Carbon capture and storage (CCS) is the process of trapping carbon dioxide before it enters the atmosphere. CCS exists today. It is possible at operating power plants, and is one of the main goals of next-generation, clean-coal facilities. However, one of the main challenges of CCS is the expense it adds to producing electricity.
Experts estimate the per-kilowatt fuel increase for capturing 90% of CO2 using the best available control technology (BACT) is 24-40% for supercritical pulverized coal plants, compared to 14-25% for coal-based integrated gasification combined cycle (IGCC) plants.
For this reason, integrating CCS into clean coal technologies rather than adding them later is one way engineers are trying to make CCS more cost-effective. The diagram to the right depicts the four most common forms of carbon capture. They are:
Post-combustion
Pulverized coal plants would likely use this technology. CO2 is removed from flue gases produced after the main fuel is combusted. About 3-15% of this flue gas is CO2.
Pre-combustion
Coal-based IGCC plants would use this technology. A synthesis gas, or “syngas,” is created in a reactor consisting of carbon monoxide (CO) and hydrogen (H2). The syngas is then mixed with steam (H2O) in a second reactor, or “shift reactor,” to form a mixture of H2 and CO2. The CO2 is easier to remove using this process than post-combustion because the CO2 content is higher (15-60%). The remaining hydrogen can be stored or combusted.
Oxy-coal combustion
This process mixes pure oxygen instead of air with fuel (such as coal) to produce flue gas during combustion. Air is about 4/5 nitrogen, and produces dangerous nitrous oxides (NOX) during the combustion process. By using oxygen instead of air, the flue gas produced has a purer stream of carbon dioxide (about 80%) and is easily captured. CCTFT member Praxair is a leading researcher in this technology.
Industrial processes
CO2 is currently removed from gas mixtures when isolating useful gases such as pure natural gas or ammonia. These processes have been used for decades. Though CO2 is rarely captured during these industrial processes, adding carbon capture to these facilities would not be as much of an adjustment as for some existing power plants.
All four involve the separation of CO2, hydrogen (H2), or oxygen (O2) from a bulk gas stream, such as a flue gas, syngas, air, or natural gas. The separation method can be achieved by:
