Scanning electron microscopic study on the catalytic gasification of coal

Scanning electron microscopic study on the catalytic gasification of coal

Scanning electron microscopic catalytic gasif ication of coal Akira Tomita. Kazutoshi Chemical Research Sendai 980, Japan (Received 24 June Instit...

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Scanning electron microscopic catalytic gasif ication of coal Akira

Tomita.

Kazutoshi

Chemical Research Sendai 980, Japan (Received 24 June

Institute

Higashiyama of Non-Aqueous

study on the

and Yasukatsu Solutions.

Tohoku

Tamai University,

Katahira,

1980)

The behaviour of nickel catalyst during coal gasification of Leopold coal (West Germany) was examined by means of scanning electron microscopy. Microscopic observations of the same field were made several times as the reaction proceeded. In addition to the uncatalysed gasification, the nickel-catalysed gasification was clearly observed under the microscope. With steam gasification, catalysts moved very actively, and they seemed to accelerate the gasification mainly by pitting holes into the char. The topological changes on char surfaces by hydrogasification were not so pronounced. The function of catalyst may not be restricted to the pit formation, for it seems to accelerate the gasification over all of the char surface. Because of the high hydrogasification temperature, agglomeration of catalyst takes place to a considerable extent. Finely dispersed nickel catalysts were observed when the coal had been pretreated with liquid ammonia. The catalytic activity of these fine particles was so large that the char beneath them was gasified rapidly.

In previous papers’ -3, it has been reported that nickel catalyses the gasification of carbon and coal, and that the pretreatment of coal with liquid ammonia enhances the catalytic activity of nicke13. The effectiveness of nickel catalyst was influenced not only by the pretreatment of coal, but also by the method of nickel loading, the kind of starting nickel salt, and the reduction condition of the salt. To utilize the maximum activity of nickel, it is essential to understand how a nickel particle catalyses the gasification reaction. For this purpose, microscopic observation is a powerful technique in the field of carbon gasification4-6. The action of a catalyst in coal gasification has also been investigated by the scanning electron microscope (SEM) in recent years 3,7- lo. These studies, however, have dealt almost exclusively with the fragmental observation of the surface of residual char after gasification. In this paper, a systematic observation of nickel catalyst during the gasification of coal is reported. To investigate the dynamic behaviour of the catalyst, the cycle of gasification and SEM observation were made several times on the same area. The motion of a particular nickel particle could thus be followed. This technique is useful because the topographical change of coal itself can be monitored, the action of nickel particles as catalyst can be observed, and the relation between activity and the size or shape of nickel particle can be determined. By using the energy dispersive analyser of X-rays (EDAX), a semi-quantitative analysis is possible with respect to the interaction of nickel with other elements. The sulphur poisoning of nickel catalyst is the most important interaction3.‘. However, the present technique has some limitations in addition to the unavoidable dangers of using microscopy, which Thomas called the ‘twin evils of eclecticism and tendentiousness’4. The first limitation is that observations were made at room temperature after each run instead of at reaction temperature. Some change may occur during the cooling process. The second limitation is the neglect of possible 001f%2361/81/020103-12$2.00 Q.1981 IPC Business Press

catalytic activities of fine particles of diameter less than 0.1 pm. Although there are limitations, it is believed that this simple technique can give important information on catalytic behaviour of nickel particles. In this paper, we report mainly the topographical change of coal surface during catalytic gasification of Leopold coal in steam and hydrogen. EXPERIMENTAL The coal used in this study was Leopold coal from West Germany, the analyses is presented in Table 1. Approximately 1 wt % of nickel was impregnated on coal from an aqueous solution of hexa-ammine-nickel (II) carbonate. In some cases, the coal was pretreated with liquid ammonia before the nickel imprgenation. Details of the method of impregnation and ammonia treatment are described elsewhere l1 . In this paper, the following abbreviations are used for coal samples with different pretreatments: UN, raw coal; UC, catalyst-bearing coal; and TC, coal pretreated with liquid ammonia and then impregnated with nickel catalyst. The same nomenclatures are also used for the resultant chars. Figure 1 shows the reactor assembly. Five coal particles of 5 l-2 mm in diameter were mounted on a cylindrical Table

1 Analysis of Leopold

coal

Proximate analysis (wt %I Moisture Volatile matter Fixed carbon Ash Ultimata analysis Carbon Hydrogen Nitrogen Sulphur

