MIL-HDBK-1003/6
accumulated ash. When firing lignite, hoppers must be cleaned continuously or
emptied within 4 hours to prevent heat damage. The inlet of the mechanical
cyclones should be installed so that the gas flow is relatively uniform; if
not, straightening vanes must be installed upstream of the collector.
Ductwork (breaching) must have gradual size transformations up-stream of the
collector to prevent turbulence in the duct and reduced collector performance.
Design shall be based on predicted flue gas temperature leaving the boiler or
economizer plus 25 degrees F (13.8 degrees C). Pressure drop will vary
according to boiler flue gas flow but will generally be from 2 to 3 inches of
water (0.5 to 0.75 kPa). Table 15 shows the particle size distribution for a
high sulfur coal with an ash content of approximately 12 percent. It can be
seen that 90 percent collection of particle sizes in the 10.0-15.0 micron
range will provide an overall collection efficiency of 60-70 percent.
Particle size distribution and the quantity of flyash will vary
according to fuel characteristics and the type of combustion process being
used.
Table 15
Typical Flyash Analysis for a Spreader Stoker
Bahco Size Distribution
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*
*
Particle Size Range
Average Particle
Percent in Gas
*
*
Microns
Size Microns
Stream
*
*
*
*
0- 7.5
3.75
28.0
*
*
7.5-10.0
8.75
5.0
*
*
10.0-15.0
12.5
7.0
*
*
15.0-20.0
17.5
5.0
*
*
20.0-30.0
25.0
8.0
*
*
30.0-40.0
35.0
6.0
*
*
40.0-60.0
50.0
7.5
*
*
60.0+
60.0+
33.5
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6.1.5
Accessories. Collectors shall be equipped with means of
periodically, and preferably automatically, removing collected ash and
cinders. Accessory equipment generally to be included in an installation are
hopper bottom vibrators, rotary air seal feeders on hopper outlet, conveyor,
expansion joints at conveyor, high level alarm, a local control panel to
operate the discharge equipment, hopper dust level indicator, hopper heater,
insulation and pressure gauge. Provide a water wash system when burning oil,
high sulfur fuels, or other corrosive fuels, in order to remove corrosive
residual ash when the unit is out of service. Provide inspection doors at the
inlet and outlet ducts of the collector.
93
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