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Glossary and Methodology

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Glossary

Contaminated coal waste: Waste from combustion of coal including fly ash, bottom ash, scrubber sludge, gypsum and other by-products. NRDC uses the term "contaminated" because toxic material is laced throughout the various waste products -- primarily in the fly and bottom ash, but also in the scrubber sludge and gypsum.

Note that in the case of new and proposed coal plants, projections for only fly and bottom ash are provided, because not enough information is available to project the other forms of solid waste.

Some new and proposed plants convert coal into a gas before combustion, a process called gasification, which results in a significantly different solid waste than that from conventional coal plants, with a much lower environmental risk. Therefore, NRDC lists gasification plants separately from conventional new and proposed conventional coal plants.

Existing plants: Power plants in operation in 2005 or earlier.

New or proposed plants: Power plants proposed, under construction or operating after 2005.

Toxic metals: Coal waste is contaminated by 10 metals classified as toxic by the Agency for Toxic Substances and Disease Registry : Antimony, Arsenic, Beryllium, Cadmium, Chromium, Cobalt, Lead, Manganese, Mercury, Nickel and Selenium. Agency for Toxic Substances and Disease Registry (ATSDR ) [1]

Fly ash: Particulate matter mainly from coal ash with a particle diameter of less than 1 x 10-4 meter. This ash is removed from the flue gas using flue gas particulate collectors such as fabric filters and electrostatic precipitators. [2]

Bottom ash: Residue mainly from the coal burning process that falls to the bottom of the boiler for removal and disposal. [3]

Scrubber sludge: Wet solid residue generated by equipment used to remove sulfur oxides from the combustion gases before discharging them into the atmosphere. The wet scrubber discharge is an off-white slurry with a solids content between 5 and 10 percent. Because scrubber systems are usually accompanied by or combined with a fly ash removal system, fly ash is often incorporated into the scrubber sludge. [4]

Gypsum: Calcium sulfate dihydrate (CaSO4 2H2O), a sludge constituent from the conventional lime scrubber process, obtained as a byproduct of the dewatering operation and sold for commercial use.

Landfill: Disposal method typically used for dry coal waste.

Pond (Surface Impoundments): Disposal method typically used for coal waste, disposed of in wet slurry form.

On-site and off-site disposal: The Energy Information Administration does not define the terms on-site and off-site disposal, but utilities report that they use both methods to dispose of considerable quantities of waste. However, since the disposal options for contaminated coal waste include landfills, ponds, or filling in of old mines, on- and off-site disposal may refer to any of these.

Sold: Sold coal waste is used, among other things, to manufacture cement and wallboard, as well as for agricultural uses. For more on how coal waste is used, see http://www.epa.gov/epawaste/partnerships/c2p2/index.htm.

Methodology

Determining the ash, toxics, and metals content of the waste stream, and the disposition of those wastes, required combining data from various sources -- waste data from 2005 and toxics estimates based on 2007 data. As a result, this analysis is intended to provide an approximate snapshot of coal waste and toxics in a given year, rather than state a definitive amount of waste each plant produces each year. These data sources and the methodology are described below. Data gathering and estimates by David Schoengold of MSB Energy Associates.

1. Coal waste amount and disposal method for existing plants

The amount and disposition of the waste stream for existing plants was calculated using data from the Energy Information Administration (EIA) Form 767, which collected a wide range of data for coal-fired boilers greater than 10 megawatts in size. Because EIA stopped gathering this data after 2005, we relied on the from that year. Form 767 is available at www.eia.doe.gov/cneaf/electricity/page/eia767.html. The form is made up of several parts. The waste stream disposition is found in the part titled F767_Plant. This table provides the amount of ash in tons (both fly ash and bottom ash) and how that ash is divided among landfills, ponds, and off-site sales. EIA is expected to begin reporting this information for coal plants again later in 2009.

2. Toxic metals in coal waste for existing plants (estimated)

The toxic metals estimates in this analysis are annual estimates based on recent coal consumption by each plant , assuming normal operations. Estimating the toxic metals in each plant's coal waste requires information on the type and consumption of coal used at each plant, as well as data on the toxics content of the different kinds of coal in use. Almost all of the toxic metal in the coal ends up in the coal ash.

The data on coal consumption was drawn from the Platts CoalDAT database for 2007. This is the most recent year for which there is a complete set of data. The Platts database indicates for each power plant the source of the coal, the type of coal, the quantity and heat content of the coal, and the ash content. CoalDAT is a proprietary database.

The toxic metals content in different coal types was obtained from the EPA Study of Hazardous Air Pollutant Emissions from Electric Utility Steam Generating Units -- Final Report to Congress, Volume 2, Appendices, EPA-453/R-98-004b, February 1998 (Table D8a). (This document no longer seems to be available at the EPA web site, though an executive summary can be found at www.epa.gov/ttn/oarpg/t3/reports/utilexec.pdf.) This table gives percentages for the toxics and metals contents of the different kinds of coal in use.

3. List of new coal plants.

Sources: New plant permit applications and Sierra Club's list of proposed new coal plants ( http://www.sierraclub.org/environmentallaw/coal/plantlist.asp). While a number of proposed coal plants have suffered legal setbacks, NRDC does not consider them "cancelled" until the developer withdraws the permit application.

4. Fly and bottom ash for proposed coal plants (estimated)

Estimates for fly and bottom ash at proposed plants were based on data from permit applications for each plant. There is not enough data in the permit applications to reliably estimate the amount of scrubber sludge and other waste products. Information from permit applications included the plant size, its heat rate and the coal type to be used. In the absence of specific information, the analysis assumed a heat rate of 10,000 BTU/kWh, a capacity factor of 80% (typical for new plants), and coal type based on location (western plants = sub-bituminous, eastern plants = bituminous, and a mix of the two for mid-western plants). Different coal types have different average ash content and heat content. If the coal type was not indicated on the application, the analysis assumed a 50/50 probability of either sub-bituminous or bituminous coal. An average of the heat content between sub-bituminous and bituminous coals was used, as well as the average ash content between bituminous and sub-bituminous.

5. Toxic metals in coal waste for proposed coal plants (estimated)

The methodology for estimating toxic metals in the ash for new plants is similar to that used for estimating it for existing plants, with the exception that the volume and type of coal in use for each plant was derived from the application permits and from the Sierra Club's on-line database of proposed new power plants rather than from the Platts database.

6. Waste from gasification plants

The amount and nature of solid waste from gasification plants varies depending on the exact processes employed. NRDC does not have sufficient information to provide reliable projections of the wastes that can be expected from new or proposed gasification plants.

[1] http://www.atsdr.cdc.gov/az/a.html

[2] http://www.eia.doe.gov/glossary/glossary_g.htm

[3] http://www.eia.doe.gov/glossary/glossary_g.htm

[4] http://www.tfhrc.gov/hnr20/recycle/waste/fgd1.htm and http://www.eia.doe.gov/glossary/glossary_f.htm

last revised 3/12/2009