STUDIES ON SPONTANEOUS HEATING LIABILITY OF SOME INDIAN COALS

STUDIES ON SPONTANEOUS HEATING LIABILITY OF SOME INDIAN COALS
STUDIES ON SPONTANEOUS HEATING
LIABILITY OF SOME INDIAN COALS
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
BACHELOR OF TECHNOLOGY
IN
MINING ENGINEERING
BY
RANJAN KUMAR SAHU
110MN0402
DEPARTMENT OF MINING ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA-769008
2013-2014
1
STUDIES ON SPONTANEOUS HEATING
LIABILITY OF SOME INDIAN COALS
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
BACHELOR OF TECHNOLOGY
IN
MINING ENGINEERING
BY
RANJAN KUMAR SAHU
110MN0402
Under the guidance of
Prof. B.K.Pal
DEPARTMENT OF MINING ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA-769008
2013-2014
2
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA
CERTIFICATE
This is to certify that the thesis entitled “STUDIES OF SPONTANEOUS HEATING
LIABILITY OF SOME INDIAN COALS” submitted by Sri Ranjan Kumar Sahu in partial
fulfilment of the requirements for the award of Bachelor of Technology in Mining Engineering at
the National Institute of Technology, Rourkela is an authentic work carried out by him under my
guidance and supervision.
To the best of my knowledge , the matter embodied in the thesis has not been submitted to any
other University/Institute for the award of any Degree or Diploma.
Dr. B.K.Pal
Professor
Department of Mining Engineering
National Institute of Technology
Rourkela-769008
i
ACKNOWLEDGEMENT
I wish to express my profound gratitude and indebtedness to Dr. B.K.Pal, Professor, Department
of Mining Engineering ,NIT ,Rourkela for introducing me this present topic and for his guidance
, constructive criticism and valuable suggestions during the course of this work. I find words
inadequate to thank him for his encouragement and effort in improving my understanding of this
project.
I am also grateful to all the faculty members and staff of Mining Department, NIT, Rourkela.
Special thanks to Mr. B.N. Naik, Mr . B.K. Pradhan, Mr . Soma Oram for their assistance and
help in carrying out various experiments in the laboratories. I am also thankful to Sri. Alok
Ranjan Mahananda, M.Tech,2nd year ,NIT,Rourkela for helping me and assisting me in carrying
out the necessary experiments.
I would take the opportunity to thank my seniors Sri Vikrant dev Singh, Sri Nishant Pati for
helping me in collection of coal samples. I acknowledge my indebtedness to all of them whose
works have been referred in understanding and completion of this project.
Last but not the least,my sincere thanks to all my well wishers who have patiently extended all
sorts of help and moral support for accomplishing this dissertation.
Ranjan Kumar Sahu
Date :
ii
CONTENTS
SL NO.
SUBJECT
PAGE NO.
ABSTRACT
v
1.0
INTRODUCTION
1.1
General
1.2
Objective
1
2-3
4
2.0
LITERATURE REVIEW
5
2.1
6-9
3.0
MECHANISM OF SPONTANEOUS HEATING OF COAL
10
3.1
Mechanism of spontaneous Heating of Coal
11-12
3.2
Factors Affecting Spontaneous Heating of Coal
12-14
3.2.1
Geological Factors
15-17
3.2.2
Mining Factors
17-20
3.2.3
Seam Factors
20-22
3.3
4.0
Literature Review
Theories of Spontaneous Combustion of Coal
22
3.3.1
Pyrite Theory
22-23
3.3.2
Bacterial Theory
23
3.3.3
Phenol Theory
23
3.3.4
Humidity Theory
23-24
3.3.5
Coal-Oxygen Complex Theory
24
EXPERIMENTAL INVESTIGATION
25
4.1
Sample collection and preparation
26
4.1.1
26-27
4.2
Channel Sampling
Proximate Analysis
28
4.2.1
Determination of Moisture Content
28-29
4.2.2
Determination of Volatile Matter Content
30-31
4.2.3
Determination of Ash Content
31-32
iii
4.2.4
4.3
5.0
6.0
Determination of Fixed Carbon
32-33
Determination of Calorific Value
35
4.3.1
35-37
Bomb Calorimetry
4.4
Crossing Point Temperature
40-42
4.5
Flammability temperature
45-46
4.6
Wet Oxidation Potential Analysis
48-49
DISCUSSION AND CONCLUSION
54
5.1
Discussion
55-56
5.2
Conclusion
61-62
REFERENCES
63
6.1
64-65
References
iv
ABSTRACT
Spontaneous heating ultimately leading to coal mine fire pose a major problem and has been an
area of concern in mining industry since long. It has drawn the attention of industry as well as
researchers to investigate regarding its cause and determine the susceptibility of various coal
seams to spontaneous heating. Majority of coal mine fires existing in different coalfields are
primarily due to spontaneous combustion of coal. Generally mine fires start very small but
gradually it expands to a large scale and causes immaculate loss of natural assets causing
pollution of land, water ,air along with huge economic losses to the concerned organisation.
Although extensive research work has been done on this topic but proper assessment and
determination of liability of various coal types to spontaneous combustion is necessary in dealing
with such problems before, during and after mining. It would help the mine operators to plan the
extraction and working of coal before spontaneous combustion takes place. Hence, determination
of spontaneous heating liability of coal samples is essential to plan the production activities and
suitably optimise it in order to do complete extraction within incubation period and prevent mine
fire and its losses.
This project deals with the study of determination of spontaneous heating liability of coal and its
variation with respect to the coal properties. Eight coal samples were collected from different
collieries across India: Mahanadi coal fields (MCL), Bharat coking coalfield Limited (BCCL),
Singreni Collieries Company Limited (SCCL), South Eastern Coalfields Limited (SECL), North
Eastern Coalfields Limited (NECL), Western Coalfields Limited (WCL) and were subjected to
different experimental techniques for assessment of their spontaneous heating liability.
v
LIST OF FIGURES
No.
Figure
Page no.
1.1.1 Curve showing Difference between Spontaneous heating and Weathering
2
4.2.1 Oven for Moisture Content Determination
29
4.2.2 Muffle Furnace for Ash Content & Volatile Matter Determination
30
4.3.1
38
4.4.1
Experimental setup of Digital Bomb Calorimeter and auto filling of Oxygen
Schematic Layout of Crossing Point Temperature(CPT) Apparatus
40
4.4.2 CPT Curve for Sample MCL1
43
4.4.3 CPT Curve for Sample NECL1
43
4.4.4 CPT Curve for Sample SECL1
43
4.4.5 CPT Curve for Sample WCL1
44
4.4.6 CPT Curve for Sample SCCL1
44
4.4.7 CPT Curve for Sample BCCL1
44
4.4.8 CPT Curve for Sample SECL2
45
4.4.9 CPT Curve for Sample MCL2
45
4.5.1 Schematic Layout of Flammability Apparatus
47
4.6.1 Experimental Set up of Wet Oxidation Potential Apparatus
49
4.6.2 Wet Oxidation Curve for Sample MCL1
50
4.6.3 Wet Oxidation Curve for Sample NECL1
51
4.6.4 Wet Oxidation Curve for Sample SECL1
51
4.6.5 Wet Oxidation Curve for Sample WCL1
51
vi
4.6.6 Wet Oxidation Curve for Sample SCCL1
52
4.6.7 Wet Oxidation Curve for Sample BCCL1
52
4.6.8 Wet Oxidation Curve for Sample SECL2
52
4.6.9 Wet Oxidation Curve for Sample MCL2
53
5.1.1 Variation of Crossing Point Temperature with Ash content
57
5.1.2 Variation of Crossing Point Temperature with Moisture
58
5.1.3 Variation of Crossing Point Temperature with Volatile matter
58
5.1.4 Variation of Flammability Temperature with Ash Content
58
5.1.5 Variation of Flammability Temperature with Moisture
59
5.1.6 Variation of Flammability Temperature with Volatile Matter
59
5.1.7 Variation of Wet Oxidation Potential Difference(∆E) with Ash content
59
5.1.8 Variation of Wet Oxidation Potential Difference(∆E) with Moisture
60
5.1.9 Variation of Wet Oxidation Potential Difference(∆E) with Volatile Matter
60
5.1.10 Variation of WITS-EHAC Index with Moisture
60
vii
LIST OF TABLES
No
Table
Page no
4.2.1 Volatile Matter(VM),Ash(A),Moisture(M) and Fixed Carbon(FC) of coal
34
4.3.1 Ultimate Heating value(UHV) of coal and its Grade
38
4.3.2 Gross Calorific Value(GCV) of coal and its Grade
39
4.4.1 Liability Index(Mahadevan et al.,1985)
41
4.4.2 Classification of CPT(Mahadevan et al.,1985)
42
4.4.3 Crossing Point Temperatue, Liability Index, WITS-EHAS and its Risk Rating
42
4.5.1 Flammability Temperature of the coal samples
47
4.6.1 Wet Oxidation Potential Difference of the coal samples
50
5.5.1 Correlation Coefficients between various Susceptibility parameters and the
57
Proximate analysis values
viii
Chapter 1
INTRODUCTION
General
Objective
1
1.INTRODUCTION
1.1 GENERAL OVERVIEW
Coal mine fire is one of the key issues in the coal mining industry in India as well as abroad.
