Fine comminution of torrefied wheat straw for energy applications: properties of the powder and energy balances of the production route

Lignocellulosic powders have a chemical composition that makes them explosive under certain conditions. In the form of ultrafine particles, they can be directly used as fuel in combustion devices to generate heat and mechanical power without the need to convert them into a liquid form. However, the production of such powder encounters two main challenges that affect engine efficiency: high energy consumption during grinding and poor flowability of the resulting powder. To tackle these challenges, one option is to modify the chemical and physical characteristics of the biomass before grinding. This can be effectively achieved through thermal pretreatment methods such as torrefaction, which alter the chemical structure of lignocellulosic biomass, making it more brittle, modifying its surface properties, and thereby improving its flowability. In this study, we investigated the impact of two torrefaction temperatures (220 °C and 280 °C) on the chemical and physical properties of wheat straw prior to fine milling. Our results indicate that the physical and chemical properties of the wheat straw were not significantly affected at 220 °C, whereas a significant change was observed at 280 °C. In particular, extensive chemical analysis and atomic force measurements show that the modifications of the properties are related to heat-induced degradation of cell wall polymers and an increase in cellulose crystallinity. For the wheat straw torrefied at 280 °C, both the grindability and flowability of the ground powder improved significantly. Meanwhile, the total energy consumption for the transformation remained constant, making the torrefaction process interesting for designing combustible powders.