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Kinetics and Energetics of High-Sulfur Northeastern India Coal Desulfurization Using Acidic Hydrogen Peroxide Samit Mukherjee* and S. K. Srivastava* Fuel Science Division, Central Fuel Research Institute (CSIR), PO FRI-828 108, Dhanbad, Jharkhand, India Received January 22, 2004. Revised Manuscript Received April 28, 2004
Studies on the kinetics and energetics of high-sulfur coals in northeastern India regions, using 15% (v/v) hydrogen peroxide mixed with (0.1 N) sulfuric acid as a desulfurizing agent, were conducted at four different temperatures, viz., 15, 25, 30, and 40 °C. The rate constants varied over a range of 6.2 × 10-5-7.8 × 10-5 for Baragolai coal and a range of 7.0 × 10-5-11.5 × 10-5 for Ledo coal. Rate constants were calculated using a second-order rate equation, as the concentration of both hydrogen peroxide and pyrite changed, with respect to time. These results, when compared with those reported earlier, indicated that, when hydrogen peroxide was mixed with 0.1 N sulfuric acid, the rate of the reaction was enhanced, as compared to that observed using hydrogen peroxide alone, when high-sulfur Meghalaya coal samples of the northeastern India region were used. The oxidation rate increased as the temperature increased from 15 °C to 40 °C. The respective activation energy and frequency factor values were 8.04 kJ/mol and 10.47 × 10-5 for Baragolai coal, and 11.49 kJ/mol and 15.85 × 10-5 for Ledo coal. The results are in excellent agreement with earlier published work [Borah and Barauh, Fuel Process. Technol., 2001, 72, 83] but are in total disagreement with the respective activation energy and frequency factor values of 19.33 × 106 J/kmol and 6.19 × 10-1 for Baragolai coal and 39.72 × 106 J/kmol and 2.33 × 10-4 for Ledo coal that were obtained by another group [Mukherjee et al., Energy Fuels, 2001, 15, 1418]. The equilibrium constant values at 15, 25, 30, and 40 °C for the formation of an activated complex were in the range of 1.03 × 10-17-1.19 × 10-17 for Baragolai coal and 1.16 × 10-171.76 × 10-17 for Ledo coal. The free energy of activation values lie in the range of 93.67-101.42 kJ/mol for Baragolai coal and 93.38-100.41 kJ/mol for Ledo coal. The positive value for the free energy of activation indicates the nonspontaneous nature of the activated complex formation reaction. The enthalpy of activation values vary from 5.64 kJ/mol to 5.44 kJ/mol for Baragolai coal and from 9.09 kJ/mol to 8.89 kJ/mol for Ledo coal. A positive enthalpy of activation value indicates that the formation of the activated complex proceeds with an absorption of heat. The entropy of activation value was -0.31 kJ/mol for Baragolai coal and -0.29 kJ/mol for Ledo coal, indicating that the number of degrees of freedom decreased when the activated complex was formed. A negative entropy of activation value is due to (i) the occurrence of collisions, (ii) a low frequency factor, (iii) nonspontaneity, and (iv) the formation of a loosely bound activated complex.
Introduction In India, high-sulfur coals/lignites are found mainly in northeastern coalfields, the Wardha valley coalfield of Central India (Maharashtra), Rajasthan, Neyveli, and Gujarat. The use of high-sulfur coals/lignites in thermal power plants causes wear and tear, as well as corrosion of the construction material/equipment, because of the formation of SO2 and finally H2SO4. It also causes acid rain, which ruins the soil fertility when SO2 is released into the atmosphere. The desulfurization of coal/lignite can be achieved through physical, chemical, and microbial processes. Among these options, chemical methods are fast and are able to remove all the forms of sulfur. Desulfurization can be achieved in any of three stages, viz, pre-combustion, in situ combustion, and post* Authors to whom correspondence should be addressed. Telephone: +91-326-2381226. Fax: +91-326-2381113, 2381560. E-mail addresses:
[email protected],
[email protected].
