Siloxanes in Producer Gas from Pyrolysis of Sewage Sludge, Operational Problems and a Solution

Pyrolysis and gasification are experiencing a renewed interest within the wastewater industry as a method to process either the biosolids or the residuals. The reasons are a) mounting evidence that per and polyfluoroalkyl substances (PFAS) are removed from the resulting biochar, b) the volume of the final material may be reduced by 90%, c) the systems (in the US) are often easier to permit than incinerators and d) they allow for sequestration of carbon within the biochar thus reducing carbon footprint. In addition to removing PFAS from the biochar, removal of microplastics and pharmaceuticals has been demonstrated (Keller et al. 2024 and Liu et al. 2023). These reasons make a compelling case for thoroughly vetting these technologies for application in wastewater industry. In Germany, a number (6 -- 8) pyrolysis units have been operating on 100% biosolids for several years and have experienced difficulties due to siloxanes in the producer gas. In the United States two pyrolysis systems and one gasification system are operating and several others have been designed and are expected to be constructed or commissioned in 2025. However, there is limited information regarding the operational requirements and challenges of these technologies operating on a feedstock of 100% wastewater biosolids or residuals. Although these technologies are well established in other industries and their applications documented by a plethora of information when operating on feedstocks such as woody or herbaceous biomass and coal; there is limited information regarding operating on a feedstock of 100% sewage sludge biomass (SSB). The primary challenge of SSB is the a) high moisture content, b) relatively low heating value and c) presence of siloxanes. The focus of this presentation is the presence of siloxanes, which manifest in the producer gas, and plug the thermal oxidizer resulting in the system being offline as much as 30 -- 40% of the time. The presence of siloxanes in wastewater is well known. In one study measuring 17 siloxane compounds, the average influent loading of siloxanes was 15.1 kg/day (Bletsou et al., 2013). In another study, in which three siloxanes were measured, their presence constituted 39 mg/L of the total influent COD (Surita & Tensel, 2014). In addition, the fate of siloxanes in wastewater has been studied especially in relation to its presence in digester gas. However, the siloxanes remaining in the sludge after digestion create different challenges when the residuals are processed in thermochemical processes such as pyrolysis or gasification. Siloxanes are manmade organosilicon compounds consisting of an Si-O backbone with a methyl, ethyl, or other organic functional group. They exist either as linear chains, designated with an L or as cyclic rings, designated with a D (Dewil et al. 2006 and Appels et al. 2008). Two main classes of widely used siloxanes that enter wastewater treatment plants are polydimethylsiloxanes (PDMS) which are non-volatile and volatile methylsiloxanes (VMS) which are volatile organic compounds (VOCs) (Xu 1999 and Traina et al. 2002). Most (greater than 90%) of the PDMS are adsorbed to the extracellular polymeric substances (EPS) of the floc. Approximately, 50% of the VMS are either volatilized or air stripped in the aeration basin (Parker et al. 1999, Xu 1999 and Traina et al. 2002). The sludge sent to anaerobic digestion; therefore, contains almost all the PDMS and 50% of the VMS. During digestion much of the PDMS is hydrolyzed to smaller VMS compounds, some of which are removed with the digester gas. The remaining VMS and approximately 34% of total PDMS entering the plant, remain in the digested biosolids (Xu 1999, Parker et al. 1999, Traina et al. 2002 and Accettola et al. 2008). These remaining siloxanes, when sent to a thermochemical process such as pyrolysis or gasification manifest as a problem within the thermal oxidizer. The siloxanes remaining in the sewage sludge will volatize in the thermochemical process and exit with the producer gas. The problem manifests downstream, in the thermal oxidizer, which typically operates at 1,100 -- 1,200˚C. At these temperatures, siloxanes precipitate to silicon dioxide (SiO2) i.e., silica. The oxidative decomposition of siloxanes may begin at temperatures from 600 -- 850˚C depending on the local environment; with nearly complete oxidation occurring at temperatures ≥ 900˚C. The combustion equations for three common cyclic siloxanes are shown below (Tansel & Surita 2014): D3: C6Si3H18O3 + 12O2 → 3SiO2 + 6CO2 + 9H2O D4: C8Si4H24O4 + 16O4 → 4SiO2 + 8CO2 + 12H2O D5: C10Si5H30O5 + 20O5 → 5SiO2 + 10CO2 + 15H2O The equations indicate a) the production of SiO2, and b) the relatively high consumption of oxygen required. The latter may result in insufficient oxygen for the combustion of the gaseous fuels within the thermal oxidizer. Silica exists as either an amorphous or a crystalline (i.e., quartz, tridymite and cristobalite) structure. In the combustion of woody biomass amorphous silica may transform to cristobalite at 800˚C and cristobalite to tridymite at 1,000˚C (Yang et al.,2022). Therefore, the silica that forms in the thermal oxidizer maybe of multiple forms all of which pose a significant operational problem. The evaluation of several systems indicated the quantity of silica generated ranged from 3.25 -- 13.5 kg/day. See Figures 1 and 2. The result was that every 5 -- 7 days the systems had to be shutdown to remove the material. To do so required 1 day for cool down, ½ - 1 day for cleaning and 1 day for warmup/startup (i.e. 2½ to 3 days) or 30 -- 40% downtime. Typically scrubbing siloxanes from digester gas is accomplished in an adsorption process using granular activated carbon (GAC). However, this is not an option with producer gas because the high concentration of other energy rich VOCs that would be removed instead of combusted in the thermal oxidizer to provide the heat for the drying process. Removal of the siloxanes from the producer gas must be selective. An alternative, to removing them from the producer gas before precipitation is a thermal oxidizer designed around the precipitation of the silica, but without requiring shutdowns. Such a thermal oxidizer exists and is designed to effectively cause the siloxanes to precipitate to silica and automatically remove the material from the process. The system, shown in Figures 2 and 3, is a dual-chamber regenerative thermal oxidizer (RTO), utilizing a regenerative ceramic material that is automatically circulated and cleaned. The technology has been operating full scale at two facilities since 2012 and 2022 and will be piloted as part of a Department of Energy grant evaluating autothermal pyrolysis. Figure 5 shows the silica removed from the gas stream. Its use in the removal of siloxanes for the producer gas is a new and innovative approach to a recent problem arising from the combustion of producer gas generated from the pyrolysis or gasification of sewage sludge. This presentation will discuss the following: a) the forms of siloxanes in the wastewater and the operational problems resulting from their presence, b) a potential solution, using a technology already operating in industrial applications and c) the challenges in applying this technology to the removal of siloxanes in the producer gas.