CO₂ Capture and Storage (CCS) is the only technology capable of abating at least 90% of the CO₂ emitted by industrial/energy sources (e.g. power plants fired with fossil fuels, cement and steel plants, refineries). First, the CO₂ is captured at source, then transported to a storage site and injected into a geological formations deep underground, where it is trapped safely and permanently ₍₁₎.

Despite the current elevated costs of CCS – which are expected to drop to commercially viable levels around 2020 ₍₂₎ – the International Energy Agency states that the cost of stabilising greenhouse gas levels without CCS would be a stark 71% higher or US$1.3 trillion per year. The European Commission also states that “the costs of meeting a reduction in the region of 30% GHG ₍₃₎ in 2030 in the EU could be up to 40% higher than with CCS.”

CCS is a suite of existing technologies which span the capture, transportation and storage of CO₂ – all of them individually tried and tested for decades. The next step is to combine and validate all these elements on a large-scale to create complete CCS value chains – as foreseen by the EU’s CCS Demonstration Programme which will be operational by 2015.

Beginning with capture – the most technically challenging component – there are three existing technologies by which CO₂ can be separated from everyday processes at major emitters: pre-combustion, post-combustion and oxy-fuel. All these technologies can capture at least 90% of the CO₂ emitted; current elevated costs are the only constraint to capturing 100% of CO₂ emissions.

If biomass (such as algae) is combined with coal to fuel a CCS-equipped power plant, you can even achieve net negative CO₂ emissions, as biomass also absorbs CO₂ as it grows!

1. Pre-combustion

The first of the three capture technologies – pre-combustion – allows operators of power plants to capture CO₂ and maximise power output.



With this method, an air separation unit produces a stream of almost pure oxygen. The oxygen then flows into the gasifier and reacts with pulverised coal to form synthetic gas, or “Syngas.” Steam is added to the Syngas in a shift reactor converting the carbon monoxide to hydrogen and CO₂. Using a physical wash, the CO₂ is then captured from the gas stream and after compression and dehydration, is ready for transport and storage. Today, the hydrogen is burnt to power turbines and make electricity; tomorrow it could also be used as fuel for transport.



Meanwhile, the flue gas that results from the hydrogen-powered turbines passes through a Heat Recovery Steam Generator which generates steam to power steam turbines, thereby optimising energy output.

2. Post-combustion

The second technology – post-combustion – has the advantage of being able to be installed on both and existing power plants – of vital importance given that the average power plant operates for 40 years.



A mixture of coal and air is blasted into the boiler and ignited. Many power stations "wash" and pulverise the coal before it is fed into the boiler. "Washing" actually refers to a process that involves passing coal through a series of liquids with varying densities. This removes many of the impurities found in coal (the impurities sink in the liquid, allowing them to be easily removed).



The heat from the combustion of the coal/air mixture generates steam, which drives the turbine. Meanwhile flue gas, a by-product of burning coal, is removed from the boiler in order to undergo a series of filtering processes.

Steam powers the turbines to generate electricity, which is transmitted into the distribution grid. Once the steam has passed through the turbine, it arrives at a condenser. This unit uses cool water to condense the steam back into water, allowing it to be piped back into the boiler and be re-heated again.



This is the first of several "cleaning" processes that the flue gas will pass through. At this point, small particles called "fly ash" are removed from the gas. Sulphur is then removed from the flue gas before it enters the CO₂ absorber where it needs to be cooled. This stage, using water, lowers the temperature of the gas. Here, the gas stream is typically passed though a liquid sorbent (the CO₂ absorber), which reacts with the CO₂, chemically binds with it and removes it from the flue gas.



Once the CO₂ is captured, the sorbent is moved to a desorber to be "regenerated", which generally involves heating the sorbent to release the captured CO₂.

3. Oxy-fuel

The third of the available capture technologies – oxy-fuel – consists of burning fuel in pure oxygen instead of air. This is done to increase the CO₂ concentration in the flue gas, thereby making it more efficient to remove ahead of processing for transport and storage.



This method deploys an air separation unit that removes nitrogen from the air, producing oxygen. This is injected – alongside the fuel – into a boiler where combustion takes place. Steam is then generated and used to power turbines and make electricity. Meanwhile, the flue gas, CO₂ and water vapour is re-circulated to control boiler temperature and gradually cooled. This leaves the captured CO₂ to be compressed and dehydrated, ready for transport and storage.



To date ₍₄₎, the only two examples of oxy-fuel combustion are Swedish utility Vattenfall’s 30 MW pilot plant in Schwarze Pumpe, Germany, and energy company Total's Lacq project in south western France.



CO₂ is preferably transported by pipeline, with ships being used when a source of CO₂ is too far from a suitable storage site and greater flexibility is required. 

For over 30 years, the oil and gas industry has been transporting and re-injecting CO₂ into oil fields all over the world to maintain or increase production. There are already about 5,000 kilometres of underground pipelines in North America used to transport CO₂ from natural reservoirs to oil fields.



The largest network supplies Permian Basin operators in Texas and New Mexico, which have been injecting CO₂ for 35 years. Shorter piping runs are used in other locations by beverage and chemical manufacturing facilities. These pipelines and industrial piping runs have operated for years without any significant safety incidents.



We can store CO₂ safely underground using a process that uses natural mechanisms which have been ‘storing’ oil, gas and CO₂ for millions of years. There are two types of CO₂ storage reservoirs: depleted oil and gas fields and deep saline aquifers – both with similar geological features.



Deep saline aquifers are porous rocks which contain undrinkable salt water and are found between 700 metres and 3,000 metres below the earth’s surface. These formations have a huge capacity for CO₂ storage worldwide and are likely to become the most widely used type of storage site. In Europe alone, it is conservatively estimated that we can store almost 60 years (5) worth of current annual CO₂ emissions from power plants and heavy industry.