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A conceptual approach to prevention of fire in coal benches
Mining Science and Technology, 8 (1989) 133-143 Elsevier SciencePublishers B.V., Amsterdam - Printed in The Netherlands
133
A CONCEPTUAL APPROACH TO PREVENTION OF FIRE IN COAL BENCHES A. Sharma and D.D. Banerjee Central Mining Research Station, Dhanbad-826001, Bihar (india)
(ReceivedJuly 4, 1988; acceptedSeptember28, 1988)
ABSTRACT The causative factors affecting fire in sub-bituminous coal benches are highlighted. A conceptual approach to prevention of fire is presented. It is suggested that the temperature of the coal wall must be brought down using a mist sprayer, allowing the coal to become weathered
at low temperature. On the basis of heat transfer processes, a mathematical formulation has been made to ascertain the optimum rate of water flow from the sprayer. Modification of the existing layouts and panel workings of the coal block is also suggested.
INTRODUCTION
minimum when the moisture level lies between 4 and 6% and the ambient air is saturated. Moisture acts as a catalyst in the coal-oxygen interaction. It helps in forming the peroky complex, an intermediate transient compound which in turn breaks down and enhances the rate of oxidation at early stages. It affects the process in other ways also. During evaporation of moisture, the internal surface area of the coal is available as fresh reactive centres. Schmidt has found that at low temperature, the rate of oxidation is proportional to the cube root of its specific internal surface area [1]. As soon as the bed is exposed, evaporation of moisture inexorably sets in. The presence of bound moisture in a hygro-
The Indian coal industry has suffered an increase in quarry fires engulfing huge areas, tropical conditions, the nature of the coal and the workings and the geological parameters all being responsible for heating in the coal benches. The mechanism of the coal oxidation process is well established [1,2]. The interaction depends on the rank of the coal, environmental parameters and the extent of exposure. The moisture in coal and the humidity of the air has a marked influence on the coal oxidation process, particularly at lower temperatures. It has been observed that the critical ignition temperature of the coal is a
134 scopic material raises the thermal conductivity and the enhanced conductivity facilitates internal transfer of heat within the coal mass. Since moisture is present in coal, the temperature gradient will cause differences in water vapour concentration in the interstitial atmosphere; it motivates mass transfer of water vapour from hot zones to cooler zones, transferring a substantial quantity of heat in the form of latent heat. Continuous evaporation from the coal face lowers its relative humidity (RH) below 100%. In such situations the mass transfer of water vapour from the hot zone to the cold zone will lower the R H in the former and raise it in the latter. It facilitates a path of almost constant water vapour concentration. At this equilibrium position the opposing forces of gaseous and thermal diffusion balance each other and transport of water vapour by diffusion then ceases altogether. When R H falls below 60%, thermal conductivity drops sharply and heat dissipation inbye becomes restricted [3]. Similarly, heat dissipates away from the exposed surface to air across the boundary layer. The difference in water v a p o u r concentration across the boundary layer causes the water vapour to diffuse away. Before the ignition of a coal bed, a thermal imbalance is established between heat generated by exothermic processes and heat dissipated to the surroundings by thermal conductivity. Since heat generation is manifested in all three dimensions of the block while the heat dissipation is two-dimensional, the possibility o f heat accumulation is facilitated. In the process of thermal explosion in a mass of reacting material, the processes identified as possible mechanisms whereby reactants are transported to the coal bed can be classified as: (1) natural convection caused by self-heating, (2) molecular diffusion caused by concentration gradients, (3) flow caused by differential wind pressure, (4) thermal breathing caused by diurnal variation in temperature, and (5) barometric breathing caused by variations in barometric pressure.
Brooks has shown that fluxes due to barometric breathing are such that they cannot sustain combustion, while the effects of thermal breathing occur so close to the surface that any heat produced is easily lost. The contribution of differential wind pressure to flow capable of causing spontaneous combustion is considered to be marginal. Initial displacement of moisture favours transport of oxygen due to molecular diffusion which is then sustained by natural convection. Apart from the availability of internal area, the crushed zone on top of the wall including powdered coal possessing highly reactive surfaces in the fissures and cracks, the heaps of powdered coal accumulated after the operation, and the curshed zone in the wall due to geological disturbances (bands and faults) [4] are in particular believed to be vulnerable to oxidation. Consequently, these factors can trigger combustion of the bench. The coal bed exhibits the characteristics of a compacted stack and can be designated as oxygen limited whereby all the oxygen entering the bed reacts very near the inlet to the bed so the heat generated can be lost to the surroundings. The conditions which can trigger heating must be present. The coal bed is essentially different from the stack: Voids do not exist, so the air permeation is restricted to low depths and the formation of a nucleus zone in which the temperature rises progressively and the threshold temperature is reached at an early stage is much smaller--in the order of centimetres, as experienced in the field study. Thermal explosion occurs in this zone and gradually envelops the area outbye where oxygen transport is facilitated. Further beyond this depth, the slow movement of air becomes depleted in oxygen in its path and correspondingly the oxidation process slows down. Should fire occur and it is not controlled, it penetrates to depth in increments. Subject to seasonal variation, the fire can create cracks in the bed and air transport becomes an imminent process.
135
DESIGN CHARACTERISTICS FOR COMBATING FIRE IN BENCHES (FIGS. 1 AND
2) Water is the commonest means of fighting fire but limitations are imposed against its use. In general, treatment of the coal face with water accelerates oxidation, and the application of insufficient quantities of water to a fire may result in explosion of water gas and producer gas. The steam generated will create a draught that will increase the rate of oxidation in the interior of the bench. The different means of water application for fire fighting depend basically on the extensivity characteristics of the fire. The degree of extensivity and intensity that create a fire zone of various dimensions can be linked as follows: (1) emergence of indications of heating at the wall of the bench, (2) origin of inflammation at the bench wall with a noticeable glow, and a (3) virulent fire covering a large area. Before a model for spraying or jet cutting is derived, the important physical processes associated with coal must be identified. The following processes are prominent: (a) drying characteristics of coal, (b) kinetics of oxidation and weathering, (c) heat of wetting, (d) geological characteristics such as slip planes and weaknesses, (e) effect of aquifers on the moisture content of coal, and (f) physicome-
b/~TOP
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EDGE OF THE H I L L
~
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~5~463452152
3o5aa6
;, ,,, ~F[
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, o,c
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L FIRE FLANE Flee
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X-Y
Fig. 2. Temperature profile of burnixagsection. Temperature ranges from 30 ° to 480 ° C. (Not to scale.) chanical properties of coal, particularly friability and grindability indices.
Drying characteristics of wet coal A study of the kinetics [5] of desorption of moisture has indicated that drying of coal in the physical phase of desorption from 100 to 607o RH is much faster than the drying of air-dried moisture from 60% RH to lower levels. It takes about a week for the moisture level to come down to the stage of 60% RH under the average humidity conditions of the atmosphere, this level being a critical value both for moisture elimination and for heat conduction.
