ricvieW OF NEEDS FOR

THERMOPHYSICAL PROPERTY DATA ON

SOLID FEEDSTOCKS. I. Coal

Jane E. Callanan

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

May 1982

f-QC'

100

. U56

82-1666

x 1982

NBSIR 82-1666

iyiOHil W);*lk*C M RiKDOM

JUL 2 1 1982 r\<A COLC . <2.€ REVIEW OF NEEDS FOR

THERMOPHYSICAL PROPERTY DATA ON

SOLID FEEDSTOCKS. I. Coal

Jane E. Callanan

Thermophysical Properties Division

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

May 1982

Final Report

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
* • *•

*c. . uv
CONTENTS

Page

1. INTRODUCTION 1

2. THERMAL PROPERTIES 2 2.1 Heat of Combustion 3 2.2 Heat Capacity 3 2.3 Thermal Conductivity 5 2.4 Heat of Wetting 6 2.5 Thermal Expansion 7 2.6 Thermal Diffusivity 7

3. COAL 8 3.1 Nature of Coal 8 3.1.1 Formation 8 3.1.2 Macerals 8 3.1.3 Types 8 3.1.4 Rank 10 3.1.5 Minerals 10 3.2 Experimental Concerns 11 3.2.1 Samples 11 3.2.2 Sample Characterizations 12 3.2.3 Reference Materials 13 3.2.4 Sample Cells and Related Problems 13 3.2.5 Sample Handling 14 3.2.6 Heterogeneity 15 3.2.7 Porosity 15 3.2.8 Weathering 15 3.2.9 Plasticity 16 3.2.10 Mineral Content ... 16 3.2.11 Subdivision 16 3.3 Possible Experimental Investigations 17 3.4 Recommendations 19 3.4.1 General 19 3.4.2 Equipment 21

iii

4.

REFERENCES 21
LIST OF TABLES

Page

1. Summary of consultations - coal 2

2. Specific heat (Cp) of Pittsburgh seam coal and cokes, J kg

-

^ K~* 5

3. Thermal conductivity (x) of Pittsburgh seam coal and cokes, W m

_1 K~1 6

4. Coal lithotypes and descriptions 9

5. Coal macerals and naceral groups recognized by the

International Committee for Coal Petrography 9

6. Classification of coals by rank 10

7. Minerals frequently associated with U. S. coals 12

8. Required characterizations for coal samples 14

v
SUMMARY

In January 1981, a comprehensive survey was undertaken to determine the need for thermophysical

properties of the following solid feedstocks/fuels: coal, oil shale, tar sands, gas hydrates. This

report deals with that portion of the survey which concerns coal and includes the results of broad

consultation with industrial, government, and academic groups as indicated in table 1.

This survey shows the need for experimental work on heat of combustion, heat capacity/enthalpy,

thermal conductivity, and heat of wetting for both wel 1 -characterized premium coal samples and for

samples of the type which will be used directly in conversion processes. Widely accepted, standardized measurement techniques do not exist for these properties, with the exception of heat of combustion, and must be developed; in addition, reliable data must be generated for efficient use of coal as

a feedstock. Theoretical studies which will allow for modeling of properties should proceed along

with the experimental investigations to allow for improvement in prediction of coal properties for

process design.

The following recommendations for coal -related properties research are substantiated in this

report.

1. Develop reliable measurement techniques for heat capacity, thermal conductivity, and heat of

wetting. Existing measurement methods are adequate for thermal expansion and heat of combustion.

2 .

3.

4.

5.

6 .

Evaluate the effects of coal handling on coal properties.

Develop a standard reference material for thermal properties of coal.

Conduct an interl aboratory testing program to evaluate the measurement techniques developed.

Deposit all data in a nationally accessible central data bank.

Develop theoretical models for both equilibrium and transport properties with possible extension to structural properties such as porosity.

v
'
Review of Needs for Thermophysical Property Data on Solid Feedstocks. I. Coal

Jane E. Callanan

Thermophysical Properties Division

National Bureau of Standards

Boulder, Colorado

This report, which presents the results of a comprehensive survey with respect to thermal properties of coal, is part of a broader study which also includes oil shale, tar sands,

and gas hydrates. This survey shows the need for experimental work on heat of combustion,

heat capacity/enthal py , thermal conductivity, and heat of wetting for both wel 1 -characterized premium coal samples and for samples of the type which will be used directly in conversion processes. Widely accepted, standardized measurement techniques do not exist for three

of these properties (heat of combustion excepted) and must be developed; in addition, reliable data must be generated for efficient use of coal as a feedstock. Theoretical studies

which will allow for modeling of properties should proceed along with the experimental

investigations to allow for improvement in ability to predict coal properties for process

design. Recommendations for work appropriate to the National Bureau of Standards and the

justification for these recommendations are included.

Key words: coal; enthalpy; heat of wetting; heat capacity; literature survey; review; solid

feedstocks; thermal conductivity; thermal properties.

1. INTRODUCTION

In January 1981 a study was initiated to determine the thermophysical properties needs for solid

feedstocks. The substances initially selected for study were coal, oil shale, and gas hydrates. An examination of various data bases indicated that tar sands also should be included. It was necessary

first to study the nature of these solid fuels/feedstocks so that appropriate judgments could be made.

For coal and oil shale, this proved a formidable task. By comparison the gas hydrates are relatively

simple materials. Following an initial familiarization with coal, oil shale, and gas hydrates, a com- puter search of data bases, which included research-in-progress and publications, was conducted.

Perhaps the most fruitful part of the study has involved personal contacts with scientists

involved in fossil-fuel research. The organizations and scientists consulted by telephone, by letter,

and by personal visit are indicated in table 1; the ideas evolving from these discussions are reflected throughout this report. Additional useful discussions took place at the Coal Sample Bank

Workshop sponsored by DOE/GRI (Atlanta, Georgia, March 1981), American Chemical Society meetings, and

the Gordon Conference on Fuel Science , (Plymouth, New Hampshire, July 1981).

This report deals with coal. By mid-1982 a second report addressing the nature of oil shale, the

status of existing thermal property data for shale, the needs of the users' community, and recommendations for experimental work on oil shale will be submitted. Another report dealing with gas hydrates

is scheduled for fall, 1982. Investigations and recommendations regarding tar sands will follow in 1983.

