Fast torrefaction versus classic torrefaction: comparison of the products on a pilot-scale continuous reactor

Biomass torrefaction –also called mild pyrolysis is a thermochemical treatment driven at low temperature (200-300°C), at atmospheric pressure and in an inert atmosphere, to produce a homogeneous solid. This torrefied solid has a higher energy content, is more hydrophobic, is easier to grind and fluidize than the raw biomass. These modifications of the biomass make it more suitable to the gasification process, particularly in an entrained flow reactor for instance. In most studies, the torrefaction is carried out at low temperature and long residence time. To the authors' knowledge, the solid and volatile products obtained after fast torrefaction i.e. at higher temperature and shorter residence time has not been studied. The aim of this study is to compare the solid and volatile products of fast torrefaction (350°C) to those of classic torrefaction (300°C) of ash tree. These torrefaction experiments were carried out on a continuous vibrating bed reactor called Vibrato. The biomass is electrically heated in five zones placed along the tube. The incondensable species are quantified online by micro GC. The condensable species are recovered separately in a three-step condensation system. The condensable species are then analysed with a GC MS, and 71 molecules are calibrated. Besides, the water content of the condensable species is determined by Karl Fischer volumetric titration. The ultimate analysis, the lower heating value and grinding energy measurement of the torrefied solids are performed. For the same anhydrous weight loss, mass balance are slightly similar as torrefaction if classic or fast. Although the torrefied biomass yield is constant at 70 wt%, the incondensable and condensable species yields vary depending on the temperature and residence time. Additionally, the lower heating value of the torrefied solid slightly varies. Besides, the composition of the condensable species, based on the GC MS quantification, varies significantly. Around 20 molecules are identified and quantified in the torrefaction condensable species. Regardless of the operating conditions, water, acetic acid, 2 propanone,1 hydroxy, glycolaldehyde dimer, methanol remain the major condensable species. The kind of torrefaction has a strong influence on torrefied solid grindability. (Texte intégral)