1. SPONTANEOUS COMBUSTION Dr. D. P. Mishra Department of Mining Engineering Indian School of Mines University, Dhanbad

2. •The process of self heating of coal or other carbonaceous material due to auto oxidation resulting eventually in its ignition is termed as “spontaneous heating”.
•It is the most important cause of fires in coal mines across the world.
•It has been a major problem in the leading coal producing countries like China, Australia and India.
•In Indian coal mines, 80% of the coal fires occur due to spontaneous combustion.
•The main reason is that the Indian coal seams are thicker and there is a tendency of coal to spontaneous heating during depillaring operation.

3. •History of coal mines fires can be traced back to the year 1865, when the first fire was reported in Raniganj Coelfields.
•Over 140 years, fires have been reported from both Jharia and Raniganj coalfields.
•Self heating of coal can occur in
–underground mines,
–opencast mines,
–shallow deposits and outcrops
–coal stockpiles,
–transportation and
–at the site of disposal of wastes from coal mine.

4. Adverse effect of spontaneous combustion
•Results in loss of lives and property.
•Causes considerable economic losses to the organizations and nation.
•Substantially increases greenhouse gas emanation levels and responsible for environmental pollution and global warming.

5. HISTORY OF COAL MINE FIRES DUE TO SPONTANEOUS COMBUSTION
World scenario:
•There are several instances of fires due to spontaneous combustion in the past history of mining which continued to burn for several decades and centuries.
China
•In China, coal fires are a severe problem, which is the world‟s largest coal producer with an annual output around 2.5 billion tons.
•It has been estimated that some 10-200 million tons of coal uselessly burn annually.
•Over 100 major fire areas are listed, which extend over a belt across the entire north China.
•They are concentrated in the states of Xinjiang, Inner Mongolia and Ningxia.

6. Germany
•In Planitz, a part of the city of Zwickau, a coal seam that had been burning since 1476 could only be quelled in 1860.
•In Dudweiler (Saarland) a coal seam fire erupted around 1668 and is still burning.
•Also well-known is the so-called Stinksteinwand (stinking stone wall) in Schwalbenthal on the eastern slope of the Hoher Meibner, where various seams caught fire centuries ago after lignite coal mining terminated; combustion gas continues to reach the surface in these days.

7. United States
•In USA many coalfields are subjected to spontaneous combustion.
•According to Federal Office of Surface Mining (OSM) database (AMLIS), 150 fire zones listed in 1999.
•There are 45 fire zones known in Pennsylvania, the most well known being the fire in the Centralia Mine in the hard coal region of Columbia.

8. Indian scenario
•Coal Mining in country was started in 1774 in Ranigunj Coalfield (RCF) in West Bengal.
•Coal mine fires in India have survived since the second and third decade of 20th century. However, the history of first happening of coal mine fires in India has not been accurately recorded.
•Over 140 years fires have been reported from both Jharia and Raniganj coal fields.
•History of coal mines fires can be traced back to the year 1865, when the first fire was reported in Raniganj Coalfields.

9. •In Jharia coalfield the first fire occurred in 1916 at Bhowra Colliery.
•In the independent India mine fires were known to have existed in the coal mines in
–Margarita in Assam,
–Venkatesh Khani Mine in Singareni in Andhra Pradesh,
–Mahakali Colliery in Chandrapura in Maharashtra,
–Talcher Colliery in Talcher Coalfield in Orissa,
–RCF in West Bengal and JCF in Jharkhand, etc.
•In 1966 there were 64 coal mine fires out of which 32 were in JCF and 19 in RCF. The number increased to 82 in 1976.
•The number of the fires increased further and was about 196 covering an area of about 30 sq km in 1994.

10. MECHANISM OF SPONTANEOUS COMBUSTION
•Auto-oxidation of coal is a complex physico-chemical process accompanied by the absorption of O2, formation of coal-oxygen complexes and their decomposition leading to the liberation of heat.
•The accurate mechanism of the reaction is still not well understood.
•However, scientists agree that the nature of the interaction between coal and oxygen at much low ambient temperatures is amply a physical (adsorption) process and changes into a chemisorption form.

11. It may be defined as a complex physico-chemical process by which the freshly exposed coal at ordinary atmospheric temperature when comes in contact with oxygen catches fire due to auto-oxidation and without the application of heat from any external source.
Fires occurring due to spontaneous combustion is a function of:
•quality of the coal and
•circumstances which it is subjected.
In coal mines, for spontaneous combustion to occur, three conditions must be satisfied:
•Coal must be present in a form which can oxidize readily at ambient temperature.
•Oxygen must be available to support oxidation.
•Favourable conditions for accumulation of heat must be present.

