INTERPRETATION OF STATUS OF SPONTANEOUS HEATING OF COAL/MINE FIRES BASED ON FIRE RATIOS

Dr.D.P.Tripathy Professor,& HeadDepartment of Mining Engineering,National Institute of Technology,Rourkela-769008, Orissa, India.E-mail: dptripathy@nitrkl.ac.in_________________________________________________________________________________

1. Introduction

In Indian coal mines, mine fires have become a very problematic issue, numbering more than 200, both surface and underground. The most significant fire-affected coalfields include: Jharia, Raniganj, Talcher, Ib-Valley, Chirimiri, Singrauli and Ramgarh etc. Due to spontaneous heating of coal in coal mines, a number of fire gases are released. Analysis and interpretation of these gases can be useful tool for the ascertaining/assessing of fire status in mines. After analyzing mine gases, different fire indices can be computed for examining the extent of fire so that suitable preventive measures can be undertaken to minimize/avoid fire hazards. The objective of this article is to present the different fire indices such as CO/O2 deficiency, CO2/O2 deficiency, C/H ratio, Willet's ratio, etc., for interpretation of status of underground (U/G) sealed off fires. Few case studies have been presented for assessing status of fire in the U/G coal mines based on these fire indices.

2. Different Fire Gas Indices Used In Indian Coal Mines

The sealed off fire is periodically monitored to check whether it is being controlled or progressing unabated. Through monitoring it is easy to establish the status and direction of fire advance and plan the steps to control the fire. Status of mine fires is generally assessed based on different fire indices in conjunction with measurement of temperature of the area. Several indices/ratios, such as production of CO and CO2, consumption of O2, unsaturated hydrocarbons, COresidual gas relationships, desorbed hydrocarbon index, Graham's ratio, Willet's ratio, CO/CO2 ratio, sub micrometer particulate detection and C/H ratio, etc., are applied to detect and assess the status of coal fire. However, to date no individual index is capable of giving a precise and definite picture of the status of heating within a sealed off area. Different fire indices are discussed below.2.1 Graham's ratio

Graham, 191415 and Graham, 1921 observed that the gases produced after oxidation are generated in relation to sorbed oxygen. He developed an index to calculate the degree of heating by comparing the rate of production of CO or CO2 with that of O2 consumed, i.e. from CO/O2 deficiency and the CO2/O2 deficiency ratio. The calculation of the CO/O2 deficiency ratio is based on the assumption (usually justifiable) that nitrogen is neither added to nor taken from the air concerned in the oxidation. This is known as Graham's ratio, and is still the most effective tool for detecting and assessing fire in underground coal mines.

As a thumb rule it can be taken that: 0.4 per cent or less indicates normal value. 0.5 per cent indicates necessity for a thorough check-up. 1 per cent indicates existence of heating. 2 per cent indicates serious heating approaches active fire. 3 per cent and above indicate active fire with certainty. 7 may occur for blazing fireExample 1:

The O2-deficiency is calculated based on the values of O2 equivalent and it can be calculated as follows:In this example, suppose a gas analysis has the following composition:CO2=5.00%O2=3.00%CO=0.05%N2=91.95%

In atmosphere 100parts of air consists of O2=20.93% and N2=79.04%

O2 equivalent to 91.95% of nitrogen=(Atmospheric O2 % / Atmospheric N2 %)N2 (analyzed)

=(20.93/79.04)91.95

=0.26591.95=24.37

Oxygen deficiency=(O2 equivalentO2 analyzed) or (Atmospheric O2 %/Atmospheric N2 %)N2 (analyzed)O2 (analyzed)

So, O2 deficiency=24.373.00

=21.37%

Example 2:

Suppose a gas analysis has the following composition:CO2=6.70 %O2=2.19 %CO=0.059%N2=90.961%Ans:

CO2 =6.79% { in 100 parts of air there are 20.93 parts of O2 and 79104 parts of N2}O2 = 2.19% {Therefore, oxygen equivalent to 90.961 parts of Nitrogen is -x 90.961=0.265x 90.96 = 24.08}CO=0.059% N2 = 90.961 % {Oxygen deficiency in this case=24.08 2.l9 = 21.89} Total = 100.00%The ratio CO/O2 def = = 0.0027, 0.27% (approx.)Similarly, CO2/O2 def. ratio (also called Youngs ratio) = 6.79/21.89 =0.31 =31%.

