lower heating value. Torrefaction makes the material undergo Maillard reactions. 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, is a promising step towards overcoming the logistical challenges in developing large-scale sustainable energy 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 200 to 320 °C. It is carried out under atmospheric pressure and in the absence of oxygen, i.e. with no air. During the torrefaction process, the water contained in the biomass as well as superfluous volatiles are released, and the biopolymers 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 and 10% of its heating value, with no appreciable change in volume. This energy 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.
Torrefied and densified biomass has several advantages in different markets, which makes it a competitive option compared to conventional biomass wood pellets. Higher energy density: More homogeneous composition: Hydrophobic behavior: Elimination of biological activity: Improved grindability:
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.
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 and sizes.
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;
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, 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 Martin, Gibson, and luthier Dana Bourgeois.
