Abstract
Biogas is a potential renewable energy resource that can reduce the current energy dependency on fossil fuels. The major limitation of utilizing biogas fully in the various applications is the presence of a significant volume fraction of carbon dioxide in biogas. Here, we used adsorption-driven CO2 separation using the most prominent adsorbents, NaX (faujasite) and CaA (Linde Type A) zeolites. The NaX and CaA zeolites were structured into hierarchically porous granules using a low-cost freeze granulation technique to achieve better mass transfer kinetics. The freeze granulation processing parameters and the rheological properties of suspensions were optimized to obtain homogenous granules of NaX and CaA zeolites 2–3 mm in diameter with macroporosity of 77.9% and 68.6%, respectively. The NaX and CaA granules kept their individual morphologies, crystallinities with a CO2 uptake of 5.8 mmol/g and 4 mmol/g, respectively. The CO2 separation performance and the kinetic behavior were estimated by breakthrough experiments, where the NaX zeolite showed a 16% higher CO2 uptake rate than CaA granules with a high mass transfer coefficient, 1.3 m/s, compared to commercial granules, suggesting that freeze-granulated zeolites could be used to improve adsorption kinetics and reduce cycle time for biogas upgrading in the adsorption swing technology.
Highlights
Biogas offers a sustainable fuel to the environment as it contributes to low carbon emissions during its combustion
NaX zeolite showed a 16% higher CO2 uptake rate than CaA granules with a high mass transfer coefficient, 1.3 m/s, compared to commercial granules, suggesting that freeze-granulated zeolites could be used to improve adsorption kinetics and reduce cycle time for biogas upgrading in the adsorption swing technology
Great efforts have been made in the biogas separation and purification technologies to procure biomethane with a low energy footprint [3,4]
Summary
Biogas offers a sustainable fuel to the environment as it contributes to low carbon emissions during its combustion. The removal of CO2 from biogas increases the methane content to above 98 vol%, which offers the use of upgraded biogas in natural gas grids and as a fuel for automotive industry [5]. In the context of biogas upgrading using the PSA technology, the structured adsorbent in the form of granules, pellets, laminates, and extrudates of zeolites, microporous silicates and activated carbon are utilized to separate CO2 from the biogas stream by exploiting equilibrium, kinetic, as well as size-selective adsorption strategies [9,10,11]. Zeolites X, A and chabazite offer promising properties, such as remarkable selectivity of CO2 over CH4 , high thermal stability, fast adsorption kinetics, and cyclic performance, which together reduce the energy penalty in the separation and purification process [14,15,16]
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