Eucalyptus and Corymbia clones: effects of harvest age on wood quality for sustainable charcoal production

Abstract The scarcity of information on the combined influence of harvest age and genetic material on the wood quality of Eucalyptus and Corymbia still limits the optimization of charcoal production for the Brazilian steel industry. This study aimed to evaluate the effect of harvest age on the quality of wood and charcoal produced from five genotypes, four Eucalyptus clones and one Corymbia citriodora × C. torelliana hybrid, at five, six, and seven years of age. Physical, anatomical, chemical and energy-related properties of the wood were characterized, in addition to muffle furnace carbonization tests. Increasing harvest age promoted higher basic density (from approximately 440 to 625 kg m -3 ), a relative reduction in sapwood proportion, and lower ash content in both wood (~ 0.2–0.6%) and charcoal (~ 0.7–1.7%), enhancing energy performance and suitability for steelmaking applications. The Corymbia citriodora × C. torelliana hybrid stood out at earlier harvest ages due to its high wood density (> 580 kg m -3 at 5 years, reaching ~ 625 kg m -3 at 7 years) and good charcoal quality, whereas tri-cross Eucalyptus clones met or exceeded the recommended industrial threshold (> 500 kg m -3 ) from six years onward. Wood higher heating value remained stable (19.0–20.0 MJ kg -1 ), while energy density increased from approximately 8,400 to over 12,400 MJ m -3 with age. Charcoal gravimetric yield remained stable (33–35%), and higher heating value ranged from 6360 to 7690 kcal kg⁻¹, with most values above the 7000 kcal kg -1 threshold required for steelmaking. The combination of higher lignin content (25–32%), cellulose crystallinity (up to 79%), and basic density resulted in charcoals with high yield and heating value, while low nitrogen (0.04–0.09%) and absence of sulfur indicated a lower potential for atmospheric pollutant emissions. In terms of productivity, the Corymbia hybrid increased charcoal production from 5.5 to 10.7 t ha -1 year -1 between 5 and 7 years, outperforming Eucalyptus clones by up to 25%. These results demonstrate that the integrated selection of clone and harvest age is decisive for maximizing charcoal yield and quality, contributing to more efficient, competitive, and environmentally sustainable bioenergy production systems in the context of the national steel industry.