Climate

A New Approach to Carbon Storage: Lakes of Liquid CO2 on the Ocean Floor

A New Zealand-based engineer says dumping CO2 in an deep ocean trench off the coast of Sumatra offers a possible storage site for captured emissions.

A novel but potentially flawed proposal to put carbon dioxide in deep ocean trenches illustrates the thorny challenge of sequestering the greenhouse gas most associated with climate change.

Scientists for years have examined whether dumping carbon dioxide in the deep ocean would be a good way of keeping the gas out of the atmosphere.

They already know they can’t put it in shallow waters. The seas are already a dumping ground for much of the carbon dioxide that humankind produces. The consequences of that pollution are clear today. The gas dissolves in the water, forming carbonic acid that is altering the ocean’s chemistry and hurting marine life.

But in most of the world, below around 2,800 meters (1.7 miles) water pressure turns carbon dioxide into a hydrate, a solid mix of the chemical and water.

Starting from that premise, independent energy analyst Steve Goldthorpe recently published a study in the journal Energy Procedia proposing to dump hydrated carbon dioxide in the Sunda Trench off the west coast of the Indonesian island of Sumatra and other oceanic valleys that are more than 6,000 meters deep.

“A lake of liquid CO2 on the seafloor at that depth would sit there,” said Goldthorpe. “The example would be to put some oil and vinegar in a jar like salad dressing.”

What’s more, the Sunda Trench could accommodate roughly 21 trillion tons of carbon dioxide, or more than the total amount now in fossil fuel reserves worldwide, argued the New Zealand-based chemical engineer who formerly worked for the National Coal Board in Britain.

“There is room,” he added.

Goldthorpe envisioned tanker ships like those that transport liquefied natural gas bringing pressurized carbon dioxide to barges in the ocean. The barges would then pipe the gas down to a depth where it turned solid and sank into yawning trenches. A network of undersea pipes running from power stations into the sea could do the same thing.

Unlike pumping carbon dioxide into the ground, like into disused gas fields or salty aquifers — currently the most widely accepted way to sequester carbon — people could easily keep tabs on the carbon dioxide lakes.

“The beauty of putting CO2 on the sea floor is that you can actually go down there with a submarine and have a look and see what’s happening,” said Goldthorpe.
  
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Other climate scientists have poked holes in the theory.

NOAA Oceanographer Christopher Sabine noted that scientists had already tested out Goldthorpe’s proposal — sort of. In 1999, researchers brought a beaker of liquefied carbon dioxide to a depth of more than 3,600 meters off the California coast and watched as it expanded and spilled over into a snowy hydrate, which became around four times larger in volume than it had in liquefied form.

Problem was, though, the snowy hydrate didn’t just sit there.

“This idea that you can put this CO2 on the bottom of the ocean and it will remain inert is wrong,” said Sabine. “The slightest bit of current that came by wafted this stuff up so that it then dissolved. It doesn’t just stay there. It’s not like a lake of C02. How do you know that water is not going to move out of the trench?”

‘Good luck with that.’

Other scientists have suggested a plastic sheath to keep the icy hydrate in place. Goldthorpe didn’t think that would be necessary, especially at the depths that he’s discussing. He believed the hydrate would at least form a crust that would keep it in place.

Harvard University geology, environmental science, and engineering professor Daniel Schrag disagreed. The hydrate would sooner or later dissolve, he said. But if it took hundreds or thousands of years to do so, that might be long enough to curb climate change or give scientists time to come up with better solutions to the problem.

For Schrag, the biggest issue with Goldthorper’s idea was logistical. Trenches are far away from power sources. The cost of capturing carbon dioxide, liquefying it, shipping it, and then pumping it into a trench are exorbitant, especially given the low cost of oil and natural gas.

“You are telling me that China is going to load its CO2 into tankers, go to the Sunda trench, and pump it down?” he asked. “Good luck with that.”

American power plants, oil and natural gas fields and other emitters could inject carbon dioxide underground at a cost of around $20 per ton of the stuff, said Schrag. Capturing it at the smokestack might cost two to four times as much, depending on estimates, he said. Today, power utilities and others have few economic incentives to invest in the technology to do either.

“The problem is not that we have good places to put it,” he said. “The problem is we don’t have a political will to impose a price on carbon high enough to make carbon capture viable.”

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Lastly, even if the hydrate stayed in trenches and it was affordable to put it there, scientists don’t know if they might harm fragile ecosystems at the bottom of the sea, Sabine added.

“We have not explored those trenches enough to know if there are life forms there,” said Sabine. “It is not safe to assume that these trenches are dead.  There could be unique life forms that we don’t see in other places that live in these trenches.”

Still, Sabine felt Goldthorpe’s proposed was a useful exercise given how many folks reject the need to sequester carbon dioxide at all.

“It’s worth having these discussed even though I don’t agree with these conclusions,” he said. “There are a lot of people who don’t want these discussions to even happen. The problem is, the discussions are going to happen anyway, and if you don’t have it in the scientific literature and respond to it, you end up with discussions based on hearsay.”