The Spontaneous Combustion of Coal. - Industrial & Engineering


Ind. Eng. Chem. , 1911, 3 (3), pp 151–158. DOI: 10.1021/ie50027a006. Publication Date: March 1911. Note: In lieu of an abstract, this is the article...
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P A R R A S D KRESSMALV OAT SPOAYTT,4AYEOYS COJIBGSTION OF COAL. object out of metal, there are only three metals which you can use, which would be cheaper than aluminum -iron, lead, and zinc. I n conclusion, I regard the bringing of aluminuniinto the rank of the cheaper metals, as one of the great metallurgical achievements of the I 9th century, and I think the historian of the future will probably class the industrial manufacture of aluminum alongside the invention of Bessemer steel. The man who takes a rare metal, and makes out of i t a common metal, a n d brings i t into every-day use, has made the entire human race his debtor. of the societies invited to sugT H E C H A I R M A NOne : gest names for the candidates for this medal is the American Electrochemical Society, and are fortunate in having with us to-night the President, Dr. William H. Walker. D R . ITTALKER: 1 have enjoyed taking part in these proceedings, and on behalf of the members of the American Electrochemical Society, I want t o endorse the action of the committee in presenting this medal t o M r . Hall. The history of this invention sounds like a story from Fairyland. On behalf of our society, I congratulate Mr. Hall, and wish for him many, many long years of life in which t o enjoy the honor and distinction. THE CHAIR MA^. : Dr. Baskerville representing the American Chemical Society, New I’ork Section. DR. BASKERVILLE: Gentlemevt: The American Chemical Society takes great pleasure when any fraction of i t is honored; consequently the members have been looking forward to this honor which he so well deserves coming to Mr. Hall. I think the committee has done itself honor in giving the medal to Mr. Hall for his great discoveries. T H E CHAIRMA;\.: Dr. Takiama, Director of the Japanese Agricultural Department, is in this country studying technical chemistry in its commercial applications, and is our guest this evening. DR. TAKIAMA:1 wish to congratulate, in the name of my countrymen, the great discoverer, Mr. Hall. THECHAIRMAK: Kow, gentlemen, the meeting will be adjourned in order t h a t you may have a n opportunity t o extend your personal congratulations t o ______ Mr. Hall.

ORIGINAL PAPERS.



THE SPONTANEOUS COMBUSTION OF COAL. B y S W PARRA N D F \V KRESSMAN

Any examination into the causes underlying the spontaneous combustion of coal resolves itself, naturally, into a study of the various forms of oxidation to which such material is subject. It involves the determination not only of the conditions which permit of such action, but also those which promote or accelerate it, as well as the interrelation of the various oxidation activites upon each other. Studies have been carried on for some time in the Department of Chemistry, in cooperation with the University of Illinois Engineering Experiment Station, which have resulted on the whole in a considerable amount of data bearing upon the topic. For example, in the

1.51

studies connected with the weathering of coal, there was found to be a drop in fuel values which occurred with marked uniformity in the first few days after the breaking out of the coal from the mine. An illustration of this fact is shown in the accompanying chart, which is typical of all of the samples in t h a t a rapid drop in values is indicated for the first ten. days or two weeks of the experiment. These studies which were published as Bulletin 38 of the University of Illinois Engineering Experiment Station, while they afforded a partial explanation for this drop in values in the fact of a loss of hydrocarbons, mainly methane, gave evidence also of a n additional factor in the avidity of the freshly-mined coal for oxygen. This phase of the matter was further taken up by Perry Barker, and the results published in Bulletin 32, entitled “The Occluded Gases in Coal.” These experiments afforded abundant confirmation of the fact t h a t freshly-mined coal very rapidly absorbs oxygen and in a manner which does not result in the formation of carbon dioxide, but by direct incorporation into the molecular structure to satisfy certain of the unsaturated compounds. Such absorption must, therefore, be looked upon as a type of oxidation of the coal. A table of results is here given which is typical of the many obtained in this study in which there is evidence of the fact t h a t the freshly-mined coal, especially in the form of drillings taken a t the face of the vein and sealed in a glass vessel, will rob the enclosed air of its oxygen even to a fraction of a per cent. T A B L E 1 -.%VlDlTY O F O L D .4ND FRESH Coal. FOR O X Y G E N . I . Atmosphere surrounding old face sample in contact with large volume of air for 2 years. I1 Old face sample sealed in fresh air 2 days. then evacuated. 111. Drillings, sealed 14 days. IT. Drillings, in vacuum 12 days. 1.. Drillings. second air in contact with coal 7 days. I. 11. ’ 111. IT, 1-. \Veight of coal. grains.. . . . . . 146 205 5 220 220 220 33 5 192.1 48.8 130.4 Volume of gas. cc.. . . . . . . . . . . 843 Per cent. by volume. C01.. ..................... 0 25 7.80 3.86 7.58 1.63 0 .. . . . . . . . . . . . . . . . . . . . . . . . 0.25 15.50 1.04 0.61 0.37 CH,.. . . . . . . . . . . . . . . . . . . . . 2.17 0 21.79 86.37 14.14 5-. . . . . . . . . . . . . . . . . . . . 97.33 76.70 73.31 5.44 83.86

