Structural and chemical changes of sludge derived pyrolysis char prepared under different process temperatures

Sewage sludge was pyrolyzed at 400–800 °C to study the influence of the pyrolysis temperature on the bulk and structural properties of the resulting sludge-chars. For this purpose, sludge-chars were subjected to proximate and ultimate analyses and the textural and porous properties were examined by the nitrogen adsorption-desorption isotherm. Raman spectroscopy was employed to correlate the results of the structural changes of the sludge-chars obtained by conventional analysis (textural properties) and together with solid state NMR it was used to characterize the organic part of the sludge-chars. Additionally, an XPS of the chars’ surfaces and inner layers of the char particles was conducted to study the phosphorus, nitrogen and sulfur speciation and its changes with regard to the change in pyrolysis temperature and the depth of the char particles. Raising the pyrolysis temperature increased the relative content of phosphates and metaphosphates and decreased the reduced P species relative content. Sulfur speciation analysis revealed an increase in sulfides content and a decrease in sulfates and sulfites content as the temperature was raised. The influence of the temperature on N speciation was marginal. The NMR and Raman spectroscopy results showed a significant contribution of graphite and (poly)aromatic hydrocarbons and a negligible contribution of the carbonyl groups to the structure of the organic part of sludge-chars. The increase in the pyrolysis temperature increased the content of (poly)aromatic hydrocarbons over the total carbon content, revealing its contribution to the porosity evolution. This is in good correlation with the microporosity and surface area analyses. Regarding the sludge-char potential soil application, the results may help to predict and better understand the sludge-char stability and N and P plant availability.