4.1 36.1 54.5 5.3

Iwt %, dafl 81.8 5.4 1.7 1.6 9.5

Oxygen

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SEM

study

on catalytic

Quartz Dish

gasification

Graphite Holder

of coal: A. Tomita

et al. RESULTS

Quartz

AND DISCUSSION

Reactor

Coal conversion

+

Figure

1

Schematic

Water

from

diagram of apparatus

The conversions for the steam and hydrogasification are summarized in Tables 2 and 3, respectively. An enhancement of reactivity by the nickel addition and the pretreatment with liquid ammonia was shown in both cases. A larger effect was observed in hydrogen, as reported previously l2 . The conversions in the steam gasification are in fair agreement with those obtained in a thermobalance3, suggesting that the cooling procedures between each stage do not affect seriously the conversion of coal. Dispersion

graphite holder which was suitable for the SEM equipment. Graphite paste was used to adhere the coal particles to the holder. Two graphite holders were placed on a rectangle quartz dish (20 x 40 mm). The dish could be moved horizontally by a quartz tube with a hook. In the quartz tube, a thermocouple was inserted to monitor the reaction temperature. The temperature difference between the tip of the thermocouple and the coal sample was estimated to be less than 5 K. The furnace temperature was controlled by another thermocouple. For the steam generation, a small evaporator with an electric heating wire was utilized to evaporate water fed from a microfeeder at a constant rate. Exposed parts of tubings were wound with a heating wire to prevent steani condensation. First, coal samples on a graphite holder were examined under a SEM, a Hitachi-Akashi MSM 4C-101, to which an EDAX, a Horiba EMAX-1500, was attached. Then the holder was put on a quartz dish and placed in the reactor. The initial position of the dish was outside of the furnace. After an evacuation of the system, nitrogen gas was introduced at the flow rate of 200 cm3 (s.t.p.) min- ‘. When the furnace temperature reached 5Oo”C, the quartz dish was pushed into the centre of the furnace. It was removed after the devolatilization for 1 h. Samples were then carefully examined by SEM and EDAX. The graphite holders were returned to the quartz dish in the reactor. The evacuation of the system was followed by the introduction of nitrogen and reactant gas. With steam gasification, the flow rate of nitrogen was maintained at 40 cm3 (s.t.p.) min - 1 and that of steam was at 150 cm3 (s.t.p.) min-‘. When the temperature and the rate of gas flow were constant, the quartz dish was placed in the furnace. After the required time of gasification, the coal sample was pulled out to be examined. These procedures were repeated several times to follow the topographical changes of a particular area on the char surface. With hydrogasification, the flow rate of hydrogen was 200 cm3 (s.t.p.) min- ‘. The conversion of coal at each stage was determined in a different series of experiments. Coal particles of m200 mg were placed in a quartz dish without graphite holders. The devolatilization and gasification were carried out under the same reaction conditions as previously described. At the end of each stage, the weight was measured and the conversion was calculated on a dry, ash-free basis. The conversion thus obtained may differ to some extent from the conversion of the particular coal particle shown in the photomicrographs.

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of catalyst

The coal sample with catalyst was prepared by mixing the coal with a solution of nickel salt which was subsequently evaporated to dryness. Thus, most crystals of the nickel salt were forced to precipitate on the external surface of the coal particle. Figure 2 shows two typical forms of the catalyst after devolatilization. An X-ray diffraction study revealed that the nickel salt was reduced to the metallic state at this stage. The dispersion mode of the catalyst is somewhat different between UC and TC samples. Usually, the nickel catalyst on the untreated coal exists as a cluster with a size of approximately several microns, as seen in Figure 2a. The coal particle appears green to the naked eye”. However, smaller spherical catalysts are generally observed on the treated coal. It can be concluded that the catalyst dispersion on the coal surface is better for the treated coal, which may be one reason why the TC sample has a higher reactivity than the UC sample. With respect to the catalyst distribution within a coal particle, the present impregnation method was found to be unsatisfactory. The SEM observation was made on a char particle, cut by a razor blade, after partial gasification. Figure 3a shows the boundary area between the Table

2

Steam gasification

of Leopold

coal Conversion

Temperature

Reaction Time

(wt %, daf)

Stage

Gas

PC)

(h)

