(458c) Effect of Selective Impurities on Carbon Capture from Biogas Using Deep Eutectic Solvents

Anaerobic digestion (AD) of food wastes is one of the most promising pathways to convert wastes to value added fuels (e.g., biogas) and products (e.g., fertilizer). As food wastes are heterogeneous and their composition changes from household-to-household, the resulting biogas composition varies significantly. Biogas from food wastes often contain 40-60 % methane (CH₄), 40-60 % carbon dioxide (CO₂), and 1-5 % impurities. Although the actual concentration may vary from the source of biomass and AD operating conditions, most common impurities found in the biogas are hydrogen sulfide (H₂S), ammonia (NH₃), octamethyltrisiloxane, and decamethylpentasiloxane. carbon capture and removal of the impurities are required prior to use or injection into a natural gas pipeline. CO₂ from the gaseous stream can be separated by physical (ionic liquid) or chemical solvents (amine-based solvents). Amine-based chemical solvents have higher adsorption capacity but are often expensive to regenerate and toxic for the environment. Physisorption solvents are generally expensive and their adsorption capacity is lower, meaning that significantly higher amounts of physisorption solvents are required. In the context of carbon capture, the impurities are likely to play a significant role in adsorptive capacity of the solvents due to an influence on the molecular environment.

In this study, deep eutectic solvents (DES) are evaluated to upgrade biogas. The benefit of DES is that they are made from the physical mixing of a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), generally made from a quaternary salt and acid/alcohols. In this study, a conductor like screening model (COSMO) is adopted to screen suitable HBA’s and HBD’s for biogas upgrading based on hydrogen bond properties using a triple-zeta valence polarization (TZVP) basis set. This model is evaluated compared to literature experimental studies to verify integrity of the results. Of the five DES studied, it was found that quaternary-octyl ammonium bromide with decanoic acid (N₈₈₈₈Br: DA) showed the highest CO₂ adsorptive capability. However, the choline-chloride (ChCl) based solvents had a much higher selectivity for CO₂ over CH₄. Through an activity coefficient analysis, it was determined that choline chloride-based DES are much more affected by the presence of contaminants than the ammonium salts with regards to adsorptive capabilities. It has been observed that the HBD is the main determinant of contaminant sensitivity rather than alkyl chain length. Due to these several trade-off factors apparent from the results, it is determined that the concentration of impurities should be accounted for when determining an appropriate DES for carbon capture from biogas.