Energy & Fuels 2004, 18, 1569-1572
1569
Conversion and Kinetics of the Oxidation of Coal in Supercritical Water Tao Wang* and Xiaofeng Zhu Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Received April 11, 2004. Revised Manuscript Received June 30, 2004
The oxidation of coal in supercritical water was explored by using H2O2 as the oxidant source in a bench-scale semicontinuous installation. The conversion and kinetics of coal oxidation in supercritical water (SCW) medium were investigated. The reaction parameter effects on the conversion and kinetics of coal supercritical water oxidation (SCWO) were determined. Increasing temperature, H2O2 concentration, or the flow rate of H2O2 solution enhances the oxidation of coal in SCW. The oxidation of coal in SCW is a pseudo-first-order process.
1. Introduction Supercritical water (SCW), which is water above its critical temperature (374.0 °C) and critical pressure (22.1 MPa), has unique properties and environmentally benignity. Using SCW as the medium for processing of coal has received increasing attention in the efforts to develop techniques for the clean and effective use of coal,1-10 including hydrolysis and pyrolysis of coal in SCW, liquefaction of coal in SCW, extraction of coal with SCW and SCW mixtures, and desulfurization of coal with SCW. In our previous works,11,12 we firstly explored the oxidation of coal using SCW as the reaction medium for developing a clean and effective coal combustion technique. The species in the gaseous and liquid effluents of the coal SCWO has been identified for investigating the transformation of the key elements in coal during the SCWO.11,12 It was disclosed that the * To whom correspondence should be addressed. Tel: +86 10 62782748. Fax: +86 10 62770304. E-mail:
[email protected]. (1) Aida, T. M.; Sato, T.; Sekiguchi, G.; Adschiri, T.; Arai, K. Extraction of Taiheiyao coal with supercritical water-phenol mixtures. Fuel 2002, 81, 1453-1461. (2) Izumiya, F.; et al. Coal decomposition by supercritical water. Fuel Energy Abstr. 2002, 43, 10-10. (3) Liu, X.; Li, B.; Miura, K. Analysis of pyrolysis and gasification reactions of hydrothermally and supercritically upgraded low-rank coal by using a new distributed activation energy model. Fuel Process. Technol. 2001, 69, 1-12. (4) Nakagawa, H.; et al. Upgrading of low rank coals by sub/ supercritical water treatment. Fuel Energy Abstr. 2002, 43, 12-12. (5) Matsumura, Y.; Nonaka, H.; Yokura, H.; Tsutsumi, A.; Yoshida, K. Co-liquefaction of coal and biomass in supercritical water. Fuel 1999, 78, 1049-1056. (6) Sasaki, A. Study of coal treatment with supercritical water. Fuel Energy Abstr. 1997, 38, 8-8. (7) Tsutsumi, A. Liquefaction of Ishikari coal using supercritical water. Fuel Energy Abstr. 1995, 36, 253-253. (8) Adschiri, T.; Sato, T.; Shibuichi, H.; Fang, Z.; Okazaki, S.; Arai, K. Extraction of Taiheiyo coal with supercritical water-HCOOH mixture. Fuel 2000, 79, 243-248. (9) Li, W.; Guo, S. Supercritical desulfurization of high rank coal with alcohol/water and alcohol/KOH. Fuel Process. Technol. 1996, 46, 143-155. (10) Timpe, R. C.; Mann, M. D.; Pavlish, J. H.; Louie, P. K. K. Organic sulfur and hap removal from coal using hydrothermal treatment. Fuel Process. Technol. 2001, 73, 127-141. (11) Zhu, X.; Wang, T. Preliminary Exploration of Coal Oxidation in Supercritical Water. Chin. J. Process Eng. 2002, 2, 177-180. (12) Wang, T.; Zhu, X. Sulfur Transformations during Supercritical Water Oxidation of a Chinese Coal. Fuel 2003, 82 (18), 2267-2272.
