1M 123 / 4-^ Illinois Minerals 123 2002 George H. Ryan, Governor Department of Natural Resources Brent Manning, Director ILLINOIS STATE GEOLOGICAL SURVEY William W.Shilts, Chief «. GtOV Equal opportunity to participate in programs of the Illinois Department of Natural Resources (IDNR) and those funded by the U.S. Fish and Wildlife Service and other agencies is available to all individuals regardless of race, sex, national origin, disability, age, religion or other non-merit factors. If you believe you have been discriminated against, contact the funding source's civil rights office and/or the Equal Employment Opportunity Officer, IDNR, 524 S. Second, Springfield, Illinois 62701-1787; 217/ 785-0067; TTY217/782-9175. This information may be provided in an alternative format if required. Contact the DNR Clearinghouse at 2 1 7/ 782-7498 for assistance. Disclaimer This report was prepared by Mei-In M. Chou of the Illinois State Geological Survey with support, in part, by grants made possible by Illinois Department of Commerce and Community Affairs through the Office of Coal Development and Marketing and the Illinois Clean Coal Institute. Neither Mei-In M. Chou, the Illinois State Geological Survey, none of its subcontractors, the Illinois Depart- ment of Commerce and Community Affairs, Office of Coal Development and Marketing, Illinois Clean Coal Institute, nor any person acting on behalf of these either (1) makes any warranty of representation, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, apparatus, method, or process disclosed in this report may not infringe privately owned rights; or (2) assumes any liabilities with respect to the use of, or for damages resulting from the use of, any information, apparatus, method, or process disclosed in this report. 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Effects of Coal-bound Chlorine on Furnace-Wall Corrosion I Under Low-N(X Conditions M-I.M. Chou and W.R. Roy Illinois State Geological Survey E.S. Robitz Jr. and S.C. Kung McDermott Technology, Inc. K.K. Ho Illinois Clean Coal Institute Illinois Minerals 123 2002 George H. Ryan, Governor Department of Natural Resources Brent Manning, Director ILLINOIS STATE GEOLOGICAL SURVEY William W.Shilts, Chief 615 E. Peabody Drive Champaign, Illinois 61820-6964 217-333-4747 http://www.isgs.uiuc.edu Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/effectsofcoalbou123chou Contents Abstract 1 Introduction 1 Methods and Materials 2 Test Material and Corrosion Probe 3 Combustion Conditions 4 Post-combustion Test Methods 4 Results and Discussion 4 Conclusions and Recommendations 9 Acknowledgments 9 References 9 Figures 1 Overall schematic of the fireside corrosion testing facility 3 2 Schematic of the stoker-fired furnace in the fireside corrosion testing facility 3 3 Appearance of tube samples before testing 4 4 Appearance of corrosion probe 4 5 Arrangement of chamfered test samples on the probe 4 6 Appearance of test sample after exposure to low-Cl coal combustion 5 7 Appearance of test sample after exposure to high-Cl coal combustion 5 8 The low-Cl coal combustion sample after chemical cleaning 6 9 The high-Cl coal combustion sample after chemical cleaning 6 10 SEM of a polished cross section at the external surface of the low-Cl coal combustion sample 6 1 1 Appearance of scale and deposit on the low-Cl coal combustion sample 7 12 Isolated chloride-rich deposits found at the tube-scale interface of the low-Cl coal combustion sample 7 13 SEM of a polished cross section at the external surface of the high-Cl coal combustion sample 8 14 Appearance of coherent scale and deposit on the high-Cl coal combustion sample 8 Tables 1 Chemical characterization of the two coals used in the prior study and in this study 3 2 Nominal chemical composition of metal alloy T22 3 3 The loss of wall thickness resulting from fireside exposure of alloy samples 5 4 EDS analysis of the chemical composition of the tube metal of a sample from the low-Cl coal test 7 5 EDS analysis of the chemical composition of the tube-scale interface of a sample from the low-Cl coal test 7 6 Analysis of combustion deposits 7 7 EDS analysis of the chemical composition of the coherent scale deposits of a sample from the low-Cl coal test 7 8 EDS analysis of the chemical composition of the tube metal of a sample from the high-Cl coal test 8 9 EDS analysis of the chemical composition of the tube-scale interface of a sample from the high-Cl coal test 9 10 EDS analysis of the chemical composition of the coherent scale deposits of a sample from the high-Cl coal test 9 Abstract Since the late 1960s, United Kingdom (U.K.) studies have reported a corre- lation between the corrosion of utility boilers