Energy & Fuels 1998, 12, 1135-1141
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Investigation of Sulfur Retention and the Effect of Inorganic Matter during Pyrolysis of South Australian Low-Rank Coals M. A. Telfer and D. K. Zhang* CRC for New Technologies for Power Generation from Low-rank Coal, Department of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia Received May 5, 1998. Revised Manuscript Received August 3, 1998
Temperature-programmed pyrolysis (TPP) experiments are conducted on two high-sulfur South Australian low-rank coals, Bowmans and Lochiel coals, and several acid-washed and ionexchanged Bowmans coal samples to study sulfur retention, changes in sulfur forms, and the influence of inorganic matter on sulfur transformations during pyrolysis. Changes in sulfur forms between 200 and 900 °C are monitored by Australian Standards wet chemical analysis and confirmed by scanning electron microcscopy (SEM) combined with energy dispersive X-ray analysis (EDX). The TPP results show retention of sulfur at low pyrolysis temperatures (400-800 °C) due to the decomposition of sulfate sulfur and its solid-state transformation to organic sulfur in the char. The retention is enhanced for higher sulfate contents and lower sulfate volatilities and suppressed by a greater proportion and decomposition of organic sulfur species between 300 and 600 °C. For acid-washed and ion-exchanged coal samples, organic sulfur decomposition above 600 °C is suppressed by the addition of organically bound sodium and calcium ions, as well as calcium carbonate constituents in the coal. The presence of organically bound calcium facilitates reactions with organic sulfur to form sulfide sulfur in the char. Organically bound sodium and discrete carbonate materials do not appear to form sulfides to any substantial degree. Hence, their effect on total sulfur retention requires further investigation. Substantial discrepancies exist between sulfide formation in Bowmans and Lochiel coals, despite similar inorganic matter.
Introduction South Australia has large and readily accessible resources of low-rank coals. The relatively high sulfur content (generally 80%) is based on a Ca:S ratio, typically greater than 2, where Ca is the content of calcium in the sorbent and S is the total amount of sulfur in the coal. However, inorganic and organic sulfur constituents in coal undergo a complex competing and opposing transformation process during FBC and FBG, where part of the sulfur is retained in the char. This implies that exorbitant amounts of sorbent material is required and high (1) Yrjas, K. P.; Zevenhoven, C. A. P.; Hupa, M. M. Ind. Eng. Chem. Res. 1996, 35, 176.
quantities of spent sorbent must be disposed of in order to reduce sulfur emissions to acceptable levels. Therefore, a key factor in developing efficient in situ sulfur removal is the ability to predict and control sulfur evolution from the combustion and gasification processes. Since a majority of sulfur evolves from the initial pyrolysis stage of combustion and gasification,2,3 it is important to obtain a fundamental understanding of sulfur transformations during pyrolysis in order to adequately apply a desulfurization strategy. Sulfur is present in coal in both organic (e.g., aliphatic thiols, aromatic sulfides, and thiophenes, etc.) and inorganic (pyritic sulfur and sulfates, etc.) forms. The organic sulfur appears relatively uniform throughout the coal structure, while inorganic sulfur is generally present as discrete mineral inclusions.2 Sulfur-bearing organic functionalities in coal decompose at various temperatures according to their complexity. In the order of increasing complexity, aliphatic thiols decompose at temperatures as low as 200-300 °C, sulfides and disulfides at around 350-400 °C, cyclic and aromatic sulfides at 700-800 °C, and thiophenes at greater than 800 °C.2,3 Retention of organic sulfur in the char also occurs via a series of cyclization reactions. Less complex sulfur compounds can transform to more complex and stable species such as thiophenes.2,3 (2) Attar, A. Fuel 1978, 57, 201-212. (3) Kahn, M. R. Fuel 1989, 68, 1439-1449.
