Energy & Fuels 1995,9, 775-781
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Ash Deposition at Low Temperatures in Boilers Burning High-Calcium Coals. 1. Problem Definition John P. Hurley* and Steven A. Benson Energy & Environmental Research Center, University of North Dakota, P.O. Box 9018, Grand Forks, North Dakota 58203-9018 Received November 2, 1994@
Utility boilers fired with high-calcium coals from the Western United States often develop massive ash-fouling deposits on steam tubes a t much lower temperatures than is observed when low-calcium coals are burned. In order to define and develop the capability to predict the problems, extensive sampling was performed at five utility boilers, including both pulverized coal- and cyclone-fired units, burning a total of four different high-calcium subbituminous coals. Four types of low-temperature deposits were observed, three types forming on the upstream side of the tubes and one type forming on the downstream side. Upstream deposits include massive ones that form toward the back of the secondary superheater and into the reheater at gas temperatures between approximately 1700” and 1900 O F , enamel-like deposits that form at lower temperatures on primary superheaters and economizers, and double-crested deposits that form in the primary superheater and economizer tubes. The deposits forming on the downstream sides of the tube occur from the reheater section t o the economizer. The common and defining characteristic of the four types of low-temperature deposit is their high concentrations of sulfur that was fixed through chemical vapor deposition afier the ash had deposited. In this article the different types of low-temperature fouling problems associated with the combustion of high-calcium coals are defined and the boiler conditions under which they occur are given. In subsequent articles the specific phenomena affecting the formation of the deposits will be addressed in detail, including the particular fractions of the ash responsible, the mechanisms of transport to the heat-exchanger surfaces, and the factors affecting the rates of deposition and development of strength in the deposits.
Introduction The rapid escalation of energy costs during the 1970s and the ensuing focus by the U.S. government on domestic energy production induced many U S . energy companies to greatly expand their domestic coal-mining operations. Some of the most explosive growth took place in Wyoming and Montana with their massive reserves of subbituminous coal in the Powder River Basin (PRB). This coal is easily surface-mined because of an overburdedcoal ratio of nearly unity, but it was not exploited earlier because of the low energy content of the coal and the meager regional demand for electricity. In the 1980s, energy prices fell, but the low emissions of sulfurous gas from boilers burning the PRB coals kept demand for them relatively high. Emissions are low not only because of the low sulfur content of the coals, but also because of the high calcium content of the ash, which fixes some of the sulfur in the ash as calcium sulfate. In most cases, the plant emissions are so minor the coals are considered “specification”fuels, meaning that plant emissions fall below levels required by law even when pollution control devices are not used. The low emissions and tightened government regulations have led many midwestern utilities to switch from high-sulfur eastern to low-sulfur western U.S.coals, even though shipping costs for the western fuels are as much as 3 times the price of the fuel. As power plant experience with the high-calcium subbituminous coals grew, reports increased about the @
Abstract published in Advance ACS Abstracts, July 15, 1995.
formation of ash-foulingdeposits on boiler heat exchangers a t much lower temperatures than had been previously encountered. The more conventional fouling deposits found a t higher temperatures are composed primarily of glassy silicate-based ash that is sticky a t temperatures above approximately 2000 oF.1-6 In contrast, temperatures in the regions where the highcalcium deposits grow are typically much lower than the melting point of silicates. Therefore, the sticky material was believed to be sodium-magnesium-calcium sulfates with low solidus temperatures, such as those described by Walsh and others7and Osborn? or possibly through sintering caused by sulfation of the lime (CaO) in the deposit, such as that described by Skrifvars and other^.^ However, low-temperature eutectic compositions are not always observed in the very-low-sodium coal ash deposits, and lime may make up only a small fraction of the calcium found in the ash. Because they form at different temperatures, the high- and low-temperature deposits form in different (1)Crossley, H. E. J. Inst. Fuel 1952, 24, 222-225. (2)Wibberley, L.J.; Wall, T. F. Fuel 1982, 61, 93-99. (3)Benson, S.A,; Jones, M. L.; Harb, J. H. Ash Formation and Deposition In Fundamentals of Coal Combustion for Clean and Eficient Use; Smoot D., Ed.; Elsevier: Amsterdam, 1993;p 299. (4)Baxter, L. L.; DeSollar, R. W. Fuel 1999, 72, 1411. (5)Walsh, P.M.;Sayre, A. N.; Loehden, D. 0.; Monroe, L. S.; Beer, J. M.; Sarofim, A. F. Prog. Energy Combust. Sci. 1990, 16, 327-346. (6)Sondreal, E. A.;Tufte, P. H.: Beckering, W. Combust. Sci. Technol. 1977, 16, 95-110. (7) Walsh, P.M.; Sarofim, A. F.; Beer, J. M. Energy Fuels 1992, 6 , 709. (8)Osborn, G.A. Fuel 1992, 71, 131. (9)Skrifvars, B. J.; Hupa, M.; Hyoty, P. J.Inst. Energy 1991, 64, 196.
