Abstract
► We used hydrous pyrolysis to study alteration products of sewage sludge (SS) OM. ► Two sets of experiments were conducted at T=200–350°C for 48h. ► The first set included only SS and water and the second with oxalic acid (OA). ► The yields and alteration products increased with temperature and presence of OA. ► The results show that the yields are higher under reductive hydrous pyrolysis.
Various pyrolysis (also termed thermolysis) methods are common techniques that have been used to convert sewage sludge organic matter (OM) into useful fuels for energy. Reductive hydrous pyrolysis of OM increases the conversion yields of the sewage sludge. Our purpose is to study the effects of low temperature (<350°C) and experimental conditions in confined hydrous pyrolysis on the chemical compound compositions of the alteration products from municipal sewage sludge. These compositions have been determined by gas chromatography-mass spectrometry. Two sets of experiments were conducted at temperatures ranging from 200 to 350°C with contact periods of 48h. The reaction mixtures of the first set included only sewage sludge and water, whereas the second set also contained oxalic acid to provide excess hydrogen to enhance reduction of the organic matter of sewage sludge. The yields and the number of observed alteration products increased with higher temperature and presence of aqueous oxalic acid in the reaction mixtures. At lower temperatures n-alkanoic acids, n-alkanols and sterols were the major compounds. Hydrocarbon concentrations increased at higher temperatures in both experiment sets, except for UCM (unresolved complex mixture), which decreased >300°C. The n-alkanes increased dramatically with temperature increase both in the presence and absence of oxalic acid, where their yields were high in the former medium. Sterane concentrations also increased at elevated temperatures, being highest in the presence of oxalic acid. The products confirm that alteration of sewage sludge biomass in aqueous medium occurs rapidly and efficiently under reductive pyrolysis conditions, and in the temperature window of 250–350°C. These results show that the product yields are higher by a factor of 1.4 under reductive hydrous pyrolysis versus hydrous pyrolysis conditions.