In the torrefaction process, biomass is heated to a temperature of approx. 250-350°C in a atmosphere with low oxygen concentrations. As a consequence, all moisture is removed as well as a fraction of the volatile matter of the dry biomass.
During the process, the viscous fibre structure of the original biomass material is largely dismantled through the breakdown of hemicellulose. In order to make the material brittle and easy to grind, it is essential to obtain an inferior degree of cellulose molecules. The material then passes from being hydrophilic to becoming hydrophobic. With the removal of the light volatile fraction, that contains most of the oxygen in the biomass, the heating value of the remaining material gradually increases from 19 MJ/kg to 21 or 23 MJ/kg for torrefied wood.
Ideally, the energy contained in the released volatiles is equal to the heating requirements of the process. In this way, a high thermal efficiency is achieved. Due to the substantial weight loss and a relatively smaller loss of calorific content, the heating value of processed biomass per mass unit increases significantly during the process.
Although there are some variations in the conditions applied in the process for the various reactor concepts, the basic concept for torrefaction and densification processes is the same and commonly incorporates heat integration.
Figure: Overview of torrefaction process with heat integration [IEA Bioenergy Task 32]
In a properly designed and operated torrefaction system, the energy contained in the torrefaction gases may be sufficient to sustain both the drying process and the torrefaction process itself. However, this strongly depends on the moisture content of the incoming biomass (latent heat requirement), and the required degree of torrefaction (the degree of mass loss and the availability of combustible volatiles). It is therefore important to dry the biomass before it enters the torrefaction reactor.
Ultimately, the net efficiency of an integrated torrefaction process is approx. 70 – 98%, depending on the reactor technology, concept for heat integration and the biomass type.
For further information about torrefaction, have a closer look to the lastest IBTC publication.