Anisotropic coke formation during coal slurry heating in the coal liquefaction process

Anisotropic coke formation during coal slurry heating in the coal liquefaction process F. F. Tao and L. D. Brown* Exxon Research and Engineering Company, PO Box 4255, Ba ytown, (Received 28 October 7986; revised 23 April 1987)

TX 77522, USA

Coke formed during early operations of the coal slurry preheater in the 250 t d - ’ EDS coal liquefaction pilot plant (ECLP) was examined by polarized light microscopy. The coke was optically anisotropic, having mosaic textures resulting from mesophase carbonization. The presence of mesophase in heated coal slurry was evidenced by the formation of micrometre-sized hydrocarbon spherules in the coal residue from laboratory tests under simulated ECLP preheater conditions. These spherules are thought to act as adhesives, causing deposit formation on the preheater wall. Pyrolysis of these wall deposits results in anisotropic coke formation. Preheater wall temperature and solvent quality control coking severity. Model solvent studies revealed that donor hydrogen plays the most important role in mitigating coke formation during coal slurry heating. Improvement in solvency (increase in Hildebrand solubility parameter) of the solvent is also effective in increasing the coal dissolution and reducing the coking tendency. (Keywords:conl liquefaction; coke de-position;nnisotropy)

The EDS (Exxon donor solvent) process is a direct coal liquefaction process to produce liquid fuels by direct hydrogenation of coal ‘. A schematic diagram of this process is shown in Figure 1. In this process, the coal is slurried with a process-derived hydrogen donor solvent and fed in admixture with molecular hydrogen to the preheater furnace, in which slurry is heated to ~425°C before entering the liquefaction reactor. The coal is hydrogenated in the reactor at 425480°C and 13.817.2 MPa total pressure. The reactor effluent, after separation from gaseous products, is fractionated into liquid products and vacuum bottoms by conventional atmospheric and vacuum distillation. This coal liquefaction process has been demonstrated successfully at the 250 t d- ’ EDS coal liquefaction pilot plant (ECLP) in Baytown, Texas’. During early ECLP operations on Illinois No. 6 coal from the Monterey mine, coke formation in the coal slurry preheater was identified as a major operating problem. The approach to eliminating this problem has been described in detail previously3. During a laboratory study of coke formation in the preheater, the presence of mesophase spherules in coal slurries was observed after coal dissolution under simulated preheater conditions. Furthermore, mesophasederived anisotropic carbon was identified as a major component of the ECLP preheater plug. Anisotropic carbon was also observed in previous inspections of coke samples from the SRC slurry preheater 4*5. In this Paper, the important factors in coke formation in the coal slurry preheater are discussed.

behaviour of coal slurries under simulated preheater conditions. The tubing bomb test is a laboratory test widely used for coal liquefaction studies and has been described previously6. The bomb was charged with the coal, hydrogenated solvent and H, gas. To simulate the preheater conditions, the bomb was heated during agitation in a preheated fluidized bed from room temperature to 427°C in 2-6min, which is about the preheater residence time. It was then rapidly quenched in a water bath. The coal residue was removed from the bomb by rinsing with cyclohexane to collect the freeflowing particles and by scraping the tube wall to collect the wall deposits. The coal residues from tubing bomb tests were analysed by C-H analysis, Fourier transform infrared (FTIR) spectrometry, scanning electron microscopy (SEM) and polarized light optical microscopy. A coking bomb test was devised to simulate coke

EXPERIMENTAL Laboratory

tests

Tubing bomb tests were used to investigate the physical * Present address: Exxon Research and Development Laboratories, PO Box 2226, Baton Rouge, LA 70821-2226, USA CO16-2361/88/010004~$3.00 0 1988 Butterworth & Co. (Publishers) Ltd.

