Re-examination of the phenolic hydroxyl contents of coals

Re-examination of the phenolic hydroxyl contents of coals Zeinab Abdel-Baset,

Peter H. Given and Richard F. Yarzab

Fuel Science Section, College of Earth and Mineral Sciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA (Received 20 January 1977)

In connection

with studies of the dependence

meters were needed that might effectively the phenolic

hydroxyl

determined,

by acetylation

contents

(dmmf).

However,

oped a linear equation nite content,

pressed in the alternative seen with carbon content. fall into three reasonably typically Statistical

acetic anhydride.

provinces

structures.

hydroxyl

content

of the USA

manner,

as fractions

However, distinct

with the carbon

calorific

content,

value and vitri-

contents

were ex-

no correlation

could be

in the wide scatter of points on the graph, the data are seen to

populations

such that at the same level of rank, hydroxyl

that coals from these areas differ

analyses showed that OH/O

reflectance,

When the hydroxyl

of the total oxygen

the three provinces

but it is not among the coal properties

have been

regression analysis to the data devel-

to the vitrinite being 92%.

para-

Accordingly,

The results, when expressed as frac-

of a stepwise multiple

of variance explained

decrease for coals from

tain, implying

on coal characteristics,

in the coals, showed a good inverse correlation

application

relating

the fraction

behaviour

the organic chemical

of 37 coals from three geological

with 14C-labelled

tions of the total organic matter contents

of liquefaction

characterize

in the order Interior

in structure

as a result of differing

has some significance found

in determining

contents

> Eastern > Rocky

Moun-

antecedents.

liquefaction

behaviour,

most significant.

In a major investigation in these laboratories we are attempting to determine the ways in which the various characteristics of coals may determine their behaviour in liquefaction processes. It is obviously likely that details of the organic chemical structure of coal macerals will be one of the factors determining processing behaviour. Our ignorance of the structures in coals makes it difficult to select parameters that will usefully characterize those structures in correlations with behaviour. Among organic substances, phenols have a particularly individual set of chemical properties. Earlier worklP4 had shown that a major fraction of the oxygen in vitrinites and vitrains is present as phenolic OH. Moreover, the C-O bond energy is relatively low, and Browns showed that decrease of the intensity of the phenolic OH band is one of the first phenomena to be observed when vitrinites are pyrolysed at temperatures of 400%550°C. These findings suggested the speculation that in liquefaction processing, dissociation of OH to produce free radicals might initiate deeper hydrogenation of the coal structures. Accordingly, we felt that it might be rewarding to accumulate information on the phenolic OH contents on coals for which we also had data on liquefaction behaviour. In addition to having this practical objective, our investigation has differed in the following respects from previous studies: (1) a much larger number of samples has been included, (2) account has been taken of the various ways in which the mineral matter in coals can interfere with valid reporting of the data, (3) the geological history of the sample has been admitted as a coal characteristic possibly determining in part coal properties, (4) correlation of the data with other coal properties has been by stepwise multiple regression analyses generated by a computer.

The second point above, interferences by the presence of mineral matter, should be enlarged upon in this introduction, since it raises serious problems that are often ignored in the literature. Firstly, ash yield is almost always appreciably less than mineral matter content, so that daf analyses are inadequately corrected for the presence of minerals. Secondly, for a variety of reasons that have been recapitulated recently 6,7, the oxygen by difference in a daf analysis does not purport to be a true organic oxygen content (and may differ by a factor of 3 in the worst case), while oxygen in a dmmf analysis does represent a best estimate of organic oxygen. Thirdly, minerals may be chemically changed in a supposedly selective organic reaction (with possible consequences to be discussed later). However, even in a properly computed dmmf ultimate analysis, oxygen-by-difference necessarily accumulates the errors in all of the direct determinations. The Mott-Spooner test is fortunately available for checking that these errors are not excessive6’s. (The authors are indebted to Dr Richard Neavel for drawing their attention to this test). The MottSpooner formula permits the calculation of the calorific value of a coal from the ultimate analysis on a dmmf basis. Experience has shown that the formula is so successful at predicting calorific values that it can be used with experimental heats to test the consistency of dmmf ultimate analyses. If the difference between calculated and observed values is small (< 150 Btu/lb*) one can have confidence that the ultimate analysis, including oxygen by difference, is most probably close to the truth. We apply this test to all our analytical data as a matter of routine.

