Pyrolysis Oils: Characterization, Stability Analysis, and Catalytic Upgrading to Fuels and Chemicals

There is a growing need to develop the processes to produce renewable fuels and chemicals due to the economical, political, and environmental concerns associated with the fossil fuels. One of the most promising methods for a small scale conversion of biomass into liquid fuels is fast pyrolysis. The liquid product obtained from the fast pyrolysis of biomass is called pyrolysis oil or bio-oil. It is a complex mixture of more than 300 compounds resulting from the depolymerization of biomass building blocks, cellulose; hemi-cellulose; and lignin. Bio-oils have low heating value, high moisture content, are acidic, contain solid char particles, are incompatible with existing petroleum based fuels, are thermally unstable, and degrade with time. They cannot be used directly in a diesel or a gasoline internal combustion engine. One of the challenges with the bio-oil is that it is unstable and can phase separate when stored for long. Its viscosity and molecular weight increases with time. It is important to identify the factors responsible for the bio-oil instability and to stabilize the bio-oil. The stability analysis of the bio-oil showed that the high molecular weight lignin oligomers in the bio-oil are mainly responsible for the instability of bio-oil. The viscosity increase in the bio-oil was due to two reasons: increase in the average molecular weight and increase in the concentration of high molecular weight oligomers. Char can be removed from the bio-oil by microfiltration using ceramic membranes with pore sizes less than 1 µm. Removal of char does not affect the bio-oil stability but is desired as char can cause difficulty in further processing of the bio-oil. Nanofiltration and low temperature hydrogenation were found to be the promising techniques to stabilize the bio-oil. Bio-oil must be catalytically converted into fuels and chemicals if it is to be used as a feedstock to make renewable fuels and chemicals. The water soluble fraction of bio-oil (WSBO) was found to contain C2 to C6 oxygenated hydrocarbons with various functionalities. In this study we showed that both hydrogen and alkanes can be produced with high yields from WSBO using aqueous phase processing. Hydrogen was produced by aqueous phase reforming over Pt/Al2O3 catalyst. Alkanes were produced by hydrodeoxygenation over Pt/SiO2-Al2O3. Both of these processes were preceded by a low temperature hydrogenation step over Ru/C catalyst. This step was critical to achieve high yields of hydrogen and alkanes. WSBO was also converted to gasoline-range alcohols and C2 to C6 diols with up to 46% carbon yield by a two-stage hydrogenation process over Ru/C catalyst (125 °C) followed by over Pt/C (250 °C) catalyst. Temperature and pressure can be used to tune the product selectivity. The hydroprocessing of bio-oil was followed by zeolite upgrading to produce C6 to C8 aromatic hydrocarbons and C2 to C4 olefins. Up to 70% carbon yield to aromatics and olefins was achieved from the hydrogenated aqueous fraction of bio-oil. The hydroprocessing steps prior to the zeolite upgrading increases the thermal stability of bio-oil as well as the intrinsic hydrogen content. Increasing the thermal stability of bio-oil results in reduced coke yields in zeolite upgrading, whereas, increasing the intrinsic hydrogen content results in more oxygen being removed from bio-oil as H2O than CO and CO2. This results in higher carbon yields to aromatic hydrocarbon and olefins. Integrating hydroprocessing with zeolite upgrading produces a narrow product spectrum and reduces the hydrogen requirement of the process as compared to processes solely based on hydrotreating. Increasing the yield of petrochemical products from biomass therefore requires hydrogen, thus cost of hydrogen dictates the maximum economic potential of the process