4

FUEL.1988,

Vol 67, January

Fire

1

EDS coal liquefaction process scheme

Anisotropic

coke formation

during coal slurry heating

in coal liquefaction:

F. F. Tao and L. 0. Brown

RESULTS AND DISCUSSION Microscopic examinations of carbon and coke deposits

A complete cross-section of the coke deposit in contact with the tube wall close to the outlet of the ECLP preheater (10 cm diameter, see Figure 3a) was obtained for examination by polarized light microscopy. Coke formation was evident at bulk temperatures exceeding 388”C3. The inside wall surface of the 316 stainless steel pipe, both in the coke-free region and in the coke deposit area, had a sulphide layer ~50,um thick. Elemental analyses by SEM confirmed that the layer was an ironchromium sulphide. The multilayered nature of the coke deposit is well illustrated by the polarized light photomicrograph shown in Figure 3b. The first layer (layer A) adjacent to the pipe was ~0.25 mm thick, had a low ash content and consisted of highly anisotropic carbon with large domains. Layer B, x0.75 mm thick, surprisingly was a high-ash zone consisting of rather large particles of mineral matter (quartz, calcite, pyrrhotite and clays) and fusinite bound together with anisotropic

Figure 2

Schematic diagram of coking bomb test: 425-WC, lO13.7 MPa H,, heating time 16-60 min, solvent-coal weight ratio 1.2-2.0

-Preheater (stainless

formation on the preheater wall while allowing solvent evaporation, as shown in Figure 2. In this test the coal and solvent are pressurized in a retort with hydrogen and plunged into a heated block which is controlled at temperatures in the range 425-540°C. The volatiles evaporated from the feed slurry are absorbed by glass wool packed in a condenser cooled by dry ice. A sample of coke residue from these tests was characterized by the same analytical procedures as used for tubing bomb residues. Coke identification

Optical microscope observations of coke samples and tubing bomb residues were performed with a Leitz Ortholux II polarized light microscope. Samples were prepared by mounting in epoxy followed by standard polishing procedures. Observations were made by vertical illumination of the polished coke surface under cross-polarized light at magnifications from 80 x to 625 x . Coke samples appear as bright anisotropic mosaic patterns and domains. The relative coking severity is determined by the extent of these anisotropic regions. For coal samples no anisotropy is observed under crosspolarized light (i.e. coal is isotropic), so coal and coke are easily distinguished by this technique. FTIR spectra were measured using a Digilab Fourier transform infrared spectrometer on coke samples prepared as KBr pellets. The relative peak sizes at 1610 cm-’ stretching) and 2920 cm - I (aliphatic (phenoxy) were also used to differentiate the coking severity. C-H analysis on the coal or coke sample was with a Hallikainen microcombustion performed instrument with a repeatability of 0.3 % for C and 0.1% for H.

wall steel )

Coke depc)bit

a

II

du-1

b

Steel pipe

Metal sulphide

Layer 8 Mineral matter and fusinite particles

Layer A low ash mesaphase

c

Layer C low ash mesophase

Figure 3 Photographs of ECLP preheater coke plug. (a) Cross Section of plug. (b) Micrograph of deposit structure adjacent to stainless steel pipe wall (incident cross-polarized light, magnification 9 80)

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1988,

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Anisotropic coke formation during coal slurry heating in coal liquefaction: F. F. Tao and L. 0. Brown Figure 5. The very low-ash layer ( z 250 pm) adjacent to

--Wall/coke interface

the preheater wall, as observed by microscopic examination, is also shown in this analysis. The discontinuity in ash buildup in the coke layer, as indicated by the dotted line, suggests that the coke was laid down in successive stages. It is also interesting to note that the HC atomic ratio shows a consistent increase from the wall side to the slurry flow side. This phenomenon may be attributable to a difference in dehydrogenation (coking reaction) as a result of the temperature gradient in the coke deposit. Adhesive nature of mesophase

