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Seam factor and the spontaneous heating of coal
Mining Science and Technology, 7 (1988) 149-159 Elsevier Science Publishers B.V., Amsterdam - Printed
149 in The Netherlands
SEAM FACTOR AND THE SPONTANEOUS OF COAL
HEATING
Ft. Morris Johannesburg
Consolidated
Investment
Company Limited, JCI House, 2001 (South Africa)
28 Harrison
Street, Johannesburg
and T. Atkinson Mining
Engineering
Department,
University
of Nottingham,
University
Received June 1, 1987; accepted February
Park, Nottingham
NG7 2RD (England)
16, 1988)
ABSTRACT Previously related publications by the authors have identified the spontaneous heating in underground coal mines as a combination of the seam factor, the geological factor and the mining factor. In this publication the authors pay
particular attention to the effects of the seam factor from research conducted from the end of the nineteenth century to the deductions of modern day workers.
INTRODUCTION
its organic matter has undergone during the period of formation. An increasing carbon content and with it a decreasing oxygen content are the most commonly accepted criteria of increasing rank. The higher the rank, e.g. anthracite, the slower the oxidation process, whilst lignite of low rank, oxidises so rapidly that it is often stated that it cannot be stored after mining without ignition. There are however numerous anomalies to a straight rank order. One part of a seam may be particularly liable to spontaneous combustion, a seam of higher rank may prove more troublesome than one of a lower rank or even the same seams in different mines may react differently.
The seam factor affecting the susceptibility of coal to spontaneous combustion may be defined by the following parameters [l]: rank; temperature; petrographic composition; available air; particle size; moisture; sulphur; other minerals; the effect of previous oxidation or heating; physical properties; heating due to crushing or bacteria. Rank The rank of a coal depends on the character of the original plant debris from which it was formed and the amount of change that
150
Petrographic
composition
The National Coal Board carried out a series of oxidation tests on hand picked petrographic constituents from five coals ranging from high ranking coking coal to low rank bituminuous coal [2]. The results of these tests, showed that in all cases fusain was the least reactive, and in general, durain was more reactive than vitrain. These results enabled calculations to be made of the reaction velocities of vitrinite, exinite and inertinite and showed that above 165°F exinite has a much greater oxidation rate than the other two constituents. Thus, it appears that a count of these “macerals” may be useful, together with rank in determining the susceptibility of coals to spontaneous combustion.
Temperature The absorption of oxygen is more rapid as the temperature increases. There is a pronounced temperature coefficient of oxidation, and the average rate of oxidation approximately doubles (1.4 to 2.3) for every rise of 18°F.
Available
air
Where there is a small amount of air, the rate of oxidation is very slow and there is no appreciable rise in temperature. Where there are large quantities of air passing over or through the coal, any heat produced will invariably be carried away so that the temperature does not rise and the oxidation rate remains at a low level. However, between these two limits there is a state when the air quantity is sufficient to promote oxidation but not sufficient to carry away the heat formed, so that there is an accelerating rate of oxidation until ignition occurs.
Particle
size
A solid coal face generally presents very little danger of spontaneous combustion, partly due to the small surface area and partly due to the very low permeability of solid coal to gases. It is, however, generally when coal is shattered in mining, or broken by roof pressure, or when falls and faulting occur that spontaneous combustion is likely to take place. It is the small coal that is mainly responsible for the heating. The air passes into the mass and oxidizes a little of the coal near the outer surface. This produces a slight rise in temperature, so that, as the air penetrates deeper and deeper, it becomes warmer and warmer and although part of its oxygen has been absorbed there is still enough to produce oxidation. Consequently, it is at some distance inside the mass that heating develops most rapidly. It should be noted that a flame will not necessarily make its appearance, even if a coal is red-hot, as flame is due to the combustion of gas and this requires that a moderately high proportion of oxygen be present. Once the oxidation process has gone beyond the early stages and heat is accumulating, it is only a matter of time before actual ignition takes place. The authors have noted in Bengal/Bihar, India and the Hunter Valley, Australia, high methane emissions from coal pillars prior to their heating. The pillars showed signs of crushing giving increased access for oxygen, reduced particle size and the release of adsorbed methane from pore surfaces, all providing the opportunity for increased oxidation. Moisture The effect of moisture on spontaneous heatings is uncertain. A small quantity seems to assist rather than retard the heating whilst large quantities of moisture retards the heat-
151
ing. However, as in a surface stockpile, alternative drying and wetting of the coal accelerates the heating process. Sulphur From the first publication on spontaneous combustion, R. Plott in 1686 [3], until about the middle of the nineteenth century it was assumed that sulphur in the form of pyrites was the main cause of spontaneous combustion. However, it was shown that coal even in the absence of sulphides would absorb oxygen and heat spontaneously [4-61. However, further research work modified this view and led to the present theory that pyrites plays a subsidiary role in promoting deterioration and spontaneous combustion [7-91. Other
minerals
Many other chemicals affect the rate of oxidation to some extent, either accelerating or retarding it. Alkalies can act as accelerators, and borates and calcium chloride as retardants. From the effects of previous oxidation on heating from experimental work on the effects of preheating coals in vacua, cooling, and then comparing their oxygen reaction with that of untreated coals, [lo] the following conclusions were drawn: (1) By preheating coals their liability to spontaneous combustion is greatly increased. (2) Lump coal previously almost impervious to air and thus without danger from the point of view of heating, may become, through being in the neighbourhood of a fire, a source of danger due to a large increase in its oxygen capacity. (3) Coal in a sealed area, even after a fire has become completely extinguished, may also have formed fine coal caused by a reduction in the strength following partial distillation.
These factors are important when attempting to re-open an area which had been sealed because of spontaneous combustion. In a number of instances, when attempts have been made to re-open workings that have been sealed sometimes for years, it has been found that there was no indication of fires, but immediately as ventilation was re-established, progressive re-heating occurred within a few days and sections had to be resealed. Physical
properties
A number of physical properties such as porosity, hardness, thermal conductivity and specific heat can affect the rate of oxidation of coal. Heating
due to earth
movement
Investigations showed that the heat generated in the crushing of rocks, such as in the goaf of a long wall face or the waste of a pillar extraction area may be sufficient to assist in starting the self heating process [ll]. Bacteria Two papers during this period deserve comment. The first, ref. [12], stated that spontaneous combustion could be caused by improper and preventable conditions or by a wrong system of mining, whilst the second [13], stated that a mining engineer has to work a mine as safely and as profitably as possible. He may then have to leave coal underground at the risk of spontaneous fires or to adopt methods of working that entail risks of heating. However, it is the contention of the authors of the present paper that the competent mining engineer will design the mine to minimize all risks to the safety of personnel and property and to maximize profitability.
