Scholarly article on topic 'Spontaneous Heating and Fire in Coal Mines'

Spontaneous Heating and Fire in Coal Mines Academic research paper on "Earth and related environmental sciences"

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{"Spontaneous heating" / "Mine fire" / Auto-oxidation / "Fire protective coating" / Inertisation / "Chemical inhibitors"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Ran Vijay Kumar Singh

Abstract Spontaneous heating and fire in coal mines is a major problem worldwide and has been a great concern both for the industry and researchers in this field. Majority of fires existing today in different coalfields are mainly due to spontaneous combustion of coal. The auto oxidation of coal ultimately leads to spontaneous combustion which is the major root cause for the disastrous of coal mine in leading and coal producing countries like USA, China, Australia, India and Germany. It is a slow process and the heat evolved is carried away by air. This process of self-heating of coal or other carbonaceous material resulting eventually in its ignition is termed as “spontaneous heating” or “auto oxidation”. Coal can interact with oxygen in the air at ambient temperature liberating heat. If the heat is allowed to accumulate the interaction rate increases and may ultimately lead to fires – known as spontaneous fires. The exact mechanism of the reaction of oxygen with coal is not completely understood as the chemical nature of coal is not yet fully established. But most of the workers agree that the reaction of oxygen with coal is a surface phenomenon and proceeds through a loosely bound coal-oxygen-water complex with subsequent steps being breakdown of the complex to simpler molecules such as CO, CO2 and H2O etc. Most popular hypothesis is that the overall reaction proceeds through a chain mechanism with moisture facilitating the formation of free radicals that act as chain carrier. Due to fire in coal mines, hues quantity of noxious gases liberated in the atmosphere which damage the ecological balance of nature. There are various technologies available in different parts of world to prevent and control spontaneous heating. Out of these technologies, chemical inhibitors play a vital role to control and combat fires. The main objective of this paper is to elaborate the causes, mechanism of spontaneous heating and technological advancement mainly development of chemical inhibitors for controlling and combating fire in coal mines.

Academic research paper on topic "Spontaneous Heating and Fire in Coal Mines"

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ScienceDirect Procedia

Engineering

ELSEVIER Procedia Engineering 62 (2013) 78 - 90

www.elsevier.com/locate/procedia

The 9th Asia-Oceania Symposium on Fire Science and Technology

Spontaneous heating and fire in coal mines

Ran Vijay Kumar Singh*

Senior Principal Scientist, Mine Fire Division, CSIR-Central Institute of Mining & Fuel Research, Barwa Road, Dhanbad, 826015, Jharkhand, India

Abstract

Spontaneous heating and fire in coal mines is a major problem worldwide and has been a great concern both for the industry and researchers in this field. Majority of fires existing today in different coalfields are mainly due to spontaneous combustion of coal. The auto oxidation of coal ultimately leads to spontaneous combustion which is the major root cause for the disastrous of coal mine in leading and coal producing countries like USA, China, Australia, India and Germany. It is a slow process and the heat evolved is carried away by air. This process of self-heating of coal or other carbonaceous material resulting eventually in its ignition is termed as "spontaneous heating" or "auto oxidation". Coal can interact with oxygen in the air at ambient temperature liberating heat. If the heat is allowed to accumulate the interaction rate increases and may ultimately lead to fires - known as spontaneous fires.

The exact mechanism of the reaction of oxygen with coal is not completely understood as the chemical nature of coal is not yet fully established. But most of the workers agree that the reaction of oxygen with coal is a surface phenomenon and proceeds through a loosely bound coal-oxygen-water complex with subsequent steps being breakdown of the complex to simpler molecules such as CO, CO2 and H2O etc. Most popular hypothesis is that the overall reaction proceeds through a chain mechanism with moisture facilitating the formation of free radicals that act as chain carrier. Due to fire in coal mines, hues quantity of noxious gases liberated in the atmosphere which damage the ecological balance of nature. There are various technologies available in different parts of world to prevent and control spontaneous heating. Out of these technologies, chemical inhibitors play a vital role to control and combat fires. The main objective of this paper is to elaborate the causes, mechanism of spontaneous heating and technological advancement mainly development of chemical inhibitors for controlling and combating fire in coal mines.

© 2013 InternationalAssociation forFireSafetyScience.Published by Elsevier Ltd.AllRights Reserved Selection and peer-review under responsibility of the Asian-Oceania Association of Fire Science and Technology

Keywords: Spontaneous heating; Mine fire; Auto-oxidation; Fire protective coating; Inertisation; Chemical inhibitors

Nomenclature

T Temperature (°C)

M3/ton Cubic metre per tonne

DSC Differential Scanning Calorimeter

Sq. Km Square Kilometer

OB Overburden

Mj Mega Joule

Jg-1 Joule per gram_

1. Introduction

The whole world stood shocked by a disaster on 8th August, 1956 in the Charleroi coalfield of Belgium where 262 miners lost their lives underground as a result of outbreak of fire in the Bois du cazier mine at Marcinelle. In India, occurrence of New Kenda mine fire disaster at Raniganj coalfield is fresh in our minds where 55 miners lost their lives. When all of a

* Corresponding author. Tel.: +91 326 2296198; fax: +91 326 2296025. E-mail address: drrvksingh@yahoo.com.

