Abstract:
Pyrolysis is a promising thermochemical technology for the utilization of biomass and other organic solid wastes. As we know, low energy-grade biomass can be converted quickly to fuel gas, bio-oil and char products through pyrolysis, but the mechanism of biomass pyrolysis is still unclear, especially for the chemical bonds breaking and re-bonding process. Char is the matrix of organic groups and the most important intermediates of biomass pyrolysis. Therefore, research on chemical composition and structure evolution of char has a great significance for the explication of biomass pyrolysis mechanism. In this paper, 0.7% ash content bamboo particles were used as raw material to prepare pyrolytic char. Pyrolysis experimental was performed on a bench-scale fixed-bed between 250 to 600 ℃ with a temperature of 50 ℃ increment, samples decomposed at the set temperature for 40 minutes and then cooled to ambient temperature with a flow of 99.999% nitrogen. The elemental composition of the char was determined using a CHNS/O elementary analyzer(Vario Micro cube, Germany), and the chemical structure of char was recorded using diffuse reflectance Fourier transform infrared spectroscopy(DRIFTS) and two-dimensional perturbation-based correlation infrared spectroscopy(2D-PCIS). The ultimate analysis of char showed that the main deoxidization reaction occurred in the temperature range of 105-300 and 350-450 ℃, while the removal of hydrogen occurred at 300-350 and 450-600 ℃. Moreover, the H/C value of char generated at 350 and 600 ℃ were around 0.6 and 0.3, respectively. It indicated that the previous char consisted of a large number of non-condensed aryl structure and the latter char contained numerous fused ring structure. The DRIFTS and 2D-PCIS results showed that H-bond, free OH, saturated hydrocarbon bond, aryl C-C/C-H, C=O, fused structure C-C, ether C-O-C, pyran ring C-O and primary alcohol C-OH groups, etc., were extremely sensitive to pyrolysis temperature, and these structures existed various correlations with each other. In the pyrolysis process, the reaction of H-bond and alkyl-CH groups occurred early before free OH and aromatic C-H groups, the two sets of groups changed the opposite direction with pyrolysis temperature increment. It demonstrated that free OH was derived from the deconstruction of H-bond network, and removal of saturated -CH structure was conductive to the aromatization process of char. The relative intensities of -CH, -CH
2, -CH
3 groups began to decrease at the temperature of 250, 300 and 350 ℃, respectively, which indicated that the reaction(scission of branched chain and pyran ring-opening reaction) of -CH occurred earlier than demethylation. Meanwhile, we found that the relative intensities of primary alcohol C-OH and pyran ring C-O decreased quickly between 250 to 350 ℃, while carbonyl C=O and aliphatic ether C-O-C structure formed at the same temperature. It revealed that the primary alcohol groups may be oxygenated to carbonyl C=O structure, and the cleavage of pyran ring C-O generated large amounts of molecular fragments which contained aliphatic ether groups. The relative intensity of aromatic structure(aryl C-C, C-H) obtained the maximum value at 350-400 ℃, which was attributed to the fracture of aryl ether(β-o-4, α-o-4) and reconstruction and condensation of small aliphatic fragments. When the pyrolysis temperature was above to 400 ℃, the relative intensities of ethers C-O-C and carbonyl C=O structure decreased sharply, while aryl-substituted structure increased rapidly, which indicated that the aryl substitution reaction has become the dominant reaction. As temperature increased to 500 ℃, more and more aryl substitutes condensed to fused structure, the char prepared at 600 ℃ was consist of large amounts of fused ring structure.