Coal: Encyclopedia II - Coal - Uses

Coal - Uses

Coal - Coal as fuel

Coal is primarily used as a solid fuel to produce heat through combustion.

World coal consumption is about 5,800 million short tons (5.3 petagrams) annually, of which about 75% is used for electricity production. The region including China and India uses about 1,700 million short tons (1.5 Pg) annually, forecast to exceed 3,000 million short tons (2.7 Pg) in 2025. ^[1] The USA consumes about 1,100 million short tons (1.0 Pg) of coal each year, using 90% of it for generation of electricity. Coal is the fastest growing energy source in the world, with coal use increasing by 25% for the three-year period ending in December 2004 (BP Statistical Energy Review, June 2005).

When coal is used in electricity generation, it is generally pulverized and then burned. The heat produced is used to create steam, which is then used to spin turbines which turn generators and create electricity. Approximately 40% of the Earth's current electricity production is powered by coal, and the total known deposits recoverable by current technologies are sufficient for 300 years' use at current rates (see World Coal Reserves, below).

A promising, more energy efficient way of using coal for electricity production would be via solid-oxide fuel cells or molten-carbonate fuel cells (or any oxygen ion transport based fuel cells that do not discriminate between fuels, as long as they consume oxygen), which would be able to get 60%-85% combined efficiency (direct electricity + waste heat steam turbine), compared to 30-40% currently possible with only steam turbines. Currently these fuel cell technologies can only process gaseous fuels, and they are also sensitive to sulfur poisoning, issues which would first have to be worked out before large scale commercial success is possible with coal. As far as gaseous fuels go, one idea is pulverized coal in a gas carrier (nitrogen), especially if the resulting carbon dioxide is sequestered, and has to be separated anyway from the carrier. A better idea is coal gasification with water, then the water recycled.

High prices of oil and natural gas are leading to increased interest in "BTU Conversion" technologies such as coal gasification, methanation, liquefacation, and solidification.

In the past, coal was converted to make coal gas, which was piped to customers to burn for illumination, heating, and cooking. At present, the safer natural gas is used instead. South Africa still uses gasification of coal for much of its petrochemical needs.

Gasification is also a possibility for future energy use, as it generally burns hotter and cleaner than conventional coal, can spin a more efficient gas turbine rather than a steam turbine, and makes capturing carbon dioxide for later sequestration much much easier.

Coal can also be converted into liquid fuels like gasoline or diesel by several different processes. The Fischer-Tropsch process of indirect synthesis of liquid hydrocarbons was used in Nazi Germany, and for many years by Sasol in South Africa - in both cases, because those regimes were politically isolated and unable to purchase crude oil on the open market. Coal would be gasified to make syngas (a balanced purified mixture of CO and H₂ gas) and the syngas condensed using Fischer-Tropsch catalysts to make light hydrocarbons which are further processed into gasoline and diesel. Syngas can also be converted to methanol: which can be used as a fuel, fuel additive, or further processed into gasoline via the Mobil M-gas process.

A direct liquefaction process Bergius process (liquefaction by hydrogenation) is also available but has not been used outside Germany, where such processes were operated both during World War I and World War II. SASOL in South Africa has experimented with direct hydrogenation. Several other direct liquefaction processes have been developed, among these being the SRC-I and SRC-II (Solvent Refined Coal) processes developed by Gulf Oil and implemented as pilot plants in the United States in the 1960's and 1970's.^[2]

Yet another process to manufacture liquid hydrocarbons from coal is low temperature carbonization (LTC). Coal is coked at temperatures between 450 and 700 °C compared to 800 to 1000 °C for metalurgical coke. These temperatures optimize the production of coal tars richer in lighter hydrocarbons than normal coal tar. The coal tar is then further processed into fuels. The process was developed by Lewis Karrick, an oil shale technologist at the U.S. Bureau of Mines in the 1920s.^[3]

All of these liquid fuel production methods release carbon dioxide (CO₂) in the conversion process. Carbon dioxide sequestration is proposed to avoid releasing it into the atmosphere. As CO₂ is one of the process streams, sequestration is easier than from flue gases produced in combustion of coal with air, where CO₂ is diluted by nitrogen and other gases.

Coal liquefaction is one of the backstop technologies that limit escalation of oil prices. Estimates of the cost of producing liquid fuels from coal suggest that domestic U.S. production of fuel from coal becomes cost-competitive with oil priced at around 35 USD per barrel ^[4], (break-even cost), which is well above historical averages - but is now viable due to the spike in oil prices in 2004-2005. ^[5].

Among commercially mature technologies, advantage for indirect coal liquefaction over direct coal liquefaction are reported by Williams and Larson (2003). Estimates are reported for sites in China where break-even cost for coal liquefaction may be in the range between 25 to 35 USD/barrel of oil.

Coal - Coking and use of coke

Coke is a solid carbonaceous residue derived from low-ash, low-sulfur bituminous coal from which the volatile constituents are driven off by baking in an oven without oxygen at temperatures as high as 1,000 °C (2,000 °F) so that the fixed carbon and residual ash are fused together. Coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. Coke from coal is grey, hard, and porous and has a heating value of 24.8 million Btu/ton (29.6 MJ/kg). Byproducts of this conversion of coal to coke include coal-tar, ammonia, light oils, and "coal-gas".

Petroleum coke is the solid residue obtained in oil refining, which resembles coke but contains too many impurities to be useful in metallurgical applications.

Coal - Harmful effects of coal burning

Combustion of coal, like any other compound containing carbon, produces carbon dioxide (CO₂), along with varying amounts of sulfur dioxide (SO₂) depending on where it was mined. Sulfur dioxide reacts with water to form sulfurous acid. If sulfur dioxide is discharged into the atmosphere, it reacts with water vapor and is eventually returned to the Earth as acid rain.

Emissions from coal-fired power plants represent the largest source of artificial carbon dioxide emissions, according to most climate scientists a primary cause of global warming. Many other pollutants are present in coal power station emissions. Some studies claim that coal power plant emissions are responsible for tens of thousands of premature deaths annually in the United States alone. Modern power plants utilize a variety of techniques to limit the harmfulness of their waste products and improve the efficiency of burning, though these techniques are not widely implemented in some countries, as they add to the capital cost of the power plant. To eliminate CO₂ emissions from coal plants, carbon sequestration has been proposed but is not yet in large-scale use.

Coal also contains many trace elements, including arsenic and mercury, which are dangerous if released into the environment. Coal also contains low levels of uranium, thorium, and other naturally-occurring radioactive isotopes whose release into the environment may lead to radioactive contamination.^[6][7] While these substances are trace impurities, enough coal is burned that significant amounts of these substances are released, paradoxically resulting in more radioactive waste than nuclear power.

If coal liquefaction or gasification is used to make petrochemicals, a great deal of carbon dioxide is produced in the process. If a carbon tax was introduced and sufficient CO₂ was not captured, the economics of such processes would be significantly less attractive. However, if sequestration or some other process were used to dispose of this by-product, fuels produced from this process would be less polluting. Some process do not have a much greater total impact on carbon dioxide levels than ones refined from petroleum. Others may be less polluting still. Research in this field is ongoing.

Adapted from the Wikipedia article "Uses", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki