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The partitioning behaviour of boron from tourmaline during ashing of coal
International Journal of Coal Geology 53 (2002) 43 – 54 www.elsevier.com/locate/ijcoalgeo
The partitioning behaviour of boron from tourmaline during ashing of coal R.J. Boyd * James Cook University, Australia Solid Energy International, New Zealand Received 25 April 2002; accepted 11 September 2002
Abstract Boron is an environmentally sensitive element that may be present in high concentrations in some coals. Three modes of occurrence for boron in coal are commonly recognised, namely, bound to the organic fraction, locked into clay minerals (mainly illite), and bound within the crystal lattice of tourmaline. The organically bound mode is generally considered to be the most likely. Following combustion in a pulverised fuel utility, boron is generally enriched in the fine fly ash waste, but in some cases, it may also escape with the flue gas, suggesting variable partitioning behaviour. There is concern that boron may be leached from fly ash disposal impounds at concentrations toxic to higher land plants. A coal sample from the D Seam of the Strongman No. 2 Mine, West Coast, South Island, New Zealand has been used to test the hypothesis that boron present in tourmaline is less volatile in coal combustion, implying that mode of occurrence is a key control on the partitioning behaviour of this element. Six sample subsets were doped with increasing amounts of ground tourmaline. One subsample from each set was analysed by prompt gamma INAA to determine the concentration of boron in the coal. Two subsamples were ashed at 1000 jC. One ash sample from each set was analysed for boron, while the other ash sample was leached according to Australian standards. It was found that the relationship between boron in the doped coal and boron in the ash is approximately linear (with some losses noted during ashing), indicating boron present in tourmaline was substantially retained in the ash. Furthermore, no relationship was found between the boron content of the ash and boron leached from the ash samples by reagent water. The results suggest boron present in coal as tourmaline is retained in the ash and is unavailable to the environment following fly ash disposal. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Coal; Boron; Mode of occurrence; Partitioning behaviour; Combustion; Leaching
1. Introduction
* James Cook University, Coalseam Gas Research Institute, School of Earth Science, Townsville QLD 4811, Australia. E-mail addresses: [email protected], [email protected] (R.J. Boyd).
The concentration of boron in coal ranges from 0.5 to 2455 ppm, with most coals having concentrations between 5 and 400 ppm (Swaine, 1990). Boron concentration in coal is commonly enriched relative to the earth’s crust (Valkovic, 1983a). Boron is con-
0166-5162/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 5 1 6 2 ( 0 2 ) 0 0 1 6 3 - 5
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sidered to be of concern in coal utilisation by the American Environmental Protection Agency and the Australian National Pollution Inventory. The concern is due to the toxic effects of high boron concentrations on land plants (Underwood, 1977; Valkovic, 1983b). High boron concentrations have been found in fly ash from pulverised coal combustion compared to bottom ash or flue gas desulphurisation wastes (Clark et al., 1999). High boron concentrations may preclude the potential use of fly ash as a soil ameliorant for acidic soils and limit disposal options. According to Swaine (1990), relatively small excesses may be detrimental; however, the susceptibility to harm is species specific. Boron is essential in low concentrations for some higher land plants, but has a narrow necessity range (Jones, 1995). For example, toxicity was suspected due to accumulation of boron in maize grown in acidic soils amended with combustion wastes from fluidised bed combustion (Clark et al., 1999). Comparative studies between soybean and sorghum found that soybeans are particularly sensitive to boron toxicity (Schwab, 1993). The species-specific tolerance to boron adds further complication in assessing the environmental impact of a particular coal. No evidence has been found for any requirement by animals (Underwood, 1977), with boron being rapidly and almost completely absorbed from food and excreted (although it is suggested by Underwood that very high intakes may cause temporary retention in tissues producing serious toxic effects). The concentration of boron in coals and sediments is commonly used as an indicator of palaeosalinity (equated to the degree of marine influence) at time of deposition (Boggs, 1987; Bohor and Gluskoter, 1973; Bouska, 1981; Curtis, 1964; Goodarzi and Swaine, 1994; Spears, 1965). Coals with boron concentrations up to 50 ppm are considered fresh water influenced; coals with a concentration of 50 –110 ppm are considered as influenced by mildly brackish water; and coals with a concentration of greater than 110 ppm boron are considered as brackish water influenced (Goodarzi, 1995; Goodarzi and Swaine, 1994; Teichmu¨ller et al., 1998). Boron may be present in coal in three main modes of occurrence, namely, organically bound, fixed into illite, and in the mineral tourmaline. The organically bound mode of occurrence is the most commonly cited [Beaton et al., 1991; Bouska, 1981 (who also sug-
gested the inorganic mode could dominate with increased coal rank); Eskenazy et al., 1994 (also illite/ muscovite); Finkelman, 1995; Goodarzi, 1987a,b, 1988; Goodarzi and Van Der Flier-Keller, 1988; Grieve and Goodarzi, 1993 (with some uncertainty); Lyons et al., 1989 (some inorganic); Newman et al., 1997; Querol et al., 1995 (also tourmaline); Querol et al., 1997a,b, 1999 (also aluminosilicates such as illite); Shearer et al., 1997; Swaine, 1990 (also sometimes in clay minerals or tourmaline); Valkovic, 1983a; Vickridge et al., 1990; Ward, 1980 (also sometimes in clay minerals)]. Finkelman (1995) estimated a moderately high confidence of 6 out of 10 that boron would be organically bound in any particular coal. The illite mode of occurrence is also noted by a number of authors, for example, Bohor and Gluskoter (1973), who stated that ‘‘most clay minerals initially adsorb boron, but only illite later ‘fixes’ boron into its lattice where it substitutes for either aluminium or silicon in the tetrahedral layers’’. Tourmaline is a complex borosilicate mineral with up to 3 wt.% boron. Finkelman (1980) suggested that tourmaline ‘‘may be more abundant in coals than heretofore suspected’’, but later noted that tourmaline is generally unlikely to be an ‘‘important’’ mode of occurrence for boron in coal (Finkelman, 1995). Some examples of Chinese coal with high boron concentrations caused by tourmaline have been found (Zhuang et al., 2000). It appears, therefore, that illite and tourmaline may be locally important modes of occurrence. In combustion, boron is considered a Group II element (Clarke, 1995) (Fig. 1) (Clarke and Sloss, 1992). Group II elements are volatilised, and later condense on and become adsorbed onto the fly ash (Meij, 1995). Between 20% and 80% of the boron present in coal will be volatilised in combustion (Clarke and Sloss, 1992). Due to the high temperatures in pulverised fuel combustion units and because slag is quickly removed, Group II elements are not incorporated into or condense on the bottom ash. Thus, the Group II elements become concentrated in the inlet fly ash compared to the slag, and in the outlet fly ash compared to the inlet fly ash. It is notable that the partitioning behaviour of boron as shown in Fig. 1 is not definitive. Boron lies in the overlap between the Group III elements (lost to flue gas) and Group II elements (condensed on fly ash particles). Boron partitioning is dependent on the
R.J. Boyd / International Journal of Coal Geology 53 (2002) 43–54
Fig. 1. Classification of elements according to their behaviour during combustion and gasification. Source: Clarke (1995), The fate of trace elements in emissions control systems.
