ARTICLE pubs.acs.org/EF
Innovative Desulfurization Process of Coal Water Slurry under Atmospheric Condition via Sodium Metaborate Electroreduction in the Isolated Slot Yafei Shen, Xueli Yang, Tonghua Sun,* and Jinping Jia School of Environmental Science and Engineering, Shanghai Jiaotong University, Dongchuan Road 800, Shanghai 200240, China
bS Supporting Information ABSTRACT: This electrochemical reductive desulfurization (ERD) method for coal water slurry (CWS), which was an offshoot of the research Novel Desulfurization Method of Sodium Borohydride Reduction for Coal Water Slurry (Shen, Y. F.; Sun, T. H.; Jia, J. P. Energy Fuels 2011, 25, 29632967.), utilized cathodic reduction to change some chemical compounds into strong reducing substances that convert various forms of sulfur (S) in coal into H2S and S2, thus releasing S from coals. It was an effective way to enhance S removal from coal by adding divalent metal catalysts (Ni2+, Mn2+, etc.). Using the atmospheric conditions of 12 g/L of NaBO2, 3.0 V of electrolytic voltage, 4 h of electrolytic time, 50 g/L of CWS, e140 mesh of coal particle size, 1.0 g/L of NaOH, and 1 mM (about 0.13 g/L) of NiCl2 catalyst, the total sulfur (TS) of CWS can be dramatically reduced by 58.5%. The ERD described here was a predominant desulfurization process, such as mild (slight improvement of the combustion characteristics), high desulfurization efficiency, convenient operation conditions, and low expense (reuse of the filtrate and by-production of hydrogen). Moreover, the realization of the filtrate recycling and the evaluation of the hydrogen releasing rates of NaBH4 were further helpful in cost reduction.
1. INTRODUCTION Sulfur dioxide (SO2) emission from coal-fired power plants and refinery operations has been implicated as a cause of acid rain and other air pollution related problems.1 According to the report on the state of the environment in China by the Ministry of Environmental Protection (MEP), national SO2 emissions decreased continuously from a maximum of 2.59 billion tons in 2006 to 2.17 billion tons in 2010; that is, a 14% reduction had been achieved in relation to 2.55 billion tons in 2005, mainly through flue gas desulfurization (FGD) installation in more than 70% of coal-fired power plants.2 Coal is still the predominant fossil fuel. Clean coal technology is being applied and popularized at an unprecedented rate in China.3 Coal water slurry (CWS) is a coal-based clean liquid fuel, which is currently prepared with refined coals. In the course of coal washing, most of the ash and inorganic sulfur can be removed; so, the concentration of ash and SO2 will be lower than that of the original coal by combustion and gasification. However, with the more restrictive environment legislation and more complex coals being used to make the slurry, the release of sulfur is larger, sometimes beyond the requirements of environment protection. As a clean liquid coal, desulfurization of CWS prior to combustion can save a lot of costs in the follow-up FGD.4 The traditional chemical desulfurizing methods, such as melting alkali treatment, high temperature gaseous treatment, and chemical oxidative treatment, can largely remove organic sulfur (OS), but the methods destroy the macromolecular structure and properties of coal.5 Sodium borohydride (NaBH4) reduction is a chemical desulfurization method that shows incomparable advantages: mild conditions, short reaction time, and high efficiency.6,7 It is confronted with the shortcoming of incomplete r 2011 American Chemical Society
reaction and excessive NaBH4 waste. However, sodium metaborate (NaBO2), a weak oxidant, is much cheaper and more stable than NaBH4. If electric power (voltage > 2.5 V) is provided, electrochemistry can realize boron (B) recycling, in theory.7 However, according to the principle of different electric charges attracting each other, the negative charge of BO2 will move to the anode, resulting in a decrease of BO2 concentration near the cathode. So, it needs to impede BO2 reduction in the cathode. Meanwhile, a produced negative charge (BH4) may be attracted to the anode, resulting in BH4 being consumed away because of anodic oxidation.8 Consequently, this isolated apparatus (Figure 1) is designed for the ERD. On one hand, it can avoid anions (BH4, BO2, etc.) moving to the anode. On the other hand, it can decrease the coal calorific value loss caused by the anodic oxidation. The objective of this study is to convert NaBO2 into NaBH4 by means of electroreduction so that NaBH4 can convert S into H2S or S2, removed from the CWS. The ERD method combines electroreduction with chemical reduction. Meanwhile, divalent metal catalysts were added to optimize the ERD condition. Thus, this paper is significantly different from the previous electrochemical desulfurization works,9,10 in which oxidative reactions have been exhaustively applied. In the present work, some variables, including the NaBO2 concentration, reaction time, and electrolytic voltage, were studied by orthogonal method to establish the level of reduction necessary for desulfurization (Table 1a). Some additional experimental variables were investigated, including CWS concentration and electrolytic solvent. Received: July 18, 2011 Revised: October 22, 2011 Published: October 23, 2011 5007
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Figure 1. Schematic of the ERD apparatus and general procedure.
