Carbon dioxide (CO2) mitigation is now seen as one of the most important areas of research around the world. Threats to our very climate from rising CO2 levels are now becoming more apparent with freak weather occurrences happening ever more frequently. Such weather occurrences have been linked to global warming and rising CO2 levels in our atmosphere. CO2 is emitted globally from many sectors, with the majority coming from fossil-fuel-burning industries and vehicles. Industry emitters are known as point source and transport emitters are known as mobile source. Mobile sources will require CO2 capture from air and advances in engine efficiencies or alternative fuels in order to reduce CO2. The fixed nature of point sources makes CO2 capture much more feasible. The ability to reduce or capture the CO2 from large point source emitters is a major challenge currently facing researchers of today.
With regards to CO2 capture from point sources, state-of-the-art processes for CO2 capture do exist but are plagued by issues hindering them from working efficiently and cost effectively. These processes often use amine-based sorbents such as monoethanolamine (MEA), which have a high affinity to CO2. Based on a temperature swing method, such processes use heat to remove chemically bonded CO2 from the sorbent. The sorbents are often ‘lost’ to vapourisation during the regeneration and be continually topped up. Funding towards further research of such processes, at least in the UK has been reduced, calling for new, more energy-efficient sorbents and processes to be investigated. With CO2 emissions ever increasing, an alternative cost-effective approach to CO2 mitigation cannot come quick enough.
As an alternative avenue for CO2 capture from point sources, room temperature ionic liquids or RTILs have been increasingly studied for their application as CO2 capture sorbents. They have shown promising results in literature, having good capacity for CO2, excellent selectivity and almost negligible vapour pressure. Even the most promising RTILs are not without their problems and generally, ionic liquids have extremely high viscosities leading to handling or potential pumping issues if used in large-scale process plants. Gas-liquid contacting methods for ionic liquids are also lacking in maturity, since diffusion coefficients of CO2 into the bulk liquid are extremely low, leading to absorption periods of up to 5 hours for capacity to be reached. The cost of the sorbent is currently not well established, with synthesis usually only taking place at small scale. It is difficult to ascertain figures for potential synthesis at industrial scale, however, most sources on the matter suggest that ionic liquids are too expensive to ever be commercialised.
Contradictorily named solid ionic liquids or SoILs are little researched in the literature, however, have many excellent properties potentially making them good capture agents for CO2. SoILs are from the same chemical family as RTILs however at room temperature, they are solids. Their capacity for CO2 was found to be marginally lower than that of RTILs but; adsorption rates are extremely fast, with full CO2 loading capacity being reached in seconds rather than hours. The ability to adsorb CO2 at such a high rate has led us to believe that capacity becomes much less of an issue when such a sorbent is twinned with a pressure swing separation process. SoILs retain the desirable traits seen in RTILs such as stability and selectivity but are able to overcome previous issues with viscosity and uptake rate based on their solid crystalline structure and surface interaction with CO2. SoILs have been observed to undergo further improvements by supporting them on simple cheap host particles, in particular, cellulose. The advantage of this is two-fold; firstly, if the SoIL layer is thin enough, the surface properties of the host particle is imparted onto that of the SoIL, increasing surface area and thus potential sites for CO2 adsorption. Secondly, due to the ‘inner core’ of the SoIL now being replaced by a much cheaper support, the sorbent volume required is vastly reduced, thus giving a potential cost reduction by way of sorbent volume used by up to 75%.
While investigating other supporting materials, it came to light that silica (SiO2) particles had an excellent capacity and selectivity towards CO2 even without any functionalisation. Silica is extremely robust, is cheap and is readily available for large-scale applications since manufacture of such particles is mature. Uptake kinetics were still viable for use in a pressure swing process. A pilot scale CO2 capture process, containing a 0.5 m tall single packed bed adsorber was designed, built and optimised to allow a 12.5 vol% CO2 simulated flue gas to be refined to give a 90 vol% CO2 output product stream. The working concept was further proven when the pilot scale plant was connected to an argon plasma reactor, designed to disassociate CO2 for further reaction.