and the total amount of chlo- rine (CI) in coals. These studies have indicated accelerated corrosion when CI content of coal was 0.3% or greater. The experience in the United States, however, has indicated that the amount of CI in high-Cl coals is not a major cause of corrosion. Neverthe- less, because of the U.K. data, many U.S. boiler manufacturers have rec- ommended a maximum CI level of 0.3% for U.S. coals. This limit, how- ever, decreases the market potential of the high-Cl coals of Illinois. In 1998, a joint study by the Illinois State Geo- logical Survey (ISGS), McDermott Technology, Inc. (MTI), Illinois Clean Coal Institute (ICO), and United States Department of Energy (U.S. DOE) focused on the high-tempera- ture superheater and reheater tube- wall corrosion that occurred in boil- ers under oxidizing conditions. Pilot- scale tests of combustion corrosion were conducted with a high-Cl Illinois coal, a high-Cl U.K. coal, and a low- Cl Illinois coal as the baseline. The results showed no correlation be- tween coal CI content and rate of corrosion, but the rate of corrosion and metal temperature were corre- lated. A joint study by PowerGen in the United Kingdom and the Elec- tric Power Research Institute (EPRI) in the United States also recently concluded that, under oxidizing conditions, increasing the CI con- tent of the coal burned was not likely to increase the tube corrosion rate, and corrosion rates were de- pendent upon metal temperatures only. The purpose of this research was to determine the effect of the CI con- tent of coal on the corrosion of the furnace wall under sub-stoichio- metric (low-NO x ) combustion con- ditions. Two pilot-scale tests of combustion corrosion were con- ducted using high-CI (0.41%) and low-Cl (0.10%) Illinois coals. The corrosion rates were measured us- ing conventional probes made of T22 metal alloy for a duration that would give reliable comparisons. Tests with the high- and low-Cl coals showed no correlation between the coal CI con- tent and the corrosion rate of the test probe samples. The maximum amount of reduction in tube- wall thickness re- sulting from combustion corrosion was the same in the low-Cl and high-Cl coals. In addition, no chloride deposition was observed in the corrosion sample ob- tained from the high-Cl coal test. The results obtained during coal com- bustion under the reducing conditions of this study were consistent with those of the previous study utilizing oxidizing conditions. Under both conditions, cor- rosion rate did not increase with in- creased CI content. The relationship be- tween corrosion rate and metal tem- perature was studied under oxidizing conditions, but the temperature depen- dence of the corrosion rate under re- ducing conditions was not a part of this study. The results of these studies could provide a comprehensive database that may help to convince boiler manufac- turers to relax the limits on the CI con- tent of coals burned in their boilers. Introduction Many U.K. studies have associated ac- celerated fireside corrosion of tubes of utility boilers with the high-Cl con- tent in coal (Bettelheim et al. 1980). Their corrosion data suggested that the corrosion rate of boiler tubes in- creased proportionately with increas- ing CI concentration in coal. Based on the results of those studies, U.S. boiler manufacturers and utilities have con- sidered coals containing more than 0.3% CI to be potentially corrosive. The 0.3% limit was based primarily on engineering studies that extrapolated the U.K. coal data to the probable corrosive behavior of U.S. coal. The 0.3% limit on CI has discouraged the burning of many Illinois Basin coals in utility boilers. A survey jointly conducted by the EPRI and ICCI (Doane et al. 1994) in- dicated that some U.S. utilities have had decades of experience burning high-Cl coals in pulverized coal-fired boilers. Although fireside corrosion problems have been reported, none of them could be directly related to the presence of CI in coal. Further- more, the corrosivity of two Illinois coals, one with a high-Cl content of 0.3 1% CI and the other with a low- Cl content of 0.16%, was deter- mined by Monroe etal. (1994) un- der pilot-scale combustion tests. Their results showed that the corro- sion rate of the high-Cl coal was ac- tually slower than the corrosion rate of the low-Cl coal. The tests used resistance corrosion probes instead of conventional probes, and corrosion rates were based on mea- surements of electrical resistance over the test period. Also, during the tests with the high-Cl coal, an equipment malfunction caused a high-temperature excursion, which increased the relative corrosion rates. Resistance probes are par- ticularly