Pyrolysis Behavior of Biologically Dried Sewage Sludge: Change in Nitrogen Form during Heat Treatment

Abstract

Biodrying processes that utilize the biological heat generated through the aerobic digestion of organic matter are believed to reduce the energy consumption for drying significantly, because supplementary fossil fuels are not required for these processes. In this study, the pyrolysis behavior of biologically dried sewage sludge used to obtain solid fuel or material is examined under heat treatment below 600 °C. We focus on the change in nitrogen form during heat treatment. Nitrogen contained in the dried sludge almost exists as Kjeldahl nitrogen, and a considerable amount of nitrogen remains in this form after heat treatment. Approximately 50% of nitrogen in the dried sludge is removed from the solid as volatile matter, including ammonia, when the sludge is heat-treated at 500 °C. The yield of ammonia is significantly larger than that of cyanides under the conditions applied for the heat treatment. It is concluded that a large amount of nitrogen in the volatile matter is present in the tar and is converted to ammonia upon further high-temperature pyrolysis of tar. The catalytic effect of minerals contained in the dried sludge is found to influence the gasification reactivity of the solid residue significantly.