152
THE EARLY RESEARCH NEOUS COMBUSTION
INTO
SPONTA-
It is possible at this stage to sum up the research into spontaneous combustion as follows: (i). By the end of the nineteenth century the capacity of coal to react with oxygen at ordinary temperatures and at an increasing rate a higher temperatures, always with the evolution of heat, had been established. Spontaneous combustion of coal in the mine or on the surface occurred when the coal presented a sufficient surface to the air and was sufficiently insulated thermally. The whole of the heating was in many cases very probably due to the oxidation of carbonaceous matter, but pyrite was thought to assist by the heat of its own oxidation and by breaking up the coal into smaller fragments which present a larger surface to the air. (ii). By the end of 1910, heating factors identified were: the kind of coal, in regard to its volatile matter; the purity of the coal; the presence of pyrite or other sulphur compounds; the temperature of the coal; the size of the coal; the presence of occluded gases in the coal; the presence of moisture; the accessibility of oxygen; and pressure on the coal. Kind
of coal
From the ignition temperatures given by Fayol [14], it may be seen that only those coals, such as lignites, bituminous and semicontaining large amounts of bituminous, volatile matter, are liable to ignite spontaneously, and that anthracite with its very low percentage of volatile matter is practically entirely excluded. The results of the work of Boudouard [U-18] on the coking power of coals also substantiates this view, for in his work he found that those coals richest in volatile matter, carbohydrates and cellulose were most liable to spontaneous combustion.
Purity
of coal
The work of Parr and collegues indicated that coals of exceptional purity are more apt to heat up then coals containing large amounts of extraneous matter [19-211. This is probably due to the fact that very pure coals are able to condense and absorb the oxygen of the air much faster than other coals and so cause an increase in temperature, which finally results in the chemical combination of the oxygen and hydrogen occluded in the coal. This view was confirmed by later work when it was found that those coals causing spontaneous combustion, or those coals which oxidized and increased in temperature most rapidly, were remarkably free from mineral matter and pyrite [22]. Presence
of pyrite
As to what part sulphur compounds, especially pyrite, play in the spontaneous ignition of coal, opinions differed greatly. Some believed pyrite to be the leading factor, while others believed it played no part at all, or, if so, ascribed to it a position of minor importance and believed its action to be merely a subsidiary one. The oxidizing action of the air upon pyrites was however admitted, and the notion seemed to be fairly general and well established that pyritic oxidation tends to raise the temperature of the coal. On the other hand, it was seen from the work of Fayol [14], Dennstedt and Bunz [22] and Threlfall [23], that coals containing pyrite in a quantity too insignificant to be noticed are very apt to ignite spontaneously. Others, however, believed that the only influence of the pyrite is a mechanical one, in which the oxidation of the thin films of pyrite in the coal serves merely to break up the coal. Temperature
of the coal
Most of the authors agreed that the temperature of the coal undoubtedly was one of
153
the main factors in the whole subject of spontaneous combustion, for cases of spontaneous combustion have occurred time and again were probably they never would have occurred if there had not been an initial heating in some way or other. The New South Wales Commission [24] thought that the initial temperature of the coal at the time of storage or loading was one of the great factors in the subject of spontaneous combustion. Sources of heat, seemingly insignificant, are frequently the cause of bringing the temperature of the coal up to the danger point, and so causing the ignition of coals to which, under carefully regulated conditions of storage, would not be at all dangerous. This increased temperature, whether coming from outside sources or from physical or chemical reactions within the coal, tends to accelerate the absorption of oxygen and thereby to raise the temperature of the coal. It also tends to drive out the inflammable gases occluded in the coal, and to greatly accentuate the danger of spontaneous ignition.
and recommended that they considered safe to transport on board ship only those coals passing a certain standard in their friability test.
Size of the coal
Opinions differed greatly as to what part moisture in the coal plays in its spontaneous combustion. Some believed the moisture content of the air-dry coal to be a direct index of its power to ignite spontaneously. Evidence given to the New South Wales Commission, showed that coal piles are more apt to take fire during warm weather following showers than at other times, although Fayol [14] as a result of experimental work, claimed that the influence of the weather on coal heaps had not been sufficiently marked to be observable, whilst others [25] believed the spontaneous combustion of coal to be due to the formation of ozone by the action of the sun on warm, sunny days following a rain, when the surface evaporation is especially great. If this is true, it was felt at the time that moisture plays a more important part in the phenomenon of spontaneous combustion than had previously been ascribed to it.
