Carbon Dioxide Capture: Part I

I am not ignoring the solar engineering forms of geo-engineering entirely in this blog but at the moment I’m focusing on carbon dioxide removal (CDR) techniques as I feel these will have fewer unknown and unintended consequences because they aim to reverse the fossil fuel trend of adding carbon to the atmosphere and do not try to reduce solar radiation reaching Earth. Atmospheric Carbon Dioxide Capture is a form of CDR geo-engineering that I feel has great potential. This involves taking carbon dioxide out of the atmosphere through chemical reactions and storing it deep underground or using it in industry. Many authors do not consider the capture of carbon dioxide at power plants to be a form of geo-engineering because it is not dealing with carbon dioxide that has already been released to the atmosphere. I personally disagree with this and see the removal of carbon dioxide from power plants as a key way to reduce the effects of climate change into the future on a large scale which is the current objective of all geo-engineering techniques. However, if you hold the view that it is not a form of geo-engineering, the procedures of carbon dioxide removal in power plants mentioned in this article are still applicable to global atmospheric carbon dioxide removal (which is a form of geo-engineering). This is why I feel it is particularly important to write a post on these techniques. This topic will be divided into two posts, the first on the different methods being used and the second on the stores, benefits, drawbacks and feasibility of these techniques.

How does it work?

According to Metz et al. (2005) there are four basic systems for capturing carbon dioxide from fossil fuels and biomass. These are as follows:

1. Capture from industrial process streams
2. Post-combustion capture
3. Oxy-fuel combustion capture
4. Pre-combustion capture

The first of these has been operational for 80 years but the CO₂ captured is still added to the atmosphere as until now there was no reason to store it. This is very unfortunate as, if the incentive to store this carbon had started 80 years earlier, the human race would not be in such a dangerous and unpredictable climate change situation. Implementation of this method therefore seems very straightforward, with few changes needed. Post-combustion capture involves the capture of carbon dioxide from flue gases produced during the burning of fossil fuels and biomass. The CO₂ is then pumped back underground. Ox-fuel combustion capture is very similar to Post-combustion but pure oxygen is used in the combustion and therefore the flue gas is mainly CO₂ and H₂O which are easier to recycle. Finally, pre-combustion capture involves ‘reacting a fuel with oxygen or air and/or steam to give mainly a ‘synthetic gas (syngas)’ or ‘fuel gas’ composed of carbon monoxide and hydrogen.’ A catalytic converter, such as those used on car exhausts, can then be used to produce carbon dioxide and more hydrogen and this can then be separated by physical or chemical absorption leaving hydrogen-rich fuel which can be used in many systems such as boilers and gas turbines. The latter three methods all revolve around the idea of making combustion less polluting to the atmosphere.

The techniques for separating carbon dioxide are varied and each have their merits. The three main processes for separation are shown in Figure 1. The first technology I wish to discuss is called ‘Separation with sorbents/solvents’ (Diagram A in Figure 1). This involves bringing the gas into contact with a liquid absorbent or a solid sorbent that separates the carbon dioxide from the rest of the gas. The sorbent that is loaded with CO₂ is then transported to another vessel where it releases the carbon dioxide following a change in the environment (e.g. heating or increased pressure). This sorbent is then recycled to collect more CO₂ in a cyclical process.

Separation with membranes is a method whereby only certain gases are able to permeate through the membrane. The flow through the membrane is driven by pressure differences. There is currently research and development ongoing to manufacture a membrane suitable for the large scale capture of carbon dioxide. This means this method is less widely used at present.

Cryogenic distillation (Diagram C in Figure 1), describes the process by which gases are converted to liquid through compression, cooling and expansion and then are distilled to capture the CO₂. This form of CDR is currently being carried out at a large scale commercially which suggests that it has great potential.

Figure 1: Main separation process for carbon dioxide capture.