Kinetics of oxidation and weathering of coal
WATER
~-~
WORKING SHOVEL
COAL
Fig. 1. Extent of exposure in a burning quarry. (Not to scale.)
The rate of oxidation increases with a rise in temperature; it has been found that the rate of oxidation doubles for every 10 ° C rise in temperature. Indian coals are subject to high ambient temperatures, (average annual temperature -- 30 ° C). In these conditions, therefore, the combination of spontaneous heating and susceptible coal leads to a fast achievement of the threshold temperature. In order to control the process of spontaneous
136 heating it is imperative to bring down the temperature by removing the heat buildup. Coal is subject to weathering when it has been saturated with oxygen, i.e., air, over a long period of time under isothermal conditions. A weathered coal is relatively unreactive to oxygen compared to fresh coal. Schmidt and Elder [1] performed many long-term experiments--up to 300 d a y s - - a t normal temperature and they established a relationship between the rate of reaction and the oxidation period which indicated that the rate of oxidation decreases with time, the rate being very fast for the first 10 days. After a period of 40 days the rate of oxidation continues to decrease, and finally it is asymptotic to the time axis. An experiment carried out on oxidation weathering of Illinois No. 6 coal also revealed that the exposure of freshly mined bituminous coal has resulted in a slow oxidation reaction which appeared to be complete within two months when oxygen uptake practically ceased. The temperature of the oxidation was not allowed to exceed 30 ° C. From laboratory studies and the experience gained so far from the field, it can be inferred that the initial phase of exposure of the bench wall warrants strict vigilance.
Heat of wetting and condensation The low-rank, high-moisture coals, which are prone to spontaneous heating, possess an open structure with random orientation of lamellae in all directions and connected by cross links. Hence the system is porous with a large internal surface area (approximately 200 m2/g). It is further characterised by a hydrophilic nature because of the presence of polar groups and high oxygen content. Coal will generate a large amount of heat when wetted with water due to interaction between the fluid and the coal surface. A secondary cause of self-heating m a y b e present when coal absorbs water either in the liquid or in the
vapour form. Although combination with oxygen is at least partially reversible, combination with water is mostly reversible, particularly with the lowest rank bituminous coals. Integral heat of wetting [6,7] may be expressed as: Uw = Q A - QL where Uw = integral heat of wetting of moist coal, QA = integral heat of absorption from the vapour phase and Q L = h e a t of condensation of water vapour to liquid at the same temperature. It has been found that Uw is not constant but depends on the water content or RH in the coal matrix. Thus, if the moisture content of the coal (weight of water/weight of moist coal) is increased from x I to x2, the heat of wetting is: oX2
Uw=G2-Gl=jx uxdx I
where Uw = differential heat of wetting/unit mass of water, Using the Clausius-Clapeyron equation the differential heat of wetting is: U~ =
R T 2 d In Px dT
d In Po dT
where Px = partial pressure of water vapour in the atmosphere, and Po = partial pressure of water vapour over water at the absolute temperature, T. The first term is the differential heat of absorption at a moisture content, x. The second term is the heat of condensation of water vapour, which is a function of temperature only and is equivalent to 2438 k J / k g of water at 25°C. It is calculated that for a normal moist coal (65% RH), the heat of wetting could cause a temperature rise of only 2 ° C which would be unlikely to contribute significantly to selfheating: For a face which has just started showing signs of heating, treating it with water spray would not invite attention on heat of wetting.
137
Geological characteristics and planes of weakness The interaction between structural defects and the susceptibility to heating may be classified into three groups: (1) bedding defect, (2) cleavage defect and (3) the banded structure of the seam profile. Bedding defect is manifested in the form of displaced bedding planes and crushed coal beds. Displaced bedding planes have residual shear strength as a result of organic mass movement and normally possess no cohesion, which causes formation of cracks during mining. Crushed coal beds fall in the category of brittle lithotypes such as fusain or vitrain, which through the action of differential tectonic stresses in coal seams have been sheared over ductile layers such as shales or durain without affecting them. The crushed bands of fusain are virtually pulverized (angle of internal friction, 0 = 15 °-20 ° ). Cleavage defect: Exogenic cleat, which is formed by external tectonic forces, is often represented by open fractures. Induced cleat which has been initiated and propagated by mining stresses creates random fractures. Banded structure: A seam profile consists of in general alternating hard (durain (dull)) and soft (vitrain (bright)) seam layers. A seam composed of differing lithotypes offers planes of weakness along which cracks extend during mining. The seam profile of a composite structure with interchange of coal beds and waste partings offers air-transportability paths. All these geological phenomena offer airentry paths and play significant roles in dust formation.
Effect of aquifers on the moisture content of coal There are certain lignite deposits, especially in the Kutch and Nevyeli region of India, which are underlain by high-pressure aquifers.
Pores in lignite act as capillary tubes and the capillary action depends on the section, shape and spatial orientation. When the fresh faces are exposed during mining, only then does capillary action come into play. The height of capillary rise is given in a general form:
/-/c
20 cos 0 rpg
where Ho = capillary height, r = radius of the capillary tube, o = surface tension at the interface between two phases, 0 - angle of wetting, and O = specific density of the liquid. The capillary rise is influenced by the salinity of the water. Saline water rises higher than fresh water. The height of the capillary rise recorded in fine-grained sand is 35-100 cm, and in clay 400-500 cm. The pore size of lignite ( < 10/L) lies between that of sand (10/~) and clay (2/x). Lignite and sub-bituminous coal are hydrophilic and the moisture is partly held by chemical bonding, while in clay the water is fixed by both molecular and electrostatic forces (10 9 Pa). Therefore, the amount of rise of water in lignite will be between that of sand and clay. Thus, in-situ lignite benches are normally wet unless the water table is appreciably lowered. Hence, occurrence of fire in lignite benches is rare, whereas in stacks it is common. Generally, the coal deposit is not underlain by an aquifer and instead a pocketed or a continuous aquifer is encountered above the deposit. The possibility of fire in coal benches is more common in this situation.
Physicomechanical properties of coal Friable coals create a large surface area amenable to heating. A relationship has been established between the rank parameters of the Indian coals and the friability characteristics [4]. The coals containing carbon in the range 84-89%, and VM in the region of 33% are highly friable. Moisture also affects the
138
friability of coal in the form of a gradual elimination of moisture, and when the moisture level declines to 2.0-2.5%, the coal becomes ver friable. Mining of such coals generates large amounts of small coal fragments and dust.