sensitive to short tempera- ture increases and to the tempera- ture gradients along the probes. Temperature gradients in the probes were unavoidable during the tests in both oxidizing and reducing zones. The overall results showed that the high-Cl Illinois coal was less corro- sive than the low-Cl coal under the test conditions. However, because of the un- certainties in the test results created by the high-temperature excursion and the sensitivity of the resistance probes to temperature variations, the results could not be used as the basis for redefining the recommended limits on CI in coal imposed by boiler manufacturers to prevent corrosion. Research teams from the ISGS, MTI, ICCI, Sandia National Laboratories, and U.S. DOE (Chou et al. 1998, 2000) recendy studied the effects of CI on high-tem- perature superheater/ reheater tube- wall corrosion during coal combustion under oxidizing conditions. Pilot-scale combustion corrosion tests were con- ducted with a high-Cl Illinois coal and a high-Cl U.K. coal; a low-Cl Illinois coal was used as the baseline. The corrosion rates were measured using conven- tional probes under the oxidizing linois State Geological Survey linois Minerals 1 23 combustion conditions that would normally be experienced by super- heater tube walls in a conventional boiler. The results showed no evidence of a correlation between coal CI con- tent and the rate of corrosion. Evi- dence suggested that high-Cl Illinois coals, like low-Cl coal, could be suc- cessfully used in utility boilers if other coal components and boiler properties were understood and controlled. These findings were further confirmed by a joint study by PowerGen in the U.K. and the EPRI in the United States (Davis et al. 2000) . They concluded that, under oxidizing conditions, in- creasing the CI content of the coal burned was not expected to increase the tube corrosion rate, and corrosion rates were dependent only upon boiler tube metal temperatures. A literature review (James and Pinder 1997) correlated the high corrosion wastage observed on the furnace wall of U.K. boilers with the high CI in U.K. coal. However, the review also sug- gested that factors other than CI should be considered as causes of ac- celerated corrosion and suggested that the accelerated corrosion took place primarily on the furnace wall where sub-stoichiometric combustion condi- tions and high heat flux coexisted. The presence of reducing gases implies that insufficient oxygen was supplied to the combustion zone by improper mixing of air and coal. Also, high heat flux results in a high metal temperature on the furnace wall. Under conditions of insufficient oxygen, the sulfur in coal is primarily converted to hydrogen sul- fide (H,S) instead of sulfur oxide (SO x ). The H 2 S gas is very corrosive and readily sulfidizes conventional furnace- wall alloys. The sulfidization of the fur- nace-wall alloys is further escalated by high metal temperature. Therefore, when a large H,S concentration and a high metal temperature coexist, accel- erated corrosion wastage on the fur- nace wall is expected. A combination of these conditions could account for the majority of the corrosion wastage experienced on the furnace walls of U.K. boilers. Contrary to the general consensus of the U.K. researchers, laboratory stud- ies by MTI (Kung et al. 1994, 1996) sug- gested that burning high-Cl coal would not cause additional corrosion on the furnace wall. In the MTI studies, HC1 was added to the combustion gases at a level equivalent to burning high-Cl coal under sub-stoichiometric com- bustion conditions. The presence of HC1 in the low-NO x combustion gas may retard sulfide attack. These results were consistent with those reported by U.K. researchers (Latham et al. 1991) for combustion under reducing condi- tions with the presence of H,S. Up to 800 ppm of HC1 (corresponding to 1.0% CI in coal) had no effect on the corrosion of steel tubes at 400°C (752°F) or 500°C (932°F) (see also Doaneetal. 1994). These findings are of potential importance to the Illinois coal industry. However, these corro- sion data from simulated laboratory studies were insufficient to allow us to evaluate the potential beneficial effects from the CI in coal on low-NO x burner applications. This study focused on pilot-scale tests conducted with conventional probes to measure corrosion rates under the re- ducing/ sulfidizing conditions that would be experienced by a water wall in a low-NO x boiler. The results we ob- tained should help clarify the signifi- cance of CI on furnace- wall corrosion when coal is burned sub-stoichio- metrically. Our study was designed to accomplish these specific tasks: • Acquire two Illinois coal