Desulfurization of Organic Sulfur from a Subbituminous Coal by

Energy & Fuels 2006, 20, 287-294

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Desulfurization of Organic Sulfur from a Subbituminous Coal by Electron-Transfer Process with K4[Fe(CN)6] Dipu Borah*,† Department of Chemistry, Pragjyotika J College, Titabar 785 630, Assam, India ReceiVed October 18, 2005

The desulfurization reaction involving direct electron transfer from potassium ferrocyanide, K4[Fe(CN)6], successfully removed organic sulfur from a subbituminous coal. The temperature variation of desulfurization revealed that increase of temperature enhanced the level of sulfur removal. Moreover, the desulfurization reaction was found to be dependent on the concentration of K4[Fe(CN)6]. Gradual increase in the concentration of K4[Fe(CN)6] raised the magnitude of desulfurization, but at higher concentration the variation was not significant. The removal of organic sulfur from unoxidized coal slightly increased with reduced particle size. Desulfurization from oxidized coals (prepared by aerial oxidation) revealed a higher level of sulfur removal in comparison to unoxidized coal. Highest desulfurization of 36.4 wt % was obtained at 90 °C and 0.1 M concentration of K4[Fe(CN)6] in the 100-mesh size oxidized coal prepared at 200 °C. Model sulfur compound study revealed that aliphatic types of sulfur compounds are primarily responsible for desulfurization. Because of higher stability, thiophene and condensed thiophene-type of compounds perhaps remained unaffected by the electron-transfer agent. Infrared study revealed the formation of oxidized sulfur compounds (sulfoxide, sulfone, sulfonic acid, etc.) in the oxidized coals. Band intensities in these oxidized compounds due to common -SdO and -SO2 units increased in their respective regions. The desulfurization reaction in different systems is well-represented by the pseudo-first-order kinetic model. The intrinsic rate constants were found to be in the range of (1.8-3.5) × 10-5 s-1, which implies that the reaction proceeds at a slow and steady rate. The activation energy of the reaction in different systems falls in the range of 5.2-8.8 kJ mol-1. Lower values of frequency factor (range: (1.9-5.5) × 10-4 s-1) revealed that during the course of the reaction an associated type of compound (activated complex) was formed. Application of the transition state theory indicated that the desulfurization reaction proceeds with the absorption of heat (endothermic reaction) and is nonspontaneous in nature.

Introduction Sulfur is primarily associated with coal in three different forms: sulfate minerals, sulfide minerals, and organic complexes. However, proportionate ratio of these forms depends on the nature and type of coal. Mostly iron sulfate and calcium sulfate account for sulfate sulfur in coal. In the sulfide mineral category, pyrite (FeS2) predominates. The organic sulfur directly bound to the coal organic matter exists in the form of sulfide, disulfide, thiol, thiophene, and cyclic sulfide.1 There are reports of the presence of trace amount of elemental sulfur in some coals.2 Baruah and Gogoi3 recently reported another form of sulfur known as secondary sulfur in coal that is neither purely organic nor purely pyritic. Combustion of high sulfur coal accounts for a sizable proportion of the total global output of anthropogenic SO2 which is toxic, corrosive, and a potential environmental polluting agent causing acid rain. This has compelled many countries to enforce regulatory measures to reduce the emission of SO2. Therefore, it is necessary to remove sulfur from coal prior to its utilization. * Corresponding author: Phone: +91 3771 248495. Fax: +91 3771 248743. E-mail: [email protected]. † Present address: Department of Energy, School of Energy, Environment and Natural Resources, Tezpur University, Napaam 784 028, Tezpur University, Assam, India. (1) IEA Coal Research, London. The Problems of Sulfur; Reviews in Coal Science; Butterworths: London, 1989; pp 5-10. (2) Richard, J. J.; Vick, R. D.; Junk, G. A. EnViron. Sci. Technol. 1977, 11, 1084. (3) Baruah, M. K.; Gogoi, P. C. Fuel 1998, 77, 979.

Presently, a number of options are available for reducing sulfur emissions, including the use of low-sulfur coals, the stack gas scrubbing, and (or) physical coal cleaning techniques. The stack gas scrubbing process is both expensive and energy-intensive. The physical coal cleaning techniques are relatively inexpensive and simple to operate yet are less effective. Different techniques of desulfurization have been developed based on physical, microbial, and chemical principles, but the former two methods are ineffective in removing sulfur completely.4-7 Chemical methods are promising desulfurization processes, are used extensively worldwide, and primarily utilize different oxidizing agents as desulfurizing reagents.8-21 Al(4) Cara, J.; Moran, A.; Carballo, T.; Rozada, F.; Aller, A. Fuel 2003, 82, 2065. (5) Martinez, O.; Diez, C.; Miles, N.; Shah, C.; Moran, A. Fuel 2003, 82, 1085. (6) Moran, A.; Cara, J.; Miles, N.; Shah, C. Fuel 2002, 81, 299. (7) Acharya, C.; Kar, R. N.; Sukla, L. B. Fuel 2001, 80, 2207. (8) Mukherjee, S.; Borthakur, P. C. Fuel 2003, 82, 783. (9) Pietrzak, R.; Wachowska, H. Fuel 2003, 82, 705. (10) Elsamak, G. G.; Oztas, N. A.; Yurum, Y. Fuel 2003, 82, 531. (11) Kozłowski, M.; Pietrzak, R.; Wachowska, H.; Yperman, J. Fuel 2002, 81, 2397. (12) Shah, C. L.; Abbott, J. A.; Miles, N. J.; Xuejun, L.; Jianping, X. Fuel 2002, 81, 519. (13) Mukherjee, S.; Borthakur, P. C. Fuel 2001, 80, 2037. (14) Borah, D.; Baruah, M. K.; Haque, I. Fuel 2001, 80, 1475. (15) Borah, D.; Baruah, M. K.; Haque, I. Fuel 2001, 80, 501. (16) Borah, D.; Baruah, M. K. Fuel Process. Technol. 2001, 72, 83. (17) Mukherjee, S.; Mahiuddin, S.; Borthakur, P. C. Energy Fuels 2001, 15, 1418. (18) Rodrigues, R. A.; Jul, C. C.; Gomez-Limon, D. Fuel 1996, 75, 606.