THE DIFFERENT WAYS OF APPLYING WATER FOR FIGHTING FIRES These include (1) maintaining the relative humidity at 60% or more in the bench, (2) dissipating away the heat by application of a mist spray, and (3) digging out the fire with the aid of a water monitor. Maintenance of relative humidity By maintaining the relative humidity at 60% or more, heat is constantly dissipated away from the face and the surface is not permitted to exceed t h e ambient temperature. In order to achieve the desired result, boreholes (diameter = 50 mm) at successive 3-m intervals (based on the area of influence of each hole) are made and the water is infused at high pressure, 7-10 bar. If water vapour in the porous coal surface is in equilibrium with the atmosphere, the partial pressure of the water vapour does not significantly affect the partial pressure of oxygen at room temperature. Thus, water vapour does not prevent the transport of oxygen to the exposed face. As long as it is occupying the pores, it will confront the entry of air into the coal. Infusion of water, however, increases heat dissipation remarkably. There are two modes of heat dissipation: heat transfer at the coal-air interface, and diffusion of heat in the coal body. The strategy is to restore the in-situ condition of moisture in order to avert the heat buildup process due to which ignition occurs. There are, however, inherent weaknesses in this strategy in that (1) the boreholes may begin to suck air, and (2) the
boreholes may fill with water which then percolates inside the macropore system sometimes constituting a grave danger of sliding along the weakness planes. An additional weakness (3) constitutes the remarkable disintegration effects such as enlargement of the original fissures and extertion of the internal stresses by expansion due to the permeation of water in the coal, hence enlarging the original fissures and producing new fissures. Heat dissipation from the bench wall by application of mist spray When the face is wetted by fine threads of water, a large proportion of water participates in carrying away the heat by evaporation. Heat generated due to auto-oxidation is robbed off, reducing the chances of ignition. Considering the dearth of water in India and a utilisation efficiency of about 25% of the water used, it is imperative to ascertain the m i n i m u m possible water flow rate [9-11]. Minimum rate of flow of mist sprayer The rate of flow of the spray must exceed the mass flux diffused under the potential of molecular diffusion and bulk motion. The bulk motion is generated due to free convection. Variation in temperature leads to differences in density and these differences are
TABLE 1 Mass of water vapour transferred at different temperatures
( o c)
Mass of vapour transferred (m3/h)
Actual requirement considering 25% water utilisation (m3/h)
50 60 70 80 90
2 2.5 2.9 4.1 6.8
8 10 12 16 28
Average air temperature
Temperature of the wall
( o c) 30 30 30 30 30
139
accentuated by a water vapour-gas mixture of varying composition. The equation governing mass transfer (Table 1) is given by:
of the air (to) and the temperature of the air near the wall (t)
Jiw= B(miw- mio)
O=t-t o
where Jiw = total mass transfer, B - - m a s s transfer coefficient, m iw-= concentration of the diffusion substance on the phase interface, and mio = concentration of the diffusion substance at a distant point.
0 = 0 w at y = 0
0w = t w - t o
O=O at y = 3 The integral mean velocity of convective air flow is given by: -
OogfiOw82
MATHEMATICAL TREATMENT In the mathematical treatment, the following assumptions are made: (1) the forces of inertia are negligible compared with those of gravity, (2) the pressure gradient is zero, and (3) the surface of the wall is smooth. Regarding this third assumption, normally the bench walls are highly irregular and rough. The height of the wall is finite (5-6 m) and the flow is considered laminar, and with laminar flow the heat transfer coefficient is independent of the degree of relative roughness. In this case, the heat transfer may increase because the heat exchange surface of a rough section is greater than thai of a plane one. As is the case in coal, if the surface irregularities are very high a stagnation zone may form between these irregularities and the wall surface, due to which heat transfer may reduce. When the face is sprayed with water, the recessess and depressions which are able to retain vapour are generally the starting points of vaporization. Heat transfer normally increases due to roughness in the vertical wall. The fourth and fifth assumptions are that heat is manifested near the wall and the temperature gradient inside is negligible, and that the total length of the wall is heated up at uniform temperature. The boundary conditions are the temperature of the wall (tw) , the average temperature
On wetting the face, an additional convective current is set up due to the density difference resulting from variations in concentration: wx =
Oog/3mS82
where /3 = volumetric expansion coefficient, = viscosity of the air at a particular temperature, po = density of the air at a particular temperature and a = thickness of the boundary layer. In convective air flow, the thicknesses of the hydrodynamic and thermal boundary layers coincide. In addition, for the case of air and water vapour mixtures, in the range of atmospheric pressure conditions, it is found that the Lewis number is approximately unity
(L e = ~ ) - - ~ r - 1 Thus, the concentration boundary layer coincides with the other two boundary layers. The resultant mean velocity, B m [a quantity analogous to air flow which indicates the variation of density with composition, i.e., (1/o)(8P/8S)1 is
poga2(BOw+ BmS) 40~ where S - - c o n c e n t r a t i o n of water vapour at particular temperature.
140
The fluid is set in motion at an initial temperature t o and is heated up in the moving layer to various temperatures ranging from t o to t w. The mean temperature of fluid in the layer is:
=0w
Once the thickness of the boundary layer is determined, the heat transfer coefficient is calculated by the relationship: OL
= 0.473{ •3p2g(flO w + tim S) w
3 The rate of fluid flow through the cross-section layer is dG = OoW~x& The quantity of heat for heating the fluid is:
dQ = aOw d x . 1 dQ = enthalpy of moist air. Heat is transferred to the air through water vapour from the wall. Enthalpy of moist air = enthalpy of dry air + enthalpy of water vapour. Enthalpy of moist air = Cpa(t - to) + SCpv( t - to) + ivo. Specific h u m i d i t y S = 0 . 6 2 2 p i p - p ~ . Where Ps = partial pressure of saturated vapour at a particular temperature and Pv = latent heat of vaporization at a particular temperature. The thickness of the boundary layer is given by:
•
× c,+
sc +
X (Cpa+Sfpw+Siv°]]-l)
I~
Cool ~hovel ~
~
: ,lilihlihhiil,hl rhlri l
l
l
t
~
/
_
/
l
where x = effective length of the wall. Using the Colburn analogy for heat and mass transfer: O~
^ ~
r2/3
Since the Lewis number is taken as approximately equal to 1, the mass transfer coefficient = a/pCp. When heat is dissipated away from the wall the temperature gradually decreases and the temperature difference does not remain the same. The mean temperature is given by the equation: A t i -- A t e Arm = I n Ati/At e
ragtine
L.I
~ 1/4
where At i --- initial temperature difference and At e = final temperature difference. For calculation purposes, a m a x i m u m temperature erring on the side of safety has been taken. Further, the temperature of the bench wall is
3 = 4.23{ l~Xx[p 2og(flOw + flm S)
............ ~
\
/vo /
i'. I
_
...... ~
,lu,l,rl,
......... |
6-",.//]'l'!J!'!I!i!i!qil'l I,I,I, II 111,IqYl I lit ,,,i l id ll Ill:
Fig. 3. Layout of quarry with dragline sidecasting.
\¢1111111qJ!!ll!qqllll!lll
141
not everywhere uniform, which again places the computation on the high side.
