CO2 Capture: Putting the Sea into CCS
Amanda Jasi and Kerry Hebden talk to companies developing technologies to capture CO2 from the ocean
"THE ocean is the single biggest carbon storage device on Earth,” said Chengxiang ‘CX’ Xiang, CTO and co-founder of direct ocean capture (DOC) company Captura. About 30% of anthropogenic CO2 emissions are absorbed by the ocean, where it is 150 times more concentrated volumetrically than in the air.
He said one of the challenges of removing CO2 directly from air is the “really, really, really low” concentration, prompting the need to build large machinery to filter a lot of air. “Leveraging the ocean to do CO2 drawdown for us as we remove CO2 from the ocean water is a unique process that is inherently scalable.”
In addition to combatting climate change, the capture route also offers the ability to restore the pH of ocean water if deployed in localised bays and inlets. The ocean is becoming increasingly acidic due to overabsorption of CO2, leading to degraded ocean health.
The Chemical Engineer spoke to Xiang and others developing ocean capture technology, to understand its potential and the different methods of removal that are being explored.
Electrochemical black box
Captura’s “electrochemical black box” technology started as a project at the US California Institute of Technology (Caltech), where Xiang is a research professor of applied physics and materials science.
“When you open up the electrochemical black box what’s inside is two functional components,” Xiang said. These are acid/base generation and CO2 removal.
Acid/base generation relies on an electrodialysis unit, a key element of which is a bipolar membrane developed at Caltech.
About 0.5% of the filtered intake water is pre-processed to generate softened sea water before it is fed into the electrodialysis unit. Charged with renewable electricity, the electrodialysis unit uses electrical currents to dissociate water molecules and generate hydrochloric (HCL) acid and the alkaline base sodium hydroxide (NaOH).
Xiang said that Caltech’s bipolar membrane produces acid and base at a rate 7 times that of commercially available alternatives. The novel membrane also makes the process efficient and low cost.
The generated acid is returned to the intake stream, swinging the pH from roughly 8.1 to 4 and transforming bicarbonate ions into molecular CO2. The CO2 is removed from the acidified flow in gas-liquid contactors, where water travels on the shell side of the contactors, outside of hollow fibre membranes. A vacuum is applied on the lumen side (inside of the hollow fibre membranes) causing the CO2 to physically diffuse and permeate onto the lumen side where it is removed at high purity.
The base is then added to the decarbonised flow to restore its pH, which is monitored before it is returned to the sea.
“I want to emphasise again that we essentially take nothing from the ocean and add nothing back to the ocean,” Xiang said. Other than the CO2, of course. It returns seawater close to its native state – except for a slight pH increase that could ultimately help address ocean acidification – preventing negative impacts on marine life and ecosystems.
The ocean-atmosphere CO2 exchange
The ocean-atmosphere CO2 exchange has profound implications for Earth’s climate, as the balance of absorbing CO2 from the atmosphere, and storing it in the deep sea where it can stay locked up for millennia, helps regulate global temperatures.
But since the industrial revolution, human activities have taken a heavy toll on this cycle. Atmospheric levels of CO2 now stand close to 420ppm, up from an average of 260ppm prior to 1750, and as atmospheric CO2 levels increase, so too do CO2 levels in the ocean, and this is causing pH levels to decrease in our seas.
Normally when CO2 enters the ocean, it dissolves and forms carbonic acid
[Eq 1: CO2 + H2O <--> H2CO3-]. Carbonic acid then dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-) [Eq 2: H2CO3- <--> H+ + HCO3-].
In turn, bicarbonate ions can also dissociate into carbonate ions (CO32-) and H+
[Eq 3: HCO3- <--> CO32- + H+]. Calcium already dissolved in seawater can react with carbonate to create calcium carbonate (CaCO3). Under normal conditions in ocean water, the balance of the ionic and non-ionic chemical species leaves enough carbonate for organisms such as mussels, clams and calcareous plankton to make shells and skeletons. But, as more CO2 is added and dissolved, H+ ions increase, and equation 3 shifts in the opposite direction to produce bicarbonate. In an attempt to maintain equilibrium, the seawater system reduces the H+ concentration by binding hydrogen and carbonate ions together, which in turn reduces the carbonate that was available for marine organisms. This lack of carbonate is a growing problem, and already scientists are seeing an adverse shift in growth patterns in molluscs due to its effects (doi.org/10.1002/ece3.4416).
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