Energy & Fuels 1998, 12, 429-442
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Ash Deposition Trials at Three Power Stations in Denmark Karin Laursen*,† Geological Survey of Denmark and Greenland, Thoravej 8, 2400 Copenhagen NW, Denmark
Flemming Frandsen Technical University of Denmark, Building 229, 2700 Lyngby, Denmark
Ole Hede Larsen I/S Fynsvaerket, Havnegade 120, 5000 Odense C, Denmark Received July 3, 1997
Six full-scale trials were conducted at three power stations in Denmark: Ensted, Funen, and Vendsyssel power stations. During these trials, pulverized coal, bottom ash, fly ash, and deposits from cooled probes were sampled and analyzed with various techniques. On the basis of SEM analyses, the deposits can be grouped into five textural types, which all possess distinct textural and chemical characteristics. Likewise, the deposition mechanisms for these five types are characteristic and they may be used for constructing a model for the buildup and maturation of an ash deposit. The deposits collected on the probes were thin (maximum 2 mm after 9 h) and the influence of operational parameters and probe temperatures on the magnitude of the deposits were minor. The probe temperatures had no influence on the composition of the ash deposits for coals with low ash deposition propensities, whereas the probe temperature did influence the composition of deposits for coals with medium ash deposition propensities. These results may indicate that coals with medium to high ash deposition propensities in existing boilers may cause increasing ash deposit formation in future boilers with higher steam temperatures (620-700 °C).
In order to increase the general knowledge of ash deposition in boilers, a three-year collaborative project on “Mineral transformations and ash deposition in pulverized coal fired systems” was initiated in Denmark
in 1993.1 The participants in the project include (1) ELSAM (project coordinator and manager of full-scale combustion trials); (2) the Geological Survey of Denmark and Greenland (coal, ash, and deposit analyses); and (3) the Combustion and Harmful Emission Control (CHEC) research program at the Technical University of Denmark (modeling of ash deposition propensities). The project focuses on existing boilers (steam parameters up to 250 bar/540 °C) and the new ultra supercritical (USC) boilers (steam parameters 290 bar/580 °C) under construction in western Denmark, but also the next generations pf-boilers (325 bar/620-700 °C) are considered. Many of the existing boilers and all the new boilers are equipped with low-NOx burners. In general, the purposes of these full-scale trials were to gain fundamental knowledge of ash deposit formation and elucidate factors controlling the growth and consolidation of deposits. The information on ash formation achieved from the collected ash deposits were combined in a classification system for ash deposits, which connect the appearance of the deposits (e.g. texture) and chemistry with the mechanisms responsible for the deposition. A major specific objective of the trials was to
† Current address: Department of Chemical and Bio-Resource Engineering, 2216 Main Mall, University of British Columbia, Vancouver B.C., V6T 1Z4, Canada. Tel: 604-822-3238. Fax: 604-822-6003. E-mail:
[email protected].
(1) Larsen, O. H.; Laursen, K.; Frandsen, F. Danish collaborative project on mineral transformations and ash deposition. In Applications of Advanced Technology to Ash-Related Problems in Boilers; Baxter, L., DeSollar, R., Eds.; Plenum Press: New York, 1996; pp 325-334.
Introduction During coal combustion, the inorganic constituents are transformed into fly ash particles which may deposit on the heat transfer surfaces of a boiler. Flame-side deposits decrease the heat transfer from the hot flue gas to the water-steam cycle leading to a reduced output efficiency for the boiler. Additionally, some types of deposits increase the corrosion of the heat transfer surfaces. Under normal operational conditions ash deposits are removed regularly from the boiler either “naturally” by gravity settling or mechanically by soot blowers. However, types or quantities of ash deposits may develop that cannot be removed with these simple methods and in severe cases of ash deposition, boilers may be damaged by sudden collapse of huge blocks of ash deposits or it may be necessary to temporarily shut down the boiler for cleaning and repair.
S0887-0624(97)00106-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/16/1998
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Figure 1. Cross sections through the Ensted, Funen, and Vendsyssel Power Stations and major plant and operational data for the three power stations. Arrows indicate the location of deposition probes and in-furnace video recordings.
