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Energy & Fuels 2001, 15, 936-942
Analysis of Inorganic Constituents Deposited in a 150 T/day Coal Liquefaction Pilot Plant Koichi Matsuoka* and Akira Tomita Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Sendai, 980-8577 Japan
Naresh Shah, Frank E. Huggins, and Gerald P. Huffman Department of Chemical and Materials Engineering, University of Kentucky, 533 South Limestone Street, Lexington, Kentucky 40506 Received January 29, 2001
A wide variety of inorganic constituents were deposited in several units of the NEDOL coal liquefaction pilot plant with a capacity of 150 tons per day. The inorganic constituents were analyzed by CCSEM, XRD and XRF. In the first liquefaction reactor, the growth of crystalline quartz and the interaction of calcium with mineral and/or catalyst were observed to be significant. The inorganic particles of larger size (> about 100 µm) were accumulated in the first liquefaction reactor while the smaller particles were flushed out to downstream. The small catalyst particles were mainly accumulated in the high-temperature separator and the other interacted particles were mainly accumulated in the outlet pipe after letdown valve.
Introduction A direct coal liquefaction pilot plant with a capacity of 150 tons per day based on the NEDOL process was built in Kashima, Japan. The plant was operated from 1996 to 1998. The operation was almost successful; however, it was reported that some problems occurred related to the solid deposition in several units in the facility.1-3 The deposit reduced the effective volume of the reactor and connecting pipe, resulting in a significant pressure drop and decrease of residence time of the slurry in the reactor. There have been several reports about the deposits in coal liquefaction plants.1-5 In these studies, the deposits in coal liquefaction plant were analyzed qualitatively. For example, Aramaki et al.2 analyzed the carbonaceous and inorganic solids in deposits of a 150 t/day pilot plant. However, the mechanism of inorganic deposit formation and other details are not fully understood. To understand the deposit formation, quantitative analysis is essential. In this study, we focused on the inorganic matter deposited in several units of the pilot plant and analyzed the inorganic deposit quantitatively by using techniques such as X-ray diffraction, X-ray fluorescence spectrom* Corresponding author. Fax: +81-22-217-5626. E-mail:
[email protected]. (1) Onozaki, M.; Namiki, Y.; Aramaki, T.; Takagi, T.; Kobayashi, M.; Morooka S. Ind. Eng. Chem. Res. 2000, 39, 2866-2875. (2) Aramaki, T.; Namiki, Y.; Onozaki, M.; Takagi, T.; Ueda, S.; Kobayashi, M.; Mochida I. J. Jpn. Inst. Energy 1999, 78, 929-941. (3) Ueda, S.; Aramaki, T.; Kobayashi, M. Proc. of 10th Intern. Conf. Coal. Sci. Taiyuan China, 1999; pp 827-830. (4) Mochizuki, M.; Imada, K.; Inokuchi, K.; Nogami, Y. J. Jpn. Inst. Energy 1997, 76, 1074-1083. (5) Okuma, O.; Yanai, S.; Yasumuro, M.; Makino, E. J. Jpn. Inst. Energy 1999, 78, 332-343.
Figure 1. Schematic diagram of a part of the NEDOL coal liquefaction pilot plant with a capacity of 150 t/day. Dotted circle indicates the points from where the deposits were collected.
