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The fate of organically bound inorganic elements and sodium chloride during fluidized bed combustion of high sodium, high sulphur low rank coals
The fate of organically bound inorganic elements and sodium chloride during fluidized bed combustion of high sodium, high sulphur lo~w rank coals A. Reza Manzoori
and Pradeep
K. AgarwaI*
The Electricity Trust of South Australia, PO Box 6, Eastwood, SA 5063, Australia *Department of Chemical Engineering, The University of Adelaide, GPO Box 498, Adelaide SA 5001, Australia (Received 18 June 1991; revised 8 November 1997 )
A series of experiments has been conducted to study the transformations of the organically bound inorganic elements and inherent sodium chloride under conditions relevant to fluidized bed combustion of low rank coals. Large coal particles (5.5-9.0 mm) were suspended in a convective gas flow in a single particle furnace operated at 700~830°C. The experiments were performed under pyrolysis as well as combustion conditions. The product particles withdrawn from the furnace at various residence times were analysed using chemical methods, X-ray diffraction and electron microscopy. It was found that the transformations of the inorganic matter in high sodium, high sulphur low rank coals result in the formation of a molten ash layer on the char surface, providing evidence of the formation of low melting point eutectics. The molten ash consisted of a matrix of mixed alkali sulphates. The findings are expected to assist in explaining the role of the inorganic matter in causing operational problems during the fluidized bed combustion of low quality, low rank coals. (Keywords: fluidized bed ; coal combustion ; inorganic matter)
Coals are complex material and contain inorganic species in numerous forms. The inorganic matter in high rank coals is mostly in the form of fine crystallites of minerals such as clays, quartz, carbonates and pyrites embedded in the coal matrix. These minerals are released from the coal matrix in the form of ash when the coal is burned in a fluidized bed combustor. The ash from high rank coals is mostly refractory and imposes minimal impact on the design of the fluidized bed combustion (FBC) systems. Low rank coals, however, along with the above minerals, contain a number of organically bound inorganic elements which are mostly alkaline in nature. These elements are likely to be intimately distributed in the coal matrix. Sodium, calcium, magnesium, potassium and, to a lesser extent, aluminium and iron are known to be present in low rank coals as cations attached to the carboxylic groups’,‘. Sulphur can be part of the coal structure3 and chlorine can be bound to the organic matter4t5. Chlorine can also be present as free ions in the inherent moisture‘j. These elements are also part of discrete mineral inclusions. However, at relatively low operating temperatures in FBC, only the organically bound inorganic elements together with low melting compounds such as sodium chloride are likely to undergo major transformation. The transformations of the mineral inclusions are likely to be less significant in terms of their impact on the design and operation of the FBC systems. The transformations of coal inorganic matter in pulverized coal fired systems are well documented’. 00162361/92/050513-10 ‘,c 1992 Butterworth-Heinemann Ltd.
However, only limited research covering this subject has been carried out under FBC conditions. The transformations of the coal inorganic matter will depend on heat and mass transfer mechanisms and their extent could well be influenced by particle size and heating rate. In comparison with pulverized coal combustion, FBC employs larger coal particle sizes (up to 10 mm) and operates at lower temperatures (about SOO-9OO“C). Hence, the research work carried out under pulverized coal combustion may not be applicable to FBC. Another feature of FBC is that the coal represents less than 5% of the total solid inventory in the furnace. Clearly, the interaction between the ash and the inert bed material comprising the bulk of the bed requires additional considerations. Limited research addressing these issues indicates that sodium in low rank coals has a detrimental effect on the operation of FBC systems : its presence was linked to the formation of agglomerates of bed particles which obstruct smooth fluidization8s9. In extreme conditions, the upward flowing gas is unable to support the weight of the bed and maintain a stable suspension, a phenomenon termed as defluidization. While some efforts have been made to address the operational problems associated with the high sodium low rank coals, the exact role of the coal inorganic matter in agglomeration and defluidization remains obscure. A series of experiments has been performed to elucidate the role of the inorganic matter in agglomeration and defluidization”. This paper reports the results of the first part of the study which examined the physico-chemical
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transformations experienced by the inorganic matter under conditions relevant to FBC. The results explain the formation of binding material responsible for agglomeration and defluidization during the FBC of high sodium, high sulphur low rank coals. The mechanisms of agglomeration and defluidization will be described in a subsequent paper”.
EXPERIMENTAL Apparatus
A single particle furnace, which has been described elsewhere”, was used to carry out the experiments. It consists of an electrical furnace enclosed inside a refractory brick insulation. Air or nitrogen, preheated in the electrical furnace to the required temperature, enters a glass cylinder of 20 mm i.d. located above the electrical furnance and fluidizes an introduced single particle of coal. A butterfly valve controls the flow of the fluidizing gas in order to retain the coal particle in the glass furnace during the test. The product particle is then withdrawn from the furnace at various residence times using nitrogen as a carrier gas. The product particle so withdrawn is then collected and quenched in a dry ice container for examination. Methodology
A lump of coal was collected from a particular zone of the Lochiel low rank coal deposit in South Australia where the coal contains only a small amount of mineral inclusions. Pyrolysis and combustion experiments were carried out on spherical coal particles of 5.5-9.0 mm diameter, carved from the coal sample, in the single particle furnace operating at 700-830°C. The product particles withdrawn from the furnace at various residence times were weighed and then analysed in terms of their water and acid solubility and mineralogical composition. Duplicate samples of the product particles were examined by electron microscopy. For water and acid solubility, the product particles were leached with demineralized water and then with dilute hydrochloric acid ( 1.ON). The leachate solutions and the residues (acid insoluble) were analysed using atomic absorption, ion chromatography and standard ash analysis techniques. The principal inorganic species in the product particles were identified by powder X-ray diffraction (XRD) using a Phillips P W 1800 microprocessor-controlled diffractometer. The polished cross-sections of the coal and product particles were analysed using an automated JOEL 733 Superprobe analyser equipped with Kevex 7000 series energy dispersive X-ray detection system (EDAX). The analysis conditions used were 15 kV accelerating voltage and 5 nA electron beam current. The morphology and qualitative analysis of the surface of the product particles were examined using scanning electron microscopy (SEM). A Phillips SEM 505 equipped with a Tracon Northern type 5500 EDAX detection system was used for this purpose. The accelerating voltage and electron beam current used were 20 kV and 5 nA respectively. Thermodynamic calculations were carried out using the CHEMIX program which forms part of the CSIRO thermodata systemi3,i4.