12. THEORIES OF SPONTANEOUS COMBUSTION OF COAL
Most important theories of spontaneous combustion:
•The Coal-Oxygen Complex Theory (Rhead and Wheeler)
•Pyrite Theory (Plot)
•The Bacterium Theory (Potter)
•The Phenol Theory (Fischer)
•The Electro-Chemical Theory

13. The Coal-Oxygen Complex Theory
•The establishment of peroxyl radical and hydro peroxides is commonly to be conceived to be the mechanism by which oxygen and moisture are initially incorporate into organic matrix.
•These species may rearrange, react or decomposed to form wide range of oxygen functionality in the gaseous product
•The following factors favours the oxidation process:
–Type of coal
–Temperature
–Moisture

14. •At ordinary atmospheric temperature, freshly exposed coal has affinity for oxygen of the air in contact with it.
•The oxygen is absorbed by coal on its surface by a purely physical process which, however, rapidly gives place to a chemical chain reaction resulting in oxidation of certain constituents of coal.
•Like all other oxidation reactions, the interaction of oxygen with coal is an exothermic with production of a small quantity of heat.
•The O2 absorption reaction is considered to take place as follows:
Coal + O2 Coal-O2 complex Oxidised coal + CO, CO2, H2O + Heat
•The heat from oxidation normally varies from 2.0 to 4.0 Cal/ml of O2 absorbed at N.T.P.
•A rise in temperature of coal, in consequence, takes place which has the effect of increasing further the rate of sorption of O2 and production of heat.

15. •The heat generated, if not dissipated by radiation or conduction or both at a rate faster than it is produced, a further rise of temperature of coal takes place which accelerates further the rate of O2 sorption and production of heat until finally the ignition point of coal is reached.
•The ignition temp. of bituminous coal is nearly 160-170°C and of anthracite coal nearly 185°C.

16. •The mechanism of reactions between O2 and coal is quite complicated.
•These reactions occur in following four steps:
•1st Step: Formation of coal-oxygen complexes and heat generation.
•2nd Step: Decomposition of these complexes, yielding of CO2 and H2O molecules and formation of more sensible groups [carboxyl (COOH), carbonyl (C=O) and phenolic (OH)] and heat generation.
•3rd Step: Decomposition of these groups too (at temp. > 100°C), production of CO, CO2, H2, H2O and high degree hydrocarbons (ethane, ethylene, propylene) and heat generation.
•4th Step: Decomposition of aliphatic structure (saturated & unsaturated hydrocarbons), production of CO, CO2, H2O.

17. Fig. Sequential stages in the
spontaneous combustion of coal
(Banerjee et.al., 1985)

18. Pyrite Theory
•Pure pyrite (polysulphide of iron) contains 46.37% Fe and 53.33% S and also reacts with oxygen of air at room temperature emancipating heat and responsible for sp. Combustion.
•Heating of coal could be caused by iron pyrites present in finely powdered and dispersed state in the presence of moisture.
•It has been established that pyrite present in coal might assist the oxidation of its carbonaceous content by breaking down coal into smaller fragments and exposing larger surface area to the air, as well as by elevating the temp. due to heat liberated from its own oxidation.

19. •The reaction of iron pyrites with oxygen and moisture gives products of larger volume than the original pyrite hence opening more pore area of exposure for oxygen, which is a exothermic reaction.
2FeS2 + 7O2 + 16 H2O = 2H2SO4 + 2FeSO4 . 7H2O + 316 × 2 kcal
•It also yields reaction product having greater volume than the actual pyrite, with the result that would break open any coal in which they are engrafted and thus exposing a greater surface of coal to the air.
•Sulphonated coal have greater reactivity towards oxygen, especially in the presence of iron oxide.
•It might have an considerable effect if its conc. in finely dispersed from exceeds 5 to 10 %. If the pyrite present is less than 5%, then its effect is negligible (Munzner, 1975).

20. The Bacterium Theory
•Bacteria also encourage self- ignition of coal due to bacterial action.
•Spontaneous heating observed in haystacks and in wood are known to be mainly due to the activity of bacteria.
•
•However, there is no conclusive proof to authenticate or dispose this theory. Hence it is concluded that bacteria could cause only a slight heating which may not play any important role.

