Carbon Engineering

A Process for Capturing CO2 from the Atmosphere

Carbon Sequestration

The notion of a “silver bullet” for solving global warming has persistent appeal: “What’s the one big thing we can do?”. However, there is no one-size-fits-all solution to a multi-layered problem, such as global warming.

Renewable energy and electric cars will not solve the imminent climate crisis alone. We have to go beyond that and use the three obvious decarbonization strategies simultaneously:

1. Replace existing fossil fuel-based energy generation with clean, renewable sources
2. Reduce wasteful consumption through technological efficiency and behavior change
3. Remove carbon by sequestering it out of the atmosphere or source of creation

I would argue that we won’t achieve net-zero carbon dioxide emissions if we just use strategy 1 (replace) and strategy 2 (reduce) because we will always add more GHG emissions than we are able to withdraw through natural carbon sinks like kelp forest and trees. You could say the primary benefit of carbon dioxide removal is that it creates space in the carbon budget for difficult-to-decarbonize activities, such as long-haul transportation, aviation, and agriculture.

Therefore, we have to focus on strategy 3 (remove) to actively remove CO2 out of the atmosphere to stop the self-feeding carbon cycle. Even if carbon dioxide emissions came to a sudden halt, the carbon dioxide already in the Earth’s atmosphere could continue to warm our planet for hundreds of years, according to Princeton University-led research published in the journal Nature Climate Change.

The natural Geocycle & Biocycle [Credit: University of Augsburg, Environmental Science Center (WZU)]

On Wednesday this week, the concentration of carbon dioxide in the atmosphere was measured at 415 parts per million (ppm). The level is the highest in human history and is growing each year. According to the IPCC, all their projections use carbon dioxide removal (CDR) on the order of 100–1000 GtCO2 over the 21st century to be consistent with the 1.5 degrees Celsius temperature increase.

Concentrations of carbon dioxide in Earth’s atmosphere have risen rapidly since measurements began nearly 60 years ago, climbing from 316 parts per million (ppm) in 1958 to more than 400 ppm today. [Credit: SCRIPPS INSTITUTION OF OCEANOGRAPHY]

X-Price for Carbon Capture

Elon Musk tweeted last Thursday that he will donate $100 million toward a prize for the best carbon capture technology. He also stated that he’ll unveil details of the competition next week. His past statements suggest that one of his key goals is to lower the price of direct-air carbon capture so it can feasibly be used to make synthetic rocket fuel, replacing the fossil fuels used now. This should enable more innovation in the carbon-capturing space.

Carbon Engineering

One promising startup that is currently working on carbon-capturing technology is Carbon Engineering. Carbon Engineering has been developing a liquid-based Direct Air Capturing (DAC) system since 2009.

Carbon Engineering uses giant fans to pull air into a tower-like structure. The air passes over a potassium hydroxide solution which chemically binds to the CO2 molecules, and removes them from the air. The CO2 is then concentrated, purified, and compressed.

The CO2 can be stored underground or used to create hydrocarbon fuels.

Carbon Engineering Planned Plant in Texas, USA [credit: Carbon Engineering]

While the challenges to direct air capture are great, the technology uses less land and water than other negative emissions technologies such as planting forests or storing CO2 in soils or oceans.

Carbon Engineering has signed a licensing deal with Occidental and 1PointFive to build megaton-scale Direct Air Capture facilities in the US. They have also secured their first corporate customers Shopify and Virgin Red in 2020.

The Carbon Sequestration Machine

Carbon Engineering’s process comprises two connected chemical loops with four reactors:

• Air Contractor (1)
• Pellet Reactor (2)
• Calciner (3)
• Slaker (4)

Figure 1. Process Chemistry and Thermodynamics [credit: Carbon Engineering]

The Process (Direct Air Capturing)

1. The Air Contractor captures Carbon Dioxide (CO2) from the ambient air using an aqueous solution of Potassium Hydroxide (KOH). This solution bids with the CO2 to form liquid Potassium Carbonate (K2CO3) and water (H2O).
2. The Pellet Reactor combines the two loops. In the Pellet Reactor, the Potassium Carbonate (K2CO3) reacts with Calcium Hydroxide (Ca(OH)2) to form Calcium Carbonate(CaCO3).
3. In the Calciner, the Calcium Carbonate(CaCO3) is heated at high temperatures in absence of air or oxygen to release pure CO2.
4. In the Slaker, the remaining Calcium Oxide (CaO) is hydrated with water to create Calcium Hydroxide for the pellet reactor.

This circular process allows for continuous operation of the plant. To learn more about the process, you can read Carbon Engineering’s article “A Process for Capturing CO2 from the Atmosphere” that they published in 2018.

A Process for Capturing CO2 from the Atmosphere

E-Fuels

Instead of storing the carbon dioxide underground, Carbon Engineering can pair CO2 from DAC with electrolytic hydrogen to synthesize liquid hydrocarbons that can be used in transportation (often referred to as “e-fuels”). This process is based on the Fischer–Tropsch synthesis.

Fischer–Tropsch (FT) synthesis is a catalytic process that converts syngas (a mixture of CO and H2) to synthetic liquid fuels and other valuable chemicals. It was first developed in the 1920s to convert coal to liquid fuels. Currently, the largest single facility employing FTS today is located in Qatar, and it produces 140 000 bpd of liquid fuels from natural gas.

Process (Fischer–Tropsch Synthesis):

1. Fuel synthesis starts with the conversion of the CO2 to CO using the Reverse Water-Gas Shift Reaction (rWGS-reaction).
2. An electrolyzer is used to produce Hydrogen (H2) for the rWGS and FTS reactions, as well as Oxygen (O2).
3. Syngas is then created by blending the CO with additional H2 to achieve the ratio required to operate the FTS. The syngas is directed to FTS to produce synthetic crude oil, which can then be hydrocracked and fractionated into middle distillates, kerosene, naphtha, and light ends (e.g., methane, ethane, propane).

Figure 2. Block flow diagram with process groups and system boundaries of the combined direct air capture (DAC) and Fischer–Tropsch synthesis (FTS) process. [credit: Carbon Engineering]

To learn more about the Fischer–Tropsch synthesis, you can read Carbon Engineering’s article “A life cycle assessment of greenhouse gas emissions from direct air capture and Fischer– Tropsch fuel production” that they published in 2020.

Cost Projections

The first peer-reviewed assessment of the technical and economic cost of CO2 removal via DAC based on the pilot plant in Squamish, B.C was in 2018. The study provided an engineering cost estimate for a commercial 1.1 MtCO2 per year plant and estimated a levelized cost of $94–232 per tCO2 captured from the atmosphere, which is lower than previously estimated. Direct air capture should become worldwide feasible and competitive at less than $30–50/ton capture price.

The baseline scenario for its newest plant in Texas assumes a capacity of 1.1 MtCO2 captured from the air per year, which is converted (along with CO2 captured from natural gas combustion) into 20 PJ of synthetic diesel (9800 bpd).

Carbon Engineering Piot Plant in Squamish, B.C. [credit: Carbon Engineering]

Outlook

With the current increased interest in carbon capturing, Carbon Engineering looks like a leader in the CO2 capturing space. Its proprietary capturing technology has the potential to scale in the future with further innovation (economy of scale).