In the furnace, chemical energy of fuel is converted into the of combustion products as pulverised fuel is
burned in suspension in the furnace space. Combustion products give up part of their heat by radiation to
water walls. Thus they leave the furnace at a safe temperature, which will not cause clinkering of the
subsequent convective heating surface.
Factors important for combustion = 3 T :
1) TIME = Residence time of fuel
2) TEMPERATURE = Furnace gas temp. (Must be > fuel ignition temp.)
3) TURBULENCE
1) => Furnace volume is large enough to give the mix. time to complete combustion. (Height of boiler is governed by the residence time of fuel) .
2) => Fuel + air mixture maintained at > ignition temp.
3) => Adequate quantity of air supply to fuel, so that O2 + fuel are thoroughly mixed.
Amount of excess air required is dependent on load.
Furnace cross-section area & design must support full load gas flow rate at acceptable velocities.
However, when load is decreased, gas flow rates and associated velocities drop off & proper mixing of the fuel with combustion air becomes difficult.
Therefore additional air is introduced as load is decreased to maintain effective combustion.
Secondary Air velocity (maintained by proper WB to furnace dp)
Air-Fuel mix. velocity
TURBULENCE
Dimension of furnace are dependent upon = 1) proper residence time 2) Avoidance of high temperature 3) Proper flue gas velocity Time required. for complete combustion also depends upon particle size of the combustible - fineness of the fuel provided other conditions are met. Combustion is a two-stage process involving ---- * Physical contact of combustible with O2 . * Chemical combustion of the two after contact. Once ignition temp. is attained -- chemical process is instantaneous but cannot proceed until physical contact has been made, i.e. rate of combustion is controlled by the rate at which contact is made between O2 & the combustibles.
Factors important for combustion = 3 T :
1) TIME = Residence time of fuel
2) TEMPERATURE = Furnace gas temp. (Must be > fuel ignition temp.)
3) TURBULENCE
1) => Furnace volume is large enough to give the mix. time to complete combustion. (Height of boiler is governed by the residence time of fuel) .
2) => Fuel + air mixture maintained at > ignition temp.
3) => Adequate quantity of air supply to fuel, so that O2 + fuel are thoroughly mixed.
Amount of excess air required is dependent on load.
Furnace cross-section area & design must support full load gas flow rate at acceptable velocities.
However, when load is decreased, gas flow rates and associated velocities drop off & proper mixing of the fuel with combustion air becomes difficult.
Therefore additional air is introduced as load is decreased to maintain effective combustion.
Secondary Air velocity (maintained by proper WB to furnace dp)
Air-Fuel mix. velocity
TURBULENCE
Dimension of furnace are dependent upon = 1) proper residence time 2) Avoidance of high temperature 3) Proper flue gas velocity Time required. for complete combustion also depends upon particle size of the combustible - fineness of the fuel provided other conditions are met. Combustion is a two-stage process involving ---- * Physical contact of combustible with O2 . * Chemical combustion of the two after contact. Once ignition temp. is attained -- chemical process is instantaneous but cannot proceed until physical contact has been made, i.e. rate of combustion is controlled by the rate at which contact is made between O2 & the combustibles.
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