The controversial machine sending CO2 to the ocean and making hydrogen

Equatic is among a wave of start-ups exploring how the ocean could be harnessed to capture and store carbon. But not everyone is sure it's such a good idea.

Many scientists now think at least some carbon capture and storage technology will be needed to prevent dangerous temperature rise.

A separate challenge, but just as relevant to climate change, is the scale up of green hydrogen, which is often viewed as the key to replacing fossil fuels in areas like industry, shipping and aviation – although current production is miniscule.

So LA-based start-up Equatic's claim to have created an ocean-based carbon removal machine that can tackle both these hurdles at once has an obvious appeal.

"We have a technology that does two things pretty well," says Edward Sanders, chief executive of Equatic. "One is we take CO2 out of the atmosphere and we store that permanently. The second thing we do is produce green hydrogen."

Equatic is among a wave of companies exploring how the ocean could be harnessed to capture and store carbon in the long term, as an alternative to the more common proposal of injecting it into rocks below the Earth's surface. It's the only company, it says, which is also producing green hydrogen in the process.

However, not everyone thinks ocean-based carbon removal is such a good idea. "Marine CO2 removal is simply too risky," says Mary Church, geoengineering campaign manager at the Center for International Environmental Law (Ciel), a non-profit environmental law firm based in Geneva, Switzerland. "It could alter ocean chemistry, causing changes in nutrient levels and species abundance, with significant consequences for ecosystems." Others are concerned about the feasibility of marine carbon removal, and whether it could really put a significant dent in emissions.

With tens of millions of dollars now pouring into companies like Equatic, marine carbon removal is rapidly moving up the climate agenda. Critics argue regulators, and the rest of us, need to catch up.

The ocean has already been a vast and often unacknowledged ally in protecting humans from climate change. It has absorbed more than 90% of the heat generated from our greenhouse gas emissions and absorbs at least a quarter of our CO2 emissions. How much more CO2 it will store naturally in the future is now a subject of intense scientific interest.

Unrelenting global emissions have led many scientists to believe we now need to intervene to take large amounts of CO2 back out of the atmosphere. So far, the bulk of attention for this has been focussed on land-based techniques, such as absorbing the CO2 using trees or other vegetation, or directly capturing it from the air, then burying it deep underground.

Ocean-based carbon removal would similarly attempt to store additional carbon in the ocean, but it has not yet been widely used or thoroughly tested. It is on the rise, however, with tens of millions of dollars pouring into the sector, including from some of the biggest names in tech, such as Microsoft and Shopify, as well as several airlines.

"The ocean is so vast, natural storage is a key advantage [over land-based techniques]," says Sifang Chen, a science and innovation advisor at Carbon180, a Washington-based non-profit which advocates for CO2 removal solutions. "It's more cost efficient to store the removed CO2, and we don't need the same infrastructure like pipelines that we do for direct air capture."

The captured carbon is also highly stable, she says, and both Equatic and Ebb Carbon, another ocean-based carbon removal company based in San Carlos in California whose technology also reduces ocean acidification, are "expected to be able to remove carbon durably for over a thousand years".

Equatic's process works like this: first, it pumps sea water into an electrolyser, a machine that uses electricity to split water into hydrogen and oxygen, which in Equatic's case is run on clean electricity such as wind, solar or hydro. This converts the seawater to hydrogen gas, oxygen gas, an acid stream and an alkaline slurry of calcium and magnesium-based materials. The alkaline slurry is exposed to air, pulling out CO2 and trapping it, then discharged into the sea. A last step is to neutralise the acid waste stream using rocks (in order to avoid ocean acidification) before this is discharged into the sea too.

The CO2 captured by Equatic ends up in the ocean as dissolved bicarbonate ions and solid mineral carbonates, forms in which the CO2 is immobilised for 10,000 years and billions of years respectively, the company says. "In electrochemical methods that convert CO2 into a stable carbon like solid carbonates, the CO2 is locked away permanently," agrees Chen. "Unless that carbonate is heated to a high temperature of around 900C (1,200K), that CO2 will not be re-released."

Other disagree, however. James Kerry, a marine and climate scientist at OceanCare, a marine conservation non-profit based in Switzerland, and James Cook University in Australia, says large-scale marine CO2 removal could harm the important role oceans play in climate, food security and oxygen production. Marine CO2 removal technologies pose "significant threats to human rights and the environment, especially at scale", he says.