Continuous research work and analysis has concluded that major part of the mine fire is due to
the soul reason of spontaneous heating. It accounts for a huge loss of beyond 300MT of per year
across the globe.
Spontaneous combustion of coal refers to the process of self heating followed by getting oxidised
in a self propellant manner resulting in ignition without any external source of heat. Coal when
exposed to air, it absorbs oxygen and evolves heat. All types of coal generally absorb oxygen and
oxidise to evolve co, CO2, and H2O along with heat. This heat slowly dissipates into air. If the rate
of oxidation is slower and the amount of heat dissipation is higher than the amount of heat
accumulation then coal doesn’t reach ignition temperature (Critical Temperature for Oxidation)
and the process is referred as weathering. But in case the oxidation rate is faster and the amount
of heat accumulation is higher than the amount of heat dissipation ,it makes the coal reach the
critical temperature for oxidation resulting in coal ignition which is referred as Spontaneous
combustion.
Fig no 1.1.1 Curve showing Difference between Spontaneous heating and Weathering
2
Spontaneous heating results in serious accidents causing environment pollution, considerable
economic losses, loss in property and life etc. Spontaneous heating in any area of a mine affects
working of other portions of the mine causing ventilation problems due to evolution of anoxic
gases. Past studies on this topic reveals that most of the mine fires resulting from spontaneous
heating could have been avoided if necessary arrangements had been done. Therefore there is a
need of assessment of the susceptibility of various coal towards spontaneous heating and its
liability along with its risk rating for quantified extraction of coal.
Different coal producing countries use various different methods to determine spontaneous
heating tendency of coal viz. 1. Crossing Point Temperature in India , 2.Olpinski Index in
Poland, 3.Russian U- Index in Russia, 4. Adiabatic calorimetry in U.S.A etc. Apart from this
various recognised researchers adopted different methods for studying the spontaneous heating
liability of different coal samples. Few of those methods are:
1.Wet Oxidation method (Singh et al., 1985; Tarafdar and Guha, 1989).
2.Differential Thermal Analysis (Banerjee and Chakravarty, 1967; Gouws and Wade, 1989a)
3.Gas Indices studies (Panigrahi and Bhattacharjee, 2004; Singh et al., 2007)
Despite of continuous research work there has been no single conclusive specific method
suggested by researchers for determination of spontaneous heating liability of coal samples.
Therefore, it is been considered that a number of methods need to be carried out in order to
conclude with fair accuracy.
3
1.2 OBJECTIVE OF THE PROJECT
1. Collection of samples from diversified coal fields.
2. Determination of Intrinsic Properties along with Calorific values of the different coal samples.
3. Determination of liability index, WITS-EHAS index of the coal samples and its risk rating by
carrying out various experimental techniques as Crossing Point Temperature, Flammability
Temperature, Wet Oxidation Potential Difference method.
4. To correlate various susceptibility indices with the intrinsic properties of coal and identifying
the property which hints majorly at the spontaneous heating tendency of the coal samples.
4
Chapter 2
LITERATURE REVIEW
5
2.1 LITERATURE REVIEW
The following is the brief overview of the various research work done by different researchers in
order to determine the liability of coal samples to spontaneous heating and their observation
regarding the same.
Mahajan,Tomita and Walker(1976) – They used DSC technique and reported DSC curves for
12 coal samples of various ranks in a helium atmosphere at a flow rate of 1ml/min and a
temperature range of 100 to 580 °C at a constant heating rate of 10 °C/min. Samples of around
12 -20 mg was used with reference material being alumina. They concluded that the thermal
effects of coal, ranging in rank from Anthracite to Bituminous were endothermic in nature. In
case of only Sub-Bituminous coal & Lignite exothermic heat was observed.
Feng et al(1973) – methodised a composite Liability Index by using various results of
CPT(Crossing Point Temperature), called FCC Index.FCC Index was calculated as per the
equation:
The lower limit was set to 110°C (Heating rate) in order to ensure that all moisture had
evaporated from the sample.
The upper limit of 220°C was chosen in order to ensure there would have been a little evolution
of Volatile matter below this temperature.
6
Gold(1980)- demonstrated the occurrence of exothermic processes associated with the
involvement of Volatile Matter in or near the plastic region of the samples. He concluded that the
temperature and the magnitude of the exothermic peak were strongly affected by the particle size
,heating rate and the sample mass.
Banerjee and Chakraborty(1967)- suggested DTA(Differential Thermal Analysis) for the
study of spontaneous combustion of coal, generally in classifying coal depending upon their
susceptibility to self heating. They prescribed particular steps for DTA studies and recommended
Calcined Alumina as reference material for DTA experiments. Heating Rate was maintained at 5
°C/min. Typical temperatures obtained from various coals were included in this phenomenon to
explain self heating process.
Mahadev and Ramlu(1985)- proposed an Index known as MR Index and objected to the
arbitrary selection of temperature range in the FCC Index.
Reciprocal of the Liability Index was found to be increasing with Self Heating Liability.
Tarafdar & Guha(1989)- reported the results of Wet Oxidation of coal be Alkaline
Permanganate Solution involving measurement of differential temperature at different base
temperature, at a constant heating rate. They experimented 7 samples of known CPT by Wet
Oxidation Method & then found out a correlation between CPT values & the corresponding
differential peak temperatures, and between CPT & the observed Potential changes.
7
They suggested that Potential Difference measurements during Wet oxidation of coal and the
differential temperature may be used as alternative techniques for the assessment of liability
towards spontaneous heating.
Banerjee(1985)- observed various experimental techniques available worldwide to assess and
analyse spontaneous heating susceptibility & summarised and organised them as follows:
MOISTURE
CHEMICAL
CONSTITUENTS
OF COAL
VOLATILE
MATTER
RANK
CORRELATION
ASH CONTENT
CPT STUDIES
FIXED CARBON
DTA STUDIES
DSC STUDIES
THERMAL
STUDIES
H2O2
EXPERIMENTAL
TECHNIQUES
PUFF
TEMPERATURE
OLPINSKI INDEX
ADIABATIC
CALORIMETER
PEROXY
COMPLEX
ANALYSIS
OXYGEN AVIDITY
STUDIES
RUSSIAN U INDEX
WET OXIDATION
8
Panigrahi et al.(1997)- They conducted experiments on 10 coal samples from Jharia. The
Carbon, Hydrogen, Nitrogen & Sulphur contents of these samples were determined by Fenton’s
method of Ultimate Analysis. CPT of above samples were also determined & attempts were
made to correlate the Russian Index & CPT of coal samples with its basic constituents viz.
Carbon, Hydrogen and Ash Content and was categorised as a handy method of coal
categorisation in Indian Context from the point of Susceptibility of Spontaneous Combustion.
Bannerjee(1972)- conducted CPT analysis of a number of Indian coal samples. He stated that
coals with Crossing Point Temperatures between 120°C & 140°C can be categorised as highly
susceptible to spontaneous heating. Coal samples with CPT from 140°C to 160°C can be
categorised as moderately susceptible ones and those above 160°C are poorly susceptible.
Nandy et al.(1972)- correlated the variation in Crossing Point Temperature values with the
Volatile Matter, Oxygen Percentage & the moisture content of coal. He observed that CPT was
inversely proportional to the above components. But beyond 4 to 6% moisture content,35%V.M
there is not much change in CPT values. Generally above 4 to 6% moisture content in coal, CPT
shows a rising trend.
Hence ,it could be noticed from the above reviews that there is no universally accepted method
for determining the spontaneous heating liability of various coal samples.Thus it was decided to
carry out different experimental techniques to reach a clear conclusion viz. Wet Oxidation
Potential Difference Analysis, Flammability Temperature Analysis, Crossing Point Temperature
Analysis in order to accomplish the objectives of this present dissertation work.
9
Chapter 3
MECHANISM OF
SPONTANEOUS HEATING OF
COAL
Mechanism of Spontaneous Heating of coal.
Factors affecting Spontaneous Heating Liability of coal.