Tubing bomb tests on Illinois No. 6 (Monterey mine) coal slurry were conducted under simulated ECLP preheater conditions. The elemental analysis of the coal sample is shown in Table 1. Coal conversion as measured by cyclohexane solubility was < 5 wt % and no sign of anisotropic coke formation in the coal residue was observed by polarized light microscopy. However, micrometre-size spherules (mesophase) in the coal residues from these tests were observed by SEM, as shown in Figure 6. These spherules were virtually mineral-free as indicated by X-ray fluorescence analysis. Since the formation of anisotropic carbon via mesophase

b Fiire 4 Photomicrographs of anisotropic coke and coke layer formation. (a) Low-ash coke layer adjacent to tube wall (magnification x200). (b) High-ash coke layer separated by a pyrrhotite band (magnification x 80)

carbon in a tight matrix. Below this high-ash layer was another relatively low-ash zone (layer C) consisting of anisotropic carbon with widely varying mosaic patterns. The multibanded structures in the coke deposit suggest that a series of independent depositional events occurred during coke formation. The large mineral matter layer, as shown in Figure 3b, was not observed in all ECLP preheater coke deposits. This is illustrated by the photomicrograph of a coke sample spalled from the preheater wall, shown in Figure 4. The thin layer of low-ash anisotropic carbon adjacent to the wall was also observed. A higher ash was found in the thick layer away from the wall. A pyrrhotite band was noted at the boundary between the low-ash and high-ash layers (see Figure 4b). Pyrrhotite bands were also observed in coke deposits from the SRC slurry preheater4*‘. Brooks and Taylor’, in their studies of mobility during semicoke formation, observed that insoluble solids were aligned along the perimeter of the mesophase spherules. It is conceivable that the pyrrhotite particles may align themselves to form a band when the mesophase coalesces to form semicoke. However, the pyrrhotite band shown in Figure 4b is relatively thick (z 150 pm). An alternative explanation is that the pyrrhotite band is originally formed on the stainless steel pipe wall together with the dense ironchromium sulphide layer. This association has been commonly observed on a stainless steel surface exposed to hydrocarbon pyrolysis or coal slurry preheater conditions. The less tightly adhering pyrrhotite layer then becomes dissociated from the iron-chromium sulphide layer to form bands in the coke layer. The spalled coke sample described above was sliced into eleven thin pieces from the preheater tube wall side to the slurry flow side for C-H-ash analysis as shown in

6

FUEL.1 988, Vol 67, January

0.6-

-60

0. 5-

-50

$ 0. 4-0

l

2 8

4o

l

E

3 0. 3-

-30

2 2

5

‘\ J

Q 0. 2-

\ I---\

A j-

b B d s .5 f a ae

20

A

0 .I -

-10 A I

OO

5

I

I

IO 15 Distatance from

I

I

I

20

25

30

35’

tube wall,mm

Figure 5 Variation in composition of ECLP coke plug with distance from tube wall

Table 1 Elemental analyses of coals (wt %)

C

H S N Ash Atomic H/C ratio

Wyoming Illinois No. 6 (Monterey mine) (Wydoak mine)

Texas Lignite (Martin Lake (mine)

68.6 4.9 4.4 1.2 9.5 0.86

64.6 4.7 0.9 1.3 11.5 0.87

70.1 4.6 0.7 1.0 7.4 0.79

Anisotropic

coke formation

during coal slurry heating

in coal liquefaction:

F. F. Tao and L. D. Brown

These wall deposits would subsequently coke over a period considerably longer than the nominal residence time of the slurry in the furnace. The tube wall temperature and solvent quality should therefore be the major factors determining coking severity in the preheater. 43

X-ray Spherule

number

fluorescence analysis (wt%) SiOp S* -A’203 -

I

0.5

0.2

2.2

2

0.3

0.3

2.5

3

0.2

0.2

2.1

* Organic

sulphur

C and H not detectable

Fire 6 SEM X-ray fluorescence analyses of mesophase spherules from tubing bomb test (427”C, 10 MPa H,, 2 min residence time, Illinois No. 6 coal, magnification x 104)

is a well-known mechanism in pyrolysis or carbonization of hydrocarbons8, these micrometre-size spherules observed in tubing bomb tests are treated as mesophase and no attempt is made to distinguish them from vitroplast as an additional intermediate stage for anisotropic coke

formation’.

