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How CCS works - capture
Energy from fossil fuels such as coal, oil and natural gas is released in the combustion (burning) process. The emission of CO2 is a by-product of this process.
Capture technology can be applied to any large–scale emissions process, including coal–fired power generation, gas and oil production, and manufacture of industrial materials such as cement, iron, steel and pulp paper. In fact, large CO2 emitter industries around the world have applied capture technology for decades. Captured CO2 is used, for example, in the food and beverage industry and in making fertiliser.
In systems where the coal is pulverised to a powder, which makes up the vast majority of coal–based power plants in North America, Australia, Europe and China, the CO2 must be separated at fairly diluted concentrations from the balance of the combustion flue gases (gas exiting via a chimney or ‘flue’). In other systems, such as coal gasification, the CO2 can be more easily separated.
There are three basic types of CO2 capture: pre-combustion, post-combustion and oxyfuel with post-combustion.
Pre-combustion processes convert fuel into a gaseous mixture of hydrogen and CO2. The hydrogen is separated and can be burnt without producing any CO2; the CO2 can be compressed for transport.
Pre-combustion capture is used in industrial processes but has not been demonstrated in much larger power generation projects. The fuel conversion steps required for pre-combustion are more complex than the processes involved in post-combustion, so the technology is more difficult to apply to existing power plants.
Pre-combustion capture increases the CO2 concentration of the flue stream, requiring smaller equipment and different solvents with lower regeneration energy requirements.
The process involves:
- partially reacting the fuel at high pressure with oxygen or air and, in some cases, steam, to produce carbon monoxide and hydrogen
- reacting the carbon monoxide with steam in a catalytic shift reactor to produce CO2 and additional hydrogen
- separating the CO2 and, for electricity generation, using hydrogen as fuel in a combined cycle plant.
Although pre-combustion capture involves a more radical change to power station design, most elements of the technology are already well proven in other industrial processes.
Post-combustion processes separate CO2 from combustion exhaust gases so that the CO2 can be captured using a liquid solvent. The CO2 is absorbed by the solvent and then released when it is heated to form a high purity CO2 stream.
The process involves scrubbing the flue with a suitable solvent, such as an amine solution, to form an amine–CO2 complex, which is then decomposed by heat to release high purity CO2. The regenerated amine is recycled to be reused in the capture process.
Post-combustion capture is applicable to coal–fired power stations but additional measures, such as desulphurisation of the gas stream, are required to prevent the impurities in the flue gas from contaminating the CO2 capture solvent.
Two significant challenges for post-combustion capture involve:
- the large volumes of gas that must be handled, requiring large–scale equipment and creating high capital costs
- the amount of additional energy needed to operate the process.
Post-combustion capture technology is used widely in the food and beverage industry.
Oxyfuel with post-combustion processes uses oxygen rather than air for combustion of fuel. This produces exhaust gas that is mainly water vapour and CO2 that can be easily separated to produce a high purity CO2 stream.
The concentration of CO2 in flue gas can be increased by using pure or enriched oxygen instead of air for combustion, either in a boiler or gas turbine. The oxygen is produced by cryogenic air separation (already used on a large scale industrially), and the CO2-rich flue gas recycled to avoid the excessively high–flame temperature associated with combustion in pure oxygen.
The advantage of oxyfuel combustion is that, because the flue gas contains a high concentration of CO2, the CO2 separation stage is simplified. The main disadvantage is that cryogenic oxygen is expensive.
Oxyfuel combustion for power generation is currently being demonstrated at a refurbished power station in Biloela, Queensland.