Energy & Fuels 2005, 19, 1673-1678
1673
Desulfurization of Coal by Pyrolysis and Hydropyrolysis with Addition of KOH/NaOH Quanrun Liu,†,‡ Haoquan Hu,*,† Shengwei Zhu,† Qiang Zhou,† Wenying Li,§ Xianyong Wei,# and Kechang Xie§ Institute of Coal Chemical Engineering, Dalian University of Technology, 129 street, Dalian 116012, People’s Republic of China, Henan Polytechnic University, Jiaozuo Henan, 454000, People’s Republic of China, Key Laboratory of Coal Science and Technology (Taiyuan University of Technology), Ministry of Education and Shanxi Province, Taiyuan 030024, Shanxi, People’s Republic of China, and School of Chemical Engineering, China University of Mining and Technology, Xuzhou 221008, Jiangsu, People’s Republic of China Received November 18, 2004. Revised Manuscript Received March 21, 2005
In this paper, a two-step desulfurization process for high-sulfur coal was investigated. Two Chinese coals with the addition of 10 wt % potassium hydroxide or sodium hydroxide were pyrolyzed under an atmosphere of nitrogen or hydrogen in a fixed-bed reactor at 600 °C, and then the obtained chars were washed with hot water. The results indicated that, without the addition of an alkali component, the sulfur removal of these two coals by pyrolysis and hydropyrolysis is ∼40%-50% and the sulfur content of chars is reduced only slightly, in comparison with the original coals; with the addition of 10 wt % potassium hydroxide or sodium hydroxide into the original coals and the chars being washed with hot water, the sulfur removal is ∼70%-80% and the sulfur content in chars is reduced dramatically. The combustion behavior of chars was also investigated, using thermogravimetric analysis. The results showed that those chars that had an added alkali component and were subjected to water-washing were more reactive and can be burned more easily than those without added alkali, which was also confirmed by a kinetics analysis of char combustion.
Introduction It is well-known that high-sulfur coals are unsuitable for direct use, which leads to serious environmental pollution and other deleterious effects. One of the feasible strategies for the utilization of high-sulfur coal is pre-desulfurization.1 Pyrolysis is one of the most important coal behaviors, because it occurs in most coal conversion processes (combustion, gasification, carbonization, liquefaction, etc.); pyrolysis is also an attractive sulfur removal method from coal prior to combustion, because it involves low cost and can remove not only inorganic but also organic sulfur. The extent of desulfurization is influenced by many factors, such as coal type, pyrolysis conditions, the quantity of sulfur, and the distribution of different sulfur forms in the coal. As we previously reported, sulfur removal during pyrolysis under mild conditions is only in the range of 15%-40%, and sulfur content of the char is higher than that of original coal for some coals.2,3 To reduce the sulfur * Author to whom correspondence should be addressed. Telephone/ fax: 86-411-88993966. E-mail address:
[email protected]. † Dalian University of Technology. ‡ Henan Polytechnic University. § Key Laboratory of Coal Science and Technology (Taiyuan University of Technology). # China University of Mining and Technology. (1) Thoms, T. Fuel Process. Technol. 1995, 43, 123-128. (2) Hu, H.; Zhou, Q.; Zhu, S.; Meyer, B.; Krzack, S.; Chen, G. Fuel Process. Technol. 2004, 85, 849-861. (3) Liu, Q.; Hu, H.; Zhou, Q.; Zhu, S.; Chen, G. Fuel Process. Technol. 2004, 85, 863-87.