I n this table all samples were from the same mine, No. I and No. I1 were partially air-dried face samples,

which had been sealed in Putnam jars for some two years. From No. I, the surrounding air in the container was collected by displacement and analyzed. No. I1 was left in one of the sealed fractionating flasks for two days. A t the end of t h a t time both the surrounding air and some of the enclosed gases were removed b y means of the air pump. No. I11 is a flask of fresh drillings from which the surrounding air and occluded gases were removed by vacuum, as above. No. IV is the analysis of the further gas given off after the surrounding air had been removed and the flask had stood in a vacuum for twelve days. No. V is the analysis of the air t h a t had been readmitted to the evacuated flask and left in contact with the coal for seven days. This is a type of many similar results obtained in this work which give evidence not only of the marked

T H E J O U R N A L OF I N D U S T R I A L A N D Eh’GIATEERlKG C H E M I S T R Y .

152

avidity of freshly-mined coal for oxygen, but also

of the fact t h a t the product of oxidation is not carbon SANGAMON COUNTY, ILLINOIS.

NUT COAL 15000

;;i

8

I

B s

c

B

E k

a d 01

14000

1

2

3

4

5

9

7

8

9

Time of Storage-Months.

Fig. 1.

dioxide, or a t most to only a limited extent. These results were substantially the same also as those obtained b y Chamberlainx in a similar line of research. A third study in which there was obtained much incidental data on oxidation was carried on b y Mr. C. K. Francis and published under the title of “The Modification of Illinois Coal by Low Temperature Distillation.” This work was published as Bulletin 24 of the University of Illinois Engineering Experiment Station. I n these studies there was obtained much interesting data upon the oxidation of coals a t temperatures above m o o . I n order t o understand the full import of t h a t study, it is necessary t o reproduce the apparatus in which these experiments were carried out. The large globe F is arranged with a n inlet and a n outlet for the circulation of a gas, while two thermometers, T and T‘, are inserted, one for indicating the change of the temperature of coal contained within the capsule C, while the other 1

United States Geological Survey, Bulktilt No. 383.

Mar.,

1911

thermometer, T, is inserted for the purpose of giving the temperature of the surrounding gas. Now upon the application of heat externally t o the flask F, and taking a log of the temperatures upon thermometers, T and T’. a t the same time continuing the circulation of the gas through ’the chamber, an indication may be had of the effect of chemical activity upon the temperature of the coal mass. A train of wash bottles is introduced t o purify the air from all traces of carbon dioxide, so that a test may be obtained of the first appearance of carbon dioxide in the discharged gases b y means of barium hydroxide solution. A few of the charts are here appended in which is given a log of the thermometer readings. I t is seen that there is a first appearance of carbon dioxide in all cases a t a point slightly above 1 2 0 O C. This activity on the part of the coal does not seem t o reach a point where its heat exceeds that of the surrounding air until a somewhat higher temperature is reached, 140’ to 160’ C. At this higher temperature two phenomena are observed: a much more evident appearance of carbon dioxide in the discharged gases and a rise in temperature on the part of the coal which carries the thermometer readings ahead of those for the surrounding atmosphere in the flask. I n all the charts an additional item of interest is t o be noted; namely, that a t a certain point the rise in temperature 10 11 12 becomes very rapid. I n Fig. 3 , with Pittsburg gas coal in an atmosphere of oxygen, this rapid rise begins a t 175’ and quickly reaches the point of combustion, which is made evident by the appearance of fire.

