Pyrolysis of Stuart oil shale in the presence of recycled shale

Pyrolysis of Stuart of recycled shale

oil shale in the presence

Nguyen V. Dung CSIRO Division of Fuel Technology, Lucas Heights Bag 7, Menai, N.S. W., 2234, Australia (Received 24 July 7989)

Research

Laboratories,

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The pyrolysis of Stuart oil shale in the presence/absence of fully combusted shale (shale ash) in superheated steam or nitrogen was conducted in the range 450 to 600°C in a 48 mm diameter semi-continuous fluidized-bed reactor. The detrimental effect of shale ash on oil yields in a recycled shale retorting system was confirmed. The coking of product oil vapour on shale ash led to oil losses that increased with increase in the ash-to-shale ratio. Reduction in surface area of the shale ash by high temperature treatment lowered oil coking losses. However, while pyrolysis in steam produced more oil than pyrolysis in nitrogen, the type of sweep gas (steam versus nitrogen) did not affect the rate of oil coking on shale ash. Certain types of clay minerals in the shale and changes in their structure during the course of pyrolysis/combustion were postulated as having a significant influence on the extent of the oil coking. Oil coking losses on shale ash, which were largely independent of temperature in the range 450 to 525°C increased significantly with temperature above 525°C. Except for methane and hydrogen, yields of gaseous products and the atomic H/C ratio and sulphur content of the oils were unchanged by the coking reactions. (Keywords: oil shale; ash; pyrolysis)

Recently developed oil shale conversion processes such as the Chevron’, Exxon2, Shel13, Lawrence Livermore National Laboratories (LLNL)4, and Battelle5 processes use hot recycled shale as the heat carrier. Compared with the hot gas Paraho6 and Union7 processes, the hot solid processes offer higher oil yields per tonne of mined shale, a simpler oil recovery system and a smaller total retort/combustor volume which can lead to lower production costs of syncrude. However, pyrolysis using shale ashes (fully cornbusted shales) as the heat carrier has caused 28 and 22% reductions in oil yields for Condor and Stuart oil shales respectively, compared with retorting of the shales using external electrical heating*. The lost oils are mainly the heavy fractions (>34O”C), which are adsorbed onto shale ash and principally converted to coke. The high specific surface areas of the Condor and Stuart ashes are speculated as the key factor causing high oil coking losses’. Wall and Udaja’ have measured the coking rates of Condor oil vapour on 15 different shale ashes from eight Australian oil shales. They have found that oil coking reactivities correlate reasonably well with the nitrogen BET surface areas of the shale ashes. Australian Tertiary oil shales have high contents of clay mineralslo,“. Experiments with pure clay minerals12 have shown that the amount of coked oil increases with the specific surface areas of the minerals. This detrimental effect of recycled shale on oil yields has been recognized recently for the US Green river (Western) and New Albany (Eastern) oil shales and Chinese Maoming shale’ 3. This paper reports the effect of various process variables such as the type of fluidizing gas (nitrogen versus steam), ash-to-shale ratio and temperature on the yields and characteristics of products from pyrolysis of Stuart oil shale in a semi-continuous fluidized-bed reactor. 0016-2361/90/040497-05 0 1990 Butterworth & Co. (Publishers)

Ltd.

EXPERIMENTAL The oil shale sample used in this work was Stuart (Kerosene Creek Member, Sub-unit A, auger No. 60 hole), which was supplied by Southern Pacific Petroleum NL and Central Pacific Minerals NL. This was the same sample used in previous studies’“15. The sample was the 0.61.4 mm fraction taken from the freshly crushed shale supplied in 4&200 kg batches, dried to below 3.5% moisture. Before each experiment, the prepared shale was fluidised with nitrogen in a cold model retort to remove lines. The Stuart shale had 20.4% organic carbon and yielded 14.4% oil in a material-balance-modified Fischer assay. Full analysis and Fischer assay results for the shale have been given elsewhere14. Shale ash was prepared by complete combustion of spent shale at 800°C in a continuous fluidized-bed combustor. The shale ash contained no carbon, hydrogen and nitrogen but 1.1% total sulphur, and had a nitrogen BET surface area of 36 m2 per gram. The semicontinuous fluidized-bed retort (48 mm i.d.) used in this work, the experimental procedure and the analytical procedures for feed and product materials have been described elsewherel&15. The retort was initially charged with 80 g of a mixture of roasted acid-washed silica sand and shale ash. The ash-to-shale ratio is defined as the ratio of the amount of shale ash initially charged into the retort to the total amount of oil shale fed. RESULTS AND DISCUSSION Effect of steam and ash-to-shale ratio

