Tuesday, June 19, 2018

Carbon Capture: Calcium Looping

I am very interested in technologies to ameliorate climate change. The looming, self-inflicted potential extinction of the human species seems important to address.

In this post we’ll examine the steps in Carbon Engineering’s Direct Air capture process, as published on their website, and explore what each step means. As I am an amateur at carbon capture technologies, anything and everything here may be incorrect. I’m writing this in an attempt to learn more about the space.


step 1: wet scrubber

A wet scrubber passes a gas containing pollutants, in this case atmospheric air containing excess carbon dioxide, through a liquid in order to capture the undesired elements. Scrubber designs vary greatly depending on the size of the pollutant being captured, especially whether particles or gaseous. In this case because CO2 molecules are being targeted, the scrubber is likely a tall cylindrical tower filled with finned material to maximize the surface area exposed to the air.

This process step uses hydroxide HO-, a water molecule with one of the hydrogen atoms stripped off, as the scrubbing liquid. Hydroxide bonds with carbon dioxide to form carbonic acid H2CO3. It is interesting to note that this same chemical process is occurring naturally at huge scale in the ocean, where seawater has acidified due to the absorption of carbon dioxide and formation of carbonic acid.


step 2: pellet reactor

The diluted carbonic acid is pumped through a pellet reactor, which is filled with very small pellets of calcium hydroxide Ca(OH)2. Calcium hydroxide reacts with the carbonic acid H2CO3 to form calcium carbonate CaCO3, which is the primary component of both industrial lime and antacid tablets. The small pellets in the reactor serve to both supply calcium for the reaction and to serve as a seed crystal to allow a larger calcium carbonate crystal to grow. In the process, hydrogen and oxygen atoms are liberated which turn back into water.

As the point of this system is a continuous process to remove carbon dioxide from air, I imagine the pellets are slowly cycled through the reactor as the liquid flows over them. The pellets with their load of newly grown crystal would automatically move on to the next stage of processing.

It is important to dry the pellets of calcium carbonate as they leave the pellet reactor. The next step collects purified carbon dioxide, where water vapor would be a contaminant. Removal of the remaining water could be accomplished by heating the pellets to somewhere above 100 degrees Celsius where water evaporates, but much less than 550 degrees where the calcium carbonate would begin to break down. Hot air would be sufficient to achieve this.


step 3: circulating bed fluid calcinator

A calcinator is a kiln which rotates. The wet pellets loaded with crystals of calcium carbonate CaCO3 slowly move through the kiln, where they are heated to a sufficient temperature for the calcium carbonate to decompose back into calcium oxide CaO and carbon dioxide CO2. A temperature of at least 550 degrees centigrade is needed for this, and the reaction works best somewhere around 840 degrees which is quite hot. There are catalysts which can encourage this reaction at lower temperatures, notably titanium oxide TiO2, but they are quite expensive and might not be economical compared with heating the kiln.

The carbon dioxide would be released as a hot gas to be collected, the calcium oxide will be left as solid grains in the calcinator. The calcium oxide can be used over and over, called calcium looping. Energy is expended at each cycle through the loop to free the carbon dioxide from the calcium oxide.


step 4: slaker

The solid output of the calcinator is calcium oxide CaO, also called quicklime. Quicklime is not stable, and will absorb other molecules from the air which would introduce impurities if put back into the pellet reactor. Therefore the calcium oxide CaO is combined with water to form calcium hydroxide Ca(OH)2.

A slaker adds controlled amounts of water to quicklime. This reaction releases a great deal of heat, so it is controlled by a feedback loop which reduces the inflow of material when the reaction gets too hot. I imagine the waste heat from this process could provide some of the heat needed for the earlier calcinator step, though additional heating would also be needed.


Companies in this technology space

  • Carbon Engineering, which builds large scale operations using the calcium loop process to capture carbon dioxide from air.
  • Calera, which captures CO2 to produce calcium carbonate and magnesium carbonate for industrial use.
  • CleanO2 builds CO2 scrubbers for HVAC systems, allowing cold air from the building to be recirculated after scrubbing carbon dioxide (and likely also scrubbing water vapor and other contaminants). As the systems produce calcium carbonate as an end-product, I'm going to assume it uses the first two steps of the calcium loop as a recovery mechanism.



At the end of the process we have a highly purified stream of carbon dioxide extracted from ambient air. The long term goal of this kind of technology would be negative carbon emissions, which would mean keeping the CO2 from immediately circulating back into the environment by utilizing it in a long-lived form like various plastics or graphene. The technology also allows carbon neutral fuels to be made for applications where energy density requirements are higher than what battery chemistries are likely to provide, such as airplanes or ocean going vessels. Using carbon which was already in the atmosphere for these applications is much better than digging more carbon out of the ground.