Fluidized Bed Combustion of a Lignin-based Slurry

Unconverted lignin is available as residual sludge with water content in the range of 40-60% by mass from 2nd generation bio-ethanol production process. Fluidized bed combustion was studied as a possible method to valorize such a biomass material, upon mixing and homogenizing with rapeseed oil for producing a fuel slurry. Combustion tests were carried out at steady state in a pilot unit with submerged feeding of the fuel slurry. The operation with the slurry resulted reliable, after the adoption of measures for improving the slurry homogeneity and flowability. To this aim, particular care was dedicated to the production and characterization of the slurry, whose rheological properties are typical of a non-Newtonian (pseudo-plastic) fluid. The combustion tests proved that C content in the ash samples at the cyclone was very low and the combustion efficiency was higher than 99.5%. A temperature increase was registered in the freeboard with respect to the bed as consequence of volatile matter combustion, typical of biomass fuels. The normalized NO emission was compliant with the Italian regulation for biogenic fuels. The bed material was enriched in K and Na from the ashes, although no bed agglomeration phenomena occurred.