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Flame transformations and burner slagging in a 2.5 MW furnace firing pulverized coal
Flame transformations and burner slagging in a 2.5 MW furnace firing pulverized coal I. Flame transformations
H. M. ten Brink,
J. P. Smart?,
J. M. Vleeskens
and J. Williamsont_
Fossil Fuels Department, Netherlands Energy Research Foundation (ECN), 7755 ZG Petten, The Netherlands * International Flame Research Foundation, IJmuiden, The Netherlands $ Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London S W7 2BP, UK (Received 22 June 1993; revised 10 January 7994)
This is the first study in which the mechanism of burner slagging in a furnace firing pulverized coal has been investigated. The objective was to relate the in-flame transformations of the coal minerals to the nature of the slags formed on deposition probes designed to simulate a burner quarl and a superheater tube. The experiments were performed in the 2.5 MW refractory-lined IFRF furnace No. 1, using a swirl-stabilized pulverized coal burner, firing a pyrite-rich coal. In-flame samples and slag deposits were extracted and analysed for pyrite and pyrite decomposition products by electron probe microanalysis and X-ray diffraction. Immediately after injection into the flame, pyrite (FeS,) decomposed to pyrrhotite (FeS), molten droplets of which were oxidized to solid iron oxide (Fe,O, and Fe,O,) particles in both the internal and external recirculation zones in the furnace. Owing to the complex flow pattern in the furnace, the kinetics of the pyrite conversion steps could not be directly determined but were indirectly derived from a comparison with the coal devolatilization and char oxidation steps. It is concluded that the decomposition of pyrite proceeds as fast as the coal devolatilization step and that the oxidation of pyrrhotite to iron oxide is as fast as the oxidation of the coal/char. (Keywords:pulverized coal combustion; slagging; pyrite)
In pulverized coal combustion, several problems are associated with the presence of the incombustible ash-forming material (mineral matter) in the coal’. Difficulties may arise when the mineral matter adheres to heat extraction surfaces in the furnace. When excessive thicknesses of deposit grow - a phenomenon known as slagging - heat extraction is reduced and the delicate heat balance in a boiler is disturbed. Slag deposits break off and may rupture tubing in their fall through the furnace or cause the boiler to ‘trip’ when they arrive hot in the bottom quench water. Boiler slagging has been recognized as one of the most troublesome problems associated with pulverized coal combustion, but is still the least understood phenomenon’. The major difficulty in predicting the severity of deposit formation from a given coal is the fact that the flame-transformed mineral impurities bear only a limited resemblance to the original mineral matter in the coal. Existing knowledge on slagging has mostly come from observations and tests in full-scale installations. However, in such tests the deposition cannot be related to the ash transformations, since the time-temperature path of the t Present Swindon,
address: National Power PLC, Windmill Wiltshire SN5 6PB, UK
0016-2361/94/11/1706-06 ;I” 1994 Butterworth-Heinemann
1706
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Volume
Hill Business
Ltd
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11
Park,
mineral matter in the furnace is not well-defined. Thus only limited insight has been gained into the deposit formation mechanism. In particular, little knowledge is available on the mineral transformations and deposition in the near-burner region’. On the laboratory scale, a knowledge of the mineral matter transformations has been obtained from fundamental studies2-4. These investigations were carried out in furnaces where both the particle heating rate and the peak temperature were lower than in p.f. boilers. The present study is the first in which p.f. particles have been made to experience the conditions of an actual flame, and in addition the complex temperature and oxygen profiles of a boiler have been compared with the laminar flow and isothermal conditions in laboratory furnaces. In this study, experiments were performed with a semi-industrial-scale furnace, with a well-defined aerodynamic and thermal environment similar to that present in full-scale pulverized coal combustors, particularly in the near-burner zone. The aim was both to generate fundamental knowledge on the relation between mineral transformations and deposit formation and to provide results on near-burner slagging directly applicable to slag abatement in full-sized furnaces. Slagging is often initiated by the formation of an iron-rich layer, This is true not only with so-called
Flame transformations
bituminous-type ashes (with Fe,O, > CaO + MgO) but even for lignite-type ashes (Fe,O, < CaO + MgO) with a relatively low iron content 2,5,6 It is beyond doubt that the initial iron-rich layer originates from the pyrite (Fe&) minerals in the coal. However, pyrite undergoes a series of reactions in the furnace and it is not certain which reaction product favours deposit formation. For this reason a pyrite-rich coal was chosen in this trial.
and burner slagging.
1: H. M. ten Brink et al.
EXPERIMENTAL Furnace characteristics and coal choice
The experiments were carried out in IFRF furnace No. 1. This furnace is a refractory chamber with internal dimensions 2.0 x 2.0 x 6.25 m; see Figure 1. The burner used was a swirl-stabilized aerodynamically air-staged burner (AASB), which is described in detail elsewhere7. The burner was operated under unstaged conditions, producing type-2 flames with a characteristic internal recirculation zone located in the near-burner field, and with reverse flow in the core of the flame in front of the burner at the primary air exit. A large external recirculation zone is also induced in the furnace. Flow directions and temperature profiles are shown in Figure 1. The coal was from the Hub seam, Prince mine, Nova Scotia. This h.v. bituminous coal has a high pyrite content and, as used in this trial, also a high ash content. The coal characteristics are given in Table 1. The coal was predried, crushed and pulverized to a standard particle size (75 wt% < 75 pm), as described in detail elsewhere7. With the available amount of coal, -40 h of firing was possible (including 10 h equilibration of the furnace). The composition of different size fractions of the milled coal is given in Table 2. Particle collection
cl
m
*o
100
&
200 “k&I
distance”
(ml)
S”prhca,rr Simullllor
__+
Figure 1 Horizontal section through IFRF furnace No. 1: A, broken line indicates region enlarged in B and C; B, sample collection sites and flow directions in relation to location of slag probes; C, temperature profile and flow boundaries
Table 1 Proximate Volatile Ash
Coal and ash analyses analysis
(wt%)
(db)
Ash analysis
matter
SiO Al,&
33.1 21.0
FeZOX CaO Ultimate analysis C H N S 0 (ditl)
Table 2 Size fraction
(daf)
Composition
MgO TiO so,Z Na,O
79.1 5.6 1.4 5.f 9.0
of sieve-classified
(flm)
Yield (wt%)
175
K,O
53.00 23.20 18.80 2.91 0.87 0.21 0.91 n.d. n.d.
