Carbon capture technologies are measures used to reduce greenhouse gas emissions. But it has often been overlooked and overshadowed by renewable energy growth – which is the most crucial strategy to eliminate our dependence on fossil fuel. Yet, carbon capture and removal’s importance has been recently recognised as an essential part to fight against climate change by the IPCC. So, what is carbon capture and storage, and what are the carbon capture technologies that we currently have?
Carbon capture and storage (CCS) refers to the action of capturing carbon dioxide and storing it, mostly underground. While most people might think CCS to be a brand-new concept since the recent emergence of direct air capture (DAC) projects, technologies that absorb carbon dioxide directly from the air have been around since the 1970s. Indeed, direct air capture only accounts for only a small part of the whole CCS industry. According to the latest Intergovernmental Panel on Climate Change (IPCC) report, carbon capture technologies are not only an alternative, but “unavoidable” to reduce global greenhouse gas emissions and limit global warming to under 1.5°C. While the costs and energy requirements of direct air capture operations remain high; the synchronous development of conventional methods of CCS is still very much relevant.
Conventional Carbon Capture and Storage Methods
Instead of capturing carbon dioxide directly from the air, conventional methods of CCS capture it in point sources of pollution. Since we know that point-source generation of electricity is the world’s largest contributor to greenhouse gas emissions, this method can be efficient in identifying and capturing emissions from fossil fuel plants, for example.
However, the disadvantage being that it’s near impossible to capture carbon from nonpoint sources, such as transportation, as a sector which accounts for a quarter of the emissions in the US. Furthermore, in order to store it underground, carbon dioxide needs to be first compressed with high pressure. And to do so, it must be separated from other gases and pollutants. Though not a particularly difficult process to do, it can be costly. Still, this method is cheaper compared to direct air capture methods.
Yet, direct air capture appears to be more attractive to governments due to the fact that the implementation of DAC is less restrictive than conventional methods, as the latter greatly requires the autonomy and cooperation of the industries. At the same time, the public opposes the idea of providing additional subsidies to these big polluters. But this indicates we have the capacity to scale up CCS technology more rapidly if incentives are given to industries to adopt these conventional methods. Here are a few existing methods to separate pure carbon dioxide from impurities.
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Absorption is a post-combustion method to capture carbon dioxide. It requires channelling the flue gas (combustion exhaust gas produced at power plants) generated during the combustion to a scrubber to separate carbon dioxide from other gases. The only chemicals that have been in commercial use are amines, derivatives of ammonia. Carbon dioxide reacts with the amines in the scrubber to form soluble carbonate salt, which will then be transferred and heated in a boiler to reverse the chemical reaction, dissolving the carbonate salt into pure carbon dioxide and amines. Afterwards, the pure carbon dioxide will be compressed and transported to the site of storage while the amines will be reused to absorb further carbon dioxide in the scrubber.
The first commercialised amine-based carbon capture began in Norway in 1996. Yet despite this, the technology has not been adopted by most power plants due to its high price of operation and the lack of public funding.
Recently, another material has been proven to be able to absorb carbon dioxide, resembling the amine-based process: plastic. Experiments done by Rice University in Houston, Texas have shown that pyrolysing plastic heated at elevated temperatures mixed with a certain chemical element, potassium acetate, produces porous particles that can hold up to 18% of its weight in carbon dioxide. An added benefit of this technology is that it can tackle two of the most pressing environmental problems simultaneously: plastic waste and carbon dioxide emissions. Estimates suggest that the cost of capturing carbon using a mixture of plastic and potassium acetate can reach up to USD$21 per ton of carbon dioxide, which is much less expensive than amine-based absorption that costs $80 to $160 per ton.
Instead of separating carbon dioxide from impurities after combustion, chemical looping is a method that aims to create pure carbon dioxide without impurity during the combustion in the first place. To create pure carbon dioxide, the fuel must be burned with pure oxygen as a substitute for air, where many other gases like nitrogen are present. Oxy-fuel combustion reduces the amount of flue gas produced by about 70%. By adding the capture of pure carbon dioxide in the equation, this process has the potential of capturing 100% of the carbon dioxide in a point source.
Traditionally, air separation is possible with the usage of a cryogenic air separation unit (ASU), a machine that is operated at extremely cool temperatures to liquify air and then distillate oxygen to the form of liquid. However, this technology is very costly and energy-intensive, reducing the incentive for the industry to adopt it.
Chemical looping, however, is an innovative form of oxy-fuel combustion that does not require the usage of an expensive ASU. Chemical looping is based on two chemical reactions – oxidation and reduction. In layman’s terms, oxidation is the gain of oxygen like rusting, while reduction is the loss of oxygen like photosynthesis. Chemical looping requires two reactors. In the first one, small particles are exposed to air, and special equipment and elevated temperatures are used to accelerate oxidation so that the rusted metal particles contain high-purity oxygen over a short period. Then, the particles will be transferred to a second reactor, where the reduction takes place. Here, the particles and the fuels meet and the oxygen that the particles carry will burn the fuels and generate high-purity carbon dioxide that is capable of being compressed. Subsequently, the rusted metal particles convert back to pure metal and are once again transferred to the first reactor to go through the processes of oxidation and reduction.
The Future of CCS
Despite the huge potential of carbon capture technologies, only about 0.1% of carbon dioxide is captured and stored worldwide each year. The underuse of CCS is a baffling phenomenon as technologies like amine-based carbon capture have been proven feasible in theory and practice, unlike some renewable energies that are at a bottleneck. Indeed, besides the lack of incentives, there are many opponents of the investment in CCS as they think that CCS is a justification for the continuation of burning fossil fuels and distracting from direct emission reduction measures.
This is a legit concern, but CCS will be necessary for the renewable energy transition as noted by the IPCC. Current renewable energy generation is not sufficient enough to bear our huge electricity demand. Moreover, CCS is also relevant in renewable energy growth since not all renewables are ‘green’; geothermal energy in some places can emit even more carbon dioxide than fossil fuels depending on the quality of the geothermal reservoirs.
Carbon capture technologies are key to reducing global greenhouse emissions and need in the era of renewable energy. With strong investment and resources, we have the potential to utilise them to achieve a net-zero future.
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Featured image by: The Climate Group/Flickr (CC BY-NC-SA 2.0)