Keywords: Spontaneous combustion; Mine fire gas indices; Sealed off fires
1. Introduction
Fire in coal mines is a serious problem for the Indiancoal industry. In Indian coal mines, most fires occur dueto spontaneous combustion, a process of coal oxidation.Coal interacts with oxygen in the air at ambienttemperature, liberating heat, which if allowed toaccumulate, ultimately would enhance the rate ofoxidation and lead to spontaneous combustion anddevastating fires. The exact mechanism of the reaction
of oxygen with coal is not clearly understood as thechemical nature of the coal is not fully established.
Plenty of work on various aspects of mine fires hasbeen done in Indian mines by various workers. Themention may be made of a few like Ghosh and Banerjee(1967), Ghosh et al. (1980), Banerjee (1985), Singh andSingh (1991, 1995), Singh and Banerjee (1993), Singh(1998), Singh et al. (1999), Singh and Sen (2003), Singhet al. (2000), Panigrahi and Sahu (2004), etc. Among theforeign examples the work done by Cudmore (1964),Chamberlin et al. (1970), Chakravorty and Feng (1978),Graham (1914, 1918, 1921), Haldane (1924), Jones andTrickett (1955), Kuchta et al. (1982), Litton (1986,1989), Morris (1988), Purshal and Ghosh (1963, 1965),
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Willet (1952, 1962), Justin and Kim (1988) and Kim(1991, 2004) are noteworthy.
The mine fire gases are the products of the oxidizedcoal. Due to mine fire activity, the gases are producedin enormous quantities, causing explosions and pollu-tion in the region. In most coal mines in the Jharia,Ramgarh and Raniganj coalfields, liberation of thesegases has become a great threat. Here, an attempt hasbeen made to use different mine fire gas indices fordetection and assessment of heating in the sealed offcoal seam fires.
The main reasons for occurrence of fire due tospontaneous heating in Indian mines are:
• Thick coal seams;• Plenty of coal fines left in the goaf;• Presence of contiguous seams, some of which areburning;
• Shallow depth;• Proximity of intake and return;• High pressure differences between the intake andreturn; and
• High spontaneous combustion susceptibility char-acteristics of most of the coal.
2. Scope of gas studies
Different gases such as CO, CO2, CH4, H2 and a fewother lower molecular weights unsaturated hydrocar-bons, which are products of the combustion due to minefire activity, are collected from the stopping in sealed offfire zones and analyzed. Their values indicate abnor-mality in the strata due to heating. Various fire indicessuch as Graham's ratio, Willet's ratio, C/H ratio, etc., arecalculated to decipher the extent of heating in the strata.
The normal air consists of N2, O2 and CO2 in theproportion of 79.04%, 20.93% and 0.03%, respectively.The deviation of the proportions of these gases in mineair is very useful for assessing the extent of heating.Combustion also gives rise to pollutants like CO, CO2,CH4, oxides of N2, H2S and other hydrocarbons. Due tothe advent of modern monitoring techniques for COdetection, carbon monoxide has become a very usefulindicator of mine fires. Depletion of O2 in the sealed offfire area is one indicator of an active fire. The rate of O2
consumption is a guide to distinguish a localized firefrom an extensive one. The CO/O2 deficiency ratio(Graham's ratio) also is an indicator of fire. The CO/O2
deficiency ratio increases as coal oxidation increases andnormally ranges between 0 and 0.40. However, in casesof serious heating, this value can reach 0.50 to 10.00,due to the formation of producer gas and water gas.
3. Methods and appliances used in mine gas analysis
In the present investigation, for collection of gassamples, the air displacement method was followed withdue precaution, so as to avoid any leakage orcontamination. Sterilized glass containers with goodpacking devices (stop cork/stopper with an inlet fordisplacement of air) were used for collecting the gassamples. Stop cork attached with grease was used in thesample tube and regularly checked to observe theleakage during sampling, which helped avoiding thecontamination with ambient air. The sample pipe in thesealed off fire area was driven to more than 5m insidethe goaf. The sampling was done from the stoppings inthe return side with the positive pressure. General bodysampling (mine air sample collected from the returnpoint of the district/mine panel) was also done toobserve the relative change in the proportion of gases inthe goaves.