10.1021/ef980117x CCC: $15.00 © 1998 American Chemical Society Published on Web 09/12/1998
1136 Energy & Fuels, Vol. 12, No. 6, 1998
Decomposition of pyritic sulfur (FeS2) to iron sulfide (FeS) occurs at 500 °C. In the presence of coal matter, FeS2 has been found to decompose at around 300 °C. The major product is elemental sulfur, which can either bond with hydrogen evolved from the coal to produce H2S or bond with active sites in the coal matrix to form various organic sulfur compounds. A number of workers3-6 have observed increases in organic sulfur in char preceding the decomposition of pyrite. Sulfate sulfur also experiences decomposition at lower temperatures between 300 and 500 °C when surrounded by coal matter. Similar to pyrite decomposition, organic sulfur increases as sulfates decompose. This has been observed by a variety of experimental and analytical techniques, such as standard wet chemical analysis,5 electron microscopy,4,5 and isotopic tracing.7 The amount of sulfur evolved from sulfate decomposition depends on the volatility of the compound. Since FeSO4 is more volatile than CaSO4, a greater amount of FeSO4 can be converted into the gas phase. Less-volatile sulfates have a greater tendency to decompose to organic sulfur via series of solid-state transformations in the coal matrix.4,7 Coal-sulfide complexes and pyrite (FeS2) have been observed as intermediate compounds of these solid-state transformations.7 FeS2 formation is more common to CaSO4 decomposition. Similar to pyrite decomposition, the evolved sulfur from sulfate decomposition may also form organic sulfur via reactions with the coal matrix.4,7 Sulfide sulfur forms in char during pyrolysis due to a number of reactions: (i) formation of sulfide as an intermediate compound during sulfate transformation to organic sulfur, (ii) decomposition of pyrite to iron sulfide, and (iii) a result of reactions involving organic sulfur or evolved sulfur from decomposition reactions with alkali and alkaline matter in the char at temperatures between 550 and 900 °C.5,7 South Australian low-rank coals such as Bowmans and Lochiel coals both contain a significant amount of inorganic elements (Na, Ca, Mg, Al, S, Cl) that are present as discrete mineral inclusions and also intimately bound and relatively evenly dispersed in the coal matrix.8 The organically bound inorganics are mostly alkaline in nature and are bonded to carboxylic and phenolic groups in the coal, which decompose between 200 and 600 °C during pyrolysis.9 Therefore, during the pyrolysis process, the bonded inorganics (e.g., Fe2+, Ca2+, Na+, etc.) are released with the volatile matter into the gas phase or form carbonate compounds in the char.9 Thus, subsequent sulfur reactions with free cations M2+ and/or mineral compounds in the char matrix can occur. The implications of the bonding nature of inorganic constituents in coal on sulfur transformation is not wellunderstood. The current work investigates the influence of the relative distribution of sulfur forms and the effect of the inherent matter on sulfur retention during pyrolysis by (4) Ibarra, J. V.; Palacios, J. M.,; Moliner, R.; Bonet, A. J. Fuel 1994, 73, 1051. (5) Gryglewicz, G.; Jasienko, S. Fuel 1992, 71, 1225-1229. (6) Cleye, P. J.; Caley, W. F.; Stewart, I.; Whiteway, S. G. Fuel 1984, 63, 1579. (7) Medvedev, K. P.; Petropolskaya, V. M. UKhIN 1966, 2, 10-13. (8) Manzoori, A. R.; Agarwal, P. K. Fuel, 1992, 71, 513. (9) Lindner, E. R. Ph.D. Thesis, Department of Chemical and Materials Engineering, University of Newcastle.
Telfer and Zhang Table 1. Analyses of Raw Bowmans and Lochiel Coals Bowmans Proximate Analysis (% db) moisture (as received) 56 ash 11.9 volatile Matter 49.3 fixed Carbon 38.8 carbon hydrogen oxygen nitrogen sulfur
31.4 10.0 49.6 40.4
Ultimate Analysis (% daf) 69.4 4.6 20.9 0.8 4.82
43.5 3.5 49.44 0.3 3.26
Sulfur Forms (% St)a 0.94 19.06 80
0.30 14.5 85.2
Inorganics (% db) 1.86 0.62 0.03 0.78 0.74 0.28 2.10
1.52 0.95 0.02 0.57 0.08 0.28 2.10
pyrite sulfate organic sodium, Na calcium, Ca potassium, K magnesium, Mg iron, Fe aluminum, Al silica, Si a
Lochiel
%St: percent of total sulfur.