1
Table 1. Summary of consultations - coal a

Contacts

1 Organi zation

Allied Chemical Corporation

Amoco Research Center

Bartlesville Energy Technology Center

Colorado School of Mines

Exxon Research Engineering:

Corporate Research, Linden, NJ Research Development, Baytown, TX Exxon Production Research, Houston, TX

Gas Research Institute

Institute of Gas Technology

Kentucky Center for Energy Research

Massachusetts Institute of Technology

NASA, Huntsville

Oak Ridge National Laboratory

Pittsburgh Energy Technology Center

Pittsburgh Mining Technology Center

Southern Illinois University

U.S. Steel

University of Tennessee

University of West Virginia

G. Hammond, A.K.S. Murthy

O. Mahajan

B. E. Gammon, N.K. Smith

R. Baldwin, A. Kidnay, V. Yesavage

C. T. Ratcliffe, S.C. Mraw

R.C. Neavel

B. Blackwell, C.H. Hewitt

W. Staats, V. Hill

K. Vorres, R. Zabransky

J. Elder

J. Boston

J. Austin, J. Lowery

L. E. McNeese, E. Fuller, E.C. Hise,

B.R. Rodgers, J. de Van, J. Ficolari,

B. Benjamin, M. Poutsma

P. Rolniak

W. Spackman, P.H. Given,

P.L. Walker, Jr., P. Dolson

H. Retcofsky, E.H. Spencer

J. Murphy, R.D. Watson, C. Lazzara,

J. Edwards, A. Kim

C. Carrel 1 R.J. Gray, S.J. Todd, R. Schoenberger

,

R. Wagner, F. Huggins

J. Larsen

L. Nice

Pace Engineers

Coal Research Group, Pennsylvania State University

a This list is representative rather than comprehensive in identifying leading coal -research centers.

This report includes a discussion of the thermal properties of interest in fuel science, a short

primer on coal, and an elaboration of experimental concerns. Wide-ranging experimental investigations

are suggested. Finally recommendations are made that will serve as a basis for delineation of the

program of thermal property measurements and studies of coal within NBS.

2. THERMAL PROPERTIES

Knowledge of thermal properties of solids, though of interest in the past, has assumed new importance today. With the emphasis on synthetic fuel development and solid fuel/feedstock conversion pro- cesses, knowledge of these properties is more widely needed than heretofore. In addition, the high cost of energy and the emphasis on environmental concerns has made the utilization of such data imperati ve.

2
Thermal properties of most interest in fuel science include heat of combustion (calorific value),

heat capacity, thermal conductivity, thermal diffusivity, heat of wetting, and to a lesser extent,

thermal expansion. Such data obtained for petroleum and petroleum-related materials from the 1930's

through the 1960's led to knowledge which has been a great asset to chemical engineers involved in petroleum refining. It is expected that corresponding information for coal and coal -related materials

will be similarly useful. A discussion of the individual thermal properties follows.

2.1 Heat of Combustion

In fossil-fuel terminology, the heat of combustion is referred to as the calorific value. For many years, for those using coal as a fuel, it was the property of greatest interest. The calorific

value of coals is determined by standard ASTM procedures in a bomb calorimeter, either isothermally or adiabatical ly. Such calorimeters are commercially available and require no extraordi nary skill for

their operation.

However, more accurate calorimeters and calorimetric techniques do exist at NBS-Gaithersburg

,

Argonne National Laboratories, and Bartlesville Energy Technology Center. Their use might allow cor- relation of loss of calorific value with degree of oxidation; also, greater accuracy in the determination of the heat of combustion would allow better estimates for the heat of formation^ of coal since

the heat of formation is calculated from measured heats of combustion. This better estimate for the

heat of formation would improve existing modeling capabilities for coal processing. In addition, a study of heats of combustion of coal fragments in order to find additive or predictive relationships

may prove useful in improving coal charges. For this purpose or for the correlation of loss of calorific value with degree of oxidation, more accurate techniques, such as those mentioned above, might

need to be utilized.

As calorific value is the most important thermal characterization parameter for fuels, measurements of this parameter do exist for a very wide spectrum of well -characterized substances. However,

unlike the heat of combustion, measurements of other thermal properties of coal have been made only on isolated or poorly-characteri zed specimens. Thus little effort has been made to correlate thermal

properties with other characteristics.

2.2 Heat Capacity

Knowledge of the heat capacity of solid fuels/feedstocks is needed for efficient design of con- version and carbonization processes and for the modeling of these processes and of properties.

Because of the scarcity of reliable measurements and the demands the present-day economy makes with

respect to energy efficiency, there is a new impetus for a better and systematic program of experimental measurements.

^The heat of formation of coal is a concept that generates some controversy. Strictly speaking it can be calculated, but the complexity of coal has tremendous implications for the actual measurements

of the heat of combustion and for the corrections which must be applied. For this reason a physical

chemist may be skeptical about the numbers assigned to heats of formation of coal; from an engineering standpoint, however, even order of magnitude information is useful and the subtleties of definitions are irrelevant.

3
Heat-capacity and enthalpy measurements of coals have been reported by several investigators

[1-14]. The techniques used are based on the method of mixtures (e.g., ASTM C351) [5], the Bunsen ice

calorimeter [7,8], a variety of adiabatic calorimeters [10,11], and drop calorimetry [11]. The best

of these measurements are accurate to about 2%. Most are either mean heat capacities (i.e., measurements over wide temperature intervals), practical heat capacities (include some heat of reaction)

[13], or were made at isolated temperatures and do not give reliable information about the temperature

dependence of the heat capacity. Some of the investigators measured the heat capacity, at room tem- perature, of coals which had been heated to elevated temperatures and report these as hi gh-temperature

data [14]. Most of the investigations report increasing heat capacity with increasing temperature

until decomposition begins within the plastic region. Clendenin [6] reports continuously increasing

heat capacities, but his measurements were made to about 120°C and then extrapolated to higher temperatures. In general heat capacities increase with increasing temperature, increasing volatile matter

content, and increasing moisture content; they decrease with increasing carbonization. However, the

scatter in the data due to variations in factors such as cell design, heating rate, and pretreatment

of the specimens make it almost impossible to draw any unified conclusions. Badzioch [4] and Kirov

[9] thoroughly review the status of these measurements up to 1965. Table 2 presents the results

obtained by Singer and Tye [11] on a Pittsburgh seam coal; these data illustrate some of the difficulties mentioned above.

In all of the above studies, very little characterization information other than elemental composition is included; it is hoped that some further information may be available upon inquiry. Though

any attempt to sort out existing heat-capacity measurements seems counterproductive at this time,

attempts to correlate the isolated data presently available with the newly-generated data will be made

as the program develops.

The differential scanning calorimeter (DSC) is capable of accuracies of 1% and allows rapid

determination of heat capacities over a wide temperature range [15]. Because the DSC requires small

specimen sizes, careful attention must be given to the homogeneity of the solid fuels and replicate

results. This drawback is to be balanced against the recognized lack of specimen temperature uniformity that results from the use of larger specimen sizes. Several recent experimental studies based on the DSC verify its utility for such studies [15-20]. We have decided to use a commercial DSC in preliminary NBS experiments.

Differential thermal analysis (DTA) and, more recently, DSC studies have used thermograms to evaluate general endothermic and exothermic tendencies of coals in various temperature regions

[21-25]. Despite some controversy, evidence does suggest the existence of an exotherm, associated

with repolymerization, in plastic or coking coals. Slow heating through the plastic region is known

to reduce the degree of swelling significantly; therefore use of low heating rates may extend the

range of useful thermal measurements, though the interpretation of results in the plastic region will

remain difficult.

Agroskin [1] has published correlations of heat capacity with rank and with characteristics of

the aromatic portions of coals. A recent paper by Benson and Schobert [18] has used high-pressure DSC

thermograms to obtain some tentative correlations with several coal characteristics.