Table 1a. Experimental Factors and Levels Selected for the Desulfurization Process factors
params
level 1
level 2
level 3
Fa
NaBO2 conc.(g/L)
6
12
18
Fb Fc
voltage (V) electrolytic time (h)
2.6 1
2.8 2
3.0 3
Table 1b. Experimental Design and Results for Desulfurization Processes According to L25 (35) Orthogonal Arraya
level 5
run no.
Fa
Fb
Fc
TS reduction (wt %)
24
30
1
1
1
1
11.6
3.2 4
3.4 5
2 3
1 1
2 3
2 3
25.2 30.5
4
1
4
4
33.7
5
1
5
5
36.2
6
2
1
2
25.5
7
2
2
3
31.3
8
2
3
4
36.6
9
2
4
5
38.0
10 11
2 3
5 1
1 3
19.5 22.4
12
3
2
4
35.6
13
3
3
5
38.8
14
3
4
1
23.1
15
3
5
2
31.5
16
4
1
4
28.2
17
4
2
5
35.0
18 19
4 4
3 4
1 2
27.5 34.1
20
4
5
3
37.3
21
5
1
5
30.2
22
5
2
1
26.4
23
5
3
2
32.5
24
5
4
3
35.8
25
5
5
4
39.6
level 4
After optimizing the reductive condition by adding a metal catalyst, the desulfurization efficiency was significantly improved. Subsequently, the calorific values and ignition temperatures of the coal samples were investigated. The filtrate after the treatment was collected for inductively coupled plasma (ICP) analysis, and the treated CWS sample (dry basis) was analyzed by Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD).
2. EXPERIMENTAL SECTION 2.1. Materials. A CWS sample was collected from Dongli Fuels Co.
Ltd. (Shanghai, China). After being dried at 110 °C in an oven, the solid residue of the CWS sample was ground and passed through a 140 mesh sieve (experimental coal particle size e 109 μm). The characterization of the original coal sample is given in Table 4. NaBO2 3 4H2O (>99%, AR), NaOH (>96%, AR), and agar powder (BR) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Preparation of Metal (Ni2+, Cu2+, Zn2+, Mn2+ and Fe2+) Catalyst Solutions (0.1 mol/L, 100 mL). 2.38 g NiCl2 3 6H2O (AR, >98%), 1.71 g CuCl2 3 2H2O (AR, >98%), 1.36 g ZnCl2 (AR, >98%), 2.70 g MnCl2 3 4H2O (AR, >98%), and 2.71 g FeCl2 3 4H2O (AR, >98%), purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China), were dissolved into 50 mL H2O, respectively, then metered volume to 100 mL. Preparation of the KCl Salt Bridge. Agar powder (3 g) was added into 97 mL of distilled water, and the mixtures was heated to dissolve the agar completely. Then, 30 g of KCl was added into the agar solution. Finally, the mixed solution was dropped or siphoned into the U-shaped glass tube. The salt-bridge can be used after agar condensation.11,12 2.2. Experimental Apparatus and Procedure. The schematic of the ERD apparatus and general procedure is shown in Figure 1. The apparatus consists of a cathode slot, anode slot, magnetic stirring
Other conditions: 50 g/L CWS concentration, coal particle size e 140 mesh, and 1.0 g/L NaOH.
a
apparatus, KCl-salt bridge, measuring cylinder, power supply, and anode/ cathode (graphite/Pb). General Procedure. First, the mixed solution (300 mL) of NaBO2 (630 g/L) and NaOH (1 g/L) was added into the cathode slot 5008
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Table 1c. Data Handling of the Orthogonal Arraya parameters Fa (NaBO2 conc.)