samples, one containing a high-Cl content (0.4% to 0.6%) and one containing low-Cl content (<0.20%), and pro cess and distribute the samples for char- acterization and combustion tests. • Characterize the properties of the coal samples used, including the occurrence of CI, sulfur, and alkali metals and their roles, if any, that could affect the chemistry and mechanism of furnace- wall corro- sion during combustion under sub- stoichiometric conditions. • Conduct two stoker-boiler burner- rig corrosion tests at MTI and collect samples for metallurgical composi- tion and the rate of corrosion exami- nations. • Perform metallographic examination of boiler scale and deposits and mea- sure rates of corrosion from speci- men cross sections. • Interpret the sampling and analytical results and compare the rates of corrosion of the low-Cl coal with those of the high-Cl coal. Methods and Materials This 2-year project focused on deter- mining the effect of CI in coal on furnace-wall corrosion under sub- stoichiometric combustion conditions. During the first year, a stoker boiler system (figs. 1 and 2) at MTI was spe- cifically modified for conducting cor- rosion-rate studies that could be re- lated to sub-stoichiometric (low-NO ) conditions. The first pilot-scale com- bustion corrosion test was conducted on the low-Cl coal, and the second test was conducted on the high-Cl coal. In our previous investigation (Chou et al. 1998, 2000), pilot-scale tests were conducted under oxidizing conditions to determine the effect of CI in coal on superheater/ reheater tube-wall corro- sion. The coal samples tested included one low-Cl and one high-Cl Illinois coal. For the current investigation, coals from the same two Illinois mines were used as the low-Cl and high-Cl Illinois coals. Before the tests, the two coal samples were ground and sieved into stoker- boiler grade (0.25 to 1.0-inch diam- eter) . Each coal sample was first screened to remove the >1 inch (>2.5 cm) material for further size reduction in a hammer mill. The size-reduced material then was screened and repro- cessed, if necessary, until all of the sample was < 1 inch in diameter. It was then remixed with the original <1 -inch material. The processed sample (roughly 20 tons) was shipped by truck to MTI, Alliance Research Center, Alli- ance, Ohio. A 40-lb ( 18.2-kg) split of the coal sample was ground to -200 mesh (74 ^m) for coal characterization at the ISGS. Chemical characterization tests of the coal samples were con- ducted using ASTM methods (ASTM 2000); total CI, total sulfur, ash, and metal oxide were among the sub- stances analyzed (table 1). linois Minerals 1 23 linois State Geological Survey solid fuel hopper Table 1 Chemical characterization ot the two coals used in the prior study and in this study (expressed as weight percentage moisture-free dry coal basis). boiler tube corrosion probe auxiliary burner solid fuel burner refractory ash hopper Low CI Low CI 1 High CI High CI 1 Total CI 0.14 0.13 0.44 0.43 Total S 4.48 3.89 1.22 1.13 Ash 9.38 9.16 7.90 6.89 Si0 2 4.17 4.27 3.88 3.60 AIA 1.63 1.60 1.85 1.63 Fe 2 3 1.75 1.85 1.00 0.84 CaO 0.48 0.43 0.24 0.19 MgO 0.07 0.08 0.10 0.08 Na 2 0.13 0.12 0.11 0.09 K 2 0.17 0.19 0.18 0.17 PA 0.02 0.02 0.02 0.02 Ti0 2 0.09 0.09 0.10 0.09 BaO <0.01 <0.01 <0.01 <0.01 SrO <0.01 <0.01 <0.01 <0.01 S0 3 0.62 0.28 0.18 0.13 screw feeder Figure 1 Overall schematic of the fireside corrosion testing facility. Data obtained for samples in this study. Test Material and Corrosion Probe The T22 metal alloy was used for the corrosion tests. The nominal composi- tion of the T22 metal alloy is given in table 2. This low-alloy steel is MTI's standard material for construction of furnace walls in utility boilers. The cor- rosion behavior of T22 also resembles that of other low-alloy steels, such as SA213-T11 and SA213-T2, making it ideal for the corrosion study. A special corrosion probe (figs. 3, 4, and 5) was designed for this study. The water-wall alloy T22 was assembled to form tube segments with 1 -inch o.d. x 1-inch length. The end of each segment was shaped to contain extruded and indented chamfers at 10° and 45°, respectively. The chamfers allowed multiple segments of samples to be connected to the probe without leak- age of the cooling air. Table 2 Nominal chemical composi- tion of metal alloy T22. Constituent Max wt% Min wt% 0.05 0.30 ND 1 ND ND 1.90 0.87 c 0.15 Mn 0.60 P 0.02 S 0.01 Si 0.50 Cr 2.60 Mo 1.13 'ND, not detected. overfire combustion air boiler tube corrosion probe natural gas burner Mechanical pressure was applied from both ends of the probe so that a tight fit could be achieved during the sample segment installation. Type K chordal thermocouples were