Production of Biocoal and Biochar - Torrefaction and Carbonization
Torrefaction and Carbonization
Torrefaction is a thermo-chemical treatment in which biomass is heated to 200 – 300°C in absence of air at atmospheric pressure. Torrefaction is a pre-treatment technology that transforms biomass into a coal-like substance in terms of heat content, grindability, and similar properties.
The torrefaction process itself has been known for centuries from coffee roasting. Much as in coffee roasting, the torrefaction process transforms the chemical and physical properties of green fibre causing it to change structure, colour, and density. What used to be a hard bean – or wood chip – ends up as a bridle piece of char-coal that can be easily ground into a fine powder.
As a consequence to the transformation the amount of electrical energy required for size reduction is cut in half compared to untreated wood fibre: rather than high-powered hammer-mills as used in pellet plants energy efficient grinders can be employed to pulverize bio-coal.
Carbonization
is – just like torrefaction - a thermal processes decomposing biomass in the absence of oxygen. The two processes can be distinguished
by the temperature they are occurring at: torrefaction happens at temperatures below, carbonization above 350°C. Below this threshold
a larger amount of volatiles and all of the feedstock fixed carbon remain in the final product.
Carbonization typically removes a larger share – above 85% -of hydrocarbons (hemicellulosis) present in wood, leaving only fixed carbon. Moreover, a part of the fixed carbon is either devolatized as well or oxidized if steam rather than an inert gas is used for heat transfer. The energy remaining in the final product (biochar) is typically lower than in biocoal and requires more energy. Gases generated during the carbonization process need to be flared or used for other processes.