21. The Phenol Theory
•Many experiments have shown that phenolic hydroxyls and poly phenols oxidize faster than other groups.
The Electro-Chemical Theory
•It describes auto-oxidation of coals as oxidation-reduction processes takes place in micro galvanic cells made by the coal components.

22. FACTORS AFFECTING SPONTANEOUS COMBUSTION OF COAL
•The magnitude of sp. combustion depends on a complex relationship of a range of internal (intrinsic) and external (extrinsic) factors:
Intrinsic Factors
(Nature of Coal)
Coal characteristics/Seam factors
Extrinsic Factors
(Atmospheric, Geological and Mining Conditions)

23. Intrinsic Factors (Coal characteristics/Seam factors)
•Composition, rank and petrographic constituents of coal.
•Friability, particle size and surface area of coal
•Moisture content
•Presence of iron pyrites
•Bacteria
•Other minerals

24. Extrinsic Factors (Atmospheric, Geological and Mining Conditions)
Climatic conditions:
•Temperature
•Moisture/relative humidity
•Barometric pressure
•O2 conc.
Geological factors
•Coal seam and surrounding strata condition
•Seam thickness
•Seam gradient
•Caving characteristics
•Faulting and other geological disturbances
•Coal outbursts
•Friability
•Depth of cover

25. Mining factors
•Mining methods
•Rate of advance
•Pillar conditions
•Roof condition
•Crushing
•Packing
•Presence of timber or other organic waste material in abandoned areas or dumps
•Leakage
•Multi-seam working
•Coal losses
•Worked out areas
•Heat from machines
•Stowing
•Ventilation system and airflow rate
•Ventilation pressure
•Method of stockpiling and stockpile compaction

26. 1. Coal Characteristics
a. Rank of coal
•Coals of different ranks have different capacities to absorb O2.
•The rate of oxidation decreases with increase in rank of coal.
•Lower rank coals containing higher moisture, O2 and volatile content are more easily oxidized and hence, the risk of sp. combustion is higher in lower rank coals.
b. Presence of pyrites
•Presence of iron pyrite (FeS2) increases the potential of coal for sp. combustion, particularly when the pyrite concentration exceeds 2% and when it is in very finely divided state.
•Pyrite accelerates sp. combustion by swelling and causing disintegration of coal mass, thereby increases the surface area available for oxidation.
•It easily oxidizes by its own in presence of O2 of the air and moisture at ordinary atmospheric temp. according to the following equation:
2FeS2 + 7O2 + 16 H2O = 2H2SO4 + 2FeSO4 . 7H2O + 316 × 2 kcal
•The oxidation of pyrites has only a promoting effect on auto-oxidation of coal.

27. c. Area of exposed coal surface
•Exposed surface area as well as internal surface area (pore spaces) of coal is a significant factor influencing its self heating.
•The entire pore surfaces in coal may be above 80 m2/g.
•The smaller the coal particle, the greater the exposed surface area in contact with the air and the greater will be the rate of oxidation and tendency towards sp. combustion.
•The rate of oxidation increases with increasing fineness of coal.
•The rate of heating has been found to be proportional to the cube root of the specific internal surface area of coal (Schmidt, 1945).
•In case of an average bituminous coal, it is generally considered that the danger of sp. combustion is slight for sizes larger than 38mm.
d. Freshness of exposed coal surface
•For any given set of constant conditions, the rate of oxidation of exposed fresh surface of coal decreases continuously during the progress of oxidation, that is, with time.
•In case of weathered coal, the exposed surface of coal reaches the stage of saturation and no further interaction with O2 at ambient temperature occurs.

28. e. Petrographic composition of coal
•Coal constituents the macroconstituents like vitrain, clarain, durain and fusain influence the liability of coal to spontaneous heating.
•Liability of spontaneous combustion decreases in the order: vitrain-clarain- durain-fusain content.
f. Volatile matter (VM) content
•Increase in volatile content of coal increases the rate of oxidation.
•Schmidt-Elder found that coal with 38% VM oxidizes 3 times as fast as coal with 18% VM.
•The research institute of Ostrava-Radvanice (Czechoslovakia) considers coals having a volatile content of 28% and more are liable to sp. heating.
g. Moisture content
•Moisture inherent in coal, present in the surrounding oxidizing atmosphere, or produced from oxidation of coal in the early stages of oxidation influences the oxidation process. It acts as a catalytic agent.
•The high moisture coals have higher tendency of spontaneous heating.
•Nandy et al. (1967) had shown that there is an optimum moisture level of around 5% in coal showing maximum spontaneous heating tendency.