It is postulated that mesophase plays the role of an adhesive in preheater coking. Some of these spherules may stick to the preheater wall to form deposits which are subsequently pyrolysed to form the low-ash coke layer. In addition, the mesophase spherules may act as a binding agent to agglomerate the coal particles. These agglomerated coal particles may stick to the preheater wall and be subsequently coked to form the high-ash layers in the deposit. Evidence of the sticky nature of coal residues and coal particle agglomeration was observed in tubing bomb tests. As illustrated previously3, ~90% of the coal residue after tubing bomb tests was observed to stick to the tube wall. The coal residue was also observed to agglomerate to a mean particle size about ten times that of the parent coal. Coke does not appear to be formed in the coal slurry flow during preheating. It is suggested that its formation is initiated with the deposition of mesophase spherules, coal slurry agglomerates or both on the preheater wall.

Eflect of tube wall temperature

To investigate the effects of temperature and solvent quality on coke formation, a series of coking bomb tests on the Illinois (Monterey) coal sample was made at 427, 454 and 482°C using four solvents ofdifferent quality. The results are given in Table 2. Statistical analysis of these data indicates that the tube wall temperature is the most significant factor affecting coke formation. A plot of atomic H/C ratio of the coke residue against tube wall temperature from coking bomb tests using different solvents is shown in Figure 7. It is seen that the atomic H/C ratio consistently decreases with increasing tube wall temperature. This consistent relation demonstrates that the coal residues from the coking bomb tests are representative of semicoke formation under thermal equilibrium. These data also indicate that the tube wall temperature plays a critical role in determining the coking tendency in the slurry preheater. The coking severity was observed to be temperatureand time-dependent, as demonstrated by the FTIR analyses in Figure 8. Painter et al.“*” have shown that FTIR can be used to identify coke formation by observing the decrease in aliphatic stretching at 2920cm-’ and phenoxy groups at 16 16 cm- ‘. The coking bomb test at 427°C and 16 min run time shows no coke formation, in that the feed coal and the coal residue from the test show no appreciable difference in FTIR spectrum. However, coke formation is quite evident when the run time is increased from 16 to 60min at 427°C or when the temperature is increased from 427 to 538°C. This suggests that coke may be formed after the deposition of sticky coal particles at a relatively low temperature with sufficient time.

TaMe 2 Effects of tube wall temperature and solvent quality on coking severity’ Coke formation (wt %

coal) Solvent

Solvent quality index (SQI)*

427°C

454°C

482°C

ECLP solvent Reference solvent 1 Reference solvent 2 Reference solvent 3

2.3 3.1 2.6 2.1

59 50 55 61

69 62 65 69

75 62 73 86

Variance analysis:

Tube temperature Solvent quality index

F

Significant at 95 % confidence level

24 8

Yes Yes

‘In coking bomb tests at 10 MPa H, and 6 min residence time with

Illinois (Monterey) coal at solvent~oal ratio of 1.2 bEDS proprietary solvent quality parameter; a higher value represents better quality c Toluene-washed residue after test

FUEL,

1988,

Vol

67,

January

7

Anisotropic

coke formation

during coal slurry heating in coal liquefaction:

0.6

I 454

I 427

0.3

I 482

I 538

I 510

Tubs walI tempemture (“C) Figure 7 EfTectof tube wall temperature on atomic H/C ratio of coal residues from coking bomb tests (Monterey coal, solventxzoal ratio 1.2, 1 h residence time): 0, ECLP solvent; 0, ECLP solvent; A, ref. 1; V, ref. 2; 0 ref. 3

