Recovery of oils with high caloric value and low contaminant content by pyrolysis of digested and dried sewage sludge containing polymer flocculants

The production of sewage sludge in Korea amounts to 2 million ton/y. It is disposed mainly by ocean dumping. At the London Convention, however, there was a strong demand for this practice to be stopped. Therefore, new treatment methods for sewage sludge should be found. One of the strong candidates for these new methods is the pyrolysis technique, which obtains oil from the organic part of sewage sludge. The pyrolysis oil, however, usually shows bad characteristics, mainly due to its low heating value and high contaminant content. In this study, digested and dried sewage sludge containing polymer flocculants was pyrolyzed in a temperature range of 446-720 °C in a pyrolysis plant equipped with a fluidized bed reactor. The mass balance was established in each experiment, and the produced gas and oil were analyzed with the aid of GCs and a GC-MS system. In the experiments, above 50 wt % pyrolysis oil was obtained with a maximum caloric value of 33 MJ/kg. In addition, the authors tried to reduce contaminants in the pyrolysis oil by using a char separation system composed of a cyclone and a hot filter and calcium oxide as an adsorbent, to capture the HCl liberated during the pyrolysis. The pyrolysis oil was almost free of hazardous metals, and its chlorine content was significantly reduced.