0887-0624/95/2509-0775$09.00/00 1995 American Chemical Society
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areas of the boiler convective pass. Conventional hightemperature deposits are usually found on the secondary superheater steam tubes. Low-temperature deposits form downstream of the secondary superheater. Unfortunately, the slow growth of the deposits and relatively frequent changes in operating conditions and fhels make it difficult to define the parameters that lead to deposit formation. To determine the mechanisms leading to the formation of low-temperature ash-fouling deposits so that their occurrence could be predicted, a governmentindustry consortium program entitled "Project Calcium: Calcium-Based Ash Deposition in Utility Boilers" was initiated at the University of North Dakota Energy & Environmental Research Center (EERC). The first task was to define the specific locations and boiler conditions at which the various types of low-temperature deposits form. Once the conditions were defined, the physical and chemical properties of the deposits, along with the fraction of the ash causing the specific problem, were delineated. Processes and rates of deposition and strength development were determined, and the factors affecting these rates were quantified. This information was then used to develop a computer code to predict the formation of key types of low-temperature deposits based on analyses of the fuel and to identify possible methods to mitigate the development of the deposits. Because of the large variety of specific issues addressed by Project Calcium, the results of the research will be discussed in a series of papers. Discussion will center on conclusions that are apparently valid for all western U.S. high-calcium low-rank coals. This article gives an overview of Project Calcium research; the various types of low-temperature fouling problems associated with the combustion of highcalcium coals are described and defined, and the boiler conditions under which they form are given. In subsequent articles specific phenomena are addressed in detail, including the fractions of the ash responsible, the mechanisms of transport to the heat-exchanger surfaces, the factors affecting the rates of deposition, and the development of strength in the deposits.
Experimental Methods Field Tests at Utility Boilers. Five utility boilers, including both pulverized coal (pc)- and cyclone-fired units, burning a total of four different coals were tested over a 2-year period. The tests involved collecting samples of coal, entrained ash, slag, convective pass deposits, and hopper ash and obtaining detailed boiler operation logs during the periods of the tests. The sampling equipment consisted of a high-velocity thermocouple for temperature profiling, pitot probes for gas velocity determinations, water-cooled probes for collecting entrained ash samples aerodynamically sized with a multicyclone during collection, and air-cooledprobes that simulated convectivepass steam tubes for deposit collection. The design and operation of the probes are given in Hurley and others.1° The definitions of the types of low-temperaturedeposits and the regions of the convective pass in which they form were determined by intensive sampling at the first three utility boilers tested. Methodologyincluded collecting deposits during a scheduled outage from the convective pass of the Northern Indiana Public Service Co. R. M. Schahfer Station Unit 14 (10)Hurley, J. P.; Benson, S. A.; Erickson, T. A.; Allan, S. E.; Bieber, J. Project Calcium Final Report, DOE/MC/10637-3292,1995.