29. h. Oxygen content
•The liability of coal to sp. heating is directly related to its O2 content.
•With coals having O2 content less than 2%, the oxidation rate is not large enough to cause any appreciable increase in temperature.
i. Ash content
•Ash present in coal generally decreases the oxidation rate which is also influenced by the mineral composition of the ash.
j. Thermal conductivity of coal
•The thermal conductivity of coal determines the rate of heat dissipation.
•Coals with low thermal conductivity are more liable to spontaneous heating.

30. 2. Geological Factors
a. Seam thickness
•Thick coal seams are more prone to sp. combustion because the working of these seams is invariably accompanied by high losses of coal in the goaf area.
•The unmined part which is left in the goaf is more prone to sp.combustion since it is exposed to sluggish ventilation flow.
•In some coal seams, coal is left in the roof or floor. Some of the roof coals drop in the gob, which will eventually be sealed off. This coal can get access to air either by waste “breathing” or by leakage.
•Coal in the floor breaks when the floor heaves, which exposes the coal to air flow.
•Also in thick coal seams, certain bands within the seam can be more liable to sp. combustion than other bands.
b. Dip of the seam
•In inclined seams control of sp. combustion becomes complicated as temperature difference causes convection air currents in the gob.
•Within the gob, flow may be due to buoyancy as a result of differing densities of CH4, CO2, and N2.
•The induced airflow in the gob increases the possibility of sp. combustion of coal in the gob or old workings.

31. c. Depth of cover
•When a coal seam under a shallow overburden is mined, the goaf areas get 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 sp. combustion.
•Also at depths, the in situ coal temperature tends to be higher making deeper seams more vulnerable to sp. combustion.
•At greater depth, the excessive rock pressure acting on coal pillars results in their crushing and thereby increases the surface area of coal exposed to oxidation, which help in increasing sp. heating.
d. Presence of multi seams
•When multi-seams in close proximity are worked, the cracks and fissures developed in the intervening strata increase the potential for sp. combustion of the surrounding unmined seams, particularly the undermined seams.

32. e. Caving characteristics
•Caving characteristics of the strata above the mining sections of the seam may considerably influence the likelihood of sp. combustion.
f. Faulting
•Presence of faults in coal seams allow air and water to migrate into the coal seams and helps in development of heating in coal mines.
•Any grinding of coal along the fault plane may lead to self-heating.
g. Coal friability
•Friable coals tend to produce coal fines which, due to larger surface area, have a greater tendency to sp. combustion.

33. 3. Mining Factors
a. Mining methods
•Longwall advancing leaves extracted areas lying between the entries serving the working places.
•The ventilating pressure differences will encourage airflow across these areas with the accompanying problem of incomplete combustion.
•In high-risk situations, a retreat system of working is preferable.
b. Multi seam workings
•In case of multi seam workings, where a seam has been mined with another virgin seam laying below, leakage paths are created from the upper seam into the lower seam with a consequent risk of heating.
c. Rate of advance
•The rate of mining and advance rate of the face determine the time in which the broken coal in the gob is exposed to ventilation air.
•If the rate of advance is slow, and time taken in entering and leaving a panel is excessive, the oxidation may occur to an unacceptable degree and a gob fire could result.

34. d. Pillar size
•Pillar size should be sufficient to prevent crushing.
•Pillars that have been standing for a long time are prone to heating, particularly when they are liable to crushing.
•Pillar crushing can create air leakage paths leading to the flow of air through the coal. Loose coal is usually produced by pillar spalling or crushing. When associated with sluggish ventilation, these areas are hazardous.
e. Regulators, doors, stoppings and air crossings
•These are the points of high air leakage. Unless they are well sealed, will tend to leak air through the fractures in the solid coal around them.
•The greater the air pressure difference across them, the more the air leakage.
•Doors between the main intake and return airway are the most vulnerable.
•Problem is more significant in air crossings as the pressure difference is high.
•Air leakage through a stopping depends on the permeability of the stopping and the pressure difference across the stopping.