4
Table 2. Specific heat (Cp) of Pittsburgh seam coal and cokes 3

, J kg" 1 K" 1

Temperature Vi rqi n*

3 HT 350°Cc of measurement, °C Test 1 Test 2 Test 1 Test 2 HT 475°C HT 625°C HT 650°C HT 850°C

30 860 850 1 ,050 1 ,090 1 ,060 1 ,050 1 ,010 870

65 920 920 NA NA NA NA NA NA 100 1 ,220 990 1 ,300 1 ,340 1 ,190 1 ,150 NA NA 125 1 ,660 NA NA NA NA NA NA NA 150 1 ,580 1 ,060 NA 1 ,520 1 ,280 NA NA NA 180 1 ,440 NA NA NA NA NA NA NA 200 1 ,360 1 ,130 1 ,600 1 ,710 1 ,380 1 ,250 1 ,200 1 ,120

240 1 ,180 NA NA NA NA NA NA NA 250 1 ,190 1 ,210 NA 1 ,910 1 ,480 NA NA NA 300 NA NA 2,100 2,120 1 ,590 1 ,370 1 ,330 1 ,270

350 NA NA NA NA 1 ,700 NA NA NA 400 NA NA NA NA 1 ,830 1 ,500 1 ,460 1 ,430

450 NA NA NA NA 1 ,920 NA NA NA 500 NA NA NA NA NA 1 ,660 NA NA 600 NA NA NA NA NA 1 ,860 1 ,830 1 ,750

800 NA NA NA NA NA NA NA 2,220

NA Not available.

a After Singer and Tye [11]

^Virgin coal sample in test 1 was undried; sample 2 for test was predried at 110°C to constant

weight to remove water.

C HT = temperature of carbonization.

2.3 Thermal Conductivity

Information on thermal conductivities is needed for both solid and powdered fuel specimens. For processes which utilize monoliths, thermal conductivity data are required for heat flow both parallel

and perpendicular to the bedding plane. Obvious difficulties are presented in preparing solid specimens by the need for exactly planar surfaces and for specimens with no defects or cracks. Information

obtained on powdered specimens is in many ways more practical. Here, however, the great difference in the thermal conductivity of the gaseous and solid phases makes the measurements very sensitive to the

packing procedures used and the gases produced.

Recommended procedures for thermal conductivity include the guarded-hot-plate method (ASTM C177)

and the heat flow meter (ASTM C518). Two other methods developed for heterogeneous and/or refractory

developed for heterogeneous and/or refractory materials are promising. The method used by Wagner [26]

on insulation materials is a variant of a hot-wire method in which resistances are measured at widelyspaced points in the specimen; such a technique seems to take microscopic heterogeneity into account more than measurements made at a single point do. Dils [27] used a modified hot-wire method with

refractory materials; with auxiliary spectroscopic measurements, values for thermal conductivity that

are consistent with those measured by the guarded-hot-plate method are obtained.

Badzioch [4,28] summarizes and critically evaluates the results of thermal conductivity measurements up to 1964. His discussion includes coal and coke monoliths and powdered specimens. One of the

principal criticisms cited in his summary is that most of the measurements were made at room temperature on coals that had been heat-treated at elevated temperatures rather than made at the elevated

temperatures themselves. The general criticism of older experimental work, that it was done with

5
Table 3. Thermal conductivity (a) of Pittsburgh seam coal and cokes 3

, W m"l K"1

Temperature Vi rqi n HT 350°C HT 475°C HT 500°C HT 625°C HT 650°C HT 850°C

of measure- 1 | | Test 1 Test 2

|| X || 1 | | 1 II ment, °C _L 1

50 0.196 0. 214 0.14 0.20 0.16 0 .185 0.125 0.137 0.21 0.29 0.31 0.74 0.76

150 .192 . 213 .17 .22 .17 .195 NA .154 NA .35 .36 NA NA 200 NA NA NA NA NA NA .18 NA .30 NA NA NA NA 250 .190 . 212 .19 .24 NA NA NA .172 NA .40 .41 .92 .95 300 NA NA NA NA .185 .21 NA NA NA NA NA NA NA 350 NA NA .21 .25 .185 .215 NA .186 NA .45 .46 1 .05 1 .09 400 NA NA NA NA .14 .16 .26 NA l .42 NA NA NA NA 475 NA NA NA NA NA NA .28 NA NA NA NA NA NA 50 (cooling) NA NA NA NA NA NA .12 NA NA NA NA NA NA 500 NA NA NA NA NA NA .29 NA NA NA NA NA NA &20 NA NA NA NA NA NA NA .215 NA .52 .52 1.19 1.24

50 (cooling) NA NA NA NA NA NA .11 NA NA NA NA NA NA 600 NA NA NA NA NA NA .45 NA .54 NA NA NA NA 500 NA NA NA NA NA NA .38 NA NA NA NA NA NA

50 (cooling) NA NA NA NA NA NA .17 NA NA NA NA NA NA 650 NA NA NA NA NA NA .54 NA NA NA NA 1 .38 1 .45 775 NA NA NA NA NA NA 1.10 NA NA NA NA NA NA 800 NA NA NA NA NA NA NA NA 1.25 NA NA 1 .60 1 .74 50 (final

cool ing) .176 . 195 .14 .19 .10 .11 .28 .14 .37 .29 .305 .75 .76

3 Singer and Tye [11].

NA Not available.

N0TE.--HT = temperature of carbonization.

_J_ = measured perpendicular to the coal bedding plane.

| = measured parallel to the coal bedding plane.

poorly-characteri zed coals, holds for the work discussed in his summary. Additional published data on the thermal conductivity of American coals include those of Clendenin [6] and Singer and Tye [11].

The recently obtained data of Singer and Tye show similar trends to those cited in Badzioch's summary

for heat-treated coals, but decreasing conductivity with increasing temperature for virgin coal.

Table 3 indicates the temperatures and temperature ranges of heat-treatment and measurements as well

as the results.

In general, thermal conductivities of powdered coal specimens are lower than those for monoliths

though some disagreement exists as to what would be expected given the nature of the devolatilized

powder. The effect on thermal conductivity of factors such as moisture content, pore shape and orientation, direction of heat flow, volatile matter and mineral content, and petrographic composition also

need evaluation. The reliability of thermal conductivity measurements of coals must be established

and such measurements applied to different coals.

2.4 Heat of Wetting

The heat of wetting of a solid is the heat generated as the dry solid is immersed in a liquid.

It is determined from the temperature rise accompanying immersion of a thoroughly evacuated, weighed,

test specimen in a known volume of a suitable liquid. If there is no chemical interaction, the effect

is one of physical adsorption and is proportional to the surface area wetted. A suitable wetting

liquid has small molecules with good wetting properties and minimum volatility at room temperature.

6
Apparatus for measuring heats of wetting vary from simply constructed systems, such as described

by Fuller [29], to sophisticated commercially available cal orimeters.