Table 1. ASTM Ash Compositions of the Coals Given on a Normalized, SO3-Free Oxide, wt % Basis
oxide
a
Shoshone 32.0 16.1 9.0 1.1 1.4 28.8 9.0 1.8 0.8 16.8
Black Thunder 31.7 21.0 5.3 2.1 1.5 28.5 8.7 1.0 0.3 19.7
Antelope 33.1 16.3 10.2 1.6 0.7 28.9 7.4 1.5 0.3 15.4
Eagle Butte 33.3 17.0 5.9 1.3 1.4 29.1 10.6 1.4 0.2 13.3
Included for comparison purposes. Primary Secondary Superheater Superheater Reheater
-\j
ToAir Preheater
I Slag
Figure 1. Locations and typical gas temperatures where offline deposits were collected. (468-MWcyclone) and collecting on-line samples at the Schahfer Station as well as at the Northern States Power Allen S. King Station Unit 1 (600-MW cyclone) and Sherburne County (Sherco) Station Unit 1 (750-MW pc). The effect of firing rate on the deposits was determined by firing the Schahfer Station boiler at two rates with one coal from the Cyprus Coal Company Shoshone mine, Hannah Basin, WY. The effects of coal type and firing mode were determined by firing the King and Sherco boilers on coals from the ARC0 Coal Co. Black Thunder and NERCO Antelope mines of the Powder River Basin in Wyoming. At each plant, particulate ash and deposits were collected in similar positions at the front, middle, and back of the convective pass. After the problem definition task, testing was performed at two other boilers to determine the rates of formation of specific kinds of deposits. These were the Otter Tail Power Co. Hoot Lake Station Unit 2 and the Southwestern Electric Power Co. (SWEPCO) Welsh Power Plant. Both of these units were burning coal from the AMAX Coal Co. Eagle Butte mine, Powder River Basin, WY. At each plant, particulate ash and deposits were collected in the middle of the convective pass where the upstream deposits seemed to be the most massive. Analytical Methods. Field test samples were characterized with a large battery of advanced analytical techniques. Computer-controlled scanning electron microscopy (CCSEM) was used to determine the size, composition, and abundance of mineral grains in the coal.ll Chemical fractionation was used to determine the abundance of organically associated elements in the coal based on their extractability with various aqueous solutions.l2 In addition, standard American Society for Testing and Materials (ASTM) methods of coal character(11)Zygarlicke, C. J.; Steadman,E. N. Scanning Electron Microsc. 1990,4 (3), 579-590. (12)Benson, S . A.; Holm, P. L. Ind. Eng. Chem.Prod. Res. Dev. 1985, 24,145-149.
Ash Deposition in Boilers Burning High-Calcium Coals
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Bo 50-
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\ 0 1000
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1400
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ZOO0
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28M)
loo0
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Figure 2. Concentrations of silicon, calcium, and sulfur in off-line deposits (SOS-free oxide basis).
ization such as proximate, ultimate, ash fusion, and calorific value were used. A scanning electron microscope point count (SEMPC) routine was used to quantify the distribution of chemically distinct phases in fly ash, deposits, and slag.13The bulk chemical compositions of the coal ash, deposits, and fly ash were determined using X-ray fluorescence (XRF)analysis, and X-ray diffraction (XRD) was used to identify the crystalline phases.
1800
1800
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Figure 3. Concentrations of phosphorus, sodium, and potassium in off-line deposits (SOs-free oxide basis).
50-
fi
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f
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.9
Definition of Low-Temperature Deposition Phenomena Figure 1 is a schematic of Schahfer Station Unit 14 showing where off-line deposit samples were collected from steam tubes and the approximate gas temperatures during the typical daytime load of the previous month. The temperatures were interpolated by comparing them with data collected during on-line testing. All deposits were taken from upstream surfaces of the lead tube in a bank, except sample 5, which was collected from the downstream side of a tube. Table 1 shows the composition of the ASTM ash of the Shoshone coal that was fired at the Schahfer station during the formation of the deposits. Figure 2 shows the concentrations of silicon, calcium, and sulfur in the deposits as a function of approximate gas temperature at the sampling point. The composition values are listed as oxide weight percents on an SO3-free basis, except for the SO3 value, which is normalized with the other major oxides in the deposits. The data for sample 5 are not shown connected with a line to the other data because sample 5 was collected from the downstream side of a tube. The figure shows that the concentration of silicon, and less so of calcium, generally increases slightly with gas temperature. This most likely occurs because calcium silicate particles become softer at higher temperatures and tend to stick more readily when they impact a steam tube. The changes in sulfur concentration are much more dramatic. The deposits collected from areas where the gas was at or below approximately 1900 O F contain 10 times as much sulfur as those collected from highertemperature regions. Although the exact temperatures of the deposits are not known, since the deposits were cooled somewhat by the underlying steam tubes and exposed to radiant heat from the front of the convective (13)Jones, M. L.; Kalmanovitch, D. P.; Steadman, E. N.; Zygarlicke, C. J.; Benson, S. A. Application of SEM Techniques to the Characterization of Coal and Coal Ash Products. In Advances in CoaE Spectroscopy; Meuzelaar, M. L. C., Ed.; Plenum Publishing Co.: New York, 1992.
