ARTICLE pubs.acs.org/EF
Novel Desulfurization Method of Sodium Borohydride Reduction for Coal Water Slurry Yafei Shen, Tonghua Sun,* and Jinping Jia School of Environment Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, China ABSTRACT: The present work is aimed at the investigation of a mild, rapid, effective, and recycling desulfurization method of NaBH4 reduction for coal water slurry (CWS). Processing parameters such as reductant concentration of NaBH4, treatment time, particle size of coal, reaction temperature, shaking rate, and initial pH were taken into consideration. The optimum running parameters have been determined as follows: 2.0 mM (0.8 g/L) NaBH4 concentration, 140 mesh coal particle size, 10 min total treatment time, 100 rpm shaking rate, and 30 °C reaction temperature. The total sulfur reduction of CWS can reach about 43% operating at the above parameters. Moreover, sulfate sulfur reduction can reach about 47.8%, 55.4% pyritic sulfur can be removed, and 23.9% organic sulfur could be removed. The sulfur content of CWS was decreased by the treatment, and combustion performance was improved. The proposed experimental method for desulfurization of coal in CWS can be used as an alternative to the traditional method.
1. INTRODUCTION China is the biggest coal consuming country in the world. The total consumption of raw coal is expected to rise to 2.6 billion tons in 2020 from 1.25 billion tons in 2000.1 In the future 20 years coal will still be the dominant energy source in China. In 2005, the emission of SO2 was 25.49 million tons.2 With the implementation of the policy of energy conservation and emissions reduction, as well as the increase of SO2 governance investment, in 2010 the emission of SO2 was reduced to 23 million tons (cut by 10%), ranking first in the world. With significant changes of population, economy, and the energy required for growth and development, in 2015 the emission of SO2 will face the trend of microgrowth. SO2 pollution can cause serious damage, not only on public health but also on soil, water, ozone layer, metal structures and concrete building, etc. Much coal has to be used at present at relatively low efficiency, so the environmental effects and energy waste are very severe. As a result, clean coal technology is being applied and popularized at an unprecedented rate in China.3 coal water slurry (CWS) is a kind of clean liquid fuel made of low gray and sulfur clean coal after washing and choosing. Although CWS is considered as a clean coal technology in China, with the increase of raw coals for pulping, the emission of sulfur dioxide is growing, indeed exceeding environmental standards.4 Therefore, as a clean fuel, sulfur removal from CWS before combustion can reduce the costs of flue gas desulfurization (FGD). Therefore, desulfurization of CWS before combustion is playing an important and practical method for air pollution control in modern times. Physical desulfurization methods for coal before combustion are low-cost, but removal of organic sulfur is difficult. Traditional chemical desulfurization methods can remove most inorganic sulfur and parts of organic sulfur, but the process conditions are rigorous and expensive. At present, there are a variety of coal chemical desulfurization methods, such as the alkali solution method, partial oxidation method, chlorinolysis method, and pyrolysis method. Compared with other nonchemical methods, they can almost remove all inorganic sulfurs and parts of organic sulfurs in coal. However, in the majority of cases, chemical methods need strong acid, strong alkali, a powerful oxidizer or deoxidizer, high temperature and pressure conditions during operation, expensive costs, as well as rigorous technology r 2011 American Chemical Society
conditions. Moreover, some chemical methods can damage the structure and properties of coal severely.5 So it is necessary to find methods of high efficiency, low expense, and mild conditions for coal desulfurization. Currently most chemical desulfurization processes employ oxidation methods, while reductive desulfurization methods are rarely studied. Reductive desulfurization methods are always catalytic hydrogenation under the conditions of high temperature and high pressure.6 Reductant costs and strict equipment and operation conditions have deeply restricted the development and utilization of chemical reduction methods. Sodium borohydride (NaBH4) is a versatile reducing agent used in many industrial processes. It releases hydrogen as a consequence of hydrolysis. This is a quick reaction at room temperature. Four moles of protonic hydrogen come from borohydride, and four moles of protons come from water.7 Also, it is the cheapest metal hydride and easy to store and dispose of.8 Recently, a method for removing sulfur compounds from gasoline was reported involving the conversion of sodium metaborate to sodium borohydride by electrochemical reduction and the incorporation of a kind of metal compound. The desulfurization efficiency was 48.1%.9 Guo10 had done research on potassium borohydride as a reducing agent in order to remove benzothiophene sulfur from gasoline. Li11 conducted a new desulfurization process with sodium borohydride reduction for high-sulfur coals of Yanzhou and Yanshan in China. These researches had revealed the potential of metal borohydride for reductive desulfurization in coals. This paper is significantly different from the previously published works, in which oxidizing agents have been exhaustively applied. It is the first time NaBH4 is used as a reductant for desulfurization in CWS. The common influencing factors such as reductant concentration of NaBH4, treatment time, coal particle size, reaction temperature, shaking rate, and initial pH value have been taken into account. Received: February 23, 2011 Revised: June 20, 2011 Published: June 23, 2011 2963
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Table 1. Analytical Results of the Original Coal parameter
content value (wt %)
PS
0.55
SS
0.33
OS
0.40
TS
1.30
carbon
73.6
nitrogen
1.6
ash
5.8
Table 2. Results of Desulfurization Processes According to L9 (33) Orthogonal Arraya NaBH4
Figure 1. The experimental installation of coal desulfurization.