Key to Equatic's process, says Xin Chen, senior scientist and co-founder at Equatic, is its development of a specific oxygen-selective anode which lets it do direct electrolysis of raw seawater to make hydrogen – without also producing chlorine gas. Chlorine is a toxic and corrosive gas usually created when seawater is electrolysed, meaning that normally seawater needs to be desalinated to remove its salt (sodium chloride) before it can be used in electrolysis. "It's like a holy grail for an electrochemist, how to do the direct seawater electrolysis without extra steps to clean up the seawater," he says.

The main focus in his lab now is to produce an electrode free of a rare and expensive collection of elements called platinum group metals. Limited supplies of these metals could disrupt the supply chain as Equatic scales up, he says. The team has already created a second-generation electrode free of these metals in lab conditions, but now needs to test it outside the lab, he says.

It's still early days for Equatic, but the company is scaling. In early 2023, it began operating two pilot plants on barges in Singapore and Los Angeles, each removing approximately 30-40 tonnes of CO2 a year (equivalent to the yearly emissions of around eight cars). It's now constructing what will be the world's largest ocean-based CO2 removal plant in Singapore. This will be 100 times larger than the pilots, removing 4,000 tonnes of CO2 and producing around 100 tonnes of hydrogen a year.

Equatic is also in the early stages of building a commercial-scale capture plant in Quebec, Canada running on hydropower, and planned to have the capacity to remove over 100,000 tonnes of CO2 and produce 3,600 tonnes of hydrogen. The plant, which will cover around 30 acres (12 hectares), will come online at the end of 2026 at the earliest, says Sanders.

Such scaling is essential for a carbon removal start-up because the pressure is on to bring down costs and prove their chosen method of carbon capture is really viable. Sanders says the company has designed its technology to comply with existing environmental regulations. The challenge, he says, has been to design a system that is robust, operates in existing planning permits and at a low enough price point for carbon credits to be affordable at scale.

Carbon credits are the big-ticket goal for companies like Equatic. They work by a scheme capturing a specific amount of CO2, permanently storing it (at least in theory), and selling this removal as a "credit" or "offset" someone else can buy.

The main customers for such credits are currently companies in the voluntary market aiming at carbon neutrality, says Asbjørn Torvanger, a researcher in climate economics and policy at the Centre for International Climate Research in Norway. But more incentives could be on the way through compliance markets, he says: the EU, for example, is developing a scheme for CO2 removal certificates.

There have been a wave of scandals about the extent to which many land-based carbon-credit projects really cut or absorb emissions, including investigations by The Guardian and Bloomberg. As a result, scepticism about carbon credits is increasingly high. This has led Equatic to do its entire process inside a closed system within its plant (it had initially planned to discharge an alkaline stream into the sea, which would in theory eventually capture carbon via air-sea gas exchange). This allows the company to more easily measure and validate how much CO2 it's really capturing, says Sanders. "[It] was another whole engineering challenge the team never envisaged they would have to do."

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Equatic is targeting the goal of achieving CO2 removal at less than $100 (£79) per tonne by 2030, says Sanders. It aims to sell credits from the Quebec plant for around $200 (£158) a tonne. "We actually think pricing needs to be below $30 (£24) a tonne to properly stimulate market demand," he says. However, "we won't get there for many years", he says: at least the early 2030s.

Still, by producing hydrogen that can be sold too, Equatic has another way of monetising its process, says Sanders. While the electrolyser itself is less efficient than a traditional electrolyser at actually producing hydrogen, it all helps, he says. Equatic already has a pre-purchase option agreement with Boeing for 62,000 tonnes of CO2 removal and 2,100 tonnes of hydrogen for the plane giant to use in aviation fuels. It is currently a semi-finalised, along with a range of other carbon removal companies, to sell credits to the US government for $460 (£364) a tonne of removal as part of an incentivising project.

So how far could it go in really removing CO2 from the atmosphere? "We would be able to take down 20% of [current] global emissions with about 1,200 of these plants," says Sanders. That's, he adds, assuming far larger 1GW plants each removing some 3.6 million tonnes of CO2 per year, 36 times more than its planned commercial plant in Quebec.

