A111DE h T b 3 E

INDS

PUBLICATIONS

NBSIR 87-3554

Self Heating Properties of Coal

Emil Braun

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Gaithersburg, MD 20899

August 1987

Sponsored in part by:

U. S. Department of the Interior

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87-3554

1987

NBSIR 87-3554

Research Information Center

Aational Bureau of Standards

Gaithersburg, Maryland 20899

SELF HEATING PROPERTIES OF COAL

Emil Braun

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Gaithersburg, MD 20899

August 1987

Sponsored in part by:

U. S. Department of the Interior

Bureau of Mines

Pittsburgh, PA

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director
_

-
CONTENTS

Page

LIST OF TABLES iv

LIST OF FIGURES v

Abstract 1

1. INTRODUCTION 3 2. SELF-HEATING FACTORS 6 2.1 Oxidation 6 2 . 2 Moisture .• 8 2.3 Trace Species 10

2.4 Geological and Mining Conditions 11

3. SYSTEMS FOR PREDICTING SELF-HEATING 11

3.1 RASCAL System 12

3.2 German System 14

3.3 Canadian System 15

4. MATERIAL EVALUATION 16

4.1 Sample Preparation 17

4.2 Adiabatic Furnace 18

4.2.1 Test Method 18

4.2.2 Test Results 20

4.3 Crossing Point Method 24

4.3.1 Test Method 25

4.3.2 Test Results 26

5. DISCUSSION 27

5.1 Mining Technique 28

5.2 Geological Characterization 29

5.3 Coal Characterization 29

5.4 Coal Evaluation 30

5.4.1 Adiabatic Furnace 30

5.4.2 Crossing Point Method 32

6. CONCLUSIONS 34

7. RECOMMENDATIONS 35

8. GLOSSARY 37

9. ACKNOWLEDMENTS 39

10 . REFERENCES 40

APPENDIX A. Summary of RASCAL System A-l

APPENDIX B. Translation of German Recommended Practices B-l

APPENDIX C. Summary of Canadian System C-l

APPENDIX D. Adiabatic Furnace Data, Individual Graphs D-l

iii
LIST OF TABLES

Page

Table 1 Heat of Wetting of Dried Coal from Experiments by

Porter and Ralston [7] 43

Table 2 Description of Bituminous Coal Samples Used in

Adiabatic and Crossing Point Method Furnace Tests.... 44

Table 3 Summary of Adiabatic Furnace Data from Eight Western

U.S. Coals 45

Table 4 Calculated Critical Heating Rates for Eight U.S.

Western Coals at Four Different Strata Temperatures

Based on Equation (4) 46

Table 5 Summary of Relative Ignition Temperature Data for

the Crossing Point Method 47
LIST OF FIGURES

Page

Figure 1 Planning Guide taken from German Evaluation System. . . 48

Figure 2 Temperature -Time curves for eight DOE coal samples

taken from western U.S. coal fields tested in the

Adiabatic Furnace 49

Figure 3 Semi-Log plot of the rate of temperature change

versus 1/T (K“ 1

) for eight samples of U.S. western

coals tested in dry air 50

Figure 4 Schematic drawing of the apparatus used in the

Crossing Point Method (CPM) 51

Figure 5 CPM test results for coal sample "A" heated at

0.59°C/min. Ignition occurred at 135°C 52

Figure 6 CPM test results for coal sample "C" heated at

0.58°C/min. Ignition occurred at 132°C 53

Figure 7 CPM test results for coal sample "B" heated at

0.53°C/min. Ignition occurred at 153°C 54

Figure 8 CPM test results for coal sample "E" heated at

0.56°C/min. Ignition occurred at 154°C 55

Figure 9 CPM test results for coal sample "D" heated’ at

0.60°C/min. Ignition occurred at 133°C 56

Figure 10 CPM test results for coal sample "H" heated at

0.57°C/min. Ignition occurred at 138°C 57

Figure 11 CPM test results for coal sample "F" heated at

0.58°C/min. Ignition occurred at 139°C 58

Figure 12 CPM test results for coal sample "G" heated at

0.61°C/min. Ignition occurred at 133°C 59

Figure 13 Plot of the CPM data for all eight coal samples

showing the relationship between the furnace/sample

temperature difference versus the furnace air

temperature 60

v
*
Self Heating Properties of Coal

Emil Braun

Abstract

Three methodologies for predicting and controlling the likely occurrence of

self -heating in underground coal mining environments were analyzed. No method

was found to be completely satisfactory for general coal mine applications.

One evaluation system was found to provide excellent guidelines for

preplanning procedures prior to initiating full scale coal mine operations

but relied heavily on past mining experience and ventilation control. Another

evaluation system used standard coal characterization parameters and

geological conditions to predict the likelihood of self -heating , while the

third system used a thermal test method and geological conditions as a predictor of self -heating potential. The self -heating properties of eight

samples of western bituminous coal from four seams were determined by two

different test methods - the Adiabatic Furnace and the Crossing Point Method.

The data from the Adiabatic Furnace indicated that for most of the coals

tested two reaction zones existed. The first temperature region extended from

25 °C to about 85° C with activation energies varying from 32 kJ/mole to 55

kJ/mole, while the region above 85 °C exhibited an activation energy ranging

from 59 kJ/mole to 82 kJ/mole. Using the lower temperature region lumped preexponential and activation energy for each coal, critical heating rates are calculated for different strata temperatures. Based on those coals exhibiting

self-heating in the field, critical heating rates in excess of 1.5°C/24 hrs at

a strata temperature of 20° C can be expected to result in self - ignition of the

coal. The Crossing Point Method yielded ignition temperatures ranging from

132°C to 154°C at an average heating rate of 0.58°C/min. An alternative

1
plotting technique is described that suitably correlates Crossing Point Method

data with end use experience. A brief review of pertinent literature is also

presented to augment existing reviews and provide a clarification of the

current understanding of those factors affecting oxidative heating of coal.

Keywords: Adiabatic furnace; coal; fire test methods; ignition temperature;

self -heating ; spontaneous combustion.

2
1. INTRODUCTION

The expansion of the United States coal mining industry into the western

continental states poses technological and environmental problems that will

have to be overcome to ensure the maximum extraction of coal under the safest

conditions. The lack of experience in working the thick seams prevalent in

these areas has raised questions regarding the self-heating characteristics of

the coal. The mining of eastern United States coal seams has been ongoing for

many years. Mining experience with these coal seams has defined the selfheating problem. Operators aware of these problems have attempted to take

precautions to lower the probability of an ignition. Similar past experience

is not available for directing the course of western coal mining.

At the request of the Department of Energy's (DOE) Office of Coal Mining, the

Center for Fire Research at the National Bureau of Standards conducted a review to locate probable methodologies for predicting the likelihood of coal

mining operations encountering self-heating problems in the course of coal

extraction. Relevant methods were identified by reviewing the literature on self-heating of coal and by correspondence with other nations expected to have

encountered similar problems. As part of this review, several coal samples

were obtained for analysis and some attention was directed to more recent

pertinent literature. The objective of this work was to identify and evaluate

currently used test procedures to determine coal susceptibility to spontaneous

combustion.

3
Mining authorities were identified in most western European countries and

Japan. Requests for information on methods they employ to predict selfheating potential of coal resulted in responses from seven countries

Countrv Method

Canada Under Development (CPM)

England Past Experience (Research)

France Past Experience

Germany Design Specification (Ventilation Control)

Italy No Method (Research)

Japan Past Experience

Spain No Method

These responses ranged from methods based on trial-and-error to methods based

on material testing and classification. While England and Italy were

continuing to conduct research in the understanding of self-heating in coal,

their approach depends largely on the past experience of local mine operators.

The Germans had the most elaborate system primarily based on past experience.

Their system relies heavily on prescribing ventilation control strategies.

The French and Japanese also rely on past mining experience, but they have not

developed a formal system of evaluation. The Canadians have developed an

evaluation system based on material testing, but appear not to have

implemented it throughout the country.

Based on this information and a review of the literature, four approaches to

predicting the likelihood of self-heating occurrences have been identified.

These are :

1) The German System;

2) The Canadian System;

4
3) RASCAL System (this had previously been developed for DOE)

4) Adiabatic Furnace.

The evaluation procedures cited in this report attempt to predict the

likelihood of self -heating occurrences based on a combination of geological,

technological, and material parameters. The Canadian Liability Index [1] uses

self - ignition temperature to characterize the coal sample, while RASCAL [2]

and the German [3] procedures employ coal rank or type as the material

characterization parameter. No evaluation scheme currently employs the data

obtained from the adiabatic furnace as fundamental input for characterizing a

coal mine's susceptibility to self -heating . However, work in the past [4,5,6]

has shown that this method coupled with additional physical parameters holds

some promise for a future evaluation system.

To assist in the evaluation of the Canadian system and the potential

usefulness of the adiabatic furnace to form the bases for a new evaluation

system, DOE provided NBS with eight coal samples taken from western coal

fields. The self-heating characteristics of these samples were determined

using these two test methods. The long term stability of these samples under

moderately elevated temperatures was determined using an adiabatic furnace.

The self - ignition temperature of these coal samples was measured by a ramped

heating method known as the Crossing Point Method (CPM) . This test method

forms an integral part of the Canadian evaluation system.

During the past half-century, several extensive reviews of self -heating or spontaneous combustion research have been conducted. [2,7-11] It is not the

intent here to repeat this procedure but to draw together some relevant

5
information as it relates to the goal of predicting spontaneous combustion

hazards. This report will first describe those factors currently identified

as influencing a coal's susceptibility to self -heat. This will be followed by

a review of the three established evaluation systems; the results of testing

coal according to the CPM and adiabatic furnace method; and a discussion of

the test results.

2. SELF -HEATING FACTORS

Research into the self-heating or spontaneous combustion of coal has been

directed primarily towards developing an understanding of the oxidative

mechanism controlling the low temperature combustion of coal. Kim [12] has

stated that, generally, spontaneous combustion is a rank related phenomenon.

As coal rank decreases, the hazard of self -heating increases. Therefore,

factors other than "simple” oxidation must account for this rank dependence.

While rank is a measure of coal fuel content, several physical properties that

effect self-heating propensity also follow coal rank. For example, higher

ranked coals rarely contain pyritic compounds or are soft which would lead to

increased granulation. Researchers have long recognized that in addition to

an oxidative mechanism, one must also understand how coal impurities (e.g.

pyrites), moisture, and ambient temperature effect the rate of coal oxidation.

2.1 Oxidation

The primary mechanism of self -heating is due to coal interaction with oxygen

and the subsequent inability of the coal to dissipate the heat generated in

6
this reaction. This area has been reviewed by several authors. [7,13-18] In

1957, Coward [7] conducted an extensive review of research data on the

spontaneous combustion of coal in the mining environment. He concluded that

spontaneous combustion of coal can occur at normal mining temperatures due to

the coal's slow oxidation by air. More recently, Yamasaki [19] conducted a study into coal oxidation that did not dramatically alter earlier conclusions.

He found that the interaction of oxygen with coal was a surface phenomenon.

The rate of oxygen consumption increased with increasing surface area per unit

weight of coal and increasing temperature. As pointed out by Guney [14], in a mining environment, this reaction mechanism would continue to accelerate until

ignition conditions were achieved unless adequate ventilation was provided to

dissipate the excess thermal energy.

Carpenter and Giddings [20,21] divided the oxidation of coal into three

stages. The various stages represent the ease with which coal can react with

diffusional oxygen. The first stage controls the rate of oxygen consumption

on the external and macropore surfaces, while stages two and three are controlled by the accessibility of oxidation sites in the micropore structure

of coal. With the exception of one coal sample, they found that the

activation energies for each stage were essentially constant, 67 kJ/mole,

throughout the temperature range investigated - 75 °C to 115° C. From mining

considerations, these three stages are indistinguishable.