2. EXPERIMENTAL METHODS 2.1. Materials. CWS (0.962% S) was obtained from Shanghai Dongli Fuels Co. Ltd. After being dried at 100 °C in an oven, the solid residue of CWS was ground and passed through differently sized (unit, mesh) sieves. Sodium borohydride (>96%, AR), stored in a dryer, was bought from Shanghai National Chemical Reagent Co. 2.2. Experimental Installation and Procedure. The experimental installation of CWS desulfurization is shown in Figure 1. Each desulfurization run was carried out in a 100 mL conical flask, which was placed on a stirring hot plate. Three gram coal samples and 30 mL of deionized water was added in. Then NaBH4 was added into the coal slurry immediately, the bottle was chocked up with a silica gel plug, and the reductive desulfurization was carried out simultaneously. The concentration of NaBH4 (0.2, 2.0, and 20 mM), coal particle size (40, 80, and 140 eyes), and treatment time (1, 10, and 30 min) were chosen as three factors, and three levels orthogonal analysis were worked out. After that, single-factor analysis was conducted by changing the shaking rate (0, 100, 200, 300, and 400 rpm), reaction temperature (4, 10, 30, 60, and 80 °C), and initial pH value (1, 4, 7, 10, and 12). Finally, a set of the best running parameters was obtained. Filtrate collections were dealt with via ICP elemental analysis and treated coal sample was analyzed for calorific value and ignition temperature. 2.3. Analyses. The industrial analysis of coal samples was accorded to the Chinese standard methods (GB/T 212-2001). The total sulfur (TS) was estimated by the Eschka method, and the sulfate sulfur (SS) was analyzed gravimetrically using BaSO4. The pyritic sulfur (PS) was determined by measuring the amount of iron combined in the pyritic state. These procedures were operated according to the Chinese standard method (GB/ T 215-2003). Organic sulfur (OS) was estimated by the difference. Analysis of coal calorific value and ignition temperature was accorded to the Chinese Standards of GB/T 213-2003 and GB/T 18511-2001, respectively. Elements of the filtrates were determined by ICP analysis (Iris Advantage 1000, Thermo King-Cord Co.). Tail gas in the air chamber was analyzed by gas chromatography (GCRAE 1000 Detection, RAE Systems Inc.). Any reported value in this work was the average of two replicates of each experiment. Concentration of NaBH4 was analyzed by iodometric titration.12,13 Table 1 shows the analytical results of the coals used for this research.
3. RESULTS AND DISCUSSION 3.1. Effects of NaBH4 Concentration, Coal Particle Size, and Treatment Time. Orthogonal experiment design provided a
run no.
concentration (mM)
particle size (mesh)
treatment time (min)
TS reduction (wt %)
1
0.20
40
1
14.5
2
0.20
80
10
18.2
3
0.20
140
30
19.8
4
2.0
40
10
24.6
5
2.0
80
30
26.2
6 7
2.0 20
140 40
1 30
31.5 28.6
8
20
80
1
32.3
9
20
140
10
35.1
a
Other conditions: room temperature, 100 rpm shaking rate, initial pH changed with concentration of NaBH4.