temperature of the combustion system, but also in part on the combustion method, mode of occurrence of the element in the coal and the chemistry of the ash (Clemens et al., 2000). Ashing experiments show significant variability in the volatility of boron between coals (Doolan et al., 1985). Boron was generally retained at temperatures up to 815 jC, but in some cases, significant loss of boron at 370 jC and, in one case, at 150 jC during low-temperature radio frequency ashing, was noted (Doolan et al., 1985). The common organic affinity of boron could be an explanation for the high emissions noted for this otherwise refractory element. Organoboron volatility is favoured at low temperatures, and oxidative retention of boron is favoured at higher temperatures (Doolan et al., 1985). Doolan et al. concluded that the variable volatility of boron is controlled by the ‘‘chemical forms of the element present in the individual coals, and possibly the oxidative conditions during the preparation of the ash residues’’. It is apparent that the partitioning behaviour of boron is not simple. However, it is likely that boron’s mode of occurrence, that is, chemical form, exerts a significant control on the partitioning behaviour of the element during combustion and this factor will be the focus of this paper. Coal combustion may serve to concentrate the boron present in coal into the waste products of combustion, particularly fly ash. Up to 83.2% of boron
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in combustion will be found in the fly ash (Valkovic, 1983b) following pulverised fuel combustion. Fly ash comprises approximately 75 –80% of ash refuse from a pulverised fuel combustion power station (the other two waste streams being bottom ash at about 20 –25% and flue gases at about 1%) (Murarka et al., 1993). (The exact proportions of waste products will vary between combustion plants depending on plant design and type) (Valkovic, 1983b). In the US alone, approximately 100 mt of fly ash per year is produced (Finkelman, 2000), making it a volumetrically important waste product that may also contain a significant concentration of boron. Fly ash waste is commonly impounded in landfills, which, if permeable, may be subject to leaching by groundwater and infiltrating rainwater. These landfills also require rehabilitation once the site reaches capacity. Group II trace elements hosted by fly-ash particles tend to be present as thin (f 0.1 Am) enriched coatings adsorbed to the outside of fly-ash particles (Fishman et al., 1999; Linton et al., 1976; Querol et al., 2001; Valkovic, 1983b). These coatings may also be enriched in sulphur from the reaction of condensed sulphuric acid aerosols and the glassy matrix (Fishman et al., 1999). An alternative explanation for the surficial enrichment of boron and other elements is that fly ash is in equilibrium with the combustion conditions rather than a transient state, so segregation of the trace elements at the surface of a particle is due to freezing out of the element during growth of mullite crystals (Hulett et al., 1980). Gaseous boron exists as boric acid B(OH)3, or boron may react with oxides at elevated temperatures to form B2O3 (Clarke and Sloss, 1992). The oxidised form of boron has a lower boiling point (1800 jC) (although this appears to conflict with the oxidative retention theory outlined by Doolan et al., 1985) than the elemental metal (BP 2550 jC), and is thought to be an important volatile species in pulverised fuel combustion systems (Clarke and Sloss, 1992). Boron present in the oxidised form on fly-ash particles is readily dissolved when mixed with water and is thought to be one of the most mobile trace elements in ash disposal impoundments (Jones, 1995; Querol et al., 2001). Up to 78% of the boron present in Spanish fly ash samples could be extracted by leaching the sample with water (Querol et al., 2001). Mobilisation of trace elements into solution by leaching of fly ash is primarily a function of pH,
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given constant Eh (Jones, 1995). Oxyanions (arsenic, boron, molybdenum, selenium) are most mobile at a pH of about 11, and cations (cadmium, copper, lead, nickel and zinc) are mobile at pH 4 –7, other controls not withstanding. Most fly ash is alkaline in nature, and the resulting leachate also tends to be alkaline. At higher pH, the availability of boron to plants is reduced (Clark et al., 1999). Fly-ash dams tend to be alkaline, implying that boron may be environmentally unavailable in these situations. However, mobilisation of the sulphur and trace element-rich fly-ash coating may result in an initial leachate that is slightly acidic (Jones, 1995). The potential impact of boron mobilised from fly ash could be significant if the element was present in sufficient quantities, but the impact is difficult to assess because of variable ash chemistries and plant susceptibilities. Boron is an element of environmental significance that, although generally organically bound in coal, has several potential modes of occurrence that exhibit very different behaviours in combustion. Boron also exhibits varying partitioning behaviour between waste phases during combustion. This paper tested the partitioning behaviour of boron from one of the less common modes of occurrence (namely tourmaline), and also leached the resultant ash to see if the boron is readily mobilised. The aim was to indicate if coals where boron is present as tourmaline have less environmental impact during combustion than coals with organically bound boron. The implication is that coals in which boron is bound into tourmaline are preferable to coals in which boron is organically bound, mode of occurrence information thus providing a useful method of ‘‘ranking’’ the environmental impact of different coals.
2. Experimental procedure A bulk sample of Strongman No. 2 D seam coal from Greymouth, New Zealand was crushed to 3 mm using a Boyd crusher. Twenty-one 50-g subsamples of the crushed coal were split off the bulk sample by cone and quartering and weighed out with a balance accurate to two decimal places. Two tourmaline crystals from the Mt. Isa area (Australia) were ground for 30 s in a ring mill to produce a tourmaline powder. Six subsets (denoted by letters ‘‘A’’ to ‘‘F’’)
comprising three subsamples were prepared by doping each subsample individually with an equal weight of the powdered tourmaline. The tourmaline powder was weighed out using a balance accurate to four decimal places, and adjustments to the doping weights were made according to the weight of the subsample. Subsamples were doped individually to ensure the consistency of doping within the subset (cf. preparing subsamples from a single doped sample). The weight of tourmaline added was increased from the ‘‘A’’ subset to the ‘‘F’’ subset; the subset labelled ‘‘U’’ was undoped (see Table 1). Each doped sample was again ground in the ring mill for 30 s to thoroughly mix the constituents. The mill was thoroughly cleaned with alcohol and fresh paper towels between each subsample. The doping procedure was designed to fit the boron concentration of the doped subset coal samples (i.e. 92– 329 ppm) into the ‘‘average’’ range of boron concentrations for world coal samples (i.e. 5 –400 ppm). Subsamples denoted with a ‘‘1’’ (i.e. samples SMU1, SMA1, SMB1, SMC1, SMD1, SME1, and SMF1) were analysed for boron concentration using prompt gamma INAA by a commercial laboratory. A duplicate sample of SMU1 (labelled Repeat 5) was also sent to determine repeatability. The repeat sample showed less than 5% difference compared to the original sample (see Table 1). A second split of the undoped coal was treated with hydrogen peroxide according to the method outlined by Ward, (1974) to remove the organic material and check for the presence of pre-existing tourmaline in the sample. The residue was analysed by XRD and the mineral proportions calculated using SIROQUANT (CSIRO, Australia). Subsamples denoted with ‘‘2’’ and ‘‘3’’ were ashed in a muffle furnace at 1000 jC. The ashed subsamples denoted ‘‘2’’ were analysed for boron concentration using prompt gamma INAA. The subsamples denoted ‘‘3’’ were leached using distilled water as outlined in the Australian Standard guidelines (Australian Standard AS 4439.1-1999, 1999). The leachates were analysed by ICP-MS for boron, calcium and aluminium. A blank sample was also run to determine the background concentration of boron in the reagent water. It should be noted that the ash generated in this study by the muffle furnace may host boron in a different fashion to the fly ash produced by pulverised
R.J. Boyd / International Journal of Coal Geology 53 (2002) 43–54
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Table 1 Analysis results for doped and ashed subsamples Whole coal Sample ID
Weight of coal (g)
Weight of tourmaline (g)
SMU1 40.46 0 SMA1 49.99 0.0255 SMB1 50.02 0.0750 SMC1 50.01 0.1500 SMD1 50.00 0.2250 SME1 50.01 0.3001 SMF1 50.01 0.5001 Tourmaline powder measured (ICP) Repeat 5 (SMU1)
B in coal (ppm)
Total sample weight
Weight percentage coal in sample
Weight percentage tourmaline in sample
Calculated B added to coal by tourmaline (ppm)
81 92 107 140 190 232 329
40.46 50.02 50.10 50.16 50.23 50.31 50.51
100.00 99.95 99.85 99.70 99.55 99.40 99.01
0.00 0.05 0.15 0.30 0.45 0.60 0.99
0 11 26 59 109 151 248 28,617 85
Ash % (arb)
Calculated ash %a
B in ash (ppm)
Calculated boron in ash (ppm) (Method 1)a
Calculated boron in ash (ppm) (Method 2)a
5.400 5.451 5.550 5.700 5.850 6.000 6.400
925 1280 1670 2350 2930 3280 4740
1500.000 1687.829 1927.890 2456.166 3247.802 3866.486 5140.786
1500.000 1738.538 2192.385 2847.113 3469.675 4061.075 5499.325
Ash Sample ID
Weight of coal (g)
Weight of added tourmaline (g)
Added tourmaline (wt.%)
SMU2 SMA2 SMB2 SMC2 SMD2 SME2 SMF2
66.61 50.01 50.03 50.01 50.01 50.01 50.02
0.0000 0.0254 0.0751 0.1500 0.2251 0.3002 0.5001
0.0000 0.0508 0.1501 0.2999 0.4501 0.6003 0.9998
a
5.4 5.6 5.9 5.5 5.8 6.3 5.8
For explanation, see text.
fuel combustion. Generally, the operating temperature of a pulverised fuel utility is f 1300 – 1600 jC. Due to the higher operating temperature, virtually all organically bound boron is volatilised, with a proportion condensing in an oxidised form on the fly ash particles upstream of the electrostatic precipitators. The condensation process is probably not occurring in a muffle furnace.