1877-7058 © 2013 International Association for Fire Safety Science. Published by Elsevier Ltd. All Rights Reserved Selection and peer-review under responsibility of the Asian-Oceania Association of Fire Science and Technology doi:10.1016/j.proeng.2013.08.046

sudden fire occurs in underground mines, then there is no sufficient time for safe withdrawal of man and equipment. Human life is being endangered due to release of noxious and poisonous gases. Fire is not only dangerous to the workers employed in the mine, but it also results in heavy loss of valuable coal, a national asset being the primary source of energy. Self heating of coal extends to spontaneous heating is the most substantial cause of fires in coal mines across the world [1]. Self heating of coal can appear in underground coal mines, opencast mines, coal stockpiles, transportation and during the disposal of barrens from coal applying industries in heap wastes.

Indian mines have a historical record of extensive fire activity for over eighty years. The scenario of fire in Indian mines is very complex because of involvement of different seams simultaneously. Such conditions do not exist elsewhere in the world. Fire is present in different coalfields of India, but fire of Jharia coalfield is very critical and needs serious attention. Jharia coalfield spread over an area of 450 sq. km. and contains 40 identified coal horizons and has one of the highest coal densities in the world. About 11000 million tonnes of coal are in proved category up to a depth of 600 meter horizon. The total coal reserves of the JCF are about 19400 million tonne. After nationalization of coal mines workings were rationalized and efforts have been made to tackle fires of Jharia coalfield. The problem of fire of Jharia coalfields is unique in itself involving no. of seams. Hence, planning should be carried out to design our own technology for fighting fire and making the sterilized coal amenable for mining.

The spontaneous heating susceptibility of different coals varies over a wide range and it is important to assess their degree of proneness for taking preventive measures against the occurrence of fires to avoid loss of lives and property, sterilization of coal reserves and environmental pollution and raise concerns about safety and economic aspects of mining etc. Although much research has been done on the subject, a proper assessment of the spontaneous heating susceptibility of coal needs to be done so that mine operators are notified well in advance and plan the working properly. Therefore, the determination of susceptibility of coals to spontaneous heating and their classification is essential to plan the production activities and optimise coal mine production within the incubation period.

Coal is the prevailing energy supply source in India and meets 56% of country's primary commercial energy supply. Mine fires in Indian coalfields is generally induced by spontaneous heating of coal. In order to determine the liability of coal to spontaneous heating different methods have been followed by various researchers of the world. A number of experiments have been done for evaluating the spontaneous heating susceptibility of coal viz., Crossing point temperature method [2], Wet oxidation potential method [3], Differential thermal analysis and Flammability temperature [4]. A number of approaches have been found over the years to evaluate the proneness of coal to spontaneous heating. The tendency to auto oxidation of coal also decide the incubation period of coal seam, which settle the size of the panel to be formed, which is a most crucial safety measure in mine planning. It is imperative that the planners of a mine finds in advance the spontaneous heating susceptibility of the seam to be mined so that either the coal has been extracted before incubation period, or advance precautionary measures are planned to rig this menace. Under these circumstances, it is necessary to develop the new chemical inhibitors to counter the problem of spontaneous heating and fire in coal mines.

2. History of coal mine fires world scenario

Coal fires occur in many countries worldwide viz. USA, China, Australia, South Africa, Indonesia, India, etc. [5-6]. Fire present in different parts of the world [7] is shown in Fig. 1.

Fig. 1. Coal fires around the world.

2.1. India

An Indian coal mine have a long history of extensive fires and is probably as old as the history of mining itself. Coal mine fires can be traced back to the year 1865 when the first fire was reported in Raniganj Coalfield. Mine fires gained wide spread attention in India in 1930 with the occurrence of major fires in the Jharia Coalfield (JCF). First fire occurred in 1916 in XII seam of Bhowrah colliery due to spontaneous heating in Jharia coalfield. In 1935, ten fires were reported. In 1937, the no. of fires reported in Jharia and Raniganj coalfield by the Coal Mining Committee was 28. In the year 1994, CMRI (Presently CIMFR), Dhanbad collected the data on fires reported to exist in different coalfields of India was about 196. In JCF along about 1864 million tonnes of coal is lying blocked in about 65 fires spread over an area of 17.32 sq. km. (The fire area includes not only the area actually under fire but also the area which has been sealed off due to heating and since then has not been reopened) affecting the prime coking coal reserve in XVII to IV seams. This itself constitutes a staggering 12% of the total coal reserves. In the year 1996, International Consultants (M/s GAI, Pittsburgh, USA and M/s MET-CHEM, Montreal, Canada under Joint Venture) studied the problem of Jharia coalfield under World Bank aided project and reported that the area under fire is 9 sq. km. (estimated from surface).