Although pyrolysis Fischer assay) than produced under both almost the same rate

in steam gave more oil (17+ 2% pyrolysis in nitrogen, oil yields retorting atmospheres decreased at as the ash-to-shale ratio increased

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Pyrolysis

oil shale in the presence

of Stuart

treated

125

l

\

0

nitrogen 100

\

ash \

steam a

of recycled

* 0

n

n 751""""""""""""' 0.5 0 0.25

Ash-to-shale

m

0.75

1

1.25

ratio

Figure 1 Influence of ash-to-shale ratio, sweep gas and treatment of shale ash on oil yield from pyrolysis of Stuart oil shale at 500°C; *, shale ash treated at 1000°C in a muffle oven for 24 h

from 0 to 1.0 (Figure I). Steam and nitrogen therefore had similar impacts on oil coking reactivities of shale ash at the very dilute oil vapour concentration (0.2 ~01%) used. This is consistent with the finding by Wall and Udaja’ that steam and carbon dioxide sweep gases have no significant effect on the rate of oil coking on Condor shale ash as measured with an argon sweep. By contrast, data for both Western and Eastern US shales16 have indicated that, as the ash-to-shale ratio increases, the conversion of organic carbon to oil-plus-gas reduces at a lower rate in steam compared with argon. Figure 1 also shows that steam pyrolysis with shale ash treated at 1000°C in a mufIle oven for 24 h gave significantly less oil loss compared with pyrolysis with untreated shale ash. This treatment reduced the specific surface areas of the shale ash from 36 to 5 m2 per gram. This indicates that high active surface areas can be considered to be one of the major factors in the reduction of oil yield. Coburn et ~1.‘~ have also found that steam gives more volatile products than an inert gas (argon). They postulated that steam must either help in the conversion of free radicals to oil or interfere with the char forming process. However, in ethylene production by cracking of naphtha and vacuum gas oil, which also proceeds by a free-radical mechanism, steam is used only as a diluent to control hydrocarbon partial pressure and is well known to have no effect on cracking17. While steam does not significantly influence the oil coking rate on recycled shale ash, the increased oil yields with steam show that it has a significant impact on the coking of oil on shale minerals during the course of retorting. There are differences between the mineral structures of shale ash and retorting shale. If active specific surface area is a major factor causing oil coking loss then clay minerals in the shale have an important role in this process. Table I shows surface areas accessible to both non-polar (e.g. nitrogen, argon, carbon dioxide) and polar (e.g. steam, glycerol) gases for various clay minerals. The Stuart oil shale sample used in this study contains 20% smectite (montmorillonite),