TOO0
1200
1400
1800
la00
2 m
22w
2400
2800
Temperature, O F
Figure 4. Concentrations of aluminum, iron, and magnesium in off-line deposits (SOa-free oxide basis).
pass, later laboratory work showed a similar phenomenon at approximately this temperature. The increase in sulfur is due to chemical vapor deposition (CVD) caused by a shift in stable chemical species and is not due to simple condensation. The laboratory work and mechanism of sulfation will be discussed in a future paper in this series. Sample 5, collected from the downstream side of a tube, has the highest sulfur content, most likely because it could not be eroded or sootblown and so had the most time to sulfate. The CaO/S03 ratio indicates that the calcium was essentially fully sulfated in sample 5. Because it presents an easy demarcation parameter, the sulfur content of the deposits is used to define the zones of high- vs low-temperature fouling. By this definition, low-temperature fouling occurs a t gas temperatures below approximately 1900 OF in a boiler firing a high-calcium subbituminous coal. Some viscous sintering of silicate glasses in the ash may occur below this temperature, but the deposits are dominated by the presence of sulfated ash. Figure 3 shows the concentrations of phosphorus, sodium, and potassium in the off-line deposits on a S03free oxide basis. As was true for sulfur, the phosphorus, sodium, and potassium are depleted in the highertemperature deposits but are relatively enriched in the low-temperature deposits. Since the gas flow patterns do not change between the high- and low-temperature zones of the furnace, the variation in concentrations of these elements indicates that, like sulfur, they vaporize during coal combustion and condense as the gas temperature drops below approximately 1900 OF. Thermochemical equilibrium modeling of the combustion of similar coals shows that sodium is present in the vapor
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Figure 5. Upstream side of the deposition probe coupon showing the presence of high-temperature fouling deposit islands formed at a gas temperature above 2000 OF.
as sodium sulfate: which condenses below this temperature in a process known as physical vapor deposition (PVD). Potassium is also known to vaporize during combustion and condense at lower temperatures as a sulfate.14J5 It is not clear at this time whether the phosphate and potassium deposit through PVD or CVD. The concentrations of aluminum, iron, and magnesium in the deposits are shown in Figure 4 on a SO3free oxide basis. Unlike sulfur, phosphorus, sodium, and potassium, the elements shown in Figure 4 do not vaporize substantially at any point in the boiler. Therefore, like silicon and calcium, their concentrations in the deposits change little with temperature, although the chemical associations of the elements may change. Like the chemical composition, the physical nature of ash-fouling deposits varies dramatically through the convective pass. Four distinct deposit morphologies were found in all boilers burning high-calcium western U.S. coals, and in some cases a fifth type was noted. They include conventional high-temperature fouling deposits and four types of low-temperature deposits. The high-temperature fouling deposits form on the upstream (windward) side of steam tubes in the secondary superheater tube banks at temperatures above 2000 OF. They are usually composed of thin inner layers of sulfur-rich material, but the bulk of the deposits contain less than 5% sulfur. These deposits resemble sandstone, although in certain cases they have completely fused into molten masses. Because they comtnonly occur when many types of coal are burned and can grow to block the flow of gas through the boiler, they have been studied extensively.1-6 Figure 5 shows sintered islands of ash on the upstream side of a deposition probe coupon that are examples of precursors to the formation of a conventional deposit. The light patches are areas from which sintered islands have fallen, exposing the under(14) bask, E.Mineml Impurities in Coal Combustion; Hemisphere Publishing Corp.: Bristol, PA, 1985. (15)Stinespring, C. D.; Stewart,G. W.A t m s . Enuimn. 1981,15, 307.
