Biosolids Pyrolysis and Gasification: Technology Advancements and an Updated Assessment

Introduction The potential for biosolids conversion into fuels and revenue-generating products has driven interest in biosolids pyrolysis and gasification for decades, yet technology adoption has been hampered by historical issues, operational complexity, and high capital outlay. A combination of market drivers, technology evolution, and an increased offering of technology manufacturers has reinvigorated interest in these technologies as a tool to address today's biosolids challenges. While previous work has been presented on the conceptual benefits of these technologies and their products, there has not yet been a biosolids pyrolysis and gasification state of the science review to provide a definitive mass and energy balance comparison among developing technologies, discuss the root cause of historical issues and define operational requirements, and outline best practices and future research for successful technology adoption. This presentation will identify key findings for each of these areas from a recent literature review and technology survey (Winchell et al., 2021) developed in collaboration with two utilities, Metropolitan Water Reclamation District of Greater Chicago (MWRD) and Great Lakes Water Authority (GLWA), who in addition provided insight into key considerations for technology evaluations in light of the evolving biosolids management landscape. Mass and Energy Balance Assessment Pyrolysis and gasification are thermochemical processes that apply high temperatures to dried wastewater solids (or biosolids when stabilized) to release organic content into a gaseous or liquid state and retain a condensed, solid product called char. The processes can be considered to proceed on a gradient with regards to oxidant mixing when assessed with a related process, incineration. While pyrolysis and gasification do produce fuel gases (or liquids when condensed) that can be considered for a variety of combustion applications, operational challenges (discussed further below), have limited current application to direct combustion in a thermal oxidizer for heat recovery and recycle to an upfront drying step (as noted below in Table 1). Consequently, the mass and energy balances, when considered with direct combustion and drying, can be compared directly to current sewage sludge incineration (SSI). Results from a technology survey of five pyrolysis and gasification manufacturers (excepts presented in Table 2) will be presented comparing targeted mass and energy balances to historical SSI operation with key insights presented regarding the amount of auxiliary fuel required and what parameters have the most critical impact (e.g. feed solids moisture content, heating value, or dryer and thermal oxidizer thermal efficiencies). Operational Requirements The primary operational concern around pyrolysis and gasification is the resulting condensable fraction of the off-gas, commonly referred to as tar, that forms as a liquid in low-temperature zones of the system (Iaconi et al., 2017). Tar is a viscous liquid that will plug downstream passages, is corrosive, and becomes increasingly viscous over time (Basu, 2013). As noted above, current operations of biosolids pyrolysis and gasification systems transfer the fuel gas at high temperatures directly into a thermal oxidizer where the volatile vapors are combusted before they have a chance to condense. However, future installations under development have proposed fuel gas or tar conditioning prior to utilization in internal combustion applications, where specialized equipment will be required to avoid the operational hazards inherent in the process. Biosolids as a feedstock also present inherent difficulties when compared to other biomass feedstocks such as woody waste, and especially fossil fuels. Example proximate analyses for a range of representative feedstocks will be compared to biosolids to demonstrate its unique qualities (comparatively high oxygen, nutrient, and ash content) and their impact on system operations. Special attention will be given to the presence of alkali and alkaline earth metals (AAEMs) which make biosolids at risk to slagging and corrosion within the system, and best practices for system operation to achieve stable temperature to avoid these issues. Variability in biosolids parameters and loading rates during operations also pose a notable challenge to pyrolysis and gasification. Coordinating operation of discreet drying, pyrolysis or gasification, and thermal oxidation parameters require upfront load leveling and adjustments to seasonal changes in solids parameters. Reactor feeding hopper and conveyor systems have been developed to address these challenges on a micro-scale and its expected that further system development will be required to optimize operational resiliency to larger changes in solids production and processing trends upstream. Furthermore, an overview of required monitoring and operational points will be presented as an indicator for operational and staffing requirements for any project. Conclusions and Next Steps The technology review findings show that pyrolysis and gasification, as deployed today with direct combustion, can address historical operating challenges, but do entail a substantial demand on operations staff. As these technologies convert all the fuel gas into usable heat onsite, they are primarily differentiated from SSI by the characteristics, and end use opportunities afforded by the solid char product. Consequently, further work to identify the environmental and economic benefits from char, and impact of upstream processing steps on its beneficial use, will be critical to determining their viability. In addition, potential exists for improved combustion conditions compared to SSI in the thermal oxidizer, but emission characterization, especially for emerging contaminants like per- and polyfluorinated alkyl substances (PFAS) will be required to verify their effectiveness. For systems considering fuel gas processing and alternative uses, such as oil production, a careful study of fundamental operating principles and requirements will be required to address challenges inherent in the biosolids feedstock. Finally, opportunities exist for process intensification, including carbon diversion in wastewater treatment, co-processing with alternative dried feedstocks, and high efficiency drying technologies. Of all these options, high-efficiency drying is shown to have the highest potential impact and further development in this area could lead to a greater breakthrough in technology adoption.