Insights into the product quality and energy requirements for solid biofuel production: A comparison of hydrothermal carbonization, pyrolysis and torrefaction of olive tree pruning

For a bio-economy establishment, understanding the energy consumption needs to produce solid biofuels is a key point. Herein, 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 (HHV) of almost 30 MJ kg-1 in most of the conditions evaluated. Chars from pyrolysis presented the greater carbon content (between 76 and 85 wt%) but also the higher ash content (ranging from 6 to 9 wt%). From an energy consumption perspective, torrefaction registered the lowest energy consumption (between 5.85 and 20.76 MJ kg-1 char). The highest energy contents per kilogram of char produced were also reflected in torrefaction samples, with values around 11 MJ kg-1 char. Although the obtained HHVs were greater for pyrolysis chars the higher mass yields obtained in torrefaction makes it more profitable. The least severe conditions allowed to obtain a positive energy balance only with the solid phase considered. Nonetheless, further room for improvement is possible since the gas and liquid phases may also be valorised. (c) 2021 Elsevier Ltd. All rights reserved.

Automated summary

In this study, olive tree pruning was subjected to dry (pyrolysis and torrefaction) and wet (hydrothermal carbonization) thermal treatments to produce solid biofuels. The products were evaluated in terms of yield, quality, and energy consumption. The results showed that chars produced from pyrolysis and torrefaction were coal-like with high heating values, while torrefaction had higher carbon content and energy content per kilogram of char produced, making it more profitable. The study also suggested potential improvements in energy balance by valorising gas and liquid phases in addition to the solid phase.