With concerns over global warming becoming an international topic for debate, carbon capture and sequestration has emerged as a possibility for limiting the effects of greenhouse gases.
Carbon capture and sequestration can be divded into three different types of process; biological, chemical, and physical.
Encouraging plankton growth
This can be achieved through iron fertilisation; this is adding iron to the oceans in order to stimulate plankton growth. Iron fertilisation occurs naturally, but geoengineering could enhance its effectiveness. As the plankton grows, it photosynthesises, using carbon dioxide to produce sugars which are stored in the plankton.
However, it is difficult to predict the effects that a phytoplankton boom could have on the ocean ecosystem, and therefore these proposals have been controversial. Another issue is that the enormous increase in plankton could cause the release of nitrogen oxides.
The planting of trees in land such as pasture land allows photosynthesis to remove carbon dioxide from the air. An advantage of reforestation is that the carbon dioxide is normally converted to biomass which, when the tree drops its leaves, is returned to the soil. However, in order to ensure that the process has the desired effect of storing carbon dioxide, the trees must not be burnt or allow to rot in open air, as this will return all the carbon dioxide to the atmosphere. This can be overcome by putting dead trees into landfill.
Agricultural methods of carbon sequestration focus on either increasing carbon removal from the atmosphere, or decreasing its emission. Increasing removal from the atmosphere can be achieved in a number of ways; for example, if livestock graze small sections of land for a couple of days at a time, they will only graze the area lightly, allowing roots to grow deeper, but will also grind manure into the ground to return carbon to the soil. If hay is laid over a bare field, the soil will retain moisture as it is sheltered from the heat of the sun; this promotes the colonisation of the soil by carbon capturing microbes.
Although increasing carbon content of the soil is beneficial in increasing crop yield, after fifteen to thirty years of increased sequestration, the soil becomes saturated and the carbon content can be enhanced no further. This suggests that globally there may be a limit to the amount of carbon that can be sequestered in soil.
Scrubbing carbon dioxide from air
Chemicals such as sodium hydroxide may be used to remove carbon dioxide frmo the air, usually using some form of the Kraft process (a technique by which sodium hydroxide is used to convert wood into wood pulp consisting purely of cellulose fibres). Lackner has proposed a method by which scrubbing could take place in ‘artificial trees’ using a polymer based ion exchange resin. This can occur at forty degrees celsius; this is an enormous advantage over other scrubbing methods that require electricity (which unless generated through sustainable techniques, will result in the release of carbon dioxide).
Basalt storage in oceans
Carbon dioxide can be injected into the ocean at sufficient depths so that it sinks (say, 2700m), where it reacts with the brine, and then basalt to release calcium and magnesium ions and form stable carbonates. An enormous advantage to this method is that it forms carbon dioxide hydrate; as this is a dense compound, the likelihood of carbon dioxide leaking back into the atmosphere is very low.
Researchers at the Earth Observatory have predicted that the Juan de Fuca plate off the western coast of the USA would provide excellent conditions for carbon dioxide storage in basalt.
Neutralisation of ocean water
The oceans have acted as natural sinks for carbon dioxide for thousands of years. However, the oceans have a limited storage capacity and as they absorb more carbon dioxide, they become more acidic. By neutralising this acidity, we could increase the carbon dioxide capacity of the oceans. This could be achieved by adding crushed limestone to the oceans.
Normal production of cement causes the release of huge amounts of carbon dioxide into the atmosphere. Companies such as Novacem and TecEco have produced cements which absorb carbon dioxide frm the surrounding air as they harden.
Bioenergy with carbon capture and storage (BECCS)
If biomass is burned in a contained that sequesters and stores the carbon dioxide produced, the net effect will be to reduce atmospheric carbon dioxide. Although the same amount of carbon present in the biomass will be sequestered and stored, the organism will have used carbon dioxide from the atmosphere in growth, and consequently the overall effect will be to remove carbon dioxide from the atmosphere.
Using carbon dioxide for enhanced fossil fuel recovery
Carbon dioxide is injected into the reserve and can enhance extraction of the fossil fuel; this method is currently in widespread use in reserves in Texas. However, the main difficulty with this method is that the cost of building the infrastructure to inject the carbon dioxide is very high.
Biomass may be buried, such as rotting trees; this prevents the release of carbon dioxide into the atmosphere as the biomass rots, and instead sequesters it in the soil. Organic household rubbish also contributes to this carbon sequestration method when put into landfill. Biochar may also be buried in landfill. Biochar is produced by pyrolysis of biomass; pyrolysis differs from burning in that it does not require the presence of oxygen, and consequently does not form carbon dioxide. The biochar produced is buried in the soil, and remains inert. As it is not exposed to air, no carbon dioxide is formed. Biomass may also be dumped in the oceans with similar effect; however, international laws on marine waste disposal would probably prevent this as a current option for carbon sequestration.
Carbon capture has become an important new area for research since global warming and greenhouse gas levels became international topics of debate. Although the ideal solution is to dramatically reduce carbon dioxide emissions, carbon capture could offer a temporary solution and way to alleviate some of the effects of greenhouse gases whilst plans for emission reduction are implemented.
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