16min

427T/

427T/60

F. F. Tao and L. D. Brown

and Martin Lake lignite are included in Table 1. The model solvents used in these tests were tetralin (hydrogen 1-methylnaphthalene (non-donor donor solvent), aromatic) and decalin (condensed ring saturate). These solvents are of similar boiling point and have Hildebrand solubility parameters” ranging from 8.8 to 11.2. Results from these tubing bomb tests are summarized in Table 3. Coal conversion to cyclohexane-solubles was minimal in these tests. The percentage of residue soluble in tetrahydrofuran (THF) is therefore used to represent the degree of coal dissolution by heating with the solvent under hydrogen pressure. These results show an increase in coal dissolution with increasing solubility parameter for the non-hydrogen-donor solvents. However, coal dissolution in the hydrogen donor solvent is appreciably higher than that in the non-donor solvent of higher solubility parameter. This indicates that coal dissolution in the preheater is not merely a physical process, but is enhanced by coal conversion in the presence of donor hydrogen. Coking bomb tests were also performed on Monterey and Wyodak coals with these model solvents. The results of these tests are summarized in Table 4. The coking tendency with two non-donor solvents of different solubility parameters is similar. Coking, however, is appreciably less severe with the hydrogen donor solvent than with the non-donor solvents. These results indicate that the presence of donor hydrogen in the solvent serves not only to enhance coal dissolution but also to reduce the coking tendency in the preheater. For further understanding of the effect of solvent properties on coke formation, coking bomb tests were also made on an asphaltene fraction (toluene-soluble) and fraction (toluene-insoluble, THFa preasphaltene soluble) derived by sequential extraction of liquefaction bottoms from Illinois No. 6 (Monterey) coal. The model solvents used in the tubing bomb tests and hexadecane (aliphatic hydrocarbon) were used for these tests. A plot of solubility parameter for these solvents against coking Table 3 Dissolution of coal in model solvent: residue soluble in THF (wt % daf)

min

Decalin Solubility parameter ((Calcm- 3)o.5) Illinois coal Wyodak coal Martin Lake lignite C-H stretch

3600

I 3000

I

I

I

2200 Wovenumber

(1610)

I

1700 (cm-’ 1

I

(1450)

To investigate the effect of solvent properties on coke formation, tubing bomb tests were carried out on Illinois No. 6 coal (Monterey mine), Wyoming coal (Wyodak mine) and Texas lignite (Martin Lake mine) with three model solvents. The elemental analyses of Wyodak coal

Vol 67, January

51 36 38

900

Eflect of donor hydrogen

FUEL,1988,

36 24 26

9.5

I

1300

Figure 8 FTIR spectra of coke samples from coking bomb tats (10 MPa H,, Illinois No. 6 coal, solvent-coal ratio 1.2)

8

13 2 4

11.2

Tetralin*

‘In tubing bomb tests at 427”C, 13.7 MPa H, and 2 min residence time, with solvent-coal ratio of 2.0 ‘Hydrogen donor solvent

(2922)

I

8.8

l-Methylnaphthalene

Table 4 coal)”

Coking severity in model solvents: coke formation (wt%

Solubility parameter ((Cal cm-3)o.5)

Decalin 8.8

Illinois coal (Monterey) 67 Wyoming coal (Wyodak) 63

l-Methyl naphthalene 11.2

Tetralin* 9.5

71 75

43 39

“Toluene-insoluble residue in coking bomb tests at 427”C, 10 MPa pressure and 60 min residence time, with solvent-coal ratio of 2.0 *Hydrogen donor solvent

Anisotropic

coke formation

during coal slurry heating

in coal liquefaction:

F. F. Tao and L. D. Brown

Table 5

Coking tendency and conversion of preasphaltenes in model solvents’

Solvent

Solubility parameter ((Cal cr~-~)O.~)

Coking tendency (wt % THF-insolubles formed)

Conversion (wt % toluene-solubles formed)

Hexadecane Decalin 1-Methyl-naphthalene Tetralin

8.0 8.8 11.2 9.5

45 35 30 16

21 35 39 60

“In coking bomb tests at 427”C, 10 MPa H, and 60 min residence time, with solvent-preasphaltenes

60

/

ratio of 2.0

donor solvent (tetralin) the conversion from preasphaltenes to asphaltenes is nearly four times the yield of the retrograde reaction to THF-insolubles. The increase in conversion with a hydrogen donor solvent should therefore contribute to a reduction of coking in the slurry preheater.