That fine coal is a more active absorbent of oxygen and more liable to ignite spontaneously than large coal was shown by Richter [4-61. Practically all of the later experimenters in this field concede this to be true, for the fine particles, having a greater surface area, can absorb much more oxygen than large lumps; and since this rapidity of absorption causes an increase in temperaturewhich in turn produces favourable conditions for further absorption and for chemical action between the oxygen of the air and the hydrogen of the coal-the danger of spontaneous combustion is greatly increased. Dennstedt and Bunz [22] in their work on this subject even went so far as to make a friability test on all the coals they worked with, for they conceded the danger arising from the fine coal with its greater avidity for oxygen,
Occluded
gases
in the coal
While it is now a well known fact that gases of an inflammable nature are occluded in coal, their relation to the spontaneous ignition of coal had not yet been clearly established by 1910. Whether the gases occluded in the coal are the real cause of spontaneous ignition is doubtful, but if the coal becomes heated up by oxidation or some other cause to a temperature high enough for the oxygen of the air to unite with these gases, then it is seen that the presence of these gases constitutes a source of danger. In this case, coals with large amounts of gases occluded in them would be more liable to ignite than coals containing smaller amounts of these gases. Presence
of moisture
154
That the presence of moisture materially assists the pyritic oxidation was generally conceded, although whether it caused an increase in temperature or merely a disintegration of the coal due to the formation of ferrous sulphate was a matter of dispute. Others believed that the only part that moisture played was a mechanical one, where alternate freezing and thawing broke up the coal into smaller particles and so exposed more surface to the oxygen of the air. Again, it is thought that aside from increasing the pyritic oxidation, moisture acts as a catalytic agent between the carbon and hydrogen or unsaturated bodies of the coal and the oxygen of the air. Perhaps, as Erdmann and Stolzenberg [25] have suggested, this reaction may consist of the formation of ozone which is immediately absorbed by the coal. The idea of catalytic action on the part of the moisture was substantiated to some extent by the fact that some coals containing minimum amounts of pyrite are nevertheless very liable to spontaneous ignition, and coals of this class have been known to cause fires. Accessibility
of oxygen
That the combination of oxygen with the constituents of the coal causes a rise in temperature seems to be firmly established. Which particular constituent is the cause of the rise in temperature has not, however, been shown with any great degree of certainty. The presence of humic acid in the oxidized coal leads one to believe that the oxygen combines with some of the unsaturated humus bodies, such as the polysaccharides. That this humus substance is an unsaturated body is shown by the fact that it absorbs large amounts of bromine without the evolution of hydrobromic acid. In fact, one researcher [26] went so far as to devise a practical test to determine the safety of a coal by means of this reaction with bromine.
The idea was also held that the oxygen of the air combined directly with the carbon and hydrogen of the coal and so caused an increase in temperature. If amorphous carbon (charcoal and lampblack) can be oxidized to carbon dioxide or exposure to the air by means of bacteria, as had been proved by Potter [27] the oxidation of the carbon of the coal is very probable. The presence of oxygen is therefore to be avoided and the old idea of thoroughly ventilating the coal piles by free access of air was gradually being dropped: at the time authorities deemed it advisable to keep the coal away from the air as much as possible, either by submerging it under water or by storing the coal in covered concrete bins. If ventilation is used to lower the temperature of the coal, it should be through pipes, so that the cooling air cannot come in contact with the coal at all. Pressure
on the coal
The belief that pressure on the coal was one of the leading factors in its spontaneous ignition seemed to be gaining ground at this time and because of this fact, it was advocated that coal heaps should not be any higher than 15 to 20 ft. (iii). The period from 1910-1930 was an exciting one but produced no drastic changes in the ideas of the researchers but rather a period of great debate, of point and counterpoint. The debate that the authors of this paper have termed the swinging pendulum between the pyrites and oxygen theories continued with publications by Lamplough and Hill [28], Wimnill [29], Lomax [30], Somermeier [31], Parr and Kressman [19], Drakely [32], Graham [33] and Haldane [34]. Graham in fact stated categorically that in some instances underground pyrites must be looked into as the primary cause of trouble; however he condeded that if checked, the majority of underground heatings would be
155
traced to the oxidation of the carbon constituents of coal. The period under discussion saw the introduction of chemical ratios as a means of both indicating the start of an underground heating and also as a means of indicating the state of the atmosphere behind fire stoppings. Rhead and Wheeler [35] introduced the first ratio CO,/CO as a means of detection, to be followed closely by Porter [36], who experimented on the relationships of the products of combustion and produced triangular diagrams representing the relationship of H,O, CO and CO, at different temperatures and found a constancy in the ratios of CO, and CO within a certain temperature range. The work of Winmill and Graham [37] was indeed a most important contribution to the period. Important conclusions included that as the temperature rises the coal absorbs 0, faster, and a larger quantity of 0, is absorbed. Further, that the ratio CO produced/O, absorption increased with temperature, and the ratio CO, produced/CO produced increased with time and decreased with increasing temperature. Porter and Ralson [38], determined that as the temperature rises, the ratio (CO, + CO)/H, increases, whilst there is a more or less constant ratio between CO, and CO. Further, when CO,, and H, are plotted on a ternary diagram the prints obtained at various temperatures fall approximately in a straight line Graham [39] introduced the ratio CO,/ oxygen absorbed, CO/oxygen absorbed and co/co,. Storrow and Graham in a continuation of this work introduced two methods of detecting underground heatings in detail and also discussed the ratio, oxygen deficiency/CO,. Haldane [41] however felt that no chemical analysis could compete with the ability of a miner to detect such heatings by his sense of smell. Graham and Jones [42] investigated fires which had occurred in 13 South Wales Collieries and Morgan [43]
introduced an important paper on the application of gas analysis to the detection of heatings. Finally the period saw the introduction of Coward’s Diagram and Graham’s Ratio. Whereas during this period, the previous nine reasons for the causes of spontaneous combustion were not queried, some additional reasons were forwarded. Stopes and Wheeler [44] felt that due to the complexity of the plant materials that had contributed to the formation of coal it was important to determine the character of the contribution made by these coal forming plants. Graham and Hill [45] stated that ulmins were the cause of spontaneous combustion. Stopes [46] determined the four constituents of coal as vitrain, clarain, fusain and durain and indicated that these needed to be considered separately. This work was continued by Lessing [47]. Sinnatt and Moore [48] indicated that there are relative temperatures of spontaneous ignition of solid fuels where there are zones of temperature at which spontaneous combusion appears. Also, for coals which are susceptible to spontaneous combustion, then spontaneous ignition occurs practically at the same instant the glow appeared. He concluded that: (1). the degree of fineness had a considerable influence upon the minimum temperature at which glowing would occur, and (2). the volatile matter of a coal had a considerable influence upon the liability of a fuel to spontaneously glow. In 1921, the Departmental Committee on Spontaneous Combustion [49] indicated that spontaneous combustion occurred in coals very high in moisture content. As moisture content is simply am easure of porosity it followed that the more porous a coal, the greater is the area exposed to oxidation. Hood [50] continued work on the fineness and temperature of stockpiles. Briggs [51] concluded that the crushing of coal under pressure converted the mechanical energy into heat. Sin-
156
natt and Slater [52] indicated that fusain posessed the property of propagating a zone of combustion. Haldane and Makgill [53] stated that the bacterial effect on coal increased its temperature whilst Francis [54] emphasized that coals differ in the proportion of ingredients (ulmins, cuticles, spore exines, resins and hydrocarbons) and modifications in the character of these ingredients determine the rank of the coal.
