IFRF Combustion Journal Article Number , October 1999 ISSN X

Transcription

1 IFRF Combustion Journal Article Number , October 1999 ISSN X ASH DEPOSITS IN A COAL-FIRED POWER STATION RELATED TO EXPERIMENTALLY MEASURED ASH CHARACTER. C W Bailey, G W Bryant and T F Wall Department of Chemical Engineering and CRC for Black Coal Utilisation The University of Newcastle Corresponding Author: Professor T F Wall Department of Chemical Engineering The University of Newcastle University Drive CALLAGHAN NSW 2308 AUSTRALIA Telephone: Fax: cgtfw@cc.newcastle.edu.au IFRF - Combustion Journal journal@ifrf.net

2 IFRF Combustion Journal - 2 ABSTRACT Boiler deposits collected from an Australian power station were characterised and the observed deposit properties correlated with combustion residues obtained from drop tube furnace experiments using two of the power station coals noted for deposition problems. The dominant growth mechanism for deposits formed from the burner region to the screen wall was found to be inertial impaction of sticky particles. A measured systematic increase in the iron content for the deposits through the boiler was found to be due to the increased iron content required for the primarily alumino-silicate ash particles to be sticky as temperature also decreases. For the economiser region, deposit growth was found to be due to fine dry alumino-silicate and iron oxide residues deposited via the thermophoresis mechanism. It was concluded that un-associated siderite residues only contribute to deposition in the radiant section of the boiler. Deposition in the convective section was found to be due to iron alumino-silicate residues formed from included siderite associated with clays, with an increasing level of iron required in the glass with decreases in the prevailing gas temperature. This trend indicates that the included siderite grains in coal are the primary cause of slagging. Key Words: Fouling; slagging; iron; sticky particles; boiler deposits.

3 IFRF Combustion Journal - 3 INTRODUCTION Operating experience on a particular power station has shown that iron in the coal used is associated with furnace slagging, but that the iron level in ash alone does not quantify its severity. A study was therefore made to attempt to relate the character of the iron minerals and its distribution through the pulverised coal to slagging in different regions of the furnace. The study involved laboratory coal combustion measurements to establish the mechanisms by which sticky ash particles are generated, followed by the relating of the chemistry of these particles to the analysis of deposits sampled in the power station. Most of the conclusions reached are particular to the coals and power station considered. The ash formation mechanisms established, however, will apply to coals containing iron in the particular mineral form of the coal studied, this being siderite. The power station studied utilises two 350MW boilers. The boilers are opposed wall fired with three banks of five swirl burners on each wall and a peak flame temperature of ~ 1600 o C. Emission control technologies, such as low NO x burners are not used, the coal is fed with excess air. Six areas of deposition within the boiler were identified and are recorded as sections A to F in Figure 1 (Worthington, 1993). From Figure 1 it can be seen that deposition problems are encountered on most surfaces in the boiler. Large differences in the prevailing gas temperature for different deposit regions would be expected. The power station is supplied with coal from nearby coal-fields. The exact details for the coals being fired when the deposits used in this study were collected are unavailable. However, the mineralogy for the coals fired at the power station defines a relatively simple chemical system as shown in Table 1 (Biggs, 1996). The coals are dominated by clay and iron minerals, in particular kaolinite and siderite. The factor that differentiates between the power station coals in respect to siderite is the mode of occurrence and the association with other minerals in the coal (in particular the clay minerals such as kaolinite and other minerals such as quartz).

4 IFRF Combustion Journal - 4 Figure 1: Regions for deposit formation at the power station(worthington, 1993)

5 IFRF Combustion Journal - 5 Table 1: Mineral analysis for the coal-fields supplying the power station (Biggs 1996) Mineral name % Total Mineral Matter All Kaolinite 62.3 Siderite 16.8 Quartz 6.0 Goethite 5.0 Haematite 3.0 Gypsum 1.0 Smectite-Kaolinite 1.0 Illite 0.6 Pyrite 0.5 Montmorillionite 0.5 Illite-Smectite 0.3 Calcite 0.3 Ilmenite 0.2 Nacrite 0.2 Fluorapatite 0.2 Dickite 0.1 Uncombusted Organics 1.2 Total 99.2