evaluate the influence of increasing steam temperatures on the morphology and chemistry of ash deposits, and also to evaluate the influence of load and general operation of the plant on the ash deposition. Experimental Methods A total of six full-scale trials were conducted: one at the Ensted, three at the Funen, and two at the Vendsyssel power stations.2 The Ensted Power Station, Unit 3, is a 600 MWe pc-fired boiler with 36 low NOx combination burners situated in three levels in a boxer configuration. Unit 7 at the Funen Power Station is a 350 MWe pc-fired unit with 16 tangential arranged low-NOx burners situated in four levels. The Vendsyssel Power Station, Unit 2, is a 300 MWe pc-fired unit with 16 retrofitted low-NOx burners situated in four levels in a boxer configuration. The major plant and operational data for the three power stations and boiler profiles are illustrated in Figure 1. Conducting full-scale trials is a very expensive way of performing tests. Thus, all the trials included in this project were carried out in collaboration with other ELSAM activities (i.e., coal test burns and research and development projects). (2) Laursen, K. Characterization of minerals in coal and interpretations of ash formation and deposition in pulverized coal fired boilers. Ph.D. Thesis, Geological Survey of Denmark and Greenland, Report 1997/65; ISBN 87-7881-022-7, 1997.
Therefore, the coals burned during the tests were mainly selected because of their general combustion behavior rather than for their ash deposition propensities; however, the coal types cover the spectrum and a major fraction (on an energy basis) of the coals consumed in power stations in Denmark. The coal burned during the trial at the Ensted Power Station was an Indonesian sub-bituminous coal (coal A). The trials at the Funen Power Station were conducted during combustion of three bituminous coals: (1) a Colombian coal (coal B), (2) a South African coal (coal C), and (3) a Polish coal (coal D). The trials at the Vendsyssel Power Station were conducted during combustion of two coal types/blends: (1) 67% coal D-33% high-S coal blend (coal E) from the USA; and (2) 100% coal E. The subbituminous coal A has a higher moisture content (15.5%) than the five bituminous coals (9.6-12.6%)(Table 1). Except for coal E, all the coals are low-sulfur coals. Coal A and coal E have a slightly higher content of volatile matter compared with the other coals. Coal A has a low ash content (6.5%), whereas the rest have medium ash contents. The heating value for coal E (28.2 MJ/kg) is slightly higher than for the other five coal types (25.16-25.88 MJ/kg). The SiO2 and Al2O3 content in the ash are similar for all coals except coal C, which has a higher amount of Al2O3, and coal B, which contains a higher amount of SiO2 (Table 2). These SiO2 and Al2O3 contents indicate that coal C contains more Al silicates or Al silicates with higher Al2O3 contents compared
Ash Deposition trials at Power Stations in Denmark
Energy & Fuels, Vol. 12, No. 2, 1998 431
Table 1. Proximate Analyses of the Six Coalsa
and an increase in the amount of Fe-Al silicates indicates that also the iron has been redistributed into the clay-derived phases with only a minor part surviving as pure iron oxide particles (i.e., hematite or magnetite). Bulk ash chemistry indexes (i.e., base/acid ratio, Rs 9) predict that all coals except coal E are low slagging coals. Coal E is classified as a medium slagging coal. In general, the ash deposition tendencies observed during the trials agreed well with these predictions. No problems with ash deposition were observed during combustion of coal A, coal B, and coal C, and small slagging problems were seen in the furnace during combustion of the blend coal C-coal D, and increasing problems with slagging in the furnace were observed during combustion of coal E. As expected, the bottom ash is enriched with Fe2O3, especially for the iron-rich coal, coal E (Table 4). All the bottom ashes are enriched in SiO2 and CaO, and depleted in Al2O3 (except for coal A) compared to the fly ashes. Trials at the Funen Power Station lasted 3 days for each of the three coal types. The operation of the plant and the excess air was constant on a given day for each coal type, but varied from day 1 to day 3 of the test as follows: day 1 (1.17%, normal coal load); day 2 (1.15%, normal coal load); and day 3 (1.17%, and higher coal load (10%) plus lower excess air (1.15%) on the lowest burner level) (Table 5). The trials at Ensted and Vendsyssel Power Stations lasted 3 and 2 days, respectively, during which full load and the same operating conditions were maintained (excess air 1.15%). Two types of probes were used for collecting deposits from the boilers: an air-cooled probe for exposure in the convective pass and a water-air-cooled probe for exposure in the furnace.2 The probes consist of a 1.5 m long stainless steel pipe with a diameter of 3.8 cm. The deposits are collected on a 10 cm exchangeable piece of tube (i.e., the test element, 10 Cr Mo 9 10 steel) located approximately 0.5-0.75 m from the boiler wall. The air-cooled probe is cooled by pressurized air let in through the walls of the probe and out through the center. The water-air-cooled probe is cooled by a central water stream surrounded by an air-cooled area. Three