6 Reactivity of British Coals in Solvent Extraction Downloaded via UNIV OF LEEDS on August 13, 2018 at 09:09:12 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
J. W. CLARKE, G. M. KIMBER, T. D. RANTELL, and D. E. SHIPLEY National Coal Board, Coal Research Establishment, Stoke Orchard, Cheltenham, Gloucestershire, England
By 1980 the rate of recovery of light crude oil from the North Sea oil fields will exceed the total demand for crude oil in the United Kingdom. Heavy crude oils from the Middle East will be used for balancing the refineries to produce the required range of petroleum products. However, by the late 1980's the diminishing supply of indigenous oil and general world shortage will result in a serious shortfall in supply. Thus it will be necessary to exploit other more abundant resources of hydrocarbons. In the United Kingdom there are large reserves of coal which could satisfy demand for at least two hundred and fifty years. The National Coal Board currently mines approximately 120 million tons of coal per annum of which 65% is used for generating electricity (1). (Combustion of the low sulphur British coals does not result in excessive atmospheric pollution.) To satisfy the increased demand for coal the National Coal Board has undertaken an investment programme which includes the development of a mining complex at Selby in Yorkshire which, it is estimated, will produce in excess of 10 million tons when full production is reached in 1988. At the Coal Research Establishment of the National Coal Board, methods for the liquefaction of coals to produce transport fuels, feedstocks for the chemical industry and high purity carbons suitable for electrode manufacture are being developed. A schematic diagram of the liquid solvent extraction process is illustrated in Figure 1. Where the production of liquid hydrocarbons is the main objective an hydrogenated donor process solvent is used, whereas in the production of needle coke this is not necessary and a coal derived high boiling aromatic solvent may be used (e.g. anthracene oil). An essential economic requirement of the process is that a high extraction yield of the coal is obtained and this will depend upon the coal used and the digestion conditions. The properties of the coals mined in the United Kingdom vary from the high carbon anthracites to the lower rank non --
0-8412-0587-6/80/47-139-1 11$05.00/0 Published 1980 American Chemical Society Whitehurst; Coal Liquefaction Fundamentals ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
112
COAL LIQUEFACTION
(
COAL
FUNDAMENTALS
( SOLVENT J
)
CD
DIGESTION
(
DIGEST
)
SOLIDS SEPARATION
COAL EXTRACT SOLUTION
HYDROGENATION
(PREMIUM LIQUID PRODUCTS*)
Figure 1.
COKING
PREMIUM
GRADE ELECTRODE COKE
The National Coal Board solvent extraction process
Whitehurst; Coal Liquefaction Fundamentals ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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CLARKE ET A L .
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Coals
113
caking coals, but with virtually no deposits of brown coals or lignites. These variations in composition can be conveniently illustrated on Seyler's coal chart (Figure 2 ) ( 2 ) . In general each mining area produces coal of a characteristic type, for example, the South Wales coal fields produce anthracites and prime coking coals; while the large mining areas of Yorkshire, Derbyshire and Nottinghamshire produce lower rank coals and i t is here that the substantial reserves of coal are found. Extensive studies into the solvent extraction of coals have revealed the mechanism controlling digestion and produced kinetic data on the reactions ( 3 - 1 1 ) . In digestion, a coalsolvent slurry is heat-treated at temperatures in the range 4 0 0 - 4 4 0 C during which most of the coal depolymerises and dissolves. The heat treatment causes some bonds in the molecular structure of the coal to rupture producing radicals which are stabilised by hydrogen transfer from the solvent. In the absence of hydrogen the radicals combine to form an insoluble high molecular weight product. This is a simplified model and more detailed studies of digestion are available (8, 9 ) . Kinetic studies have mainly supported this model but the interpretation of the reaction orders and activation energies of the reactions appears complex ( 1 0 , 1 J L ) . In a study of the digestion of a low rank British coal in anthracene o i l i t was found that the initial decomposition of the coal was of first order with an apparent activation energy of 4 0 kJ mole-1. This was followed during further heat-treatment of the digest by a second order polymerisation with an activation energy of 1 2 0 kJ mole"-'-. For a solution prepared from a high rank coal the polymerisation was again second order but with an activation energy of 1 9 0 kJ mole (12). Q
-1
Most studies on the mechanism and kinetics of solvent extraction have necessarily used only a limited selection of coals. In a commercial environment where coals with widely varying properties are available, i t is necessary to develop a generalised system of grading the coals with respect to their suitability for liquefaction. To classify coals for the production of metallurgical cokes the National Coal Board adopted a system of ranking based upon the Gray King coke type and volatile matter (Figure 2 ) ( 1 3 ) . Unfortunately, this method of ranking coals was found unsuitable for solvent extraction ( 1 4 ) . In this work consideration has been given to methods of ranking coals based upon reactivity during liquefaction. Technique Samples of digest were prepared using either the maxibomb digester shown in Figure 3 or a smaller digestion vessel (minibomb) described elsewhere (12). The maxibomb consisted of a length of 2 5 mm o.d. stainless steel tube closed at one end
Whitehurst; Coal Liquefaction Fundamentals ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
114
COAL LIQUEFACTION
FUNDAMENTALS
Figure 2. Classification of British coals
Whitehurst; Coal Liquefaction Fundamentals ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
6.
