Advantages and disadvantages of major CO2 capture technologies
Post combustion capture advantages
- Can be retrofitted to existing plants allowing the continued operation of valuable resources
- In either new build or retrofit application it enables the continued deployment of the well established Pulverized Coal (PC) technology familiar to power industries worldwide
- The continued development of improved materials for Ultra Supercritical (USC) plants will increase the efficiency and reduce the CO2 emissions of future PC plants
- The widespread R&D on improved sorbents and capture equipment should reduce the energy penalty of PCC capture
- Sub-scale demonstration of PCC is proceeding. The 110 MW Boundary Dam project of Saskatchewan Power with PCC using the Cansolv process is under construction with planned operation in 2014.
Post combustion capture challenges
- processes are commercially available at relatively small scale and considerable re-engineering and scale-up is needed
- The addition of capture with current amine technologies results in a loss of net power output of about 30% and a reduction of about 11 percentage points in efficiency. In the case of retrofit this would imply the need for replacement power to make up for the loss.
- Most sorbents need very pure flue gas to minimize sorbent usage and cost. Typically < 10 ppmv or as low as 1 ppmv of SO2 plus NO2 is required depending on the particular sorbent
- Steam extraction for solvent regeneration reduces flow to low-pressure turbine with significant operational impact on its efficiency and turn down capability.
- Water use is increased significantly with the addition of PCC particularly for water cooled plants where the water consumption with capture is nearly doubled per net MWh. For air cooling the water consumption is also increased with capture by about 35% per net MWh.
- Plot space requirements are significant. The back-end at existing plants is often already crowded by other emission control equipment. Extra costs may be required to accommodate PCC at some more remote location.
Pre combustion capture advantages
- Pre combustion capture using the water-gas shift reaction and removal of the CO2 with AGR processes is commercially practiced worldwide.
- Pre combustion capture of the CO2 under pressure incurs less of an energy penalty (∼20%) than current PCC technology (∼30%) at 90% CO2 capture.
- Ongoing R&D on improved CO shift catalysts, higher temperature gas clean up and membrane separation technology for and CO2 has the potential to produce a step-change reduction in the energy penalty of capture
- Water use, while still substantial, is lower than with PCC
- The ongoing continued development of larger more efficient gas turbines can markedly improve the efficiency of future plants
- The Kemper County plant in Mississippi, an IGCC plant with pre combustion capture, is under construction with planned operation in 2014.
Pre combustion capture challenges
- While the energy loss with addition of pre-combustion capture is lower than with the addition of PCC the energy loss is still significant
- The commercial demonstration of large F or G gas turbines firing hydrogen has yet to be demonstrated in an IGCC plant with capture
- In the event of a need to vent the CO2 additional purification may be needed
- IGCC is not yet very widely used in the power industry
- The capital costs of IGCC without capture are much higher than SCPC without capture. The IGCC costs need to be reduced to compete more effectively.
Oxy combustion advantages
- Oxy-combustion power plants should be able to deploy conventional, well-developed, high efficiency steam cycles without the need to remove significant quantities of steam from the cycle for CO2 capture.
- The added process equipment consists largely of rotating equipment and heat exchangers; equipment familiar to power plant owners and operators. (No chemical operations or significant on-site chemical inventory).
- Ultra-low emissions of conventional pollutants can be achieved largely as a fortuitous result of the CO2 purification processes selected, and at little or no additional cost.
- On a cost per tonne CO2 captured basis, it should be possible to achieve 98+% CO2 capture at an incrementally lower cost than achieving a baseline 90% CO2 capture.
- Development of combustion with advanced ultra-supercritical steam cycles could result in an oxy-combustion power plant (with CO2 capture) that is higher efficiency than air-fired power plants being built today (without CO2 capture).
- The best information available today (with the technology available today) is that oxy-combustion with CO2 capture should be at least competitive with pre- and post-combustion CO2 capture and may have a slight cost advantage.
Oxy combustion challenges
- It is not possible to develop sub-scale oxy-combustion technology at existing power plants. An oxy-combustion power plant is an integrated plant and oxy-combustion technology development will require commitment of the whole power plant to the technology. Thus, the technology development path for oxy-combustion may be more costly than that for either pre-combustion or post-combustion capture which can be developed on slip streams of existing plants.
- The auxiliary power associated with air compression in a cryogenic unit and CO2 compression in the CO2 purification unit will reduce net plant output by up to 25% compared to an air fired power plant with the same gross capacity (without CO2 capture).
- There is no geological or regulatory consensus on what purity levels will be required for CO2 compression, transportation and storage. For this reason, most oxy-combustion plant designs include a partial condensation CO2 purification system to produce CO2 with purity comparable to that achieved by amine post combustion capture. Oxy-combustion costs may be reduced if the purity requirements could be relaxed.
- Air-fired combustion is commonly anticipated for start-up of oxy-combustion power plants. The very low emissions achieved by oxy-combustion with CO2 purification cannot be achieved during air-fired start-up operations without specific flue gas for air-fired operations that are redundant during steady state oxy-fired operations. If a significant number of annual restarts are specified, either these added flue gas quality controls will be required (at additional capital cost) or provisions must be made to start up and shut down the unit only with oxy-firing and without venting significant amounts of flue gas.
- Plot space requirements are significant for the air separation unit and CO2 purification units.