In-flame samples of reacting coal and associated mineral matter were collected with a standard IFRF B-probe* at positions in a horizontal plane through the furnace on a level with the burner, mostly in the near-burner region. With this device, the hot particles are rapidly quenched before collection on a ceramic filter located in the probe head. It is estimated that the characteristic quenching time is 10 ms. Rapid quenching is a prerequisite for an experiment of the present kind in which flame samples are collected from within the burner quarl, almost immediately after injection of the coal and mineral particles (Figure I), and the transformations appear to be very fast (see later). Samples were taken after the furnace had reached thermal equilibrium, between 10 and 20 h after the start of the trial. Figure I shows the locations of the deposit probes superimposed on the flow field and temperature distribution. Analytical methods
The two methods used for particle analysis were scanning electron microscopy combined with elemental analysis by X-ray spectroscopy, better known as electron probe microanalysis (EPMA), and X-ray diffractometry. Supplementary data were obtained by classical chemical analysis.
coal
Sulfur (wt% db)
Ash (wt% db)
Pyrite (wt% of minerals)
Silicates (wt% of minerals)
Fe,O, in ash (wt%)
23.7
3.7
11.0
72
28
6.6
63-75
6.9
4.0
13.9
66
34
19.1
45-63
9.3
4.3
14.2
64
36
24.6
3745
5.6
3.4
18.5
54
46
18.2
54.6
3.4
20.6
34
66
9.1
<37
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Flame transformations
and burner slagging.
Table 3 Composition (wt%,) of ‘in-flame’ and flue gas ashes for comparison
particles
80cm
1: H. M. ten Brink et al. from the side-wall
(H=80)
and at different
Na,O
MgQ
(cm) 0.6
0.9
22.6
53.3
1.1
20
0.5
0.9
23.8
54.1
1.1
50
0.6
0.9
22.2
50.4
1.1
1.4
0.5
0.7
19.9
46.5
1.0
0.3
0.2
0.6
1.4
0.1
0.6
0.8
22.6
47.7
1.0
0.8
1.0
23.0
51.8
1.1
‘In-flame’ 0
particles
at distance
150 Standard Pulverized
deviation” fuel ash”
Flue gas ash
SiO,
AlA
K,Q
distances
CaO
from the burner
wall, and of p.f.
TiO,
MnO
Fe,&
1.2
1.1
0.1
19.1
1.4
1.0
0.1
16.9
1.0
0.2
22.1
1.5
0.9
0.1
28.8
0.1
0.1
0.1
2.1
1.3
1.0
0.1
24.9
1.1
1.0
0.2
19.9
‘n 5 10 for each sample *Standard ash analysis
Table 4 Distribution flame samples
(%) of silicon
between
silicate
Quartz ‘Illite’ ‘Kaoline’ ‘Mixed silicates’
minerals
in the
21 i4.1 35* 5.0 llk1.5 31 k3.7
EPMA. The EPMA method uses a combination of data on size and composition of individual particles for characterization. An automated coal mineral analysis (CMA) programg*” was used to classify each particle; details of this method can be found elsewhere”. To obtain statistical reliability, between 600 and 900 particles from each (flame) sample were analysed. The powdery samples were mixed with epoxy resin, cold-hardened, ground and polished. Final fine polishing was done with 0.5 pm diamond paste, and the samples were coated with evaporated carbon (since carbon does not show peaks in the X-ray emission spectrum). The method was also used to analyse size fractions of the milled coal. X-ray dlfiactometry. This was performed to assess the proportions of the crystalline compounds in the samples. Though the method is qualitative, it was a powerful tool in the present study, since it discriminates between the various forms of iron sulfide and iron oxide’.
RESULTS
AND PRELIMINARY
DISCUSSION
Coal composition Table 2 shows that whereas the ash content decreased
with increasing size, the pyrite content of the minerals increased. Thus the pyrite concentration in each size fraction remained virtually constant. The estimated proportion of sulfur associated with pyrite is in the region of 65% of the total sulfur. Hence the fate of the total sulfur in the furnace should be reflected to a good approximation by the fate of the pyritic sulfur. The results of the analysis of the size fractions by CMA agreed very well with XRD analysis of the raw coal with respect to pyrite, quartz and clays. Only the classification of the clay-type minerals, i.e. illite and kaolinite, differed between the two methods, CMA indicating that illite was the major clay mineral present, whereas normative analysis suggested the reverse.
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EPMA of furnace particles
The compositions of particles taken from the flame are shown in Table 3. They are seen to be closely similar to that of the p.f. ash, except that there are significant variations in Fe,O, level with distance from the burner. Silicate-derived particles should show only phase transitions; the composition of the particles would not be expected to change in the furnace or flame. Pyrite (FeS,), in contrast, undergoes a series of compositional changes in the furnace in which sulfur is lost and iron oxides are formed (see Appendix). Silicates. Analytical data for the distribution of silicon between the main silicates in the furnace samples are given in Table 4. The names in quotation marks have no crystallographic meaning, since most particles are essentially glassy droplets, but refer to the chemical composition. The data did not show any compositional change in the furnace, which confirms the above hypothesis that the chemical composition of silicate particles does not change during passage through the furnace or flame. Most of the iron in the coal is present as pyrite, but some is associated with silicates. Iron silicates remained constant in the furnace, as discussed in more detail in the full report on this trial12. Pyrite. Because the distributions of pyrite and its flame products form an irregular pattern, the data are presented graphically, rather than tabulated, as a function of distance from the burner wall. Pyrite and its products have been classified into three categories: pyrite (S/Fe = 2); pyrrhotite (S/Fe = 1); and iron oxide. Figure 2 shows the relative proportions of iron present in these forms in each sample, as a function of axial distance A at three distances H from the side-wall. It is seen that the pyrite and pyrrhotite concentrations are greatest near the burner at the H = 80 level. Regions with high iron oxide concentration are found at the far end of the flame (H = 100-I 50 cm) and at short distances (A = O-30 cm) from the burner at the H = 60 and H = 100 levels. The total sulfur content of the samples showed the same trend as observed here for the pyritic sulfur12. As a further reference, the carbon content of the near-burner samples is also reported (Table 5, Figure 3). Significant carbon burnout (low carbon concentration) is observed where the iron oxide concentration is high, i.e. close to the burner at the H = 60 and H = 100 levels and at the far end of the flame (compare Figure 3 with Figure 2). The consistently high carbon content at H = 80
Flame
1001%
I-
I,
transformations
and burner
slagging.
