Before carbon dioxide (CO2) from power plants can be permanently stored, it must be captured as a relatively pure gas. There are three technology routes to capturing CO2: pre-combustion, post-combustion and oxyfuel combustion. The Office of Fossil Energy's Innovations for Existing Plants (IEP) program is focused on post-combustion and oxyfuel combustion technologies that can be retrofitted to today's coal plants. Both technologies are feasible, safe and have the potential to be cost-effective. The challenge lies in developing the processes so that they can be deployed economically on a large scale. 

While carbon capture is relatively new to power generation, it is not an uncommon industrial practice. CO2 is routinely separated and captured as a useful by-product from industrial processes such as synthetic ammonia production, hydrogen production, and limestone calcination.

MORE INFO Read the NETL report Carbon Dioxide Capture from Existing Coal-Fired Power Plants

Existing CO2 capture technologies are not cost-effective when considered in the context of large power plants. Economic studies indicate that carbon capture will add over 30 percent to the cost of electricity for new integrated gasification combined cycle (IGCC) units and over 80 percent to the cost of electricity if retrofitted to existing pulverized coal (PC) units. A recent study from the National Energy Technology Laboratory (NETL) confirms that additional alternatives need to be pursued to bring the cost of carbon capture down. In addition, the net electricity produced from existing plants would be significantly reduced - often referred to as parasitic loss - since 20 to 30 percent of the power generated by the plant would have to be used to capture and compress the CO2.

In order to achieve the IEP program goal of 90 percent CO2 capture at no more than a 35 percent increase in the cost of electricity, a number of technical challenges need to be addressed. These include:

• Improving methods for increasing the CO2 concentration of flue gas;
• Increasing the durability of furnace materials;
• Mitigating the impacts of flue gas contaminants;
• Lowering parasitic power demands;
• Improving the efficiency of solvent/sorbent adsorption cycles; and
• Improving sorbent stability.

Some of these challenges may be addressed by evolutionary changes, but for others, advanced technologies compatible with existing power plants must be developed. 

DOE's R&D in Carbon Control Technologies for Existing Plants

The Office of Fossil Energy is engaged in several innovative schemes that could significantly reduce CO2 capture costs, compared to conventional processes. These include:

• Oxyfuel Combustion processes use oxygen rather than air for combustion of fuel. This produces exhaust gas that is mainly water vapor and CO2. The exhaust gas has a relatively high CO2 concentration (greater than 80 percent by volume). Oxyfuel combustion represents an opportunity to improve the economics of CO2 capture. Click here for more information about DOE-supported projects (link to NETL oxy-fuel website).
  
• Solvents and Sorbents for CO2 separation from flue gas (both physical and chemical) can be further enhanced to reduce cost, improve reaction rates and regeneration loads, and eliminate contamination from other pollutants. This includes technologies such as aqueous ammonia, advanced amines, ionic liquids, metal organic frameworks, and amine-enriched sorbents. Click here for more information about DOE-supported projects (link to NETL post-combustion website).

                                       Post-Combustion Capture Using Solvents
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• Advanced CO2 Compression from atmospheric pressure (around 15 psi) to pipeline pressure (1,200 to 2,000 psi) represents a large parasitic load. Novel compressors can reduce this loss. Click here for more information about DOE-supported projects (link to NETL post-combustion website). 

• Advanced Membranes for both oxygen-separation and CO2 capture are key enabling technologies. This effort will evaluate needs for advanced membranes applicable to pulverized coal systems and other conventional combustion systems that will minimize the cost and efficiency losses for CO2 separation. Click here for more information about DOE-supported projects (link to NETL post-combustion website). 

• Chemical Looping processes that prevent direct contact of air and fuel offer the ability to produce a relatively pure stream of CO2 that does not need to be separated from flue gas. Technical challenges remain in key areas such as solids handling and oxygen carrier capacity, reactivity, and attrition. Click here for more information about DOE-supported projects (link to NETL oxy-combustion website).

Alternative Uses of CO2

Recycling or reusing CO2 from energy systems would be an attractive alternative to permanent sequestration. The goal of this program area is to reduce the cost and energy required to chemically and/or biologically convert CO2 into either commercial products that are inert and long-lived, or stable solid compounds. Two promising chemical pathways are magnesium carbonate and CO2 clathrate, an ice-like material. Both provide quantum increases in volume density compared to gaseous CO2. As an example of the potential of chemical pathways, all of the U.S. CO2 emissions in 1990 could be contained as magnesium carbonate in a volume of approximately one cubic mile.

Concerning biological systems, incremental enhancements to the carbon uptake of photosynthetic systems could have a significant effect. Also, harnessing naturally occurring, non-photosynthetic microbiological processes capable of converting CO2 into useful forms, such as methane and acetate, represents a potential technology breakthrough. An important advantage of biological systems is that they do not require pure CO2 and thus, would not incur costs for separation, capture, and compression of CO2. This program area will seek to develop novel and advanced concepts for reuse of CO2 from energy production and utilization systems.