Thermal Valorization of Pulp Mill Sludge by Co-processing with Coal

At the pulp and paper industries, large volumes of primary and secondary sludge are generated from wastewater treatment. Aiming to assess the co-processing of each type of sludge with coal, thermogravimetric (TG) analysis has been used in this work. First, combustion and pyrolysis TG curves of coal (C), primary (L1) and secondary (L2) pulp mill sludge were obtained under oxidizing and inert atmosphere, respectively. Then, under identical experimental conditions, TG analysis was carried out on the primary sludge–coal (CL1) and the secondary sludge–coal (CL2) blends. The corresponding differential thermogravimetric (DTG) curves showed evident differences between C, L1 and L2, which were mainly related to the relative large fixed carbon and low volatiles content of C. However these differences, the combustion of CL1 and CL2 mostly resembled that of C. Meanwhile, the pyrolysis of CL1 and CL2 showed lower devolatilization temperatures and char yields than that of C. Non-isothermal kinetic analysis of TG data showed slightly lower apparent activation energy (E) for the combustion of C than for the combustion of either CL1 or CL2. Contrarily, much lower E values were determined for the pyrolysis of CL1 and CL2 than for the pyrolysis of C. Yet, lower E values than the weighted calculated ones were determined for the combustion and, especially, for the pyrolysis of the blends.