OXIDATION OF COAL AND

TEMPERATURE

MeansuaaMeNrs

Fig 2

I n Fig. 4, this rapid rise begins a t 240°, the coal being an anthracite. In Fig 5 , the coal is of the Illinois type and in

PARR AAYD K R E S S M A S O K SPOXTA1VEOL.S C O M B U S T I O K OF COAL.

153

a n atmosphere of air. Here the rapid rise is more of temperature, the first oven being carried a t 40°, or less in evidence after 210’ has been passed. In the second at 6 0 ° , the third a t S o o and the fourth a t this latter chart a further observation is made a t a approximately I I 5 ’. Aside from external temperatemperature of 2 I O O where the external source of .heat ture, the other variables which were introduced were was withdrawn, whereupon the activity within the those of size of particles, the content of sulphur and coal ceased and there was an indication that the tem- the effect of moisture. In the matter of size three perature would fall to that of the surrounding gas. divisions were made use of. The first or 0 size was However, upon a reapplication of the heat, the oxidiz- pulverized by a disc machine to such an extent that ing condition continued until a point was reached a t all of the coal would pass through an 80-mesh sieve. Other sizes were prepared by crushing the coarse 28 j O where the removal of t h e external source of heat was without effect in so f a r as a cessation of oxidation coal in a jaw crusher and passing the material through was concerned. We have evidence, therefore, that a revolving screen. The second division or size oxidation of carbon with formation of carbon dioxide o - ~ / g l included all the material passing the I,”‘’ holes. begins a t about 125-135O, and that an PITTSBURGGAS COAL- PONDERED autogenous stage is reached a t about 280 O. ATMOSPHERE OF O X Y G E N Further experiments were next taken 240 up as hereinafter described‘ with a view to a more definite determination, not only of the fundamental causes of oxidation, but the relative importance of the 200 various phases involved, including especially the influence of iron pyrites which numerous foreign investigators have pronounced of little or no concern in the 160 matter.3 The apparatus is illustrated in Fig. 6 and is explained as follows: A large box with dimensions 3’ X 3’ X 3’ had within it a second box of smaller dimensions, I20 approximately 2 1 / ~x~ 2 1 / ~ ‘X 2 1 / ~ ’ , thus affording about 4” of space upon all sides. This space was filled with mineral wool as an insulating medium. Within the 80 inner box was then placed an earthenware jar capable of holding from 35-40 pounds of coal and having perforaA APPEnRnNCE O F FIRE tions in the bottom to facilitate the cir40 + A P P E n R A N C E O F co, culation of air. Across one corner of TEMPtRnTURE O F C O n L the inner chamber a board 7’’ wide was fastened in such a manner as to enclose TCMPLRnTURC O F G n s a triangular space about 5” X 5” X 7 “ , 0 and extending from near the bottom 0 0 16 e4 32 40 to near the top of the chamber. A chimney was thus formed for both heatTIME -MINUTES ing and circulating the air. I t was lined Fig 3 with asbestos and fitted with a number of electric lights. These were operated by a relay con- The third division or size 1 / 8 - ~ / 4 was made up of such trolled by a mercury column which would cut the coal as would pass over the z/~’’and through the lights out a t a given temperature. I n this manner perforations. Coarser sizes were used in a few of the the air surrounding the coal mass was maintained a t a preliminary experiments, but the results were not fixed temperature. Thermometers were inserted, one essentially different from the third division already in the air space about the jar and the other penetrating described and they were consequently dropped out the coal mass. An indication of temperatures from of the series. In order to indicate a t a glance the reboth thermometers was thus obtained, the readings sults obtained the charting of a few of the logs is being taken twice daily for three days. Four such presented herewith as follows: ovens were arranged in order to introduce the variable In Fig. 7 there is shown the activity of oxidation l To be printed in full in Bulletin, No. 47, of the University of Illinois for each size and also as influenced by the variations Engineering Experiment Station. ’ Richter, Dzngler’s Poly. J . , 190, 3 9 8 ; 195, 3 1 3 , 449; 193, 54. 264. in temperature, the greatest rise over the oven temLewes, Journ. Gas Lighting, 5 5 , 645; also 1906, 33. Fayol, Bulletin de perature being shown by the finer coal and in the la Socie‘te‘ de L’lndustrie ,WMinerale, 8 , [Z] P a r t 3, 1879. Report of the First higher oven. This order of variation is consistent New South W a l e s Commission, inquiring into the causes of the firing of throughout the series where the two variations are coal cargoes, etc.