Fb (voltage)
Fc (electrolytic time)
V1
27.44
23.58
21.62
V2
30.18
30.70
29.76
V3
30.28
33.18
32.52
V4
32.42
32.94
34.74
V5
32.90
32.82
35.64
5.46
9.60
14.02
Vi
range a
Note: Vi refers to average of the TS reductions at the same level; range = (Vi)max (Vi)min. max{total sulfur reduction (wt %)} = (range)/(max{Vi, level i}) 100. (500 mL). Meanwhile, the prepared raw CWS powder (330 g) and metal chloride catalyst (0.1 mol/L, 3 mL) was added. Then, the blended solution was stirred at the speed of 300 rpm for one minute. After the power was turned on, the stirring rate was adjusted to 100 rpm until the electrolysis ended. At last, the clean CWS was obtained by filtering and leaching the coal slurry in the cathode slot. 2.3. Analysis Methods. Total sulfur (TS) was determined using an elemental analyzer (Elementar Vario EL III, Germany); the sulfate sulfur (SS) was analyzed gravimetrically, using the Eschka method; pyritic sulfur (PS) was analyzed by measuring the amount of iron combined in the pyritic state (GB/T 215-2003); and organic sulfur (OS) was estimated by the difference. The proximate analysis of the solid CWS sample followed the Chinese standard method (GB/T 212-2001); analysis of the calorific value followed the Chinese standard method (GB/T 213-2003); and analysis of the ignition temperature followed the Chinese standard method (GB/T 18511-2001). The concentration of NaBH4 was measured by iodometric titration;13,14 gas-collecting and measurement of hydrogen was done by the drainage method. The elemental analysis of the filtrate was determined using an inductively coupled plasma (ICP, Iris Advantage 1000, Thermo King-Cord Co., U.S.A.) instrument; the OS structures were analyzed using a Fourier transform infrared (FTIR, EQUINOX 55, Bruker, Germany) instrument; and the minerals in coal were analyzed using an X-ray diffractometer (D8 ADVANCE, Germany). The data was presented as the average of two replicates (data error < 5%) in each treatment. The desulfurization efficiency was calculated by the following formula, where TS1 is sulfur content in the original solid CWS sample and TS2 is sulfur content in the treated solid CWS sample. desulf urization ef f iciency ðwt %Þ ¼
TS1 TS2 100 TS1
3. RESULTS AND DISCUSSION 3.1. Optimization of the NaBO2 Concentration, Electrolytic Time, and Voltage by L25 Orthogonal Array. Electric
potential is an essential factor to electrochemical reaction rate. Although increasing the electric potential probably improves the reaction rate, higher voltage will destroy the structure of organic matter in coal, dividing it into a large number of oxidative fragments, which influence the energy adsorption capacity of electrolyte.15 Process time reflects the desulfurization efficiency, while electrolytic time determines the process time and energy consumption. Tables 1b and 1c present the experimental and analytical results of the orthogonal array for the ERD process. For each factor in the Vi row, the results of the three experiments consisting of level i (15) were added and then divided by 5, which gave the mean values of Vi. For example, the value of V2 for
Figure 2. Effect of electrolytic time on desulfurization efficiency and NaBH4 concentration. Process conditions: 12 g/L NaBO2, 3.0 V electrolytic voltage, 50 g/L CWS, e140 mesh coal particle size, and 1.0 g/L NaOH.
