CHARACTERISTICS

Steam coal can be cleaned prior to burning to remove impurities and increase the coal's heating value. This added step adds a front-end cost for power generators, but this extra cost is outweighed by the savings achieved in fuel handling, plant efficiency, availability, and environmental controls.

Physical coal cleaning involves crushing the coal and screening it into differently-sized particles to separate out impurities that are not chemically bound to the coal. Water sprayed over the coal particles loosens the remaining mineral matter. Specific gravity differences are used to separate the coal from other constituents such as ash. Physical cleaning typically removes 10-30% of the sulfur and 60% of the ash-forming minerals. The coal "fines" are either discarded or can be cleaned using froth flotation. With coal-water slurry technologies, these coal "fines" can be utilized as a cost-effective fuel instead of being discarded as waste, which also reduces emissions. Physical cleaning is now commercially viable; more complicated chemical and biological coal cleaning processes under development may also soon be used commercially.

Subbituminous and lignite coals have lower heat contents and high moisture contents. These coals can be upgraded by physical cleaning, followed by drying to lower the moisture content; this process, called beneficiation, alters the coal so that it has a higher heat content, and lower ash and sulfur contents. Coal drying (or briquetting) is promoted in some countries, particularly China, where over 1000 coal briquetting kilns have been constructed. There are three basic technology types of coal beneficiation, of which one, physical beneficiation, is commercially available; two other types, chemical cleaning–which uses chemical reactions to remove impurities that are organically bound to the carbon in coal–and biological cleaning–that uses microbes to attack and break down impurities in coal–are under development.

Coal pretreatment and beneficiation marginally reduces GHG emissions, but significantly reduces other air pollutants (sulfur) and combustion byproducts (ash). Also, by cleaning the coal prior to combustion, boiler availability is improved and maintenance reduced, the heat content of the coal is increased corresponding to lower CO₂ emissions/kWh of electricity generated, and SO₂ and dust emissions are significantly reduced as well. In addition, washed coal reduces transportation needs since it is of lower weight and volume than run-of-mine coal. This reduces carbon emissions from transportation.

SIZE:
100-1,000 tons per hour throughput. A 500 MW conventional steam-coal plant requires 200-250 tons per hour of bituminous coal.

FEATURES:
Pulverized coal steam plants using bituminous coals with high sulfur and/or ash contents and plants using subbituminous and lignite coals with high moisture content.

COST:
The feasibility of coal cleaning is very specific to the requirements of the user and the characteristics of the coal. Capital costs of the preparation plant range from $25,000 to $100,000 per metric ton per hour of cleaning capacity. Related non-fuel O&M costs range from $1-5/metric ton of coal cleaned. These costs are offset by reduced ash disposal requirements, increased generation per metric ton of coal, reduced SO₂ and particulate emissions, decreased transportation costs, reduced equipment wear and tear, and lower operating and maintenance costs. Coal washing in China was estimated to increase the value of the coal from $5.05 per ton to $8.45 per ton.

CURRENT USAGE:
About 75% of U.S. steam coal is cleaned prior to combustion, by 700 facilities with capacities ranging between 200-20,000 tons per day. Far less pretreatment and beneficiation currently takes place in developing and emerging market countries.

POTENTIAL USAGE:
Chemical and biological cleaning methods are still under development.

ISSUES ASSOCIATED WITH IMPLEMENTING ACTION

• Beneficiation increases the cost of coal because of processing costs as well as cost for the disposal of solid and liquid waste products produced. However, use of the waste products as fuel in a non-pulverized boiler (i.e., fluidized bed) can its increase cost-effectiveness.
• There are potential problems with collection of fly ash using electrostatic precipitators due to changes in the ash resistivity.
• Lignite coals have a tendency to degrade or even spontaneously combust over time.
• Most coal beneficiation systems require large quantities of water, although some of the water may be recycled.

CLIMATE CHANGE IMPACT

EMISSION EFFECT:
    

CONDITIONS FOR EMISSIONS MITIGATION:

• Reduces transportation-associated emissions as less ash and more Btus per pound are transported. Boiler thermal efficiency is generally marginally improved, resulting in a corresponding reduction in greenhouse gas emissions.

EMISSION ESTIMATE:
Improves power plant efficiency by 0.5%-2%

COST-EFFECTIVENESS:
$20-110/ton of carbon removed

SECONDARY EFFECTS:
Reduces SO₂ and particulate emissions from combustion as well as transport of coal. Can also benefit boiler operations through reduced O&M.

RESOURCES

• EPRI/DOE Research, Advanced Physical Fine-Coal Cleaning: Spherical Agglomeration.
• DOE Clean Coal Technology Program has a number of resources and technical documents on technologies for coal beneficiation. Information is available online at http://www.fe.doe.gov/coal_power/cct.html
• United States Department of Energy, Office of Fossil Energy, 1997, Sustainable Development with Clean Coal, 1997, DOE/FE-0363.

CONTACTS

CQ Inc.
Clark Harrison
Homer City, PA
Tel: (412) 479-6016

Rosebud SynCoal Partnership
Ray W. Sheldon
Director of Development
Billings, MT
Tel: (406) 748-2366

U.S. Department of Energy
Joseph B. Renk
Federal Energy Technology Center
Pittsburgh, PA
Tel: (412) 892-6249
renk@fetc.doe.gov
http://www.fetc.doe.gov

U.S. Department of Energy
Douglas Archer
Office of Clean Coal Technology
Germantown, MD
Tel: (301) 903-9443
douglas.archer@hq.doe.gov
http://www.doe.gov