The following analytical techniques are generallyused in gas detection and measurement in Indiancollieries.
3.1. Gas volumetric method
In this method various processes are applied (i.e.solution, adsorption, combustion or catalytic oxidation)and the volumes of gas before and after each operationare noted.
A measured volume of gas mixture at knowntemperature and pressure is step-wise subjected toselective chemical reagents in proper sequence forremoving the various constituents. The diminution involume is the record of the quantities of gas removed.The combustible gases having no absorbent (e.g. CH4)are burnt with excess oxygen in presence of a Pt-catalyst, and contraction in volume or estimation ofproduced CO2 is the record of combustible gas.
Various gas appliances are presently used for gasanalysis
• Hempel gas analyzer,• Orstat gas analyzer (sensitivity±0.1%),• Haldane gas analyzer (sensitivity±0.02%),• Graham–Lawrence gas analyzer
In the present investigation, the Haldane gas analyzerwas used, as it is a conventional type with high accuracyand is cheaper for periodic checking. An addedadvantage is that it has a provision for keeping thecompensating burette attached to it for annulling theerror caused by fluctuation in the atmospheric pressure.
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The apparatus consists of a graduated burette and pipettewith a regulator for temperature adjustments.
Reagents
(a) KOH solution of 10%w/v CO2 absorption.(b) Alkaline solution of Potassium–Pyrogallol forabsorption of O2.
Principle
Volumetric method of gas absorption and combustion. Procedure The analysis involves two steps:
1. First step – This step is an absorption technique forcalculation of CO2 and O2. The known volume ofsample is taken in burette and the adjustment is madethrough a compensator. The burette reservoir is tiltedupward and downward several times to make thesample repeatedly contact the KOH solution. Thegas sample is then passed through an alkalinePyrogallol pipette repeatedly and the diminution involume due to the absorption of O2 in Pyrogallol isnoted. This is repeated until a constant volume isreached. CO2 and O2 are calculated from theabsorption values.
2. Second step – The gas mixed with excess air is burntin the combustion pipette fitted with a coiledplatinum wire, electrically connected for heating upthe coil. Combustible gases like CO, H2, and CH4 areestimated from the measurement of post burningvolume contraction and CO2 formation. The CO andH2 burn at a lower temperature (i.e. below 450°C)causing a faint glow in the platinum wire called dullcombustion.
In the bright combustion (temperature higher than450°C) the platinum wire produces a red glow, whileburning out the remaining combustible gases (i.e. CH4
and other hydrocarbons). The above techniques involvethe following combustion reactions
2H2 þ O2↔2H2O
2CO þ O2↔2CO2
CH4 þ 2O2↔CO2 þ 2H2O
Limitations – With the help of this instrument onlyCO2, O2, H2 and CH4 are being analyzed. For analysisof CO more quantity of gas is required and is analyzedwith the help of Graham–Lawrence apparatus.
CO2, O2, CO, H2 and CH4 in the mine air wereanalyzed by the above technique using a Haldaneapparatus and N2 was calculated by difference. Henceany analytical error was adjusted in the N2
percentage.
4. Different fire gas indices used in Indian coalmines
The sealed off fire is periodically monitored to checkwhether it is being controlled or progressing unabated.Through monitoring it is easy to establish the status anddirection of fire advance and plan the steps to control thefire. Status of mine fires is generally assessed based ondifferent fire indices in conjunction with measurementof temperature of the area. Several indices/ratios, suchas production of CO and CO2, consumption of O2,unsaturated hydrocarbons, CO–residual gas relation-ships, desorbed hydrocarbon index, Graham's ratio,Willet's ratio, CO/CO2 ratio, sub micrometer particulatedetection and C/H ratio, etc., are applied to detect andassess the status of coal fire. However, to date noindividual index is capable of giving a precise anddefinite picture of the status of heating within a sealedoff area. Different fire indices are discussed below(CMRI S&T report, 1991).