3. Results and discussion 3.1. Ashing Analysis results for the doped and ashed subsamples are presented in Table 1. Analysis of the undoped subsample (SMU1) of Strongman No. 2 D seam coal gave a boron concentration of 81 ppm; the duplicate analysis (Repeat 5) found a concentration of 85 ppm in coal. Regression of weight percentage tourmaline
against boron in coal (see Fig. 2) suggests a boron concentration of 74.58 ppm in the undoped coal. The residue resulting from the hydrogen peroxide dissolution was found to contain 52% quartz, 15% muscovite and 33% kaolinite. The result of the XRD analysis suggests tourmaline was not present in this sample of Strongman No. 2 D seam coal, or is present in quantities below the limit of detection (about 0.1 – 1% depending on crystal properties of the analyte) (Lewis and McConchie, 1994). Another crystal of tourmaline from the Mt. Isa area was analysed by ICP-AES according to the method of Walsh (1985). ICP analysis gave a figure of 28,617 ppm (Table 1). Furthermore, Fig. 2 indicates a concentration of 25,600 ppm (2.56 wt.%) boron in tourmaline. Although the concentration derived from Fig. 2 may be prone to analytical errors due to the small tourmaline powder weights involved, the derived concentration is only 11% different to the ICP analysis result. It is suggested the concentration
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Fig. 2. Weight percentage tourmaline in coal versus boron in coal.
of boron in tourmaline used in this experiment probably lies between 2.56 and 2.86 wt.%. The doped coal subsamples (SMA1 – SMF1) show increasing boron concentrations with increasing tourmaline content, as expected given the high weight percentage of boron in tourmaline. A graph of the boron concentration in subsamples SMA1 – SMF1 plus the undoped coal subsample SMU1 against the respective ashed subsamples (i.e. SMA2– SMF2 plus SMU2) also shows a strong positive trend (see Fig. 3), indicating that boron added as tourmaline resulted in a progressive increase in the concentration of boron in the 1000-jC ash.
A graph of the weight percentage of added tourmaline versus the ash content of the coal for the ashed subsamples showed no discernable relationship. Given the clear increases of boron content in the ash, it is considered unlikely that the lack of a relationship between tourmaline and ash content is due to variable loss of tourmaline during ashing. All the determined ash figures are less than 10% (as-received basis), and theoretically should range from 5.4% (arb) to 6.4% (arb) (see calculated ash percentage column, Table 1). Ash determination by BS 1016, Part 3 and AS 1038 Part 3 has a repeatability of 0.15% for coals with less than 10% ash. ISO 1171– 1981 quotes a repeatability
Fig. 3. Boron in coal subsamples versus boron in ash subsamples.
R.J. Boyd / International Journal of Coal Geology 53 (2002) 43–54
of 0.2% absolute for coals with less than 10% ash. ASTM D3174 quotes a repeatability of 0.2% absolute when no carbonates are present, and 0.3% when carbonates are present for coals with less than 10% ash (Gray, 1983). Given the Strongman No. 2 coal often has millimetre-sized nodules of siderite, it is suggested that analytical errors are exacerbated by the presence of carbonate resulting in the lack of correlation between measured ash increase and the weight of tourmaline added to the coal. An expected concentration of boron in ash was calculated assuming all the boron in the parent coal (81 ppm) and in the tourmaline reports to the ash at 1000 jC. The measured ash values were seen as a source of error in the calculations outlined below so the calculated ash figures from Table 1 are used. The calculated ash figures assume a constant parent coal ash content of 5.4%, and that all the tourmaline reported to the ash. The expected boron in ash was calculated using two methods. Method 1 uses the measured boron concentration in the doped coal samples (SMU1– SMF1) and the calculated ash values. Method 2 uses a boron concentration of 271,00 ppm (the average of the two indicated concentrations of boron in tourmaline) added to the boron concentration of the undoped coal (81 ppm) and the calculated ash values. The calculated boron in ash is the value expected if there is no loss of boron due to volatilisation during the ashing process.
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A graph of the weight percentage of added tourmaline versus the calculated boron in ash is presented in Fig. 4. Fig. 4 shows a relatively constant difference between the regression lines for the expected and measured concentration of boron in ash. In particular, the slope of the regression line for values calculated by Method 1 (the middle regression line data) is very similar to the slope of the regression line for measured values (the bottom regression line data). However, the measured concentration of boron in ash is lower than either of the expected values calculated using Methods 1 and 2. The variation between the expected and measured boron in ash for the undoped coal is 25% (Method 1) to 31% (Method 2). For the samples where 0.9998 wt.% of tourmaline was added to the coal, the difference is 7.6% (Method 1) to 13.8% (Method 2). (Regression figures are used instead of actual measurements for calculation of percentage differences.) The variation between the measured and expected values suggests that some of the boron present in the coal is volatilised at 1000 jC. Fig. 4 suggests that the volatile boron fraction makes up the highest proportion of the total coal boron in the undoped sample (25 – 31%), and reduces significantly as the proportion of tourmaline –boron increases. Furthermore, there appears to be an excellent relationship between the weight percentage of tourmaline added to the coal before ashing and the boron measured in the ash (Fig. 3), suggesting the
Fig. 4. Weight percentage of added tourmaline versus calculated boron in ash (Methods 1 and 2) and measured boron in ash.
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addition of tourmaline is the dominant control on the boron content of the ash. Two implications follow from the observations of boron retention in ash. (1) If the mode of occurrence of boron is the primary control on the volatility of boron exhibited in this experiment, then boron in the undoped coal sample must be present in one or more modes of occurrence, one or more of which is volatile. Organically bound elements are generally considered to exhibit greater volatility than elements bound into minerals (Clarke and Sloss, 1992; Finkelman et al., 1990; Galbreath et al., 2000; Querol et al., 1995; Valkovic, 1983b; Vassilev et al., 2000). It is inferred here that the volatilised boron lost from each subsample is the proportion of the total boron in the Strongman No. 2 coal that is organically bound. Two possible modes of occurrence for the nonvolatile boron in the Strongman D seam coal are envisaged. First, although tourmaline was not detected by XRD analysis of the dissolution residue, it has previously been observed in associated sandstones in the area of the Strongman No. 2 Mine (Boyd and Lewis, 1995). The presence of sparse microscopic tourmaline grains at concentrations below the limit of detection cannot be discounted. Second, the XRD analysis of the undoped coal sample found 15% muscovite. Newman (1988), in a study of the mineral matter of the New Zealand West Coast coals, noted that illite is often very
well ordered in comparison to overseas examples of illite in coal, being close to the muscovite composition. Therefore, a second possibility is that non-volatile boron is present in the Strongman No. 2 D seam sample locked into well-ordered illite/muscovite (cf. Eskenazy et al., 1994). Two alternative explanations for the incomplete volatilisation of boron from the undoped coal could be proposed. First, it is possible that calcium present within the mineral siderite or crandallite (noted by Newman, 1988) may have captured a proportion of the boron by a calcination mechanism, as outlined by some authors (Clemens et al., 1999, 2000). Second, ‘‘primary coal’’ boron may have been incompletely volatilised due to diffusion constraints, that is, not all the boron present in minerals has had time to escape from those minerals. Further directed research work would be required to assess if these controls are a factor in this experiment. (2) Whatever the cause of the partial volatilisation of boron from the coal fraction, Fig. 4 suggests that the additional boron added to the coal by tourmaline doping was retained in the sample during ashing at 1000 jC. In contrast, a large fraction of the boron originally present in the coal sample is volatilised by ashing at 1000 jC. It is considered most likely that the boron contained within the tourmaline crystal structure has not been liberated at the ashing temperature employed in this experiment.
Table 2 Ash leachate analysis results Ash leaching Sample ID
Weight of coal (g)
Weight of added tourmaline (g)
Added tourmaline (wt.%)
Ash % (arb)
Sample weight (g)
B in leachate (ppm)
Al in leachate (ppm)
Ca in leachate (ppm)
SMU3 SMA3 SMB3 SMC3 SMD3 SME3 SMF3 Blank
79.61 50.00 50.03 50.02 50.02 50.02 50.01
0.0000 0.0254 0.0750 0.1502 0.2252 0.3002 0.5001
0.0000 0.0508 0.1499 0.3003 0.4502 0.6002 1.0000
5.6 5.6 5.7 5.6 6.1 6.4 7.3
4.3262 2.7021 2.7409 2.7051 2.9445 3.0792 3.5710
7.7 8.2 11.0 6.5 5.9 5.9 9.3 0.064
1.6 1.9 2.0 2.3 2.1 2.4 1.6 < 0.1
130 110 150 160 100 110 110 0.27
R.J. Boyd / International Journal of Coal Geology 53 (2002) 43–54
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Fig. 5. Boron in ash versus boron in the leachate (ppm).
3.2. Ash leaching Table 2 presents the analysis results for the ash leachates from subsamples SMA3 to SMF3 plus SMU3. Analysis of the blank sample of reagent water found a very low boron concentration of 0.064 ppm, proving that the background boron concentration of the reagent water is not a significant error in this experiment. Two implications follow from the leachate observations.
(1) Fig. 5 shows a graph of boron in ash versus boron in the leachate. In spite of the increasing concentrations of boron in the ash contributed by the tourmaline, the concentration of boron in the reagent water remains relatively consistent, showing no relationship with the concentration of boron in ash. Fig. 5 strongly suggests that boron contributed to the ash by tourmaline is not mobilised by the leaching procedure. It does appear that there is some relationship between the aluminium content of the leachate and the
Fig. 6. Weight percentage added tourmaline versus aluminium in the leachate (ppm).
