Coal Power

Coal Power refers to electric power produced from direct combustion of coal, or combustion following gasification. Coal supplies the majority of the world's electricity due to its low relative cost and the global distribution of coal reserves. Unlike natural gas and crude oil, which tend to be concentrated heavily in specific regions, coal is available in all inhabited continents, and given the magnitude of the estimated reserves, coal will likely remain abundant for the foreseeable future.

World Primary Energy Supply by Fuel/ World Electricity Generation by Fuel

Source: World Coal Institute. Author: World Coal Institute. Permission: National standards on internet accessibility.

In 2007, the United States produced 1,145.6 million short tons of coal, and consumed 1,128.8 million short tons.¹ The small amount of net international trade is typical for coal, which is generally unprofitable for long-distance trade. In the U.S., about 93% of the total primary energy supply of coal is consumed for electricity generation.² The remainder is mostly used in the manufacture of iron, steel, cement and other nonmetallic minerals. In China, which uses more than twice as much coal as the U.S., the electric power sector only accounts for 52% of the total primary energy supply of coal.³

Coal power technologies

Regardless of the generation technology used, coal power plants tend to provide baseload and intermediate power. In terms of grid dynamics, coal power plants are more flexible for dispatch than nuclear power plants, but are not turned on and off like gas and oil combustion turbines. In the U.S. in 2004, the generation-weighted average capacity factor of all coal power plants was 62%; for comparison, nuclear was 85%, and gas and oil were 20% and 24%, respectively.⁴

In a pulverized coal power plant, coal is ground into a fine powder, mixed with hot air and transported to a furnace for combustion. The heat from the combustion vaporizes water in pipes into steam.

Saturated steam is superheated before entering a high-pressure steam turbine. Steam released from the boiler powers a turbine, transforming heat energy from burning coal into mechanical energy that spins the turbine engine. The spinning turbine is used to power a generator.⁵ The generator is around 15-25 Kv, which is insufficient for long distance transportation of the energy. The voltage is increased and simultaneously the current is reduced by a transformer at the generation station to permit long distance energy transport. The combustion reaction produces H2O and CO2, in addition to a range of other gases. These flue gases are dissipated into the atmosphere through the stack. A condenser cools the steam moving through the turbine, condensing it back into water. The water then returns to the boiler to be reheated.

A second, more efficient technology which is not currently in use for producing electric power is integrated gasification combined cycle (IGCC). To this point, IGCC has not been used to generate electricity, as the higher energy efficiency (and therefore lower fuel costs) does not offset the higher capital costs, and absent a price on carbon emissions, likely will not for the foreseeable future. However, IGCC plants may be built in the near term because of the possibility of installing IGCC that have the capacity to perform carbon capture and storage (CCS). Additionally, because the coal is cleaned prior to gasification, emissions of non-CO2 pollutant gases (NOx, CO, SO2, and black carbon) are very low, so IGCC may become the standard for coal electric power plants in some areas because of the benefits for human and environmental health.

Advantages

Compared with other fossil fuels, coal is ubiquitous, relatively cheap, and the prices have historically been stable.⁶ This stability is important because expected future prices are an important factor in the decision to invest in electric power plants, which have long expected lifetimes (typically 50 years). Because most coal produced worldwide is used domestically, coal is also associated with energy security. For electric utilities, coal power plants are practical for meeting both base and intermediate loads.

Disadvantages

The use of coal raises many safety and environmental issues, most of which are related to its extraction and combustion, and the disposal of the tailings from each of these processes.

Extraction

Much of the coal produced at present is done by "mountaintop removal," a highly destructive process which involves removal of all vegetation on a mountain and the top layers of soil, in order to expose a coal seam which can then be easily accessed by large equipment. Mountaintop removal has a very high ratio of tailings to coal extracted, but is relatively low-labor, and per unit of coal produced, is often cheaper than traditional subsurface mining of coal veins.

Subsurface mining is also problematic; issues include the short-term and long-term health and safety risks posed to miners, impact on local water quality, and global warming effects from the methane that escapes from coal mines. While coal-bed methane may one day be an important source of natural gas, today it is responsible for about 28 Mt of CH4 emissions⁷, equivalent in global warming potential to about 600 Mt of CO2.⁸

Air Pollution

The environmental impact of burning coal has drawn attention for decades. Coal power plants are responsible for 83% of the CO2, 95 percent of the SO2, and 88 percent of the nitrogen oxide (NOx) emissions generated by the electric power sector in the U.S.⁹ According to the Union of Concerned Scientists, a typical 500-MW coal plant generates 3.7 Mt of CO2, 10 kt of sulfur dioxide SO2, 500 t of small airborne particles, 10.2 kt of nitrogen oxide (NOx), 720 t of carbon monoxide (CO), 220 t of volatile organic compounds (VOCs), and 225 pounds of arsenic.¹⁰

Solid Waste

Waste created by a typical 500-MW coal plant includes more than 125 kt of fly ash and 193 kt of sludge from the smokestack scrubber each year. In the U.S., more than 75% of this waste is disposed of in unlined, unmonitored onsite landfills and surface impoundments. Toxic substances in the waste -- including arsenic, mercury, chromium, and cadmium -- can contaminate drinking water supplies and damage vital human organs and the nervous system.¹¹

Cooling Water Discharge

A typical 500-MW coal power plant uses about 2.2 billion gallons of water each year, discharging the water back into local lakes and rivers after increasing the temperature by up to 20-25°F. This "thermal pollution" can decrease fertility and increase heart rates in fish. Typically, power plants also add chlorine or other toxic chemicals to their cooling water to decrease algae growth.¹²

Inefficiency

In any thermal electric power plant, most of the energy content of the input fuel is not converted to electrical energy. The stock average efficiency of coal power plants in the U.S. in 2005 was 37%, with the remainder of this energy lost, mostly to heating water and air. ¹³ There are further system losses associated with long-distance transport of electricity, as most coal power plants tend to be sited far from electricity demand centers. The total system losses for the US electric sector are about 12%.¹⁴

Clean coal technology

The U.S. Department of Energy started the Clean Coal Technology Program and the Clean Coal Power Initiative in the late 1980s and early 1990s. The program originally focused on mitigating the impact of acid rain on forests and watersheds. It then evolved to provide co-financing for new coal technologies that can help utilities cut sulfur, nitrogen and mercury pollutants from power plants. ¹⁵

Areas of research have included:

Carbon Capture & Storage: Carbon capture and storage (CCS) technologies allow emissions of carbon dioxide to be sequestered underground. It is one of the most promising options for large-scale reductions in CO2 emissions from coal use. The technologies for geological CCS are already proven and there are a number of large-scale CCS projects in operation today. Current projects include: Sleipner in Norway;Weyburn in Canada;In Salah in Algeria.¹⁶
Advanced Combustion: A range of advanced coal combustion technologies have been developed to improve the efficiency of coal-fired power generation. Efficiency improvements include the most cost-effective and shortest lead time actions for reducing emissions from coal-fired electricity.¹⁷
IGCC: Integrated Gasification Combined Cycles uses a gasifier to convert coal to syngas, which drives a combined cycle turbine. Coal is combined with oxygen and steam in the gasifier to produce the syngas, which is mainly H2 and carbon monoxide (CO). The gas is then cleaned to remove impurities, such as sulphur, and the syngas is used in a gas turbine to produce electricity.
Coal Cleaning – is also known as coal preparation, beneficiation or coal washing. It is the cleaning process in which mineral matter is removed from mined coal to produce a cleaner product.¹⁸