Then, CO/O2 deficiency ratio will be: 0.27 % and CO2/O2 deficiency ratio will be :0.31 %.

Graham developed two indices, which are summarized below.CO/O2 deficiency ratio

The ratio relates the production of CO with oxygen utilized by the burning coal and indicates average intensity of the heating. This remains one of the most effective indices for estimation of intensity of heating. However, due to limitations in calculations of total amount of CO produced and the total amount of coal involved the extent of heating can not be calculated accurately. Active fire can be predicted if the CO/O2 deficiency ratio exceeds 0.5.Advantages:1. Since both numerator and denominator are affected, the ratio is independent of dilution of fire area by air or methane. 2. CO is not produced by factors other than fire, so increase in CO level and the CO/CO2 ratio is a sure test to assess fire intensity.Disadvantages:1. This ratio provides only an average value, so sometimes maximum heating in a particular area may be underestimated. 2. If the products of combustion are diluted by black damp (N2) or O2-deficient air, the ratio would be affected.3. Sometimes the CO may disappear due to bacterial action, although this is not an indicator of decay of fire.

In the Indian coal mining industry, the general trend is to use only the CO/O2 deficiency ratio for determination of fire status. But, in actual practice this is not the only ratio that can be used to interpret the fire position. After examining the different case studies in the fire areas, it was observed that even if CO and CO/O2 deficiency is nil, a fire may still be inside the sealed off area. In these cases, the other ratios will play a vital role in better assessing the fire status and extent. In the above cases, use of the different fire indices was very helpful.2.2 CO2/O2 deficiency ratio (Youngs ratio)

Carbon dioxide produced as a percentage of oxygen absorbed is considered as Youngs ratio or CO2/O2 def. Ratio. As the fire progresses from smouldering to open flame, the burning of carbon monoxide will produce an increase in carbon dioxide. Hence, a simultaneous rise in [CO2]/ O2 and fall in [CO]/ O2 indicates further development of the fire. However, as both ratios have the same denominator, the straightforward plot of carbon monoxide and carbon dioxide show the same trends. If the value of this ratio is below 25 it is considered to be indicative of superficial heating. If it is more than 50 it should be corroborated with other fire indices to rule out or confirm a high intensity fire7. This ratio suffers from various limitations Its accuracy becomes suspect if very little oxygen has been consumed, i.e. the oxygen deficiency, O2 is less than 0.3 per cent. The concentration of carbon dioxide may have been influenced by adsorption, its solubility in water, strata emissions of the gas and other chemical reactions.

In case of failures of the CO/O2 deficiency ratio, where CO extinction is not indicative of fire status, the CO2/O2 deficiency ratio can be applied. Higher values of the CO2/O2 deficiency ratio indicate a change from heating to actual fire.Advantages:In severe fires, involving the combustion of coke-like materials, where an enormous amount of CO2 is produced, and this index is effective.Disadvantages:The ratio sometimes gives anomalous results due to extraneous origin of CO2 and its solubility in water.2.3 CO/CO2 ratio

It has been established that the CO/CO2 ratio of product of combustion, under a particular combustion situation, attains equilibrium, from the thermodynamic and gasification point of view. This ratio, [CO]/ [CO2] of products of combustion may be considered to attain an equilibrium in a particular combustion situation. Thus attempts have been made to understand the fire situation from this ratio. It indicates the completeness of the combustion or oxidation. If the value of this ratio is more than 2 per cent it indicates active fire in the adjacent zone. If the value of this ratio is equal to or more than 13 it indicates a blazing fire. This ratio has the significant advantage that it is unaffected by inflows of air, methane or injected nitrogen. It is, however, subject to variation in carbon monoxide and carbon dioxide that are not caused by fire. Limitations are chances of appearance of blackdamp and disappearance of CO2 (dissolution in water, etc.) in mines. This method is as good as the CO/O2 deficiency ratio (Kuchta et al., 1982) to assessing an active fire. The erratic values of CO2, i.e. its origin from various sources and tendency to dissolve in water, cause the limitations to this mine fire index.Thermodynamically from gasification point of view, CO/ CO2 ratio of P.O.C (Products of combustion) may be considered to attain an Equilibrium in a particular combustion situation, Thus, attempts have been made to understand the fire situation from CO/ CO2 ratios as well Kuchta et al. (1982) collected a large number of data on P.O.C. and the heat developed by burning definite amount of coal in a model mine gallery. They then compared three fire indices, viz., CO/ CO2 ratio, CO/ O2 deficiency and Jones and Tricket ratio. They found the CO/ CO2 ratio and CO/ O2 deficiency to be more dependable than Jones and Tricket ratio. Values of both the former ratios were sensitive to coal bed temperature and increased with time. The maximum values of CO/ O2 deficiency percentages were between 0.15 and 0.20 after sealing, with corresponding values of CO/ CO2 between 0.30 and 0.50. But CO2 detection appliances being less sensitive than CO detectors, they advocated CO analysis for early detection of fire/ heating and CO/ CO2 analysis for fire assessment. 2.4 Willet's ratio