SUGGESTIONS BENCH FIRES
FOR
PREVENTING
It has already been suggested that the area of exposure of the coal bench should be minimal. However, this is only possible as long as the first bench is not exposed. Winning of the first bench exposes the underlying coal to auto-oxidation. The following measures are suggested: (1) While quarrying, the seam must be taken in sequence. The seams should not all be opened simultaneously. As well as making mining difficult, this makes combating fire a laborious task. (2) As far as possible the coal must be taken up to the floor, although this does require simultaneous working of benches. (3) The workings should be divided into fairly large blocks. The length of the slice in the block should be such that the time during which it is finished off must be commensurate with the period in which the exposed wall becomes weathered. The movement of the face must be away from the face which is to be kept exposed, i.e., from dip to rise (Fig. 5). In multiple benching, the length of slice to be worked out should be comparable to the last bench. It defines the length of the block along the dip. The layout of the workings should be based around judicious combination of advance along the dip and the strike simultaneously. Further, in coal deposits with an along-strike extent in the order of 2-3 km it is more appropriate to divide the entire working into panels and each panel should be worked independently (Fig. 4) [14]. This will facilitate early return to the initial position. The width of the panel is influenced by the size of the surface mine field, the thickness of the overburden, the equipment used and the method of haulage (Fig. 3) [121.
!:? :.i
;: W' "'
""" v
Fig. 4. Layout of quarry with panel division (shovel-dumper combination). L e = width of panels; LB1 = average haulage distance of overburden from the first panel at the working benches (excluding the haulage distance around the pit-end slope) (m); Ld, = average haulage distance of overburden from the first panel around the pit-end slope (m); Lp~ = average haulage distance of overburden from the first panel on the inner dump (excluding the haulage distance around the pit-end slope) (m); open arrow - - working front advance direction; closed arrow - - overburden haulage direction; Kp = swell coefficient of the overburden in the inner dump; H B - average thickness of overburden (m); fl = stabilized slope angle of loose dumping material in the inner dump ( o ); "r = pit-end slope angle of overburden in situ ( ° ) ; c = unit haulage cost of overburden; Q = loading and dumping cost of rehandling overburden. Number of panels,
4cfl 2 -- cflHBKp( ctg fl + c t g ' / ) + 2 c H c t g n
4KpHB( etg3' + c t g f l ) ( c L + 1000Q)
Width of the panels, L c = -n The width of the panel is related to the layout of the quarry with the shovel-dumper combination.
Spraying must be gentle so that the layer is not dislodged creating fresh exposure of the surface. (4) Overhang and a crushed zone on top of the bench must be avoided. The height of the bench must be maintained within reach of the boom height of the loading equipment. (5) Scooping out of blasted coal must be thorough and any remaining material should
142
Ori~ional qround level Top bench 1B ...............
....
1
~
~
~
,/ /., <-
......
/ . ~ o~ 7 ; ,'~ r~2 ;'~ . )1
Scraper 4.6m3 ~'-k~.:.-,.._ ' "i" Foce 4 5m _L.?B 3-jim3
3dvcmcle
I
"??,
Romp ~i
~" , ,.
~75m~
11N6
4.6rn3
DecoQled QreQ
.~,~_~ ~, ,~1L ,
"2."J L . . ~75m ~~75m~
tr\,t
.
i 'J
,ebgm [ [ COEl[ ~1"9.." ',L~"
~1
"9
3kin
L
- \ r
Ir
'
J
.
-
,t
/
I
_
.
.
Fig. 5. Layout of quarry with dip to rise face.
be removed. A small-capacity loader with a hopper for collection of small quantities would meet this requirement.
tation of air entry by water vapour anchor is not a plausible approach as it does not affect the partial pressure of oxygen near the exposed face. In such circumstances, if the exposed coal is allowed to become oxidised and heat buildup is dissipated, thermal explosion will not occur since water application is to be carried out at the time of temperature rise in the exposed face, and not regularly. This technique helps to conserve water. Additionally, on the basis of heat transfer processes, the rate of flow of water spray has been calculated so that the water can be used effectively. The irregularity of the coal wall may affect the theoretical model to some extent. The assumption implicit in this exercise is that a fresh face of coal will not be exposed, and so attention to the face during initial exposure is important. Spalling from the weathered face is not ruled out, however. The layout of the bench is also proposed: the direction of movement of the face should be away from the face which is to be kept exposed, i.e., from dip to rise end, and the length of each slice should be such that the working remains confined in a particular block until the period of cessation of oxygen uptake.
ACKNOWLEDGEMENTS CONCLUSION The coal bench represents a compacted stack, but its susceptibility to fire in Indian quarries is a common phenomenon. The critical role of moisture balance in the coal pores, geological disturbances, induced fracture along the weakness planes, and the banded structure of the seam profile are all envisaged as initiators o f heating. Which of these plays a predominant role is difficult to assess at this stage. The different proposed models, such as maintaining relative humidity at its original level, have been critically analysed. Confron-
The authors thank Dr. B. Singh, Director of the Central Mining Research Station, for permission to publish this paper, and Dr. S.C. Banerjee, Scientist-in-Charge at the Mine Fire Laboratory, CMRS. The opinions expressed in this paper, however, are solely the authors' and they are not necessarily those of the CMRS or of the laboratory.
REFERENCES 1 Schmidt, L.D. and Elder, J.L., Atmospheric oxidation of coal at moderate temperatures, Ind. Eng. Chem. Feb. (1940): 249-252.
143 2 Sevenster, P.G., Studies on the interaction of oxygen with coal in the temperature range 0 ° C - 9 0 ° C , parts I and II. Fuel (1940): 7-32. 3 Walker A.K., The role of water in spontaneous combustion of solids. Chem. Div. D.S.I.R., Petone, New Zealand. 4 Roy, L.C. and Lahiri, A., The role of friability and grindability in mechanical coal winning. J. Mines Metals Fuels (1962). 5 Majumdar, B.K., Banerjee, A. and Nandy, H.C., Spontaneous combustion of coal--an approach to the problem. Fuel Sci. Technol., 2 (1983): 93-102. 6 Nardon, P. and Bainbridge, N.W., Heat of wetting of a bituminous coal. Fuel (1983). 7 Berkowitz, N. and Schein, H.G., Heat of wetting and spontaneous ignition of coal. Fuel 30 (1951): 94-96.
8 Jones, R.E. and Townsend, D.T.A., The role of oxygen complexes in the oxidation of carbonaceous materials. Trans. Faraday Soc. (1942): 297. 9 Arora, O.P., Heat and Mass Transfer. Khanna Publ. 10 Kert, E.R.G. and Drake, R.M., Heat and Mass Transfer. McGraw-Hill, New York. 11 Schlichting, Boundary Layer Theory. McGraw-Hill, New York. 12 Rzheusky, V.V., Opencast Mining Technology and Integrated Mechanization. 13 McKinstry, H.E., Mining Geology, Prentice Hall (1960). 14 Yang, Rong-Xin, Parameters of dividing the surface mine field into panels. China Inst. Min. Technol. (1988).
