Tuesday, June 26, 2018

Carbon Capture: ocean plants

The ocean has absorbed approximately a third of the extra carbon released since the industrial age. When carbon dioxide is absorbed by seawater it becomes carbonic acid, leading to the gradual acidification of the oceans.

There are several methods proposed by which the carbon stored in the ocean can be more rapidly sequestered, reducing carbonic acid levels (though the ocean would promtly absorb more carbon from the atmosphere):

  • alkalinization: to counteract the carbonic acid by adding huge quantities of alkalines to the ocean, such as bicarbonate. Quite usefully, bicarbonate can be produced via the Calcium Loop, by breaking apart the calcium carbonate into bicarbonate instead of heating it to release concentrated carbon dioxide.
  • fertilization: the carbon in the oceans could be handled by encouraging phytoplankton to grow. Different parts of the ocean contain phosphorous, nitrogen, and iron in differing amounts. By adding these three in the correct ratio, phytoplankton will be enabled to consume more carbon.

The main issue with these ideas is that they are not self-funding, they do not produce an output which can be used to generate revenue to continue the effort. These kinds of projects would depend on massive external support, as by governments or the (very) wealthy.

Sunday, June 24, 2018

Carbon Capture: reforestation

Pre-industrialization, forests covered approximately 5.9 billion hectares across the planet. Today that figure is 4 billion hectares, and still dropping. The deforestation has reduced the ability of the terrestrial plants to sink carbon in their yearly growth.

The basic idea in reforestation is straightforward: plant trees and other long-lasting plants in order to take up and store carbon from the atmosphere. Development of mechanisms to plant trees in large enough scale and short enough time frame to be useful in ameliorating climate change is the difficult part. This requires automation, most obviously by use of flying drones.

Biocarbon Engineering and Droneseed are two firms building technologies for rapid planting of trees. They use largish drones loaded with seed pods. The drones do require pilots, as most jurisdictions now require licensed pilots for dones, but where possible the drones are set to fly in a formation to allow a single pilot to control many at a time.

Biocarbon Engineering provides more details about their planting technology, utilizing seed pods loaded with nutrients fired from the drones toward the ground. A good percentage of the seed pods will embed into the ground, and the outer packaging will rapidly biodegrade and allow the seed to germinate.

The cost efficiency of this automated seeding method is not clear from publicly available information. Each reseeding project is a unique bid, and the bids are mostly not made public. Estimates of the cost of manual planting average $4940 per hectare using manual methods. Rough estimates of the cost of a Biocarbon Engineering project to reseed Mangrove trees in Myanmar is about half of what a manual effort would be.

Companies in this technology space

  • Propagate Ventures works with farmers and landowners to implement regenerative agriculture, restoring the land while keeping it productive.

 

musings on plants

In real deployments the type of plant life seeded will be chosen to fit the local environment by the client, such as the choice of Mangrove trees in Myanmar. If we were only concerned with the rapidity of carbon uptake, and did not care about invasive species, I think there are two species of plants we would focus on:

  • Paulownia trees which grow extremely rapidly, up to 20 feet in one year. These are native to China, and an invasive species elsewhere.
  • Hemp: "Industrial hemp has been scientifically proven to absorb more CO2 per hectare than any forest or commercial crop and is therefore the ideal carbonsink." (source). I find it amusing that hemp may be crucial in saving humanity after all.

Saturday, June 23, 2018

Carbon Capture: Biochar

biochar is charcoal made from biomass, from agricultural waste or other plant material. If left to rot or burned, the carbon trapped in this plant material would return to the atmosphere. By turning it into charcoal, a large percentage of the carbon is fixed into a stable form for decades.

Turning plant material into charcoal is a straightforward process: heat without sufficient oxygen to burn. This process is called pyrolysis (from the Greek pyro meaning fire and lysis meaning separating). In ancient times this was accomplished by burying smoldering wood under a layer of dirt, cutting it off from air. More recently, a kiln provided a more efficient way to produce charcoal by heating wood without burning it. Modern methods generally use sealed heating chambers in order to capture all of the produced gases.

