sp comb factors&stages

8/3/2019 Sp Comb Factors&Stages

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I. Factors related to Coal Characteristics

II. Environmental Factors

III. Geological Factors

IV. Factors Related to Mining Practice

V. Time factor


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I. Factors related to Coal Characteristics:

Susceptibility to spontaneous combustion of various coal

depends on their characteristics. These characteristicsinclude:

a) Rank of coal

b) Presence of pyrite

c) Area of coal exposure

d) Moisture content

e) Freshness of coal surface

f) Volatile Matter content

g) Petrographic composition of coal

h) Ash contenti) Oxygen Content of Coal


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I.a. Rank of Coal

Coals of different ranks have different capacities to

sorb oxygen. Experience with a large number ofmine fires has shown that the rank of coal is only of

secondary importance in judging the liability of a

coal to spontaneous combustion, although the

general observation made is that lower rank coalsare more easily oxidized than higher rank coals.

Hence liability to spontaneous combustion reduces

from Lignite to Anthracite


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I.b. Presence of PyritePyrites, which frequently occurs in coal seams as very smallconcentrations, is one of the most important factor. It oxidizes

easily in the presence of oxygen of the air and moisture at ordinaryatmospheric temperatures according to the following equation:

2FeS2+ 7 O2 + 16 H20 = 2H2S04 + 2FeS04.7 H20 + Heat

It results into the increase in volume and liberation of heat.

The swelling contributes to disintegration of coal mass therebyincreasing the surface available for oxidation.

Hence the oxidation of pyrites or any oxidizable iron sulphide hasa promoting effect on auto-oxidation of coal.

It is observed that if pyrite concentration is >2% and is finelydivided, it has prominent effect.

If it is found in lumps or highly crystalline form, its contributesvery little to initial heating


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I.c. Area of coal exposure

The rate of oxidation of coal and, hence, the amount

of heat liberated from it depends on the surface areaof coal. Greater the area of surface in contact with

the air, greater will be the rate of oxidation and

susceptibility to spontaneous combustion.

It is generally considered in the case of an average

bituminous coal that the danger of spontaneous

combustion is slight for sizes larger than 38 mm.

Friable coals which produce a considerable amountof fines when mined are more vulnerable to

spontaneous combustion.


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I.d. Moisture contentMoisture inherent in coal, present in the surroundingoxidizing atmosphere, or produced from oxidation of coal in

the early stages of oxidation influences the oxidationprocess.

The changes in moisture content of the coal affect thepotential of coal for spontaneous combustion.

It has been found that the rate of oxidation increases with anincrease in moisture content up to a certain value. After thismoisture content it may have retarding effect

Also wetting is an exothermic process and drying is anendothermic process.

Highly wet coal seams, where water is continuously drippingthrough the seam walls, are safer than partially humid seams


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I.e. Freshness of coal surfaceFor any given set of constant conditions, the rate of

oxidation of exposed fresh surface of coal decreasescontinuously during the progress of the oxidation, thatis, with time.

I.f. Volatile ContentIncrease in volatile content of coal increases the rate ofoxidation. It has been found that coal with 38% volatilematter oxidizes three times as fast as coal with 18%

volatile matter.

It is normally considered that coals having a volatilecontent of 28% and more as liable to spontaneousheating.


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I.g. Petrographic Composition of CoalIt has been observed that liability of spontaneous

combustion decreases in the order vitrain-clarain-durain-fusain-content

I.h. Ash ContentAsh in coal generally decreases the oxidation ratewhich is also influenced by the mineral compositionof the ash.

I.i . Oxygen ContentThe oxidation rate decreases with decreasing oxygencontent of coal.

With coals having an oxygen content less than 2%, theoxidation rate is not large enough to cause any

appreciable increase in temperature.


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These include:

a. Ambient Temperature

b. Humidity in airc. Gas emission

d. Oxygen content in contact with air

e. Air flow rates


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II.a. Ambient Temperature

Warm air in contact with exposed surface of coal

increases the oxidation rate considerably. There isless loss of heat by conduction to the atmosphere.

The temperatures of the strata increase with depth(Geo-thermal Gradient). Therefore the oxidation

rate will increase with depth making deeper seams

more vulnerable to spontaneous combustion.


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II.b. Humidity

Higher RH facilitates spontaneous heating

This is due to wetting of coal thereby vapours give

off latent heat of condensation to coal on absorption.

It has been found that heat evolved during oxidation

is 2.5 times greater for exposure in moist air thandry air


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II.c. Gas emission:High gas emissions are advantageous in reducing theoxygen content in goaves by replacement of air with

methane or blackdamp.

Hence the incidences of goaf fires from spontaneousheating are reduced.

II.d. Concentration of Oxygen in Contactwith CoalAt any point on a coal surface, the rate of oxidation is

directly proportional to the oxygen concentration at thatpoint.

For an atmosphere devoid of oxygen, the oxidation rateis zero regardless of all other factors


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II.e. Air Flow RatesFor spontaneous combustion to develop, the rate of heatgeneration should be more than the rate of heat dissipation.