content in pyrolytic chars, more-efficient measurements must be taken during coal pyrolysis. In this study, a two-step desulfurization process for high-sulfur coal was investigated. Coal pyrolysis was performed in an atmosphere of nitrogen or hydrogen with the addition of potassium hydroxide (KOH) or sodium hydroxide (NaOH), and then the pyrolytic char was washed with hot water. Experimental Section Coal Samples. Two Chinese coal samples, Yima (YM) and Yanzhou (YZ), which were prepared according to Testing Standard of China No. GB474-96 and ground to -120 mesh, were used in this study. The approximate, ultimate, and sulfur form analyses of the coal samples are listed in Table 1, and the composition of coal ash is listed in Table 2. In some experiments, 10 wt % of KOH or NaOH were added to the coal samples via impregnation before pyrolysis. Coal Pyrolysis. Coal pyrolysis was performed in a vertical fixed-bed reactor. Approximately 10 g of sample were used in each experiment and placed in the center of the reactor. Nitrogen or hydrogen was introduced from the top of the reactor, using a mass-flow controller that was operating at a flow rate of 0.5 L/min at atmosphere pressure. The heating time from ambient temperature to reaction temperature (that is, 600 °C) was ∼10 min, and the sample was held at the reaction temperature for 30 min. Water-Washing of Char. The chars obtained from pyrolysis with an addition of alkali were washed with water at 60 °C, filtered, and then dried at 100 °C in a vacuum oven for
10.1021/ef0497053 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/23/2005
1674
Energy & Fuels, Vol. 19, No. 4, 2005
Liu et al.
Table 1. Analyses of Yima (YM) and Yanzhou (YZ) Coal Samples Proximate (wt %) sample YM YZ a
Ultimate (wt %,daf)
moisture, ash, volatile Mad Adb matter, Vdaf 9.2 3.8
21.8 10.9
40.9 45.4
C
H
N
Ob
76.16 4.62 1.09 16.67 78.41 5.38 1.47 13.09
Heating Value (MJ/kg, daf) lower
higher
28.3 30.6
29.4 31.9
Sulfur Form (wt %, db) pyritic sulfatic organic total sulfur, St sulfur, Sp sulfur Ss sulfur, Soa 2.58 3.78
1.22 1.92
0.22 0.39
1.14 1.47
By difference. Table 2. Analysis of Coal Ash Composition (wt %, db)
sample
SiO2
Al2O3
Fe2O3
CaO
SO3
K2O
MgO
YM YZ
46.6 35.6
18.7 14.6
15.2 26.6
7.8 11.9
5.6 9.0
2.2 0.4
1.9 1.3
8 h. The amount of total sulfur in the char was analyzed using the Coulomb method (Testing Standard of China No. GB214-77). Calculation of Desulfurization. The desulfurization was calculated using the following expression:
desulfurization ) 1 -
ScharWchar × 100% ScoalWcoal
where Schar and Scoal represent the content of a specific sulfur form in the char and original coal, respectively, and Wchar and Wcoal represent the mass of char and original coal, respectively. Char Combustion. To estimate the combustion behavior of the char, temperature-programmed combustion (TPC) tests were conducted in a thermogravimetric analyzer (Mettler Toledo model TGA/SDTA851e). Approximately 10 mg of coal sample was placed in a ceramic crucible and burned under an air flow of 50 mL/min at a heating rate of 10 K/min from 25 °C to 700 °C. In this study, the coal pyrolysis, sulfur determination, and char combustion experiments were conducted at least twice and the experimental results were shown to be well-reproducible.
