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

Hary Demey,Sebastien Thiery,Thierry Melkior,Maguelone Grateau,Elie Lacombe,Elvira Rodriguez-Alonso,Nicolas Jaricot,Muriel Marchand,André Chatroux

doi:10.3390/met11121905

Hary Demey, Sebastien Thiery + Show 7 more

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https://doi.org/10.3390/met11121905

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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.

Highlights

The over-exploitation of fossil-based energy worldwide has progressively increased in the last century and has led to an increase in CO2 anthropogenic emissions
There were no significant differences between the properties of almond shells and olive stones after torrefaction at pilot scale at 350 ◦ C
The different torrefaction conditions tested in the laboratory-scale oven have shown that the thermal treatment needs to be quite severe to meet the requirements of the metallurgy industry

Summary

Introduction

The over-exploitation of fossil-based energy worldwide has progressively increased in the last century and has led to an increase in CO2 anthropogenic emissions. EnergyIntensive Industries (EIIS), which include metallurgical industries, are some of the main contributors to this increase in CO2 emissions. The International Energy Agency reported in 2018 that for every ton of steel produced, 1.9 t of CO2 is released into the environment [1]. Environmental solutions are urgently required to mitigate the impact of using fossil fuels. Green energy schemes have been worldwide developed for achieving this goal, which are essentially associated with solar, wind, ocean, geothermal and biomass resources [2]. Special consideration has to be given to biomass-based energy, as it supplies 10% of the primary energy demand [3]

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