Research Of High-value Utilization Of Pyrolyzed Rice Husk

Fast pyrolysis is one of the most promising technologies to utilize biomass. However, poor economy greatly limits its further development and industrialization. On the other hand, large amounts of bio-char will be produced as a by-product in the biomass pyrolysis. This bio-char contains high carbon content, and can be effectively used by processing and modifying, so that the economy of biomass pyrolysis will be improved. As a raw material to pyrolyze, rice husk can be massively obtained from rice milling factory, and thus it has an enormous advantage in the collecting cost and convenience. Due to high SiO2 content, the yield of rice husk char in pyrolysis is usually above 30%. However, currently few researches have been carried out on the high value-added utilization of rice husk char. Based on this background, solid acid catalyst and activated carbon were prepared from rice husk char in this thesis.(1) Preparation of sulfonated carbon-based solid acid catalyst and its applicationSulfonated carbon-based solid acid catalyst was prepared by sulfonating rice husk char with concentrated sulfuric acid. The effects of sulfonation temperature and time on the solid acid catalyst were investigated. The catalyst as-prepared was characterized, and its catalytic performance and stability was investigated with the esterification of oleic acid and methanol as the probe reaction. Besides, its catalytic activity for the transesterification was studied, and it was used to catalyze waste cooking oil to produce bio-diesel. The results showed that the preparation of solid acid catalyst from rice husk char was simple, timesaving and energy efficient, and the best sulfonation temperature and time were 90℃ and 0.5h, respectively. The catalyst prepared under these conditions was amorphous carbon structure with less developed porosity. The sulfuric acid and phenolic hydroxyl groups were 0.9 mmol/g and 1.6 mmol/g, respectively. The as-prepared catalyst had an excellent thermal stability, and the sulfuric acid group was decomposed at above 180℃. The as-prepared catalyst exhibited a high catalytic performance. The conversion rate of oleic acid was above 98% under the optimal reaction conditions with catalyst loading of 5%, methanol to oleic acid molar ratio of 4:1, reaction temperature and time of 110℃ and 2 h. Moreover, the as-prepared catalyst showed a good catalytic activity for the transesterification of triolein and methanol, and the yield of methyl oleate after reaction 15 h was around 80%. In the preparation of bio-diesel from waste cooking oil, the as-prepared catalyst could simultaneously and efficiently catalyze the esterification of free fatty acid and the transesterification of triglyceride in waste cooking oil, and it showed a higher activity than the typical solid acid catalyst Amberlyst-15. In the presence of the as-prepared catalyst, the free fatty acid conversion reached above 98% after 3 h, and the fatty acid methyl ester yield reached around 90%. The as-prepared catalyst showed good stability in the esterification of oleic acid and preparation of bio-diesel from waste cooking oil, and in the latter process, after 5 cycles of successive reuse, the free fatty acid conversion and fatty acid methyl ester yield were still 95.75% and 80.20%, respectively. The as-prepared catalyst showed a higher catalytic activity for esterification than that for transesterification, thereby having potential use as a heterogeneous catalyst for biodiesel production from materials with a high free fatty acid content.(2) Preparation of activated carbon by leaching out ash content after CO2 activation and its applicationThe effects of re-carbonization on the preparation of activated carbon from rice husk char were investigated, and activated carbon was prepared by leaching out ash content after CO2 activation. The effects of KOH or K2CO3 solution concentration and leaching time on the textural properties and yield of activated carbon were investigated. The porosity developing mechanism in the leaching process was studied according to the ash content and component of activated carbon. Besides, the activated carbons prepared under the best conditions was characterized, and their iodine number was measured. The as-prepared activated carbon was used a sorbent, and its adsorption characteristics for methylene blue and CO2 were investigated. The results showed that re-carbonization had little influence on the preparation of activated carbon. The best concentration for KOH and K2CO3 solution was 1 mol/L and 4 mol/L, respectively, and the optimal leaching time was 1 h. The activated carbons obtained by KOH and K2CO3 under the best leaching conditions had the same physicochemical and textural properties, and their specific surface areas and mesopore percentages were around 1100 m2/g and 60%, respectively, pore size distributions were mainly in the range of 1.5～15 nm, and iodine numbers were above 1100 mg/g. The leaching process could effectively removed the ash in the sample, and the silica mass was decreased in the best leaching process by more than 99%. In addition, the mass reduction ration and the ash mass reduction ratio in the leaching process were almost identical, which implied the porosity developing mechanism in the leaching process was attributed to the reaction of the leaching reagents with silica. The preparation of activated carbon from rice husk char by leaching out ash content with KOH and K2CO3 after CO2 activation is an efficient method, and compared with physical and chemical activation, this method has advantages of relatively weak corrosion and high porosity of the prepared activated carbon. The adsorption equilibrium time of methylene blue for the as-prepared activated carbon was 20 h. The equilibrium adsorption capacity was increased with the increase of the initial concentration of the methylene blue solution, and temperature has almost no influence on the equilibrium adsorption capacity when the initial concentration of the methylene blue solution was in the range of 200-300 mg/L, while the equilibrium adsorption capacity was increased with the increase of temperature when the initial concentration of the methylene blue solution was in the range of 400～500 mg/L. The adsorption of methylene blue for the as-prepared activated carbon was an entropic-adding process, and could be best described by the pseudo-second order model. The equilibrium data were best represented by the Langmuir isotherm model, and the maximum monolayer adsorption capacity at 30℃ was 404.86 mg/g, which implied that micropores were also involved in the adsorption of methylene blue. The as-prepared activated carbon had a high CO2 adsorption ability, and the CO2 adsorption capacity was decreased with the increase of temperature, and increased with the increase of the CO2 concentration. There was a good linear relationship between its CO2 adsorption capacity and the CO2 concentration. The as-prepared activated carbon had an excellent CO2-over-N2 selectivity and fast CO2 adsorption and desorption rates. In addition, the as-prepared activated carbon had a good stability, and its regeneration was simple.