Theories on Spontaneous Heating
10
3.0
MECHANISM OF SPONTANEOUS HEATING OF COAL
3.1. MECHANISM:
Coal is a stratified organic heterogeneous rock with carbon content varying from 95% in
Anthracite to 60% in Lignite stage(young coal) with Bituminous and Sub Bituminous in
between. The automatic oxidation of coal is a complex physio- chemical process accompanied
by the absorption of oxygen followed by formation of coal oxygen complex and further its
decomposition to liberate heat. Because of the enormous diversity of composition of mineral
matter in coal the complexity of the above process is tremendous. Various overlapping
simultaneous reactions do take place during oxidation of the heterogeneous coal mass. Generally
the proneness of coal to auto oxidation is proportional to the rate of oxidation of coal at ambient
temperature. This low temperature aerial oxidation consists of various structural alteration of the
coal mass resulting in the formation of numerous stable chain reaction due to large number of
oxidation states of carbon followed by formation of a variety of strong carbon oxygen bonds.
The observable, structural and compositional changes hint that the above process is a time
dependent dynamic process. The entire process is heterogeneous:
1. Intrinsically- due to presence of two phases: solid and gas.
2. Extrinsically- due to diversified structural changes.
Generally coal gets heated up when it absorbs oxygen and the decomposition procedure can be
explained as follows:
The rate of oxidation is petty sluggish below 50°C and it accelerates above 50°C but after it
crosses 80°C a steady state is attained for a short interval of time probably due to removal of
moisture. Following it removal of oxides of carbon begins at 120°C. The rate of interaction of
11
oxygen with coal accelerates up to 180°C and next to decomposition starts between 120°C and
180°C. Finally the self sustained process of combustion begins somewhere around 220°C to
275°C with erotic rise in temperature until ignition point is reached.
3.2. Factors affecting spontaneous heating of coal
There are numerous methods which affect the Spontaneous heating of coal. The classification of
the factors as per categories is as follows:
SEAM THICKNESS
SEAM GRADIENT
CAVING
CHARACTERISTICS
FAULTING
GEOLOGICAL
FACTORS
COAL OUTBURSTS
FRIABILITY
DEPTH OF COVER
GEOTHERMAL
GRADIENT
12
MINING
METHODS
RATE OF
ADVANCE
PILLAR
CONDITIONS
ROOF
CONDITIONS
CRUSHING
PACKING
EFFECT OF
TIMBER
ROADWAYS
LEAKAGE
MINING
FACTORS
MULTI SEAM
WORKINGS
COAL LOSSES
MAIN ROADS
WORKED OUT
AREAS
HEAT FROM
MACHINES
STOWING
VENTILATION
PRESSURE
BAROMETRIC
PRESSURE
CHANGE IN
HUMIDITY
13
RANK
ASH/MINERAL
MATTER
PETROGRAPHIC
COMPOSITION
EFFECT OF
PREVIOS
OXIDATION
TEMPERATURE
AVAILABLE AIR
PARTICLE SIZE
SEAM FACTORS
MOISTURE
SULPHUR
PHYSICAL
PROPERTIES
HEATING DUE TO
EARTH
MOVEMENT
BACTERIA
PYRITE CONTENT
THERMAL
CONDUCTIVITY
14
3.2.1. GEOLOGICAL FACTORS
Seam thickness: whenever the seam thickness is more than the parts of the seam which can be
mined in one go, such an area is more susceptible to spontaneous heating because the un-mined
area will be exposed to a sluggish ventilation flow. It was found that spontaneous heating was
invariably dependent on the method of working, friability of coal, type of ventilation and
thickness of the seam. Generally the bands present with section of thick seam are more liable to
spontaneous heating. The more thicker the seam, it becomes more difficult to avoid leaving high
risk coal within the goaf region. Precisely, it is advisable to do selective mining in order to leave
the lowest risk coal as waste. But such a thing is not always practically possible. On few
occasions a coal floor or roof can be left un-mined where:
The natural roof or floor tends to be weak.
The seam is thick.
Presence of inferior coal below or above the mined area.
Seam gradient: Seams which are flat in gradient are less prone to spontaneous heating. But in
case of steeper seams there arises convection currents due to difference in temperature leading to
air currents in the goaf region. Also within the extracted areas there may be flow due to
buoyancy due to difference in densities of methane, carbon dioxide, nitrogen at different
elevation leading to flow of air influencing the development of spontaneous heating in waste,
goaf or old workings.
Caving characteristics: In mines where partial extraction is done leaving sufficient pillars
behind to support the super incumbent strata , the caving characteristics are of least significance.
But in such a situation it is desirable to fill the waste as fine material in order to reduce the
amount of leakage air flow within the region of extraction. Thus the falling material occupies the
15
maximum possible volume in order to fill the void created. It should be kept in mind the friable
strata to fall should not be of carbonaceous type as it may lead to spontaneous heating and
catching of fire of the left over pillars. Thus in such case the pillars should be extracted
judiciously in order that the entire strata caves down.
Faulting: Spontaneous combustion is immensely influences by faulted ground. The grinding
action along the fault plane with the fine coal formations may lead to spontaneous combustion.
Faults generally slows down the pace of face advance leading to attendant risk of heat
development.
Coal outbursts: It is majorly found in hard coal formation rather than soft and low grade coals.
In coal outbursts generally fine coal is formed and is thrown away which might lead to passing
of it through any place of active heating resulting in spontaneous combustion. Chances of coal
outbursts are more with increasing depth. Factors associated with outbursts of spontaneous
heating are:
1. Volcanic acivities
2. Mining method
3. Geological conditions
4. Characteristics of roof and floor
5. Characteristics of rock and coal
6. Contained gas
7.Overburden stress
8.Permeability of the gas reservoir
Coal friability: Friable coals expose larger surface area resulting in higher rate of oxidation
tending to achieve more heat per unit volume.
16
Depth of cover/Geothermal Gradient: With increase in depth, the natural strata temperature
increases along with the in-situ base coal temperature. The geothermal gradient is 40 metre per
degree centigrade. Thus high depth ensures high temperature of exposed strata after extraction
which might lead to increased susceptibility to spontaneous combustion.
3.2.2. MINING FACTORS
Mining methods: Advancing long wall mining method leaves an extracted area lying between
the entries to serve the working places. Generally the creation of ventilating pressure difference
encourages air to pass though these areas leading to chances of spontaneous heating. Retreating
system of extraction avoids above problem in cases where continuous stowing is done along with
the advancement of the coal face, except the case when bleeder entry system is made to ventilate
the waste.
Rate of advance: There is always entry of air to waste areas in close proximity to the working
face by ventilating pressure or by bleed action. In such cases the rate of flow at times may be
critical. So generally in a working face a piece of coal is exposed to air for the time period
required in the advancing of the face. This time should not be excessive in order to avoid
spontaneous heating.
Pillar size: It has a direct impact on liability of coal to spontaneous heating. Pillars should be of
optimum size in order to avoid crushing at the edges leading to spontaneous heating. The depth
of cover, strength of coal and other parameters determine the size of coal. Also crushing of
pillars is another sign of methane emission and spontaneous heating.
Roof conditions: Poor roof generally allow shock waves to pass through it subsequently leading
to have cracks within it. As a result the fallen areas of the roof are supported by timber supports.
17
These cavities accumulate methane which acts as a source of ignition after localised spontaneous
heating.
Crushing: It is primarily found at pillar edges and at worked out areas. As a result of crushing at
pillar edges it may lead to leakage paths via itself tending to interaction of loose coal with air and
may lead to auto oxidation. Also in worked out areas loose coal is present due to spalled pillars
or roof falls and sluggish ventilation at such places may lead to spontaneous combustion.
Packing: Low quality packs often used in coal mines pose a big threat and are more liable to
spontaneous combustion.
Effect of timber: In past generally timber props or other timber equipments were generally
found in close proximity to spontaneous combustion regions, which gave rise to a school of
thought that timber, is responsible for spontaneous heating.
Roadways : Roadways always pose a sign of threat and mostly leads to spontaneous combustion
due to availability of loose coal as well as leakage air is available which adds up to auto
oxidation of coal. More the amount of exposed coal more are the chances of liability of
spontaneous heating. The common points of such incidents are:
1. Juntions
2. Air Crossings
3. Doors, Regulators, Connecting roads
4. Obstruction in roads
5. Old roadways
Leakage: For spontaneous heating to take place there must be a steady supply of oxygen and an
area should be available where a build-up of heat is possible. Such a case is attained by air leaks
through fissures available in solid coals resulting in shallow –seated heating circumstance.
18
Above situation is found when leakage paths are available around air regulators, doors, air
crossings etc or elsewhere high pressure gradient is available across the stopping’s leading to
drivage of air across it.
Multi seam workings: In case of multi seams where an un- mined seam is available underneath
another totally mined seam, it may lead to a situation where leakage of air may be from upper
seam to lower seam leading to spontaneous heating chances of un-mined underlying coal seam.
