February, 1925
INDUSTRIAL A N D ENGINEERING CHEMISTRY Effect on Boiler Operation
The ash from stored coal has a greater tendency to fuse to stoker links and stick in air-ways of same to interfere with passage of air. As coal deteriorates the percentage of “fines” increases, the coal bed on stoker grates “packs” more, and greater resistance to air passage is encountered. There is always a carry-up of fine ash particles into boiler tubes, and since storage coal ash has a lower fusion point, the amount of slag fusing on the front banks of tubes is greatly increased. Operation is therefore affected adversely through interference with grates and increase in labor to keep heating surfaces of boilers in clean condition. When burning storage coal the method of operation of the stokers must be altered, the fuel bed being cut down in depth, and draft increased. Ordinarily it is impossible to carry as high a capacity on the boilers; therefore, one or two extra boiler units must be kept in operation. The storage coal burns unevenly, and often a boiler that apparently has a good fire may drop half its load within five minutes because the fire bed has burned in two. Boiler operation must therefore be watched very closely.
125
Spontaneous Combustion of Characteristics Shown by an Adiabatic Calorimeter By J. D. Davis and J. F. Byrne PITTSBURGH EXPERIMENT STATION, BUREAU
OF
MINES, PITTSBURGH,
PA.
KNOWLEDGE of the phenomena connected with spontaneous heating of coal is important because of its bearing on the Btorage of coal. Until the reactions accompanying spontaneous heating and the conditions favoring them are understood, it will be difficult to devise storage methods for any given coal so that the minimum loss from heating in storage may be realized. At present we know that the reactions involved are primarily oxidation reactions. Further, it has been shown fairly conclusivelythat the oxygen first combines with the coal to form an unstable solid, which gradually breaks down with increasing temperature, forming the normal gaseous oxidation products.3 Presumably it is this initial solid reaction product, sometimes characterized as Loss in Efficiency a peroxide, which, owing to its ready formation and instaIn actual operating efficiency the loss while burning storage bility, is the chief cause of spontaneous heating. In this coal is very much greater than is indicated by the heat value loss discussion it will be shown that for temperatures up to 130” G. on a B. t. u. basis. Loss in efficiency is due, not alone to loss in the rate of oxygen absorption of a bituminous coal is about ten times the rate of evolution of oxygen as gaseous products. heat value of coal, but also to necessity of carrying extra boiler It will be shown further that the absorbed oxygen is unstable, units with their radiation losses, losses from inability to keep heating surfaces clean, and most of all through necessity of and that most of it may be easily removed as water and oxides carrying higher drafts, resulting in “carry-off” of a higher of carbon by heating in an inert atmosphere at 160”C. What this compound or group of compounds which is sensitive to percentage of heat with the increased volume of stack gases. Apparently one of the surest ways to prove a coal has been oxidation is chemically, we do not know, but it is reasonable stored is through the volatile combustible. In many cases to believe that spontaneous heating is directly caused by the where volatile has been found low in samples tested, investi- amount and distribution of these substances in the coal. The rate at which coal absorbs oxygen is commonly taken as gation has shown that coal had been shipped from storage. a measure of its tendency to heat spontaneously, and considMany samples from storage piles belonging to this company have indicated volatile loss from 1to 5 per cent, depending on erable experimental work done in the past has been based on that a~sumption.~The conditions influencing spontaneous length of storage. A bulletin of the University of Illinois2 contains results of heating of coal have also been studied with its affinity for boiler tests using fresh coal and weathered coal, and gives oxygen as a basis, but nowhere has it been shown, so far as the conclusions that boiler capacity and efficiency are not affected writers know, that the rate of generation of heat is proporby weathering. These tests were run using fresh screenings tional to the rate of absorption of oxygen. I n the present from Danville, Ill., District, whereas weathered coal used paper the method used to study spontaneous heating is based consisted of nut coal and screenings from Springfield and on the rate at which heat is developed rather than on the rate Southern Illinois Districts. Coals from these districts do not of absorption of oxygen, although one typical coal waa tested weather alike, and nut coal weathers at a different rate than by both methods for the sake of comparison. screenings. Checks on boiler operat.ion when stored coal is Apparatus and Method being burned almost invariably show higher draft, lower There are many factors which are known to effect the sponcarbon dioxide readings, and higher uptake temperature than when fresh screenings from the same mine are being burned, taneous heating of coal, the most important perhaps being fineness of the coal, moisture content, air supply, and heat even though this coal has been in storage but a short time. Efficiency may not be greatly affected under ideal testing conductivity of the coal. In the method employed by the conditions, but observations covering a number of years writers these factors were either kept constant or eliminated, indicate a loss of efficiency, including loss in storage piles so that the heat development by the coal was a11 used to through the double handling, plus losses in the plant covered raise its temperature; the rate a t which the temperature rose under these conditions was therefore taken as a measure of its by their local operating conditions, of a t least 10 per cent. to heat spontaneously. All coals tested were re* Bull. 9 1 (May 23, 1917). See also Parr, “Spontaneous Combustion tendency duced to the same heness (100 mesh) as accurately as posof Coal in Storage,” p. 120, this issue. sible without undue exposure to air, all had the same previous exposure to air (except where noted), and all were previously China Chemists’ and Druggists’ Review dried at 140’ C. in a current of natural gas. The drying Trade Commissioner A. V. Smith, Shanghai, has submitted temperature was above any attained in the heating experito the Chemical Division of the Department of Commerce a ments, and the oxygen supplied during the tests was dried over copy of the initial issue of the China Chemists’ and Druggists’ sulfuric acid. Review. This periodical has been started by H. Schloten, a Ger-
man chemist of long residence in China, to assist in the development of pharmacy and of the chemical and drug trades on modern lines in China. The journal will be loaned to interested American firms upon application to the Chemical Division, Bureau of Foreign and Domestic Commerce, Washington, D. C.