* <0.35 MJ/kg

FUEL,

1978,

Vol 57, February

95

Phenolic

hydroxyl

contents

of coals: Z. Abdel-Baset,

P. H. Given and R. F. Yarzab

EXPERIMENTAL Hydroxyl determination We have relied on acetylation as a means of determining hydroxyl contents, using the conditions described by Blom et al9 However, these authors determined the acctyl uptake by a laborious procedure involving hydrolysis of the acetates and titration of the acetic acid. We have used instead the radiochemical method used by Hill and Given” for carbon blacks, in which 14C-labelled acetic anhydride is used. The acetylated coal is washed and dried, and an aliquot is then burnt in oxygen in a modified Schoeniger flask. This is a llitre spherical flask with a test tube of about 25 ml capacity fused to the bottom. The absorbent for CO2 is pipetted into the test tube and kept cold in ice during the combustion. A I:4 mixture of benzylamine and dimethylformamide is used to absorb the CO2. and the radioactivity of an aliquot of the solution is measured in a liquid scintillation counter, an internal standard being used to determine counting efficiency.* A relation between disintegrations per minute and acetyl content was established with the aid of acetyl derivatives of pure compounds (aniline, 2-naphthol). Cogls selected and coal analyses The 37 samples studied were drawn from the Penn State Coal Collection, which has been accumulated by Dr William Spackman and his associates. Ultimate and proximate analyses, including forms of sulphur, were by Commercial Testing Laboratories, Charleston, W. Va. All other analyses were performed in these laboratories. Mineral-matter contents were either determined directly by acid demineralization”~‘* or calculated by a modification of the Parr formula6. Where directly determined CO2 values were not available (for correction of carbon contents in the ultimate analysis) they were estimated by an approximation formula. The methods used and the nature of the approximations used, if any, are fully described in a recent report6. Petrographic analyses are due to W. Spackman, A. Davis and their associates. Earlier work3>4>‘3showed that the hydroxyl contents of the macerals of European coals of Carboniferous age differ appreciably, the highest contents being found in vitrinites. In this work, we selected samples that had contents of vitrinite plus pseudovitrinite of at least 70%. We also selected the samples to give a fairly uniform representation of the Eastern, Interior and Rocky Mountain Provinces of the USA. (Some comments on the distinctions between the coals of these provinces appear in another paper recently submitted to Fue/14). Analytical data characterizing the samples and the experimental results for OH are presented in Table 1. Checks on the reliability of the data Duplicate determinations of OH were made on eight of the samples studied, with results shown in the Appendix. The __.__ * In earlier work we used a commercial solution known as hyamine hydroxide as absorbent. This is a solution in methanol of a longchain quaternary ammonium hydroxide. On one occasion, after the combustion had been completed and while the flask was being shaken to aid the gas absorption, a violent explosion occurred. This was pre sumably due to the combustion of methanol, initiation of the chain reaction being catalysed by the platinum gauze of the basket used to contain the sample. The benzylamine/DMF mixture now used should be perfectly safe, and has proved satisfactory except that if a refrige rated liquid scintillation counter is used, a solid may crystallize out during counting and the data become erratic. We assume the solid is the benzylamine salt of benzyl carbamic acid. We have in fact used a non-refrigerated counter

96

FUEL,

1978, Vol 57, February

standard deviation of the duplicate determinations is 0.05. Clay minerals contain OH groups, and we could find no information as to whether they were acetylatable. Accordingly, samples of pure kaolinite and montmorillonite were subjected to the same procedure as were the coals. The acetylatable OH contents found were: kaolinite, 0.03% and montmorillonite, 0.7%. Kaolinite is a major constituent of the mineral matter of many coals, but montmorillonite is not of common occurrence in coals, and is never, as far as we know, a major constituent. Calculations making various assumptions showed that error due to acetyl uptake by clays in determinations with coals is never likely to exceed 0.1 in a hydroxyl content of 3% (worst case). In computing the hydroxyl content of the coal from the acetyl content of its acetylated product, it is convenient to assume that the weight of mineral matter present in the coal sample taken is unchanged by the reaction procedure. Yet pyridine might leach ions from the clays, and carbonates might be at least partly extracted by acetic acid during washing of the coal after hydrolysis of excess acetic anhydride. To check properly this possibility, the weight of mineral matter in each product would have to be determined; we have not done this*. Experiments with clays showed that weight losses in pyridine are measurable but not significant as an interference. We attempted to determine whether carbonates had been solubilized by examining the filtrates from some acetylated coals. Insignificant amounts of Ca++ and Na+ were detected, but small amounts of particulate matter appeared to have passed through the filters. From low- and high-temperature ashing, the particulates appeared to contain both organic and inorganic matter, but we could not further interpret our observations. Calculations indicated that in the worst case the reported OH content of the coal could be too high by about 0.2%. but in most cases the error would be considerably less. RESULTS AND DISCUSSION All of the results obtained are plotted in Figure 1, where oxygen as OH as a percentage of dmmf organic matter is shown as a function of dmmf carbon content. The line through the points is a least-squares fit, of equation: OH(%) = 33.2 - 0.35 C(%) The correlation coefficient for the equation is 0.93. Thus if carbon content be accepted as a parameter defining coal rank, so also, to a high degree of probability, is the OH content (as defined above). A stepwise multiple regression analysis was applied to the data, in which the computer was supplied with six characteristics of the samples as independent variables. As each new variable entered, an F-test of the significance of the additional explained variance was applied, using an F-value of 2.5. The linear equation developed was: OH(%) = -2.49 R, ~ 0.0010 CV+ 0.018 Vt 19.7