combustion. In situ combustion desulfurization can be achieved when high-sulfur coal is mixed with lime, followed by combustion in a fluidized-bed reactor. Postcombustion desulfurization can be achieved by absorbing SO2 in an alkali adsorbent; however, the SO2 must pass through the various parts of the plants after combustion, which corrodes the construction material. Flue gas desulfurization unit is very costly. There are only a few fluidized-bed thermal power plants; therefore, pre-combustion desulfurization is preferred to enhance the life of the plants and equipment and to minimize the sulfur-associated problems. Most high-sulfur coals contain high concentrations of pyritic sulfur; therefore, a large amount of research worldwide has been conducted on the removal of pyritic sulfur. In chemical desulfurization, several oxidative and reductive reagents have been tried for the removal of pyritic sulfur; hydrogen peroxide (H2O2) was deter-
10.1021/ef0499731 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/25/2004
High-Sulfur Coal Desulfurization Using Acidic H2O2
Energy & Fuels, Vol. 18, No. 6, 2004 1765
Table 1. Sulfur Distribution, Calorific Value, and Ultimate and Proximate Analyses of Baragolai, Ledo, and Meghalaya Coal Samples proximate analysis, as received (%) moisture ash volatile matter fixed carbon ultimate analysis, db (%) carbon hydrogen sulfur nitrogen oxygen (difference) sulfur distribution, db (%) pyritic sulfur sulfate organic sulfur calorific value (kcal/kg) a
Baragolai coala
Ledo coala
Meghalaya coalb
5.4 8.4 41.4 44.8
4.9 10.4 41.5 43.2
6.0 12.9 41.5 39.6
68.8 5.1 4.3 1.5 20.3
70.0 5.2 4.3 1.5 19.0
80.7c 5.8c 5.7c 1.2c 8.7c
0.64 0.52 3.11 7526
0.52 0.41 3.38 7326
0.6 1.5 3.6
Data taken from Mukherjee et al.3 b Data taken from Borah and Baruah.2 c Data are dmmf values.
Table 2. Effect of H2O2 Concentration (with 0.1 N H2SO4) on Demineralization and Desulfurization of Baragolai and Ledo Coalsa Sulfur Distribution (%) H2O2 concentration (%)
heating value (kcal/kg)
ash (%)
unleached coal 2.5 5.0 10.0 15.0
7526 7849 7758 7773 7787
8.8 5.3 5.2 5.0 4.8
unleached coal 2.5 5.0 10.0 15.0
7326 7705 7622 7638 7659
11.0 6.8 6.7 6.5 6.3
a
pyritic sulfur
sulfate sulfur
Baragolai Coal 0.64 0.52 0.11 0.05 0.09 0.03 0.08 0.01 0.07 0.00 0.52 0.08 0.06 0.05 0.04
Ledo Coal 0.41 0.06 0.04 0.02 0.01
organic sulfur
total sulfur
reduction in total sulfur (%)
reduction in ash (%)
3.11 2.48 2.41 2.33 2.28
4.27 2.64 2.53 2.42 2.35
38.2 40.7 43.3 45.0
40.0 41.1 42.9 45.0
3.38 2.51 2.44 2.37 2.32
4.31 2.65 2.54 2.44 2.37
38.5 41.1 43.4 45.0
37.6 39.3 41.0 43.0
Coal samples were leached for 4 h at a temperature of 25 °C. Data taken from Mukherjee et al.3
mined to be effective in reducing >90% of the pyritic sulfur from high-sulfur coal. Ali et al.1 performed work on the removal of pyritic sulfur using hydrogen peroxide. Borah and Baruah2 performed kinetic and energetic studies on the hydrogen peroxide-pyrite (H2O2-FeS2) (in coal) reaction and reported an activation energy of 0.22 kJ/mol. Mukherjee et al.3 reported an activation energy of 39.72 × 10 6 J/kmol for the pyritic sulfurhydrogen peroxide reaction of Ledo coal. The two values are diametrically opposite, although the coals used were from the northeastern region of India only. Hence, the study of the kinetics and energetics of the removal of pyritic sulfur, using hydrogen peroxide, became of interest. This condition forms the basis for the present paper.