Coal losses: Basically all mining methods lead to spillage of some remmant coal.It is never
possible to extract 100% of coal by any mining method. So if there is availability of air and there
is presence of some remaining crushed waste coal and there also exists a place where potential
accumulation of heat is possible then it may pose a major threat resulting in spontaneous
combustion.
Working out areas: Within the area of ventilating air-screen worked out areas pose a potential
area for spontaneous combustion. Also addition of heat by working of machinery may demand
additional air to be circulated requiring a high ventilating pressure with consequently increased
risk of leakage.
Stowing: It is an weapon to control spontaneous combustion by sealing off the extracted area.
Ventilating pressure: In any mines the flow of air is generated by creating pressure difference.
Such pressure difference are created by mine fans or by natural ventilation, but the distribution of
pressure is primarily dependent on the distribution of the air quantity and resistance of the
various mine paths. Wherever there is a pressure difference it leads rise to leakage or flow of air
via strata or pillars etc. Thus ventilating pressure is one of the prime causes of spontaneous
heating.
19
Barometric pressure: Generally air at any cost finds a way into a sealed –off area as a result of
the underlying cases:
1. Barometric changes.
2. Continous leakage resulting from a pressure difference between the intake and return
stopping’s.
3. Fluctuations in ventilating pressure resulting from the opening of doors and the movement of
cages and mine cars.
Humidity :It is the property of coal to absorb moisture and when it does so from the ventilation
air it gets heated up due to release of latent heat of condensation and due to chemisorptions
effects and the reverse happens when it losses moisture by evaporation. Thus humidity plays a
major role in case of any misbalance.
3.2.3. SEAM FACTORS
Rank: The rank of coal is a function of the original plant debris from which it is formed and the
amount of it has undergone. The increase in carbon content and decrease in oxygen content
signifies high rank coal. Basically high rank coals oxidise very slowly in comparision with low
rank coals. Low rank coal poses more prone to spontaneous heating.
Petro graphic composition: Various petro graphic tests do suggest that fusain is least reactive,
and Durain is more reactive than Vitrain. Calculations of the reaction velocities of
exinite,inertinite and vitrinite do suggest that beyond 165 degree Fahrenheit exinite has a much
higher oxidation rate than others. Hence a quantitative count of macerals may help in
determining susceptibility of coals towards spontaneous combustion.
20
Temperature : With increase in temperature, the rate of absorption of oxygen increases
substantially. With every 18 degree Fahrenheit rise in temperature the average rate of oxidation
doubles.
Available air: In case of inadequate availability of oxygen, the rate of oxidation is very slow and
there is no appreciable accumulation of heat. In case if the rate of air flow is high then the
oxidation rate rises but at the same time the accumulated heat gets carried away by the flowing
air. Thus if the flow rate is optimum then at that time it comprehensively supports spontaneous
heating.
Particle size: Powdered coal poses more threat than solid coal. Solid coal has low permeability
to allow passage of air through it. Thus small particles will show more proneness to spontaneous
heating.
Moisture: Its effect is uncertain. Presence of moisture in small quantity adds up to spontaneous
heating rate but large amount of moisture retards spontaneous heating rate. But in case of
stockpiles drying and wetting continuously add up to rate of spontaneous heating.
Sulphur: Initially it was believed that pyrites add up to spontaneous heating due to the presence
of sulphur but later research work justified that coal absorbs oxygen in the absence of sulphides
also which changed the belief.
Ash/Mineral Matter: Chemicals play a major role in accelerating or retarding spontaneous
heating. Alkalis act as accelerators where as borated and calcium chlorides act as retardants. Also
preheating improves spontaneous heating liability of coal samples. High ash content retards
spontaneous heating process while silica detoriates the rate.
21
Effect of extent of previous oxidation: Fresh coals are relatively more reactive to oxygen rather
than weathered coal which was justified from the fact that fresh coals had much lower ignition
temperature than similar weathered coal.
Physical properties: Physical properties as porosity, thermal conductivity, hardness, specific
heat can also do affect the rate of oxidation.
Heating due to earth movement: Heat evolved during crushing of rocks, goaf fall or pillar
crushing may act as a cause of starting of self heating process.
Bacteria: Bacterias such as thiobaccillus ferro-oxidants and ferro-baccillus thio-oxidants play a
major role during the auto oxidation process of coal. These bacteria’s are inactive at -193°C and
560 °C.
Pyrite contents: The sulphides present in coal are pyrite, marcasite ,sparelite, galena,
chalcopyrite and melnikovite-pyrite. Pyrites generally show catalytic effect as their oxidised
product accelerates the rate of oxidation of organic compounds present in coal. The pyrite
oxidation leads to formation of ferric ions which catalyzes the reaction. Also pyrite oxidation
results in swelling which in turn causes breakage of the coal particles increasing the surface area
for enhanced oxidation.
Thermal conductivity: In solid coals heat transfer is by conduction where as in broken coal heat
transfer is by conduction and convection and it predominates at higher compactions. Convection
gets enhanced at high temperature and low compactions, when moisture is evaporated and
condensed. Moist coals with moisture around 5-7% are highly conductive.
3.3. THEORIES OF SPONTANEOUS COMBUSTION OF COAL
3.3.1 Pyrite Theory-There has been quite a few cases of heating due to oxidation of pyrites in
pyrite mines. Heating can be availed in coal by presence of pyrite in considerable amount in
22
finely powdered and dispersed state being comprehended by moisture. The reaction of pyrites
with oxygen in presence of moisture is exothermic and yields product of greater volume which
leads to increase in surface area of the coal thus ensuring increased rate of oxidation.
2Fe S2+ 702+ 16H2O = 2 H2SO4+ 2 Fe S04.7 H2O + 316 kcal
The oxidation of pyrite during weathering of coal seam may be represented by
Fe S2+ 15/4 O2+ 9/2 H2O = Fe O.OH + 2 H2SO4
Above equations suggest that oxygen and moisture are two prime weathering agents,which
contribute to the pyrite alteration shown and it also leads to formation of sulphuric acid as by
product of the alteration process. Presence of moisture doubles the reactivity rate of coal and
presence of pyrite in dispersed form 10 folds the actual reaction rate.
Presence of pyrite in less of 5% showed negligible effect.
3.3.2.Bacterial Theory-Earlier bacteria was considered to add up to self- ignition of coal but
further investigations clarified that bacteria’s hardly had any influence on self heating of coal.
The contribution of bacteria towards heating cannot be totally ruled out due to observance of
spontaneous heating in haystacks due to bacterial action. Still, there is no justified fact to obey or
discard the above concept.
3.3.3.Phenol Theory-Experimental research and investigations have justified that poly phenols
and phenolic hydroxyls oxidize faster than many other organic groups. This theory is convincing
as well as interesting as it drives a way in determining liability of coal to spontaneous heating.
3.3.4 Humidity theory-It stated that the quantity of heat required in removing water from coal is
much high than the quantity of heat liberated by atmospheric oxidation of coal. The temperature
of heating would definitely decrease if the evaporation is to be done at the cost of heating. But as
a matter of fact water is one of the oxidation product formed during low temperature oxidation of
coal along with co and co2.
23
3.3.6 Coal-Oxygen Complex Theory-The native radical sight is the point of initiation of
oxidation of coal. The formation of peroxyl radical and hydro peroxides justifies the fact that
during the formation mechanism oxygen and moisture are initially incorporated into an organic
matrix. In fact these species may react, decompose, reform, alter to form a wide range of
complex showing oxygen functionality in matrix or gaseous product form.
Kroger and Beier claimed that coal oxygen interaction occurs via formation of peroxy complex.
The peroxy radical along with water lead to the formation of OH and OOH radicals. The above
process consisted 3 stages:
1. The physical adsorption of oxygen which takes place at low temperatures and it requires low
activation energy.
2. The Chemisorption step where motion of complex containing active form of oxygen called per
oxygen occurred and it occurred between temperature range 70°C to 80°C.Much amount of heat
is involved in this process.
3. Rapid chemical reaction resulting in decomposition of per-oxygen formed leading to release
of CO,CO2 and H2O and finally active combustion taking place.
24
Chapter 4
EXPERIMENTAL INVESTIGATION
Sample Collection and Preparation
Proximate Analysis
Determination of Calorific Value
Crossing Point Temperature
Flammability Temperature
Wet Oxidation Potential Analysis
25
4.0
EXPERIMENTAL INVESTIGATION
To carry out the experimental study, samples were collected from different collieries across India
by channel sampling method. The intrinsic properties of the collected samples were determined
by proximate analysis and bomb calorimetry experiment. Also in addition to that various
Liability Index and susceptibility indices were determined by carrying out Crossing Point
Temperature, Flammability Temperature and Wet Oxidation Potential analysis Experiments.
