Making Energy Work: Building a Sustainable Energy Economy in the Southeast

Coal

Coal is the most abundant fossil fuel in the world with global recoverable reserves estimated to be over a trillion short tons. The United States holds almost 25% of the world’s reserves of the non-renewable resource with 250 billion short tons of recoverable coal reserves. At current production rates, the U.S. coal reserves will last almost 250 years. Other countries that possess large reserves of coal are Russia, China, India, and Australia.

Coal is divided into four categories, or ranks, depending upon the age, percent carbon, and energy content of the rock. From lowest to highest rank, the coal hierarchy goes lignite, sub-bituminous, bituminous, and anthracite. Within the United States, most coal production comes from one of three large regions. The oldest coal region in the country, found in the Appalachian Mountains, is comprised mainly of bituminous coal. In recent years, a large amount of growth has been seen in the production of coal from the eastern Rocky Mountains region where large reserves of sub-bituminous and lignite coal reside. In between these two coal regions is the Interior Basin which extends from Michigan, Illinois, and Iowa down to the Gulf Coast. The northern half of this basin produces bituminous coal while the Gulf coast region has large reserves of lignite coal.

How It’s Used

In the United States, the vast majority of coal use is for the generation of electricity with much smaller amounts being used for industrial process heat and in the production of steel. 50% of the electricity generated in the U.S. comes from the combustion of coal. The most common method of generating electricity from coal is to pulverize the coal rock and burn it in a boiler to generate heat. This heat is used to generate steam which in turn drives a turbine to generate electricity. This method of generating electricity has existed in the United States since the 1920s. The typical pulverized coal power plant manages to convert between 30% and 34% of the energy content of coal into electrical power to be delivered to end-use consumers.

The combustion of coal generates 60% of the electricity consumed in the state of North Carolina currently1.  None of the coal used for North Carolina electricity generation comes from inside the state.

Environmental Impact

While coal resources are abundant, the negative environmental impacts of coal production and combustion are numerous. Most notably, while coal provides only 23% of the primary energy use in the United States, it accounts for 37% of the carbon dioxide emissions that result from the energy sector and 81% of the emissions from the electricity sector2.  Consequently, coal combustion is the second largest contributor to U.S. greenhouse gas emissions behind oil consumption for the transportation sector.

Additional atmospheric impacts from the combustion of coal include the large emissions of sulfur dioxide, nitrogen oxides, mercury, and other trace elements such as arsenic. These pollutants are associated with a wide array of environmental problems including acid rain, smog, increased respiratory illnesses, decreased visibility, and negative impacts on the health of fish and other wildlife. Burning coal also creates large amounts of ash. Much of this ash is used as an ingredient in cement but significant amounts are also placed in landfills or ash ponds.

Additional concerns exist with the large amounts of water used with coal power plants as a coolant. The large water needs of a coal power plant may negatively affect neighboring plants and wildlife that depend upon access to water. Wastewater returned to neighboring bodies of water may be contaminated with hazardous pollutants and will have a significantly higher temperature than the surrounding water, potentially harming local plants and wildlife. Rainwater runoff from coal piles adjacent to power plants can flush heavy metals like arsenic and lead out of the coal and into surface and groundwater resources.

Along with the emissions and waste products from burning coal, the process of mining of coal has a significant environmental impact as well. The removal of mountaintops in the Appalachians to access the coal resources that lie underneath and the subsequent dumping of millions of tons of earth into neighboring valleys has a massive impact on the local ecosystems as well as threatening the water quality of the region from the potential leeching of heavy metals and acidic liquors into waterways and groundwater. Likewise, the open pit mining common to western U.S. coal mining disturbs large stretches of land, results in increased dust, and threatens the quality of groundwater.

Advanced Technologies

Most coal-fired electricity generated in the U.S. comes from older, “subcritical”, pulverized coal power plants. While this technology has proven to be highly reliable at generating electricity at a low financial cost, its low efficiency and high emissions rate for various pollutants has left it out of favor among electricity utilities looking to build new, baseload power plants. If new coal-fired facilities are to be built in the future, they are likely to utilize more advanced coal technologies such as supercritical and ultra-supercritical pulverized coal, integrated gasification combined cycle (IGCC), carbon capture and sequestration, and coal-to-liquids.