These three capture technologies highlight the different ways carbon dioxide is removed in power plants and during biomass burning. The methods here deal with the carbon dioxide at the source and therefore prevent the carbon from reaching the atmosphere and influencing climate change. Achieving this reduces the need for other extreme methods of geo-engineering that could lead to unintended negative consequences for climate. These systems can also be applied to the removal of carbon dioxide already in the atmosphere. However, due to the lower atmospheric concentrations of carbon dioxide in the atmosphere than in power plants suggests that the efficacy will be less for atmospheric carbon dioxide capture.

This concludes the first part on the topic of how carbon dioxide removal is carried out in power plants and from the atmosphere. The next post will continue on this topic but turn the focus to where the carbon would be stored once it has been captured. In addition to this, the post will look into the benefits and drawbacks to this form of geo-engineering and whether it is feasible as a method to reduce the effects of climate change at a large enough scale to be considered by governments worldwide. 

Geo-Engineering in the news

Geo-Engineering literature is widespread in the academic field with papers exploring a wide variety of different techniques within the two main areas of Solar Geo-Engineering and Carbon Dioxide Removal but is there much of a focus in the newspapers and media? This is as important as the academic research in my opinion as the only way Geo-Engineering methods will be adopted is if there is a global consensus to implement them. The best way of achieving this is making the methods widely known using global news channels. By carrying out a quick look at the term ‘climate engineering’ on Google Trends suggests that, between 2004 and present, there has been an increase in interest in this topic.

An article in the Financial Times titled ‘Scientists grapple with geoengineering plans’ shows that geo-engineering is appearing in well-known newspapers. The article provides a concise overview of geo-engineering in the two main forms. Despite initially being pessimistic about the effectiveness of these methods, the article does highlight some promising news. Experiments in Iceland discovered that, when carbon dioxide was pumped into underground volcanic basalt rock, the conversion to solid carbonate minerals took just two years compared to previous estimates of hundreds to thousands of years.  This is promising for the storage of carbon underground as basalt strata is found all around the world. It is encouraging to see a major newspaper write an extensive article on this topic as it helps motivate people to take geo-engineering seriously.

The New York Times has also discussed the idea of climate by design. This article highlights the issues with human directly managing the climate, stating ‘Who gets to set the global thermostat?' This is particularly important for the politics behind geo-engineering and the coming together of many different countries to instigate it. Some countries at higher latitudes may want the global climate to be warmer to boost food production and access shipping routes in the Arctic whereas others may want climate to be cooler to reduce the chance of droughts. This illustrates why is may be extremely difficult to come to a global agreement.  Another important point raised in this article, quoted from Oliver Morton who is a senior editor at The Economist and has written a book titled ‘The Planet Remade: How Geoengineering Could Change the World’ is that that different academics have different views on the time periods that climate change should be addressed and this means that there are many debates surrounding the validity of geo-engineering.

A final article comes from Bloomberg titled ‘Geoengineering to Alter Climate Moves Closer to Reality.’ The article states, ‘A United Nations body is investigating controversial methods to avert runaway climate change by giving humans the go-ahead to re-engineer the Earth’s oceans and atmosphere’. To achieve the climate change reductions agreed during COP21 last December it seems that geo-engineering is needed. This article points out some strong arguments against geo-engineering including the unpredictable consequences, the fact that it can be hazardous and it is costly to implement. It is interesting that this article puts across the most pessimistic view of geo-engineering for the future of the climate.

It is clear to see that there are mixed opinions on geo-engineering methods in the news but the good thing is that it is being increasingly discussed. Many of the articles provide useful information on the different techniques and the issues surrounding them which is positive as it allows people to judge for themselves. It is vitally important that geo-engineering is discussed further in public circles as it could be very beneficial when attempting to deal with climate change.

The easiest geo-engineering technique?

Geo-Engineering can be divided into to two main categories, solar geo-engineering and carbon dioxide removal (CDR) geo-engineering. The previous post on ocean acidification falls into the latter category. Next I wish to look at another form of CDR geo-engineering, one which I feel can have obvious impacts, is relatively cheap and easy to implement and the barriers to implement this are relatively small. This method is called afforestation/reforestation. Below, I will outline this method and why I feel it is a Carbon Dioxide Removal scheme that we as a planet should be introducing now because it has very few negative repercussions.