THE WORK SEARCHERS
OF
MODERN
DAY
RE-
gases, then an additional reaction must take place to increase its quantity. The mechanism suggested by Chamberlain et al. was as follows. The first step is the formation of peroxide which then decomposes to give compounds which are similar to those which have been found during other experiments. The formation and decomposition of peroxides occur mainly within the aliphatic structure of the coal and may proceed via the following reaction: R-H+O,+R’+HO,
.
R’+ 0, + R-O-O’ It is generally agreed by modern researchers that following an initial physical absorption of oxygen, a variety of more or less stable coal-oxygen complexes are formed with the release of some gaseous products. The oxidation reaction of coal at moderate temperatures is generally of two types: (1). The formation of oxy-functional groups on the surface of the coal; and (2). production of gases such as CO,, CO and H,O. The total consumption of oxygen is the sum of the oxygen consumed by these two reactions. Due to the extremely complex heterogeneous nature of coal, the many oxidation reactions of coal that occur make it difficult to examine all the reactions on an experimental level. Coal in its natural state contains varying amounts of oxy-functional groups. During the oxidation process the quantities of these groups increase. The initial chemical process is considered to be chemisorption. The oxy-functional groups include: carboxyl, hydroxyl, carbonyl, methoxyl, ester, ethers, peroxides and hydroperoxides [55]. Chamberlain, Barrass and Thirlaway [2] found that an increased temperature produced a much greater increase in carbon monoxide than any other gas. Therefore, they assumed that if carbon monoxide was being produced by a similar mechanism to the other
R-O-O’+
HO; + R-O-OH
R-O-O’+
RH + R-O-OH
+ 0, + R
The peroxide can then decompose in different ways according to its structure. Although there is universal agreement that oxygen is adsorbed on the coal surface during the oxidation, the literature presents many views of the dominant process. Physical adsorption, which may be in single or multiple layers, is due to dispersion forces of van der Waals. This type of adsorbed oxygen is recoverable by physical means such as evacuation and therefore may be considered a reversible process. On the other hand in chemisorption the oxygen combines to form ionic or covalent bonds on the coal surface, this process is considered irreversible, i.e. the oxygen is not recoverable at reduced pressures. Several authors have pointed out that the oxygen adsorption is the result of irreversible chemisorption. Walker et al. [56] found in their oxidation studies that the oxygen chemisorbed on the carbon could not be removed by evacuation up to 10e9 torr. Van Krevelen [57] reported that oxygen consumption rate is first order with respect to oxygen concentration, and concluded that consumption rate increases with the square root of oxygen concentration and suggests
157
that this is due to dissociation of oxygen molecules into atoms prior to chemisorption. Coal moisture and the humidity of the atmosphere in which oxidation occurs have been found to influence both the rate of oxidation and products that are formed. Tests made by Woldwezyk [58] indicate that when atmospheric humidity reaches a maximum, the greatest number of fires occurred underground. Winmill [7] noted that dried coal absorbs oxygen at a slower rate than when it contains moisture. Another assumption made by Rees [59] is that moisture could exert a catalytic effect on the coal oxidation, but he stated that such catalysis is difficult to prove experimentally. Measurements made in the laboratory with a wide range of coals has led to the conclusion that the normal heat of oxidation may cause a limited temperature rise, but ignition can only occur in a pile if it is first partially dried by exposure to dry weather and subsequently rapidly wetted. This is due to the heat of wetting of dry or partially dry coal. The effect of humidity on the rate of heat release due to oxidation of coal has been investigated by Bhattacharyya [60]. In his work it has been shown that under conditions where a coal is likely to absorb water vapour the chance of self-heating is greater. In a humid atmosphere where simultaneous sorption of water vapour and oxidation takes place, the rate of heat generation in coal due to adsorption of water vapour becomes the rate-determining factor. For a given coal the rate of heating has been found to reach a maximum within a few hours of the start of the process and to increase with the increase in the equilibrium deficiency of water in the coal. Many interesting deductions were made by Hodges, Hinsley, Bhattacharyya and Guney at Nottingham University during the 60’s on the influences of both moisture and humidity on spontaneous combustion. Hodges and Hinsley [61] showed that there were two im-
portant sources of moisture to be considered: these are, the moisture associated with the oxygen or air, and the inherent moisture in the coal. Bhattacharyya et al. 1621 indicated that in a moist atmosphere the coal is faced with any of the following three phenomena: (1) oxidation, (2) oxidation and sorption of water vapour by the coal, and (3) oxidation and description of water vapour from the coal.
CONCLUSIONS The authors have attempted to build up a comprehensive picture covering the total spectrum of the research into the seam factor affects on spontaneous combustion. This publication used in conjunction with earlier works of the authors of this paper [l] completes the theory behind the whole phenomenon of spontaneous combustion as an aggregate effect of these situations, which have been classified as: (1) seam factor, (2) geological factor, and (3) mining factor.
