Physical–chemical properties and hygroscopicity of Brazilian metallurgical charcoal

Wood is exposed to different atmospheric conditions in the production of charcoal due to the occurrence of rainfall and variation in relative humidity. However, there is a lack of scientific information related to charcoal hygroscopicity and desorption capacity depending on water content. Thus, in the present study, we analyzed the influence of carbonization temperature in brick kilns on the hygroscopic capacity of charcoal from a 7-year-old Eucalyptus sp. wood. Charcoal was produced at final temperatures of 340, 380, 420, and 460 °C. Raman spectroscopy, Fourier-transform infrared absorption spectroscopy, scanning electron microscopy, and surface area measurement were performed to identify structural changes in charcoal. The charcoal samples were exposed to six different saline solutions to simulate the relative humidity of the environment, ranging from 33 to 98%, for the determination of the moisture adsorption capacity. The charcoal surface area values ranged from 7.9 (340 °C) to 12.3 m² g⁻¹ (460 °C). Charcoal porosity increased by 14.4% with increasing temperature. The adsorption capacity decreased with the rising final carbonization temperature. An average reduction of 9.9% between the moisture adsorbed by the charcoal samples produced at 340 °C and 460 °C was observed. The increase in surface area and porosity of charcoal as a function of temperature resulted in the loss of environmental moisture adsorption capacity due to the removal of carboxyl and hydroxyl groups in the temperature range analyzed. Physical mechanisms were more relevant in the water–charcoal relationship, which can directly influence the drying process of the bioreducer in stockyards.