This article reports the effect of ash content in raw coal on the energy performance of coal fired thermal power plants of capacity range 30-500 MW The focus is of? primary energy effects-combustion, heat transfer, and flow hydrodynamics. The effects Of variation of ash in coal from 6% (taken as standard) up to 75% oil component performance are studied and overall unit performance is quantified based oil experimental data. When the ash content increases front 6% to 75%, the effects oil the system (without fuel oil support) are as follows: (a) total boiler area requirement (excluding air preheater [APH]) increases by 69%, (b) efficiency of boiler fans drop by 22%-27%, (c) drum mills show an increase in SEC of 115% while ball-race mills and bowl mills show an increase in SEC of 30%, (d) induced draft (ID), forced draft (FD), and primary air (PA) fans show an increase in specific energy consumption (SEC) of around 30%, 6%-14%, and 2%-7% respectively, (e) the gross and net overall efficiencies are reduced to 77% and 66% of their original values, (J) the ratio of the specific fuel consumption at a given ash content to that at standard ash of 6% increases from 1 to 10, and (g) specific fuel consumption (SFC)(grass) increases front 0.35 to 3.0. When the ash content of coal goes beyond 57%, limitations in combustion space and flow arise, and beyond this the unit has to be operated only at partial load. Whet? the ash in coal exceeds 75%, its UHV is reduced to zero.