United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-84-003 Jan. 1984
v>EPA Project Summary
Long-Term Optimum
Performance/Corrosion Tests of
Combustion Modifications for
Utility Boilers. Host Site:
Louisville Gas and Electric
Company, Mill Creek No. 3
P. S. Natanson
Corrosion in utility boilers, as
possibly affected by combustion
modifications (CM) for decreased NOx
emissions, was studied at large boilers
burning high sulfur coal. Initially, each
boiler was characterized to determine
the short term effects of various
combustion modifications on boiler
operation and emissions. Later, a Level
1 environmental assessment (EA) of
boiler operation was performed, as well
as tests to measure corrosion rates in
the furnace. Also, two 30-day continu-
ous emission monitoring (CEM) tests
were performed. This report discusses
the work performed on Boiler No. 3 at
Louisville Gas and Electric (LG&E) Co.'s
Mill Creek generating station in
Louisville, KY. During the short-term
characterization tests, full load NOx
emissions (as equivalent NO2) were
reduced (by CM) by approximately 20%
from about 235 ng/J without adverse
side effects. NOx emissions during the
30-day monitoring tests were log-
normally distributed with a mean of
202-220 ng/J and a geometric disper-
sion of 1.02-1.07. The Level 1 EA
revealed no unusual environmental
hazards resulting from low-NOx
operation. For the nearly 2-year study.
waterwall corrosion rates (measured
ultrasonically) were about 2 mils/yr.*
This Project Summary was developed
by EPA's Industrial Environmental Re-
search Laboratory, Research Triangle
Park, NC. to announce key findings of
the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back),
Introduction
For coal-fired utility boilers, NOx
emission regulations can often be met by
using combustion modification tech-
niques such as decreased total excess
air flow. However, this could lead to a
chemically reducing atmosphere in the
furnace and an increased potential for
corrosion. Under this program, methods
of decreasing NOx emissions were
studied (characterization testing) from
environmental and corrosion points of
view. To this end, the program included
three parts: boiler characterization. Level
1 environmental assessment (EA), and
corrosion testing. Several large' utility
boilers were studied during this long-
'Readers more familiar with the metric system may
use the conversion factors at the back of this
Summary.
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term performance/corrosion study. This
report deals with the No. 3 boiler at
Louisville Gas and Electric Company's
Mill Creek generating station.
Background
NOx has long been considered an
undesirable component of the earth's
atmosphere. In parts per million
concentrations, NOx is considered a
precursor to acid precipitation,
photochemical smog, and irritation in
human respiratory systems. Since the
early 1970's, after coal-fired utility
boilers were identified as a major NOx
source, EPA has supported research for
NOx control at these large boilers.
Under the sponsorship of the U.S. EPA,
Exxon Research and Engineering
Company (ER&E) studied the effects of
boiler operating conditions on NOx emis-
sions. These and similar studies led to
improved operating procedures for NOx
reduction. The new procedures (known
as combustion modifications [CM])
included decreased excess air flow and
staged combustion. However, these
modifications could increase the
chemically reducing nature of the gases
in certain regions of the furnace,
resulting in an increased corrosion rate
on the fireside of the boiler's waterwall
tubes, especially in boilers fueled by high-
sulfur coal.
The program reported here was a
follow-on to this earlier work. Under this
program, furnace wall corrosion was
studied at several coal-fired utility boilers
designed to use CM for reducing NOx
emissions. Some of the combustion
modifications included decreased excess
air flow, and use of special low-NOx
burners to meet the applicable new
source performancestandards(NSPS)for
NOx emissions (0.7 lb/106 Btu, as NO2).
Work Plan
The test program included:
• Boiler Characterization
• Corrosion Testing by
—Probes
—Panels
—Wall measurements
• Level 1 Environmental Assessment
• 30-day Continuous NOx Monitoring
During boiler characterization, effects
of various boiler combustion controls
were assessed and their ability to reduce
NOx emissions without causing short-
term adverse side effects (such as
increased slagging) were evaluated.
