2.1 System to capture CO2
A wide range of technologies exists for separation and capture of CO2 from gas streams. Current commercial processes employ a variety of physical and chemical mechanisms including absorption, adsorption, membranes and cryogenics (Hendriks 1994; Mimura et al. 1997).
The choice of a suitable technology depends upon the characteristics of the CO2-laden gas stream, which in turn depends mainly on the type of power plant technology. There are four basic systems for capturing CO2 from use of fossil fuels and/or biomass (IPCC 2005) .
• Capture from industrial process streams
• Pre-combustion capture
• Post-combustion capture
• Oxy-fuel combustion capture
2.1.1 Capture from industrial process streams
Current examples of CO2 capture from process streams are purification of natural gas and production of hydrogen-containing synthesis gas for the manufacture of ammonia, alcohols and synthetic liquid fuels. Most of the techniques employed for CO2 capture in the examples mentioned are also similar to those used in pre combustion (IPCC 2005).
2.1.2 Pre combustion capture
The technology for pre-combustion can be widely applied in fertilizer, chemical, gaseous fuel (H2, CH4), and power production. In these cases, the fossil fuel is partially oxidized, such as in a gasifier. The resulting syngas (CO and H2) is shifted into CO2 and more H2, and then the resulting CO2 can be captured from a relatively pure exhaust stream.
The H2 can now be used as fuel; the carbon is removed before combustion takes place.
Figure 2-1 shows the pre combustion system, which involves several steps. At the beginning the air separator removes nitrogen from the air that is pumped into the unit, the remaining product is an almost pure stream of oxygen. In the gasifier the coals with oxygen and steam form a "syngas”. Then the syngas is then passes through the first filtering process, where small "fly ash" particles are removed. The shift reactor injects steam, causing a chemical reaction that converts the CO into hydrogen (H2) and carbon dioxide (CO2). Next, sulphur is removed from the syngas. In CO2 the syngas stream is passed though a CO2 capture device such as a CO2 absorber, which captures the CO2 from the gas stream. For CO2 sorbent/solvent processes the sorbent is moved to the desorber to be "regenerated". Regeneration in the CO2 desorber generally involves heating the sorbent, which releases the CO2 ready for compressing and transporting. Once the CO2 has been removed, the syngas consists primarily of hydrogen. This is then used to power gas turbines to generate electricity. In the heat recovery stage, the excess heat from the combustion of the syngas is captured to generate steam and this in turn is used to power a steam turbine which also generates electricity.
At the end of the process, the residual water vapour and air is released into the atmosphere (Miliband 2009).
Figure 2-1 Pre combustion system for CO2 capture (Vattenfall 2008) 2.1.3 Post combustion capture
In post-combustion, the CO2 is removed after combustion of the fossil fuel; this is the scheme that would be applied to conventional power plants. Instead of being discharged directly to the atmosphere, flue gas is passed through equipment which separates most of the CO2 (IPCC 2005).
The CO2 is fed to a storage reservoir and the remaining flue gas is discharged to the atmosphere.
Figure 2-2 shows the diagram of the post combustion system, first in the fuel injection stage, the coal is washed and pulverized before entering into boiler. The heat generated from the combustion of the coal/air mixture in the boiler creates steam, which is then pumped to the turbine. The turbines 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.
The particle removal is the first of several cleaning processes that the flue gas will pass through. The stage of sulphur removal involves a process called flue gas desulphurization (FGD). Before the flue gas enters the CO2 absorber, it needs to be cooled. In this stage, using water lowers the temperature of the gas. In the CO2 absorber, the gas stream is passed though a CO2 captured by a device, typically an absorber, which reacts with the CO2. Then after purged from the flue gas, the almost pure stream of CO2 can now be compressed into a liquid state (Miliband 2009).
Figure 2-2 Post combustion system for CO2 capture (Vattenfall 2008)
2.1.4 Oxy fuel combustion capture
In oxy-fuel combustion, nearly pure oxygen is used for combustion instead of air, resulting in a flue gas that is mainly CO2 and H2O. If fuel is burnt in pure oxygen, the flame temperature is excessively high, but CO2 and/or H2O-rich flue gas can be recycled to the combustor to moderate this. Oxygen is usually produced by low temperature (cryogenic) air separation and novel techniques to supply oxygen to the fuel, such as membranes and chemical looping cycles are being developed (IPPC 2005).
Figure 2-3 Oxyfuel combustion system for CO2 capture (Vattenfall 2008) Figure 2-3 shows the oxyfuel combustion system that involves burning the coal in nearly pure oxygen rather than the air/coal mix currently used in conventional power plants.