T H E J O U R - V A L OF I S D G S T R I A L -.ZA\-D E - V G I l Y E E R I S G C H E - I I I S T R Y .

154

Mar.,

1911

those of size and temperature. In the course of the first experiments the exhaustion of the oxygen of the chamber was made evident so t h a t a method of changing the air was introduced, thereby securing uniform conditions as t o the supply of oxygen. The next variable which was introduced was that of moisture, wherein the sample of coal was thoroughly saturated with all the water i t could hold, being simply drained free from the excess of water. The effect of this variable upon both the size and temperature

content, wherein the conditions were otherwise the same. In the next test, Fig. I O , the variable of moisture was introduced and a consistently higher indication of temperatures was shown throughout. I t is interesting t o note that the rise in temperature begins soon after 80' has been reached instead of waiting until a total volatilization of the water a t the boiling point had been attained. Here in the o and o-I// sizes the condition of autogenous oxidation was reached a n d the coal continued its activity after being ANTHRACITE P O W D E R E D removed from the last oven until reduced to ash. A thin zone of unburned coal, howATMOSPHERE OF OXYGEN ever, remained around the sides and over the top of the jar where the cooling effect had brought the material down below the ignition temperature. Identical results are obtained in the next two succeeding tests, Figs. 11 and 12, where the amount of pyrites was brought u p t o j per cent., showing a thoroughly consistent behavior throughout in regard to the four fundamental items; namely, external temperature, fineness of division, sulphur content and moisture. In the last test, with the introduction of water, sizes o and o - ' / ~ ' ' (see Fig. 1 1 ) continued their oxidation activity after removal from the oven until all of the interior was reduced to ash. In connection with the oxidation of iron pyrites, an extended examination of t h e residual product was made t o determine the actual amount of oxidation which had taken place in connection with the iron sulphide. In all cases the total amount of oxidation of this material in the period of time, 15 days, in which each sample was C O A L No. 542-1 under observation very closely approximated 2 0 per cent. of the total sulphur present. A APPEARGNCE O F F I R E The amount of heat generated by transAPPEARANCE OF C 0 2 ferring this amount of FeS, to FeS, is calcu---TlMPCRL\TURE OF COnL lated as follows:

-

340

300 W

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u

260

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220

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a ts

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140

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2FeSz+l10 = Fez03+4S0z+3i3K1 ( 1 2Fe 3 0 = Fez03+ f198K ( 2

+

24

32

40

2FeS2-2Fe

4S02+1i5K (1) ( 2 ) = (2a)' 4S02 + 4 0 +4HzO =4HzSO4 +256K (3 )

TIME -MINUTES

ZFeS2+2Fe+ 1 2 0 + 4 H : 0

Fig 4

conditions is shown in Fig. 8, wherein a consistent rise slightly above the temperature increase as indicated in Fig. 7 is shown throughout. I n Fig. 9 there is introduced the variable of iron pyrites. The amount of iron pyrites, FeS,, in the first two series was 1.65 per cent, I n the second series this was increased t o 3 per cent. b y the addition of sufficient material ground t o pass a zo-mesh sieve t o bring the percentage up t o the equivalent of 3 per cent. pure FeS,. The first test with this modification was carried through upon the dry samples with a very vident rise in temperature over the lower sulphur

+ 80

) )

=

= 4HzSOa+431K

(2a)+(3) = (3a) (4 ).

+ 2HzSO4 = 2FeSOa + 2Hz + 94K (3a) + (4) = = 2FeS04+2H2+S2SK 2Fe

2FeS2+ 120+4Hz0-2H1S0n 2 0 ZHz

+

-

= 2HsO

2FeS2+140+4H20-2H9S04 = ZFeSOa or 2FeSz+iO2+ZHlO = 2FeSOa+2H?SO4

+642K

(4a) (5

+11iK (4a)+(S) =

)

(Sa)

+642K

It will thus be seen that the total volume of heat discharged would equal 6 4 2 Calories per gram molecule ( 2 mols.). N o account is made in this calculation of the heat of solution of the sulphuric acid nor of t h e hydration of the ferrous sulphate. If we assume the weight of a jar of coal as being 40 pounds (18.12 1

"K" is used t o represent the large Calorie.