Willet (1952) analyzed gas samples collected from sealed off fire areas, incorporating CO, black damp (a term commonly applied to only carbon dioxide but strictly speaking it also includes residual nitrogen) and combustible gases produced as an index to detect the heating in coal seams. He concluded that the CO produced by oxidation does not disappear at all with progressive extinction of fire. The magnitude and extent of fire can be understood by the followingratio:Willets ratio = CO produced/ black damp(residual N2 and CO2)+ combustiblesThis ratio is applicable to the specific situations where there is a higher than usual evolution of carbon monoxide by ongoing low temperature oxidation. This ratio can be used as a criterion of progress to-wards the extinction of fire behind the seals under circumstances where the Grahams index does not apply. It may be pointed out here that the absolute figures vary from seam to seam and even from district to district, thus it is only the trends that can be universally applied. Some researcher emphasized that the constancy of the Willetts ratio is not an indication that a fire is extinct but only that the fire activity has greatly decreased. 2.5. C/H ratio

Ghosh and Banerjee (1967) introduced carbon/hydrogen ratio determination of the mine gases and claimed to assess the intensity character of the fire, along with oxygen consumption values to decipher the character of sealed off fires and the nature of fuel participating in the actual oxidation process. The CO2, CO and hydrocarbons produced by combustion facilitate the calculation of carbon, while the available hydrogen is calculated from evolved hydrogen, hydrocarbons and from hydrogen utilized in the formation of water as calculated from the reduction in used oxygen. The index is calculated from the product gases asfollows:

C/H ratio up to a maximum of 3, indicate only superficial heating, values above 5 of active fire and from around 20 of a blazing fire.

Based on C/H ratio and oxygen consumption rate they have given a method to determine the nature of fire in a sealed area.

Table 1. Application of C/ H Ratio in the Assessment of Fire (Ghosh & Banerjee, 1967) Sl.No.C/H values, from analytical data of fire areaRate of oxygen consumption as observed from periodic analysisRemarks on the nature of fire

1High and very near to that of coal i.e. nearly 20FastBlazing and extensive burning of coal

SlowBlazing but localised burning of coal

2Appreciably higher than that of coalFastBlazing and extensive fire associated with burning of props etc.

SlowBlazing fire associated with burning of props etc., but a localised one

3Appreciably lower than that of coal i.e. much below 10FastSuperficial fire but covering an extensive area

SlowSuperficial fire and a localised one

They also showed from a comparative statement between C/ H and CO/O2 deficiency ratios, of the fire area gases in Iharia field that this C/H ratio gives a better indication than even CO/O2 deficiency ratios, particularly for long standing fires in old abandoned mines (Table 2). Along with other fire indices like Grahams ratio and oxygen consumption data, C/H ratio is a useful guide to assess the status of heating. Of course it has got certain limitation as given below. Table 2 Comparing Grahams Ratio with C/H Values

Sl.No.Description of the fire areasCO/O2 def.%C/ H Ratio

1Fire of recent origin, slight heating noticed, area kept sealed off 0.20-0.900.25-0.75

2Heating in an advanced stage, area kept sealed off1.75-3.001.01-1.45

3An old abandoned long-standing fire and above hot fumes observed coming out from overhead surface cracks0.22-1.1620