A
Received October 30, 1924. Published by permission of the Director, U. S. Bureau of Mines. 8 Porter and Ralston, Bur. Mines, Tech. Paper 66 (1914). 4 Winmill and Graham, J . SOC.Chem. I n d . , 53, 1000 (1914);Trans. Inst. Min. Eng ,48,503,535 (1914-15), 61, 493,510(1915-16). 1
2
.
E
126
I,VDUSTRIAL AL4’D ElVGINEERIA-G C H E M I S T R Y
The apparatus used has already been de~cribed,~ so a brief description will suffice here. Referring t o Figure 1, 30 to 35 grams of the coal to be tested are placed in the vacuumjacketed tube, b, which is provided with a side tube, f , for admitting oxygen and a 24-element copper-constantan thermel, c, for temperature control. A stirrer, e, a heating coil, d, and the tube b are supported by a wood cover in the Dewar bottle, a, which is filled with heavy vacuum oil. The temperature of the oil bath is automatically kept equal to that of the coal by means of the thermel, c, which is connected to the reflecting
Vol. 17, No. 2
content. The lignite, for example, contains about twice as much oxygen as the Sharon and Illinois coals, and it would be expected that it would heat more rapidly. The sample tested, however, was not so fresh as the other samples, and it is possible that it had undergone some oxidation previous to the test. It would also be expected that the Pocahontas coal would behave about the same as the Kittanning coal, since both are of about the same rank. The percentages of volatile matter and oxygen shown in Figure 2 refer to the “moisture and ashfree” basis.
Figure 1-Apparatus for Adiabatic Control
galvanometer,g. A beam of light reflected by the galvanometer mirror varies the current in the selenium cell, i, sufficiently to operate relays j and k,which control the current to heating coil d. With the sensitive galvanometer used and a 24-element thermel, the temperature of the oil bath could be made to follow that of the coal to within 0.013’ C. Considering that the coal container was vacuum-jacketed, and that the oxygen supplied was preheated to the temperature of the bath, there could have been very little heat exchange between the coal and its surroundings during an experiment. In other words, any rise in temperature of the system was entirely due to spontaneous heating of the coal-adiabatic conditions were maintained. The recorder, I, served to give a continuous record of the temperature of the system. The heating results which follow were obtained in pure oxygen, and in no case was air used. This was done in order to have maximum sensitivity. Tendencyof Coals of Different Rank to Heat Spontaneously
’
Porter and Ralston,a however, found the same behavior for Pocahontas coal as compared with that of lower rank coals by a heating method based on elevation of the coal temperature above that of a surrounding bath maintained a t constant temperature. In view of these results, the reason that Pocahontas coal does heat occasionally in storage may be that it is a very friable coal and ordinarily there is a much larger proportion of fines than in other stored coals. It has been shown repeatedly that other things being equal, fine coal heats much more rapidly than coarse coal; this is to be expected, since with the fine particles the surface exposed to oxidation is e-?rmously greater than with the coarse coal Repeated attempts to obtain for peat the heating curve characteristic of high-oxygen coals failed, probably because peat has not yet developed into the rank of a true coal. The difference between the heating rates of Illinois and Sharon coals may be due to the higher sulfur content of the former, although it is questionable whether the difference is great enough to be regarded as significant. Heating curves for four coals of approximately the same rank plotted to the same scale in Figure 3 indicate the same tendency to heat spontaneously. The Freeport coal heated somewhat slower4than was expected, but this sample stood exposed to the air of the laboratory in lump form for several months before being ground. Previous oxidation may therefore account for slow heating in this case.