UN

UC

TC

1 2 3 4 5

NZ HZ0 Hz0 Hz0

500 750 750 750

1 .o 0.5 0.5 0.5

25 36 39 41

23 36 40 43

20 35 40 44

"20

800

0.5

48

54

55

+'20

800

1 .o

60

66

70

6

Table

3

Hydrogasification

Stage

Gas

1 2 3 4 5 6

N2

Temperatuce PC) 500

"2

1000

"2

1000 1000 1000 1000

Hz "2 '42

of Leopold

coal Conversion (wt %, daf)

Reaction Time (h)

UN

UC

TC

1 .o 0.5 0.5 2.0 6.0 10.0

26 38 38 39 42 48

22 34 35 37 46 58

21 34 34 37 52 63

SEM

study on catalytic

gasification

b

Q Figure 2

Nickel

on char surface after devolatilization.

Figure 3

Distribution

of coal: A. Tomita et al

1

IOwl

(a) UC; (b) TC

of nickel on a cleaved coal particle.

(a) SEM; (b) X-ray analysis for nickel

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SEM study

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cut surface (lower side) and the external surface (upper side). The EDAX analysis (Figure 3b), shows that few catalyst particles are found within the char particle. This unbalanced distribution is not due to the accumulation of nickel on the external surface which can be expected at a high conversion level. The conversion in this case was only *50’& including the volatile matter. The ash particles containing iron, in fact, are observed more uniformly on both sides. The catalytic action of nickel might therefore be restricted to only the external surface of coal particles. To maximize the activity of added catalyst, it is necessary to develop a new impregnation method which can give a better dispersion of the catalyst on the internal surface of coal particles. Behuciour

of catalyst

in the steum gusijication

Figure 4 shows a typical gasification mode in steam. The stage numbers specified in this Figure, and also in the following Figures, correspond to the stages indicated in Tubles 2 or 3. Figure 4a is an example of a devolatized char surface, which has many cracks with different widths. Relatively large amounts of catalyst are seen along large cracks. At the second stage (Figure 4b), catalyst particles start to agglomerate with each other. Many particles can be seen on the flat surface in the upper right corner, where there was little indication of the presence of catalyst at the first stage. Small cracks in this area, which had been clearly recognized in Figure 4a had almost disappeared in Figure 4b. Some cracks of medium size also disappeared, and the others became shallow. This indicates that the gasification occurred all over the surface, and that the level of the surface gradually receded towards the centre of particle. The pit formation around a catalyst indicates the higher gasification rate in the vicinity of catalyst. At the third stage (Figure 4c), nickel particles further submerged into the char and most of them became invisible. A few cracks, which has been seen in the second stage, disappeared. At the fifth stage (Figure 4d), the roughening of the surface proceeded significantly. The width of the large cracks increased. White particles still remaining on the surface were usually non-spherical. Judging from EDAX data, these nickel particles invariably contain a certain amount of iron. The interaction of nickel catalyst with the mineral matter then began to occur. These particles seem to be less active than the pure nickel catalysts, and they tend to stay on the surface instead of pitting into the char. Gasification through pit formation undoubtedly is catalysed. It is difficult to assign whether or not the gasification through gradual surface recession is also catalysed, however, we consider that this is an uncatalysed reaction, and that the effect of the catalyst, therefore, is restricted to its vicinity. Behaviour of’cutalyst in the hydrogusijicatiorl A typical movement of the nickel catalyst in hydrogenis shown in Figure 5. Large nickel lumps observed at the first stage (Figure Su) split into a number of small spherical particles (Figure 5b), with diameters in the range ;tO.I&5 pm. The shape of the particles clearly indicates that these particles have melted during the course of gasification. Although the reaction temperature is higher than that for the steam gasification, it is still ~450 K lower than the melting point of nickel metal. The mobility of nickel atoms might be increased either by the local high