Afforestation/reforestation are two distinct terms and it is vital to explain the difference now. Caldeira et al. (2013) explain the differences clearly and concisely:

“Afforestation is the direct human-induced growth of forest on land that has not historically been forested. Reforestation is the direct human-induced conversion of nonforested land to forested land on land that had been previously converted from forest to other uses.”

The planting of new young trees in areas previously covered before human activity is an easy way to increase the storage of atmospheric carbon dioxide because trees take in CO₂ during the process of photosynthesis that takes place during the day. This means that forests are a natural carbon sink (store of carbon). Further to this, trees absorb more carbon dioxide during their early development phase for growth and this means that planting new forests would have a greater impact than existing forests. The planting of new trees is therefore seen to have quick impacts on the atmospheric carbon dioxide levels.

The processes of afforestation and reforestation have clear benefits. In the instance of reforestation, there are relatively few drawbacks as it is just the process of returning land to the habitat that existed there around 300 years ago. Because of this, it is unlikely that reforestation will have any unexpected or dramatic impacts on the planet as a whole which is a particular concern for many of the other geo-engineering schemes proposed. There is also clear understanding of the processes involved and therefore predictions on carbon uptake by trees is more reliably calculated. The ecological benefits for endangered species are noticeable as many of the species now endangered were put in this situation by human clearance of their habitats in the past. Reforestation could help many species recover.

The use of afforestation of new areas would also be beneficial. Research has found that afforestation at low latitudes in tropical climates could produce an added cooling benefit due to increased formation of low clouds that could increase the amount of solar radiation reflected back into space. One ambitious estimate suggested that the use of forest planting could restore all the carbon lost through human deforestation and potentially decrease atmospheric carbon dioxide levels by 40 to 70 ppm by 2100. This is a very optimistic estimate however with other research estimating that if the USA and European countries planted forests over 0.5% of their areas, around 40Mt CO₂ per year could be removed from the atmosphere which is just 0.2% of the annual world anthropogenic CO₂ emissions. Other papers have suggested that a feasible estimate for atmospheric carbon dioxide reductions would be 15-30ppm by the end of the century. Despite this, I still feel that the planting of forests could have other as yet unknown beneficial climate feedbacks that could aid cooling to restore the climate to pre-industrial revolution levels.

There are unfortunately some drawbacks to this method of geo-engineering as there are with all methods. Much of the land that would be used for reforestation was cut down to make way for crops and livestock and this land is still being used for this purpose today. Farmers are unlikely to want to make way and lose their livelihoods to plant some trees and this highlights the main reason why reforestation has not been employed more aggressively in the past. On top of this, the world is under ever growing food insecurity and the demand for food is endlessly increasing. It therefore seems implausible that it will be easy to implement reforestation schemes without harming other sectors.

Afforestation also has some drawbacks with regard to climate. Afforestation in higher latitude areas that are seasonally or permanently covered by snow would lower the albedo (reflectivity) of the area causing more solar radiation to absorbed, actually leading to increases in temperature despite reductions in carbon dioxide. This highlights that the use of afforestation needs to be more carefully studied compared to reforestation as there may be less well understood adverse impacts that alter the climate of a region possibly affecting other systems such as rainfall patterns, monsoons, flooding, etc. These impacts may also occur far away from the afforestation programme meaning larger scale research is necessary.

To end this post, it is useful to consider the extent to which afforestation and reforestation could be implemented. A 2010 paper studied how humans have altered the biomes (different habitat types) between 1700 and 2000. The map below (Figure 1) shows the different anthromes (human biomes) worldwide. The 2000 map shows that a large proportion of the land that used to be forested is now dominated by croplands and rangelands (livestock). This illustrates the potential for reforestation on a large scale. In 1700 over 90% of the world was classed as wild or semi-natural but in 2000, less than 50%of the land was classified in such a way. Personally, I am not advocating a total reforestation of all the land that was previously woodland – that would be completely unachievable in the current global situation with the high demand food but a gradual and progressive reforestation of cropland and rangelands as well as careful afforestation of suitable areas would have an influence on atmospheric CO₂ (albeit small) with relatively few serious drawbacks. What’s not to like?