133
A CONCEPTUAL APPROACH TO PREVENTION OF FIRE IN COAL BENCHES A. Sharma and D.D. Banerjee Central Mining Research Station, Dhanbad-826001, Bihar (india)
(ReceivedJuly 4, 1988; acceptedSeptember28, 1988)
ABSTRACT The causative factors affecting fire in sub-bituminous coal benches are highlighted. A conceptual approach to prevention of fire is presented. It is suggested that the temperature of the coal wall must be brought down using a mist sprayer, allowing the coal to become weathered
at low temperature. On the basis of heat transfer processes, a mathematical formulation has been made to ascertain the optimum rate of water flow from the sprayer. Modification of the existing layouts and panel workings of the coal block is also suggested.
INTRODUCTION
minimum when the moisture level lies between 4 and 6% and the ambient air is saturated. Moisture acts as a catalyst in the coal-oxygen interaction. It helps in forming the peroky complex, an intermediate transient compound which in turn breaks down and enhances the rate of oxidation at early stages. It affects the process in other ways also. During evaporation of moisture, the internal surface area of the coal is available as fresh reactive centres. Schmidt has found that at low temperature, the rate of oxidation is proportional to the cube root of its specific internal surface area [1]. As soon as the bed is exposed, evaporation of moisture inexorably sets in. The presence of bound moisture in a hygro-
The Indian coal industry has suffered an increase in quarry fires engulfing huge areas, tropical conditions, the nature of the coal and the workings and the geological parameters all being responsible for heating in the coal benches. The mechanism of the coal oxidation process is well established [1,2]. The interaction depends on the rank of the coal, environmental parameters and the extent of exposure. The moisture in coal and the humidity of the air has a marked influence on the coal oxidation process, particularly at lower temperatures. It has been observed that the critical ignition temperature of the coal is a
134 scopic material raises the thermal conductivity and the enhanced conductivity facilitates internal transfer of heat within the coal mass. Since moisture is present in coal, the temperature gradient will cause differences in water vapour concentration in the interstitial atmosphere; it motivates mass transfer of water vapour from hot zones to cooler zones, transferring a substantial quantity of heat in the form of latent heat. Continuous evaporation from the coal face lowers its relative humidity (RH) below 100%. In such situations the mass transfer of water vapour from the hot zone to the cold zone will lower the R H in the former and raise it in the latter. It facilitates a path of almost constant water vapour concentration. At this equilibrium position the opposing forces of gaseous and thermal diffusion balance each other and transport of water vapour by diffusion then ceases altogether. When R H falls below 60%, thermal conductivity drops sharply and heat dissipation inbye becomes restricted [3]. Similarly, heat dissipates away from the exposed surface to air across the boundary layer. The difference in water v a p o u r concentration across the boundary layer causes the water vapour to diffuse away. Before the ignition of a coal bed, a thermal imbalance is established between heat generated by exothermic processes and heat dissipated to the surroundings by thermal conductivity. Since heat generation is manifested in all three dimensions of the block while the heat dissipation is two-dimensional, the possibility o f heat accumulation is facilitated. In the process of thermal explosion in a mass of reacting material, the processes identified as possible mechanisms whereby reactants are transported to the coal bed can be classified as: (1) natural convection caused by self-heating, (2) molecular diffusion caused by concentration gradients, (3) flow caused by differential wind pressure, (4) thermal breathing caused by diurnal variation in temperature, and (5) barometric breathing caused by variations in barometric pressure.
Brooks has shown that fluxes due to barometric breathing are such that they cannot sustain combustion, while the effects of thermal breathing occur so close to the surface that any heat produced is easily lost. The contribution of differential wind pressure to flow capable of causing spontaneous combustion is considered to be marginal. Initial displacement of moisture favours transport of oxygen due to molecular diffusion which is then sustained by natural convection. Apart from the availability of internal area, the crushed zone on top of the wall including powdered coal possessing highly reactive surfaces in the fissures and cracks, the heaps of powdered coal accumulated after the operation, and the curshed zone in the wall due to geological disturbances (bands and faults) [4] are in particular believed to be vulnerable to oxidation. Consequently, these factors can trigger combustion of the bench. The coal bed exhibits the characteristics of a compacted stack and can be designated as oxygen limited whereby all the oxygen entering the bed reacts very near the inlet to the bed so the heat generated can be lost to the surroundings. The conditions which can trigger heating must be present. The coal bed is essentially different from the stack: Voids do not exist, so the air permeation is restricted to low depths and the formation of a nucleus zone in which the temperature rises progressively and the threshold temperature is reached at an early stage is much smaller--in the order of centimetres, as experienced in the field study. Thermal explosion occurs in this zone and gradually envelops the area outbye where oxygen transport is facilitated. Further beyond this depth, the slow movement of air becomes depleted in oxygen in its path and correspondingly the oxidation process slows down. Should fire occur and it is not controlled, it penetrates to depth in increments. Subject to seasonal variation, the fire can create cracks in the bed and air transport becomes an imminent process.
135
DESIGN CHARACTERISTICS FOR COMBATING FIRE IN BENCHES (FIGS. 1 AND
2) Water is the commonest means of fighting fire but limitations are imposed against its use. In general, treatment of the coal face with water accelerates oxidation, and the application of insufficient quantities of water to a fire may result in explosion of water gas and producer gas. The steam generated will create a draught that will increase the rate of oxidation in the interior of the bench. The different means of water application for fire fighting depend basically on the extensivity characteristics of the fire. The degree of extensivity and intensity that create a fire zone of various dimensions can be linked as follows: (1) emergence of indications of heating at the wall of the bench, (2) origin of inflammation at the bench wall with a noticeable glow, and a (3) virulent fire covering a large area. Before a model for spraying or jet cutting is derived, the important physical processes associated with coal must be identified. The following processes are prominent: (a) drying characteristics of coal, (b) kinetics of oxidation and weathering, (c) heat of wetting, (d) geological characteristics such as slip planes and weaknesses, (e) effect of aquifers on the moisture content of coal, and (f) physicome-
b/~TOP
/
EDGE OF THE H I L L
~
E
10 m
~5~463452152
3o5aa6
;, ,,, ~F[
OFCOAL
, o,c
,,,
L FIRE FLANE Flee
SUMP SECTION
X-Y
Fig. 2. Temperature profile of burnixagsection. Temperature ranges from 30 ° to 480 ° C. (Not to scale.) chanical properties of coal, particularly friability and grindability indices.
Drying characteristics of wet coal A study of the kinetics [5] of desorption of moisture has indicated that drying of coal in the physical phase of desorption from 100 to 607o RH is much faster than the drying of air-dried moisture from 60% RH to lower levels. It takes about a week for the moisture level to come down to the stage of 60% RH under the average humidity conditions of the atmosphere, this level being a critical value both for moisture elimination and for heat conduction.
