Stand-alone ocean capture systems are expensive

Photo by Matt Hardy on Unsplash

Ocean-based capture technology focuses on extracting carbon dioxide (CO2) directly from seawater, as the ocean naturally holds 16X times more carbon than the atmosphere. The CO2 in the ocean is primarily present as bicarbonate ions at the ocean's normal pH of 8.1. The process relies on a series of steps to extract CO2 as a gas, which can be pumped downhole or captured and used. It roughly follows the following steps:

1. Ocean water intake: In the first step, seawater is drawn into the CO2 capture facility. This water has a higher concentration of dissolved carbon than the atmosphere.

2. Pre-treatment: Seawater is subjected to pre-treatment to remove impurities, algae, organic particles, and sand particles, ensuring the water is clean before it enters the next stage.

3. Acidification and degasification: The pre-treated seawater is acidified, ideally by electrochemically creating independent acid and base streams electrochemically from water. The acid stream converts bicarbonate ions (HCO3-) and carbonate ions (CO3 2-) into dissolved CO2. The acidified water is then passed through a membrane contactor or other degasification systems to separate CO2 as a gas.

4. The CO2 gas is collected, and then the acid and base streams are recombined and returned to the ocean.

The idea of removing CO2 directly from seawater in this way is enticing: There is, for example. no need to build a contactor when the ocean is already pulling CO2 from the air, and naturally mixing it into the water. The problem is cost.

A 2020 paper, A direct coupled electrochemical system for capture and conversion of CO2 from oceanwater, provides a useful breakdown of these costs, the most important of which are related to the pumping of the water itself. Water is about 1000 times denser than air, and the energy cost of pumping scales linearly with this density, while the accessible carbon is only 16X higher. The energy cost of pumping and pre-treating the water adds up to maybe $500/ton CO2 captured at an energy cost of $0.06/kWh. This is before consideration of the capex required, which could add 3X even more in payments. If a stand-alone system of water pumping is required for ocean capture, the economics simply can't work out.

There are a couple solutions to this. The first is to develop only solutions such as seaweed growth, which do not require exogenous pumping. The second is to build any carbon capture system into existing pumps, such as those already built for reverse osmosis plants. This second approach is being taken by companies such as Ebb Carbon, which hopes to gain additional efficiencies by working only with recovered RO brines, whose high conductivity gives the electrochemical acidification process higher efficiency. Building a carbon capture system on the back end of a reverse osmosis plant eliminates all of the filtering and pumping costs, provides a more concentrated fluid which requires less energy to handle, and valorizes a brine product that is otherwise considered waste. If there is hope for ocean capture, it is very likely going to aligned to integration with existing water treatment. This reduces the overall amount of scaling that can be done, but could make for compelling economics.