Torrefaction is a thermal pre-treatment technology, which produces a solid biofuel product that has a superior handling, milling and co-firing capabilities compared to other biomass fuels.
The advantages of torrefied biomass are widely recognized: better transportation characteristics and compatible properties to coal such as heating value, grindability, bulk energy density and hydrophobicity. Torrefaction is seen as a breakthrough technology to decrease the handling and storage costs and reduce investment for co-firing application.
Biomass has the right potential to become a powerful energy source at global level. To create highly efficient biomass-to-energy chains, torrefaction of biomass, in combination with densification, is a promising step to overcome the logistic economics in large-scale sustainable energy solutions. It is clear that from forestry waste products to wide scale carbon farming, torrefaction is now seriously taken into consideration for a role in the future energy scenarios.
Due to the compression strength used in the torrefaction process, the pellets do not disintegrate easily during the handling and storage phases, as they have a 1.5 to 2 times impact load. Simultaneously, all biological activity is stopped, reducing the risk of fire and stopping biological decomposition like rotting.
Figure: Variety in fuels suitable for biomass co-firing [IEA Bioenergy Task 32]
Many different factors determine the quality level of combustion. This, can be achieved when burning a certain fuel in a fixed installation, such as heating value, moisture content, ash content, reactivity and particle size. The calorific value of torrefied wood can reach a calorific value close to coal and is very dry (moisture content lower than 5%). It contains less ash than coal (0.7 to 5% db, compared to 10 to 20% db for coal), and has a higher reactivity, largely by the virtue of high amounts of volatile matter (55 – 65% db compared to 10 – 12% db for coal).
Handling and Storage Characteristics
As a result of the whole process, pelletised torrefied material can count on a significantly reduced volume for both handling and storage features. Due to the higher energy density of torrefied pellets, less mass is required for the same energy production, especially if compared to wood pellets.
In this regard, another important factor is the hydrophobic nature of torrefied material, which make the fuel less sensitive for degradation (rotting), self heating and moisture uptake. After torrefaction comes to an end, the adsorption of moisture and water will decrease as a function of degree of torrefaction.
Subsequently, there is a significant achievement in savings for the handling and storage at the power plant, particularly if the weather protected storage is not required.
During torrefaction, the bulk density decreases due to the decrease in mass (moisture and volatiles), while almost maintaining the original volume. Pelletising torrefied biomass makes the product significantly better for long distance transportation, as more energy is transported with the same volume.
In addition, freshly pressed torrefied biomass pellets are less sensitive to degradation than wood pellets and the risk for self heating / self ignition decreases.
Based on the current economics of regular wood pellets trade, the added costs of pelletisation are compensated by the reduction in transportation costs (e.g. from Eastern Europe or North America to Western Europe). Transportation distances are, therefore, an important factor for the design of the torrefaction installation and the business case.
Homogeneous output from mixed biomass
Most types of biomass contain hemicelluosic and cellulosic polymers. For this reason, torrefaction can be performed on any lignocellulosic type of biomass, virtually too. In fact, it is possible in theory to design a torrefaction plant for a wider diversity of feedstock, in order to produce a more homogeneous product.
Torrefied biomass becomes more homogeneous with increased physical and chemical properties. It also allows the sourcing of different types of woody biomass for pelletizing in a single apparatus, improving the economics of pelletization. The chance to use different types of local woody biomass for energy use in a single combustion equipment, considerably improves the fuel availability, the supply reability, and it reduces the cost of the fuel.