Characterization, hygroscopicity, and NIR modelling of charcoals from logging wastes under sustainable forest management for steelmaking use

Abstract

From the removal of charcoal from the oven to its final use, several steps are involved, exposing the material to various atmospheric conditions, including variations in relative humidity. In the Amazonia, climatic conditions such as high humidity and high temperatures aggravate this scenario, directly influencing the quality of charcoal produced in the region. The water content in charcoal intended for steelmaking varies from 5% in the dry season to 10% in the rainy season, reflecting the influence of climatic conditions on the quality of the bioreductant. This variation affects the energy density and calorific value of the material, as well as the efficiency of steelmaking processes, which is essential for optimizing energy consumption and reducing costs. This study investigates the hygroscopicity of charcoal produced from the residues of three Amazonian wood species: Dinizia excelsa Ducke (angelim-vermelho), Inga spp. (inga), and Parkia pendula (faveira), carbonized at two final temperatures, 400 °C and 800 °C. The samples were conditioned in a climatic chamber under six controlled levels of relative humidity, ranging from 40% to 90% at 30 °C. The analyses performed on the charcoal included bulk density, proximate and ultimate chemical composition, higher heating value (HHV), surface area (BET), porosity, scanning electron microscopy (SEM), FTIR, thermogravimetric analysis (TGA), and equilibrium moisture determination. Near-infrared (NIR) spectral signatures were collected using the portable MicroNIR spectrometer, and multivariate chemometric models were developed to classify the samples based on wood species and moisture conditions. The results showed that gravimetric yields were highest for Dinizia excelsa carbonized at 400 °C (average of 37%) and lowest for Parkia pendula carbonized at 800 °C (average of 33%). The higher heating value increased with carbonization temperature, reaching 33.86 MJ/kg for Dinizia excelsa at 800 °C. Ultimate analysis revealed an increase in carbon content with temperature, up to 93.52% for Dinizia excelsa at 800 °C. Equilibrium moisture varied with pyrolysis temperature and species: charcoal carbonized at 400 °C showed lower hygroscopicity (average of 5.64% for Dinizia excelsa). C charcoal at 800 °C absorbed more moisture, with maximum values observed in Parkia pendula (20.60%). This increase in moisture content is likely related to structural changes in porosity caused by high carbonization temperatures. Principal component analysis (PCA) indicated spectral variation associated with these factors, and partial least squares discriminant analysis (PLS-DA) models achieved high classification accuracy for the species studied (above 89%) and moisture conditions (above 94%). Cross-validation confirmed the robustness of the model, with over 97% correct classification for relative humidity levels. This study demonstrates the feasibility of using portable NIR spectroscopy combined with chemometric modeling for rapid and non-destructive assessment of charcoal moisture content. Understanding the hygroscopic behavior of Amazonian charcoal contributes to the improvement of production practices and the more efficient use of these materials in the pig iron and steel production chain, with industrial relevance especially in regions with high relative humidity and ambient temperature.