Do you even know how a power plant works? The steam that turns the turbine condenses into condensate water which stays in the system. The cooling water passes through the condenser separate from the condensate then goes right back out. Neither of these separate systems  even come in contact with coal or ash. Do you really think any plant would use ashy water to make steam and then throw that dirty steam at their turbine blades? The only ashy water is in the bottom of the furnace which goes through a separator to get the solids out then goes to a holding pond to further clarify before being sent to the river.