Pyrolysis of sewage sludge : product analysis, upgrading and utilization

Abstract

Pyrolysis, thermal decomposition, is applied to simultaneously treat and stabilize sewage sludge. Liquid and solid products are generated and be able to utilized for providing energy and valuable chemicals. Thermal decomposition of sewage sludge was carried out under atmospheres N₂ and CO₂ by means of thermogravimetric analysis (TGA). The results show that the thermal decomposition of sewage sludge under N₂ and CO₂ atmospheres are quite similar and can be described by a pseudo bi-component separated state model (PBSM). The decomposition shows two decomposition steps under both N₂ and CO₂ atmospheres. Under CO₂ atmospheres, however, the primary reaction was significantly accelerated whereas the secondary reaction temperature was shifted to a lower temperature. The apparent activation energies for the first reaction corresponded to the main decomposition temperature under N₂ and CO₂ atmospheres at 305°C is 72 kj mol⁻¹, while that of the second decomposition at ca. 400-500°C is found to decrease from 154 to 104 kj mol⁻¹ under CO₂ atmosphere. Typical reaction order is in the range of 1.0-1.5. In the presence of CO₂ the solid yield is slightly reduced while the gas and liquid yields are improved. Furthermore, CO₂ influenced the liquid product by increasing the oxygenated compounds and lessoning the olefins through the insertion of CO₂ to the unsaturated compounds. To improve the pyrolytic liquid to meet the requirement of the conventional diesel fuel, the upgrading via deoxygenation of pyrolytic liquid is required. The result showed that the deoxygenation of pyrolytic liquid model compound, oleic acid (C₁₇H₃₃COOH), over C₁xeZrxO₂ catalyst can be achieved by direct removal of the carboxylic part of oleic acid and generating CO and CO₂ as major product in the gas phase. The upgraded liquid contained mainly C₁₇ hydrocarbons. Moreover, the utilization of pyrolytic solid (sewage sludge char) to provide energy in the form of heat and as a cheap adsorbent for dye removal were also studied.