the factor Fc is (25.2 + 25.5 + 31.5 + 34.1 + 32.5)/5 = 29.76. The higher mean value is indicative of the better desulfurization efficiency. The range value for the each factor was obtained by subtracting the minimum value from the corresponding maximum value among the Vi rows. Table 1c shows that the range value of factor Fc is the highest among the three factors, implying the electrolytic time had the most significant influences. The length of the factor Fa has the lowest effect on the TS reduction (Table 1c). This is significant, because it is suggests that an increase in NaBO2 concentration from 12 to 30 g/L may not be necessary for enhanced levels of desulfurization. As the NaBO2 concentration increased from 6 to 30 g/L, the electrolytic voltage increased from 2.5 to 3.3 V, the electrolytic time increased from 1 to 5 h, and the maximum values of TS reduction increase by 16.6%, 29.1%, and 39.8%, respectively. Indeed, the desulfurization efficiency increases with the electrolytic time from 1 to 5 h, and the TS reduction is about 38% in 5 h (Figure 2). Considering economical and environmental efficiency, the process conditions of NaBO2 concentration, electrolytic time, and voltage are optimized to be 12 g/L, 4 h, and 3.0 V, respectively, and the accompanying TS reduction is 36.6% (Table 1b). The results are utilized in the following sections. Moreover, the NaBH4 concentration increases with the increase of electrolytic time in accordance with the empirical curve shown (red dotted line in Figure 2). Namely, the production yield of NaBH4, as a result of electrochemical reduction is in accordance with first order reaction kinetics. The empirical equation is as follows: C = 0.7663 exp(t/3.343) 0.8445 (R2 = 0.9986), where C is the concentration of NaBH4 (mM) and t is the electrolytic time (15 h). 3.2. Effect of CWS Concentration. CWS concentration has an effect on the mass transfer and the contact probability of coal particles with the electrode surface. A low concentration of CWS is beneficial for mass transfer, but it decreases the contact frequency between the electrode and coal particles, resulting in the extent reduction of electrochemical reaction, while a high concentration of CWS inhibits mass transfer, reducing the desulfurization efficiency. As shown in Figure 3, the desulfurization efficiency is little affected when the CWS concentration is below 50 g/L, beyond that, the desulfurization efficiency greatly decreases with the increase of CWS concentration. The 5009
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Energy & Fuels maximum value (42%) of desulfurization efficiency is observed at 40 g/L CWS concentration. In the ERD process, 50 g/L is the optimum CWS concentration where the desulfurization efficiency and the gross of TS removal reached about 40% and 260 mg/L, respectively.
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3.3. Effect of Water Content. In the mixed solvent system (CH3CH2OH and H2O), compared with alcohol, water is the prime substance of providing the protons. A mass of protons in the electrolyte is beneficial for the coal hydrogenation in the cathode, while NaBH4 reacts more easily with water than with sulfides in coal. Therefore, the water content in the electrolyte has a significant effect on desulfurization efficiency. Oxygen in water molecules and sulfur in sulfides have one pair electron, but the polarity of a water molecule is stronger than that of a sulfide, so NaBH4 is prone to react with water.7 In Figure 4a, TS reduction presents a negative correlation with the water content. However, the water content is close to zero, accompanied by the decrease of TS reduction. That result suggests that water is necessary, and low electrolyte water content is beneficial for the ERD process. On the contrary, the hydrogen yield shows a
Table 2. Experimental Results of Desulfurization Efficiency and Hydrogen Yield by Addition of Metal Catalysts a
Figure 3. Effect of CWS concentration on desulfurization efficiency. Process conditions: 12 g/L NaBO2, 3.0 V electrolytic voltage, 4 h electrolytic time, e140 mesh coal particle size, and 1.0 g/L NaOH. Total sulfur removal (mg/L) = total sulfur content (%) desulfurization efficiency (%) CWS concentration (g/L) 103 (mg/g).
metal catal.
desulf. effic. (wt%)
H yield (ml)b
FeCl2
41.5
9.8
CuCl2
45.2
12.5
ZnCl2
43.6
11.0
MnCl2
49.4
16.6
NiCl2
58.5
20.2
a
Other conditions: 12 g/L of NaBO2, 3.0 V of electrolytic voltage, 4 h of electrolytic time, e140 mesh of coal particle size, 50 g/L of CWS and 1.0 g/L of NaOH. b Approximate value.