The next technological step beyond subcritical pulverized coal is the development of “supercritical” technology. While this technology has been around for decades, it is only in recent years that some of its difficulties have been resolved. Most new coal power plants being proposed or constructed in the U.S. are to use supercritical technology. The main benefits of this technology it has the potential to operate at efficiencies as high as 45%, reducing the amount of coal and consequent emissions of harmful pollutants needed to generate each kilowatt-hour of energy.

The next step beyond supercritical technology is “ultra-supercritical” pulverized coal power plants. Few of these plants operate around the world today and few are being planned, largely because the benefits from the increased efficiency of the technology does not outweigh the increased cost of constructing and operating the more advanced technology power plant. A next generation ultra-supercritical pulverized power plant could potentially achieve an efficiency of 50% but would likely require the use of more exotic materials in the construction of the power plants core components which would drive up the cost of the power plant3.

Unlike advanced pulverized coal technologies which can be seen as a refinement of the well-established subcritical technology, IGCC technology involves a markedly different method of generating electricity from coal. The value of IGCC lies in its potential to significantly reduce many of the persistent problems associated with pulverized coal combustion. IGCC technology begins with pulverized coal being “gasified”, broken down into largely carbon monoxide and hydrogen, in a high-pressure, steam-filled boiler. As a high-pressure gas, all the contaminates in the coal gas, or syngas, can be removed before the combustion process, improving the percentage of harmful pollutants that can be removed from the energy generation process while also significantly reducing the costs of achieving these reductions. Once cleaned, the syngas can then be fully combusted in order to drive a gas turbine. The hot gas leaving the gas turbine can then used to generate steam for use in a steam turbine to generate additional electricity. This combined cycle process allows for IGCC technology to achieve efficiencies above 40% with the potential for efficiencies as high as 45% or even 50%.

It is estimated that the cost of electricity generated from an IGCC power plant will be 10-20% greater than for a conventional pulverized coal power plant because of the newer and more complicated technology involved. The U.S. federal government is currently developing a next generation IGCC power plant through its FutureGen program and several other private corporations and coalitions are planning or constructing IGCC power plants to demonstrate the potential of the technology.

Beyond its general versatility, much of the current interest in IGCC spurs from its potential to play in integral role in addressing the problem of Global Warming. Currently, much of the carbon present in coal is emitted out the smokestack in the form of carbon dioxide. If the threat of Global Warming is to be addressed, these carbon dioxide emissions will need to be drastically reduced. One method of achieving this goal is to capture the carbon for sequestration in geologic formations. It is believed that carbon capture technology can be more easily and inexpensively applied to IGCC technology after the gasification process but before the resulting syngas is combusted than in conventional pulverized coal power plants.

Once captured, the carbon dioxide can be sequestered in oil and gas reservoirs, deep coal seams, and saline aquifers among other locations. How well each of these carbon sinks will work to hold sequestered carbon dioxide is an area of intense government and industry research.

Beyond using coal for the generation of electricity, coal is also beginning to be used to be used for the production of liquid fuels such as gasoline and diesel. High world oil prices along with concerns about energy independence of made it appealing to turn coal into refined petroleum products. Coal-to-liquids (CTL) technology is similar to an IGCC power plant except that it is optimized to create diesel or gasoline instead of electricity.

The main concern with CTL technology is its potential to increase the amount of carbon dioxide emitted from the production and consumption of gasoline and diesel fuel by as much as 60% or 100%.  As with IGCC technology, CTL has the potential to cost-effectively employ carbon capture and sequestration. If these technologies were used together, carbon dioxide emissions from the production and consumption of CTL fuels could be comparable or even slightly better than conventional fuels derived from petroleum.
  1. EIA-906
  2. http://www.eia.doe.gov/oiaf/1605/ggccebro/chapter1.html
  3. Viswanathan, R; Coleman, K; Rao, U. “Materials for ultra-supercritical coal-fired power plant boilers” 2006.
  4. http://www.csmonitor.com/2007/0302/p02s01-ussc.htm