Comparison of inorganic constituents in bottom and fly residues from pelletised wood pyro-gasification

Wood biomass is a source of renewable energy and is now recognised as a potential alternative for sustainable energy production. During the pyro-gasification process, the mineral components of the biomass accumulate under the grate of the pyro-gasifier (bottom residue) or become volatile and are deposited as fly residue in the cyclone filters. This work analyses the constituents of the two residues produced by the pyro-gasification process of pellets in a pilot scale downdraft system. The residues are composed of char and ash. The ash makes up 7% of the bottom residue and 38% of the fly residue. The Ca, Mg, Na, S, P, Si and Cu contents in the two residues are almost comparable. Fe, Al, Cr, Zn, Ni, Pb, Cd, Mn and B are concentrated in the fly residue. Potassium is a particular case and is present in low concentration in the fly residue. The mass balance has revealed that some elements can pass the first filtration phase (cyclones) and they are trapped in the gas which must be successively cleaned. Therefore, knowledge of the composition of the biomass (macro and microelements), and of the behaviour of their transformation products, is important for the plant design and functioning. It is also equally important for the disposal or the utilisation of the solid by-products.