Experimental results obtained by the oxygen absorption method have indicated that coals of low rank-that is, of high oxygen content-are more liable to heat spontaneously than high rank coals. The curves of Figure 2, in which the time of exposure of typical coals under adiabatic conditions is plotted against the temperature rise, show in general that true coals of high oxygen content do heat more readily than coals of low oxygen content. Their relative heating tenden- Effect of Different InitialTemperatureson theHeating Rate cies, however, are not strictly proportional to their oxygen The curves in Figure 4 show the time required for Illinois coal to heat in oxygen from various temperatures-37 O , 42 O, 8 Davis and Byme, J . A m . Ceramic Soc., 7 , 809 (1924).
February, 1925
INDUSTRIAL A N D ENGINEERING CHEMISTRY
51", and 72" to 130" C. The results show clearly the enormous effect of temperature on spontaneous heating. The only other point which should be mentioned in this connection relates to the close agreement of the shape of all these curves within the range 90" to 130" C. Short-time Curves 1 and 2 are somewhat' flatter than the other two, doubtless 1
1
1
Pi
E-
& ! E
3,
TIME, HOURS
Figure 2-Heating
Rates of Coals of Different Rank
127
this result was attempted by two methods-namely, (1) the adiabatic method with apparatus (Figure 1) with more sensitive adjustment, and (2) the Bunsen ice calorimeter method. In the first case it was found impossible to induce heating a t temperatures below 35" C., and in the second case no measurable quantity of heat was developed on exposure of the coal to excess oxygen a t 0" C. for 200 hours. Clearly, the above result obtained by mathematical extrapolation is correct for this coal to within the sensitivity limits of the test apparatus now available. Other coals of this rank give adiabatic heating curves of the same shape (Figures 2 and 3), so the result should apply to them also. For the Pocahontas coal, anthracite, and peat (adiabatic curves, Figure 2) the heating is so slow at 70" C. that the temperature seems to be a straight-line function of the time. If there is curvature, the sensitivity of the apparatus is not great enough to indicate it. From the behavior of the other coals some curvature at least would be expected for the Pocahontas coal. Referring again to Equation 1 (Figure 5), it w ill be seen that by making T = b, e becomes infinity. This means that after starting the adiabatic heating a t 42" C., b hours, or 61.7 hours, will be required to heat the coal to the ignition point, where e = infinity. By extrapolating back, the time required for heating from 25.26" to 42" C. is found to be 82 hours. The total time required to heat this coal from the point where heating will start to the ignition point is therefore approximately 82 plus 62, or 144 hours.
owing to high initial sensitivity of the coal to oxygen; this effect seems to be lost in Curves 3 and 4, where oxidation has taken place for some time at low temperatures. Amount of Heat Generated, during Spontaneous Heating of Coal It will be observed on inspection of Figures 2 and 3 that for
bituminous coals the curves all have the same general shape, indicating the possible existence of a spontaneous heating law, expressible mathematically, which perhaps with different constants For different coals would be applicable generally. It was realisled a t the outset that care should be exercised in applying mathematics to data representing the behavior of a substance so heterogeneous as coal; nevertheless, it was worth while to find an equation, empirical or otherwise, which would express the time-temperature relationship, since, knowing the equation, calculation of the spontaneous reaction heat was readily possible. The heat capacity of fine coal was known, having beten determined previously in the Bureau of Mines fuels laboratory.6 From the equation, the first derivative of the temperature with respect to time multiplied by the heat capacity of the coal per gram gave the heat developed per gram of coal per unit of time by the oxidation reactions. Pittsburgh coal was chosen for development of the equation and for the calculation, since the time-temperature relation was more accurately known for that coal. Figure 5 shows the curve, the equation which fits it, and the values of the constants used. The significance of the asymtote equations, e = 25.26 and 1' = 61, is that they define the limits within which spontaneous heating takes place. I n the first case, by making T = - infinity, takes the indeterminate
:&:
infinity
form infinity' -- which, however, can be evaluated by the wellknown method involving differentiation of both numerator and denominator. So evaluated, the expression is equal to -1. Whence e = a constant, = k or 25.26" C. In other words, the temperature a t this point does not vary-there is no spontaneous heating. Experimental confirmation of 6 Davis and Byme, Carnegie I n s t . Tech and Bur Mines, Bull 8 , CoalMining Investigations (1922)
Figure 3-Heating
TIME, HOURS Rates of Coals of Similar Rank
Table I shows the method used to calculate the heat of reaction of oxygen with the coal based on the heating rate equation given above, and also a method of calculating the heat dissipated through a coal covering 5 feet thick and inactive. The maximum amount of heat that could be generated by a cubic foot of reacting fine coal under adiabatic conditions is compared with what would be conducted through a covering of 5 feet of inactive coal. The figure used for conductivity of the coal was found experimentally6 by the method based on flow of heat in a sphere; for this reason a spherical covering is assumed here.