1: H. M. ten Brink
et al.
calcite or anhydrite. Table 6 provides an overview of the results. Results for pyrite and its products are given in Figure 4. The ratio of crystalline to amorphous glassy material could not be determined for these samples. Nevertheless the XRD results are of importance, since it is seen that both magnetite and haematite as oxidized pyrite products (see Appendix} are present in the furnace. The distribution of pyrite and its products over the furnace as determined by XRD accords well with the EPMA results. Kaolinite and illite, the original clay-type minerals, are found only at collection sites close to the burner, whereas mullite is mainly present at locations further along the furnace. Mullite is a decomposition product of the clay-type minerals, and a finite time is required for nucleation and growth to give particles detectable by XRD. Thus mullite can be present only in particles that have had sufficient residence time to allow this phase to grow.
I
FURTHER
DISCUSSION
In this section the implications of the mineral transformations for deposit formation in the furnace are discussed. In the evaluation of the results it has been taken into account that the furnace flame samples were examined after cooling, during which process, phase changes occur which modify the appearance of the particles. Silicates 0
20
50
100
150
100%
Quartz. The quartz grains retained their crystalline appearance during passage through the flame. This conclusion stems from microscopic observation but is
Table 5 zone
Carbon
content
(wt%) of flame samples
Distance from burner wall, A
Distance 60
(4 0 10 20
0
20
50
100
150
in the near-burner
from side-wall,
H (cm)
80
100
41 47 49
10 12
15 20 16
52
cm 0
pyriteH
pyrrhotile
0
iron oxide
Figure 2 Distribution ofiron between pyrite, pyrrhotite and iron oxide as a function of distance from burner wall, at different distances (H) from side-wall
shows that this is the level of the forward coal and mineral flow in the furnace, X-ray
difSraction
The major crystalline compounds found by XRD of the furnace samples were the silicate minerals quartz (SiO,) and mullite (3A1,0,.2Si02), and magnetite/spine1 (Fe,O,) as the iron containing phase. Close to the burner, traces of pyrite/pyrrhotite and kaolinite/illite were detected, and in some samples haematite (Fe,O,) was found. Most samples contained traces of a Ca-bearing phase: gypsum,
4 0
10 20 30
50
60
100
150
em A Figure 3 Carbon content of flame samples as a function from burner wall, at different distances (H)from side-wall
of distance
Fuel 1994 Volume 73 Number 11
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Flame
transformations
and burner
Table 6
Crystalline
phases
identified
Distance from sidewall, H (cm)
Distance from burner wall, A (cm)
Pyrite
Illite
100
0 10
slagging.
I: H. hl. ten Brink
in ‘in-flame’ particles”
Kaolinite
W W
Gypsum
Calcite
Pyrrhotite
W
W W
W W
30
0
w
M
w
w
20
w
w
30
W
vs vs
S
W W
S
M
vs
M
M
vs
W
vs
vs
M
vs
M
?
W
M
M
W
vs
W
M
S
W
vs
S
M
W
vs
W
vs
M
vs
?
W
W
W
?
W
W
M
M
W
W
W
M
vs
W
W
M W
W
M
M
W
vs
W
W
W
?
M
M
W
vs
W
M
W
W
S
M
M
vs
100
W
W
?
M
M
W
vs
150
W
W
?
M
S
W
vs
10
W
W
W
S
vs
W
vs
20
W
W
W
S
S
S
vs
M
W
M
M
W
vs
M
W
M
M
W
vs
W
W
30
W
50
W
W
80
M
W
100 M
150 “W = weak, M = medium,
S = strong,
Empty
0
20
60
symbols
denote small amounts
100
150
“Axial distance” (cm) -+ Figure 4 Iron species present in particles sampled at the positions shown. Fuel injection flow indicated by broad arrows
also substantiated by the XRD results. Quartz is a refactory material and it is presumably associated with the particles that were seen to bounce off the superheater probe on impact’ 3. Clay-type minerals. The aluminosilicate-based claytype minerals are transformed from crystalline grains into viscous entities during flame passage, as evidenced by the spherical form and glassy appearance of the particles after collection (see Figures 4.9 and 5.20 in ref. 12). This vitrification of the clay minerals was expected3, given the relatively high temperatures in the flame. Resolidification of the particles takes place after flame passage, as evidenced by the crystallization ofmullite from the glass.
Fuel 1994
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M
M
W
M
M
W
vs
W
M
M
W
vs
vs
VS = very strong
iJ
1710
vs
M M M
80
60
Quartz
M
W
50
Haematite
W
W
W
Spine1
W
W
10
?
Anorthite
M ‘7
100 150
?
Mullite
?
50 80
Anhydrite
W
20
80
et al.
73 Number
11
Pyrite
transformations
In the present study the same pyrite transformation products were detected as in model experiments2v4*‘4. It is not easy to assess the sequence of the reactions of pyrite, because of the uncertainty in the time-temperature history of the particles. However, the conversion sequence as well as the rate at which the various conversion steps take place have been identified in the following indirect way, using the better-known fate of the coal particles as a reference. Initially, it is assumed that the pyrite particles follow trajectories identical to those of the coal particles; this is a valid assumption, as most of the pyrite in the test coal was present within the coal particles and not in discrete form. Also it has been shown from model experiments under controlled conditions (see Appendix or ref. 15) that pyrite particles undergo a series of transformations comparable to those of the coal, i.e. thermal decomposition/ devolatilization and subsequent oxidation of the resulting products. Furthermore, it has been shown that these transformations take place over comparable timescales. These assumptions allow the decomposition and oxidation of the pyrite particle to be traced with respect to the comparable transformations of the coal. For the present coal - 65 wt% of the combustibles are released in the initial devolatilization stage16. This volatile release corresponds to an ash content of the flame samples of -50 wt%. It is thus concluded that samples in which the carbon content is between 40 and 50 wt% are collected at positions where devolatilization is complete but further oxidation of the remaining char is
Flame transformations
not significant. The carbon analyses (Table 5) suggest that the coal/mineral flow is as follows. Injection is along the line H = 80 with an extension of the flow to the H = 60 level and recirculation of the (fuel) flow on to the front wall (A = 10 and 20 at H = 60). A further extension occurs along H = 100, also with flow reversal. Figure 4 gives an overview of the coal/mineral flow. At the first collection point in the coal/mineral flow in the furnace (A =0 at H = 80) the ratio of pyrrhotite to pyrite is > 2. This means that at this location pyrite decomposition is approaching completion. Since the coal devolatilization is also virtually complete there, it is concluded that the pyrite decomposition is as fast as the coal devolatilization. In the recirculation zone the carbon content is low, indicating extended progress in the coal (char) oxidation process. In this zone pyrrhotite is apparently largely converted to iron oxide. It is therefore concluded that the oxidation of pyrrhotite to iron oxide proceeds at a rate comparable with that of the carbon (char) oxidation. Haematite (Fe,O,) is found near the burner wall in the external recirculation zone, and this pyrite product is known to be formed rather slowly”. This suggests that the particles spend a considerable time in the furnace before arriving at this location. The presence of the conversion product mullite at this site suggests a similar long residence time in the furnace for the silicates.
