Biomass as Fuel in Small Boilers (PDF 6.4 - APO Asian Productivity ...

Biomass as Fuel in Small Boilers
v. Percentage heat loss due to unburnts in fly ash
% heat loss due to unburnts in fly ash
vi. Percentage heat loss due to unburnt particles in bottom ash
% heat loss due to unburnts in bottom ash
= Total bottom ash collected per kg of fuel burnt ×GCV of bottom ash×100
= Total ash collected per kg of fuel burnt ×GCV of fly ash×100
GCV of Fuel
vii. Percentage heat loss due to radiation and other unaccounted losses
The actual radiation and convection losses are difficult to assess because
of particular emissivity of various surfaces, its inclination, its flow pattern,
etc. In a relatively small boiler, with a capacity of 10 TPH, the radiation and
unaccounted losses could amount to between 1% and 2% of the gross calorific
value of the fuel, while in a 500 MW boiler, value between 0.2% to 1% are
typical. The loss may be assumed appropriately depending on the surface
condition.
Efficiency of boiler (η) = 100–(i + ii + iii + iv + v + vi + vii)
Example:
The following are the data collected for a typical rice husk-fired boiler. Find the
efficiency of the boiler by the indirect method.
- Type of boiler : Rice husk
- Ultimate analysis of rice husk
Carbon - 38.5%
Hydrogen - 5.7%
Oxygen - 39.8%
Nitrogen - 0.5%
Sulphur - 0%
G.C.V. - 3200 K.Cal/kg
Moisture in fuel - 8%
Ash in fuel - 15%
GCV of bottom ash - 900 K.Cal/kg
GCV of fly ash - 650 K.Cal/kg
Ratio of bottom ash to fly ash - 80:20
Percentage of oxygen in flue gas - 7%
Flue gas temperature - 200 ºC
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GCV of Fuel