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Posted: Jul 24, 2013

Want to capture carbon? Look to nanomaterials

(Nanowerk News) Much of Australia’s large scale electricity generation comes from coal-fired power plants. Given the cost of building alternative electricity infrastructure and Australia’s large reserves of coal, this is likely to continue in the near future.
Unfortunately, the CO2 produced from burning coal is a major source of Australia’s greenhouse gas emissions and a major contributor to man-made climate change.
Carbon capture and storage (CCS) schemes are an ideal stop-gap technology to mitigate these environmental problems, while allowing the country to continue reaping the benefits of the coal industry. But currently devised schemes are highly energy-intensive and costly.
Recently, a research project that I’m involved in has developed a new nanomaterial that could offer a more efficient alternative to current carbon capture technologies.
The problem with capturing carbon
Pilot demonstrations of carbon capture schemes are already underway, including an Australian project at a site in Victoria, where CO2-rich waste gas is being stored in a depleted natural gas field.
However, two major challenges need to be addressed to make the technology feasible on large scales.
The first is the need to find ways to separate CO2 after burning the coal. The most popular method is using adsorbents (materials that can chemically absorb other materials) that can separate CO2 from flue gas. Flue gas, the gas produced by coal combustion, is a mix of nitrogen, oxygen, oxides of sulphur, and water. Flue gas is passed through the adsorbent, where chemicals in the solution bind to CO2 and separate it from the other gases.
But separating CO2 from the flue gas mixture presents a major challenge – it uses a lot of energy. The adsorbent can be reused, but to do so the chemical reaction has to be reversed, and this takes heat. In fact this process can consume a quarter of the energy produced by the power plant in the first place.
The second challenge is the processes required to store or convert the large volumes of CO2-rich gas that gets left over. As noted above, storage in depleted gas fields is being considered but this is not an easy process.
The most attractive idea to deal with the vast quantities of CO2 – from a chemist’s perspective – are to convert it into something useful.
This could be done by recycling it into a fuel or transforming it into a building material. Using the concentrated CO2 stream to generate fuels is one approach that is being pursued.
A solid solution
Using nanomaterials might lower the energy needed for recycling. Nanomaterials are solids – these adsorbents aren’t a solution of chemicals in water. Considerable research has focused on identifying new solid-state materials that selectively capture CO2.
In our recent work, described in the Journal of the American Chemical Society ("Post-synthetic Structural Processing in a Metal–Organic Framework Material as a Mechanism for Exceptional CO2/N2 Selectivity"), we report a new metal-organic framework (MOF) that has a remarkable capacity for separating CO2 from nitrogen. These frameworks are sponge-like nanomaterials with an extraordinary range of properties.
honeycombs
Metal-organic frameworks resemble honeycombs because they have regular pore structures. It’s this structure that makes them so efficient at sifting out gases like carbon dioxide.
Previous research into these frameworks has identified materials with the potential to separate CO2. But these approaches were not without pitfalls.
Sieving out the carbon
Our new approach is to find materials that can act like a sieve. CO2 has a slightly smaller kinetic diameter than nitrogen. This means that if a material can be made with pores sufficiently small then these gases could be separated.
This is exactly what occurs for our framework material. The solid adsorbs considerably more CO2 than nitrogen at room temperature. An added advantage of using a sieving mechanism is that recycling the material for reuse is made easier.
Membranes are a third option that could be used for carbon capture and storage. Membranes are routinely used in separation, in water and gas purification, and could provide considerable advantages for capturing CO2.
Membranes operate in a continuous flow manner where one component can permeate out through the membrane and be separated from the original mixture. One approach, that we are pursuing, is to add our selective frameworks to a membrane and, in turn, enhance that membrane’s performance.
Still a long way to go
There is still a considerable amount of research that needs to be conducted before any of the nanomaterial adsorbents described could feature in commercial carbon capture and storage systems.
Solvent-based systems remain the industry benchmarks and any system will have to outperform the existing technology. Additionally, the cost of any system is a major consideration.
A report for the US Department of Energy ("Carbon Dioxide Removal from Flue Gas Using Microporous Metal Organic Frameworks") indicates that nanomaterial adsorbents like metal-organic frameworks are a next generation technology and will likely see more widespread use in the future.
Nonetheless, emerging technologies based on nanomaterials like the one we identified represent one possible route to satisfy Australia’s energy demands while reducing our environmental footprint.
Source: By Christopher Sumby, Deputy Director, Centre for Advanced Nanomaterials; Associate Professor; ARC Future Fellow at University of Adelaide, via The Conversation
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