Novel Hot Vapor Filter Design for Biomass Pyrolysis

Fast pyrolysis is a mature technology for the conversion of solid biomass into a liquid intermediate, fast pyrolysis bio-oil (FPBO). FPBO has so far been used mainly as heating fuel, but the target is to use it for the production of sustainable fuels and chemicals in the future. In the pyrolysis process, inorganic materials (ash) from the biomass are mostly sequestered in char particles, which can be separated with cyclones. Small particles (<10 μm) escape the cyclones and condense with vapors. In bio-oils, these solid materials are unfavorable because they can cause erosion, corrosion, and blockages at injection nozzles in power generation systems and deactivate the catalyst in bio-oil upgrading. Hot vapor filtration using either moving bed or barrier filters has been tested on a small scale for the removal of fine particles from the pyrolysis vapors. The challenges with these filter types have been the increased pressure drop across the filter with time, inefficient solid removal, and loss in organics. A new hot vapor filter combining both a barrier filter and a moving bed filter was constructed and tested to overcome these problems. In this filter system, the filter elements are inserted in a vessel, where hot sand flows through to continuously remove the cake over filter candles. The filter was successfully tested with stem chips, contaminated wood, and forest residue for 6-8 h without any pressure increase. The organic liquid yield decreased in the best case only by 3 wt % using the lowest filtration temperature and shortest residence time. The oil properties were slightly affected by cracking of the sugar fraction, which decreased the oxygen content, microcarbon residue, and carbonyl content but increased the acidity. Only minor improvements in metal removal were seen due to the high detection limits of metal analysis.