comparison of two South Australian coals, Bowmans coal (raw and acid-washed), and Lochiel coal. The individual role of particular inorganic constituents, such as calcium and sodium, on sulfur transformations is also assessed using acid-washed (AW) Bowmans coal followed by Ca and Na ions exchanged to the coal matrix, respectively. Pyrolysis of AW Bowmans mixed with pure calcium carbonate and Caroline Limestone enables the comparison of the physical nature of the calcium on sulfur retention. Experiments were conducted using a temperature-programmed pyrolysis (TPP) technique to investigate the changes in individual sulfur forms in the temperature range 200-900 °C. Chemical and electron microscopic analysis techniques are employed to determine the sulfur forms and quantities remaining in the pyrolyzed char. The results reveal the mechanism of sulfur transformations in coal and the compounds responsible for sulfur retention during pyrolysis. Experimental Section Sample Preparation. Two South Australian low-rank coals, Bowmans coal and Lochiel coal, are employed in the current study. The proximate, ultimate, elemental, and sulfur form analyses of the raw coals are featured in Table 1. Acid washing of Bowmans coal is performed by mixing the coal with 0.5 M HCl solution to remove inorganic sulfur and inorganic minerals.10 In essence, organic sulfur is the only sulfur form residing in the acid-washed coal. The calcium- and sodiumexchanged coals, denoted as AW-Ca2+ and AW-Na+, are prepared by mixing the acid-washed coal with 0.5 M solutions of calcium acetate and sodium acetate, respectively. Samples are then washed with demineralized water to remove excess calcium and sodium ions. Sulfur analyses of the acid-washed and ion-exchanged samples are featured in Table 2. To investigate further the effect of calcium additives during the pyrolysis experiments, pure calcium carbonate and Caroline Limestone (95% CaCO3, 60-90 µm) are also added to acidwashed Bowmans coal and are denoted as AW-PCC and AW(10) The Science of Victorian Brown Coal. Structure, Properties and Consequences for Utilisation; Durie, R. A., Ed.; Butterworth-Heinemann Ltd.: Oxford, U.K. 1991; p 528.
Investigation of Sulfur Retention
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Table 2. Sulfur Analysis of Acid-Washed and Ion-Exchanged Bowmans Coal Samplesa sulfur analysis
AW coal
AW-Ca2+
AW-Na+
total sulfur (%daf) pyrite (% St) sulfate (% St) organic (% St)
4.08 0.22 3.31 96.47
3.92 0.15 3.05 96.80
4.27 0.30 3.58 96.12
a % S : percent of total sulfur. AW-Ca2+: Acid-washed calciumt exchanged Bowmans coal. AW-Na+: Acid-washed sodium-exchanged Bowmans coal.
LST in the paper, respectively. The samples are prepared by physically mixing the additives to achieve a calcium concentration equivalent to that of the AW-Ca2+ sample (i.e., 4 wt %). All samples employed in the experiments are pulverized to a size fraction of 150-212 µm and dried in a nitrogen atmosphere at 110 °C until there is no further loss in sample mass. Temperature-Programmed Pyrolysis. Temperatureprogrammed pyrolysis (TPP) experiments are conducted in a muffler furnace continuously flushed with a flow of nitrogen at 3 L min-1. Pulverized coal samples, of about 3 g each, are placed in crucibles located in the furnace, which is then electrically heated with a constant heating rate of 17.7 °C min-1 from the ambient temperature. This heating rate, being limited by the equipment employed, is slow enough so that a general trend of various sulfur forms in the samples may be observed. Samples are removed from the furnace at different final temperatures between 200 and 900 °C with a 100 °C interval, quenched with dry ice, and then stored for subsequent analyses. Preliminary experiments with different sample mass and crucible sizes offered the same sulfur retention and form distributions, confirming that there are no interparticle mass diffusion effects on the measurements. Analysis Methods. Total sulfur, sulfate, and pyrite sulfur contents of the coal chars collected are determined using chemical analysis methods following the Australian Standards procedures, specifically developed for Australian low-rank coals.11,12 Organic sulfur is calculated by difference. H2S evolved from HCl treatment with a sample is captured by an iodine solution to indicate the sulfide content in the char, a method similar to that used by Ibarra4 and Gryglewicz.5 Direct analysis of organic sulfur in the raw and acid-washed Bowmans chars is performed using a Philips XL 20 scanning electron microscope with an energy dispersive X-ray detector (SEM-EDX). TPP chars produced between 250 and 500 °C are mounted in epoxy resin as 10 mm cylindrical pellets,4,13 while the rest of the chars (600-900 °C), which are too fine to pelletize, are mounted on slides. An EDXauto package is employed to conduct multiple spot analysis for sulfur and other inorganic constituents (Na, Ca, Si, Mg, Fe, Al) on the char pellets and individual char particles on the slides. The atomic weight percent is recorded along with the concentration measurement, and atomic ratios of Na, Ca, and Fe to sulfur are calculated. Inorganic sulfur constituents are identified and eliminated from the analysis on the basis of their stoichiometric ratio with sulfur. The organic sulfur value is then taken as an average of over 100 spot analysis.