Table 1. Composition of the Coal Sample moisture (wt %)
ash (wt %)
volatile components (wt %)
nitrogen (wt %)
sulfur (wt %)
4.0
7.3
29.2
0.8
0.3
sulfur and nitrogen contained in coal could be conversed into SO42- and N2, respectively, by the oxidation in SCW with H2O2 as the oxidant source under suitable reaction conditions.11,12 The information on the kinetics is necessary for further research and development of this novel coal SCWO process. In this work, the conversion and kinetics of coal solid during SCWO was investigated. The reaction parameter effects on the conversion and kinetics of coal SCWO were experimentally determined. 2. Experimental Section 2.1. Materials. Coal with the particle size 0.5-1.0 mm and the composition given in Table 1 was used for this study. The oxidant hydrogen peroxide was purchased as the 30.0 wt % aqueous solution from Beijing Chemicals Corp. (Beijing, China) and diluted to the required concentrations with tridistilled water. 2.2. Setup and Procedure. A bench-scale semicontinuous system, shown in Figure 1, was used for the experiments involving SCWO of coal. H2O2 aqueous solution in the reservoir (1) was continuously charged into the preheater (3) by a high-pressure pump (2) (LB-10C, Xingda Corp., Beijing, China). In the preheaters (3 and 4), H2O2 decomposed and released O2 to be dissolved in supercritical water. The coal sample was oxidized by O2 dissolved in supercritical water in the horizontal reactor (5), which is a Φ6 × 4 Inconel 625 tube with an effective length of 230 mm. The temperatures of the reactor and preheaters, which were both electrically heated, were measured with K type thermal couples and controlled with an accuracy of (1 °C. The temperature of the reactor outlet was reported as the reaction temperature. The reaction pressure was controlled by a back-pressure regulator (BPR) (8) with an accuracy of (0.1 MPa. After being passed through a filter (6), the supercritical water solution from the reactor was cooled in a cooler (7), depressurized to atmosphere pressure by BPR (8), and separated into gaseous and liquid effluents in the separator (9).
10.1021/ef0499123 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/04/2004
1570 Energy & Fuels, Vol. 18, No. 5, 2004
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Figure 1. Schematic diagram of the bench setup for SCWO of coal. Figure 3. Conversion of coal vs reaction temperature. P ) 25.0 MPa, t ) 17 min, F ) 5.0 mL min-1, C ) 5.0 wt %.
Figure 2. Conversion of coal vs reaction time. P ) 25.0 MPa, T ) 420 °C, F ) 5.0 mL min-1, C ) 5.0 wt %. To conduct the experiment, a 1.5 g coal sample was packed in the clean and dry reactor. Tridistilled water was introduced into the system by the high-pressure pump during heating and pressurizing. Once the desired temperature and pressure were reached, the H2O2 aqueous solution replaced tridistilled water and was continuously pumped into the system. The reaction time was set to be zero at this point. After the desired reaction time, heating and pumping were shut down, and the system was quickly depressurized. The reactor was immediately taken away from the system and quenched in water. The solid residue inside the reactor was dried and weighted. The moisture in the sample of the coal and solid residue was determined by a standard test method (ASTM D3173-00). Each experiment was repeated three or more times, and the average result is reported with a standard deviation of less than (1.5%.
3. Results and Discussion 3.1. The Conversion of Coal SCWO. The conversion of coal was defined according to the weight loss of coal by SCWO as
x)
W0 - W × 100% W0
(1)
Where x is the conversion of the coal by SCWO, W0 is the initial moisture-free weight of coal sample (g), and W is the moisture-free weight of the solid sample (g). The conversion of coal was increased as reaction time increased. Under the experimental conditions at 420 °C, 25.0 MPa, and 5.0 mL/min of 5.0 wt % H2O2 solution, the conversion of coal was respectively 26.7%, 68.1%, and 82.1% for the reaction times of 5, 15, and 20 min, as shown in Figure 2.
Figure 4. Conversion of coal vs concentration of H2O2. T ) 400 °C, P ) 25.0 MPa, t ) 17 min, F ) 5.0 mL min-1.
The conversion of coal was strongly dependent on the reaction temperature. The conversion of coal at different reaction temperatures is shown in Figure 3 for a coal sample of 1.5 g oxidized at 25.0 MPa with 5 mL/min of 5.0 wt % H2O2 solution for 17.0 min. As shown in Figure 3, the conversion increased as temperature increased. When the reaction temperature was higher than 380 °C, it went up rapidly with the temperature increase. Figure 4 represents the conversion for a coal sample of 1.5 g oxidized at 25.0 MPa and 400 °C with 5.0 mL/ min of different concentrations of H2O2 solutions. The conversion depended on the H2O2 concentration and was enhanced by using a higher concentration of H2O2 solution. The dependence of the conversion on the flow rate of H2O2 solution is illustrated in Figure 5. Obviously, it increased as the flow rate of H2O2 solution increased. It was experimentally verified that the conversion of the coal by SCWO was enhanced with high temperature, long reaction time, high concentration, and high flow rate of H2O2 solution. The reaction temperature is the most significant parameter affecting the conversion of coal SCWO. 3.2. The Kinetics of Coal SCWO. The coal SCWO was assumed to be a pseudo-first-order process. So that
dW/dt ) -kW
(2)
ln W ) -kt + ln W0′
(3)
Then
Oxidation of Coal in Supercritical Water
Energy & Fuels, Vol. 18, No. 5, 2004 1571
Figure 5. Conversion of coal vs flow rate of H2O2 solution. T ) 400 °C, P ) 25.0 MPa, t ) 17 min, C ) 5.0 wt %. Figure 7. ln k vs 1/T. P ) 25.0 MPa, F ) 5.0 mL min-1, C ) 5.0 wt %.
