The pyrolysis of biosolids in a novel fluidized bed heat exchanger reactor: Pilot plant trials, biochar properties, gas emissions testing, and fate of PFAS

There are limited pilot-scale investigations of biosolids pyrolysis, particularly utilising an autothermal fluidised bed reactor system. This study reported the pilot trials of biosolids pyrolysis using the novel integrated fluidised bed heat exchanger, PYROCO™ reactor technology. The pilot plant was operated for 14 days at an average feed rate of 15 kg/h dry biosolids. The produced biochar was comprehensively characterized for its physicochemical properties and residual contaminant profile. Gas effluent streams sampled from various locations within the plant were analysed for traditional pollutants, including CO, SOx, NOx, dioxins, and furans, along with particulate matter, hydrocarbons, chlorides, fluorides, and metal(loid)s through independent gas emission testing. Additionally, the fate of per- and polyfluoroalkyl substances (PFAS) was evaluated through PFAS analysis in all solid, liquid, and gas streams, as well as through fluorine mass balance. The biochar produced exhibited a high carbon content (20–30 wt%) and retained beneficial agronomic properties, such as low O/C and H/C atomic ratios, along with high macronutrient contents. While concentrations of heavy metals and polycyclic aromatic hydrocarbons increased in the biochar, most persistent organic contaminants were effectively removed, resulting in non-detectable levels of PFAS, microplastics, pharmaceuticals, hormones, estrogens, and pesticides. The PFAS and fluorine mass balance analysis indicated that PFAS concentrations in the influent biosolids (58.1 ng/g) were reduced to non-detectable levels across all effluent streams, including biochar, scrubber water, and flue gas, demonstrating effective PFAS removal. Gas emissions testing revealed that all measured gas pollutants, except for particulate matter and mercury, were within acceptable industrial emission thresholds for thermal plants. Overall, the findings underscore that the PYROCO™ technology is a promising solution for the energy-efficient conversion of biosolids into high-quality biochar.