6 IFRF Combustion Journal - 6 Iron minerals are known to contribute to deposition, the chemical type, form and associations for the iron mineral determining the deposition potential. Previous work identified the transformations of included and excluded siderite under oxidising (Bailey, Bryant et al, 1998: Bryant, Bailey et al, 1997) and reducing conditions (McLennan, Bryant et al, 1999: McLennan, Bryant et al, 1999). The interaction of siderite and clay minerals was also investigated (Bryant, Bailey et al, 1997: McLennan, Bryant et al, 1999: McLennan, Bryant et al, 1999). These mineral transformation mechanisms were used to determine the deposit formation mechanism(s) for the boiler regions studied. Deposits Studied The deposit samples used in this study were collected for a previous investigation at the power station (Worthington, 1993). The locations within the boiler where the samples were collected are shown in Table 2. From the table it can be seen that the samples obtained cover most regions within the boiler. For characterisation purposes two main techniques were used: scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and x-ray fluorescence (XRF). Elemental mapping using the electron microprobe was performed for a small number of the deposit samples (samples with a clear deposit/surface interface) in order to investigate deposit initiation. For characterisation using XRF ~1g of selected deposit samples were ground to a powder and fused into a borate glass bead at 1050 o C. XRF analysis utilised a Philips PW1404 wavelength dispersive sequential XRF spectrometer controlled by Philips X40 software. This technique provides bulk deposit composition as elemental oxides. For characterisation using SEM-EDS deposit samples were cross-sectioned and mounted in resin. Samples were polished and carbon coated prior to analysis using a JEOL JSM- 840 SEM with an attached Oxford Isis EDS system. The typical operating parameters were: beam current = 3 nano-amps, accelerating voltage = 15KeV, working distance varying from 15-38mm. This technique allows the correlation of deposit morphology with chemical composition.

7 IFRF Combustion Journal - 7 Table 2: Deposit Sample Locations Sample Set Sample Sample Set Sample Burner Regions Unit 2 A side Top Burner Secondary Superheaters (cont) Unit 2 A side Bottom Burner Unit 2 B side Top Burner; Sec SH 2 Top Sec SH 2 Middle Sec SH2 Bottom Unit 2 B side Bottom Burner Tertiary Superheaters Tertiary SH Middle Furnace walls Unit 2 5 th level Powdery fly ash Tertiary SH Bottom Unit 2, 2 nd and 3 rd level sintered deposit Unit 1 3 rd level Running Slag Front Screen Wall Top Middle Nose Unit 2 Tip of nose Bottom Secondary Superheaters Sec SH 1 Element 13 Top Economiser A side Sec SH 1 Element 7 Middle B Side Sec SH 1 Element 14 Bottom Elemental mapping was performed using a Cameca Camebax electron microprobe with a wavelength dispersive spectrometer. A Kevex 7000 EDS facility was also available. Mapping was performed for resin mounted specimens using an array consisting of iron, silicon, aluminium, calcium and magnesium. This technique was used in an investigation of deposit initiation for superheater deposit samples.

8 IFRF Combustion Journal - 8 Deposit Characterisation In respect to chemical composition, the XRF analyses indicated the same components for all deposit samples when analysed in bulk, differences lie in the levels of particular components. This will be discussed in a later section. SEM-EDS analysis indicated that all deposits were due to iron and clay mineral residues. The mineralogy of the coals used at the power station is dominated by clay (~65%) and iron minerals (~25%) and this is reflected in the deposit composition (SiO 2 + Al 2 O 3 + Fe 2 O 3 > 90wt%). The initiation of deposits was determined for the superheater region as these samples had clear deposit/superheater surface interfaces. SEM imaging and elemental mapping indicated that for this region the initiation of deposits is via fine dry alumino-silicate particles much less than one micron in diameter, as shown on Figure 2 for SEM analysis and Figure 3 for elemental mapping. Iron rich particles of a larger diameter (~1-5 microns) were found to be a minor component. These particles are most likely transported to the surface via thermophoresis. The dominant growth mechanism for each set of deposit samples was determined from observations of the deposit structure using the SEM. For the deposits formed in the radiant section, on the superheaters and the screen wall, growth is via inertial impaction of sticky particles greater than microns, as shown by Figure 4 for the tip of nose deposit sample.