35. f. Obstruction in roads
•All roads in thick coal seams have breaks/fractures associated with them. Any obstruction in the road, viz. a stockpile of materials, mine cars, etc. can force air into the fractures, causing heating.
g. Ventilation/air flow rates
•For sp. combustion to develop, the rate of heat generation should be more than the rate of heat dissipation.
•In case of a strong ventilating current or at very high airflow rates, almost unlimited O2 for the oxidation of coal is available, but the dissipation of the heat generated by oxidation is very efficient.
•The strong ventilating current conducts away the heat produced from oxidation without materially rising the temperature of coal.
•On the other hand, a week ventilating current or a low flow rate might supply air just sufficient for oxidation but not sufficient to keep down the temperature.
•A critical flow rate is one that provides sufficient O2 for widespread oxidation but does not dissipate the heat generated.
•A good rule to be observed by mining men would be “No ventilation at all is better than a deficient ventilation”.

36. Table: Set elements of mining conditions influencing sp. Combustion ( Banerjee ,1985)

39. STAGES OF SPONTANEOUS COMBUSTION
One recognizes three stages of spontaneous combustion of coal in coal mines:
•the incubation period
•the indication period
•open fire
1. The incubation period
•It is the period between the onset of first oxidation and the timepoint when one can detect by the senses.
•In a broader sense, it denotes the period between the beginning of coal extraction in a district or panel and the appearance of first signs of heating.
•During incubation period, one does not detect heating during one’s passage through mine workings.

40. The incubation period varies widely depending on
•characteristics of the coal
•seam thickness
•nature of the immediate roof
•method of working and method of roof control
•regularity and continuity of working
•air leakage and the heat accumulation in the environment
For low-rank coals, the incubation period generally varies between 3 and 6 months.
With high-rank coals it varies between 9 and 18 months.
Under adverse conditions, the period can be less than 2 weeks, especially with low-rank coals.

41. 2. The indication period
•The end of the incubation period is the beginning of the indication period.
• It is marked by ‘sweating’ and haze which is caused by the warmed up air from a fire area cooling on coming in contact with the cooler coal, rock and metallic surfaces and depositing moisture.
•The indication period is often of very small duration lasting sometimes only a few hours and end with the appearance of ‘fire stink’.
•The fire stink can be easily recognized by its characteristic petrolic smell.
3. The open fires
•After the indication period, open fire with visible active combustion breaks out.
•In open fires, seams seldom burn with a bright flame but they glow developing bluish-white clouds of smoke.

42. DETECTION OF SPONTANEOUS COMBUSTION
•Detection of sp. combustion in its early stage is very much essential for its prevention. Earlier a fire is detected; easier it is to deal with.
•Sp. combustion of coal in mine workings may be detected by physical and chemical indications given by the heating itself.
There are essentially 3 classes of detection for sp. heating.
Early detection of physical symptoms through the human senses
Conducting thermal survey using thermal devices
Monitoring gas concentrations in return airways and sealed-off areas and making compositional analysis of mine environment

43. 1. Early detection of physical symptoms through the human senses
•This is the oldest technique.
•Reliance has been placed on these indications in the past for early detection of sp. heating.
•Development of heating in a goaf is generally accompanied by the appearance of
–haze,
–sweating of the strata,
–gob stink or fire stink,
–sound,
–smoke and
–fire.
•These physical symptoms of heating can be identified through human senses.

44. GOB STINK
•Usually the first indication of a heating that can be detected by the human senses is the characteristic odour of the gob stink in the air coming from the suspected area.
•This tarry smell (sometimes described as petrolic or aromatic smell), termed as Gob stink or Fire stink, is considered as a sure warning to the initiation of spontaneous heating since the earliest times.
•In the initial stages of heating, the smell resembles that of petrol but as the heating progresses it changes to kerosene and later to a tarry odour.
•A strong smell indicates that the heating has made sufficient headway.
•This smell is caused by distillation gases like saturated hydrocarbons and higher hydrocarbons produced during the oxidation phases.
•The characteristic smell normally found in a coal fire area, once experienced can never be forgotten and can not be missed.

45. HAZE
•In the initial stage of heating, as the moisture released from coal come in contact with the cooler ventilating air and thereby condense, making haze like formation.
•Haze is visible a little away from the seat of fire i.e. near the intake side of the ventilation.
•Typical signs are poor visibility of the mine environment in the vicinity of heating.
SWEATING OF THE STRATA
•This symptom is observed at an advanced stage after formation of haze.
•Caused due to condensation of water vapour released from coal due to sp. heating & formation of water droplets on the roofs, walls, timber supports or any other cooler surfaces in its vicinity.

46. SOUND
•At times typical creaking sound (from collapse of strata or due to crack formation in them) may be heard behind the stopping or barriers, which could be indicative of advancement of fire.
•Sometimes increased activity and chirping of cockroaches and crickets (possibly caused by increase in temperature) may be considered as warning of the advancement of fire.
SMOKE IN AIRWAYS
•Smoke can only be observed above the ignition temperature of coal when fire has set in.