They endanger not only worthful lives of men in mines but also induce considerable economic losses to the organization involved by them. These fires are not only carrying on to bedspread to adjoin areas, adding to the losses but also combat economic exploitation of the seam in the locality. Again an open fire in these fields induces environmental pollution by emission of vast quantities of steam, smoke and deadly gases posing a serious health hazards. In Indian coal mine 75% of the coal fire appears due to spontaneous combustion [8].

2.2. China

In China, the world's biggest coal producer with an annual output around 2.5 billion tons, coal fires are a dangerous problem. It has been calculated that some 10-20 million tons of coal uselessly burn annually, and that the same amount again is built inaccessible to mining. They are cantered in provinces of Xinjiang and Inner Mongolia and Ningxia, except exits from burned and inaccessible coal. These fires add to air pollution and substantially increased level of green house gas and have thereby become a problem which has gained International attention. China underground coal fire are bedspread within a region debasing 5000 km east - west and 750 km north - south. It is accepted that fires in northern China consume a calculated amount of 100 - 200 MT of underground coal which is about 2 - 3% of world CO2 production [9].

2.3. Germany

In Planitz, now a part of the city of Zwickau, a coal seam that had been burning since 1476 could only be allayed in 1860. In Dudweiler (Saarland) a coal seam fire caught fire around 1668 and is still burning today. Also known is so-called Stinksteinwand (stinking stone wall) in Schwalbenthal on the eastern slope of the Hoher Meibner, where various seams ignites fire centuries ago after lignite coal mining ended; combustion gas continues to reach the surface today . Up to 10 coal fires per year in the Ruhr area of Germany are induced by spontaneous heating [10].

2.4. Indonesia

Coal and peat fires of Indonesia are often caught fires by forest fires near outcrop alluviations at the surface. No exact numbers of coal seam fires has been realized in Indonesia. Only a minuscule 5 fraction of the country has been reviewed for coal fires. The best data available come from a brief study based on systematic, on-the-ground notice. In 1998, a total 125 coal fires were placed and mapped within a 2-kilometer strip either side of a 100-kilometer extend of road north of Balikpapan to Samarinda in East Kalimantan, using hand-held Global Positioning System (GPS) equipment. Inferring the data to areas on Kalimantan and Sumatera underlain by known coal deposits, it was considered that more than 250,000 coal seam fires have been burning in Indonesia in 1998.

2.5. United States

Many coalfields in USA are leads to spontaneous heating. The Federal Office of opencast Mining (OSM) asserts a database (AMLIS), which in 1999 named 150 fire zones. In Pennsylvania, 45 fires zones are known, the most famed being the fire in the Centralia mine in the hard coal region of Columbia County. In Colorado coal fires have been found as a consequence of variations in the groundwater level, which can increase the temperature of the coal up to 30 °C, enough to induce it to spontaneously ignite. The Powder River Basin of Wyoming, Montana has some 800 billion tons of brown coal,

and already the Lewis and Clark Expedition (1804 to 1806) described fires there.

Coal mining in Pennsylvania began in the mid 1700's in response to colonial America's demand for iron. Since then, Pennsylvania coal has supplied energy to the United States and countries abroad (Pennsylvania Department of Environment Protection). Coal fires in Pennsylvania have been recorded since 1772 [11], but the major fire occurred in 1869 when a ventilating furnace ignited wooden supports in the Avondale mine in Plymouth, suffocating 110 men trapped below ground [12]. Since then, coal fires across Pennsylvania have destroyed floral and faunal habitats, consumed buildings, emitted toxic fumes into houses, contributed to different illness, induced land subsidence and many more. One of the underground mine fire in the US is the Centralia mine fire. The Centralia mine fire began when the Centralia Borough Council decided to burn trash to reduce volume of and control rodents in an abandoned strip-mining cut used as an unregulated dump at the edge of the town. Burning trash ignited anthracite in the Buck Mountain seam concealed behind the refuse and the fire spread along the seam to tunnels beneath Centralia [13].

3. Mechanism of spontaneous heating

The oxidation of coal, alike all oxidation reactions, is exothermic in eccentric. The exact mechanism of the reaction is still not well observed. However, scientists accord that the nature of the interaction between coal and oxygen at very low temperatures is fully physical (adsorption) and changes into a chemisorptions form beginning from an ambient temperature [2]. When coal is disclosed to air it occupies oxygen at the disclosed surface. Some fraction of the exposed coal substance gains oxygen at a faster rate than others and the oxidation outputs in the production of gases. Mainly CO, CO2, water vapour along with the release of heat during the chemical reaction. The rate of oxygen consumption is extremely high during the first few days (particularly the first few hours) abiding the exposure of a fresh coal surface to the atmosphere. It then downs very slowly except causing problems unless produced heat is allowed to assemblage in the environment. Under certain conditions, the accumulation of fire cannot be combated prevented, and with sufficient oxygen (air) supply, the process may reach higher stages. The coal-oxygen-water complex made during the initial stage (peroxycomplexes) breaks down above 70-85 °C, producing CO, CO2 and H2O molecules. The rate of chemical reactions and exothermicity change with the rise in temperature, and radical changes take place, beginning at about 100 °C, major due to loss of moisture [2]. This process leads with the rise in temperature, producing more stable coal-oxygen complexes until the critical temperature is achieved. The ignition temperature of bituminous coal is about 160-170 °C and of anthracite coal nearly 185 °C. Once the coal achieves it ignition point, the air supply to it will only increase the Combustion.