etry and computer-controlled scanning electron microscopy to shed light on the mechanism of deposit formation. Experimental Section Figure 1 shows a schematic flow of a part of the 150 t/day coal liquefaction pilot plant. The units related to the present study are three liquefaction reactors, a high-temperature separator and a letdown valve (LDV). In this study, we analyzed the deposits in the first liquefaction reactor, the hightemperature separator and the outlet pipe after LDV. The sampling points are represented by dotted circles. The deposits studied in the present paper were formed under the following conditions using Tanitoharum coal: temperature, 455 °C;
10.1021/ef0100202 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/07/2001
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Table 1. Analyses of Tanitoharum Coal Ultimate analysis (wt %, daf) C
H
N
O
S
76.3
5.6
1.4
16.5
0.2
Proximate analysis (wt %, dry) Ash Volatile matter Fixed carbon 4.8
46.6
48.6
Ash analysis (wt %, ash) SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
TiO2
SO3
37.3
21.0
11.0
10.2
4.2
1.3
1.9
0.8
12.0
pressure, 170 kg/cm2; ratio of gas to liquid (G/L), 710 Nm3/tslurry; coal concentration, 40 wt %; catalyst, natural pyrite (FeS2); mean diameter of the catalyst, 0.7 µm; catalyst loading, 2.7 wt %-coal. The properties of Tanitoharum coal are listed in Table 1. The deposits in the units of the pilot plant were extracted with THF to minimize the effect of the presence of organic matter in the following analyses. Low-temperature ash (LTA) was also prepared from the raw coal to examine the mineral matter composition in the raw coal. The THF-insoluble fraction of the deposits and LTA of the raw coal were examined by X-ray diffraction analysis (XRD; Shimadzu XD-D1), X-ray fluorescence spectrometry (XRF; Shimadzu XRF 1700), and computer-controlled scanning electron microscopy (CCSEM). The CCSEM system consists of SEM (Topcon ABT60) and energy-dispersive X-ray analyzer (EDAX Phoenix). Morphological information for individual particles, such as diameter, perimeter, aspect ratio, etc., was obtained from backscattered images, and the constituent elements in each particle were determined by EDX analysis. In the EDX analysis, we checked the effect of the analysis area on the results by changing the area of from 80 wt %, Al < 5 wt %), Si + Al (S + Al > 80 wt %), Fe + S (Fe + S > 80 wt %), Ca (Ca + Mg > 80 wt %), Ca + Si (Ca +Si > 80 wt %), Ca + Si + Al (Ca + Si + Al > 80 wt %), Si + Fe + S (Si + Fe + S > 65 wt %), Si + Al + Fe + S (Si + Al + Fe + S > 65 wt %), Ca + Si + Al + Fe + S (Ca + Si + Al + Fe + S > 80 wt %), if a particle does not meet the above definition then it is classified as “others”. We classified the inorganic deposits consecutively on the basis of the above sequence. The categories of Si, Si + Al, Ca and Fe + S are ascribed to quartz, kaolinite + montmorillonite + illite, calcite + dolomite, and pyrite + pyrrhotite, respectively. With increasing analysis area, the Si fraction of inorganic deposit in the first liquefaction reactor decreased while that of Ca + Si + Al + Fe + S increased. The fraction of other minerals was more or less independent of the analysis area. On the other hand, analysis area significantly affected the analyzed composition of deposits in the LDV outlet pipe. The fraction of Si, Ca, and Fe + S decreased with increasing analysis area, while those of Ca + Si, Ca + Si + Al, Ca + Si + Al + Fe + S increased. It suggests that the contribution of calcium from the peripheral area increased by increasing the analysis area. The analysis results determined by taking 80% of the particle area into consideration are in better agreement with that determined from XRF compared with those obtained under other assumptions. Therefore, the discussion hereafter will be made for the results determined by using 80% area for analysis. Figure 8 compares the fraction of each category deposited in the first liquefaction reactor and the LDV outlet pipe with that of raw coal. More than 50% of the inorganic deposit in the first liquefaction reactor consists of quartz, and this percentage is about four times that of the quartz content in the raw coal. Clearly, quartz selectively accumulates in
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Energy & Fuels, Vol. 15, No. 4, 2001 941
Figure 8. Weight fraction of inorganic materials in deposits and in raw coal.
