Effect of temperature on characteristics products derived from bamboo chips pyrolysis based on pyrolytic polygeneration

Abstract: Bamboo is one of the most important forestry resources, and a large amount of waste is produced during its utilization, such as bamboo chips and tailing. To improve the recycling of bamboo waste, pyrolysis technology for polygeneration was employed. The experiment was carried out in a fixed bed reactor at 250-950℃, and the effect of temperature on products yields, compositions and characteristics was investigated. Micro-GC (3000, Agilent, USA) and GC-MS (7890A/5975C, Agilent, USA) were used to analyze the compositions of bio-gas and bio-oil, respectively. The evolution of bio-char structure was studied with automatic adsorption equipment (ASAP 2020, Micromeritics, USA) via nitrogen adsorption at 77 K. The specific surface area was calculated from the adsorption isotherms using the Brunauer-Emmett-Teller (BET) equation. The pore size distribution was estimated by the Barrett-Joiner-Halenda (BJH) method from the desorption isotherms. In addition, the fractal theory was applied to characterize the fractal properties of pore structure of bio-char. With the temperature increasing, bio-char yield was decreased and bio-gas yield was increased significantly, while bio-oil yield was not changed much. Change of products yields was mainly due to the three components (hemicellulose, cellulose, and lignin) decomposing at different temperatures, and volatiles secondary cracking at high temperature. Bio-gas was mainly composed of H2, CH4, CO, and CO2. Cellulose and hemicellulose decomposed at lower temperature, which resulted that CO and CO2 were released. After the temperature increased over 450℃, lignin began to decompose, and the content of H2 rose sharply, while the content of CH4 rose slowly. After 750℃, volatiles secondary cracking intensified to release more H2. Liquid oil mainly consists of acetic acid, furfural, furan, ketone, aldehyde, and phenol. At 250℃, hemicellulose decomposed predominantly, which generated acetic acid, 2-furanmethanol, hydroxyacetone, and small molecular organic compounds. When the pyrolysis temperature was increased from 250 to 550℃, cellulose decomposed significantly, which resulted that furfural and pentene compounds appeared. With the lignin decomposed, phenol class materials increased quickly, while indene and naphthalene appeared after 650℃. The N2 absorption-desorption isotherms showed that bio-char pore structure was slits pore at lower temperature in comparison with conical pore at higher temperature. With the temperature increasing, the BET specific surface area and pore volume of bio-char increased significantly first, and then decreased gradually. However, the trend of the mean pore size was reversed. This phenomenon could be explained by that the number of micropores significantly increased with the removal of volatiles in bio-char, and some of them might be blocked as a result of ash melting at high temperature. At 650℃, the BET specific surface area and pore volume reached the maxima (307.65 m2/g and 16.416 mL/g, respectively), while the mean pore size was the minimum (2.11 nm). Besides, micro-pores accounted for about 83%. The pore structure of bio-char had doubled fractal characteristics with the pore surface and pore volume. With the increase of pyrolysis temperature, both the surface fractal dimension and volume fractal dimension firstly increased and decreased later. Surface fractal dimension and volume fractal dimension reached the highest value (2.93 and 2.97, respectively) at 650℃. This phenomenon reflected that the pore structure of bio-char developed gradually and then tended to be uniformity.