4. Factors/Causes affecting spontaneous heating of coal

Spontaneous heating appears only in certain seams and these capable/susceptible seams are frequently adjacent to seams acted by the same method and submitted to the same tectonics, but which have never been the site of combustion. Some seams have only been susceptible in one area and, again, some seams are more susceptible than others. Every cause of spontaneous combustion, however small, if not deal with effectively and efficiently in the early stages can produce into open fires or into an explosion of gas or dust, with ravaging results. Most of the major fires were in shallow workings originating from workings along the outcrops and abandoned opencast mines. In case of thick seams invariably, coal is left in the goaf after the extraction of pillars. The goaf is generally connected through cracks to surface following subsidence. These cracks feed fresh air to the goaf where coal oxidizes. Coal catches fire due to spontaneous heating and the fire spreads out over bigger areas.

Fires in coal mines originate from spontaneous combustion occurring either underground or on the surface. 75 % of the fires occur due to spontaneous combustion. The main reason for occurrence of fires due to spontaneous heating in Indian coal mines are:

• coal seams are thicker,

• plenty of coals are left in the goaf,

• presence of contiguous seams which are affected due to fires,

• shallow depth,

• proximity of intake and return,

• sometimes high pressure difference exists between intake and return and

• in many cases due to high spontaneous combustion susceptibility of coal.

Particularly in surface (opencast) mines the main causes of occurrence of spontaneous heating in coal benches are:

• presence of micro and macro cracks in the bench walls,

• accumulation of loose coal lying in the toe of the benches,

• long exposure of the coal bench to the open atmosphere.

In overburden dumps and in coal rejects, fire occurs due to the presence of carbonaceous materials. It is practically not possible to dispatch all mined coal within its incubation period. Thus coal is stacked on the ground. Due to improper stacking of coal, fire starts in this stacked coal.

Besides this, some other factors that have contributed to mine fires are:

• Bantulsi conflagration,

• Illicit distillation, and

• Dumping of hot ash.

Basic cause of initiation of mine fire is self heating of coal which depends on the intrinsic characteristics of coal and other extrinsic causes like mining related technical factors, which may facilitate or retard this initiation, Intrinsic property of coal includes its chemical composition of coal, moisture, volatile matter, presence of pyrites, geological factors etc. The extrinsic causes include the system of winning coal and the system of ventilation etc. Moreover, past unscientific mining were also responsible for initiating mine fires during the pre-nationalisation time.

Many underground mines have been converted into opencast to augment the production. In such cases heated up areas catch fire after exposure to the atmosphere. Besides these, there are fires in coal stack, in developed galleries worked by opencast method, washery rejects and overburden dumps etc. The whole phenomenon of the spontaneous combustion as combine effects of the situation which may be classified as:

(A) Seam factors

(C) Mining Factors

(1) Rank

(2) Petrographic Composition

(3) Temperature

(4) Available Air

(5) Particle Size

(6) Moisture

(7) Sulphur

(8) Effect of previous oxidation

(9) Physical properties

(10) Heating due to earth movement

(11) Bacteria

(12) Other Minerals

(B) Geological factors

(1) Seam Thickness

(2) Seam Gradient

(3) Caving Characteristics

(4) Faults/dykes

(5) Coal Outbursts

(6) Friability

(7) Depth of Cover

(1) Mining Methods

(2) Rate of Advance

(3) Pillar Conditions

(4) Roof Condition

(5) Crushing

(6) Packing

(7) Effect of Timber

(8) Leakage

(9) Multi-Seam Working

(10) Coal Losses

(11) Main Roads

(12) Worked Out Areas

(13) Stowing

(14) Ventilating Pressure

(15) Humidity

5. Present fire combat efforts

The strategy adopted for dealing with the fires is a two phase operation - first the fires must be contained and secondly it should be extinguished. The methods adopted for dealing with these fires as per fire situation are as follows:

• Hydro-pneumatic stowing;

• Fly ash flushing;

• Trench cutting and its filling;

• Water circulation under pressure;

• Cementation;

• Stowing;

• Blanketing;

• Digging out;

• Nitrogen/Carbondioxide injection;

• Pressure balancing;

• Sealing by construction of isolation stoppings;

• Remote sealing - concrete cement injection;

• Cement water slurry spraying;

• Infusion of silicic acid and application of Fire Protective coating material and

• Chemical inhibitors for control and combating fire

The above mentioned techniques are being applied for control of underground as well as surface fires, depending upon the situation of fire problem. It will vary case to case and site to site basis.

6. Technological problem

There are some of the difficulties faced during implementation of the above technologies to control the fire in Indian coal mines.

• Fires travel from one seam to the other which is in close proximity to each other, through caved areas, developed galleries/partings.