the first liquefaction reactor. On the contrary, the fraction of quartz is quite small in the LDV outlet pipe, while the fraction of Ca + Si, Ca + Fe + S, Ca + Si + Al + Fe + S is relatively large. The fraction of the mineral matter containing Ca + Si + Al + Fe + S is 30%, indicating significant interaction among the catalyst, kaolinite, montmorillonite and calcium. Discussion Mechanism for Growth of Quartz. As described above, many quartz particles of large size were deposited in the first liquefaction reactor. Ueda et al.3 suggested that growth of quartz might be due to hydrothermal reaction. We also conclude that the increase in the size of quartz is due to the hydrothermal reaction by the following reasons. The conditions in the liquefaction reactor are quite similar with that in the autoclave to form the man-made quartz crystal. Manmade quartz crystal is commercially formed by the hydrothermal reaction in the autoclave.12,13 In the top of autoclave, seed quartz crystals are suspended and quartz with an aqueous alkali hydroxide or carbonate solution is placed on the bottom of the autoclave. The temperature of the seed quartz is kept a little bit lower than that of the solution. The alkali metal compounds increase the solubility of soluble silicate complexes. The temperature difference between the top and bottom of the liquefaction reactor is about 30 °C.3 Furthermore, a small amount of alkali salts such as Na, Ca and K from the raw coal are present in the reactor. In the liquefaction reactor, a part of original small quartz particles were dissolved in water evolved during the liquefaction. Dissolved quartz deposited on nondissolved quartz that might behave as seeds could easily result in the formation of larger quartz. Interaction of Inorganic Matter. In addition to the growth of quartz, interaction between calcium and mineral matter or catalyst was significant. Evidence for the interaction between calcium and mineral matter (12) Corwin, J. F.; Swinnerton, A. C. J. Am. Chem. Soc. 1951, 73, 3598-3601. (13) Hosaka, M.; Taki, S. J. Crystal Growth 1981, 52, 837-842.
is as follows. As described in the discussion related to Figure 2, Tanitoharum coal is considered to have much organically associated calcium in addition to mineral calcium. It is reported that organically associated calcium readily forms calcium carbonate in the liquefaction process and that the calcium carbonate causes scale formation.5,6 The calcium envelope is considered to be calcium carbonate on the basis of the above reports and the XRD patterns shown in Figure 3. From the above results, it is likely that the organically associated calcium played the main role in formation of the calcium envelope. As shown in Figure 6, catalyst particles (about 100 µm) with a calcium envelope were found in the liquefaction reactor and the LDV outlet pipe despite the fact that the average size of the loaded catalysts was only 0.7 µm. Pyrite particles aggregated and formed large particles, because pyrite is easy to aggregate.6 After the aggregation of pyrite particles, calcium interacted with the aggregated catalyst. The behavior of the inorganic matter in the liquefaction plant can be summarized as follows. An extensive growth of quartz particles was observed in the first liquefaction reactor. Another important reaction was the interaction of calcium with other inorganic matter. Since some Ca-coated particles were found even in the first liquefaction reactor, the interaction of inorganic matter with Ca might occur mainly in the first reactor. The inorganic particles with larger size (> about 100 µm) accumulated in the first liquefaction reactor and the other particles with smaller size were flushed out to downstream. The small catalysts were mainly accumulated in the high-temperature separator and the Ca-coated particles (mainly < 100 µm) were mainly accumulated in the LDV outlet pipe. Based on the above results, we would like to propose the following procedure to minimize the deposit formation and to operate the plant smoothly. 1) Minimize the temperature difference between the upper and lower parts of the liquefaction reactor to suppress the growth of quartz particles. 2) Spontaneous removal of inorganic deposits from the bottom of the first reactor before their growth.
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Conclusions A wide variety of inorganic constituents were accumulated in several units in the NEDOL coal liquefaction pilot plant. In the first liquefaction reactor, growth of mineral matter and interaction of mineral matter mainly occurred. The size of quartz in the reactor was about 10 times that in the raw coal and the amount of quartz corresponded to 50% of the inorganic deposit. To suppress the growth of quartz, the minimization of the temperature difference across the liquefaction reactor
Matsuoka et al.
is important. Catalyst and Ca-interacted particles with the size of around 100 µm were mainly deposited in the high-temperature separator and the LDV outlet pipe, respectively. Acknowledgment. The deposits and Tanitoharum coal were provided from NCOL Co. Ltd. This work was partially supported by COMSACT. EF0100202
