Activated carbons derived from Victorian brown coal as adsorbents for CO₂ capture
2017-03-03T06:03:45Z (GMT) by
Atmospheric levels of CO₂ have increased from 275-285 ppm prior to the industrial revolution, to 400 ppm in 2013. CO₂ and other greenhouse gases (GHGs) have been implicated in global warming, and climate change. Due to increasing environmental, social and economic pressures, the release of anthropogenic CO₂ must be minimised. The work described in this thesis focuses on the development of solid carbon adsorbents and adsorbent supports for the post-combustion capture of CO₂ from coal fired power plants. One of the major hurdles for the implementation of adsorbent-based carbon capture technology is the cost of the adsorbent. With this in mind, the objective of this research was to investigate the potential to convert inexpensive and readily available Victorian brown coal (VBC) into activated carbons (AC) for application as CO₂ adsorbents. Three series of microporous ACs (miACs) were produced from VBC and VBC-derived chars by either chemical or physical activation at 1073 K in a fixed-bed reactor. Physically activated miACs were produced using mild steam gasification conditions for extended time periods to encourage the development of micropores. Chemical activation was undertaken using KOH as the activating agent at doping levels < 10 wt%, followed by carbonisation. A further two series of mesoporous ACs (meACs) were prepared by catalysed steam activation using lanthanoid oxide catalysts of the form LnxOy (where Ln was Ce or La), to produce materials with larger pore diameters amenable for subsequent amine-modification. Several of the most promising ACs were further investigated as supports for polyethylene imine (PEI). All the materials prepared using these methods were thoroughly characterised in terms of their chemical composition, surface chemistry, physicochemical structure and surface morphology using a combination of analytical techniques, including x-ray photoelectron spectroscopy (XPS), near edge x-ray adsorption fine structure (NEXAFS) spectroscopy, Raman spectroscopy, powder x-ray diffraction (P-XRD), gas physisorption and transmission electron microscopy (TEM). The gas separation behaviour of the carbon adsorbents was evaluated using a thermogravimetric analyser equipped with a gas dosing manifold to simulate the partial pressure conditions that would occur in a vacuum swing adsorption (VSA) process. The measurements were undertaken using gas mixtures of Ar and CO₂, in a procedure that is referred to as partial pressure swing adsorption (PPSA). Several promising miACs were identified through the PPSA studies. Two of the miACs in particular (AC-90 and AC-K10), outperformed a leading commercial carbon (AC-N) for CO₂ adsorption at all temperature and CO₂ partial pressure conditions investigated, despite having smaller pore volumes. Intensive surface chemistry studies using XPS and NEXAFS spectroscopy revealed that the surface of AC-90 and AC-K10 possessed much higher concentrations of hydroxyl and carboxylic acid species than AC-N, which increased the affinity of the surfaces of these ACs for CO₂. This clearly highlighted the importance that AC surface chemistry can play in adsorption processes, a fact that is often overlooked. Importantly, PPSA studies performed in the presence of H₂O, revealed very little competitive adsorption over AC-90, whilst AC-K10 exhibited significant competitive adsorption with H₂O. The results for AC-90 were interesting, as ACs that adsorb appreciable amounts of CO₂ also adsorb significant amounts of H₂O. This may make the VBC derived carbons attractive for industrial scale CO₂ separation, on account of their relatively high CO₂ adsorption capacity and low cost. In an attempt to further develop the mesoporosity of the ACs, LnxOy was used to catalyse partial oxidation reactions during the gasification of VBC. However, this approach was not as successful as had been desired. Through the development of protocol to post-synthetically remove the LnxOy using H₂SO₄, it was found that much of the mesoporosity of similar materials presented in the literature was actually due to the porosity inherent to the LnxOy itself. From the PPSA studies, the Ln-meACs exhibited lower CO₂ adsorption relative to the miACs. This was not surprising on account of the differences in surface chemistry and pore structure existed between the miACs and meACs. Selected carbons were used as supports for polyethylene imine (PEI). All of the PEI-AC composites exhibited higher CO₂ capacities than their corresponding support materials. However, post-synthetic vacuum treatment of the PEI-AC composites reduced the CO₂ adsorption capacity, possibly by drawing the PEI deeper into the carbon pores, leading to stronger interactions between the PEI and the carbon such that the PEI mobility is restricted. The ACs presented in this thesis were produced from VBC which is a readily available, easily accessible and inexpensive starting material. Several promising carbon adsorbents for CO₂ were identified and found to outperform a leading commercial carbon. Moreover, many of the ACs that showed high CO₂ adsorption capacity did not undergo competitive adsorption with H₂O which may indicate their potential for industrial scale CO₂ capture. The viability of these materials should be investigated further in bench scale studies. It is likely that further improvements to both the development of the pore structure and the surface chemistry of the ACs could be achieved through adjustments to the activation process, such as gasification rate.