William Fang, Deputy General Counsel and Climate Issue Director and
Eric Holdsworth, Director, Climate Programs, Edison Electric Institute
As of August, close to a dozen bills had been introduced in Congress to further reduce the country’s carbon dioxide (CO2) and other greenhouse gas (GHG) emissions. Some proposals have called for cutting the country’s emissions by as much as 90 percent from 1990 levels by 2050. The electric power industry climate change principles can help guide the legislative debates over lowering the country’s GHG emissions, while at the same time maintaining an affordable and reliable electricity supply.
These principles emphasize that to be effective, any long-term climate strategy must be economy-wide. It must minimize cost increases to customers and do no harm to the U.S. economy. And it must be technology driven. Policy-maker support of technologies that can lower coal-based power plant CO2 emissions, and then eventually capture and store those emissions, will be especially important.
Achieving the CO2 emissions reductions targets that are being discussed on Capitol Hill will require a sustained commitment to the development and cost-effective deployment of a full suite of CO2 -reducing technologies. Nuclear power, for example, is the only zero-emitting, new baseload generation option currently available.
Technologies that improve energy efficiency, such as advanced combustion technologies, must also play a role in reducing the country’s CO2 emissions. To derive significant emissions reductions from efficiency, though, will require new regulatory and business models for states and electric utilities. These models will be needed to ensure that energy efficiency can stand alone as a business for utilities in both regulated and unregulated states.
Edison Electric Institute (EEI) has commissioned a study that analyzes a variety of business model prototypes. Some models include methods of sharing energy savings with consumers and shareholders; others simply treat energy efficiency like any other expenditure, such as power plant construction or other infrastructure improvements.
Advanced coal plant technologies
But to really reduce the power sector’s CO2 emissions will require that the nation make an investment in advanced coal plant technologies. A suite of technology options will be needed to improve the thermal efficiency of existing coal plants, create a new generation of advanced coal combustion and gasification technologies, and eventually capture coal plant CO2 emissions and permanently store them.
Making efficiency upgrades to the turbines, boilers and the milling systems used to grind coal in existing plants is a practical way to reduce CO2 emissions in the near term. The potential efficiency opportunities at a given plant can range as high as 10 to 12 percent, with typical efficiency opportunities that are perhaps half that level. A 5 percent improvement in the efficiency of the nation’s overall coal fleet would lower the industry’s CO2 emissions by about 100 million metric tons every year.
Unfortunately, concerns over the impact of new source review (NSR) regulations can discourage power plant owners from initiating these efficiency upgrades. The industry is encouraged by recent efforts from the U.S. Environmental Protection Agency to revise portions of its NSR regulations in order to achieve progress on this issue.
The thermal efficiency of today’s traditional pulverized coal plants average between 33 to 35 percent. Investing in plants with advanced combustion technologies”supercritical” and “ultra-supercritical” pulverized coal, and circulating fluidized bed (CFB)along with gasification technologies (commonly known as integrated gasification combined cycle, or IGCC), can raise these efficiency levels to as high as 45 percent.
There will certainly be a cost premium associated with these more efficient plants. Many are estimated to cost 20 to 50 percent more than traditional plants. Importantly though, besides helping to lower CO2 emissions, the higher efficienciesthat is, producing the same electrical output with lower fuel inputwill also help to greatly reduce sulfur dioxide (SO2), nitrogen oxide (NOX), particulate matter and mercury emissions from these new plants.
Carbon capture and storage
Along with more efficient coal plants, carbon capture and storage (CCS) will also be critical for reducing the industry’s CO2 emissions. CO2 can be captured either pre- or post-combustion. In an IGCC plant, the CO2 is removed pre-combustion. The CO2 is separated from a relatively small volume of synthetic gas (syngas) at high pressure before combustion. Since the CO2 is already under pressure, less energy is required to compress it for pipeline transport. The pre-combustion capture technologies can use a variety of physical solvents to capture the CO2. These are commercially available today, but they have never been integrated with a coal-based IGCC plant.
The traditional pulverized coal plant uses air as an oxidant for combustion. This creates a number of challenges for capturing CO2. Key among them:
•A large volume of flue gas with a low concentration of CO2 (typically 12 to 14 percent), which means more energy is required to remove it.
•Air is 80 percent nitrogen, which leaves trace impurities in the flue gasthese may reduce the effectiveness of the processes used to absorb the CO2
•Compressing the captured CO2 for pipeline transportation—moving from atmospheric pressure to pipeline pressuretakes a lot of energy.
The use of oxygen instead of airoxycombustionin a pulverized coal plant would help to reduce the amount of energy required to capture the CO2 post-combustion. Oxycombustion does this by:
•Producing a higher concentration (90 percent) of CO2 in the flue gas
•Eliminating the need for energy-intensive gas separation
•Significantly reducing need for costly “clean-up” equipment.
Oxycombustion, however, is more costly due to the expense of the oxygen, and the oxygen is combustible, which requires more training and caution to use.
To date, chemical absorption is the only technique that has been used to capture CO2 from flue gas. Analysis conducted at the National Energy Technology Laboratory shows that CO2 capture using this method would raise the cost of electricity from a newly built supercritical coal power plant by 75 percent or more. DOE’s goal is to cut that penalty to just 20 percent for an existing pulverized coal facility by 2012, and to only 10 percent for a new IGCC plant or another type of pre-combustion capture technology.
There are a number of longer-term technology strategies for capturing CO2 emissions post-combustion from coal plants. These efforts are important because they may allow power generators to build zero or near-zero emissions plants using current technology.
One such post-combustion CO2 capturing technology being studied is chilled ammonia. The chilled ammonia process takes flue gas, which is typically at about 130 degrees Fahrenheit, and cools it to 32 to 50 degrees Fahrenheit. After it cools, it passes through a CO2 absorber, where it is absorbed with ammonia carbonate. The absorption occurs at low temperature, preventing ammonia release but achieving high capture efficiency. In chilling the flue gas, the system also recovers large quantities of water for recycling.
Smaller-scale carbon-separation systems are available to serve the industrial market for CO2, but none is ready for the size of a power plant, in terms of either cost or performance. Some, though, represent potential “breakthrough technologies.” One of these involves metal organic frameworks (MOFs). MOFs are nano-scale organic/inorganic structures to which CO2 in flue gases will stick. Often referred to as crystal “sponges” because of their ability to capture other substances, these new materials are constructed so that, incredibly, just one gram has the surface area of a football field.
Electro-catalytic oxidation, which has proven effective in reducing sulfur dioxide, nitrogen oxides, mercury, and fine particulate matter, is another potential technology for capturing CO2 in the flue gas. So too is a research project using what are known as ionic liquids to capture CO2 from post-combustion flue gases. Discovered only about 12 years ago, ionic liquids are salts that are liquid at room temperature and do not evaporate. According to a 2005 DOE presentation, they have demonstrated the ability to absorb large amounts of CO2.
Accelerated technical and financial support could make a number of these technologies commercially available within the next 15 years, with full-scale implementation taking another decade or so. Of course, the captured CO2 must then be permanently stored or used so it does not enter the atmosphere. Right now, most experts believe the bulk of captured CO2 could be stored underground in a variety of geologic formations, including depleted oil and gas reservoirs, deep saline formations and basalts.
Part two of this article appears in the September 4 enewsletter.