47. Limitations:
•Haze or sweating of the strata may give misleading information giving false alarm at places where there are chances for the humid return air to meet the comparatively cooler intake air. For this reason haze formation or sweating of the strata are taken more seriously in dry mines and with coals of lower moisture content (< 5%).
•The early physical symptoms may be totally missed, even in dry mines depending on the mine environment and the rate of progress of heating.
•Even today gob stink is considered a very useful warning for recognising the onset of incipient heating or the advancement of fire in coal mines. But physical symptoms like gob stink also, can not detect heating at a very early stage, at least not up to 120°C or so.
•One can not access the degree of heating from smell alone which demands measuring quantitative changes that occur during the heating of coal.

48. 2. Conducting thermal survey using thermal devices
•The second class of detectors are thermal devices used to determine increase in temperature.
•Infra-red scans of roadway sides employed to identify the emission points of warm gases into airways and are useful for localized heatings in pillars or around stoppings.
•Thermocouples or thermistors left in gob areas. However, they too have met with very little success to the present time with following demerits:
–First, their wiring is unlikely to withstand the mechanical stresses of an active caving zone, even when sheathed.
–Secondly, the thermal conductivity of crushed rock is low. Hence, the temperature even within a metre of an active centre of heating may indicate no abnormal condition.

49. 3. Monitoring gas concentrations in return airways and sealed-off areas and their trend analysis
•Monitoring the quality of air in a mine is the modern, most widespread and dominant method of detecting sp. heating.
•This method of early detection is based on monitoring gas concentrations in return airways and analysing the changes in air composition.
•Sampling the air downstream from a fire or from within a newly sealed area and plotting the trends is the primary method of tracking the behaviour of fire.
•As the gases emitted vary with the phases of oxidation, time and temperature, it is necessary to employ skilled interpretation of those trends.

50. FIRE INDICES / Susceptibility indices of spontaneous combustion
Based on the compositional analyses of mine environment, a no. of ratios or fire indices have been suggested, to detect not only the onset of heating in mines, but the degree of it as well.
They assist in the interpretation of fire gases.
Some of these gas ratios or indices used in interpreting trends of gas concentrations produced by mine fires are given as follows:
Ratio Name
•CO/ ΔO2 Graham's Ratio or Index for Carbon Monoxide (ICO)
•CO2 / ΔO2 Young's Ratio
•CO/(Excess N2 + CO2 + combustibles) Willett's Ratio
•(CO2 + 0.75CO - 0.25H2 )/ ΔO2 Jones and Trickett Ratio
•CO/ CO2 Oxides of Carbon Ratio

51. •A feature of several of the ratios is the oxygen deficiency, ΔO2. This is a measure of the O2 that has been consumed and is based on two assumptions;
First, that the air has been supplied with 20.93% O2 and 79.04% inert gases (excepting 0.03% CO). That 79.04 per cent contains traces of other gases but is referred to simply as N2.
Secondly, it is assumed that no N2 has been consumed or added (except from the air) through the area under consideration.
If no O2 is consumed, then the O2 /N2 ratio would remain at 20.93/79.04 = 0.2648 ≈ 0.265 irrespective of the addition of other gases. For any measured values of O2 and N2, the conc. of O2 that was originally in place can be calculated as
N2
Hence, the amount of O2 that has been consumed, or O2 deficiency is given as
Δ O2 = 0.265 N2 – O2 per cent
04.7993.20

52. Graham's Ratio
•It is the most widely used indicator of an incipient heating in coal mines and has often given warnings several weeks before any odour could be detected.
•It is recognised that coal on being exposed to air, even at room temperature, consumes O2 and gives rise to gases like CO and CO2 as oxidation products.
•As CO2 may be produced from other sources or may be lost in air due to sorption by coal or by solution in water, CO is the most reliable indicator of heating.
•By 1914, Ivon Graham recognized the importance of CO as an early indicator of sp. heating of coal and the equally vital influence of O2 that was consumed.
•He first suggested the index CO/ΔO2, now known as Graham's Ratio or Graham's Index or the Index for Carbon Monoxide (ICO).
•It has the significant advantage that it is almost independent of dilution by leakage of air as this affects both numerator and denominator equally.

