Managing siloxanes in biogas-to-energy facilities: Economic comparison of pre- vs post-combustion practices Berrin Tansel ⇑ , Sharon C. Surita Department of Civil and Environmental Engineering, Florida International University, College of Engineering and Computing, 10155 West Flagler St, Miami, FL 33174, United States article info Article history: Received 6 March 2019 Revised 8 July 2019 Accepted 10 July 2019 Keywords: Siloxanes Biogas Gas purification Carbon adsorption Siloxane removal Biogas-to-energy abstract Siloxanes present in small concentrations in biogas interfere with the operation of biogas-to-energy facil- ities. During biogas combustion, siloxanes form white deposits on engine components (engine heads, spark plugs, valves) in crystals or amorphous forms depending on the temperature. The purpose of this study was to evaluate the economic feasibility of biogas-to-energy systems for managing the operational challenges due to siloxanes in biogas. The facility maintenance cost data were compiled by a survey of biogas-to-energy facilities in the United States. Economic analyses were performed to compare the oper- ational costs due to increased maintenance for removing the white deposits forming on the engine com- ponents and the installation of a pretreatment system (carbon adsorption) to remove siloxanes prior to combustion. Numerical analyses showed that for the facilities with operating capacities less than 1300 m 3 /h (750 scfm), the costs for installation and operation of the carbon adsorption system exceeded the maintenance costs for removal of deposits from the engine components. The maintenance costs cor- related well with the reported maintenance needs which were between 120 and 800 man hours per year. On the basis of siloxane removal costs alone, it is not economically feasible to install a carbon adsorption system for siloxane removal prior to combustion for small facilities processing less than 1300 m 3 /h (750 scfm) of biogas. However, using a process for siloxane removal prior to gas engines (e.g., carbon adsorp- tion) would be improve the overall performance of the gas engines and reduce maintenance need at all facilities. Ó 2019 Elsevier Ltd. All rights reserved. 1. Introduction Biogas from anaerobic digesters and landfills is a sustainable alternative for conventional fuels. Biogas has a heating value of approximately 500 Btu per cubic foot which is about half of the heating value natural gas (LMOP, 2009). Landfill gas (LFG) pro- duced by degradation of municipal solid waste (MSW) and anaer- obic digester gas produced during degradation of biosolids at wastewater treatment facilities contain about 40–60 percent methane and 40–45 percent carbon dioxide as well as water vapor, nitrogen and small amounts of volatile organic compounds (i.e., hydrocarbons, hydrogen sulfide, siloxanes). The composition of the biogas varies depending on climate as well as the composition of materials that are decomposing under anaerobic conditions. The estimated biogas yield ranges between 150 and 265 cubic meters per ton of MSW (5300 and 9400 cubic feet per ton) (Bhide et al., 1990). Deficiency in mineral nutrients (carbon, oxygen, hydrogen, sulfur, phosphorus, potassium, calcium, magnesium and other trace elements); presence of heavy metals, antibiotics, detergents; pH levels below 6.2 can have inhibitory effects on biogas generation. Presence of siloxanes in biogas from landfills and anaerobic digesters create operational challenges at the biogas-to-energy facilities. During combustion of biogas, siloxanes form white deposits (in crystal or amorphous state) on engine components (e.g., engine heads, spark plugs, valves) depending on the temper- ature (Sevimoglu and Tansel, 2013a, 2013b; Piechota et al., 2013; Álvarez-Flórez and Egusquiza, 2015) as well as in other units (e.g., boiler tubes, intercooler radiator) (Sevimoglu and Tansel, 2013a). The accumulation of deposits affects the engine perfor- mance by causing detonation in combustion chambers and decrease in efficiency of gas engines (Xu et al., 2013; Surita and Tansel, 2014; Surita and Tansel, 2015; Elwell et al., 2018; Eichler et al., 2018). Volatile methyl siloxanes (VMS) are man-made compounds containing silicon and oxygen with organic side groups (typically methyl groups) attached to the silicon atoms. Siloxane based com- pounds are used in health care, personal hygiene and industrial products as well as in paper and construction industry (due to their https://doi.org/10.1016/j.wasman.2019.07.019 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: tanselb@fiu.edu (B. Tansel). Waste Management 96 (2019) 121–127 Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman