Heating value of coal, Btu/lb of coal x lb. of coal/Hour x Boiler efficency equals the steam enthal.Btu/lb of steam x lb.of steam/Hour.
simple cal. but need to know Btus of saturated and superheated steam, and of feed water.
You can find info by check "Steam Its Generation and Use" by Babcock & Wilcox co. Good Luck.