United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-83-054 Feb. 1984
Project Summary
Evaluation of Tubewall Corrosion
Rates on a Coal-Fired Utility
Boiler Using Staged Combustion
for NOX Reduction
P.S. Natanson, E.H. Manny, and A.R. Crawford
Nitrogen oxide (NOX) emissions at a
coal-fired utility boiler have been
decreased by combustion modifications
(CMs) using the boiler's existing hard-
ware for combustion control. The CMs
studied included decreased excess air
and a variety of fuel/air ratios at
selected burner elevations (i.e., biased
combustion). Side effects of these
operating conditions were also studied,
including: changes in boiler efficiency
(but not heat rate), other emissions, and
(for the first time) detailed studies of
boiler wall corrosion rate. Wall thickness
measurements revealed that, averaged
over the total measured area, the boiler
walls lost 2.2 mils (5.2 mils/yr) during a
5-month baseline period: 2.6 mils (6.2
mils/yr) in the nonburner zones com-
pared to 1.7 mils (4.0 mils/yr) in the
burner zone. During a 12-month low-
NOx period, the non-burner elevations
lost only 2.11 mils/yr, while the burner
area lost 5.05 mils/yr; a slightly higher
loss rate in the burner zone than for the
same zone during the baseline period.
Typically, near full load, CMs were used
to achieve NO, reductions of about 20%
without significant side effects. The
CMs included biased burner firing and
decreased boiler excess air. The corro-
sion rates during extended low-NO*
operation would not appear to apprecia-
bly reduce the average expected life of
the furnace tubes.
This Project Summary was developed
by EPA's Industrial Environmental
Research Laboratory. Research Triangle
Park. NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering
information at back).
Introduction
For coal-fired utility boilers, nitrogen
oxide (NOX) emission regulations can
often be met by using combustion modifi-
cation (CM) techniques such as decreased
total excess air. However, this could
result in a chemically reducing atmosphere
within the furnace and increased corrosion
potential. Consequently under EPA
Contract 68-02-1415, Exxon Research
and Engineering Company (ER&E) studied
CMs for decreased NOX emissions at
several boilers and gas turbines. At the
last boiler tested (Crist No. 7), special
attention was given to corrosion. Thus, in
addition to evaluating CM effects on NO«
emissions, the study was expanded to
include the effects of CMs on boiler wall
wastage rates. This report addresses the
study of Boiler No. 7 at Gulf Power
Company's Crist Generating Station in
Pensacola, Florida.
Background
NOx is not only injurious to human
health, but also reacts chemically (with
organic gases and other pollutants in the
presence of sunlight) to produce a brown
"smoggy" haze commonly observed in
densely populated areas. NOX also reacts
in the atmosphere to form nitric acid
(HNOa), and is thus partially responsible
for "acid rain." Hence, N0« has been
classified as a major pollutant, and a
-------
great deal of effort has been aimed at its
abatement. Typically, IMOX is a product of
combustion. It is formed either from
nitrogen in the fuel (fuel-NOx) or from the
high temperature reaction of nitrogen in
the combustion air (thermal-NOx).
Since 1971, EPA regulations have
limited NOX emissions from coal-fired
steam generators. The National Ambient
Air Quality Standards (NAAQS) set
maximum limits on the concentration of
NOx in air, requiring NO* emission
reductions. Further NOX reduction was
required by the New Source Performance
Standards (NSPS) of 1971. In 1978, more
stringent NSPS for NOX emissions from
electric-utility coal-fired steam generators
became effective. CMs (e.g., low excess
air and staged firing) are ways of meeting
regulations requiring decreased NOx
emissions. However, low-NOx operation
by CM brings with it the possibility of
increased slagging and furnace tubewall
corrosion, especially with high-sulfur
coals.
Work Plan
The study at Crist No. 7 was designed to
develop boiler operating guidelines for
low-NOx emissions and then to evaluate
the possibly corrosive effects of such
operation. To this end, the program was
divided into several parts.
In the first part, "Boiler Characteriza-
tion," about 100 short (1-hour) tests were
performed to evaluate effects of CMs on
stack emissions and other immediate
side effects. These tests helped develop
boiler operating guidelines for long-term
low-NOx operation. They also helped to
define baseline conditions and establish
an improved low-NOx operating mode.
