COMBUSTION EFFICIENCY - BOILER
Fuel combustion in boiler i.e. Triple-T = TIME, TEMPERATURE & TURBULENCE. Normally major boiler efficiency game changer values during design calculations are predicted parameters like Unburnt Carbon Losses, Flue Gas Temperature at APH outlet, Excess Air requirement, Ambient Temperature, Air temperature increment in Fan, Fuel Ultimate & Proximate Analysis, Fuel Sieve Analysis for fines %
Residence Time – depend on Furnace height, Furnace volume, Fixed Carbon in fuel, Flue gas mass quantity, Gas Velocity and Unburnt carbon losses
- Residence time is influenced by Furnace temperature, Furnace volumetric loading, Flue gas velocity in furnace, Particle density, Particle size distribution, Excess Air
- Residence Time is inversely proportional to Furnace Exit Gas Temperature-FEGT, Furnace Pressure, Unburnt Carbon Losses and Carryover in fly ash, Furnace exit gas velocity
- Unburnt Carbon Losses is directly proportional to higher fines% in fuel, higher fixed carbon% in fuel, lower volatile matter% in fuel, higher furnace gas velocity,
- Unburnt Carbon Losses is inversely proportional to Furnace / Bed temperature, Excess Air, Furnace Height, Front Ash / Bed Ash% discharge, Residence time
- Unburnt Carbon in kg/kg of Fuel = (GCV X Unburnt Carbon Loss %) /(8079 x 100) = e.g. (2175 x 3)/(8079 x 100) = 8.08 gm/Kg of Fuel
- LOI & Unburnt Carbon Loss should be clearly identified to know the exact deviation from design performance
- Higher excess air reduces unburnt carbon loss but increases dry fuel gas loss. Fuel with higher fixed carbon to volatile ratio, more excess air is required
- Combustion Efficiency is directly proportional to Residence time and High Turbulence
- Combustion efficiency is relate with Un-burnt Carbon Losses based on Fixed carbon v/s Volatile Matter OR Fuel Reactivity:
- Loss from Steam Coil Air Preheater- SCAPH due to supply of steam is not considered in performance calculation. This loss is calculated as Loss due to preheating the air by steam = Mass flow of steam used to preheat the air X (Enthalpy of steam supplied to air preheater - Enthalpy of water/steam discharged from air preheater)
- Incomplete combustion of the carbon in the form of Carbon Monoxide– CO in Flue gas leaving the furnace is generally considered NIL i.e. ZERO.
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