Finally correlation coefficients were determined for dependence of susceptibility indices over
intrinsic properties of coal.
4.1 Sample Collection and Preparation
Sampling is a method of collecting a small section of a large unit (i.e- in this case collecting a
coal sample of a coal seam). For collection of sample for this project Channel Sampling was
done at various mines. The seams were chosen of various collieries and a section of 10 cm width
and 10 cm depth was marked and was drilled or picked out.
4.1.1Steps of channel sampling
Under written steps were followed during the process of Channel Sampling :
1. The face was prepared.
2. Channel was demarked through.
3. The Channel was cut through.
4. Sample was collected.
5. The channel was labelled.
6. Finally stored in plastic bags.
26
1. Preparation of the surface: The surface was thoroughly cleaned by using scrubbers or brushes
in order to remove dirt, dust, oxidised part of coal being exposed, other soluble salts. Also at
times the upper layer is chipped up to 10 cm thickness in order to expose a fresh face to continue
with.
2. Demarcation of the channel: After cleaning the surface, a channel was demarked by drawing
two parallel lines 12-15 cm apart by the use of chalk or paint.
3. Cutting the channel: Following it the channel was cut by the use of a hand pick (prospectors
pick). Although in case of cutting channel in soft coal mines hand picks are used but in case of
hard coal generally a light weight air operated drill machine is used. In case of underground
mines drill machines are used to cut channels in case where more no of samples are to be
collected in a single shift.
4. Collection the sample: A sheet of canvas was spread on the floor in order to collect the coal
chips as they fall through.
5. Labelling the sample: After collecting the coal sample the canvas bag was wrapped through
and was tagged .Then the tagged sac was brought out of the mine.
The collected samples from different coalfields were brought to the laboratory. The collected
samples were then crushed to smaller pieces. Further Coning and quartering procedure was done
in order to get a representative sample of the entire coal seam. Finally the samples were
grounded and screened (sieving) to a size of - 212 (micron). Then the samples were stored in
sealed packets for further analysis process.
8 samples were collected by the following procedure from different colliery belonging to NECL,
MCL, BCCL, SECL, SCCL, and WCL.
27
4.2. PROXIMATE ANALYSIS
Proximate analysis id primarily done to determine the intrinsic properties of coal such as:
1. Moisture Content
2. Ash Content
3. Volatile Matter Content
4. Fixed Carbon
For determination of Proximate Analysis the method specified by IS(Indian Standard) 1350(Part1)-1969 was followed.
4.2.1 Determination of Moisture Content
Coal, due to its nature, origin and occurrence, is always associated with some amount of
moisture, which is both physically and chemically bound. It is differentiated between external
and inherent moisture. When a wet coal is exposed to atmosphere, the external moisture
evaporates, but the apparently dry coal still contains some moisture, which can be removed only
on heating above 100°C. External moisture is also called accidental or free moisture, where as
inherent moisture is termed as equilibrium or air-dried or hygroscopic moisture. The quantity of
external moisture depends mainly on the mode of occurrence and handling of coal, but the airdried moisture is related to the inherent hygroscopic nature of the coal.
Experimental Procedure -1g of finely powdered air-dried coal sample (-212μ) was weighed in
a silica crucible and was placed inside an electric hot air oven (Fig no 4.2.1) maintained at
108°C. The crucible with the coal sample was kept in the oven for 1.5 hours and was then taken
out with a pair of tongues, cooled in a desiccator for about 15 minutes and then weighed. The
28
loss in weight was reported as moisture (on percentage basis). The calculation was done as per
the following.
Where X = weight of empty crucible, g
Y = weight of crucible + coal sample before heating, g
Z = weight of crucible + coal sample after heating, g
Y - X = weight of coal sample, g
Y- Z = weight of moisture, g
Fig 4.2.1 Oven for Moisture Content Determination
29
Fig no 4.2.2 Muffle Furnace for Ash Content & Volatile Matter Determination
4.2.2 Determination of Volatile Matter Content
The loss of mass, corrected for moisture, which results when coal is heated in specified
equipment under prescribed conditions, is referred to as volatile matter. The matter lost is
composed of materials that form upon the thermal decomposition of the various components of
coal. Some of the constituents of coal volatile matter are hydrogen, carbon monoxide, methane,
other hydrocarbons, tar vapours, ammonia, some organic sulphur, oxygen containing compounds
and some incombustible gases, such as carbon dioxide and water vapour.
Experimental Procedure
For the determination of volatile matter a special volatile matter silica crucible (38mm height,
25mm external diameter and 22mm internal diameter) was used. The empty volatile matter
crucible was weighed. Approximately 1g of coal sample (-212 size) was weighed in the volatile
matter crucible and it was placed inside a muffle furnace maintained at 925°C with the lid
30
covering the crucible. The heating was carried I the muffle furnace (Fig no. 4.2.2) out exactly for
seven minutes, after which the crucible was removed, cooled in air, then in a desiccator and
weighed again. The calculation was done as per the following.
Where X = weight of empty crucible, g
Y = weight of crucible + coal sample before heating, g
Z = weight of crucible + coal sample after heating, g
Y -X = weight of coal sample, g
Y- Z = weight of volatile matter + moisture, g
4.2.3 Determination of Ash Content
Coal ash is the residue remaining after the combustion of coal under specified conditions. It does
not occur as such in the coal, but is formed as the result of chemical changes that take place in
the mineral matter during the combustion process. Ash and mineral matter of coal are therefore
not identical.
There are two types of ash forming materials in coal: extraneous and inherent mineral matters.
The extraneous mineral matter consists of materials such as calcium, magnesium and ferrous
carbonates, pyrite, marcasite, clays, shales, sand and gypsum. The extraneous mineral matter
owes its origin to i) the substances which got associated with the decaying vegetable material
during its conversion to coal, which is difficult to remove by mechanical methods, and ii) rocks
and dirt getting mixed up during mining and handling of coal. Inherent mineral matter represents
the inorganic elements combined with organic components of coal. The origin of such materials
is probably the plant materials from which the coal was formed.
31
Ash from inherent mineral matter is insignificant as far as the total quantity of ash is concerned.
But Indian coals suffer from the major disadvantage, that the mineral matter content is not only
high, but of intimately associated type, due to its drift origin.
Experimental Procedure
The empty crucible was cleaned by heating in a muffle furnace for one hour at 800°C so that
other mineral matter if presents get burnt. It was taken out, cooled to room temperature and the
weight is taken. Approximately 1gm of coal sample was weighed in the crucible and placed in a
muffle furnace at 450°C for 30 minutes and the temperature of the furnace was raised to 850°C
for 1hour. The crucible was taken out and placed in a desiccator and weighed.
Where X = weight of empty crucible in grams
Y = weight of coal sample + crucible in grams (Before heating)
Z = weight of coal sample + crucible in grams (After heating)
Y - X = weight of coal sample, g
Z - X = weight of ash, g
4.2.4 Determination of Fixed Carbon (FC)
Fixed carbon (FC) is by definition, the mathematical remaining after the determination of
moisture, volatile matter and ash. It is, in fact a measure of the solid combustible material in coal
after the expulsion of volatile matter. Fixed carbon plus ash represent the approximate yield of
coke from coal. The fixed carbon value is determined by subtracting from 100 the resultant
summation of moisture, volatile matter and ash, with all percentage on the same moisture
reference base.
32
FC= 100-(VM+M+A)
The moisture (M), volatile matter (VM), Ash and Fixed carbon content of coal determined by
following the above procedure is presented in Table 4.1.
33
Table no 4.2.1 Volatile Matter(VM),Ash(A),Moisture(M) and Fixed Carbon(FC) of coal
Sl no.
Sample
Volatile Matter(VM%)
1.
MCL1
32.015
2.
NECL1
39.2
3.
SECL1
4.
Moisture(M%)
13.575
Ash(A%)
Fixed Carbon(FC%)
11.3
43.11
3.41
2.005
55.385
29.115
10.215
26.01
34.66
WCL1
31.56
12.32
11.915
44.205
5.
SCCL1
27.795
6.95
25.535
39.72
6.
BCCL1
17.23
1.3
26.845
54.625
7.
SECL2
29.905
6.75
23.46
39.885
8.
MCL2
24.94
6.955
33.83
34.275
34
4.3. Determination of Calorific Value
4.3.1 Bomb Calorimetry
The energy value of a coal sample refers to the amount of potential energy stored in coal which
can be effectively used as actual heating ability. Depending upon the Gross Calorific Value
(GCV) coals are classified into different grades. And to measure the same Digital Bomb
calorimeter was used.
Calorific Value
The energy value of coal, or the fuel content, is the amount of potential
energy in coal that can be converted into actual heating ability. The process of measuring the
heat of chemical reactions or physical changes as well as heat capacity is called calorimetry and
the object used for measuring the energy value is called as calorimeter. The value differs with
grades of coal or materials as different materials of different grades produces differing amounts
of heat for a given mass.