Sevenster [22,23], beginning at a temperature of 30°C, found that coal

oxidation could be described by three temperature regimes. The calculated

activation energies differed for each regime. The low temperature regime,

7
30°C to 50°C, and the high temperature regime, 70°C to 90°C, had higher

activation energies than the middle 50°C to 70°C temperature range. In

general, the activation energies for the high and low temperature regimes were

similar, 14 to 40 kJ/mole and 19 to 42 kJ/mole, respectively. The activation

energy for the middle regime was 8 to 19 kJ/mole. The value of the high

temperature activation energy was probably due to chemical' reactions of

combustion, while the low temperature regime was controlled by oxygen

adsorption. The middle regime was controlled by transport phenomena as described by Carpenter and Giddings. [20]

In analyzing research data on the kinetics of low temperature coal oxidation,

Resnik and Chekhovskoi [24] found that two temperature regimes existed, each

of which could be fitted by an Arrhenius equation. These yielded activation

energies of 21 kJ/mole at 20°C to 70°C and 126 kJ/mole at 70°C to 90°C. These

predicted values are somewhat higher than previously cited.

Because a critical heat balance exists during each stage of reaction, the

existence of a reaction zone and its characteristic activation energy depend

on the test apparatus. Differences in equipment design, sample size, sample

granularity, and exposure time may explain the discrepancies cited above.

2.2 Moisture

The effects of atmospheric moisture on the self-heating tendency of coal have

been demonstrated to be rank- dependent . While not directly addressing this

8
question, Coward [7] does cite some data that show that the heat of wetting of

dry coal is a decreasing function of increasing coal rank (table 1)

Hodges and Acheijer [25] and Bhattacharyya et al [26] showed that the presence

of moisture in the atmosphere can accelerate the heat generation rate and,

therefore, decrease the time to ignition by self -heating. Giiney [27],- using

an adiabatic furnace, came to similar conclusions. These researchers found

that when water vapor pressure in the air was greater than at the coal

surface, moisture is adsorbed on the coal surface producing a heating effect.

This self -heating increases the rate of oxidation leading to ignition. A

cooling effect due to evaporation was induced if the water atmospheric vapor

pressure was lower than the vapor pressure of the water in the coal.

More recently, work by Bhattacharyya [28] indicated that, for the coals

tested, the rate determining factor was the adsorption of water vapor and that

the rate of heat release during adsorption of water vapor was related to the

hygroscopicity of coal. Mahajan and Walker [29] experimentally measured the

heat of adsorption of water on coal and arrived at a value of 44.4 kJ/mole.

This was comparable to the activation energy of dry coal, previously cited, in

the temperature range of 50° C to 70° C and 70° C to 90° C. Their work also

showed that the extent of adsorption depended on coal rank.

Panaseiko [30] investigated the ability of moisture to accelerate or retard

the oxidation of coal. He found that the amount of oxygen adsorbed by coal

decreased with increases in the amount of adsorbed water. More importantly,

he found that the rate of oxidation had a maximum value over a narrow range of

9
coal moisture content. The critical moisture content increased with

decreasing coal rank.

Bacharach et al [2] , in reviewing spontaneous combustion problems for American

coal mines, asserted that the major environmental difference between the

American and British mining conditions were the low ventilation humidities

present in the western American coal fields. Therefore, coals mined in

Britain that pose serious self -heating problems may be safely mined in the

United States, although they may present certain difficulties in handling and

storage once outside the mine environment.

2.3 Trace Species

The effects of trace species to induce or retard self-heating in coal has been

studied for many years. Coward [7] presented data that showed that the

presence of pyrites, while not necessary to induce self -heating , greatly

increase the likelihood of developing self-heating conditions. Two

mechanisms have been proposed to account for increased self-heating in coals

containing pyrites. First, in the presence of moisture, pyrite oxidation

increases the coal temperature which in turn increases the rate of coal

oxidation. [2] Alternatively, the pyrite may swell and cause the coal to

disintegrate increasing the rate of oxidation by increasing coal surface area

[12] Research has also shown that other trace species can effect selfheating propensity. Alkalies are known to act as accelerators, while borates

and calcium chloride have exhibited retarding actions. [2]

10
2.4 Geological and Mining Conditions

Those geological features of a coal seam that increase the surface area

exposed to oxygen will increase the coal's susceptibility to self -heating

Therefore, highly pitched seams, very thick seams, and seams with weak

disturbed strata, faults or shallow overburden are the most vulnerable to

self-heating. [5]

Mining techniques that increase oxygen accessibility to coal beds can also

lead to a greater probability for self -heating. Also, the specific method of

coal extraction can enhance unwanted air leakage into a gob or coal bed.

Therefore, retreating long wall mining is preferable to advancing long wall

mining because the air leakage paths do not occur across gob areas. [2,5]

Ventilation conditions must be carefully controlled to minimize air leakage

under all mining conditions. [3]

3. SYSTEMS FOR PREDICTING SELF-HEATING POTENTIAL

Traditionally, self-heating susceptibility of a coal field has been determined

by experience. In newly developing fields this experience is lacking and coal

extraction proceeds cautiously until sufficient data have been collected

characterizing the coal field. This is a somewhat subjective process that

could be proved wrong with changes in the geological conditions within the

coal field as well as changes in the mining techniques . With a more

structured understanding of the self-heating characteristics of a coal seam,

11
extraction rates might be improved without unwittingly increasing the risk of

self-heating occurrences.

Since many variables enter into a determination of self-heating susceptibility

of a coal seam, several attempts have been made to develop systems that

integrate the material, environmental, geological, and technological elements

into an index of self -heating propensity. Changes in any parameter that can

effect the probability of self-heating should be reflected by corresponding

changes in the index. Using this index as a guide, optimum mining techniques,

ventilation rates, detection systems, etc. can be implemented. Three such

systems have been located and are reviewed. Two systems, the RASCAL System

and the German System, consider coal rank as the determining material

characteristic and place greater emphasis on mining and geological conditions.

Only the Canadian System relies on a test method to measure coal self-heating

propensity.

3.1 RASCAL System

The system known as RASCAL, Rapid Appraisal of Spontaneous Combustion Assessed

Liability, was previously described and recommended for use in western United

States coal fields by Bacharach et al. [2] It is summarized in Appendix A.

RASCAL combines six factors that include coal characteristics and mining

environment into an overall risk factor that is a measure of the potential

risk of self-heating. The overall risk factor is divided into three

categories: low risk (less than 10); medium risk (10-20); and high risk

(greater than 20)

Of the six factors considered, three define the nature of

12
the coal, one defines the geological conditions,' and two define the mining

conditions

Coal rank, as measured by oxygen content, the presence of pyrites, and

hardness, which determines the extent of gob granulation, are measures of the

oxidative potential of a coal bed. This system places the greatest emphasis

on coal rank. This factor alone can place a coal bed in the medium risk

category. Based on these three factors alone, a sub -bituminous coal seam,

having a high oxygen content, will always have no better than a medium risk

for developing self-heating problems.

Strata temperature is used by RASCAL to classify geological conditions. The

hardness of the coal, roof pressure causing falls, faulting, and, as previously explained, pyrite concentration are among some of the factors that

determine the surface area available for oxidation. As the rate of oxidation

increases, thermal energy must be dissipated in order to prevent critical

self -heating . Since the rate of oxidation and, thereby the rate of heat

generation, is temperature dependent, elevated strata temperatures can produce

an oxidation rate sufficiently high to prevent adequate cooling of the coal

bed. According to RASCAL, strata temperatures in excess of 21° C can begin to

have an accelerating effect on coal oxidation rates.

RASCAL describes mining conditions by two factors. The first is concerned

with the transfer of moisture from the ventilating air stream to the coal bed.

As the moisture content of the ventilating air stream increases the amount of

heat liberated in to the coal bed during condensation of water can be

13
sufficient to increase the oxidation rate above a critical value causing selfheating. Ventilating air stream moisture content in excess of 60% RH are considered to be significantly hazardous and will increase the susceptibility

of a coal bed to spontaneous combustion. Unmined roof coal is the second

factor used by RASCAL to describe mining conditions. The thicker the roof

coal the more likely are occurrences of self -heating

3.2 German System

Appendix B is a translation of a German Manual for Work Safety for the

Prevention of Spontaneous Combustion Fires. [3] The document contains three

sections plus an appendix. Section one is a general review of those factors

affecting self -heating of coal. Sections two and three describe measures to

be taken during the planning and operational phases of coal mining. In

addition, the appendix contains a planning aid for the evaluation of the

potential for self -heating . In general, this document relies on practical

experience rather than a quantitative assessment of hazard to control decision

making

The planning aid is divided into two categories (figure 1) . The first

includes those parameters that cannot be controlled by the mining operation

(i.e., geological, tectonic, petrographic) that effect self-heating potential

of coal. The second are those parameters that are designed into the mining

operation. They include fire protection measures and ventilation techniques.

The weighting factors used in the example at the end of Appendix B

sequentially increase down the form. However, since the weighting factors for

14
each parameter are arbitrary, the document recommends that each site develop

its own set of factors. This can only be accomplished through experience.

This appears to be an iterative process where initial designs are implemented

and, as mining conditions change, the design guidelines for a particular site

are modified. The general utility of this approach as a predictive tool is,

therefore, severely limited. It is interesting to note that the parameters

cited in this document are considered critical by other evaluation systems and

by researchers in the field of self -heating of coal.

3 . 3 Canadian System

The Canadian system [1] for evaluating the self -heating potential of a coal

seam is composed of a Liability Index for coal and a Mine Environment Index.

The product of these two indices yields a Risk Index for a given mine . A

summary of this system can be found in Appendix C. Like the RASCAL system,

the Risk Index places a mine into one of three risk categories; low (0-10),

medium (10-20), high (20-40). The Liability Index relies on data from an ignitability test method that measures the relative ignition temperature of

coal. This method is commonly called the Crossing Point Method (CPM) for

determining coal ignitability [31] and it will be described in more detail

later in this report.

The Mine Environment Index depends on a qualitative evaluation of the mining

environment. This index considers coal loss, fissuration, and ventilation

pressure differential as the variables controlling the potential for selfheating of a given coal seam. The Mine Environment Index ranges from 1 to 4.

15
The lower the index the less likely is an occurrence of self -heating. Worse

case conditions are achieved when the three controlling variables are considered high. A low index indicates that these three variables are within

normal limits and that the likelihood of self-heating is thereby controlled by

the characteristics of the coal.

The CPM is used to determine the self-heating characteristics of a coal sample

by determining the ignition temperature under ramped heating conditions. The

relative ignition temperature is defined as the temperature at which the

center temperature of a coal sample exceeds or crosses the external ambient

temperature of the air in the furnace. The Liability Index is then defined as

the ratio of average heating rate, °C/min, between 110°C to 220°C to the

relative ignition temperature, °C. This ratio is arbitrarily multiplied by

1000. This index is used to classify a coal sample as to its self -heating

potential. An index of 0 to 5 indicates a low probability for self -heating

while indices of 5 to 10 and greater than 10 represent medium and high,

respectively, probabilities for self -heating . Eight coal samples were tested

according to this method and the results will be discussed later in this

report

4. MATERIAL EVALUATION

The self-heating tendencies of eight coal samples taken from four western

United States coal fields were determined using an adiabatic furnace and a

rate of temperature rise apparatus, CPM. The adiabatic furnace provides a means for determining basic thermochemical parameters, while the CPM

16
represents, in at least- one evaluation system, a critical element in assessing

self -heating potential.

For any given coal sample, the most important factors determining

susceptibility to self-heating are the thermophysics and oxidation kinetics.

The adiabatic furnace represents a method by which heat transfer from the

sample can be reduced, thereby allowing for a clearer study of only the heat

generating process within the sample itself. Environmental and technological

factors would have to be combined with the results of the adiabatic furnace

before a determination could be made of the likelihood of encountering selfheating problems in a given coal field. With regard to the CPM test, the

Canadians have already attempted to construct an evaluation system.

4.1 Sample Preparation

The eight coal samples evaluated in this report are described in table 2. The

source of the samples was established by DOE. They monitored the collection

and shipment of the samples to NBS . The samples were drawn from eight

different mining sites representing four different coal seams. Samples "A"

and "C", from seam number B-2, represent the only samples taken from a mine

with a known occurrence of self -heating induced fire. The fire was discovered

in a sealed gob area.