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Warwickshire thick coal. Part I: The application of gas analysis to the detection of heatings. Inst. Min. Eng., 71 (1926): l-32. M.C. Stopes and R.V. Wheeler, Monograph on the constitution of coal. Dept. Sci. Ind. Res., (1918): l-58. J.I. Graham and J. Hill, The oxidisable constituents of coal, Part 1. Trans. Inst. Min. Eng., 54 (3) (1917): 197-221. M.C. Stopes, On four visible ingredients in banded bituminous coal; studies in the composition of coal, I. Proc. Roy. Sot., B 90 (1919): 470. R. Lessing, The behaviour of the constituents of banded bituminous coal on coking: Studies in the composition of coal. Trans. Chem. Sot., 117 (1920): 247-256. F.S. Sinnatt and B. Moore, A method of determining the relative temperatures of spontaneous ignitions of solid fuels. Trans. Sot. Chem. Ind., 39 (1920): 72-78. The Final Report of the Committee on the Spontaneous Combustion of Coal in Mines, H.M.S.O., (1921). O.P. Hood, Factors in the spontaneous combustion of coal. Chem. Abstr., 107 (1922): 110661115. H. Briggs, Possibility of spontaneous combustion being initiated by the heat produced in crushing. Trans. Inst. Min. Eng., 64 (1922). F.S. Sinnatt and L. Slater, Propagation of a zone of
53
54
55 56 57 58
59 60
61
62
combustion in coal. Fuel and Science and Practice, (1922): 211-216. J.S. Haldane and R.H. Makgill, The spontaneous combustion of hay. Fuel in Science and Practice, 2 (1924): 380-387. W. Francis and R.V. Wheeler, The composition of coal in relation to spontaneous combustion. Proc. Amer. Gas Assoc., (1927): 1392-1398. W.N. Adams and G.J. Pitt, Examination of oxidised coal by absorption methods. Fuels, 34 (1955). P.L. Walker, R.C. Bansel and F.J. Vestola, The structure and chemistry of coal. 81 (1) (1969). D.W. Van Krevelen, Coal (2nd edn.) Elsevier, Amsterdam, 1981. P. Woldwezyk, The influence of meteorological effects on the origin of fire due to spontaneous combustion in coal mines. Bergakaolemie, 12 (1915). W. Rees, Spontaneous combustion of coal. Kohle and Eng., 23 (1926). K.K. Bhattacharyya, The role of desorption of moisture from coal in its spontaneous heating. Fuel, 51 (1972). D.J. Hodges and F.B. Hinsley, The influence of moisture on the spontaneous heating of coal. Min. Eng., (January 1964). K.K. Bhattacharyya, D.J. Hodges and F.B. Hinsley, The influence of humidity on the initial stages of the spontaneous heating of coal. Min. Eng., (February 1968).
149 in The Netherlands
SEAM FACTOR AND THE SPONTANEOUS OF COAL
HEATING
Ft. Morris Johannesburg
Consolidated
Investment
Company Limited, JCI House, 2001 (South Africa)
28 Harrison
Street, Johannesburg
and T. Atkinson Mining
Engineering
Department,
University
of Nottingham,
University
Received June 1, 1987; accepted February
Park, Nottingham
NG7 2RD (England)
16, 1988)
ABSTRACT Previously related publications by the authors have identified the spontaneous heating in underground coal mines as a combination of the seam factor, the geological factor and the mining factor. In this publication the authors pay
particular attention to the effects of the seam factor from research conducted from the end of the nineteenth century to the deductions of modern day workers.
INTRODUCTION
its organic matter has undergone during the period of formation. An increasing carbon content and with it a decreasing oxygen content are the most commonly accepted criteria of increasing rank. The higher the rank, e.g. anthracite, the slower the oxidation process, whilst lignite of low rank, oxidises so rapidly that it is often stated that it cannot be stored after mining without ignition. There are however numerous anomalies to a straight rank order. One part of a seam may be particularly liable to spontaneous combustion, a seam of higher rank may prove more troublesome than one of a lower rank or even the same seams in different mines may react differently.
The seam factor affecting the susceptibility of coal to spontaneous combustion may be defined by the following parameters [l]: rank; temperature; petrographic composition; available air; particle size; moisture; sulphur; other minerals; the effect of previous oxidation or heating; physical properties; heating due to crushing or bacteria. Rank The rank of a coal depends on the character of the original plant debris from which it was formed and the amount of change that
150
Petrographic
composition
The National Coal Board carried out a series of oxidation tests on hand picked petrographic constituents from five coals ranging from high ranking coking coal to low rank bituminuous coal [2]. The results of these tests, showed that in all cases fusain was the least reactive, and in general, durain was more reactive than vitrain. These results enabled calculations to be made of the reaction velocities of vitrinite, exinite and inertinite and showed that above 165°F exinite has a much greater oxidation rate than the other two constituents. Thus, it appears that a count of these “macerals” may be useful, together with rank in determining the susceptibility of coals to spontaneous combustion.
Temperature The absorption of oxygen is more rapid as the temperature increases. There is a pronounced temperature coefficient of oxidation, and the average rate of oxidation approximately doubles (1.4 to 2.3) for every rise of 18°F.
Available
air
Where there is a small amount of air, the rate of oxidation is very slow and there is no appreciable rise in temperature. Where there are large quantities of air passing over or through the coal, any heat produced will invariably be carried away so that the temperature does not rise and the oxidation rate remains at a low level. However, between these two limits there is a state when the air quantity is sufficient to promote oxidation but not sufficient to carry away the heat formed, so that there is an accelerating rate of oxidation until ignition occurs.
Particle
size
A solid coal face generally presents very little danger of spontaneous combustion, partly due to the small surface area and partly due to the very low permeability of solid coal to gases. It is, however, generally when coal is shattered in mining, or broken by roof pressure, or when falls and faulting occur that spontaneous combustion is likely to take place. It is the small coal that is mainly responsible for the heating. The air passes into the mass and oxidizes a little of the coal near the outer surface. This produces a slight rise in temperature, so that, as the air penetrates deeper and deeper, it becomes warmer and warmer and although part of its oxygen has been absorbed there is still enough to produce oxidation. Consequently, it is at some distance inside the mass that heating develops most rapidly. It should be noted that a flame will not necessarily make its appearance, even if a coal is red-hot, as flame is due to the combustion of gas and this requires that a moderately high proportion of oxygen be present. Once the oxidation process has gone beyond the early stages and heat is accumulating, it is only a matter of time before actual ignition takes place. The authors have noted in Bengal/Bihar, India and the Hunter Valley, Australia, high methane emissions from coal pillars prior to their heating. The pillars showed signs of crushing giving increased access for oxygen, reduced particle size and the release of adsorbed methane from pore surfaces, all providing the opportunity for increased oxidation. Moisture The effect of moisture on spontaneous heatings is uncertain. A small quantity seems to assist rather than retard the heating whilst large quantities of moisture retards the heat-
151
ing. However, as in a surface stockpile, alternative drying and wetting of the coal accelerates the heating process. Sulphur From the first publication on spontaneous combustion, R. Plott in 1686 [3], until about the middle of the nineteenth century it was assumed that sulphur in the form of pyrites was the main cause of spontaneous combustion. However, it was shown that coal even in the absence of sulphides would absorb oxygen and heat spontaneously [4-61. However, further research work modified this view and led to the present theory that pyrites plays a subsidiary role in promoting deterioration and spontaneous combustion [7-91. Other
minerals
Many other chemicals affect the rate of oxidation to some extent, either accelerating or retarding it. Alkalies can act as accelerators, and borates and calcium chloride as retardants. From the effects of previous oxidation on heating from experimental work on the effects of preheating coals in vacua, cooling, and then comparing their oxygen reaction with that of untreated coals, [lo] the following conclusions were drawn: (1) By preheating coals their liability to spontaneous combustion is greatly increased. (2) Lump coal previously almost impervious to air and thus without danger from the point of view of heating, may become, through being in the neighbourhood of a fire, a source of danger due to a large increase in its oxygen capacity. (3) Coal in a sealed area, even after a fire has become completely extinguished, may also have formed fine coal caused by a reduction in the strength following partial distillation.