A bomb calorimeter consists of a small cup to contain the sample, oxygen,
a stainless steel bomb, water, a stirrer, a thermometer, the insulating container (to prevent heat
flow from the calorimeter to the surroundings) and ignition circuit connected to the bomb. By
using stainless steel for the bomb, the reaction will occur with no volume change observed. `It is
a type of constant-volume calorimeter used in measuring the heat of combustion of a particular
reaction. They withstand a large pressure within the calorimeter as the reaction is being
measured. Electrical energy is used to ignite the fuel; as the fuel is burning, it will heat up the
surrounding air, which expands and escapes through a tube that leads the air out of the
35
calorimeter. When the air is escaping through the copper tube it will also heat up the water
outside the tube. The temperature of the water allows for calculating calorie content of the fuel.
Experimental Procedure
The Calorific value of the coal in digital bomb calorimeter can be calculated by five steps:
Step – 1 (Initials)
About 1gm of sample is taken in a crucible. The desired weight can be done carried out
manually or entered digitally. Two liters of distilled water is filled in the outer jacket..5 to 10 ml
of water is placed inside the bomb.
Step -2 (Connection of Igniting Wires)
The sample (powder form) is placed in the crucible in the bomb by the help of tripod stand. The
two electrodes are connected by nichrome wire touching the coal sample. The bomb is now
tightened by the lid.
Step – 3 Filling of Oxygen
After tightening the lid of the bomb, oxygen is to be filled into it. It takes 1 minute to fill
desired amount oxygen in the bomb. It is filled into the bomb by connecting it into the gas pipe
socket of the bomb head and oxygen cock to the cylinder. The cock is then unscrewed to fill the
oxygen of 420 psi (400 – 450).
Step -4 Run of program
The bomb is placed inside the outer jacket which gets immersed in distilled water. The electrodes
are fitted to the bomb before the cover of the bomb is to be shut down. Sufficient care is to be
36
taken to avoid the contact of stirrer to the bomb. The device is now ready to run which on
running will give us the result of calorific value and temperature rise of coal.
Step -5 Reaction
The whole bomb is now pressurized with excess pure oxygen (typically at 30atm) which contains
a weighed mass of a sample (1-1.5 g) and a small fixed amount of water (to saturate the internal
atmosphere, thus ensuring that all water produced is liquid, and removing the need to include
enthalpy of vaporization in calculations), is submerged under a known volume of water (2 l)
before the charge is electrically ignited. The bomb, with the known mass of the sample and
oxygen, form a closed system does not allows gases to escape during the reaction. The weighted
reactant put inside the steel container is then ignited. Energy is released by the combustion and
heat flow from this crosses the stainless steel wall, thus raising the temperature of the steel bomb,
its contents, and the surrounding water jacket. The temperature change in the water is then
accurately measured with a thermometer. This reading, along with a bomb factor (which is
dependent on the heat capacity of the metal bomb parts), is used to calculate the energy given out
by the sample burn. A small correction is made to account for the electrical energy input, the
burning fuse, and acid production (by titration of the residual liquid). After the temperature rise
has been measured, the excess pressure in the bomb is released.
37
Fig no 4.3.1 Experimental setup of Digital Bomb Calorimeter and auto filling of Oxygen in
Bomb.
Table no 4.3.1 Ultimate Heating Value (UHV) of coal and its Grade
Sl no.
Sample
Ultimate Heat Value(Kcal/Kg)
Grade
1.
MCL1
5467.25
C
2.
NECL1
8152.73
A
3.
SECL1
3900.95
E
4.
WCL1
5555.57
C
5.
SCCL1
4417.07
D
6.
BCCL1
5015.99
C
7.
SECL2
4731.02
D
8.
MCL2
3271.67
F
38
Table no 4.3.2 Gross Calorific Value(GCV) of coal and its grade
Sl no.
Sample
Gross Calorific Value(Kcal/Kg)
Grade
1.
MCL1
5574.3665
D
2.
NECL1
7167.3499
A
3.
SECL1
3946.8717
F
4.
WCL1
6122.5100
B
5.
SCCL1
5071.3672
E
6.
BCCL1
6049.2439
B
7.
SECL2
5510.1490
D
8.
MCL2
4574.2311
E
39
4.4.Crossing Point Temperature
It refers to the minimum temperature at which the coal temperature coincides with that of the
bath temperature. This method is primarily used in India for determination of liability of coal
samples towards spontaneous heating.
Fig no 4.4.1 Schematic layout of Crossing Point Temperature Apparatus
Experimental procedure
In this method initially coal sample is prepared of size -212 micron. Next to that 4 gram of coal
sample was taken and was placed inside a helical test tube over some glass wool initially placed
inside the tube. Then the helical glass tube was place inside the furnace and one end of the
reaction tube was connected to a valve which supplies constant oxygen flow to the reaction tube
at a rate of 80ml/min.Following it the reaction tube was placed inside the air bath furnace and a
constant heating rate of 1 degree celcius rise per min was maintained via a rheostat. Continuous
reading were noted of the coal and the bath temperature at interval of 3-4 minutes and finally the
40
experiment is stopped when the coal temperature crosses the bath temperature. Same procedure
was applied for all the 8 samples and the liability to spontaneous heating and the risk rating was
determined using the table 4.4.1,4.4.2.
(Mahadevan et al,.,1985) proposed a new Liability Index or MR Index after analysing several
CPT curves. He divided the heating curve into 3 stages.
The 1st stage was considered upto the point of inflexion( i.e upto a point at which the rate of
heating rapidly increased called the inflextion point).
The 2nd stage started from the inflexion point and ended at the Crossing Point Temperature.
The 3rd stage was considered to start from the crossing point temperature and ended at active
combustion.
The risk rating with respect to the liability index :
Table no 4.4.1 Liability Index (Mahadevan et al.,1985)
Liability Index
Risk Rating
0-10
low
10-20
Medium
>20
High
41
Table no 4.4.2. Classification of CPT(Mahadevan et al.,1985)
CPT (°C)
Risk Rating
120-140
Highly susceptible
140-160
Moderately susceptible
>160
Poorly susceptible
WITS-EHAC Index(Uludag,2007) of self- heating liability of coal is calculated from the
following formula:
Area of the triangle= 0.5* Stage 2 slope *
*1000
Table no 4.4.3 Crossing Point Temperature, Liability Index,WITS-EHAS and its Risk Rating.
Sl no.
Sample
CPT (°C)
1.
MCL1
159
2.
NECL1
193
3.
SECL1
161
4.
WCL1
155
5.
SCCL1
126
6.
BCCL1
222
7.
SECL1
158
8.
MCL2
160
Risk
Liability
Rating
Index
Moderately
18.39
susceptible
Poorly
8.47
susceptible
Poorly
6.18
susceptible
Moderately
11.27
susceptible
Highly
10.09
susceptible
Poorly
7.06
susceptible
Moderately
15.64
susceptible
Moderately
14.17
susceptible
Risk
Rating
Medium
WITSEHAC
4.77
Low
3.435
Low
3.86
Medium
3.87
Medium
4.55
Low
2.34
Medium
4.101
Medium
4.71
42
TEMP IN
CELCIUS
180
160
140
120
100
80
60
40
20
0
BATH TEMP
COAL TEMP
0
50
100
150
TIME IN MINS
Fig no 4.4.2 CPT Curve for Sample MCL1
250
200
150
BATH TEMP
TEMP IN
CELCIUS
100
COAL TEMP
50
0
0
50
100
150
200
TIME IN MINS
TEMP IN
CELCIUS
Fig no 4.4.3 CPT Curve for Sample NECL1
180
160
140
120
100
80
60
40
20
0
BATH TEMP
COAL TEMP
0
50
100
150
TIME IN MINS
Fig no 4.4.4 CPT Curve for Sample SECL1
43
180
160
140
120
100
80
60
40
20
0
TEMP IN
CELCIUS
BATH TEMP
COAL TEMP
0
50
100
150
TIME IN MINS
Fig no 4.4.5 CPT Curve for Sample WCL1
140
120
100
80
TEMP IN
CELCIUS
60
BATH TEMP
40
COAL TEMP
20
0
0
20
40
60
80
100
120
TIME IN MINS
Fig no 4.4.6 CPT Curve for Sample SCCL1
250
200
150
BATH TEMP
TEMP IN
CELCIUS
100
COAL TEMP
50
0
0
50
100
150
200
250
TIME IN MINS
Fig no 4.4.7CPT Curve for Sample BCCL1
44
TEMP IN CELCIUS
180
160
140
120
100
80
60
40
20
0
BATH TEMP
COAL TEMP
0
50
100
150
TIME IN MINS
TEMP IN CELCIUS
Fig no 4.4.8 CPT Curve for Sample SECL2
180
160
140
120
100
80
60
40
20
0
BATH TEMP
COAL TEMP
0
50
100
150
TIME IN MINS
Fig no 4.4.9 CPT Curve for Sample MCL2
4.5.Flammability Temperature(Nimaje et al., 2010)
It basically refers to the minimum temperature at which coal ignites. Here the concept is based
on the ideology that flammability temperature of a coal sample decreases with increasing
oxidation of coal and this concept of difference between the ignition temperature of coal after
and before oxidation can be used as a tool
to determine the liability of coal samples to
spontaneous combustion.It is used to determine the efficiency of coal dusting . The experimental
45
set up consists of a vertical tubular furnace of internal diameter 50 mm, length 300 mm which is
open at both ends having a a dust dispersing unit along a solenoid valve and a reservoir for air.It
also consists of a mercury manometer, a drying tower and an aspirator bulb. Coal dust sample is
kept in the helical dust disperser. Air at an pressure of 50 mm of Hg from the reservoir is made
to pass through the disperser in order to disperse the powdered coal forming a uniform air-dust
mixture within the furnace. The minimum temperature at which this mixture catches fire, which
is indicated by the appearance of flame coming out of the bottom of the tubular furnace is known
as the flammability temperature of the coal dust.
