Carbon Capture, Usage and Storage (CCUS): what, why, how?

Carbon dioxide is one of many gases in the exhaust streams of power stations and industrial processes. To be used or stored, it first needs to be separated. This can be done in a number of ways, such as passing the exhaust fumes through solvents or across catalysts and other substances that selectively bind to CO₂.

One way to strip the carbon out is through chemical absorption.

This is when waste gases are passed through solvents like ammonia, which absorb the CO₂ in a chemical reaction. This absorption is then reversed by blasting the ammonia with high temperature steam, releasing the purified CO₂. This type of capture can be retrofitted onto existing buildings. This meakes it the most popular CCUS technology trialled to date.

Another way to strip out CO₂ is by oxyfuel combustion.

This is when the fossil fuels are burned (e.g. during power generation) in a purer mixture of air. This mix contains just oxygen and carbon dioxide, with the nitrogen removed. Burning the fuel in purer air means that the exhaust gases are more concentrated with CO₂, making it easier to capture and store the waste gases after burning.

Oxyfuel combustion is slightly simpler than chemical absorption as no solvent is required. The unit that carries out this process is also relatively small. This means there is a possibility of retrofit to an existing plant with some alterations.

CCUS will be useful in decarbonising industries that are likely to have residual emissions to 2050. Obvious examples are heavy industries like steel making and power generation that uses natural gas or biomass. CCUS capture rates are not 100% efficient, so there will still be some residual emissions.

Today, around 5kg of CO₂ can be captured per person per year across the world, to a total of around 39.5 million tonnes. Yet, global CO₂ emissions are around 33 billion tonnes. So a significant gap remains. Although the current planned pipeline will double the amount that can be captured per person, a much greater scale up is needed – around a thousand times the current level per person – to sequester the global average person’s carbon emissions of five tonnes (5000kgs) per year.

Before it can be used or stored, captured CO₂ must be transported, usually via pipelines. For this, it is compressed into a liquid state and can then be moved through the pipelines, by ships or in road tankers.

Sharing infrastructure such as pipelines between multiple emissions sources, creating ‘industrial clusters’, is becoming increasingly attractive. Clustering can combine emissions from power stations and industry into a single pipeline, cutting costs and providing the scale needed to make CCUS projects viable.

Captured CO₂ can be used to make a multitude of materials. It can be converted to building materials such as concrete (mineralisation), used to make plastics via polymerisation as a feedstock for microalgae that is converted to biofuels, among others.

These processes can be very energy intensive. The additional cost of using CO₂, as well as the waste produced in making new materials, can impact the viability of CCU projects.

CO₂ can also be used directly in commercial processes such as food and drink, horticulture and in enhanced oil recovery. In food and drink, CO₂ can be used to carbonate drinks, freeze and chill food and in packaging. In horticulture, it can be added to greenhouses to enhance the production of crops that use CO₂ in photosynthesis.

The main use for CO₂ today is enhanced oil recovery, a process to increase the maximum amount of oil and gas that can be extracted from a site. By injecting CO₂ into the reservoir, more hydrocarbons are forced out than would be otherwise. The additional revenue of these fuels imparts a value on the CO₂ used to extract them; to compete other uses of CO₂ will also need to produce economic value.

Storing captured CO₂ – geologically, minerally or in oceans – prevents it from entering the atmosphere and contributing to climate change.

Geological storage, in sedimentary rocks in old oil and gas fields or saline formations (porous rocks), can include re-using existing oil and gas infrastructure. Europe has several areas that have significant storage capacity potential, such as Spain (up to 14 GtCO₂) and Norway (134 GtCO₂).

One often-cited concern around geological storage is that CO₂ will leak out over time. However, studies have found this is unlikely, stating that even if there is a 50% probability of 0.0008% leakage per year, if ‘well-regulated’ over 98% of the stored CO₂ would remain in the subsurface for over 10,000 years. For this reason, geological storage is the most credible option CO₂ examined to date.

Ocean storage is another option. While technically possible, injecting CO₂ into the ocean and either letting it diffuse or trapping it in a specific location, would lead to the formation of carbonic acid, a direct cause of ocean acidification. For this reason it is not seen as a credible means of storing CO₂.

Mineral storage is also used. this is where CO₂ is chemically bound to calcium and magnesium-rich rocks, holding it in place at specific sites. As calcium and magnesium are stable and abundant, the CO₂ will not be released into the atmosphere. However, this reaction can be slow under normal temperatures and so would ned to be heated (using more energy) to increase the speed, or minerals would have to be pre-treated.

The UK has had targets for CCUS for many years.

For example, the 2003 Energy White Paper aimed to develop large scale CCUS projects. Competitions to support CCUS demonstrations were carried out throughout the 2000s and 2010s.

• The first demonstration competition was cancelled because it could not be funded within the £1bn budget agreed.
• A second £1bn competition was launched in 2012. The White Rose oxyfuel combustion project was due to take place in a coal power station owned by Drax in North Yorkshire, with capture rates of around 90%. But it was cancelled by Government in 2016.

In the Clean Growth Strategy in 2017, the Government committed to demonstrating ‘international leadership in CCUS’ and investing up to £100 million in the technology. This was backed by a CCUS action plan and CCUS cost challenge taskforce in the years that followed, all aiming to help reduce costs and accelerate large scale roll out.

More recently, investment into CCUS has been re-pledged; up to £1bn was announced in the PM’s 10-point-plan for supporting four industrial clusters to decarbonise using CCUS by 2030, up from the previous target of ‘one low-carbon cluster by 2030’ that was announced in 2019. This aims to support a target of capturing 10MtCO₂ each year by 2030, the equivalent of four million car’s annual emissions.

Many projects in these areas are underway, such as Net Zero Teesside, HyNet and Acron CCS in Scotland. The Government have estimated that its £1bn investment in CCUS could support 50,000 jobs by 2030.

In the UK, the captured CO₂ could be stored in existing infrastructure such as the 300 platforms and 1000 pipelines that are found in the North and East Irish Seas. The Energy Technologies Institute estimate that the UK has ‘the CO₂ storage capacity to meet its needs out to 2050 and beyond’ and indicated that ‘there are no technical barriers’ to storing CO₂ offshore.

A large number of scenarios to net zero emissions include significant CCUS use. The Climate Change Committee (CCC) has stated that 100Mt CO₂e will need to be captured per year in 2050 in the UK. Close to 60% of this will be via engineered greenhouse gas removals (58MtCO₂/yr).