• Fire is to be extinguished to exploit coal below by opencast method to immense heat trapped in the strata causes the fire to flare up as soon as any excavation is attempted.

• Multiple openings from the surface directly through inclines from the outcrops.

• The use of top soil cover for surface sealing is not sufficient.

• Surface sealing with sand adopted over the past few years, which has been not effective on the view of permeability of air.

• The main constraint are drilling and blasting in hot strata/coal seam.

Besides this cause of failure of many of the previous attempts to deal with these fires, particularly in the Jharia coalfield of Indian coal mines were mainly inadequate resource mobilization, paucity of fund and/or lack of speedy operation with time target strategy.

7. R&D work for technological advancement on chemical inhibitors

Researchers from different parts of the world carried out Research and development work on chemical inhibitors to counter spontaneous heating/fire in coal mines. Tarpagosova tested the oxidation inhibiting action of various reagents over coal and found petroleum products and their emulsions to be fairly effective, while fatty acids and a few other agents showed only minor improvements [14]. Nakano and Yamski claimed to have obtained encouraging result by applying a thick coat of asphalt emulsion over the coal surface [15].

A trial with a coat of tar and burnt mobile emulsion, in the ratio 1:3 over a quarry bench wall in Jhingurda seam MP, India was carried out and claimed to have increased the incubation period considerably [16]. But the chief drawback of such agents is that they induce greater risk from contact fire (exogenous fire) and cannot be used in a hot spot/fire nearby. These agents have however been improved upon making them fire resistant, which offers good promise as a protective agent [17]. Accordingly, a cationic bitumen emulsion based fire protective coating material (under S&T project, funded by Ministry of Coal, Govt. of India) for preventing spontaneous heating in coal mines was developed by Central Institute of Mining & Fuel Research, Dhanbad, India. It would, therefore help not only in saving a huge amount of coal lost every year due to fire in coal mines but also in minimising environmental pollution. The efficacy of the coating has been well proved by rigorous laboratory scale studies as well as field trials in coal benches at Karkatta opencast project, Dakra (CCL), Jagannath opencast project, Talcher (MCL) and Jhingurdah opencast project, Singrauli (NCL) having coal of high susceptibility of spontaneous heating.

These studies have confirmed that a fire protective coating material has following advantages: (i) offers good fire resistance (ii) prevents air permeation and (iii) does not washed down by water/rain (iv) it forms no cracks, remains intact for a long period of time (more than one year) and does not create the problem of scaling-off the material. Moreover, it is easy to spray and is very much compatible with coal. A patent application has been filed for the fire protective coating and the technology has been commercialised to M/s Signum Fire Protection (India) Pvt. Ltd., Nagpur, India for manufacturing and marketing. The said firm is manufacturing & marketing this product under the trade name SIGNUM-CMRI Fire Protective Coating/Sealant [18].

Polish workers claim to have achieved encouraging results using plastic/PVC sheet layering over vulnerable patches and hot spots on coal stacks, thereby preventing ingress of fire. Among the two groups, CaCl2 used by Enterbrink and Lewer [19] and NH3 suggested by Kroger and Beier [20] have showed promise as oxidation heating inhibitors.

Chinese workers claim to have achieved good results by infusing inhibiting agent such as 10-15% CaCl2/MgCl2.6H2O in

the goaf [21]. Czechoslovakian and Hungarian researchers have confirmed 10% calcium chloride to be suitable for reducing the incidence of spontaneous combustion in pits. Australian researchers have suggested use of ammonium hydrogen tetraborate for preventing spontaneous heating. In Japan Diapon T (Na-N methyl-N olieltuarte) is claimed to be surface active agent [22].

The search for suitable Chemical inhibitors to spontaneous heating is in progress in many countries, but a foolproof solution has not yet to be achieved. Above mentioned Chemical inhibitors of different countries have been found suitable only as preventive material for spontaneous heating. Here author want to describe about the recent research and development work of chemical inhibitors for control and combating spontaneous heating/fire in surface coal mines. The dream of researchers has been to prevent spontaneous fires by discovering suitable oxidation inhibitors that will hinder the process of oxidative heating of coal, perhaps by blocking the formation of free radicals, or otherwise. The present study includes three approaches:

• Using agents that are likely to interact with the coal matrix so restricting ingress of oxygen at the coal surface - termed oxidation inhibitors;

• examination of the chemicals that may start evolving inert gases at slightly elevated temperature and thereby make the oxidizing atmosphere inert, restricting advancement of heating;

• using agents that may initiate endothermic reactions at the onset of oxidation so neutralizing the oxidative heating process itself and combating and control of fire.

Chemical inhibitors were selected for investigating such as barium hydroxide (likely to form a barium carbonate coat on the micropores of the coal matrix at the onset of oxidation of coal as soon as CO2 is liberated): boric acid (likely to liberate moisture at the onset of heating so restricting the progress of oxidation): and mercuric chloride, which is a bactericidal agent.

