(351i) Assessment of Low Cost Adsorbents for Removal of Siloxanes from Landfill Gas

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 [1-2]. Siloxanes are an increasing contaminant of LFG as many consumer products being land-filled contain this compound [3]. Siloxanes in biogas cause damage to machines if not removed because it thermally decomposes to silica [3-5]. This leads to high maintenance cost of WTE technologies thereby increasing the cost of the process. Current purification techniques available for siloxanes removal are too expensive [5]; it costs less to repair damaged engine parts than to adopt current siloxane purification techniques. In order to accelerate adoption of WTE processes, we need more economical methods for removing siloxanes from LFG.

The goal of this research project is to develop low cost strategies for siloxane removal from LFG. This research will adopt two approaches towards its goal of developing low cost strategies for siloxane removal from LFG. First, natural zeolite that can be purchased for as low as $100/ton, as compared to activated carbon that is factors of 3+ more expensive, as an adsorbent and other low cost adsorbents such as diatomaceous earth, crushed glass and biochar will be evaluated on the performance, in terms of capacity, selectivity, regeneration ability, and stability, and compared to activated carbon which is the current state of technology. Secondly, taking advantage of the siloxanes inclination to undergo thermal degradation, low cost/waste inorganic materials will be evaluated as catalytic adsorbents to capture only the silicon/silica of the siloxanes. An advantage of this approach is that the siloxanes permanently change phases into one that will remain trapped. The performances of these materials will be evaluated in terms of capacity and longevity. In both cases, the experimental results will be used to conduct a techno-economic feasibility study that can be compared to previous results found in literature using activated carbon as the adsorbent.

This presentation will focus on the first strategy which involves testing selected low-cost adsorbents for the adsorption capacity of selected siloxanes. Four low cost and/or waste materials were selected for examination as adsorbents for this study: clinoptilolite, diatomaceous earth (DE), biochar, and crushed glass. Selected siloxanes for this project are hexamethyldisiloxane (L2) and octamethylcyclotetrasiloxane (D4). Characterization techniques such as N₂ physisorption and XRD analysis were adopted to determine the physical characteristics (such as surface area) and chemical composition of the samples. Water vapor sorption experiments and CO₂ chemisorption experiments were used to examine the competition of adsorption sites by these compounds. Results of the experiments stated above for clinoptilolite are displayed in figure 1.

From the adsorption isotherms derived for clinoptilolite as displayed in figure 1, clinoptilolite was observed to possess a water vapor adsorption capacity of 102.9 mg H₂O/g clinoptilolite at room temperature and a CO₂ adsorption capacity of 96.3 mg CO₂/g clinoptilolite at room temperature. The N₂ adsorption isotherm was used to determine the BET surface area of clinoptilolite which resulted in 17 m²/g. N₂ physisorption, CO₂ chemisorption and water vapor sorption experiments will be carried out on all other selected low cost adsorbents. We will also present results of vapor sorption experiments to generate breakthrough curves and adsorption isotherms to determine the adsorption capacity and regeneration ability of selected low cost adsorbents for selected siloxanes in inert and surrogate LFG. Finally, we will present results of a techno-economic analysis to compare the cost in comparison with conventional adsorbents.

References

[1] Ajhar, M., Travesset, M., Yüce, S., & Melin, T. (2010). Siloxane removal from landfill and digester gas–a technology overview. Bioresource technology, 101(9), 2913-2923.

[2] Kuhn, J. N., Elwell, A. C., Elsayed, N. H., & Joseph, B. (2017). Requirements, techniques, and costs for contaminant removal from landfill gas. Waste Management, 63, 246-256.

[3] Finocchio, E., Garuti, G., Baldi, M., & Busca, G. (2008). Decomposition of hexamethylcyclotrisiloxane over solid oxides. Chemosphere, 72(11), 1659-1663.

[4] Sonoc, A. C., Thurgood, C., Peppley, B., & Kelly, D. G. (2017). Kinetic study of the thermal decomposition of octamethylcyclotetrasiloxane on activated gamma alumina. Journal of environmental chemical engineering, 5(5), 4858-4865.

[5] Urban, W., Lohmann, H., & Gómez, J. S. (2009). Catalytically upgraded landfill gas as a cost-effective alternative for fuel cells. Journal of Power Sources, 193(1), 359-366.