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RESULTS
AND DISCUSSION
Raw coal characterization
The coal sample collected from the Lochiel deposit was analysed using standard chemical methods and the extraction method described in the previous section. The results, presented in Tables I to 3, show that it contains high levels of moisture, sulphur, sodium and chlorine. The coal contains only a small amount of acid-insoluble minerals, indicating that the inorganic matter in this coal is predominantly organically bound. Earlier work6 suggests that most of the chlorine and part of the sodium in this coal are present as sodium chloride dissolved in the coal’s inherent moisture. Using the analysis given in Table 2, it is calculated that about 42% of total sodium is present as dissolved sodium chloride. About 30% of total sodium, which is acid soluble, is organically bound. The remaining sodium (about 28%) is suggested to be weakly bonded to the coal surface6. Microscopic examination of the coal samples showed only a small number of discrete mineral inclusions within the coal matrix. These minerals were mostly silica and clay with some calcium sulphate and pyrites. Sodium chloride was not detected as a discrete mineral inclusion. EDAX analysis of the carbonaceous matter (typical composition is presented in Table 4) showed that sodium, calcium, magnesium, aluminium, sulphur and chlorine are intimately dispersed in the coal matrix and have a relatively even distribution. Besides being present in silicon-bearing minerals, silicon was also found to be intimately-albeit non-uniformly-distributed in the coal matrix, probably as superfine particles. The even
Table 1 Coal analysis Proximate analysis Moisture (% ) Ash (%) Volatiles (% ) Fixed carbon (% ) Total chlorine (%db) Total sulphur (%db)
Table 2
62.1 3.6 18.2 15.5 0.6 3.4
Inorganic analysis by extraction method
Inorganics (%db)
Water soluble
Water and acid soluble
Acid insoluble
Na Ca Mg K Fe Al Cl S
0.65 0.01 0.00 0.01
0.93 1.33 0.92 0.02 0.01 0.36 0.60 <0.07
0.00
Table 3
FeA 40, SiO, MgO CaO K,O Na,O TiO, SO,
0.03 0.02 _ 0.01 0.03 _ 3.3
Ash analysis (% ) 0.2 8.3 1.5 17.5 21.5 0.2 11.9 0.1 38.7
Fate of organically Table 4
Typical
microprobe
analysis
(%)
of the inorganic
matter
in coal 0.1
FeA AlP,
5.5 1.2 7.8 10.4 0.1 11.1 0.0 62.8
SiO, MgO CaO K,O Na,O TiO, SO,
"0
20
40
60
80
100
bound
elements:
A. R. Manzoori
and P. K. Agarwal
Similar plots for calcium and magnesium are presented in Figures 2 and 3. These plots show that the acid-soluble (organically bound) calcium and magnesium transform to acid-insoluble forms. The results for magnesium agree with those reported by Murray and Ledger2’; for calcium, however, they found the acid solubility to increase with temperature. Aluminium could not be analysed due to the limitation of the atomic absorption technique in analysing small quantities of this element. While the above results give an indication of the transformations undergone by a number of inorganic elements, they do not assist in predicting the chemical composition of the species formed. XRD analysis carried out on the product particles identified the main phases, such as organic carbon, sodium chloride, calcium sulphide and magnesium oxide, but some of the diffraction lines could not be assigned, at the present state of knowledge, to minerals or compounds. The results suggest that complex compounds may have been formed. These compounds may account for the presence
120
Time (5)
Figure 1 Transformations of sodium in a single coal particle (7.5 mm) during pyrolysis. ---, Water soluble; --, acid soluble; -.-, acid insoluble; x . 700°C; m , 770°C ; 0, 830°C
distribution of sodium and chlorine in dry coal samples suggests that sodium chloride crystallizes as submicrometre particles which become intimately distributed in the coal matrix.
0 0
20
40
60
80
100
9 120
Time(s)
Figure 2
Transformations
of the inorganic
matter
during pyrolysis
Work on low rank coals’5*‘6 has indicated that the carboxylic groups decompose very early during devolatilization. In order to gain an insight into these transformations, the first series of experiments was carried out under pyrolysis conditions. Chemical transformations. The inorganic matter retained in the product particles was characterized in terms of its water and acid solubility. In Figure 1, the quantities of water-soluble, acid-soluble and acid-insoluble sodium retained in the coal particle (normalized with respect to 100 mg of the raw coal sample) are plotted as a function of residence time in the furnace. It is evident from this plot that a portion of sodium transforms to acid-insoluble compounds. At 7oo”C, a significant portion of the acid-soluble (organically bound) sodium transforms to water-soluble compounds. It appears that the extent of these transformations decreases with increasing temperature. However, this could be the result of sodium chloride transformation to acid-soluble sodium compounds ” . Low temperature transformation (dissociation ) of sodium chloride to sodium carbonate’8”9 does not affect the solubility of sodium.
Transformations of calcium in a single coal particle (7.5 mm) during pyrolysis. ---, Acid soluble; -~ -, acid insoluble; x ,e, 700°C ; +, +, 770°C; 0, ., 830°C
Time (51
Figure 3
Transformations of magnesium in a single coal particle (7.5 mm) during pyrolysis. P-P1 Acid soluble; Pm_, acid insoluble; x , .,7oo”C; *, +, 770°C; 0, ., 830°C
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Fate of organically Table 5 lations
Species
included
bound elements: A. R. Manzoori in the equilibrium
thermodynamic
calcu-
Gas phase N,,O,,NH,,H,O,HC1,CO,CO,,NO,NO,,H,S,SO,SO,,SO,, NaOH, Na,O, Na, NaCI, Na,CI,, Na,SO,, AICI, Liquid phases Melt 1: Na,O, Na,SiO,, Na,Si,O,, NaAlSiO,, NaAlSi,O,, Casio,, Ca,SiO,, CaO, MgSiO,, Mg,SiO,, MgO Melt 2:NaOH, NaCl,Na,SO,,Na,CO,,CaSO,,CaCl,,Ca(OH),, Cat%, MgSO,, MgCl,, IWOH),, M&O,
SiOt,
Solid phases Na,O, NaOH, NaCI, Na,SO,, Na,CO,, Na,SiO,, Na,Si,O,, SiO,, A&O,, A&S,, NaAlSiO,, NaAlSi,O,, CaO, CaCO,, Ca(OH),, CaCI,, Ca,SiO,, CaAl,Si,O,, CaSO,, Casio,, MgS, MgO, MgCl,, MgSiO,, Mg,SiO,, MgCO,, MgSO,, Mg(OH),, AI,Si,O,
of acid-insoluble compounds of sodium, calcium and magnesium in the product samples. Equilibrium thermodynamic calculations were carried out in an attempt to predict the chemical composition of the species formed during pyrolysis. The possible species considered in these calculations are presented in Table 5. The results indicated that, under pyrolysis conditions and with a large proportion of sulphur, sodium carbonate, sodium sulphide, sodium hydroxide, sodium chloride, calcium sulphide, aluminium oxide and magnesium oxide are likely to be formed. While the thermodynamic calculations did not assist in describing the formation of acid-insoluble compounds, they predicted those species detected by XRD analysis. It may be noted that such calculations give equilibrium compositions. The experimental results show that the extent of transformations depend on time as well as temperature, confirming the importance of rate processes. It is interesting to note the impact of sulphur on the decomposition products of metal carboxylates. Work reported on low sulphur Victorian brown coalsZo has shown that sodium carbonate, calcium oxide and magnesium oxide are likely to be formed under pyrolysis conditions at temperatures up to 800°C. The results reported in this paper suggest that, with high sulphur coals, sulphides of sodium and calcium may be formed. Although the equilibrium calculations show that sodium carbonate can be formed with high sulphur coals, XRD analysis was unable to detect this compound, perhaps due to its small proportion in the char. The high level of sulphur in the coal sample may also have been responsible for the formation of acid-insoluble compounds of sodium, calcium and magnesium. Further research is necessary to investigate the nature of these compounds. Physical transformation (vaporization). The percentages of the inorganic elements lost from the coal particles during pyrolysis were calculated from the chemical analysis of the raw coal (Table 2) and that of the product particles. Besides vaporization and formation of volatile species there is another possible way, specific to the experimental techniques employed in this investigation, by which the inorganic elements may be lost from a coal particle: small fragments could break off from the char and get entrained in the convective gas stream in the single particle furnace. Analysis of calcium and magnesium indicated that these elements were not lost from the coal particles. Since char fragmentation would result in loss of all elements, including those forming refractory
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FUEL, 1992, Vol 71, May
and P. K. Agarwal
compounds, these analyses indicate that loss of the inorganic matter through char fragmentation has not been significant. The analyses of sodium and chlorine, presented in Figure 4, show that these elements are lost from the coal particles and that the percentage loss of these elements increases with both time and temperature. The loss of sulphur could not be determined by chemical methods due to the small amount present in a single coal particle. Sodium can be lost from the coal particles through vaporization of sodium chloride and/or through formation of volatile species from the transformations of the organically bound sodium. The former has been reported to be the dominant mechanism2r. It should be noted that the vapour pressure of sodium species which are likely to be formed from transformations of the organically bound sodium is markedly lower than that of sodium chloride22. Chlorine can be lost from the coal particles through vaporization of sodium chloride as well as the formation of gaseous hydrogen chloride from sodium chloride reactions and/or dissociation. The latter could lead to the disproportionate release of sodium and chlorine. About 42 wt% of the sodium in the raw coal sample is associated with chlorine. Assuming that sodium is lost due to transformations of sodium chloride, then its loss can be calculated as a percentage of that portion of sodium present in coal as sodium chloride. The results, presented in Figure 5, indicate that sodium and chlorine present in the raw coal sample as sodium chloride are evolved disproportionately. It is concluded that a portion of sodium chloride in coal either dissociates or reacts with other compounds resulting in the formation of non-volatile sodium species and gaseous hydrogen chloride. The reactions could occur inside the coal particles and on the char surface after the release of sodium chloride into the gas phase. For instance, sodium chloride could react with silicon-bearing minerals in coal. However, the extent of these reactions is expected to be limited due to small proportions of these minerals in the coal sample used.