Figure 4. Effect of water content on desulfurization efficiency (a) and hydrogen yield (b). (c) Empirical formula: V = 17.68 exp(t/2.083) + 13.26 (R2 = 0.9967), where t is the electrolytic time (15 h) and V is the hydrogen yield (ml). Process conditions: 12 g/L NaBO2, 3.0 V electrolytic voltage, 5 h electrolytic time, 50 g/L CWS, e140 mesh coal particle size, and 1.0 g/L NaOH. 5010
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Table 3. ICP Analytical Result of the Filtrates (100 Times Diluted) a element
before react. (mg/L)
Table 4. Analytical Results of Physicochemical and Combustion Characteristics a
after react. (mg/L) params
original
treated coal 1
treated coal 2
coal
(w/o Ni)
(with Ni)
Anode Slot S
0b
Ultimate Analysis
2.20
(wt %, dbb)
Cathode Slot 0b
C
73.66
71.38
70.24 (4.6%)
S
1.52
H
9.48
9.82
9.30
B
22.00
21.48
N
1.56
1.50
1.48
Fe
b
0
1.36
S
1.30
0.76 (41.5%)
0.54 (58.5%)
Na
159.05
158.33
Oc
14.00
16.62
18.44
Ni
1.40
1.32
Sulfur Content of Different Forms
a
(wt%, dafd)
Process conditions: 12 g/L NaBO2, 3.0 V electrolytic voltage, 4 h electrolytic time, 50 g/L CWS, e140 mesh coal particle size, 1.0 g/L NaOH, and 1 mM NiCl 2 ; desulfurization efficiency, 58.5%. b Theoretical value.
PS
0.48
0.22 (54.2%)
0.16 (66.7%)
SS OSc
0.30 0.52
0.08 (73.3%) 0.46 (7.7%)
0.04 (86.7%) 0.34 (34.6%)
Combustion Characteristics
positive correlation with water content (Figure 4b). As well, Figure 4c presents a positive correlation of the hydrogen yield with the electrolytic time, which is in accordance with the first order reaction kinetics. It is possible that water contributes to the NaBH4 hydrolysis but impedes the NaBH4 production. Furthermore, it can be concluded that only if water and sodium borohydride coexisted in the solution can the ERD proceed under the perfect state. Namely, for example, the pyritic sulfur (FeS2), widely existing in coals, may be removed according to the following equations:
calorific value (J/g)
32 190
32 580
32 750 (+1.74%)
ignition temp. (°C)
348
342
339 (9)
Proximate Analysis (wt %, db) ash
12.6
8.2
8.4 (33.3%)
volatile matterc moisturee
23.8 3.2
19.6 2.4
20.4 2.0
fixed carbon
60.4
69.8
69.2 (+14.6%)
2H2 O þ BH4 þ FeS2 f Fe þ H2 S v þ S þ BO2 þ H2 v ðprimary reactionÞ
Process conditions: 12 g/L NaBO2, 3.0 V electrolytic voltage, 4 h electrolytic time, 50 g/L CWS, e140 mesh coal particle size, 1.0 g/L NaOH, and 1 mM NiCl2; b db: dry basis. c The contents were calculated by mass difference. d daf: dry ash free basis. e Atmospheric condition.