(2)

* As we shall see. the suspected effect was less serious than had been feared. However, in other reactions, such as an oxidation under acid or alkaline conditions, valid interpretations of analyses would emphatically need mineral matter determinations on coal and reaction product. These problems of possible interference by mineral matter in organic reactions have never been even considered in the previous literature, to the best of our knowledge

Phenolic Table I

Source and characteristics

hydroxyl

contents

of coals: Z. Abdel-Baset,

P. H. Given and R. F. Yarzab

of coals used Vitrinite and pseudovitrinite f%)

Oxygen Carbon

(diff)

OasOH

OasOH

(wt %, dmmf)

(wt %, dmmf)

(wt%of dmmf coal)

total dmmf 0)

Ohio No. 2, Coal Township, Oh. Lyons, Duncan Gap, Va. Imboden, Big Stone Gap, Va. Ohio No. 8, Cadiz, Oh. Ohio No. 9, Cumberland, Oh. Pittsburgh, Thomas, Pa. Ohio No. 11, Smithfield, Oh. Ohio No. 12, Yorksville, Oh. Ohio No. 1 ZA, Yorksville. Oh. Ohio No. 5, Hamden, Oh. Middle Kittanning, Edinburg, Pa. Brookville, Sharon, Pa.

A. Coals from the Eastern Province HVB 10.9 90.7 79.8 HVC 3.0 78.3 79.3 HVA 5.5 75.9 86.3 HVA 7.1 79.0 86.0 HVA 13.8 80.7 83.5 HVA 23.0 77.1 80.5 HVA 12.0 84.0 84.3 HVB 27.5 86.2 82.3 HVA 33.1 61.9 83.8 HVA 21.2 87.0 82.3 HVB 13.5 79.8 80.2 HVB 8.0 84.2 83.5 HVA 5.8 80.5 84.6

11.5 13.1 4.7 6.5 7.8 9.1 7.6 9.1 7.4 8.9 11.1 8.5 7.5

5.7 5.8 2.5 3.1 4.2 4.7 2.8 4.7 3.4 3.9 5.0 4.0 4.0

49.9 44.1 54.6 48.2 53.9 51.2 37.2 51.7 46.4 44.2 44.9 47.1 54.1

213 215a 216 2i7a 219 272 273 28Oa 284= 288a 289a 29oa

Kentucky No. 9, Owensboro, Ky. Kentucky No. 9, Sturgis, Ky. Kentucky No. 14, Madisonville, Ky Kentucky No. 9, Madisonville, Ky. Kentucky No. 4, St. Charles, Ky. Kentucky No. 9, Drakesboro, Ky. Kentucky No. 11, Drakesboro, Ky. Indiana No. 6, Dugger, In. Lower Dekoven, Stonefort, Ill. Illinois No. 6, Victoria, III. Illinois No. 4, S. Wilmington, III. Illinois No. 4, S. Wilmington, III.

B. Coals from the Interior HVB 11.5 75.0 HVA 16.2 86.1 HVB 10.5 84.0 HVB 21.4 86.2 HVB 9.8 84.7 HVB 11.9 82.7 HVB 19.5 85.7 HVC 18.3 92.1 HVA 25.1 87.1 HVC 10.7 87.7 HVB 11.4 84.0 HVB 15.9 82.5

80.6 83.9 81.3 82.3 82.1 82.1 79.3 81.6 83.7 79.6 80.6 81.3

9.9 6.6 9.3 8.2 10.9 8.0 10.5 9.6 7.3 11.7 9.9 9.7

5.3 4.1 5.1 4.3 4.9 4.7 5.5 5.1 4.1 5.8 5.2 5.1

53.7 61.5 55.3 52.9 44.7 58.2 51.9 53.4 56.6 49.9 52.3 52.7

23Oa

Rosebud,

76.3

17.1

6.5

37.9

23ia

Laramie

76.6

16.3

6.4

39.3

233a 236

Wadge, Energy, Co. Basin 8, Redstone, Co.