thermocouples, placed adjacent to and on the wall side of the exchangeable test element, are used for control and registration of the metal temperature of the probe and the test element. One of the thermocouples, usually on the upstream side, is chosen as a control temperature (set temperature). The metal temperature is maintained by adjusting the air flow through the inside of the probe. The test elements exposed at Ensted Power Station were all preoxidized at 600 °C for 200 h, whereas the test elements used at Funen and Vendsyssel power stations were unoxidized. The locations of the probes are indicated in the boiler profiles in Figure 1. During the trials at Ensted and Vendsyssel power stations, the probe metal temperatures (furnace 450-650 °C, convective pass 500-700 °C) and exposure time (2-56 h) were varied in order to evaluate the effects of the two parameters on the morphology, texture, and chemistry of the deposits (Table 5). During the trials at the Funen Power Station, the effects of boiler operation were evaluated and the metal temperatures for all the probes were held at 570 °C. Additionally, at the Funen Power Station deposits were also collected on an uncooled ceramic probe inserted next to probe 2. The deposits from the uncooled probes would simulate a deposit which is not influenced by the cooling from the tubes and indicate the appearance and chemistry of a thick deposit in the consolidation phase. In addition, during the trials at the Vendsyssel Power Station, an in-furnace video recording system was used for recording ash deposition on the probe from the convective pass and in the furnace in general. The probes were generally covered with a loose powder deposit on the downstream side (shelter side) and to a lesser
Ensted PS coal A moisture (%) ash (%) volatile matter (%) total sulfur (%) heating value (Qgross) (MJ/kg) a
15.5 6.5 35.0 0.81 25.16
Funen PS
Vendsyssel PS
coal B
coal C
coal D
12.6 11.4 30.3 0.77 25.50
8.4 14.3 22.2 0.47 25.80
11.7 11.8 27.5 0.80 25.88
coal D/ coal E 11.4 11.1 37.6 1.35 26.38
coal E 9.6 8.8 38.3 2.45 28.20
All the parameters are given on an as-received basis.
Table 2. Bulk Ash Analyses of the Six Pulverized Coalsa SiO2 Al2O3 Fe2O3 CaO MgO Na2O Na2O TiO2 P2O5 SO3 a
coal A
coal B
coal C
coal D
coal D/coal E
coal E
48.5 25.4 11.1 2.13 2.24 0.41 2.13 1.12 0.52 2.40
61.0 20.4 7.6 3.07 1.61 0.63 1.96 1.03 0.30 2.69
47.1 30.8 3.15 6.73 1.65 0.14 0.66 1.78 1.44 3.76
48.4 26.6 9.11 4.32 2.85 1.17 2.39 1.15 0.61 3.18
48.0 22.8 11.1 4.97 2.50 2.19 0.79 1.01 0.56 3.91
46.1 20.9 16.6 4.87 1.11 1.82 0.88 1.00 0.37 3.88
All data are on a weight percent basis.
with the other coals while coal B has a higher content of quartz. The Fe2O3 content varies significantly for the coals, from very low in coal C (3.15%) to high in coal E (16.6%). Table 3 lists the results of computer-controlled scanning electron microscopy (CCSEM) analyses of the six coal and fly ashes collected from the electrostatic precipitator during the trials. The CCSEM technique is used to determine the size, shape, quantity, and semiquantitative composition of mineral grains in coal.3-7 The components in the SEM system used in the CCSEM technique include the following: the backscattered electron (BSE) detector, the stage control, the beam control, and the energy-dispersive X-ray (EDX) detector. As expected, clay minerals (e.g., kaolinite, illite, montmorillonite) account for the majority of the mineral matter present in the coals, comprising from 42% (coal B) to 66% (coal C). Likewise, the main phases present in the fly ashes are clay-derived phases (they are classified under the clay minerals, but will occur as transformed phases) comprising from 41% (coal B) to 69% (coal A). The amount of clay-derived particles in the fly ash from coal C is low compared to the amount of clay minerals in the coal, probably due to a redistribution of calcium from dolomite and calcite into the clay-derived particles evidenced by a high content of Ca-Al silicates in the fly ash. coal E and coal D show signs of a similar redistribution of calcium. As indicated by the bulk chemical ash analyses, coal B is significantly richer in quartz (19%) than the other coals (39%), whereas coal E is richer in pyrite (22%) than the rest (2-10%). Manual investigations of the particles classified as iron oxide in coal B revealed that these particles are mainly siderite (FeCO3) and ankerite (Ca(Mg,Fe,Mn)(CO3)2). This Indonesian coal apparently has mineral characteristics similar to Australian coals, where siderite and ankerite are the most common iron-containing minerals, and pyrite only occurs as a minor phase.8 As expected, no pyrite is found in the fly ashes, (3) Lee, R. J.; Huggins, F. E.; Huffman, G. P. Scanning Electron Microsc. 1978, 1, 561-568. (4) Huggins, F. E.; Kosmack, D. A.; Huffman, G. P.; Lee, R. J. Scanning Electron Microsc. 1980, 1, 531-540. (5) Straszheim, W. E.; Yousling, J. G.; Younkin, K. A.; Markuszewski, R. Fuel 1988, 67, 1042-1047. (6) Zygarlicke, C. J.; Steadman, E. N. Scanning Microsc. 1990, 4 (3), 579-590. (7) 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 coal spectroscopy. Meuzelaar, H. L. C., Ed.; Plenum Press: New York, 1992; 1-27. (8) Couch, G. Understanding slagging and fouling during pf combustion; IEA Coal Research Report No. IEACR/72, London, 1996.
