Brief description of major technologies for CO2 capture
The main competing technologies for CO2 capture from fossil fuel usage are:
- Post Combustion Capture (PCC) from the flue gas of Combustion-based plants;
- Pre Combustion Capture from the in based plants; and
- Oxy Combustion – the direct combustion of fuel with Oxygen.
These three approaches are shown diagrammatically for coal based power systems in Figure 1-1.
Post combustion capture (PCC) at near atmospheric pressure can be applied to newly designed plants or retrofitted to existing coal plants after suitable flue gas clean up. Absorption processes are currently the most advanced of the PCC technologies. The PCC technologies can also be used in other industries besides power e.g. cement, oil refining, and petrochemicals.
Pre-combustion capture in the IGCC power application comprises gasification of the fuel with oxygen or air under high pressure, the use of the shift reaction followed by CO2 removal using Acid Gas Removal (AGR) processes withrich syngas supplied to the gas turbine based power block. Pre combustion capture can be added to existing IGCC plants but in the future IGCC plants will almost certainly be designed with capture from the start. The pre-combustion capture of CO2 using AGR processes is also practiced commercially in natural gas processing, natural gas reforming and coal gasification plants.
Oxy combustion is the combustion of fuel with oxygen. In an Oxy coal power plant, flue gas is recycled to the oxygen fired boiler to keep the boiler temperature at the level acceptable for boiler tube material integrity. The flue gas containing mostly CO2 is purified, dried and compressed. The Oxy technology may also be applied to existing plants but in most cases a new boiler and steam turbine would probably be justified.
Within each of the three major capture categories there are multiple pathways using different technologies which may find particular application more favourably in certain climate conditions, locations, elevations and coal types.
The importance of improved efficiency
The addition of CO2 capture incurs a very significant loss of efficiency and power output that has a large effect on theeconomics since the capital cost has to be spread over less MWh and the fuel cost per MWh is increased. This document is focused on the CO2 capture technologies and potential improvements to reduce the energy losses and capital costs associated with capture. However, a major contribution to the reduction of CO2 from fossil based plants will be achieved through increases in the efficiency of the basic technologies of pulverized coal combustion and combustion (gas) turbines.
For example, considerable work is underway to develop and qualify advanced materials that will enable the use of ultra supercritical steam conditions with higher temperatures (up to 700-750°C) and pressures (up to 350 bar). This, in turn, will lead to higher plant efficiencies and lower CO2 emissions per MWh. As illustrated in Figure 1-2 a 20% reduction in CO2 emissions can be achieved through efficiency improvement. EPRI studies indicate that this CO2 emissions reduction from efficiency improvement can be accomplished at lower cost per tonne of CO2 removed than from CO2 capture.
For PCC, the major energy losses are incurred in sorbent regeneration and CO2 compression. Current PCC R&D is focused on improved sorbents that require less energy for regeneration and/or could be regenerated at pressure, thereby reducing the CO2 compression energy required.
There are also major developments underway to increase the firing temperatures (up to 1600°C) and efficiencies of gas turbines. These developments will in turn reduce the CO2 emissions from natural gas combined cycle (NGCC) and(IGCC) plants.
For IGCC pre combustion capture, the major energy losses are incurred in theunit (ASU), water-gas shift, gas cooling and CO2 separation areas. The IGCC R&D is focused on improvements to the ASU, gasification, shift catalysts, and in the processes and equipment that reduce the energy loss of the separation of hydrogen from CO2 and of CO2 compression. The use of higher firing temperature higher efficiency gas turbines will further increase plant efficiency and reduce the CO2 emitted per MWh. These gas turbines will also be of larger sizes that will provide further economies of scale and improve economics.
For oxy combustion the major energy penalty is in the ASU area. Current oxy combustion R&D is focused on energy improvements to the ASU, potential reduction of recycle gas and CO2 purification energy losses. . The use of higher temperature materials in the boiler and steam turbine will further increase efficiency and reduce the CO2 emitted per MWh.