I” __--__--
60
50
0
____------__
0
20
40
60
80
100
1
Time(s)
Figure 4 pyrolysis, coal. -,
Sodium and chlorine losses from a single coal particle during presented as a percentage of total sodium and chlorine in Na; ---, Cl; f, 830°C; 0, 770°C; *, 700°C
Fate of organically bound 70
60
50
3
40
L? 3
30
20
I
10
0
0
20
40
60
80
100
120
Time(s)
Figure 5 Sodium and chlorine losses from a single coal particle during pyrolysis, presented as a percentage of their respective amounts present in coal as sodium chloride. Pp. Na; ---, Cl; f, 830°C; 0, 770°C: m, 700°C
Transformations of the inorganic matter during combustion
Once exposed to oxygen on the char surface, the inorganic species formed during devolatilization are expected to undergo further transformation resulting in the formation of ash. In the single particle furnace, small fragments of ash break off from the char and get entrained in the convective air stream. Thus, the inorganic matter remaining in the product particles consists of an unknown mixture of species formed under reducing and oxidizing environments. As these species are expected to have different characteristics in terms of their water and acid solubility, the results of extraction tests cannot give an indication of the transformations undergone by the inorganic matter during combustion. The losses of sodium and chlorine relative to those of calcium and magnesium, presented in Figure 6, give only a qualitative indication of the formation of volatile Na and Cl species. The transformations of the inorganic matter under combustion have been studied by electron microscopy and XRD. The back-scattered electron image of a polished cross-section of a char particle after 38% total weight loss (based on the initial wet coal containing 62% moisture) is shown in Figure 7. The bright band surrounding the char surface shows the formation of ash. EDAX analysis, presented in Figure 7, indicates that the char near the surface has a similar composition to the raw coal: it contains a significant proportion of S, Na, Cl, Ca, Mg and Al. However, the ash formed on the char surface (positions 1 and 2) contains very little Cl indicating that halite is not present in the ash in significant proportions. The content of Si in the ash also appears to be less than that in the char. The proportion of Si is very small compared with other inorganic elements in ash, providing evidence that the ash consists mostly of sulphates and oxides. Figure 8 shows the back-scattered electron image of a polished cross-section of a product particle after a total weight loss of 89%. The extent of burn-out at this stage is estimated to be about 50 wt% (on dry, volatile-free basis). It is evident that the ash has developed further on the char surface. EDAX analyses given in Figure 8
elements:
A. R. Manzoori and P. K. Agarwal
show that, compared with the analyses given in Figure 7, the char near the char-ash interface is depleted in Cl and S. This comparison suggests that these elements are released into the gas phase. The proportion of sodium in the char appears to remain unchanged. The ash composition is similar to that given in Figure 7; that is, it contains Na, Ca, Mg, Al and S but almost no Cl. Some of the analyses show traces of potassium and phosphorus. Figure 9 shows the formation of a relatively thick ash matrix around the char surface at about 80% char burn-out. Clearly, the thickness of the ash matrix increases with increasing char burn-out. The ash matrix appears to be broken at some locations. EDAX analyses of char and ash, given in Figure 9, indicate that the freshly formed ash adjacent to the char’s surface (position 1) has the same composition as that formed in the earlier stages of combustion. However, after its formation the ash undergoes further transformation resulting in the depletion of sodium and sulphur (positions 2 and 3) towards the surface of the ash matrix. Compared with the analyses given in Figures 7 and 8 for lower burn-outs, the char near the char-ash interface (position 4) appears to be further depleted in sulphur and chlorine. The analyses of char at positions 5 and 6 show that the concentrations of sulphur and chlorine increase towards the centre of char particles. The proportion of sodium in the char remains relatively constant. The presence of chlorine and sulphur in char at high burn-outs suggests that, compared to pulverized coal combustion23, these elements are evolved from the coal particles at slower rates. Clearly, the rate processes between the two combustion systems are different. The morphology of the ash was examined by scanning electron microscopy. The scanning electron micrograph, presented in Figure 10, shows the formation of a sintered sponge-like ash on the char surface at the early stages of combustion (49% total weight loss). The qualitative EDAX analyses of the char (position 1) and ash (position 2) are also presented. The surface of a product particle after 74% total weight loss is shown in Figure II. The extent of char burn-out cannot be calculated as the total weight loss suggests that
20 Time(s)
Figure 6 Loss of inorganic particle size 5.5 mm, furnace Mg; 0, Cl
matter during combustion (initial coal temperature 700°C). 0, Na; *, Ca; +.
