LONDON – According to the popular narrative, coal is locked in a fight to the death with natural gas and renewables to supply clean electrical energy.
Promoters of gas, wind and solar often talk about coal as if it were not just a rival but an enemy. Environmental campaigners want to phase out coal-fired power plants and leave most of the world’s coal reserves below ground.
Coal-fired power plants produce much higher carbon dioxide (CO2) emissions and make the largest single contribution to global warming. For their part, coal producers are battling new environmental regulations that they see as a threat to the survival of their firms and communities.
In practice, however, there is no way to meet growing global demand for electricity that does not rely on large amounts of coal-fired power generation for the foreseeable future. Cheap, abundant and widely distributed coal reserves will remain an essential component of the global energy mix for the next 50 years.
The challenge is to burn coal more cleanly, producing more electricity with fewer emissions of CO2 and other harmful pollutants.
With current technology, that outcome is possible, but it will be expensive and require heavy capital investment.
In the long term, the goal is to fit coal-fired plants with carbon capture and storage (CCS) systems that would separate nearly all the carbon dioxide from exhaust gases and trap it underground in saline aquifers and depleted oil fields.
But while CCS has been successfully demonstrated on a small scale at various facilities, nowhere has it been implemented at a big utility-scale power plant. The engineering and commercial challenges are significant. CCS is at least a decade, and maybe two, away from being a mature commercial technology.
Gasification is another technology that could radically cut emissions. By converting coal into hydrogen and carbon monoxide, rather than burning it directly, and using these gases to drive first a jet turbine and then a steam one, the efficiency of the process can be improved enormously.
The byproduct of coal gasification is a concentrated stream of carbon dioxide, which is much easier to capture and store.
Coal gasification technology is more than a century old. Modern plants that integrate gasification with combined-cycle turbine technology, however, are expensive to build and difficult to operate. But there are other technologies already in use that could cut emissions by as much as 40 percent — mostly by improving the efficiency with which the coal is turned into steam.
In a conventional coal-fired plant, only a third of the energy contained in the fuel is turned into electricity.
The rest is lost, mostly as heat, from the steam generator, turbines, and exhaust system as well as in the cooling water. The waste of energy is prodigious, but it is possible to raise the thermal efficiency of a coal-fired power plant from around 33 to 37 percent to 40 percent, or even 45 percent, using fairly well-established technology.
By squeezing more electrical energy from the same amount of coal, more-efficient power plants can slash carbon emissions. Every 1 percentage point gain in thermal efficiency equates to a 2 to 3 percent reduction in CO2 per kilowatt-hour. The main efficiency gains come from operating the steam generator and turbines at higher temperatures and pressures.
In a conventional subcritical power plant, water is boiled first and then turned into steam, and the temperature of the steam is raised further in a superheater. But in a super-critical power plant, water is converted directly to steam without passing through the boiling stage, which is much more efficient.
Thomas Edison’s first electric power plant, Pearl Street Station in New York, employed steam at a pressure of just 60-160 pounds per square inch (psi) and operated at a maximum temperature of 185 degrees Celsius.
Pearl Street Station was just 2.5 percent efficient. Since then, steam generation technology has improved enormously.
In a modern subcritical power plant, steam pressure is below 3,200 psi and temperature is under 550 degrees Celsius. In a super-critical plant, however, pressure is raised to over 3,500 psi and temperature to about 565 degrees. More than 200 supercritical units were operating worldwide by 2011.
Even higher pressures and temperatures are possible. Ultra-supercritical (USC) plants have been installed that operate at 4,600 psi and 600 degrees.
Siemens, for example, has installed large ultra-supercritical steam plants in Japan, China, Germany and the Netherlands since the turn of the century.
Power plant designers now aim to build advanced ultra-supercritical (A-USC) plants that would operate at 700 to 730 degrees. Boosting power plant efficiency by using supercritical or ultra-supercritical technology is not new.
The first supercritical power plant was built near Zanesville, Ohio, by American Electric Power, Babcock & Wilcox and General Electric. Philo Unit 6 began operating in 1957 and ran until 1975. A second supercritical plant was built in 1960 at Eddystone, Pennsylvania, by companies that are now part of Siemens, ABB and Exelon.
The idea of a supercritical steam generator had been around for decades. Yet in the 1960s, ’70s and ’80s, most coal-fired power plants built in the United States and around the world were still installed with subcritical boilers.
The constraint on supercritical, ultra-supercritical and now advanced ultra-supercritical systems has always been the state of technology and cost of materials. To withstand tremendous pressure and temperature, the steam generators, turbines and pipework require tremendously strong metals that are highly corrosion-resistant.
Super-strength steels and alloys of the type needed contain large amounts of expensive metals such as nickel, chromium and cobalt, which push up the cost considerably.
For example, in the advanced ultra-supercritical steam generators of the future, power plant designers are considering employing a super-alloy called INCO 740. It is being developed by Special Metals Corp, and is astronomically expensive. The challenge is how to build a power plant using as little of this alloy as possible.
Among other problems for designers is how to move the turbines closer to the steam generator to minimize the amount of expensive piping needed to carry steam between the two.
John Kemp is a Reuters market analyst. The views expressed are his own.
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