Ash from wood may be compost-able, but I think the leftovers from burning coal can contain arsenic, mercury and lead among other things. There are also instances of the fly ash being radioactive. u/leetrout hints that they are from WV/EKY area, where hilly terrain can make stashing fly ash in slurry ponds less visible (but more susceptible to contaminating small streams undetected). Specifically in the eastern KY/OH regions, there massive impoundments <500 yards from navigable rivers (since barges transport to plants and water used in generation), so if they break, the sludge won't have far to travel to contaminate major waterways.

https://pubs.usgs.gov/fs/1997/fs163-97/FS-163-97.html - information regarding some of the byproducts.

Power plant and incinerator fly ash contains heavy metals and other poisonous substances so needs to be separated to prevent environment contamination.

But I guess you could turn it into bricks and isolate those instead or even use them as building materials in some form of infrastructure.

Ash is not organic material.