Technologies for capturing CO2 from gas streams have been used for many years to produce a pure stream of CO2 from natural gas or industrial processing for use in food processing and chemical industries. Both post-combustion CO2 capture and oxy-combustion technology provide retrofit options for existing coal-fired power plants. Methods currently used or developed for CO2 separation include but not limited to:
- Physical and chemical solvents, particularly monoethanolamine (MEA)
- Various types of membranes
- Adsorption onto solids
- Cryogenic separation
- Other novel technologies include ionic liquids, Nanoparticle organic hybrid materials, and chemical looping sorbents
These methods can be used on a range of industrial processes; however, their use for removing CO2 from high-volume, low-CO2 concentration flue gases, such as those produced by coal-fired power plants, is more problematic. The high capital costs for installing post-combustion separation systems to process the large volume of flue gas is a major impediment to post-combustion capture of CO2. In addition, a large amount of energy is required to release the CO2 from solvents or solid adsorbents after separation. Major technical and cost challenges need to be overcome before retrofit of existing power plants with post-combustion capture systems becomes an effective mitigation option. Capture technologies will likely improve in the coming years, thereby improving economics.
Integrated CO2 capture designs
A number of the integrated carbon capture technologies are designed and built around existing or new energy conversion systems (e.g., H2 production from fossil fuels). One such approach is to use limestone sorbents to capture CO2 from the gas mixture. Limestone, or calcium carbonate, is calcined to form calcium oxide, which is then used to react with and continuously remove the CO2 product formed during the WGS reaction. Because the CO2 absorption proceeds at very high temperatures, the substantial heat of carbonation is useful and is actually incorporated into the overall process.(22-24) This basic idea has led to development of a number of integrated carbon-capture processes that enhance H2 production via the carbonation-calcination loop of the calcium-based sorbents, such as the Zero Emission Coal Alliance (ZECA) process(25), HyPr-RING process(26), ALSTOM process, GE process(27), and calcium looping process.(28)
Another approach to producing carbon-free H2 employs a different looping technology based on an oxidation-reduction cycle. These technologies, including coal-direct chemical looping and syngas redox processes, use looping particles made from metal or a low oxidation state metal oxide as oxygen carriers. The metal reacts with steam to form the metal oxide, and it is converted back into the metal in the presence of a carbonaceous fuel that needs the oxygen for its combustion. By separating the process steps into two reactors, one for the oxidation of carbonaceous fuels (e.g., CO, coal, and biomass) and the other for the reduction of steam, inherent gas separation can be achieved while producing high-purity H2. The exit stream of the fuel reactor contains only sequestration-ready CO2 along with water, which can easily be condensed out. The advantages of this approach to H2 production are that it eliminates the separation of H2 from the fuel gas and does not require the cleanup of fuel gas prior to H2 production.(29) Moreover, chemical looping processes produce a sequestration-ready CO2 stream at high pressure, which eliminates a costly pressurization step.
CO2 Conversion and Utilization
With an introduction of the utilization portion of the carbon management scheme, this area of research requires rapid and efficient technological development. While the current view on the CO2 utilization in the United States is limited to EOR (Enhanced Oil Recovery), there are many other options for CO2 conversion to useful products (e.g., chemicals, fuels and construction materials). Thus, one of the main questions to be addressed by the RCN-CCUS is what should be considered as sustainable CO2 utilization and what are the technological and economic challenges to be overcome.