TM 5-810-15
rapid load swings are expected, stoker-fired units
negotiating fuel supply contracts.
e. Fuel selection considerations. The use of
may be eliminated because of their inferior response
to these conditions. When economics dictate the
natural gas has the lowest first cost provided there
is adequate supply in a nearby supplier*s pipeline.
use of low grade fuels including those of high or
variable ash content or high sulfur content then
Natural gas does not require storage facilities;
stoker-fired and PC systems may be eliminated in
however, it is subject to interruption and possible
favor of ACFB systems. If none of these firing
curtailment. Although diesel oil burns more effi-
systems are excluded by these factors, then the
ciently than natural gas, oil requires on site storage
choice between firing systems must be made on the
and pumping facilities. Because oil has the potential
basis of a life cycle cost analysis (LCCA).
to contaminate ground water, storage facilities are
b. Base capital cost. The base capital cost of a
required to include spill containment and leak
dual firing system is the total price of purchasing
detection systems. Coal can be stored in piles
and installing the entire system, including the boiler,
outdoors. Steel tanks and spill containment are not
furnace, either stoker, pulverizer or fluidization
required. Coal pile runoff (coal fines in rain water)
system, fans, flues, ducts, and air quality control
into surrounding waters and airborne fugitive dust
equipment. PC fired and ACFB boilers are more
emissions are concerns that have to be addressed.
expensive than stoker-fired boilers of a given
Transportation of coal from stockpiles to the
capacity, in part because they have a larger furnace
bunkers requires dedicated labor to operate
to provide space and time for the combustion
unloading, storage, reclaim, and handling systems.
process to go to completion. Approximately 60 to
These needs along with sizing, ash handling, and
90 percent of the ash content of the coal passes
particulate emissions reduction requirements make
through the unit along with the gases of combus-
coal firing the highest capital investment
tion. Tube spacing within the unit has to be
alternative.
f. Solid fuel considerations. Due to the special
provided in order to accommodate this condition
and the ability of this ash to cause slagging and
coal sizing requirements of stoker and ACFB fuel
fouling of the heating surface. These factors can
for such a unit may cost (5 to 15 percent) more
increase the size of the boiler and its cost. PC fired
than the unsized coal that could be purchased for a
units have been replaced by packaged boilers in
PC fired unit. However, if unwashed or run-of-
capacity ranges below 100,000 pph. Gas, oil and
mine (ROM) coal is purchased for a PC fired unit,
PC boilers require a flame failure system which
a crusher and motor should also be included in the
increases their cost. The total cost of an ACFB
coal handling system in order to reduce the coal
boiler addition is offset by not requiring flue gas
particle size to approximately 1-1/4 by 0-inch.
desulfurization (FGD) or selective catalytic reduc-
Another consideration is that it may be difficult to
tion (SCR). Selective noncatalytic reduction
obtain the size stoker or ACFB coal due to either
(SNCR) is required on ACFB boilers in place of
transportation difficulties or lack of equipment at a
SCR.
mine site. When using the same bituminous quality
c. Average boiler duty. The remaining expenses
coal, PC and ACFB fired units have a higher
calculated for an LCCA are all functions of the
thermal efficiency (86 to 88 percent) compared to
average boiler duty. This value is based on the
stoker fired units (80 to 84 percent) that effectively
estimated annual boiler load during the expected
lowers their fuel usage costs. The primary reason
life of the plant. It is calculated as follows:
for these differences is unburned carbon loss and
dry gas (exit gas temperature) losses and amount of
average load (pph)
hours of operation
' averageboilerduty
fly ash reinjection. These efficiency percentages will
X
rated capacity
8,760 hours
vary with the quality of the fuel. With low sulfur
For example, if a 100,000 pph boiler operates at an
western fuels having a high moisture content (20 to
average load of 75,000 pph for 8,000 hours per
30 percent), a PC fired unit efficiency may be as
year out of a possible 8,760 hours, the average
low as 82 to 85 percent. A particular ACFB boiler
boiler duty is 68 percent.
can fire a wide range of low grade inexpensive
d. Fuel flexibility. When economically feasible
fuels. These include high sulfur coal, petroleum
the ability to satisfy steam requirements with more
coke, refuse derived fuel, waste water plant
then one type of fuel offers significant advantages.
treatment sludge and mixtures of coal with various
Problems with only one fuel*s source, trans-
scraps such as shredded tires, wood chips and
portation, handling or firing system will not stop
agricultural waste. Another feature of PC and
steam production. The flexibility of alternate fuel
ACFB fired units that results in increased costs and
supplies can be a powerful bargaining tool when
must also be considered in the overall evaluation is
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