Thermal valorisation of pulp mill sludge by co-processing with coal

The pulp and paper industry, which is a strategic economic sector in Europe, is very demanding in terms of energy and water. The high water consumption results in the generation of large volumes of wastewater, which must be treated to accomplish with regulations. Therefore, sludge is unavoidable generated and its management is a key issue for the pulp and paper industry. Given prohibitions on landfilling and land application, thermal co-processing with coal may be a viable management and valorisation option for such wastes. In this work, the separate combustion and pyrolysis of coal (C), primary (L1) and secondary (L2) pulp mill sludge and their respective blends (10 wt.% of either L1 or L2) was assessed by thermogravimetric (TG) analysis. Differential Thermogravimetric (DTG) curves made 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, combustion of CL1 and CL2 mostly resemble that of C. Meanwhile, pyrolysis of CL1 and CL2 showed lower devolatilization temperatures and char yields, as compared with that of coal. Non-isothermal kinetic analysis evidenced only slightly lower apparent activation energy (E) for the combustion of C than for the combustion of either L1 or L2. Contrarily, much lower E were determined for the pyrolysis of CL1 and CL2 than for the pyrolysis of C. Yet, lower E than the weighted calculated ones where determined for the co-combustion and, especially, for the co-pyrolysis of the blends.