Report: APP CMHS Project 1

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3.4.5. Spontaneous Combustion and Fire Control

Indian coal mining is predominantly dominated by bord and pillar method of mining. The recovery rate of coal in the bord and pillar mining is only 60-80 %. The developed pillars are extracted by means of caving or stowing which by default leaves 20-40 % of coal in the form of crushed coal, left over stooks, hard coal in the roof and floor and coal in the barrier pillars. All are major causes of spon com related fires in underground coal mines in India.

There are numerous incidents of occurrence of fires in Indian coal mines and extensive knowledge is gained through research into the causes for the fires. Several innovative methods are used to prevent and control such fires.

Current Technologies

The prevention of spontaneous combustion is through the implementation of good ventilation practices, the proper system of panel extraction, early detection of heatings, and proactive use of fire retarding chemicals.

In accordance with CMR (1957) mining panels are required to be isolated from each other and ventilated with an independent air current. Consideration shall be given to the incubation period of coal in designing panel sizes and full panel extraction.

All loose coal should be removed from the mine or sealed off. Preparatory seals and seals for unused roadways are required to be installed prior to commencement of pillar extraction. Panels must be isolated by adequate stoppings as soon as they are mined. The seals are to be constructed with non-flammable materials.

Comprehensive studies are carried out to determine spontaneous combustion propensity based on geothermic properties such as Crossing Point Temperature (CPT) and Inflexion Point Temperature (IPT); maceral, volatile matter and moisture contents. Detection is achieved through physical inspection and CO monitoring. Recent developments are occurring in some mines where gas samples are drawn from inside goaf areas and analysed with chromatographs.

Sponcom Prediction Techniques

Laboratory experiments are used in order to determine the spon com propensity, viz., Crossing point temperature (CPT), Differential thermal analysis (DTA) and Differential scanning calorimetric (DSC) studies.

    Crossing Point Temperature (CPT) Test

The CPT test is the most commonly used method for determining spon com propensity of coal seams since 1974. The method was prescribed by the Director General of Mines Safety (DGMS) and mandated for all coal mines where depillaring operations are likely to be carried out. As per the DGMS study, the assessment of the proneness of coal seams to spon com could be made by studying the Crossing point temperature and moisture content of the coal seams.

A coal seam with CPT 160C and moisture content of 2 % has low susceptibility; coal with CPT 140C - 160C and moisture content 2-5 % is moderately susceptible and coal with CPT 120C - 140C and moisture content more than 5 % is highly susceptible.

The standard procedure for determination of CPT and Moisture content as per DGMS technical Circular 3/1994 is:

  • Determination of moisture content in an air oven with 60 % relative humidity.

  • Determination of Crossing point temperature:

  • 20 grams coal size -72 to +200 B.S.mesh.

  • Heating of bath 10C per minute in air/glycerine media.

  • Oxygen flow rate of 80 ml/minute

  • Reactor tube - 20 cm X 2 cm internal diameter with double water spiralling around the reactor (8 spirals) in the bath of Oxygen.

  • Fresh Run off Mine (ROM) coal of about 3 kg collected and placed under water. Outer part of the lump scraped off before testing the sample. Sample thus prepared to be used within 24 hours.

Differential Thermal Analyser (DTA)

Differential thermal analysis procedure was first developed by Banerjee and Chakravorty (1967). A thermogram of coal can be divided into three segments or stages. In the initial stage of heating (stage I), the endothermic reaction predominates, probably due to the release of inherent moisture in coal. In the second stage (stage II), the exothermic reaction becomes significant, but the rate of heat release is not steady all through, as it changes with temperature. A steep rise in heat evolution is observed in the third stage (stage III). A sample thermogram developed by Panigrahi and Sahu et. al for a coal sample in India is given in Figure 51, which shows all the three stages as explained above. Later the thermograms were analysed for finding out the Onset temperature or characteristic temperature - a significant parameter to show spon com propensity. It is considered that the lower the onset temperature the more susceptible the coal seam will be for spontaneous heating.