Table 3. Experimental Datum Handling of the L9 (33) Orthogonal Arraya influence factor Vi (wt %)
NaBH4 concn (mM)
particle size
treatment time
V1
17.5
22.6
26.1
V2
27.4
25.6
26.0
V3
32.0
28.8
24.9
range
14.5
6.2
1.1
a
Vi refers to an average of the experimental values at the same level; range = (Vi)max (Vi)min.
fast, systematic, and simple way to optimize the benefit of efficiency and cost. Table 2 enumerates the concentration of NaBH4, treatment time, and coal particle size as three economical and operable factors, and every factor consisted of three levels. The experimental results were as follows. Results of the processing of the data of Table 2 are shown in Table 3. Obviously, the significant influence factor was NaBH4 concentration. Fine particles of coal were evidently beneficial for desulfurization, ranking only second to NaBH4 concentration. However, the processing time had less effect on desulfurization. So this desulfurization method of NaBH4 reduction cut down treatment time greatly and provided an obvious economical performance. 3.2. Effect of Concentration of NaBH4. At room temperature, 140 mesh particle size, 100 rpm shaking rate, and 10 min treatment time, the TS reduction increased upon increasing 2964
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Figure 4. The hydrolysis rate of NaBH4. Black line, experimental data; red line, fitting curve.
Figure 2. Effect of NaBH4 concentration.
Table 4. TS Reduction with Shaking Rate, Reaction Temperature, and Initial pH Valuea run no.
shaking
reaction
initial
TS reduction
rate (rpm)
temp (°C)
pH value
(wt %)
1
0
30
9.88
20.4
2 3
100 200
30 30
9.88 9.88
37.7 38.0
4
300
30
9.88
38.6
5
400
30
9.88
38.9
1#
200
4
9.88
28.5
2#
200
10
9.88
36.4
3#
200
30
9.88
37.6
4#
200
60
9.88
35.8
Figure 3. Effect of treatment time on TS reduction.
5# 1*
200 200
80 30
9.88 1
34.3 34.1
NaBH4 concentration. The TS reduction was 34.738.3% when the concentration of NaBH4 was 100800 mM (Figure 2). Considering the cost, we used 200 mM NaBH4 concentration. 3.3. Effect of Treatment Time. At room temperature, 2.0 mM NaBH4 concentration, 140 mesh particle size, and 100 rpm shaking rate, the TS reduction increased with increasing time. The hydrolysis rate of NaBH4 and sulfur reduction changed rapidly from 2.00 to 1.43 mM and 0 to 31.5% in 1 min, respectively. The TS reduction was almost inconspicuous 10 min later (Figure.3). In view of the economical efficiency, 2.0 mM NaBH4 concentration, 140 mesh particle size, and 10 min treatment time were taken in later experiments. The hydrolysis rate of NaBH4 gradually reduced as the treatment time increased. NaBH4 hydrolysis in CWS accorded with first-order reaction kinetics (eq 1). The hydrolysis of NaBH4 basically completed more than 80% after 10 min. This result proved that the treatment time of coal desulfurization coincided closely with the hydrolysis rate of NaBH4 (Figure 4). Also, it provided a reasonable time for hydrogen collection. The kinetic equation of NaBH4 hydrolysis (empirical equation) in CWS is .
2*
200
30
4
36.5
3*
200
30
7
37.3
4*
200
30
10
37.8
5*
200
30
12
37.0
C ¼ 1:87 expð t=4:63Þ þ 0:09
ð1Þ
where C is the concentration of NaBH4 (mM) and t is the treatment time (min). 3.4. Effects of Shaking Rate, Reaction Temperature, and Initial pH Value. Table 4 showed that the TS reduction did not increase obviously with the improvement of shaking rate. When
a
Other conditions: 2.0 mM NaBH4 concentration, 140 mesh coal particle size, 10 min treatment time.