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temperature due to the heat evolution upon exothermic reaction or by the reaction with sulphur. As the gasification proceeds to the fourth (Figure 5c) and the sixth stage (Figure 5d), nickel particles agglomerate not only in a crowded region but also in a less crowded region (upper left). Pitting was caused particularly by the smaller catalysts. Figure 6 shows both agglomeration and pit formation at a larger magnification. Again, smaller particles submerge more quickly than larger ones. Figure 7 shows a group of catalysts which seem to be less active. There was no appreciable pit formation, although agglomeration was observed to some extent. These catalysts contain considerable amounts of silicon and aluminium as a result of interaction with the mineral matter, which may be one possible explanation for the smaller catalytic activity. Another possibility is that the char substrate beneath these catalysts has an extremely low reactivity. Semi-quuntitutiue

analysis

The observations described previously were assessed more quantitatively by analysing the size of the nickel particles. Using Figures 5-7, the number of particles, their mean diameter, and the surface density, which was calculated from the former two values by assuming that the shape of particle is spherical, were measured. The results for three cases are summarized in Table 4. A, B and C in the Table correspond to the region surrounded by white lines in the last photographs of Figures 5-7, and the areas were 60, 110 and 80 pm2, respectively. In region A, the surface density of the nickel particles decreased quickly. This implies that the nickel rapidly disappears as a consequence of pit formation. However, the density in the region C remained almost constant between stages 3 and 6, indicating that the decrease in the number of particles is possibly only due to the agglomeration. Case B is between these two extremes. Although the number of particles decreased rapidly, the density decreased slowly. Only the smaller particles formed pits, the larger particles tended to remain on the surface. The increase in diameter can be ascribed mainly to the agglomeration, and partly to the preferential disappearance of smaller particles. The detail of the pit-forming action of a nickel particle is shown at the centre of photographs in Figure 8. After * 18 h the particle advanced l-2 pm toward the inside. The volume of the pit formed by such a movement may be related to the difference in the char conversion between the catalysed and the uncatalysed gasifications. The approximate total pit volume was estimated from the pitforming rate (Figure 8) and the surface density of the particle (Table 4), and was found to be too small to account for the increase of conversion by catalyst. This apparent discrepancy partly results from an underestimation of the pit-forming rate. The pit-forming rate observed in Figure 8 is measured relative to the level of the surface, which itself may also move inward. This is supported by the observation that two nickel particles at the upper left corner in Figure 8 appeared again from the inside at the fifth stage (Figure 8~). Thus, it is apparent that the influence of the catalyst is not restricted to its vicinity but extends all over the char surface. The activated hydrogen can migrate from the catalyst to the carbon substrate. Another possible explanation for the above discrepancy is that the contribution by fine nickel catalysts, which are invisible under an SEM, may have

SEM study on catalytic

gasification

of coal: A. Tomita et al.

a

d Figure 4

Typical

gasification

mode of TC coal in steam.

(a) Stage 1; (b) Stage 2; (c) Stage 3; (d) Stage 5

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1981,

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SEM study on catalytic gasification of coal: A. Tomita et al.

b

Figure 5

108

Typical

FUEL,

gasification

1981,

mode of UC coal in hydrogen.

Vol 60, February

(a) Stage 1; (b) Stage 2; (c) Stage 4; (d) Stage 6

SEM

13lJy

C Figure 6

Agglomeration

and pit formation

on UC coal in hydrogen.

study on catalytic

gasification

d

of coal: A. Tomita et al

13

rq

(a) Stage 3; (b) Stage 4; (c) Stage 5; (d) Stage 6

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1981,

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109

SE/W study on catalytic gasification of coal: A. Tomita et al

13 Figure 7

110

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Less active catalyst

1981,

in the hydrogasification

Vol 60, February

wq of UC coal.

d (a) Stage 3; (b) Stage 4; (c) Stage 5; (d) Stage 6

13lJ9

SEM study on catalytic gasification

b

a

Figure 8

Details of pitting for the hydrogasification

of coal: A. Tomita et al.

of TC coal.

(a) Stage 3; (b) Stage 4; (cl Stage 5; (d) Stage 6

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1981,

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SEM study on catalytic gasification of coal: A. Tomita et al.

I

Figure 9

112

Gasification

FUEL,

1981,

3Om

I

mode of TC coal with fine catalysts in steam.

Vol 60, February

b

aI), (c) Stage 1; (b), (dJ Stage 2

SEM study on catalytic

a

Fiwre

10

gasification

b

Gasification

mode of TC coal with fine catalysts in hydrogen.

of coal: A. Tomita et al.