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amount of tourmaline added to the coal (see Fig. 6) if the SMF3 sample measurement is ignored, although the trend is not strong. It is therefore possible that tourmaline may contribute to the aluminium concentration in leachates. No relationship was found between calcium and the weight percentage of tourmaline added to the coal. (2) The concentration of boron in all leachates was higher than measured for the blank reagent water sample. As discussed above, a high proportion of the boron in the original coal sample was lost due to volatilisation. It is possible that some boron is liberated from the coal or the structure of boronbearing minerals in the coal but does not escape to the atmosphere. It is suggested that the liberated but retained boron is responsible for the increased concentration of boron in the leachate. Whatever the explanation for the partial volatilisation of boron from the undoped coal (see above), it is hypothesised that the additional boron in the leachate has come from the parent coal (see also above) and not from the tourmaline used to dope the coal samples.
1600 jC, but the residence time of the individual fuel particles in the burning zone is only a few microseconds. In the experimental situation, the residence time of the coal is several hours but the operating temperature is considerably lower. However, it is expected that the results of the present study would be indicative of the behaviour of tourmaline in a pulverised fuel furnace given the previously noted variability of the partitioning behaviour of boron, that is, the boron present as tourmaline would be retained in the ash. However, it is likely that all the organically bound boron would be volatilised in actual combustion. Complete volatilisation of organically bound boron is likely to cause a higher proportion of boron to be leached from the ash due to condensation of soluble boron oxides on the surface of fly ash particles downstream of the burner zone. The current test results are considered to be directly relevant to fluidised bed combustion situations where the operating temperature of the plant is considerably lower (750 – 900 jC).
Acknowledgements 4. Conclusions and implications The following conclusions are made as a result of the present experimental study. (1) Boron present in coal as tourmaline is retained in ash at a temperature of 1000 jC, while organically bound boron is volatilised. Therefore, mode of occurrence exerts a major control on the partitioning behaviour of boron from coal during utilisation. (2) Boron from tourmaline that is retained in the coal ash at 1000 jC is not mobilised by leaching with reagent water and therefore will not cause adverse environmental impacts. The implication of this experiment is that combustion of coals in which boron is bound in tourmaline will have a lesser environmental impact than combustion of coals where boron is organically bound. It is difficult to extrapolate the current results directly to a pulverised fuel combustion situation. Generally pulverised fuel furnaces operate at temperatures of 1300 –
John McNee provided access to Strongman No. 2 coal for sampling. Prof. Nick Oliver (James Cook University) kindly provided tourmaline crystals for pulverisation. Prompt Gamma INAA analysis was undertaken by ACTLABS, Perth, Australia. CRL Energy, Wellington, New Zealand ashed the coal samples and provided invaluable assistance with the leaching tests. ICP-MS analysis was undertaken by Agriquality, Wellington, New Zealand. Particular thanks to Dr. Peter Crosdale, Dr. Bob Finkelman and Dr. Curtis Palmer who provided helpful comments on earlier drafts of the manuscript. Dr. F. Goodarzi and another anonymous reviewer further improved the manuscript with helpful reviews.
References Australian Standard AS 4439.1-1999, 1999. Wastes, Sediments and Contaminated Soils: Part 1. Preparation of Leachates—Preliminary Assessment. Published by Standards Australia. Beaton, A.P., Goodarzi, F., Potter, J., 1991. The petrography, mineralogy and geochemistry of a Paleocene lignite from Southern Saskatchewan, Canada. Int. J. Coal Geol. 17, 117 – 148.
R.J. Boyd / International Journal of Coal Geology 53 (2002) 43–54 Boggs Jr., S., 1987. Principles of Sedimentology and Stratigraphy. Merrill Publishing Company, Columbus, OH. 784 pp. Bohor, B.F., Gluskoter, H.J., 1973. Boron in illite as an indicator of paleosalinity of Illinois coals. JSP 43, 945 – 956. Bouska, V., 1981. Geochemistry of Coal. Elsevier, Amsterdam. 284 pp. Boyd, R.J., Lewis, D.W., 1995. Sandstone diagenesis related to varying burial depth and temperature in Greymouth Coalfield, South Island, New Zealand. NZJGG 38, 333 – 348. Clark, R.B., Zeto, S.K., Ritchey, K.D., Baligar, V.C., 1999. Boron accumulation by maize grown in acidic soil amended with coal combustion products. Fuel 78, 179 – 185. Clarke, L.B., 1995. The fate of trace elements in emissions control systems. In: Swaine, D.J., Goodarzi, F. (Eds.), Environmental Aspects of Trace Elements in Coal. Kluwer Academic Publishers, Dordrecht, pp. 128 – 145. Chap. 8. Clarke, L.B., Sloss, L.L., 1992. Trace Elements—Emissions from Coal Combustion and Gasification. IEA Coal Research, London, London. 111 pp. Clemens, A.H., Damiano, L.F., Gong, D., Matheson, T.W., 1999. Partitioning behaviour of some toxic volatile elements during stoker and fluidised bed combustion of alkaline sub-bituminous coal. Fuel 78, 1379 – 1385. Clemens, A.H., Deely, J.M., Gong, D., Moore, T.A., Shearer, J.C., 2000. Partitioning behaviour of some toxic trace elements during coal combustion—the influence of events occurring during the deposition stage. Fuel 79, 1781 – 1784. Curtis, C.D., 1964. Studies on the use of boron as a palaeoenvironmental indicator. Geochim. Cosmochim. Acta 28, 1125 – 1135. Doolan, K.J., Turner, K.E., Mills, J.C., Knott, A.C., Ruch, R.R., 1985. Volatilities of Inorganic Elements in Coals During Ashing. BHP Central Laboratories. Shortland, NSW, Australia 13 pp. Eskenazy, G., Delibaltova, D., Mincheva, E., 1994. Geochemistry of boron in Bulgarian coals. Int. J. Coal Geol. 25, 93 – 110. Finkelman., R.B., 1980. Modes of Occurrence of Trace Elements in Coal. PhD Thesis, University of Maryland. Also USGS Open File Report No. OFR-81-99 (1981), 301 pp. Finkelman, R.B., 1995. Modes of occurrence of environmentallysensitive trace elements in coal. In: Swaine, D.J., Goodarzi, F. (Eds.), Environmental Aspects of Trace Elements in Coal. Kluwer Academic Publishers, Dordrecht, pp. 24 – 50. Chap. 3. Finkelman, R.B., 2000. Current topics on trace elements and toxic metal ion studies: environmental legislation, management and use of some waste products containing toxic metals. Conference Proceedings—Metals, Health and the Environment. University of Canterbury, New Zealand. Finkelman, R.B., Palmer, C.A., Krasnow, M.R., Aruscavage, P.J., Sellers, G.A., Dulong, F.T., 1990. Combustion and leaching behaviour of elements in the Argonne premium coal samples. Energy Fuels 4, 755 – 766. Fishman, N.S., Rice, C.A., Breit, G.N., Johnson, R.D., 1999. Sulphur-bearing coatings on fly ash from a coal-fired power plant: composition, origin, and influence on ash alteration. Fuel 78, 187 – 196. Galbreath, K.C., Toman, D.L., Zygarlicke, C.J., Pavlish, J.H., 2000. Trace element partitioning and transformations during combus-
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tion of bituminous and subbituminous U.S. coals in a 7-kW combustion system. Energy Fuels 14, 1265 – 1279. Goodarzi, F., 1987a. Concentration of elements in lacustrine coals from Zone A Hat Creek Deposit No. 1, British Columbia, Canada. Int. J. Coal Geol. 8, 247 – 268. Goodarzi, F., 1987b. Elemental concentrations in Canadian Coals 2. Byron creek collieries, British Columbia. Fuel 66, 250 – 254. Goodarzi, F., 1988. Elemental distribution in coal seams at the fording coal mine, British Columbia, Canada. Chem. Geol. 68, 129 – 154. Goodarzi, F., 1995. Geology of trace elements in coal. In: Swaine, D.J., Goodarzi, F. (Eds.), Environmental Aspects of Trace Elements in Coal. Kluwer Academic Publishers, Dordrecht, pp. 51 – 75. Chap. 4. Goodarzi, F., Swaine, D.J., 1994. Paleoenvironmental and environmental implications of the boron content of coals. Geol. Surv. Can. Bull. 471. Geological Survey of Canada. Canadian Communication Group-Publishing, Ottawa, Canada. 76 pp. Goodarzi, F., Van Der Flier-Keller, E., 1988. Distribution of major, minor and trace elements in Hat Creek Deposit No. 2, British Columbia, Canada. Chem. Geol. 70, 313 – 333. Gray, V.R., 1983. Coal Analysis in New Zealand. New Zealand Energy Research and Development Committee. Report No. 97, 75 pp. Grieve, D.A., Goodarzi, F., 1993. Trace elements in coal samples from active mines in the Foreland Belt, British Columbia, Canada. Int. J. Coal Geol. 24, 259 – 280. Hulett Jr., L.D., Weinberger, A.J., Northcutt, K.J., Ferguson, M., 1980. Chemical species in fly ash from coal-burning power plants. Science 210, 1356 – 1358. Jones, D.R., 1995. The leaching of major and trace elements from coal ash. In: Swaine, D.J., Goodarzi, F. (Eds.), Environmental Aspects of Trace Elements in Coal. Kluwer Academic Publishers, Dordrecht, pp. 221 – 262. Chap. 12. Lewis, D.W., McConchie, D., 1994. Analytical Sedimentology. Chapman & Hall, New York. 197 pp. Linton, R.W., Loh, A., Evans Jr., C.A., Williams, P., 1976. Surface predominance of trace elements in airborne particles. Science 191, 852 – 854. Lyons, P.C., Palmer, C.A., Bostick, N.H., Fletcher, J.D., Dulong, F.T., Brown, F.W., Brown, Z.A., Krasnow, M.R., Romankiw, L.A., 1989. Chemistry and origin of minor and trace elements in vitrinite concentrates from a rank series from the eastern United States, England, and Australia. Int. J. Coal Geol. 13, 481 – 527. Meij, R., 1995. The distribution of trace elements during the combustion of coal. In: Swaine, D.J., Goodarzi, F. (Eds.), Environmental Aspects of Trace Elements in Coal. Kluwer Academic Publishers, Dordrecht, pp. 111 – 127. Chap. 7. Murarka, I.P., Mattigod, S.V., Keefer, R.F., 1993. An overview of Electric Power Institute (EPRI) research related to effective management of coal combustion residues. In: Keefer, R.F., Sajwan, K.S. (Eds.), Trace Elements in Coal and Coal Combustion Residues. Lewis Publishers, Boca Raton, pp. 11 – 24. Newman., N.A., 1988. Mineral Matter in Coals of the West Coast, South Island, New Zealand. Canterbury University, unpublished PhD Thesis, 293 pp.