1: H. M. ten Brink et al.
ACKNOWLEDGEMENTS The authors are grateful to Mr G. Hamburg and Miss M. Campbell for the analysis of the samples. The present study was performed within Annex II to the IEA Implementing Agreement on Coal Combustion Sciences (Part I). The participants in this task were Canada, the Federal Republic of Germany and The Netherlands. The study was financed by NOVEM (Netherlands Agency for Energy and the Environment), in the framework of the National Coal Research Programme. REFERENCES I InternationalEnergy 2 3
7
Implications for deposit formation
The transformations of pyrite may be expected to have the following consequences for deposit formation on the simulation probes. In the forward flow pyrite is present as pyrrhotite, which will be molten, in view of its low melting point. That the pyrrhotite particles were indeed molten is evidenced by their spherical shape, contrasted with the crystalline nature of the original pyrite particles1 2. Pyrrhotite arrives at the deposit probe in a liquid state and may be expected to adhere to the surface of the probe. In the external recirculation zone, along H = 60, the pyrite particles are virtually completely oxidized to iron oxide particles before arrival at the reverse side of the quarl probe. Iron oxide is a solid2 at normal furnace temperatures; hence particles of iron oxide would not be expected to adhere to the probe. It is shown in Part 2 of this paper” that the mechanism of actual deposit formation is in accordance with this hypothesis.
and burner slagging.
8 9 10
11 12
13 14
15
16
Agency. ‘Coal Quality and Ash Characteristics: a Study by the IEA Coal Industry Advisory Board’. OECD/IEA, Paris, 1985 Groves, S. J., Williamson, J. and Sanyal, A. Fuel 1987, 66, 461 Raask, E. ‘Mineral Impurities in Coal Combustion: Behaviour, Problems and Remedial Measures’, Hemisphere, New York, and Springer, Berlin, 1985 Huffman, G. P., Huggins, F. E., Levasseur, F. E.. Lytle, F. W. and Greegor, R. B. Fuel 1989, 68, 486 Bryers, R. W. J. Eng. Power 1976, 98, 517 Nakabayashi, Y., Yugami, H., Iritani, J., Haneda, H., Namiki, T. and Masuyama, F. In ‘Coatings and Bimetallics for Aggressive Environments’, Conference Proceedings, American Society of Mechanical Engineers, New York, 1985, p. 103 Smart, J. P. and Knill, K. J. ‘Detailed Characterisation of the Near Burner Field of a Low NO. Burner Firing Coal of Two Different Ranks: Report on the CC 2-4 Experiments’. IFRF Document F088iai8. International Flame Research Foundation. IJmuiden, 1988 International Flame Research Foundation. ‘Technical Services to Members’, IFRF Document C72/a/93, IJmuiden, 1985 Huggins, F. E., Kosmack, D. A., Huffman, G. P. and Lee, R. J. In ‘Scanning Electron Microscopy 1980’, Vol. I, 1980, pp. 531-540 Huggins, F. E., Huffman, G. P. and Lee, R. J. In ‘Coal and Coal Products: Analytical Characterization Techniques’ (Ed. E. L. Fuller Jr), American Chemical Society, Washington, DC, 1982, pp. 239-258 Casuccio, G. S., Gruelich, F. A., Hamburg, G., Huggins, F. E., Nissen, D. A. and Vleeskens, J. M. Scanning Microsc. 1990,4.227 Ten Brink, H. M., Hamburg, G., Vleeskens, J. M., Smart, J. P. and Dug&, J. ‘Mineral Matter and Slagging in a Semi-Industrial Furnace’, ECN-222, Netherlands Energy Research Foundation. Petten, 1989 Ten Brink, H. M., Smart, J. P.. Vleeskens, J. M. and Williamson, J. Fuel 1994, 73, 1712 Simpson, D. R. and Bond, R. M. In ‘Mineral Matter and Ash Deposition from Coal’ (Eds R. W. Brvers and K. S. Vorresl. I Engineering Foundation; New York, 1990, p. 237 Ten Brink, H. M., Hamburg, G., Vleeskens, J. M., Smart, J. P. and Dug&, J. ‘Mineral Matter Transformations in Combustion of Size-Reduced Coal’, ECN-223, Netherlands Energy Research Foundation, Petten, 1989 Ten Brink, H. M. and Heere, P. T. ‘Early Reactivity and Nitrogen Release in Pulverized Coal Combustion’, ECN-225, Netherlands Energy Research Foundation, Petten, 1990 Srinivasachar, S. and Boni, A. A. FurI 1989, 68, 829
CONCLUSIONS
17
The near-burner transformations of minerals in a semi-industrial furnace can indeed be followed by means of the analysis techniques used here. The transformations of pyrite in the furnace are in accordance with results of bench-scale experiments with pure pyrite. The decomposition of pyrite (FeS,) to pyrrhotite (FeS) is as fast as the devolatilization of the coal. The oxidation of pyrite is at least as fast as the residual char oxidation. With respect to deposition on the probes the results should have the following consequences. At the front side of the quarl simulation probe an adherent deposit is formed because of deposition of molten pyrrhotite particles. On the reverse side of the quarl probe solid iron oxide particles will deposit and are not expected to give rise to an adherent deposit.
APPENDIX In-furnace transformations of pyrite (FeS2)
To serve as a reference for the discussion on pyrite transformations in the text, the following simplified chemical reaction sequence for pyrite is provided: FeS, -+ FeS(1) (decomposition) (+S) pyrite 0, FeS(1) + Fe,O,(s) (+ SO,) (oxidation) pyrrhotite Fe,O,(s) 3 Fe,O,(s) magnetite haematite where (1) and (s) denote liquid and solid phases respectively, present under the furnace/flame conditions.
