Thermal characterisation of the products of wastewater sludge pyrolysis

The aim of this work was to characterise fundamental properties of the products of wastewater sludge pyrolysis and determine if the pyrolysis process of this material can be energy neutral. Wastewater sludge samples from different origin, including domestic, commercial and industrial sludges, were applied in the study. All samples were pyrolysed at a heating rate of 10 °C/min in a fixed bed reactor. The major gas species of pyrolysis, CO, CO2, CH4, C2H4, C2H6 and H2, were monitored with gas chromatograph. Among the released species, hydrocarbons comprised half of the bio-gas fraction (50%) which suggests high potential for energy recovery through their combustion. Thermal properties of sludge samples were investigated using computer aided thermal analysis technique. The results showed that the energy required to pyrolyse wastewater sludge samples from room temperature to the carbonisation temperature of 550 °C varies according to the source and origin of the wastewater sludge and ranges from 1180 kJ/kg for the domestic to 730 kJ/kg and 708 kJ/kg for the commercial and industrial sludges, respectively. This study confirmed that in case of the commercial and industrial sludge samples, the recoverable calorific value from stoichiometric combustion of the pyrolysed bio-gas is sufficient enough to self-maintain the pyrolysis process. In case of the sample from domestic origin, the recoverable energy from combustion of the bio-gas compounds was lower than the energy required to heat the sample to the temperature of carbonisation. To pyrolyse this sample, excess energy will be required, possibly through combustion of the bio-oil fraction.