Carbon capture and storage: our last resort?

The conversation about climate change and global warming has been happening for decades; specifically, since the 80s. Since 1985 many global efforts have been placed into maintaining a “sustainable development”, starting with the United Nations Environment Program, the Intergovernmental Panel on Climate Change, the Kyoto Protocol and the Paris Agreement. These programs aim mainly at restraining greenhouse gas emissions, which increase temperatures in the Earth’s surface; and consequently, establishing a carbon budget to ensure that the average global temperature does not exceed a 2 ⁰C increase.

Carbon capture and storage has gained relevance as an instrument for mitigating anthropogenic greenhouse gas emissions, principally carbon dioxide. The relevance it has taken is mainly because what else are we going to do? We keep producing CO₂ and the Earth, even though it naturally removes it from the air, cannot handle the rate we produce it alone so we need to accelerate the process. It takes a lot of energy too. The amount of energy needed to remove a ton of CO₂ in a given year is equivalent to a 500 MW power plant.

Carbon capture and storage is as the name says, a method to capture carbon emissions from the air and store it somewhere. Almost all methods happen at the source of the emissions like manufacturing facilities and power plants. There are two methods to do so: pre and post combustion.

In pre-combustion carbon capture, the CO₂ is trapped before it is diluted by other flue gases. The fossil fuel is heated in pure oxygen resulting in a mix of carbon monoxide and hydrogen. These two are then separated and the carbon monoxide is reacted with water to form CO₂ which can be compressed for transport and storage. On the other hand, the hydrogen can be burnt without producing any CO₂. The processes for pre-combustion carbon capture are more complex than those for post-combustion carbon capture, which is explained next.

In post-combustion carbon capture, the CO₂ is captured after the fossil fuel is burnt. The combustion of fossil fuels releases a blend of different gases, mainly carbon dioxide, sulfur dioxide, methane and nitrous dioxide; therefore, the CO₂ needs to be separated from the flue gas. This is achieved by bubbling the gas through an absorber column packed with liquid solvents. Once absorbed by the solvent, the CO₂ is released by heating to form a high purity CO₂ stream, which is then transported for storage elsewhere. 

Pre and post-combustion carbon capture can prevent up to 90% of the power plants’ carbon emissions from entering the atmosphere; but it needs to be stored somewhere. The captured CO₂ is often stored underground in a process called geological sequestration, which involves injecting CO₂ into rock formations. Another option is to inject it very deep into the ocean (below 3500 m) as a liquid or as hydrates where it will remain due to its density and the tremendous pressure at those depths. Both pose risks as a big leak can negatively affect the environment and wild life in its vicinity.

Capturing CO₂ in power plants or manufacturing facilities is a relatively easy concept to implement; but what about the emissions from all the other sources like cars, agriculture, farming and even people?

Direct air capture has been up to recently a theoretical way for removing CO₂ directly from the atmosphere largely due to its expensiveness (about 600 $/metric ton of CO₂). The concept includes building a structure with great surface area but thin in thickness to capture CO₂ called direct air capture contactor. It is like building a “synthetic forest”, as Jennifer Wilcox called it in her TED Talk, except that a forest like the Amazon is able to capture 1.6 billion tons of CO₂ a year in a 5.5 km² area; whereas, the synthetic forest can capture the same but is 500 times smaller.

Incidentally, scientists from Harvard University and the Bill Gates funded company Carbon Engineering have found the way to achieve the process more cheaply, starting at 94 $/metric ton of CO₂. Their technology uses a solution of hydroxide to capture CO₂ and the mixture is then heated to high temperatures to release the CO₂, which is later stored and the hydroxide reuse. This new process is a combination of direct air capture, water electrolysis and fuels synthesis. They say the process can also transform the carbon back to gasoline and jet fuel creating a neutral carbon based fuel.

A new study from the Grantham Institute for Climate Change at Imperial College London uses two different models to determine how direct air capture could help meet global climate goals. The results of some of the modeled scenarios show that the “technology can play an important role in a long-term climate change mitigation strategy”. However, the study also points to the challenges; one of them being the amount of energy needed to run direct air capture machines. According to the results, the energy needed in 2100 would be 300 Exajoules per year, which is more than half of overall global demand today.

So what else can we do? Maybe all the countries should take a page out of Bhutan’s book and go carbon negative. Meanwhile, carbon capture and storage is a good solution to at least slow down increasing temperatures. More efforts are needed and luckily, several companies are working on developing new technologies or improving CCS.

Sources:

Real Engineering – Carbon capture: Humanity’s last hope? Video, CarbonBrief.com, J. Wilcox. A new way to remove CO2 from the atmosphere. TED Talk (2018)

Featured image: ResearchGate