The Pyrolysis of Biosolids in a Novel Closed Coupled Pyrolysis and Gasification Technology: Pilot Plant Trials, Aspen Plus Modelling, and a Techno-Economic Analysis

Pyrolysis is gaining recognition as a sustainable solution for biosolid management, though scaling it commercially presents challenges. To address this, RMIT developed a novel integrated pyrolysis and gasification technology called PYROCO (TM), which was successfully tested in pilot-scale trials. This study introduces PYROCO (TM) and its application to produce biochar, highlighting the biochar properties of the results of the initial trials. In addition, an energy analysis using semi-empirical Aspen Plus modelling, paired with a preliminary techno-economic assessment, was carried out to evaluate the feasibility of this technology. The results show that the PYROCO (TM) pilot plant produced biochar with a similar to 30 wt% yield, featuring beneficial agronomic properties such as high organic carbon (210-220 g/kg) and nutrient contents (total P: 36-42 g/kg and total N: 16-18 g/kg). The system also effectively removed contaminants such as PFASs, PAHs, pharmaceuticals, and microplastics from the biochar and scrubber water and stack gas emissions. An energy analysis and Aspen Plus modelling showed that a commercial-scale PYROCO (TM) plant could operate energy self-sufficiently with biosolids containing >30% solids and with a minimum calorific value of 11 MJ/kg. The process generates excess energy for drying biosolids and for electricity generation. Profitability is sensitive to biochar price; prices rise from AUD 300 to AUD 1000 per tonne, the NPV improves from AUD 0.24 million to AUD 4.31 million, and the payback period shortens from 26 to 12 years. The low NPV and high payback period reflect the use of a relatively high discount rate of 8%, chosen to be on the conservative side given the novel nature of the technology.