These factors are important when attempting to re-open an area which had been sealed because of spontaneous combustion. In a number of instances, when attempts have been made to re-open workings that have been sealed sometimes for years, it has been found that there was no indication of fires, but immediately as ventilation was re-established, progressive re-heating occurred within a few days and sections had to be resealed. Physical
properties
A number of physical properties such as porosity, hardness, thermal conductivity and specific heat can affect the rate of oxidation of coal. Heating
due to earth
movement
Investigations showed that the heat generated in the crushing of rocks, such as in the goaf of a long wall face or the waste of a pillar extraction area may be sufficient to assist in starting the self heating process [ll]. Bacteria Two papers during this period deserve comment. The first, ref. [12], stated that spontaneous combustion could be caused by improper and preventable conditions or by a wrong system of mining, whilst the second [13], stated that a mining engineer has to work a mine as safely and as profitably as possible. He may then have to leave coal underground at the risk of spontaneous fires or to adopt methods of working that entail risks of heating. However, it is the contention of the authors of the present paper that the competent mining engineer will design the mine to minimize all risks to the safety of personnel and property and to maximize profitability.
152
THE EARLY RESEARCH NEOUS COMBUSTION
INTO
SPONTA-
It is possible at this stage to sum up the research into spontaneous combustion as follows: (i). By the end of the nineteenth century the capacity of coal to react with oxygen at ordinary temperatures and at an increasing rate a higher temperatures, always with the evolution of heat, had been established. Spontaneous combustion of coal in the mine or on the surface occurred when the coal presented a sufficient surface to the air and was sufficiently insulated thermally. The whole of the heating was in many cases very probably due to the oxidation of carbonaceous matter, but pyrite was thought to assist by the heat of its own oxidation and by breaking up the coal into smaller fragments which present a larger surface to the air. (ii). By the end of 1910, heating factors identified were: the kind of coal, in regard to its volatile matter; the purity of the coal; the presence of pyrite or other sulphur compounds; the temperature of the coal; the size of the coal; the presence of occluded gases in the coal; the presence of moisture; the accessibility of oxygen; and pressure on the coal. Kind
of coal
From the ignition temperatures given by Fayol [14], it may be seen that only those coals, such as lignites, bituminous and semicontaining large amounts of bituminous, volatile matter, are liable to ignite spontaneously, and that anthracite with its very low percentage of volatile matter is practically entirely excluded. The results of the work of Boudouard [U-18] on the coking power of coals also substantiates this view, for in his work he found that those coals richest in volatile matter, carbohydrates and cellulose were most liable to spontaneous combustion.
Purity
of coal
The work of Parr and collegues indicated that coals of exceptional purity are more apt to heat up then coals containing large amounts of extraneous matter [19-211. This is probably due to the fact that very pure coals are able to condense and absorb the oxygen of the air much faster than other coals and so cause an increase in temperature, which finally results in the chemical combination of the oxygen and hydrogen occluded in the coal. This view was confirmed by later work when it was found that those coals causing spontaneous combustion, or those coals which oxidized and increased in temperature most rapidly, were remarkably free from mineral matter and pyrite [22]. Presence
of pyrite
As to what part sulphur compounds, especially pyrite, play in the spontaneous ignition of coal, opinions differed greatly. Some believed pyrite to be the leading factor, while others believed it played no part at all, or, if so, ascribed to it a position of minor importance and believed its action to be merely a subsidiary one. The oxidizing action of the air upon pyrites was however admitted, and the notion seemed to be fairly general and well established that pyritic oxidation tends to raise the temperature of the coal. On the other hand, it was seen from the work of Fayol [14], Dennstedt and Bunz [22] and Threlfall [23], that coals containing pyrite in a quantity too insignificant to be noticed are very apt to ignite spontaneously. Others, however, believed that the only influence of the pyrite is a mechanical one, in which the oxidation of the thin films of pyrite in the coal serves merely to break up the coal. Temperature
of the coal
Most of the authors agreed that the temperature of the coal undoubtedly was one of
153
the main factors in the whole subject of spontaneous combustion, for cases of spontaneous combustion have occurred time and again were probably they never would have occurred if there had not been an initial heating in some way or other. The New South Wales Commission [24] thought that the initial temperature of the coal at the time of storage or loading was one of the great factors in the subject of spontaneous combustion. Sources of heat, seemingly insignificant, are frequently the cause of bringing the temperature of the coal up to the danger point, and so causing the ignition of coals to which, under carefully regulated conditions of storage, would not be at all dangerous. This increased temperature, whether coming from outside sources or from physical or chemical reactions within the coal, tends to accelerate the absorption of oxygen and thereby to raise the temperature of the coal. It also tends to drive out the inflammable gases occluded in the coal, and to greatly accentuate the danger of spontaneous ignition.
and recommended that they considered safe to transport on board ship only those coals passing a certain standard in their friability test.
Size of the coal
Opinions differed greatly as to what part moisture in the coal plays in its spontaneous combustion. Some believed the moisture content of the air-dry coal to be a direct index of its power to ignite spontaneously. Evidence given to the New South Wales Commission, showed that coal piles are more apt to take fire during warm weather following showers than at other times, although Fayol [14] as a result of experimental work, claimed that the influence of the weather on coal heaps had not been sufficiently marked to be observable, whilst others [25] believed the spontaneous combustion of coal to be due to the formation of ozone by the action of the sun on warm, sunny days following a rain, when the surface evaporation is especially great. If this is true, it was felt at the time that moisture plays a more important part in the phenomenon of spontaneous combustion than had previously been ascribed to it.
