> CO₂ Capture - understanding the three technologies

There are three existing technologies by which CO₂ can be separated from everyday processes at major emitters: pre-combustion, post-combustion, oxyfuel.

All of these technologies can capture at least 90% of the CO₂ emitted; current elevated costs are the only constraint to capturing 100% of CO₂ emissions using CCS.


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 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.

The flue gas that results from the hydrogen-powered turbines passes through a Heat Recovery Steam Generator which powers steam turbines re-using this energy and optimising energy output.


2. Post-combustion

Post-combustion has the advantage of being able to be installed on both existing and future power plants – of vital importance given that the average power plant operates for 40 years, sometimes longer.

A mixture of coal (or gas) and air is blasted into the boiler and ignited. Many power stations "wash" the coal before it is fed into the boiler. "Washing" actually refers to a process that involves passing coal through a series of liquids of varying densities which removes many of the impurities.

The heat from the combustion of the coal and air mixture generates steam, which drives the turbine. The flue gas, a by-product of burning coal, is removed from the boiler and filtered. The steam powers turbines to generate electricity which is then transmitted to 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.

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 also 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, 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 usually involves heating the sorbent to release the captured CO₂.


3. Oxyfuel

The third of the available capture technologies – oxyfuel – consists of burning fuel in a mixture of pure oxygen and recirculated flue gas instead of air. This is done to increase the CO₂ concentration in the flue gas, thereby making it more efficient to remove before 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 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, the only two examples of oxyfuel 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.

Source: European Technology Platform for Zero Emission Fossil Fuel Power Plants

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