Cleaning up Coal: Kemp

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, so the conflict has a moral dimension.

For their part, coal producers are battling new environmental regulations which they see as a threat to the survival of their firms and communities.

All sides employ confrontational rhetoric.

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.

CLEAN-UP OPTIONS

In the long term, the goal is to fit coal-fired plants with carbon capture and storage (CCS) systems, which would separate nearly all the carbon dioxide from exhaust gases and trap it underground in saline aquifers and depleted oilfields.

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 by-product 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.

RAISING 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 33-37 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 2-3 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 sub-critical 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 sub-critical 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 (SIEMENS.NS), 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 700-730 degrees.

ADVANCED MATERIALS

Boosting power plant efficiency by using supercritical or ultra-supercritical technology is not a new idea.

The first supercritical power plant was built near Zanesville, Ohio by American Electric Power (AEP), Babcock & Wilcox and General Electric. Philo Unit 6 began operating in 1957 and ran until 1975 ("Philo Unit 6: Advancement of a Technology", August 2003).

A second supercritical plant was built in 1959-60 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 earlier. Yet in the 1960s, 1970s and 1980s, most coal-fired power plants built in the United States and around the world were still installed with sub-critical 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.

INCO 740 consists almost entirely of nickel (48 percent), chromium (25 percent) and cobalt (20 percent) with small amounts of carbon, molybdenum, aluminium, titanium, niobium, manganese, iron and silicon added.

INCO 740, being developed by Special Metals Corp, part of Precision Castparts, is astronomically expensive. Refined nickel currently costs around $20,000 per tonne. Cobalt is $30,000 per tonne. And chromium is almost $9,000 per tonne.

The challenge is how to build a power plant using as little of this expensive alloy as possible. Among other problems that designers have to solve is how to move the turbines closer to the steam generator to minimise the amount of expensive piping needed to carry steam between the two ("Advanced ultra-supercritical power plant design for Indian coal", October 2012).

FLEET UPGRADE

Steam generators have become progressively more efficient as the state of materials science and costs have allowed.

The challenge is how to replace the old fleet of ageing and inefficient sub-critical power plants around the world with the most modern and efficient ultra-supercritical units.

For example, nearly half the coal-fired power-generating capacity in the United States dates from before 1973. Most of those plants are subcritical. The average capacity is just 172 megawatts. Efficiency is generally below 35 percent.

Maximum efficiency is generally thought to require power plants rated around 800 megawatts in Europe and up to 1,000 megawatts in China.

Trianel's ultra-supercritical power plant at Lunen in Germany, which began operating in December 2013, is rated at 750 megawatts and is almost 46 percent efficient, making it the most efficient in Europe.

If coal is to play a useful role in future power generation, while limiting emissions, the global coal fleet must be upgraded to supercritical and ultra-supercritical standard, old subcritical units must close, and eventually the fleet must be coupled with CCS.

The coal-fired plants of the future will have to compete with gas, nuclear and increasingly efficient wind and solar.

Modern coal-fired plants will be expensive to build, though there are efficiency savings, and coal is relatively cheap.

Some commentators imply there will be no role for coal. The optimum mix (from a cost, security and environmental standpoint) is unlikely to have a zero coal share, however. Coal will still supply a significant amount of electricity. But the power plants of the future will be radically re-engineered to be far more efficient and much cleaner.

By John Kemp