Composição química monomérica da lignina da madeira de Eucalyptus spp. para produção de carvão vegetal

Abstract

The purpose of this work was to assess the effect of clones, and planting sites in the concentration of guaiacyl (G) and syringyl (S) type structural units (monomers), as well as the S/G ratio in the lignin. Besides, we estimated the correlation of these characteristics with those found in the wood, charcoal, and thermal degradation profile of wood. We used wood obtained from seven clones of the Eucalyptus genus about six growing years, sampled at four planting sites (farms) in southern Minas Gerais, Brazil. The wood thermogravimetric analysis and the estimate of lignin monomeric composition were carried out by the alkaline oxidation, using nitrobenzene, followed by the highperformance liquid chromatography. The chemical, structural and elemental composition, and the high calorific value of wood were already known, as well as the carbonization yields, and the charcoal characteristics. The experiment was performed in completely randomized design with 12 treatments and four replications. The analysis of variance was carried out for three situations: (i) analysis for planting sites, separately; (ii) 3×2 factorial design analysis, which was three clones × two planting sites; and (iii) 2×3 factorial design analysis, which was two cones × three planting sites. The Pearson correlation was used to assess the association of lignin characteristics with characteristics of wood and charcoal, and weight losses during pyrolysis. The S and G type units, and the S/G ratio in the lignin ranged, respectively, from 1.57 to 2.06 mmol.L -1 , from 0.46 to 0.64 mmol.L -1 , and from 2.85 to 3.78. The S/G ratio was affected by both planting sites and genetic material. The clones showed lesser S/G ratio in the most productive sites. The G type unit was found to show significant correlation with content of N (-0.41) and the C/H ratio (0.58), among wood characteristics; while the S type unit was significantly correlated with the basic density measured at breast height (0.48), wood ash content (-0.53), and the C/H ratio (0.66). Beyond these, the S/G ratio was significantly correlated with the content of N (0.42). Regarding the charcoal characteristics, the G type unit showed significant correlation with the gravimetric yields of the carbonization (0.43) and fixed carbon yield (0.42); the S type unit was significantly correlated with the carbonization yields (0.70) and content of volatile materials (0.42); and the S/G ratio was significantly correlated with the wood ash content (-0.49) and apparent relative density (-0.42). The wood with lower concentration of the G type unit showed greater weight losses from 450 to 600ºC (6.68%) and from 600 to 800ºC (5.16%), resulting in lower residual weight at 1000 o C (4.45%). Samples with greater concentration of the G type unit showed lower weight losses from 315 to 390ºC (48.10%), resulting in the greatest residual weight at 450 o C (22.40). There was a significant and negative correlation between G type units and weight losses from 315ºC to 390ºC, showing the potential of these structures to increase the gravimetric yield in charcoal.