Samples were prepared for testing by grinding and sieving in a glove box

initially purged with nitrogen. Coal samples were separated into three ranges

of grain sizes: less than 150 ^m; between 150 fj, m and 180 n ; greater than 180

17
/zm. Samples* with grain sizes between 150 /z and 180 /zm were used for testing.

This ensured that test were conducted on granular material with little if any

possibility for the presence of voids within the test sample. -The interior of

the glove box was cleaned between samples to minimize intersample

contamination

4.2 Adiabatic Furnace

4.2.1 Test Method

The adiabatic furnace used in this work and its basic operation have

previously been described. [4,32] In general, an adiabatic apparatus is

designed to minimize heat losses from a sample. This is accomplished by

maintaining a sample and its environment at the same temperature.

Operationally, once a sample has been brought to an elevated equilibrium

temperature with its surroundings, the apparatus control system increases or decreases the temperature of the surrounding air and the sample surface in

response to the thermal behavior of the center of the sample. Temperature

time data are recorded and analyzed to determine kinetic information about the

test sample.

The analysis of self -heating data [4,33] has generally been done by assuming

that the rate of chemical heat release follows the chemical reaction law of

Arrhenius

Q = AHVpA exp (-E/RT) (1)

18
This is a first order law, where;

A = pre- exponential (min-2

),

E = Activation energy (kJ/mole),

T = Sample temperature (K)

AH= Heat of combustion (kJ/kg),

V = Volume of sample (m3

)

,

p = Density of sample (kg/m3

)

R = Gas constant (8.314 X 10“ 3 kJ/mole/K).

A heat balance on a coal sample yields

qs = Q ’ 4l (2)

where the rate of energy storage in the sample, qs (kW) , is the rate of heat

generation within the sample, Q (kW) minus the rate of heat loss from the

surface of the sample, qL (kW) . The rate of energy storage in a sample is

given by

„ dT / o v qs “ Vpc — (3)

where

c = heat capacity (kJ/kg/K)

During an adiabatic process, negligible heat is lost by the sample.

Therefore, setting qL =0, combining equations (1), (2) and (3), and rearranging

terms yields a relationship that describes the rate of temperature increase in

the sample as a function of the heat generation rate.

19
( 4 ) dT = AAH -E/RT

dt c

6

Since the adiabatic furnace is designed to minimize heat losses, equation (4)

represents a suitable method for analyzing the data. The coals were tested in

the adiabatic furnace after being ground and sieved in a dry nitrogen

atmosphere, as previously described. A 76 mm high by 76 mm diameter cylinder

made from a number 80 stainless steel mesh was filled with coal and placed in

the furnace. Dry air flowed through the sample for the entire test. The

effects of moisture on ignition delay time was not investigated in this study

because it has been reasonably well documented in the studies previously

cited.

4.2.2 Test Results

Figure 2 is the temperature- time data for the eight western United States coal

samples collected by DOE. Initial furnace temperatures were between 50° C and

70° C. Samples "B" , "C", and "D" all began at 50° C, while "H" had a starting

temperature of 70° C. The remaining samples were tested beginning at

approximately 60° C. Qualitatively, it can be seen from figure 2 that samples

"B"

"E" , and "D" had the longest time delay to thermal runaway (i.e.,

spontaneous combustion) . Since two coal samples were drawn from each seam,

comparative observations are possible. For example, comparing samples "D" and

"H" demonstrates, as previously observed [25,28,30], that initial sample

temperature can have a dramatic effect on the time delay to thermal runaway.

Sample "D" was tested at 50° C, while sample "H" was tested at 70° C. The 20°

20
difference in starting temperature translated into a 3.5 fold increase in

delay time (40 hrs to 140 hrs)

By taking logarithms of both sides of equation (4) , it is apparent that

plotting the temperature- time data in figure 2 as the ln(dT/dt) versus 1/T the

resultant line has a slope of -(E/R) and an intercept along the ln(dT/dt) axis

at the ln(AAH/c) . The data were evaluated in this way by plotting the rate of

temperature change, (dT/dt) , as a function of the reciprocal of the

temperature, (1/T), on semi -logarithmic paper. If our assumption of first

order reaction is correct then the data should form a straight line with a negative slope. Least squares fitting of the experimental data would yield

the activation energy, E, and a lump parameter pre- exponential factor

combining A, AH, and c. The values for AH and c must be determined by another

method.

Figure 3 shows the results of such a plot for the eight western United States

coals. These plots indicate that the coals can be separated into two groups.

Five of the coal samples ("A" through "E") show that at least two rate

controlling processes are present. Samples "F" , "G"., and "H" show that a single rate parameter can be used to describe the their oxidative process.

Since samples "H" and "D" were taken from the same coal seam, one would have

expected to see comparable results. These two samples, however, were tested

at different initial temperatures. While sample "D" was tested at 50°C and

exhibited a two-stage reaction process, sample "H" reached initial equilibrium

at 70° C. This was apparently too high a starting temperature to observe the

first- stage reaction. Samples "F" and "G" also show no indication of the

21
presence of multiple processes. These samples were taken from the same coal

seam and, since the starting temperature for these two samples were comparable

to the other low temperature tests 55° C and 60° C, they represent a distinctively different class of behavior.

Least squares fitting over the range of linear response yielded values for an apparent activation energy and lumped pre - exponential . These values and the

temperature regimes over which they correspond are tabulated in table 3. ' For

those samples that indicated a two step reaction, the activation energy varied

from 32 kJ/mole to 55 kJ/mole with an average of 44 ± 9 kJ/mole for the first

reaction mechanism and 58 kJ/mole to 82 kJ/mole with an average of 71 ± 10

kJ/mole for the second. Those samples with only one reaction mechanism had an

apparent average activation energy of 77 ± 3 kJ/mole with a range of 74

kJ/mole to 79 kJ/mole. This was within the range of the second reaction

mechanism of the two-stage results. The apparent activation energy for the

second reaction mechanism appears to be consistent with other published values

for bituminous coal. [5] Reznik and Chekhovskoi [24] have theoretically

deduced the presence of two reaction zones for coal self -heating . They

calculated that at approximately 70° C the oxidation mechanism radically

changes with an increase in the activation energy. From table 3, it can be

seen that the observed transition point between reaction mechanisms occurred

within the range of 70°C to 90°C and that the activation energies increased.

However, the measured values for activation energy computed based on these

experiments differed by 50% from the predicted values of Reznik and

Chekhovskoi. [24] According to their calculations, the first reaction zone

should have an activation energy of 21 kJ/mole, while the activation energy in

22
the second reaction zone was computed to be 126 kJ/mole. Heusinger and

Miinzner [34] experimentally determined the adiabatic self -heating

characteristics of coal extracted from the Katharina, Prosper II seam and

observed a two-step reaction. They found that the activation energy for the

first reaction zone was 48 kJ/mole and 66 kJ/mole for the second reaction

zone. In addition, the transition temperature between reaction zones occurred

at approximately 84°C. These data appear to be consistent with our observations

It was suggested [35] that the frequency factor determined in tests like the

Adiabatic Furnace could be correlated to self -heating potential. From the

data currently available, it is not possible to determine the value of the

frequency factor. The pre- exponential occurring in equation (4) is a combination of several coal parameters. These are the heat capacity, heat of

combustion, and frequency factor. These additional thermal and physical

parameters have not been determined for the coal samples submitted for

analysis. The use of handbook values for these parameters would result in a simple scaling of the pre -exponential values listed in table 3. Generally,

any analytical method developed for predicting self-heating potential will

require both the frequency factor and the activation energy. The preexponential for the first reaction zone ranged from 103 to 10 6

, while the

second reaction zone had values between 10 7 and 10 10

.

Based on information supplied by DOE, samples "A" and "C" were taken from coal

seams that had experienced self -heating problems. Ignoring for the moment

environmental and technological effects associated with the particular coal

23
mines, based on the work of Banerjee et al [35], one would therefore expect to

see differences in either activation energy or pre- exponential between these

coal samples and the other six samples. No obvious correlation appears to

exist between propensity to self -heat and the activation energy or the preexponential factor. However, the use of actual thermal property data for each

coal sample may result in a positive correlation between the frequency factor

and susceptibility to self -heat. An alternative explanation for the lack of

correlation between test data, as applied by Banerjee et al [35], and field

experience may be due to the fact that all of these coal samples had similar

thermal characteristics, but that technological, geological, and environmental

factors varied significantly from mine to mine. With the information supplied

by DOE, this possibility could not be assessed. However, there is no reason

to suspect that either the activation energy or frequency factor alone is

sufficient to correlate laboratory data to end-use hazard. These are only

parameters that empirically describe a complex reaction mechanism. They,

therefore, must be used together to determine the rate characteristics of

coal. The use of these two parameters to provide a limited correlation will

be discussed later.

4.3 Crossing Point Method

The Crossing Point Method (CPM) apparatus employed in this study was

functionally similar to the CPM equipment previously described in the

literature. [1,31] Basically, a CPM apparatus is a controlled furnace that

linearly increases the temperature of the environmental air around a coal

sample at a fixed rate. In the CPM system, heated air is passed over a

24
prepared sample of coal. Recording equipment monitors both the temperature

rise of the center of the sample and the surrounding air. The sample

temperature lags the air temperature until the ignition temperature of the

sample is approached. The rate of change in sample temperature begins to

exceed the rate of change in the surrounding air temperature. The

intersection of the center sample temperature and the surrounding air

temperature is defined as the relative ignition temperature of a coal sample.

This crossing point temperature is a function of sample thermal properties,

mass of material, heating, and sample diffusion characteristics (i.e.,

uniformity of granulation and grain size)

4.3.1 Test Method

In previous furnace designs, pre-heated air was required to pass through a

coal sample. [31] This resulted in an increase in the pressure across the

upstream face of the coal sample. Pre-heated air was driven by this force

through the sample circumventing the samples diffusional characteristics.

This introduced an additional variable. Karn et al [36] has shown that flow

through a coal bed is not only dependent on the pressure drop across the coal

bed in the direction of gas flow, but also depends on the direction of fissure

formation and granulation. In general, the actual flow through the coal bed

differed among the coals they studied. Furthermore, once oxidation began,

downstream coal was exposed to a reduced oxygen concentration. The system

reported on here differs from those previously used by maintaining a more

uniform oxygen fraction around the entire sample with a minimum pressure force

applied to any sample face. This was accomplished by using low air flow rate,

25
500 cm3 /min, and a large furnace to sample volume ratio of about 110. Since

the total air flow rate was not forced through the sample, the oxidation

mechanism was primarily controlled by the diffusion of air. The diffusion of

air through the sample, while not typical of coal mines, has been standardized

by testing coal samples with grain sizes between 150 /im and 180 pm. In

addition, downstream material was not necessarily exposed to vitiated air.

Air vitiation, normal to the sample surface, was directed towards the sample

center. The sample was a 76 mm high by 38 mm diameter cylinder, similar to

the samples used in the adiabatic furnace tests. Figure 4 is a schematic

drawing of the furnace and control/data acquisition system.

4.3.2 Test Results

Sample preparation was as previously described. Approximately 28 gm of ground

coal was placed in the center of the furnace. The furnace was programmed to

increase the air temperature flowing around the sample at a rate of 0.5°C/min.

The actual rate was a little higher than this value and varied between runs from 0.53°C/min to 0.61°C/min with an average of 0.58°C/min. Typical runs of

all eight coal samples are shown in figures 5 to 12.

The average heating rate and the relative ignition temperature for each coal

sample are listed in table 4. The average heating rate was determined by a least squares fit of the furnace air temperature from approximately 90°

to

170° C. Relative ignition temperatures were defined as the crossover point of

sample temperature and furnace air temperature. Because the heating rate and

the sample mass did not greatly vary, direct comparisons of relative ignition

26
temperatures were possible. Relative ignition temperatures were found to vary

from 132° C to 154° C. Six of the eight samples tested had relative ignition

temperatures that did not differ by more than 7°C (132°C to 139°C). Only two

samples, "B" and "E" , had higher relative ignition temperatures, 153 °C and

154° C, respectively.