In the past, heat of wetting measurements, particularly those using methanol, have been used to evaluate the open porosity and surface areas of coal. Because of interactions between the OH groups

in methanol and the oxygen functional group on coal surfaces, such measurements have often been seriously in error. As a result of the development of other techniques for surface area measurements,

heats of wetting are used less often for such determinations. A recent study [30] of an alumina and a silica-alumina used thermometric titration procedures to determine surface area and heats of adsorption with considerable success.

Liquefaction processes and the proposed use of slurries have generated a pressing need for reliable information on heats of wetting, in water, in hydrocarbon liquids, and in mixtures of hydrocarbon

liquids. For conversion processes, heats of interaction with liquids are needed both upstream and

downstream, i.e., for solid feedstocks going into the process and for the separation of the solids at the conclusion of the process. To say that the development of suitable measurement techniques is a challenge is perhaps an understatement; nevertheless the problems of sample homogeneity, moisture content, surface area, and evaluation of interactions must be addressed in an attempt to reduce the

experimental error of these measurements from an extreme of a 50% level to one that will allow for

reasonable use of the data.

2.5 Thermal Expansion

There is no strong expressed need for thermal expansion data other than that available through

free swelling index (ASTM 0720), the Gray-King assay, or dilatometric measurements. There is some interest in the possible contribution of the micropore system to anisotropy in thermal expansion.

2.6 Thermal Diffusivity

The heat transport property of most interest for solid fuels is the thermal diffusivity (a).

Though methods are available for direct measurement, this quantity is often calculated by means of the

relationship

X

where X is the thermal conductivity, Cp, the heat capacity, and p, the bulk density. For a solid

sample, thermal diffusivity is measured directly by imposing a heat pulse on the front side of a disk

through the relationship

a = 0.139 L^/to.g,

where L is the thickness of a disk and to .

5

is the time required for the rear surface temperature to rise to half its maximum value.

7
3. COAL

Some readers of this report will be familiar with the nature of coal and its complexities and how

they bear on the problems and challenges confronting the experimentalist; others will not. Section

3.1, dealing with the nature of coal, is included for the latter group of readers. Sections 3. 2-3.

4

deal with experimental difficulties, research opportunities, and recommendations for work by the NBS.

3.1

Nature of Coal

Coal is essentially a sedimentary rock which accumulates as peat. It is composed of macerals and

minerals and contains water and gases in submicroscopic pores.

3.1.1 Formation

Macerals are the organic components of coal and arise from the partial decay of plant material in the swamp or forest which generated the coal deposit. Decaying vegetation is covered by mineral

matter brought in and deposited by wind and/or water. These strata alternate in an irregular fashion

to build up the coal seam. The sedimentary strata are subjected to natural geologic processes which

alter them physically and chemically.

The product of these processes has been defined by Neavel [31] as a heterogeneous aggregate of

microscopically distinguishable, physically distinctive, and chemically different macerals and minerals. He has compared coal to a fruitcake formed initially of a mixture of diverse ingredients and baked to a usable consistency.

3.1.2 Macerals

The macerals in coal can be traced to the specific plant tissues from which they arose and are characterized by their appearance, chemical composition, and optical properties. As the macerals

develop from varied tissues such as wood, spores, cuticle, and resins, they manifest the heterogeneity

of plant tissue. Thus, portions of a coal seam separated by microns will have vastly different compositions and characteristics. This heterogeneity presents tremendous challenges to the experimentalist

in obtaining representati ve samples and repeatable measurements and data.

3.1.3 Types

The terminology for the classification of macerals is not uniform. In general, lithotypes refer

to macroscopic classification and petrographic groupings to microscopic classification. The scheme

suggested by Stopes, as outlined in Mackowsky [32], for lithotypes is shown in table 4.

The microscopic examination of coal, either in thin section by transmitted light or of polished

faces by reflected light, allows for the petrographic classification of coal. Table 5 lists coal macerals and groups as recognized by the International Committee for Coal Petrography. The suitability of coals for various processes is related to their maceral composition. For example, vitriniteand exinite-rich coals are useful for processes such as liquefaction or coking; an inertinite-rich

coal, which is largely carbon, has no utility for such processes.

8
Table 4. Coal lithotypes and descriptions 3

Lithotype Descri ption

Vitrai

n

brilliant; glossy; uniform whole. >

Cl arai

n

smooth surface if broken at angles to Bright Coals

bedding plane; banded surface luster. J

Durai

n

close firm texture; granular; broken

face has fine lumps or a mat rather than a smooth one.surface

Fusai

n

powdery patches; detachable fibrous

strands.

Dull Coals

Cannel Coal dull black; preponderance of fungal

spores and pollen.

Boghead Coal dull brown; large concentrations of

algal and plankton exinite. J

a After Mackowsky [32].

Table 5. Coal maceral s and maceral groups

International Committee for Coal

recognized by

Petrography 3 the

Maceral Group Maceral Composed of or derived from

Vitrinite Col 1 i nite Humic gels; colloidal matter

Tel 1 inite Wood, bark, cortical tissue

Vitrodetriniteb

Exi nite Spori nite Fungal and other spores

Cut inite Leaf cuticles

Resinite Resins and waxes

A1 ginite Algal remains

Li ptodetri nite b

Inerti nite Micrinite < 10 ym Macrinite 10 - 100 ym Unspecified detritus

Semi f us inite 'i Fusinite J

Carboni zed woody tissues

Sclerotenite

Inertodetri ni te b

a After Berkowitz [33],

b These terms refer to small entities assigned to the maceral groups on the basis of reflectivity,

but not able to be unequivocally identified with any particular maceral within the group.
3.1.4 Rank

The rank of coal is related to the extent of coal i f ication or carbonization which has taken place

as a result of the geologic processes it has undergone. The classification is made first on carbon

content; within some ranges of carbon content, a further classification is made on the amount of volatile matter present. For coals with a carbon content less than 69%, a further division is made on the

basis of the heat of combustion, or calorific value, of the coal. Table 6 illustrates the classification of coals according to rank. The classical H/C versus 0/C diagram used by van Krevelen [35] is a representation of the classification of coals according to rank.

3.1.5 Minerals

The mineral content of coals, considered to be the sum of all elements that are not part of the

organic coal substance, varies from less than 5% to 35%, though it is generally less than 20% for

American coals. This is true whether one considers a narrow definition of a mineral as given here,

the broader concept of non-coal inorganic material [36], or all inorganic constituents [37]. The

importance of mineral content to coal science and technology comes from the ability of some mineral

components to affect processing to a remarkable degree. The conversion or pyrolysis reactions may be

catalyzed, thus allowing for treatment and conversion under less rigorous conditions; alternatively,

the mineral component may poison an added catalyst or adversely affect materials used in the processing equipment. In addition, mineral residues present potential waste disposal problems and environmental hazards.