10.1021/ef050340b CCC: $33.50 © 2006 American Chemical Society Published on Web 12/10/2005

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Table 1. Proximate and Ultimate Analysis of the Coal proximate

(wt %)

ultimate

(wt %)a

moisture ash volatile matter fixed carbon

6.0 12.9 41.5 39.6

carbon hydrogen nitrogen sulfur oxygen (by diff) total sulfur sulfate sulfur pyritic sulfur

80.7 5.8 1.2 3.6 8.7 5.7 1.5 0.6

a

maf basis.

though chemical processes can successfully remove almost the entire inorganic sulfur component of coal, removal of organic sulfur is achieved to a certain extent. Chemically controlled electron-transfer processes of desulfurization are also known in the literature.22-31 In coal chemistry, Sternberg et al.22 first used an electron-transfer process that utilized an admixture of alkali metal and naphthalene in tetrahydrofuran. Chatterjee et al.25 reported the removal of 5090% organic sulfur from coal accomplished by single electrontransfer process with potassium naphthalenide in tetrahydrofuran. Metal naphthalenide prepared by treating transition metal ion with naphthalene in ethanol was also used as an electrontransferring agent in the desulfurization of coal organic sulfur but the level of desulfurization was achieved only to a certain extent.29,30 The present investigation is an attempt to desulfurize organic sulfur from a subbituminous Meghalaya (India) coal with potassium ferrocyanide, K4[Fe(CN)6], as the electron-transfer agent. Effect of temperature, concentration of K4[Fe(CN)6], and particle size on the extent of desulfurization were studied. The desulfurization process was also accomplished with oxidized coals. Infrared spectra of the oxidized coals were taken to verify the formation of oxidized sulfur compounds. Kinetic and thermodynamic approaches were also undertaken to understand the nature of the sulfur removal reaction. Experimental Section Coal Sample. The coal under investigation is a subbituminous Meghalaya coal of northeast India. The sample was ground to three different size fractions viz. 100, 150, and 200 meshes using standard stainless steel Tyler sieves. The ground samples were washed with distilled water, dried in air atmosphere at laboratory temperature (30 °C) for 48 h, and then stored in a desiccator. The proximate and ultimate analyses of the coal sample are presented in Table 1. The proximate analysis of coal sample was done by following Indian standard methods [IS: 1350 (Part-I)-1984]. The ultimate analysis was carried out using a Perkin-Elmer elemental analyzer. Total sulfur was estimated by using Eschka method (ASTM D 3177). The percentage of oxygen was calculated by difference. The forms of sulfur were determined by following the ASTM D 2492 method. (19) Palmer, S. R.; Hippo, E. J.; Dorai, X. A. Fuel 1994, 73, 161. (20) Sain, B.; Saikia, P. C.; Baruah, B. P.; Bordoloi, C. S.; Mazumder, B. Fuel 1991, 70, 753. (21) Boron, D. J.; Taylor, S. R. Fuel 1985, 64, 219. (22) Sternberg, H. W.; Delle Donne, C. L.; Pantags, P.; Moroni, E. C.; Markby, R. E. Fuel 1971, 50, 432. (23) Ignasiak, B. S.; Gawlak, M. Fuel 1977, 56, 216. (24) Ignasiak, T.; Kemp-Jones, A. V.; Strausz, O. P. J. Org. Chem. 1977, 42, 312. (25) Chatterjee, K.; Wolny, R.; Stock, L. M. Energy Fuels 1990, 4, 402. (26) Chatterjee, K.; Stock, L. M. Energy Fuels 1991, 5, 704. (27) Chatterjee, K.; Stock, L. M.; Gorbaty, M. L.; George, G. N.; Keleman, S. R. Energy Fuels 1991, 5, 771. (28) Stock, L. M. Energeria 1992, 3, 1. (29) Borah, D.; Baruah, M. K. Fuel 1999, 78, 1083. (30) Borah, D.; Baruah, M. K. Fuel 2000, 79, 1785. (31) Cook, P. S.; Cashion, J. D. Fuel 1987, 66, 661.