attached to one of the tube segments from inside the probe. The thermocouple junctions were embedded in the tube wall in a hole drilled in the sample's inner surface and Figure 2 Schematic of the stoker-fired furnace in the secured by silver soldering. fireside corrosion testing facility. boiler tube corrosion probe upper furnace reduction zone lower furnace reduction zone - underfed grate stoker linois State Geological Survey linois Minerals 1 23 Combustion Conditions Room-temperature compressed air was supplied to the corrosion probe to maintain the metal temperature. The airflow rate was regulated electroni- cally by a controller that responded to signals from the probe thermocouples. To reproduce the conditions for fur- nace-wall corrosion, the metal tem- perature was maintained at 850°F (454 °C), which is typical for a pulver- ized- coal-fired supercritical boiler with a steam temperature of 1005°F (541 °C) at the steam outlet. Post-Combustion Test Methods Prior to the corrosion probe testing, the initial outer diameter of each tube sample was carefully measured. Fol- lowing 800 hours of exposure in the fireside corrosion testing facility, the T22 corrosion samples were chemically cleaned. The chemical cleaning re- moved the coal ash that had been de- posited on the sample surface as well as the corrosion scale formed during the fireside exposure. After the clean- ing was completed, the final outer diameter was again measured. Metallographic examinations were performed on the test samples after corrosion exposure. Each sample was cross sectioned, mounted, and pol- ished using standard metallographic procedures. The morphology of the ash deposit and scale were examined using an optical microscope and a scanning electronic microscope (SEM) equipped with energy dispersive x-ray spectroscopy (EDS). Elemental map- ping was performed on the sample with SEM-EDS when necessary to reveal the distribution of various corrosion species. A sample of the coal ash deposit formed on the air-cooled corrosion probe was analyzed to determine the major constituents that might affect the corrosion mechanism in the coal ash layer adjacent to the alloy surface. The results of this analysis were helpful in identifying the amount of CI in- volved in water-wall corrosion under conditions of sub-stoichiometric com- bustion. Figure 3 Appearance of tube samples before testing. air-cooled probe extension air cooled probe extension TC output Figure 4 Appearance of corrosion probe. Results and Discussion Table 1 lists the results of analyses for total CI, total sulfur, and ash contents as well as for metals contents, reported in oxide form, obtained for the coal samples during the prior study and for this study. The samples used in this study were very similar to those used in the previous study. Notably, the low- Cl coal contained a much higher sulfur content than did the high-Cl coal. Data from the x-ray absorption near edge spectrum (XANES) analysis indi- cated that the CI in these coals occurred in an ionic form. The effects of tem- perature and atmosphere on the forms of CI in coals were followed by XANES analysis, and the results obtained were reported elsewhere (Chou et al. 1995). During the first test on the low-Cl coal, an effort was made to recondition the stoker boiler by removing the excess slag that accumulated. The time and cost spent on this extra effort were compensated by our finishing the test in 806 hours instead of the originally planned 1,000 hours. According to MTI, the 800-hour test was sufficient to complete the project goals. The second combustion corrosion test was also conducted for a duration of 800 hours. The corrosion samples obtained from both tests were analyzed, and the re- sults were compared. J g g 3? Figure 5 Arrangement of chamfered test samples on the probe. linois Minerals 1 23 linois State Geological Survey Figure 6 Appearance of test sample after exposure to low-CI coal combustion. Figure 7 Appearance of test sample after exposure to high-CI coal combustion. Photographs of the corrosion samples obtained from the first test of the low- CI coal and the second test of the high- CI coal are shown in figures 6 and 7. The corrosion sample from the test fir- ing with the low-CI coal (fig. 6) had a much thicker surface deposit than did the sample (fig. 7) obtained from the test firing with the high-CI coal. One sample from each of the tests was chemically cleaned to remove scale be- fore the wall thickness was remeasured. Photographs of these two chemically cleaned samples are shown in figures 8 and 9. The wall thickness measurements before and after the combustion tests are compared in table 3. For both tests, the thickness of metal