Sewage Sludge Torrefaction in an Auger Reactor

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 degrees 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.