Atmospheric Oxidation of Coal at Moderate Temperatures Rates of the Oxidation Reaction for
Representative Coking Coals L. D. SCHMIDT AND J. L. ELDER Central Experiment Station, U. S. Bureau of Mines, Pittsburgh, Penna. Rates of oxidation of samples representative of eight coal beds of commercial importance were measured at temperatures below 100" C. The characteristic rates of oxidation of the coals increased with increasing volatile matter content. A coal of 38 per cent volatile matter oxidized three times faster than a coal of 18 per cent volatile matter (dry basis). The rate of oxidation of each fresh coal decreased rapidly as the coal consumed oxygen, the rate being inversely proportional to the one fourth root of the amount
of oxygen consumed. For all coals tested, the rate of oxidation was proportional to the 0.61 power of the oxygen concentration in the gas phase. The rate of oxidation of coal from the Pittsburgh bed increased by a factor of about 1.7 for each 10" C. rise in temperature. In general, the relative storing qualities of coals depend upon their friabilities as well as their characteristic rates of oxidation. For the coals tested, higher rates of oxidation tended to be compensated by lower friabilities.
HEX the phenomenon of spontaneous ignition was
determined. In addition, coals from seven other coinnit'rcially important beds have been investigated. This wvork falls naturally into two parts-(a) the rates of oxidation of different coals and the effect of temperature, time, osygeii concentration, and coal composition on these rates and ( b ) the effects of oxidation on carbonizing properties. The present paper covers only the rates of oxidation.
not well understood, coal often was regarded as a n unstable substance subject to large losses in heating value and even so unstable as to burst into flame when stored. An early investigation (1909) of the Bureau of Mines showed that it is not subject to large losses in heating value if properly stored. Measurements of the loss in heating value of coal stored in small experimental piles where spontaneous heating was avoided proved definitely that for many coals there is no serious loss in heating value during several years' exposure (16). Since that time the Bureau of Mines has from time to time investigated various phases of the problems connected with the changes that occur when coal is stored ( 8 ) . For example, laboratory studies (17) proved that coal exposed to air reacts continuously n-ith oxygen a t slow but measurable rates, even at room temperatures, and that appreciable quantities of heat are evolved. Studies were continued t o determine the effect on the rate of oxidation of such variables as temperature ( 1 7 ) , concentration (1'7), and availability (11, 16) of oxygen, moisture (content ( 3 , 1 2 ) , the extent of previous weathering ( I @ , and coal composition (9). The primary aim of the present investigation was the determination of the effect of oxidation on carbonizing properties. A previous publication (19) showed that mild oxidation of Pittsburgh bed coal a t 30" C. was accompanied by a progressive change in carbonizing properties before the oxidation had progressed far enough to affect significantly other properties of the coal, such as agglutinating value, chemical analysis, and heating value>. This investigation has been continued, with some modification of procedure, and the effect of temperature of oxidation on the rate of change of the carbonizing properties of Pittsburgh bed coal has been
Apparatus The coal was oxidized in drums maintained at constant temperature while being rotated slowly. The slow rot'ation of a drum less than half filled with crushed coal expo5es each piece of coal to the same atmosphere periodically, and the motion makes it possible to maintain constant temperature throughout the mass of coal. Figures 1 and 2 show t,he apparatus usec1 for osidizing 182-kg. samples of coal: A jacketed drum, 1.55 meters long and 0.66 meter diameter, is made of mild steel 1.6 mm. thick. The entire drum, including the ends, is enclosed by a jacket which in operation is filled with the vapors from a boiling liquid to maintain the interior of the drum a t constant temperature. Sixteen t o twenty liters of the liquid, chosen so that its boiling point under atmospheric pressure corresponds to the temperature desired, is placed in the jacket, and heat is applied with a gas burner. The jacket is vented to the atmosphere through a reflux condenser. Vapor in excess of that required t o fill the annular space between the drum and jacket is condensed and returned t o the boiling liquid. The vapor jacket is covered with rock-wool insulation (8 em. thick), except for a band (15 em. wide) which is left bare. -4 small gas flame under this exposed band keeps the liquid in the jacket boiling. The interior surface of the drum is protected from corrosion by two coats of an enamel baked at 170' C. by using ethylene glycol in the vapor jacket. 249
In all the testa described in this report, water %-asused in the vapor jacket in order to maintain tlie coal rat an average temrvr,?tnre nf 94 x o
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remahder of the oil. From tbe ohareoal filter it is passed tbrough a wet test meter. Excess water vapor is removed ins. condenser, and the air at a definite relative humidity passes through a packing box to the far end of the d n m . As it leaves the drum, it pas.%s tlirougb a filter which removes coal dust and thcn through 5 packing box to a condenser that removes excess water. An average sample of the exit air is obtained by the use of an antomatie solenoid sample pump and a gas holder. The residual exit air is vented to the atmosphere. Water condensed from the exit air returns through a water leg to the inlet pipe and re-enters the drum with the fresh air. Cooling water llorving through the two condensers in series maintains equal moisture content in the entering and exit sir. This arrangement prevents loss of water from the drum throughout the test. Figure 2 shows the design of the gas holder. By means of this simple holder the back pressure on the sample pump can be held constant at zero or at any podtive or negative pressure that may be desired. Since an ~ u o O u solutionr\.as s uied &s a conJking liquid it w&6 desirable to remove the carbon dioxide from the gases before storage. A tube containing potassium hydroxide solution was inserted between the sampler and the holder, and the amount of carbon dioxide removed from the gas wns determined by titration.
Proaedure Tho samples for oxidation Rem tuken from coals received for carboniaatibn tests in the Survey of Carbonizing Properties of American Coals ( 7 ) . Immediatcly after being mined, these samides are nlaced in steel drums made airtieht bv means of a rubbcr'gnsket under the cover. Preoxidation of the samples, wliiclr would nffcct the rate of oxidation observed in the test, is kept as l o w as possiblc. Within about a week after arrival at the laboratory, tho coal is reduced in size by stage crushing to pass through 6.4mm. ('/
