Oxidative torrefaction of biomass nutshells: Evaluations of energy efficiency as well as biochar transportation and storage

Oxidative torrefaction of three biomass nutshells (walnut shell, xanthoceras sorbifolia shell, and sapindus mukorossi shell) were analyzed in the present study where the influences of torrefaction temperature (250 and 300 °C), torrefaction duration (10–30 min), and oxygen concentration (0–21%) on torrefaction performance were taken into consideration. The results suggested that the oxidatively torrefied nutshells also exhibited strongly linear distribution in the van Krevelen diagram, and the carbon enrichment was a feasible index to describe weight loss (torrefaction severity) and the enhancement factor of biochar calorific value. To evaluate the industrial potential of oxidative torrefaction for biochar production, the energy efficiency and energy-mass co-benefit index were also examined. The analysis indicated that torrefaction with a higher oxygen concentration accompanied by a shorter duration rendered a higher energy efficiency, and vice versa. However, from biochar delivery and storage points of view, an opposite trend was observed, especially at the torrefaction temperature of 250 °C, but the influence of oxygen concentration was relatively small. Overall, it appeared that the biomass oxidatively torrefied for 20 min along with using air as the carrier gas could reach the balance of energy efficiency and fuel delivery.