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 much better fuel quality for combustion and gasification applications. Torrefaction leads to a dry product with no biological activity like rotting. Torrefaction combined with densification leads to a very energy-dense fuel carrier of 20 to 21 GJ/ton lower heating value (LHV). Torrefaction makes the material undergo Maillard reactions.
Biomass can be an important energy source. However, nature provides a large diversity of biomass with varying characteristics. To create highly efficient biomass-to-energy chains, torrefaction of biomass in combination with densification (pelletisation or briquetting) is a promising step to overcome logistic economics in large-scale sustainable energy solutions, i.e. make it easier to transport and store it. Pellets or briquets are lighter, drier and stable in storage as opposed to the biomass they are made of.
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 (cellulose, hemicellulose and lignin) 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 (dry bone 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 from which it is made.
The history of torrefaction goes back to the beginning of the 19th century, and it was also 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.
Higher energy density:
- An energy density of 18–20 GJ/m³ — compared to the 26 to 33 gigajoules per tonne heat content of natural anthracite coal — 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 allows the material to be more easily pelletized or briquetted.
More homogeneous composition:
- Torrefied biomass can be produced from a wide variety of raw biomass feedstocks while yielding similar product properties. Most woody and herbaceous biomass consists of three main polymeric structures: cellulose, hemicellulose and lignin. Together these are called lignocellulose. Torrefaction primarily drives moisture and oxygen-rich and hydrogen-rich functional groups from these structures, resulting in similar char-like structures in all three cases. Therefore, most biomass fuels, regardless of origin, produce torrefied products with similar properties with the exception of the ash properties, which largely reflect the original fuel ash content and composition.
- Torrefied biomass has hydrophobic 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.
- Torrefaction of biomass leads to improved grindability of biomass. This leads to more efficient co-firing in existing coal-fired power stations 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 of denser material 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 (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.
- 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.
- 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.
- Torrefied biomass results in lower handling costs;
- Torrefied biomass serves as a ‘clean’ feedstock for production of transportation fuels (Fischer–Tropsch process), which saves considerably on production costs of such fuels.
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