(9) Winegartner, E. C. Coal fouling and slagging parameters; Am. Soc. Mech. Eng.: New York, 1974.
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Table 3. Results of CCSEM Analyses of Coal Minerals and Fly Ashes From the Six Full-Scale Trialsa Ensted PS
Funen PS
coal A aluminosilicate apatite Ca silicate Ca-Al silicate calcite clay-pyrite dolomite Fe silicate Fe-Al silicate Fe-Cr oxid gypsum gypsum/Al silicate illite iron oxide kaolinite mixed silicate montmorillonite Na-Al silicate pyrite quartz Si-rich unclassified
coal B
Vendsyssel PS
coal C
coal
fly ash
coal
fly ash
4.7
6.9
3.8 1.4
4.8
coal
coal D
coal D/coal E
coal E
fly ash
coal
fly ash
coal
fly ash
coal
fly ash
2.3
1.3
1.0
2.2
3.2
2.3
8.1
1.3
5.7 1.1
3.8
2.2 4.3
1.0 3.8 1.4
0.8 3.4 5.6 1.3
30.4
5.7
6.3 1.0
5.9 1.8
1.4
5.4
4.5 1.3
3.5
3.8 1.8 25.7
38.8
17.9
4.2 12.6 4.9
28.0
25.7
25.5
6.6
2.0
4.3 8.5 6.5
41.7
32.0
7.5 1.2 5.1
27.2 1.7 15.3
34.9 2.8 10.6 2.0 10.9
15.3 9.2 9.2 2.1 17.6
8.8 5.2 3.1
6.6 19.3 3.7 14.1
13.8 5.9 19.7
4.6 1.7 13.1
6.0 5.0 1.3 13.5
17.6 2.4 3.5 11.9
1.7 1.2 5.8 2.4
2.0 1.3 1.3
3.0 5.9 3.7 26.8
9.9 1.7
8.4
2.4
9.0
5.6 1.7 20.7
11.2
15.4 1.6 22.1 1.3 15.1
10.4 6.6 1.2 12.3
7.9 2.6 12.3
1.5 7.6 3.3
1.8 2.0
1.7 4.6 2.4 20.3 1.9 18.6
12.0 26.0 6.2 1.2 22.5 7.7 1.4 13.3
6.3 4.5 9.5
a
Categories with concentrations below 1% for all coal and ash samples are excluded from the table. The data are given in weight percent on a mineral basis. Table 4. Results of Bulk Chemical Analyses of Fly Ashes (Electrostatic Precipitator) and Bottom Ashesa Ensted PS coal A SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O TiO2 P2O5 SO3 a
Funen PS coal B
coal C
Vendsyssel PS coal D
coal D/coal E
coal E
fly ash
bt. ash
fly ash
bt. ash
fly ash
bt. ash
fly ash
bt. ash
fly ash
bt. ash
fly ash
bt. ash
50.7 26.0 10.2 2.42 2.05 1.92 0.36 1.30 0.59 0.32
52.2 26.3 11.4 2.84 2.05 1.92 0.36 1.30 0.59 0.32
58.5 20.7 7.4 2.98 1.61 1.97 0.61 0.99 0.26 0.54
64.8 17.1 8.1 3.66 1.66 1.53 0.58 0.87 0.25 0.15
49.5 33.0 3.3 6.78 1.71 0.69 0.14 1.76 1.31 0.40
52.0 27.8 3.9 7.42 1.71 0.54 0.07 1.51 1.23 0.21
50.0 27.0 8.7 4.18 2.71 2.29 1.14 1.14 0.60 0.81
51.7 23.3 11.2 4.32 2.75 2.00 0.73 0.99 0.45 0.16
46.1 24.9 10.4 5.26 2.68 2.40 0.93 1.07 0.58 1.17
50.1 22.3 13.4 5.23 2.65 2.14 0.65 0.94 0.39