Along with other indices (Grahams ratio etc.), C/H ratio with oxygen consumption rate proves to be a very useful guide to assess the status of heating of a fire area. Advantages:1. When used in conjunction with O2-consumption data, it defines the extent and intensity of a fire;2. its range is larger, as compared to Graham's ratio, providing better sensitivity; and 3. it can distinguish a coal fire from a wood fire.Disadvantages:1. Unlike CO/ O2 def.% it is not independent of dilution 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. There are some practical difficulties to determine oxygen consumption rate in sealed off fire, in view of undefined air leakages situations.4. in cases of low O2-deficiency values, the C/H ratio may give misleading results.2.6 Jones and Trickett RatioJones & Tricket Ratio, giving the relationship between the products of combustion with O2 def. is defined as below: [CO2] + 0. 75[CO] 0. 25[H2]_____________________________________{[0. 265[N2] [O2]} i. e. Oxygen deficiency

Obviously, like CO/O2 def. ratio, it is independent of dilution by air or CH4.Different authors tried to use this ratio in mine fire diagnostics.Limitations:

Kuchta et al. (1982), Delverny & Chaiken (1991) and others attempted to use this ratio in mine fire diagnostics along with other fire indices. But it did not meet with much success. It could be observed that with oxygen Concentration < 17%, JTR values from fire area bore hole samples showed Values between 0.65 and 0.85. Fuel rich combustion process however, showed higher values of JTR between 1.0 and 1.5. For certain types of combustion with 50% conversion from CO to CO2, JTR values may go as high as 7.0. It was thereafter inferred that JTR values by itself is not much dependent on temperaturethough it may be dependent on combustion process (Delverny 8: Chaiken, 1991). This ratio is rather more important to distinguish between coal dust explosion from methane explosion-from examination of post explosion gases JTR Values 0.5 indicates methane explosion, whence values around 0.85018 indicates coal dust explosion. Values in between are indicative of both methane and coal dust explosion.

2.7 Litton RatioThis ratio was designed to eliminate the potential for recognition of a mine fire when oxygen was reintroduced into the mine during reopening. Only four gases are required to measure to determine this ratio. They are oxygen, carbon monoxide, methane, and ethane. The atmosphere is divided into three parts: air, methane, ethane, and residual gas. Residual gas (Rg ) is the volume per cent of gas within a sealed coal mine after air, methane and ethane have been eliminated from the sample. Rg contains excess nitrogen (not part of air), Products of Combustion (POC) (CO, CO2 , H2 and hydrocarbons), along with products formed by chemical processes not related to combustion and is presented as: Rg = 100 air CH4 C2 H6, where air = Alfa*O2 and Alfa = 100/20.946 = 4.774.

The CO concentration in parts per million of air within the residual gas is: (CORg) = [(COs) / Rg ] X 100, where (CO)S is the concentration of CO, ppm in the original sample.

Utilizing the concentration of CO within the residual gas a new parameter called the R-index (R1 ) is derive d I as: From mathematical point of view, if CO within a sample remains constant while Rg either increases or decreases the R-index is the absolute value of the rate of change of the CO within the residual gas with respect to change in the residual gas. If R1 is constant, either sustained low temperature combustion or a low rate of ambient CO production is occurring. When combustion takes place the average R1 will be greater than if ambient CO production is occurring. From any sample of gas, Rg can be calculated from the measured O2 , CH4 and C2H6. From the measured value of CO (in ppm) and the calculated value of R the actual value of R-index can be determined. This value is de-noted by (RI )act . Further from the measured O2 concentration and the calculated Rg value, a maximum equilibrium value for the R-index can be calculated using following equation.

This equation has been obtained by Litton from the graph plotted RI vs the ratio Rg /O2 . The graph has been plotted after collecting samples from behind stoppings when it was known that no combustion was present. An equation RATIO determines the state of a sealed area: If RATIO is equal to one it serves only as a benchmark that divides the region between unsafe and possibly safe conditions. If it is greater than one, temperatures are above ambient and either smoldering combustion or above ambient temperature oxidation is taking place. When RATIO is less than or equal to one it represents that the temperature is ambient and a state of equilibrium exists. However equilibrium continues only if RATIO stabilizes at a value less than one and remains there for at least 30.