At very high airflow rates almost unlimited oxygen for theoxidation of coal is available but dissipation of the heat generatedby oxidation is very efficient.

A low Row rate restricts the amount of oxygen available, but doesnot allow the heat generated to be dissipated.

A critical Row rate is one that provides sufficient oxygen forwidespread oxidation but does not dissipate the heat generate

RISKY FLOW RATES 0.1-0.9 m/ min. with O2>17%

A good rule to be observed by mining men would be "Noventilation at all is better than a deficient ventilation".


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The factors are:

a. Presence of Faults

b. Thickness of coal seams

c. Depth of working

d. Presence of Multi seams

e. Presence of Dykes


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III.a. Presence of Faults

The presence of faults in coal seams often contributes tothe development of heating in coal mines by allowing

air and water to migrate into the coal seams.

Zones of weakness which usually develop in the areaaround the faults also aid in the development of heating.

III.b. Thickness of coal seams

Thick coal seams are often considered to have morepotential for spontaneous combustion because theworking of these seams is invariably accompanied by

high losses of coal in the goaf areas.

The low thermal conductivity of coal compared withthat of shale or sandstone is also a contributory factor.


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III.c. Depth of working

When a coal seam at shallow depth (< 50 m) is

mined, the goaf areas become connected to the

surface by cracks and fissures. Air and water from

the surface can gain access to the coal and increase

the potential for spontaneous combustion.

Higher depths >300 m, may be responsible for

producing more fractures in the coal seam and

thereby aggravate spontaneous combustion risks.


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III.d Presence of Multi Seams

when multi-seams in close proximity are worked,

the cracks and fissures developed in the interveningstrata increase the potential for spontaneous

combustion of surrounding unmined seams

(particularly the undermined seam)

III.e Presence of Dykes

Presence of dykes or intrusion of igneous rocks are

also risky as they usually cause more fractures in thecoal seam in the vicinity and also cause more coal

loss during mining


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Some of the most common places, wherespontaneous heating occurs, are :

goaf areas and unconsolidated wastes,

pack walls with a high proportion of coal

the edges of goaves where high strata pressure causes

crushing, roof falls and floor heave ,

crushed pillars

regulators, doors and air crossings and

constrictions in the roadways.


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a. Coal left in goaf areas is very liable to spontaneous

combustion as the air movement, there, is very

sluggish, and any heat generated as a result of oxidationwill not be removed.

b. In coal mines, coal is left in the roof and/or floor to

support the weak adjoining strata or bands of inferiorquality coal which are left unmined. The roof falls and

floor heaves occur causing large-scale crushing of the

left coal and creating conditions susceptible for heating


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c. Pillars that have been standing for a long time are prone to

heating, particularly when they are liable to crushing.

d. Regulators, doors and air crossings are points of high airleakage due to the air moving through the fractures in thesolid coal around them. The greater the pressure difference

across them, the greater is the leakage.

e. Constrictions of mine roadways also cause leakage of air.Changes in ventilation, either intentional or accidental, maycause excessive air leakage or may suddenly bring moist air

into contact with dry coal.

f. Goaf areas, where a large amount of coal is left , presentoptimal conditions for spontaneous heating


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It is common experience that a certain time must

elapse before the first signs of heating appear.

The term Incubation Period' generally implies the

time required for the oxidation of coal, in suitablecircumstances, to cause a rise in temperature to its

ignition point.

It is also defined as the period between the onset of

first oxidation and the time point when one can

detect fires by senses or time of first sign of

heating.


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It depends on the characteristics of the coal, the air

leakage and the heat accumulation in the environment.

For low-rank coals, the time period generally variesbetween 3 and 6 months, but with high-rank coals the

period varies between 9 and 18months.

The incubation period can be extended by reducingfissuration and/or air leakage.

Under adverse conditions. the period can be less than2 weeks: especially with low-rank coals.


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On recognizes three phases or stages of spontaneous

combustion of coal in coal mines :

1) The incubation period

2) The indication period

3) Open fire


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I. Incubation Period

The incubation period is the period between the onset of first

oxidation and the time point when one can detect by thesenses.

From the practical point of view, the term is, however, usedin a broader sense to denote the period between the

beginning of coal extraction in a district or panel and theappearance of first signs of heating.

The incubation period varies widely depending on seamthickness, nature of the immediate roof, method of working,

method of roof control, regularity and continuity of working,and liability of coal to spontaneous combustion.

During this period, one does not detect heating during one'spassage through mine workings.


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II. Indication Period

The end of the incubation period, that is, the beginning of theindication period is marked by "sweating" which is caused

by the warmed up air from a fire area cooling on coming intocontact with the cooler coal, rock and metallic surfaces anddepositing moisture.

The indication period is often of very small duration lasting

sometimes only a few hours.

III. Open Fire:

Indication period comes to an end with the appearance of"firestink" when open fire with visible active combustion

breaks out. The firestink can be easily recognized by itscharacteristic petrolic smell.

Seams seldom burn with a bright flame but they glowdeveloping bluish-white clouds of smoke.

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