Removal of char particles from fast pyrolysis bio-oil by microfiltration

Bio-oil formed by the fast pyrolysis of biomass has tremendous potential as a renewable feedstock to make liquid transportation fuels. However, the presence of char particles in the bio-oil causes problems in storage and end-use. Currently there is no well-established technology to remove char particles less than 10 μm in size. This study focuses on the application of a liquid-phase microfiltration process to remove char particles from bio-oil down to slightly sub-micron levels. Tubular ceramic membranes of nominal pore sizes 0.5 and 0.8 μm were employed to carry out the microfiltration, which was conducted in the cross-flow mode at temperatures ranging from 38 to 45 °C and at three different trans-membrane pressures varying from 1 to 3 bars. Microscopic and ash content analysis of the feed and permeate streams were conducted to determine the efficacy of the process. The results demonstrated the removal of the major quantity of char particles with a significant reduction in overall ash content of the bio-oil. Water extraction and gas chromatography were employed to characterize the chemical composition of the bio-oil, and no significant change was observed due to the microfiltration process. Results of fouling analysis obtained from longer runs of bio-oil through the membranes are also presented in this study. The results clearly showed that the cake formation mechanism of fouling is predominant in this process. A membrane cleaning protocol for the fouled membrane was developed, comprising successive treatments with methanol, sodium hydroxide, and acetic acid.