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Fate of organically bound elements: A. R. Manzoori and P. K. Agarwal
b Positions analysed
Na,O
1
11.2
2
9.1 11.6
7.4
3 (char)
CaO
AW,
SiO,
P,O,
Cl
0.1
_
0.3
_
_
0.2
_
8.9
MgO
SO,
K,O
17.8
14.2
49.9
6.5
_
18.5
13.0
52.5
6.0
0.1
6.9
60.7
3.5
0.7
0.3
Figure 7 (a) Back-scattered electron electron image and (b) EDAX analysis of a polished cross-section of a char particle loss during combustion. Bright band around the char surface shows the formation of ash. Scale bar = 100 ,um
after 38% total weight
b Positions analysed
Na,O
CaO
MgO
Al@,
SiO,
K,O
P,O,
0.4
1 (char)
12.7
12.8
10.6
52.0
5.3
1.1
_
2 3
10.3 15.2
18.7 20.4
15.0 10.6
49.1 48.2
6.3 5.2
0.1
0.1 _
Figure 8 (a) Back-scattered electron image and (b) EDAX analysis 50% char burn-out). Scale bar = 100 pm
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FUEL, 1992, Vol 71, May
of a polished
cross-section
of a char particle
Cl
5.1 0.4 0.3
after 89% total weight loss (about
Fate of organically bound elements: A. R. Manzoori and P. K. Agarwal
b Positions analysed
Na,O
CaO
MgO
SO,
SiO 2
~~
K,O
1 (ash)
14.7
20.3
11.1
48.0
5.3
0.1
2 (ash)
7.6
23.5
15.0
46.9
6.6
0.2
3 (ash)
3.9
26.2
18.5
43.8
7.0
0.1
4 (char)
13.7
12.8
14.6
46.0
1.4
1 .o
5 (char)
11.3
1.9
11.6
53.1
5.3
2.3
0.1
6 (char)
12.5
6.2
10.5
56.6 ~__~~_
4.2
I .o
0.3
.~___
Cl
0.1
0.4 0.2
0.2
Figure 9 (a) Back-scattered electron image and (b) EDAX analysis of a polished cross-section of a char particle 80% char burn-out) showing the formation of a porous ash layer on the char surface. Scale bar = 100 pm
devolatilization is not yet complete. EDAX analyses indicate that the ash is highly sulphated and contains Ca, Mg and Al. Due to its low atomic number, sodium could not be detected. The morphology of the ash matrix on a product particle having 87% total weight loss (about 50% char burn-out) is shown in Figure 12. The ash appears to consist of a molten matrix in which solid phases are embedded. While the surface of the ash matrix appears to be molten, the ash near the char-ash interface has a sintered sponge-like morphology. EDAX analyses confirmed that the proportions of sodium and sulphur decrease towards the surface of the ash matrix. The analyses of the samples obtained at various temperatures and with different initial coal particle sizes indicated that the nature of the transformations undergone by the inorganic matter is the same for the same burn-outs in the range considered. However, the time scale of the transformations is undoubtedly influenced since the extent of char burn-out depends on both particle size and temperature. As the product particles obtained from combustion experiments had only a small amount of ash, a sample of ash was prepared by low temperature ashing in a crucible. The ash was subsequently analysed by XRD which identified sodium sulphate, calcium sulphate, quartz, aluminates and oxides of Al, Ca and Mg. The results also showed a shifted diffraction pattern of Na-Ca-sulphate, suggesting the presence of a foreign element such as Mg in its crystalline structure. It should
P,Os
0.3 0.6
3.8
0.2
8.2 8.7
after 95% total weight loss (about
be noted that the sulphates of Na, Ca and Mg can form eutectics in the temperature found in FBC24. Thermodynamic equilibrium calculations were carried out to determine the species which can form under FBC conditions. The species considered in these calculations are presented in Table 5. The results indicate that for high sulphur coals with very low content of silicon, sodium sulphate is the dominant sodium compound in the combustion product. Only a small proportion of the sodium was predicted to be present as sodium chloride. The proportion of sodium silicate was predicted to be insignificant. Sodium and calcium sulphates, aluminates (spinel), oxides of Al and Mg and a small proportion of magnesium sulphate were predicted. The results of thermodynamic calculation are in agreement with those from XRD analysis. These calculations carried out for higher temperatures, similar to those found in pulverized coal fired systems, indicated that sodium silicate and sodium chloride are the dominant sodium species. The formation of sulphates is not thermodynamically favoured at higher temperatures. CONCLUSIONS From the detailed analysis of experiments it is concluded that, with high sodium, high sulphur low rank coals, the inorganic matter undergoes the following transformations. Under pyrolysis conditions particles :
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rb
Position 1
Position 2
C
S
AI
S
1
Ca
Figure 10
(a) Secondary
electron image and (b, c) EDAX analysis
of the surface of a char particle after 49% total weight loss. Scale bar = 0.1 mm
Under combustion conditions : 0 Oxidation and coalescence of the inorganic matter on the char surface results in the formation of a molten ash matrix which consists of sulphates of Na, Ca and possibly Mg. The solid phases embedded in the molten phase include aluminates, oxides of Al, Mg and part of Ca as well as mineral inclusions such as silica and clay. While the ash is likely to include sodium species formed from sodium chloride transformations, it contains only a small proportion of chlorine (less than 0.4% ). 0 Within the range considered, particles of different sizes undergoing combustion at different temperatures
FUEL, 1992, Vol 71, May
1 Ca
0 The organically bound calcium, magnesium and part of the sodium transform to acid-insoluble compounds. 0 Sodium chloride dissociates and/or reacts with other compounds resulting in the disproportionate release of sodium and chlorine. 0 Although sodium content in the char remains unchanged, sulphur and chlorine are progressively depleted.
520
Ca
experience similar transformations outs.
for similar burn-
The results presented in this paper are considered to be of significance in explaining the role of the inorganic matter in the formation of binding material (molten ash) responsible for agglomeration and defluidization during FBC high sulphur, high sodium low rank coals. The mechanisms are described in a subsequent paper. ACKNOWLEDGEMENTS The authors wish to acknowledge and thank the following for their assistance : the South Australian State Energy Research Advisory Committee for funding the project ; the Electricity Trust of South Australia (ETSA) and the University of Adelaide for their support ; Dr E. R. Lindner, Chief Scientist, and Dr M. Bosio, former Research Director of ETSA, for their invaluable contributions ; Dr R. A. Durie for editorial comments and invaluable advice; and CSIRO, Division of Soils, Adelaide for XRD analysis of samples.