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Newman, N.A., Moore, T.A., Esterle, J.S., 1997. Geochemistry and petrography of the Taupiri and Kupakupa coal seams, Waikato coal measures (Eocene), New Zealand. Fuel 33, 103 – 133. Querol, X., Ferna´ndez-Turiel, J.L., Lo´pez-Soler, A., 1995. Trace elements in coal and their behaviour during combustion in a large power station. Fuel 74, 331 – 343. Querol, X., Alastuey, A., Lopez-Soler, A., Plana, F., FernandezTuriel, J.L., Zeng, R., Xu, W., Zhuang, X., Spiro, B., 1997a. Geological controls on the mineral matter and trace elements of coals from the Fuxin Basin, Liaoning Province, Northeast China. Int. J. Coal Geol. 34, 89 – 109. Querol, X., Whateley, M.K.G., Ferna´ndez-Turiel, J.L., Tuncali, E., 1997b. Geological controls on the mineralogy and geochemistry of the Beypazari Lignite, Central Anatolia, Turkey. Int. J. Coal Geol. 33, 255 – 271. Querol, X., Alastuey, A., Lopez-Soler, A., Plana, F., Zeng, R., Zhao, J., Zhuang, X., 1999. Geological controls on the quality of coals from the West Shandong Mining District, Eastern China. Int. J. Coal Geol. 42, 63 – 88. Querol, X., Uman˜a, J.C., Alastuey, A., Ayora, C., Lopez-Soler, A., Plana, F., 2001. Extraction of soluble major and trace elements from fly ash in open and closed leaching systems. Fuel 80, 801 – 813. Schwab, A.P., 1993. Extractable and plant concentrations of metals in amended coal ash. In: Keefer, R.F., Sajwan, K.S. (Eds.), Trace Elements in Coal and Coal Combustion Residues. Lewis Publishers, Boca Raton, pp. 185 – 211. Shearer, J.C., Moore, T.A., Vickridge, I.C., Deely, J.M., 1997. Tephra as a control on trace element distribution in Waikato coals. Author Preprint, 7th New Zealand Coal Conference, Wellington, New Zealand, 1997. Spears, D.A., 1965. Boron in some British carboniferous sedimentary rocks. Geochim. Cosmochim. Acta 29, 315 – 328.
Swaine, D.J., 1990. Trace Elements in Coal. Butterworths, London. 278 pp. Teichmu¨ller, M., Taylor, G.H., Littke, R., Swaine, D.J., 1998. The nature of organic material—macerals and associated minerals. In: Taylor, G.H., Teichmu¨ller, M., Davis, A., Diessel, C.F.K., Littke, R., Robert, P. (Eds.), Organic Petrology. Gebru¨der Borntraeger, Berlin, pp. 175 – 274. Chap. 4. Underwood, E.J., 1977. Trace Elements in Human and Animal Nutrition. Academic Press, San Diego. 545 pp. Valkovic, V., 1983a. Trace Elements in Coal, vol. I. CRC Press, Florida, USA, p. 210. Valkovic, V., 1983b. Trace Elements in Coal, vol. II. CRC Press, Florida, USA, p. 281. Vassilev, S.V., Eskenazy, G.M., Vassileva, C.G., 2000. Contents, modes of occurrence and behaviour of chlorine and bromine in combustion wastes from coal-fired power stations. Fuel 79, 923 – 937. Vickridge, I.C., Sparks, R.J., Bibby, D.M., 1990. Nuclear microprobe studies of boron and calcium distributions in Waikato coals, New Zealand. Fuel 69, 660 – 662. Walsh, J.N., 1985. Determination of boron in trace levels in rocks by inductively coupled plasma spectrometry. Analyst 110, 959 – 962. Ward, C., 1974. Isolation of mineral matter from Australian bituminous coals using hydrogen peroxide. Fuel 53, 220 – 221. Ward, C., 1980. Mode of occurrence of trace elements in some Australian Coals. Coal Geol. 2, 77 – 98. Zhuang, X., Querol, X., Zeng, R., Xu, W., Alastuey, A., LopezSoler, A., Plana, F., 2000. Mineralogy and geochemistry of coal from the liupanshui mining district, Guizhou, South China. Int. J. Coal Geol. 45, 21 – 37.
The partitioning behaviour of boron from tourmaline during ashing of coal R.J. Boyd * James Cook University, Australia Solid Energy International, New Zealand Received 25 April 2002; accepted 11 September 2002
Abstract Boron is an environmentally sensitive element that may be present in high concentrations in some coals. Three modes of occurrence for boron in coal are commonly recognised, namely, bound to the organic fraction, locked into clay minerals (mainly illite), and bound within the crystal lattice of tourmaline. The organically bound mode is generally considered to be the most likely. Following combustion in a pulverised fuel utility, boron is generally enriched in the fine fly ash waste, but in some cases, it may also escape with the flue gas, suggesting variable partitioning behaviour. There is concern that boron may be leached from fly ash disposal impounds at concentrations toxic to higher land plants. A coal sample from the D Seam of the Strongman No. 2 Mine, West Coast, South Island, New Zealand has been used to test the hypothesis that boron present in tourmaline is less volatile in coal combustion, implying that mode of occurrence is a key control on the partitioning behaviour of this element. Six sample subsets were doped with increasing amounts of ground tourmaline. One subsample from each set was analysed by prompt gamma INAA to determine the concentration of boron in the coal. Two subsamples were ashed at 1000 jC. One ash sample from each set was analysed for boron, while the other ash sample was leached according to Australian standards. It was found that the relationship between boron in the doped coal and boron in the ash is approximately linear (with some losses noted during ashing), indicating boron present in tourmaline was substantially retained in the ash. Furthermore, no relationship was found between the boron content of the ash and boron leached from the ash samples by reagent water. The results suggest boron present in coal as tourmaline is retained in the ash and is unavailable to the environment following fly ash disposal. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Coal; Boron; Mode of occurrence; Partitioning behaviour; Combustion; Leaching
1. Introduction
* James Cook University, Coalseam Gas Research Institute, School of Earth Science, Townsville QLD 4811, Australia. E-mail addresses: [email protected], [email protected] (R.J. Boyd).