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shale: N. V. Dung 7% kaolinite and 4% illite”. Among these minerals, smectites are the most important because of their structure and high contents (15-30%)’ ’ in Tertiary shales from seven Australian deposits. The smectite lattice consists of two sheets of tetrahedrally co-ordinated silicon (with oxygen) and one sheet of octahedrally co-ordinated aluminum (with oxygen and hydroxyl group), hence a 2:l or three-sheet structure”. The structural three-sheet layers are bound together by relatively weak attractive forces allowing polar molecules to penetrate easily the spaces between the layers. Neutral molecules such as nitrogen, carbon dioxide and argon probably penetrate only at the edges of the layers. This gives rise to a significant difference in specific surface areas obtained using nitrogen and glycerol (Table 1). Below lOO”C, all the interlayer surface area is occupied by water molecules through sorption and/or solvation of exchangeable cations (Na, Ca, K) which compensate for the excess negative charge of the layers. As smectite is being heated to about 400°C in a dry atmosphere, the interlayer water is continuously released and therefore free surface increases1 9. In a retort swept with nitrogen, some of the free interlayer surface areas would be accessible to oil vapours for absorption and coking, particularly polar compounds which are present at high concentrations in high boiling point fractions2’. However, in steam, oil coking losses would be less because the free interlayer surface areas are expected to be less and the oil vapour molecules have to compete with water molecules for the free areas. Smectite, dehydroxylated, in the combusted shale ash has a completely different structure. As dehydroxylation proceeds, the four hydroxyl groups in the smectite lattice react to produce two water molecules, the structure is rearranged, and the interlayer spaces disappear. Novak and Novakoval’ have shown that treating three smectite samples at 800°C in a muffle furnace for one hour reduces their total surface areas (by ethylene glycol monoethyl ether) from the original 74G780 m2 per gram to about 100 m2 per gram. The areas reduce further to about 25 m2 per gram after heating the samples at 900°C. Small interlayer surface areas in shale ash are therefore to be expected. This would explain the finding by Wall and Udaja’, supported by the present work, that, compared to a non-polar gas, steam has no significant effect on the rate of oil coking on shale ash. Figures 2 and 3 show that for both nitrogen and steam, the oils lost by coking on shale ash were the heavy oil fraction (> 350°C). This is consistent with data from a continuous fluidized-bed retort using nitrogen for Condor and Stuart oil shales*. Measurements of the rate of coking for various Stuart oil fractions on Condor ash

Table 1

Specific surface areas (m* per gram) of various clay minerals”

Sample

Nitrogen adsorption

Glycerol adsorption

Kaolinite

10

16

Smectites montmorillonite, Texas montmorillonite, Arizona hectorite, California

82 92 58

599 820 486

101

NA

Illite NA: Not available

Pyrolysis of Stuari oil shale in the presence

Pyrolysis

with

nitrogen

0.00 0.25 0.50 0.75

Ash-to-shale 0 m

Heavy oil (>340 Light

oil

(<340

shale: PI. V. Dung

those obtained at an ash-to-shale ratio of 0.5. For convenience, a yield ratio is defined as the ratio of the yield of a product from steam pyrolysis with an ash-to-shale ratio of 0.5 to that from the pyrolysis without shale ash. Oil yields, which were not significantly influenced by temperature in the range 450 to 525°C decreased markedly as temperature increased from 525 to 600°C (Figure 4~). Yield ratios for oil of less than one indicated drops in oil yield when retorting with shale ash over the whole temperature range (Figure 4b). The ratios reduced very slightly between 450 and 525°C but at a higher rate between 525 and 600°C. Furthermore, the organic carbon conversion to char/coke increased as temperature increased above 525°C (Figure 5). In contrast, the carbon

125

FA

of recycled

i--

1.00

loo ; -----La

ratio

75

C) C)

Figure 2 Influence of ash-to-shale ratio on yields of oil fractions pyrolysis of Stuart oil shale in nitrogen at 500°C

50400

from

450

500

550

600

6 i0

450

500

550

600

650

1.1

Pyrolysis 125

with

steam

1:

I

0.9

7

0.8 : Oa7400

Temperature,

’C

on a, oil yield; b, oil yield ratio, Figure 4 Influence of temperature from pyrolysis of Stuart oil shale in steam at an ash-to-shale ratio of0.5

Steam

IT? ’

Pyrolysis

of

Shale

Stuart

with Ash

25-

z! -I+

2 *

ul

i

n -

FA

0.00 0.25 0.50 0.75

Ash-to-shale

&___

1.00

ratio 2

0 m

Heavy oil (>340 C) Light oil (<340 C)

3

Figure 3 Influence of ash-to-shale ratio on yields of oil fractions pyrolysis of Stuart oil shale in steam at 500°C

have shown it cokes9.

40

that the heavier

the oil fraction,

0

25

0 0

0

0

00

20

from

the faster

Effect of temperature The yields and characteristics of products from the steam pyrolysis of Stuart oil shale at various temperatures in the absence of shale ash have been reported elsewhere15. This section compares these results with

?

