Global warming is a topic of widespread public concern nowadays. Industrial development over past hundred years has led to a huge increase in fossil fuel consumption and CO2 emissions, which causes increase in atmospheric CO2 concentration. A survey report portrays that between the year 2004 – 2011, global CO2 emissions were contributed by energy uses (26%), power plant (55%), transportation (23%) and industry (19%). This increased CO2 is believed to be responsible for a significant rise in global temperature over the past several decades .
There are three options to reduce total CO2 emission into the atmosphere:
- Reducing energy intensity,
- Reducing carbon intensity, and
- Enhancing the sequestration of CO2.
The first option requires efficient use of energy. The second option requires switching to using non-fossil fuels such as hydrogen and renewable energy. The third option involves the development of technologies to capture and sequester more CO2.
The high capital cost of implementing capture technologies means that they are best suited to plants that generate a high volume of concentrated CO2 emissions. Depending on the type of plant, CO2 capture may use one of three different technologies: pre-combustion, post-combustion and oxy-fuel combustion.
The idea of pre- combustion is to remove carbon from the fuel before combustion. This is done by transforming the fuel into a synthetic gas comprising mainly carbon monoxide and hydrogen. Then water vapor is added, which reacts with the carbon monoxide converting it into CO2. The CO2 and the hydrogen are then separated using an amine-type solvent. The hydrogen is used to produce the required energy, without any CO2 emissions.
Post combustion capture consists in extracting CO2 diluted in the flue gases produced by combustion in air of a fossil fuel or biomass. This technique is important because:
- It offers flexibility
- It can be implemented on installations, which are already in operation
- It is the leading candidate for gas fired power plants
Oxy combustion is combustion in pure oxygen rather than air. In a conventional combustion scheme air is used, but this generates a large volume of smoke and fumes, where the CO2 is much diluted. This technique is better than the other two in terms of both cost and energy efficiency.
There are the three basic technologies for CO2 separation: Separation with sorbent/solvent, cryogenic distillation and membrane processes. Adsorption is a heterogeneous process where CO2 molecules are attracted and trapped by surface groups of the sorbent or physisorption due to interactivity between sorbent and guest molecules. These processes do not give sufficient experimental data on performance after multiple adsorptions/desorption cycles. Also pressure and temperature swing desorption approaches have not been adequately researched. Absorption (i.e. solvent scrubbing) is a well-established CO2 separation approach which is divided into two categories:
- Physical (absorption occurs at high pressures and low temperatures) and
- Chemical where absorption of CO2 depends on the acid–base neutralization reaction
But this process has some disadvantages like the solvent cannot be fully regenerated, the reclaimed stage is energy intensive and the waste stream can be hazardous. Cryogenic distillation uses a principle of separation based on cooling and condensation, and has been used in liquid separations for a long time. It enables direct production of liquid CO2 at a low pressure, so that the liquid CO2 can be stored or sequestered via liquid pumping instead of compression of gaseous CO2 to a very high pressure, thereby saving on compression energy. During the cryogenic separation process, the components of gas mixtures are separated by a series of compression, refrigeration, and separation steps which makes the process very expensive. The heart of membrane technology is the membrane which acts as a filter to selectively remove one or more gas components from a mixture. Among all three processes membrane separation technology is preferred as it consumes less energy, uses environment friendly materials and easy to handle. The first successful industrial membrane gas separation systems were built by Monsanto in 1979 −1980 which is used to separate hydrogen from the nitrogen, argon, and methane in ammonia synthesis plant purge gas [2,3]. Based on the material properties the membranes can be classified as ceramic, polymeric and mixed matrix membranes.
Ceramic membranes are a type of artificial membranes made from inorganic materials (such as alumina, titania, zirconia oxides, silicon carbide or some glassy materials). They are used in membrane operations for liquid filtration. By contrast with polymeric membranes, they can be used in separations where aggressive media (acids, strong solvents) are present. They also have excellent thermal stability which makes them usable in high-temperature membrane operations.
Typically, polymers have the advantages of desirable mechanical properties and economical processing capabilities. Pure polymer membranes for CO2 separation have a few distinct advantages over other materials, such as very low production cost and generally high gas fluxes. Plasticization (depression of the glass transition temperature usually accompanied by drastic changes in membrane properties) in the presence of CO2 is also a serious issue for polymeric materials. Now-a-days most of the asymmetric polymeric membranes used in membrane separation processes are prepared by phase inversion process.
The recent development of polymeric and inorganic materials has seemingly reached a limit in trade off between selectivity and permeability. The deficiencies of these materials have in turn switched the focus of researches towards a new class of membrane material, namely mixed matrix membrane (MMM) which is expected to play important role in CO2 separation in future .