The curves in Figure 6 show the relation between the amount of heat generated and that which would be dissipated by conduction, the condition being as assumed above. It is realized, of course, that, in view of the assumptions necessary
-
0 temperature,
O
T: time. hours
g ;;'
"C.
-
0.3608
hour
b
p
44 46 8 12 53.74 49.74 2888 2474 22 24
b - T
K ( a -t b ) ?d (b T)* dT
+ b ) a -103; (m,;
K(a
I
dT
C.
small amount of heat required (roughly calculated) to raise the air required to the temperature of the coal, the critical temperature is shifted from 127" to about 137O C . Such considerations as these, though not based on rigid mathemati-
Table I-Heat
e - K ( a + T);?d
48
16
45.74 2092 26
0.4213
Transfer Calculations Heat dissipated 4 sK(@i =m -25.26; Cu. ft. perhaur ' 1% 55 59 68 76 84 24 28 32 63 36 40 44 37.74 33.74 29.74 26.74 21.74 17.74 1424 1138 884 663 473 315 33 37 41 46 54 62 p
0.4982
- Wrm; -n
61.74; K
0.7319
0.9158
1.179
~
103 48 13.74 189 81
o.1541 137 52 9.74 94.9 116
160 54 7.74 59.9 138
1.572
2.203
3.309
5.514
10.98
17.36
Calories per gram per hour
K (a -I-b) 0.27
cal. gram hour Calories per cc. (factor 0.523) (b
Vol. 17, No. 2
INDUSTRIAL A N D ENGINEERING CHEMISTRY
128
-
- T)*
0.097
0.113
0.134
0.197
0,247
0.318
0.425
0.595
0.893
1.49
2.96
4.7
0.051
0.059
0.070
0.103
0.129
0.166
0.222
0.311
0.467
0.780
1.65
2.46
1. u. Per cu. ft.
Heat generated-B.
pev hour (" C.)
B. t. u. per cu. ft. (" C . ) hour 6.73 6.63 7.68 11.57 14.50 18.7 factor 28320 252 Heat dissipated per h y r (el - el) 1.376 B. t. U. 30.2 33.0 35.8 45.4 50.9 66.4 a Assumed that 1 cu. ft. of coal is heating uniformly in a sphere of coal of 6 f t . radius.
-
to the derivation of these results, their practical value becomes largely problematical. In practical storage, however, a pocket of fine coal might well be covered with relatively coarse coal, which for this low-temperature range might be considered as practically unacted upon by oxygen. The tendency of the coal pile to heat from various init,ial temperatures is then indicated approximately by the curves in Figure 6. It will be observed that the heat dissipated is practically a straightline function of the temperature, whereas the heat generated is an increasing function of the temperature, but that the angle of their intersection is very acute. This means that a very slight variation in the rate a t which heat is generated, or in the rate a t which it is dissipated (thickness of covering), will have a large effect on the temperature limit above which heat will accumulate in the coal. For example, considering the problem from the standpoint of conduction alone: heat would accumulate in the coal above 127" C.; if we add the
1
0
1
1
1
1
10
1
1
1
20
1
1
1
1
30
Figure 4-Influence
1
1
~
40
~
1
1
~
5o
~
~
1
1
24.9
34.9
52.6
87.6
174
276
63.3
74.3
85.3
111.4
158
190
cal treatment, and being of restricted application as mentioned above, do give a rough quantitative indication of the behavior of coal undergoing spontaneous heating. They help to explain the erratic behavior of coal in storage-for example, why a given coal will not heat under one set of storage conditions in one locality and will heat in another locality under conditions not preceptibly different from those prevailing in the first case. The conclusion is that a very slight variation in storage conditions may determine whether heat will accumulate in a coal pile. 'In other words, storage conditions may well have a more pronounced effect on heating in storage than the inherent heating tendency of the coal itself. Reaction Heat and Rate of Oxygen Absorption Rate of oxygen absorption and rate of formation of oxidation products were determined for the same sample of Pitts-
1
1
1
60 70 TIME, HOURS
1
1
1
1
80
1
1
1
1
1
1