0

400

450

500

550

Temperature,”

600

650

C

Figure 5 Influence of temperature on organic carbon conversion to char/coke from pyrolysis of Stuart oil shale in steam at an ash-to-shale ratio of 0.5

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Pyrolysis

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of recycled

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Al. V. Dung

conversion to char/coke in the absence of shale ash is constant at about 22% over the whole temperature range15. These results indicate that low oil yields in the higher temperature range were due to increased oil coking losses on shale ash.

Carbon

monoxide

Methane :j;;,

450

500

Ethane 1.5 b 1.25 _*_ t -* 1 -.El

600

550

~ _@_ -.-

1-

650

- -

0.75

1 450 o%oo

k.

o-3oo

I.

”

”

450

500

600

550

Butane 1.5 t 1.25 W --t----_--_~____. 1 -0 0.75

O*200 F.

450 I

500 *

I

5

Temperature,

600 ”

650 I

f!fyzq 550

At all temperatures, the presence of shale ash did not appear to affect the yields of gases, except for methane and hydrogen (Figures 6-8). The yield ratios for methane and hydrogen were greater than one, which is to be expected if these gases are by-products from coking. However, the effects of temperature on the yield ratios for these gases were very different from those on the oil yield ratio and the organic carbon conversion to char/coke. The yield ratio for methane increased linearly in the whole temperature range (Figure 6). The hydrogen yield ratio exhibited a maximum at about 5W525”C 8).

600

650 CONCLUSIONS

Temperature,

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At all temperatures, an ash-to-shale ratio of 0.5 did not appear to influence the atomic H/C and the sulphur content of oil; however, it did reduce the nitrogen content (Figure 9).

“C

Figure 7 Influence of temperature on yield ratios for C,-C, alkenes from pyrolysis of Stuart oil shale in steam at an ash-to-shale ratio of0.5

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Figure 8 Influence of temperature on yield ratios for carbon monoxide, C,-C, hydrocarbons, hydrogen and alkenes-to-alkanes ratio from pyrolysis of Stuart oil shale in steam at an ash-to-shale ratio of 0.5

(Figure

Butene 1.5 F ~. --1

500

0

I

650

“C

Ethene

500

so

600

Temperature,“C

0

550

550

650

Figure 6 Influence of temperature on yield ratios for C,-C, alkanes from pyrolysis of Stuart oil shale in steam at an ash-to-shale ratio of 0.5

450

Hydrogen 1.5 b 0 1.25 : a 1 i ---------------.

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The detrimental effect of shale ash on oil yield in a recycled shale retorting system was confirmed. As the ash-to-shale ratio increased, oil yields reduced due to oil coking on shale ash. Reducing the surface area of the shale ash by high temperature treatment lowered oil coking losses. However, the type of sweep gas (steam versus nitrogen) did not affect oil coking rates on shale ash despite pyrolysis in steam producing more oil than pyrolysis in nitrogen. Differences in the ability of steam and nitrogen to absorb into the interlayer spaces of clay

Pyrolysis

Stuart

Shale

of Stuart

Oil

oil shale in the presence

N. V. Dung

500

450

Sulphur

REFERENCES 1 2

__-__-__--*_2-- * * * * 6

**

0.75

O-5400

shale:

N.L./Central Pacific Minerals N.L.. It was supported by grant no. 931 administered by the National Energy Research Development and Demonstration Council (NERDDC). The author gratefully acknowledges the contributions of B.H. Stratten who assisted with the fluidised-bed experiments, and P. Udaja, A. R. Tibbett, A. K. Hutchings and L. T. Roberts who analysed the feed and product samples.

;;;_.::1!

I.........

of recycled

500

450

*

....,

.‘..““.

