Clean Coal

Why do we need clean coal?

Coal is the dirtiest form of all fossil fuels when burned, producing more pollutants and releasing more carbon dioxide than other energy forms. Due to its low cost, however, it is one of the most widely used.

The Clean Coal Technology Program began in 1985 because the US and Canada wanted to end the “acid rain” that was damaging the environment in both countries. Many pollutants that form “acid rain” came from coal-burning power plants in the US, so the national government stepped in to find a solution.

Since then, knowledge of the rising health and environmental costs associated with burning coal, not to mention concern over climate change, have further encouraged utility companies to look for cleaner ways to utilize coal for our energy needs.

Controversy?

There has been much debate in recent years between utility companies and activists who say the term “clean coal” is a way of appearing to help the environment while not being fully environmentally friendly. Activists say that the commodity’s association with mountaintop removal, health problems, and the high carbon emissions that have augmented climate change make coal, even when treated, anything but a “clean” source of energy. Others point to increasing technological advances that are working toward zero emission coal technology.

What is clean coal and how does it work?

While there is no one way to make coal burn completely clean, there are several ways in which utility companies are making coal burn cleaner by addressing the sulfur and nitrogen oxides existent in the coal.

Sulfur is found in coal but the amount varies depending on where the coal is mined. Sulfur must be removed before it is burned. One way to do this is to break up the coal and wash it to get the sulfur out. Coal floats in water while sulfur sinks and is removed.

But washing does not remove all of the sulfur. “Organic sulfur” is chemically connected to the coal’s carbon molecules so it won’t wash away in the water. No simple, inexpensive option exists to remove this sulfur but scientists are continuing to work on this.

Modern power plants (built after 1978) all have special processes to remove sulfur from coal’s combustion gases before these go into the smokestack. These are called “scrubbers” because they “scrub” the sulfur out of the smoke.

Scrubbers

Scrubbers work by using limestone or a similar material (lime) mixed with water to pull the sulfur from the coal gases. When the limestone and sulfur combine they form a powder or paste that is left behind from the gas.

NOx (Nitrogen Oxides) occur naturally but when heated it can bond with oxygen creating nitrogen oxide, which can cause smog and also is a part of acid rain. Anything that heats to a high enough temperature can cause NOx, like a car, but power plants are one of the main generators of NOx.

Stopping NOx before it starts

Scientists have found ways to burn the coal in chambers that have more fuel than air so the nitrogen does not have a chance to bond with oxygen. The coal is burned in one chamber then sent to another, in a process called “staged combustion.”

Staged combustion can cut the amount of NOx produced in half, and there are new technologies that act like scrubbers to break apart the NOx.

New bed methods

Traditional coal burners ignite coal in a boiler forming a long flame or the coal would rest on grates. A relatively new type of burner, created in 1979, uses jets of air to create a floating mass of coal that bubble like a volcano. This style of burning uses a “fluidized bed boiler.”

The tumbling of the coals in this process allows scientists to put limestone into the area more easily (think back to the sulfur section).

This type of boiler also burns “cooler” than older boilers. The cooler temperatures mean less NOx is created.

These two elements combined mean fluidized boilers can burn dirtier coal and still remove about 90-percent of the sulfur and nitrogen pollutants, according to the U.S. Department of Energy.

New fluidized boilers are popping up around the country and a new form of the boiler, a “pressurized fluidized bed boiler” creates energy from coal and steam.

Coal as a gas

Though most people think of coal as a solid black material, it is just a mixture of atoms that can be manipulated to make a new substance, like a gas. Using heat and water, scientists can break coal into its component parts: hydrogen and carbon monoxide.

This gas can be used to spin a turbine and generate electricity. The gas that leaves the gas turbine is hot enough to boil water making steam which can spin yet another turbine making more electricity.

Carbon Sequestration

About one third of the U.S.'s carbon emissions come from power plants and other major points. Carbon capture and storage technology (CCS) exists to stabilize and reduce the carbon concentration. This process involves taking power plants' carbon dioxide emissions and storing it away from the atmosphere.

But today's CCS technologies are not cheap. They add about 75-percent to the cost of electricity produced by a pulverized coal plant (the most common type of coal-burning power plant) and 35-percent to the cost of electricity created by advanced gasification-based plants.

Carbon sequestration is a geologic technique of storing carbon dioxide emissions permanently in geologic formations. This includes areas that have stored oil, natural gas and brine. This technique is not a vailable everywhere and often depends on a plant's location near usable geologic formations.