Kinetics of oxidation and weathering of coal
WATER
~-~
WORKING SHOVEL
COAL
Fig. 1. Extent of exposure in a burning quarry. (Not to scale.)
The rate of oxidation increases with a rise in temperature; it has been found that the rate of oxidation doubles for every 10 ° C rise in temperature. Indian coals are subject to high ambient temperatures, (average annual temperature -- 30 ° C). In these conditions, therefore, the combination of spontaneous heating and susceptible coal leads to a fast achievement of the threshold temperature. In order to control the process of spontaneous
136 heating it is imperative to bring down the temperature by removing the heat buildup. Coal is subject to weathering when it has been saturated with oxygen, i.e., air, over a long period of time under isothermal conditions. A weathered coal is relatively unreactive to oxygen compared to fresh coal. Schmidt and Elder [1] performed many long-term experiments--up to 300 d a y s - - a t normal temperature and they established a relationship between the rate of reaction and the oxidation period which indicated that the rate of oxidation decreases with time, the rate being very fast for the first 10 days. After a period of 40 days the rate of oxidation continues to decrease, and finally it is asymptotic to the time axis. An experiment carried out on oxidation weathering of Illinois No. 6 coal also revealed that the exposure of freshly mined bituminous coal has resulted in a slow oxidation reaction which appeared to be complete within two months when oxygen uptake practically ceased. The temperature of the oxidation was not allowed to exceed 30 ° C. From laboratory studies and the experience gained so far from the field, it can be inferred that the initial phase of exposure of the bench wall warrants strict vigilance.
Heat of wetting and condensation The low-rank, high-moisture coals, which are prone to spontaneous heating, possess an open structure with random orientation of lamellae in all directions and connected by cross links. Hence the system is porous with a large internal surface area (approximately 200 m2/g). It is further characterised by a hydrophilic nature because of the presence of polar groups and high oxygen content. Coal will generate a large amount of heat when wetted with water due to interaction between the fluid and the coal surface. A secondary cause of self-heating m a y b e present when coal absorbs water either in the liquid or in the
vapour form. Although combination with oxygen is at least partially reversible, combination with water is mostly reversible, particularly with the lowest rank bituminous coals. Integral heat of wetting [6,7] may be expressed as: Uw = Q A - QL where Uw = integral heat of wetting of moist coal, QA = integral heat of absorption from the vapour phase and Q L = h e a t of condensation of water vapour to liquid at the same temperature. It has been found that Uw is not constant but depends on the water content or RH in the coal matrix. Thus, if the moisture content of the coal (weight of water/weight of moist coal) is increased from x I to x2, the heat of wetting is: oX2
Uw=G2-Gl=jx uxdx I
where Uw = differential heat of wetting/unit mass of water, Using the Clausius-Clapeyron equation the differential heat of wetting is: U~ =
R T 2 d In Px dT
d In Po dT
where Px = partial pressure of water vapour in the atmosphere, and Po = partial pressure of water vapour over water at the absolute temperature, T. The first term is the differential heat of absorption at a moisture content, x. The second term is the heat of condensation of water vapour, which is a function of temperature only and is equivalent to 2438 k J / k g of water at 25°C. It is calculated that for a normal moist coal (65% RH), the heat of wetting could cause a temperature rise of only 2 ° C which would be unlikely to contribute significantly to selfheating: For a face which has just started showing signs of heating, treating it with water spray would not invite attention on heat of wetting.
137
Geological characteristics and planes of weakness The interaction between structural defects and the susceptibility to heating may be classified into three groups: (1) bedding defect, (2) cleavage defect and (3) the banded structure of the seam profile. Bedding defect is manifested in the form of displaced bedding planes and crushed coal beds. Displaced bedding planes have residual shear strength as a result of organic mass movement and normally possess no cohesion, which causes formation of cracks during mining. Crushed coal beds fall in the category of brittle lithotypes such as fusain or vitrain, which through the action of differential tectonic stresses in coal seams have been sheared over ductile layers such as shales or durain without affecting them. The crushed bands of fusain are virtually pulverized (angle of internal friction, 0 = 15 °-20 ° ). Cleavage defect: Exogenic cleat, which is formed by external tectonic forces, is often represented by open fractures. Induced cleat which has been initiated and propagated by mining stresses creates random fractures. Banded structure: A seam profile consists of in general alternating hard (durain (dull)) and soft (vitrain (bright)) seam layers. A seam composed of differing lithotypes offers planes of weakness along which cracks extend during mining. The seam profile of a composite structure with interchange of coal beds and waste partings offers air-transportability paths. All these geological phenomena offer airentry paths and play significant roles in dust formation.
Effect of aquifers on the moisture content of coal There are certain lignite deposits, especially in the Kutch and Nevyeli region of India, which are underlain by high-pressure aquifers.
Pores in lignite act as capillary tubes and the capillary action depends on the section, shape and spatial orientation. When the fresh faces are exposed during mining, only then does capillary action come into play. The height of capillary rise is given in a general form:
/-/c
20 cos 0 rpg
where Ho = capillary height, r = radius of the capillary tube, o = surface tension at the interface between two phases, 0 - angle of wetting, and O = specific density of the liquid. The capillary rise is influenced by the salinity of the water. Saline water rises higher than fresh water. The height of the capillary rise recorded in fine-grained sand is 35-100 cm, and in clay 400-500 cm. The pore size of lignite ( < 10/L) lies between that of sand (10/~) and clay (2/x). Lignite and sub-bituminous coal are hydrophilic and the moisture is partly held by chemical bonding, while in clay the water is fixed by both molecular and electrostatic forces (10 9 Pa). Therefore, the amount of rise of water in lignite will be between that of sand and clay. Thus, in-situ lignite benches are normally wet unless the water table is appreciably lowered. Hence, occurrence of fire in lignite benches is rare, whereas in stacks it is common. Generally, the coal deposit is not underlain by an aquifer and instead a pocketed or a continuous aquifer is encountered above the deposit. The possibility of fire in coal benches is more common in this situation.
Physicomechanical properties of coal Friable coals create a large surface area amenable to heating. A relationship has been established between the rank parameters of the Indian coals and the friability characteristics [4]. The coals containing carbon in the range 84-89%, and VM in the region of 33% are highly friable. Moisture also affects the
138
friability of coal in the form of a gradual elimination of moisture, and when the moisture level declines to 2.0-2.5%, the coal becomes ver friable. Mining of such coals generates large amounts of small coal fragments and dust.