He admits this is ambitious. In theory, though, Equatic would be able to scale up to this kind of level by the mid-2040s, he says. "From an infrastructure point of view… we've seen that sort of coastal deployment before," he says, noting there are more than 20,000 desalination plants dotted around coastlines across the planet.

But even as companies like Equatic are polishing off their processes and doubling down on huge scale-up plans, many are concerned that a rapid increase in the use of ocean carbon removal technologies is not such a great idea.

"At a scale to meaningfully impact the climate, marine CO2 removal would be inherently unpredictable and pose significant, new and unprecedented risks to the fragile ecosystems that sustain life on Earth," says Church. "It does nothing to address the root causes of the climate crisis. Instead, it creates the illusion of a quick 'fix', delaying real solutions to the climate crisis and prolonging reliance on fossil fuels."

Kerry is concerned that Equatic's process would "create 'dead water', killing marine life by processing seawater", noting that it would process at least 350 tonnes of seawater for each tonne of CO2 removed, according to Equatic's own estimates.

And even if the industry could scale up by a thousandfold, he argues, it would still play a negligible role in mitigating climate change, noting that even far more advanced land-based CO2 removal methods still capture only minimal CO2. "We cannot, and should not, rely on these types of technologies as a solution to the climate crisis," he says.

In 2023, the non-profit Ocean Visions published an open letter, which has now been signed by over 400 scientists, warning about the risks of ocean-based CO2 removal. "While [these] approaches have enormous potential, there are also risks," it says. "Society does not yet have nearly enough information about the effectiveness or impacts of any specific approach and so cannot make informed decisions about their use at scale." It called for "rigorous and transparent monitoring and evaluation frameworks" to be developed for these techniques.

While Torvanger says he finds Equatic's technology "interesting", he also cautions that a substantial up-scaling could have potential negative environmental impacts. "The ocean chemistry will be affected, as well as some environmental effects of mining the required amounts of rocks."

However, Sanders says process does not create "dead water", and restores the chemical balance of the sea water before discharge. He notes that Equatic comes in below globally established standards and local regulations for the concentration of discharges and will be monitoring discharge areas on an ongoing basis using buoys. It will also need to conduct marine ecosystem studies ahead of opening any plants, he adds. And while the process requires around a tonne of rock for every tonne of CO2 removed, he says, there are "significant quantities of waste rock… worldwide for this process", such as rock that has already been mined to access and ore body.

Another enormous barrier is resource and energy use, say Church, Kerry and Torvanger. Equatic estimates it would need 2.3MWh for each tonne of carbon removed, This means that drawing down 20% of the CO2 emitted globally in 2023 would use some 50% of the global electricity supply that year However, the hydrogen produced returns around a third of that energy, according to Sanders. It's also worth noting the system would be able to use flexible energy, he adds – for example, it could use extra wind or solar at times of excess.

Sanders says it is "incorrect to say Equatic does nothing to address the root cause of climate change" since it takes CO2 out of the atmosphere and stores it permanently, while also producing clean hydrogen which can replace high-emissions fossil fuels. "We need technologies that both remove CO2 and reduce CO2," he says. "Doing nothing is not an answer. We must remove legacy CO2 emissions from the atmosphere to reduce the climate-induced stress (heat, acidification) that is occurring in our oceans." Similar to industries such as solar, he adds, CO2 removal will take time to develop.

Ben Tarbell, co-founder and chief executive of Ebb Carbon, meanwhile, tells the BBC that work at its demonstration plant was designed to ensure that "every step we take, from research to deployment, is grounded in rigorous science."

"Critically, CO2 removal is not a substitute for emissions reductions," he added, pointing to scientific findings that both reduction emissions and CO2 removal will be needed. "If we're going to have a shot at keeping warming in check, we need carbon removal solutions that can meet the urgency of the climate crisis."

As Sifang Chen sums it up, there are four major challenges which need to be addressed on ocean carbon removal: insufficient governance, a small knowledge base, underdeveloped monitoring and verification processes, and uncertain environmental and social impacts. "Good policies will be critically important to safeguarding our ocean's ecosystems and coastal communities," she says.

Ultimately, says her colleague Charlotte Levy, managing science and innovation advisor at Carbon180, we will need carbon removal at scale to come back from overshoot to 1.5C temperature rise. Ocean carbon removal is "one tool in our box" to do this, she says.

However, she adds, "no removal solution later is as good as mitigation now".

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