Evaluating the Sustainability Impacts of Replacing Multiple Hearth Furnaces with High Temperature Fluidized Bed Incinerators and PFAS Regulation Implications on Process Design

Introduction In general, the two options for biosolids incineration are multiple hearth furnaces and FBI units. In the 1970s, tighter emission regulations were introduced, which encouraged the replacement of multiple hearth furnaces with fluidized bed incinerators. Veolia Water Technologies & Solutions (Veolia) is an established industry leader in high temperature fluidized bed (HTFB) incineration, also known as fluidized bed incineration (FBI). Over the years, Veolia has replaced several multiple hearth furnaces (MHF) with specialized FBI units. These include: Ypsilanti (1 MHF replaced with 1 FBI), TZ Osborne Greensboro (1 MHF replaced with 1 FBI), Green Bay (2 MHF replaced with 1 FBI), Mill Creek (6 MHF replaced with 3 FBI), Southerly (4 MHF replaced with 3 FBI), and Sutton (3 MHF replaced with 3 FBI). Emerging PFAS air and solids regulations have impacted the process design of FBI systems. This presentation will review: - Drivers of multiple hearth replacement by fluidized bed incinerators - The greenhouse gas emissions quantification of a current fluidized bed incinerator and associated air pollution control compared to that of a multiple hearth furnace - Impacts of PFAS regulations on process design of FBI systems Why are Multiple Hearth Furnaces Being Replaced by Fluidized Bed Incinerators? Multiple hearth furnaces (MHF) are circular steel units with multiple hearths (shelves) where biosolids are fed. The solids are raked towards the center of the hearth where the combustion air rises. Biosolids burn in the MHF at a range of 572-1796oF (Vallero, 2008). Biosolids fall through holes in the hearths where they drop to the hearth below, until they reach the bottom of the furnace (as ash), where it is removed (United States Environmental Protection Agency, 2003). Veolia provides a teardrop shaped fluidized bed incinerator, where biosolids are fed directly into or above a fluidized sand bed. The bed is fluidized via preheated air that is injected into a windbox, which then forces the air through tuyeres that are embedded in a refractory arch dome. The tuyeres evenly distributes the combustion air through the sand bed to ensure homogenous temperature throughout the bed. Veolia FBI reactors are operated with a freeboard temperature of ~1550oF. The emissions regulation that prompted the replacement of MHF with FBIs was the Clean Air Act of 1970. Two distinct advantages that FBIs have over MHF are that there is significantly less excess air required (40% versus 75-100%, respectively) and the ability to operate autogenously (United States Environmental Protection Agency, 2003) (Winchell, et al., 2021). Greenhouse Gas Emissions Impacts on Replacements of MHF with FBIs Veolia has developed a methodology to quantify greenhouse gas emissions, utilizing ISO 14064-2 as a guide and applied it on multiple projects, including that of fluidized bed incinerators. Initially, a base case and a project case are determined. For the purposes of this presentation, the base case is a multiple hearth furnace application, and the project case is a fluidized bed incinerator application. Water, electricity, natural gas/fuel, and chemical consumption are all taken into consideration. Using emissions factors to convert these values into tonnes of CO2 equivalents per year, we can quantify the GHG impacts of a MHF replacement with an FBI. This quantification is ongoing, but results will be presented if this abstract is selected. The results of these quantifications can be used to justify selecting one unit process over another, such as choosing a granular activated carbon unit over a sorbent polymer composite unit. PFAS Regulation Impact on FBI Process Design Similar to regulations influencing multiple hearth replacements in the 1970s, emerging PFAS regulations will also influence what unit processes will be used in incinerator systems. Currently, there is limited published data on the PFAS treatment ability of a high temperature fluidized bed incinerator. There are air pollution control unit processes however, used in FBI systems that have proven effectiveness on the treatment of PFAS compounds. In the United States, if the incineration system is new, it must be designed to meet MACT LLLL requirements, which includes the removal of mercury, dioxins, furans, cadmium, lead, NOx, SO2, HCl, and particulate matter (Figure 1). The main addition in a MACT LLLL system is a granular activated carbon (GAC) unit. In drinking water applications, the use of a GAC unit for PFAS treatment is common (Winchell, et al., 2021). Using a GAC unit, PFAS treatment efficiencies can exceed 99% in water streams (Barr Engineering Co., Hazen and Sawyer, 2023). It is important to note that the efficiency of PFAS treatment using a GAC unit is dependent on the chain length and the presence of competitors (total organic carbon, volatile organic compounds) (Barr Engineering Co., Hazen and Sawyer, 2023). There is proven ability to treat PFAS in water streams using GAC units, but limited data available for the treatment in gas streams. However, there is much data on the removal of acid gases (PFAS thermally converts to hydrogen fluoride (HF)) using wet scrubbers, which are used on almost all our incinerator trains (Wang, et al., 2022). If the incinerator system is existing and a furnace replacement or retrofit is being performed, the system must be designed to meet MACT MMMM requirements (Figure 2). In this system, instead of a GAC unit, a sorbent polymer composite system (SPC) is used for mercury removal. However, the PFAS treatment ability of SPC units has not yet been studied (Barr Engineering Co., Hazen and Sawyer, 2023). Summary Multiple hearth furnaces have been replacing by fluidized bed incinerators since the early 1970s, however, the greenhouse gas emissions impacts have not been measured. Using Veolia methodology, we can quantify the impacts of these replacements, while also identifying areas where emissions could potentially be reduced (for example, selecting certain air pollution control unit processes over another). Depending on if the fluidized bed incinerator is replacing a MHF or is part of a brand new plant, the air pollution control equipment may be different. This may also impact the PFAS treatment ability of the system, as GAC units have been proven to treat PFAS, whereas SPC units have yet to be studied. It is imperative that studies be conducted to analyze not only flue gas PFAS concentration (before and after air pollution control units), but to treat PFAS-laden ash slurry as well.