Fate of organically bound elements: A. R. Manzoori and P. K. Agarwal
b
Position
Position
1
2
S I
S
Ca
Mg
h Al
Figure 11 (a) Secondary formation of a sponge-like
i
electron image and (b, c) EDAX ash. Scale bar = 10 pm
Figure 12 Secondary electron ash. Scale bar = 0.1 mm
image of the surface
analysis
of a char particle
of the surface
of a char
particle
after 74% total
after 87% total weight loss showing
the formation
weight
loss showing
the
of a more consolidated
FUEL, 1992, Vol 71, May
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Fate of organically bound elements: A. R. Manzoori and P. K. Agarwal REFERENCES
11 12
9 10
52,2
Durie, R. A. Fuel 1961,40, 407 Murray, J. B., State Electricity Commission of Victoria, Miscellaneous Report no. Mr-145, 1968 Attar, A. ‘Analytical Method of Coal and Coal Products’, Vol. 4, Academic Press, 1980, p. 585 Hodges, N. J., Ladner, W. R. and Martin, T. G. J. Inst. Energy 1983, 157 Gibb, W. H. in ‘Corrosion Resistant Materials for Coal Conversion’ (Eds. D. B. Meadowcraft and M. I. Manning), Applied Science Publishers, 1983, p. 25 Readett, D. and Quast, K. Proceedings Australian Coal Science Conference, Adelaide, May 1988, A2: 3.1 Raask, E. ‘Mineral Impurities in Coal Combustion’, Hemisphere, New York, 1985 Goblirsch, G. M., Benson, S. A., Karner, F. R., Rindt, D. K. and Hajicek, D. R. 12th Biennial Lignite Symposium, USA, 1983, DOE and FE/60181 Atakul, H. and Ekinci, E. J. Inst. Energy 1989, 56 Manzoori, A. R. PhD Thesis The University of Adelaide, 1990
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13 14 15 16 17 18 19 20
21 22 23 24
Manzoori, A. R. and Agarwal, P. K. Fuel submitted for publication Wildegger-Gaissmaier, A. E. and Agarwal, P. K. Fuel 1990,69, 44 Turnbull, A. CALPHAD 7 1983, 2, 137 Turnbull, A. and Wadsley, M. Thermodata Service, CSIRO Division of Mineral Chemistry, 1986 Murray, J. B. and Evans, D. G. Fuel 1972,51, 290 Schafer, A. N. S. Fuel 1979, 58, 667 Brinsmead, K. H. and Kear, R. W. Fuel 1956, 35, 84 Edgecombe, L. J. Fuel 1956, 35, 8 Daybell, G. N. and Pringle, W. J. S. Fuel 1958, 37, 283 Murray, J. B. and Ledger, R. C. ‘The State of Combination of the Inorganic Constituents of Brown Coal Chars’, State Electricity Commission of Victoria, Report no. 264, 1972 Murray, J. B. Fuel 1973, 52, 105 Jackson, P. J. in ‘The Mechanism of Corrosion by Fuel Impurities’, Butterworth, London, 1963, p. 427 Lindner, E. R. PhD Thesis University of Newcastle, 1988 Levin, E. M., Robbins, C. R. and McMurdie, M. E. ‘Phase Diagrams for Ceramists’, 4th Edn., The American Chemical Society, Ohio, 1979
and Pradeep
K. AgarwaI*
The Electricity Trust of South Australia, PO Box 6, Eastwood, SA 5063, Australia *Department of Chemical Engineering, The University of Adelaide, GPO Box 498, Adelaide SA 5001, Australia (Received 18 June 1991; revised 8 November 1997 )
A series of experiments has been conducted to study the transformations of the organically bound inorganic elements and inherent sodium chloride under conditions relevant to fluidized bed combustion of low rank coals. Large coal particles (5.5-9.0 mm) were suspended in a convective gas flow in a single particle furnace operated at 700~830°C. The experiments were performed under pyrolysis as well as combustion conditions. The product particles withdrawn from the furnace at various residence times were analysed using chemical methods, X-ray diffraction and electron microscopy. It was found that the transformations of the inorganic matter in high sodium, high sulphur low rank coals result in the formation of a molten ash layer on the char surface, providing evidence of the formation of low melting point eutectics. The molten ash consisted of a matrix of mixed alkali sulphates. The findings are expected to assist in explaining the role of the inorganic matter in causing operational problems during the fluidized bed combustion of low quality, low rank coals. (Keywords: fluidized bed ; coal combustion ; inorganic matter)
Coals are complex material and contain inorganic species in numerous forms. The inorganic matter in high rank coals is mostly in the form of fine crystallites of minerals such as clays, quartz, carbonates and pyrites embedded in the coal matrix. These minerals are released from the coal matrix in the form of ash when the coal is burned in a fluidized bed combustor. The ash from high rank coals is mostly refractory and imposes minimal impact on the design of the fluidized bed combustion (FBC) systems. Low rank coals, however, along with the above minerals, contain a number of organically bound inorganic elements which are mostly alkaline in nature. These elements are likely to be intimately distributed in the coal matrix. Sodium, calcium, magnesium, potassium and, to a lesser extent, aluminium and iron are known to be present in low rank coals as cations attached to the carboxylic groups’,‘. Sulphur can be part of the coal structure3 and chlorine can be bound to the organic matter4t5. Chlorine can also be present as free ions in the inherent moisture‘j. These elements are also part of discrete mineral inclusions. However, at relatively low operating temperatures in FBC, only the organically bound inorganic elements together with low melting compounds such as sodium chloride are likely to undergo major transformation. The transformations of the mineral inclusions are likely to be less significant in terms of their impact on the design and operation of the FBC systems. The transformations of coal inorganic matter in pulverized coal fired systems are well documented’. 00162361/92/050513-10 ‘,c 1992 Butterworth-Heinemann Ltd.
However, only limited research covering this subject has been carried out under FBC conditions. The transformations of the coal inorganic matter will depend on heat and mass transfer mechanisms and their extent could well be influenced by particle size and heating rate. In comparison with pulverized coal combustion, FBC employs larger coal particle sizes (up to 10 mm) and operates at lower temperatures (about SOO-9OO“C). Hence, the research work carried out under pulverized coal combustion may not be applicable to FBC. Another feature of FBC is that the coal represents less than 5% of the total solid inventory in the furnace. Clearly, the interaction between the ash and the inert bed material comprising the bulk of the bed requires additional considerations. Limited research addressing these issues indicates that sodium in low rank coals has a detrimental effect on the operation of FBC systems : its presence was linked to the formation of agglomerates of bed particles which obstruct smooth fluidization8s9. In extreme conditions, the upward flowing gas is unable to support the weight of the bed and maintain a stable suspension, a phenomenon termed as defluidization. While some efforts have been made to address the operational problems associated with the high sodium low rank coals, the exact role of the coal inorganic matter in agglomeration and defluidization remains obscure. A series of experiments has been performed to elucidate the role of the inorganic matter in agglomeration and defluidization”. This paper reports the results of the first part of the study which examined the physico-chemical
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Fate of organically bound elements: A. R. Manzoori and P. K. Agarwal
transformations experienced by the inorganic matter under conditions relevant to FBC. The results explain the formation of binding material responsible for agglomeration and defluidization during the FBC of high sodium, high sulphur low rank coals. The mechanisms of agglomeration and defluidization will be described in a subsequent paper”.
EXPERIMENTAL Apparatus
A single particle furnace, which has been described elsewhere”, was used to carry out the experiments. It consists of an electrical furnace enclosed inside a refractory brick insulation. Air or nitrogen, preheated in the electrical furnace to the required temperature, enters a glass cylinder of 20 mm i.d. located above the electrical furnance and fluidizes an introduced single particle of coal. A butterfly valve controls the flow of the fluidizing gas in order to retain the coal particle in the glass furnace during the test. The product particle is then withdrawn from the furnace at various residence times using nitrogen as a carrier gas. The product particle so withdrawn is then collected and quenched in a dry ice container for examination. Methodology
A lump of coal was collected from a particular zone of the Lochiel low rank coal deposit in South Australia where the coal contains only a small amount of mineral inclusions. Pyrolysis and combustion experiments were carried out on spherical coal particles of 5.5-9.0 mm diameter, carved from the coal sample, in the single particle furnace operating at 700-830°C. The product particles withdrawn from the furnace at various residence times were weighed and then analysed in terms of their water and acid solubility and mineralogical composition. Duplicate samples of the product particles were examined by electron microscopy. For water and acid solubility, the product particles were leached with demineralized water and then with dilute hydrochloric acid ( 1.ON). The leachate solutions and the residues (acid insoluble) were analysed using atomic absorption, ion chromatography and standard ash analysis techniques. The principal inorganic species in the product particles were identified by powder X-ray diffraction (XRD) using a Phillips P W 1800 microprocessor-controlled diffractometer. The polished cross-sections of the coal and product particles were analysed using an automated JOEL 733 Superprobe analyser equipped with Kevex 7000 series energy dispersive X-ray detection system (EDAX). The analysis conditions used were 15 kV accelerating voltage and 5 nA electron beam current. The morphology and qualitative analysis of the surface of the product particles were examined using scanning electron microscopy (SEM). A Phillips SEM 505 equipped with a Tracon Northern type 5500 EDAX detection system was used for this purpose. The accelerating voltage and electron beam current used were 20 kV and 5 nA respectively. Thermodynamic calculations were carried out using the CHEMIX program which forms part of the CSIRO thermodata systemi3,i4.
