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

Biomass-based Fluidized Bed Combustion (FBS) Boilers
3.5 BOILER BLOW-DOWN
When water is boiled and steam is generated, dissolved solids that were
contained in the water remain in the boiler. After a period of time their
solubility in water is exceeded and they are deposited from the solution.
Above certain levels of concentration, these solids encourage foaming and
cause carryover of water into steam. The deposit also leads to scale formation
in boiler tubes and reduces efficiency, localized heating, and tube failure.
It is therefore necessary to control the level of concentration of the solids and
this can be achieved by the process known as blow-down, in which a certain
amount of water from the boiler is blown off. The maximum amount of total
dissolved solids (TDS) concentration permissible in various types of boilers is
shown in Table 3-3.
Table 3-3 Recommended TDS levels for various boilers
Boiler Type Maximum TDS (ppm)
1. Lancashire 10,000 ppm
2. Smoke and water tube boilers (12 kg/cm 2 ) 5,000 ppm
3. Low-pressure water tube boiler 2000–3,5000 ppm
4. Package and economic boiler 3,000 ppm
5. Coil boilers and steam generators 2000 (in the feed water)
Note: Refer guidelines specified by manufacturer for more details
3.5.1 Blow-down calculations
The quantity of blow-down required to control boiler water solids concentration
is calculated using the following formula:
Blow-down (%) =
If maximum permissible limit of TDS in a package boiler is 3000 ppm,
percentage that makes up water is 10%, and TDS in feed water is 300 ppm,
then the percentage of blow-down is given as
=
= 1%
Feed water TDS x % Make-up water
Maximum Permissible TDS in Boiler Water
300x10
3000
If boiler evaporation rate is 3000kg/hr, then the required blow-down rate is
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