With a view to ensure fuel efficient performance of all future thermal power stations, the CEA has prescribed a formula to determine the minimum efficiency that must be maintained by the boilers used in such installations.
--Minimum steam generator efficiency (%) = 92.5 - [ 50xA + 630(M+9xH) ]/ HHV,
where A = % ash in fuel; M = % moisture in fuel; H = % hydrogen in fuel and; HHV = high heat value of fuel in kcal/kg.
- The central agency has also stipulated that the maximum continuous rating (BMCR) for boilers shall correspond to at least 102% of the steam flow at turbine inlet under VWO (valves wide open) condition plus continuous steam requirement for auxiliary systems of the unit (e.g. fuel oil heating, etc.) when unit is operating above control load.
- The steam generator shall be capable of BMCR output at the worst fuel quality stipulated.
- Further, the furnace should be able to withstand pressures corresponding to +/- 660 mmwc at 67% yield strength or maximum expected pressure/draft of fans, whichever is higher.
- The maximum average gas velocity, when using indigenous coal, in any zone of furnace, superheater, reheater, economizer, shall be 10 m/sec to prevent erosion of pressure parts. However, maximum local velocity can be up to 12 m/s.
- Moreover, if indigenous coal is proposed as main source of fuel, its typical characteristics, like high abrasive ash, slow burning, high ash resistivity, etc. shall be given due consideration while designing the steam generator and auxiliaries.
- While pulverized fuel combustion based steam generator should not require oil support above 40% unit load, FBC-based steam-generators should be designed in a manner that oil support is not needed beyond 25% load.
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