THE DIFFERENT WAYS OF APPLYING WATER FOR FIGHTING FIRES These include (1) maintaining the relative humidity at 60% or more in the bench, (2) dissipating away the heat by application of a mist spray, and (3) digging out the fire with the aid of a water monitor. Maintenance of relative humidity By maintaining the relative humidity at 60% or more, heat is constantly dissipated away from the face and the surface is not permitted to exceed t h e ambient temperature. In order to achieve the desired result, boreholes (diameter = 50 mm) at successive 3-m intervals (based on the area of influence of each hole) are made and the water is infused at high pressure, 7-10 bar. If water vapour in the porous coal surface is in equilibrium with the atmosphere, the partial pressure of the water vapour does not significantly affect the partial pressure of oxygen at room temperature. Thus, water vapour does not prevent the transport of oxygen to the exposed face. As long as it is occupying the pores, it will confront the entry of air into the coal. Infusion of water, however, increases heat dissipation remarkably. There are two modes of heat dissipation: heat transfer at the coal-air interface, and diffusion of heat in the coal body. The strategy is to restore the in-situ condition of moisture in order to avert the heat buildup process due to which ignition occurs. There are, however, inherent weaknesses in this strategy in that (1) the boreholes may begin to suck air, and (2) the
boreholes may fill with water which then percolates inside the macropore system sometimes constituting a grave danger of sliding along the weakness planes. An additional weakness (3) constitutes the remarkable disintegration effects such as enlargement of the original fissures and extertion of the internal stresses by expansion due to the permeation of water in the coal, hence enlarging the original fissures and producing new fissures. Heat dissipation from the bench wall by application of mist spray When the face is wetted by fine threads of water, a large proportion of water participates in carrying away the heat by evaporation. Heat generated due to auto-oxidation is robbed off, reducing the chances of ignition. Considering the dearth of water in India and a utilisation efficiency of about 25% of the water used, it is imperative to ascertain the m i n i m u m possible water flow rate [9-11]. Minimum rate of flow of mist sprayer The rate of flow of the spray must exceed the mass flux diffused under the potential of molecular diffusion and bulk motion. The bulk motion is generated due to free convection. Variation in temperature leads to differences in density and these differences are
TABLE 1 Mass of water vapour transferred at different temperatures
( o c)
Mass of vapour transferred (m3/h)
Actual requirement considering 25% water utilisation (m3/h)
50 60 70 80 90
2 2.5 2.9 4.1 6.8
8 10 12 16 28
Average air temperature
Temperature of the wall
( o c) 30 30 30 30 30
139
accentuated by a water vapour-gas mixture of varying composition. The equation governing mass transfer (Table 1) is given by:
of the air (to) and the temperature of the air near the wall (t)
Jiw= B(miw- mio)
O=t-t o
where Jiw = total mass transfer, B - - m a s s transfer coefficient, m iw-= concentration of the diffusion substance on the phase interface, and mio = concentration of the diffusion substance at a distant point.
0 = 0 w at y = 0
0w = t w - t o
O=O at y = 3 The integral mean velocity of convective air flow is given by: -
OogfiOw82
MATHEMATICAL TREATMENT In the mathematical treatment, the following assumptions are made: (1) the forces of inertia are negligible compared with those of gravity, (2) the pressure gradient is zero, and (3) the surface of the wall is smooth. Regarding this third assumption, normally the bench walls are highly irregular and rough. The height of the wall is finite (5-6 m) and the flow is considered laminar, and with laminar flow the heat transfer coefficient is independent of the degree of relative roughness. In this case, the heat transfer may increase because the heat exchange surface of a rough section is greater than thai of a plane one. As is the case in coal, if the surface irregularities are very high a stagnation zone may form between these irregularities and the wall surface, due to which heat transfer may reduce. When the face is sprayed with water, the recessess and depressions which are able to retain vapour are generally the starting points of vaporization. Heat transfer normally increases due to roughness in the vertical wall. The fourth and fifth assumptions are that heat is manifested near the wall and the temperature gradient inside is negligible, and that the total length of the wall is heated up at uniform temperature. The boundary conditions are the temperature of the wall (tw) , the average temperature
On wetting the face, an additional convective current is set up due to the density difference resulting from variations in concentration: wx =
Oog/3mS82
where /3 = volumetric expansion coefficient, = viscosity of the air at a particular temperature, po = density of the air at a particular temperature and a = thickness of the boundary layer. In convective air flow, the thicknesses of the hydrodynamic and thermal boundary layers coincide. In addition, for the case of air and water vapour mixtures, in the range of atmospheric pressure conditions, it is found that the Lewis number is approximately unity
(L e = ~ ) - - ~ r - 1 Thus, the concentration boundary layer coincides with the other two boundary layers. The resultant mean velocity, B m [a quantity analogous to air flow which indicates the variation of density with composition, i.e., (1/o)(8P/8S)1 is
poga2(BOw+ BmS) 40~ where S - - c o n c e n t r a t i o n of water vapour at particular temperature.
140
The fluid is set in motion at an initial temperature t o and is heated up in the moving layer to various temperatures ranging from t o to t w. The mean temperature of fluid in the layer is:
=0w
Once the thickness of the boundary layer is determined, the heat transfer coefficient is calculated by the relationship: OL
= 0.473{ •3p2g(flO w + tim S) w
3 The rate of fluid flow through the cross-section layer is dG = OoW~x& The quantity of heat for heating the fluid is:
dQ = aOw d x . 1 dQ = enthalpy of moist air. Heat is transferred to the air through water vapour from the wall. Enthalpy of moist air = enthalpy of dry air + enthalpy of water vapour. Enthalpy of moist air = Cpa(t - to) + SCpv( t - to) + ivo. Specific h u m i d i t y S = 0 . 6 2 2 p i p - p ~ . Where Ps = partial pressure of saturated vapour at a particular temperature and Pv = latent heat of vaporization at a particular temperature. The thickness of the boundary layer is given by:
•
× c,+
sc +
X (Cpa+Sfpw+Siv°]]-l)
I~
Cool ~hovel ~
~
: ,lilihlihhiil,hl rhlri l
l
l
t
~
/
_
/
l
where x = effective length of the wall. Using the Colburn analogy for heat and mass transfer: O~
^ ~
r2/3
Since the Lewis number is taken as approximately equal to 1, the mass transfer coefficient = a/pCp. When heat is dissipated away from the wall the temperature gradually decreases and the temperature difference does not remain the same. The mean temperature is given by the equation: A t i -- A t e Arm = I n Ati/At e
ragtine
L.I
~ 1/4
where At i --- initial temperature difference and At e = final temperature difference. For calculation purposes, a m a x i m u m temperature erring on the side of safety has been taken. Further, the temperature of the bench wall is
3 = 4.23{ l~Xx[p 2og(flOw + flm S)
............ ~
\
/vo /
i'. I
_
...... ~
,lu,l,rl,
......... |
6-",.//]'l'!J!'!I!i!i!qil'l I,I,I, II 111,IqYl I lit ,,,i l id ll Ill:
Fig. 3. Layout of quarry with dragline sidecasting.
\¢1111111qJ!!ll!qqllll!lll
141
not everywhere uniform, which again places the computation on the high side.