Preparation of Inorganic Sulfur-Free Coal. Sulfate sulfur and pyritic sulfur, the two typical forms of inorganic sulfur, were eliminated from the coal samples chronologically by treating the ground sample with 4.8 N HCl and 2 N HNO3 as reported.29 The inorganic sulfur-free coal (feed coal) sample was further treated with CCl4 with the objective to remove the residual pyrite in accordance to the method described by Cook and Cashion.31 This was done because the presence of traces of pyrite might affect the desulfurization reaction by electron-transfer process. Prior to the desulfurization reaction, the feed coal sample was tested for elemental sulfur likely to be formed during acid treatment. This was done by mixing a small amount of the sample with CS2. The admixture was then stirred at room temperature for about 3 h and filtered, and the filtrate was treated with piperidine. Red coloration due to elemental sulfur was not observed.32 Preparation of Oxidized Coal. The inorganic sulfur-free coal samples were oxidized in air atmosphere in a temperature controlled oven by linearly increasing the temperature at a rate of 5 K min-1 from room temperature (30 °C) to 200 °C. Samples were withdrawn at 100 and 200 °C, cooled to room temperature, and preserved in a desiccator. Desulfurization Method. The desulfurization reaction was accomplished by treating feed coal of 100-mesh size weighing 2.0 g with 50 mL of distilled water and 5 mL of 0.01 M K4[Fe(CN)6] in a 100-mL conical flask. The materials in the flask were shaken to ensure homogeneous mixing. The flask was then kept at room temperature (30 °C) in an isothermal shaker bath for 1 h. An aliquot of 5 mL was withdrawn carefully to avoid coal contamination as much as possible, filtered, and then washed repeatedly with distilled water. The filtrate and washings were collected in a 100-mL beaker. The temperature of the shaker bath where the conical flask containing the residual mixture was placed was increased to 45 °C at a heating rate of 2 K min-1. After keeping the mixture at this temperature for 15 min; an aliquot of 5 mL was withdrawn. The same procedure was adopted to study the temperature dependence on desulfurization at 60, 75, and 90 °C. The entire process was repeated afresh by varying the concentration of K4[Fe(CN)6] (0.1 and 1.0 M) to evaluate the concentration dependence on desulfurization. Similar experiments were accomplished to study the effect of coal particle size on desulfurization at a particular concentration of K4[Fe(CN)6]. Oxidized samples were also treated with the desulfurizing agent under identical reaction conditions. Experiments were also undertaken to understand the response of an aromatic sulfur compound (dibenzothiophene) toward desulfurization reaction under similar reaction conditions. Estimation of Sulfur. The volume of the collected filtrate and washings was reduced by placing the beaker in a water bath at ∼70 °C. The materials were then treated with 2 mL of concentrated HNO3 and heated strongly to almost dryness. The process was repeated twice. This was followed by the extraction of 3 mL of concentrated HCl and then heated strongly. The HCl extracted solution was diluted by adding 25 mL of distilled water, warmed at 70 °C for 15 min, and finally filtered. The filtrate was then treated with 10 mL of 5% BaCl2 solution, and the beaker was kept overnight for complete precipitation of BaSO4. Sulfate sulfur was estimated gravimetrically using the standard method.33 The chemicals used in the experiments were analytical reagent grade. Moreover, all the experiments were repeated twice for reproducibility, and the average values are reported so as to minimize the experimental error. In most of the experiments identical results were obtained. Infrared Spectra. Spectra of both unoxidized and oxidized coal samples were recorded in KBr pellets in correct proportion (1:150) using a Perkin-Elmer infrared spectrophotometer (model 983) in the range of 4000-400 cm-1. (32) Remy, H. In Treatise on Inorganic Chemistry; Kleinberg, J., Ed.; Elsevier: Amsterdam, 1956; Vol. I, p 700. (33) Vogel, A. I. A Textbook of QuantitatiVe Inorganic Analysis; Longmans: London, 1962.

Desulfurization of Organic Sulfur with K4[Fe(CN)6]

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Figure 1. Removed organic sulfur (wt %) against reaction temperature for the desulfurization reaction accomplished with 100-mesh size unoxidized coal and 0.01 (b), 0.1 (2), and 1.0 M ([) K4[Fe(CN)6].

Figure 3. Removed organic sulfur (wt %) against reaction temperature for the desulfurization reaction accomplished with 0.1 M K4[Fe(CN)6] and 100-mesh size unoxidized coal (b) and oxidized coal prepared at 100 °C (2) and 200 °C ([).

Figure 2. Removed organic sulfur (wt %) against reaction temperature for the desulfurization reaction accomplished with 0.1 M K4[Fe(CN)6] and 100-mesh (b), 150-mesh (2), and 200-mesh ([) size unoxidized coal.

Figure 4. Infrared spectra of unoxidized coal (a) and oxidized coal prepared at 100 °C (b) and 200 °C (c).

Results and Discussion Analysis of Desulfurization. The results of desulfurization of organic sulfur from feed coal of particle size 100 mesh by electron-transfer process accomplished with potassium ferrocyanide, K4[Fe(CN)6], at different temperatures are shown in Figure 1. The sulfur extrusion process has been found to be continuous and gradually increases with increase of temperature. Furthermore, the figure reveals that with gradual increase in the concentration of K4[Fe(CN)6] the level of desulfurization is enhanced, yet at higher concentrations the difference of removal is not pronounced. The effect of particle size on desulfurization is presented in Figure 2. The figure clearly demonstrates that particle size has no significant impact on sulfur removal. The minor increase in the rate of desulfurization with reduced particle size is due to the increase in external surface area of the coal particles. On the basis of the conclusions drawn from Figures 1 and 2, the desulfurization reaction was also carried out in oxidized coals of particle size 100 mesh with 0.1 M K4[Fe(CN)6] solution, and the results are depicted in Figure 3. The extent of sulfur removal is more in oxidized coals in comparison to unoxidized coal. Moreover, in oxidized coals an oxidation temperature difference of 100 °C has no profound impact on the level of desulfurization as the difference of removal is not very high. This process has succeeded in removing a maximum of 36.4 wt % of the total organic sulfur from 100-mesh size oxidized coal prepared at 200 °C with 0.1 M K4[Fe(CN)6] solution. The higher level of desulfurization in oxidized coals could be due to the formation of oxidized sulfur compounds formed during aerial oxidation. Depending on the nature of the sulfur compounds and oxidizing conditions, organic sulfur molecules