Results and Discussion To observe and compare the behavior of sulfur for various coal samples during the TPP experiments, the amounts of various sulfur species remaining in the char (11) Australian Standard 1038.6.3.1. Part 6.3.1sUltimate Analysis of Higher Rank CoalsDetermination of Total Sulphur (Eschka Method), 1986. (12) Australian Standard 1038.11. Part 11sCoalsForms of Sulphur, 1993. (13) Maijgren, B.; Hubner, W.; Norrgard, K.; Sundvall, S. B. Fuel 1988, 62, 1076.
Figure 1. Retention of various forms of sulfur for raw Bowmans (s) and Lochiel coals (- - -) during temperatureprogrammed pyrolysis.
are divided by the amount of total sulfur in the starting coal sample, according to
(Sform)char/(Stotal)coal ) (Swt %)char/(Swt %)coalMchar/Mcoal (1) where Sform indicates the amount of sulfur in a specific form, Swt % is experimentally determined sulfur concentration, and M is the mass of the coal or char. Effect of the Distribution of Sulfur Forms. Temperature-programmed pyrolysis employs a relatively low heating rate (17.7 °C min-1) to reveal the onset decomposition temperatures for the individual sulfur compounds in coal. It also helps to unfold the general nature and relative reactivity of the organic sulfur species and the degree of volatility of the coal sulfate compounds. It must be noted here that reabsorption of volatile sulfur can occur immediately preceding its evolution. Thus, results are only suggestive of the type and nature of sulfur forms as they could present a net result of decomposition and absorption of sulfur. Figure 1 displays the percentage of sulfur species remaining in the char as a function of final temperature during the temperature-programmed pyrolysis experiments on Bowmans and Lochiel coals. A noticeable decrease in the organic sulfur content at low temperatures (200-300 °C) in Figure 1 indicates that a small proportion of the organic sulfur present in both Bowmans and Lochiel coals is of very high volatility. The rapid loss of organic sulfur between 300 and 500 °C is indicative of a large proportion of organic sulfur which decomposes in this temperature region. The amount and rate of organic sulfur decomposition in the temperature region is greater for Lochiel coal. Though Lochiel coal has a higher distribution of organic sulfur than Bowmans coal, due to the lower total sulfur value, the concentration of organic sulfur in Lochiel coal is only 2.8 wt % compared with 3.8 wt % for Bowmans. In the temperature range between 400 and 500 °C, sulfate sulfur decomposition commences for both coals and corresponds to an increase in pyrite sulfur after 400 °C, with a maximum peak at 500 and 600 °C for both Bowmans and Lochiel coals, respectively. The sulfide
1138 Energy & Fuels, Vol. 12, No. 6, 1998
content of Lochiel coal also increases to a maximum at 500 °C, where it levels off around 600 °C before increasing thereafter. For Bowmans coal, the sulfide sulfur remains relatively negligible throughout and is therefore not shown in Figure 1. The transformations of the organic, sulfide, and pyritic sulfur in this temperature region are indicative of the nature of sulfate transformation that occurs and can thus be used to deduce the mechanism of sulfate decomposition. From 500 to 600 °C, the rate of organic sulfur decomposition drops significantly for Lochiel coal and the percent of organic sulfur actually begins to increase in the Bowmans char. This suggests that for both coals (i) the proportion of organic sulfur decomposing in this temperature region is small and/or (ii) the decomposition of sulfate sulfur results in an immediate solid-state transformation to organic sulfur in the char, with pyrite and sulfide sulfur as intermediate compounds. The latter is in agreement with previous findings on highsulfate coals.4,7 The TPP results also reveal that the sulfate compound present in Bowmans coal is not very volatile in nature due to its preferred solid-state transformation rather than conversion to H2S into the gas phase. Confirmation of this is the fact that the initial sulfate decomposition is small. Between 400 and 700 °C, one-half of the sulfate has decomposed, while between 700 and 800 °C, the rest of the sulfate decomposes. The latter is signified by a steep decent in the last stage of the sulfate curve. The corresponding increase in organic sulfur in Bowmans coal continues to 800 °C, coinciding with complete decomposition of sulfate sulfur. The reincorporation of organic sulfur in Lochiel coal is not as dramatic due to a number of reasons. Lochiel coal has a lower sulfur content and lower proportion of sulfate than Bowmans. It is also possible that Lochiel contains sulfate compounds of higher volatility contributing to more sulfur release into the gas phase than to organic sulfur formation in the char. This is