summarizes the data on the calorific value, sulfur distribution, and ultimate and proximate analyses of the Baragolai, Ledo, and Meghalaya coals on an as-received basis and on a dry basis (db). For desulfurization work, ∼100 g of coal samples (-212 µm) was treated with 250 mL of hydrogen peroxide (H2O2) solution of different concentration in the presence of 0.1 N H2SO4 in a three-necked round-bottom flask that was fitted with a condenser and a stirrer. The mixture was stirred at different temperatures for various periods. After the end of the reaction, the treated coal was recovered by filtration, followed by washing until a neutral pH was obtained (according to the litmus paper), drying at 90 °C, and analysis for ash, calorific value, and identification/content of the forms of sulfur. Detailed experimental procedures have been reported in earlier publications.3,4
Results and Discussion Experimental Section The coal samples used in the investigation were collected from Baragolai and Ledo collieries of the Makum coalfield in Assam, India. The proximate analysis (using IS 1350 (part I)1984), the ultimate analysis (using an elemental analyzer), the total sulfur content (using ASTM D 3177), the oxygen content (determined by difference), the forms of sulfur (using ASTM D 2492), and the calorific value (using ASTM D 3286) were determined by applying standard test methods. Table 1
Table 2 shows the results of a study on the effect of hydrogen peroxide concentration (with 0.1 N sulfuric acid) on demineralization and desulfurization of Baragolai and Ledo coals (on a dry basis) at 25 °C (leaching time of 4 h). Table 3 presents data on the removal of pyritic sulfur from Baragolai and Ledo coals using 15% (v/v) hydrogen peroxide, mixed with 0.1 N sulfuric acid, at 15, 25, 30, and 40 °C and for different leaching
(1) Ali, A.; Srivastava, S. K.; Haque, R. Fuel 1992, 71, 835. (2) Borah, D.; Baruah, M. K. Fuel Process. Technol. 2002, 72, 83. (3) Mukherjee, S.; Mahiuddin, S.; Borthakur, P. C. Energy Fuels 2001, 15, 1418.
(4) Mukherjee, S. Characterization of Inorganic Constituents Associated with Assam Coal and Their Removal. Ph.D. Thesis, Department of Chemistry, Dibrugarh University, Dibrugarh, Assam, India, 2001.
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Table 3. Pyritic Sulfur Removal from Baragolai and Ledo Coals Using H2O2 (15% H2O2 + 0.1 N H2SO4) at Different Temperatures Pyritic Sulfur Removed (%) time (s)
at 15 °C
at 25 °C
at 30 °C
at 40 °C
900 1800 3600 5400 7200 10800 14400
23.15 34.25 49.63 61.24 70.50 83.46 87.50
Baragolai Coal 29.52 40.35 55.10 66.42 75.25 85.45 89.06
37.28 49.15 63.50 74.23 80.56 86.65 90.62
42.20 62.45 71.62 82.45 86.57 92.45 92.99
900 1800 3600 5400 7200 10800 14400
36.15 45.68 59.72 70.82 79.08 88.15 90.38
Ledo Coal 41.53 51.65 66.36 78.28 85.42 91.36 92.31
51.20 62.25 76.18 85.23 91.15 94.36 95.23
61.45 73.20 86.13 91.28 94.65 96.35 98.08
periods, viz, 900, 1800, 3600, 5400, 7200, 10 800, and 14 400 s. Table 3 shows that, as the temperature was increased from 15 °C to 40 °C, the percentage of pyritic sulfur that was removed increased at a fixed leaching time. Similarly, this table also shows that keeping the reaction temperature constant as the leaching period (or residence time) was enhanced from 900 s to 14 400 s, the percentage of pyritic sulfur removal increased. The maximum pyritic sulfur removal achieved was 93% with Baragolai coal. Similarly, in the case of Ledo coal, it can be noticed from Table 3 that as the temperature was raised from 15° to 40 °C, the percent pyritic sulfur removal enhanced at the same leaching time. This table also shows that keeping the reaction temperature constant, as the residence time or the leaching period was enhanced from 900 s to 14 400 s, the percentage of pyritic sulfur that was removed increased. The maximum amount of pyritic sulfur removed was 98%, with the Ledo coal. Borah and Baruah2 performed an oxidative desulfurization of FeS2, using excess air, and correctly used the following equation to calculate the rate constant:
Kr )
2.303 a log t a-x
(
)
(1)
The aforementioned equation is for a pseudo-unimolecular first-order reaction. In eq 1, Kr is the rate constant, t the time (in seconds), a the initial concentration of pyritic sulfur in coal, and x the concentration of pyritic sulfur removed at time t. Mukherjee et al.3 leached 100 g of