During this time, flue gas emissions
and fuel and ash composition were
monitored periodically. Also, furnace gas
composition was monitored at various
locations along the furnace walls
(furnace gas tap sampling) to identify
locally corrosive environments for further
study.
Corrosion testing was initiated after
boiler characterization. Three methods
(corrosion probes, corrosion panels, and
wall thickness mapping) were used to
evaluate corrosion.
Corrosion probes were used to
evaluate short-term (30- to 1000-hour)
corrosion effects. In this method, pieces
of wall tube type materials (called
corrosion coupons or test rings) were
inserted at several different locations in
the furnace for various times and then
weighed to determine the rate of metal
loss. This qualitative indirect method did
not give a true reading of actual wall loss,
but was used as an economically
attractive alternative to other methods
which require entering the furnace
during an outage to measure the tubes
directly, as discussed below.
In the corrosion panel method,
probably the most reliable, several
sections of the boiler wall were removed,
and replaced by new sections (called
corrosion panels) on which the tube wall
thickness had been carefully measured.
About 2 years later, the furnace was
reentered (during an outage) and the
panels were remeasured. Wall thickness
was measured ultrasonically, a nonde-
structive test in which high frequency
sound waves are bounced off the inner
wall of the tube and the thickness is
determined from the echo time delay.
The third method used to measure
corrosion was the wall thickness
mapping method, involving ultrasonic
thickness (UT) measurements of all walls
throughout the furnace. The first set of
such wall measurements occurred
during the outage when the corrosion
panels were installed, and the second set
was about 2 years later (coincidental with
the final panel measurements).
During these long-term corrosion tests,
an EPA Level 1 EA test was performed: it
included physical, chemical, and biologi-
cal (toxicity) analysis of all major streams
entering and leaving the boiler. The
Source Assessment Sampling System
(SASS) train was used to sample the flue
gas stream, while EPA Method 5 was
used to measure particulate removal
efficiency of the electrostatic precipitator
(ESP). Other streams sampled included
sluice water, coal, and ESP hopper-ash.
Twice, during the 2-year corrosion
exposure period, NOx and other flue
gases were monitored continuously for
30 days using a continuous emission
monitoring (CEM) system. The 30-day
CEM tests quantified flue gas emissions
over normal boiler load cycles and for a
longer period than was possible during
the characterization tests. These tests
also indicated how a CEM may be
expected to perform (reliability, accuracy,
maintenance needs, etc.) in similar
situations.
Boiler Description
Mill Creek No. 3 is the third boiler
constructed at Louisville Gas and Electric
(LG&E) Company's Mill Creek generating
station in Louisville, KY. Designed by
Babcock and Wilcox Company (B&W), it
burns about 200 tons/hr of high-sulfur (3
to 4 wt% S) coal to generate more than
400 MW of electric power. Its 40 wall-
fired burners are horizontally opposed in
a single-compartment, balanced-draft,
waterwall boiler which produces about
3x106 Ib/hr of superheated steam
(1000°F, 2800psi). Itwasdesignedtouse
CMs (such as flue-gas recirculation and
dual-register low-NOx burners) to meet
the New Source Performance Standards
(NSPS)for NOxemissions(0.7 lb/106 Btu,
as N02), and is one of the first boilers to be
governed by this NSPS regulation.
Boiler Characterization
Under Phase I of the long-term per-
formance/corrosion study (involving
several large utility boilers), Exxon
Research and Engineering Company
(ER&E) evaluated CMs for decreased NOx
emissions. This report describes test
work conducted at LG&E's Mill Creek
Generating Station, Boiler No. 3. The
following combustion modifications were
among those selected for detailed study:
• Decreased total excess air flow
• Biased firing (bottom burners fuel-
rich)
• Flue-gas recirculation
The operator's freedom to vary certain
control parameters (such as excess air)
was tightly constrained by programmed
limits in the boiler's control system. Even
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so, the limited changes that could be
made resulted in a 10-20% decrease in
NOx emissions (as N02) from the as-found
case of about 0.55 lb/106Btu (400 ppm at
3% O2, dry), without adverse short-
term side effects. (The applicable NSPS
maximum allowable emission rate is 0.7
lb/106 Btu.) By increasing the range of
control on the excess oxygen, and with
additional instrumentation (e.g., a CO
monitor), NOx emissions might be further
reduced.