Experimental Procedure:
-72 micron (-200 mesh BSS) was placed in a helical
tube.
The aspirator bulb was squeezed continuously to make the mercury column difference
maintained at a pressure of 80 mm.
ing on the solenoid valve, at the desired temperature of furnace, the air passes at a
very fast rate and carries away the coal dust along with it to show signs of flame, smoke or spark.
If flame appears then in order to determine the exact temperature experiment is carried out in
lower temperature range or else we move on to high temperature range. But in all it’s a hit and
trial method of identifying the exact Flammability Temperature. Precaution should be taken to
pre dry the circulated air in order to avoid error due to humidity.
46
Fig no 4.5.1 Schematic Layout of Flammability Apparatus
Table no 4.5.1 Flammability Temperature of the Coal Samples
Sl no.
Sample
Flammability Temperature(°C)
1.
MCL1
505
2.
NECL1
500
3.
SECL1
540
4.
WCL1
515
5.
SCCL1
415
6.
BCCL1
450
7.
SECL2
545
8.
MCL2
530
47
4.6 Wet Oxidation Potential Analysis:
Coal molecules constitutes two part:
1.The condensed aromatic structure which in totally resistant tooxidation.
2. The hydro-aromatic structure or the aliphatic(open chain) part that are highly prone to
oxidation.
Basically presence of hydroxyl groups in the aromatic part adds up to the reactivity rate of the
coal structure and oxidises it faster. Above mentioned reason is one of the prime cause of fast
oxidation of low rank coals. In addition to that also low rank coals have lower degree of
condensation of aromatic structure in it. On oxidation low rank coals produce huge amount of
aliphatic acids mainly formed from base aliphatic presence in low rank coals. But higher rank
coals have have structure similar to that of graphite which helps it in formation of aromatics
rather than aliphatics. Hence ,lower the potential difference lower is the liability of coal towards
spontaneous heating.
Experimental Procedure
Initially a solution of 100ml mixture was formed by mixing 0.1 N solution of potassium
permanganate and 1N potassium hydroxide. Then this solution was placed inside the beaker and
the magnetic stirrer is allowed to stir smoothly the solution. Then 0.5 gram of coal sample of 212
micron size was weighed and kept aside. Next to that e carbon and calomel electrodes were
dropped into the solution and the millivoltmeter was turned on. After that the weighed coal
sample was put inside the solution and the resultant coal oxidation suspension was continuously
stirred by the magnetic stirrer and the potential difference(EMF) between the carbon and calomel
48
electrode was noted down within every 1 minute upto 30 minutes until the potential difference
value becomes constant. The higher the Potential Difference ,the higher the liability towards
spontaneous heating.
Carbon electrode
Calomel electrode
Magnetic stirrer
Millivoltmeter
Fig no 4.6.1 Experimental set up of Wet Oxidation Potential Apparatus
49
Table no 4.6.1 –Wet Oxidation Potential Difference of the coal samples
Sl no.
Sample
Wet Oxidation Potential Difference(mV)
1.
MCL1
65.2
2.
NECL1
22.4
3.
SECL1
43.4
4.
WCL1
48
5.
SCCL1
73
6.
BCCL1
17.4
7.
SECL2
36.3
8.
MCL2
56.3
EMF(mv)
EMF(
mv)
380
370
360
350
340
330
320
310
300
0
5
10
15
20
25
30
35
Time(min)
Fig no 4.6.2 Wet Oxidation Curve for Sample MCL1
50
EMF(mv)
405
400
EMF(mv)
395
390
385
380
375
0
5
10
Time(min)
15
20
25
30
35
40
EMF(mv)
Fig no 4.6.3 Wet Oxidation Curve for Sample NECL1
380
375
370
365
360
355
350
345
340
335
330
0
5
10
15
20
25
30
35
30
35
Time(min)
Fig no.4.6.4 Wet Oxidation Curve for Sample SECL1
380
370
EMF(mv)
360
350
340
330
320
0
5
10
15
20
25
Time(min)
Fig no.4.6.5 Wet Oxidation Curve for Sample WCL1
51
EMF(mv)
378
EMF(mv)
368
358
348
338
0
5
10
15
20
25
30
35
40
Time(min)
Fig no.4.6.6 Wet Oxidation Curve for Sample SCCL1
EMF(mv)
405
EMF (mV)
400
395
390
385
380
0
5
10
15
20
25
30
35
TIME IN MINS
Fig no 4.6.7 Wet Oxidation Curve for Sample BCCL1
EMF(mv)
420
400
EMF(mv)
380
360
340
320
0
5
10
15
20
25
30
35
40
Time(min)
Fig no . 4.6.8 Wet Oxidation Curve for Sample SECL2
52
EMF(mv)
380
370
360
350
340
330
320
310
0
5
10
15
20
25
30
35
Time(min)
Fig no 4.6.9 Wet Oxidation Curve for Sample MCL2
53
Chapter 5
DISCUSSION AND CONCLUSION
Discussion
Conclusion
54
5.1 Discussion
1.The moisture content of the various coal samples varied from 1.3% to 13.575% with MCL1
being the highly moist coal and BCCL1,the least moisture content. High moisture content in
MCL1 is because the seam is watery in nature which justifies field observation.
2.High moisture content and low fixed carbon in SECL1 conveys the coal seam is highly prone
to spontaneous heating.
3.Low fixed carbon content along with high volatile matter content in SCCL1 shows the coal
seam is highly susceptible to spontaneous heating.
4.Low moisture content and high fixed carbon content in BCCL1 & NECL1 conveys these seam
do not pose any spontaneous heating problem.In fact they are poorly susceptible to spontaneous
heating.
5.The average values of moisture,relatively high value of volatile matter in case of
MCL2,SECL2,WCL,MCL1 shows they are moderately to poorly susceptible to spontaneous
heating.
6.High Gross Calorific Value of the samples NECL1,BCCL1 shows that they are good rank coal
and hence very less susceptible to spontaneous heating.
7.The highest Wet Oxidation Potential Difference in case of SCCL1 shows that it is highly prone
to rapid oxidation resulting in highly susceptible to spontaneous heating problems.
8.Averagely high value of Wet Oxidation Potential Difference in case of MCL1,MCL2 and
WCL1 suggests these respective coal seams are moderately susceptible to spontaneous heating.
55
9.Low values of Wet Oxidation Potential Difference in case of BCCL1,NECL1 claims that these
seams are less susceptible to spontaneous heating.
10.Lowest Flammability temperature of SCCL1 shows that it is highly prone to spontaneous
heating.
11.High Crossing Point Temperature of BCCL1 & NECL1 shows that they are poorly
susceptible to spontaneous heating.
12.SCCL1 has the lowest Crossing point temperature value which claims that it is highly
susceptible to spontaneous heating and field investigation also supports the argument.
13.Relatively
average
values
of
Crossing
point
temperature
of
samples
from
MCL1,WCL1,SECL2,MCL2 shows that they are moderately susceptible to spontaneous heating.
Above discussion shows that most of the Indian coals are moderately to poorly susceptible to
spontaneous heating where as few seams are totally free of spontaneous heating related issues.
Correlation studies were carried out between various susceptibility indices and the coal
properties as obtained from proximate analysis. The susceptibility parameters were taken as
dependent variables where as the the coal properties were taken as independent variables and the
correlation coefficients were found out. Also the trendline curves were modelled to show the
dependence of susceptibility indices on the various coal intrinsic properties.