Based on ignition temperature alone , the CPM method does not appear to

distinguish between those samples, "A" and "C" , that were taken from coal

seams that had experienced self -heating and those samples that have not

reported self -heating occurrences. In order to normalize the data for

differences due to heating rate, Feng et al [1] developed a figure of merit

(table 4) to evaluate CPM data. These values are also used in determining a mine's liability to self -heating in the Canadian Evaluation System. The

figure of merit is computed by

Liability Index = Heating Rate

Ignition Temperature

x 1000 (5)

The Liability Index values for these coal samples were all below 5. This

indicates that these coal samples have an approximately equivalent, but low,

potential for self-heating given identical geological and technological

conditions

5. DISCUSSION

To develop a reliable predictive system for the quantification of self-heating

potential of a coal mine, several factors must be considered and combined into

27
a working model. These include mining technique, geological characterization,

and coal characterization. In order to maximize the rate of coal extraction

and maintain, at a minimum, the likely occurrence of self-heating incidents, a mining technique must be designed consistent with the type of coal and

geological characteristics found in a specific coal field. A good predictive

system would allow for the evaluation of alternative extraction methods as they impact on self -heating.

The three systems reviewed in this report address each of the three parameters

to a varying degree of detail. In total, the German system is the most

comprehensive design tool controlling mining techniques to reduce the

occurrence of self -heating , but it has no means to measure quantitatively coal

susceptibility to self -heat. It requires that consideration be given to selfheating problems in the planning stage prior to actual full-scale production

mining. However, because of the arbitrary weighting method used in the German

system, the objective importance attributed to any one factor in the overall

system is not clearly defined and relies heavily on past experience. This

severly reduces the general utility and transportability of the German system

to other coal fields that have limited use experience.

5 . 1 Mining Technique

Of the three parameters necessary for an evaluation system, mining technique,

or the method used to extract the coal, is the only one that can be

controlled. It is only indirectly addressed, if at all, by the RASCAL and

Canadian systems. This oversight on the part of the Canadian system may be

28
understandable, since it was intended for use in a specific coal field under

presumably identical extraction conditions. The RASCAL system, however, is a general purpose evaluation system intended for use in western United States

coal fields where several extraction methods can be employed. RASCAL does not

consider the selection of an extraction method, sealing method, or ventilation

method as a means to control the self -heating of coal, while the German system

goes into great detail regarding the type of mining operation. Factors such

as the direction of ventilation air movement with regard to extraction

direction, the nature of the extraction method (e.g., advancing or retreating

long wall, continous mining, etc.), the presence of coal pillars, and the type

of sealing method are all addressed in the German system.

5.2 Geological Characterization

The geological components considered by the RASCAL system are nearly as detailed as in the German system, while the Canadian system combines many

elements into four broad categories

5.3 Coal Characterization

The elements included in the coal characterization parameter are also more

detailed in the RASCAL and German systems than in the Canadian System.

Although this may appear to be a shortcoming of the Canadian system, it may in

fact be its virtue. The other systems rely on an analysis of the coal (i.e.,

coal rank, pyrite concentration, hardness, etc.) as an indirect measure of the

coal's self -heating potential. The presence of other accelerating or

29
inhibiting factors are ignored. By relying on a thermal test method for

measuring a coal's susceptibility to self -heat, the Canadian system represents

a more general measure of a coal's self -heating potential and is less

susceptibile to oversight of critical parameters not considered in a limited

physical characterization. The thermal test method used in this evaluation

system is the CPM. It measures the relative ignition temperature of a coal

sample when it is heated at a fixed rate. This provides the necessary

information for the determination of a coal's liability index . and

distinguishes this evaluation system from the others.

5.4 Coal Evaluation

Eight coal samples were evaluated by two test methods: the adiabatic furnace

and the CPM apparatus. While no evaluation system uses the data provided by

the adiabatic furnace, the CPM apparatus is an integral part of the Canadian

evaluation system. The data from the CPM apparatus is used to develop a Liability Index that is combined with other mining and geological conditions

to arrive at a measure of coal susceptibility to self -heat.

5.4.1 Adiabatic Furnace

The data from the adiabatic furnace has traditionally been used to study

fundamental kinetic properties of a broad range of materials. The data on the

eight coal samples showed that coal has two reaction zones. One regime below

85 °C had an activation energy between 32 kJ/mole and 55 kJ/mole. The second

reaction above 85 °C had an activation energy range of 58 kJ/mole to 82

30
kJ/mole. This was consistent with prevaling theories of coal oxidation;

however, neither the activation energy nor the pre- exponential were able to

distinguish between coals from sources with known and unreported occurrences

of self -heating . Work by Enig et al [37] showed that the data from the

adiabatic furnace could be used to calculate a critical size for self -heating

thermal ignition of a slab, sphere, or cylinder of material.

2 7kRT2 (E/RT)

B

s e (6)

Where : Bs = radius or half thickness of critical size

7 = geometric parameter

It can be seen from equation ( 6 ) that one requires, in addition to the data

obtained from the adiabatic furnace, the thermal conductivity, k, of the

material. Obtaining the additional information and applying equation ( 6 ),

would allow one to use B s as a measure of self -heating potential. It is still

necessary to determine appropriate ranges for Bs before an evaluation system

can be developed based on data from the adiabatic furnace.

As an alternative to the use of Bs , it is possible to use equation (4) to

compute heating rates at different strata temperatures for the eight coal

samples tested in this study, table 5. Heating rate data can be combined with

field experience to determine a critical heating rate above which self-heating

can be expected to occur. Since samples. "A" and "C" were reported to have

been taken from coal seams with previous occurrances of self -heating , heating

rates equal to or above their minimum at a given strata temperature would be

an appropriate critical heating rate. At 20° C, heating rates above 1.5° C/24

31
hrs , table 5, could be expected to result, in self -heating , while raising* the

strata temperature to 40°C increases the critical heating rate to 5.4°C/24

hrs

5.4.2 Crossing Point Method

The use of the CPM based Liability Index on coals tested in this report

indicated a lack of correlation between self-heating occurrence and relative

ignition temperature. Noting that the rate at which a coal sample approached

the furnace temperature differed between samples, an alternative method for

analyzing CPM data can be developed. A measure of the rate of heat generation

by the sample can be calculated by taking the difference between the rate of

heating of the furnace and the sample core. Ideally, once the sample and

furnace air are in dynamic equilibrium, sample thermal lag should prevent the

sample from crossing the furnace temperature. In fact, samples of inert sand

tested in the CPM apparatus showed that the difference between furnace

temperature and sample temperature remained constant, only depending on sample

diameter. Therefore, a sample heating rate in excess of the furnace heating

rate is indicative of the ignition process. The difference between the

instantaneous sample heating rate and the instantaneous air heating rate is

plotted as a function of furnace air temperature in figure 13, for all eight

samples. It is interesting to note that of the four samples that follow the

same trajectory, two of these samples, "A" and "C"are from a coal seam with

a previous history of self -heating incidents. Samples "D" and "H" have a somewhat shallower trajectory, while samples "B" and "E" appear to have a nearly flat profile with an anomalous drop in the differential heating rate as

32
the ignition temperature was approached. This may be due to the volatile

content of these two coal samples. They were the only medium volatile

bituminous coals tested in this series. Because of the grouping of the data,

samples "F" and "G" should have similar field performance to samples "A" and

"C". A review of the mining operations between these two coal seams may

explain the lack of self-heating problems associated with the former samples.

This alternative analysis based on the rate of change in the difference

between furnace air temperature and sample core temperature showed a positive

correlation to observed self-heating characteristics of the coal samples.

Furthermore, this analysis method differentiated between coals of different

rank. (Samples "B" and "E" were medium volatile coals, while the other

samples were high volatile coals.) However, due to the limited test

population, it is not possible to extrapolate the usefulness of this method of

analysis to all coals. Nor is it possible to build an evaluation system based

on this method without further work.

Based on the self-heating history of the eight coals investigated, both test

methods appear capable of distinguishing coals with a high susceptibility to

self- ignite. The CPM method has the advantage of speed (approximately one day

per sample compared to 3-4 days per sample using the Adiabatic Furnace method)

and provides for the development of a simple ranking of coals

Howeverthe

CPM method is conducted at high heating rates without regard for

characteristic coal mining temperatures. While the performance of the

Adiabatic Furnace method is more time consuming, the fundamental data obtained

(i.e., activation energy and lumped pre-exponential) can be used to directly

33
evaluate coal heating rates at any chosen mine operating temperature

Critical heating rates, however, are determined by comparison to coals of

known susceptibility to self- ignite.

6. CONCLUSION

A review of three evaluation systems for the prediction and control of selfheating incidents in coal mines indicates that:

• the German system is the most comprehensive, but relies heavily on past

experience

• the German system provides detailed planning guidelines that may be

adaptable to the development of other coal fields;

• the German system and RASCAL system lack a means for the thermal

assessment of coal;

• the RASCAL system is the only system that includes atmospheric moisture

as an evaluation element;

• the Canadian system includes a method for the thermal assessment of

coal

All three systems have not been tested against a common set of coal fields

with known self-heating properties.

The adiabatic furnace appears to be a useful instrument upon which to base a thermal assessment system for determining the potential for coal to self -heat.

To take full advantage of this data, additional measurements need to be made

and incorporated into equations (4) and (6) before absolute estimates of the

propensity of coal to self -heat can be calculated. However, because it

34
provides fundamental kinetic information about low temperature thermal

oxidation of coal, the data can be applied to other areas of coal

manufacturing such as coal storage facilities and transportation methods as well as coal mining. Kinetic data indicate that the oxidation of coal

undergoes two thermal regimes. One regime was below 85 °C with an activation

energy of approximately 32 kJ/mole to 55 kJ/mole. The thermal regime above

85°C had an approximate activation energy of 58 kJ/mole to 82 kJ/mole. Using

the kinetic data associated with the low temperature thermal region and

equation (4) , the self -heating potential of coal at any chosen mine operating

temperature can be determined.

The Crossing Point Method was used to test the eight samples at an average

heating rate of 0.58°C/min. The relative ignition temperature varied from

132°C to 154°C. The Liability Index specified for the Canadian analysis was not sufficiently sensitive to discriminate among the tested coals. An

alternative plotting method that suitably correlated CPM data to end-use

experience was developed. While a positive correlation was achieved with the

CPM apparatus, the data are not readily useful at characteristic mining

temperature changes

7. RECOMMENDATIONS

None of the three evaluation systems appear to be general enough to use in

evaluating western United States coal fields for susceptibility to selfheating. An evaluation system needs to be developed that is based on the best

features of each system reviewed in this report. The new evaluation system

35
should incorporate a thermal assessment test, like CPM or the adiabatic

furnace, to determine coal characteristics. These should be combined with

geological and technological data more consistent with western United States

coal fields.

The new system should be carefully monitored and updated as additional

information is made available by the coal mining companies and basic

researchers.

36
8. GLOSSARY

Gob: The space from which coal has been removed and broken waste or filling materials have been left or placed.

Overburden: A consolidated or unconsolidated material overlying a deposit of

coal

Mining Techniques

• Advancing Long Wall: A method of coal removal that is accomplished

while advancing in the direction of the coal seam. With each pass

of the cutting machine across the face of the seam, mining is

halted to advance the cutting machine.

• Continuous Mining: A method of coal extraction that is

accomplished by a continuous process of driving a mining machine

into the coal seam.

• Retreating Long Wall: Similar to advancing long wall except that

the cutting machine is installed such that with each pass of the

cutting machine the mining equipment is drawn towards the entrance

to the seam.

Pillars: Unmined coal in the form of columns left in a mine to provide roof

support and air pathways

Pyrite: Any of various metallic sulfides found in underground mining. A

37
common sulfide found in underground coal mines consists of iron

Rank:

Seam:

Slope

disulfide, FeS2

.

A measure of a coals volatile, carbon, or calorific content.

A stratum or layer of coal representing the coal bed.

An inclined entry in a dipping coal bed or an inclined tunnel, to a

coal bed.