4.1. Production of CO
With modern monitoring techniques, even minutetraces of CO can be detected. CO, a specific gaseousproduct formed during the entire coal and oxygenreaction process, is considered to be the very effectiveindicator of status of fire. Rate of production of COalong with its ratio to the consumption of oxygen is auseful guide for studying the extent of heating.However, there are studies indicating the fact that ifsufficient O2 is available, CO2 is the primary combus-tion product; measurable CO is not produced until theO2 concentration is at least below 15%.
Chamberlin et al. (1970) upheld the importance ofCO over unsaturated hydrocarbons as coal fire indicator,provided accurate analysis of CO using IR-techniquehas been carried out with sensitivity up to 1ppm.Through laboratory investigations they confirmed thatCO is evolved much earlier than ethylene or otherunsaturated hydrocarbons. Under controlled conditionsfrom heating the coal in the laboratory they established arelationship between CO-evolution and temperaturerise, i.e. the CO/O2 deficiency ratio rose from 0.2% at50°C to 1.6% at 150°C. Similar observations weremade by Chakravorty and Feng (1978) throughlaboratory experiments.
4.2. Disappearance of CO
Like the presence of CO, the disappearance of COmay also be used as indicator of heating, although this is
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not always effective as the reasons for depletion arediverse. Banerjee et al. (1965) observed the cause to bedue to bacterial action (aerobic and anaerobic) inbringing down the level as much as 1%. In waterymines, even with a pH value of 7, observation of COdisappearance (even a slight decrease in CO content) isindicative of heating.
4.3. Oxygen consumption
If the level of O2 is reduced to 12.4% (Banerjee et al.,1965; Banerjee, 1985), the flaming condition on coalceases. But it has been observed that coal can sustainheating after a considerable period and reignite when O2
is readmitted. The fire is extinguished only whenadmission of sufficient O2 can not start reignition.There are also some examples showing smolderingcombustion continuing in atmospheres with as little as3% O2, both retained heat and continued combustioncontribute to the reignition problem. If the likelihood ofleakages is controlled, accurate measurement of O2
depletion can be effectively used to distinguish a localfire from an extensive fire (Willet, 1962; Ghosh andBanerjee, 1967; Justin and Kim, 1988).
In the present case studies during initial oxidationphases, production of CO was found, and eventuallyCO2 was detected, that is why formation of CO wasconsidered to be most effective indicator of fire status.With the help of CO formation, CO/O2 deficiency ratioor Graham ratio was determined.
4.4. CO/CO2 ratio
It has been established that the CO/CO2 ratio ofproduct of combustion, under a particular combustionsituation, attains equilibrium, from the thermodynamicand gasification point of view. This can be used as indexto assess the fire situation. The values of CO and CO2
are sensitive to coal bed temperature and increase withtime. This method is as good as the CO/O2 deficiencyratio (Kuchta et al., 1982) to assessing an active fire. Theerratic values of CO2, i.e. its origin from various sourcesand tendency to dissolve in water, cause the limitationsto this mine fire index.
4.5. Willet's ratio
Willet (1952) analyzed gas samples collected fromsealed off fire areas, incorporating CO, black damp (aterm commonly applied to only carbon dioxide butstrictly speaking it also includes residual nitrogen) andcombustible gases produced as an index to detect the
heating in coal seams. He concluded that the COproduced by oxidation does not disappear at all withprogressive extinction of fire. The magnitude andextent of fire can be understood by the followingratio:
Willet's ratio
¼ CO2 produced
Black damp ðresidual N2 and CO2Þ þ combustibles
ð1Þ
4.6. C/H ratio
A team of researchers (Ghosh and Banerjee,1967, 1980) at Central Mining Research Institute,India developed a new index using the ratio ofcarbon and available hydrogen, along with oxygenconsumption values from the mine gases, to decipherthe character of sealed off fires and the nature offuel participating in the actual oxidation process. TheCO2, CO and hydrocarbons produced by combustionfacilitate the calculation of carbon, while theavailable hydrogen is calculated from evolvedhydrogen, hydrocarbons and from hydrogen utilizedin the formation of water as calculated from thereduction in used oxygen. The index is calculatedfrom the product gases as follows:
C=H ratio
¼ 6ðCO2 þ COþ CH4 þ other hydrocarbonsÞ2ðN2=3:78−O2−CO2 þ CH4Þ−CO− other hydrocarbonsþ H2
ð2Þ(C/H ratio up to max. 3–4 indicates superficialheating, 4–20 active fire and above 20 blazing firewith the possibility that wooden props may alsoburn).