78.3 90.4

13.9 2.2

5.5 1 .o

39.3 44.4

237a 238a 239 241

Rock Canyon, Price, Ut. Seam A, Hiawatha, Ut. Seam 8, Hiawatha, Ut. Monarch, Sheridan, Wy.

81.9 80.5 80.3 73.9

10.2 10.6 11.5 19.0

4.7 4.7 4.5 6.4

45.6 44.4 39.3 33.8

249a 3iia 3i3a 3t4a

Rock Canyon, Wellington, Ut. No. 6 Seam, Fruitland, N.M. Hiawatha, Wattis, Ut. Blind Canyon, Huntington, Ut.

81.3 78.9 80.2 81.5

11.1 13.0 11.4 10.1

4.2 5.5 4.7 4.3

38.1 42.3 41.4 42.3

PSOC

ASTM

Mineral

Sample No.

Seam and location

rank class

matter (w-t %, dry)

203=

Ohio No. 7, Wellston, Oh.

212 268a 269 276a 278a 295= 305a 306a 307a 308a 330 331

a

Conversion

Colstrip,

Mt.

No. 3, Erie, Co.

(wt%of

Province

C. Coals from Western Provinces SUBBIT 10.5 78.2 B SUBBIT 6.1 76.1 8 HVC 10.3 88.8 MED 9.1 92.2 VOL HVB 14.1 79.9 HVA 10.8 85.2 HVA 11.5 83.9 SUBBIT 8.1 85.7 B HVB 12.3 70.2 HVC 20.0 80.9 HVB 17.3 78.9 HVA 11.5 81.8

data available; see ref. 14

where R, = mean maximum reflectance of vitrinite under oil immersion (70). CV= calorific value (dmmf, Btu/lb*), and V = vitrinite content (%). The multiple correlation coefficient had the very high value of 0.96 (which corresponds to a frac, tion of variance explained equal to 92%) and the partial correlation coefficients were: R,, -0.74; CV, -0.84; V, 0.36. The experimental data are plotted against values calculated from equation (2) in Figure 2. In view of the known differences in the OH contents of the different macerals, the inclusion of the vitrinite content is not surprising (except that, having regard to the principles on which samples were selected for study, the range of vitrinite content in the sample set was restricted). However, we do find it particularly interesting that two different rank parameters were selected as significant in the correlation. The process of coal metamorphism is, presumably, extremely complex, so that * 1 Btu/lb = 2.326 kJ/kg

one would not a priori expect any single parameter adequately to express its results. This inference (or guess) is confirmed by the finding that best correlation of OH contents requires two parameters describing the rank of the samples. Inspection of the plot in Figure 1 suggests that perhaps coals from the Rocky Mountain Province may tend to have a lower OH content than coals from the other provinces, but the conclusion is obviously of uncertain validity. However, in Figure 3, we plot the same experimental data, recalculated as (0 as OH)/(total 0). On this basis the OH content is obviously not a rank parameter: a great deal of scatter is observed in the plot. This implies that (0 as OH)/ (organic matter) and 0 total/(organic matter) are quite different and independent functions of rank. It appears to follow that although (0 as OH)/(total organic matter) varies rectilinearly with rank over a considerable range, other- forms of oxygen (i.e. 0 rota/ -00~) vary in a rather random fashion with rank.

FUEL,

1978, Vol 57, February

97

Phenolic hydroxyl contents of coals: Z, Abdel-Baset, P. H. Given and R. F. Yarzab 8.0

I

I

1

7.2

I

I

I

1

tetralin in our microautoclave, total reactives is the sum of the contents of vitrinite, pseudovitrinite and the liptinite suite of macerals expressed on a dmmf ~01% basis, and Srotal is the percent total sulphur (dry basis). The multiple correlation coefficient is 0.85; the partials are: (OH) OSO,(S,,I) 0.76 and total reactives 0.5 1. The best regression that can currently be generated with the subset of conversion data is, when the computer’s choice of variables is unrestricted14:

1

OHi%) = -0.35 x Carbon +33.2

5.6 -

Conversion (%) = -14.0 R, + 0.3 1 total reactives + 1.7 Sroral t 5 1.6

3.2 -

1.6 A Eastern Rovince 6 Interior Province C Rocky Mountain Province

0,80 7L

Figure

7

I

I

76

78

I

I

I

86 88 (wt % dmmf 1

90

1

I

60 82 Carbon content

Phenolic OH, % of dmmf

I

8L

92

coal, related to carbon content

This equation has a very slightly better multiple correlation coefficient of 0.86. Clearly the essential difference in these two equations is the substitution of one rank parameter, OH (%), for another, vitrinite reflectance. Attempts to relate liquefaction yields to OH oxygen as a percentage of total organic oxygen met with more success (it will be recalled that OH reported in this way correlates poorly with rank and so may be considered an independent parameter). The best regression obtained was: Conversion (%) = 0.4 (OH/oxygen) - 22.8 R, + 69.0

7.2 -

The multiple correlation coefficient is 0.72; the partials are OH/oxygen, 0.57 and R,, - 0.64. It therefore appears that although OH/O may be of some significance in determining liquefaction behaviour, it is not among the most significant properties of coals. Hence other parameters characterizing the organic structures of coals must be examined for their relevance.

m c”6.4 ._ g 65.6* = a&.0ou 54.0 E m $3.2 r G2.L

-

ACKNOWLEDGEMENTS -

0 In

‘J1.6-

0

0

04

I

1

I

I

I

I

I

1

1.6

2.4

3.2

L.0

4.8

5.6

6.4

7.2

0 as OH Iwt% of dmmf coal figure 2 Regression analysis, dependence characteristics. *

Twu coincidental

1calculated

This study was supported by Grant No. AER 73-07837 from the RANN Division of the National Science Foundation. to which the authors are indebted. They also are grateful to Dr William Spackman for the coal samples, to Joann Roebuck for the mineral matter contents, and to Dr Roger Granlund, Professor of Biophysics, for the use of the liquid scintillation counter under his charge.

of phenolic OH on coal

points

A further point of interest in Figure 3 is that the points for the coals from different provinces fall into certainly two and probably three populations. In the range 79-85%C, there is a marked tendency for OH/O of coals from the different provinces to fall in the order, Interior > Eastern > Rocky Mountain. Thus there is here reasonably convincing evidence that significant differences in an important structural characteristic can result from differences in the origins and conditions of metamorphism of coals. Also determined was the liquefaction yield of 25 of the 37 coals studied in this work14. Attempts to relate these liquefaction yields to OH content met with moderate success. Using OH as percentage of total organic matter as an independent variable, the following regression was generated:

c t

Conversion (%) = 1.8 OH (%) + 0.29 (total reactives) + 2.1 Storat + 35.1 Here % conversion

98

FUEL,

refers to the liquefaction

1978, Vol 57, February

conversion

with

6

AAC

c

c

c c

AA

t C

A

I

1

80 82 8L Carbon content fwtV.dmmf Figure 3

Phenolic contents of coals, as fractions

I

1

86

86

90

I of total oxygen

Phenolic hydroxyl

contents of coals: Z. Abdel-Baset, P. H. Given and R. F. Yarzab

REFERENCES

APPENDIX Duplicate hydroxyl determinations studied

for 8 of the samples

PSOC Sample No.

OasOH (wt % of organic matter)

213 230 239 249 273 280 284 288

5.3,5.2 6.5, 6.4 4.5,4.5 4.2,4.2 5.5,5.5 5.1,5.2 4.1.4.1 5.9, 5.8

1

2

4 6

8 9 10 11 12 13 14

Vaughan, G. A. and Swithenbank, J. J. Analyst 1970, 95, 890 Friedman, S., Kaufman, M. L., Steinger, W. A. and Wender. 1. Fuel 1961,40,33 Blom, L., Edelhausen, L. and van Krevelen, 1). W. Fuel 1959, 38,537 Given, P. H., Peover, M. E. and Wyss. W. F. Fuel 1960, 39, 323 Brown, I. K. J. Chem. Sot. 1955, p 752 Given, P. H. and Yarzab, R. F., ‘Problems and Solutions in the Use of Coal Analyses’, Tech. Report 1 from Coal Research Section, Pennsylvania State University to Energy Research and Development Administration, Rep. FE-0390-1, 1975 Given, P. H. Fuel 1976,55, 256 Mott, R. A. and Spooner, C. E. Fuel 1940, 19, 226 Blom, L., Edelhausen, L. and van Krevelen, D. W. Fuel 1957, 36, 135 Hill, L. W. and Given, P. H. Carbon 1969, 7, 649 Radmacher, W. and Mohrhauer, P. Gllickauf 1953. 89,503 Bishop, M. and Ward, D. L. Fuel 1958, 37, 191 Given, P. H., Peover, M. E. and Wyss, W. F. Fuel 1965,44,425 Abdel-Baset, M., Yarzab, R. F. and Given, P. H. Fuel 1978,57, 89

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