The concentration of boron in coal ranges from 0.5 to 2455 ppm, with most coals having concentrations between 5 and 400 ppm (Swaine, 1990). Boron concentration in coal is commonly enriched relative to the earth’s crust (Valkovic, 1983a). Boron is con-
0166-5162/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 5 1 6 2 ( 0 2 ) 0 0 1 6 3 - 5
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R.J. Boyd / International Journal of Coal Geology 53 (2002) 43–54
sidered to be of concern in coal utilisation by the American Environmental Protection Agency and the Australian National Pollution Inventory. The concern is due to the toxic effects of high boron concentrations on land plants (Underwood, 1977; Valkovic, 1983b). High boron concentrations have been found in fly ash from pulverised coal combustion compared to bottom ash or flue gas desulphurisation wastes (Clark et al., 1999). High boron concentrations may preclude the potential use of fly ash as a soil ameliorant for acidic soils and limit disposal options. According to Swaine (1990), relatively small excesses may be detrimental; however, the susceptibility to harm is species specific. Boron is essential in low concentrations for some higher land plants, but has a narrow necessity range (Jones, 1995). For example, toxicity was suspected due to accumulation of boron in maize grown in acidic soils amended with combustion wastes from fluidised bed combustion (Clark et al., 1999). Comparative studies between soybean and sorghum found that soybeans are particularly sensitive to boron toxicity (Schwab, 1993). The species-specific tolerance to boron adds further complication in assessing the environmental impact of a particular coal. No evidence has been found for any requirement by animals (Underwood, 1977), with boron being rapidly and almost completely absorbed from food and excreted (although it is suggested by Underwood that very high intakes may cause temporary retention in tissues producing serious toxic effects). The concentration of boron in coals and sediments is commonly used as an indicator of palaeosalinity (equated to the degree of marine influence) at time of deposition (Boggs, 1987; Bohor and Gluskoter, 1973; Bouska, 1981; Curtis, 1964; Goodarzi and Swaine, 1994; Spears, 1965). Coals with boron concentrations up to 50 ppm are considered fresh water influenced; coals with a concentration of 50 –110 ppm are considered as influenced by mildly brackish water; and coals with a concentration of greater than 110 ppm boron are considered as brackish water influenced (Goodarzi, 1995; Goodarzi and Swaine, 1994; Teichmu¨ller et al., 1998). Boron may be present in coal in three main modes of occurrence, namely, organically bound, fixed into illite, and in the mineral tourmaline. The organically bound mode of occurrence is the most commonly cited [Beaton et al., 1991; Bouska, 1981 (who also sug-
gested the inorganic mode could dominate with increased coal rank); Eskenazy et al., 1994 (also illite/ muscovite); Finkelman, 1995; Goodarzi, 1987a,b, 1988; Goodarzi and Van Der Flier-Keller, 1988; Grieve and Goodarzi, 1993 (with some uncertainty); Lyons et al., 1989 (some inorganic); Newman et al., 1997; Querol et al., 1995 (also tourmaline); Querol et al., 1997a,b, 1999 (also aluminosilicates such as illite); Shearer et al., 1997; Swaine, 1990 (also sometimes in clay minerals or tourmaline); Valkovic, 1983a; Vickridge et al., 1990; Ward, 1980 (also sometimes in clay minerals)]. Finkelman (1995) estimated a moderately high confidence of 6 out of 10 that boron would be organically bound in any particular coal. The illite mode of occurrence is also noted by a number of authors, for example, Bohor and Gluskoter (1973), who stated that ‘‘most clay minerals initially adsorb boron, but only illite later ‘fixes’ boron into its lattice where it substitutes for either aluminium or silicon in the tetrahedral layers’’. Tourmaline is a complex borosilicate mineral with up to 3 wt.% boron. Finkelman (1980) suggested that tourmaline ‘‘may be more abundant in coals than heretofore suspected’’, but later noted that tourmaline is generally unlikely to be an ‘‘important’’ mode of occurrence for boron in coal (Finkelman, 1995). Some examples of Chinese coal with high boron concentrations caused by tourmaline have been found (Zhuang et al., 2000). It appears, therefore, that illite and tourmaline may be locally important modes of occurrence. In combustion, boron is considered a Group II element (Clarke, 1995) (Fig. 1) (Clarke and Sloss, 1992). Group II elements are volatilised, and later condense on and become adsorbed onto the fly ash (Meij, 1995). Between 20% and 80% of the boron present in coal will be volatilised in combustion (Clarke and Sloss, 1992). Due to the high temperatures in pulverised fuel combustion units and because slag is quickly removed, Group II elements are not incorporated into or condense on the bottom ash. Thus, the Group II elements become concentrated in the inlet fly ash compared to the slag, and in the outlet fly ash compared to the inlet fly ash. It is notable that the partitioning behaviour of boron as shown in Fig. 1 is not definitive. Boron lies in the overlap between the Group III elements (lost to flue gas) and Group II elements (condensed on fly ash particles). Boron partitioning is dependent on the
R.J. Boyd / International Journal of Coal Geology 53 (2002) 43–54
Fig. 1. Classification of elements according to their behaviour during combustion and gasification. Source: Clarke (1995), The fate of trace elements in emissions control systems.
temperature of the combustion system, but also in part on the combustion method, mode of occurrence of the element in the coal and the chemistry of the ash (Clemens et al., 2000). Ashing experiments show significant variability in the volatility of boron between coals (Doolan et al., 1985). Boron was generally retained at temperatures up to 815 jC, but in some cases, significant loss of boron at 370 jC and, in one case, at 150 jC during low-temperature radio frequency ashing, was noted (Doolan et al., 1985). The common organic affinity of boron could be an explanation for the high emissions noted for this otherwise refractory element. Organoboron volatility is favoured at low temperatures, and oxidative retention of boron is favoured at higher temperatures (Doolan et al., 1985). Doolan et al. concluded that the variable volatility of boron is controlled by the ‘‘chemical forms of the element present in the individual coals, and possibly the oxidative conditions during the preparation of the ash residues’’. It is apparent that the partitioning behaviour of boron is not simple. However, it is likely that boron’s mode of occurrence, that is, chemical form, exerts a significant control on the partitioning behaviour of the element during combustion and this factor will be the focus of this paper. Coal combustion may serve to concentrate the boron present in coal into the waste products of combustion, particularly fly ash. Up to 83.2% of boron
45
in combustion will be found in the fly ash (Valkovic, 1983b) following pulverised fuel combustion. Fly ash comprises approximately 75 –80% of ash refuse from a pulverised fuel combustion power station (the other two waste streams being bottom ash at about 20 –25% and flue gases at about 1%) (Murarka et al., 1993). (The exact proportions of waste products will vary between combustion plants depending on plant design and type) (Valkovic, 1983b). In the US alone, approximately 100 mt of fly ash per year is produced (Finkelman, 2000), making it a volumetrically important waste product that may also contain a significant concentration of boron. Fly ash waste is commonly impounded in landfills, which, if permeable, may be subject to leaching by groundwater and infiltrating rainwater. These landfills also require rehabilitation once the site reaches capacity. Group II trace elements hosted by fly-ash particles tend to be present as thin (f 0.1 Am) enriched coatings adsorbed to the outside of fly-ash particles (Fishman et al., 1999; Linton et al., 1976; Querol et al., 2001; Valkovic, 1983b). These coatings may also be enriched in sulphur from the reaction of condensed sulphuric acid aerosols and the glassy matrix (Fishman et al., 1999). An alternative explanation for the surficial enrichment of boron and other elements is that fly ash is in equilibrium with the combustion conditions rather than a transient state, so segregation of the trace elements at the surface of a particle is due to freezing out of the element during growth of mullite crystals (Hulett et al., 1980). Gaseous boron exists as boric acid B(OH)3, or boron may react with oxides at elevated temperatures to form B2O3 (Clarke and Sloss, 1992). The oxidised form of boron has a lower boiling point (1800 jC) (although this appears to conflict with the oxidative retention theory outlined by Doolan et al., 1985) than the elemental metal (BP 2550 jC), and is thought to be an important volatile species in pulverised fuel combustion systems (Clarke and Sloss, 1992). Boron present in the oxidised form on fly-ash particles is readily dissolved when mixed with water and is thought to be one of the most mobile trace elements in ash disposal impoundments (Jones, 1995; Querol et al., 2001). Up to 78% of the boron present in Spanish fly ash samples could be extracted by leaching the sample with water (Querol et al., 2001). Mobilisation of trace elements into solution by leaching of fly ash is primarily a function of pH,
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R.J. Boyd / International Journal of Coal Geology 53 (2002) 43–54
given constant Eh (Jones, 1995). Oxyanions (arsenic, boron, molybdenum, selenium) are most mobile at a pH of about 11, and cations (cadmium, copper, lead, nickel and zinc) are mobile at pH 4 –7, other controls not withstanding. Most fly ash is alkaline in nature, and the resulting leachate also tends to be alkaline. At higher pH, the availability of boron to plants is reduced (Clark et al., 1999). Fly-ash dams tend to be alkaline, implying that boron may be environmentally unavailable in these situations. However, mobilisation of the sulphur and trace element-rich fly-ash coating may result in an initial leachate that is slightly acidic (Jones, 1995). The potential impact of boron mobilised from fly ash could be significant if the element was present in sufficient quantities, but the impact is difficult to assess because of variable ash chemistries and plant susceptibilities. Boron is an element of environmental significance that, although generally organically bound in coal, has several potential modes of occurrence that exhibit very different behaviours in combustion. Boron also exhibits varying partitioning behaviour between waste phases during combustion. This paper tested the partitioning behaviour of boron from one of the less common modes of occurrence (namely tourmaline), and also leached the resultant ash to see if the boron is readily mobilised. The aim was to indicate if coals where boron is present as tourmaline have less environmental impact during combustion than coals with organically bound boron. The implication is that coals in which boron is bound into tourmaline are preferable to coals in which boron is organically bound, mode of occurrence information thus providing a useful method of ‘‘ranking’’ the environmental impact of different coals.