are oxidized to sulfoxide, sulfone, sulfonic acid, sulfenic acid, and sulfinic acid.15,34 To verify the presence of oxidized sulfur compounds, infrared spectra of both unoxidized and oxidized (100 and 200 °C) coals were taken and a comparison was made about the extent of formation of oxidized compounds (Figure 4). Whatever the oxidized sulfur species (sulfoxide, sulfone, sulfonic acid, sulfenic acid, and sulfinic acid), they have one unit in common: -SdO. The absorption band found in the range of 1070-1030 cm-1 is due to -SdO in sulfoxides.35 A broad band is observed in the range of 1080-1000 cm-1 in both unoxidized and oxidized coals and can be assigned to -SdO. The presence of this band in the unoxidized coal might be due to oxidation of some of the organic sulfur compounds of coal during its preparation. The intensity of this band has increased in the oxidized coal. Additionally, the increase is more in the oxidized coal prepared at 200 °C. This reveals that a certain fraction of the sulfur compounds has been oxidized during the oxidation stage. Moreover, both sulfone and sulfonic acid contain a common -SO2 unit and absorption bands for both these two compounds appear almost in the same range. The asymmetric and symmetric stretching vibration of -SO2 in sulfone occurs in the range of 1350-1300 cm-1 and 11601120 cm-1, respectively, whereas the corresponding ranges are 1350-1342 cm-1 and 1165-1150 cm-1 in sulfonic acid.35 In both unoxidized and oxidized coals, one sharp band is found at 1130 cm-1 and another weak band is identified at 1335 cm-1. These two bands are due to symmetric and asymmetric vibrations of -SO2. The intensities of these bands are comparatively lower in the unoxidized coal. (34) March, J. AdVanced Organic Chemistry; Wiley: New Delhi, 1994. (35) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds; Wiley: New York, 1981.

290 Energy & Fuels, Vol. 20, No. 1, 2006

Sulfur removal would not be possible unless the C-S bond, the common linkage of all organic sulfur molecules, is activated and ruptured. In this desulfurization process, obviously K4[Fe(CN)6] acts as the activator and induces C-S bond cleavage by electron transfer. K4[Fe(CN)6] is a versatile electron-transfer agent and is involved in redox reactions through direct electrontransfer processes such as from [Fe(CN)6]4- to [Fe(CN)6]3- in which only the formal valence states of the metal ions have changed without appreciable change of atomic arrangement during the time of electronic transition. In the absence of ferricyanide ion, [Fe(CN)6]3-, the redox properties of K4[Fe(CN)6] might possibly be due to the transference of electron from it to some reactive sites of coal molecule that could display oxidation-reduction reactions or could expand their valence states. The divalent sulfur atom present in coal molecule has the potential to expand its valency due to the presence of a vacant d-orbital. This electrophilic site upon acceptance of electron becomes negatively charged and exerts a bond weakening effect on the C-S bond. This results in the cleavage of the C-S bond releasing sulfur in its soluble form. Considering these aspects, a mechanism is proposed to explain the observed desulfurization denoting organic sulfur molecule of coal as R-S-R. Dissolution

K4[Fe(CN)6] f 4K+ + [Fe(CN)6]4[Fe(CN)6]4- + R-S-R f [Fe(CN)6]3-R-S--R f [Fe(CN)6]3- + R-S--R Activated complex -

R-S -R f R-SSoluble sulfur Moreover, another mechanism can be suggested that involves [Fe(CN)6]3- ion produced in the above mechanism for the formation of soluble sulfur. The divalent sulfur atom of the coal sulfur molecule contains two lone pairs of electrons. Expulsion of an electron from the valence shell of sulfur by [Fe(CN)6]3will induce a positive charge creating an acrimonious situation around it facilitating the rupture of the C-S bond.

[Fe(CN)6]3- + R-S-R f [Fe(CN)6]4-R-S+-R f [Fe(CN)6]4- + R-S+-R Activated complex R-S+-R f R-SSoluble sulfur The proposed mechanisms demonstrate that the breakaway sulfur species are charged and their molecular weight is decreased substantially. Therefore, extensive solvation might have facilitated their dissolution process. Moreover, R-Sproduced through the above mechanisms could also react with available K+ ion generated from the dissolution of K4[Fe(CN)6] forming K2S, which is soluble in the medium.

R-S- + 2K+ f K2S f 2K+ + S2Infrared spectra of oxidized coals revealed the presence of oxidized sulfur compounds (sulfone, sulfonic acid, etc.). Hydrolysis of these compounds under the reaction conditions might

Borah

have brought sulfur from these compounds into soluble form as sulfate causing desulfurization.

R-SO3H + H2O f R-H + H2SO4 R-SO2-R + H2O f R-H + H2SO4 Formation of elemental sulfur (S°) under the reaction conditions cannot be ruled out.

R-S--R/R-S+-R f S° But diagnostic tests relating to the detection of S° in the solution gave negative results. This reveals that even if some amount of S° has formed, a fraction might have reacted with S2- to form di- or polysulfides.