53. •Like other indices, a normal range of Graham's Ratio should be established for any given mine. This will usually be less than 0.5%. Any consistently rising values in excess of 0.5% is indicative of a heating.
Typical values of the carbon CO/ O2 deficiency ratio for underground coal mines are given below:
•0.4% or less – normal value
•0.5% – necessary for thorough check-up
•1.0% – heating is almost certain
•2.0% – heating is serious with or without the presence of active fire
•3.0% – active fire surely exists

54. Drawbacks of Graham's Ratio
•Its accuracy becomes suspect if very little O2 has been consumed, i.e. Graham's Ratio is unreliable if the oxygen deficiency, ΔO2, is < 0.3 %.
•This is a weakness shared by the other indices that involve O2 deficiency.
•It is affected by sources of CO other than the fire, e.g.
–use of diesel equipment, or
–if the air supplied to the fire is not fresh: it occurs if the fire is fed by air that has migrated through old workings and contains blackdamp (de- oxygenated air).

55. Young's Ratio
•CO2 is the most prolific of the gases produced in mine fires. Hence, the values of CO2 /ΔO2 will be much higher than CO/ ΔO2.
•As a fire progresses from smouldering to open flame, the burning of CO will produce an increase in CO2.
•Hence a simultaneous rise in CO2 / ΔO2 and fall in CO/ ΔO2 indicates further development of the fire.
•However, as both ratios have the same denominator, the straightforward plots of CO and CO2 show the same trends.
•Young's Ratio is nearly independent of dilution by fresh air.
•It suffers from similar limitations to Graham's Ratio. Additionally, the conc. of CO2 may have been influenced by adsorption, its solubility in water, strata emissions of the gas and other chemical reactions.
•Thus, the extraneous origin of CO2 as well as its solubility in water make the interpretation anomalous at times.

56. Willett's Ratio
•This ratio was introduced by Dr. H.L. Willett in 1951 with specific reference to situations where there is a higher than usual evolution of CO by ongoing low temp. oxidation.
•In these cases, gradual extinction of a fire in a sealed area may not be reflected well by the CO trend alone but as a percentage of the air-free content of the sample.
•In these cases, Willet suggested to use the ratio
%
besides the analysis of CO, to understand the magnitude and extent of fire.
•This ratio can be used only as a supplementary index along with other fire indices.
gas eCombustibl dampBlack produced CO 

57. Jones and Tricket Ratio (JTR)
•Jones-Tricket Ratio is used as a measure of reliability of sample analysis and also as an indicator of the type of fuel involved.
•It can be used for the gaseous products of both fires and explosions.
•Jones and Tricket Ratio (JTR), giving the relationship between the products of combustion with O2 deficiency is defined as below:
(CO2 + 0.75 CO - 0.25 H2) / ΔO2
•Dilution by fresh air has no effect on the Jones-Tricket Ratio. However, it is subjected to the limitations of O2 deficiency.
•This ratio helps in distinguishing coal dust explosions from methane explosion-from examination of post-explosion gases.
–JTR values ≤ 0.5 indicates methane explosion,
–values around 0.85 ± 0.18 indicates coal dust explosion.
–Values in between are indicative of both methane and coal dust explosion.

58. Oxides of Carbon Ratio
•The Oxides of Carbon Ratio, CO/CO2 or the ratio of P.O.C (Products of Combustion) is a useful pointer to the progression of fire, rising during the early stages and tending to remain constant during flaming combustion.
•However, the CO/CO2 rises rapidly again as a fire becomes fuel-rich and is an excellent indicator of this condition.
•This ratio may also be favoured because it is unaffected by inflows of air, methane or injected nitrogen (Mitchell, 1990).
•It is, however, subject to variations in CO and CO2 that are not caused by the fire.

59. C/H Ratio
•C/H ratio of the mine gases was introduced by Ghosh and Banerjee (1967) for assessing the intensity of fire, along with O2 consumption values that indicate extensity character of it.
•They argued that in case of burning of fuel (mainly compound of C and H2 in varied proportion) the temp. determines the extent to which the C and H2 part of it would burn.
•At low temp, the H2 part may burn completely but there would be a lot of unburned C deposited as soot.
•In case of rapid burning of CH4, the C/H ratio calculated from the product gases is always less than 3, which is the maximum value for complete combustion of CH4.
•Likewise, the maximum attainable C/H value for complete burning of bituminous coal lies between 16-20, depending on the type of coal.
•Higher C/H values indicate burning of the cellulose bodies (i.e. timber etc. having higher C/H values) while lower values indicate partial burning of coal.
•Thus, the degree or intensity of burning of coal may be adjudged from the value of C/H ratio of the product gases obtained.

