Torrefaction of oak and olive stones in a semi-industrial multiple hearth furnace: Products yields and composition

Multiple Hearth Furnace (MHF) is a promising technology for biomass torrefaction. However, experimental data from large-scale units are missing to understand process parameters and biomass composition effects on torrefaction yields. In the present study, a deciduous wood (oak) and an agricultural biomass (olive stones) were torrefied in a semi-industrial multiple hearth furnace at 250 and 300 degrees C for a mean residence time of 40 min. Permanent gases and biomass streams entering and exiting the furnace were quantified on-line and analyzed. Steady state regime was reached and main torrefaction yields were replicable along torrefaction tests. Large axial dispersion of particles in this MHF was inferred by residence time distribution measurements (8 < Pe < 16). As expected, for both olive stones and oak, the solid mass loss and the carbon content of the torrefied biomass increase when increasing the process temperature. Moisture content of raw biomass has a determining impact on torrefaction severity. At 250 degrees C, dry mass losses of two oak batches having a moisture content of 12.6 and 27.4 %w are respectively equal to 11 and 4 %w-db. Condensable gases were identified by gas chromatography coupled to mass spectrometry (GC-MS). Most of the species detected are the same as those reported in the literature but some additional ones are present such as polycyclic aromatic hydrocarbons, benzene and toluene due to hot spots near burner flames. Temperature distribution along the MHF is not fully homogeneous and can diverge from 20 to 40 degrees C from the temperature set point.