Figure 51 Stages of oxidation of coal in a differential thermal analyser

    Differential Scanning Calorimetry

In the differential scanning calorimetric technique, a coal sample and a reference material are subjected to a controlled temperature program and the energy inputs are measured and compared as a function of temperature. The technique is used at the Central Institute of Mining and Fuel Research (CIMFR) laboratory with the experimental setup using the Mettler-Toledo DSC 821 e differential scanning calorimeter. The complete experimental set up is shown in Figure 52. and comprises a Differential Scanning calorimeter , sample holder, Crucible sealing press, purge gas supply arrangement , a computer with STAR R software which calculates the calibration, control, data display, standard calculations and curve comparisons.

Figure 52 Set up of the differential scanning calorimeter at CIMFR

Sponcom Control Techniques

Fire hazards in underground coal mining in India are controlled with several available technologies and fundamentally depend upon the extent of the fire, local conditions and availability of resources. Fire conditions in underground mines vary extensively - sealed off areas, working panel goafs, crushed pillars, abandoned mine workings etc. Several mine fires in India have been successfully controlled under the guidance of the apex research agency Central Institute of Mining and Fuel Research (CIMFR) which has an exclusive mine fire laboratory where several mine fire prevention technologies are tested. Some of the techniques are:

  • Water based fire control with mist, fog or foam additives

  • High expansion foam

  • Infusion of silica gel into the coal pillars
    Effective in controlling the leakage of air through the crushed shaft and barrier pillars. The methodology consists of infusing silica gel which contains 10 % solution of sodium silicate admixed with 25 % of a 10 % diammonium phosphate (DAP) solution at a pressure of 3 -4 kg/cm2. The setting of the silica gel is around 45 minutes at a pH of 9.)

  • Fire protective sealant coating
    Lime and bitumen emulsion based coatings. One such coating sealant based on cationic Bitumen emulsion is extensively used for control of spon com

  • Inertisation by combination of CO2 and N2 behind the sealed off areas.

Jharia Coal Fire

Jharia coal field is the most known name in the Indian coal mining history where mining started 120 years ago and has the premium coking coal deposits in India. There are 28 major coal seams in the Jharia coal field consisting of 23 large underground and nine large open cast mines. The history of coalmine fires in the Jharia coalfield can be traced back to 1916 when the first fire was detected. Fires in Jharia coalfield have originated from spontaneous combustion occurring either underground or along the outcrops and the top seams which are thick, with shallow depth of less than 40 m. There are about 20 fires covering an area of 17.35 sq km in Jharia coalfield. This is likely the largest complex of surface and underground coal fires in an actively mined area in the world (Michalski et al., 1997).

These fires have considerable effected the coal mining, health and environment of about one million people residing in the vicinity. Many studies were initiated nationally and internationally to combat the Jharia fires but the cost involved and effort required has lead to inaction resulting in the coal continuing to burn.

Some of the measures adopted by Bharat Coking Coal Limited (BCCL), Dhanbad (lease holder of the coal seams) for arresting coal fires and restricting the propagation of fires are explained below:

  • Removal and dumping of the fire-affected coal seams by selective opencast mining operations.

  • Surface blanketing by incombustible materials e.g. fly-ash, soil and sand followed by dozing, compaction and dense plantation. Over 22 million m3 of surface blanketing work has been carried out so far by BCCL for combating coal fire.

  • Quenching coal fire by water-pooling of fire affected opencast mines.

  • Isolation of fire by trenching around the fire affected coal seams.

  • Isolation of fire by construction of sand/cement grout cut-off barriers.

  • Hydraulic stowing/blind flushing through boreholes. More than 50 million m3 of sand has been stowed below ground.

  • Infusion of inert gas. Over 3 million m3 of nitrogen gas has been flushed below the ground to control fires.

Application Sites

The tests on propensity of all coal samples from the coal mines in India are undertaken at the laboratory of CIMFR. Advanced goaf gas monitoring and analysis are currently practised in some of the Singareni coal mines.

Technology Gaps/Needs

Coal characterisation studies are required for geothermic and petrographic properties of coal for sponcom proneness. Apart from the determination of CPT and IPT; Differential Scale Calorimetric (DSC), gas evolution and R70 tests should be introduced to the Indian coal sample testing process.

Introduction of real time monitoring of sponcom indicator gases and proactive inertisation techniques is necessary to reduce the risk of sponcom in the Indian coal mines.

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