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FUEL, 1992, Vol 71, May
RESULTS
AND DISCUSSION
Raw coal characterization
The coal sample collected from the Lochiel deposit was analysed using standard chemical methods and the extraction method described in the previous section. The results, presented in Tables I to 3, show that it contains high levels of moisture, sulphur, sodium and chlorine. The coal contains only a small amount of acid-insoluble minerals, indicating that the inorganic matter in this coal is predominantly organically bound. Earlier work6 suggests that most of the chlorine and part of the sodium in this coal are present as sodium chloride dissolved in the coal’s inherent moisture. Using the analysis given in Table 2, it is calculated that about 42% of total sodium is present as dissolved sodium chloride. About 30% of total sodium, which is acid soluble, is organically bound. The remaining sodium (about 28%) is suggested to be weakly bonded to the coal surface6. Microscopic examination of the coal samples showed only a small number of discrete mineral inclusions within the coal matrix. These minerals were mostly silica and clay with some calcium sulphate and pyrites. Sodium chloride was not detected as a discrete mineral inclusion. EDAX analysis of the carbonaceous matter (typical composition is presented in Table 4) showed that sodium, calcium, magnesium, aluminium, sulphur and chlorine are intimately dispersed in the coal matrix and have a relatively even distribution. Besides being present in silicon-bearing minerals, silicon was also found to be intimately-albeit non-uniformly-distributed in the coal matrix, probably as superfine particles. The even
Table 1 Coal analysis Proximate analysis Moisture (% ) Ash (%) Volatiles (% ) Fixed carbon (% ) Total chlorine (%db) Total sulphur (%db)
Table 2
62.1 3.6 18.2 15.5 0.6 3.4
Inorganic analysis by extraction method
Inorganics (%db)
Water soluble
Water and acid soluble
Acid insoluble
Na Ca Mg K Fe Al Cl S
0.65 0.01 0.00 0.01
0.93 1.33 0.92 0.02 0.01 0.36 0.60 <0.07
0.00
Table 3
FeA 40, SiO, MgO CaO K,O Na,O TiO, SO,
0.03 0.02 _ 0.01 0.03 _ 3.3
Ash analysis (% ) 0.2 8.3 1.5 17.5 21.5 0.2 11.9 0.1 38.7
Fate of organically Table 4
Typical
microprobe
analysis
(%)
of the inorganic
matter
in coal 0.1
FeA AlP,
5.5 1.2 7.8 10.4 0.1 11.1 0.0 62.8
SiO, MgO CaO K,O Na,O TiO, SO,
"0
20
40
60
80
100
bound
elements:
A. R. Manzoori
and P. K. Agarwal
Similar plots for calcium and magnesium are presented in Figures 2 and 3. These plots show that the acid-soluble (organically bound) calcium and magnesium transform to acid-insoluble forms. The results for magnesium agree with those reported by Murray and Ledger2’; for calcium, however, they found the acid solubility to increase with temperature. Aluminium could not be analysed due to the limitation of the atomic absorption technique in analysing small quantities of this element. While the above results give an indication of the transformations undergone by a number of inorganic elements, they do not assist in predicting the chemical composition of the species formed. XRD analysis carried out on the product particles identified the main phases, such as organic carbon, sodium chloride, calcium sulphide and magnesium oxide, but some of the diffraction lines could not be assigned, at the present state of knowledge, to minerals or compounds. The results suggest that complex compounds may have been formed. These compounds may account for the presence
120
Time (5)
Figure 1 Transformations of sodium in a single coal particle (7.5 mm) during pyrolysis. ---, Water soluble; --, acid soluble; -.-, acid insoluble; x . 700°C; m , 770°C ; 0, 830°C
distribution of sodium and chlorine in dry coal samples suggests that sodium chloride crystallizes as submicrometre particles which become intimately distributed in the coal matrix.
0 0
20
40
60
80
100
9 120
Time(s)
Figure 2
Transformations
of the inorganic
matter
during pyrolysis
Work on low rank coals’5*‘6 has indicated that the carboxylic groups decompose very early during devolatilization. In order to gain an insight into these transformations, the first series of experiments was carried out under pyrolysis conditions. Chemical transformations. The inorganic matter retained in the product particles was characterized in terms of its water and acid solubility. In Figure 1, the quantities of water-soluble, acid-soluble and acid-insoluble sodium retained in the coal particle (normalized with respect to 100 mg of the raw coal sample) are plotted as a function of residence time in the furnace. It is evident from this plot that a portion of sodium transforms to acid-insoluble compounds. At 7oo”C, a significant portion of the acid-soluble (organically bound) sodium transforms to water-soluble compounds. It appears that the extent of these transformations decreases with increasing temperature. However, this could be the result of sodium chloride transformation to acid-soluble sodium compounds ” . Low temperature transformation (dissociation ) of sodium chloride to sodium carbonate’8”9 does not affect the solubility of sodium.
Transformations of calcium in a single coal particle (7.5 mm) during pyrolysis. ---, Acid soluble; -~ -, acid insoluble; x ,e, 700°C ; +, +, 770°C; 0, ., 830°C
Time (51
Figure 3
Transformations of magnesium in a single coal particle (7.5 mm) during pyrolysis. P-P1 Acid soluble; Pm_, acid insoluble; x , .,7oo”C; *, +, 770°C; 0, ., 830°C
FUEL, 1992, Vol 71, May
515
Fate of organically Table 5 lations
Species
included
bound elements: A. R. Manzoori in the equilibrium
thermodynamic
calcu-
Gas phase N,,O,,NH,,H,O,HC1,CO,CO,,NO,NO,,H,S,SO,SO,,SO,, NaOH, Na,O, Na, NaCI, Na,CI,, Na,SO,, AICI, Liquid phases Melt 1: Na,O, Na,SiO,, Na,Si,O,, NaAlSiO,, NaAlSi,O,, Casio,, Ca,SiO,, CaO, MgSiO,, Mg,SiO,, MgO Melt 2:NaOH, NaCl,Na,SO,,Na,CO,,CaSO,,CaCl,,Ca(OH),, Cat%, MgSO,, MgCl,, IWOH),, M&O,
SiOt,
Solid phases Na,O, NaOH, NaCI, Na,SO,, Na,CO,, Na,SiO,, Na,Si,O,, SiO,, A&O,, A&S,, NaAlSiO,, NaAlSi,O,, CaO, CaCO,, Ca(OH),, CaCI,, Ca,SiO,, CaAl,Si,O,, CaSO,, Casio,, MgS, MgO, MgCl,, MgSiO,, Mg,SiO,, MgCO,, MgSO,, Mg(OH),, AI,Si,O,
of acid-insoluble compounds of sodium, calcium and magnesium in the product samples. Equilibrium thermodynamic calculations were carried out in an attempt to predict the chemical composition of the species formed during pyrolysis. The possible species considered in these calculations are presented in Table 5. The results indicated that, under pyrolysis conditions and with a large proportion of sulphur, sodium carbonate, sodium sulphide, sodium hydroxide, sodium chloride, calcium sulphide, aluminium oxide and magnesium oxide are likely to be formed. While the thermodynamic calculations did not assist in describing the formation of acid-insoluble compounds, they predicted those species detected by XRD analysis. It may be noted that such calculations give equilibrium compositions. The experimental results show that the extent of transformations depend on time as well as temperature, confirming the importance of rate processes. It is interesting to note the impact of sulphur on the decomposition products of metal carboxylates. Work reported on low sulphur Victorian brown coalsZo has shown that sodium carbonate, calcium oxide and magnesium oxide are likely to be formed under pyrolysis conditions at temperatures up to 800°C. The results reported in this paper suggest that, with high sulphur coals, sulphides of sodium and calcium may be formed. Although the equilibrium calculations show that sodium carbonate can be formed with high sulphur coals, XRD analysis was unable to detect this compound, perhaps due to its small proportion in the char. The high level of sulphur in the coal sample may also have been responsible for the formation of acid-insoluble compounds of sodium, calcium and magnesium. Further research is necessary to investigate the nature of these compounds. Physical transformation (vaporization). The percentages of the inorganic elements lost from the coal particles during pyrolysis were calculated from the chemical analysis of the raw coal (Table 2) and that of the product particles. Besides vaporization and formation of volatile species there is another possible way, specific to the experimental techniques employed in this investigation, by which the inorganic elements may be lost from a coal particle: small fragments could break off from the char and get entrained in the convective gas stream in the single particle furnace. Analysis of calcium and magnesium indicated that these elements were not lost from the coal particles. Since char fragmentation would result in loss of all elements, including those forming refractory