9 IFRF Combustion Journal - 9 Figure 2: SEM image of secondary superheater 2 deposit/surface interface

10 IFRF Combustion Journal - 10 Figure 3: Elemental mapping of secondary superheater 2 deposit/surface interface

11 IFRF Combustion Journal - 11 For the economiser region growth was found to be via a different mechanism. These deposits consisted of fine dry alumino-silicate particles less than one micron as the major component and larger dry iron oxide particles (up to microns) as a minor component. The size of the alumino-silicate particles and the uniformity of the deposit suggested thermophoresis as the dominant mechanism for deposit growth for this case. Figure 4: SEM image of deposit structural components for the tip of nose deposit sample Figure 5 presents the normalised XRF data for the deposit samples on the SiO 2 -Al 2 O 3 - FeO n phase diagram with liquidus lines for the system in equilibrium with metallic iron (Scheel, 1975). Also shown on the figure is the bulk ash compositional region for a range of the power station coals (Biggs, 1996). The points on the diagram have been assigned a number, an explanation for each coded point is shown in the figure caption. From the figure there is a clear trend in respect to the bulk iron content and distance from the burner region for deposits identified as being formed via inertial impaction of sticky particles (points 1,4,5,6 and 7). A similar trend in respect to bulk iron content for deposits was observed in the study by Juniper et al (1993), although the data did not

12 IFRF Combustion Journal - 12 extend as far through the furnace due to the study focusing on deposition in the radiant section and not including the convective section. Figure 5: Normalised XRF data for deposit samples presented on the FeO n -SiO 2 - Al 2 O 3 ternary phase diagram with liquidus isotherms for the system in contact with metallic iron 3. Point 1: Burner deposits (fused) Gas Temperature ~ 1600 o C; Point 2: line of constant SiO 2 /Al 2 O 3 ratio; Point 3: Bulk coal ash compositional range for the coalfields; Point 4: Wall deposits (fused/semi-fused) Gas temperature ~1400 o C; Point 5: Screen wall deposits (semi-fused/sintered) Gas temperature < o C; Point 6: Superheater deposits (semi-fused/sintered) Gas temperature o C; Point 7: Tip of nose deposit (fused/semi-fused) Gas temperature o C; Point 8: Economiser deposits (powder) Gas temperature <1000 o C.

13 IFRF Combustion Journal - 13 The increase in bulk iron content for deposits suggested a more dominant role for the iron mineral residues with decreases in temperature. While only a small difference in iron content is observed for samples from the burner (point 1) and furnace wall (point 4) regions there is a large increase for samples in the superheater region (point 6). This indicates more selective deposition for the cooler boiler regions. While the bulk ash (point 3) may be sticky at burner temperatures only certain particle types are sticky at the lower temperatures. It is expected from the SEM-EDS analysis of particle compositions that the included mineral residues play a more dominant role as temperature decreases. It is for this reason that the power station coals used in the study were separated into included and excluded mineral rich fractions prior to experiments. ASH CHARACTER MEASUREMENTS 1) Experiments with power station coals Two of the coals noted for deposition problems (Biggs, 1996), (coals DC and BH) were separated into sink (excluded mineral rich) and float (included mineral rich) fractions at a specific gravity of 2.8. The fractions obtained were size graded to microns and used in drop tube furnace experiments performed at 1600 o C in an oxidising environment in order to allow a comparison between ash character and observed deposit properties. The modes of occurrence and associations for the siderite in the coal samples were determined using SEM analysis prior to use in combustion experiments. The SEM investigation indicated that both included and excluded siderite may be present as an associated (with another mineral type) or un-associated (only siderite) mineral. For the excluded case, the dominant form is un-associated nodular siderite for coal DC and unassociated cleat material for coal BH. Other excluded forms apparent in both samples include physical association with quartz and clay minerals such as kaolinite. The included forms were observed to be large un-associated siderite inclusions (50-80 microns) spread out over an entire coal particle and smaller siderite inclusions (20-50 microns) present in association with clays.

