h sub 2 osodium perborate nabo sub 3 u00b7 h sub 2 onon boratelimestone caco sub 3dolomite mgco sub 3 u00b7 caco sub 3cryolite na sub 3 alf sub 6______________________________________ the darkening agent or mix may be used in solid or liquid form and may be applied in any appropriate fashion if a liquid is used the liquid preferably should be an oil or water based dispersion the darkening agent or mix may be applied to the coal before the coal is placed in the furnace preferably the darkening agent or mix is blown into the furnace near the part of the boiler wall that is coated with the reflective ash the invention will be more completely understood with reference to the following examples a 600 mw coal fired boiler originally designed to burn western coal was switched to low sulfur powder river basin coal the following description refers to 34 levels 34 at a specific height above sea level in this particular boiler the burners were positioned between 970 8 m 3185 ft and 979 9 m 3215 ft above sea level and the nose was at 1004 m 3294 ft above sea level after the switch to powder river basin coal the water.