2. Experimental procedure A bulk sample of Strongman No. 2 D seam coal from Greymouth, New Zealand was crushed to 3 mm using a Boyd crusher. Twenty-one 50-g subsamples of the crushed coal were split off the bulk sample by cone and quartering and weighed out with a balance accurate to two decimal places. Two tourmaline crystals from the Mt. Isa area (Australia) were ground for 30 s in a ring mill to produce a tourmaline powder. Six subsets (denoted by letters ‘‘A’’ to ‘‘F’’)
comprising three subsamples were prepared by doping each subsample individually with an equal weight of the powdered tourmaline. The tourmaline powder was weighed out using a balance accurate to four decimal places, and adjustments to the doping weights were made according to the weight of the subsample. Subsamples were doped individually to ensure the consistency of doping within the subset (cf. preparing subsamples from a single doped sample). The weight of tourmaline added was increased from the ‘‘A’’ subset to the ‘‘F’’ subset; the subset labelled ‘‘U’’ was undoped (see Table 1). Each doped sample was again ground in the ring mill for 30 s to thoroughly mix the constituents. The mill was thoroughly cleaned with alcohol and fresh paper towels between each subsample. The doping procedure was designed to fit the boron concentration of the doped subset coal samples (i.e. 92– 329 ppm) into the ‘‘average’’ range of boron concentrations for world coal samples (i.e. 5 –400 ppm). Subsamples denoted with a ‘‘1’’ (i.e. samples SMU1, SMA1, SMB1, SMC1, SMD1, SME1, and SMF1) were analysed for boron concentration using prompt gamma INAA by a commercial laboratory. A duplicate sample of SMU1 (labelled Repeat 5) was also sent to determine repeatability. The repeat sample showed less than 5% difference compared to the original sample (see Table 1). A second split of the undoped coal was treated with hydrogen peroxide according to the method outlined by Ward, (1974) to remove the organic material and check for the presence of pre-existing tourmaline in the sample. The residue was analysed by XRD and the mineral proportions calculated using SIROQUANT (CSIRO, Australia). Subsamples denoted with ‘‘2’’ and ‘‘3’’ were ashed in a muffle furnace at 1000 jC. The ashed subsamples denoted ‘‘2’’ were analysed for boron concentration using prompt gamma INAA. The subsamples denoted ‘‘3’’ were leached using distilled water as outlined in the Australian Standard guidelines (Australian Standard AS 4439.1-1999, 1999). The leachates were analysed by ICP-MS for boron, calcium and aluminium. A blank sample was also run to determine the background concentration of boron in the reagent water. It should be noted that the ash generated in this study by the muffle furnace may host boron in a different fashion to the fly ash produced by pulverised
R.J. Boyd / International Journal of Coal Geology 53 (2002) 43–54
47
Table 1 Analysis results for doped and ashed subsamples Whole coal Sample ID
Weight of coal (g)
Weight of tourmaline (g)
SMU1 40.46 0 SMA1 49.99 0.0255 SMB1 50.02 0.0750 SMC1 50.01 0.1500 SMD1 50.00 0.2250 SME1 50.01 0.3001 SMF1 50.01 0.5001 Tourmaline powder measured (ICP) Repeat 5 (SMU1)
B in coal (ppm)
Total sample weight
Weight percentage coal in sample
Weight percentage tourmaline in sample
Calculated B added to coal by tourmaline (ppm)
81 92 107 140 190 232 329
40.46 50.02 50.10 50.16 50.23 50.31 50.51
100.00 99.95 99.85 99.70 99.55 99.40 99.01
0.00 0.05 0.15 0.30 0.45 0.60 0.99
0 11 26 59 109 151 248 28,617 85
Ash % (arb)
Calculated ash %a
B in ash (ppm)
Calculated boron in ash (ppm) (Method 1)a
Calculated boron in ash (ppm) (Method 2)a
5.400 5.451 5.550 5.700 5.850 6.000 6.400
925 1280 1670 2350 2930 3280 4740
1500.000 1687.829 1927.890 2456.166 3247.802 3866.486 5140.786
1500.000 1738.538 2192.385 2847.113 3469.675 4061.075 5499.325
Ash Sample ID
Weight of coal (g)
Weight of added tourmaline (g)
Added tourmaline (wt.%)
SMU2 SMA2 SMB2 SMC2 SMD2 SME2 SMF2
66.61 50.01 50.03 50.01 50.01 50.01 50.02
0.0000 0.0254 0.0751 0.1500 0.2251 0.3002 0.5001
0.0000 0.0508 0.1501 0.2999 0.4501 0.6003 0.9998
a
5.4 5.6 5.9 5.5 5.8 6.3 5.8
For explanation, see text.
fuel combustion. Generally, the operating temperature of a pulverised fuel utility is f 1300 – 1600 jC. Due to the higher operating temperature, virtually all organically bound boron is volatilised, with a proportion condensing in an oxidised form on the fly ash particles upstream of the electrostatic precipitators. The condensation process is probably not occurring in a muffle furnace.
3. Results and discussion 3.1. Ashing Analysis results for the doped and ashed subsamples are presented in Table 1. Analysis of the undoped subsample (SMU1) of Strongman No. 2 D seam coal gave a boron concentration of 81 ppm; the duplicate analysis (Repeat 5) found a concentration of 85 ppm in coal. Regression of weight percentage tourmaline
against boron in coal (see Fig. 2) suggests a boron concentration of 74.58 ppm in the undoped coal. The residue resulting from the hydrogen peroxide dissolution was found to contain 52% quartz, 15% muscovite and 33% kaolinite. The result of the XRD analysis suggests tourmaline was not present in this sample of Strongman No. 2 D seam coal, or is present in quantities below the limit of detection (about 0.1 – 1% depending on crystal properties of the analyte) (Lewis and McConchie, 1994). Another crystal of tourmaline from the Mt. Isa area was analysed by ICP-AES according to the method of Walsh (1985). ICP analysis gave a figure of 28,617 ppm (Table 1). Furthermore, Fig. 2 indicates a concentration of 25,600 ppm (2.56 wt.%) boron in tourmaline. Although the concentration derived from Fig. 2 may be prone to analytical errors due to the small tourmaline powder weights involved, the derived concentration is only 11% different to the ICP analysis result. It is suggested the concentration
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Fig. 2. Weight percentage tourmaline in coal versus boron in coal.
of boron in tourmaline used in this experiment probably lies between 2.56 and 2.86 wt.%. The doped coal subsamples (SMA1 – SMF1) show increasing boron concentrations with increasing tourmaline content, as expected given the high weight percentage of boron in tourmaline. A graph of the boron concentration in subsamples SMA1 – SMF1 plus the undoped coal subsample SMU1 against the respective ashed subsamples (i.e. SMA2– SMF2 plus SMU2) also shows a strong positive trend (see Fig. 3), indicating that boron added as tourmaline resulted in a progressive increase in the concentration of boron in the 1000-jC ash.
A graph of the weight percentage of added tourmaline versus the ash content of the coal for the ashed subsamples showed no discernable relationship. Given the clear increases of boron content in the ash, it is considered unlikely that the lack of a relationship between tourmaline and ash content is due to variable loss of tourmaline during ashing. All the determined ash figures are less than 10% (as-received basis), and theoretically should range from 5.4% (arb) to 6.4% (arb) (see calculated ash percentage column, Table 1). Ash determination by BS 1016, Part 3 and AS 1038 Part 3 has a repeatability of 0.15% for coals with less than 10% ash. ISO 1171– 1981 quotes a repeatability
Fig. 3. Boron in coal subsamples versus boron in ash subsamples.
R.J. Boyd / International Journal of Coal Geology 53 (2002) 43–54
of 0.2% absolute for coals with less than 10% ash. ASTM D3174 quotes a repeatability of 0.2% absolute when no carbonates are present, and 0.3% when carbonates are present for coals with less than 10% ash (Gray, 1983). Given the Strongman No. 2 coal often has millimetre-sized nodules of siderite, it is suggested that analytical errors are exacerbated by the presence of carbonate resulting in the lack of correlation between measured ash increase and the weight of tourmaline added to the coal. An expected concentration of boron in ash was calculated assuming all the boron in the parent coal (81 ppm) and in the tourmaline reports to the ash at 1000 jC. The measured ash values were seen as a source of error in the calculations outlined below so the calculated ash figures from Table 1 are used. The calculated ash figures assume a constant parent coal ash content of 5.4%, and that all the tourmaline reported to the ash. The expected boron in ash was calculated using two methods. Method 1 uses the measured boron concentration in the doped coal samples (SMU1– SMF1) and the calculated ash values. Method 2 uses a boron concentration of 271,00 ppm (the average of the two indicated concentrations of boron in tourmaline) added to the boron concentration of the undoped coal (81 ppm) and the calculated ash values. The calculated boron in ash is the value expected if there is no loss of boron due to volatilisation during the ashing process.
