Chemical Engineering February 2016 - 40

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Figure 2. Completely watercooled
furnaces, such as the
one shown here, help lower
NOx emission and increase
furnace effectiveness
tory-lined boilers have to be started up more
slowly than a water-cooled furnace due to
potential formation of cracks in the refractory
if started up quickly.
The completely water-cooled furnace
(Figure 2) has several advantages over refractory-lined
boilers, such as lower nOx
emission rates, better utilization of furnace
surface and lower FEGT. This results in lower
tube-wall temperatures in the heat transfer
surfaces located downstream of the furnace,
such as the superheater, which, as a result,
enjoys longer life. The FEGT is lower due to
the higher furnace effective area for a given
volume of furnace; hence the ratio of net
heat input to furnace divided by the effective
furnace area is smaller (Figure 3) [1].
The refractory-lined front wall reradiates
Figure 3. This chart shows
the effect of heat input
divided by effective surface
area versus FEGT [sometimes
known as furnace outlet temperature
(FOT)]
1400
1350
1300
1250
1200
1150
1100
1050
1000
150
40
200
250
300
250
400
450
Neat heat release rate, kW/m2
500
550
600
energy back to the flame, increasing its local
temperatures and thus increasing the nOx
levels. A furnace with water-cooled front wall,
side walls and floor, as shown in Figure 2 (left),
offers a benign enclosure to the flame and
lowers the nOx emissions, besides lowering
the heat flux and area heat-release rate.
In order to meet nOx and CO emissions
standards of today, steam generators widely
use fluegas recirculation (FGr) ranging from
10 to 25% depending on the combustion
temperature of the fuel and nOx emission
limits; excess air ranges from 12 to 18% to
limit the CO emissions in natural-gas- and
oil-fired boilers. In a completely water-cooled
furnace that offers a cooler enclosure all
around the flame, the nOx emissions are
lower compared to a refractory-lined boiler.
refractory used in the front wall and areas
close to the floor and side walls reradiates
energy back to the flame, increasing nOx
formation. Hence lower FGr is required to
limit the nOx in a fully water-cooled furnace
compared to the refractory-lined boiler.
Higher excess air and FGr simply inGas
Oil
crease
the fluegas mass flow through the
boiler for the same steam parameters, thus
increasing the fan power consumption. remember
that the size of a boiler depends on
the fluegas quantity handled by the boiler
heating surfaces and not the steam parameters.
Custom-designed boiler manufacturers
consider this fact and redesign their
heating surfaces (by changing tube spacing,
increasing the size of economizer) and
ensure high efficiency and low fluegas pressure
drop through the unit. unfortunately, a
few boiler suppliers still offer standard, offthe-shelf
designs selected based on steam
parameters alone, which will have lower efficiency
or higher fan power consumption if
FGr has to be used.
Convective superheater design versus
radiant superheater. Superheaters of boilers
designed decades ago were located at
the furnace exit, irrespective of steam temperature
required; the superheater was in
the radiant zone and hence exposed to high
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Chemical Engineering February 2016

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