S2- + S° f S22There is also the possibility of the fixation of S°. It is known that S° interacts with organic compounds forming sulfides by dehydrogenation processes36 at elevated temperatures. Moreover, Weitkamp37 reported the formation of 3,6-dimethlbenzo[b]thiophene by the interaction of S° with terpenes. Interestingly, this compound has been detected in coal extracts.38 During the course of the desulfurization reaction, a light blue color is observed. The depth of the color increased with increase of temperature. The appearance of blue color is due to the formation of ferric ferrocyanide, Fe4[Fe(CN)6]3, whose formation would be possible provided the system contains Fe3+ ion. Both [Fe(CN)6]4- and [Fe(CN)6]3- are not further degradable, and therefore, Fe3+ ions are produced from the coal matrix. It is understood that coal contains metal bound to the organic matter where iron predominates. Under the prevailing reaction conditions, a fraction of bound iron might have released in the form of Fe3+ ion and combined with [Fe(CN)6]4- to form Fe4[Fe(CN)6]3. Apart from this, the generated Fe3+ ion might have also participated in the desulfurization reaction. A literature survey reveals that Fe3+ ion in the form of ferric sulfate39,40 or ferric chloride41,42 is a well-known desulfurizing agent. The level of desulfurization achieved in this process clearly demonstrates that a certain type of organic sulfur molecules is attacked at the C-S bond. To understand the type of sulfur molecules that underwent desulfurization, a model study was undertaken taking an aromatic compound (dibenzothiophene) and treating it with K4[Fe(CN)6] under identical reaction conditions. The aromatic sulfur compound failed to respond to the desulfurization process. This could be due to the higher C-S bond dissociation energy of dibenzothiophene. It is understood that the thiophene type of compounds are resistant toward oxidation in air up to 500 °C.43 Friedman et al.44 also reported that heterocyclic sulfur compounds such as dibenzothiophene are thermally very stable and cleavage of the C-S bond under the conditions generally applied for oxidative chemical desulfurization processes is not possible. Moreover, a comparison (36) Mazumdar, B. K.; Choudhuri, S. S.; Chakraborty, S. K.; Lahiri, A. J. Sci. Ind. Res. 1958, 17B, 509. (37) Weitkamp, A. W. J. Am. Chem. Soc. 1959, 81, 3430. (38) White, C. M.; Douglas, L. J.; Perry, M. B.; Schmidt, C. E. Energy Fuels 1987, 1, 222. (39) Meyers, R. A. Hydrocarbon Process. 1975, 53, 93. (40) Meyers, R. A. Hydrocarbon Process. 1979, 57, 123. (41) King, W. E., Jr.; Perlmutter, D. D. AIChE J. 1977, 23, 679. (42) Oshinowo, T.; Ofi, O. Can. J. Chem. Eng. 1987, 65, 481. (43) Ignasiak, T. M.; Strausz, O. P. Fuel 1978, 57, 620. (44) Friedman, S.; Lacount, R. B.; Warzinski, R. P. Prepr. Pap.-Am. Chem. Soc., DiV. Fuel Chem. 1977, 21, 100.

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Energy & Fuels, Vol. 20, No. 1, 2006 291

of C-S bond dissociation energy in aliphatic and aromatic sulfur compounds revealed that C-S bond dissociation energy is relatively lower in aliphatic sulfur compounds than in their aromatic analogues.45 The subbituminous coal under study is located in the northeastern part of India, where the presence of mercaptan, sulfide, disulfide, thiophene, and ring compounds is reported.17,46-49 Therefore, it can be inferred that the desulfurization reaction involving K4[Fe(CN)6] is primarily due to aliphatic sulfur compounds and is consistent with the type of sulfur compounds present in the coal. Furthermore, the aliphatic component of coal can be estimated using the following relationship.50

Fixed carbon ) 1.023 Car where Car is the aromatic carbon. The investigated coal contains 39.6 wt % fixed carbon (Table 1). Therefore, Car and Cal (aliphatic carbon) are 38.7 and 61.3 wt %, respectively. Thus, it is obvious that the larger fraction of organic sulfur is fixed in the aliphatic network and that the component could have predominantly suffered desulfurization. Desulfurization Kinetics. Desulfurization by electron-transfer process involving K4[Fe(CN)6] was found to be continuous, as ascribed by Figures 1 and 2. To represent the sulfur extrusion process, a model based on pseudo-first-order kinetics was applied. The rate of the desulfurization reaction can be represented in the following form with respect to the concentration of coal organic sulfur denoted as Org-S on the considerations that the concentration of the electron-transfer agent remained unaltered. This is because the mechanistic approach revealed that the generated [Fe(CN)6]3- can further participate in the sulfur loss reaction.

Rate ) -d[Org-S]/dt ) k[Org-S]

(1)

where k is the intrinsic rate constant and is related to the initial concentration (a) of organic sulfur and concentration at time t (x) as

log(a - x) ) -(k/2.303)t + log a

(2)

k ) (2.303/t) log(a/a - x)

(3)

or

The global kinetic parameters viz. activation energy (E) and pre-exponential factor (A) are estimated by applying the classical Arrhenius equation which predicts that a plot of log k against 1/T produces a straight line.

log k ) -E/2.303 RT + log A

(4)

where R is the universal gas constant. The pseudo-first-order kinetic model (eq 2) is tested with a representative run carried out with 100-mesh size unoxidized coal and 0.01 M K4[Fe(CN)6] at 30 °C. It has been found that desulfurization increases continuously with the progress of time (45) Benson, S. W. Chem. ReV. 1978, 78, 23. (46) Bhatnagar, S. S.; Dutta, N. L. Report, Board of Scientific and Industrial Research; Department of Commerce, Govt. of India, 1940-1941; p 16. (47) Chowdhury, J. K.; Dutta, P. B.; Ghosh, S. R. J. Sci. Ind. Res. 1952, 118, 146. (48) Barooah, P. K.; Baruah, M. K. Fuel Process. Technol. 1996, 46, 83. (49) Iyenger, M. S.; Guho, S.; Bari, M. L.; Lahiri, A. In Proceedings of the Symposium, The Nature of Coal; CFRI (India): Dhanbad, 1959, Chapter 26. (50) Solomon, P. H. Fuel 1981, 60, 3.

