Upscaling Severe Torrefaction of Agricultural Residues to Produce Sustainable Reducing Agents for Non-Ferrous Metallurgy

Torrefaction of almond shells and olive stones, which are typically considered agricultural waste in the southern regions of the European Union, was investigated in this work for application as reducing agents in the metallurgical industry. Four different temperatures were tested: 250, 280, 300 and 350 degrees C. The evolution of the solid yields with the temperature was determined with TGA measurements. This showed that the duration of torrefaction should not exceed 45 min. The kinetic profiles were successfully fitted using the pseudo-first-order rate equation (PFORE). Then, torrefaction for 45 min was systematically carried out at every temperature and for each resource in a laboratory-scale batch device. The raw and torrefied biomasses were characterized using proximate, ultimate and calorific analyses. The carbon/oxygen ratio and the heating values were increased as a result of the torrefaction severity (from 20 MJ/kg for both raw biomasses to 30 MJ/kg at 350 degrees C). The highest mass losses were obtained at the highest temperature (67.35 and 65.04 %w for almond shells and olive stones, respectively, at 350 degrees C). The fixed carbon value also increased, being higher than 67 %w for torrefaction at 350 degrees C. The large-scale torrefaction at 350 degrees C (45 min) of these biomasses was carried out in a continuous pilot plant. The solids were characterized as well, and their properties were close to those of the biomasses torrefied in the laboratory-scale batch reactor under the same conditions. This thermal treatment provided biochars with all the required properties to be used as reducing materials in metallurgy.

Automated summary

The torrefaction of almond shells and olive stones showed that treatment time should not exceed 45 minutes, with the highest temperature leading to increased heating values and fixed carbon values, as well as maximum mass loss. Large-scale torrefaction produced biochars with properties similar to those obtained in the laboratory-scale batch reactor, suitable for use as reducing materials in metallurgy.