Characteristic Time Analysis of Biomass Torrefaction Phenomena - Application to Thermogravimetric Analysis Device

Torrefaction is a promising mild thermal treatment for biomass waste, typically between 473 and 573 K during a few tens of minutes in a default-oxygen atmosphere. The product is a solid, with properties close to coal. Some volatile species are released at the same time as by-products, with an interesting potential as source of "green" chemicals. In this context, the behaviour of the biomass during torrefaction is studied in a thermogravimetric analyzer (TGA). In order to build a robust kinetic model, chemical regime must be ensured. This can be assessed thanks to a characteristic time analysis of physical and chemical processes involved in torrefaction. The results show that chemical regime is generally achieved at particle-scale under classical conditions of torrefaction in TGA device. However, this is not always the case at bed-scale, limitations may appear when bed thickness becomes too high (>4 mm). The proposed methodology can be applied to other torrefaction devices, in order to determine the regime as a function of the experimental conditions.