P A R R A S D K R E S S l f A S 0-Y SPOIYT.AA\-EO L-S COAIIIBL-STIOA- OF COAL, kilograms), and if the coal contains 5 per cent. of pyrite, then i t will contain 2 pounds (906 grams) of pyrite. The latter is 19.3 per cent. oxidized, then

coal. If we assume the specific heat of the coal used to be 0.35,' then the heat generated b y the oxidation of the pyrite would be sufficient to raise the tempera-

B~TUMINOUSCORL-POWDERED ATMOSPHERE

OF

155

ture of the coal

AIR

413

18.12

._

X 0.35

or

74.5'

which it is seen is sufficient in amount to account for the larger part of the increase as indicated by the curves. This fact has a very direct bearing upon the conclusions u 320 arrived a t and must be considered directly contradictory to the results obtained by those foreign investigators who have de280 clared the influence of sulphur to be inoperative or too small in amount to bear any part in the matter of spontaneous E40 combustion. A very considerable number of experin ments were carried out intended t o deter200 mine the effect of various possible detera rents, such as the saturation of the coal [IT with a strong salt brine, solutions of ferrous Irl Q 160 sulphate, sodium bicarbonate, etc. A satuI rated solution of lime water and a I O per cent. solution of NaHCO, were about IEO equally effective in preventing a rise of 3. more than 45' above the oven temperature. Under corresponding conditions, i. e . , 80 with 5 per cent. pyrites in size 0-1,'~'' in the 115' oven, the increase over the oven t temperature was over IOO', making a total A0 ERRANCE O F of 220°, which was well up to the point of PERRTURE O F autogenous oxidation. While the coal as treated with the alkaline solutions, under 0 like conditions, only reached a temperature 0 8 16 24 52 40 46 56 of 165' and still below the autogenous stage, there does not seem to have been a sufficient check to chemical action to warTIME-MINUTES rant the conclusion that such treatment Fig 5 would be of practical value. I t is, how19.3 Per cent. of 906 grams will enter into the reaction! ever, suggestive and warrants further experiment along or 177 grams. The molecular weight of pyrite is 1 2 0 , t h a t line, a n d two molecules entering into the reaction would

360

F: $ 5 $

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1 7 7 X 642 K or 473 K as the 240 heat liberated b y the oxidized pyrite in 40 pounds of

be 240, so we will have

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40 60 OVEN T E M P E R A T U R E Fig Fig

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7

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156

T H E J O U R N A L OF Ii\-DUSTRIAL

Other experiments looking to remedial measures were tried, sych as a seasoning or artificial oxidation

4,VD ENGINEERII\-G

CHEJUSTRY.

temperature approaching the point of autogenous action, t h a t is, to somewhere near 200' C. They were then cooled down t o ordinary temperature and

O V E N TEMPERATURE

Fig

40

60

80

/OO

/20

Mar., 191 I

DEQ

C

11

returned t o their respective ovens which were maintained for three days a t the same temperature as before. The increase for the first heating and that for

l C W P € q A T U R E D E G C.

Fig 8

80

b y a process of preheating. Three sizes were used, A being a mixture of coal passing a I/,'' screen, B 60

40

20

0

40 80 OVEN TEmPERATURE D E C . Fig

120 C.

12

the second are joined b y dotted lines in Fig. 13, which shows a t a glance the marked difference in oxidation activities in the treated coal.

being the usual 0-1// size and C being coal ground t o a n impalpable powder in the ball mill. Each size was placed in a n oven and the mass brought t o a

OWEM TEMPERATURL 0 E G . c .

Fig. 10.

Fig. 13.

