POWER February 2011 - 23

Table 2. Boiler field data summary.
Source: Cleaver-Brooks Engineered
Boiler Division
Performance parameter
Steam flow (tons/h)
Steam pressure (kpa)
Feedwater in (C)
Oxygen in flue gas (%)
NOx (ppm)
Flue gas recirculation (%)
Ambient air temp (C)
Air temp after airheater (±5C)
Exit gas temperature (±5C)
100%
load
160
3,751
110
2.3
46
10
21
66
103
120%
load
192
3,751
110
2.2
43.8
11
22
82
108
culation calculations were performed to
ensure proper sizing and location of downcomer
and riser piping.
Cleaver-Brooks has used this concept in
several waste heat boilers but for the first
time applied it to a packaged steam generator.
Use of properly sized downcomers
along the furnace also ensures minimum
variation in drum water level between the
front and rear of the furnace. Large holdup
times are possible in the drum, which
minimizes drum level fluctuations and
swell/shrink during load transients.
Designing for High Efficiency
The second novel feature was the use of
a closed-loop gylcol recirculation system
to improve boiler efficiency. The typical
economizer exit gas temperature in natural
gas-fired package boilers is about
300F with 230F feedwater temperature.
On this project, the exhaust gas temperature
was reduced to 215F (uniquely lower
than the 230F feedwater temperature)
using a closed-loop glycol heating system
that transfers heat from the exhaust
gas to the incoming combustion air (Figure
7). The glycol heat-recovery system
offers two advantages:
■ It improves boiler efficiency by about
2.3%, as each 40F drop in boiler exit
flue gas temperature increases boiler
efficiency by about 1%.
■ It reduces capital cost and improves
operational reliability of the combustion
air fan by raising the air inlet
temperature to a minimum of 50F even
during the harsh winter ambient conditions
of -40F.
The first finned tube air heater preheats
cold ambient air entering the fan
and maintains it at a minimum temperature
of about 50F after mixing with the
recirculated flue gas. The second air
heater, located after the fan, preheats
the mixture of flue gas recirculated for
NOx
control and air from the fan. This
glycol bypass system maintains the
stack gas temperature at about 215F. If
the ambient air temperature is high, as
it is in summer, then either air heater
may be shut off and the glycol flow bypassed
on the tube side of either air
heater, as very hot combustion air will
increase NOx
production. The as-tested
performance of the plant is summarized
in Table 2.
The boiler exit gas temperature may
also be controlled to less than 215F at any
load by varying the glycol flow through the
scavenger exchanger and through each air
heater. The glycol flow was set to a nominal
value of 70 ton/h at full load. ■
-Contributed by M. Vasudevan (mvasu@
cleaverbrooks.com), vice president, operations,
and V. Ganapathy (v_ganapathy@
yahoo.com) boiler consultant, CleaverBrooks
Engineered Boiler Systems.
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