1) fines
2) slack
3) fine coal
the coal having a maximum particle size usually less than 4 mm, and having no lower limit; the upper limit may vary widely
GRM: pl.n. (1), n. (2,3)
USG: to avoid confusion, the term should always be qualified by stating the nominal size (1), term not recommended (3)
References:
SEE coal

ENERGY TERM COURTESY OF ENERGY.EU