516
FUEL, 1992, Vol 71, May
and P. K. Agarwal
compounds, these analyses indicate that loss of the inorganic matter through char fragmentation has not been significant. The analyses of sodium and chlorine, presented in Figure 4, show that these elements are lost from the coal particles and that the percentage loss of these elements increases with both time and temperature. The loss of sulphur could not be determined by chemical methods due to the small amount present in a single coal particle. Sodium can be lost from the coal particles through vaporization of sodium chloride and/or through formation of volatile species from the transformations of the organically bound sodium. The former has been reported to be the dominant mechanism2r. It should be noted that the vapour pressure of sodium species which are likely to be formed from transformations of the organically bound sodium is markedly lower than that of sodium chloride22. Chlorine can be lost from the coal particles through vaporization of sodium chloride as well as the formation of gaseous hydrogen chloride from sodium chloride reactions and/or dissociation. The latter could lead to the disproportionate release of sodium and chlorine. About 42 wt% of the sodium in the raw coal sample is associated with chlorine. Assuming that sodium is lost due to transformations of sodium chloride, then its loss can be calculated as a percentage of that portion of sodium present in coal as sodium chloride. The results, presented in Figure 5, indicate that sodium and chlorine present in the raw coal sample as sodium chloride are evolved disproportionately. It is concluded that a portion of sodium chloride in coal either dissociates or reacts with other compounds resulting in the formation of non-volatile sodium species and gaseous hydrogen chloride. The reactions could occur inside the coal particles and on the char surface after the release of sodium chloride into the gas phase. For instance, sodium chloride could react with silicon-bearing minerals in coal. However, the extent of these reactions is expected to be limited due to small proportions of these minerals in the coal sample used.
I” __--__--
60
50
0
____------__
0
20
40
60
80
100
1
Time(s)
Figure 4 pyrolysis, coal. -,
Sodium and chlorine losses from a single coal particle during presented as a percentage of total sodium and chlorine in Na; ---, Cl; f, 830°C; 0, 770°C; *, 700°C
Fate of organically bound 70
60
50
3
40
L? 3
30
20
I
10
0
0
20
40
60
80
100
120
Time(s)
Figure 5 Sodium and chlorine losses from a single coal particle during pyrolysis, presented as a percentage of their respective amounts present in coal as sodium chloride. Pp. Na; ---, Cl; f, 830°C; 0, 770°C: m, 700°C
Transformations of the inorganic matter during combustion
Once exposed to oxygen on the char surface, the inorganic species formed during devolatilization are expected to undergo further transformation resulting in the formation of ash. In the single particle furnace, small fragments of ash break off from the char and get entrained in the convective air stream. Thus, the inorganic matter remaining in the product particles consists of an unknown mixture of species formed under reducing and oxidizing environments. As these species are expected to have different characteristics in terms of their water and acid solubility, the results of extraction tests cannot give an indication of the transformations undergone by the inorganic matter during combustion. The losses of sodium and chlorine relative to those of calcium and magnesium, presented in Figure 6, give only a qualitative indication of the formation of volatile Na and Cl species. The transformations of the inorganic matter under combustion have been studied by electron microscopy and XRD. The back-scattered electron image of a polished cross-section of a char particle after 38% total weight loss (based on the initial wet coal containing 62% moisture) is shown in Figure 7. The bright band surrounding the char surface shows the formation of ash. EDAX analysis, presented in Figure 7, indicates that the char near the surface has a similar composition to the raw coal: it contains a significant proportion of S, Na, Cl, Ca, Mg and Al. However, the ash formed on the char surface (positions 1 and 2) contains very little Cl indicating that halite is not present in the ash in significant proportions. The content of Si in the ash also appears to be less than that in the char. The proportion of Si is very small compared with other inorganic elements in ash, providing evidence that the ash consists mostly of sulphates and oxides. Figure 8 shows the back-scattered electron image of a polished cross-section of a product particle after a total weight loss of 89%. The extent of burn-out at this stage is estimated to be about 50 wt% (on dry, volatile-free basis). It is evident that the ash has developed further on the char surface. EDAX analyses given in Figure 8
elements:
A. R. Manzoori and P. K. Agarwal
show that, compared with the analyses given in Figure 7, the char near the char-ash interface is depleted in Cl and S. This comparison suggests that these elements are released into the gas phase. The proportion of sodium in the char appears to remain unchanged. The ash composition is similar to that given in Figure 7; that is, it contains Na, Ca, Mg, Al and S but almost no Cl. Some of the analyses show traces of potassium and phosphorus. Figure 9 shows the formation of a relatively thick ash matrix around the char surface at about 80% char burn-out. Clearly, the thickness of the ash matrix increases with increasing char burn-out. The ash matrix appears to be broken at some locations. EDAX analyses of char and ash, given in Figure 9, indicate that the freshly formed ash adjacent to the char’s surface (position 1) has the same composition as that formed in the earlier stages of combustion. However, after its formation the ash undergoes further transformation resulting in the depletion of sodium and sulphur (positions 2 and 3) towards the surface of the ash matrix. Compared with the analyses given in Figures 7 and 8 for lower burn-outs, the char near the char-ash interface (position 4) appears to be further depleted in sulphur and chlorine. The analyses of char at positions 5 and 6 show that the concentrations of sulphur and chlorine increase towards the centre of char particles. The proportion of sodium in the char remains relatively constant. The presence of chlorine and sulphur in char at high burn-outs suggests that, compared to pulverized coal combustion23, these elements are evolved from the coal particles at slower rates. Clearly, the rate processes between the two combustion systems are different. The morphology of the ash was examined by scanning electron microscopy. The scanning electron micrograph, presented in Figure 10, shows the formation of a sintered sponge-like ash on the char surface at the early stages of combustion (49% total weight loss). The qualitative EDAX analyses of the char (position 1) and ash (position 2) are also presented. The surface of a product particle after 74% total weight loss is shown in Figure II. The extent of char burn-out cannot be calculated as the total weight loss suggests that
20 Time(s)
Figure 6 Loss of inorganic particle size 5.5 mm, furnace Mg; 0, Cl
matter during combustion (initial coal temperature 700°C). 0, Na; *, Ca; +.