CLARKE ET A L .
British
115
Coals
Figure 3.
Maxibomb digester
Whitehurst; Coal Liquefaction Fundamentals ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
116
COAL LIQUEFACTION FUNDAMENTALS
1
with a 'Gyrolok compression coupling. The top of the tube was closed with a modified coupling containing a packed gland and stirrer. Approximately 20 g of a coal-solvent slurry were accurately weighed into the barrel of the reactor. The complete bomb was immersed in a fluidised sandbath controlled at the required operating temperature. After a given residence time the bomb was removed from the sandbath and water quenched. The volume of gas produced during digestion was measured. For short residence times i t was necessary to compensate for the heating up period. Heating curves were constructed at each digestion temperature by using a modified digestion vessel into which a thermocouple had been sealed. From these curves and a knowledge of the activation energies i t was possible to calculate the time interval to be allowed (seven minutes for maxibomb) after immersion of the bomb in the sandbath before digestion was effectively started (12). This method of compensating for the heating up period introduces a small error in the short residence time tests. The extraction yield of the coal was determined from the concentration of residual solids in the digest and the yield of gas. The solids concentration of the digest was determined from the measurement of the solubility of the digest in quinoline. The addition of quinoline does not precipitate the dissolved coal in the digest and thus the yield of quinoline insolubles can be directly equated to the concentration of solids in the digest (12). In some tests the samples of digest were filtered at constant temperature and pressure through a heated pressure filter. The yield of f i l t e r cake was measured and the solubility in quinoline determined in order to calculate the extraction yield. Materials Data relating to the coals used are listed in Table I. The coals were crushed 80% less than 75 jim. The solvents used were anthracene o i l (ex British Steel Corporation), hydrogenated process solvent (produced in a continuous coal extract hydrogénation plant) and several pure organic compounds (ex Koch-light). Results The results from the extraction of coals in anthracene o i l and phenanthrene are given in Tables III to VII, while Table II gives extraction yields of Annesley coal when digested in a range of solvents including a selection of organic compounds. The influence of digestion conditions upon extraction is shown in Figures 4 to 6.