Table 6. Classification of coals by rank 3

Class / Group

Fixed carbon

limits (%)

[dry, mineral - matter-free

basis)

Volatile matter Calorific value

limits (%) limits, (Btu/lb)

(dry, mineral- (moist mineral

matter-free matter-free

basis) basis)

agglomerating

> < > < > < character

I. Anthracitic

1. Meta-anthracite 98 — — 2 — --

1 non2. Anthracite 92 98 2 8 — agglomerati ng

3. Semi anthracite 86 92 8 14 — - J

II. Bituminous

1. Low volatile bituminous 78 86 14 22 — - "I 2. Medium volatile bituminous 69 78 22 31 — “ 1 commonly

3. High volatile A bi tumi nous -- 69 31 — 14,000 > agglomerating

4. High volatile B bitumi nous — — — — 13,000 14,000

5. High volatile C bituminous — — — — 11,500 13,000 J

10,500 11,500 agglomerating

III. Subbituminous

1. Subbituminous A -- -- — — 10,500 11,500

^

2. Subbituminous B -- — — — 9,500 10,500

3. Subbituminous C — -- -- -- 8,300 9.500 nonf agglomerating

IV. Li gnitic

1. Lignite A — — — — 6,300 8,300

2. Lignite B -- -- -- — -- 6,300 >

a Table adapted from Montgomery [34].

10
The spectrum of minerals present in coals varies with the geologic history of the seam and exhibits the same heterogeneity as do the macerals. The principal minerals found in United States coals

are listed in table 7. One of the sources of confusion in interpreting and comparing literature data

on coals is the manner in which mineral matter is, or is not, accounted for. The effect of the pres- ence of the minerals on the measurements themselves must also be considered.

3.2 Experimental Concerns

3.2.1 Samples

A major barrier in a coal research plan is the identification, selection, and availability of

wel 1 -character!

- zed coal samples. The most extensive coal sample bank in existence is that at Pennsylvania State University. However, its coals were not mined or processed under conditions that

would prevent oxidation or deterioration of the coals. Only the more recently obtained samples have

been stored in an inert atmosphere. Given the potentially serious but relatively unquantified effects

of oxidation on properties and the uncertain degree of oxidation of these coals, their use is less

than desirable for our purpose.

Samples from the bank^ containing vitrinite-rich coal described by Neavel et al . [38] are closer

to premium, undegraded, samples than any others available in the U.S. today. Though no program of

monitoring sample quality presently exists for this bank, such a program is under consideration and

should be initiated in the near future. Three principal drawbacks exist, however, with the use of

these samples. The first is that the bank contains only vitrinite-rich samples, which are those most

useful for synthetic fuel development. A complete study should include a wider spectrum of coals, some of which are of great importance to other industrial users. Secondly, a minimum of twenty specimens must be run; this is beyond the number necessary for the development of measurement methods.

Most important, however, is the question of how much information about the samples, other than the

usual characterization information, will be in the public domain.

The proposed GRI/DOE Coal Sample Bank will contain a wide spectrum of coals considered to be premium samples and the information obtained will be available to all. However, the existence of the

Bank is still under study; even if approved, the characterized samples will not be available for, at best, close to two years.

Another alternative would be to obtain samples through the cooperation of USGS or the Pittsburgh

Technology Center. Desirable sampling techniques would have to be assured and proper preparation and

characteri zations obtained. While this procedure might be feasible for one or two coals, any broader

use of it would mean effectively establishing and maintaining ones own Coal Sample Bank. This is an unacceptable solution because of the tremendous financial outlay that would be required for equipment

and qualified personnel; also, it would represent an attempt to duplicate, unnecessarily, existing

excellent characteri zation facilities.

^Coal Library, Baytown Research and Development Division, Exxon Research and Engineering Company,

Baytown, Texas.

11
Table 7. Minerals frequently associated with U.S. coals 3

Silica Minerals:

Chlorite:

Clay mineral s:

Feldspar group:

Sul fates

:

Sul fides

Carbonates

:

Quartz*3 (trigonal). Si O2

(Mg, Al, Fe)i 2 C(Si, A1 ) 8

0 20 ](0H) 16

Kaolinite*3 group, Al4[Si 40 iq](OH)q

I llite, b K1_1.5Al4CSi7_5.5Al i_i. 5°20](°h )4

Montmoril Ionite*3 group,

(l/2Ca, Na) 0 . 7 (Al, Mg, Fe) 4 [(Si, Al

)

8

0 20 ](0H) 4 .nH 2 0

Alkali feldspars, (K, Na ) [Al Si 8 08 ] PI agiocl ase , Na[ Al Si 8

08 ]-Ca[Al 2

Si 2 0

8 ]

Gypsum, CaS04 .2H20

Anhydrite, CaS04 Hemihydrate, CaS04 .l/2H20 (bassanite)

Barytes, BaS04 Hydrated Iron sulfate, FeS04 *nH20

Jarosite (Na, K, Fe) sulfates

Pyrite*3 (cubic), FeS2

Marcasite (orthorhombic), FeS2

Ankerite, Ca(Mg, Fe2+

S Mn)(C0 8 )2

Calcite (trigonal), CaC0 8 Magnesite, MgC0 8 Rhodochrosite , MnC0 8

Si derite, FeC0 8 Dolomite, CaMg(C0 8 )2

Mixed carbonates of Ca, Mg, Mn and Fe

a Jenkins and Walker [36].

b Most commonly occurring species.

Currently the best approach seems to be to obtain well-preserved samples of wel 1 -characterized

coals which come from industrially important seams and are in sufficient supply to be commercially

useful for many years. The selection of coals suitable for various processes for the experimental

program will help to ensure the general applicability of the methods developed.

3.2.2 Sample Characterizations

Much of the difficulty in utilizing existing data on coals stems from the fact that measurements

have been made on poorly-characteri zed specimens. While each coal scientist may wish for knowledge of very specific properties, it has been generally agreed that the properties listed in table 8 should be

supplied with samples used in coal research. The accepted procedures for determining many of these

properties are those specific by ASTM. However, more modern instrumental methods give equivalent or

12
improved results for some of these properties though their use is not yet standardi zed. Slow but

perceptible progress is being made with respect to the substitution of new methods.

Of the properties listed in table 8, the calorific value of coal, or more precisely, the heat of

combustion, is the only thermal property included. As indicated earlier the heat qf combustion is measured in standardized commercial equipment based on ASTM test methods.

3.2.3 Reference Materials

Another problem which must be addressed is that of a standard reference material suitable for use

in calibrating apparatus used to make thermal property measurements on coals and related materials.

Either graphites or chars are most often used; both are carbonaceous materials. Graphite has a distinctive structure and as such may not be as closely related to the carbon structure of coal as is char. Also graphites of different origin have different heat capacities. While coal does show a lamellar arrangement of aromatic moieties and the connecting aliphatic elements are the first to be expelled, the carbon backbone of the char should be more similar in structure to that of coal than is

graphite, and thus be more suitable for comparison.

3.2.4 Sample Cells and Related Problems

The choice of sample cells and sample cell materials for use with the DSC presents many problems.

The aluminum cells commercially available will not remain sealed at the pressures that we anticipate

may develop at elevated temperatures. The reusable stainless steel cells modeled after those used at Bartlesville Energy Technology Center and those presently available commercially have the advantage of

being able to withstand relatively high pressures and are machined in such a way as to make good thermal contact with the sample cavity in the instrument. However, they also suffer from several disadvantages. Their weight is large compared to the weight of the specimen; stainless steel has low thermal diffusivity; and the sulfur in coals reacts with the steel and leads to subsequent generation of

hydrogen sulfide even in empty cells [40]. The last of these difficulties can be overcome by goldplating the cells. The significance of the other disadvantages is being evaluated.