Figure 1: Anthropogenic biomes (1900 and 2000)

Ocean fertilisation for carbon capture

For this blog post I am going to look at the concept and implementation ocean fertilisation for carbon capture as a form of geo-engineering to reduce carbon levels in the atmosphere – one of the key greenhouse gases that is causing climate change.

So what exactly is ocean fertilisation? Ocean fertilisation refers to the adding of iron or other nutrients to the ocean at a large scale to enhance algal growth in order to increase the uptake of atmospheric CO2. When the algae die, they sink to the sea bed, taking the carbon with them and storing it in the deep ocean. This would benefit the planet as a reduction in atmospheric CO2 would reduce the amount of heat reflected back to Earth thus reducing or reversing the current trend of a warming planet. In order to boost this algal growth, nutrients such as nitrogen (N), phosphorous (P) and iron (Fe) could be added to the ocean waters to stimulate this growth and thus trigger the uptake of more Carbon Dioxide. Figure 1 below shows the biological pump, the process by which carbon dioxide is transferred between the atmosphere and the ocean. Stimulation of this cycle with nutrients is the purpose of ocean fertilisation.

Figure 1: The Biological Pump

One major question with regards to the implementation of ocean fertilisation is the feasibility of it. A study from 2014 looked into the feasibility of ocean iron fertilisation in the Southern Ocean using simulations with a Global Climate Model. This paper concluded that the Southern Ocean would be one of the best areas of ocean iron fertilisation worldwide which could have a noticeable effect on atmospheric carbon dioxide levels. In addition to this, other studies have involved data from a Southern Ocean-Iron Experiment (SOFeX). From this research, ocean fertilisation is a geo-engineering method that is difficult to achieve as the amount of iron required and the frequency to achieve the level of carbon sequestration (uptake of carbon by oceans) that is necessary to influence the climate is difficult to achieve. There are suggestions that commercial ships could deposit iron along the major shipping routes but these ships would be unable to carry the amount of iron needed.

The concept of ocean fertilisation sounds promising but there are a number of other drawbacks that need to be considered. One of the key concerns is ocean acidification. Carbon is a key cause of ocean acidification and by using ocean fertilisation, the oceans would be taking in more carbon, lowering the pH of the water. This can have severe consequences of marine wildlife and coral. In addition, there is a problem known as outgassing which refers to the fact that not all the carbon transferred to the deep ocean will remain there. Carbon is returned to the surface through ocean upwelling and evaporation transfers it into the atmosphere. One piece of research, found that within 100 years, around two-thirds of the carbon transferred to the deep ocean by ocean fertilisation returns into contact with the atmosphere because it is redistributed through the world’s oceans. This emphasises the fact that ocean fertilisation is not a long-term geo-engineering solution unless it is sustained. The process of ocean fertilisation is also slow and therefore it may not be the solution necessary to quickly react to anthropogenic climate change. The use of ocean fertilisation can also reduce oxygen levels in the ocean. This can lead to a loss of marine wildlife and a reduction in biodiversity because they are not well adapted to low oxygen (anoxic) waters. This can kill fish which has a knock on effect for commercial farmers.

So is ocean fertilisation a suitable Geo-Engineering solution? On the surface, ocean fertilisation seems to be a reasonable method of addressing the effects of climate change at a small scale. It is possible to implement quickly and effectively, and at relatively little cost. However, when looking at this method in greater depth, it is clear that there are a great number of problems with it and the suitability of this technique is questionable. In my own personal opinion, I do not feel that ocean fertilisation is the best possible geo-engineering as it is slow, has relatively little effect on global climate and has a number of negative environmental impacts that are difficult to avoid. The focus should be on protecting current ecosystems not destroying one to save the others.