Figure 5. (a) Removed organic sulfur (wt %) against reaction time. (b) Kinetic plot of log(a - x) against t for the representative run accomplished with 100-mesh size unoxidized coal and 0.01 M K4[Fe(CN)6] at 30 °C.

as shown in Figure 5a. The validity of the pseudo-first-order kinetic equation is justified by constructing the plot of log(a x) against t as shown in Figure 5b and finding that the plot is almost linear. This proves the applicability of the model to the desulfurization reaction. The kinetic parameters viz. intrinsic rate constant, activation energy, and pre-exponential factor are summarized in Table 2. The intrinsic rate constants calculated using eq 3 are sufficiently low in all the systems. This implies that the desulfurization reaction has proceeded in a slow and steady manner. This might be due to a number of factors that have governed the overall rate of the reaction. The first one is the statistical factor that arises from the fact that though there is a huge number of organic sulfur molecules in the coal sample, only a minor fraction is in the activated state to undergo desulfurization. The second one is the stereochemical aspect of the reactant molecules. The consequence of this effect is more pronounced as interaction in terms of electron transfer would be possible provided the reactant molecules are properly oriented and the sulfur atom that receives the donated electron is exposed in the right direction. The third one is the diffusional consequences of the interacting species as it governs the rate of a reaction occurring in solution phase. Coal is a porous material, and electron transfer by [Fe(CN)6]4- to the sulfur atom would be possible only if it is mobilized to the surface of the pores. Because of pore heterogeneity, the entire electron-transfer agent might have failed to penetrate into the pores limiting the level of desulfurization. The activation energies estimated from the slope of the Arrhenius plots (Figure 6) fall in the range of 4.6-8.8 kJ mol-1 (Table 2). The values are sufficiently low and do not differ much. This implies that the dependence of the concentration of K4[Fe(CN)6] and particle size on the desulfurization reaction cannot be described elegantly by activation energy. It is

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Table 2. Kinetic Parameters of the Desulfurization Reaction Accomplished with K4[Fe(CN)6] nature of coal unoxidized

unoxidized

unoxidized

unoxidized

unoxidized

oxidized at 100 °C

oxidized at 200 °C

temp, °C 30 45 60 75 90 30 45 60 75 90 30 45 60 75 90 30 45 60 75 90 30 45 60 75 90 30 45 60 75 90 30 45 60 75 90

particle size

concn of K4[Fe(CN)6]

100 mesh

0.01 M

100 mesh

0.1 M

100 mesh

1.0 M

150 mesh

0.1 M

200 mesh

0.1 M

100 mesh

0.1 M

100 mesh

0.1 M

understood that direct electron-transfer reactions are very rapid and in the related mechanism each complex retains its full coordination shell in the activated complex, even though activation energy is required.51 Desulfurization reaction accomplished by direct electron-transfer process envisages activation energy (i) to overcome electrostatic energy, (ii) to distort the coordination shell of the complexes, and (iii) to modify the solvent structure around the species. Emeleus and Sharpe51 reported that [Fe(CN)6]4--[Fe(CN)6]3- exchange reaction is catalyzed by positively charged species, the effect being pronounced for alkali metals. This is because a partly desolvated cation (M+) accelerates the electron exchange process through the formation of a transition state that minimizes the electrostatic repulsion.

M+‚‚‚[Fe(CN)6]3Lower activation energy observed for the desulfurization reaction could be perhaps due to the occurrence of such a transition state. This is highly probable because of the presence of a sufficient amount of K+ ions generated through the dissociation of K4[Fe(CN)6]. Moreover, Fe3+ ions released by the coal organic matter could have also been involved in the formation of the transition state that comprises [Fe(CN)6]4- and R-S-R/R-S--R/R-S-.

K+/Fe3+‚‚‚R-S-R/R-S--R/R-SThe physical interpretation of pre-exponential factor is that it describes the amount of successful collisions among the (51) Emeleus, H. J.; Sharpe, A. G. Modern Aspects of Inorganic Chemistry; Universal Book Stall: New Delhi, 1992.

k, s-1 10-5

1.8 × 2.2 × 10-5 2.2 × 10-5 2.7 × 10-5 2.9 × 10-5 2.2 × 10-5 2.4 × 10-5 2.8 × 10-5 3.0 × 10-5 3.2 × 10-5 2.1 × 10-5 2.4 × 10-5 2.9 × 10-5 3.1 × 10-5 3.3 × 10-5 2.2 × 10-5 2.6 × 10-5 2.8 × 10-5 3.3 × 10-5 3.5 × 10-5 2.4 × 10-5 2.5 × 10-5 2.8 × 10-5 3.3 × 10-5 3.4 × 10-5 3.3 × 10-5 3.4 × 10-5 3.8 × 10-5 4.5 × 10-5 4.7 × 10-5 3.8 × 10-5 4.0 × 10-5 4.1 × 10-5 4.9 × 10-5 5.0 × 10-5