THF lnsolubleson preospholtenx feed

CONCLUSIONS Toluene lnsdubles

on

0.6

0l Donor

0.4 co

I 8.8

8 Solubility

I 9 parameter

CD

I 9.5 of solvents

I IO

I II

II Il.2

(co1 cm-3)0.5

Figure 9

Coking tendency of asphaltenes and preasphaltenes from Illinois bottoms in model solvents in coking bomb tests (427°C 10 MPa H,, 60 min residence time, solventfeed ratio 2.0 by weight)

tendency is shown in Figure 9. The relative coking severity was determined by measuring toluene-insolubles formed with the asphaltene feed and THF-insolubles formed with the preasphaltene feed. As expected, the preasphaltenes showed a higher coking tendency than the asphaltenes for all the model solvents used. The coking severity of asphaltenes in non-donor solvents appears more dependent on the solubility parameter of the solvent than that of preasphaltenes does, as indicated by the different slopes in Figure 9. Because the preasphaltenes are less soluble in these solvents than are asphaltenes, the coking tendency is less dependent on the solvency of the solvents. As is also shown in Figure 9, the coking tendency using a donor solvent (tetralin) is appreciably less than that for a non-donor solvent with the same solubility parameter of 9.5. Coal dissolution is enhanced by greater coal conversion in the presence of a hydrogen donor. The increase in coal dissolution results in a lower coking tendency. An appreciable portion of preasphaltenes was converted to asphaltenes (toluene-soluble) in these coking bomb tests, as shown in Table 5. This indicates that conversion and retrograde reactions occur simultaneously under the simulated coking conditions. In a

The micrometre-size spherules formed during coal dissolution in the slurry preheater are responsible for coke formation via mesophase. These spherules act as adhesives which cause wall deposits of hydrocarbons or coal agglomerates that are subsequently coked over a period of time. Since the coking occurs on the preheater wall, the tube wall temperature is the most significant factor in coking severity, but solvent quality also plays a role. An improvement in solvency is effective in decreasing the coking tendency for asphaltenic material, but its effectiveness is less for preasphaltenic material. The presence of donor hydrogen in the solvent is effective in reducing the coking tendency for both asphaltenes and preasphaltenes. In the slurry preheater, coal undergoes initial conversion to preasphaltenes. The donor hydrogen content of the solvent is therefore the more important solvent property controlling coking severity. ACKNOWLEDGEMENTS Financial support for the EDS coal liquefaction project was provided by the US Department of Energy, Exxon Co. USA, Electric Power Research Institute, Japan Coal Liquefaction Development Co. Ltd, Phillips Coal Co., Anaconda Mineral Company, Ruhrkohle AG and ENI. REFERENCES Wade, D. T., Ansell, L. L. and Epperly, W. R. Chemtech 1982, 242

10 11 12

Vick, G. K. and Epperly, W. R. Science 1982,217,217 Sharp, D. W., Stober, B. K.,Tao, F. F., Dimopoulos, A. I., Pierz, E., Potter, M. J. and Pate& R. D. ‘Coal slurry preheater coking in the EDS coal liquefaction pilot plant’, presented at AIChE 1983 Spring National Meeting, 1983 Wakeley, L. D., Davis, A., Jenkins, R. G., Mitchell, G. D. and Walker, P. L. Jr. Fuel 1979, Ss, 379 Harris, L. A., Kennedy, R. and Yust, C. S. Fuel 1979,58, 59 Maa, P. S., Neavel, R. C. and Vernon, L. W. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 242 Brooks, J. D. and Taylor, G. H. Carbon 1965, 3, 185 Marsh, H. and Walker, P. L. Jr. in ‘Chemistry and Physics of Carbon’, Dekker, New York, Vol. 15, 1979, p. 229 Mitchell, G. D., Davis, A. and Spackman, W. in ‘Liquid Fuels from Coal’ (Ed. E. T. Ellington), Academic Press, New York, 1977, p. 255 Painter, P. C. and Coleman, M. M. Fuel 1979,58,301 Painter,P. C.,Yamada, Y., Jenkins, R. G., Coleman, M. M. and Walker, P. L. Jr. Fuel 1979, Ss, 293 Hildebrand, J. H. and Scott, R. L. ‘The Solubility of Nonelectrolytes’, 3rd Ed., Reinhold, New York, 1950

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