550

600

I

I

650

Temperature, “C Figure 9

Influence of temperature on atomic H/C ratio, sulphur and nitrogen contents of oils from pyrolysis of Stuart oil shale in steam at an ash-to-shale ratio of 0.5 relative to those from steam pyrolysis in the absence of shale ash

minerals in the pyrolysing shale could explain the differences in oil coking losses with steam and nitrogen pyrolysis. However, following combustion, the reorganized structure, now devoid of interlayer spaces in the dehydroxylated clay minerals of the shale ash, results in neither steam nor nitrogen having any role to play in absorption processes. Oil coking losses on rcycled shale ash showed only a small temperature dependence in the range 450 to 525°C but an increased dependence above 525°C. Except for methane and hydrogen, yields of gaseous products and the atomic H/C ratio and sulphur content of oils were unchanged by the coking reactions over the whole temperature range.

8 9 10 11

12 13

14 15 16

17

18 19 20

ACKNOWLEDGEMENTS 21

This work was funded partly through a collaborative research agreement with Southern Pacific Petroleum

Chevron Research Company, US Palent 4 448 666, 1984 Bauman, R. F., Mitchell, W. N., Eakman, J. M. and Koveal, R. J. Proc. of the 4th Australian Workshop on Oil Shale, Brisbane, NSW, December 1987, p. 198 Voetter, H., Poll, I. and van Wechem, H. M. H. 20th Oil Shale Symposium Proceedings, Golden, CO, USA, April 1987, p. 122 Lewis. A. E.. Braun. R. L. and Diaz. J. C. 17th Oil Shale Symposium Proceedings, Golden, CO, ‘USA, April 1984, p. 16 Shirley, F. W., Steedman, W. G. and Appelbaum, H. R. Proceedings of 1983 Eastern Oil Shale Symposium Lexington, KY, 1983, p. 83 Jones, J. B. Jr. and Glassett, J. M. in ‘Oil Shale Technology’ (Ed. V. D. Allred), Center for Professional Advancement’, East Brunswick, NJ, USA, 1982, p. 107 Barnet, W. I. in ‘Oil Shale Technology’, (Ed. V. D. Allred), Center for Professional Advancement, East Brunswick, NJ, USA, 1982, p. 169 Dung, N. V., Wall, G. C. and Kastl, G. Fuel 1987, 66, 372 Wall, G. C. and Udaja, P. Fuel 1988,67, 1340 Lindner, A. W. ACS Symposium Series 230, American Chemical Society, Washington, D.C., USA, 1983, p. 97 Patterson, J. H.-in ‘Comparative Mineralogy in Processing of Australian Tertiarv Oil Shales’, (Eds. J. H. Patterson and A. W. Lindner), Southern Pacitic Petroleum N.L., Central Pacific Minerals N.L. and CSIRO Division of Fuel Technology, NERDDP Project 1001, Final Report, Department of Resources and Energy (Australia), 3 volumes, February 1989, Annex 5 Espitalie, J., Madec, M. and Tissot, B. American Association of Petroleum Geologists Bull. 1980, 64, 59 Cena, R. J. and Taylor, R. W. Proceedings International Conference on Oil Shale and Shale Oil, Beijing, China, May 1988, p. 361 Dung, N. V. Fuel 1989,68, 1570 Dung, N. V. Fuel 1990,69, 368 Coburn, T. T., Taylor, R. W. and Morris, C. J. ‘Isothermal Pyrolysis and Combustion of Oil Shale in Steam’, Report UCRL-1011002, Lawrence Livermore National Laboratory, Livermore, CA, USA, 1989 Rebick, C. in ‘Pyrolysis -Theory and Industrial Practice’, (Eds. L. F. Albright,, B. L. Crynes and W. H. Corcoran), Academic Press, New York, USA, 1983, Chapter 4 Brown, G. (Ed.) ‘The X-ray Identification and Crystal Structures of Clay Minerals’, Mineralogical Society, London, UK, 1961 Novak, I. and Novakova, L. Chem. Zvesti 1980, 34(3), 348 Berga, J. 0. in ‘Relining Synthetic Liquids from Coal and Shale’, National Academy Press, Washington, DC., USA, 1980, Chapter 5 van Olphen, H. and Fripiat, J. J. in ‘Data Handbook for Clay Minerals and other Non-metallic Minerals’, Pergamon Press, Headington Hill, Oxford, UK, p. 209

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