3.4. Addition of Metal Catalysts. Noble metals such as Pt and Ru have been reported to exhibit high catalytic activity for the hydrolysis of NaBH4.16,17 Thus, suitable metal catalysts are conducive to the hydrolysis efficiency and the reducibility of NaBH4. Divalent transitional metal chlorides such as FeCl2, ZnCl2 and NiCl2 are investigated in this research. Table 2 shows that the TS reduction increases with the addition of metal chlorides (1 mM). Moreover, using NiCl2 as a catalyst, the maximum desulfurization efficiency and hydrogen yield are up to 58.5% and 20.2 mL, respectively, among the different formulation systems of metal catalyst and cathode electrolyte. Also, the increase of desulfurization efficiency and the decrease of treatment time are attributed to the addition of metal catalysts. It is possible that S in coals contains lone pair electrons, and Ni2+ had an empty d orbital, so S reacts easily with Ni2+. In the Ni based catalyst, a stabilizing agent (NaOH) promoted the production of hydrogen. Additionally, the hydrogen evolution ability of metal chloride in the ERD process is as follows: NiCl2 > MnCl2 > CuCl2 > ZnCl2 > FeCl2. In the reaction, the activated hydrogen is adsorbed on the surface of nickel boride (NiB), forming a kind of NiMH intermediate product. Then, highly activated hydrogen attacks S bonded with αC, resulting in the cleavage of CS and CC bonds. Meanwhile, the NiB produced by the reaction of NiCl2 and the deoxidizer has a high NiB/Ni rate, so it is beneficial for the activated adsorption of in situ hydrogen and organic sulfides, which possess higher desulfurization activity compared to aromatic organic sulfides.1821 Nickel boride is a
fine black solid that is easily prepared by the reduction of Ni2+ salts with NaBH4, usually in protic solvents, as shown in the following equation: 4NaBH4 + 2NiCl2 + 9H2O f Ni2B + 3H3BO3 + 4 NaCl + 12.5 H2v.19 Using catalysts of NiCl2 can greatly improve the desulfurization efficiency of organic sulfur (OS). The OS decreases from 0.52% to 0.48% (7.7%, w/o Ni) and 0.34% (34.6%, with Ni), as shown in Table 4. 3.5. Mass Balance. Table 3 presents the results of ICP analysis for the filtrates. The mass of TS removal: experimental value (mg/L) = desulfurization efficiency (wt %) total sulfur content (wt %) CWS concentration (mg/L) = 58.5% 1.3% 50 g/L 1000 = 380 mg/L; estimated value (mg/L) = S1 (sulfur in the anode slot, mg/L) + S2 (sulfur in the cathode slot, mg/L) = (2.20 + 1.52) 100 = 372 mg/L. Obviously, the estimated value of TS reduction is mainly in accordance with its corresponding experimental value (a small amount of incalculable S covers the surface of electrodes), which demonstrated that the removed S is practically converted into H2S. After the treatment, the concentration of boron (B) in the filtrate from 2200 to 2148 mg/L changes a little; so, it realized B recycling as a result of the electrochemical reaction. The concentrations of Na and Ni are reduced from 15905 to 15833 mg/L, and 140 to 132 mg/L, respectively. (See Table 3 in the Supporting Information.) It is suggested that Na and Ni residues in treated coals are below 0.14 and 0.16 mg/g after the filtering and leaching. Because a small amount of metal ion is adsorbed in the coal particles,
a
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Figure 5. (a) FTIR analysis of the solid CWS samples. (b) XRD analysis of the solid CWS samples.
most of the metal ions in the filtrate can still be recycled and reused. Additionally, the Na and Ni residues in the treated coal have almost no effect on the following-up combustion process. 3.6. Characterization. Ignition temperature and calorific value are two of the most important coal combustion characteristics. Oxidative desulfurization of coal tends to decrease its calorific value because some CC bonds are broken and part of the carbon is washed away.22 However, the calorific value of the coal can also increase after oxidation, because oxidation can lead to its demineralization.23 Thus, how the calorific value changes after desulfurization may be depend on the decarbonitation/ demineralization ratio of the treatment.6,22 Quantitative Analysis. In the present work, the ratios of decarbonization and de-ashing were 4.6% and 33.3%, respectively. The calorific value of the solid CWS sample increases from 32190 to 32750 J/g (or by 1.74%). Moreover, there is a slight decrease in the ignition temperature (by 9 °C) and ash content (by 33.3%)
