Biochar production via slow pyrolysis of biosolids
Creators: name: PATEL, Savankumar
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400403
subject: Chemical engineering design
classificationCode: 400499
subject: Chemical engineering not elsewhere classified
classificationCode: 401103
subject: Global and planetary environmental engineering

description: Biosolids, or stabilised sewage sludge, obtained as the byproduct of wastewater treatment plants, contain organics as well as macro- and micro- nutrients. Australia produced ~1.5 million wet tonnes of biosolids in 2017 of which nearly 75% are beneficiated to the agricultural land. The presence of heavy metals, pharmaceuticals, pesticides and emerging contaminants such as polyfluoroalkyl substances (PFAS) and micro-plastics may limit the direct application of biosolids to agricultural land in the future. The research work in this thesis is focused on the slow pyrolysis of biosolids with the aim to convert them into value-added biochar. In particular, this thesis is looking at understanding the role of the addition of minerals, such as lime and zeolite, in the pyrolysis of biosolids. The intention of using minerals in the pyrolysis of biosolids is two-fold: 1. minerals act as a catalyst in pyrolysis and 2. their use reduce the concentration of heavy metals in the resultant biochar. Firstly, TGA-FTIR experiments were carried out to derive mass loss as well as gas profiles during the pyrolysis of biosolids with and without the presence of minerals. Hereafter, a hybrid kinetic modelling approach, consisting of model-free and model-fitting methods, was adopted to derive the kinetic parameters. The kinetic parameters obtained from the pyrolysis of biosolids in the presence of minerals were then compared with a synthetic catalyst in order to understand the differences between the mineral-based and synthetic catalyst-based pyrolysis. Several analytical tools such as XRD, SEM and FTIR-solids were used to analyse the physico-chemical properties of the resultant biochars. It has been concluded that minerals, despite their low catalytic activity, offer various process and morphological advantages. Furthermore, a bench-scale fluidised bed reactor was employed to investigate the effect of temperature and bed material on the pyrolysis of biosolids. The bed materials used in this study were lime, biochar and activated char. The main objective of this work was to identify the most suitable bed material and optimum operating temperature which yield biochar with favourable properties while cracking down the unwanted nitrogenated, oxygenated, polycyclic aromatic compounds (PACs) and aliphatic compounds present in the oil and gas vapours which are difficult to combust or have the potential to cause secondary level of emissions. The biochar bed material and temperatures above 700 C demonstrated the best performance. Biochar produced in a fluidised bed using biochar or activated char as a bed material is expected to have higher heavy metal content. Although it is established in the literature that bioavailabilty of heavey metals in the resultant biochar will be very low, in the absence of any soil application guidelines, biosolids derived biochar application as a soil amadement material might be questionable. Therefore, efforts have been made to demonstrate the application of biochar and activated char made from biosolids pyrolysis for the catalytic decomposition of methane. The effect of temperature and initial methane concentration on the methane conversion (or hydrogen production) as well as carbon nanomaterials production on the surface of biochar and activated char has been investigated in detail. Reasonable level of methane conversions were achieved above 700 C with a potential of producing carbon nanofibres at 700 C and carbon nanospheres at 900 C. Lower initial methane concentrations were fevourable. Finally, a techno-economic assessment has been carried out to examine the feasibility of biosolids pyrolysis. ASPEN modelling suggested that moisture can significantly affect the energy balance of the pyrolysis process. The overall process can be energy positive for moisture contents lower than 50 wt.%. It was concluded that a payback period of 4-6 years is achievable.