FUEL,
1992, Vol 71,
May
517
Fate of organically bound elements: A. R. Manzoori and P. K. Agarwal
b Positions analysed
Na,O
1
11.2
2
9.1 11.6
7.4
3 (char)
CaO
AW,
SiO,
P,O,
Cl
0.1
_
0.3
_
_
0.2
_
8.9
MgO
SO,
K,O
17.8
14.2
49.9
6.5
_
18.5
13.0
52.5
6.0
0.1
6.9
60.7
3.5
0.7
0.3
Figure 7 (a) Back-scattered electron electron image and (b) EDAX analysis of a polished cross-section of a char particle loss during combustion. Bright band around the char surface shows the formation of ash. Scale bar = 100 ,um
after 38% total weight
b Positions analysed
Na,O
CaO
MgO
Al@,
SiO,
K,O
P,O,
0.4
1 (char)
12.7
12.8
10.6
52.0
5.3
1.1
_
2 3
10.3 15.2
18.7 20.4
15.0 10.6
49.1 48.2
6.3 5.2
0.1
0.1 _
Figure 8 (a) Back-scattered electron image and (b) EDAX analysis 50% char burn-out). Scale bar = 100 pm
518
FUEL, 1992, Vol 71, May
of a polished
cross-section
of a char particle
Cl
5.1 0.4 0.3
after 89% total weight loss (about
Fate of organically bound elements: A. R. Manzoori and P. K. Agarwal
b Positions analysed
Na,O
CaO
MgO
SO,
SiO 2
~~
K,O
1 (ash)
14.7
20.3
11.1
48.0
5.3
0.1
2 (ash)
7.6
23.5
15.0
46.9
6.6
0.2
3 (ash)
3.9
26.2
18.5
43.8
7.0
0.1
4 (char)
13.7
12.8
14.6
46.0
1.4
1 .o
5 (char)
11.3
1.9
11.6
53.1
5.3
2.3
0.1
6 (char)
12.5
6.2
10.5
56.6 ~__~~_
4.2
I .o
0.3
.~___
Cl
0.1
0.4 0.2
0.2
Figure 9 (a) Back-scattered electron image and (b) EDAX analysis of a polished cross-section of a char particle 80% char burn-out) showing the formation of a porous ash layer on the char surface. Scale bar = 100 pm
devolatilization is not yet complete. EDAX analyses indicate that the ash is highly sulphated and contains Ca, Mg and Al. Due to its low atomic number, sodium could not be detected. The morphology of the ash matrix on a product particle having 87% total weight loss (about 50% char burn-out) is shown in Figure 12. The ash appears to consist of a molten matrix in which solid phases are embedded. While the surface of the ash matrix appears to be molten, the ash near the char-ash interface has a sintered sponge-like morphology. EDAX analyses confirmed that the proportions of sodium and sulphur decrease towards the surface of the ash matrix. The analyses of the samples obtained at various temperatures and with different initial coal particle sizes indicated that the nature of the transformations undergone by the inorganic matter is the same for the same burn-outs in the range considered. However, the time scale of the transformations is undoubtedly influenced since the extent of char burn-out depends on both particle size and temperature. As the product particles obtained from combustion experiments had only a small amount of ash, a sample of ash was prepared by low temperature ashing in a crucible. The ash was subsequently analysed by XRD which identified sodium sulphate, calcium sulphate, quartz, aluminates and oxides of Al, Ca and Mg. The results also showed a shifted diffraction pattern of Na-Ca-sulphate, suggesting the presence of a foreign element such as Mg in its crystalline structure. It should
P,Os
0.3 0.6
3.8
0.2
8.2 8.7
after 95% total weight loss (about
be noted that the sulphates of Na, Ca and Mg can form eutectics in the temperature found in FBC24. Thermodynamic equilibrium calculations were carried out to determine the species which can form under FBC conditions. The species considered in these calculations are presented in Table 5. The results indicate that for high sulphur coals with very low content of silicon, sodium sulphate is the dominant sodium compound in the combustion product. Only a small proportion of the sodium was predicted to be present as sodium chloride. The proportion of sodium silicate was predicted to be insignificant. Sodium and calcium sulphates, aluminates (spinel), oxides of Al and Mg and a small proportion of magnesium sulphate were predicted. The results of thermodynamic calculation are in agreement with those from XRD analysis. These calculations carried out for higher temperatures, similar to those found in pulverized coal fired systems, indicated that sodium silicate and sodium chloride are the dominant sodium species. The formation of sulphates is not thermodynamically favoured at higher temperatures. CONCLUSIONS From the detailed analysis of experiments it is concluded that, with high sodium, high sulphur low rank coals, the inorganic matter undergoes the following transformations. Under pyrolysis conditions particles :
FUEL,
prevailing inside the coal
1992,
Vol 71, May
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Fate of organically bound elements: A. R. Manzoori and P. K. Agarwal
rb
Position 1
Position 2
C
S
AI
S
1
Ca
Figure 10
(a) Secondary
electron image and (b, c) EDAX analysis
of the surface of a char particle after 49% total weight loss. Scale bar = 0.1 mm
Under combustion conditions : 0 Oxidation and coalescence of the inorganic matter on the char surface results in the formation of a molten ash matrix which consists of sulphates of Na, Ca and possibly Mg. The solid phases embedded in the molten phase include aluminates, oxides of Al, Mg and part of Ca as well as mineral inclusions such as silica and clay. While the ash is likely to include sodium species formed from sodium chloride transformations, it contains only a small proportion of chlorine (less than 0.4% ). 0 Within the range considered, particles of different sizes undergoing combustion at different temperatures
FUEL, 1992, Vol 71, May
1 Ca
0 The organically bound calcium, magnesium and part of the sodium transform to acid-insoluble compounds. 0 Sodium chloride dissociates and/or reacts with other compounds resulting in the disproportionate release of sodium and chlorine. 0 Although sodium content in the char remains unchanged, sulphur and chlorine are progressively depleted.
520
Ca
experience similar transformations outs.
for similar burn-
The results presented in this paper are considered to be of significance in explaining the role of the inorganic matter in the formation of binding material (molten ash) responsible for agglomeration and defluidization during FBC high sulphur, high sodium low rank coals. The mechanisms are described in a subsequent paper. ACKNOWLEDGEMENTS The authors wish to acknowledge and thank the following for their assistance : the South Australian State Energy Research Advisory Committee for funding the project ; the Electricity Trust of South Australia (ETSA) and the University of Adelaide for their support ; Dr E. R. Lindner, Chief Scientist, and Dr M. Bosio, former Research Director of ETSA, for their invaluable contributions ; Dr R. A. Durie for editorial comments and invaluable advice; and CSIRO, Division of Soils, Adelaide for XRD analysis of samples.
Fate of organically bound elements: A. R. Manzoori and P. K. Agarwal
b
Position
Position
1
2
S I
S
Ca
Mg
h Al
Figure 11 (a) Secondary formation of a sponge-like
i
electron image and (b, c) EDAX ash. Scale bar = 10 pm
Figure 12 Secondary electron ash. Scale bar = 0.1 mm
image of the surface
analysis
of a char particle
of the surface
of a char
particle
after 74% total
after 87% total weight loss showing
the formation
weight
loss showing
the
of a more consolidated
FUEL, 1992, Vol 71, May
521
Fate of organically bound elements: A. R. Manzoori and P. K. Agarwal REFERENCES
11 12
9 10
52,2
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