4.6.1. Advantages(i) When used in conjunction with O2-consumption
data, it defines the extent and intensity of a fire; (ii) itsrange is larger, as compared to Graham's ratio,providing better sensitivity; and (iii) it can distinguisha coal fire from a wood fire.
4.6.2. Disadvantages(i) The C/H ratio is not independent of the dilution by
firedamp emitted from strata; (ii) accuracy is affecteddue to various sources of CO2 and its nature to dissolvein water; (iii) on reduction in the amount of O2 duringlow temperature oxidation, it is difficult to accurately
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evaluate the available hydrogen; and (iv) in cases of lowO2-deficiency values, the C/H ratio may give misleadingresults.
4.7. Graham's ratio
Exposed coal, even at ambient temperature, givesrise to gases like CO, CO2, etc. (Haldane andMeacham, 1898–99). As the temperature increases,more O2 is absorbed by the coal during oxidation.Graham (1914–15, 1921) observed that the gasesproduced after oxidation are generated in relation tosorbed oxygen. He developed an index to calculate thedegree of heating by comparing the rate of productionof CO or CO2 with that of O2 consumed, i.e. from CO/O2 deficiency and the CO2/O2 deficiency ratio. This isknown as Graham's ratio, and is still the most effectivetool for detecting and assessing fire in undergroundcoal mines.
The O2-deficiency is calculated based on the valuesof O2 equivalent and it can be calculated as follows:
In this example, suppose a gas analysis has thefollowing composition:
Graham developed two indices, which are summa-rized below.
4.7.1. CO/O2 deficiency ratioThe ratio relates the production of CO with oxygen
utilized by the burning coal and indicates averageintensity of the heating. This remains one of the mosteffective indices for estimation of intensity of heating.However, due to limitations in calculations of total
amount of CO produced and the total amount of coalinvolved the extent of heating can not be calculatedaccurately. Active fire can be predicted if the CO/O2
deficiency ratio exceeds 0.5.
4.7.1.1. Advantages.
(1) Since both numerator and denominator areaffected, the ratio is independent of dilution offire area by air or methane.
(2) CO is not produced by factors other than fire, soincrease in CO level and the CO/CO2 ratio is asure test to assess fire intensity.
4.7.1.2. Disadvantages.
(1) This ratio provides only an average value, sosometimes maximum heating in a particular areamay be underestimated.
(2) If the products of combustion are diluted by blackdamp (N2) or O2-deficient air, the ratio would beaffected.
(3) Sometimes the CO may disappear due to bacterialaction, although this is not an indicator of decay offire.
4.7.2. CO2/O2 deficiency ratioIn case of failures of the CO/O2 deficiency ratio,
where CO extinction is not indicative of fire status, theCO2/O2 deficiency ratio can be applied. Higher valuesof the CO2/O2 deficiency ratio indicate a change fromheating to actual fire.
4.7.2.1. Advantages. In severe fires, involving thecombustion of coke-like materials, where an enormousamount of CO2 is produced, this index is effective.
4.7.2.2. Disadvantages. The ratio sometimes givesanomalous results due to extraneous origin of CO2 andits solubility in water.
5. Case studies from Indian underground coal mines
Four underground mine fires from three importantcoalfields were studied by collecting gas samples fromstoppings and galleries, which were sealed after theegress of fire (Fig. 1). The collected samples wereanalyzed using a Haldane apparatus to determine thecomposition (percentage) of gases like CO2, CO, O2,H2, CH4 and N2, etc. Different mine fire indices werecalculated based on the proportion of the detectedgases.
Fig. 1. Location of studied coalfileds.
