coal ash

Well, not really. Ash is what's left after all the combustibles have been consumed.

Biogas comes mainly from sewage treatment and is a valuable source of process heat within the works. A well-run major sewage installation should be a net exporter of electric power as a result.

ash possesses some percentage of sulphur, nitrogen, carbon and some other elements. cant they be realesed and used for other purpose???

Fly ash can be blended with cement to reduce the amount of cement needed.

Basically it reacts with the free lime liberated when the cement sets and forms a product very similar to set cement.

Also reduces the sensitivity of concrete to acid attack if enough is used to react with all, or nearly all the free lime.