TM 5-810-15
the natural gas or No. 2 fuel oil burner lighters
vanadium in the oil. ACFB boilers have higher
which are normally in the range of 3 million to 10
maintenance when compared to PC boilers. The
million Btu per hour for each device. Stoker fired
abrasive action of the solids circulating through the
units are normally started by spreading kerosene or
combustor and solids separator causes wear. ACFB
other waste oil and scrap wood over a coal bed and
systems are more complicated with more
lighting it. Annual fuel cost is based on the cost of
components which add to the maintenance cost. As
the fuel, in dollars and cents per million Btu
more ACFB experience is gained, maintenance
multiplied by the hours of operation and average
costs can be expected to decrease as improvements
load and divided by the percent boiler efficiency.
are made. Maintenance costs for a PC fired unit are
generally higher than that for a stoker fired boiler
electrical energy requirements. Oil fired units are
due to the higher duty requirements of such items
next due to oil pumping and heating needs. Auxil-
as the pulverizers, primary air fans or exhausters,
iary power requirements for gas and oil boilers are
electric motors, coal lines and greater number of
considerably less than coal fired units because ash
sootblowers. Maintenance costs are also a
handling, coal handling, sorbent handling and
reflection of the hours of operation and average
pollution control systems are not needed. A com-
boiler duty.
i. Operating costs. These expenses include man-
parison can be approximated by listing the fan and
drives with their respective duties and the sizes of
power, sorbent, and other costs incurred on a
each. For example, on a PC fired unit there are
continuing basis while the plant is in operation.
forced draft (FD) and induced draft (ID) fans and
Manpower requirements for oil fired boilers are
drives, primary air (PA) fan(s) and exhauster(s) and
somewhat higher than gas boilers, because of the
drive(s) and pulverizer drive motor(s). It is possible
fuel storage and increased handling concerns asso-
the primary air fan or exhauster drive and
ciated with oil. Coal fired technologies require
pulverizer drive motor may be combined so there is
considerably more manpower than either oil or gas.
a single motor driving both devices. Normally,
Fuel handling, ash handling and pollution control
there would be two or more pulverizers and PA or
systems account for the majority of the increase in
exhauster fans per boiler unit. For the stoker fired
operating costs. Even though stoker fired and PC
unit, there are also an FD and ID fan drive, and an
boilers are less complicated than ACFB boilers,
overfire air and ash reinjection system that likewise
stoker and PC boilers, unlike ACFB boilers, must
may be combined as a single piece of equipment.
include scrubbers. The evaluation of operating
The pulverizer drive and primary air fan and
costs among coal firing technologies is site specific
exhauster drive are relatively high duty or
and must include all relevant factors.
horsepower (hp) consumers compared to the stoker
overfire air and ash reinjection system fan drive
3-7. Pulverizers (Mills).
together with the stoker drive motor. Annual
a. Types. There are four basic types of pulveriz-
power costs, kilowatthours per year, is directly
ers frequently used on industrial sized boilers. They
related to average boiler duty. Sootblower motors
are commonly referred to by the type of grinding
are fractional horsepower and generally are not
elements, i.e., ball and tube, attrita, ball-and-race,
included in any power comparison. ACFB boilers
and ring-and-roll.
also have high electrical power requirements. The
(1) The ball and tube type mill is commonly
fluidized bed is suspended on air that is provided by
used on boilers that use coal as the principle fuel.
a primary air fan. The solids reinjection device has
They require more space and use more power input
fluidizing air needs which are provided by blowers.
than the other types, so they are at an economic
Limestone handling is another electrical user which
disadvantage unless only one mill is used.
is unique to ACFB. Inert bed handling is also in this
(2) Attrita type mills are usually used on
category. ACFB boilers, however, unlike PC and
boilers that use gas or oil as the primary fuel with
stoker boilers, may not require sulfur removal
coal as a backup fuel. These mills are subject to
equipment such as scrubbers. This must be
high maintenance due to the use of unwashed
considered when evaluating power cost.
(ROM) coal and foreign objects (rail, spikes, rebar,
h. Maintenance costs. Gas fired boilers have the
wood) getting into the coal stream. This mill
lowest maintenance cost. Oil fired boiler installa-
combines the pulverizer and the exhauster in a
tions are higher than gas fired systems due to oil
single package.
pumping needs, oil storage requirements and boiler
(3) The following information and
corrosion and external deposits on heat transfer
illustrations primarily pertain to the more frequently
surfaces resulting from sulfur, sodium and
used ball-and-race and ring-and-roll type mills.
3-11
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