In order to study the effect of biomass components on the low temperature (30-400 °C) pyrolysis, the thermogravimetric analysis (TGA) of corn stover (CS) and its three detergent fibers (extracted by Van Soest method) were studied, and the model compounds of cellulose, hemicellulose and lignin were also tested as comparison. The results shows the low temperature pyrolysis index (P) is significantly different (PADL < Plignin < PCS < PNDF-CS < Pxylan < PADF-CS < Pcellulose). Klason-lignin has stronger thermal stability and decomposes more difficult than alkali lignin. The original cross-linked structure and interaction of the three components inhibited volatiles releasing, especially significant below 300 °C. The synergistic effect between cellulose and lignin promoted devolatilization and decreased the initial temperature of cellulose decomposition. At last, the low temperature pyrolysis kinetics parameters (apparent activation energy and pre-exponential factor) of CS and its detegent fibers were calculated via the Coats-Redfern methods.
The pyrolysis of corn stalk (CS) was carried out to investigate the effect of wet torrefaction (WT) pre-treatment and Zn/Ni-HZSM-5 on the production of bio-oil characteristics. The synergy between WT and the loaded metal catalyst was also analyzed. The oxygen content of the CS was reduced from 50.5% to 40.8% with WT, and hemicellulose was almost removed. WT pretreatment also significantly reduced the oxygenated compounds of bio-oil and increased the selectivity of phenols, aromatics, and anhydro-sugars. The addition of catalyst improved the deoxygenation, oligomerization, and aromatization during pyrolysis. The loading of Zn and Ni could optimize the pyrolysis reaction path and increased the relative content of monocyclic aromatics (MAHs) from 3.58% to 9.67% and 6.44% during the pyrolysis of CS-WT, respectively, and bimetallic catalyst further enhanced the relative content of MAHs to 11.1%. The relative content of aromatics below C9 was higher than other groups (14.8%). Thus, the WT pretreatment of raw materials and synergistic effect of catalysts can jointly optimize the biomass pyrolysis reaction.
This study compared the pyrolysis behaviors of corn stalk (CS) and its torrefied biomass after inert torrefaction (IT), oxidative torrefaction (OT), and wet torrefaction (WT), focused on the kinetic parameters and reaction mechanisms. Inert and oxidative torrefaction reduced volatile matter while increasing ash content and fixed carbon. Wet torrefaction reduced both volatile matter and ash content while increasing fixed carbon. Three pretreatment methods decreased oxygen content, increased carbon content, and had a higher heating value. The materials were pyrolyzed in a thermogravimetric analyzer. For CS, the average activation energy (E) values calculated by the Flynn–Wall–Ozawa and Kissinger–Akahira–Sunosen methods were 62.5 and 60.07 kJ/mol. IT and WT showed increased trend, with values of 81.58, 81.48 kJ/mol and 69.75, 67.58 kJ/mol respectively. Conversely, OT decreased with the E values of 57.39 and 56.2 kJ/mol. Pyrolysis was divided into two stages based on various conversion rates (α) using Malek and Coats–Redfern methods. When α was below 0.5, a one-dimensional diffusion mathematical model described the pyrolysis process. When α was beyond 0.5, the pyrolysis of CS conformed to the cylindrical symmetric three-dimensional diffusion mathematical model, while IT, OT, and WT better fit the spherical symmetric three-dimensional diffusion mathematical model. However, the torrefaction atmosphere's impact on the pyrolysis kinetic mechanism was limited, exhibiting no alterations in the diffusion model. Different torrefaction samples demonstrated a degree of homogeneity, considering the lower pretreatment temperatures and the economic feasibility of torrefaction atmospheres in oxidative torrefaction, coupled with the lowest activation energy of oxidative torrefaction products indicating more efficient pyrolysis, oxidative torrefaction was recommended as the torrefaction pretreatment process before pyrolysis engineering.
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