Rising high above a reclaimed phosphate strip mine in southern Florida is a preview of coal's future.  Within the gleaming steel towers of Tampa Electric Co.'s Polk Power Station, near Lakeland, Florida, are the latest – and to date, one of the most technologically advanced – of a series of innovations that make the plant one of the cleanest and most efficient coal plants operating anywhere in the world.

Many of these innovations can trace their engineering roots back a quarter century or more. In fact, for much of coal's modern history, technological progress has been defined by efforts to reduce pollutants released when coal is burned.

Most efforts to date have focused on cleaning pollutants from a coal boiler's “flue gas,” the smoke produced by coal combustion. In recent years, however, scientists and engineers have developed entirely new ways to produce electricity from coal – for example, by first converting it to gas. This “new breed” of coal-fueled power plant offers plant owners unprecedented levels of operating efficiency – a benefit that, in turn, could help keep the costs of electricity to consumers affordable. Equally important, these new power plants are incorporating revolutionary new emission control equipment that could lead to a day when energy from coal can be produced pollution-free.

Eliminating Soot
In the early part of the 20th century, the primary concern was soot – the tiny particles of fly ash and dust that are expelled from coal-burning power plants. In 1923, the first electrostatic precipitator was used. It used electrical fields to remove particulate matter from a boiler's flue gas, much in the way that static electricity causes dust to cling to certain types of materials.  Electrostatic precipitators, along with baghouses (which work like large industrial-scale vacuum cleaners to capture ash and dust particles in felt or woven fabric bags), have been able to reduce the release of soot-forming particulate matter by 99 percent or more. Today, all coal burning power plants employ one, or in some cases, both of these devices.

Scrubbing Out Sulfur
In the 1970s, sulfur and nitrogen oxides became the pollutants of most concern. Both combine with water vapor in the air to form dilute acids that can fall to earth as “acid rain.”

Initially, some coal companies and power plant operators removed sulfur (as well as some of the ash-forming impurities in coal) from coal primarily by pre-cleaning it before it was burned; however, such “coal preparation” techniques were generally effective in reducing the sulfur content of coal by about only 25 percent. The sulfur that was present in coal in distinct particles (often bound with iron in the form of pyrite) could be removed by washing the coal, but such techniques had little effect on sulfur that was chemically bound to coal's carbon molecules.

The Clean Air Act of 1970 imposed more stringent pollution control requirements on coal-fired power plants. Although many coal-fired power plants attempted to comply with the new standards by building taller smokestacks to disperse the flue gases over a wider area or by burning lower sulfur coal, the legislation also accelerated research on a new type of pollution control device called a “flue gas desulfurization unit” or “scrubber.”  Rather than removing sulfur from coal before it was burned, scrubbers worked at the “back end” of a power plant, removing sulfur in the form of sulfur dioxide (or SO₂) that was present in the flue gas exiting the coal boiler.

Scrubbers can reduce sulfur emissions by 90 percent or more. They are essentially large towers in which aqueous mixtures of lime or limestone “sorbents” are sprayed through the flue gases exiting a coal boiler. The lime/limestone absorbs the sulfur from the flue gas.

Although some scrubbers had been built in Great Britain in the 1930s, it wasn't until 1967 that the first full-scale scrubber began operating in the United States at a coal-burning power plant. Early scrubbers experienced operating difficulties and cost overruns, but as the technology matured, their reliability and economics improved.  Also, the first scrubbers converted the sulfur into a sludge-like waste product that could be difficult to handle; more recently, new treatment processes have been developed that produce a dry powder that can be used to make wallboard and for other commercial purposes.

In 1977 Congress passed a new Clean Air Act that essentially mandated that all new coal-fired power plants install scrubbers. By 1981, 52 of the Nation's 380 coal-burning utility plants had installed 84 scrubber systems. Today, more than 190 scrubbers are operating at 110 U.S. coal-fired power plants. Modern scrubbers have also shown varying degrees of effectiveness in reducing other pollutants, including particulates, acid gases, and in some cases, mercury and other heavy metals.

