Physicochemical characterisation of torrefied biomass

Torrefaction is a mild pyrolysis treatment under inert atmosphere that has been found to improve the physical and chemical properties of biomass as a fuel. In this study, several wood fuels: willow, eucalyptus, a mixture of hardwoods (oak and birch) and a mixture of softwoods (pine, larch and spruce) were torrefied at 270 and 290 °C with two residence times (30 and 60 min). Physicochemical properties of the torrefied materials, such as grindability, density, hydrophobicity and surface area were investigated. Furthermore, information was also obtained from microscopic and spectrometric studies, namely transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), to gain an insight into any changes in morphology and chemical composition due to the treatments. Temperature plays a major role during torrefaction and choosing an optimum condition is fundamental to attaining a satisfactory energy yield. Results show the lower temperature and shorter residence time was the best treatment to achieve good physical properties with a relatively high energy yield. When treated at these conditions, the softwood mixture had the highest energy (95%), followed by the hardwood mixture (80%), then willow (79%), and finally eucalyptus (75%). Increasing the severity of the torrefaction conditions greatly improved the physical characteristics of the torrefied biomass, in terms of grindability properties and hydrophobicity. While little difference in porosity and surface area were detected by the methods used, the XPS and FTIR studies showed that torrefaction results in a decrease in the ROH groups and an increase in CO groups within the fuels. This results in a decrease in the affinity of the fuels to absorb water, and therefore improves their hydrophobicity. For all the fuels, there was a critical temperature for torrefaction, above which abrupt changes in many properties occurred. These include hydrophobicity, energy yield, and grindability. The critical temperatures were fuel dependent and therefore the results indicate that careful optimisation is required for all fuel types to maximise the benefits of torrefaction whilst maintaining a good energy yield.

► We study solid properties of torrefied willow, eucalyptus, softwood, hardwood. ► Changes in chemical groups upon torrefaction lead to hydrophobicity and grindability. ► Gradual changes in solid properties occur below a critical torrefaction temperature. ► Abrupt changes in solid properties occur above a critical torrefaction temperature. ► Optimisation maximises benefits of torrefaction whilst maintaining good energy yield.