Study Of Bio-oil Catalytic Cracking Based On The Molecular Distillation

The utilization of biomass reliefs the shortage of fossil fuels, as well as the air pollution. Fast pyrolysis can transform solid biomass into liquid fuel bio-oil. However, the crude bio-oil has an inferior fuel property, so upgrading is necessary. Catalytic cracking is an important bio-oil upgrading technology. Because of the complicated composition of bio-oil, the coke yield is high during the catalytic cracking of whole bio-oil. The distilled fraction from bio-oil molecular distillation enriches the reactive acids and ketones in bio-oil, thus it is more suitable for cracking than whole bio-oil. Therefore, in this paper, the study of catalytic cracking of bio-oil distilled fraction and its model compounds is presented.In the study of catalytic cracking, in order to overcome the coking problem caused by the low effective hydrogen to carbon ratio of bio-oil components, alcohols were introduced as the co-cracking reactant. First, cyclopentanone, hydroxypropanone, and acetic acid were selected as the model compounds, and the cracking of individual model compound and the co-cracking of model compound with methanol or ethanol were investigated over HZSM-5 catalyst. It was found that the addition of alcohols could significantly improve the yield of liquid hydrocarbons. Meanwhile, the promotion of liquid hydrocarbon formation by ethanol was much more obvious than that by methanol, indicating that ethanol was more suitable as the co-cracking reactant. It was supposed that the surplus hydrogen generated from alcohol cracking could participate in the deoxygenation of model compounds, which benefited their conversion to hydrocarbons with high H/C ratios and thus increased the liquid hydrocarbon yield and suppressed the coke formation. Subsequently, the influence of mixing ratio of model compound mixture and ethanol was studied, and the composition of 30% model compound mixture and 70% ethanol was found to be the most suitable.On the basis of co-cracking of model compound and ethanol, the co-cracking of bio-oil distilled fraction and ethanol was then researched over HZSM-5 catalyst. It was found that both increasing reaction temperature and pressure benefitted the formation of liquid hydrocarbons. Under the optimum reaction conditions, the yield of oil phases reached above 25.9%, and the contents of hydrocarbons were greater than 98%. However, some C3-C4 gaseous hydrocarbons were also generated, which lowered the selectivity of liquid hydrocarbons. In view of this phenomenon, HZSM-5 catalyst was modified by loading metal oxides, and the catalytic activities of these catalysts were tested. It was found that for the co-cracking of model compound mixture containing acid, ketones and phenols with ethanol over ZnO/HZSM-5 or CuO/HZSM-5, because of the quick deactivation of catalyst, the yield of liquid hydrocarbon was very low. Ga2O3/HZSM-5 catalyst showed the best performance, because the presence of Ga2O3 could promote aromatization and better maintain the activity of catalyst. Then the co-cracking of model compound mixture and ethanol was studied systematically over Ga2O3/HZSM-5. It was found that increasing the loading amount of Ga2O3, raising the reaction temperature and reducing the weight hourly space velocity of reactant could promote the formation of liquid hydrocarbons. Finally, the co-cracking of bio-oil distilled fraction and ethanol over Ga2O3/HZSM-5 was also carried out, and the yield of oil phase was 33.1%, in which the content of hydrocarbon was above 98%, while the yield of coke was as low as 2.2%.Hydrogenation pretreatment is another way to improve catalytic cracking. It is proposed that part of bio-oil distilled fraction can be used for catalytic reforming to provide hydrogen for the hydrogenation pretreatment of other distilled fraction, which then undergoes cracking. As a primary exploration of this technical route, the catalytic reforming of bio-oil model compounds over a novel Ni/nano-Al2O3 catalyst was also studied. Higher reaction temperature and steam to carbon ratio of feedstock, as well as higher catalyst amount and Ni loading, favored the conversion of reactant and increased the hydrogen yield.