Definition and Properties
Torrefied pellets are created through a process called torrefaction. The biomass is roasted, ground and afterwards pelletized.
One of the main reasons for torrefaction is the increase of the energy content. Further advantages of torrefied pellets are:
- Improved combustion properties (energy content, heating value)
- Increased grindability (requires less energy during processing)
- Increased resistance to water absorption (outdoor storage is possible)
- Reduced biological activity (avoid problems with decomposition of biomass during storage)
- Increased energy density (reduces transport costs
Hence the benefits of torrefaction may outweigh the additional costs for the torrefaction process in many cases.
The common raw material for torrefied pellets is woody biomass, but every raw material which can be pelletized can also be torrefied before or after pelletizing. It is also possible to use low quality feedstocks, like bark. However the use of alternative raw materials, for example hay or straw, can cause operational difficulties.
Straw has, for example, a low ash melting point. Pelletizing has no positive effect on this combustion property. Therefore, if low-quality raw materials are pelletized, combustion properties are not necessarily improved. How far properties of low-quality raw materials can be improved by torrefaction is still under research.
There is a draft European standard (DIN EN ISO 17225-8:2016: Solid biofuels – Fuel specifications and classes – Part 8: Graded thermally treated and densified biomass fuels) which defines the application and specification of torrefied pellets.
Torrefaction can be applied to raw materials, but also to finished pellets. Research is being carried out on various technologies and raw materials. It is a thermochemical process, typically occurring at 200-350 °C in the absence of oxygen, at atmospheric pressure with low particle heating rates and a reaction time of one hour. Because of the high temperature, during the torrefaction process, the components of the biomass decompose. Low-energy components of the biomass are transferred to the gas phase. First the water is expelled, then hemicellulose decomposes, and partly also lignin. The process is creating torrefied biomass or char, also referred to as 'biocoal'.
At present torrefied pellets are produced mainly for large scale co-firing in coal power plants, they can be used in every plant which normally burns pellets or coal. Because of the roasting, the grindability of the biomass is increased and the grounded pellets can be co-fired with coal. Therefore and because of the reduced costs for transportation, torrefied pellets can become a mass product for industrial application. Nevertheless the commercialization of torrefaction technologies is more difficult than expected. Torrefied pellets and their properties and advantages are still under research. The currently known advantages of the torrefied pellets are mainly of interest for the industry, not for the end costumer. For this reason, the end costumer usually buys conventional pellets.