Potential energy generation of sludge from a thermomechanical pulp (TMP) mill

Pressure to increase the renewable energy matrix motivates the search for new energy sources. Pulp and paper mills have been constantly expanding in Brazil, and to ensure their competitiveness, new processes should be established both in terms of reducing the waste discarded in landfills, as well as the substitution of fossil fuels used in the mills. The present work aims to evaluate the energy potential of primary sludge (PS), secondary sludge (SS), a mixture of primary and secondary sludge (MS) and tree bark (B) generated in a Brazilian TMP mill. Characterization of all biomasses was performed. Moisture content, higher, lower, and net heating value, elemental, structural, and immediate composition, thermogravimetric analysis and quantification of minerals and metals were determined The bark presented the highest higher heating value (HHV) (18.90 MJ kg⁻¹), while the secondary sludge had the lowest HHV (12.45 MJ kg⁻¹). The high concentration of ash in the sludges PS (26.58%), SS (40.46%) and MS (25.78%) negatively affected their heating value. The sludges also showed high moisture content, which made their burning without a previous drying stage unattractive. Nevertheless, the sludges and the bark are rich in carbon which makes them attractive for use as biomasses for energy generation.