Knocking Out NOx
Nitrogen oxides also posed perplexing environmental challenges. Nitrogen oxides – or NOx – not only contribute to acid rain, they can also form harmful levels of ozone and reduce visibility. When coal burns, NOx forms from two sources: some nitrogen impurities embedded in coal's chemical structure combine with oxygen from the air to form NOx. In addition, the heat of combustion also causes nitrogen molecules in the air to break apart and undergo the same pollutant-forming reaction. In the late 1970s and 1980s, power plant engineers tested a new type of coal burner that fired coal in stages and carefully restricted the amount of oxygen in the stages where combustion temperatures were the highest. This concept of “staged combustion” led to “low-NOx burners.”  Low-NOx burners have been installed on nearly 75 percent of large U.S. coal-fired power plants. They have typically been effective in reducing nitrogen oxides by 40 to 60 percent.

In 1990, new amendments to the Clean Air Act mandated that nationwide caps be placed on the release of sulfur dioxide and nitrogen oxides from coal-burning power plants. In some areas of the United States – particularly the eastern portion of the Nation – many states must implement plans to reduce nitrogen oxides to even greater levels than those mandated by the nationwide cap. To reduce NOx pollutants to these levels, scientists have developed devices that work similar to a catalytic converter used to reduce automobile emissions. Called “selective catalytic reduction” systems, they are installed downstream of the coal boiler. Exhaust gases, prior to going up the smokestack, pass through the system where anhydrous ammonia reacts with the NOx and converts it to harmless nitrogen and water.

Some power plants are also injecting ammonia (or urea) directly into the coal furnace to reduce NOx. This technique, called “selective non-catalytic reduction,” is much less expensive but also less effective that selective catalytic reduction.

Tackling Mercury
Today, mercury has joined the list of air pollutants of concern. Mercury is found in coal only in minute quantities, but when released during combustion, it can find its way into nearby water systems and accumulate in fish, creating a potential human health problem. The challenge is that mercury can be released from a coal plant in several different chemical forms, depending on the type of coal burned and the type of power plant equipment. 

In some cases, flue gas desulfurization units, or baghouses and electrostatic precipitators can help reduce mercury emissions; but with certain coals and plant configurations, these devices may be completely ineffective. Scientists are working on new ways to modify existing pollution control devices or develop new ones that use special sorbents for reducing mercury emissions. For example, activated carbon – a powdery substance commonly used to remove odors and contaminants in drinking water systems – has also been shown to be effective in absorbing mercury from the flue gases of coal plants.  The mercury clings to the activated carbon particles and can be removed by downstream particulate control devices such as electrostatic precipitators. Research is also underway to study other chemicals that could be added to the coal or injected into the flue gas to enhance mercury capture.

A New Way to Burn Coal
For much of coal's modern history, technological innovations have focused on ways to reduce pollutants after they have been released from the coal boiler. But beginning in the mid-1960s, engineers began studying an entirely different approach to controlling emissions – reducing them inside the boiler itself.  The concept came to be known as fluidized-bed combustion.

Rather than the traditional way of burning coal – blowing pulverized particles of coal into a super-hot (approx. 3,000 degrees F) combustion chamber – fluidized-bed combustors suspend larger chunks of coal (about the size of your fingernail) on upward-blowing jets of air. Suspended on this cushion of air, the “bed” of coal tumbles as it burns, taking on many of the characteristics of a boiling liquid, hence the name “fluidized bed.”

Pressurized combustion air is supplied by the turbine compressor to fluidize the bed material, which consists of a coal-water fuel paste, coal ash, and a dolomite or limestone sorbent. Dolomite or limestone in the bed reacts with sulfur to form calcium sulfate, a dry, granular bed-ash material, which is easily disposed of or is usable as a by-product. A low bed-temperature of about 1,600°F limits NOx formation.

The turbulent mixing process allows coal to be burned efficiently at lower temperatures, reducing the formation of nitrogen oxides. The lower temperatures (1,200–1,400 degrees F) also are ideal for mixing limestone (or a similar substance called dolomite) in with the coal to absorb sulfur dioxide and convert it into a dry powdery form that can be removed with the coal ash. Fluidized-bed combustors are effective in reducing sulfur and nitrogen oxide pollutants by more than 90 percent, they eliminate the need for a post-combustion scrubber, and they can burn almost any grade of coal.

