The most important step in coal-fired unit design is to
properly size the furnace. Furnace size has a first-order influence on the size
of the structural-steel framing, the boiler building and its foundations, as
well as on the sootblowers, platforms, stairways, steam piping, and duct work.
The fuel-ash properties that are particularly important when
designing and establishing the size of coal-fired furnaces include
The ash fusibility temperatures (both in terms of their
absolute values and the spread or difference between initial deformation
temperature and fluid temperature)
The ratio of basic to acidic ash constituents
The iron/calcium ratio
The fuel-ash content in terms of pounds of ash per million
British thermal units
The ash friability
These characteristics and others translate into the furnace
sizes in Fig. 5-5, which are based on the six coal ranks shown in Table 5-2.
This size comparison illustrates the philosophy of increasing the furnace plan
area, volume, and the fuel burnout zone (the distance from the top fuel nozzle
to the furnace arch), as lower-grade coals with poorer ash characteristics are
fired.
Wide variations exist in fuel properties within coal ranks,
as well as within several subclassifications (e.g., subbituminous A, B, C),
each of which may require a different size furnace.
Among the most important design criteria in large
pulverized-fuel furnaces are net heat input in British thermal units per hour
per square foot of furnace plan area (NHI/PA) and the vertical distance from
the top fuel nozzle to the furnace arch.
Furnace dimensions must be adequate to establish the
necessary furnace retention time to properly burn the fuel as well as to cool
the gaseous combustion products. This is to ensure that the gas temperature at
the entrance to the closely spaced convection surface is well below the
ash-softening temperature of the lowest-quality coal burned.
Heat-absorption characteristics of the walls are maintained
using properly placed wall blowers to control the furnace outlet gas
temperature by removing ash deposited on the furnace walls below the furnace
outlet plane.
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