Three important methods
Several CO2 capture technologies exist, and additional methods are being developed. All CO2 capture methods require large investments and so far, the technology has only been tested on a small scale.
Post-combustion
This method is easiest to implement because the capture plant can be constructed independently of the power plant. Several gas power plants with post-combustion CO2 capture are being planned in Norway. The flue gas emitted by a gas power plant only contains 3.5-4 volume % CO2. In other words, large volumes of gas need to be treated.
The most commonly used capture method is amine scrubbing. When flue gas is passed through a tower filled with amine solvent, CO2 is 'scrubbed', i.e., absorbed by the amines. The remaining flue gas, consisting mainly of air, water vapour and a residual amount of CO2 (0.6 % of the initial contents are not captured) is discharged into the atmosphere. In a subsequent step, the amine solvent containing the captured CO2 is heated. This process releases the CO2 again, which then can be compressed and transported to a storage site. The amines can be re-used. The absorption tower required for a 420 MW gas power plant would cover an area of about 240 m2, and reach a height of more than 50 m. The power plant would annually generate about 1.2 million tonnes of CO2.
Pre-combustion
In a pre-combustion plant, natural gas or coal is reformed to so-called synthesis gas (syngas), which consists of carbon monoxide (CO) and hydrogen. CO is then transformed to CO2 using steam and heat. The CO2 is then captured, while the hydrogen is fired in the gas turbine.
The actual power plant based on fuel reformation is more expensive than a conventional gas-fired power plant. The total cost of a gas-fired power plant using pre-combustion carbon capture is thus higher than of a power plant using post-combustion carbon capture. Another disadvantage of pre-combustion is the challenge posed by nitrogen oxide emissions resulting from high-temperature combustion of hydrogen. This model is relevant for operators wishing to produce both power and hydrogen fuel, e.g., for transportation.
Oxy-fuel
In an oxy-fuel plant, oxygen is for combustion instead of air. This gives a nearly clean combustion, with the flue gas consisting of CO2 and water. Thus the CO2 gas can be captured by condensation of the water, a simpler process than conventional post-combustion capture. However, the initial separation of oxygen requires a large energy input.