14 IFRF Combustion Journal ) Results for experiments using power station coals The combustion residues obtained from the drop tube experiments were analysed using SEM-EDS for semi-quantitative particle compositions. The compositions obtained were then adjusted to 100wt% for one of two ternary systems depending on relative proportions of components. The two systems are: the FeO n -MgO-CaO system or the FeO n -SiO 2 -Al 2 O 3 system. Residues containing < 5wt% SiO 2 + Al 2 O 3 and > 95wt% FeO + MgO + CaO + MnO were plotted on the FeO n -MgO-CaO phase diagram with the iron and manganese contents combined. Residues with higher proportions of SiO 2 + Al 2 O 3 were normalised to 100wt% FeO + SiO 2 + Al 2 O 3 and plotted on the FeO n -SiO 2 -Al 2 O 3 phase diagram. For all residues the CaO + MnO + MgO content was less than approximately 7wt%. The normalised SEM-EDS compositional regions for residues obtained from the drop tube furnace experiments are presented on Figure 6. The points of interest on Figure 6 have been assigned a number, an explanation for the coded points is shown in the figure caption. For the un-associated included and excluded siderite residues the FeO n -MgO- CaO system, with isotherms for the system in contact with metallic iron (Muan and Osborn, 1965) (Diagram A), shows a narrow compositional region for the residues (point 3). The siderite present in the coal is known to have little variation in composition (Patterson, Corcoran and Kinealy, 1994) and this is reflected in the compositions obtained for un-associated siderite residues. Depending on the level of oxidation these residues are expected to be solid at temperatures less than ~1400 o C. Residues resulting from included and excluded siderite associated with clays are plotted on the FeO n -SiO 2 -Al 2 O 3 ternary phase diagram with isotherms for the system in contact with metallic iron (Scheel, 1975), (Diagram B). For this case a larger spread is observed for the residue compositions (areas 1,2 and 3). A similar compositional spread was observed for coal BH, as well as other coals, by McLennan et al (1999). The residue compositions for the associated excluded particles are generally clustered around the dominant parent mineral composition, as shown by regions one and three on the figure.

15 IFRF Combustion Journal - 15 Figure 6: SEM-EDS chemical compositions for residues obtained from experiments using power station coals. Diagram A: FeO n -CaO-MgO system with isotherms for the system in contact with metallic iron; Point 1: Eutectic 2370 o C; Point 2: Eutectic 1160 o C; Point 3: Compositional area for pure included and excluded siderite residues. Diagram B: FeO n -Al 2 O 3 -SiO 2 system with isotherms for the system in contact with metallic iron; Area 1: Un-associated siderite residues; Area 2: Products of the coalescence of siderite and clay mineral residues, primarily as included minerals; Area 3: Un-associated clay residues.

16 IFRF Combustion Journal - 16 For the included associated residues a much larger compositional range is evident, with residue compositions covering the entire triangular area marked on the figure (regions one, two and three). Compositions falling in region two were solely due to included minerals. These residues (as Fe 2+ ) will be sticky at considerably lower temperatures (<1200 o C) when compared to the un-associated siderite residues (as Fe 2+ ). The reduced rate of oxidation for included minerals compared to excluded minerals may also be a factor when considering formation of sticky particles. McLennan et al indicate that reducing conditions do prolong the existence of Fe(II) oxides, such as wustite from pure siderite (McLennan, Bryant et al, 1999: McLennan, Bryant et al, 1999). Residues containing iron 2+ will be sticky at a lower temperature than residues containing iron 3+. DISCUSSION From the analysis of deposits and combustion experiments using the power station coals the important particles types contributing to deposition, their proposed transformations during combustion and their proposed role in deposition is summarised in order of importance on Figures 7a-7c. Figures 7a and 7b indicate that the most troublesome particle types are those where the siderite is associated with clays. A similar conclusion was made from studies of iron glass formation by McLennan et al (1999). The lower liquidus and solidus temperatures for iron alumino-silicates (as low as 1200 o C) when compared to iron oxides (~1400 o C) results in an increased extent of deposition for the associated siderite residues when compared to un-associated siderite residues. The included siderite associated with clays is expected to be more troublesome than excluded siderite associated with clays due to the higher temperatures and more reducing environment experienced by the included form prolonging the existence of sticky Fe 2+ phases. Figure 7c details the transformations for un-associated included and excluded siderite. For this case, the siderite residues are only expected to contribute to deposition in the radiant section of the furnace.

