Clean Tech 101

What is Clean-Coal Technology?

Introduction
Impurities Trapped Inside Coal
The Clean-Coal Technology
Getting Rid of Sulfur in Coal
The Nitrogen Pollutants in Coal
The Cleanest Coal Technology

Introduction

Coal is the workhouse of the electric power industry of the United States and supplies 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 coal is burned, and can combine with water vapor to form droplets which falls back to Earth as a weak form of sulfuric and nitric acid, common known as acid rain.

Apart from impurities, there are also tiny fragments of minerals and common dirt mixed in coal. The ashes left behind in a coal combustor are made up of these tiny fragments because they don’t burn. These tiny fragments also form 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, a greenhouse gas responsible for global warming.

The Clean-Coal Technology

At present, technologies are available which can filter out about 99 percent of the tiny impurity fragments as well as removing more than 95 percent of the acid rain pollutants in coal. Technologies are also 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 in 1985 when the United States and Canada decided that a course of action needs 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 a step to combat this problem. Scientists are testing new methods to make coal to burn much cleaner. This became 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 yellowish substance that exists in tiny amounts in coal. Nevertheless, it is still important that most of this sulfur be removed before it goes up a power plant's smokestack.

One approach is simply crush the coal into small pieces and washing it before arriving 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 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 clean 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, which has proved to be more effective, lower cost, and more reliable than older scrubbers. The program also tested other types of devices that sprayed limestone inside the tubing of a power plant to absorb sulfur pollutants.

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 and form nitrogen oxides.

Nitrogen oxides can be produced by any fuel that burns hot enough. A lot 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 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". These 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 way to convert coal into electricity. Rather than burning coal directly, coal is broken down to its basic chemical constituents during gasification. Under these conditions, 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 buildup of greenhouse gases. If oxygen is used in a coal gasifier instead of air, 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 percent of which is nitrogen, the resulting carbon dioxide is diluted and more costly to separate.

The ability to produce electricity, hydrogen or various combinations while eliminating nearly all air pollutants and potentially greenhouse gas emissions makes coal gasification a promising technology for energy plants of the future.

Source: AZoCleantech
Last Update 30th December 2007

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