Table 1Composition of gases and calculated mine fire gas indices in theisolation stopping of North-rise top section, Pit-6, seam XI/XII,Madhuban colliery
Time of sampling: mid day. Gases such as H2 and CH4 were notdetected.
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5.1. Case study I
5.1.1. Madhuban colliery fire (Borehole-1), JhariaCoalfield, Dhanbad, Jharkhand
A fire was detected in the year 1988 in the Madhubancolliery borehole number-1 and immediately effortswere made to seal off the area to apply combattechniques through boreholes. The analytical results ofthe mine fire gas samples and temperature are shown inTable 1 and Fig. 2. Samples were taken from July 1989to October 1993. The sampling was done generallyduring mid-day (noon). The percentage of CO2 was 9.63in July 89, declined to 8.80 in May 92 and finally rose to10.45 in Oct. 93. The oxygen percentage was 10.72 inJuly 89, increased to 12.17 in Dec. 89 and fell to 5.80 inDec. 92. The presence of CO was observed only in July89 and Dec. 91 i.e. 0.246 and 0.020. The H2 and CH4
were not detected. The amount of nitrogen gas wasobserved fluctuating between 79.40% in July 89 and83.37% in Dec. 92 and finally 79.91% in Oct. 93. Therecording of the temperature from the stopping was alsocarried out regularly by thermometers. In July 89 thetemperature was 42°C and rose to 46°C in June 91 andafter a fluctuating trend finally 46°C in Oct. 93.
In the year 1986, a fire was detected in the 5thstopping in the 10-incline of seam XIV, Lodna colliery
Fig. 2. Calculated mine fire gas indices in the isolation stopping of North-rise top section, Pit-6, seam XI/XII, Madhuban colliery (CO2/O2 deficiencyon secondary axis).
Table 2Composition of gases and calculated fire indices in the samplescollected from the isolation stopping-5, incline-10, seam XIV, Lodnacolliery
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of Bharat Coking Coal Ltd, a subsidiary of Coal IndiaLtd in Jharia coalfield. Colliery authorities took theprompt action to seal off the area and arrangements weremade for sampling the gas from isolation stoppings. Afew gas samples were collected from the stopping-5,seam XIV (Incline-10) isolation stopping. Gas sampleswere analyzed to determine the proportion of CO2, O2,CO, H2, CH4 and N2, etc. Fire indices were calculatedand compared with the previous analysis available forthe fire site in the Lodna fire project. The gas sampleswere analyzed with the help of a Haldane gas analyzeravailable at the Lodna complex.
In this case gas analysis results from 27 Sept. 85 to 18Oct. 93 were analyzed (six times monthly). Thedistribution of CO2 was 12.40% in Sept. 85, increasedto 17.13% in June 87, fell to 10.65% in Oct. 92 andagain showed stabilized amount (10.67%) in Oct. 93.The amount of O2 was 5.18% in Sept. 85, fell to about1.55% in June 87, increased to 4.75% in Dec. 87 andfinally reached 8.18% in Oct. 93. The presence of CO,H2 and CH4 was also noticed. CO was detected fromSept. 85 to June 89, ranging from 0.02% to 0.81%.Hydrogen was detected from June 87 to June 89,ranging from 0.38% to 2.34%. Methane gas wasdetected only in Sept. 85 (0.35%), June 88 (0.53%)and June 89 (0.09%). Throughout the period the N2
ranged from 79.25% to 89.35% (Oct 92), and camedown to 81.16% in Oct. 93. The record of temperaturewas very much encouraging; as it was 73°C in Sept. 85,increased to 80.0°C in June 87 and finally came down to33°C in Oct. 93 (Table 2; Fig. 3).
Sayal ‘D’ colliery is situated in the Ramgarhcoalfield and belongs to Central Coalfields Ltd. It is
an underground mine, being worked by the board andpillar method of mining. In this mine, the first sign ofheating was detected in May 1990 in the No. 1 inclinebetween 2nd and 3rd levels and was considered to bethe outburst of spontaneous heating initiated at thefractured surface of the standing pillar by return air/surface leakage as well as leakage from the goaf at thebottom (CMRI S&T report, 1992). After some timethe fire traveled through the pillars and reached inclineNo. 2. Due to multiple fractures, the collieryauthorities attempted to isolate the fire from rest ofthe workings by erecting isolation stoppings. Toaccomplish this, the return air was diverted fromincline No. 1 to incline No. 3 by erecting isolationstoppings 1, 2, 3, 4, 5, and 6. However, in spite of thedifferent measures, the heating kept on advancing,threatening the closure of the mine (Table 3; Fig. 4Aand B).
