Torrefaction is a mild thermo-chemical treatment method carried out at atmospheric pressure. It was originally developed in the 1820’s to roast or torrefy Coffee and Cacao beans. It occurs in an near oxygenless environment between 250 and 310 degrees Celsuis, where the biomass is partially decomposed giving off various types of volatiles, which are re-utilized to auto-thermally drive the process.
The process is characterized by relatively long reactor residence times, with the final product being a solid uniform product called tor-coal, which can be transformed into tor-briquettes or tor-pellets.
Once torrified, the material will have a higher energy density in order of 22.000 kJ/ KG (or 9.600 BTU/lb). Overall the mass of the feedstock will reduce with 25% and it will retain 90% of the original biomass feed energy content.
Feedstock for torrefaction is not limited to wood and lumber waste, but includes also forrest debris, agriculture-and-industry waste, animal-and-biohousehold waste, seaweed, algea, peat, energy grasses (elephant grass, miscanthus) among many others.
The torrefaction process can in fact produce the optimal coal substitue from a variety of biomass feedstock, as discussed above.