Valorisation of Pulp and Paper Mill Sludge by Hydrothermal Carbonisation

Abstract

Substantial amounts of waste residues in the form of sludge consisting of high moisture, organic and inorganic matter are continuously generated at the South African pulp and paper mills and disposed of through landfills, land applications and incineration. The methods are detrimental to the atmosphere, surface and groundwater resources, thereby threatening public health. The lignocellulosic composition in the paper sludge offers enormous potential for energy recovery. Common thermochemical technologies for energy recovery from lignocellulosic wastes such as pyrolysis, gasification and torrefaction, require feedstock pre-drying and elevated processing temperatures, thus making the overall process energy-intensive. Hydrothermal carbonisation (HTC) is an attractive technology to address problems associated with moisture-rich unconventional feedstock and convert it into high-value solid products for various applications. This study evaluates the effect of HTC operating conditions on the physicochemical properties, fuel characteristics and combustion performance of the resulting hydrochar fuels from different recycling paper mill streams to select the best feedstock for the process optimisation and potential energy assessment. 10 g raw fibre rejects (RF), sludge generated at the primary clarifier (PS) and final waste sludge (FS) were individually mixed with 90 mL and hydrothermally carbonised in batch reactor autoclave at 205, 225 and 245 °C for 3 hours. The resulting hydrochars were analysed in terms of proximate, elemental composition, functional groups, mass yield and energy yield and combustion performance. Based on the evaluated results, the RF was considered the most suitable feedstock due to factors including relatively low ash, increased fixed carbon, energy content, and adequate combustion performance in the hydrochar recovered compared to the hydrochars from the other substrates. Thus, consequently selected for process optimisation and further investigations. Response surface methodology (RSM) was used to evaluate the interaction of the HTC process operating parameters, i.e. temperature, residence time and solid load, as well as to develop models for the process to predict the optimum condition to produce hydrochar from paper sludge with maximal mass yield and calorific value. The fuel properties and thermal behaviour of the resulting hydrochars were further assessed to determine their potential for energetic applications. Results showed that temperature and residence time were the most significant model variables and highly influenced the hydrochar yield and calorific values, while the solid load had no significant effect on both responses. The highest calorific value recorded was 22.9 MJ/ kg on a dry basis, approximately equivalent to the HHV of coal used for commercial utility in South Africa and corresponded to an increase of 50 % of the HHV of the original feedstock. Under the design space investigated, at the optimal condition of 232 °C and 2 h, a maximal hydrochar yield of 74.4 % and a calorific value of 18.5 MJ/ kg are feasible. The variation in HTC operating conditions resulted in hydrochars with improved physicochemical characteristics for the most part. The influence of carbon on calorific values showed strong linear positive correlations. In contrast, the relationship between oxygen and calorific values showed a negative correlation. Both correlations had p-values less than 0.0001, confirming that increased carbon and lower oxygen lead to increased calorific values. The coalification diagram showed that dehydration and decarboxylation were the dominant reactions during HTC. The H/C and O/C were reduced by 35.5 and 64 %, respectively. The energy assessment of hydrochar production from paper sludge showed that up to 58.3 % of the energy generated from hydrochar fuel combustion could be generated. Furthermore, considering the current waste of 0.5 million tonnes generated annually, over 2.4 million gigajoules could be generated from the pulp and paper wastes generated, which is equivalent to 20.9 % of the electricity currently generated at one of South Africa’s coal-fired power plants, thus contributing to the diversification of energy resources, thereby reducing environmental emissions