Inhibiting effect of the chemicals was studied by admixing them with powder coal of -72BS mesh, in suitable proportions, and keeping them in 60% RH, in a desicator. For HgCl2, a 0.1% solution was used, the salt being sparingly soluble in water.

The inhibiting effect was measured by noting the degree of oxygen absorption using a manometer as well as measuring depletion of the oxygen percentage and oxygen product gases such as CO and CO2 in the enclosed desicator (Table 1).

Table 1. Evaluation of gases on heating of chemical inhibitor in sampling tube

Sl. No. Quantity of chemical inhibitor Temp (°C) N2 (%) O2 (%)

1. 10 ml 40 84.48 15.51

2. 10 ml 60 96.35 03.64

3. 10 ml 80 88.21 11.78

4. 10 ml 100 86.00 13.99

Coal samples studied were from the highly susceptible one Kenda Seam, Raniganj coal field, ECL, India. Results show that the inhibiting effect compared with the blank is marginal through the oxidation process is retarded, with the bactericidal agent mercuric chloride showing a slightly higher retarding effect than the boric acid and barium hydroxide.

7.1. Evolution of nitrogen gases under heat

10 ml of hydrazine hydrate (80%) was held in a 250 ml conical flask fitted with two stoppers (one for placement of thermometer and the other for allowing evolved gases to escape). It was allowed to be heated in different regions from 30 °C to 100 °C in a water bath.

Gases evolved on heating were collected in sampling tubes and measured for Nitrogen (N2) content using Gas Chromatography techniques of analysis. The results obtained (Table 2 and Fig. 2) show that maximum evolution, more than 95% N2 was produced at a temperature in the region of 60°C. Interestingly, when the compound is mixed with celite, sand and bentonite compounds, it liberates ammonia, (free radical chain arrestor) along with N2 (88.34%) at room temperature. But when admixed with stone dust instead of celite it did not liberate ammonia only N2 was evolved.

7.2. Good potential

Chemical agents that have the ability to liberate inert gases have wide scope for use in preventing spontaneous heating in mines. If such chemicals are injected in the goaf area, mixed with celite, sand and bentonite material, the oxidation inhibiting agent NH3 (ammonia) is also liberated in addition to N2, helping create an inert environment. So they have good potential for preventing spontaneous fires underground (goaf etc).

Chemical inhibitor such as NH4Cl, showing endothermic reaction may advantageously be used for cooling down the

effect of oxidative heating, so annulling the process of spontaneous heating as soon as it is initiated. Of the oxidation inhibitors, bactericidal agents show promise in a lab scale investigation.

Table 2. Inhibiting effect of chemical inhibitor on highly susceptible coal

Sl. No. Samples Gas collection after O2 (%) CO2 (%) CO (%)

1. Coal (blank) 15 days 20.20 0.10 0.00

2. Coal with barium hydroxide 15 days 19.79 0.10 0.00

3. Coal with boric acid 15 days 19.31 0.40 0.00

4. Coal with mercuric chloride 15 days 18.73 0.41 0.00

Fig. 2. Evolution of N2 gas and absorption of O2 gas after heating the chemical inhibitor.

7.3. Endothermicity of chemical inhibitors

The endothermic characteristics of chemicals have been determined for prevention & control of coal mine fire using Differential Scanning Calorimeter (DSC) equipment. Chemical inhibitors like NH4Cl, CaCl2, NaCl and diammonium phosphate - when mixed with coal show varied degree of endothermicity on heating at Different temperature regions. NH4Cl shows pronounced endothermicity at 250 °C; CaCl2 indicates minor endothermicity at 50 °C and 150-170 °C; and NaCl shows at around 400 to 450 °C. NH4Cl may find use for preventing goaf fires because of liberation of NH3 as its endothermic effect at the onset of heating.

7.4. Differential Scanning Calorimeter

Differential Scanning calorimeter (DSC) is a technique in which the difference in energy inputs into a substance and a reference material is measured as a function of temperature while the substance and reference materials are subjected to a controlled temperature programme.

In Differential Thermal Analysis (DTA) equipment, 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).

But in the DSC technique, the ordinate value of an output curve at any given temperature is directly proportional to the differential heat flow between a sample and reference material in which the area under the measured curve is directly proportional to the total differential calorific input. By this technique, coal samples can be studied under experimental conditions that simulate spontaneous heating process of materials [23]. Author used DSC equipment named METTLER TOLEDO DSC 821e and LINSEIS L 62 H 1550. The temperature range of the METTLER TOLEDO DSC 821e equipment is 0 ( room temperature) to 600 °C and LINSEIS L 62 H 1550 is 0 to 1450 °C.

In Mettler Toledo STARe system, the DSC 821e module is designed in modular concept ( basic module with the facility of later extension. The basic module consists of:

• DSC 821e measuring cell with ceramic sensor,

• Module electronics

• Manual furnace lid

• Operating status indicator

• Air cooling (cooling at room temperature by ventilator).