49
A graph of the weight percentage of added tourmaline versus the calculated boron in ash is presented in Fig. 4. Fig. 4 shows a relatively constant difference between the regression lines for the expected and measured concentration of boron in ash. In particular, the slope of the regression line for values calculated by Method 1 (the middle regression line data) is very similar to the slope of the regression line for measured values (the bottom regression line data). However, the measured concentration of boron in ash is lower than either of the expected values calculated using Methods 1 and 2. The variation between the expected and measured boron in ash for the undoped coal is 25% (Method 1) to 31% (Method 2). For the samples where 0.9998 wt.% of tourmaline was added to the coal, the difference is 7.6% (Method 1) to 13.8% (Method 2). (Regression figures are used instead of actual measurements for calculation of percentage differences.) The variation between the measured and expected values suggests that some of the boron present in the coal is volatilised at 1000 jC. Fig. 4 suggests that the volatile boron fraction makes up the highest proportion of the total coal boron in the undoped sample (25 – 31%), and reduces significantly as the proportion of tourmaline –boron increases. Furthermore, there appears to be an excellent relationship between the weight percentage of tourmaline added to the coal before ashing and the boron measured in the ash (Fig. 3), suggesting the
Fig. 4. Weight percentage of added tourmaline versus calculated boron in ash (Methods 1 and 2) and measured boron in ash.
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R.J. Boyd / International Journal of Coal Geology 53 (2002) 43–54
addition of tourmaline is the dominant control on the boron content of the ash. Two implications follow from the observations of boron retention in ash. (1) If the mode of occurrence of boron is the primary control on the volatility of boron exhibited in this experiment, then boron in the undoped coal sample must be present in one or more modes of occurrence, one or more of which is volatile. Organically bound elements are generally considered to exhibit greater volatility than elements bound into minerals (Clarke and Sloss, 1992; Finkelman et al., 1990; Galbreath et al., 2000; Querol et al., 1995; Valkovic, 1983b; Vassilev et al., 2000). It is inferred here that the volatilised boron lost from each subsample is the proportion of the total boron in the Strongman No. 2 coal that is organically bound. Two possible modes of occurrence for the nonvolatile boron in the Strongman D seam coal are envisaged. First, although tourmaline was not detected by XRD analysis of the dissolution residue, it has previously been observed in associated sandstones in the area of the Strongman No. 2 Mine (Boyd and Lewis, 1995). The presence of sparse microscopic tourmaline grains at concentrations below the limit of detection cannot be discounted. Second, the XRD analysis of the undoped coal sample found 15% muscovite. Newman (1988), in a study of the mineral matter of the New Zealand West Coast coals, noted that illite is often very
well ordered in comparison to overseas examples of illite in coal, being close to the muscovite composition. Therefore, a second possibility is that non-volatile boron is present in the Strongman No. 2 D seam sample locked into well-ordered illite/muscovite (cf. Eskenazy et al., 1994). Two alternative explanations for the incomplete volatilisation of boron from the undoped coal could be proposed. First, it is possible that calcium present within the mineral siderite or crandallite (noted by Newman, 1988) may have captured a proportion of the boron by a calcination mechanism, as outlined by some authors (Clemens et al., 1999, 2000). Second, ‘‘primary coal’’ boron may have been incompletely volatilised due to diffusion constraints, that is, not all the boron present in minerals has had time to escape from those minerals. Further directed research work would be required to assess if these controls are a factor in this experiment. (2) Whatever the cause of the partial volatilisation of boron from the coal fraction, Fig. 4 suggests that the additional boron added to the coal by tourmaline doping was retained in the sample during ashing at 1000 jC. In contrast, a large fraction of the boron originally present in the coal sample is volatilised by ashing at 1000 jC. It is considered most likely that the boron contained within the tourmaline crystal structure has not been liberated at the ashing temperature employed in this experiment.
Table 2 Ash leachate analysis results Ash leaching Sample ID
Weight of coal (g)
Weight of added tourmaline (g)
Added tourmaline (wt.%)
Ash % (arb)
Sample weight (g)
B in leachate (ppm)
Al in leachate (ppm)
Ca in leachate (ppm)
SMU3 SMA3 SMB3 SMC3 SMD3 SME3 SMF3 Blank
79.61 50.00 50.03 50.02 50.02 50.02 50.01
0.0000 0.0254 0.0750 0.1502 0.2252 0.3002 0.5001
0.0000 0.0508 0.1499 0.3003 0.4502 0.6002 1.0000
5.6 5.6 5.7 5.6 6.1 6.4 7.3
4.3262 2.7021 2.7409 2.7051 2.9445 3.0792 3.5710
7.7 8.2 11.0 6.5 5.9 5.9 9.3 0.064
1.6 1.9 2.0 2.3 2.1 2.4 1.6 < 0.1
130 110 150 160 100 110 110 0.27
R.J. Boyd / International Journal of Coal Geology 53 (2002) 43–54
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Fig. 5. Boron in ash versus boron in the leachate (ppm).
3.2. Ash leaching Table 2 presents the analysis results for the ash leachates from subsamples SMA3 to SMF3 plus SMU3. Analysis of the blank sample of reagent water found a very low boron concentration of 0.064 ppm, proving that the background boron concentration of the reagent water is not a significant error in this experiment. Two implications follow from the leachate observations.
(1) Fig. 5 shows a graph of boron in ash versus boron in the leachate. In spite of the increasing concentrations of boron in the ash contributed by the tourmaline, the concentration of boron in the reagent water remains relatively consistent, showing no relationship with the concentration of boron in ash. Fig. 5 strongly suggests that boron contributed to the ash by tourmaline is not mobilised by the leaching procedure. It does appear that there is some relationship between the aluminium content of the leachate and the
Fig. 6. Weight percentage added tourmaline versus aluminium in the leachate (ppm).
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R.J. Boyd / International Journal of Coal Geology 53 (2002) 43–54
amount of tourmaline added to the coal (see Fig. 6) if the SMF3 sample measurement is ignored, although the trend is not strong. It is therefore possible that tourmaline may contribute to the aluminium concentration in leachates. No relationship was found between calcium and the weight percentage of tourmaline added to the coal. (2) The concentration of boron in all leachates was higher than measured for the blank reagent water sample. As discussed above, a high proportion of the boron in the original coal sample was lost due to volatilisation. It is possible that some boron is liberated from the coal or the structure of boronbearing minerals in the coal but does not escape to the atmosphere. It is suggested that the liberated but retained boron is responsible for the increased concentration of boron in the leachate. Whatever the explanation for the partial volatilisation of boron from the undoped coal (see above), it is hypothesised that the additional boron in the leachate has come from the parent coal (see also above) and not from the tourmaline used to dope the coal samples.
1600 jC, but the residence time of the individual fuel particles in the burning zone is only a few microseconds. In the experimental situation, the residence time of the coal is several hours but the operating temperature is considerably lower. However, it is expected that the results of the present study would be indicative of the behaviour of tourmaline in a pulverised fuel furnace given the previously noted variability of the partitioning behaviour of boron, that is, the boron present as tourmaline would be retained in the ash. However, it is likely that all the organically bound boron would be volatilised in actual combustion. Complete volatilisation of organically bound boron is likely to cause a higher proportion of boron to be leached from the ash due to condensation of soluble boron oxides on the surface of fly ash particles downstream of the burner zone. The current test results are considered to be directly relevant to fluidised bed combustion situations where the operating temperature of the plant is considerably lower (750 – 900 jC).
Acknowledgements 4. Conclusions and implications The following conclusions are made as a result of the present experimental study. (1) Boron present in coal as tourmaline is retained in ash at a temperature of 1000 jC, while organically bound boron is volatilised. Therefore, mode of occurrence exerts a major control on the partitioning behaviour of boron from coal during utilisation. (2) Boron from tourmaline that is retained in the coal ash at 1000 jC is not mobilised by leaching with reagent water and therefore will not cause adverse environmental impacts. The implication of this experiment is that combustion of coals in which boron is bound in tourmaline will have a lesser environmental impact than combustion of coals where boron is organically bound. It is difficult to extrapolate the current results directly to a pulverised fuel combustion situation. Generally pulverised fuel furnaces operate at temperatures of 1300 –
John McNee provided access to Strongman No. 2 coal for sampling. Prof. Nick Oliver (James Cook University) kindly provided tourmaline crystals for pulverisation. Prompt Gamma INAA analysis was undertaken by ACTLABS, Perth, Australia. CRL Energy, Wellington, New Zealand ashed the coal samples and provided invaluable assistance with the leaching tests. ICP-MS analysis was undertaken by Agriquality, Wellington, New Zealand. Particular thanks to Dr. Peter Crosdale, Dr. Bob Finkelman and Dr. Curtis Palmer who provided helpful comments on earlier drafts of the manuscript. Dr. F. Goodarzi and another anonymous reviewer further improved the manuscript with helpful reviews.
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