What is the cause of odor during papermaking? The main sources of odor in pulp and paper mills are the exhaust gas discharged during the pulping process and the volatile gas discharged from the auxiliary process, and its cooking system. Black liquor extraction systems, alkali recovery evaporation stations, alkali recovery furnaces, lime kilns and sewage treatment stations all produce different degrees of odor, but the main source of odor is still in the cooking system.