Pyrolysis produces three outputs:

  • the solid char, which has a much higher concentration of carbon than the original plant material.

  • a thick tar referred to as bio-oil, which is much higher in oxygen than petroleum but otherwise similar.

  • a carbon-rich gas called syngas. It is flammable, though it contains only about half the energy density of methane. In earlier times the gas generally just escaped, while modern processes capture and usually burn it as heat to continue the pyrolysis process.

The temperature and length of pyrolysis determines the relative quantity of char, bio-oil, and syngas. Baking for longer time at lower temperature emphasizes char, shorter times at higher temperature produces more gas and oil.

The idea of biochar for carbon capture is to intercept carbon about to return to the atmosphere, primarily agricultural waste, and turn it into a form which both sequesters carbon and improves the soil into which it is tilled. The very fine char produced from agricultural waste is quite porous and makes soil retain water more effectively. It can also improve the soil health of acidic soils, balancing the pH and making the soil more productive.

Carbon Capture: Temperature Swing Adsorption

Adsorption: the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent.

Temperature Swing Adsorption (TSA) for carbon capture relies on a set of materials, called carbon dioxide sorbents, which attract carbon dioxide molecules at low temperature and release them at a higher temperature. Unlike the Calcium Loop described previously, there is no chemical reaction between the sorbent and the CO2. Adsorption is purely a physical process, where the CO2 sticks to the sorbent due to the slight negative charges of the oxygen atoms and positive charge of the carbon.

There are a relatively large number of materials with this sorbent property for carbon dioxide, enough to have a dedicated Wikipedia page. These materials contain porous gaps. The gaps in the most interesting materials for our purpose are the right size to hold a CO2 molecule, with a slight charge at the right spot to attract the charges of different points on the CO2. To be useful for carbon capture, the sorbent has to attract CO2 molecules but readily release them with a change in temperature. They can be cycled from cold to hot to repeatedly grab and release carbon dioxide.

Unfortunately most of the known materials have drawbacks which make them unsuitable for real-world use, such as being damaged by water vapor.

The most recent class of sorbents developed are Metal-Organic Frameworks (MOFs), which are chains of organic molecules bound up into structures with metals. Metal-Oxide Frameworks are interesting because they are much more robust than the previously known sorbents, not being easily damaged by compounds found in the air and capable of being cycled in temperature without quickly wearing out.


 

Companies in this technology space

  • Inventys in Canada builds a carbon capture system using Temperature Swing Adsorption materials. Their system uses circular plates of a sorbent material, stacked vertically, and rotates the plates within a cylindrical housing. At different parts of the revolution the plates spend 30 seconds adsorping CO2, 15 seconds being heated to 110 degrees Celsius to release the concentrated CO2, and 15 seconds cooling back down to 40 degrees to do it again.

    Inventys goes to some length to explain that their technology is in the whole system, not tied to any particular sorbent material. I suspect this is emphasized because Metal Oxide Frameworks are innovating rapidly, and indeed the entire class of MOF materials was developed after Inventys was founded, so they ensure that the system can take advantage of new sorbent materials as they appear.


  • Skytree in the EU is a patent licensing firm which is fairly coy about the technologies it licenses but says they were developed as part of the Advanced Closed Loop System for the International Space Station. One of the main innovations in the ACLS is the development of a solid resin adsorbent Astrine, which means the technology is adsorption-based.

  • Climeworks in Switzerland describes their process as a filter which is then heated to release the carbon dioxide. This is clearly an adsorption process, and almost certainly using Metal-Organic Frameworks as it is described as being reusable for a large number of cycles.

  • Global Thermostat in New York describes their process as an amine-based sorbent bonded to a porous honeycomb ceramic structure.

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 for industrial use.

 

Postscript

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.