56
Table no.5.1.1 Correlation coefficients between various susceptibility parameters and the
proximate analysis values.
Sl no.
Dependent variable
Independent
Correlation
coefficient(r)
variable
1.
CPT
Moisture
-0.63628
2.
CPT
Volatile matter
-0.29151
3.
CPT
Ash Content
-0.14875
4.
Flammability
temperature
Flammability
temperature
Flammability
temperature
Wet Oxidation
Potential Difference
Wet Oxidation
Potential Difference
Wet Oxidation
Potential Difference
WITS-EHAC
Moisture
0.3869
Volatile Matter
0.3328
Ash Content
-0.0494
Moisture
0.662
Volatile Matter
0.0874
Ash Content
0.199
Moisture
0.66
5.
6.
7.
8.
9.
10.
CPT
CPT
250
200
150
100
50
0
y = -0.4036x + 174.87
R² = 0.0221
CPT
Linear (CPT)
0
10
20
30
40
ASH CONTENT
Fig no 5.1.1 variation of CPT vs Ash Content
57
CPT
CPT
250
200
150
100
50
0
y = -4.3425x + 200.12
R² = 0.4048
CPT
Linear (CPT)
0
5
10
15
MOISTURE
Fig no 5.1.2 variation of CPT vs Moisture
CPT
250
y = -1.3288x + 205.25
R² = 0.085
CPT
200
150
100
CPT
50
Linear (CPT)
0
0
10
20
30
40
50
VOLATILE MATTER
Fig no 5.1.3 Variation of CPT vs Volatile Matter
FLAMMABILITY TEMP
FLAMMABILITY TEMPERATURE
y = -0.213x + 504.28
R² = 0.0024
600
400
FLAMMABILITY
TEMPERATURE
200
0
0
10
20
ASH CONTENT
30
40
Linear (FLAMMABILITY
TEMPERATURE)
Fig no 5.1.4 Variation of Flammability temperature vs Ash Content
58
FLAMMABILITY TEMPERATURE
FLAMMABILITY TEMP
600
y = 4.1914x + 467.79
R² = 0.1497
FLAMMABILITY
TEMPERATURE
400
200
Linear (FLAMMABILITY
TEMPERATURE)
0
0
5
10
15
MOISTURE
Fig no 5.1.5 Variation of Flammability temperature vs Moisture
FLAMMABILITY TEMPERATURE
FLAMMABILITY TEMP
600
y = 2.4115x + 430.14
R² = 0.1111
400
FLAMMABILITY
TEMPERATURE
200
Linear (FLAMMABILITY
TEMPERATURE)
0
0
10
20
30
40
50
VOLATILE MATTER
Fig no 5.1.6 Variation of Flammability vs Volatile Matter
∆E(in mV)
80
y = 0.3699x + 37.811
R² = 0.0399
∆E(in mV)
60
40
∆E(in mV)
20
Linear (∆E(in mV))
0
0
10
20
30
40
ASH CONTENT
Fig no 5.1.7 Variation of ∆E vs Ash Content
59
∆E(in mV)
80
y = 3.0832x + 21.558
R² = 0.4385
∆E(in mV)
60
40
∆E(in mV)
20
Linear (∆E(in mV))
0
0
5
10
15
MOISTURE
Fig no 5.1.8 Variation of ∆E vs Moisture
∆E(in mV)
80
y = 0.2719x + 37.374
R² = 0.0076
∆E(in mV)
60
40
∆E(in mV)
20
Linear (∆E(in mV))
0
0
10
20
30
40
50
VOLATILE MATTER
Fig no 5.1.9 Variation of ∆E vs Volatile Matter
6
y = 0.1261x + 2.9859
R² = 0.4359
5
WITS-EHAC INDEX
4
3
Series1
2
1
0
0
5
10
15
MOISTURE
Fig no. 5.1.10 Variation of the WITS-EHAS vs Moisture
60
5.2 Conclusion
1. The results shown by Crossing Point Temperature value and Wet Oxidation Potential
Difference value regarding liability of the coal samples towards spontaneous heating were quite
similar. Both the experimental results suggested SCCL1 sample as highly susceptible and
BCCL1 as least susceptible.
2.Wet Oxidation Potential Method showed productive results in determining spontaneous
heating liability of coal samples showing similar results as CPT. Thus it can be used as a handy
experimental tool in predicting spontaneous heating susceptibility of both high and low moisture
coals. In addition to that it takes just 30 minutes for completion of the entire experiment in
comparison to 2 hrs used in case of Crossing Point Temperature.
3.Flammability temperature cannot be considered as a sound experimental technique for
determining spontaneous heating liability of various coal samples as it showed poor correlation
with all the coal properties.
4.Both Wet Oxidation Potential Difference and Crossing Point Temperature
showed high
correlation value with moisture content of coal justifying their use in determining spontaneous
heating liability of coal samples.
5. Crossing Point Temperature is used as a handy experimental technique in prediction of
spontaneous heating liability in India but many a times it fails in predicting the spontaneous
heating liability of high moisture coals. Also it is a time consuming process and reproducibility
of similar results is seldom.
61
6. Wet Oxidation Potential Difference is an effective tool for determining spontaneous heating
tendency for all types of coals and it produces faster results but still no proper guideline has been
mentioned till date in order to standardise the potential difference values with respect to liability
of coal towards spontaneous heating.
62
Chapter 6
REFERENCES
63
6.1 References
1.Banerjee, S. C., Nandy, D. K. , Banerjee, D. D., Chakravorty, R. N., 1972, Classification of
coal with respect to their Susceptibility to Spontaneous Combustion, MGMI, July, Vol. 69, No.
2, pp. 15-28.
2.Banerjee, S. C., 1985, Spontaneous Combustion of Coal and Mine Fires, Oxford & IBH
Publishing Co. Pvt. Ltd., 1st Ed., pp. 1-38.
3.Feng, K. K., Chakravarty, R. N., Cochrane, T. S., 1973, Spontaneous combustion – a coal
mining hazard, CIM Bulletin. Oct., pp. 75-84.
4.Gouws, M.J., Wade, L., 1989, The self-heating liability of coal: Prediction based on composite
indices, Mining Science and Technology. 9, pp. 81-85.
5.I.S. (Indian Standard): 1350 (Part-I) – 1984, Methods of Test for Coal and Coke: Proximate
Analysis, Bureau of Indian Standards, New Delhi, pp. 3-28.
6.I.S. (Indian Standard): 1350 (Part II) - 2000, Methods of Test for Coal and Coke:
Determination of Calorific Value, Bureau of Indian Standards, New Delhi, pp. 3-24.
7.Mahadevan, V., Ramlu, M. A., 1985, Fire risk rating of coal mines due to spontaneous heating,
Journal of Mines, Metals and Fuels. August, pp. 357-362.
8.Nandy, D. K., Banerjee, D. D., Chakravorty, R.N., 1972, Application of crossing point
temperature for determining the spontaneous heating characteristics of coal, Journal of Mines,
Metals and Fuels, pp. 20-41.
9.Nordon, P., Young, B. C. and Bainbridge, N.W., 1979, The rate of oxidation of char and coal
in relation to their tendency to self-heat, Fuel, Vol. 58, p. 443-449.
10.Ramlu, M.A., 1997, Mine Disasters and Mine Rescue, Oxford & IBH Publishing Co. Pvt.
Ltd., 1st Ed., pp. 1-19.
64
11.Tarafdar, M. N., Guha, D., 1989, Application of wet oxidation processes for the assessment of the
spontaneous heating of coal, Fuel. 68, March, pp. 315-317.
12.Panigrahi et al, 2001, A study of susceptibility of Indian coals to spontaneous combustion and
its correlation with their intrinsic properties, Mine Environment and Ventilation, Oxford & IBH
Publishing Co., New Delhi, pp. 247-254.
13.Panigrahi, D. C., Udaybhanu, G., Ojha, A., 1996, A comparative study of wet oxidation
method and crossing point temperature method for determining the susceptibility of Indian coals
to spontaneous heating, Proceedings of Seminar on Prevention and Control of Mine and
Industrial Fires – Trends and Challenges. Calcutta. India. Dec., pp. 101-107.
14.Tripathy,D.P. & Pal,B.K.(2001):Spontaneous Heating Susceptibility of coals-evaluation
based on experimental techniques,Journal of mines,Metals and Fuels, Vol. 49,pp.236-243
15.Morris,R. & Atkinson,T.(1986):Geological and Mining Factors Affecting Spontaneous
Heating of coal,Mining Science & Technology,Vol.3,pp.217-231.
16. Morris,R. & Atkinson,T.(1988):Seam Factors and the Spontaneous Heating of coal,Mining
Science & Technology,Vol.7,pp. 149-159.
65
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