Baseline operation was defined as the
boiler's normal operating procedure
before these tests began. The characteri-
zation tests led to an optimized operating
procedure (low excess air, staged combus-
tion, etc.) for decreased NOX emissions.
This became known as the low-NOx
operating mode. For the operating modes
showing the greatest NOX reduction,
samples of combustion gas were with-
drawn from along the furnace walls (by
furnace gas taps) and analyzed for
potentially corrosive local environments.
In the next part of the study, small
pieces of boiler tube material (called
corrosion probes) were inserted into the
furnace through inspection doors to study
local, short-term (30-1000 hours) corro-
sion effects. Also, longer term effects (a
few months to 2 years) were studied by
entering the boiler during scheduled
outages and ultrasonically measuring
tubewall thicknesses at thousands of
points inside the furnace. Additionally,
selected sections of boiler wall were
removed during the outages and replaced
with specially characterized new sections
of wall tubes (called corrosion panels)
which were later removed and analyzed
for corrosion.
Finally, detailed measurements were
made of stack emissions at both baseline
and low-NOx operating modes. The
measurements included trace metals,
organics, and dust loading in the flue
duct.
Boiler Description
Crist No. 7, a pulverized-coal-fired
utility boiler designed by Foster Wheeler
Energy Corporation, uses horizontally
opposed firing in a natural circulation,
radiant, reheat steam generator to
produce steam for generating electric
power. The furnace has 24 burners (12 on
each of the front and rear walls),
arranged in three rows of four on each
wall. Its rated capacity is 1.64 x 106 kg of
superheated steam per hour (3.6 x 106
Ib/hr) at 17.2 MPa (2500 psi) and 81 OK
(1000°F with an electric generating
capacity of 500 MW.
Crist No. 7 is of pre-NSPS design and,
under normal operation, was not designed
to meet the NSPS for NOX emissions.
However, by use of appropriate combus-
tion modifications, NOX emissions were
decreased significantly during the test
period.
Results
Boiler Characterization
In the first part of this program (boiler
characterization) several boiler CMs were
investigated for NOX reduction, including:
• Secondary air registers. (These
were used to change the air distribu-
tion within the combustion zone.)
• Firing pattern. (Fuel distribution was
changed by varying the coal feed
rates to the six burner rows.)
• Load. (NO, emission rates can be
reduced by operating the boiler at
less than full capacity.)
• Total air flow. (Firing with decreased
excess air decreases NOX emissions.)
Secondary air register setting (air
distribution) and firing pattern (fuel
distribution) were combined for maximum
combustion staging. Staged firing was
found to be an effective method of NOX
control. In this CM'technique, fuel and air
distribution were adjusted so that the
bottom of the combustion zone was
operated fuel-rich, while the upper
•burners were supplied with excess air
(fuel-lean). Or, in the extreme case, when
load demand permitted, fuel to the upper
burners was shut off completely, and they
were operated on air only. Figure 1 shows
that decreased fuel in the upper burner
rows results in decreased NOXemissions.
However, in Figure 1, the effect is
somewhat exaggerated because staging
was accompanied by a slight drop in load
and total excess air. (Reductions in load
and excess air are known to decrease NOX
emissions somewhat, and the effect seen
in Figure 1 may not be entirely due to
staging.)
Load reductions had a beneficial effect
on NOX emissions. However, decreased
load was not considered practical for NOX
control because load, usually determined
by demand, is not conveniently under the
operator's control.
Control of the total air flow was
perhaps the most appropriate method
found for controlling NOX emissions. In
this method, NOX'emission rates were
reduced by using the fan-speed and
damper controls to limit the amount of
excess air available in the furnace's
combustion zone. (Excess air is measured
as percent oxygen in the flue gas.)
Trace Metals and Other
Emissions
Evaluating the side effects of Iow-N0x
operation included measurements of
paniculate mass and size distribution in
the flue duct, as well as volatile organics,
trace elements, sulfur oxides, boiler
performance, and furnace tube corrosion.
Details of each may be found in the full
report. However, as an example, some of
the metal data are presented on Table 1,
which shows metal data on stack emis-
sions measured upstream of the precipita-
tor under both baseline and low-NOx
conditions. At the levels of NOX control
practiced in this program, the analysis
indicates that differences between base-
line and low-NOx firing conditions are
minimal and that (from this point of view)
no appreciable effect on boiler operation
would be expected.