SUGGESTIONS BENCH FIRES
FOR
PREVENTING
It has already been suggested that the area of exposure of the coal bench should be minimal. However, this is only possible as long as the first bench is not exposed. Winning of the first bench exposes the underlying coal to auto-oxidation. The following measures are suggested: (1) While quarrying, the seam must be taken in sequence. The seams should not all be opened simultaneously. As well as making mining difficult, this makes combating fire a laborious task. (2) As far as possible the coal must be taken up to the floor, although this does require simultaneous working of benches. (3) The workings should be divided into fairly large blocks. The length of the slice in the block should be such that the time during which it is finished off must be commensurate with the period in which the exposed wall becomes weathered. The movement of the face must be away from the face which is to be kept exposed, i.e., from dip to rise (Fig. 5). In multiple benching, the length of slice to be worked out should be comparable to the last bench. It defines the length of the block along the dip. The layout of the workings should be based around judicious combination of advance along the dip and the strike simultaneously. Further, in coal deposits with an along-strike extent in the order of 2-3 km it is more appropriate to divide the entire working into panels and each panel should be worked independently (Fig. 4) [14]. This will facilitate early return to the initial position. The width of the panel is influenced by the size of the surface mine field, the thickness of the overburden, the equipment used and the method of haulage (Fig. 3) [121.
!:? :.i
;: W' "'
""" v
Fig. 4. Layout of quarry with panel division (shovel-dumper combination). L e = width of panels; LB1 = average haulage distance of overburden from the first panel at the working benches (excluding the haulage distance around the pit-end slope) (m); Ld, = average haulage distance of overburden from the first panel around the pit-end slope (m); Lp~ = average haulage distance of overburden from the first panel on the inner dump (excluding the haulage distance around the pit-end slope) (m); open arrow - - working front advance direction; closed arrow - - overburden haulage direction; Kp = swell coefficient of the overburden in the inner dump; H B - average thickness of overburden (m); fl = stabilized slope angle of loose dumping material in the inner dump ( o ); "r = pit-end slope angle of overburden in situ ( ° ) ; c = unit haulage cost of overburden; Q = loading and dumping cost of rehandling overburden. Number of panels,
4cfl 2 -- cflHBKp( ctg fl + c t g ' / ) + 2 c H c t g n
4KpHB( etg3' + c t g f l ) ( c L + 1000Q)
Width of the panels, L c = -n The width of the panel is related to the layout of the quarry with the shovel-dumper combination.
Spraying must be gentle so that the layer is not dislodged creating fresh exposure of the surface. (4) Overhang and a crushed zone on top of the bench must be avoided. The height of the bench must be maintained within reach of the boom height of the loading equipment. (5) Scooping out of blasted coal must be thorough and any remaining material should
142
Ori~ional qround level Top bench 1B ...............
....
1
~
~
~
,/ /., <-
......
/ . ~ o~ 7 ; ,'~ r~2 ;'~ . )1
Scraper 4.6m3 ~'-k~.:.-,.._ ' "i" Foce 4 5m _L.?B 3-jim3
3dvcmcle
I
"??,
Romp ~i
~" , ,.
~75m~
11N6
4.6rn3
DecoQled QreQ
.~,~_~ ~, ,~1L ,
"2."J L . . ~75m ~~75m~
tr\,t
.
i 'J
,ebgm [ [ COEl[ ~1"9.." ',L~"
~1
"9
3kin
L
- \ r
Ir
'
J
.
-
,t
/
I
_
.
.
Fig. 5. Layout of quarry with dip to rise face.
be removed. A small-capacity loader with a hopper for collection of small quantities would meet this requirement.
tation of air entry by water vapour anchor is not a plausible approach as it does not affect the partial pressure of oxygen near the exposed face. In such circumstances, if the exposed coal is allowed to become oxidised and heat buildup is dissipated, thermal explosion will not occur since water application is to be carried out at the time of temperature rise in the exposed face, and not regularly. This technique helps to conserve water. Additionally, on the basis of heat transfer processes, the rate of flow of water spray has been calculated so that the water can be used effectively. The irregularity of the coal wall may affect the theoretical model to some extent. The assumption implicit in this exercise is that a fresh face of coal will not be exposed, and so attention to the face during initial exposure is important. Spalling from the weathered face is not ruled out, however. The layout of the bench is also proposed: the direction of movement of the face should be away from the face which is to be kept exposed, i.e., from dip to rise end, and the length of each slice should be such that the working remains confined in a particular block until the period of cessation of oxygen uptake.
ACKNOWLEDGEMENTS CONCLUSION The coal bench represents a compacted stack, but its susceptibility to fire in Indian quarries is a common phenomenon. The critical role of moisture balance in the coal pores, geological disturbances, induced fracture along the weakness planes, and the banded structure of the seam profile are all envisaged as initiators o f heating. Which of these plays a predominant role is difficult to assess at this stage. The different proposed models, such as maintaining relative humidity at its original level, have been critically analysed. Confron-
The authors thank Dr. B. Singh, Director of the Central Mining Research Station, for permission to publish this paper, and Dr. S.C. Banerjee, Scientist-in-Charge at the Mine Fire Laboratory, CMRS. The opinions expressed in this paper, however, are solely the authors' and they are not necessarily those of the CMRS or of the laboratory.
REFERENCES 1 Schmidt, L.D. and Elder, J.L., Atmospheric oxidation of coal at moderate temperatures, Ind. Eng. Chem. Feb. (1940): 249-252.
143 2 Sevenster, P.G., Studies on the interaction of oxygen with coal in the temperature range 0 ° C - 9 0 ° C , parts I and II. Fuel (1940): 7-32. 3 Walker A.K., The role of water in spontaneous combustion of solids. Chem. Div. D.S.I.R., Petone, New Zealand. 4 Roy, L.C. and Lahiri, A., The role of friability and grindability in mechanical coal winning. J. Mines Metals Fuels (1962). 5 Majumdar, B.K., Banerjee, A. and Nandy, H.C., Spontaneous combustion of coal--an approach to the problem. Fuel Sci. Technol., 2 (1983): 93-102. 6 Nardon, P. and Bainbridge, N.W., Heat of wetting of a bituminous coal. Fuel (1983). 7 Berkowitz, N. and Schein, H.G., Heat of wetting and spontaneous ignition of coal. Fuel 30 (1951): 94-96.
8 Jones, R.E. and Townsend, D.T.A., The role of oxygen complexes in the oxidation of carbonaceous materials. Trans. Faraday Soc. (1942): 297. 9 Arora, O.P., Heat and Mass Transfer. Khanna Publ. 10 Kert, E.R.G. and Drake, R.M., Heat and Mass Transfer. McGraw-Hill, New York. 11 Schlichting, Boundary Layer Theory. McGraw-Hill, New York. 12 Rzheusky, V.V., Opencast Mining Technology and Integrated Mechanization. 13 McKinstry, H.E., Mining Geology, Prentice Hall (1960). 14 Yang, Rong-Xin, Parameters of dividing the surface mine field into panels. China Inst. Min. Technol. (1988).