Torrefaction Technology
Torrefaction is the thermochemical treatment of biomass at 200 to 300 °C, carried out under atmospheric conditions and in the absence of oxygen. During the process the biomass partly decomposes, giving off various types of volatiles. The final product is the remaining solid, which is often referred to as torrefied biomass, or torrefied wood when produced from woody biomass.
Typically, 70% of the mass is retained as a solid product, containing approximately 90% of the initial energy content. The remaining 30% of the mass is converted into torrefaction gases, but contains only approximately 10% of the energy content of the biomass. Hence a considerable energy densification can be achieved, typically by a factor of 1.3 on mass basis. This example points out one of the fundamental advantages of the process, which is the high transition of the chemical energy from the feedstock to the torrefied product, while concurrently the fuel properties are improved.
In the 1930’s, the principles of torrefaction were first reported in relation to woody biomass. Research in France proved this application to produce a gasifier fuel. Since then, the process received only attention again when it was discovered that torrefied wood could be used as a reducing agent in metallurgic applications. This led to a demonstration plant, which was operated during the 1980's, but was dismantled again in the beginning of the 1990's. During the last five years, torrefaction has received new attention - now as pretreatment technology to upgrade biomass for energy production chains (co-combustion and gasification).
The key property that makes torrefied biomass attractive for co-firing in existing coal-fired power stations is its superior grindability compared to untreated or fresh biomass. After torrefaction biomass has lost its tenacious nature and partly its fibrous structure. Through torrefaction, biomass becomes more alike coal and so its size reduction characteristics. Torrefied biomass is more brittle of nature compared the biomass it was derived from. This is crucial for establishing the desired grindability, allowing torrefied biomass to run simultaneously with coal in existing coal-fired facilities.
Torrefaction can potentially be applied to a wide variety of biomass (softwood, hardwood, herbaceous, wastes) so that the range of biomass feedstock for biopellets can be enlarged seriously. Sea 2 Sky plans to obtain and apply a torrefaction technology to increase the energy output in biomass products and to provide a coal-like product with significant environmental advantages.
Torrefaction is a scientifically proven method for improving the properties of biomass as a fuel. The torrefied biomass has also proven to have hydrophobic (resistant to or avoiding wetting ) properties which are welcome during storage. From the pelletisation viewpoint, the implementation of torrefaction within the pelletisation process offers theoretical solutions to the problems encountered with the durability and biological degradation of biopellets.