Technologies For Direct Removal Of Atmospheric Carbon

Two weeks after the release of the report from IPCC Working Group Two, the  report from Working Group Three was released. While the previous report dealt with the likely effects of future climate change, this new report focuses on mitigation strategies. And while considerable emphasis is put on reducing greenhouse gas emissions, this report also details strategies for the removal of carbon from the atmosphere. With this in mind, it seems like a good time to review the various technologies which may help us to reduce atmosphere CO₂ concentrations in future years.

When we think of carbon capture, we often think of capturing CO₂ from high-density point sources, such as power stations, cement factories, and vehicle exhausts. It is likely that this will be an important intermediate step in helping us to reduce our emissions, prior to the establishment of a clean energy infrastructure. There are a number of technologies which are able to extract a high percentage of the CO₂ from such emissions. These include pre-combustion, and post-combustion processes, which have been used to supply CO₂ to industry for a number of years.

More recently a technique known as oxy-firing has been the subject of considerable interest. This process involves burning fuel in a pure oxygen environment, and makes it possible to recover a stream of highly-concentrated CO₂. A number of pilot schemes which use oxy-firing are currently being evaluated. However the disadvantage of all current forms of CO₂ capture from power generation facilities is that there is an energy overhead, meaning that between ten and forty percent of the energy generated has to be used in the carbon capture process, with the result that more fossil fuels need to be burned in the first place.

One promising technology for carbon capture from high concentration streams of CO₂ is through the use of algae. A number of strains of algae have been identified which are very effective at removing CO₂ from power station residues, and removal rates of eighty percent have been reported. It is expected that genetically-engineered strains of algae can be developed which will further improve the efficiency of this process. However doubts remain as to whether algae can be effective at the scales required. A 2010 study estimated that to remove eighty percent of the CO₂ from the emissions of an average 200 MW coal fired power station would require an algal pond some 7,000 acres (2,800 ha) in extent, an area equivalent to approximately 4,500 soccer fields. Algae will also only absorb CO₂ during daylight hours, meaning that only half the emissions could be captured.

While the use of algae can be an effective way to absorb CO_2, much of the focus of research to date has been geared towards the development of algae-based petroleum substitutes. These alternative fuels are often labelled as being carbon neutral. However when they are burned, all the CO₂ that was originally removed from the power station emissions is released back into the atmosphere. The fuel itself may therefore technically be classified as being carbon neutral, but it is generated as an add on to carbon-intensive fossil fuel generation. Many misleading claims are made about carbon capture, and despite the greenwash it is apparent that many of the processes being developed for CO₂ capture are primarily focused on energy-generation, rather than on true carbon sequestration.

Direct Air Capture

While capturing emissions from point sources can help to limit new emissions of CO₂, such solutions only apply to new emissions and do nothing to reduce current CO₂ concentrations. Absolute reduction of CO₂ levels can only be achieved through direct capture of carbon from the atmosphere. There are a number of ways to do this, with the simplest being reforestation. Forests are estimated to absorb around three billion tons of anthropogenic carbon each year, which is approximately one third of total human emissions. By planting fast growing trees on unused land, it may be possible to increase this fraction. However, unless the corresponding rates of deforestation occurring elsewhere can be curbed, it is doubtful that we will be able to significantly increase the percentage of CO₂ absorbed by the world’s forests over the next few decades.

Other forms of biomass may also be used to absorb carbon, such as the unused parts of food crops and specially planted fast-growing crops, such as switchgrass. These sources of biomass may be used to produce biochar through a process known as pyrolysis, in which the plant materials are burned in an oxygen free environment. Biochar is a stable form of charcoal which can be spread over existing croplands, potentially keeping CO₂ locked up in the soil for thousands of years, while also helping to enrich the soil. A side benefit of this process is the ability to produce synthetic fuels in a sustainable way. It is estimated that biochar production has the potential to absorb a significant percentage of anthropogenic emissions each year.

One method of carbon removal which has considerable potential is through the use of artificial trees. This concept has been developed by Klaus Lackner, a geophysicist at Columbia University. Artificial trees are specially developed filters, which are coated with a resin which is able to capture the CO₂ directly out of the air, one thousand times more efficiently than natural trees are able to. To release the trapped CO₂, the filters are soaked in water, allowing them to be reused. Lackner himself estimates that ten million of these artificial trees could potentially drop atmospheric CO₂ by 0.5 ppm per year. The cost of each unit is projected to be similar to that of a domestic car, and to put this into perspective, there were approximately 60 million cars produced worldwide last year. So such a program would be possible if the political will existed.

A number of other researchers are working on their own ideas for carbon capture. David Keith of Harvard has built a machine which uses liquid sodium hydroxide to capture CO₂. The CO₂ is then released through heating; a process which allows for continuous operation. Another concept has been developed by the technology startup TerraLeaf. This makes use of chlorophillin, a salt derived from chlorophyll. When this is combined with an electrically-conducting polymer it has the ability to pull greenhouse gasses directly from the air and use them to form carbon-based chemicals. There are also several other concepts which have been developed for direct capture of atmospheric CO₂, such as the system developed by Climeworks, a Swiss company which has developed their own process for direct extraction of CO₂ from the atmosphere.

Carbon Storage 

Unfortunately the challenge does not stop at the removal of CO₂ from the atmosphere. There is also the small problem of where we are going to put the estimated five billion tons of unsequestered carbon that we are adding to the atmosphere each year, after we have captured it. Also, if we want to achieve a net reduction of CO₂ concentrations over time, we will need to do considerably better than that, perhaps capturing and storing ten billion tons of carbon per year. Storage is the second part of the equation, and in many ways it is more challenging than carbon capture.