tube wall lost on the hot-side centerline was 1 mil, and the maximum wall thickness lost was 3 mils, which would extrapolate to 33 mils per year of con- tinuous running. The average wall thickness lost for the sample from the low-CI coal test was 1 .88 mils, which would extrapolate to an average of 20 mils per year. The average thickness lost for the sample from the high-CI coal test was 1.13 mils, which would extrapolate to 12 mils per year. The data indicated that the maximum cor- rosion rate of the T22 samples was the same whether burning the low-CI or high-CI coal. However, the average corrosion rate obtained from burning the low-CI coal was slightly greater than that for the high-CI coal. Examination of polished cross sections of the sample from the test with the low-CI coal (figs. 10 and 1 1) showed that the sample had a coherent scale at the interface with the underlying metal. The average thickness of this scale was approximately 2.5 mils. Coal ash and other combustion deposits were found attached to, and above, the coherent scale. The EDS was used to provide a semi- quantitative analysis of the metal com- position. The tube metal composition of the sample from the low-CI coal test was examined. The data obtained at location 1, shown in figure 1 1, are given in table 4. The composition of the deposit at the interface between the scale and the underlying metal was also determined (table 5). At that loca- tion, the sulfur content was 5.36 wt%, and no chloride deposition was ob- served. An additional investigation was undertaken to determine whether chloride deposition might be found elsewhere at the interface. Some small, isolated pockets containing chloride- rich deposits were found, and one of the pockets (fig. 12) showed a CI con- tent of about 0.73 wt% (table 6). Additional analyses were performed to evaluate the remainder of the deposit. The composition of the coherent scale (table 7) showed a sulfur content of Table 3 The loss of wall thickness resulting from fireside exposure of alloy samples. Wall thickness (inches) at o'clock position 12:00 1:30 3:00 4:30 6:00 7:30 9:00 10:30 Low-CI coal: 806 h r, 850°F Initial 0.121 0.121 0.121 0.121 0.122 0.122 0.122 0.122 Final 0.120 0.119 0.118 0.120 0.120 0.120 0.120 0.120 Change 0.001 0.002 0.003 0.001 0.002 0.002 0.002 0.002 High-CI coal: 800 r ir, 850° F Initial 0.119 0.118 0.118 0.118 0.119 0.120 0.120 0.120 Final 0.119 0.119 0.119 0.118 0.118 0.117 0.117 0.118 Change 0.000 0.000' o.ooo 1 0.000 0.001 0.003 0.003 0.002 Final measurement exceeding initial measurement may have resulted from residual scale, small axial thickness variations, and/or rounding effects. In any case, these cir- cumstances were counted as zero thickness loss. In the low-CI coal test, the 12 o'clock position was at the hot-side centerline, whereas in the high-CI test, the 6 o'clock position was the hot-side centerline. Illinois State Geological Survey Illinois Minerals 1 23 12 o'clock view (fireside center-line) 3 o'clock view 6 o'clock view 9 o'clock view mill L-..a«l U.aalf Figure 8 The low-CI coal combustion sample after chemical cleaning. 12 o'clock view 3 'clock view 6 o'clock view (fireside center-line) 9 o'clock view ..iiIUIhiIII Figure 9 The high-CI coal combustion sample after chemical cleaning. F22-3 sample 50 x -500 urn 1 100x -500 fitn- Figure 10 SEM of a polished cross section at the external surface of the low-CI coal combustion sample. linois Minerals 1 23 linois State Geological Survey F22-3 sample showing EDS locations 1-7 Wt$£@M Table 4 EDS analysis of the chemical composition of the tube metal (at location 1 , shown in fig. 1 1 ) of a sample from the low-CI coal test. Element Atom % Weight % Error (±) Norm % Si 1.54 0.78 0.10 0.78 Cr 2.31 2.16 0.13 2.16 Mn 0.72 0.72 0.11 0.72 Fe 94.74 95.15 0.66 95.16 Mo 0.69 1.19 0.20 1.19 Total 100.00 100.00 100.00 250x (backscattered image) ■100 /m- 250 x Figure 11 Appearance of scale and deposit on the low-CI coal combustion sample. Table 5 EDS analysis of the chemical composition of the tube-scale interface (at location 2, shown in fig. 11' of a sample from the low-CI coal test. Element Atom % Weight % Error (±) Norm % O 78.34 53.80 0.73 53.80 Al 0.23 0.26 0.02 0.26 Si 0.86 1.04 0.09 1.04 S 3.90 5.36 0.11 5.36 Cr 2.51 5.60 0.16 5.60 Fe 14.16 33.93 0.41 33.93 Total 100.00 100.00 100.00 Table 6 Analysis of combustion deposits (pocket loca- tion indicated by arrow in fig. 12). Element Atom % Weight % Error (±) Norm % 74.67 48.11 0.69 48.11 Al 0.26 0.28 0.02 0.28 Si 0.82 0.93 0.09 0.93 S 3.41 4.40 0.10 4.40 CI 0.51 0.73 0.06 0.73 Cr 1.09 2.27 0.12 2.27 Fe 19.25 43.27 0.45 43.27 Total 100.00 100.00 100.00 Table 7 EDS analysis of the chemical composition of the