confirmed by a intermediate pyrite peak (though too small to be shown in the figure) occurring at 600 °C, compared to 500 °C for Bowmans, suggesting a delayed solid-state transformation during sulfate decomposition. Unfortunately, due to lack of chemical means, comparative analysis of the specific sulfate forms could not be performed to elucidate this. Due to the faster decomposition of organic sulfur in the lower temperature region, it is also quite likely that a significant amount of organic sulfur is still decomposing in Lochiel coal between 500 and 600 °C. This is consistent with previous studies9 which have found that the bulk of the volatiles evolve between 200 and 600 °C. This could also suppress solid-state sulfate transformations. Sulfate decomposition in Lochiel coal continues to 800 °C, but the majority of sulfate conversion to organic sulfur occurs between 600 and 700 °C, whereafter organic sulfur remains constant. An increase in organic sulfur after the decomposition of inorganic sulfur (pyrite and sulfate) has been observed by a number of researchers employing a variety of analytical techniques such as chemical analysis,5 electron microscopic tracing,4,6 and isotopic tracing.7 In the current study, the Australian Standards (AS) wet chemical method is employed which determines organic
Telfer and Zhang
Figure 2. Comparison of the chemical and SEM analyses of organic sulfur during temperature-programmed pyrolysis of raw and acid-washed Bowmans coal samples.
sulfur from the difference between total sulfur and inorganic sulfur forms (pyrite, sulfate, and sulfide). As a result, the organic sulfur measurement is subject to errors of the other four measurements. Furthermore, for Bowmans and Lochiel coals and their chars, where it is quite common to have negligible amounts of pyritic and sulfide sulfur, the organic sulfur content becomes solely dependent on the sulfate sulfur measurement. Therefore, to confirm the accuracy of the chemical analysis and the effect of sulfate sulfur on organic sulfur, wet chemical and electron microscopic analysis is conducted on raw and acid-washed Bowmans coal chars. Figure 2 displays the Australian Standards method (Chemical) and electron microscopic (SEM-EDX) analyses of organic sulfur for the raw and acid-washed Bowmans coal samples during the TPP experiments. An average of over 100 spot analyses for each char sample represents the organic sulfur values present in the SEM-EDX curves in Figure 2. Both coal samples displayed close agreement between the analysis methods in the temperature range between 25 and 500 °C. Between 500 and 900 °C, large discrepancies are present due to the fact that the SEM-EDX char samples analyzed in this temperature region were prepared on glass slides because they were too fine and too difficult to pelletize. Since the slides were much thinner than the epoxy-mounted pellets for char samples collected at 25-500 °C, penetration of electrons occurred beyond the char layer, giving rise to lower values of organic sulfur than the chemical method, as observed between 500 and 900 °C in Figure 2. However, in the 25-400 °C range, it can been concluded that there was a good agreement between SEM-EDX and AS wet chemical results, confirming the accuracy of the organic sulfur values from the chemical analysis. Comparison of the organic sulfur transformations during TPP experiments of the raw and acid-washed Bowmans coal samples from the wet chemical analysis curves alone shows that despite the variation in the initial distribution of organic sulfur in the coal (100% for acid-washed and 80% for raw coal), the rate of organic sulfur decomposition is similar for temperatures up to 400 °C. This indicates that the low-temperature
Investigation of Sulfur Retention
organic sulfur decomposition is independent of the presence of inorganic sulfur and inorganic mineral matter in the coal. Between 400 and 600 °C, the TPP transformations for the acid-washed sample reveal a much steeper organic sulfur than for the raw coal. The discrepancy in organic and, hence, total sulfur content between the two coals is further acknowledged between 600 and 800 °C. The contribution from the formation of sulfide sulfur due to the presence of inorganic constituents in the raw Bowmans char to the total sulfur increase is negligible (1% between 600 and 800 °C, see Figure 1). This implies that the organic sulfur increase can only be attributed to the presence of sulfate in the raw coal and its subsequent solid-state transformation to organic sulfur in the char. Bowmans and Lochiel coals undergo similar sulfur transformations between 200 and 700 °C due to similar distributions of organic and inorganic sulfur forms. However, even though discrepancies in distribution are subtle, by 700 °C there exists a 15% difference