During the initial short-term studies,
furnace gas sampled from along the
furnace walls confirmed that corrosion
was most likely to occur (low O2, high CO)
in the chemically reducing environment
around the burner zone. Longer term
effects (including corrosion), studied in
detail as the program continued, are
described below.
Level 1 Environmental
Assessment (EA) Testing
The Level 1 EA test performed at Mill
Creek No. 3 involved the major inlet and
effluent streams crossing the plant's
process boundaries. The streams
sampled included the inlet and outlet of
the ESP, the ESP hopper catch, the fuel
feed, and the sluice water flushing the
furnace hopper.
This test was performed as a screening
study to identify potential pollutants in
the various streams and included three
types of analyses. Chemical analysis
determined the chemical composition of
the various streams. Physical analysis on
particles entrained in various streams
provided information on shape and size.
Finally, biological analysis provided
information on the mutagenic and/or
toxic effects of the various streams on
living matter.
The test showed that, as expected,
most of the flue gases leaving the
combustor and going to cleanup devices
are fixed gases (02, C02, N2, etc.(.Other
components of this stream include sulfur
compounds, hydrocarbons, and
entrained particulate matter.
Measurements taken at the ESP showed
that the particulate matter (fly ash
entering the ESP) represents most of the
total ash leaving the furnace (the rest is
slag or bottom ash and exits the furnace
by another stream). More than 98% of the
fly ash (mostly the larger particles) is
removed by cleanup devices before the
flue stream is discharged through the
stack. The particulate matter and
entraining flue gases contained low
levels of organics.
Fly ash particles sampled from the ESP
hopper were less spherical than the
sample collected at the ESP inlet, and had
a bimodal size distribution with peaks at
about 3-4 fjm and at 20 fjm. The composi-
tion was mostly silicates.
The various streams were also
analyzed for metals. Table 1 presents the
metals data for the solid and liquid
samples, with EPA priority metals for
wastewater shown first.
The effluent sluice water samples
contained chlorides and sulfates, as did
the coal and ash extracts.
The coal extract was separated into
seven fractions, but the spectra of each
fraction were generally too complex to be
analyzed by EPA Level 1 procedures.
Instead, the major peaks (listed in the full
report) show that the extract was highly
aromatic, highly oxygenated material
containing phenolic material and
carboxylic acids.
Biological tests (using living matter)
were used to assess the environmental
effect of process effluents on life forms.
The results, summarized on Table 2,
show that the coal, ESP hopper ash, and
sluice water are not mutagenic. The Rat
Alveolar Macrophage (RAM) Assays
were also negative. The Chinese
Hamster Ovary (CHO) Cell Assay yielded
toxic responses with coal leachates and
with sluice water. While moderate to high
Table 1. Trace Metal Concentrations in Various Louisville Process Streams: Level 1 Test at
Louisville Gas and Electric Company's Mill Creek No. 3 Unit
(micrograms of metal per gram of sample)
Element
Ag
As
Be
Cd
Cr
Cu
Hg
Ni
Pb
Sb
Se
Tl
Zn
Al
Ba
Ca
Co
Fe
K
Li
Mg
Mn
Mo
Na
P
Si
Sn
Sr
Ti
V
Flue Gas
10 and 3 ftm
Fly Ash
Panicles
<4.2
67*
(2.7)*
57.
140.
110.
0.058
240.
(47.)*
(69.)*
<17.
(22.)*
820. .
98000.