Corrosion Studies
Since the influence of NOX controls on
corrosion rates in the boiler was of
special interest in this test program,
several methods were used to assess
corrosion under both baseline and low-
NOx operation, including:
• Furnace gas taps - Analysis of
furnace gases (0? and CO) to identify
chemically reducing atmospheres I
-------
700
I
on
i
600
500
(Note: 1000lb/hr =
454 kg/hrl
Direction
of Increased
..Staging^
1E
4C
4E
Test
No.
IE
4C
40
4E
Fuel to
Top Burners
1000 Ib/hr
150
130
110
90
Load
MW
510
490
456
440
Oz in
Flue Gas
%
2.9
2.7
2.5
2.5
/VO,
ppm
690
657
591
520
0 90 110 130
Coal Flow Rate to Top Burners. 1000 Ib/hr
Figure 1. Staged combustion: N0tvs. degree of staging.
150
(i.e., increased corrosion potential)
near furnace walls.
• Corrosion probes - Exposure of
small samples of boiler tubewall
material inserted into the furnace
through inspection doors.
• Corrosion panels - Installation of
specially characterized sections of
boiler wall which were later removed
for corrosion analysis.
• Ultrasonic mapping - Non-destructive
(echo sounding) measurements of
tubewall thickness during periodic
outages.
As expected, the furnace gas analysis
showed that CO concentrations were
high, where 02 concentrations were low.
Generally, one would expect decreasing
CO concentrations (depletion through
burnout) with increasing elevation (further
downstream) in the boiler. However, in
many of the tests, staged combustion was
used to elongate the flame to achieve
lower flame temperatures. This caused
CO concentrations to remain high (>
1,000 ppm) further downstream in the
flue. Nevertheless, even with the most
extreme staging tested, CO depletion was
always completed upstream of the
boiler's economizer outlet. No other
strong relationship between CO and
height in the boiler was apparent.
The CO/02 ratio within the furnace
was generally higher for Iow-N0x firing
patterns than for baseline conditions,
indicating a greater potential for corrosion
at low-NOx conditions than at baseline.
For "as-measured" furnace gas composition,
the difference between baseline and low-
NOx conditions was most noticeable at
the top burner elevation. At this elevation.
the CO readings (along the wall) were
higher during low-NOx operation than
during baseline tests. Therefore, it was
not surprising that, for this region of the
furnace, corrosion rates were higher
under low-NOx conditions than under
baseline.
Wall thickness measurements (Table 2)
showed that under low-NOx operation,
the burner area experienced more metal
loss than the non-burner zone. Further-
more, the burner zone loss was greater
under low-NOx than under baseline.
However, in the non-burner zone no
explanation was found for the corrosion
rate under baseline being almost triple
the low-NOx rate.
In the test panels, two types of metals
were used so that the corrosion resistance
of each could be evaluated. These
included the normal boiler tube material
as well as a material thought to be more
resistant to corrosion. However, in this
application, the analysis showed an
insignificant difference in their corrosion
rates.
During low-NOx operation, burner-
zone panels corroded faster than non-
burner-zone panels. Also, for the panels
in the burner zone, more corrosion
occurred during low-NOx than during
baseline operation. The opposite was true
for panels in non-burner zones.
Corrosion probe data (Figure 2) show
that the rate of metal loss is fastest when
the probe is first exposed in the furnace,
but slows down as exposure time
Table 1.
Summary of Metal Data on Stack Emissions'
(ug/m3)
Baseline Test
Low NO, Test
Silver
Arsenic
Beryllium
Cadmium
Chromium
Copper
Mercury
Nickel
Lead
Antimony
Selenium
Titanium
Zinc
Barium
Bismuth
Cobalt
Iron
Manganese
Molybdenum
Tellurium
Thallium
Tin
Uranium
Vanadium
Zirconium
First
Analysis
/W?"
151.5
28.1
2.01
4363
13 EPA 249
Priority 7.3
Pollutants 2702
257
39.9
52.4
10,174
1596
3727
<8.3
223
50,471
2011
386
6.2
<791
11.7
<402.549
743
<40,255
Second
Analysis
NR
151.2
28.1
2.01
4349
249
7.3
2696
255
38.2
52.2
10,507
1542
3860
<8.3
226
41.830
1931
386
6.2
791
11.7
<402,549
730
40.255
NR
215
45.7
22.8
1153
369
<19.9C
699
505
49.6
67
17.938
3012
8152
<12.7
264.5
106,216
3465
292
<6
<1477
18.2
<32 1,648
1063
32,163
" Sample is from flue duct just downstream of air heater and just upstream of precipitator.
b Not reported.