in the amount of total sulfur retained in the char. This is due to a lower percent of organic sulfur and a greater percent of sulfate in Bowmans coal, resulting in lower decomposition and greater incorporation of organic sulfur in the char, respectively. Also observed from the comparative TPP results is that Lochiel has a greater amount of more reactive and more volatile sulfate sulfur. After 700 °C, however, the behavior of the two coals deviates substantially, though the final total sulfur values converge. The organic sulfur content in Bowmans coal decomposes after 800 °C. The stability and nature of the latter organic form is unknown at this stage, and hence, its contribution to decomposition cannot be deduced from the TPP experiments in Figure 1 alone. Figure 2 reveals a noticeable amount of organic sulfur decomposition for the acid-washed char after 800 °C but to a lesser extent than that of raw Bowmans coal. This suggests that only a small amount of original sulfur contributes to the decomposition observed after 800 °C in the raw Bowmans coal. It also implies that a significant amount of sulfate-derived organic sulfur is responsible for the decomposition observed. For Lochiel coal, both the organic sulfur and total sulfur remain relatively constant after 700 °C. Between 700 and 800 °C, the less volatile sulfate component decomposes but the characteristic increase in organic sulfur is not observed. Instead, sulfide sulfur is formed. The possibility of sulfate sulfur reducing directly to sulfide sulfur in this temperature region is low. Sulfur transformations in Lochiel coal between 400 and 600 °C and that in Bowmans coal between 400 and 800 °C have already confirmed that sulfate preferentially transforms to organic sulfur in the char, as consistent with Medvedev’s7 findings. Furthermore, the bulk decomposition of sulfate sulfur and formation of sulfide sulfur do not correspond. Sulfide is still being formed even after complete decomposition of sulfate sulfur. The possibility of the low-volatile sulfate compound converting to sulfur in the gas is also low since decomposition or devolatilzation is low at this stage of pyrolysis. Therefore, it is suggested that organic sulfur is still formed in this region due to sulfate transformation but some part of the organic sulfur decomposes and reacts with inorganic matter in the char to form sulfide sulfur.
Energy & Fuels, Vol. 12, No. 6, 1998 1139
Figure 3. Total sulfur retention in char during temperatureprogrammed pyrolysis of raw, acid-washed (AW), and acidwashed calcium-exchanged (AW-Ca2+) and acid-washed sodiumexchanged (AW-Na+)-Bowmans coal samples.
As a result, the organic sulfur remains relatively constant between 700 and 900 °C. An increase in sulfide sulfur also occurs in the TPP of Bowmans coal between 700 and 900 °C but to a much lesser degree. The total ash and inorganic compositions of Lochiel and Bowmans coals are very similar, as shown in Table 1. Lochiel coal contains only a slightly higher percent of calcium, which has the greatest affinity for reactions with sulfur. Acid solubility tests8 show that the calcium is primarily organically bound, and therefore, it is likely to play a significant role in sulfur retention in both coals. The fact that this does not occur in Bowmans coal however, could be due to the dramatic decomposition of organic- and sulfate-derived sulfur, which may prevent the sulfur uptake reactions by inorganic matter. It has also been observed in numerous pyrolysis studies that retention of organic sulfur in char results from less complex organic sulfur species rearranging or undergoing cyclization reactions to form complex and stable organic sulfur compounds such as thiophenes.2,3,14,15 This increases the overall stability of the organic sulfur compounds. Cernic-Simic14 investigated the sulfur retention during coal carbonization processes and found that the presence of previously established carbonsulfur bonds inhibits C-S bond formation during carbonization. The Role of Calcium and Sodium. TPP experiments were also conducted on acid-washed calcium ionexchanged (AW-Ca2+) and sodium ion-exchanged (AWNa+) Bowmans coal samples to investigate the individual role of inorganic matter such as Ca and Na on the sulfur retention in coal chars during pyrolysis. The results are compared with those of the raw and acid-washed coals in Figure 3. Despite slight variations in decomposition between 200 and 400 °C, the acid-washed and Ca2+- and Na+-exchanged samples reach a similar total sulfur value at 500 °C. The evolution of sulfur is greater than that of the raw coal between 400 and 600 °C due to the (14) Cernic-Simic, S. Fuel 1994, 41, 141. (15) Yperman, J.; Franco, D.; Mullens, J.; Van Poucke, L. C.; Gryglewicz, G.; Jasienko, S. Fuel 1995, 74, (9), 1261-1266.