38
9. ACKNOWLEDGMENTS

The author gratefully acknowledges the assistance provided by William J.

Parker in reviewing this report and making constructive recommendations that

improved its technical content. The author would also like to acknowledge the

work of G. E. Mitchell and W. G. Mallard on a previous project relating to the

characterization of the self-heating properties of coal using an adiabatic

furnace and to R. G. Gann for the design of the furnace used in the CPM

method.

39
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No. 2, 1970.

[36] Karn, F.S., Friedal, R.A.

and Sharkey, A.G., Jr., Mechanism of Gas Flow

Through Coal, Fuel, Vol. 54, No . 5

1975.

[37] Enig, J.W., Shanks, D., and Southworth, R.W.

The Numerical Solution of

the Heat Conduction Equation Occurring in tne Theory of Thermal

Explosions, NAVORD Report 4377, NTIS No. AD 116 873, 1956.

42
Table 1. Heat of Wetting of Dried Coal

from Experiments by Porter and Ralston [7

Coal J /gm D

Brown lignite (North Dakota) 107

Sub -bituminous (Wyoming) 80

Bituminous (Illinois) 29

Coal

43
Table 2. Description of Bituminous Coal Samples Used in

Adiabatic and Crossing Point Method Furnace Tests

Samnle ID Mine Name Seam Number Ventilation Area Coal Rank1

A Somerset Mine B-2 3 Dip 9 West Hi Vol B

1 C Somerset Mine B-2 Main Ext 5 West Hi Vol B

B L. SWood #3 Coal Basin Slope Med Volc E L. S

Wood #3 Coal Basin 301 Wall Med Vol

D Sunnyside Mine #1 Lower Sunnyside 17 LT. Lw. Hi Vol B

H Sunnyside Mine #1 Lower Sunnyside

(upper split)

19 LT. Tailgate Hi Vol B

F Hawksnest Mine E 7-h West Hi Vol B

G Hawksnest Mine E 9 West Hi Vol B

a) Classification of Coals By Rank - ASTM D-388

b) High Volatile B Bituminous Coal

c) Medium Volatile Bituminous Coal

44
Table 3

Summary of Adiabatic Furnace Data from Eight Western U. S . Coal,

Initial Temperature Lumped Activation

Temperature Range Pre - Exponential Energy

SanvDle ID (°C) rc) ( ° C/min') (kJ /mole')

A 60 60- 90 3.9 X 10 5 48 + 11

90-130 4.5 X 10 9 76 + 7

C 50 50- 70 1.3 X 10 3 32 + 9 70-120 9.2 X 10 6 58 + 10

B 50 50- 80 1.1 X 10 4 40 + 7

80-130 2.2 X 10 7 64 + 9 E 60 60- 80 5.1 X 10 4 47 + 5 80-130 1.2 X 10 9 74 ± 9

D 50 60- 70 1.2 X 10 6 55 ± 7

70-130 1.4 X 10 10 83 ± 9 H 70 70-130 8.9 X 10 9 79 + 10

F 60 60-130 3.8 X 10 9 77 + 9

G 55 55-130 2.1 X 10 9 74 + 12

45
<

O

PQU

Qffi

Pmo

Table 4. Summary of Relative Ignition Temperature Data for

the Crossing Point Method

Sample ID

Ignition Temperature Heating Rate Liability Index

("O’) ( °C/min) (min' 1

)

135 0.59 4.4

132 0.58 4.4

153 0.53 3.5

154 0.56 3.6

133 0.60 4.5

138 0.57 4.1

139 0.58 4.2

133 0.61 4.6

46
o*ri

tco

ww

o>

Table 5 Calculated Heating Rates for Eight U.S. Western Coals

at Four Different Strata Temperatures Based on Equation (4)

Sample ID

Heating Rate (

, °C/24 hrs)

Strata Temperature f°C)

10 20 30 40

0.8 1.5

2.3 3.7

0.6 1.2

0.2 0.3

0.1 0.3

<0.1 0.1

<0.1 0.1

0.1 0.2

3.0 5.4

5.7 8.6

2.1 3.4

0.6 1.1

0.6 1.1

0.3 0.8

0.3 0.8

0.5 1.3

47
Figure

1

Planning

Guide

taken

from

German

Evaluation

System.

48
TEMPERATURE

(°

C)

TIME (hrs)

Figure 2 Temperature -Time curves for eight DOE coal samples

taken from western U.S. coal fields tested in the

Adiabatic Furnace

49
RATE

OF

TEMPERATURE

CHANGE

(°C/min)

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0

TEMPERATURE (1/T x 10 3 K)

Figure 3 Semi-Log plot of the rate of temperature change

versus 1/T (K 1 ) for eight samples of U.S. western'

coals tested in dry air.

50

Mil I

I

I

I

~[TTT II

I

I

I

|l11I

I

I
Top heater

Top heater

power

supply

Figure 4

Exhaust

Digital to

analog

Sample

Main heater

Main heater

power

supply

TCAir TCsample

Analog to

digital

Control

computer

Disk drive Printer

Terminal

Schematic drawing of the apparatus used in the

Crossing Point Method (CPM)

51
TIME (min)

Figure 5 CPM test results for coal sample "A" heated at

0 . 59°C/min. Ignition occurred at 135°C.

52
TEMPERATURE

(°C)

Figure 6 CPM test results for coal sample "C" heated at

0.58°C/min. Ignition occurred at 132°C.
TEMPERATURE

(°C)

100 120 140 160 180 200 220 240 260 280 300

TIME (min)

Figure 7 CPM test results for coal sample "B" heated at

0.53°C/min. Ignition occurred at 153°C.

54
TEMPERATURE

(°C)

100 120 140 160 180 200 220 240 260 280 300

TIME (min)

Figure 8 CPM test results for coal sample "E" heated at

0.56°C/min. Ignition occurred at 154° C.

55
TEMPERATURE

(°C)

Figure 9 CPM test results for coal sample "D" heated at

0.60°C/min. Ignition occurred at 133°C.

56
TIME (min)

Figure 10 CPM test results for coal sample "H" heated at

0.57°C/min. Ignition occurred at 138°C.

57
130 140 150 160 170 180 190 200 210 220 230

TIME (min)

Figure 11 CPM test results for coal sample "F" heated at

0.58°C/min. Ignition occurred at 139°C.

58
TEMPERATURE

(°C)

150 160 170 180 190 200 210 220 230 240 250

TIME (min)

Figure 12 CPM test results for coal sample "G" heated at

0.61°C/min. Ignition occurred at 133°C.

59
DIFFERENTIAL

HEATING

RATE

(°C/min)

Figure 13 Plot of the CPM data for all eight coal samples

showing the relationship between the furnace/sample

temperature difference versus the furnace air

temperature.

60
Appendix A

Rapid Appraisal of Spontaneous Combustion Assessed Liability

(RASCAL) [2]

RASCAL provides a preliminary generalized indication of the degree of risk

that can be obtained by considering the following parameters:

I -Rank of coal, as a factor of oxygen content;

II-Presence of pyritic sulphur minerals;

III -Hardness

IV-Humidity of ventilating air;

V- Strata temperature;

VI -Unworked roof coal thickness.

These parameters are combined by taking the sum of parameters I, II, III, and

IV. The sum of these parameters is multiply by thefactors from parameter V

and VI.

Rank of Coal Oxvsen Content.Points

Sub -bituminous +15 18

15 - 12 16

Bituminous 12 - 10 14

Weakly coking 10 - 8 12

Coking 8 - 6 8

S emi - anthrac i t e s 6 - 4 3

Antracites 4 - 2 1

<2 0

A- 1
II Presence of Pvritic Sulphur Minerals. % Points

+3.0 7

3.0 - 2.0 5

2.0 - 1.0 3

1.0 - 0.5 2

0.5 - 0.25 1

<0.25 0

III Hardness Type Points

Very Friable + 4

Friable + 2

Medium Hard 0

Hard - 2

IV Humidity of Ventilation Air Saturation . % Points

+60 + 2

60-40 0

40-20 - 2

20-0 - 5

V Virgin Strata Temperature (°F) Factor

+105 5.0

105 - 85 2.0

85 - 70 1.25

70 - 50 1.0

32 - 50 1.0

A- 2
VI Unmined Roof Coal Thickness ( ft) Factor

+10 4.0

8-10 3.5

6-8 3.0

4-6 2.5

2-4 2.0

>0-2 1.5

The overall Risk Factor (RF) is determined by:

RF=(I + II + III + IV) * V * VI,

Where

Low Risk = Less than 10;

At Risk =10 - 20;

High Risk= Greater Than 20.

Two Examples

Coal A Coal B

Parameters Data Points Data Points

I Coal Rank (oxygen) 12 % 14 5 % 3 II Pyrite Content 1.1 % 3 0.5 % 1

III Hardness Friable 2 Medium Hard 0

IV Humidity 65 % 2 40 % 0

Total 21 4

V Roof Coal 2 ft x 1.5 4 ft 6 in x 2.0

VI Strata Temperature 60 ° F x 1.0 60 ° F x 1.0

RISK FACTOR 31.5 8

A- 3
* . ' . .
Appendix B

Translation of German Recommended Practices

Handbuch Arbeitssicherheit

Ruhrkohle AG

Nr.: 801 302 Blatt: 1 Stand: 12/78

Sachgebiet: Brandschutz unter Tage

Titel: Verhiitung von Selbstentzundungsbranden

B-l
No. 801 302

Dec ' 78

Manual For Work Safety Area: Fire Protection Below Ground

Prevention of Spontaneous Combustion Fires

Contents

0 Foreword

1 General Basis and Definition of Terms

1.1 Explanation and Definition of Spontaneous Combustion

1.2 Criteria and Influence on Spontaneous Combustion

1.3 Points of Origin and Causes

2 Measures for the Prevention of Spontaneous Combustion Fires during

Planning

2 . 1 General

2.2 Measures Based on Lode Evaluation

2.3 Measures Based on Geological Situations

2.4 Measures to be Considered When Designing Underworkings

2.5 Measures to be Considered in Ventilation Technology

2.6 Measures to Secure Approaching and Trailing Edges

2.7 Measures

Planning

for the Prevention of Spontaneous Combustion Fires During

3 Measures for the Prevention of Spontaneous Combustion Fires During

Operations

3 . 1 General

3.2 Securing Approaching Edges

3.3 Ventilation in Operating Mine

3.4 Sealing Off Wall Trenches

3.5 Observation of Geological Disturbances

3.6 Treatment of Residual Pillars

3.7 Sealing of Abandoned Mines

3.8 Preventive Intertization

3.9 Monitoring Fire Protection Parameters

3.10 Information and Instruction

4 Publications

Appendix: Planning Aids for the Evaluation of Danger of Spontaneous

Combustion

B- 2
0 Foreword

In collaboration with the Main Institute for Life Saving Systems for Mines in

Essen, the Ruhrkohle AG has once more discussed the danger to underworkings

stemming from spontaneous combustion, in a working committee of fire

protection experts. The insights gained and the suggested measures are contained in the foregoing recommendations for the prevention of spontaneous

combustion fires and they should serve as a contribution to a reduction of

dangers to mine underworkings from spontaneous combustion of the coal.

The listed measures may be taken as needed. They represent various

alternative solutions and, in this wide-range form, should be seen as a collection of possibilities of which the various enterprises may select

effective measures, depending on the circumstances prevailing in their mines.

The effective official regulations issued by mining authorities for the

prevention of spontaneous combustion fires are not affected by these

recommendations

These recommendations will be updated in accordance with technical

developments by later publications.

1 General Basis and Definitions of Terms

1.1 Explanation and Definition of Spontaneous Combustion

Coal oxidizes physically by the absorption or the accumulation of oxygen at

the coal's surface. With a further penetration of oxygen into the coal, the

temperature rises, unless the heat is drawn off. Oxygen enters chemical

compounds and the results are oxidation products such as water (condensation

spots) and low- temperature carbonization gases of carbon dioxide and carbon

monoxide; later, these gases will also contain aromatic hydrocarbons (the

smell of benzene and benzol)

Definition: The spontaneous combustion of coal is a physical-chemical process,

producing oxidation products, with the temperature rising to such

a level that smoldering fires will be created, which, if exposed

and increased air having access to them, will begin to glow or to

flare up

1.2 Criteria and Their Influence on Spontaneous Combustion

Even Though all types of coal may attract and chemically combine with oxygen,

not all lodes are in danger of spontaneous combustion. A spontaneous

combustion will always depend on several criteria, the influence of which

should be evaluated as follows

Type of Coal

Shiny coal (vitrite and clarite) particularly tends to oxidation. This

tendency increases with an increase of the volatile components and the

humidity contents.

