Article pubs.acs.org/EF
Sewage Sludge Torrefaction in an Auger Reactor María Atienza-Martínez,* José Francisco Mastral, Javier Á brego, Jesús Ceamanos, and Gloria Gea Thermochemical Processes Group (GPT), Aragón Institute for Engineering Research (I3A), Universidad de Zaragoza, Edificio I+D, C/Mariano Esquillor s/n, 50018 Zaragoza, Spain S Supporting Information *
ABSTRACT: A lab-scale auger reactor was used for the study of dry sewage sludge torrefaction. The influence of the torrefaction temperature (between 250 and 300 °C) and the solid residence time (between 13 and 35 min) on the product distribution and properties was investigated. The results have shown that both parameters affect dry sewage sludge torrefaction products to a similar extent within the ranges of study. The yield of torrefied sewage sludge decreases when increasing the torrefaction temperature and the solid residence time, while the yields of liquid and noncondensable gases show the opposite trend. Carbon dioxide and hydrogen sulfide are the major noncondensable products. The yield of water is higher than the initial moisture content of sewage sludge. Organic compounds are also released during torrefaction, especially under severe conditions. Torrefaction liquid separates into an organic phase and an aqueous phase. The former is rich in oxygen-containing aliphatic compounds and steroids and their derivatives. The latter is rich in oxygen-containing aliphatic compounds and oxygen- and nitrogen-containing aliphatic compounds. Torrefaction pretreatment eases sewage sludge grindability and improves some of its fuel properties. O/C and H/C molar ratios of the torrefied solid are lower than those of the dry sewage sludge, while the higher heating value (daf) is higher. The energy density is higher under specific torrefaction conditions. 300 °C, a few of these compounds from the cracking of proteins can also be present in the liquid product.16 Polysaccharides in sewage sludge may be present in bacterial cell walls and extracellular polymeric substances, and also in nondegraded plant and paper wastes present in the wastewater.17 The thermal degradation of polysaccharides occurs through a large number of reactions, such as rearrangement, dehydration, decarboxylation, and fragmentation, which take place simultaneously and consecutively.18 Triglycerides, free fatty acids, and steroids are the main lipids present in sewage sludge.19 One of the first steps in the thermal cracking of triglycerides is the degradation into free fatty acids.20 The thermal decomposition of triglycerides could also originate alkanes, alkenes, and aromatic compounds.21 Saturated fatty acids first undergo decarboxylation reactions during the thermal treatment and after that cracking products, such as alkanes and alkenes, are formed.22 Both deoxygenation (through decarboxylation and decarbonylation) and cracking of allylic C−C bonds (with lower bond dissociation energy than C−H and C−O bonds) take place simultaneously during the thermal decomposition of unsaturated fatty acids. The thermal degradation of unsaturated fatty acids is affected by the position of the double C−C bond. At low degradation temperatures (
