Effect of Ash in Coal on the Performance of Coal Fired Thermal Power Plants. Part II: Capacity and Secondary Energy Effects

This article reports the secondary energy effects (wear/erosion/abrasion, slagging, and fouling) of ash in coal on the energy performance of coal fired thermal power plants of capacity range 30–500 MW. It also gives the extent of capacity reduction in equipment due to firing of coals with higher ash contents.

At an ash content of 75% in coal, the effects on the system (without fuel oil support) follow: (a) decrease in Hardgrove index from 80 to 44; (b) 20% of the specific energy consumption (SEC) of induced draft (ID) fans, 10%–12% of that of forced draft (FD) and primary air (PA) fans, 17% of that of drum mills, and 12%–13% of that of ball-race mills and bowl mills, are accounted for by wear/erosion/abrasion effects; (c) decrease of fan efficiencies by 5%–6% points due to wear/erosion/abrasion effects; (d) capacity loss originating from wear/erosion/abrasion effects alone is 8% due to ID fans, 1% due to PA fans, and 6% due to mills; (e) fouling effects are high fouling factor, decrease in boiler efficiency by 3%, and capacity reduction of 2%; and (f) CFs based on overall unit performance are 31% for units below 210 MW, 26% for 210 MW units, and 40% for 500 MW. Considering the capacity restrictions due to individual equipment, CF at an ash content of 57% is 85% due to the boiler fans, 84% due to Raymond bowl mills and drum type ball mills, 71% due to slow speed large ball and race mills, and 88% due to ash slurry pumps. When the coal exceeds 70% and tends toward 76%, the heating value of coal tends toward zero. The effects of slagging (independent of ash content in coal) area 20% decrease in boiler water wall loading, a 3.5% points decrease in boiler efficiency, and capacity reduction of 14%.

Keywords:

• ash content
• equipment efficiency
• specific energy consumption
• capacity reduction
• efficiency under rating