of the solid CWS sample. (Table 4) These results indicate this ERD process is extremely mild, owing to no oxidation applied. Specifically, it can not only remove S but also decrease the ash content of CWS. Qualitative Analysis. The majority of the peak time and shape of treated coal coincide with the original one, while the transmittance and diffracted intensity are slightly reduced. (See Figure 5 in theSupporting Information.) It demonstrates that the structures of organic (Figure 5a) and inorganic (Figure 5b) matter in coal basically have not been destroyed. These results prove that the ERD process described here is a secure process on the sulfur reduction along with the combustion characteristics improved of CWS. Significantly, the FTIR spectrum of CS (696 cm 1) disappears after the treatment, which may be caused by S reduction. Meanwhile, minerals in the solid CWS sample become multifarious, possibly because of the slight reduction in the ash and volatile content by the ERD process. 5012
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BH4 þ 2H2 O f BO2 þ 8Hð4H2 Þ
ð3Þ
Although it has proven difficult to characterize, nickel boride (Ni2B) is not only a useful reducing agent for a variety of functional groups19,24 but also a fine catalyst for the hydrogen production from NaBH4 hydrolysis.25 (See eqs 4, 5,26 6,6 712.) Ni2þ þ 2BO2 f NiðBO2 Þ2
ð4Þ
3NiðBO2 Þ2 þ 2Ni2 B f 7NiðpolymolecularityÞ þ 4B2 O3
ð5Þ
FeS2 þ 2H f Fe þ H2 S v þ S
ð6Þ
f2H2 O þ FeS2 þ BH4 f Fe þ H2 S v þ S þ BO2 þ 3H2 v g ðprimary reactionÞ
ð7Þ
2H þ R SH f R H þ H2 S v
ð8Þ
à à 4H þ R S R A f R H þ R A H þ H2 S v
ð9Þ
Anode : 4OH 4e f 2H2 O þ O2 v
ð10Þ
H2 S þ 2NaOH f Na2 S þ 2H2 O
ð11Þ
2H2 O þ 2Na2 S þ O2 f 2S þ 4NaOH
ð12Þ
Figure 6. Schematic diagram of the ERD process.
4. CONCLUSIONS 4.1. Summary of the ERD Process. The ERD under atmo-
spheric conditions was an effective method of CWS desulfurization. According to the optimum conditions of 12 g/L NaBO2, 3.0 V of electrolytic voltage, 4 h of electrolytic time, 50 g/L CWS, coal particle size e 140 mesh, and 1.0 g/L NaOH, the total desulfurization efficiency was about 41.5%. The desulfurization efficiencies of TS (58.5%), PS (66.7%), SS (86.7%), and OS (34.6%), were improved by the addition of NiCl2 (1 mM) catalyst, and the low water content of the electrolyte was beneficial for improving desulfurization efficiency. Furthermore, when water and sodium borohydride coexist in the solution, the ERD can proceed under the perfect state. Additionally, the method described here is so mild that the CWS combustion characteristics of calorific value and ignition temperature were slightly improved after the treatment. Meanwhile, the realization of the filtrate recycling and the evaluation of the hydrogen release rates are further helpful in cost reduction. So, it can be used as an alternative to traditional methods, according to the industrial schematic below in Figure 6. 4.2. Reaction Mechanism Analysis. According to the related literature and the analytical results of the present research, the reaction mechanism of the ERD process can be essentially inferred (eqs 112 and Figure 6). Cathode : BO2 þ 6H2 O þ 8e f BH4 þ 8OH ð1Þ
Note that H* refers to the strong reducibility of the active hydrogen atom produced from NaBH4 hydrolysis. Significantly, this ERD method can only use cheap electricity (valley electricity, unavailable electricity, etc.) and green solutions (BH4 as a role of catalyst by electrolysis) to realize B recycling; so, it reduces the additional pollution, realizing reasonable application and sustainable development of resources. The future work will investigate effective assisted methods for improving conversion efficiency of NaBH4 and H2 to cut the processing expense.
’ ASSOCIATED CONTENT
bS
Supporting Information. Details of ICP analysis of the filtrates (Table 3) and the FTIR and XRD (Figure 5) analyses of the solid CWS samples. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is financially supported by the Chinese High Technology Research and Development Program (“863”, Grant No. 2009AA062603). The thoughtful comments and suggestions provided by reviewers and editors are greatly appreciated. ’ REFERENCES
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