Pyrolyis Gas Thermal Oxidation: PFAS and Emissions Control

Introduction Biosolids pyrolysis and gasification are gaining interest as a means to remove per- and polyfluoroalkyl substances (PFAS) from biosolids, however PFAS destruction has yet to be proven from the gas phase emissions. In 2020, the USEPA demonstrated that PFAS were removed to below reporting limits in biochar, the solid phase residual product from the Silicon Valley Clean Water (SVCW) biosolids pyrolysis system, but analytical techniques had not matured at the time for gas phase analysis (Thoma et al., 2021). Later bench scale studies showed that appreciable amounts of PFAS could be transferred from biochar to the gas phase at typical pyrolysis temperatures (500°-700°C) (McNamara et al., 2023; Williams et al., 2021). Pyrolysis gas contains an appreciable energy density and can be combusted for energy recovery and the industry has long postulated that gas-phase PFAS would be destroyed in the gas phase when combusted (Winchell et al., 2022). However, no studies had been conducted to date to demonstrate the extent of destruction. This presentation presents results from a Water Environment Federation (WEF) research study lead by Brown and Caldwell with five utility and industry partners to develop a bench-scale biosolids pyrolysis and thermal oxidation laboratory scale reactor to evaluate PFAS inputs and outputs, including flue gas emissions from the pyrolysis gas thermal oxidizer. The lab-scale reactor was created in partnership with Western University and BioForceTech, the pyrolysis system supplier for SVCW, to mimic the pyrolysis and thermal oxidation parameters from the full-scale installation. This presentation provides results from the study and lessons learned from operational issues and proposed mitigation steps for thermal oxidation at full-scale. Methodology The bench scale biosolids pyrolysis and thermal oxidation system were manufactured and commissioned by Western University over a twelve-month period starting Spring of 2022. The thermal oxidizer was configured by installing a forced air gas burner in a steel-lined combustion chamber. The burner and combustion chamber were configured with a propane pilot system that ignited the pyrolysis gas mixture, creating a self-sufficient flame when pyrolysis gas is available. The chamber was configured to provide sufficient residence time and turbulence to match the SVCW system parameters (1,000°C at two seconds). Once the system was found to meet key performance requirements the PFAS sampling event was conducted May 2023. Dried biosolids from SVCW were processed in the lab-scale biosolids pyrolysis and thermal oxidation system over three operating days for a continuous four-hour sampling campaign each operating day. Biosolids were loaded at a rate of 15 grams per minute at a solids residence time of 20 minutes and processed at a pyrolysis reactor temperature of 620°C. PFAS were analyzed in the biosolids, biochar and gas phase extracts by Eurofins. Gas-phase sampling was conducted by a commercial stack sampler, Ortech, using a modified OTM-45 PFAS sampling train. The PFAS data was received in August and September 2023 and are currently being evaluated using the data quality analysis plan developed under the Brown and Caldwell lead Water Research Foundation Tailored Collaboration program project (WRF 5111). Results and Discussion PFAS sampling data is under review and will be available for detailed comparison by 2024 WEF RBC. An initial review of preliminary findings and their comparison to a 2019 BioForceTech study is provided below. - No clear trend was observed between PFAS in the dewatered and dried biosolids. A wide range of variability in PFAS concentrations was observed indicating potential PFAS transformation in the dryer, some PFAS transfer to gas-phase, or sampling variability. - Except for PFBA at 0.7 ng/g, PFAS were below reporting limits in biochar. The reporting limit for the 2023 data was 0.4 ng/g, an order of magnitude lower than the 2-4 ng/g reporting limits available in 2019. - The loading for each PFAS to the bench-scale system ranged from 0.1 - 15 ng per second and the individual PFAS emissions rates in the flue gas ranged from 0.0001 - 0.008 ng per second, indicating individual destruction and removal efficiencies (DRE) will generally exceed 99% for PFAS in the thermal oxidizer flue gas. Conclusions This study is the first to demonstrate PFAS destruction and removal from combustion of biosolids pyrolysis gas. This data can support PFAS air permitting from biosolids pyrolysis projects under consideration by state permitting agencies such as Maine Department of Environmental Protection. While the study demonstrates the efficacy of thermal oxidation for PFAS treatment at bench scale, several lessons learned have been identified from operational issues at the full scale SVCW pyrolysis system that should be considered before widespread adoption. The main issue faced by SVCW is excessive dust from the pyrolysis reactor transferred into the combustion chamber, coating the interior of the burner and chamber. This requires a regular shutdown and cleaning cycle that takes two to three days for cooling, cleaning and reheating. This presentation will provide insights gained into dust creation and mitigation measures under consideration by SVCW and BioForceTech to improve operational uptime.