Grain Size

B- 3
Oxygen absorption increases with decreasing grain size, so that fine'

coal is the most endangered type.

Pvrite Contents

Pyrite oxidizes only in the presence of water and influences spontaneous

combustion only in the lower ranges up to 325 K « 50° C *) 1 Available Oxygen

The speed of chemical reaction and inclination to oxidation as a rule

increases with increasing availability of oxygen.

Temperature and Temperature Curve

Combustion temperature is between 380 K (107°C) and 500 K (227°C)

dependent on the thickness of collected coal dust layers. Development

of temperature in a spontaneous combustion fire is not in proportion

with time. Beginning with a certain temperature threshold, which

depends on the type of coal, the temperature rises much faster. The

higher the initial temperature, the lower the difference to the critical

temperature threshold. The temperature difference to the critical

temperature threshold therefore becomes less and less as the depth and

the rock temperatures increase.

Lode Dips and Lode Thickness

Observations made during a number of years have shown that the tendency

to spontaneous combustion increases with increasing thickness and

greater lode dips

Ventilation

In addition to the effective ventilation currents, slow currents are caused because of the pressure difference. These slow currents, by

introducing oxygen to residue coal remaining in abandoned workings,

heighten the danger of spontaneous combustion. The less heat is drawn

off by these slow currents and the more oxygen is introduced by them,

the greater this danger becomes. The volume of critical slow currents

is within the limits in which a steady oxygen supply to the center of

oxidation is guaranteed, while not enough heat is removed so that the

temperature increases steadily until the point of spontaneous ignition

of the coal is reached.

Type of Workings

Compact stowing counteracts the formation of slow air currents and,

therefore, spontaneous combustion. Given otherwise identical

conditions, working in thrusts will further spontaneous combustion

dangers

Type of Mining

Retreating mining operations, because of fewer slow air current

formations, is more advantageous than forward operations when it comes

to prevent fires by spontaneous combustion.

Organization of Recovery Operations and of Barraging

The faster anf the more efficiently abandoned underworkings are stripped

and sealed, the lower the danger of spontaneous combustion.

1.3 Points of Origin and Causes

1 *) t= T-T0 n(°C) = n + 273 . 15 [K] 0°C = +273.15 K 0 K = -273.15°C

B- 4
1.3.1

Points of Origin

Many years of experience have shown that the points of a possible spontaneous

combustion are located wherever several criteria combine to favor the

spontaneous combustion of coal. These are mostly:

- Leading and Trailing Edges

- Coal Islands and Residual Coal Pillars;

- Thrust -working Operations with Parallel Lodes;

- Recovery- stripping Operations. 1.3.2

Causes

The folowing are possible causes:

- Slow air currents in connection with;

• Residual Coal in Abandoned workings,

• Crushed Coal in Rim Zones

• Gas-removal Drill Holes with Connections to Parallel Lodes,

• Test and Relief Drill Holes,

• Disturbanceareas with connections to open workings.

2 Measures for the Prevention of Spontaneous Combustion Fires During Planning

2 . 1 General

With a correct estimate and evaluation of the geological geometric situations,

important measures can be agreed upon and can be initiated in the planning

stage, by which the danger of spontaneous combustion can be reduced.

2.2

Measures Based on Lode Evaluation

When planning the mining operations, it should first be determined whether, in

a certain lode,

based on experience during operations

or because of coincidence of several characteristics favorable to

spontaneous combustion as to origin and cause,

there is reason to believe that the danger of spontaneous combustion will be

expected.

It is therefore advisable to take the following measures, at the beginning of

the planning phase:

compilation and evaluation of experiences during operations

• Entering fire -areas in plans of the lode to be worked and in

the neighboring lodes

• Experiences made in installations neighboring the lode to be

worked

• Evaluation of documents recording the CO-contents in

previous elevations of the same lode.

Compilation and evaluation of values influencing spontaneous

combustion

B- 5
• Type of coal (shiny coal, fibrous coal)

• Grain sizes

• Share of volatile components

• Moisture contents

• Sulfur contents , pyrite contents

• Tendency to oxidize

If the danger of spontaneous combustion must be expected, the following is to

be observed during the planning stage:

the selection of suitable mining equipment with proper machinery

to completely mine the lode

the selection of suitable face support methods for the complete

mining of the lode

the selection of the packing method, e.g. pneumatic packing in

case of coal slippage from an adjacent roof

the selection of suitable sealing methods to prevent slow air

currents, e.g. securing of leading edges and sealing mines

the barraging of the entire height after completion of mining

(mining plan, stripping plan)

2.3 Measures Based on Geological Situations

Disturbance zones require special measures to protect them from

spontaneous combustion, since this danger is greater in their

areas. These measures must be planned and prepared at an early

stage (sealing, injections, cement injections after the torcret

system)

Coal of non-mined lodes in the roof which may come within the

thrust range because of the short distance to the lode, increases

the danger of spontaneous combustion. The measures for the

reduction of this danger must be planned and initiated early: e.g.

pneumatic packing, sealing of mines and leading edges.

2.4 Measures to be Considered when Designing Underworkings

For many reasons, retreat mining operations are less subject to

dangers of spontaneous combustion than forward mines.

The leading edges of mining operations should not be located in

disturbance areas. The distances between base mines and other mines should either be

sufficient (= 10 -M at least 25 m) or be kept as little as possible

If this or other measures should create residual pillars, steps to

prevent fires caused by spontaneous combustion must be taken.

These measures include:

• injection of solutions of hygroscopic salts containing

surface - active agents

• treatment of surfaces with hygroscopic salts,

• sealing by means of vaulting, backfilling, and packing (more

information see 3.6).

B- 6
Long mine dips require effective measures because of the great air

pressure differences at the abandoned workings during the advance

to

• seal the leading edge;

• seal the mining drifts

• maintain the cross sections for ventilation.

2.5 Measures to be Considered in Ventilation Technology- Planning

The danger of spontaneous combustion can be highly reduced if the following

demands are taken into consideration in the planning phase:

the avoidance of slow air currents

the reduction of pressure differences at abandoned workings

The following measures are suited to meet these demands:

sufficiently large ventilation cross sections

avoiding throttles and narrow passages

air-tight sealing of leading edges (No. 3.2)

selection of suitable winning- and ventilation systems

The following viewpoints should be observed:

B- 7
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B-8
2.6 Measures to Secure Approaching and Trailing Edges

The approaching and the trailing edges of supports have increasingly become

points of origin for spontaneous combustion.

The following are the causes:

residual coal with a high share of fine coal

crushed fine coal residue from the faces

steady introduction of fresh air by slow air currents

reduced heat evacuation by choked ventilation.

In order to reduce the danger of spontaneous combustion, therefore, it is

necessary to plan and organize suitable measures to secure the approaching and

the trailing edges . Dependent on the evaluation of the danger of spontaneous

combustion, the following measures could be provided:

Sealing

• of the headings

• of the stripped longwalls

• of the lode trenches in front of and behind the approaching

and the trailing edges, respectively.

For this sealing operation, the following are particularly

suitable materials:

• hydraulically settling material

• wooden posts with insulating foam back-fill

• rock dust barriers

In regions where there is particular danger, the sealing

operations could be complemented by a back-filled collar accross

the entire mine circumference.

Treatment of approaching and trailing edges with hygroscopic

salts

• by applying it in powdered form on the coal face;

• by injection of solutions of hygroscopic salts containing

surface -acyive ingredients into the approaching and the

trailing edges;

• by sprinkling the floor with flaked material.

2.7 Measures for the Prevention of Spontaneous Combustion Fires

During the Planning Stage

If the criteria which further spontaneous combustion are properly weighed and

evaluated in the planning phase, such possible danger can be discovered early

(see Planning Aids in the appendix to these recommendations) . As soon as these insights have been gained, measures can be planned by which spontaneous

combustion can either be prevented or greatly reduced.

3 Measures for the Prevention of Spontaneous Combustion Fires

During Operations

3 . 1 General

B- 9
Basically, two types of measures must be taken to prevent fires caused by

spontaneous combustion:

measures which have been planned because of foreseeable influences

immediate steps which may become necessary because of

unforeseeable and suddenly occurring events

Quite generally, measures are planned prior to the start of operations, and

dealing with the various phases of the operation, which can prevent fires by

spntaneous combustion, given normal conditions and a careful execution of

these measures. If these conditions should change, or unforeseen influences

should develop which favor spontaneous combustion, it will be necessary to

provide supplemental or additional measures during the operations and in due

time, so that this danger of spontaneous combustion can be banned even under

these altered conditions. Depending on the evaluation of the danger of

spontaneous combustion, the measures listed in numbers 3.2 to 3.10 are suitable for both of these cases.

3 . 2 Securing the Approaching Edges

3.2.1 Sealing

When sealing the approaching edge,

normally three sections must be

observed:

Headings

Approaching Edge

3. 2. 1.1 Sealing Trench of Lode Walls

The Trench of the lode walls should

be sealed about 5 - 10 m behind the

approaching edge, by providing airtight insulation against the coal

outcrop

i i

The following materials are suitable, with their value increasing as follows:

wooden posts with a rock-dust barrier

wooden posts with foam backfill (e.g. Iso- foam)

a dyke of hydraulically settling material (B 2 M)

collars of hydraulically settling material across the entire

circumference of the mine.

3. 2. 1.2 Sealing of Heading Entrances

B- 10
The heading- entrances should be

sealed off across the entire width

of the heading.

The following materials are suitable, with their value increasing as follows:

Closing plugs made of

• rock dust (L~5 M)

• hydraulically settling material (L~3 M)

A dyke of hydraulically settling material extending the seal of

the heading

3. 2. 1.3 Sealing Trench of Lode Walls Against the Thrust -working Field

in the Area of the Approaching Edge

The following materials and means are suitable for this purpose:

rock dust barriers (B~3 M)

wooden posts with iso-foam back-fill

a parallel dam made of hydraulically settling material (B 2 M)

The wall trench of the approaching

longwall must be sealed off along a distance of a least 15 m immediately

adjoining the sealing of the

heading.

3. 2. 1.4 Securing the Approaching Edge Along Lode -parallel Mines

Whenever longwalls are developed

immediately out of the base mine or lode-mine, similar steps to the

measures listed in 3. 2. 1.3 must be

taken to provide a ventilation seal

across the entire approaching edge,

with hydraulically settling

materials being given preference for

reasons grounded in ventilation- and

fire -prevention technology.

3.2.2 Special Measures

3. 2. 2.1 Collar with Back-fill

If the sealing measures enumerated thus far should not guarantee an air-tight

seal of the surrounding edges, (for instance in very loose roof lodes in the

area of the trench) the insertion of a collar across the entire mine

circumference is recommended in the area of the approaching edge insulation,

and back-filling it with hydraulically settling material.

B-ll
3. 2. 2.

Treatment of Approaching Edge with Hygroscopic Salts

such as CaCl2 or MgCl2 , Containing Surface -Active Agents

Whenever the danger of spontaneous

combustion has already been

identified, additional safeguards

are recommended for the approaching

edge by treatment with saturated

solutions of hygroscopic salts such

as CaCl2 or MgCl2 containing

surface -active agents.

The solutions with the surface -active agents should be injected into the

approaching edge which is to be protected by means of drill holes

at a distance of 3 -M from each other

of at least 3 -M depth.

The injection must be performed with pressures which are adapted to counterpressure of the coal. This should prevent a loosening of the approaching edge

and should result in the highest possible degree of saturation in the injected

area. Experience has shown that these pressures range from 20 to 100 bar.

