In this study, activated carbons (ACs) were synthesized and tested as CO2 sorbents. In-house ACs were prepared starting both from a traditional biomass (i.e. oak wood) and from an unconventional macroalgal seaweed (i.e. Laminaria hyperborea). In addition to this, a biomass-derived commercial AC was studied as a sorbent on which polyethylenimine (PEI) was impregnated.
Biochars were produced both by pyrolysis at 800 °C and by hydrothermal carbonization (HTC) at 250 °C. Pyrolysis chars generally had higher fixed carbon and lower volatile content compared to hydrochars. Moreover, seaweed-derived chars exhibited significantly larger ash content than that measured for oak wood-based chars. Pyrolyzed and HTC-treated biomass were then activated either by physical (CO2) or chemical (KOH) treatment. Limited texture development of the biochars was observed after CO2 activation, yet this treatment proved to be more suitable for the creation of narrower micropores. By contrast, KOH activation, followed by HCl washing, led to a more dramatic texture enhancement (but to lower narrow micropore volumes) and higher purity of the ACs due to a significant demineralization of the chars. The morphology of all materials was examined by Scanning Electron Microscopy (SEM) which revealed the creation of larger pores after KOH activation, whereas chars and CO2-ACs generally showed an undeveloped porous matrix along with particles anchored onto the carbon structure. Furthermore, Energy-Dispersive X-ray spectroscopy (EDX) analyses corresponding to the SEM micrographs proved that these particles were inorganic. In particular, Ca compounds predominated in oak wood-based samples. For macroalgae-derived materials, a significant proportion of alkali (i.e. Na, K), alkaline-earth (i.e. Ca, Mg) metal ions and Cl was detected, along with high levels of Cl. Conversely, reduced or negligible levels of inorganic fractions were detected for all KOH-ACs, which confirmed that demineralization occurred upon HCl washing. The identity of inorganic species was revealed by X-Ray Diffraction (XRD) patterns. In particular, calcium oxalate and Ca(OH)2 were identified in oak wood chars, whereas CO2-activated derivatives had CaCO3 as their main crystalline phase. For macroalgae-based materials, KCl and NaCl were found to be the dominant crystalline phases. In addition, MgO was also identified in pyrolyzed seaweed and in its CO2-activated counterpart. By contrast, a partial or total lack of crystalline phases was found for all KOH-ACs, thus offering further evidence of the loss of inorganic species after HCl rinsing. The intrinsic alkalinity of biomass-derived chars and CO2-ACs was corroborated by the great amount of basic surface groups, whose number was lower for KOH-ACs.
CO2 sorptions by chars and ACs were initially measured at T=35 °C, PCO2=1 bar, and Ptot=1 bar by using Thermogravimetric Analysis (TGA). Sorbents showing promising behaviour were then tested for capture of CO2 under simulated post-combustion conditions (T=53 °C, PCO2=0.15 bar, and Ptot=1 bar). Unmodified ACs showed relatively high sorption capacity (up to 70mg CO2∙g-1) at higher partial pressure and lower temperature. Nonetheless, the ACs’ sorption capability dramatically decreased at lower partial pressure and higher temperature. However, the biomass feedstocks included in this work proved to be advantageous precursors for sustainable synthesis of CO2-selective sorbents under post-combustion conditions. In particular, Ca(OH)2 and MgO intrinsically incorporated within the raw materials enabled production of highly basic “CO2-philic” sorbents without applying any chemical modifications. The best virgin ACs also exhibited fast adsorption kinetics, excellent regeneration capacity and good durability over ten Rapid Temperature Swing Adsorption (RTSA) cycles. On the other hand, the CO2 uptake of optimally-PEI modified commercial AC was up to 4 times higher than that achieved by the best performing unmodified AC. PEI impregnation was optimized to maximize post-combustion uptakes. In particular, the influence of various parameters (i.e. PEI loading, stirring time of the PEI/solvent/AC mixture, solvent type and sorption temperature) on the post-combustion capture capacity of the PEI-modified ACs was assessed. Interestingly, longer agitation engendered efficient dispersion of the polymer through the porous network. Additionally, a more environmentally friendly (i.e. aqueous) impregnation enabled uptakes nearly as large as those attained when the impregnation solvent was methanol, despite using lower amounts of polymer and shorter impregnation runs. In addition, when measuring uptakes under simulated post-combustion conditions but at 77 °C, optimization of aqueous PEI impregnation led to a sorption capacity larger than those achieved by the best performing PEI-loaded ACs impregnated using methanol as solvent. The use of an oak wood-derived carbon support or monoethanoloamine (MEA) as impregnating agent did not lead to any significant improvement of the CO2 sorption capacity. On the other hand, tetraethylenepentamine (TEPA)-impregnated AC slightly outperformed the optimally-PEI loaded sorbent, but the use of PEI was preferred because of its thermal stability. The addition of glycerol to the PEI/solvent/AC blend resulted in lower CO2 uptakes but moderately faster adsorption/desorption kinetics along with comparable “amine efficiency”. In addition, PEI-loaded AC showed larger CO2 uptakes and faster kinetics than those attained, for comparison purposes, by Zeolite-13X (Z13X). Furthermore, amine-containing ACs were found to be durable and easy to regenerate by RTSA at 120 °C. This CO2 desorption required ca. one third of the energy needed to regenerate a 30% MEA solution (i.e. the state of the art capture technique), thus potentially implying a lower energy penalty for the PEI-based technology in post-combustion power plant.
Overall, at higher partial pressure of carbon dioxide, textural properties were the dominant parameter governing CO2 capture, especially at lower temperatures. This CO2 physisorption appeared to be governed by a combination of narrow microporosity and surface area. In contrast, at increased temperature and lower partial pressure, basic (alkali metal or amine-containing) functionalities were the key factor for promoting selective chemisorption of CO2.
