Characterization of sewage sludges by primary and secondary pyrolysis

An apparatus formed by the combination of a primary pyrolyzer (Pyroprobe 1000) and a secondary reactor was used to study the thermal decomposition of three different chemical sewage sludges, using temperatures in the secondary reactor between 290 and 650 degrees C and 700 degrees C in the primary pyrolzer. Yields of 12 pyrolysis products were determined (methane, ethylene, ethane, propylene, propane, methanol, acetic acid, acetaldehyde, C4-hydrocarbons, CO, CO2 and water). The pyrolysis of chemical sewage sludge is an alternative to incineration because it presents the advantage of concentrating the heavy metals in the final residue, and of avoiding the formation of toxic organic compounds. This technique allows the liquids formed in primary pyrolysis to be transformed almost completely into light gases that do not present significant environmental problems. Only about 1-2% of liquids remains without cracking. The gases formed are mostly CO, CO2 and water. Hydrocarbons yields are only around 1-2% wt%, on a dry basis of the sludge. The paper presents the characterization of three sewage sludges by pyrolysis using simple equipment that can be useful for studying any type of wastes. (C) 1997 Elsevier Science B.V.