Injection should continue until the solution emerges from the face of the

coal. The ijected solution should - if possible - be no less than 10 i/m3 coal, which corresponds to a value of 2.5 kg salt/t coal.

In addition to this injection of salt solutions with surface -active agents, a surface treatment of the approaching edge with suitable hygroscopic salts

(CaCl2

) may be performed. In this operation, at least 4 kg powder or flakes

for each m

2 surface of exposed coal floor and the face of the leading edge

within the heading are needed for a sprinkling operation.

C

4 kg/m2

CaCIgOderMgC^

B- 12
3.3 Ventilation in Operating Mine

During the operations, all possible measures must be applied and made

permanently effective to keep the pressure difference at the abandoned

workings as small as possible and to prevent slow air currents.

Some of these measures are:

maintaining the mine -cross -sections

maintaining the face openings as planned

avoiding unnecessary obstructions to the ventilation (door frames,

material)

sealing of trenches so that as little slow air currents can reach

the abandoned workings . This measure is better suited for

maintaining of the mine-cross-sections, and a better flow of

service ventilation through the mine is guaranteed (No. 3.4)

maintaining the necessary service ventilation (not to be exceeded)

adapting the service ventilation to altered mining or gas -drainage

conditions

3.4 Sealing of Wall Trenches

In order to prevent slow air currents, and supposing a recognized existing

danger of spontaneous combustion, the wall trenches should be sealed directly

subsequently to the securing of the approaching edges. The following

processes are suitable for this purpose, and their effectiveness is increased

in the following order:

Filling hollow spaces in the

wall trenches with foam

material (e.g. Iso-foam)

Filling wall trenches with

parallel dams of dam setting

building material. Methods

permitting a close connection

with the hanging lode and

guaranteeing early load

bearing properties should be

preferred

Additional sealing of the

shore seam by vaulting

(panelling, sprayed mortar or similar methods) and backfilling with subsequent

compression with dam setting

material

B- 13
Sealing of entire

circumference of mine, in

some special cases even

including the floor portion,

by vaulting, back-filling and

subsequent compression with

dam setting material.

3.5 Observation of Geological Disturbances

Geological disturbances increase the danger of spontaneous combustion and

require additional preventive measures. Some of these are:

leaving as little as possible residual coal in abandoned workings;

filling cavities in the area of disturbance, not with wood but

with non-flammable material, e.g. with foam, hydraulically setting

dam materials, light-weight concrete barriers;

in the case of geological faults in the mine area, a predetermined stretch will be selected, along which stretch - and in

zones preceeding and following it - special steps will be

necessary. It is recommended to use vaulting, bak- filling and

compression with setting dam materials;

treatment of the loose rock

area in the disturbance zone

with saturated solutions of

hygroscopic salts , such as CaCi2 or MgCl2 , with these

solutions containing surface

active ingredients. The

solution can be introduced

into the abandoned workings

by means of lances from the

direction of the longwall

opening, so that the residual

coal is wetted. For this

purpose, suitable, closeable,

openings (50 mm in diameter)

in a shield- type support

should be provided, with

proper suitable connection

means

3.6 Treatment of Residual Pillars

Residual pillars which are the result of mining activities tend to contribute

to the danger of spontaneous combustion because of their less compact

composition due to the mining pressure.

Residual pillars of this kind are:

geological coal islands

B- 14
safety pillars caused by support construction and ventilation

techniques (mostly parallel to the lode axis)

residual pillars caused by the mining course, for instance because

of slewing methods.

Residual pillars require special measures to prevent spontaneous combustion:

injection of saturated

solutions of hygroscopic

salts such as CaCl2 or MgCl2 with surface-active

• components into the remaining

residual pillars.

When doing this, the

following must be observed:

• the drill holes should

be inserted from all

sides, if possible;

• the distance between

boreholes and their

depth must be adapted

to the solidity and the

looseness of the coal,

• the highest possible

saturation should be

achieved with the

lowest possible

pressures (see No.

3. 2. 2. 2)

Sealing of residual pillars

with setting insulating

substances, e.g. by vaulting,

or filling of neighboring

mines

Surface treatment by covering with powdered suitable hygroscopic

salts (CaCl2

) the coal faces of residual pillars in several

repeated operations, using 4 kg/m2 of powder.

Surface treatment by repeated application of pasty dust- fixing

materials

After taking these preventive measures, these residual pillars must be

specially supervised to prevent fires.

The following are suitable methods, among other things:

the introduction of sensor blow pipes and the taking of probes at

regualr intervals

monitoring the temperature by means of infra-red and drill hole

thermometers

B-15
observation of CO-generation in the free cross section of the

neighboring mine

3.7 Sealing of Abandoned Underworkings

Abandoned underworkings which are still open have proven to be points of

origin for spontaneous combustion fires. Insufficient heat drainage because

of insufficient ventilation will, if they last long enough, lead to

spontaneous combustion with a very high degree of probability. Underworkings

.of this kind, therefore, require particular attention.

In principle the following measures are deemed necessary:

adherence to the guidelines of the LOBA NW for barraging and

sealing of underworkings

sufficient ventilation during the stripping phase,

timely start of the stripping phase after abandoning the

underworkings to be stripped,

expedient stripping operations,

selection of a suitable location for the sealing, so that the

underworkings can be sealed permanently, solidly and tighly,

selection of suitable buttresses and dam building materials

selection of suitable dam length dependent on the pressure

difference in the ventilation and on the degree of looseness of

the surrounding rock formation,

observation of instructions regarding the use of dam building

materials and the use of the required operational means

observation of instructions regarding long-range sealing.

3.8 Preventive Inertization

When mining for topcoal or winning operations in parallel lodes with roof

coal, from which coal can reach the loose rock area, there is great danger of

spontaneous combustion. Based on experiences gained abroad, this danger can

be limited by the preventive continuous insertion of inert gas into the loose

rock area.

Thus far, the experiences with

inertization relate to retreating mining

operations only.

The liquid nitrogen is vaporized in a cold process above ground, inder normal

atmospheric conditions and taken into the

mining site by means of steel pipes with

diameters up to 150 mm. The pipeline along the ventilation

channel takes the nitrogen gas to the

loose rock area, flowing at 500 m

3 per

hour and, by reducing the oxygen contents

- t £ ' ' KOpfsire Offe Mc.(hrc~-ire_ 60_ooer80&

Flexible

B- 16
to 10rl5 %by volume, reduces the danger

of spontaneous combustion. Within the

loose rock area, perforated steel pipes

are installed every 50 m, having

diameters of 50 to 80 mm, from which air

samples can be drawn by means of flexible

plastic tubes.

3.9 Monitoring Fire Protection Parameters 3.9.1

General

The measuring, compiling and the evaluation of parameters for an early

detection of spontaneous combustion fires have become ever increasingly

important

It is therefore advisable that operations which must consider a possible

spontaneous combustion of the coal be monitored at regular intervals.

3.9.2

Possibilities of Supervision

The following parameters must be monitored especially, so that beginning

spontaneous combustion fires can be detected:

CO-generation (ppm- contents , or, respectively, amount of CO

generated, in 2/min)

Ventilation current (slow air current)

Pressure difference

Temperature development

The required measurements can be conducted by means of

portable measuring devices

complete analysis

automatic and, if needed, recording measuring devices.

Details regarding methods and equipment can be found in the following

bulletins and publications:

Bulletin dealing with the use of CO-metering devices and pertinent

amendments 1975 and 1977

Bulletin for the evaluation of foul gas analysis

Training of Mine Rescue Teams Parts I and II (1973 and 1974)

Training for Gas Teams (1975)

3.9.3

Special Supervisory Measures

Especially endagered regions with increased danger of spontaneous combustion

require special supervisory measures.

These regions include:

Approaching and trailing edges,

Zones of disturbances,

Residual pillars and,

Partially mined coal and parallel hanging lodes

B-17
The following are suitable special measures:

the taking of samples from sensor blow pipes,

the taking of samples from gas drainage pipes,

measuring of temperatures (infra-red or drill hole thermometer)

shortening the time intervals of sample taking,

additional use of stationary metering devices.

These measures should be decided on in each individual case and should be well

coordinated.

3.10 Information and Instruction

The effectiveness of the planned and executed measures for the prevention of

spontaneous combustion fires will depend on thorough instruction and exchange

of information among the responsible personnel.

These are:

the plant management,

the staffs for planning and operational areas,

the safety and mining units

operational supervisory personnel (fire bosses, gas men, foremen),

supervisory personnel in the affected mines.

The instruction of this personnel is the responsibility of the plant

management, in cooperation with the responsible staff and expert personnel

offices. It should be repeated in set intervals.

Newly hired persons must be advised by instructed personnel of the special

dangers of spontaneous combustion before they take over certain duties.

For a coordination of the planned measures to be taken, comprehensive

information of the responsible persons is absolutely necessary. It is

recommended that, for this purpose, a complete information system and a written information service regarding planned or applied changes be set up.

4 Publications

Babokin:

Mining losses in coal mining as cause for the generation of spontaneous

combustion fires

International Conference Czechoslovakia, October 1966

Both and Weinheimer:

A remarkable case of a spontaneous combustion fire in the cooperative mine

HausAden

Glueckauf, 5 Feb 76

Both and Stark:

Fighting a hidden mine fire by pressure equalization

Glueckauf, No. 13 (1974) p. 514-517.

Both:

B- 18
Prevention of spontaneous combustion fires in mines by cutting off of slow air

currents

Glueckauf, 30 Mar 72

Buecher

Spontaneous combustion fires in the Ruhr area, their dependence on geological

and operational conditions

Glueckauf, 21 Nov 62

Bykov

Evaluation and forecasting of fire hazards in the mines of the Moscow region

Izvest.VUZ, Gom z 11 (1968) No. 8, p. 62/64

Chamberlain and Hall:

Examination of Factors preventing or furthering spontaneous combustion of coal

Special Printing of the National Coal Board

Chamberlain:

Spontaneous Combustion of Coal; Examination of Inhibitors and Preventors

Colliery Guardian 1974, March No. 3, p. 79/82

Externbrink and Lewer:

Reduced danger of spontaneous combustion by use of calcium chloride mineral

powder

Glueckauf, 1971, No. 17, p. 652/653

Flachowsky and Voelkel:

Some methods for the fire prevention and fighting in the VED coal mine

Oelsnitz

Bergakademie 19 (1967) No . 1 , p. 16/21

Foelfoeldy

Results in the field of protection from mining fires

Information of Hungarian Institute for Mining Research, 1970, No. 13,

p. 153/159

Gorbacev:

The application of clay-chloride paste in the prevention of spontaneous

combustion of coal

Bezop. Truda, Moscow, 11 (1967) p. 31-33

Lewer

Means for the prevention of spontaneous combustion of coal

Disclosure Document 2 129 462 (German Patent Application 4 Jan 73)

Lewer

Prevention of spontaneous combustion by the use of calcium and magnesium

chloride with surface -active agents

(Disclosure document 2 419 144, German Pat. Application 30 Nov 75)

Lewer

B-19
Method for the dust control when packing in coal stripped spaces and for the

simultaneous prevetion of fires in packing areas in coal mining

Disclosure document 2 440 175, 25 Nov 76

Lewer

Prevention of spontaneous combustion of coal

Chemical Works Kalk GmbH, 30 Aug 1977

Lindenau and Maerskaja:

The determination of fire hazards in the winning areas of coal mines and their

ventilation techniques

Institut Vost N II

(Scientific Research Institute for Mine Safety East)

B- 20
Matusek:

Model for the criteria to evaluate hazards of spontaneous combustion of coal

in longwalls

Paper D 4, International Conference Varna 1977

Mueller, R.