Fuel 1994
Volume
73 Number
11
1711
H. M. ten Brink,
J. P. Smart?,
J. M. Vleeskens
and J. Williamsont_
Fossil Fuels Department, Netherlands Energy Research Foundation (ECN), 7755 ZG Petten, The Netherlands * International Flame Research Foundation, IJmuiden, The Netherlands $ Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London S W7 2BP, UK (Received 22 June 1993; revised 10 January 7994)
This is the first study in which the mechanism of burner slagging in a furnace firing pulverized coal has been investigated. The objective was to relate the in-flame transformations of the coal minerals to the nature of the slags formed on deposition probes designed to simulate a burner quarl and a superheater tube. The experiments were performed in the 2.5 MW refractory-lined IFRF furnace No. 1, using a swirl-stabilized pulverized coal burner, firing a pyrite-rich coal. In-flame samples and slag deposits were extracted and analysed for pyrite and pyrite decomposition products by electron probe microanalysis and X-ray diffraction. Immediately after injection into the flame, pyrite (FeS,) decomposed to pyrrhotite (FeS), molten droplets of which were oxidized to solid iron oxide (Fe,O, and Fe,O,) particles in both the internal and external recirculation zones in the furnace. Owing to the complex flow pattern in the furnace, the kinetics of the pyrite conversion steps could not be directly determined but were indirectly derived from a comparison with the coal devolatilization and char oxidation steps. It is concluded that the decomposition of pyrite proceeds as fast as the coal devolatilization step and that the oxidation of pyrrhotite to iron oxide is as fast as the oxidation of the coal/char. (Keywords:pulverized coal combustion; slagging; pyrite)
In pulverized coal combustion, several problems are associated with the presence of the incombustible ash-forming material (mineral matter) in the coal’. Difficulties may arise when the mineral matter adheres to heat extraction surfaces in the furnace. When excessive thicknesses of deposit grow - a phenomenon known as slagging - heat extraction is reduced and the delicate heat balance in a boiler is disturbed. Slag deposits break off and may rupture tubing in their fall through the furnace or cause the boiler to ‘trip’ when they arrive hot in the bottom quench water. Boiler slagging has been recognized as one of the most troublesome problems associated with pulverized coal combustion, but is still the least understood phenomenon’. The major difficulty in predicting the severity of deposit formation from a given coal is the fact that the flame-transformed mineral impurities bear only a limited resemblance to the original mineral matter in the coal. Existing knowledge on slagging has mostly come from observations and tests in full-scale installations. However, in such tests the deposition cannot be related to the ash transformations, since the time-temperature path of the t Present Swindon,
address: National Power PLC, Windmill Wiltshire SN5 6PB, UK
0016-2361/94/11/1706-06 ;I” 1994 Butterworth-Heinemann
1706
Fuel 1994
Volume
Hill Business
Ltd
73 Number
11
Park,
mineral matter in the furnace is not well-defined. Thus only limited insight has been gained into the deposit formation mechanism. In particular, little knowledge is available on the mineral transformations and deposition in the near-burner region’. On the laboratory scale, a knowledge of the mineral matter transformations has been obtained from fundamental studies2-4. These investigations were carried out in furnaces where both the particle heating rate and the peak temperature were lower than in p.f. boilers. The present study is the first in which p.f. particles have been made to experience the conditions of an actual flame, and in addition the complex temperature and oxygen profiles of a boiler have been compared with the laminar flow and isothermal conditions in laboratory furnaces. In this study, experiments were performed with a semi-industrial-scale furnace, with a well-defined aerodynamic and thermal environment similar to that present in full-scale pulverized coal combustors, particularly in the near-burner zone. The aim was both to generate fundamental knowledge on the relation between mineral transformations and deposit formation and to provide results on near-burner slagging directly applicable to slag abatement in full-sized furnaces. Slagging is often initiated by the formation of an iron-rich layer, This is true not only with so-called
Flame transformations
bituminous-type ashes (with Fe,O, > CaO + MgO) but even for lignite-type ashes (Fe,O, < CaO + MgO) with a relatively low iron content 2,5,6 It is beyond doubt that the initial iron-rich layer originates from the pyrite (Fe&) minerals in the coal. However, pyrite undergoes a series of reactions in the furnace and it is not certain which reaction product favours deposit formation. For this reason a pyrite-rich coal was chosen in this trial.
and burner slagging.
1: H. M. ten Brink et al.
EXPERIMENTAL Furnace characteristics and coal choice
The experiments were carried out in IFRF furnace No. 1. This furnace is a refractory chamber with internal dimensions 2.0 x 2.0 x 6.25 m; see Figure 1. The burner used was a swirl-stabilized aerodynamically air-staged burner (AASB), which is described in detail elsewhere7. The burner was operated under unstaged conditions, producing type-2 flames with a characteristic internal recirculation zone located in the near-burner field, and with reverse flow in the core of the flame in front of the burner at the primary air exit. A large external recirculation zone is also induced in the furnace. Flow directions and temperature profiles are shown in Figure 1. The coal was from the Hub seam, Prince mine, Nova Scotia. This h.v. bituminous coal has a high pyrite content and, as used in this trial, also a high ash content. The coal characteristics are given in Table 1. The coal was predried, crushed and pulverized to a standard particle size (75 wt% < 75 pm), as described in detail elsewhere7. With the available amount of coal, -40 h of firing was possible (including 10 h equilibration of the furnace). The composition of different size fractions of the milled coal is given in Table 2. Particle collection
cl
m
*o
100
&
200 “k&I
distance”
(ml)
S”prhca,rr Simullllor
__+
Figure 1 Horizontal section through IFRF furnace No. 1: A, broken line indicates region enlarged in B and C; B, sample collection sites and flow directions in relation to location of slag probes; C, temperature profile and flow boundaries
Table 1 Proximate Volatile Ash
Coal and ash analyses analysis
(wt%)
(db)
Ash analysis
matter
SiO Al,&
33.1 21.0
FeZOX CaO Ultimate analysis C H N S 0 (ditl)
Table 2 Size fraction
(daf)
Composition
MgO TiO so,Z Na,O
79.1 5.6 1.4 5.f 9.0
of sieve-classified
(flm)
Yield (wt%)
175
K,O
53.00 23.20 18.80 2.91 0.87 0.21 0.91 n.d. n.d.
