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

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