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Saturday, 20 July 2019

Trapping CO2 in natural "molecular cages" to produce electricity

Methane in flames at the end of a methane hydrate

In view of the tremendous increase in atmospheric CO2 levels, scientists have embarked on a race to effectively reduce the concentration of this greenhouse gas. Several techniques have been proposed in recent years, but all have proved relatively expensive and without any real counterpart. Recently, researchers have proposed a method to trap CO2 in methane hydrates; the methane thus hunted could be burned to produce electricity in return.

A method explored over the last decade could be a step forward, according to a new computer simulation. The process would involve pumping atmospheric CO2 into methane hydrates - large pools of chilled water and methane under the sea floor, under water at a depth of 500 to 1000 meters - where the gas would be stored or sequestered Permanently.

Inbound CO2 would release methane, which would be channeled to the surface and burned to produce electricity. This would fuel the sequestration operation or generate income to pay for it. There are many deposits of methane hydrate along the coast of the Gulf of Mexico and other coasts. Large power plants and CO2-emitting industrial facilities also border the Gulf Coast.

Methane hydrates form naturally at the bottom of the seas and oceans. They constitute cages trapping methane molecules escaping from geological cracks. Credits: Janet Kimantas

An option would be to capture the gas directly from the nearby chimneys, preventing it from reaching the atmosphere. And factories and industries themselves could provide a direct market for the electricity produced.

Trapping atmospheric CO2 in methane hydrates

A methane hydrate is a deposit of frozen water molecules, similar to a crystal lattice. The unstable network includes many empty molecular-size pores, or "cages," that can trap methane molecules rising through cracks in the rock below. The computer simulation shows that the extraction of methane with CO2 is greatly improved if a high concentration of nitrogen is also injected and the gas exchange is a two-step process.

In one step, the nitrogen enters the cages; this destabilizes the imprisoned methane, which escapes from the latter. In a separate step, nitrogen helps CO2 to crystallize in empty cages. The disturbed system " seeks to reach a new equilibrium; the balance goes to more CO2 and less methane e "explains Kris Darnell, lead author of the study published in the journal Water Resources Research.

A methane hydrate forms a real molecular cage that can trap several types of gas. The method proposed by the researchers aims to drive the methane out of the cage and replace it with CO2; the methane is then burned out to produce electricity. Credits: AMU

A group of laboratories, universities and companies tested the technique in a limited 2012 feasibility test on the North Slope of Alaska, where methane hydrates are formed in sandstone under deep permafrost. They sent CO2 and nitrogen through a pipe in the hydrate. Part of the CO2 was eventually stored and methane was released in the same pipe. " It's good that Kris Darnell can make progress, " says Ray Boswell of the US Department of Energy's National Energy Technology Laboratory.

Brine: an alternative to methane hydrates

The new simulation also showed that CO2 exchange for methane would probably be much larger and faster if CO2 penetrated at one end of a hydrate pool and methane was collected at a far end. . The concept of the technique is quite similar to that of Steven Bryant and other researchers at the University of Texas, presented in the early 2010s.

In addition to numerous deposits of methane hydrates, the Gulf Coast includes large pools of warm salt brine in sedimentary rocks beneath the coast. In this system, the pumps would send CO2 down through one end of the deposit, forcing the brine into a pipe at the other end and rising to the surface. There, the hot brine would circulate in a heat exchanger, where the heat could be extracted and used for industrial processes or to generate electricity.

The upstream brine also contains methane that could be siphoned and burned. CO2 dissolves in the underground brine, becomes dense and flows further underground, where it remains theoretically trapped.

Interesting methods still too economically unviable 

Both systems face great practical challenges. A concentrated flow of CO2 is created; gas represents only 0.04% of air and about 10% of stack emissions from a power plant or industrial plant. If an efficient system using methane hydrate or brine requires a 90% CO2 input, for example, the gas concentration will require a huge amount of energy, which makes the process very expensive. " But if you only need a 50% concentration, it could be more interesting, " says Bryant. " You have to reduce the cost of capturing CO2 ".

Another major challenge for the methane hydrate approach is to collect released methane, which could simply escape the deposit through numerous cracks, and in all directions.

Given these realities, there is little economic incentive to use methane hydrates to sequester CO2. But with increasing atmospheric concentrations and global warming, systems capable of capturing gas while providing the energy or revenue needed to run it could become more viable than techniques that simply extract CO2. from the air and trap him without offering anything in return.

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