2.8 Hydrogen/Methane Ratio This ratio was developed by Mitchell and used as an indicator of flaming combustion. The progression of gas liberation is normally: carbon monoxide, carbon dioxide, hydrogen, ethylene, propylene, and acetylene. Combustion occurs as the temperature rises above the liberation point of hydrogen. Therefore, if gases above hydro-gen are detected, an active fire is indicated. 2.9 Hydrocarbon RatioJustin and Kim have suggested a method by which drilling a large number of boreholes in a fire area from surface and drawing out gas samples from within the location of a fire as well as its state of heating could be determined. Methane is the primary hydrocarbon liberated at ambient temperatures. They have shown that the desorption of low molecular weight hydrocarbon coal gases is directly related to temperatures. The hydrocarbon ratio equation was derived as:

where THC = total hydrocarbon concentration, ppm; CH4= methane concentration, ppm ; and C = constant, 0.01 ppm. RI equals to zero when no hydrocarbons are present in the sample, 10 when methane is the only hydrocarbon measured and about 1010 at the upper limit. The only limiting factor is that methane must be greater than 20 ppm. 2.10 Desorbed Hydrocarbon Index Iustin and Kim (1988) suggested a novel method by which drilling a large number of boreholes in a fire area from surface and drawing out gas samples from within, the location of a fire as well as its state of heating could be determined. They observed that the desorption of low molecular weight hydrocarbons (C1 C5) from coal is temperature dependent. At ambient temperature the desorbed gas is primarily methane, but with rise in temperature other hydrocarbons are also evolved. They defined the concentration ratio RI as: (1.01[THC] - [CH4])RI= -----------------------------------x 1000 ([THC] + C) Where THC = Conc. of total hydrocarbons in ppm . CH4 = Conc. of methane in ppm C = A constant ,The value of RI, which of course is dependent on type of coal, decreases with cooling and increases with the heating of coal. .Merits:In this method, sampling is done by collection of the system of moving gases within the fire area, applying suction at boreholes. By plotting the g s analysis results from different borehole points as vectors, the seat of heating may be located from the fact that the above desorbed gas will not appear on applying suction, if the borehole sampling point is outside the fire/ heat zone. Thus, it enables to locate the seat of heating in a sealed zone. The index can also be used to characterize a fire from RI values,viz. for the bituminous coal RI index would be as below:0-50 for normal temperature,50-100 for possible source of heating and, more than 100 indicates hot zone. The value increases with rise in intensity of heating.Demerits: Drilling a large number of boreholes (4" dia) for monitoring as required in this method, may at times be a difficult exercise in view of topographical constraints besides the prohibitive cost, particularly for deep mines. On the other hand at shallow depths, the sample may get vitiated, due to air leakage from surface on applying suction particularly in cases of not thoroughly well contained fires. Emission of methane in a sealed fire area as well as its normal evolution as combustion product gases (during fire), may mask the hydrocarbon content of the desorbed gases, affecting accuracy of the analysis (ppm range) required for RI estimation.3.0 Case Studies3.1 CASE-1 (Low Intensity Fire Covering A Small Area) Khus Kajora Cclliery, ECL, India the trend of values of different fire indices for a prolonged period of 5 months, as monitored from sample points from three stoppings of a small isolated fire areaare shown in Table 3.Fire Status ln this particular fire CO/O2 def. values came down from 0.03% to nil, CO2/O2 def. % around 50% and above, CO/CO2 came down from 0.02 to nil, C/ H ratio value from 3-4, CO-residual gas content from 0.01 to nil, Surface temperature over stoppings were in ambient level with pressure fluctuation in all the stoppings varying a little, in both positive and negative direction along with baromatric fluctuations. It is evident from the above results that a fire covering a small area started cooling down gradually with not much signs of air leakages. It was later on confirmed by the rescue team who entered the fire area breaching a stopping, that average temperature of inbye galleries were between 38-44C. Table 3. Comparison of different fire indices of Panel P at Khas Kajora Colliery, ECL