Fig. 3. Calculated mine fire gas indices in the samples collected from the isolation stopping-5, incline-10, seam XIV, Lodna colliery (CO2/O2
deficiency on secondary axis).
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5.4. Case study IV
5.4.1. New Kenda colliery, Raniganj Coalfield, WestBengal
The New Kenda colliery is situated in the southeast-ern part of Raniganj Coalfield under the leasehold ofEastern Coalfields Ltd. The seam mined here is knownas the Kenda seam. A fire problem was reported to haveoriginated in an isolated old stowed goaf in panel No. 3,burst out through the roof between the 29th and 30thlevel of the 1st dip of the main return air ways and wasthreatening the main dip. The area was isolated by a 2nd
Table 3Composition of gases and calculated fire indices in the samplescollected from stopping-1 and -2, Sayal ‘D’ colliery (Mine No. 9)
row of stoppings. The colliery authorities tried severalmeasures without much success. After that CentralMining Research Institute, Dhanbad studied the prob-lem in detail and suitable measures were suggested forimplementation to prevent advance of the fire. Analyt-ical details are given in Table 4 and Fig. 5(A–E). (CMRIS&T report, 1993).
6. Result and discussion
In the first case study (Case study – I), the fire wassealed off and the extraneous O2 supply was checked.Pipes were fixed in the stoppings to monitor the fire bycollecting gas samples and recording the temperatureregularly. Mine fire gas indices CO/O2 deficiency andCO2/O2 deficiency were calculated (Fig. 2). The CO/O2 deficiency ratio was only determined as 2.38 inJuly 89 and 0.17 in Dec. 91. The CO2/O2 deficiencyratio was 93.3 in July 89. It decreased in Dec. 89 andslowly increased up to Dec. 91. A marked reduction inDec. 92 was observed with final stabilization at 90.33in Oct. 93. An active fire was proved, which wasunder control due to various measures including liquidN2 infusion at last. The temperature was almoststabilized.
In the second case, the fire was sealed off andisolated, and O2 supply from outer sources was checked.Gases from the mine fire zones were sampled.Sometimes it was observed that there is negligible orno change in the value of fire indices due to leakage ofair through the stopping. Wherever it was establishedthat there is no leakage from the stoppings, thepercentage of gases decreased.
A decreasing trend of CO2 and absence of CO, H2
and CH4 is indicative of a fire under control. Graham'sratio (CO/O2 deficiency) ranged from 0.01 to 4.81; theCO2/O2 deficiency ratio was 74.98 in Sept. 85,increased to 95.67 in Dec. 89, reduced to 67.0 in Oct.92 and again increased to 80.11 in Oct. 93. Due to
Fig. 4. (A) Calculated mine fire gas indices in the samples collected from stopping-1, Sayal ‘D’ colliery (Mine No. 9); (B) calculated mine fire gasindices in the samples collected from stopping-2, Sayal ‘D’ colliery (Mine No. 9) (CO2/O2 deficiency on secondary axis).
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precautionary and timely controlling measures such asisolation, sealing-off, and continuous monitoring of thegas samples, the fire was controlled and the temperaturecame down to 33°C in Oct. 93 from 73°C in Sept. 85and 80°C in June 87.
