Physicochemical properties of product liquid from pyrolysis of sewage sludge

Pyrolysis of sewage sludge to produce liquid for fuel use is under study as a promising way to derive value from this waste. The physicochemical properties of such a liquid were studied in this work. The total liquid and organic yields were 49 and 31 wt % of dry ash free feedstock, respectively. The gas and char yields were 32 and 20 wt % of dry ash free feedstock. The liquid was nonhomogenous, dark, bad-smelling, and sticky. By centrifugation, three phases were obtained: 11 v % top, 52 v % middle, and 37 v % bottom phase. The top phase did not contain water, had a high heating value (LHV ) 41.1 MJ kg^-1) and the lowest nitrogen and sulfur contents (2.4 and 0.2 wt %) of the three phases. Some nitrogen compounds were water-soluble and could be washed out with water, obtaining a 12% reduction in the nitrogen content. The phase contained mainly alkanes and alkenes (C₁₂-C₂₄), triglycerides, and steroids (C₂₇-C₂₉). It showed miscibility with diesel in the proportion 1:10, indicating good cofiring properties with diesel. The middle phase had the highest water content (34 wt %), and the lowest heating value (LHV ) 15.5 MJ kg-1). It contained mainly water-soluble compounds such as amino-sugars, acids (C₃-C₉), phenols, nitrogen-containing compounds (some of them with thiol groups), and levoglucosan. The bottom phase had low water content (12 wt %) and high heating value (LHV = 30.6 MJ kg ^-1). Most of it (57 wt %) was water-insoluble material. It contained phenols, fatty alcohols, nitrogen-containing compounds (some of them with thiol groups), fatty acids (C12-C18), triglycerides, and amino-sugars. In all of the three phases the amount of GC-eluted compounds was low. The high nitrogen and sulfur contents of the middle and bottom phases make their use as a fuel oil difficult without any secondary treatment. These phases could probably be used as fuel for a lime kiln.