A third cell design has been considered, but cells have not been fabricated as yet. These would

be disposable aluminum cells made of sufficient thickness to withstand the anticipated higher pres- sures and would be sealed by cold welding. Preliminary calculations have been done on the thickness of

aluminum which would be needed to prevent distortion of the shape due to the build-up of pressure; the

integrity of the seal must be evaluated experimentally.

Rowlinson [41] discusses measurement of heat capacity when a liquid is in equilibrium with its

vapor and differentiates between Cp, C

v , and C a

(C sa ^ or the heat capacity of a liquid maintained at

all temperatures in equilibrium with its vapor). Similar considerations should be applied for solids

if the solid being studied has a vapor pressure^ in the range of those of organic liquids, which is

^Reference to a vapor pressure for coal does not imply the existence of an equilibrium vapor pressure

from which precise thermodynamic quantities can be obtained. However, the non-negl igible vapor

pressure, equilibrium or not, in the sample container does require the evaluation of vaporization

effects

13
Table 8. Recommended characterizations for coal samples 3

Proximate analysis

Ultimate analysis

Calorific value

Equilibrium moisture

Sulfur forms analysis

Maceral analysis

Vitrinite reflectance analysis

Mineralogical analysis

Direct determination of mineral content

Analysis of major elements in ash

Trace element analysis

Surface area Density (helium and mercury)

Free swelling index

Carbon dioxide content

Soluble alkali and calcium (lower rank coals)

Calcium/barium exchange

Total oxygen (neutron activation)

Gieseler fluidity

Geologic and geographic information on sampling site

Full history of sample handling since collection

Proceedings of Coal Sample Bank Workshop [39].

presumed to be true for coal in certain temperature regions. For this reason, the effects of specimen

volume to cell volume on the measured heat capacities must be evaluated.

Mahajan and coworkers found no significant temperature gradients within their DSC specimens [20].

A similar evaluation should be made for the stainless steel cells because of the relatively large

volumes (45 microliters) and the poor thermal diffusivity of the cell materials and of the samples.

3.2.5 Sample Handling

The handling of the coal samples should be carried out as far as is possible in an inert atmo- sphere to prevent weathering. Through the use of a controlled-atmosphere glove box, sample preparation can be done under nitrogen. With some samples, though, the weathering process will have started

because of mining and transportation procedures.

The drying of the coals is another experimental procedure that presents problems which may vary

to some extent with the coal and its degree of subdivision. The as-received coal is pulverized under

water to prevent local heat effects from altering its properties. Water will be removed by prolonged

evacuation at room temperature; if this is not sufficient, as preliminary work seems to indicate, the

coal will have to be heated during evacuation until it attains a constant weight. Several

14
investigators follow such procedures and feel that they do not alter the coal structure significantly

by this treatment [42,43], Others maintain that both the grinding and drying processes alter the pore

structure to a significant extent [44], As indicated in sections 2.4 and 3.2.7, more experimental

confirmation is needed.

3.2.6

Heterogeneity

The heterogeneity of coals presents an experimental problem which is considered by some to be insurmountable. Experienced experimentalists have estimated the degree of subdivision required, given

certain total sample size and individual specimen-size limits, to assure that a sample is representative. As the DSC and TGA require very small specimens, it is necessary to evaluate the reproducibility of the measurements and determine the number of replicates needed to establish a reliable average

value rather than to estimate the accuracy of individual measurements. The existence of this extreme

heterogeneity in coals supports the selection of the DSC for heat capacity measurements and will be an important consideration in the development of other measuring techniques. It would seem more appropriate to have a simple, fairly rapid, technique that will allow for many replicate measurements so that an average may be obtained rather than an inherently more accurate technique that is so time-consuming that replicate measurements are not feasible.

It should be noted that many of the newer methods used in coal characterization, particularly

those applied in minerological analysis, require specimen sizes as small or smaller than those used by

the DSC. Thus the general use of specimens of small size and fine subdivision is becoming more common.

3.2.7 Porosity

The porosity of coal has widespread consequences with respect to properties, coal preparation and

storage, and processing. The pore structure of coals contains both micropores (<2 nm) and macropores

(>30 nm) in addition to pores of intermediate size [42]. The total pore volume can be determined

readily from moisture content or helium and mercury densities and porosimetry. Surface areas have

been determined historically from heat of wetting and gas adsorption measurements. More important

than the pore volume and pore surface area, however, is the size distribution of the pores; these are determined most frequently through mercury porosimetry and nitrogen adsorption isotherms [43]. A recent study by Mraw and Naas-0' Rourke has used heat-capacity measurements at subambient temperatures

to evaluate pore size and pore-size distribution [45,46]. The whole spectrum of questions concerning

the surface properties and porosity of coals and chars is an active field of research today. Further

study is needed in this area, particularly with reference to the applicability of thermal methods to the solution of these problems [43,47,48,49].

3.2.8 Weatheri ng

The term weathering can be used to describe the many changes, physical and chemical, which occur

in coals as a result of exposure to natural elements such as air, wind, rain, snow, and freezing.

However, it is used here to refer to the oxidation of coal by natural processes. As weathering causes

rapid deterioration of the heating value and suitability for processing, it has serious industrial

consequences. The chemical species generated in the oxidation of coals, as a result of exposure to

15
air, may differ from those generated by deliberate addition of specific chemical agents. In both

instances, oxidation proceeds from the surface, with surface being understood to include microfissures

and accessible pores. As reactivity increases with decreasing rank, the problems associated with

weathering are more severe for lower-rank coals. Indeed the combined effects of heat of wetting and

weathering can readily induce spontaneous combustion, as manifested by the occurrence of mine fires.

3.2.9 Plasticity

Plasticity refers to the tendency of some coals, usually bituminous coals with moderate amounts

of volatile material, to go through a softening process, become fluid, and, after expulsion of some volatile material, to resolidify. This phenomenon is essential to coke manufacture and and is beneficial also in some conversion processes; in others it is detrimental. Swelling, which may T>e measured

with a dilatometer, and fluidity, measured most frequently with a Gieseler plastometer, are the pertinent properties most readily measured. Rank, degree of subdivision, thermal treatment, degree of oxidation, maceral content, and porosity all affect the range of the plastic region and the degree of

plasticity. Thus by blending and by varied treatments, one can do much to obtain a coal with the

desired plastic properties.

Many mechanisms have been proposed to explain plasticity; a critical consideration of them is not relevant to this report. A sufficient and acceptable explanation is that physical softening or melting occurs shortly before the plastic region is reached. Within the plastic region, an initial

depolymerization results in smaller, more readily mobile, molecular fragments; these leave the main

structure of the coal and, in most instances, eventually volatilize. Following the removal of these

fragments, repolymerization and resolidification occur.

