Tailoring of adsorptive properties of zeolites for biogas upgrading

Abstract: Biogas is a promising alternative fuel with low CO2 emissions and high market potential due to the abundance of organic biomass. Despite being a renewable form of energy, biogas consists of 40-45% CO2, which lowers its calorific value [1]. Various porous materials have been tailored to adsorb CO2 gas from the biogas stream to obtain 95-97% biomethane. Zeolites are one of the promising porous materials that can contribute remarkably in the upgrading process by selectively adsorbing the CO2 gas from biogas [2].The aim of this thesis is to develop hierarchical structures by novel approaches to enhance their CO2 adsorptive properties. The first part of the study addresses the tailoring of zeolites NaX and CaA binderless beads using the ion-exchange process to acquire high CO2 adsorption capacities of 5.1 mmol/g and 4.3 mmol/g at 298 K with the high mechanical strength of 2 MPa and 1.3 MPa respectively. The ion-exchange process was optimized for NaX and CaA zeolite to obtain high CO2-over-CH4 selectivity of 525 and 1775 respectively. The breakthrough experiments show that the partially ion-exchanged zeolite NaX has high mass transfer kinetics with a CO2 uptake rate of 2.8 mg of CO2/g/s as compared to the zeolite CaA binderless beads.The second part dealt with the structuring of zeolites using freeze granulation and electrospinning techniques. The freeze granulation process was optimized to form granules of 2-3 mm in diameter from NaX and CaA zeolite powder. The CO2-over-CH4 selectivities were investigated using Henry’s law and it shows that the NaX granules offer high selectivity of 214 than the CaA granule, 172 at 273 K and 100 kPa. No physical damage was observed when the granules were subjected to five cyclic breakthrough adsorption-desorption experiments at 4 bar. In addition, NaX granules offer a high uptake rate of 3.6 mg of CO2/g/s with a mass transfer coefficient of 1.3 m/s as compared to the CaA granules.To move further in structuring techniques, electrospinning was used to fabricate hierarchical porous structures. ZSM-5 nanofibers composites were developed from the ZSM-5 nanopowder and polyvinylpyrrolidone (PVP) polymer. Two-step post thermal treatments were carried out: Pre-oxidation and carbonization on ZSM-5 nanofibers composites to form mechanically strong composite structures. The post-carbonized structures showed a 30.4% increase in specific BET surface area than the ZSM-5 nanopowder with the CO2 uptake of 2.15 mmol/g. To investigate the CO2 separation properties, secondary pellet structures were developed with a tensile strength up to 6.46 MPa. The CO2 uptake rate for pellets was 2.3 mg of CO2/g/s without any performance decay after the first cycle with the simulated mass transfer coefficient of 1.24 m/s.[1] J Wang. Decentralized biogas technology of anaerobic digestion and farm ecosystem: Opportunities and challenges, Front.Energy Res. 2 (2014).[2] RV Siriwardane, M- Shen, EP Fisher, J Losch. Adsorption of CO 2 on zeolites at moderate temperatures, Energy Fuels. 19 (2005) 1153-1159.