) 3wJmj

(a), (c) Stage 1; (b), (d) Stage 2

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1981,

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SEM study on catalytic gasification of coal: A. Tomita et al. Table 4

Change of nickel particle

size during hydrogasification

Number

of particles

Mean volume diameter

(rm-2)

Surface densitv (9 prnb2 x 10-14)

(pm) *

Stage

Case A

Case B

Case C

Case A

Case B

Case C

Case A

Case B

Case C

3 4 5 6

6.8 3.2 2.0 1 .l

3.0 1.7 0.5 0.3

3.1 2.9 2.4 1.7

0.21 0.27 0.21 0.22

0.52 0.61 0.86 1.02

0.67 0.58 0.62 0.70

30 28 7 5

190 180 160 160

270 270 260 260

been missed. However, the transmission electron microscopic observation revealed that line particles (~3 nm in diameter) which existed at the first stage, agglomerated at the later stages, owing to the high reaction temperature, and became large enough to be recognized under a SEM. This semi-quantitative analysis is applicable only to the hydrogasification. The catalyst in the steam gasification resulted in more drastic changes on the topology of the char surface, as shown by comparing the last photographs in Figures 4 and 5. The topological change shown in Figure 4 is larger, in spite of its smaller magnification, than that of Figure 5, the conversion being almost the same in both cases. The catalyst may have a different function in these two reactant gases, i.e., the main function of catalyst in steam is the gasification of char in its vicinity, whereas in hydrogen the catalyst can accelerate the gasification of char somewhat apart from it. To clarify this point, further work is required. Finely dispersed nickel catalyst on the coal pretreated liquid ammonia

with

The earlier discussions have not made any distinction between UC and TC, because no special differences, except that TC char has peculiar nickel catalysts on its surface, were observed. This section is concerned with this point. After devolatilization, line nickel particles with diameters of less than 100 nm gathered to form a long, narrow stripe. A coal particle with a size of 1 mm usually has a couple of these stripes. The chemical modification of coal with liquid ammonia might result in such a special interaction with nickel. The detailed mechanism whereby this occurs is unclear. The stripes always existed in association with narrow cracks, which had not been present before the devolatilization. Some examples are shown in Figure 9a and Figure 1Oa. Close-up views of the centre of these photographs are shown in Figures 9c and 10~. After the gasification for 0.5 h either in steam (Figure 9) or in

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hydrogen (Figure ZO), a deep ditch appeared where fine particles had existed. Some agglomerated nickel particles remained along the ditch. The distance between two particular points separated by the stripe was measured before and after the second stage gasification. If the breaking of char particle is responsible for the ditch formation, the distance would expand. However, the distance did not increase, but slightly decreased, indicating that the ditch formation was a result of rapid gasification by catalyst. Comparing the depth of holes made by ordinary nickel particles with the depth of the ditches suggests that the line particles are much more active as catalysts. Unfortunately these active nickel catalysts cover only a fractional area of the surface. It is therefore necessary to find an impregnation method which can disperse a large portion of nickel in such an active state13.

REFERENCES 1 2 3 4 5 6 I 8 9 10

11 12 13

Tomita, A., Sato, N. and Tamai, Y. Carbon 1974, 12, 143 Tamai, Y., Nishiyama, Y. and Hagiwara, H. J. Chem. Sot. Jpn. 1978, 1670 Tomita, A., Oikawa, Y., Kanai, T. and Tamai, Y. Fuel 1979,58, 614 Thomas, J. M. ‘Chemistry and Physics of Carbon’ (Ed. P. L. Walker, Jr.) 1965, 1, p 121 Tomita, A. and Tamai, Y. J. Phys. Chem. 1974, 78,2254 Baker, R. T. K. Chem. Eng. Prog. 1977, Apr., 97 Gardner, N., Samuels, E. and Wilks, K. ‘Coal GasifKation’(Ed. L. Massey), Am. Chem. Sot. Chem. Ser. 1974,131, p 217 Rai, C. and Hoodmaker, F. AIChE Symp. Ser. 1976,72, 332 Mochida, I., French, M. and Marsh, H. Proceedings, 5th Conf. on Industrial Carbon and Graphite, London, 1978 Otto, K., B&tosiewicz, L. and Shelef, M. Fuel 1979, 58, 85, 565 Tomita, A., Tano, T., Oikawa, Y. and Tamai, Y. Fuel 1979,58,609 Nishiyama, Y. and Tamai, Y. Fuel 1979,s 366 Tomita, A., Takarada, T. and Tamai. Y. J. Chem. Sot. Japan 1980,

959