Experimental investigations of thermochemical phosphorus release behavior from sewage sludge under inert and reducing conditions

In this work, five different sewage sludge (SS) samples were investigated regarding phosphorus release behavior under reducing conditions using a thermogravimetric analyzer coupled with mass spectrometry (TG-MS). Results show that primary pyrolysis representing mainly hydrocarbon fragmentation and tar release is finished at temperatures higher than 550 °C. The phosphorus release starts at temperatures ranging from 969 to 1052 °C depending on the specific SS composition. The analyses reveal that the phosphorus is predominantly released in elemental form. The observed phosphorus release ratios (PRR) range between 81.9 and 98.2% and are correlated positively with the aluminum content and negatively with the iron content of the samples. In order to investigate the influence of the reduction potential of the gas atmosphere versus the reduction potential of the carbon, the original SS, a SS coke and sewage sludge ash (SSA) were treated in forming gas (FG) atmosphere containing 3.75% H2 in N2 or Ar. Results show clearly that hydrogen in the reaction atmosphere does not increase the phosphorus release for SS and SS coke and it can be concluded that the main reduction reaction occurs due to the carbon from the feedstock. However, phosphorus cannot be released from SSA at temperatures up to 1450 °C applying an inert gas atmosphere. In this case, applying a forming gas atmosphere as reducing agent is necessary.