That fine coal is a more active absorbent of oxygen and more liable to ignite spontaneously than large coal was shown by Richter [4-61. Practically all of the later experimenters in this field concede this to be true, for the fine particles, having a greater surface area, can absorb much more oxygen than large lumps; and since this rapidity of absorption causes an increase in temperaturewhich in turn produces favourable conditions for further absorption and for chemical action between the oxygen of the air and the hydrogen of the coal-the danger of spontaneous combustion is greatly increased. Dennstedt and Bunz [22] in their work on this subject even went so far as to make a friability test on all the coals they worked with, for they conceded the danger arising from the fine coal with its greater avidity for oxygen,
Occluded
gases
in the coal
While it is now a well known fact that gases of an inflammable nature are occluded in coal, their relation to the spontaneous ignition of coal had not yet been clearly established by 1910. Whether the gases occluded in the coal are the real cause of spontaneous ignition is doubtful, but if the coal becomes heated up by oxidation or some other cause to a temperature high enough for the oxygen of the air to unite with these gases, then it is seen that the presence of these gases constitutes a source of danger. In this case, coals with large amounts of gases occluded in them would be more liable to ignite than coals containing smaller amounts of these gases. Presence
of moisture
154
That the presence of moisture materially assists the pyritic oxidation was generally conceded, although whether it caused an increase in temperature or merely a disintegration of the coal due to the formation of ferrous sulphate was a matter of dispute. Others believed that the only part that moisture played was a mechanical one, where alternate freezing and thawing broke up the coal into smaller particles and so exposed more surface to the oxygen of the air. Again, it is thought that aside from increasing the pyritic oxidation, moisture acts as a catalytic agent between the carbon and hydrogen or unsaturated bodies of the coal and the oxygen of the air. Perhaps, as Erdmann and Stolzenberg [25] have suggested, this reaction may consist of the formation of ozone which is immediately absorbed by the coal. The idea of catalytic action on the part of the moisture was substantiated to some extent by the fact that some coals containing minimum amounts of pyrite are nevertheless very liable to spontaneous ignition, and coals of this class have been known to cause fires. Accessibility
of oxygen
That the combination of oxygen with the constituents of the coal causes a rise in temperature seems to be firmly established. Which particular constituent is the cause of the rise in temperature has not, however, been shown with any great degree of certainty. The presence of humic acid in the oxidized coal leads one to believe that the oxygen combines with some of the unsaturated humus bodies, such as the polysaccharides. That this humus substance is an unsaturated body is shown by the fact that it absorbs large amounts of bromine without the evolution of hydrobromic acid. In fact, one researcher [26] went so far as to devise a practical test to determine the safety of a coal by means of this reaction with bromine.
The idea was also held that the oxygen of the air combined directly with the carbon and hydrogen of the coal and so caused an increase in temperature. If amorphous carbon (charcoal and lampblack) can be oxidized to carbon dioxide or exposure to the air by means of bacteria, as had been proved by Potter [27] the oxidation of the carbon of the coal is very probable. The presence of oxygen is therefore to be avoided and the old idea of thoroughly ventilating the coal piles by free access of air was gradually being dropped: at the time authorities deemed it advisable to keep the coal away from the air as much as possible, either by submerging it under water or by storing the coal in covered concrete bins. If ventilation is used to lower the temperature of the coal, it should be through pipes, so that the cooling air cannot come in contact with the coal at all. Pressure
on the coal
The belief that pressure on the coal was one of the leading factors in its spontaneous ignition seemed to be gaining ground at this time and because of this fact, it was advocated that coal heaps should not be any higher than 15 to 20 ft. (iii). The period from 1910-1930 was an exciting one but produced no drastic changes in the ideas of the researchers but rather a period of great debate, of point and counterpoint. The debate that the authors of this paper have termed the swinging pendulum between the pyrites and oxygen theories continued with publications by Lamplough and Hill [28], Wimnill [29], Lomax [30], Somermeier [31], Parr and Kressman [19], Drakely [32], Graham [33] and Haldane [34]. Graham in fact stated categorically that in some instances underground pyrites must be looked into as the primary cause of trouble; however he condeded that if checked, the majority of underground heatings would be
155
traced to the oxidation of the carbon constituents of coal. The period under discussion saw the introduction of chemical ratios as a means of both indicating the start of an underground heating and also as a means of indicating the state of the atmosphere behind fire stoppings. Rhead and Wheeler [35] introduced the first ratio CO,/CO as a means of detection, to be followed closely by Porter [36], who experimented on the relationships of the products of combustion and produced triangular diagrams representing the relationship of H,O, CO and CO, at different temperatures and found a constancy in the ratios of CO, and CO within a certain temperature range. The work of Winmill and Graham [37] was indeed a most important contribution to the period. Important conclusions included that as the temperature rises the coal absorbs 0, faster, and a larger quantity of 0, is absorbed. Further, that the ratio CO produced/O, absorption increased with temperature, and the ratio CO, produced/CO produced increased with time and decreased with increasing temperature. Porter and Ralson [38], determined that as the temperature rises, the ratio (CO, + CO)/H, increases, whilst there is a more or less constant ratio between CO, and CO. Further, when CO,, and H, are plotted on a ternary diagram the prints obtained at various temperatures fall approximately in a straight line Graham [39] introduced the ratio CO,/ oxygen absorbed, CO/oxygen absorbed and co/co,. Storrow and Graham in a continuation of this work introduced two methods of detecting underground heatings in detail and also discussed the ratio, oxygen deficiency/CO,. Haldane [41] however felt that no chemical analysis could compete with the ability of a miner to detect such heatings by his sense of smell. Graham and Jones [42] investigated fires which had occurred in 13 South Wales Collieries and Morgan [43]
introduced an important paper on the application of gas analysis to the detection of heatings. Finally the period saw the introduction of Coward’s Diagram and Graham’s Ratio. Whereas during this period, the previous nine reasons for the causes of spontaneous combustion were not queried, some additional reasons were forwarded. Stopes and Wheeler [44] felt that due to the complexity of the plant materials that had contributed to the formation of coal it was important to determine the character of the contribution made by these coal forming plants. Graham and Hill [45] stated that ulmins were the cause of spontaneous combustion. Stopes [46] determined the four constituents of coal as vitrain, clarain, fusain and durain and indicated that these needed to be considered separately. This work was continued by Lessing [47]. Sinnatt and Moore [48] indicated that there are relative temperatures of spontaneous ignition of solid fuels where there are zones of temperature at which spontaneous combusion appears. Also, for coals which are susceptible to spontaneous combustion, then spontaneous ignition occurs practically at the same instant the glow appeared. He concluded that: (1). the degree of fineness had a considerable influence upon the minimum temperature at which glowing would occur, and (2). the volatile matter of a coal had a considerable influence upon the liability of a fuel to spontaneously glow. In 1921, the Departmental Committee on Spontaneous Combustion [49] indicated that spontaneous combustion occurred in coals very high in moisture content. As moisture content is simply am easure of porosity it followed that the more porous a coal, the greater is the area exposed to oxidation. Hood [50] continued work on the fineness and temperature of stockpiles. Briggs [51] concluded that the crushing of coal under pressure converted the mechanical energy into heat. Sin-
156
natt and Slater [52] indicated that fusain posessed the property of propagating a zone of combustion. Haldane and Makgill [53] stated that the bacterial effect on coal increased its temperature whilst Francis [54] emphasized that coals differ in the proportion of ingredients (ulmins, cuticles, spore exines, resins and hydrocarbons) and modifications in the character of these ingredients determine the rank of the coal.
