Capturing CO2 that would otherwise be emitted to the atmosphere, cleaning it, and compressing it to the point where it can be transported represents the greatest additional costs for applying CCS to power generation. In some other processes, for example gas processing, the CO2 is already captured as part of the process, so the greatest cost is that of compressing, transporting, and storing the CO2 instead of venting it to the atmosphere. This chapter provides an update on the progress made in capture technology, its challenges, and an outlook across the different sectors where CCS can be applied.
The most advanced technology options for CO2 capture from fossil fuel usage are:
- pre-combustion capture from gas streams;
- post-combustion capture from combustion flue gas; and
- oxyfuel combustion – the direct combustion of fuel with oxygen.
These three approaches are shown for coal-based power systems in Figure 49. These technologies can also be applied to gasfired power systems and are also applicable to certain non-power generation applications.
FIGURE 49 Technical options for CO2 capture from coal-fired power plants
Pre-combustion capture inpower plants requires a partial reaction of the fuel with oxygen or air under high pressure. This produces a synthetic gas consisting of CO2, CO, and H2. Further can be produced through a water-gas shift reaction. The CO2 from the resulting gas can be removed using an acid gas removal (AGR) process which uses solvents. The separation of CO2 produces a hydrogen-rich gas that is burned in a gas turbine to produce electricity.
Pre-combustion capture of CO2 using AGR processes is already practised commercially at full-scale in oil and gas processing, and chemicals plants where CO2 is separated as part of the standard industrial process. This process is slightly different to precombustion for power generation.
The second main process for separating CO2 from flue gases is post-combustion capture. This involves the removal of the CO2 from the flue gas after the fuel has been completely combusted. It can be applied to newly designed fossil fuel power plants, or retrofitted to existing plants. Processes using liquid(absorption) are currently the most advanced options for postcombustion capture, but there is research and development underway to investigate other technologies such as membranes and solid adsorbents. Post-combustion capture can also be applied to other industries producing flue gases containing CO2 such as cement production, oil refining, and petrochemicals.
A third technology is oxyfuel combustion, where the fuel is burned with high-purity oxygen instead of air. This eliminates the nitrogen in the flue gases and produces a flue gas with a high concentration of CO2The oxygen is sourced through anunit (ASU). The resulting flue gas contains mostly CO2 and is then cleaned, dried, and compressed.
Oxyfuel combustion can be applied to both new plants and can also be retrofitted to existing plants. In a coal-fired oxyfuel power plant, some flue gas (mainly consisting of CO2) is recycled to use in the oxygen-fired boiler, effectively replacing nitrogen from air to keep the temperature at a level acceptable for boiler tube materials. Oxyfuel technologies can also be used in other industries including cement, steel manufacturing, and oil refining.
Within each of these three advanced capture technologies are multiple pathways such as solvents or membranes. The selection of the technology and its pathway needs to consider the fuel being used, the climate conditions, the availability of resources (such as water) at the chosen locations, and the operational requirements of the plant.
Figure 50 illustrates the spread of technologies for power generation from the existing portfolio of LSIPs. The majority of projects apply post-combustion capture, which reflects retrofitting of existing power stations with capture technology. New plants are favouring pre-combustion technology. There is a mix of retrofit and new plants for the six oxyfuel projects.
FIGURE 50 Number of power generationby capture technology and stage
Pre-combustion is the technology used for all the projects that are currently operational. This covers natural gas processing andor fertiliser production.
In industries such as steel mills and cement plants, capture processes are still in early stages of development in comparison with power generation and gas processing projects. However, it is possible that an existing capture technology can be tailored to suit the particular production process. For example,production may require only simple capture technologies, as almost pure CO2 is produced from fermentation and it often only requires dehydration and compression before being transported.