The DSC measurement in the instrument is based on the heat flux principle. It can only be operated in conjunction with the personal computer. LINSEIS DSC equipment comprising a programme for higher temperature range data evaluation to display a large amount of data in a very short time on a screen. Any energy difference in the independent supplies to the sample and the reference is then recorded against the programme temperature. Thermal events in the sample in DSC equipment shows endothermic or exothermic direction (downward and upward peak), depending upon whether more or less energy has to be supplied to the sample relative to the reference material. Downward peak represent the endothermic and upward peak represent the exothermic behaviour of the chemical inhibitors.

7.5. Experimental procedure for DSC thermogram

Firstly the instrument was calibrated using standard indium samples. Sample below 10 mg (chemical inhibitors) was taken in the aluminium crucible. After taking the sample, the lid of the sample crucible was sealed through giving pressure. In the sample lid one small hole was being done through needle and another blank crucible was also taken using two small holes as reference. Both the sample and reference crucible was kept in the sample holder. Purging gas such as N2 was flow at 80 cc/minute in the equipment. DSC thermogram was programmed in the software for analysis upto maximum 600 °C in the equipment with heating rate 20 cc/minute. After analyzing the sample, the thermogram was evaluated to find out the normalized value, onset and peak.

7.6. Analysis of results from DSC thermogram

The thermogram represents the various reactions occurring during oxidation process. It was followed by endothermic and exothermic reaction took place in the sample during analysis. Particularly in the coal sample, the temperature of initiation of exothermic reaction can be considered as an indicator of spontaneous heating susceptibility of coal sample, which is known as the onset temperature. The onset temperature obtained from differential scanning calorimeter may be the better indication for determining spontaneous heating susceptibility. Similarly, different composition of identified chemical inhibitors has been analysed in the DSC equipment. The degree of endothermicity represents the absorbing capacity of the chemical inhibitor at different temperature ranging from 50 °C to 550 °C comprising different composition (Figs. 3 & 4). After that some of the composition has been applied successfully in the field application for controlling and combating fire.

Fig. 3. Thermogram of chemical inhibitors using Mettler Toledo DSC 821e

Fig. 4. Thermogram of chemical inhibitors in different proportion.

8. Field application

Author is presenting the successful field application study at Shatabdi Opencast coal mines, Barora Area, BCCL, India, where blazing fire was present in the exposure point of developed gallery presently worked by opencast mines. The Shatabadi Opencast Project forms a part of opencast Block-III, one of the nine large opencast blocks envisaged in the F.R. for reconstruction of Jharia coalfield in 1978. The project area comprises parts of Muraidih Colliery of Barora Area (mainly) and Jogidih and Maheshpur Collieries of Govindpur Area (partly). The project area is about 30 km west of Dhanbad town and is well connected by rail and road. The project area is easily approachable from Hirak Ring road (Dhanbad -Chandrapura) from Dhanbad town. The project area is bounded by

1. North - Southern surface limit of sanctioned Muraidih opencast.

2. South - Dhanbad - Chandrapura railway line with a minimum safety margin of 100 m

3. East (dip side limit) - HZL factory complex and its residential areas with minimum safety margin of 100 m

4. West (rise side limit) - Existing railway siding with minimum safety margin 60 m and exposed coalface of quarry in crop region of V/VI/VII seam.

The base of the quarry has been fixed at the floor of V/VI/VII seam as the marginal stripping ratio of underlying I to IV seam is very high (7.0 m3/te approx.) and seams are of poor quality. The total mineable reserves from seam are 19.74 Mt of coal within the area. The total volume of OB to be removed has been estimated at 44.87 Mm3. The average stripping ratio works out to 2.27 m3 /te. The V/VI/VII seam is the major seam contributing about 87.7% of mineable reserves considered for exploitation. The in-band thickness of the seam is 14.89 to 22.06 m. It is developed in three section goaved at places and virgin over 35% of the area in dip side. The other seams namely VIIIA/VIIIB contribute about 12.3% of mineable reserve. The VIIIA and VIIIB seams are partly virgin and partly developed in one section each. The V/VI/VII seam quarried in the outcrop zone, developed by underground method in three sections viz. 2nd, 3rd and 4th section and goaved in patches. About 65% of area has been worked by underground/ opencast method of mining and 35% of area is lying virgin. The Shovel-Dumper system of mining has been deployed taking into consideration the prevailing geomining condition, mineable reserve, stripping ratio and production capacity of the project. The coal of V/VI/VII seam is non-coking [24]. The detail of the mine is as follows:

Coal reserve Life of Mine Annual production Present man power Coal grade Seam worked Thickness Stripping ratio HEMM deployed

Drilling Blasting

18.19 Mt

23 year 0.8 Mt/year 620

Washery-IV V/VI/VII combined 24.5 m 1:2.25

EKG Shovel/ 5 m3- 4 nos.

35 m3 capacity dumper- 22 Nos.