THE WORK SEARCHERS
OF
MODERN
DAY
RE-
gases, then an additional reaction must take place to increase its quantity. The mechanism suggested by Chamberlain et al. was as follows. The first step is the formation of peroxide which then decomposes to give compounds which are similar to those which have been found during other experiments. The formation and decomposition of peroxides occur mainly within the aliphatic structure of the coal and may proceed via the following reaction: R-H+O,+R’+HO,
.
R’+ 0, + R-O-O’ It is generally agreed by modern researchers that following an initial physical absorption of oxygen, a variety of more or less stable coal-oxygen complexes are formed with the release of some gaseous products. The oxidation reaction of coal at moderate temperatures is generally of two types: (1). The formation of oxy-functional groups on the surface of the coal; and (2). production of gases such as CO,, CO and H,O. The total consumption of oxygen is the sum of the oxygen consumed by these two reactions. Due to the extremely complex heterogeneous nature of coal, the many oxidation reactions of coal that occur make it difficult to examine all the reactions on an experimental level. Coal in its natural state contains varying amounts of oxy-functional groups. During the oxidation process the quantities of these groups increase. The initial chemical process is considered to be chemisorption. The oxy-functional groups include: carboxyl, hydroxyl, carbonyl, methoxyl, ester, ethers, peroxides and hydroperoxides [55]. Chamberlain, Barrass and Thirlaway [2] found that an increased temperature produced a much greater increase in carbon monoxide than any other gas. Therefore, they assumed that if carbon monoxide was being produced by a similar mechanism to the other
R-O-O’+
HO; + R-O-OH
R-O-O’+
RH + R-O-OH
+ 0, + R
The peroxide can then decompose in different ways according to its structure. Although there is universal agreement that oxygen is adsorbed on the coal surface during the oxidation, the literature presents many views of the dominant process. Physical adsorption, which may be in single or multiple layers, is due to dispersion forces of van der Waals. This type of adsorbed oxygen is recoverable by physical means such as evacuation and therefore may be considered a reversible process. On the other hand in chemisorption the oxygen combines to form ionic or covalent bonds on the coal surface, this process is considered irreversible, i.e. the oxygen is not recoverable at reduced pressures. Several authors have pointed out that the oxygen adsorption is the result of irreversible chemisorption. Walker et al. [56] found in their oxidation studies that the oxygen chemisorbed on the carbon could not be removed by evacuation up to 10e9 torr. Van Krevelen [57] reported that oxygen consumption rate is first order with respect to oxygen concentration, and concluded that consumption rate increases with the square root of oxygen concentration and suggests
157
that this is due to dissociation of oxygen molecules into atoms prior to chemisorption. Coal moisture and the humidity of the atmosphere in which oxidation occurs have been found to influence both the rate of oxidation and products that are formed. Tests made by Woldwezyk [58] indicate that when atmospheric humidity reaches a maximum, the greatest number of fires occurred underground. Winmill [7] noted that dried coal absorbs oxygen at a slower rate than when it contains moisture. Another assumption made by Rees [59] is that moisture could exert a catalytic effect on the coal oxidation, but he stated that such catalysis is difficult to prove experimentally. Measurements made in the laboratory with a wide range of coals has led to the conclusion that the normal heat of oxidation may cause a limited temperature rise, but ignition can only occur in a pile if it is first partially dried by exposure to dry weather and subsequently rapidly wetted. This is due to the heat of wetting of dry or partially dry coal. The effect of humidity on the rate of heat release due to oxidation of coal has been investigated by Bhattacharyya [60]. In his work it has been shown that under conditions where a coal is likely to absorb water vapour the chance of self-heating is greater. In a humid atmosphere where simultaneous sorption of water vapour and oxidation takes place, the rate of heat generation in coal due to adsorption of water vapour becomes the rate-determining factor. For a given coal the rate of heating has been found to reach a maximum within a few hours of the start of the process and to increase with the increase in the equilibrium deficiency of water in the coal. Many interesting deductions were made by Hodges, Hinsley, Bhattacharyya and Guney at Nottingham University during the 60’s on the influences of both moisture and humidity on spontaneous combustion. Hodges and Hinsley [61] showed that there were two im-
portant sources of moisture to be considered: these are, the moisture associated with the oxygen or air, and the inherent moisture in the coal. Bhattacharyya et al. 1621 indicated that in a moist atmosphere the coal is faced with any of the following three phenomena: (1) oxidation, (2) oxidation and sorption of water vapour by the coal, and (3) oxidation and description of water vapour from the coal.
CONCLUSIONS The authors have attempted to build up a comprehensive picture covering the total spectrum of the research into the seam factor affects on spontaneous combustion. This publication used in conjunction with earlier works of the authors of this paper [l] completes the theory behind the whole phenomenon of spontaneous combustion as an aggregate effect of these situations, which have been classified as: (1) seam factor, (2) geological factor, and (3) mining factor.
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