Effects of red mud on emission control of NO_x precursors during sludge pyrolysis: A protein model compound study

The nitrogen-containing gases pyrolyzed from sewage sludge can be converted into NO_x compounds, which would cause severe environmental pollution. This study developed a new strategy to reduce the emission of NO_x precursors such as ammonia (NH₃) and hydrogen cyanide (HCN) using red mud. The highest reduction efficiencies (15.10% for NH₃ and 24.72% for HCN) were achieved at 900 °C while compared with those pyrolyzed from raw sludge without the addition of red mud. The transformation and distribution of nitrogenous compounds in three-phase pyrolysates were studied at 400-800 °C for pyrolysis process of a model soybean protein compound. The nitrogenous compounds, i.e., amine-N, heterocyclic-N, and nitrile-N, were identified as the three main intermediates related with the production of NO_x precursors. Ferric oxide (Fe₂O₃) and calcium oxide (CaO) presented in red mud were identified as the driving force which facilitated nitrogen stabilization in char (e.g., at 800 °C, 21.63% increase of char-N after addition of Fe₂O₃, and 41.54% increase of char-N after addition of CaO). These metal oxides possibly reacted with protein-N to form Fe_xN and CaC_xN_y, inhibited the secondary cracking of amine-N compounds in tar (e.g., at 800 °C, 2.33% increase of amine-N after addition of Fe₂O₃, and 0.38% increase of amine-N after addition of CaO), and reduced the production of nitrile-N (e.g., at 800 °C, 30.41% reduction of nitrile-N after addition of Fe₂O₃, and 27.40% reduction of nitrile-N after addition of CaO) and heterocyclic-N compounds (e.g., at 800 °C, 21.60% reduction of heterocyclic-N after addition of Fe₂O₃, and 13.98% reduction of heterocyclic-N after addition of CaO). Hence, the emission of NH₃ and HCN in gas phase can be controlled. Moreover, Fe₂O₃ showed better capability in controlling the emission of NO_x precursors than CaO (higher reduction of NH₃-N and higher reduction of HCN-N). These results indicate that red mud is an efficient catalyst to reduce emission of NO_x precursors through controlling intermediates at 400-800 °C.