In-flame samples of reacting coal and associated mineral matter were collected with a standard IFRF B-probe* at positions in a horizontal plane through the furnace on a level with the burner, mostly in the near-burner region. With this device, the hot particles are rapidly quenched before collection on a ceramic filter located in the probe head. It is estimated that the characteristic quenching time is 10 ms. Rapid quenching is a prerequisite for an experiment of the present kind in which flame samples are collected from within the burner quarl, almost immediately after injection of the coal and mineral particles (Figure I), and the transformations appear to be very fast (see later). Samples were taken after the furnace had reached thermal equilibrium, between 10 and 20 h after the start of the trial. Figure I shows the locations of the deposit probes superimposed on the flow field and temperature distribution. Analytical methods
The two methods used for particle analysis were scanning electron microscopy combined with elemental analysis by X-ray spectroscopy, better known as electron probe microanalysis (EPMA), and X-ray diffractometry. Supplementary data were obtained by classical chemical analysis.
coal
Sulfur (wt% db)
Ash (wt% db)
Pyrite (wt% of minerals)
Silicates (wt% of minerals)
Fe,O, in ash (wt%)
23.7
3.7
11.0
72
28
6.6
63-75
6.9
4.0
13.9
66
34
19.1
45-63
9.3
4.3
14.2
64
36
24.6
3745
5.6
3.4
18.5
54
46
18.2
54.6
3.4
20.6
34
66
9.1
<37
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1707
Flame transformations
and burner slagging.
Table 3 Composition (wt%,) of ‘in-flame’ and flue gas ashes for comparison
particles
80cm
1: H. M. ten Brink et al. from the side-wall
(H=80)
and at different
Na,O
MgQ
(cm) 0.6
0.9
22.6
53.3
1.1
20
0.5
0.9
23.8
54.1
1.1
50
0.6
0.9
22.2
50.4
1.1
1.4
0.5
0.7
19.9
46.5
1.0
0.3
0.2
0.6
1.4
0.1
0.6
0.8
22.6
47.7
1.0
0.8
1.0
23.0
51.8
1.1
‘In-flame’ 0
particles
at distance
150 Standard Pulverized
deviation” fuel ash”
Flue gas ash
SiO,
AlA
K,Q
distances
CaO
from the burner
wall, and of p.f.
TiO,
MnO
Fe,&
1.2
1.1
0.1
19.1
1.4
1.0
0.1
16.9
1.0
0.2
22.1
1.5
0.9
0.1
28.8
0.1
0.1
0.1
2.1
1.3
1.0
0.1
24.9
1.1
1.0
0.2
19.9
‘n 5 10 for each sample *Standard ash analysis
Table 4 Distribution flame samples
(%) of silicon
between
silicate
Quartz ‘Illite’ ‘Kaoline’ ‘Mixed silicates’
minerals
in the
21 i4.1 35* 5.0 llk1.5 31 k3.7
EPMA. The EPMA method uses a combination of data on size and composition of individual particles for characterization. An automated coal mineral analysis (CMA) programg*” was used to classify each particle; details of this method can be found elsewhere”. To obtain statistical reliability, between 600 and 900 particles from each (flame) sample were analysed. The powdery samples were mixed with epoxy resin, cold-hardened, ground and polished. Final fine polishing was done with 0.5 pm diamond paste, and the samples were coated with evaporated carbon (since carbon does not show peaks in the X-ray emission spectrum). The method was also used to analyse size fractions of the milled coal. X-ray dlfiactometry. This was performed to assess the proportions of the crystalline compounds in the samples. Though the method is qualitative, it was a powerful tool in the present study, since it discriminates between the various forms of iron sulfide and iron oxide’.
RESULTS
AND PRELIMINARY
DISCUSSION
Coal composition Table 2 shows that whereas the ash content decreased
with increasing size, the pyrite content of the minerals increased. Thus the pyrite concentration in each size fraction remained virtually constant. The estimated proportion of sulfur associated with pyrite is in the region of 65% of the total sulfur. Hence the fate of the total sulfur in the furnace should be reflected to a good approximation by the fate of the pyritic sulfur. The results of the analysis of the size fractions by CMA agreed very well with XRD analysis of the raw coal with respect to pyrite, quartz and clays. Only the classification of the clay-type minerals, i.e. illite and kaolinite, differed between the two methods, CMA indicating that illite was the major clay mineral present, whereas normative analysis suggested the reverse.
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EPMA of furnace particles
The compositions of particles taken from the flame are shown in Table 3. They are seen to be closely similar to that of the p.f. ash, except that there are significant variations in Fe,O, level with distance from the burner. Silicate-derived particles should show only phase transitions; the composition of the particles would not be expected to change in the furnace or flame. Pyrite (FeS,), in contrast, undergoes a series of compositional changes in the furnace in which sulfur is lost and iron oxides are formed (see Appendix). Silicates. Analytical data for the distribution of silicon between the main silicates in the furnace samples are given in Table 4. The names in quotation marks have no crystallographic meaning, since most particles are essentially glassy droplets, but refer to the chemical composition. The data did not show any compositional change in the furnace, which confirms the above hypothesis that the chemical composition of silicate particles does not change during passage through the furnace or flame. Most of the iron in the coal is present as pyrite, but some is associated with silicates. Iron silicates remained constant in the furnace, as discussed in more detail in the full report on this trial12. Pyrite. Because the distributions of pyrite and its flame products form an irregular pattern, the data are presented graphically, rather than tabulated, as a function of distance from the burner wall. Pyrite and its products have been classified into three categories: pyrite (S/Fe = 2); pyrrhotite (S/Fe = 1); and iron oxide. Figure 2 shows the relative proportions of iron present in these forms in each sample, as a function of axial distance A at three distances H from the side-wall. It is seen that the pyrite and pyrrhotite concentrations are greatest near the burner at the H = 80 level. Regions with high iron oxide concentration are found at the far end of the flame (H = 100-I 50 cm) and at short distances (A = O-30 cm) from the burner at the H = 60 and H = 100 levels. The total sulfur content of the samples showed the same trend as observed here for the pyritic sulfur12. As a further reference, the carbon content of the near-burner samples is also reported (Table 5, Figure 3). Significant carbon burnout (low carbon concentration) is observed where the iron oxide concentration is high, i.e. close to the burner at the H = 60 and H = 100 levels and at the far end of the flame (compare Figure 3 with Figure 2). The consistently high carbon content at H = 80
Flame
1001%
I-
I,
transformations
and burner
slagging.
