Carbon capture and sequestration (CCS) is a process that helps to reduce the carbon dioxide emissions from primary sources, such as fossil fuel power plants and industrial processes, by collecting the produced gas and storing it such that the amount of atmospheric greenhouse gases does not increase.
The most well known use of CCS is within the energy sector. This sector – which encompasses extracting resources, generating energy, producing fuels and transporting energy – produces the most emissions of any sector at 35% of global greenhouse gas emissions.
Within current CCS usage, there are three main capture processes used within fossil fuel plants: pre-combustion, post-combustion and oxyfuel combustion. These involve techniques such as partially oxidising the fuel before combustion, conducting the combustion itself in pure oxygen and using a solvent or membrane to absorb the carbon dioxide from the flue gases – the mixture of gases produced through the combustion process.
These processes have been shown to reduce emissions by up to 90% but unfortunately come with a number of heavy limitations. The first of these is that only oxyfuel combustion can be relatively easily retrofit to existing power plants, potentially incentivising the production of new ‘low emission’ fossil fuel plants rather than focusing investment on renewable sources. Also, the initial energy cost of, for example, producing a supply of pure oxygen, means that the total offset in emissions may actually be minimal.
The other main part of CCS is the sequestration (or storage). This necessity of this stage poses the question of where and how to safely store the carbon dioxide long term, as for CCS to have an impact on greenhouse gas emissions, it must be prevented from returning to the atmosphere after capture. Geological storage is the most common form and involves injecting carbon dioxide as a supercritical fluid into underground geological formations.
There is a reasonably large availability of areas that could act as potential storage sites. For example, the UK has a number of suitable reservoirs in areas such as the north and central North Sea basin. Overall, it is estimated that the UK has the capacity for around 16-20 gigatonnes (Gt) of storage in abandoned hydrocarbon fields and 19-716 Gt of storage in saline aquifers.
There are also two other main fields in which CCS is predicted to have an invaluable impact even after the transition away from fossil fuels: CCS with biomass fuel production and CCS within hard to decarbonise industrial processes.
BECCS (bioenergy with carbon capture and sequestration) is a process combining the renewable energy source of burning biofuel with CCS technology. Biomass is the name of any organic matter used as a fuel in order to generate electricity. It can take the form of substances such as wood, agricultural waste and energy crops such as elephant grass and switch grass. This process presents the possibly for a net negative carbon emission system due to the carbon intake that occurs through photosynthesis when the plants are growing. This idea of net negative emissions is very important to the mitigation of global warming, as some processes are very difficult or even impossible to fully decarbonise, so the use of negative emission technologies is imperative in order to ensure a safe level of atmospheric greenhouse gas emissions is maintained.
The main issue relating to this process is the large amount of land use needed to produce biomass on a scale that will be able to make a significant impact on emissions. This raises problems such as a decrease in local food security due to a reduction in land available for growing edible crops which will drive up food prices. In fact, in order to capture 10 Gt of carbon dioxide, nearly 40% of current global cropland would need to be used. The use of such large areas of land in this way would also significantly decrease biodiversity if not properly managed and could potentially release carbon that had been trapped in the soil for many years, negating any positive environmental impact. The combustion of the biomass itself would also cause air pollution, meaning adverse effects on the health of local populations.
However, this form of carbon capture does present the possibility of very large reductions in global emissions. The US National Academy of Sciences predicts that BECCS could result in the capture of between 3.4-5.2 Gt of carbon dioxide per year by 2050 whilst avoiding many of these large scale issues. The IPCC also estimates that BECCS could be capturing 5-10 Gt of carbon dioxide annually by 2100 but this has met some dispute simply due to the sheer amount of land that this would require.
Arguably the most important use of CCS going forward is its potential use within industrial processes. This is because they are one of the hardest areas to decarbonise both due to the emission of carbon dioxide from the chemical and physical reactions required for the production of the materials and from the need for these reactions to be conducted in such high temperatures. This heating alone is the cause for one third of total industrial energy consumption. In the past 40 years, the most energy intensive industrial processes have seen quite significant increases in efficiency. However, this does mean that the thermodynamic limits of the reactions are very close to being reached, so could mean little to no future decrease. This would leave CCS a very important task of helping the sector reach close to net zero carbon emissions going forward.
CCS is predicted to be most helpful in the cement, iron and steel and chemical sectors and is one of the most cost-effective ways to reduce industry emissions with costs as low as 15-29 USD per tonne of carbon dioxide captured. It is currently being used on a large scale at 16 different industrial facilities and is able to capture over 30 Mt of carbon dioxide per year. However, as the industry is a very competitive one with very low margin markets it makes it less appealing to invest in innovation or low carbon routes as companies are likely to lose out financially, especially in sectors where there are no carbon emission prices or regulations.