Here are a couple of links:
http://evolutionwiki.org/wiki/Carbon-14_in_Coal_Deposits_indicates_a_young_earth
http://web.archive.org/web/20090131074436/http://toarchive.org/indexcc/CD/CD011_6.html
Explanations given for why 14C is found in coal are as follows:
Secondary carbonates from groundwater that form on fracture surfaces.
Whewellite, a carbon-containing mineral, that often forms as coal weathers.
the radioactive decay of the uranium-thorium isotope series that is naturally found in rocks. The uranium-thorium isotope series is found in different amounts in different rocks which accounts for the variation in the amounts of 14C in different coals.
microorganisms and fungi (previously) and currently living and dying in coal beds.
carbon-14 entering into coal deposits through contamination from the atmosphere, especially when the coal is being minded and exposed to the air.
