Effect of pyrolysis temperature on physicochemical properties of biochar produced from cellulose, hemicellulose and lignin

Abstract: Biochar is produced from slow pyrolysis at high temperature in the atmosphere of limited or without oxygen, which is a infusible, stable, highly aromatic, and porous carbon-rich material. It could be widely used in the energy, agriculture, environment, and material industry. In this paper, the biochar, derived from biomass three major components (cellulose, hemicellulose, and lignin), was produced by the tube furnace with programmed temperature controlling. Then, the effect of pyrolysis temperature (250, 350, 450, 550, 650, 750, and 850 ℃) on the properties of biochar was investigated by several instruments, such as elementary analyzer, bomb calorimeter, Fourier transform infrared spectrometry (FTIR), X-ray diffractometer (XRD), Nuclear magnetic resonance spectrometer (13C NMR), Thermogravimetric analyzer (TG), and Scanning electron microscope (SEM). The results indicated that, as the pyrolysis temperature increased from 250 to 850 ℃, the mass yields of cellulose char, hemicellulose char, and lignin char were gradually decreased from 94.23%, 63.06% and 87.14% to 17.01, 20.67%, and 41.40%, respectively, and the energy yields cellulose char, hemicellulose char, and lignin char were gradually decreased from 94.23%, 55.7%, and 77.82% to 58.69%, 12.91%, and 31.09%, respectively. As the pyrolysis temperature increased, the content of C in biochar derived from biomass three major components all increased, while the content of H and O, the value of H/C and O/C all decreased, especially more remarkably in the temperature of 250-450 ℃. The FTIR analysis indicated that as the pyrolysis temperature increased, the absorbance of functional groups (-OH, -CH3, -CH2, C=O, C=C, benzene ring skeleton, C-O, C-H, etc.) on the biochar derived from biomass three major components all decreased. The XRD analysis indicated that as the pyrolysis temperature increased, the intensity of crystalline structure of the cellulose Iα(triclinic) and Iβ(monoclinic) in the cellulose char and hemicellulose char decreased, while the graphite structure of (002) and (101) in cellulose char, hemicelluloses char and lignin char increased. The 13C NMR analysis indicated that as the pyrolysis temperature increased, the content of alkyl-C, O-alkyl-C, and carboxylic-C all decreased in the biochar derived from three major components, while the Aryl-C all increased. As the pyrolysis temperature increased, the weight loss rate at peaks of DTG curves from cellulose char and hemicellulose char gradually decreased, while that of lignin char increased, meanwhile the temperature at peaks of DTG curves shift towards the side of high temperature. As the pyrolysis temperature increased, the shrinking of cellulose char occurred, and the diameter of cellulose char decreased; melt phase with substantial foaming formed in the hemicellulose char, the hemicellulose char became laminar structure material; the pore structure of lignin char became stronger, and spherical metallic crystal formed on the surface of lignin char. This study will provide basic data for the production and application of biochar.