RECP-650 (160 mm dia)

Using explosives (SMS, Emulsion and heat

resistant explosive)

8.1. Scientific investigation

After visit the Shatabdi OCP mine site, it has been observed that presently mine is working between grids 74000-74500 M and 107000-106500 M. The whole area has been developed in three sections with Bord and Pillar method. Extraction is being carried out using Shovels in three benches of overburden (OB) and one in coal top. The area is confined by the boundary line, which is at 100 m from the village existing towards north, and OB dumped closed to extraction edge of coal up to the complete height at south side. This narrows the approach for deployment of shovels and for stabilising haul road. It has taken a shape of trough. For removing the formation of trough in between two lines, removal is to be done, of hot OB filled up along the coal edge, which has fire all along the developed gallery and throughout the length of edge. Exposed mouth of the developed gallery is under fire. Huge quantity of fallen coal is lying in the gallery due to the mining operation and sufficient air is entering through the opened developed mouth to initiate the spontaneous heating/fire. After some time blazing flame is coming from the developed mouth and due to this flame OB gets also heated as well as multi cracks are developed. Maximum temperature (540 °C) was found in the mouth of the developed gallery using Infra-red scanner and Heat-spy Infra-red thermometer showing in (Fig. 5 and 6). The mouth of the gallery is being filled up with the burnt and subsided overburden material due to extensive fire. It is also observed that the idle faces of the exposed coal catch fire within a week. Temperature was measured in different locations and it is mentioned in Table 3:

Table 3. Temperature mentioned in different locations of the fire area

locations temperature (°C) ( max.)

burning loose overburden 410

hot overburden 450

flame temp. at the mouth of the developed gallery 540

borehole temp. interface between coal and OB up to 6 m depth 145

borehole temp. in the virgin coal up to 7 to 8m depth 91

borehole temp in the coal above the gallery mouth 141

borehole drilled in the loose OB near coal top bench up to 8m depth 143

Fig. 5. Thermal image of fire area of Shatabdi opencast mine, India.

Fig. 6. Status of blazing fire in Shatabdi opencast mine, India liberating smoke and gases.

Application of chemical inhibitors have been carried out on the basis of the laboratory R&D experiments using high pressure water jet to reduce the flame, to extract the heat of the hot zone and found that temperature has been drastically reduced from 540 °C to 44 °C after continuous operation for five to six hours.

Another experiment for application of chemical inhibitors were also carried out in West Bokaro colliery, Tata Iron and Steel Company (TISCO), India to control and combating fire existing in the ecological park of dump yard. Due to presence of carbonaceous shale or shaley material in the dump yard, fire occurred (Fig. 7). Immediately, as per the site specific fire situation, chemical inhibitors were applied in the hot zone and it was tried to reduce about ambient level (Fig. 8).

Fig. 7. Status of surface fire on ecological park made up on overburden dump in West Bokaro mine, TISCO, India.

Fig. 8. Status of fire after application of chemical; inhibitors in quarry SE mine of West Bokaro colliery, India.

9. Discussions

Suitable technique was used for prevention and control of spontaneous heating/fire as per site specific situation. Solutions would differ from case to case and mine to mine. Fire in opencast benches and coal stacks spread more rapidly than underground. For all types of fire (underground and surface) where fires are detected at an early stage and the necessary steps are taken promptly, the fires are successfully dealt with and no further damage is caused. Speed is of prime importance in dealing with mine fires. If the fire is allowed to grow, it becomes very difficult to control. It has also been suggested that occurrences of spontaneous heating should be detected using Heat spy Infra-red thermometer and Thermovision camera at an early stage. After detection of fire, the following technology such as surface sealing, trench cutting, digging out of the hot masses, inertisation, use of inert material and endothermic chemicals, water injection under pressure, application of fire protective coating as well as chemical inhibitors should be adopted for prevention and control of underground and surface fire as per site situation.

10. Conclusions

Author wants to conclude that chemical inhibitors will be more suitable and effective to control and combat fire in surface coal mines. The different technologies mentioned above are not suitable for controlling and combating all types of underground/surface fires. Every fire problems are different in nature. Under the circumstances, before tackling any fire problem one should carefully examine the status of fire and to decide the appropriate technology before application for controlling the fire. It is not possible to eliminate fires completely, but it is possible to bring the situation under control. Particularly, the problem of fire of Jharia coalfield is unique in itself involving no. of seams. No such condition exists elsewhere in the world, hence we have to design our own technology for fighting fire and making the sterilized coal amenable for mining. Existing fires need to be studied individually after that suitable technology should be selected. The main attention should be given for detection, assessment, prevention and control of mine fire of Jharia coalfield to deal with the existing fires and to avoid future occurrences of fire in Indian coal mines. The incidence of mine fire is mostly attributable to spontaneous heating in Indian coal mines. It is not possible to eliminate fires completely, as the very process of mining which introduces factors contributable to spontaneous heating of coal. Suitable techniques should be used for prevention and control of fire as per fire situation.

Acknowledgements

Author is grateful to Dr. A. Sinha, Director, CSIR-Central Institute of Mining & Fuel Research, Dhanbad, India for his keen interest in carrying out the work and for the kind permission to publish this paper. Thanks are due to the officials of mine and staff of Mine Fire Division of CSIR-CIMFR for their assistance in various stages during laboratory & field work. A view expressed in this paper is of author and not necessarily of CIMFR.

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