In the third case study, the CO/O2 deficiency ratiowas as high as 2–3, particularly behind stopping Nos. 1and 2 during the pre-gel infusion period in May 1991.Post-gel infusion results showed improvements, bring-ing down the CO/O2 deficiency as low as in samestopping. Other ratios show no changes, but extent ofthe fire was reduced by implementation of suitabletechnology. In the gel infusion technique, the solution ofsodium silicate 10% (w/v) and di-ammonium phosphate10% (w/v) in the ratio of 4 :1, having pH value around 9and the setting time of about 40min, was applied in thepillars to fill up the micro cracks present in the coalmatrix. By using the technology of Gel infusion in theSayal-D colliery, the extent of fire was not totallycontrolled but it worked as a preventive technique fromleakage of air through the sides and corners of thestopping, which were the vital points where the chancesof leakage of air in the goaf was observed. Throughapplication of this technique the chance of leakage wasreduced within a period of six months and not muchchange was observed in fire ratio. CO/O2 deficiencyratio remained apparently same during this period (Fig.4A and B).
In the last case study, the compositional analyses ofthe fire area prior to March 1992 from differentstoppings show that the oxygen percentage variedfrom 7% to 16%, CO2 from 4% to 10% and CO from0.01% to 0.06%. The CO2/O2 deficiency ratio and C/Hratio vary from 0.21 to 1.00 and from 3.00 to 8.00,respectively, which signifies that the fire was still in anactive condition. See Table 4 and Fig. 5A–E (CMRIS&T report, 1993).
The distribution of CO2 in the stopping 26L, 2 dipshowed a decreasing trend from 3.84% to 3.76% duringFeb. 92 to Oct. 92, the O2 was nearly constant, while theCO was generally 0.01% with minute departure (0.14%)in the month of June and Oct. 92. H2 was not detectedduring that period, CH4 was observed only in Feb. 92(0.50%), while N2 showed an increasing trend through-out the period. CO/O2 deficiency ratio showed anincreasing trend from 0.21 to 0.27 during Feb. 92 to Oct.92. Willet's ratio was about 16.00 throughout the periodand C/H ratio about 7.00, but CO2/O2 deficiency ratioshowed decreasing trend from 76.109 in Feb. 92 to71.237 in Aug. 92 (Table 4; Fig. 5A).
In the stopping 30L, 2 dip a gradual increasing trendfrom 3.77% to 9.67% for CO2 was observed duringApril to Oct. 92. The O2 showed a decreasing trend from15.30% to 7.02% during the same period, the COincreased from 0.04% in April 04 to 0.12% in Oct. 92.H2 and CH4 were not detected. The distribution of N2
Table 4Composition of gases and calculated fire indices in the samplescollected from New Kenda colliery
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showed an increasing trend. CO/O2 deficiency showedan increasing trend from 0.62 in April 92 to 0.80 in Oct.92. The C/H ratio first increased from 4.86 to 5.72 fromApr. 92 to June 92, followed again by a decline up to5.54 in Oct. 92. Willet's ratio first increased from 13.95to 19.85 from April 92 to June 9. CO2/O2 also followedthe same trend (Table 4; Fig. 5B).
The distribution of CO2 in stopping 26D showed adecreasing trend from 9.70% to 9.48% during July toOct. 92, while the O2 decreased from 7.86% to 7.74%during the same period. The CO was almost constant,while H2 and CH4 were not detected. N2 showed aconstant trend, CO/O2 deficiency ratio was 0.46 in July92, 0.33 in Sep. 92 and 0.49 in Oct. 92 with a fluctuatingtrend. The C/H ratio, Willet's ratio, and CO2/O2 ratiowere almost constant (Table 4; Fig. 5C).
In stopping 19L, 2 dip the CO2 first showed anincreasing trend from 0.22% in March 92 to 0.24% in
June 92. CO2 and O2 were almost constant; CO wasnegligible during the period. C/H ratio was 3.57 inMarch 92 and increased to 4.83 in Aug. 92, followed bya reduction up to 3.26 in Oct. 92. Willet's ratio was11.87 in March 92 and increased to 14.00 in Aug. 92,followed by a reduction up to 11.33 in Oct. 92. CO2/O2
also followed the same trend. H2 and CH4 were notdetected during the above period (Table 4; Fig. 5D).
