What is clean coal technology?

Coal is a fossil fuel composed primarily of carbons and hydrocarbons. Its ingredients help make plastics, tar and fertilizers. A coal derivative called coke melts iron ore and reduces it to create steel. But most coal — 92 percent of the U.S. supply — goes into power production [source: Energy Information Administration]. Electric companies and businesses with power plants burn coal to make the steam that turns turbines and generates electricity.

When coal burns, it releases carbon dioxide and other emissions in flue gas, the billowing clouds you see pouring out of smokestacks. Some clean coal technologies purify the coal before it burns. One type of coal preparation, coal washing, removes unwanted minerals by mixing crushed coal with a liquid and allowing the impurities to separate and settle.

Other systems control the coal burn to minimize emissions of sulfur dioxide, nitrogen oxides and particulates. Wet scrubbers, or flue gas desulfurization systems, remove sulfur dioxide, a major cause of acid rain, by spraying flue gas with limestone and water. The mixture reacts with the sulfur dioxide to form synthetic gypsum, a component of drywall.

Low-NOx (nitrogen oxide) burners reduce the creation of nitrogen oxides, a cause of ground-level ozone, by restricting oxygen and manipulating the combustion process. Electrostatic precipitators remove particulates that aggravate asthma and cause respiratory ailments by charging particles with an electrical field and then capturing them on collection plates.

­­Gasification avoids burning coal altogether. With integrated gasification combined cycle (IGCC) systems, steam and hot pressurized air or oxygen combine with coal in a reaction that forces carbon molecules apart. The resulting syngas, a mixture of carbon monoxide and hydrogen, is then cleaned and burned in a gas turbine to make electricity. The heat energy from the gas turbine also powers a steam turbine. ­

Next, we'll learn about the most ambitious of all clean coal technologies and what needs to happen before clean coal can become commercially feasible.

Carbon capture and storage — perhaps the most promising clean coal technology — catches and sequesters carbon dioxide (CO2) emissions from stationary sources like power plants. Since CO2 contributes to global warming, reducing its release into the atmosphere has become a major international concern. In order to discover the most efficient and economical means of carbon capture, researchers have developed several technologies.

Flue-gas separation removes CO2 with a solvent, strips off the CO2 with steam, and condenses the steam into a concentrated stream. Flue-gas separation renders commercially usable CO2, which helps offset its price. Oxy-fuel combustion, another process, burns the fuel in pure or enriched oxygen to create a flue gas composed primarily of CO2 and water — this ­sidesteps the energy-intensive process of separating the CO2 from other flue gasses. A third technology, pre-combustion capture, removes the CO2 before it's burned as a part of a gasification process.

After capture, secure containers sequester the collected CO2 to prevent or stall its reentry into the atmosphere. The two storage options, geologic and oceanic, must contain the CO2 until peak emissions subside hundreds of years from now. Geologic storage involves injecting CO2 into the earth. Depleted oil or gas fields and deep saline aquifers safely contain CO2 while unminable coal seams absorb it. A process called enhanced oil recovery already uses CO2 to maintain pressure and improve extraction in oil reservoirs.

Ocean storage, a technology still in its early stages, involves injecting liquid CO2 into waters 500 to 3,000 meters deep, where it dissolves under pressure. However, this method would slightly decrease pH and potentially harm marine habitats. All forms of CO2 storage require careful preparation and monitoring to avoid creating environmental problems that outweigh the benefits of CO2 containment.

­­Since alternative forms of energy cannot yet replace a power source as cheap and plentiful as coal, clean coal technology promises to mitigate the increasingly severe climactic effects ­of coal emissions. Utility companies and businesses do not, however, always accept technology purely for the sake of the environment — the technology must first make economic sense.

Cleaning coal and sequestering its emissions significantly raises the per-BT­U price of what would otherwise be an inexpensive fuel. While selling byproducts like gypsum or commercial CO2­ for sodas and dry ice can offset the price of clean coal technologies, a charge on carbon could make emission reduction financially realistic.