1: H. M. ten Brink
et al.
calcite or anhydrite. Table 6 provides an overview of the results. Results for pyrite and its products are given in Figure 4. The ratio of crystalline to amorphous glassy material could not be determined for these samples. Nevertheless the XRD results are of importance, since it is seen that both magnetite and haematite as oxidized pyrite products (see Appendix} are present in the furnace. The distribution of pyrite and its products over the furnace as determined by XRD accords well with the EPMA results. Kaolinite and illite, the original clay-type minerals, are found only at collection sites close to the burner, whereas mullite is mainly present at locations further along the furnace. Mullite is a decomposition product of the clay-type minerals, and a finite time is required for nucleation and growth to give particles detectable by XRD. Thus mullite can be present only in particles that have had sufficient residence time to allow this phase to grow.
I
FURTHER
DISCUSSION
In this section the implications of the mineral transformations for deposit formation in the furnace are discussed. In the evaluation of the results it has been taken into account that the furnace flame samples were examined after cooling, during which process, phase changes occur which modify the appearance of the particles. Silicates 0
20
50
100
150
100%
Quartz. The quartz grains retained their crystalline appearance during passage through the flame. This conclusion stems from microscopic observation but is
Table 5 zone
Carbon
content
(wt%) of flame samples
Distance from burner wall, A
Distance 60
(4 0 10 20
0
20
50
100
150
in the near-burner
from side-wall,
H (cm)
80
100
41 47 49
10 12
15 20 16
52
cm 0
pyriteH
pyrrhotile
0
iron oxide
Figure 2 Distribution ofiron between pyrite, pyrrhotite and iron oxide as a function of distance from burner wall, at different distances (H) from side-wall
shows that this is the level of the forward coal and mineral flow in the furnace, X-ray
difSraction
The major crystalline compounds found by XRD of the furnace samples were the silicate minerals quartz (SiO,) and mullite (3A1,0,.2Si02), and magnetite/spine1 (Fe,O,) as the iron containing phase. Close to the burner, traces of pyrite/pyrrhotite and kaolinite/illite were detected, and in some samples haematite (Fe,O,) was found. Most samples contained traces of a Ca-bearing phase: gypsum,
4 0
10 20 30
50
60
100
150
em A Figure 3 Carbon content of flame samples as a function from burner wall, at different distances (H)from side-wall
of distance
Fuel 1994 Volume 73 Number 11
1709
Flame
transformations
and burner
Table 6
Crystalline
phases
identified
Distance from sidewall, H (cm)
Distance from burner wall, A (cm)
Pyrite
Illite
100
0 10
slagging.
I: H. hl. ten Brink
in ‘in-flame’ particles”
Kaolinite
W W
Gypsum
Calcite
Pyrrhotite
W
W W
W W
30
0
w
M
w
w
20
w
w
30
W
vs vs
S
W W
S
M
vs
M
M
vs
W
vs
vs
M
vs
M
?
W
M
M
W
vs
W
M
S
W
vs
S
M
W
vs
W
vs
M
vs
?
W
W
W
?
W
W
M
M
W
W
W
M
vs
W
W
M W
W
M
M
W
vs
W
W
W
?
M
M
W
vs
W
M
W
W
S
M
M
vs
100
W
W
?
M
M
W
vs
150
W
W
?
M
S
W
vs
10
W
W
W
S
vs
W
vs
20
W
W
W
S
S
S
vs
M
W
M
M
W
vs
M
W
M
M
W
vs
W
W
30
W
50
W
W
80
M
W
100 M
150 “W = weak, M = medium,
S = strong,
Empty
0
20
60
symbols
denote small amounts
100
150
“Axial distance” (cm) -+ Figure 4 Iron species present in particles sampled at the positions shown. Fuel injection flow indicated by broad arrows
also substantiated by the XRD results. Quartz is a refactory material and it is presumably associated with the particles that were seen to bounce off the superheater probe on impact’ 3. Clay-type minerals. The aluminosilicate-based claytype minerals are transformed from crystalline grains into viscous entities during flame passage, as evidenced by the spherical form and glassy appearance of the particles after collection (see Figures 4.9 and 5.20 in ref. 12). This vitrification of the clay minerals was expected3, given the relatively high temperatures in the flame. Resolidification of the particles takes place after flame passage, as evidenced by the crystallization ofmullite from the glass.
Fuel 1994
Volume
M
M
W
M
M
W
vs
W
M
M
W
vs
vs
VS = very strong
iJ
1710
vs
M M M
80
60
Quartz
M
W
50
Haematite
W
W
W
Spine1
W
W
10
?
Anorthite
M ‘7
100 150
?
Mullite
?
50 80
Anhydrite
W
20
80
et al.
73 Number
11
Pyrite
transformations
In the present study the same pyrite transformation products were detected as in model experiments2v4*‘4. It is not easy to assess the sequence of the reactions of pyrite, because of the uncertainty in the time-temperature history of the particles. However, the conversion sequence as well as the rate at which the various conversion steps take place have been identified in the following indirect way, using the better-known fate of the coal particles as a reference. Initially, it is assumed that the pyrite particles follow trajectories identical to those of the coal particles; this is a valid assumption, as most of the pyrite in the test coal was present within the coal particles and not in discrete form. Also it has been shown from model experiments under controlled conditions (see Appendix or ref. 15) that pyrite particles undergo a series of transformations comparable to those of the coal, i.e. thermal decomposition/ devolatilization and subsequent oxidation of the resulting products. Furthermore, it has been shown that these transformations take place over comparable timescales. These assumptions allow the decomposition and oxidation of the pyrite particle to be traced with respect to the comparable transformations of the coal. For the present coal - 65 wt% of the combustibles are released in the initial devolatilization stage16. This volatile release corresponds to an ash content of the flame samples of -50 wt%. It is thus concluded that samples in which the carbon content is between 40 and 50 wt% are collected at positions where devolatilization is complete but further oxidation of the remaining char is
Flame transformations
not significant. The carbon analyses (Table 5) suggest that the coal/mineral flow is as follows. Injection is along the line H = 80 with an extension of the flow to the H = 60 level and recirculation of the (fuel) flow on to the front wall (A = 10 and 20 at H = 60). A further extension occurs along H = 100, also with flow reversal. Figure 4 gives an overview of the coal/mineral flow. At the first collection point in the coal/mineral flow in the furnace (A =0 at H = 80) the ratio of pyrrhotite to pyrite is > 2. This means that at this location pyrite decomposition is approaching completion. Since the coal devolatilization is also virtually complete there, it is concluded that the pyrite decomposition is as fast as the coal devolatilization. In the recirculation zone the carbon content is low, indicating extended progress in the coal (char) oxidation process. In this zone pyrrhotite is apparently largely converted to iron oxide. It is therefore concluded that the oxidation of pyrrhotite to iron oxide proceeds at a rate comparable with that of the carbon (char) oxidation. Haematite (Fe,O,) is found near the burner wall in the external recirculation zone, and this pyrite product is known to be formed rather slowly”. This suggests that the particles spend a considerable time in the furnace before arriving at this location. The presence of the conversion product mullite at this site suggests a similar long residence time in the furnace for the silicates.
