Introduction to Carbon Capture From Coal Fired Powerstations
Our Fossil fueled power stations produce the biggest percentage of carbon dioxide gas, emitting it straight to the atmosphere.
The CO2 is formed when we burn a fossil fuel,
Fuel + Air = Energy + CO2 + H2O
The amount of CO2 produced is proportional to the type of fossil fuel used,
- Coal -1.00T of CO2/MWh
- Oil – 0.75T of CO2/MWh
- Gas – 0.50T of CO2/MWh
As we can see from the above figures, coal is the worst CO2 producing culprit, but ironically we have much more coal reserves- enough to last us 300 to 400 years- compared to current estimates that show oil and gas supplies running out this century.
In order to use coal as a fuel in power stations without polluting the atmosphere, we need to remove the CO2 from the coal combustion gasses and store it in some safe location where it cannot escape back into the atmosphere.
In this article we will examine the methods used in the capture of CO2 from coal-fired power stations combustion gasses as part of my series on fossil fuelled power stations.
We shall begin by recapping how power is produced from coal, and then examine a current method of capturing the CO2 from the gasses after they have passed through the normal fume treatment unit.
Operation of a Coal Fired Power Stations
The coal is delivered to the stockyard from where it is supplied by conveyor to the coal pulverizers. The pulverizers crush the coal to a fine dust, and from here it is blown into the boiler furnace along with the optimum combustion air, is combusted, and the heat energy is used to raise steam in the boiler.
The steam is superheated and supplied to a three stage steam turbine which drives the power generator.
The combustion gasses passed through a gas treatment plant where the SOx, NOx, and particles are removed, with the remaining gasses including CO2, passing out of the flue into the atmosphere.
Methods Used to Remove CO2 From Coal Fired Power Plants.
There are several methods for removal of CO2,
- Removal before combustion, such as in coal gasification
- Removal during combustion, using an enriched oxygen combustion process
- Removal from the combustion gasses by absorption tower or by using a PTFE micro membrane
In our example of a coal-fired power station we will use an absorber tower process to remove the CO2 from the combustion gasses.
Removal of CO2 Using a Chemical Absorbent
The process of the removal of CO2 from the combustion gasses forms part of the fume treatment unit of a conventional coal-fired power station. This process consists of first removing the NOx, SOx and particles from the gas (see my article on coal-fired power stations) then passing the combustion gasses through a packed absorber tower where the CO2 is removed. Different types of chemical absorbents can be used in the tower, the most popular being monoethanolamine (MEA).
MEA Absorber Tower Operation
The absorber tower is a tall cylindrical pressure vessel, containing several sections of packing. The packing is normally made from ceramic rings, which are inert, providing a good contact between the MEA and the fume gasses. The fumes enter the tower moving up through the packed sections. The MEA is introduced at the top of the tower being dispersed downwards over the packed sections by a distributor plate. The MEA meets the gasses passing upwards which bubble through the packed sections removing the CO2 from the gas which drops to the bottom suspended in the liquid MEA. The resultant liquid mixture then gathers at the bottom of the tower from where it is withdrawn and pumped into the desorber tower.
This is another pressure vessel of similar design to the absorber tower including the ceramic packed sections. The liquid from the absorber tower is fed into the top of this tower, falling downwards through the packed sections. Steam is introduced into the bottom of the desorber, bubbling up through the liquid mixture cascading downward, and passing through the packed sections. The CO2 is stripped from MEA, and passes out the top of the desorber tower for dehydration and cooling and then is sent to storage facilities or to further processing. The MEA gathers at the bottom and is pumped into a reboiler where it is regenerated, and from here it is recycled into the absorber tower along with make-up MEA.
Uses of Captured CO2
- Dry ice manufacture
- Carbonated beverages
- Inert gas welding
- Oilfield enhancement
- Raw material used in chemical industry
- Firefighting as a blanketing gas and fire extinguisher medium
Note: The various methods for storage of CO2 will be covered in the next article on fossil fuelled power plants.