3.2.10 Mineral Content

As the specific mineral matter present in coal has such a dramatic effect on its behavior, careful attention must be given to its nature and presence. While an evaluation of the effects of mineral

content in any experimental study of coal is worthwhile, it may be particularly important in the study

of thermophysical properties because of its recognized effects (e.g., the presence of a small amount

of pyrite greatly increases the heat capacity). Careful attention must be given to the possible

effects of mineral composition on the thermophysical property measurements. It would be useful to evaluate its effect by complete removal of mineral matter and the subsequent addition of known species

and amounts.

3.2.11 Subdivision

While finely-divided specimens may be suitable for research use, they are not always realistic

for application to industrial processes. At some point in the development of an adequate measuring

program attention should be given to comparisons between results on specimens of different degrees of

subdivision. While it is true that some equilibrium thermal properties, e.g., heat capacity, should

not be size-dependent, it is possible that the actual numerical results obtained will show size-dependence, as an artifact of the measuring technique. If this is so, measurements on various-sized speci- mens will be required in order to make adequate allowance for the phenomenon. Other properties, e.g.,

16
thermal conductivity, are expected to be affected by specimen size and subdivision. Equipment that

will allow for measurements on appropriate specimens may need to be developed.

3.3 Possible Experimental Investigations

Acceptable measurement techniques for coals to determine thermal conductivity, heat capacity and

heat of wetting must be selected.

In the course of this investigation several possible studies that would be of interest have presented themselves. The carrying out of even a portion of these would require many investigators, a long period of time, and experimental facilities that are not available presently at NBS. These studies indicate the variety of unanswered questions relating to the thermophysical properties of coal.

Heat capacity relationships of interest:

Heat capacity as a function of temperature for a diverse group of coals, to as high a temperature for each coal as is practical given variations in plastic properties.

Heat capacity as a function of rank.

Heat capacity as a function of type or maceral composition. In particular, look for additive

or predictive relationships that can be used to obtain properties of blends.

Heat capacity as a function of subdivision. This investigation would require some use of an adiabatic calorimeter since the size of the sample cells in the DSC limits the size of particle that can be used. Also, the poor thermal conductivity of larger particles requires long

equilibrium times; the scanning mode would not be suitable for such particles.

Heat capacity as a function of the ratio of organic to mineral matter.

Heat capacity in the presence or absence of specific minerals. For such a study, minerals

could be added to demineralized coals.

Heat capacity as a function of the atmosphere surrounding the specimen. Atmospheres of interest include nitrogen, oxygen, air, pyrolysis and combustion products, and air and nitrogen

with varying degrees of moisture.

Heat capacity as a function of degree of weathering. Such information is needed for combustion models and for industrial users who must work with weathered coals.

Heat capacity as a function of oxygen content and/or functionality.

Heat capacity of coal fragments formed by solvent swelling and emerging fractionation techniques.

17
Heat capacity of representative model compounds to provide the basis for modeling of thermal

behavi or.

Heat capacity of chars; of ash.

Variations of heat capacity relationships mentioned as a function of thermal history.

Thermal conductivity studies of interest:

Thermal conductivity as a function of temperature for virgin coals; for heat-treated coals,

for chars, and ash.

Thermal conductivity as a function of pressure, particularly realistic mine and processing

pressures.

Thermal conductivity as a function of direction of heat flow with respect to the bedding

plane.

Thermal conductivity as a function of degree of subdivision.

Thermal conductivity of powdered, packed samples.

Techniques for high-temperature studies (to 1200°C) for poorly conducting materials; apply

such techniques to carbonaceous materials; to insulating materials.

The variation of thermal conductivity as a function of many of the variables listed in the

section on heat capacity such as weathering, maceral and mineral content, and atmosphere. The

modeling of thermal conductivity is as necessary and desirable as is the modeling of heat

capacity.

Heat of wetting studies:

Heat of wetting of coal as a function of subdivision.

Heat of wetting as a function of solvent (water, hydrocarbon liquids, mixtures of such

liquids, typical coal processing fluids).

The variation of thermal properties with the variables mentioned has not been measured for the

most part; thus no judgment can be made as to significant differences in such properties, e.g., maceral s are all organic materials, though the mode of incorporation of mineral matter often differs. Mea- surement may show that the differences in thermal properties of the macerals are not significant or do not vary in a regular way. However, such evidence must be obtained, not guessed at.

As noted previously, the studies indicated here would require many staff -years of work. In the

formulation of the recommendations which follow, the charter of the Bureau of Standards to provide

18
support in satisfying the most pressing needs of industry, external consultations, and NBS staff

expertise were considered.

3.4 Recommendati ons

3.4.1 General

Indications of research needs in thermophysical properties of solid feedstocks and the implied

generic rationale for their performance by the Thermophysical Properties Division are interspersed

throughout this report. However, the recommendations are summarized here and the rationale restated

for the convenience of the reader and for emphasis.

There was a consensus among those consulted that a strong need exists for thermophysical property

data for coal. The needs identified in this report were considered urgent by some, but are only now being recognized by others.

Knowledge of these properties is needed to reduce energy consumption and its associated expense;

to minimize capital expenditure for processing and transport equipment; to tailor coal feedstocks for

various processes; to minimize safety and environmental hazards; and most importantly, to permit

intelligent and operative design of synthetic fuel processes and to facilitate use of solid feedstocks

for direct synthesis of industrially important chemicals. For the determination of the heat balance

in conversion processes, these properties must be known for the substances going into the process as well as for those in the product stream.

An in-depth knowledge of the thermophysical properties of coals is important to the detailed

understanding of normal combustion processes as well as solid-liquid conversion and gasification

processes. Knowledge of these properties will aid in formulating acceptable correlations between

process factors and properties and characteri sties of coals. Measurements are needed for heat capacities, thermal conductivities, and enthalpies for raw coals, chars, and ash in various temperature

regions for above-ground and in situ processes. There is also a need to know these same properties as

a function of a) realistic mine pressures and b) the higher pressures encountered in processing (up to 20 MPa (3000 psi )

) . In addition, heats of interactions with various liquids are needed.

It is recommended that satisfactory procedures be developed for the measurement of the heat

capacity of coals throughout as much of the temperature range, 0 to 700°C, as is possible. Methods

for chars and mineral matter in this temperature range should be standardized. As the temperature

range for useful measurements on coals may be extended significantly by slow heating, an attempt

should be made to evaluate thermophysical properties as far as possible into the plastic region.

It is desirable to make thermophysical property measurements on coals in which weathering has

been curtailed as far as possible. At first glance, this may seem impractical from the standpoint of

industrial users. However, it is important for such users, who work with coals in various stages of

weathering, to be able to relate the properties measured to the degree of weathering and thus account,

and compensate, for changes in properties due to weathering as processing occurs. For .this reason, it

is recommended that the first study undertaken, subsequent to the development of measurement techniques, be the variation of thermophysical properties as a function of degree of weathering.

19
It is desirable to be able to predict the thermophysical properties of blends of coals from the

properties of the individual components. Because of the importance of maceral content to suitability

for processing, it would be useful to be able to predict the thermophysical properties of blends from

the thermophysical properties of the component macerals. As maceral enrichment and separation techniques are presently available [50,51], a second recommended study is that of the variation of the

thermophysical properties mentioned above as a function of maceral type and composition.