The idea that has been most widely discussed is to store CO₂ in underground geological formations. Suitable formations include depleted oil and gas fields, deep coal seems, and saline formations. The CO₂ is then injected under pressure into porous rocks deep underground. Suitable sites need to have at least one layer of cap rock to prevent CO₂ leakage. The US Department of Energy estimates that suitable sites for underground carbon storage within the continental US are large enough to hold the equivalent of at least six hundred years of US carbon emissions.

However there are a number of problems associated with underground CO₂ storage. The first problem is that this is an enormously expensive process. In order to be considered viable from the viewpoint of conventional economics, the price of carbon would have to rise considerably. Secondly, safeguards would need to be put in place to ensure that the CO₂ placed in underground storage would remain there indefinitely. In high concentrations CO₂ is dangerous, and can cause suffocation. In 1986, a natural leak of CO₂ from volcanic Lake Nyos in Cameroon lead to the death of almost two thousand people. The safety concerns of storing large volumes of CO₂ underground would therefore obviously need to be addressed. It is worth noting that in spite of widespread discussion of this option, there are  only a few small pilot schemes which have been developed to date, most of which have ironically been used for enhanced oil recovery from deep geological reservoirs.

A better long-term solution is to convert CO₂ into an inert solid, taking it out of circulation for millions of years. This is how the Earth has regulated CO₂ levels over geological time, through the process of rock weathering. The idea is that it should be possible to speed up this natural process considerably. Mafic rocks such as olivine are capable of absorbing large amounts of CO₂, forming of stable carbonates in the process. One idea is therefore to locate direct CO₂ capture devices, such as the artificial trees described above, close to large olivine formations. The country of Oman has the largest concentration of olivine rocks in the world, and a high concentration of CO₂ collecting devices located in this region could potentially provide a permanent way of removing much of the additional carbon we have introduced to the atmosphere.

A simpler, more direct approach is taken by the Dutch company “Smart Stones”, one of the eleven finalists in the Virgin Earth Challenge. The company has proposed mining formations of olivine, crushing the rock into olivine sand, and simply spreading it across large parts of the Earth, as an additive to fertilizer on agricultural lands, across beaches, and as construction materials. By focussing primarily on tropical regions, the natural weathering process can be greatly enhanced. According to the company, this should be sufficient to drive the process of CO₂ removal, without any further intervention. One ton of olivine is capable of absorbing 1.25 tons of CO₂. Much of the olivine used will also wash into the ocean, where it has the potential to help counteract ocean acidification. This simple solution appears to have considerable potential for lowering atmospheric CO₂ concentrations, at a fraction of the cost of underground storage. However to have a significant impact on atmospheric CO_2, somewhere close to five billion tons of olivine would need to be mined and processed each year. To put this into perspective, this represents about ten percent of the current annual worldwide production of sand and gravel.

The Virgin Earth Challenge

The Virgin Earth Challenge was launched in 2007, with the objective of finding practical technologies for the reduction of atmospheric carbon levels. From thousands of entries, the field has now been reduced to eleven finalists, who are competing for a prize of $25 million. The finalists offer a variety of potential solutions, many of which have been described above. These include the production of biochar, habitat restoration, mechanisms for direct atmospheric carbon capture and storage, and solutions which promote enhanced rock weathering. While a number of the finalists appear to be focused on energy production, rather than large-scale CO₂ reduction, all of the technologies are able to remove at least some of the CO₂ from the atmosphere.

Should we consider taking such drastic action?

Many people see the direct removal of atmospheric CO₂ as a form of geoengineering, and thus regard it with suspicion. However, given our current level of emissions, it is highly unlikely that we will be able to stay under two degrees of additional warming, without resorting to such methods. The IPCC Working Group Three report acknowledges that some form of atmospheric carbon removal is likely to be necessary in the future. There is a danger that many in industry will see such projects as an invitation to continue with business as usual; however nothing could be further from the truth. The main priority is to develop a low-carbon economy and to phase out fossil fuels as fast as possible. However it is likely that CO₂ reduction strategies will also be necessary to stabilize the climate over the next century if we are to avoid catastrophic climate change. Our current levels of emissions already represent an unplanned form of geoengineering. Carefully researched and controlled programs for atmospheric carbon reduction are likely to cause considerably less harm than carrying on along our current path.

References

Microalgae: The Potential for Carbon Capture, Richard Sayre BioScience (2010) 60 (9): 722-727.

Oxy-fuel combustion process, http://en.wikipedia.org/wiki/Oxy-fuel_combustion_process

Carbon capture and storage, http://en.wikipedia.org/wiki/Carbon_capture_and_storage

Biochar, http://en.wikipedia.org/wiki/Biochar

Al Gore, Our Choice: A plan to sole the Climate Crisis, 2009, Rodale Books

Scientific American, May 16 2013, 400 PPM: Can Artificial Trees Help Pull CO2 from the Air?http://www.scientificamerican.com/article/prospects-for-direct-air-capture-of-carbon-dioxide/

New York Times, March 29, 2014, The Artificial Leaf Is Here. Again.  http://www.nytimes.com/2014/03/30/technology/the-artificial-leaf-is-here-again.html?_r=0

EPA, Carbon Dioxide Capture and Sequestration, http://www.epa.gov/climatechange/ccs/

The Virgin Earth Challenge, http://www.virginearth.com/

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