Results and Discussion Effect of KOH and NaOH on Desulfurization during Pyrolysis and Hydropyrolysis. Panels a and b in Figure 1 show the sulfur content in coal and chars obtained from the pyrolysis and hydropyrolysis of YM and YZ coal samples, respectively, with and without added KOH or NaOH and water-washing. Without the addition of KOH or NaOH, the sulfur content of coal decreases slightly after pyrolysis: by ∼18% for YM and ∼16% for YZ. After hydropyrolysis, the sulfur content in YM char decreases from 2.58% of the original coal to 1.76%, and that in YZ char decreases from 3.78% of the original coal to 2.81%. Water-washing has little effect on the sulfur content of the char in this case. With the addition of KOH or NaOH, the sulfur content in the chars experiences a slight increase, in comparison with those without the addition of KOH or NaOH during pyrolysis and hydropyrolysis. This is reasonable, because KOH or NaOH can capture H2S, generating K2S or Na2S, which is retained in the chars. After the waterwashing, the sulfur content in the chars decreases dramatically both for pyrolysis and hydropyrolysis. This observation is due to the fact that K2S and Na2S are easy to dissolve in water. In this case, the sulfur content in the char of the YM coal decreases to 1.08% in pyrolysis and 0.82% in hydropyrolysis. The sulfur content in the char of the YZ coal is 1.16% in pyrolysis
Figure 1. Effect of different procedures on the sulfur content in chars of Yima (YM) and Yanzhou (YZ) coals. Table 3. Effect of KOH/NaOH on Char Yield and Desulfurization of Coal during Pyrolysis and Hydropyrolysis samplea YM-NA-N YM-NA-H YM-KOH-N-W YM-NaOH-N-W YM-KOH-H-W YM-NaOH-H-W YZ-NA-N YZ-NA-H YZ-KOH-N-W YZ-NaOH-N-W YZ-KOH-H-W YZ-NaOH-H-W
char yield sulfur content desulfurization (wt %, db) in char (wt %, db) (wt %) 69.5 64.6 66.7 66.0 60.1 62.1 68.3 66.4 65.8 64.6 61.0 61.8
2.13 1.76 1.08 1.00 0.82 0.78 3.18 2.81 1.16 1.04 1.20 1.14
43.1 56.3 72.3 74.6 81.0 80.4 42.5 50.6 79.8 82.2 80.4 81.4
a Char sample code A-B-C-W means that the char was obtained by pyrolysis of coal A (YM ) Yima coal, YZ ) Yanzhou coal) in atmosphere C (N ) N2, H ) H2) with the addition of B (KOH, NaOH, or NA, where the latter code means without the addition of alkali). The code W, when present, denotes that the char sample measurements were obtained after water-washing.
and 1.20% in hydropyrolysis. This observation indicated that the process of water-washing is vital for the desulfurization of coal. The experimental results of the conversion and sulfur removal, under different conditions and procedures, are listed in Table 3. From Table 3, KOH and NaOH clearly have some catalytic effect on coal conversion during pyrolysis and hydropyrolysis and the char yield de-
Desulfurization of Coal by Pyrolysis with KOH/NaOH
Energy & Fuels, Vol. 19, No. 4, 2005 1675
potassium and sodium, in the char can be easily washed by water. When the coal is subjected to heat treatment at >300 °C, KOH could react with silicon dioxide (SiO2) in the coal as follows:8
Table 4. Analyses of Chars Ultimate (wt %, db)
Proximate ash volatile matter, (wt %, db) (wt %, daf)
samplea YM-NA-N YM-NA-H YM-KOH-N-W YM-NaOH-N-W YM-KOH-H-W YM-NaOH-H-W YZ-NA-N YZ-NA-H YZ-KOH-N-W YZ-KOH-H-W YZ-NaOH-N-W YZ-NaOH-H-W a
30.1 35.8 24.8 24.8 26.7 26.3 18.1 19.8 15.0 17.7 15.9 18.1
18.5 14.4 26.4 25.4 21.7 22.2 16.8 14.7 21.5 18.4 21.2 17.6
C
H
N
63.86 59.66 70.12 69.88 68.04 68.10 74.47 74.02 77.92 76.13 77.36 74.99
1.98 1.62 1.42 1.50 1.46 1.86 2.06 1.94 1.96 2.07 1.59 1.72
1.03 0.67 0.95 0.95 0.77 0.77 1.03 1.09 1.07 1.04 1.06 1.03
Sample code is the same as that in Table 3.
creases slightly. With the addition of KOH or NaOH, and the associated water-washing, the sulfur content in the char evidently decreases and >70% and ∼80% of the total sulfur in YM and YZ coals, respectively, could be removed by pyrolysis. Using hydropyrolysis, desulfurization is ∼80% for both coals. Being exposed to heat treatment, the possible reactions that may occur between the sulfur component in the coal and KOH are as follows:4,5
2KOH + H2S f K2S + 2H2O 8FeS2 + 30KOH f 4Fe2O3 + 14K2S + K2S2O3 + 15H2O R-SH + 2KOH f K2S + 2H2O + R′CHdCH2 RCH2SCH2R′ + 2KOH f RCHdCH2 + R′CHdCH2 + K2S + 2H2O
It can be seen that KOH cannot only transfer pyrite sulfur into K2S effectively, but it also can catalyze organic sulfur decomposition. Thiophene, which cannot decompose during pyrolysis at