540.
30000.
<3.4
1 10000.
24000.
79.
5500.
330.
1OO.*
4100.
900.
210000.
<13.
200.
4800.
410.
Stream
1 fjm
Fly Ash
Particles
<5.0
350t
20.*
110.
260.
190.
0.0021
330.
190.
(72J*
280*
87*
3100.
120000.
800.
26000.
40. t
79000.
26000.
100.
7900.
430.
320.*
4600.
4200.
120000.
(38.1*
300.
6200.
1200.
Coal
<6.
<25.
(3.2)*
<11.
<1.8
9.7
0.0020
<15.
<19.
(58)*
<25.
<11.
62.
14000.
58.
5400.
(9.2)
1 1000.
3200.
6.8
770.
45.
<9.8
800.
<54.
66000.
<19.
20.
330.
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Table 2. Louisville Gas and Electric Co., Mill Creek No. 3 Boiler Bio Assay Results (Summary)
Bottom Ash/
Water Slurry
Ames Mutagenicity (Salmonella/ Microsomal
Mutagenesis Assay)
Rabbit Alveolar (Lung) Macrophage
Cytotoxicity Assay (RAM)
Rodent Cell Clonal Toxicity Assay
(Chinese Hamster Ovary - CHOI
In Vivo Oral Rodent (Whole Animal
Acute Toxicity - Rat)
Fresh-Water Fish (Fathead Minnow)
Fresh-Water Daphnia Magna (Water Flea)
Fresh Water Algae
U
._
M
U
U
U
U
ESP Hopper Ash Coal
U
U
__
U
M
H
H
U
U
M
U
U
U
H
Key:
H = High toxicity
M = Moderate toxicity
U = Undetected
- = Not tested
A detailed explanation
may be found in Appendix 1
of the full report.
situatons. A summary of the data is
presented in Table 4.
Although the boiler steam load varied
widely (e.g., from about 23% to 94% of
maximum capacity during the first CEM
test), the NO emission rate was fairly
steady with mean values (Table 4) of 208
and 220 ng/J for the first and second
tests, respectively. The standard
deviation was less than about 10% of the
mean value. The statistical parameters
shown in Table 4 are in good agreement
with the probability plots shown in the full
report.
The continuous monitor system
(utilizing an extractive sampling system)
provided accurate gaseous emissions
data for the test duration and passed all
EPA performance specifications.
However, calibration of the instruments
on a daily basis is necessary, as is
maintenance of the sampling system.
toxicity was also recorded for some of the
aquatic assays, the sluice water caused
no toxicity to any of the aquatic species
studied, but was stimulatory to the growth
of algae.
Corrosion Tests
To more fully evaluate the longer term
side effects of low-NOx operation,
corrosion testing began at the conclusion
of the characterization period. As dis-
cussed earlier, corrosion rate was
measured by three methods:
• Corrosion probes
• Corrosion panels
• Wall thickness mapping
For the corrosion probes, the rate of
metal loss decreased (from more than 30
to less than 10 mils/yr) as the exposure
time increased, approaching the
expected (and actually measured) loss
rate for the furnace walls. The corrosion
panels had an average loss rate of 1.3
mils/yr which compares well with the
average wall loss rate of 2.3 mils/yr
(Table 3).
No correlation was found (in any of the
methods) between corrosion rate and
location in the furnace. On the average,
the rate of corrosion within this boiler
was such that it would require more than
30 years for the tubes to lose half their
thickness. Therefore, corrosion rates,
while operating in compliance with NSPS
NOx regulations, appear to be acceptable.
Thirty-Day CEM Tests
Twice during the long-term operating
period, flue gas emissions were
measured for 30-day periods using
CEMs. The data proved useful for
evaluating boiler emissions over normal-
load cycles for longer periods than during
the earlier characterization tests. These
CEM tests also helped to evaluate the
abilities, potential problems, and
performance limitations that might be
associated with CEMs in similar
Conclusions
Combustion modification on coal-fired
utility boilers is an effective method of
reducing NOx emissions without adverse
side effects. On the average, for the
Louisville boiler, tubewall corrosion rates
resulting from low NOx operation will not
decrease or limit the useful life of the
boiler. The Level 1 test confirms that no
unusual environmental hazards result
from low-NOx operation.