E, kJ mol-1

A, s-1

8.8

5.5 × 10-4

5.2

1.9 × 10-4

7.2

3.8 × 10-4

6.8

3.3 × 10-4

5.9

2.4 × 10-4

7.0

4.7 × 10-4

4.6

2.2 × 10-4

reacting constituents to activate the reacting species. The preexponential factor determined for the desulfurization reaction in all systems is considerably low (Table 2). This reveals that collisions have succeeded in activating only a minor fraction of sulfur molecules. Moreover, a low pre-exponential factor is in favor of an associated type of reaction.52 The mechanisms suggested for the desulfurization reaction indicate the formation of a precursor activated complex that is an associate in nature. This is in conformity with the estimated values of the preexponential factor. Apart from this, a low pre-exponential factor suggests a nonspontaneous type of reaction. However, the nature of the desulfurization reaction can be properly explained with the help of thermodynamic parameters. Desulfurization Thermodynamics. Thermodynamic parameters are versatile physical functions that can describe the nature of a chemical reaction adequately and appropriately. Therefore, a semiquantitative thermodynamic approach has been adopted based on the transition state theory. This theory predicts the formation of a precursor activated complex of sufficient energy that remains in equilibrium with the reacting constituents. On the basis of a statistical thermodynamic approach,53 the equilibrium concentration of the activated complex (AC) can be expressed as

[AC]* ) K*[Org-S]

(5)

where K* is the equilibrium constant and Org-S is the coal organic sulfur. The rate of the desulfurization reaction with (52) Moore, W. J. Basic Physical Chemistry; Prentice Hall: New Delhi, 1989. (53) Banerjea, D. Coordination Chemistry; Tata McGraw-Hill: New Delhi, 1993.

Desulfurization of Organic Sulfur with K4[Fe(CN)6]

Energy & Fuels, Vol. 20, No. 1, 2006 293

where ∆H* and ∆S* are the enthalpy change of activation and entropy change of activation, respectively. Substitution of ∆G* in eq 8 gives

-d[Org-S]/dt ) (kT/h) exp(-∆H*/RT) exp(∆S*/RT)[Org-S] (10) This approach enables one to relate ∆S* with the pre-exponential factor, A, of the Arrhenius equation as

∆S* ) R[ln A + ln(h/kT) - 1]

(11)

Moreover, the relationship between E and ∆H* is

E ) ∆H* + RT

Figure 6. Arrhenius plots of -log k against 1/T. (a) Coal particle size of 100-mesh and K4[Fe(CN)6] concentration of 0.01 (b), 0.1 (2), and 1.0 M ([). (b) K4[Fe(CN)6] concentration of 0.1 M and coal particle size of 150 mesh (b) and 200 mesh (2). (c) Coal particle size of 100 mesh, K4[Fe(CN)6] concentration of 0.1 M, and oxidized coal prepared at 100 °C (b) and 200 °C (2).

respect to Org-S can be shown to be equal to (kT/h)[AC]*.

-d[Org-S]/dt ) (kT/h)[AC]* ) (kT/h)K* [Org-S] ) k[Org-S] (6) where k, k, and h are the pseudo-first-order rate constant, Boltzman’s constant, and Plank’s constant, respectively. Furthermore, K* can be correlated to the Gibbs free energy of activation (∆G*) in the following manner.

∆G* ) -RT ln K*

(7)

-d[Org-S]/dt ) (kT/h) exp(-∆G*/RT)[Org-S]

(8)

Therefore,

The Gibbs-Helmholtz relationship is

∆G* ) ∆H* - T∆S*

(9)

(12)

The difference between thermodynamic functions of the complete reaction (∆H, ∆S, and ∆G) and thermodynamic functions of activation (∆H*, ∆S*, and ∆G*) is very low,30,52 and therefore, data are given in the form of ∆H, ∆S, and ∆G. The values of all the thermodynamic parameters viz. enthalpy change (∆H), entropy change (∆S), and Gibbs free energy change (∆G) of the desulfurization reaction in different systems carried out with K4[Fe(CN)6] are assembled in Table 3. It is apparent that ∆H values are all positive irrespective of the concentration of K4[Fe(CN)6], particle size, nature of coal, and leaching temperature. This further reveals that the desulfurization reaction proceeds with the absorption of heat. Unless heat is supplied or some allied exothermic reactions occur in the system, the reaction is difficult to accomplish, and even if it occurs, the rate would be considerably low. The observed desulfurization at laboratory temperature (30 °C) is due to easily removable sulfur compounds. An increase in temperature might have activated certain stable sulfur compounds with consequent increase of desulfurization. There might be some exothermic reactions occurring concurrently as well, and they probably drove the desulfurization reaction. Moreover, lower activation energy could be another driving force that might have compelled those sulfur compounds that are thermally less staggered. Usually in any chemical reaction a rise in temperature enhances the rate of reaction and a fall in temperature declines the rate, and it is immaterial whether the reaction is exothermic or endothermic. The entropy change of desulfurization reveals that values are all negative irrespective of temperature, concentration of K4[Fe(CN)6], particle size, and nature of coal. This is due to a low pre-exponential factor and arises because of the presence of a lower amount of activated sulfur molecules in the coal. Moreover, a negative value of entropy change is in favor of the nonspontaneity of the desulfurization reaction which, however, can be appropriately described by the Gibbs free energy change function. In fact, entropy change of a reaction in solution phase measures the total change in entropy of the reactants and the solvent on formation of the activated complex. This is, however, very difficult to estimate in systems that comprise coal and other substances because of complex and varied composition of coal. The values presented in Table 3 are derived for coal sulfur molecules only; influences of other substances in the formation of activated complex are neglected. The entropy change parameter further predicts the type of complexes formed during the path of desulfurization reaction. The estimated values unveiled that during the course of reaction an associated type of complex was formed in the form of activated complex. The negative value of ∆S is due to the formation of this transition state because its formation leads to the total decrease in disorderliness in the system. The sign and magnitude of ∆S

