Spont Heat Detection

8/6/2019 Spont Heat Detection

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DETECTION AND ASSESSMENT OF HEATING INCOAL MINES

Earlier is the fire is attended to,easier would it be to deal with

Methods based on:

A. Observation of the physical

symptomsB. Thermal surveys

C. Mine air analysis


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A. Observation of physical symptoms:

i. Appearance of faint haze

ii. Sweating of the strata

iii. Gob stink or fire stink

iv. Sound

v. Smoke

vi. Fire


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i. HazeInitial stage of heating-- moisture released from coal

gets converted into vapour on coming into contactwith cooler ventilating air = haze like formation

Signs: Poor visibility in the vicinity of heatingAppear little away from seat of fire towards intakeside

ii. Sweating of strata:Advanced stage after formation of haze.

Water vapour, on coming into contact with coolerventilating air/surface, forms water droplets onroofs, walls and timber supports

This may be misleading symptom in humid mines,hence considered only in dry mines and with coalshaving low moisture content


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iii. Gob stink or fire stink:Characteristic smell normally found in coal fire area

Tarry smell is considered as a sure warning to theinitiation of spontaneous heating

iv. Sound (Strata or insects):Cracking sound, from collapse of strata or due tocrack formation, may be heard behind stopping orbarriers indicates advancement of fire

Increased activity and chirping of cockroaches andcricket may be considered as warning

v. Smoke:

Observed beyond the ignition temperature of coalwhen fire has set in


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Limitations of Physical Symptoms:

One can not assess the degree of heating,nor can one always detect heating earlystages.

At times the physical symptoms may give

misleading informatione.g lubrication of wire ropes on haulageroads give smell similar to tarry smell,decay of wood in warm damp places,

grinding rocks under roof weight, tarredbrattice cloth, pine like odour of newtimber


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Symptoms indicating different stages of heating

I. Incipient or initial stage

a. faint haze

b. Sweating of roof, sides, timber and metal surfaces

c. Typical faint odour- slightly oily, some times sweetsometimes like decaying of timber

d. Increased activity of cockroaches and crickets if

presente. Slight discomfort and uneasiness to men near the

site of heating due to increased temp and humidity.Sense of tickling in nose, increased secretion ofsaliva and dry throat


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II. Intermediate or smouldering stage (>130 deg)

Intensified symptoms of Incipient stagea. Smell now becomes petrol like (distillation of coal)

b. Men may feel headache, increased restlessness anddulling of senses. Wobbling of knees in some cases

III. Heating approaching ignition (>330 deg)

a. Petrol smell changes to tarry odour

b. Smoke, incandescence or even flame may bevisible

III. Cooling stage

Characteristics odour and unpleasant smell of stale garlic


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B. Thermal surveys:

Much before actual fire, temperature

tends to show some rise due tospontaneous heating

Systematic check on temperature risein areas susceptible to spontaneous

heating may help in the early detectionof spontaneous heating

Methods:

B.1. Direct methodsB.2 Indirect methods


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B.1. Direct methods

Principles:

Expansion of confined volume of material (Hg-in-glasstype thermometers, bimetallic strips)

Melting point of suitable material (fusible plugs)

Thermo-electric effect (thermocouples)

Resistance change of semi-conducting

material( thermistor type units)

Limitations of Direct methods:

Connecting wires may get damaged due to roof falls, or

floor heaving or similar disturbance underground


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B.2. Indirect methods:

Thermal response can be relayed without wired

connection (wireless systems)i. Radio SondeHumid atmosphere obstruct their successfuloperations in goaves

ii. Stentch agents:Capsules containing highly odourous liquid of

low boiling point-> quite useful when capsules are placed atsuitable points. Human nose is sensitive torespond 0.001 ppm of odorant in air


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iii. Paints:Emit strong smelling vapour when heated

iv. Infra red Radiometry (HeatSpy, infra-red camera)

Picking the infra-red rays from any hot surface andconverts the received energy into an electric signal

It can measure temperature differential of a surface from adistance of >30 m with an accuracy of 10C.

Limitations of Indirect methods:

If hot surfaces can not be sighted by the equipment, thenthis technique becomes in-effective

Thermal surveys has to be supplemented with gas surveystudies or analysis of products of combustion


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C. Mine Air Analysis:

Gaseous products generated from heating of coal are

likely to be carried to places far off from the sources of

heating due to convection current as well as from mine

ventilation. Hence examination of change in composition of

the constituents of mine air becomes very useful in early

detection as well as in assessing the state of heating in themine

Mine Air Composition vary over wide range depending

upon the amount and nature of pollutant gasesemitted/produced in mines e.g.CO2, CH4, CO,

Hydrocarbons, traces of N2. H2 may be present in fire area.