The distribution of CO2 in the stopping MR 4 dipshowed almost constant trend from March to July 92,the O2 was detected 19.80% in March 92, whereas20.04% in July 92. The CO and CH4 were almostconstant. H2 was not detected during that period, whilethe CO was negligible. N2 and CO/O2 deficiency ratiofollowed almost constant trend throughout the period.The C/H ratio was 0.41 in March 92, which increased to0.43 in July 92. Willet's ratio was 2.93 in March 92 andincreased to 3.83 in July 92. CO2/O2 ratio was 10.71 inMarch 92 and increased to 14.61 in July 92. (Table 4;Fig. 5E).
In the Indian coal mining industry, the general trendis to use only the CO/O2 deficiency ratio fordetermination of fire status. But, in actual practice thisis not the only ratio that can be used to interpret the fireposition. After examining the different case studies inthe fire areas, it was observed that even if CO and CO/O2 deficiency is nil, a fire may still be inside the sealedoff area. In these cases, the other ratios will play a vitalrole in better assessing the fire status and extent. In theabove cases, use of the different fire indices was veryhelpful.
A number of fire indices, based on the compositionalanalysis of mine environment, are in use in differentcountries to detect not only the onset of heating inmines, but the degree of it as well. In this paperadvantages and limitations of different fire indices arepresented and some of the observations have also beenrepresented based on mine case studies. Each and everyratio has its own limitations that may not be used in allthe cases. Generally, Graham's ratio is the common ratioused for early detection of heating, but when the heatingis in advanced stage the formation of CO is negligible.In this case the value of Graham's ratio (CO/O2) is zero.But heating is present in the sealed off panel; in that caseother ratios come in picture depending upon thepresence of thermo-compositional analyses. At anadvanced stage of fire, it is mostly the combustion ofcoke like materials producing more of CO2. Usually, COproduced during heating progressively decreases as thefire dies down. At times, the disappearance of CO is notconsistent with the decay of fire. Sometimes, it goes onpersisting in a sealed off area or may disappear fast even
Fig. 5. (A) Calculated mind fire gas indices in the samples collected from stopping-1, 26L, 2 dip (CO2/O2 deficiency on secondary axis); (B) Calculated mine fire gas indices in the samples collectedfrom stopping-2, 30L, 2 dip (CO2/O2 deficiency on secondary axis); (C) calculated mine fire gas indices in the samples collected from stopping-3, 26D (CO2/O2 deficiency on secondary axis); (D)calculated mine fire gas indices in the samples collected from stopping-4, 19L, 2 dip (CO2/O2 deficiency on secondary axis); (E) calculated mine fire gas indices in the samples collected from stopping-5, MR, 4 dip (CO2/O2 deficiency on secondary axis).
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when the fire is not completely extinct. This anomalousdisappearance of CO may pose a problem in the correctinterpretation of mine air analysis.
After carrying out the detailed thermo-compositionalstudies as shown in Figs. 2–5, it has been observed thatinitially the ratio showed at some places superficialheating and at some paces blazing ones. Due toapplication of suitable site specific fire control technol-ogies, the ratio reduced and indicated the status of fire,which helped colliery authorities in controlling the fire.
7. Conclusion
Fire ratios play a very important role in interpretingthe status of a sealed off coal fire. Not all ratios can beused in all cases. The ratios used will vary case to casedepending upon the extent and condition of the fire.Finally, it is concluded that when using any ratioattention must be given to understand its limitations andfactors that can affect its applicability. Different ratiosand indicators will always give a more reliableinterpretation than one ratio alone.
Acknowledgments
The authors wish to thank the Director, Central FuelResearch Institute and Central Mining Research Insti-tute, Dhanbad for kindly permitting us to publish thispaper. The typological support provided by Mr. P. Boral,and Mr. Vivek Singh of CFRI for this paper is thankfullyacknowledged. The cooperation from the officers andstaff members of Bharat Coking Coal Ltd, Dhanbad,Central Coalfield Ltd, Ranchi and Eastern CoalfieldsLtd., Asansol, during fieldwork and providing thenecessary permission and support for gas analysis isduly acknowledged.
The authors would like to acknowledge the contribu-tions in the form of very constructive comments andsuggestions by Professor J. C. Hower, Professor A. G.Kim and Dr E. L. Heffern through reviewing andupdating the manuscript.
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