Fan, booster- A large fan installed in the main air current, and thus in tandem with the main fan.

Fan signal- Automation device designed to give alarm if the main fan slows down or stops.

Fault- A slip-surface between two portions of the earth's surface that have moved relative to each other. A fault is a failure surface and is evidence of severe earth stresses.

Fault zone- A fault, instead of being a single clean fracture, may be a zone hundreds or thousands of feet wide. The fault zone consists of numerous interlacing small faults or a confused zone of gouge, breccia, or mylonite.

Feeder- A machine that feeds coal onto a conveyor belt evenly.

Fill- Any material that is put back in place of the extracted ore to provide ground support.

Fire damp- The combustible gas, methane, CH4. Also, the explosive methane-air mixtures with between 5% and 15% methane. A combustible gas formed in mines by decomposition of coal or other carbonaceous matter, and that consists chiefly of methane.

Fissure- An extensive crack, break, or fracture in the rocks.

Fixed carbon�C The part of the carbon that remains behind when coal is heated in a closed vessel until all of the volatile matter is driven off.

Flat-lying- Said of deposits and coal seams with a dip up to 5 degrees.

Flight- The metal strap or crossbar attached to the drag chain-and-flight conveyor.

Float dust- Fine coal-dust particles carried in suspension by air currents and eventually deposited in return entries. Dust consisting of particles of coal that can pass through a No. 200 sieve.

Floor- That part of any underground working upon which a person walks or upon which haulage equipment travels; simply the bottom or underlying surface of an underground excavation.

Flue Gas Desulfurization�C Any of several forms of chemical/physical processes that remove sulfur compounds formed during coal combustion. The devices, commonly called "scrubbers," combine the sulfur in gaseous emissions with another chemical medium to form inert "sludge" which must then be removed for disposal.

Fluidized Bed Combustion�C A process with a high degree of ability to remove sulfur from coal during combustion. Crushed coal and limestone are suspended in the bottom of a boiler by an upward stream of hot air. The coal is burned in this bubbling, liquid-like (or "fluidized") mixture. Rather than released as emissions, sulfur from combustion gases combines with the limestone to form a solid compound recovered with the ash.

Fly ash�C The finely divided particles of ash suspended in gases resulting from the combustion of fuel. Electrostatic precipitators are used to remove fly ash from

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