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Points to be remembered:

1. In case of breathing of men, the burning of lights and the decay of timber, no CO is

produced and the amount of CO2 produced is roughly equal to O2 deficiency.

CO2/O2Deficiency > 80% & CO/O2 deficiency = 0

2. In case of iron pyrite, no CO is produced. The amount of CO2 produced indirectly bythe action of sulphuric acid on carbonates in the coal, is usually less than half the O2

absorbed, CO2/O2Deficiency < 50% & CO/O2 deficiency = 0

3. On oxidation of coal variable amount of CO2 and CO is produced depending upon the

temperature of coal. At normal temp CO2/O2Deficiency = 4% & CO/O2 deficiency


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Indicators:

1. Presence of various gases as indicator of fires: CO,

unsaturated hydrocarbons, H22. Oxygen consumption

3. Various Indices:

a) Grahams Ratio

b) Willets Ratio

c) Pertington Ratio

d) C/H ratio

e) CO/CO2Ratio

Presence of various gases:


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Presence of various gases:

CO, CO2, O2, N2


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Presence of Unsaturated Hydrocarbons


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2. Oxygen Consumption

Flaming combustion ceases in an atmosphere


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Indices/Ratios

a) Grahams Ratio

b) Willets Ratio

c) Pertington Ratio

d) C/H ratio


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3. Indices/Ratios

a. Grahams Ratio

CO/O2 deficiency %, CO2/O2 deficiency %

CO/O2 deficiency ratio

Most useful tool in detection and assessment of state of

heating in sealed off areas.

= CO*100/( 0.265N2

- O2

)

Relates the production of CO with oxygen

used up by oxidizing material and

indicates the average intensity of

oxidizing/ burning of coal mass. It doesnot provide any information on the extent

of the fire


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CO2= 6.79, O

2= 2.19%, CO= 0.059%, N

2= 90.961%

CO/O2 def 0.059*100/(0.265*90.961-2.19)

= 0.27%

CO2/ O

2Def = (6.79-.03)*100/(0.265*90.961-2.19)

= 30%


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As CO2can be emitted by strata and it is soluble in water, the results

are sometimes anomalous

Limitations:

1. Only the average value is given , and thus maximum intensity of

heating in a sealed zone is often under-estimated

2. The analytical errors accrued in the gas analysis may considerably

affect its accuracy for low oxygen deficiencies3. The ratio will be affected if products of combustion are diluted with

blackdamp or oxygen deficient air

4. Usually CO produced progressively reduces as the fire cools. At

times CO disappearance takes place even if the fire is not

completely extinct, particularly in wet mines (due to bacterialdisappearance)


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Willets Ratio

Analysis of samples collected behindsealed off area shows that certain amount

of CO produced by oxidation does not

disappear at all or falls at a very slow rate

with progressive extinction of fire

egasesCombustiblBlackDamp

COWR

+

=

100

P ti t R ti


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Pertington Ratio

N2/(CO+CO

2)

It increase with temperature , peak atabout 1000 C then reduces until

temperature reaches 2500C. It shows

increasing trend if fire rekindles.


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C/H ratio

Assess the intensity of fire and along with oxygen

consumption rate values it indicates extensivity of fire

For complete burning of bituminous coal- 16-20

High values burning of cellulosic material (Timber etc)

Lower values partial burning of coal

C/H with O2 consumption rate gives a good assessment ofa fire in a sealed off area

]))04.79/73.20((2[

)]2(6[/

2442222

2242

COHCHHCCOON

HCCHCOCOHC

+++

+++=


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Limitations:

1. It depends on dilution of area with fire damp

emitted from the strata

2.The various sources of generation of CO2 and

chances of its escape from dissolution with

water also affect its accuracy

3.But in combination with O2 consumption rate andgrahams ratio it gives good indication


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CO2/CO

Ratio

Sensitive to temperature of coal bed,

decreases with the increase intemperature and decreases with time. It

becomes constant beyond a certain

temperature


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Conclusions:

1. Physical symptoms like gob stink and persistent rise in CO concentration

level ( as well as rise in C2H

4and CO

2values) in the return air are indicative

of heating in mine. A check on change of gas composition and also of

thermal changes at vulnerable places would assist in early detection andlocation of heating

2. With present day innovation of tube bundle system of gas sampling,

collecting gases from vulnerable areas and introduction of continuous

monitoring devices with gas analysis appliances like IR gas analysers, early

detection of spontaneous heating and location of the site are no longer

arduous work3. With outbreak of fire mine air composition varies:

O2--

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Spont Heat Detection