The first fluidized bed combustors were used in smaller industrial plants and for local heating systems; today, the technology has been scaled up and is used in several multi-hundred-megawatt commercial power plants. More than 170 fluidized-bed combustion units now operate in the United States.

These technological advances helped coal-burning utilities sharply reduce air pollutants even as they have substantially increased their use of coal. From 1980 to 2003, the amount of coal used to generate electricity in the United States increased by 75 percent; however, during the same time period, sulfur dioxide and nitrogen oxide emissions declined by 40 percent. At the same time, existing pollution controls reduced mercury emissions by 40 percent below levels that would have been emitted had there been no pollution controls on power plants.

The Coal Plant of the Future
A new breed of coal plant that relies on coal gasification represents an important trend in coal-fired units – distinctly different from the conventional coal combustion power station. Rather than burning coal, such plants first convert coal into a combustible gas. The conversion process – achieved by reacting coal with steam and oxygen under high pressures – produces a gas that can be cleaned of more than 99 percent of its sulfur and nitrogen impurities using processes common to the modern chemical industry. Trace elements of mercury and other potential pollutants can also be removed from the coal gas; in fact, the coal gas can be cleaned to purity levels approaching, or in some cases, surpassing those of natural gas.

These new plants also achieve unprecedented efficiencies by generating two sources of electricity. Once cleaned, the coal gases are burned in a gas turbine-generator – again much like natural gas – to produce one source of electricity. Exhaust gases exiting the turbine are hot enough to boil water, creating steam that drives a conventional steam turbine-generator, producing a second source of electricity. This dual combination of gas and steam turbine systems accounts for the technology's name: integrated gasification combined-cycle.

Integrated gasification combined-cycle power plants are one of the cleanest and most efficient coal-fueled power stations. Not only will they be able to eliminate virtually all of coal's pollutants, they will be able to generate considerably more power from a given quantity of coal. Today's power plants, for example, extract only about 33–35 percent of the energy value of coal. Because of their dual means for generating electric power, future integrated gasification combined-cycle plants may be capable of extracting up to 60 percent of coal's energy. Higher coal-to-electricity efficiencies mean that less coal is used to generate power; and when less coal is used, less carbon dioxide is emitted.

A Climate Change Solution
Carbon dioxide is the latest – and certainly the most challenging – of coal's environmental concerns. Boosting power plant efficiencies is currently the most cost-effective way to reduce carbon dioxide, but it will likely not be sufficient to substantially reduce the threat of global climate change. For large-scale carbon dioxide reductions, it will likely be necessary to capture these emissions from the exhausts of coal plants and safely prevent them from entering the atmosphere.

Within the last decade, many of the world's coal and power plant researchers have begun studying ways to capture and dispose of carbon dioxide. Their efforts have led to a new family of promising carbon sequestration technologies. With carbon sequestration, it may be possible to safely and permanently store carbon dioxide from coal plants directly in deep geologic formations, perhaps in unmineable coal seams or in depleted oil or gas fields, or indirectly in forests and soils. Longer-range research is also showing exciting possibilities for converting carbon dioxide into environmentally safe solid minerals that could be returned to the earth, perhaps to the same mines from which the coal was extracted.

A key to successful carbon sequestration will be to find affordable ways to separate carbon dioxide from the exhaust gases of coal plants. Techniques are being developed that can be applied to conventional combustion plants, but it is likely that capture methods will be even more effective when applied to integrated gasification combined-cycle plants. Integrated gasification combined-cycle plants release carbon dioxide in a much more concentrated stream than conventional plants, making its capture more effective and affordable.

Today, as a result of these technological advancements, the concept of a pollution-free, highly efficient coal-fueled power plant is no longer confined to an engineer's drawing board. The basic equipment for this new breed of coal plant is being developed and tested, much of it at large scales. No longer is the coal plant of the future just a utility company's dream; today such plants are taking shape, and because of the new technology they will employ, the future of coal – like our Nation's air – is becoming clearer.