Chemicals by Biomass Pyrolysis in a Fluidized Bed

Abstract

Dried, digested sewage sludge and sludge from a paper mill have been pyrolyzed in an indirectly heated fluidized bed reactor at temperatures ranging from 500 °C to 800°C. A quantity of up to 3 kg of sewage sludge per hour has been converted into pyrolysis gas, oil, product water and carbonaceous residues in a continuously working pilot plant. The pyrolysis gas has a calorific value of approximately 30 MJ/kg for sewage sludge and 18 MJ/kg for cellulose sludge from a paper mill. The main constituents of the pyrolysis gas are hydrogen, methane, ethane, ethene and propene as well as carbon monoxide and carbon dioxide. The amount of the oil fraction obtained is up to 10 % (on dried organic input) and contains more than 98 % aromatic compounds mainly benzene, toluene, styrene, and naphthalene. The residue consists mainly of carbon black and increases up to 35 wt% at low pyrolysis temperatures. The heavy metals from sewage sludge are to a great extent enriched in the solid residue on account of the reducing conditions of the pyrolysis process. Elution experiments with water show that heavy metals are more strongly incorporated in the matrix of pyrolysis residue than in incineration ash or the sewage sludge itself.