P,1RR izAVD KRES.5-1IA.V 0.Y SPONTAiYEOL-.5 C O X B C S T I O + Y OF C O A L . coiicLusIo~s. From this brief rCsume of oxidation conditions, the conclusions arrived a t are as follows : I . The oxidation of coal is continuous over a wide range of time and conditions, and begins with the freshly-mined coal a t ordinary temperatures. A number of oxidation processes are involved which are more or less distinct in character, some being relatively slow and moderate in form, while others are rapid and vigorous in their action. 2 . I n general, we may say t h a t for a given coal a point exists as indicated b y the temperature, below which oxidation is not ultimately destructive and its continuance is dependent upon certain accessory conditions which, if withdrawn, the oxidation ceases. On the other hand, above this critical point, which is best indicated b y temperatures, oxidation is ultimately destructive and is characterized by the fact that it does not depend for its continuance upon external conditions, but is self-propelling or autogenous. 3. The point of autogenous oxidation, while varying for different conditions, may be indicated b y temperatures of the mass ranging from 200-275' C., depending t o a great extent upon the fineness of division. The phenomenon of fire or actual kindling does not occur until a much higher temperature is reached, usually beyond 350' C. 4. The temperature a t which autogenous oxidation begins is the sum of numerous temperature components, each one of which, either because of its own contribution t o the total heat quantity or because of its function as a booster for chemical activities, must be looked upon as a dangerous factor tending directly to the ultimate result of active combustion throughout the mass. An enumeration of the more important elements which contribute towards this end are the following : A . Exferizal Sources o j Heat.-Oxidation, especially of the lower or moderate form, is greatly accelerated and in certain phases directly dependent upon a n increase of temperature. What may be termed external or physical sources of heat, and thus presumably avoidable, are suggested b y the following: ( I ) Contact of the mass with steam pipes, hot walls or floors under which are placed heat conduits of any sort. ( 2 ) The heat of impact or pressure due to the method of unloading or t o the depth of piling. (3) Climatic or seasonal temperature a t the time of storage. ( 4 ) The direct absorption of heat from the sun or from reflecting surfaces. B. Fineness of Division.-Coal in a fine state of division presents a very much larger surface and brings a much larger quantity of reacting substances in contact with oxygen than when in solid masses. Under these conditions, with a condensation or accumulation of relatively large amounts of oxygen immediately surrounding or in contact with the particles of carbonaceous matter, the circumstances are exceedingly favorable for rapid oxidation upon the arrival of the mass to a suitable temperature.

I57

But, more especially does this fineness of division facilitate the initial form of oxidation described under C below. C. Easily Oxidizable Compouuzds.-A first or initial stage of oxidation exists in bituminous coals which does not result in the formation of carbon dioxide. There are present in coals of this type unsaturated compounds which have a marked avidity for oxygen a t ordinary temperatures, the products being humic acid or other fixed constituents of the coal texture. Coals vary widely in this matter and it has been proposed by some to regard this property as an index of the liability to spontaneous combustion. I t is, however, very largely dependent upon the freshness of the coal and upon the fineness of division (see under B above), and should be looked upon as a contributing factor, though in coals of the Illinois type a t least, with their high per cent. of sulphur, this action should doubtless be considered second in importance to that of iron pyrites. D . Iron Pyrites.-The presence of sulphur in the form of iron pyrites is a positive source of heat due t o the reaction between sulphur and oxygen. This may be conveniently referred to as the second stage in the process of oxidation. Here again rapidity of oxidation is directly dependent upon fineness of division. Since coals of the midcontinental field especially have a much higher earthy or ash content in the fine material, and since iron pyrites is a large component of this substance, i t follows that the presence of dust or duff in all coals of the Illinois type is a positive source of danger. Since coals of the Illinois or midcontinental field have in the large number of cases iron pyrites averaging over j per cent. or as sulphur above zl/* per cent., the heat increment from the oxidation of only of this material is sufficient to raise the temperature of the mass approximately 7 0 ° , assuming that there is no loss by radiation. Under usual conditions and especially considering the greatly accelerated rate of chemical activity accompanying a rise of temperature, this oxidation may proceed with such rapidity that the heating up of the mass will be but little affected by loss of heat due to radiation, except in relatively shallow piles. Coals of low sulphur content or such as do not have sulphur greatly in excess of, say, I I / ~per cent. are popularly supposed to be immune from heating, but no method of selection or hand-picking a t the mine can eliminate all of the iron pyrites. Lumps of coal, to all outward appearance of good texture, may have nodules or bands of iron pyrites. These become centers of activity and with the addition of moisture such coal will have many detached spots where heating begins. If fine coal is mixed in with the coarse, the difficulty is accentuated. Doubtless a complete separation of fine and lump material in such cases would lessen the danger. E. Moisture.-Moisture, while essential to pyritic oxidation, is given separate mention because its importance is apt t o be underestimated. Any coal with pyritic conditions as above mentioned will be facilitated in that action b y moisture. It is t o be noted in this

\

158,

T H E J O C R S A L OF IiYDL;STRIAL AA;D EA-GIKEERIKG C H E M I S T R Y .

