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

Abstract

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 °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 °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 °C). The fixed carbon value also increased, being higher than 67 %w for torrefaction at 350 °C. The large-scale torrefaction at 350 °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.

Development of a Sustainable Metallurgical Process to Valorize Copper Smelting Wastes with Olive Stones-Based Biochar

“…It was processed in the multiple-hearth furnace facility of CEA-Grenoble in France (so-called CENTORRE furnace, Figure 1). As explained by Demey et al [18], two different agricultural residues were studied for their application as reducing agent in the metallurgical industry. The study concluded that both materials had similar values of fixed carbon content and higher heating value (HHV) after being torrefied at 350 • C. However, torrefied olive stones were selected due to their higher availability and lower cost.…”

“…The ignition of 1 g of dry sample under oxygen atmosphere was performed at a pressure of 30 bar. Benzoic acid (Merck, Darmstadt, Germany) was used for calibrating the heat capacity of the calorimeter [18].…”

A Crucial Step Toward Carbon Neutrality in Pyrometallurgical Reduction of Nickel Slag

“…Since biomass absorbs CO 2 from the atmosphere during their growth, they release the absorbed CO 2 in the reduction process, thus creating a closed loop carbon cycle [15,16]. Several research groups have investigated the use of biomass or biochar as reductant in metal production [10,[17][18][19][20][21][22][23][24][25].…”

“…From an economic perspective, however, incorporating biomass reducing agents in metallurgical industries cannot currently compete with fossil reductants. On the other hand, this economic disadvantage decreases when higher carbon taxes are imposed [17,26,27].…”