Insights into the Product Quality and Energy Requirements for Solid Biofuel Production: A Comparison of Hydrothermal Carbonization, Pyrolysis and Torrefaction of Olive Tree Pruning

Abstract

Understanding the energy consumption needs to produce solid biofuels is a key point in the path towards a bio-economy establishment. In this paper, olive tree pruning was treated by both dry (pyrolysis and torrefaction) and wet (hydrothermal carbonization) thermal treatments. Product yield, solid quality and energy consumption were assessed. The solids were characterized by means of chemical and thermogravimetric analysis. For all treatments, coal-like solid products were obtained, with higher heating values of almost 30 MJ/kg in most of the conditions evaluated. Chars obtained from pyrolysis present the greater carbon content (between 76 and 85 wt.%) but also the higher ash content (ranging from 6 to 9 wt.%). The values obtained from torrefaction and HTC are lower, ranging from 60 to 76 wt.% in terms of carbon content. From an energy consumption perspective, torrefaction is suggested to be the most advantageous treatment. The least severe conditions allow to obtain a positive energy balance only with the solid phase considered. Nonetheless, further room for improvement of the processes are at sight as the gas and liquid phases can also be valorised. These results at lab-scale open new avenues for optimization of the thermal treatments proposed at industrial-scale.