connection t h a t the normal water content or vein moisture of coals in this central region is rarely below I O per cent. and ranges usually from 12-1 j per cent. The presence of such water must be borne in mind in considering the likelihood of chemical activity on the part of the pyrites present. F . The Oxidation of Carbon and Hydrogen.-A third stage of oxidation of the carbonaceous material exists b y reason of the property of certain of the hydrocarbon compounds of coal to oxidize with the formation of CO, and H,O a t temperatures in excess of 120-140'. Though this type of oxidation does not take place appreciably a t ordinary temperatures, i t must be looked upon as a n exceedingly dangerous stage in the process of oxidation, owing t o the very much higher quantity of heat which is discharged b y the oxidation of carbon and hydrogen; so that the temperature of autogenous action, though ordinarily occurring a t a higher point b y 100' or more, may be quickly attained as a result of this form of oxidation. Any initial heat increments, therefore, which threaten to bring the chemical activities along t o the point where the oxidation processes invade the carbonaceous material in this manner must be looked upon as dangerous. For example, any of the initial or contributory processes which result in raising the temperature of the mass 50° above the ordinary temperature would; in all probability, have enough material of the sort involved in, such action t o continue the activity until another 50' had been added, which would thereby attain t o the condition wherein this third stage of oxidation would begin. G. The fourth stage of oxidation may be indicated as occurring a t temperatures above 200-275' and differs from the previous stages in t h a t the action is autogenous and not dependent upon other sources of heat to keep up the reacting temperature. Activity in this stage is further accelerated b y the fact that above 300' the decomposition of the coal begins, which is exothermic in character, thereby contributing somewhat to a further increase in temperature. The ignition temperature is reached a t a point still further along, usually in excess of 300-400' C. The above formulation of the various stages and types of oxidation clearly indicate the principles which must be observed in any attempt a t the prevention of spontaneous combustion. The following enumeration, therefore, of preventive or precautionary measures is to be considered as suggestive rather than complete in character. First, the avoidance of external sources of heat which may in any way contribute toward increasing the temperature of the mass is a first and prime essential. Second, there must be an elimination of coal dust or finely-divided material. This will reduce to a minimum the initial oxidation processes of both the carbonaceous matter and the iron pyrites. These lower forms of oxidation are t o be looked upon as boosters, without which it would be impossible for the more active and destructive activities to become operative.

Mar.,

1911

Third, dryness in storage and a continuation of the dry state, together with a n absence of finely-divided material, would practically eliminate the oxidation of the iron pyrites. The drenching down with water of heating piles, where the sulphur content is high and uniformly distributed, accentuates the difficulty. Where pyritic activity is localized in spots or is so small in amount as to reach a possible exhaustion, the drenching with water may check the heating or prolong the action so that oxidation of the carbonaceous matter does not get under way t o a serious extent. I n such cases, however, there is no ultimate safety except in the removal of the heated zones. Fourth, artificial treatment with specific chemicals or solutions intended to act as deterrents does not offer great encouragement, though some results seem to warrant further trial in this direction. Fifth, by means of a preliminary heating the low or initial stages of oxidation are effected. These sources of contributory heat being removed the forms of destructive oxidation are without the essential of a high starting temperature and are, therefore, inoperative. Whether such preliminary treatment is within the realm of practical or industrial possibility could not, of course, be determined within the scope of these experiments. Sixth, the submerging of coal, i t is very evident, will eliminate all of the elements which contribute towards the initial temperatures. As to its industrial practicability, like the conditions under 5 above, it can best be determined by actual experience. Other processes may be suggested b y the formulation of the principles involved. Such, for example, would be the distribution throughout the coal of cooling pipes through which a liquid would circulate having a lower temperature than the mass. This would serve to carry away any accumulation of heat and confine the oxidation to the lower stages only. On the contrary, the proposition sometimes made t o provide circulating passages for the transmission of air currents is of questionable value, since it may result i n , t h e contribution of more heat b y the added accessibility of oxygen than will be carried away by the movement of the air. UNIVERSITY O F ILLINOIS

THE CENTENARY OF GLUCOSE AND THE EARLY HISTORY OF STARCH. B y B . HERSTEIN. Received December 14, 1910.

The year 1 9 1 1 is the centenary of the discovery of the inversion of starch t o glucose b y means of dilute acids. During these hundred years, glucose has become one of the most important products of chemical manufacture, a n importance which is only emphasized b y the very close relation of glucose to agriculture. It is the simplest and best known representative of t h a t great group of substances known as carbohydrates, which form the body structure of all vegetable growth, and the main sustenance of animal life, a -fact in itself sufficient t o give glucose a most prominent place in chemistry.