the reaction temperature changed from 4 to 30 °C, desulfurization efficiency increased and the maximal sulfur reduction was 37.6% at 30 °C. However, the TS reduction decreased when the temperature was over 60 °C. It can thus be inferred that high temperature can accelerate the hydrolysis of NaBH4, reducing the generation of active hydrogen atoms. Accordingly, coal desulfurization could proceed at the temperature of 30 °C. TS reduction varied with the initial pH value of the solution. Under conditions of room temperature and partial alkaline, NaBH4 could conduct the process of hydrolysis mildly. So it was not appropriate for desulfurization under the conditions of strong acid or alkali. Considering economy and environmental protection, the desulfurization process need not adjust the initial pH value of solution. Under the conditions of 2.0 mM NaBH4 concentration, 140 mesh particle size, 10 min treatment time, 100 rpm shaking rate, and 30 °C reaction temperature, the TS reduction can reach about 38%. 3.5. Removal of Organic and Inorganic Sulfurs. Inorganic sulfur accounted for about 70% of TS (Table 1). This reductive desulfurization method had an effect on both inorganic and organic 2965
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Table 5. Removal of Inorganic and Organic Sulfur with Different Particle Sizesa sulfur reduction (wt %) coal particle size (mesh)
PS
SS
OS
TS reduction (wt %)
140 (109 μm) 80 (180 μm) 40 (380 μm)
55.4 50.0 36.2
47.8 38.7 33.6
23.9 15.4 10.5
43.0 35.8 27.1
a
Process conditions: 2.0 mM NaBH4 concentration, 10 min treatment time, 100 rpm shaking rate, 30 °C reaction temperature.
Table 6. Analytical Results of the Filtrates
a
process
pH value
B (mg/L)
Fe (mg/L)
S (mg/L)
Na (mg/L)
water water + NaBH4 CWS + NaBH4
6.97 9.90 4.75
0 20.75 20.32
0 0.14 23.27
0 1.06 158.16 (277.50a)
0 47.25 44.80
Theoretical value.
sulfurs. TS reductions, varied from 27.1% to 43.0%, were observed for CWS. Thus, decreasing the particle size from 380 to 109 μm increased the TS reduction by 16%. For the different size fractions, the PS and the SS reductions were 36.255.4 and 33.647.8%, respectively. It should be noted that the wide removal range of the OS, namely from 10.5 to 23.9%, was obtained when their particle sizes decreased from 380 to 109 μm (Table 5). OS removal may be due to the active hydrogen atoms produced by NaBH4 hydrolysis, which attack the CS bond and result in CS bond fracture. OS enwrapped in the coal matrix previously was exposed to the reagent attack when the coal sample was ground to finer size. So the removal of OS can be improved by decreasing the coal particle size. 3.6. ICP Analysis for the Filtrates. As Table 6 showed, the pH value increased from 6.97 to 9.90 only when NaBH4 was added into the water. After the reductive desulfurization experiment, the pH value reduced to 4.75, which proved that during the hydrolysis of NaBH4, the pH value changed rapidly accompanied by the reaction proceeding because of the temporary reduction process. The total sulfur removal in theory was 277.50 mg/L. Assuming that all removed sulfur remained in the filtrate. The actual remaining sulfur in the filtrate was 158.16 mg/L in Table 6. Obviously, the actual values were greatly lower than their corresponding theoretical values, which suggested that parts of the removed sulfur escaped from the aqueous media in their volatile forms. After treatment, concentration of boron in the filtrate ranged from 20.75 to 20.32 mg/L, almost unchanged. So it was necessary to deal with the filtrates and realize the recycling of boron. The concentration of sodium reduced from 47.25 to 44.80 mg/L (by 5%). Namely, the content of sodium residue in treated coals was only 0.0245 mg/g, and it had almost no effect on the following combustion process. 3.7. Reaction Mechanism Analyses. The hydrolysis of NaBH4 proceeds14,15 by eqs 2 and 3 NaBH4 þ 2H2 O f NaBO2 þ 8H ð4H2 Þ
ð2Þ
4NaBO2 þ 11H2 O f Na2 B4 O5 ðOHÞ4 3 8H2 O ðsodium borateÞ þ 2NaOH
ð3Þ
In the process of desulfurization, the pyritic sulfur (FeS2), widely existing in coals, may be removed according to eqs 46 below. However, after calculating the Gibbs free energy (eqs 4
Figure 5. GC analysis of tail gas from the air chamber. Process conditions: 2.0 mM NaBH4 concentration, 10 min treatment time, 100 rpm shaking rate, 140 mesh coal particle size, and 30 °C reaction temperature.