1: H. M. ten Brink et al.
ACKNOWLEDGEMENTS The authors are grateful to Mr G. Hamburg and Miss M. Campbell for the analysis of the samples. The present study was performed within Annex II to the IEA Implementing Agreement on Coal Combustion Sciences (Part I). The participants in this task were Canada, the Federal Republic of Germany and The Netherlands. The study was financed by NOVEM (Netherlands Agency for Energy and the Environment), in the framework of the National Coal Research Programme. REFERENCES I InternationalEnergy 2 3
7
Implications for deposit formation
The transformations of pyrite may be expected to have the following consequences for deposit formation on the simulation probes. In the forward flow pyrite is present as pyrrhotite, which will be molten, in view of its low melting point. That the pyrrhotite particles were indeed molten is evidenced by their spherical shape, contrasted with the crystalline nature of the original pyrite particles1 2. Pyrrhotite arrives at the deposit probe in a liquid state and may be expected to adhere to the surface of the probe. In the external recirculation zone, along H = 60, the pyrite particles are virtually completely oxidized to iron oxide particles before arrival at the reverse side of the quarl probe. Iron oxide is a solid2 at normal furnace temperatures; hence particles of iron oxide would not be expected to adhere to the probe. It is shown in Part 2 of this paper” that the mechanism of actual deposit formation is in accordance with this hypothesis.
and burner slagging.
8 9 10
11 12
13 14
15
16
Agency. ‘Coal Quality and Ash Characteristics: a Study by the IEA Coal Industry Advisory Board’. OECD/IEA, Paris, 1985 Groves, S. J., Williamson, J. and Sanyal, A. Fuel 1987, 66, 461 Raask, E. ‘Mineral Impurities in Coal Combustion: Behaviour, Problems and Remedial Measures’, Hemisphere, New York, and Springer, Berlin, 1985 Huffman, G. P., Huggins, F. E., Levasseur, F. E.. Lytle, F. W. and Greegor, R. B. Fuel 1989, 68, 486 Bryers, R. W. J. Eng. Power 1976, 98, 517 Nakabayashi, Y., Yugami, H., Iritani, J., Haneda, H., Namiki, T. and Masuyama, F. In ‘Coatings and Bimetallics for Aggressive Environments’, Conference Proceedings, American Society of Mechanical Engineers, New York, 1985, p. 103 Smart, J. P. and Knill, K. J. ‘Detailed Characterisation of the Near Burner Field of a Low NO. Burner Firing Coal of Two Different Ranks: Report on the CC 2-4 Experiments’. IFRF Document F088iai8. International Flame Research Foundation. IJmuiden, 1988 International Flame Research Foundation. ‘Technical Services to Members’, IFRF Document C72/a/93, IJmuiden, 1985 Huggins, F. E., Kosmack, D. A., Huffman, G. P. and Lee, R. J. In ‘Scanning Electron Microscopy 1980’, Vol. I, 1980, pp. 531-540 Huggins, F. E., Huffman, G. P. and Lee, R. J. In ‘Coal and Coal Products: Analytical Characterization Techniques’ (Ed. E. L. Fuller Jr), American Chemical Society, Washington, DC, 1982, pp. 239-258 Casuccio, G. S., Gruelich, F. A., Hamburg, G., Huggins, F. E., Nissen, D. A. and Vleeskens, J. M. Scanning Microsc. 1990,4.227 Ten Brink, H. M., Hamburg, G., Vleeskens, J. M., Smart, J. P. and Dug&, J. ‘Mineral Matter and Slagging in a Semi-Industrial Furnace’, ECN-222, Netherlands Energy Research Foundation. Petten, 1989 Ten Brink, H. M., Smart, J. P.. Vleeskens, J. M. and Williamson, J. Fuel 1994, 73, 1712 Simpson, D. R. and Bond, R. M. In ‘Mineral Matter and Ash Deposition from Coal’ (Eds R. W. Brvers and K. S. Vorresl. I Engineering Foundation; New York, 1990, p. 237 Ten Brink, H. M., Hamburg, G., Vleeskens, J. M., Smart, J. P. and Dug&, J. ‘Mineral Matter Transformations in Combustion of Size-Reduced Coal’, ECN-223, Netherlands Energy Research Foundation, Petten, 1989 Ten Brink, H. M. and Heere, P. T. ‘Early Reactivity and Nitrogen Release in Pulverized Coal Combustion’, ECN-225, Netherlands Energy Research Foundation, Petten, 1990 Srinivasachar, S. and Boni, A. A. FurI 1989, 68, 829
CONCLUSIONS
17
The near-burner transformations of minerals in a semi-industrial furnace can indeed be followed by means of the analysis techniques used here. The transformations of pyrite in the furnace are in accordance with results of bench-scale experiments with pure pyrite. The decomposition of pyrite (FeS,) to pyrrhotite (FeS) is as fast as the devolatilization of the coal. The oxidation of pyrite is at least as fast as the residual char oxidation. With respect to deposition on the probes the results should have the following consequences. At the front side of the quarl simulation probe an adherent deposit is formed because of deposition of molten pyrrhotite particles. On the reverse side of the quarl probe solid iron oxide particles will deposit and are not expected to give rise to an adherent deposit.
APPENDIX In-furnace transformations of pyrite (FeS2)
To serve as a reference for the discussion on pyrite transformations in the text, the following simplified chemical reaction sequence for pyrite is provided: FeS, -+ FeS(1) (decomposition) (+S) pyrite 0, FeS(1) + Fe,O,(s) (+ SO,) (oxidation) pyrrhotite Fe,O,(s) 3 Fe,O,(s) magnetite haematite where (1) and (s) denote liquid and solid phases respectively, present under the furnace/flame conditions.
Fuel 1994
Volume
73 Number
11
1711