The 30-day CEM tests show that
through a good maintenance and
calibration program, CEMs can be made
Table 3. Louisville Gas and Electric Co., Mill Creek No. 3 Boiler — Summary of Metal Loss Data
for Furnace Walls (On-Line Exposure Time Between Measurements = 15,000 Hours)
Tube Wall Thickness Loss (mils)
Elevation
F
(Near Nose)
5
D
C
B
Left
Wall
+5.0
+3.1
+2.0
+4.4
+ 1.6
Right
Wall
+3.1
+2.5
+3.1
+6.7
+5.0
Front
Wall
+4.3
+5.2
+6.5
+3.6
+5.4
Rear
Wall
+3.7
+3.0
+5.9
+ 1.1
+2.1
Average
(mils) (mils/year)
+4.0
+3.4
+4.4
+3.7
+3.5
+2.3
+2.0
+2.6
+2.2
+2.0
A
(In Hopper)
Average
+6.3
+3.7
-1.7*
+3.1
+ 10.1
+5.9
+ 1.1
+2.7
+4.8
+3.9
+2.8
+2.3
* Negative values (thickness gains) have sometimes been seen in data sets that are too small to
average out random errors. A more detailed explanation may be found in the full report.
4
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Table 4. Louisville Gas and Electric Co.. Mill Creek No. 3 Boiler - Normal Distribution
Statistical Parameters
Mean
Standard
Deviation
Maximum
Minimum
Range
1st Test
2nd Test
1st Test
2nd Test
1st Test
2nd Test
1st Test
2nd Test
1st Test
2nd Test
Load
(MWthJ
478.3
557.7
182.6
129.7
773.4
718.1
191.3
207.3
582.1
510.8
02
(%)
7.3
6.1
2.4
1.7
11.8
12.0
4.2
4.2
7.6
7.8
C02
<%)
11.7
11.5
2.5
2.0
15.2
14.0
6.4
4.9
8.8
9.1
CO
ng/J
2.6
15
4.6
9.2
19.0
41
0
0
19.0
41
NO* fas NOi)
(lb/10e BtuJ
0.48
0.51
0.05
0.03
0.58
0.57
0.36
0.43
0.22
0.14
to meet the EPA performance
specifications.
Recommendations
When possible, CMs should be
considered as an economical approach to
NOx control on coal-fired utility boilers.
However, slagging and other possible
side effects should be monitored to
ensure that satisfactory operation
continues.
Because CM for NOx control requires
close observation and tight control on
boiler operations, a well-maintained CEM
for CO should be used as an aid for
decreasing excess air to optimum levels.
Conversion Factors
EPA policy is to use the metric system
in all its documents; however, for
convenience, nonmetric units are used
several times in this Summary. Readers
more familiar with metric units may use
the following conversion factors.
Nonmetric
Btu
°F
Ib
fb/10*Btu
mil
psi
ton
Times
1055.1
5/9 (°F-32>
0.454
427.3
0.0254
6894.8
907.2
Yields
Metric
J
°C
kg
ng/J
mm
Pa
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P. S. Natanson is with Exxon Research and Engineering Co., Florham Park, NJ
07932.
David G. Lachapelle is the EPA Project Officer (see below).
The complete report, entitled "Long-Term Optimum Performance/Corrosion
Tests of Combustion Modifications for Utility Boilers; Host Site: Louisville Gas
and Electric Company, Mill Creek No. 3," (Order No. PB 84-128 966; Cost:
$23.50, 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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
PS 0000329
U.S. GOVERNMENT PRINTING OFFICE: 1964-758-102/842
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