MIL-HDBK-1003/7
Coal Scales. See MIL-HDBK-1003/6, regarding the use of rail car scales and
4.5.6
the use of truck scales for coal delivery. Coal scales are also used to measure coal
feed to stokers or pulverizers. These are located at the in-plant bunker outlet and may
be of the batch weigh bucket, volumetric (volume rate of flow measurement) belt, or
gravimetric (weight rate of flow measurement) belt type.
Wood Firing. Combustion systems for wood are usually designed specifically
4.6
for the material and mixture of fuels to be burned. When the moisture content is high,
over 60 to 65 percent, supplemental firing of coal, oil, or gas can be used or the wood
must be mixed with low-moisture fuels so that enough energy enters the boiler to support
combustion. Dry wood may have a heating value of 8,750 Btu/lb (20 353 kJ/kg); but at 80
percent moisture, a pound of wet wood has a heating value of only 1,750 Btu/lb (4071
kJ/kg). The usual practice when burning wood is to propel the wood particles into the
furnace through injectors, along with preheated air, with the purpose of inducing high
turbulence in the boiler. The wood is injected high enough in the combustion chamber so
that it is dried, and all but the largest particles are burned before they reach the
grate at the bottom of the furnace. Spreader stokers and cyclone burners work well for
this application. For burning wood as a fuel to produce steam or high temperature water
(HTW), methods should be researched thoroughly and their successful operation, adequate
source of fuel, and economics evaluated.
Suspension Burning. Small wood chips or saw dust are blown into the furnace
4.6.1
chamber and burned in suspension. The ash or unburned particles are collected on
traveling grate and transported to ash pit. In wood burning applications, heat releases
have been as high as 1,000,000 Btu/ft2/hr. (11 357 373 kJ/m2/hr) of active grate area.
4.7
Soot Blowers. Soot blowers are required for No. 6 fuel oil, coal,
and wood and may or may not be required for No. 2 fuel oil. Additional information
regarding soot blowers and blowing mediums are presented in MIL-HDBK-1003/6.
Economizers. Economizers are located in the boiler flue gas outlet duct and
4.8
are used to heat the incoming feedwater by reducing the flue gas temperature. The
result is an increase in boiler efficiency. For application and design considerations,
see MIL-HDBK-1003/6.
Air Heaters. Air heaters should be used to burn bark and wood chips and may
4.9
be used for other fuels if economically justified or required for combustion. For
additional information see MIL-HDBK-1003/6.
Forced Draft Fans. For forced draft fan size, types, drives, and general
4.10
requirements, refer to MIL-HDBK-1003/6.
Induced Draft Fans. For induced draft fan size, types, drives and general
4.11
requirements, refer to MIL-HDBK-1003/6. When flue gas scrubbers are used, the induced
draft fans must be able to accommodate the boiler full test steam load when the
scrubbers are not in operation. In addition, allowances must be made for leakage and
pressure requirements for air pollution control equipment.
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