Figure 6. Massive upstream deposits on lead tubes in the reheater tube banks of a boiler burning a high-calcium western
U.S.coal.
.
lying powder layer of the deposit. As the islands grow, they contact each other and then grow primarily in the windward direction. They are composed of ash particles consisting primarily of aluminosilicate glasses that are fluxed with sodium, magnesium, calcium, or iron. The islands are believed to be initiated by the deposition of one relatively large, very sticky ash particle that changes the local aerodynamics and provides a capture surface for further growth of the island. The process is discussed in more detail by McCollor and others? As the ash passes back into the convective pass and cools, it forms deposits with distinctly different physical (16)McCollor, D. P.; Zygarlicke, C. J.; Allan, S. E.; Benson, S. A.
Energy Fuels 1993,7,761.
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SWEPCO Welsh Power Station Unit 1, January 1992 Test 2 , 5 day, Cycle 5-250 MW Gas Flow 4
1
Figure 7. Long-duration deposition probe coupon showing both upstream enamel and downstream powder low-temperature -
.
deposits.
and chemical properties from those of the high-temperature deposits. Four main types of low-temperature deposits have been widely observed: 1. Massive deposits that form on the upstream side of tubes at the rear of the secondary superheater and into the reheater region at gas temperatures ranging from approximately 1700 to 1900 O F . 2. Enamel-like deposits that form on the upstream sides of tubes in the primary superheater and economizer at temperatures below approximately 1700 O F . 3. Powder deposits that form on the downstream sides of tubes at the rear of the secondary superheater and back through the economizer. 4. Double-crested deposits that form on the upstream sides of tubes in the primary superheater and economizer. The massive deposits forming on the upstream side of the reheater tubes are less lustrous than hightemperature fouling deposits. They are often dull reddish or brownish in color. They can be very massive and hard, almost always covering the front of the tube and extending a foot or more into the gas stream and forming a sharp ridge. Figure 6 shows a side view of such deposits. The cleared area at the bottom of the figure where the person is standing is the path of a sootblower. Such paths are common in boilers burning high-calcium western U.S.coals. They tend to be less than 10 Et in total height and indicate that sootblowers need to be more closely spaced than this to prevent the accumulationof these massive upstream deposits. These deposits do not normally grow dramatically in width and close off the gas flow unless a region of gas turbulence makes the direction of growth transverse to the rest of the gas stream or unless a piece of a deposit breaks off and lodges between plenums. If this occurs, the broken piece acts as a seed that initiates rapid growth of blocking deposits. The deposition mechanisms and rates and the strength development of these deposits will be addressed in subsequent papers. Farther back in the pass, the ash particles cool and harden so that they cannot deform and stick when they
collide with a tube, preventing the formation of the massive upstream dep0sits.l' Instead, a thin, hard, enamel-like coating forms on the upstream sides of the tubes. Figure 7 shows the coating on the top of the removable coupon from the long-duration deposition probe after a deposit at the SWEPCO Welsh Station was collected. These deposits become well sintered over sampling periods as short as 15 min. Therefore, sootblowing for these deposits is less effectivethan for most other types. Effectiveness could possibly be improved if thermal shocking were significant, such as when water is used as the blowing medium. Off-line removal of the deposits is equally difficult. One Project Calcium sponsor reports great difficulty in removing them with high-pressure water sprays, although another reports that an overnight soaking with acidic river water sprayed with lawn sprinklers made them easily removable. Because these deposits build slowly, they do not dramatically impede heat transfer. However, they can act as a capture surface to which incoming ash particles will stick. In this way, the enamel layer can initiate the formation of a larger deposit. This is believed to be the initiation process of the massive upstream lowtemperature deposits. In addition, because the enamel layers are rich in sulkr, they can corrode the tubes. Figure 8 shows a cross section of a stainless steel steam tube that has, on the upstream side, a thick enamel deposit. The black band between the deposit and the tube is the corrosion layer. Figure 9 shows a scanning electron micrograph of the backscatter signal from the deposit-tube interface. It shows that the black layer, which appears light in the micrograph, is actually composed of an inner iron oxide layer overlain by the sulfur-rich ash deposit into which the iron oxide has dissolved. The third type of low-temperature deposition occurs on the downstream sides of steam tubes all through the (17)Tsai, C. J.; Pui, D.Y. H.;Liu, B. Y. H.Aerosol Sci. Technol. 1991,15, 239.