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Figure 4. Percent of sulfide sulfur retained in char during temperature-programmed pyrolysis of raw, acid-washed calcium-exchanged (AW-Ca2+), and acid-washed sodium-exchanged (AW-Na+) Bowmans coal samples.
absence of sulfate sulfur. Between 500 and 600 °C, the extent of organic sulfur decomposition for AW-Na+ and AW-Ca2+ coals is somewhat less than that for AW coal, resulting in greater total sulfur retentions thereafter. The acid-washed coal continues to decompose between 600 and 900 °C by approximately 8%. However, neither of the ion-exchanged coals display any sulfur decomposition in this temperature region. This implies that decomposition of both organic sulfur and sulfides between 500 and 600 °C and decomposition of complex organic sulfides at 800 °C is suppressed in the ionexchanged samples or results in conversion to other sulfur forms in the char. As a consequence, the final total sulfur value at 900 °C for the AW-Na+ and AWCa2+ chars is very close to that of raw Bowmans coal. Measurements of the percent of sulfide sulfur retained in the raw, AW-Ca2+ and AW-Na+ TPP chars are presented in Figure 4. Very small quantities of sulfide sulfur are retained in the AW-Na+ chars over the duration of the experiment, which implies that reactions of organically bound Na+ with organic sulfur in the char does not occur to any significant extent. This is confirmed further by the fact that at 900 °C the quantity of sulfide is one-half that of raw Bowmans coal, despite the greater concentration of sodium ions than in the raw Bowmans sample. This too suggests that the absence of Ca2+ ions in AW-Na+ samples reduces the sulfide formation and that the presence of Ca2+ has a greater influence on sulfur retention than Na+ during pyrolysis of raw Bowmans coal. It is possible, however, that Na+ does undergo reactions with sulfur in the char to form sodium sulfide (Na2S) and sulfide complexes, which are not detected using the sulfide sulfur analysis method employed. It has been found in previous studies that unreactive iron sulfides of complex structure can form during pyrolysis from sulfate material, which are not determinable by chemical analysis.16,17 Further analysis using SEM-EDX of the pyrolyzed char samples and residue from the HCl treatment during sulfide detection (16) Markuszewski, R. Coal Qual. 1988, 7, 1. (17) Ibarra, J.; Palacios, J. M.; Garcia, M.; Gancedo, J. R. Fuel Process. Technol. 1994, 21, 63.
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is required to determine whether a reaction between sulfur and sodium has occurred. This will also help to explain why a greater sulfur retention occurs for the AW-Na+ coal than the AW and will help to elucidate any inhibiting effect sodium may have on sulfur evolution during pyrolysis between 500 and 900 °C. Comparisons of the sulfide formation in Figure 4 reveal that AW-Ca2+ coal forms a much greater amount of sulfide sulfur during the TPP experiments, which confirms its role in the sulfur retention observed at temperatures above 600 °C in Figure 3. The bulk of sulfur decomposition between 600 and 900 °C for the AW coal, however, does not correspond to the bulk formation of sulfide in the AW-Ca2+ char. Furthermore, the sulfur retained in the AW char decreases by approximately 8%, while approximately 14% sulfide sulfur is formed in the AW-Ca2+ char. This suggests that in order for calcium sulfide to form, Ca2+ must catalyze the decomposition of organic sulfur in the char and facilitate a reaction with sulfur. This is quite possible since trends in the TPP experiment of the AW-Ca2+ sample show a slightly lower total sulfur percent between 25 and 400 °C than the other coal samples, suggesting the possible catalytic nature of calcium on organic sulfur decomposition. Atmospheric-pressure temperature-programmed reduction experiments (AP-TPR) were conducted by Maes,18 which measured the evolution of sulfur in the gas phase from pyrolysis of raw coal and a variety of calcium-doped coals. The results showed that coals with ion-exchanged calcium caused a significant reduction of sulfur released into the gas phase, which occurred at temperatures as low as 200 °C. This corresponds to the temperature for which calcium-bound carboxylic groups begin to decompose during the pyrolysis process. In the present study, it appears that the ion-exchanged calcium in coal catalyzes the decomposition of more complex sulfur but prevents its evolution to the gas phase by reacting with the sulfur to form calcium sulfide in the char. Comparison of Calcium Additives. Figures 5 and 6 compare the total