By John Kemp
LONDON Feb 11 Following President Barack Obama's second inaugural address, which reaffirmed his commitment to clean energy, there has been mounting speculation the administration will impose stricter emission standards on existing coal-fired power plants across the United States.
Cutting emissions from existing coal-fired plants is essential to the administration's goal of limiting carbon dioxide (CO2) emissions.
New power plants built after 2014 will already be subject to tougher emissions limits under draft regulations published by the Environmental Protection Agency (EPA).
EPA's Carbon Pollution Standard for New Power Plants, published in 2012, will in effect prohibit construction of new coal-fired plants unless they are eventually fitted with carbon capture and storage (CCS) technology.
However, the standard's impact is likely to be limited. Even before it was published, few if any power generating companies were planning to add coal plants because burning natural gas is currently much cheaper. Its main practical effect will be to prevent new coal plants being built if the price of gas eventually rises.
The new rules will not apply to the existing fleet of 1,400 coal-fired power plants across the United States that accounted for 37 percent of all electricity generated in the 12 months to November 2012.
These long-lived plants will still account for nearly a quarter of the country's generating capacity by 2040, according to the Energy Information Administration (EIA)'s 2013 "Annual Energy Outlook".
In theory there are a wide range of options for cutting emissions from coal-fired power stations, ranging from the installation of integrated gasification and combined cycle (IGCC) systems and highly-efficient supercritical boilers to post-combustion CO2 scrubbers and oxyfuel combustion systems.
But IGCC systems are largely unproven, complex, and would require the construction of an entirely new generation of coal-fired power plants. Replacing subcritical boilers with a new generation of supercritical ones would essentially mean rebuilding existing power plants from scratch.
So in practice the options for retrofitting the existing fleet come down to two: post-combustion CO2 capture and oxyfuel systems.
Post-combustion CO2 capture has a proven track record, but only in natural gas processing plants, steam methane reformers, ethanol plants and other chemicals facilities, which produce a relatively concentrated stream of CO2 in their exhaust gases or as a by-product.
In contrast, the CO2 in the exhaust gases from a utility-scale power plant is much more diluted, mixed with large amounts of other gases, particularly nitrogen, which makes up almost 80 percent of the air the plant takes in to burn the coal.
The low concentration of CO2 in power station exhaust requires large absorption systems that use a lot of energy and water and substantially reduce the net energy output of the plant.
Oxyfuel combustion plants solve this problem by burning coal in a concentrated stream of oxygen rather than air.
Oxyfuel requires an expensive and energy-intensive air separation unit (ASU) to produce the oxygen for combustion, but the exhaust gases contain a much higher concentration of CO2, without nitrogen, so the post-combustion absorption system can be smaller, cheaper and use less energy.
Oxyfuel is the least-developed of the main options for carbon capture. There are just two small pilot plants in operation at Vattenfall's Schwarze Pumpe coal-fired power station in Germany and Total's pilot project in Lacq, France.
"Despite being the least developed of the CO2 capture options, oxyfuel combustion is perhaps the CO2 capture option that the traditional electric power industry likes best," according to the authors of the "Global Energy Assessment 2012."
"This is most likely due to the lack of chemical processing compared to the simple chemical processing of post-combustion capture (systems) and especially the complex chemical processing of pre-combustion CO2 capture," they wrote.
Oxyfuel continues to gain interest and development spending. More than two dozen projects, ranging from laboratory scale tests to pilot plants, have been undertaken or are planned, examining how to make oxyfuel boilers work efficiency and separate out the oxygen needed using less energy.
The Department of Energy is funding FutureGen 2.0, a demonstration project that involves retrofitting an existing coal-fired power plant in Illinois with oxyfuel technology to capture more than 90 percent of its CO2 emissions, and store them underground in a nearby saline aquifer.
FutureGen was originally conceived as an IGCC plant but changed to oxyfuel in part because it could provide more useful experience for retrofitting the rest of the country's existing coal fleet.
On Feb 4, the Energy Department announced that it was moving ahead with Phase II of the FutureGen project.
"In cooperation with the FutureGen project partners, the Department of Energy is investing in the upgrade of a coal-fired power plant in Meredosia, Illinois, with oxy-combustion technology ... This project will test oxygen separation technology and exhaust processing technology ... at power plant scales," the Department explained.
Phase II will including preliminary design, pre-construction and engineering for the retrofitted power plant.
The main barrier to the commercial deployment of oxyfuel combustion at utility scale is the ability to make large amounts of oxygen cheaply and without using too much energy in the process.
"For oxy-combustion to be a cost-effective power generation option, a low-cost supply of pure oxygen is required," according to the U.S. Department of Energy's National Energy Technology Laboratory (NETL).
"In the most frequently proposed version of this concept, a cryogenic air separation unit is used to supply high purity oxygen to the boiler. This commercially available technology is both capital and energy-intensive and could raise the cost of electricity from coal-fired plants considerably, in addition to degrading the overall plant efficiency," NETL explains on its website.
New supercritical coal-fired power plants can achieve a thermal efficiency of 40 percent (40 percent of the energy originally present in the coal is turned into usable electricity).
But if it is fitted with oxyfuel technology, even the most advanced supercritical power plant will see thermal efficiency drop to just 32 percent the Congressional Research Service (CRS) explained in a report to Congress ("Carbon Capture: A Technology Assessment" July 2010).
Oxyfuel plants would consume up to 25 percent more coal per kilowatt of electricity produced. The main source of extra fuel consumption is the air separation unit, which CRS estimates would account for about 60 percent of the additional energy used.
"The most compelling need -- and a major focus for research and development -- is for improved, lower-cost processes to deliver large quantities of high-purity oxygen, the major cost in current oxyfuel schemes," CRS wrote.
Praxair and BOC Group, both of which are major manufacturers of industrial gases and equipment, have been working on projects aimed at separating oxygen much more cheaply and using less energy.
But oxyfuel plants are still a long way from commercial deployment, let alone being cost-competitive with natural gas-fired plants.
New EPA regulations could give oxy-combustion technology a much-needed push towards realisation. But the Obama administration needs to take care its race to mandate CCS does not outpace development of the technology.