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Figure 8. Cross section of a stainless steel steam tube and thick enamel-like deposit on the upstream side showing the dark corrosion layer at the deposit-tube interface.
I
I
c
II
Figure 9. Backscattered electron micrograph of the corrosion layer at the interface between the upstream enamel layer and the steam tube shown in Figure 8.
convective pass. An example can be seen on the downstream side of the sample coupon shown in Figure 7. Such deposits occur all through the convective pass of a boiler, but usually become much thicker in the horizontal steam and water tube banks of the primary superheater and economizer. These deposits do not bond to the tube surface and are poorly sintered, so they often shed from vertical tube banks under their own weight. In cases where they do not shed, they can completely fill the interstitial spaces between steam tubes in a plenum, in line with the gas flow. If sootblower coverage is not available, and if some turbulence exists or a seed consisting of a piece of upstream deposit is lodged between the plenums, then these deposits will grow between plenums and may block gas flow, forcing a boiler trip. Also, because these deposits are poorly sintered and can cover well over one-half of
the tube surface in a bank, they can significantly reduce heat transfer from the gas. Since the lead tubes in a bank protect deposits from the sootblower blast, they can form in the immediate vicinity of sootblowers, although in such cases they will not grow between Dlenums. One Project Calcium sponsor reports that the ieposits can be removed three to five tubes deep in a bank by angling the sootblower nozzles 15" from the perpendicular to the sootblower path. The last type of low-temperature deposit does not often form and usually would not significantly impede heat flow, although they are very hard and exceptionally difficult to remove. The deposits are composed of two crests, or humps, that form on either side of the center line of the tube on the upstream side, often leaving the center line bare. The crests sometimes extend into the gas flow for several inches, the center line remaining bare. They most likely form from a specific size range of ash particles that diverge slightly with the gas around the tube but then separate from the gas flow and impact the tube. They were not studied under Project Calcium because of their very slow growth, but problems associated with their presence and removal methods are expected to be similar to those of the upstream enamel deposits. Conclusions Extensive sampling and analysis of coal, ash, and deposits from a variety of boilers burning high-calcium western U.S.coals has revealed significant similarities in the properties of ash deposits that form at low temperatures. Four main types of low-temperature deposits have been widely observed: upstream massive deposits that form toward the back of the secondary superheater and into the reheater at gas temperatures between approximately 1700 and 1900 O F , upstream enamel deposits that form at lower temperatures on primary superheaters and economizers, downstream powder deposits that form from the reheater section back into the economizer, and double-crested deposits that form on the upstream sides of primary superheater
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Ash Deposition in Boilers Burning High-Calcium Coals
and economizer tubes. Their common and defining characteristic is their high concentrations of sulfur and that sulfation occurs after ash particle deposition through a CVD process. In subsequent articles, the specific phenomena affecting the formation of the deposits will be addressed in detail, including the particular fractions of the ash responsible, the mechanisms of transport to the heat-exchanger surfaces, and the factors affecting the deposition rates and development of strength in the deposits.
Acknowledgment. The authors thank the members of Project Calcium for their support for this work. They
include the U.S.Department of Energy (DOE), the Electric Power Research Institute (EPRI), Northern States Power Co. (NSP), Northern Indiana Public Service Co. (NIPSCO), Cyprus Coal Co., Wisconsin Power and Light Co., AMAX Coal Sales Co., Otter Tail Power Co., Ontario Hydro, Peabody Coal Co., and Babcock & Wilcox Co. In addition, the authors thank the Southwestern Electric Power Co. (SWEPCO) for allowing us to collect samples at their Welsh Power Plant, and Stan Selle, president of Northwest Research, for his helpful discussions of ash-fouling phenomena. EF940207B