and 5), the reductive FeS2 seemingly prefers to proceed through reaction 4.11 FeS2 þ 2H f Fe þ H2 S v þ S ðΔGr m ¼ 56:3 kJ=molÞ
ð4Þ FeS2 þ H2 f Fe þ H2 S v þ S ðΔGr m ¼ 132:7 kJ=molÞ
ð5Þ H2 S þ NaOH f Na2 S þ H2 O hydrolysis
H2 S sf Hþ þ HS
ð6Þ ð7Þ
Note that H* refers to the strong reducibility of active hydrogen atom produced by NaBH4 hydrolysis. According to reports in the literatures and experimental theory analysis,16,17 two chemical reaction equations can be determined NaBH4 þ 2H2 O f NaBO2 þ 8H ð4H2 Þ ðas aboveÞ electrolysis
NaBO2 þ 2H2 O sfNaBH4 þ O2 v
ð8aÞ ð8bÞ
From the mechanism equations above, it may be good to combine the chemical and electrochemical methods of NaBH4 reduction for coal desulfurization. It can resolve the problem of deoxidizer costs and realize the recycle of filtrate. Besides, this kind of clean reduction method only uses electricity and green solutions, so it reduces the additional pollution, realizing reasonable application and sustainable development of resources. 3.8. Analysis of Tail Gas by GC. H2S was identified as a gaseous product of the desulfurization process. Slight amounts of H2S and organic gases were detected by GC analysis from the air chamber (inexact concentration shown in Figure 5). It showed that trace amounts of organic gases were produced because of carbon loss in the desulfurization process, and it may effect the combustion characteristics of CWS. 3.9. Analysis of Ignition Temperature and Calorific Value. Ignition temperature and calorific value are the most important parameters of coal combustion. Chemical desulfurization usually decreases coal calorific value because some CC bonds are broken or part of the carbon is washed away.18 Figure 5 showed that some organic gases are produced because of the carbon loss in the desulfurization process. However, coal calorific value could 2966
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Table 7. Analytical Results of Ignition Temperature and Calorific Valuea combustion character samples
ignition temperature (°C)
calorific value (J/g)
original coal
355
28 788
treated coal
348
29 263
a
Process conditions: 2.0 mM NaBH4 concentration, 10 min treatment time, 100 rpm shaking rate, 140 mesh coal particle size, and 30 °C reaction temperature.
also increase after treatment because desulfurization generally leads to its demineralization.19 Table 7 showed that coal calorific value increased by 475 J/g and ignition temperature reduced by 7 °C after the process. These results proved that NaBH4 reductive desulfurization was a mild process for sulfur reduction that increased the combustion characteristics of CWS.
4. CONCLUSIONS Experimental results showed that the efficiency of coal desulfurization could be improved by the method of sodium borohydride reduction for CWS, and it affected both organic and inorganic sulfurs. Under the conditions of 2.0 mM (0.8 g/L) NaBH4 concentration, 140 mesh coal particle size, 10 min treatment time, 100 rpm shaking rate, and 30 °C reaction temperature, the total sulfur reduction of CWS can reach about 43%. Oscillation and reaction proceeded simultaneously, so it took only 10 min for the whole process. Moreover, sulfate sulfur can reach about 47.8% and pyritic sulfur can be stripped by 55.4%. Also, organic sulfur could be removed by 23.9%. Besides, both the calorific value and ignition temperature were improved by this method. In conclusion, compared with conventional chemical oxidation processes, this desulfurization method was superior, using little reagent, less time and energy, and mild and convenient operation conditions. For low sulfur CWS, sodium borohydride reduction is a considerable desulfurization method. It should be mentioned that H2S produced by reductive desulfurization can be absorbed by alkali liquor or collected to reuse in other ways. Future work will include the realization of the filtrate recycling and the evaluation of the hydrogen releasing rates of NaBH4, which are also helpful in cost reduction.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is financially supported by the Chinese High Technology Research and Development Program (“863” program, No. 2009AA062603). The authors thank Prof. M. Sheng, College of Resource and Environmental Engineering, East China University of Science & Technology, for the determination of the calorific value and ignition temperature of the coal samples. The thoughtful comments and suggestions provided by reviewers and editors are greatly appreciated. ’ REFERENCES (1) You, C. F.; Xu, X. C. Coal combustion and its pollution control in China. Energy 2010, 35, 4467–4472. 2967
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