Method for the prevention of spontaneous combustion fires in abandoned

workings and loose rock areas of Saarbergwerke AG

Glueckauf 103 (1967) No. 22, p. 1125/31

Dr. Muenzner:

The Influence of foreign substances on the spontaneous combustion behavior of

hard coal

Glueckauf Research Magazine, April 1972, Part I and Part II

Dr. Muenzer and Peters:

Adiabatic oxidation of hard coal to characterize its spontaneous combustion

behavior

Brennstoff -Chemie , Vol. 50 (1968)

Dr. Muenzer and Peters:

Results of recent investigations regarding the spontaneous combustion behavior

of hard coal, petroleum and coal, natural gas

Petrochemie, 22nd Year

Price

Information regarding spontaneous combustion in coal mines

The Wigan Coal Corporation

Kirkless, Wigan

Rudenko

The prevention of spontaneous combustion of coal in the mines of Combine

"Artemugol"

Besopasnost Truda 1975, No . 7 , p. 54/55

Schewe and Kugler:

Fighting a fire in the mine of Osterfeld by introduction of nitrogen

Glueckauf 1975, p. 467/472

Dr. Sebor and Hofbauer:

Chemical methods for the prevention and fighting of fires in mines

VEB Deutscher Verlag Fuer Grundstoffindustrie 1962

Siepmann:

The saturation of residual pillars with solutions of hygroscopic salts

(unpublished; Herne 15 Feb 77)

N.N. : Measures for the prevention of spontaneous combustion fires in mines

Information of BAG

Niederrhein, September 1973

B- 21
Appendix

Planning Aids for the Evaluation of Danger of Spontaneous Combustion

1 General

Spontaneous combustion of coal is more or less influenced by several

geological conditions and measures taken in the course of mining operations

singly or in combinations thereof. In the presence of several combined

criteria, the danger of spontaneous combustion increases. We have practical

experiences regarding the influence of certain criteria and their effect on

the spontaneous combustion and also regarding the influence of a combination

of them, but proven scientific knowledge is still outstanding.

As a planning aid for the evaluation of a possible danger of spontaneous

combustion we have, for the time being, supplied a system whereby, with the

aid of a form (see enclosure 1) the experiences gained may be utilized while

there is no claim made that the statements made are scientifically proven. As

soon as additional experience and insights have been gained, based on

continuous observation and intensive research, this planning aid will be

examined from time to time as to its applicability and, if needed, will be

updated to represent the state of the latest knowledge.

2 Instructions for Use of the Planning Aid

The evaluation of a possible danger of spontaneous combustion for a mining

operation begins with the selection of design alternatives. After selecting

one or several possibilities, a check is made how conditions furthering

spontaneous combustion and other conditions affect these alternatives.

The form (enclosure 1) contains a listing of 10 criteria which, according to

experience, have a more or less pronounced influence on the spontaneous

combustion of coal. Their vertical arrangement represents their relative

weight and does not claim general applicability. Dependent on experience and

the evaluation of special conditions in the individual mines , the experts

responsible may weigh these criteria with numbers 1 to 10, according to their

own judgement. The example given in the form of Enclosure 2 which has been

filled out, therefore, is no more than a rough sketch of possible evaluations.

The criteria are divided into two categories. Those who cannot be influenced:

geological, tectonic, and petrographic conditions (group 1) and those which

can be influenced by measures of fire protection and ventilation techniques

with effects on the spontaneous combustion of coal (group 2)

Group 1

Depending on their type and extent, the conditions which are beyond influence,

influence the spontaneous combustion hazard and must be evaluated accordingly,

ere, too, the horizontally arranged evaluation figures from 1 to 10 in the

form for different areas within the criteria represent no more than a rough

B- 22
suggestion. With different experiences in a mine, the responsible personnel

may set them according to their own judgement.

When evaluatin the influence of the type of coal on the spontaneous combustion

behavior

the degree of carbonization,

the petrographic structure, and

the share of volatile compounds

must be taken into consideration. In individual cases, certain types of coal

may call for a higher evaluation, if there exist corresponding experiences

made in the mine

Disturbances and small -area tectonics should be evaluated, so that

characteristics

thickness of the fault area,

residual coal in abandoned workings,

structure of coal in the fault area,

situation of the fault areas relative to the mining lodes and

approaching edges

increased pyrite contents in the neighborhood are considered

accordingly.

Parallel hanging lodes must be evaluated according to:

the distance from the supported lode

their thickness and

their pyrite content.

Group 2

This group includes fire protection measures and ventilation techniques which

exert an important influence on the spontaneous combustion, but which may be

decisively influenced by the operations. Their evaluation will be made after

qualitative judgement of the measures taken. The responsible technical

personnel will have to determined for each design alternative whether, because

of ventilation or geometrical conditions,

the treatment of residual pillars,

the execution of sealing operations,

the securing of approaching and trailing edges

will be required.

According to the prevailing opinions here and abroad, slow air currents are an important criterion for the generation of spontaneous combustion fires. The

knowledge regarding the extent and the nature of dangerous slow air currents

is still negligble and should be increased by purposeful investigations. The

defineable air currents in drill holes must be seen as a special form of slow

air currents, with the holes serving the purpose of gas drainage and testing,

as well as for the relaxing of dangers of pressure bursts. Because of their

intentional introduction of oxygen and the insufficient drainage of

underground heat, they are a latent hazard of spontaneous combustion.

B-23
The differentiated evaluation of the danger of spontaneous combustion based on

the sums and the sub-division into four danger groups is based on past

operational experience. After the system has been tested and now experiences

have been gained, the limits for danger groups are to be re-examined and, if

necessary, newly defined.

B- 24
Planungshilfe

ffUr

die

Bewertung

der

Selbstentzundungsgefahr

B-25
Planning

Aid

for

the

Evaluation

of

Danger

of

Spontaneous

Combustion

<S>

Direction

of

Mining

Operation

Retreating

Advancing 3 <4-1 i

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u o <o o o o o MOM 3 M'3

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A A

ft O' V V

B-26
Planungshilfe

(Or

die

Bewertung

der

SelbstentzUndungsgefahr

B-27
Translation of Additional Terms from Enclosure 2

The second form of the Planning Aid is filled out, as an example, with data

for a fictitious mine. Translation applies to lower portion of the form.

<A> Alternative

retreating and advancing

<B> Hydraulically setting material

<C> Pillars (old ties) foam

<D> Not applicable

<E> Iso-Foam

<F> UR

- Uv <G> with UR = little danger of spontaneous combustion

with Uv — medium danger of spontaneous combustion

must be expected

B-28
Appendix C

The Canadian System

Evaluation of Coal Mine Susceptibility to Spontaneous Combustion

This system for predicting the likelihood of spontaneous combustion in coal

mines is based on the calculation of a Risk Index that is the combination of

two factors - the nature of the coal and the mine environment. The product of

these factors is the Risk Index. It is a single valued index that corresponds

to the anticipated likelihood of spontaneous combustion incidents.

Risk Index = (Liability Index) X (Environmental Index) (1)

The nature of the coal to self -heat is measured by the Crossing Point Method

developed by Feng, et al [1] . This method experimentally determines the

relative ignition temperature at which measureable self -heating occurres . A

Liability Index or Figure of Merit is calculated by

Ignition Temperature

Liability Index = x 100 (2)

Furnace Heating Rate

Liabilitv Index for Coal

Index Rating

>10 High

5-10 Medium

0-5 Low

The ignition temperature is the temperature at which the furnace air and the

sample core are equal. The furnace air is increased at a steady rate until

the sample core temperature crosses the furnace air temperature.. The results

of equation (2) are indicative of the coal sample's susceptibility to selfheat

The Mine Environment Index is determined by classifying the mine environment

according to three parameter: coal loss; fissuration; and ventilation pressure

differential. A Mine Environment Index is obtained by reading a value from a chart the lists the three parameters and the index value.

C- 1
The Mine Environment Index is determined by:

Mine Environment Index

Grout) Coal Loss Fissuration

Ventilation

Pressure

Differential Index Ratine

A Normal Natural Normal 1 Low

B High

Normal

Normal

Natural

High/Induced

Natural

Normal

Normal

High

2 Medium

C Normal

High

High

High/Induced

Natural

High/Induced

High

High

Normal

3 High

D High High/Induced High 4 Very H.

This index and the Liability Index are combined according to equation (1) . The resulting Risk Index translates into a self-heating susceptibility rating

factor according to:

Risk Index for Mines

Index Ratine

0 - 10 Low

10 - 20 Medium

20 - 40 High

A "Low” rating indicates a low susceptibility to self -heat and thereby

initiate smoldering combustion within a coal bed or gob. A "High" rating

indicates that mine operators should implement precautionary procedures to

detect spontaneous combustion hot spots.

References

[1] Feng, K.K.

Chakravorty, R.N.

and Cochrane, T.S., Spontaneous

Combustion - A Coal Mining Hazard, CIMM Bulletin, October 1973.

C- 2
Appendix D

Adiabatic Furnace Data

Individual Graphs
RATE

OF

TEMPERATURE

CHANGE

(°C/min)

Adiabatic Furnace Test Results for the Determination

of the Reaction Rate of Coal Sample "A" in Dry Air

D-2
RATE

OF

TEMPERATURE

CHANGE

(°C/min)

Adiabatic Furnace Test Results for the Determination

of the Reaction Rate of Coal Sample "C" in Dry Air

D-3
RATE

OF

TEMPERATURE

CHANGE

(°C/min)

Adiabatic Furnace Test Results for the Determination

of the Reaction Rate of Coal Sample "B" in Dry Air
RATE

OF

TEMPERATURE

CHANGE

(°C/min)

Adiabatic Furnace Test Results for the Determination

of the Reaction Rate of Coal Sample "E" in Dry Air

D-5
RATE

OF

TEMPERATURE

CHANGE

(°C/min)

Adiabatic Furnace Test Results for the Determination

of the Reaction Rate of Coal Sample "D" in Dry Air

D-6

HTT

'mr—
RATE

OF

TEMPERATURE

CHANGE

(°C/min)

Adiabatic Furnace Test Results for the Determination

of the Reaction Rate of Coal Sample "H" in Dry Air
RATE

OF

TEMPERATURE

CHANGE

(°C/min)

Adiabatic Furnace Test Results for the Determination

of the Reaction Rate of Coal Sample "F" in Dry Air

D-8
RATE

OF

TEMPERATURE

CHANGE

(°C/min)

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

TEMPERATURE (1/T x 1000 K)

Adiabatic Furnace Test Results for the Determination

of the Reaction Rate of Coal Sample "G" in Dry Air

D- 9
FORM NBS-1 14A (REV. 1 1-84) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. x NBSIR-87/3554

2. Performing Organ. Report No. 3. Publication Date

August 1987

4. TITLE AND SUBTITLE

Self Heating Properties of Coal

5. author(S) Emil Braun

S. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

U.S, DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No. X

8 . Type of Report & Period Covered

9.

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

U. S. Department of the Interior

Bureau of Mines

P.0. Box 18107

Pittsburgh, PA 15236

10.

SUPPLEMENTARY NOTES

j

1 Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11.

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

bi bliography or literature survey, mention it here)

Three methodologies for predicting the likely occurrence of self -heating in

underground coal mining environments were analyzed. No method was found to be

completely satisfactory for general coal mine applications. One evaluation system

was found to provide excellent guidelines for preplanning procedures prior to

initiating full scale coal mine operations, but it relied on past mining experience.

Another evaluation system used standard coal characterization parameters, while the

third system used a thermal test method a predictor of self-heating potential. The

self-heating properties of eight samples of western bituminous coal were determined

using the Adiabatic Furnace and the Crossing Point Method. The Adiabatic Furnace

indicated that, in general, two reaction zones existed. The first temperature

region extended from 25°C to about 85°C with activation energies varying from 32

kJ/mole to 55 kJ/mole, while the reaction zone above 85 °C exhibited an activation

energy ranging from 59 kJ/mole to 82 kJ/mole. Using this data, critical heating

rates in excess of 1.5° C/24 hrs at a strata temperature of 20° C were found to result

in self -heating. The Crossing Point Method yielded ignition temperatures ranging

from 132° C to 154° C at an average heating rate of 0.58°C/min. An alternative

plotting technique is described that suitably correlates Crossing Point Method data

with end use experience. A brief review of pertinent literature is presented to

provide an understanding of those factors affecting oxidative heating of coal.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Adiabatic furnace; coal; fire test methods; ignition temperature; self -heating;

spontaneous combustion.

13.

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