0 Below detection limit.
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Table 2. Wall Metal Loss Summary*
Gulf Power Company's Crist No. 7 Boiler
mils (mils/yr)
Baseline
(5 months)
Low-NO*
(12 months)
Non-burner zone
(All walls)
2.6
(6.2)
2.1
(2.1}
Burner zone
(A II walls)
1.7
(4.0)
5.0
(5.0)
Furnace
average
2.2
(5.2)
3.4
13.4)
'Data is from ultrasonic thickness measurements of boiler tubes performed inside the boiler during
annual and regularly scheduled outages.
increases. During very short exposures
(30 hours), burner-zone probes corroded
faster than non-burner-zone probes
under both baseline and low-NOx condi-
tion. But longer term exposures (240-
1000 hours) did not show significant
corrosion rate differences between
burner and non-burner zones, or between
baseline and low-NOx operation.
Conclusion
At Gulf Power Company's Crist No. 7
boiler, in one of the first studies of its kind,
corrosion rates were related to CM
techniques for controlling NOX emissions.
The CMs tested made use of existing
boiler controls (no retrofits) to achieve a
20% decrease in NOX emissions at full
load, and up to about 50% decrease at
lower loads during short-term tests.
Operation for decreased NOX was accom-
plished by staged combustion (with low
excess air), by biasing the secondary air
flow and dedicating the pulverizers
serving the lower burners to full load
operation. Although there were no
significant side effects from decreased
NOx operation, the tube wastage rate in
the burner zone was increased enough
that the boiler manufacturer feels that
staged firing by itself with high-sulfur
coal may be a questionable means of
controlling NO« from pre-NSPS boilers.
Other than NOX, CO was the only
gaseous emission that was seen to be
significantly affected by CMs for NOX
control. However, adverse effects of NOX
controls on CO emissions were overcome
by proper choice of air feed rate. This was
especially easy when a continuous CO
monitor was installed on the boiler.
As just mentioned, staged firing is an
effective method of NOX control, and is
accomplished by adjusting fuel and air
flows so that the bottom burners are
operated fuel-rich, and the top ones fuel-
lean. However, the effect of secondary air
140
120
too
X
42
6
«
80
60
40
20
100 200 300
400 5OO 600
Exposure Time, hr
700
800 9OO 1000
register settings on NOx was weak, and
adjustment of fuel feed was the more
effective method for staging.
Under this program, the CMs tested did
not significantly affect the operation or
performance of the boiler. However, the
NOx reduction methods used (fuel/air
biasing and reduced excess air firing)
require tight control of air feed rates, fuel
distribution, etc., and will thus limit boiler
flexibility. Furthermore, the NOX control
methods applied may not be as applicable
with other coals.
Finally, the boiler's manufacturer feels
that wastage rates on the boiler's side
walls indicate that long-term operation
under the reduced-NOx biased-firing
conditions could potentially result in a
corrosion problem.
Recommendations
For existing boilers, decreased excess
air is one of the most effective methods of
controlling NOx emissions. A CO monitor
would help to establish the lowest safe
level of excess air under most conditions.
Where practical, it is recommended that
such a monitor be installed. For Crist No.
7, the staging effect of the secondary air
register settings was only marginally
effective over the range tested. However,
that range might be extended if the
registers were controlled automatically.
Additional staging could be accomplished
by use of specially installed overfire air
nozzles. However, this is not often viable
as a retrofit. While effective on the boiler
tested, the results obtained in this
program may have limited applicability to
other types of boiler designs. Therefore,
before any general conclusions can be
made, additional data should be gathered
on NSPS boilers from several manufacturers.
Figure 2.
Corrosion probe data (comparison of corrosion rates using probes at Gulf Power Co.,
Crist Station, Boiler No. 7 pulverized-coal firing).
-------
P. S. Natanson, E. H. Manny, and A. R. Crawford are with Exxon Research and
Engineering Co., Florham Park, NJ 07932.
Robert E. Hall is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of Tubewall Corrosion Rates on a Coal-
Fired Utility Boiler Using Staged Combustion for A/0, Reduction," (Order No. PB
84-118 231; Cost: $26.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
-------
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Environmental Protection
Agency
Center for Environmental Research
Information
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