(510j) Techno-Economic Analysis and Life Cycle Assessment of Contaminant Removal from Landfill Gas for Electricity Generation.

Landfill gas (LFG) is increasingly used and researched as a feedstock for a variety of traditional and proposed Waste-to-Energy (WTE) technologies, which includes electricity generation, compressed natural gas, or liquid hydrocarbon fuels. In these various scenarios, contaminants in the LFG can have substantial economic and environmental consequences in the WTE processes. There are several classes of contaminants contained in varied quantities in LFG such as siloxanes, sulfur compounds, chlorides, halogenated volatile organic compounds (VOCs), nitrogen, oxygen, carbon monoxide, alkanes/alkenes and mercury compounds (Andriani et al 2013). The two most important classes of contaminants requiring further treatment are the sulfur species and siloxanes, as these contaminants lead to substantial processing challenges and irreversible damage even at low concentrations and require removal technologies. The EPA has regulated the emission of harmful gases like hydrogen sulfide from landfills since it oxidizes to sulfur dioxide in the atmosphere (Public Lab n.d.).

Damages caused by hydrogen sulfide includes corrosion of equipment such as pipelines, compressors, engines and storage tanks; poisoning of fuel cells and catalysts; conversion to harmful sulfur dioxide during combustion of LFG composed of H₂S, thereby raising environmental concerns (Andriani et al 2013). Siloxanes are an emerging component in many consumer products that are landfilled and have a high enough vapor pressure such that a substantial amount is contained in the LFG. Siloxanes decompose to silica causing equipment damage that results in process downtime and reoccurring maintenance costs. The current purification techniques available are too expensive, in that it costs less to repair damaged engine parts than to adopt the current gaseous siloxane scrubbing technology. In order to accelerate adoption of waste to energy processes, a desire for more economical ways for removing siloxanes from LFG exists.

The goal of this research project is to conduct the techno-economic analysis (TEA) and life cycle assessment (LCA) of LFG cleaning for the generation of electricity using internal combustion engines, solar turbines and microturbines. The result from this research will identify the process unit with the highest economic and environmental impact in the LFG to energy process, thereby informing decision making when improving the entire process. Literature available have either focused on just the TEA of the removal of key contaminants from LFG (Hill 2014, Kuhn et al 2017, Tansel and Surita 2019) or the environmental assessment of biogas upgrading technologies that involves the removal of CO₂ for applications such as biomethane for gas grid injection, and compressed natural gas for transportation fuels (Cozma et al 2013, Lombardi and Francini 2020, Lorenzi et al 2019, Starr et al 2012). Although LFG is being converted into several end products (electricity, biomethane, bio-CNG, liquid fuels), about 70 percent of currently operational LFG energy projects in the United States generate electricity (EPA 2021). For this reason, only the assessment of contaminant removal from LFG for electricity generation was conducted in this study.

In this work, a natural zeolite (clinoptilolite), which costs as low as ~$100/ton, as compared to activated carbon that is factors of 3+ more expensive, and four other low-cost materials (diatomaceous earth, crushed glass, biochar and hydrochar), were characterized using various techniques such as N₂ physisorption, x-ray diffraction, CO₂ adsorption and water vapor sorption analysis. Clinoptilolite showed the most favorable characteristics for siloxane adsorption among all samples characterized and hence was used in further analysis. TEA and LCA were conducted for siloxane removal units with clinoptilolite as an adsorbent and then compared with the current state of technology, activated carbon and another low-cost material that had been tested for siloxane adsorption in literature - 13X zeolite. The TEA results revealed that clinoptilolite as an adsorbent for siloxane removal is too expensive as compared to activated carbon and 13X zeolite. Sensitivity analysis was conducted to study the effect of the adsorption capacity of activated carbon on the total annual cost. The result from the sensitivity analysis revealed that 13X zeolite becomes a more cost-effective adsorbent for siloxane removal from LFG when compared with activated carbon that possesses an adsorption capacity less than 500 mg D4/g media. Assessment was also conducted to find out the effect on the total annual cost (TAC) for facilities with and without siloxane removal units installed. It was found that it becomes economically feasible for facilities with LFG siloxane concentration of 15 mg/m³, to install a siloxane removal unit for biogas flowrates above 1300 scfm. The result from the life cycle assessment revealed that the CO₂ emissions from all siloxane removal units analyzed are approximately equal and therefore, only the cost of siloxane adsorption systems can be used in making decisions on adoption.

Fe oxide-based media is the most common adsorbent for H₂S. LFG is usually cooled down to about 5⁰C to remove moisture. Activated carbon is usually used as the adsorbent in the halide removal unit. The TEA and LCA of the process to remove all contaminants from LFG prior to electricity generation will be conducted. Comparisons will be made between impact generated from engines with varied contaminant tolerance limits. Sensitivity analysis will be conducted to understand the impact of varying certain factors.

References

Andriani, Y., Morrow, I.C., Taran, E., Edwards, G.A., Schiller, T.L., Osman, A.F., and Martin, D.J. "In vitro biostability of poly(dimethyl siloxane/hexamethylene oxide)-based polyurethane/layered silicate nanocomposites," Acta Biomater. 9: 8308-17 (2013).

Cozma, P., Ghinea, C., Mămăligă, I., Wukovits, W., Friedl, A., and Gavrilescu, M. "Environmental Impact Assessment of High Pressure Water Scrubbing Biogas Upgrading Technology," CLEAN - Soil, Air, Water. 41: 917-27 (2013).

EPA "Landfill Methane Outreach Program (LMOP)," (2021).

Hill, A. Conduct a Nationwide Survey of Biogas Cleanup Technologies and Costs; 2014.

Kuhn, J.N., Elwell, A.C., Elsayed, N.H., and Joseph, B. "Requirements, techniques, and costs for contaminant removal from landfill gas," Waste Manag. 63: 246-56 (2017).

Lombardi, L., and Francini, G. "Techno-economic and environmental assessment of the main biogas upgrading technologies," Renewable Energy. 156: 440-58 (2020).

Lorenzi, G., Gorgoroni, M., Silva, C., and Santarelli, M. "Life Cycle Assessment of biogas upgrading routes," Energy Procedia. 158: 2012-8 (2019).

Public Lab "Hydrogen Sulfide Regulations " (n.d.).

Starr, K., Gabarrell, X., Villalba, G., Talens, L., and Lombardi, L. "Life cycle assessment of biogas upgrading technologies," Waste Management. 32: 991-9 (2012).

Tansel, B., and Surita, S.C. "Managing siloxanes in biogas-to-energy facilities: Economic comparison of pre- vs post-combustion practices," Waste Management. 96: 121-7 (2019).