Figure 6. ln W vs reaction time at different reaction temperatures. P ) 25.0 MPa, F ) 5.0 mL min-1, C ) 5.0 wt %. Table 2. Kinetic Parameters of the Coal SCWO at Different Reaction Temperatures T (°C)
W0′E (g)
W0′ (g)
k (min-1)
r
380 390 400 410 420
1.16 1.21 1.30 1.36 1.47
1.18 1.20 1.31 1.38 1.48
0.01159 0.01742 0.03139 0.04736 0.05977
0.996 0.993 0.989 0.990 0.984
where W0′ is the moisture-free weight of solid residue at zero reaction time when the H2O2 aqueous solution was continuously pumped into the system to replace the tridistilled water (g), W is the moisture-free weight of the solid sample (g), t is the reaction time (min), and k is the first-order rate constant (min-1). The plotting of ln W vs t is shown in Figure 6 for different reaction temperatures at 25.0 MPa with 5.0 mL/min of 5.0 wt % H2O2 solution. The parameters in eq 3 were found by fitting the experimental data and were listed in Table 2. The data in Figures 6-8 have good linearity. This disclosed that coal SCWO is a pseudo-first-order process. As shown in Figure 6, the weight of the solid sample decreased more quickly at higher temperature. The rate constant of the coal SCOW, k, listed in Table 2 increased as the reaction temperature increased. These data evidenced that the rate of the coal SCWO increased as temperature increased.
Figure 8. ln W vs reaction time at different concentrations of H2O2. T ) 400 °C, P ) 25.0 MPa, F ) 5.0 mL min-1. Table 3. Kinetic Parameters of the Coal SCWO with Different H2O2 Concentration C (wt %)
W0′E (g)
W0′ (g)
k (min-1)
r
3.0 5.0 7.0
1.27 1.30 1.21
1.28 1.31 1.22
0.01317 0.03139 0.03162
0.996 0.989 0.986
Table 4. Kinetic Parameters of the Coal SCWO with Different Flow Rates of H2O2 Solution F (mL/min)
W0′E (g)
W0′ (g)
k (min-1)
r
5.0 7.0
1.30 1.30
1.31 1.29
0.03139 0.04107
0.989 0.991
As shown in Tables 2-4, the fitted parameter W0′ coincides well with the experimentally determined one, which is denoted as W0′E. The value of W0′E indicates the conversion of coal during heating and pressurizing, in which the hydrolysis and pyrolysis of coal take place without the oxidation. The conversion of coal during heating and pressurizing, which could be calculated with W0′ and W0 according to eq 1, declined as the reaction temperature elevated. This conversion was 22.7% at 380 °C but only 2.0% at 420 °C. These values disclosed that the conversion contributed by the hydrolysis and pyrolysis of coal taking place during heating and pressuring was small at the reaction
1572 Energy & Fuels, Vol. 18, No. 5, 2004
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The flow rate of H2O2 solution also affected the kinetics of coal SCWO. As shown in Figure 9 and Table 4, the rate constant depended positively on the flow rate of H2O2 solution. 4. Conclusion Under the experimental conditions, the conversion and kinetics of coal SCWO are strongly dependent on the temperature, H2O2 concentration, and the flow rate of H2O2 solution. Increasing any of those enhances the oxidation of coal in SCW. The oxidation of coal in SCW is a pseudo-first-order process with an activation energy of 154.65 kJ/mol at 25 MPa pressure. Nomenclature
Figure 9. ln W vs reaction time at different flow rates of H2O2 solution. T ) 400 °C, P ) 25.0 MPa, C ) 5.0 wt %.
temperature above 420 °C, although it was significant at lower temperatures. The dependence of the rate constant on the temperature is shown in Figure 7 and could be expressed by Arrhenius’ relationship, with the activation energy being 154.65 kJ/mol for the data at 25.0 MPa and 5.0 mL/min of 5.0 wt % H2O2 solution. The kinetics was also dependent on the H2O2 concentration. As shown in Figure 8 and Table 3, the rate constant increased as H2O2 concentration increased.
C ) concentration of H2O2 aqueous solution, wt % F ) flow rate of H2O2 aqueous solution, mL/min k ) first-order rate constant, min-1 P ) reaction pressure, MPa r ) linear correlation coefficient T ) reaction temperature, °C t ) reaction time, min x ) conversion of the coal by SCWO W ) moisture-free weight of the solid sample, g W0 ) initial moisture-free weight of coal sample, g W0′ ) moisture-free weight of the solid when t is zero, determined by data fitting, g W0′E ) experimentally determined moisture-free weight of the solid when t is zero, g EF0499123