coherent scale deposits (at location 3, shown in fig. 1 1 ) of a sample from the low-CI coal test. Element Atom % Weight % Error (±) Norm % O 75.11 50.79 0.67 50.79 Al 0.22 0.26 0.02 0.26 Si 0.66 0.78 0.09 0.78 S 8.27 11.20 0.13 11.20 Cr 1.21 2.66 0.11 2.66 Fe 14.53 34.31 0.39 34.31 Total 100.00 100.00 100.00 Figure 12 Isolated chloride-rich deposits found at the tube- scale interface of the low-CI coal combustion sample. linois State Geological Survey linois Minerals 123 F22-2 sample / £;r'M 50x 100x -200 nm- Figure 13 SEM of a polished cross section at the external surface of the high-CI coal combustion sample. F22-2 sample showing EDS locations 1-4 viiv. 250x (backscattered image) ' 100 //m 1 ioOx Figure 14 Appearance of coherent scale and deposit on the high-CI coal combustion sample. -100 ^m- 1 1.2 wt%, but again, no chloride deposition was detected. Analyses to verify the composition of the combus- tion deposits that covered the coherent deposit also showed no evidence of chloride deposition. Polished cross sections through the tube-metal sample exposed to test fir- ing with the high-CI coal (figs. 13 and 14) were also examined. This sample had a coherent scale that was some- what thinner than the sample obtained from the low-Cl firing test ( 1 .0 mil in- stead of 2.5 mils). Much less coal ash and smaller other deposits were found on top of the coherent scale. The tran- sition to these deposits was more gradual than that for the sample from the low-Cl coal test. Again, the EDS analysis was used to provide a semi- quantitative measure on the metal tube composition of the sample from the high-CI coal test. The data obtained at location 1 of the sample shown in figure 5 are given in table 8. The composition of the deposit at the interface between the scale and the underlying metal (table 9) showed an enrichment of sulfur (3.54 wt%), but no chloride deposition. Table 8 EDS analysis of the chemical composition of the tube metal (at location 1, shown in fig. 14) of a sample from the high-CI coal test. Element' Atom % Weight % Error (±) Norm % Si 1.18 0.59 0.10 0.59 Cr 2.34 2.18 0.12 2.18 Fe 95.55 95.63 0.61 95.63 Mo 0.93 1.60 0.19 1.60 Total 100.00 100.00 100.00 Mn was not detected at this location. Illinois Minerals 1 23 linois State Geological Survey Table 9 EDS analysis of the chemical composition of the tube-scale interface (at location 2, shown in fig. 14) of a sample from the high-CI coal test. Element Atom % Weight % Error (±) Norm % O Al Si S Cr Fe Total 81.09 57.38 0.66 57.68 0.26 0.31 0.02 0.31 0.76 0.94 0.08 0.94 2.50 3.54 0.08 3.54 1.05 2.42 0.09 2.42 14.34 35.41 0.34 35.41 100.00 100.00 100.00 Table 10 EDS analysis of the chemical composition of the coherent scale deposits (at location 3, shown in fig. 14) of a sample from the high-CI coal test. Element Atom % Weight % Error (±) Norm % O 82.10 57.06 0.64 57.06 Al 0.18 0.21 0.02 0.21 S 0.24 0.34 0.05 0.34 Fe 17.47 42.39 0.36 42.39 Total 100.00 100.00 100.00 Additional searching revealed no sig- nificant evidence of chloride deposi- tion at the scale-metal interface. Analy- sis of the coherent scale itself revealed a sulfur content (0.34 wt%) for the high-CI coal test (table 10) that was sig- nificantly lower than that for the sample from the low-Cl coal test (1 1.2 wt%) (table 7). Again, in no case was chloride deposition found either in the scale or in the combustion deposits on this sample. The results of this study showed that the maximum corrosion rate of the T22 metal samples was the same whether samples were exposed to low- Cl or high-CI coal combustion. How- ever, the average corrosion rate of the test alloy exposed to combustion gases from the low-Cl coal was greater than that from the high-CI coal. Sulfur con- tent was greater in the scale produced from burning the low-Cl coal. This dif- ference appeared to be related to the sulfur content in the coal. The sulfur content of the low-Cl coal was much greater than that of the high-CI coal. There was no evidence of chloride deposition at the interface or in the scale produced from burning the high- CI coal. Some isolated chloride-rich pockets were observed at the interface of the sample obtained from the low- Cl coal test. The sources of these chlo- ride deposits were not obvious. There was, however, no indication of increased corrosion in the areas near and adja- cent to these deposits. This observation supports the view that chloride depos- its may not be a source for increasing the rate of metal surface corrosion. Conclusions and Recommendations Under low NO v conditions, combustion of the high-CI coal resulted in essen- tially the same corrosion rate of the tube metal alloy as with the low-Cl coal. Indeed, the corrosion