Hydrothermal carbonization of different recycling paper mill waste streams

Abstract

Hydrothermal carbonization (HTC) is an attractive technology to address problems associated with moisturerich pulp and paper sludge generated at South African mills and wastewater treatment plants. In this study, sludge generated from the repulping process, recycling paper process, and primary clarifier wastewater treatment were selected to investigate the potential of HTC to upgrade the feedstock into solid fuel. Experiments were performed in a batch reactor autoclave at different operating temperatures (200 °C, 220 °C and 240 °C), residence time of 3 h, and wet biomass to deionized water ratio of 1:9. Data on the physicochemical characteristics and calorific values of the solid yield, as well as carbon mass distribution, were reported. Results revealed that higher temperatures resulted in hydrochar with higher calorific values from 15.26 to 22.57 MJ/kg, 15.33 to 20.32 for repulping paper sludge and recycling paper sludge, respectively, but lower calorific values (from 9.79 MJ/kg to 7.02 MJ/kg) for sludge from primary clarifier process. The proportions of fixed carbon for hydrochars obtained at 245 °C were 5-7 times greater than the raw samples, but solid mass yield decreased with an increase in reaction temperatures for all feedstocks. Analysis of the carbon mass balance revealed that, for all HTC treatments, the majority of the carbon fraction was retained in the hydrochar. HTC was proven to be an efficient technology for upgrading sludge generated at pulp and paper mills in South Africa into hydrochar with enhanced properties with a variety of applications including solid fuel, energy storage, soil amendment, adsorbent and catalyst.