Selected coals are used to manufacture coke; the high pure carbon content, porous structure, and high resistance to crushing of coke have made it a necessary material in the production of molten iron (or hot metal, in iron/steel industry parlance). Coke also serves as a fuel in blast furnaces, as well as serving several nonfuel functions. In a typical blast furnace, iron ore, coke, and other materials, including sinter and fluxes, are loaded in layers. Air, preheated to 9001C, is blasted through the bottom of the furnace through nozzles called tuyeres. Oxygen in the air reacts with coke to form carbon monoxide and generate heat. (more…)

air pollution problems created by coal combustion. Meanwhile, coal-fired power plants and industrial boilers spewed out tons of gaseous and particulate pollutants into the atmo- sphere. During combustion, the small amounts of sulfur and nitrogen in coal combine with oxygen to form sulfur dioxide (SO2), sulfur trioxide (SO3), and the oxides of nitrogen (NOx). (more…)

Although underground coal mines operations are not as visible as surface mining, their overall environmental impact can be greater than that of the typical surface mine. A key environmental problem is subsidence. Underground mines are large cavities in the rock, and depending on the strength of the intervening strata, the depth of the mine, and the type of mining and roof support, the rock walls can fail, causing cracks and land collapse at the surface. Typically, coal seams at depths greater than about 200 feet are extracted by underground mining methods rather than by surface mining, with the exact depth principally based on the relative amount of coal and overburden. However, before improved technology made surface mining so affordable, the trade-off occurred at much shallower depths; some abandoned underground mines are only 35 feet below the land surface. (more…)

A combination of legislation and technology has helped clean up many of the world’s coal-burning plants. Both developed and developing countries have adopted increasingly stringent environmental regulations to govern emissions from coal-fired power plants. In the United States, all coal-fired power plants built after 1978 must be equipped with postcombustion cleanup devices to capture pollutants before they escape into the atmosphere. Cyclones, baghouses, and electrostatic precipitators filter out nearly 99% of the particulates. Flue gas scrubbers use a slurry of crushed limestone and water to absorb sulfur oxides from flue gas. The limestone reacts with the sulfur dioxide to form calcium sulfate, which may be used to produce wallboard. Staged combustion and low-NOx burners are used to burn coal to minimize NOx formation. Another strategy, selective catalytic reduction, reacts ammonia with NOx over a catalyst to produce nonpolluting nitrogen and water vapor.

Conventional coal-fired power plants capture pollutants from the flue gas after it leaves the boiler. Circulating fluidized bed (CFB) combustors capture most of the pollutants before they leave the furnace. Crushed coal particles and limestone circulate inside the CFB combustor, suspended by an upward flow of hot air. Sulfur oxides released during combustion are absorbed by the limestone, forming calcium sulfate, which drops to the bottom of the boiler. The CFB combustor operates at a lower temperature (14001F) compared to pulverized coal (PC) boilers (27001F), which also helps reduce the formation of NO x .

Precombustion coal cleaning is another strategy to reduce sulfur emissions by cleaning the coal before it arrives at the power plant. Sulfur in coal is present as pyrite (FeS2 ), which is physically bound to the coal as tiny mineral inclusions, and as ‘‘organic sulfur,’’ which is chemically bound to the carbon and other atoms in coal. Pyrite is removed in a coal preparation plant, where coal is crushed into particles less than 2 inches in size and is washed in a variety of devices that perform gravity-based separations. Clean coal floats to the surface, whereas pyrite and other mineral impurities sink. Additional cleaning may be performed with flotation cells, which separate coal dust from its impurities based on differences in surface properties. Precombustion removal of organic sulfur can be accomplished only by chemical cleaning. So far, coal combustion emissions and chemical cleaning has proved to be too costly, thus flue gas scrubbers are often required to achieve near-complete removal of sulfur pollutants.

The tightening of environmental regulations is likely to continue throughout the world. In the United States, for example, by December 2008, it is anticipated that coal-fired power plants will have to comply with maximum emission levels for mercury. Emissions of mercury and other trace metals, such as selenium, are under increasing scrutiny of coal combustion emissions because of suspected adverse effects on public health.

Coal is sometimes combusted with waste material as a combined waste reduction/electricity production strategy. The disposal of waste from agriculture and forestry (biomass), municipalities, and hospitals becomes costly when landfill space is limited. Some wastes, particularly biomass feedstock, are combustible, but their low energy density (compared with coal) limits their use as an electricity production fuel. Blending coal with these fuels provides an economical method to produce electric power, reduce waste, and decrease coal plant emissions. Most wood wastes, compared to coal, contain less fuel nitrogen and burn at lower temperatures. These characteristics lead to lower NO x formation. In addition, wood contains minimal sulfur ( o 0.1% by weight) and thus reduces the load on scrubbers and decreases scrubber waste biomass.

Numerous electric utilities have demonstrated that 1–8% of woody drying biomass can be blended with coal with no operational problems. Higher blends may also be used, but require burner and feed intake modifications as well as a separate feed system for the waste fuel. Cofiring in fluidized bed boilers may avoid some of these drawbacks, but the economics of co-firing are not yet sufficiently attractive to make it a widespread practice.

Coal use today is no longer evocative of dirty power plants with polluting black smoke billowing from their smokestacks. Many of these plants have been transformed through technology to operate more efficiently and with significantly lower emissions. Some fire coal with other waste materials and others produce both electric power and heat transmission. Cases of plant retrofits and their new performance statistics are documented by various institutions, including the Energy Information Administration (http:// www.eia.doe.gov) and the World Coal Institute (http://www.wci-coal.com). The following examples highlight clean coal use throughout the world: (more…)

The largest use of coal today is in the generation of electric power by electric utilities. According to the U.S. Energy Information Administration, about 65% of coal mined in the world is used for power generation. The next largest use of coal is in the production of coke for the iron and steel industries. Coal is still used for industrial heating and even commercial heating and residential heating in certain countries. (more…)

Natural gas and oil are common source energy used to give electricity. How efficient is coal if we compare to these other source of energy? With respect to security of supply, coal has a clear advantage. The United States has about 300 million recoverable tons of coal. This amount is sufficiency to last 300 years if we are consuming coal in the same ratio that we used today. In addition, carbon is a versatile and cheap source of fuel. Coal can be used as a solid fuel or converted into a gas to replace expensive imported fuels. (more…)

Power consumption from electricity is an essential element of the U.S. economy since the beginning of the century. Coal power plants, a power source of electricity are available to deliver large quantities of low cost and reliable power supply. It has become more important than the supply of petroleum and natural gas, as the latter supply is decreasing over the time. In 1995, the burning of coal produces about 55% of the electricity produced in the U.S. We also know that coal reserves are expected to last for centuries at current use. (more…)

Coal is still used to a small extent for home heating and cooking. In the homes of more affluent nations, coal is used for recreational or nostalgic reasons, rather than for routine heating and cooking. Coal stoves designed for this purpose are aesthetically attractive and are engineered for efficient operation with near-zero indoor air pollution. In contrast, coal used for cooking and heating in developing countries is burned in crude stoves with inadequate ventilation. In more developed countries, it is common to see coal source of energy. (more…)

Anyone who wants to save money on gas should be interested in the process of underground coal gasification, or UCG. It is an alternative way of obtaining clean fuels, one that has been explored more recently due to increasing oil prices. Considering that the process benefits everyone, it is surprising how few individuals have any idea what it is. Not only do its advantages affect everyone, but the ideas and technology behind UCG are also fascinating. For this reason, it is important to at least have an idea of what goes on underground during the coal gasification process. (more…)