60. PREVENTION OF SPONTANEOUS COMBUSTION
Prevention of spontaneous combustion is based on two factors:
•Elimination of coal from the area and
•Control of ventilation:
–to exclude O2 entirely from the area, or
–to supply a sufficient flow of air to dissipate the heat efficiently as it is generated and before a critical temp. is reached.

61. Several methods and strategies which can be adopted to prevent and control spontaneous combustion in coal are given as follows:
A.Mining layout
•Proper mine layout design is very important for prevention of sp. heating.
•Design of general mine layout should be simple, so that each area/section can be quickly and effectively sealed off or isolated at short notice without affecting production in other districts.
•Panel system of working: appropriate for mining seams liable to sp. heating, which facilitates effective sealing with a few stoppings.
•The panels must be laid as to minimize severe crushing of coal during extraction.

62. •Size of each panel should be based on the incubation period of coal and the rate of extraction.
•Panels must be of a size which would permit complete extraction within the incubation period.
•The size of the panel barriers needs to be sufficient for stability.
•Panels with independent ventilation should be formed.
•When working seams by bord and pillar method, the size and shape of the pillars must be sufficient to avoid excessive crushing at edges and corners.
•Side bolts can be used and low viscosity grouts may be injected to maintain the integrity of pillars.
•In case of longwall mining, face lengths smaller than 60 to 65 m should not be made to ensure good closure and compaction of the goaf.

63. B. Coal winning
•Probability of sp. combustion can be reduced by minimizing the amount of coal, timber, paper, oily rags or other combustible materials left in gob areas.
•The coal should be won as completely as practicable especially in disturbed zones, thick seams and strongly folded steep seams.
•Efficient clearance of the fragmented coal from the face and good housekeeping should be practiced in mines that have a history of sp. combustion.
•A high rate of extraction should be adopted to prevent fire due to spontaneous heating.
•The advance or retreat of a coal face should never be interrupted.

64. •If any local fall of a roof or a fault is encountered, immediate steps should be taken to get over the obstacle or reduce its effect on the face advance.
•It is essential to follow descending order of extraction when mining multiple seams.
•When winning a group of seams, the workings in an upper seam should be in advance of those of lower seam and the coal of the upper seam should be extracted as complete as practicable.
•In case of longwall working, retreating longwall method should be preferred as it eliminates leakage currents through the goaf area due to large difference in ventilation pressure.
•In case of longwall advancing, the gate side packs should be made airtight.
•On completion of production from a panel, reclamation of material should be completed without delay and the panel adequately sealed as quickly as possible.

65. C. Ventilation and air leakage control
The points which should be considered for ventilation and leakage control to prevent sp. heating are as follows:
•The layout of the ventilation network should be designed to minimize pressure differentials between adjoining airways and across caved areas.
•Branch resistances in the surrounding ventilation network should be kept as low as practicable by means of larger cross-sections or driving parallel entries. Furthermore, obstructions in those airways should be avoided.
•All active mine workings and roadways in coal should be adequately ventilated and unused roadways sealed off.
•Ventilation pressure should not be unduly high so that air leakage through crushed pillars or defective stoppings or seals of sealed areas does not take place.

66. •Doors, stoppings, and regulators should be properly sited. Unnecessary stopping and starting of main and booster fans should be avoided.
•Short-circuiting of air as well as its uncontrolled circulation must be eliminated.
•As far as practicable, the formation of leakage paths should be minimized by providing adequately sized pillars and good gate side packs. If this is not sufficient to prevent air leakage, leakage paths should be sealed off by sealant coating or injection.
•Air leakage into sealed areas through fractures extending from the surface should be prevented by artificial sealing using sand.
•When a panel has ceased production and is to be sealed off, the ventilation pressure difference should be balanced across the old panel.

67. D. Inhibitors
•In storage areas and surface stock piles, certain chemical agents can be applied to the coal surface to hinder the penetration of O2 into coal by sealing the surface pores and thereby stopping initiation of auto-oxidation of coal at ambient temperatures.
•Stock piles should be properly designed to reduce air movement through them.
•Surface stock piles can also be sealed off by consolidation and bitumen.

68. E. Inspections
•Every working place underground should be inspected by a supervisory official or competent person at least once during each working shift.
•During inspection, attention should be paid to the presence of and means of eliminating fire risk arising from sp. combustion and other causes.
•On idle days, all districts liable to sp. heating should be inspected at least once by competent persons.