rate based on average wall-thickness loss from the burning high-CI Illinois coal was actu- ally slightly less than that from burning low-Cl coal. Also, there was no evi- dence of chloride deposition in the scale that formed during the high-CI coal test. The results of this study, ob- tained from combustion of coal under reducing conditions, were consistent with those of the previous study con- ducted using oxidizing conditions. Un- der either oxidizing or reducing condi- tions, corrosion rate did not increase as the CI content of the coal increased. The results of this study suggest that the CI content limits on coal set by boiler manufacturers are not necessar- ily applicable in all cases and that futher investigations to build a com- prehensive database that confirms these findings may help boiler manu- facturers to relax the limits on the CI content of coals burned in their boilers. Acknowledgments The authors thank Murray Abbott of CONSOL Inc. and Lisa Duffy and Charles Smith of Freeman United Coal Mining Co. for providing coal samples and other in-kind contributions. Technical assistance from the Analyti- cal Chemistry, Combustion Technol- ogy, and MetallurgicalTechnology Sections at McDermott Technology, Inc., is also acknowledged. Thanks also go to Elliot Doane (retired, Kerr- McGee Co.) for his review and valuable discussions of this publication. References ASTM, 2000, Proximate analysis, ulti- mate analysis, forms of sulfur in coal, and total chlorine content in coal, in Annual book of ASTM stan- dards: West Conshohocken, Pennsyl- vania, American Society for Testing and Materials. Bettelheim, J., W.D. Halstead, D.J. Les, and D. Mortimer, 1980, Combustion problems associated with high chlo- rine coals: Erdol and Kohle-Erdgas Petrochemie, v. 33, p. 436. Chou, M-I., J.M. Lytle, Y.C. Li, F.E. Huggins, G.P Huffman, and K.K. Ho. 1995, Chlorine in five Illinois coals and three British coals — An x-ray absorbance near edge structures (XANES) spectroscopic investigation: Initial preprints of the American Chemical Society Fuel Chemistry Di- vision Meeting, Chicago, Illinois, Au- gust 20-24. Chou, M.-L, J.M. Lytle, S.C. Kung, and K.K. Ho, 2000, A comparative study on the corrosivities derived from a British coal and an Illinois coal both with a high-chlorine content: Journal of Fuel Processing Technology, v. 64, p. 167-176. Chou, M-I., J.M. lytle, R.R. Ruch, K.C. Hackley, S.C. Kung, D.T. Davison, L.L. Baxter, and K.K. Ho, 1998, Effects of linois State Geological Survey linois Minerals 1 23 chlorine in coal on boiler corrosion, in Final Report to the Illinois Coal Development Board, Illinois Clean Coal Institute: Illinois State Geologi- cal Survey. Davis, C.J., RJ. James, L.W. Pinder, and A.K. Mehta, 2000, Furnace wall fire- side corrosion in PF-fired boilers — The riddle resolved, in Effect of coal quality on power plant manage- ment — Ash problems, management and solutions: Conference, Electric Power Research Institute, May 8-1 1 , 2000, Park City, Utah. Doane, E.R, J.A.L. Campbell, and M.F. Abbott, 1994, Combustion of high- chlorine Illinois Basin coals in utility boilers: Proceedings of the Effect of Coal Quality on Power Plants, Fourth International Conference, Electric Power Research Institute, August 1994. James, P.J., and L.W. Pinder, 1997, The impact of fuel chlorine on the fire- side corrosion behavior of boiler tubing: A UK perspective, Corrosion 97 Symposium, New Orleans, Louisi- ana, March 1997. Kung, S.C., R Daniel, and R.R. Seeley, 1996, Effect of chlorine on furnace wall corrosion in utility boilers: Cor- rosion 96 Symposium, Denver, Colo- rado, March 1996. Kung, S.C., R Daniel, and J. Tanzesh, 1994, Effects of chlorine in coal on corrosion under reducing combus- tion gases: Proceedings of the Elev- enth Annual International Pittsburgh Coal Conference, Pittsburgh, Penn- sylvania, September 12-16, 1994. Latham, E., D.B. Meadowcroft, and L. Pinder, 1991, The effects of coal chlorine on fireside corrosion, in J. Stringer and D.D. Bannerjee, eds., Chlorine in coal: Elsevier Science Publishers B.V., Amersterdam, p. 225-246. Monroe, L.S., R.J. Clarkson, and I.G. Wright, 1994, Pilot-scale corrosion tests of high chlorine coal using novel resistance probes: Proceedings of the Eleventh Annual International Pittsburgh Coal Conference, Pitts- burgh, Pennsylvania, September 12- 16, 1994. 10 Illinois Minerals 1 23 linois State Geological Survey ID o en Z F O CD 3" — fc BO r- cd f— »■ c — ^ 03 3 cn m c Z^ 3 T3 CD s O ^ Cfl CD o CD — \ CD -O st Peabody aign, IL618 J3 CD 0) O C O CD to Si m ? CD |\0 O 03 m5 Cfl ~^ c O 21 O CD CD CD CD < CD 3' *» (O O c > 3 r - cd 03 w C m < z "0 c m c CO z 03 1 "D s CD > 5 "0 ID O O H ~1 to Tl 2 O > 3 H O m O