Abstract:

Heavy bio-oil (HB) is rich in phenols, esters and alcohols, but it faces great challenges in refining to obtain high commodity chemicals and high calorific value fuels due to its complex composition, high viscosity, low calorific value, and poor thermostability. The current catalysts for bio-oil upgrading are expensive and prone to deactivation, thereby requiring types of cheap and reasonable catalysts to valorize heavy bio-oils. Metal oxides (MOs), as mature and cheap catalysts, promotes the generation of more stable products in the catalytic pyrolysis of biomass, but the comparative performances on catalytic pyrolysis characteristics of heavy bio-oils by MOs are hardly investigated before. Therefore, this paper selected four metal oxides (Fe₂O₃, Al₂O₃, CaO and TiO₂) and investigated catalytic effects of four different MOs on pyrolytic characteristics, bio-oil composition, and pyrolysates emissions and distributions, so as to provide a reference for catalysts selection.The above four MOs with 5%(mass) blend ratio were in-situ pyrolyzed with HBs. The catalytic pyrolysis behaviors of HBs were characterized by a thermogravimetric analyzer (TGA), and the pyrogenic product distribution was carried out by a TGA coupled with a Fourier transform infrared spectrometer (FTIR). Additionally, the multi-scale challenges for HB re-utilization were outlooked by a fixed bed reactor for the sake of estimating the bio-oil composition. The distribution characteristics of the pyrolysis products were analyzed by GC/MS. The temperature range in the experiment was set at 20—1000℃ and the heating rate was set at 20℃/min.The results showed that Fe₂O₃ and Al₂O₃ have better catalytic effects at low temperature, but there was a general inhibitory effect above 200℃. Al₂O₃ was the most aggressive in both promoting and inhibiting effects. The four catalytic pyrolysis processes above promoted the deoxidation of heavy bio-oil and CaO performed best. Al₂O₃ showed good activity for reducing the reaction temperature and making the main reactions available below 400℃. Thus they effectively reduced the energy consumption of the refining process. Fe₂O₃ greatly promoted the depolymerization of heavy bio-oil, and the weight of coke decreased by 21.23%. TiO₂ had the most obvious inhibition effect on the formation of CO₂, and can reduce the final temperature of the reaction. At low temperature, Fe₂O₃, Al₂O₃ and TiO₂ can promote the formation of products, but they had different emphasis on different kinds of substances. CaO increased the temperature needed for the reaction and had a strong selectivity for the enrichment of guaiacol. The catalytic effect of CaO and TiO₂ were better at medium temperature. In addition, the catalytic pyrolysis above effectively promoted the enrichment of phenols, especially Al₂O₃ increased the relative content of phenols by 31.10%.Therefore, Al₂O₃ could be used in the low temperature pyrolysis of heavy oil to obtain a strong promotion effect, while CaO or TiO₂ is more suitable at medium temperature. CaO has the best catalytic effect when the goal is to enrich guaiacol or deoxygenate heavy oil. In terms of utilization efficiency, Fe₂O₃ or TiO₂ could be used to obtain a high mass conversion rate. All three metal oxides except Fe₂O₃ reduce the order of biochar. The biochar prepared by adding CaO has the most disordered carbon structure and the highest solid phase yield.

Key words: heavy bio-oil, TG-FTIR, GC/MS, metal oxides, catalyst, pyrolysis

摘要：

通过TG-FTIR、GC/MS和XRD等分析手段，研究了Fe₂O₃、Al₂O₃、CaO和TiO₂四种金属氧化物催化下重质生物油的热解特性及产物差异。结果表明：应用上述四种催化剂的再裂解实验均促进了重质生物油的脱氧，其中CaO催化下脱氧效果最好，Al₂O₃能够有效降低反应温度，Fe₂O₃有效促进了重质生物油成炭前的解聚、固相产物质量降幅达21.23%，TiO₂对CO₂的生成有最明显的抑制效果、同时可以降低反应结束温度；在低温下，除CaO外的三种催化剂均对有效产物的生成有促进作用，但对不同种类的物质各有侧重，而CaO则会使反应所需温度升高且对愈创木酚的富集有很强的选择性；在中温下，CaO和TiO₂表现出较好的催化效果。上述催化热解过程有效促进了酚类的富集，效果最好的是Al₂O₃，酚类相对含量增幅达31.10%。除Fe₂O₃外的三种金属氧化物均降低了生物炭的有序度，添加CaO制备的生物炭具有最无序的炭结构和最高的固相产率。

关键词: 重质生物油, TG-FTIR, GC/MS, 金属氧化物, 催化, 热解