Characterization of chars from biomass-derived materials: pectin chars

Abstract

The effect of pyrolysis conditions on the yield and composition of char from pectin was studied. Pectin is a component of the carbohydrate fraction of biomass-derived materials. The pyrolysis was done at atmospheric pressure under oxidative and non-oxidative (inert) atmospheres and at temperatures ranging from 150 to 550°C. The volatile product was analyzed by gas chromatography/mass spectrometry (GC/MS). The solid product, i.e. char, was characterized by solid-state 13C nuclear magnetic resonance (NMR) and Fourier-transform infrared (FTIR) spectroscopy. The char was also analyzed for its elemental composition and surface area. The surface morphology of char was studied by scanning electron microscopy (SEM). The results were compared to those from chlorogenic acid (CA), a component of the phenolic fraction of biomass-derived materials. The behavior of pectin was similar to that of CA. For both pectin and CA, the char yield decreased with increasing temperature before leveling-off at ca. 20% of the starting substrate in non-oxidative runs. In oxidative runs, the char yields from both substrates became negligible at 550°C. NMR analysis indicated that the aromatic character of char increased as the pyrolysis temperature increased. The oxygen functionality was progressively lost and the resonance bands corresponding to carbonyl groups mostly disappeared above 350°C. FTIR analysis also suggested the loss of hydroxyl and carbonyl groups from chars at high temperatures. The H/C and O/C ratios of chars decreased continuously with increasing temperature. The oxidative chars showed characteristics essentially similar to those of the non-oxidative chars. The surface area of char was negligible at low temperatures, but increased dramatically to a maximum of 70 m 2 g −1 at 450°C before decreasing at 550°C. SEM analysis indicated that the pyrolysis of pectin occurred via softening and melting of the substrate followed by bubble formation. At high temperatures, surface etching followed by the appearance of crystal deposition on the char surfaces were also observed. The results are consistent with the analysis of the evolved gases