Given the strongly expressed consensus that heat of wetting measurements are urgently needed for

coal slurries, for liquefaction processes, and for the control of spontaneous combustion, and in light

of the existing uncertainties regarding methods for such measurements, it seems advisable to undertake

methods research on heats of wetting. Such measurements are within the domain of the Thermophysical

Properties Division.

A strong case can be made for wide-ranging investigations of a reasonable number of commercially

important coals rather than a more academic study of great numbers and varieties of coals, though the

latter could lead to some interesting and worthwhile correlations. From the standpoint of the industrial user the selection of coals for study should be practical, i.e., the coals studied must be those

which are suitable for processing, either alone or as a component of blends, and should exist in sufficient quantity to be commercially useful for many years. Consequently it is recommended that a few

coals important to coking, combustion, liquefaction, and gasification processes be sought.

That the development of satisfactory procedures for sampling and sample handling for solid fuels

is integral to the development of satisfactory measurement procedures is a truism. Many years of coal research have generated recommended procedures; as the experimental program defining measurement pro- cedures progresses, the necessity and/or effects of coal sample handling procedures should be evaluated systematically so that the final methods adopted will be sufficient for their purpose, but not precise beyond reason. More directly, sampling and sample handling techniques must be a component of our experimental program. It is recommended that such research be undertaken in cooperation with a)

groups from NBS having appropriate expertise, and b) research programs recently instituted by groups

such as the Gas Research Institute.

When measurement procedures are satisfactory, the development of standard reference materials for

thermophysical properties will be feasible and should be undertaken. Following this, an interlaboratory testing program should be set up and, from the results, necessary refinements of measurement

techniques made.

Banks of data coming into existence on wel 1 -characteri zed coals contain few thermophysical property data. The attempts to correlate coal properties with suitability for industrial processing have

not led to generally satisfactory results. Since many of these processes depend on heat transfer, the

addition of heat capacity, thermal conductivity, and heat of wetting data to these banks will enhance

the possibility of improving such correlations. As the experimental work progresses, efforts should

be made to develop correlations among properties and general (non-proprietary) process requirements.

The probability of success in deriving correlations increases as the available data increase. It is advisable to strengthen consultation and cooperation with as broad a group of coal scientists as possible and to establish a reputation for sound experimental work in solid feedstocks/fuels with a view

to encouraging the release of as much non-proprietary data as possible. The general charter and

20
reputation of the National Bureau of Standards should be used to facilitate, direct, and perhaps to carry out, the compilation of these data.

As the compilation of characteri zation and physical data proceeds, attempts should be made to develop models or predictive relationships. Such relationships do exist for heats of combustion;

Kirov [9] and Gomez [8] have developed predictive relationships for specific heat and/or enthalpy

based on proximate analysis and temperature, respectively. Of these Kirov's is more generally applicable, but admittedly is used in lieu of reliable data or a better model (Aspen program). Continued

attempts to derive predictive relationships should be made as the matrix of reliable characteri zation

and physical property data expands. The possibility of relating thermal properties to the structure

of coals and to structural changes is intriguing and should be pursued.

Thermophysical properties of fuels may also be modeled from similar properties of selected com- pounds. A recent attempt by Benson and Schobert [18] to reproduce thermograms of coal with one or two

compounds shows remarkable promise. Research of this sort, which represents an alternative attempt at modeling, should be conducted in conjunction with the measurement of the properties of the coals as soon as the measurement techniques are determined to be reliable.

3.4.2 Equipment

Instrumental choices for the initiation of the experimental program have been made with consideration of the nature of the materials to be tested, available instrumentation, and the needs of potential users in mind.

For heat capacity, the differential scanning calorimeter (DSC) has been selected because of its

speed and simplicity of operation along with an adequate accuracy (better than 2% with pure materials

over wide temperature ranges). The rapidity with which scans can be made will allow for several

replicate measurements in the course of a determination; this is desirable given the variety of coals

and their inhomogeneity. The need is for a simple, rapid, readi ly-automated method in contrast to methods inherently more accurate, but difficult, time-consuming, and expensive to carry out.

Numerous thermal conductivity techniques are described in the literature. The most suitable ones

are those that involve specimens with a small thickness to cross-sectional area ratio due to the relatively low conductivity of coal specimens. The hot-wire technique and the guarded-hot-plate technique

appear to be most suitable, but other longitudinal or radial heat-flow methods can be applied with success. The hot-wire technique is tentatively selected for initial investigations. Measurement

uncertainties of 10% are probably acceptable for general characteri zation ; for the establishment of

standards related to solid feedstocks an uncertainty of no more than 2% is recommended.

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21
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[15] Mraw, S. C. and Naas, D. F. , "The Measurement of Accurate Heat Capacities by Differential Scanning Calorimetry. Comparison of d.s.c. Results on Pyrite (100 to 800 K) with Literature Values

from Precision Adiabatic Cal orimetry ," J. Chem. Thermodynamics 11 , 567-584, 1979.

[16] Mraw, S. C. and Naas-0' Rourke , D. F. , "Thermodynamic Properties of Cyclohexane-d]^: Low-Temperature Heat Capacity and Enthalpies of Transition and Fusion From Accurate Differential Scanning

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22
[17] Mraw, S. C. and O'Rourke, D. F., "Accuracy of Differential Scanning Calorimetry for Heat Capacities of Organic Compounds at High Temperatures. New Method for Enthalpy of Fusion by d.s.c.,"

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Coals", paper presented at Lignite Conference, San Antonio, Texas, June 1981.

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294-309, 1954.

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23
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24
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25
NBS-114A irev. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 82-1666

2. Performing Organ. Report No. 3. Publ ication Date May 1982

4. TITLE AND SUBTITLE

Review of Needs for Thermophysical Property

Data on Solid Feedstocks. I. Coal

5. AUTHOR(S)

Jane E. Callanan

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s) 7. Contract/Grant No. national bureau of standards

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

10. SUPPLEMENTARY NOTES

j Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

This report, which presents the results of a comprehensive survey with respect

to thermal properties of coal, is part of a broader study which also includes oil

shale, tar sands, and gas hydrates. This survey shows the need for experimental work

on heat of combustion, heat capacity/enthalpy, thermal conductivity, and heat of

wetting for both wel 1 -character!' zed premium coal samples and for samples of the type

which will be used directly in conversion processes. Widely accepted, standardized

measurement techniques do not exist for three of these properties (heat of combustion

excepted) and must be developed; in addition, reliable data must be generated for

efficient use of coal as a feedstock. Theoretical studies which will allow for

modeling of properties should proceed along with the experimental investigations to

allow for improvement in ability to predict coal properties for process design.

Recommendations for work appropriate to the National Bureau of Standards and the

justification for these recommendations are included.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Coal; enthalpy; heat of wetting; heat capacity; literature survey; review; solid

feedstocks; thermal conductivity; thermal properties.

13. AVAILABILITY

[~lf] Unlimited For Official Distribution. Do Not Release to NTIS

~] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

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