sulfur and sulfide sulfur transformations, respectively, for the TPP experiments of acidwashed Bowmans coal mixed with pure calcium carbonate (AW-PCC) and Caroline Limestone (AW-LST), both to a calcium content of 4 wt %. The results are compared with acid-washed (AW) and acid-washed calcium-exchanged coal (AW-Ca2+) to observe how the physical nature of the calcium additives affects sulfur transformations and retention during pyrolysis. Figure 5 shows that both AW-LST and AW-PCC samples undergo similar total sulfur transformations to the AWCa2+ coal, undergoing a steady decomposition of sulfur between 25 and 600 °C, which stabilizes between 600 °C and 900 °C. In the lower temperature region, all three calcium-doped coals experience faster sulfur decomposition than AW coal. It appears that the presence of calcium promotes decomposition of sulfur functionalities in this temperature range. However, as mentioned previously, calcium has been found to prevent sulfur evolution at temperatures as low as 200 °C.18 Thus, further work is required to understand the observed increase in sulfur evolution by the addition of (18) Maes, I. I.; Gryglewicz, G.; Yperman, J.; Franco, D. V.; Mullens, J.; Van Pouke, L. C. Fuel 1997, 76, (2), 143.
Investigation of Sulfur Retention
Figure 5. Total sulfur retention during temperature-programmed pyrolysis of acid-washed Bowmans coal and acidwashed Bowmans coal mixed with ion-exchanged calcium (AWCa2+), Caroline limestone (AW-LST), and pure calcium carbonate (AW-PCC).
Figure 6. Comparison of the percent of sulfide sulfur retained in char during temperature-programmed pyrolysis of acidwashed Bowmans coal mixed with ion-exchanged calcium (AWCa2+), Caroline limestone (AW-LST), and pure calcium carbonate (AW-PCC).
calcium. At 600 °C, AW coal has a slightly higher total sulfur content than the calcium-doped coals but continues to decompose between 600 and 900 °C. For AWPCC, AW-LST, and AW-Ca2+, decomposition of sulfur in the higher temperature region is not observed, suggesting that the presence of calcium actually inhibits decomposition of complex sulfur forms. As a result, the
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total sulfur contents in AW-LST and AW-PCC level off to a value similar and slightly below that of AW char, respectively. The total sulfur content of the AW-Ca2+ coal, however, is substantially greater than the others. Comparison of the percentage of sulfide sulfur formed during pyrolysis is featured in Figure 6 and shows that relatively negligible amounts of sulfide are formed from the samples with carbonate additives. Furthermore, there is little difference between pure calcium carbonate and limestone, with the latter facilitating only slightly higher sulfide formation and, hence, total sulfur retention. The AW-Ca2+ sample forms a significantly greater percent of sulfide sulfur, which corresponds to the higher total sulfur retention observed. This implies that calcium intimately bound to the coal reacts with sulfur more effectively than crystalline carbonate inclusions, which is in agreement with observations from AP-TPR experiments conducted on a variety of calcium-doped coals.18 Conclusion Temperature-programmed pyrolysis experiments of Bowmans and Lochiel coals reveal that slight variations in the distribution of organic and inorganic sulfur forms and variation in the type and nature of inorganic matter have a significant effect on the sulfur transformations during pyrolysis between 200 and 900 °C. Sulfur retention in the intermediate temperature range (500700 °C) is enhanced by a greater percent of sulfate sulfur due to its solid-state transformations to organic sulfur in the char between 400 and 800 °C. Organically bound calcium is found to facilitate reactions with the organic sulfur to form sulfide sulfur in the char. Pure calcium carbonate limestone additives, which are physically mixed into the coal samples, produce negligible amounts of sulfide sulfur and, hence, a lower sulfur retention. Acknowledgment. The authors gratefully acknowledge the financial and other support received for this research from the Cooperative Research Centre (CRC) for New Technologies for Power Generation from Lowrank Coal, which is established and supported under the Australian Government’s Cooperative Research Centres program. Marnie Telfer would like to thank the CRC for a postgraduate scholarship. The authors also wish to gratefully thank the Editor Prof. John Larsen and the reviewers for their constructive and professional comments on the earlier version of the manuscript, which led to a great improvement of this paper. EF980117X