17 IFRF Combustion Journal - 17 It is clear from the evidence presented that for this case the mode of occurrence for the siderite is the most important factor when considering the deposition potential. The use of average iron levels will predict deposition problems in the radiant section for this case. However, for the convective section average iron levels will not indicate deposition problems. CONCLUSIONS The conclusions for the study in terms of deposit initiation, deposit growth and the relationship with the mineral character of the parent coal are summarised below: Deposit initiation involves a fine particle layer derived from kaolinite and siderite fragments. Deposit growth for regions extending from the burners to the screen wall is due to inertial impaction of sticky particles derived from iron and clay minerals. For the economiser it is due to fine particles deposited via thermophoresis. Excluded siderite grains will only contribute to deposition as a sticky particle in the radiant section. Residues from excluded and included siderite associated with clays will contribute to deposition as a sticky particle in the radiant and convective sections of the boiler. Therefore, deposit growth in the radiant section is a function of the mineral matter in the whole coal, whereas for the convective section it is a function of the included minerals with small contributions from the associated excluded minerals. The burner deposit chemistry is the same as the bulk ash chemistry, therefore, the bulk ash analysis obtained using XRF can indicate deposition in the radiant section of the boiler for these coals The average iron in coal will not determine the deposition behaviour in the regions removed from the burners. The deposition is due to products from the coalescence of iron and clay minerals, hence it is the associations of the iron that are important when considering deposition in these regions.

18 IFRF Combustion Journal - 18 Figure 7a: Proposed mechanisms for the transformations of important particle types present in the power station coals and their role in deposit formation 1: Transformations of included siderite associated with clay minerals Figure 7b: Proposed mechanisms for the transformations of important particle types present in the power station coals and their role in deposit formation 2: Transformations of excluded siderite associated with clay minerals

19 IFRF Combustion Journal - 19 Figure 7c: Proposed mechanisms for the transformations of important particle types present in the power station coals and their role in deposit formation 3: Transformations of un-associated included and excluded siderite ACKNOWLEDGEMENT The authors wish to acknowledge the financial support provided by the Co-operative Research Centre for Black Coal Utilisation, which is funded in part by the Co-operative Research Centres Program of the Commonwealth Government of Australia. The assistance provided by the SEM unit here at the University of Newcastle is also gratefully acknowledged. REFERENCES Bailey, C.W., Bryant, G.W., Matthews, E.M., Wall, T.F., Investigation of the High Temperature Behaviour of Excluded Siderite Grains During Pulverised Fuel Combustion, Energy and Fuels, 12, pp , 1998.

20 IFRF Combustion Journal - 20 Biggs,M. S., The Distribution and Significance of Iron Minerals in the Callide Coal Measures East Central Queensland, Masters Thesis, Queensland University of Technology, Bryant, G.W, Bailey, C.W., Wu, H., McLennan, A.R., Stanmore, B.R., and Wall, T.F., Iron in Coal and Slagging: The Significance of the High Temperature Behaviour of Siderite Grains During Combustion, in Proceedings of the Engineering Foundation Conference on Impact of Mineral Impurities in Solid Fuel Combustion, November 2-7, Kona Hawaii, Juniper L., Pohl J., Ottrey A., Creelman B., Wall T., Kent J., and Griss R., Slagging Investigations at Callide 'B' Power Station, Queensland Electricity Commission Contract No. CC/92/269, McLennan, A.R., Bryant, G.W., Stanmore, B.R., and Wall, T.F., Ash Formation Mechanisms During PF Combustion in Reducing Conditions, submitted to Energy and Fuels, McLennan, A.R., Bryant, G.W., Stanmore, B.R., and Wall, T.F., An Experimental Comparison of the Ash Formed from High Iron Coals in Oxidising and Reducing Conditions, submitted to Energy and Fuels, Muan A., and Osborn E.F., Phase Equilibria Among Oxides in Steelmaking, Addison Wesley Publishing Company Inc., Reading, Massachusetts, Patterson, J.H., Corcoran, J.F., Kinealy, K.M., The Chemistry and Mineralogy of Carbonates in Australian Bituminous and Sub-bituminous Coals Fuel, 73, 11, pp , Scheel, R., Sprechsaal Keram, Glas, Baustoffe 108, 23-24, pp , Worthington N, Unit One Boiler Fouling Survey, Internal Report, Austa Electric File No. 660/120/10, 1993.