(622a) Carbon Dioxide Removal from Biogas Using Amine Based Silica Sorbents

Biogas is a source of methane that can be used directly as fuel for combustion engines, gas turbines, and fuel cells as a renewable energy source after its purification. With CO₂ being the major contaminant (30–50%), its removal from methane becomes one of the critical steps in upgrading to increase the energy content of the biogas. Commercial techniques such as scrubbing and pressure swing adsorption (PSA) which uses high pressure for selective adsorption of gas molecules on solid surfaces has major disadvantages like corrosion problems, heavy use of water and/or decreased efficiencies due to the high temperatures/pressures used in the regeneration step [1]. Production of compressed natural gas (CNG) requires high pressures in order to separate CO₂ from CH₄ and compressor costs can be high [2]. Compressors contribute to more than 50% of the costs for a waste to energy processes. CO₂ adsorption using amine functionalized silica is a low pressure process and using this reduces the capital and operating expenses of compressors required for biogas upgrading. Mesoporous silica functionalized with amine groups have proven to be good adsorbents of CO₂ with high selectivity, low energy utilization and low regeneration costs [3]. APTES (3-Aminopropyl triethoxysilane) modified silica has been studied before [4] [5] but never tested for CO₂ separation with actual biogas.

In the work reported here, APTES immobilized on SBA-15 was prepared using conventional grafting techniques. Techniques including XRD, N₂ physisorption, FTIR, TPO were used for characterization. We tested a series of APTES modified SBA-15 with different loadings of amines for adsorption experiments of CO₂ at room temperature and 1 atm for a dry 50% CO₂ in He feed. Results show that with an increase in APTES loading (from 12 wt % APTES to 26 wt% APTES) the CO₂ adsorption capacity increases. The adsorption capacity of 12 wt % APTES and 26 wt% APTES was 0.07 mmol CO₂/g_ads and 0.85 mmol CO₂/g_ads, respectively. The adsorbents were regenerated at 100 ^oC. Preliminary CO₂/CH₄ selectivity study for a 50% He and dry CO₂/CH₄ feed in 1:1 ratio was done. It was seen that the adsorbent has a high selectivity ratio ~ 5 for the CO₂/CH₄ binary mixture. Currently, we are investigating the CO₂ separation in simulated biogas containing water. Following this the regeneration for large number of cycles will also be tested.

References:

1. Sun, Q., et al. (2015). "Selection of appropriate biogas upgrading technology-a review of biogas cleaning, upgrading and utilisation." Renewable and Sustainable Energy Reviews 51: 521-532.
2. Jaramillo, P. and H.S. Matthews, Landfill-gas-to-Energy projects: analysis of net private and social benefits. Environ Sci Technol, 2005. 39(19): p. 7365-73.
3. Liu, X., et al. (2017). "Different CO₂ absorbents-modified SBA-15 sorbent for highly selective CO₂ capture." Chemical Physics Letters , 676: 53-57
4. Mafra, L., et al. (2018). "Amine functionalized porous silica for CO₂/CH₄ separation by adsorption: Which amine and why." Chemical Engineering Journal 336: 612-621.
5. Vilarrasa-Garcia, E., et al. (2015). "CO₂/CH₄ adsorption separation process using pore expanded mesoporous silicas functionalizated by APTES grafting." Adsorption-Journal of the International Adsorption Society 21(8): 565-575.