Whitehurst; Coal Liquefaction Fundamentals ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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Coals
111
Table I Analysis of Coals
NCB Coal Coal
rank code (CRC)
Cynheidre beans Penalta Garw
Proximate analysis (ar)
% Moisture
101 202 204
1.1 0.5
204 204
0.5
Tilmanstone Tymawr Beynon Windsor
301a 301a
Marine Oakdale Cwm (a) (b) (c) (d) Bedwas Herrington
0.5
%
7.4 7.3 9.4
0.5 1.0 1.2
7.3 5.8 8.1 7.4
301a
1.3
301a 301a
1.3 0.5
301a 301a 301a
0.5 0.8 0.8
301b 401
0.9 1.0
%
Volatile Ash matter 5.4 14.2 18.1 17.7 18.8 24.1
0.9 0.8
0.9 0.6 0.8
25.1
90.4
20.9
2,3 6.7 2.7
21.3 21.6 21.7
91.2 4.7 1.6 1.6 91.0 4.8 1.9 1.5 90.8 4.7 2.6 1.5 90.3 4.9 2.3 1.6
4.2
22.9 28.5 32.8
90.6 4.9 2.3 1.6 0.6 88.8 5.1 1.5 3.8 1.7 87.2 5.2 4.8 1.9 0.5
39.2 36.0 35.5 37.0
85.7 5.2 6.0 1.7 86.0 5.2 5.8 1.8 86.8 5.4 5.3 1.8 85.7 4.7 6.1 1.4
36.3
86.6
84.5 5.3 7.1 1.9 1.8 85.4 5.6 6.1 1.8 1.3 85.1 5.3 6.3 1.9 1.8 84.5 5.4 8.0 1.9 0.7
10.7 8.2 4.5 5.4
Barrow
2.3 2.3 1.1
7.3 3.7 6.6
Brodsworth (a)
602
2.6
Swallowood
602
3.8
4.3 5.7
38.1 36.9
Brodsworth (b) Annesley
702
2.0 1.8
14.2
38.9
Rufford (a) Rufford (b) Blidworth
702 702
Markham
Bestwood high Linby (fresh) Linby (aged)
0.9
6.7
1.5
Whitwell
1.5 1.6 1.4 4.5 1.9 1.6 4.9 3.2 1.4 4.8 1.5 3.3 5.0 1.6 2.7
10.2
3.4
5.4
6.3 5.6
702
5.3 6.9
5.9 8.1
37.9 38.8 38.7 40.4
702
5.5
5.0
39.2
902 902 802
3.1
5.7
38.6
4.2
6.2
39.0
6.9
902
3.1
7.5 7.5
34.3 38.0
702
1.2 1.0
26.1
502 502 502 502
Hawthorn Manton
93.7 3.1 1.2 91.3 4.3 2.0 91.6 4.4 1.3 91.7 4.3 1.8 91.2 89.4 89.7
502
Bersham Wearmouth
Ultimate ana!.ysis % (dmmf) S 1 Ν (to H Θ C tal
84.1 84.3 83.6
1.0 1.5 0.8
0.4 0.9
0.9
0.9 1.5 2.5 5.4 5.1 1.8 1.9
5.3 7.7 2.0 2.0 5.2 7.6 1.9 1.3 5.0 8.3 1.8 1.6
83.2 5.0 8.7 2.0 0.9 84.6 82.5
5.5 7.3 1.9 0.8 5.3 9.5 1.9 0.8
Whitehurst; Coal Liquefaction Fundamentals ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
118
COAL LIQUEFACTION
FUNDAMENTALS
Table II Solvent Power Coal: Annesley Digestion: 400°C, 60 minute residence time Coal:Solvent ratio 1:4
Solvent
(*)
Diphenyl
13
Naphthalene
14
Anthracene
32
1-Methylnaphthalene
48
Dibenzofuran
51
2-Me thylnaphthalene
51
Phenanthrene
55
Fluorene
66
Dibenzothiophene
66
Anthracene o i l
71
9,10-Dihydroanthracene
77
Pyrene
83
Acenaphthene
85
Tetralin
86
Hydrogenated anthracene o i l
89
Indoline
95
1,2 ,3 ,4-Tetrahydroquinoline
95
Note 1 d.a.f. basis. m
^ -,
^
· -, -,
Total extraction yield = J
w
t
- of (dry coal-residue) 7—TTTTî wt. of d.a.f. coal
,
( 0 / λ
x 100
Whitehurst; Coal Liquefaction Fundamentals ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
(%)
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Coals
119
Table III Extraction of Coals in Anthracene Oil Digestion conditions 400°C, 60 minute residence time Coal:solvent ratio 1:3
Characterisation NCB coal rank code
Coal
(CRC)
Cynheidre beans Penalta Garw Tilmanstone Tymawr
101 202 204 204 204
Beynon
301a
Windsor
301a
Marine
301a
Oakdale
301a 301a
Cwm (a)
of products % % gas
coal in solution
2
% total extraction
*1
6
6