3.2 CASE-2 (LOW INTENSITY FIRE COVERING AN EXTENSIVE AREA) Laikdih Dip Colliery, 4 dip, top section, BCCL, India The trend of fire indices of this sealed zone covering a large area are shown in Table. 4. Fire Status It could be observed that CO / O2 def. percentage mostly recorded zero values. CO2/ O2 def. percentage between 20-50%, CO-residual gas content Showed 0.1 to nil, C / H ratio 1-3, CO/CO2 mostly nil, at times little less than 2. Surface temperature profile of stoppings showed maximum values between 40-60C. Differential pressure measurement through stoppings was mostly positivethough air leakages did exist in stopping 7 with high oxygen percentage. Table 4. Comparison of different fire indices of Laikdih Deep Colliery

Case-3 . Madhuban colliery fire (Borehole-1), Jharia Coalfield, Dhanbad, Jharkhand

A fire was detected in the year 1988 in the Madhuban colliery borehole number-1 and immediately efforts were made to seal off the area to apply combat techniques through boreholes. The analytical results of the mine fire gas samples and temperature are shown in Table 5 and Fig. 1. Samples were taken from July 1989 to October 1993. The sampling was done generally during mid-day (noon). The percentage of CO2 was 9.63 in July 89, declined to 8.80 in May 92 and finally rose to 10.45 in Oct. 93. The oxygen percentage was 10.72 in July 89, increased to 12.17 in Dec. 89 and fell to 5.80 in Dec. 92. The presence of CO was observed only in July 89 and Dec. 91 i.e. 0.246 and 0.020. The H2 and CH4 were not detected. The amount of nitrogen gas was observed fluctuating between 79.40% in July 89 and 83.37% in Dec. 92 and finally 79.91% in Oct. 93. The recording of the temperature from the stopping was also carried out regularly by thermometers. In July 89 the temperature was 42C and rose to 46C in June 91 and after a fluctuating trend finally 46C in Oct. 93.

Table 5. Composition of gases and calculated mine fire gas indices in the isolation stopping of North-rise top section, Pit-6, seam XI/XII, Madhuban collieryDateCO2 (%)O2 (%)CO (%)N2 (%)Temperature (C)

31.7.899.6310.720.24679.4042.00

17.12.897.6512.170.080.1842.00

27.9.9011.209.030.079.7742.50

10.11.9011.129.210.079.6744.00

21.6.9110.669.580.079.7746.00

25.12.9111.209.400.02079.3845.45

27.5.928.809.350.0081.8544.00

14.12.9210.835.800.0083.3745.00

15.6.9311.467.480.0081.0644.00

20.10.9310.459.640.0079.9146.00

Time of sampling: mid day. Gases such as H2 and CH4 were not detected.Fig. 1.Calculated mine fire gas indices in the isolation stopping of North-rise top section, Pit-6, seam XI/XII, Madhuban colliery (CO2/O2 deficiency on secondary axis).

Fig.1

In this case, the fire was sealed off and the extraneous O2 supply was checked. Pipes were fixed in the stoppings to monitor the fire by collecting gas samples and recording the temperature regularly. Mine fire gas indices CO/O2 deficiency and CO2/O2 deficiency were calculated (Fig.1). The CO/O2 deficiency ratio was only determined as 2.38 in July 89 and 0.17 in Dec. 91. The CO2/O2 deficiency ratio was 93.3 in July 89. It decreased in Dec. 89 and slowly increased up to Dec. 91. A marked reduction in Dec. 92 was observed with final stabilization at 90.33 in Oct. 93. An active fire was proved, which was under control due to various measures including liquid N2 infusion at last. The temperature was almost stabilized.7. Conclusion

Fire ratios play a very important role in interpreting the status of a sealed off coal fire. Not all ratios can be used in all cases. The ratios used will vary case to case depending upon the extent and condition of the fire. Finally, it is concluded that when using any ratio attention must be given to understand its limitations and factors that can affect its applicability. Different ratios and indicators will always give a more reliable interpretation than one ratio alone.ReferencesBanerjee, S.C. (2000) Prevention and combating mine fires, Oxford and IBH publishing Co. Pvt. Ltd, pp.147-180.Singh, A.K. et al. (2007) Mine fire gas indices and their application to Indian underground coal mine fires, Int. Journal of Coal Geology, Vol.69, Issue 3, February, pp.192-204.Ray, S.K. et al (2004) Assessing the status of sealed fire in underground coal mines, JSIR,Vol.63.__________________________________________________________________________________