Thermochemical valorization of argan nutshells: Torrefaction and air–steam gasification

Abstract

The upswing of argan’s oil for the cosmetic industry has increased the farming of this plant and originated an unexplored residue of complex treatment, its nutshells. This work deals with the characterization of this feedstock, its pretreatment via torrefaction and its gasification either with or without the pretreatment stage. Torrefaction was carried out in a continuous auger reactor at two temperatures, 220 and 250 °C. Hemicellulose was almost totally removed from argan nutshells after torrefaction. Compared to the raw argan shells, the torrefied solids showed an increased content of fixed carbon, a noticeable reduction in the O/C ratio and a significant increase in the HHV, the higher the torrefaction temperature. Chemical thermodynamic equilibrium calculations were implemented to the gasification stage to calculate the equivalence ratio for auto-thermal operation and to assess the effect of temperature and steam-to-biomass ratio on gas yield and composition. Air-steam gasification was also experimentally tested for the raw and torrefied materials at the operational conditions drawn from the simulation results. The torrefied material gasification yielded four times more char than the raw material, while tar production from the torrefied material was only reduced in the presence of steam (anyway starting from a low value: 0.7 g/m3STP for raw argan shells vs 0.3 g/m3STP for the torrefied material). Nor gas production, neither the H2/CO ratio nor the energy content in syngas were improved by gasifying the torrefied material, so torrefaction does not appear to be a profitable pretreatment stage for the gasification of argan nutshells from the point of view of syngas quality.