294 Energy & Fuels, Vol. 20, No. 1, 2006

Borah

Table 3. Thermodynamic Parameters of the Desulfurization Reaction Accomplished with K4[Fe(CN)6] nature of coal unoxidized

unoxidized

unoxidized

unoxidized

unoxidized

oxidized at 100 °C

oxidized at 200 °C

temp, °C

particle size

concn of K4[Fe(CN)6]

∆H, kJ mol-1

∆S, JK-1mol-1

∆G, kJ mol-1

30 45 60 75 90 30 45 60 75 90 30 45 60 75 90 30 45 60 75 90 30 45 60 75 90 30 45 60 75 90 30 45 60 75 90

100 mesh

0.01 M

100 mesh

0.1 M

100 mesh

1.0 M

150 mesh

0.1 M

200 mesh

0.1 M

100 mesh

0.1 M

100 mesh

0.1 M

6.3 6.2 6.1 6.0 5.8 2.7 2.6 2.5 2.3 2.2 4.7 4.5 4.4 4.3 4.2 4.3 4.2 4.1 4.0 3.8 3.4 3.3 3.1 3.0 2.9 4.4 4.3 4.2 4.1 3.9 2.1 2.0 1.9 1.7 1.6

-315.8 -316.2 -316.6 -317.0 -317.3 -324.8 -325.2 -325.6 -325.9 -326.3 -318.8 -319.2 -319.6 -319.9 -320.3 -320.0 -320.4 -320.8 -321.2 -321.5 -322.8 -323.2 -323.6 -324.0 -324.3 -317.1 -317.5 -317.9 -318.2 -318.6 -323.5 -323.9 -324.2 -324.6 -325.0

102.0 106.8 111.5 116.3 121.0 101.1 106.0 110.9 115.8 120.6 101.3 106.0 110.8 115.6 120.4 101.3 106.1 110.9 115.7 120.5 101.2 106.0 110.9 115.7 120.6 100.5 105.3 110.0 114.8 119.6 100.1 105.0 109.8 114.7 119.6

furthermore very much depend on the charge of the activated complex. This is because the overall charge of the complex determines the extent of solvation. Activated complexes that are highly charged suffer increased solvation of the transition state which results in the decrease in the value of ∆S and becomes negative. The mechanisms of desulfurization reveal that the activated complexes formed are negatively charged and their extensive solvation might have caused substantial decrease in the value of ∆S and made it negative. Furthermore, it is understood that a loosely bound activated complex has more entropy compared to a tightly bound one.52 In the present investigation, during the course of desulfurization, it seems likely that tightly bound activated complexes have dominated. These complexes are sluggish to eliminate sulfur in the soluble form, and that might have suppressed the desulfurization reaction from proceeding at a desired rate. The feasibility of a chemical process is eloquently described by the Gibbs free-energy function. For the desulfurization reaction, all the ∆G values presented in Table 3 are found to be positive, no matter what the temperature, concentration of K4[Fe(CN)6], particle size, and nature of coal. This is in conformity with the nonspontaneous nature of the reaction. The low pre-exponential factor and negative value of entropy change of desulfurization could have conferred nonspontaneity to the reaction. Moreover, the nature of the activated complexes formed under the reaction conditions could also govern whether the reaction is thermodynamically favorable or not. On a molecular basis, coal organic sulfur can be classified into two groups: aliphatic and aromatic. Obviously during the course of the desulfurization reaction two types of activated complexes might have formed. Because of complex stereochemical properties of bulky aromatic sulfur compounds, the [Fe(CN)6]4- ion might have suffered greater hindrance to transfer its electron to the sulfur atom and that could have prohibited the desulfurization reaction from creating a greater degree of nonspontaneity.

Conclusions Desulfurization by an electron-transfer process involving K4[Fe(CN)6] effectively removes organic sulfur from a subbituminous coal. Though the effect of particle size on the level of desulfurization is not pronounced, an increase in temperature and concentration of K4[Fe(CN)6] enhanced the amount of removal to a considerable level. The level of desulfurization has been found to be higher in the oxidized coals. This is due to the formation of oxidized sulfur compounds during the preparation of oxidized coal in air atmosphere. Formation of oxidized sulfur compounds such as solfoxide, sulfone, sulfonic acid, and so forth are confirmed by infrared spectra. The model sulfur compound study revealed that, during desulfurization, primarily an aliphatic type of sulfur compound is removed from the coal. With aromatic compounds, the desulfurization reaction is not feasible because of the higher C-S bond dissociation energy and associated stability. The pseudo-first-order kinetic model adopted to represent the desulfurization reaction reveals that the model properly fits with the reaction in different systems. The activation energy and pre-exponential factor are estimated for the reaction using the Arrhenius equation and are found to be very low in all the systems. This favors an associated type of reaction and justifies the formation of a precursor activated complex during the course of the reaction. Moreover, this kinetic approach is in good agreement with the proposed mechanisms of desulfurization. The semiquantitative thermodynamic approach reveals that the reaction proceeds with the absorption of heat and is nonspontaneous in nature. Furthermore, there is a decrease in the disorderliness in the systems due to the occurrence of the desulfurization reaction. Acknowledgment. The author offers his sincere thanks and gratitude to CFRI-Jorhat Unit for providing the coal sample and the director of RSIC-Shillong for recording the infrared spectra. EF050340B