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Coal is the workhorse of the electric power industry of the United States and provides the supply for more than half of the electricity consumed. Coal-fired electric generating plants are the cornerstone of America's central power system. To preserve this economically vital energy foundation, low-cost environmental compliance technologies and efficiency-boosting innovations are being developed.
Impurities Trapped Inside Coal
Traces of impurities such as sulfur and nitrogen are trapped inside coal. These impurities are released into the atmosphere when the coal is burned. Combined with water vapor, these impurities form droplets that fall back to Earth as a weak form of sulfuric and nitric acids, commonly known as acid rain.
Apart from impurities, there are also tiny fragments of minerals and common dirt mixed into coal. The ashes left behind in a coal combustor are made up of these tiny fragments because they don’t burn. These tiny fragments from the smoke that comes out of a coal plant’s smokestack when caught up in the swirling combustion gases.
Like all fossil fuels, coal is formed out of carbon. When coal burns, its carbon combines together with the oxygen in the air to form carbon dioxide. As almost everyone nowadays knows, carbon dioxide is the number one greenhouse gas responsible for global warming.
At present, technologies are available to filter out about 99% of the tiny impurity fragments, as well as removing more than 95% of the acid rain pollutants in coal. Furthermore, specialized technologies are available to reduce the amount of carbon dioxide released into the atmosphere, by burning coal more efficiently.
A number of these technologies belong to a family of energy systems referred to as "clean coal technologies". The Clean Coal Technology Program began back in 1985 when the United States and Canada decided that a course of action needed to be taken to reduce, or eliminate, the "acid rain" that was believed to be damaging rivers, lakes, forests, and buildings in both countries.
Since many of the pollutants that formed acid rain were coming from big coal-burning power plants in the United States, the U.S. Government took the lead in finding a solution.
A partnership program between the U.S. Government, several States, and private companies was created by the U.S. Department of Energy as an initial step in combating this problem. Scientists are now testing new methods to make coal-burning a much cleaner process. This became known as the Clean-Coal Technology Program.
Getting Rid of Sulfur in Coal
As stated earlier, traces of impurities such as sulfur and nitrogen are trapped inside coal. Sulfur is a yellow chemical element that can be found in tiny amounts in coal. Nevertheless, it is still important that most of this sulfur be removed before it goes up into a power plant's smokestack.
One approach for extracting sulfur from the coal is to simply crush the coal into small pieces and wash it before being used at the power plant. Sulfur, which exists in tiny pieces in coal, can be washed out using a water-filled tank. These facilities are referred to as “coal preparation plants”.
Some of the sulfur in coal is actually chemically connected to coal's carbon molecules instead of existing as separate particles. This type of sulfur is known as organic sulfur, and it cannot be washed away. Most modern power plants have special devices installed that filter the sulfur from the coal's combustion gases before the gases go up the smokestack. The technical name for these devices is "flue gas desulfurization units," or commonly called "scrubbers".
Several new types of scrubbers were tested under the Clean Coal Technology Program. They have proved to be more effective, less expensive, and more reliable than older scrubbers. The program also tested another type of sulfur filtering device – it sprayed limestone inside the tubing of a power plant to absorb sulfur pollutants. So far results look promising.
The Nitrogen Pollutants in Coal
Nitrogen is the most common part of the air we breathe. But when air is heated, for example in a coal boiler's flame, these nitrogen atoms break apart and join with oxygen to form nitrogen oxides.
Nitrogen oxides can be produced by any fuel that burns hot enough. Plenty of nitrogen oxide comes from coal-burning power plants, so the Clean Coal Technology Program developed new ways to reduce this pollutant.
Scientists have discovered ways to burn coal in burners where there is more fuel than air in the hottest combustion chambers. Under these conditions, most of the oxygen in the air combines with the fuel, rather than with the nitrogen.
The burning mixture is then sent into a second combustion chamber where a similar process is repeated until all the fuel is burned. This concept is called "staged combustion". Burners can reduce the amount of nitrogen oxides released into the air by more than half.
The Cleanest Coal Technology
Coal gasification offers one of the most versatile and cleanest ways to convert coal into electricity. Rather than burning coal directly, coal is broken down into its basic chemical constituents during gasification. Under these conditions, the carbon molecules in coal break apart, setting off chemical reactions that typically produce a mixture of carbon monoxide, hydrogen, and other gaseous compounds.
Coal gasification may offer a further environmental advantage in addressing concerns over the atmospheric build-up of greenhouse gases. If oxygen is used in a coal gasifier instead of in an aerial environment, carbon dioxide is emitted as a concentrated gas stream in syngas at high pressure. In this form, it can be captured and impounded more easily and at lower costs. On the other hand, when coal burns or is reacted in air, 80% of which is nitrogen, the resulting carbon dioxide is diluted and is more costly to separate.
The ability to produce electricity, while eliminating nearly all air pollutants and potentially reducing greenhouse gas emissions, makes coal gasification a promising technology for energy plants of the future.
This article was updated on 29th January, 2020.