February, 1925
INDUSTRIAL AND ENGINEEHNG CHEMISTRY
115
STORAGE OF COAL AND SPONTANEOUS COMBUSTION ROUND TABLE DISCUSSION Papers presented before the Section of Gss and Fuel Chemistry at the 08th Meeting of the American Chemical Society, Ithaca, N. Y.. September S to 13,1024
The Oxidation of Coal at Storage Temperatures' By S . W. Parr and R. T. Milner rrlrrvsasrrv OP
ILLINOIS. UEBdN.4,
ILL.
UMEROW investigators have noted the avidity of coal for oxygen.* Much discussion has arisen as t.o the mechanism of the chemistry of this combination. Is it an occlusion, an ab- or an adsorption, or a true chemical combination? A discussion of this nhase of thenroblem woulddoubt-
N
by-pass to the flask. Mercury-seal cups are used for connections, so the apparatus can be easily dismounted. Coal to the extent of 20 grams is put into an aluminium boat and placed in the inner tube. Telescoping tubes for housing the aluminium boat and sample are employed during the time of the removal of the boat for weighing. With the outfit thus assembled any gas can be made to circulate over thesample, and absorption trains are attached for purification or collection of by-products. Moreover, the outfit readily lends itself to the maintenance of a constant temperature over long periods of tim+t.hat is, from 1 to 5 months. For example, acetone vapors maintaiii a temperature of 55" C., carbon tetrachloride maintains t,he 7.5" C. stam. water 100" C , . toluene 110" C.. and xylene 140" C. In &der to parallel operations, ten of these instruments, set up as shown in Figure 2, were available. For the preliminary preparation of the sample an atmosphere of nitrogen, purified by washing through pyrugallate, caustic potash, and concentrated sulfuric acid, was used. After passage of the nitrogen at the determined temperature until equilibrium was established, or as nearly so as possible, oxygen was passed through the same train-omitting, of course, the pyrogallate solution.
Ffpure 2
Great care was taken in choosing the absorbents for carbon dioxide and water. For carbon dioxide, Ascarite was used. This is a commercial prepartion of soda lime and asbestos; it is light and a very efficient absorbent. For water, magnesium perchlorate trihydrate was used. This is a compare-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
116
tively new drying reagent and has been described by Smith, Brown, and Ross.' There are many advantages favoring its use. The perchlorate is a solid, much cleaner, and more easily handled than phosphorus pentoxide; it is more efficient than calcium chloride, since its vapor pressure is lower and it is very
Vol. 17, No. 2
after the coal had ceased to lose weight. The tubes and boat were weighed daily. I n weighing, the boat was placed in the large telescoping capsule immediately after removal from the apparatus, allowed to cool in a desiccator, and weighed in the capsule. These precautions were taken to avoid the absorption of moisture from the air by the dry, finely divided coal. The absorption tubes were weighed at the same time, the ends
6
/Z /t? 24 30 7ime o f O?idu//on /i7 Dogs
36
42
Figure 3
Figure 4
stable, absorbing as much as 30 per cent of water by weight. The perchlorate tubes were made as described by Smith and his collaborators, and as shown in the illustration. The gas was passed through the train of wash bottles, through a tube of perchlorate to remove last traces of water, over the coal, and then through a train consisting of perchlorate and Ascarite tubes, with a guard tube containing both at the very end. A little perchlorate was placed in the Ascarite tube to catch any water formed in the absorption of carbon dioxide. The final guard tube had a small rubber tube attached to it, which led into a beaker of water so that the rate of flow of gas through the apparatus could be followed and, by comparison with the amount of gas entering, the possibility of leaks could be covered.
of the perchlorate tube being closed with policemen to exclude the air. The temperatures in this series of tests were first normal or room temperature, indicated as at 25' C., then a t various stages as 55", 75", 100°, and 140' C. After a preliminary period of from 3 to 5 days, oxygen was admitted to replace the nitrogen. Results
The manipulation of the apparatus was very simple. The tubes and wash bottles were filled with reagents and the apparatus was connected with tight-fitting rubber tubing. Nitrogen was passed through the empty apparatus, kept a t a temperature at which it was to be used, until the tubes were at constant weight (as shown by daily weighing). If the corks
The accompanying charts show the results of typical tests a t various temperatures. Figure 3 shows the amount of moisture and carbon dioxide discharged a t 100" C. over a period of 44 days. Notwithstanding the notable amount of these constituents discharged there is a steady increase in weight throughout theperiod. These constituents represent a loss in weight, calculated to carbon and hydrogen only, of approximately 3 per cent of the original coal. The increase in weight due to oxygen absorbed is greater than this loss and is, presumably, at first taken on in an adsorbed state, from which it gradually proceeds to a chemical combination resulting in the weights of moisture and carbon dioxide as shown.
Figure 5
Figure 6
(not rubber) for the inner tubes were oven-dried, this occurred in about 2 days. The coal, ground to 100 mesh, was then weighed into the aluminium boat and placed in the apparatus. The passage of nitrogen was continued until about 5 days
Figure 4, with temperature maintained a t 75" C., shows similar results, differing only in amount. Charts for temperatures at 55" and at 25" C. are not reproduced here, since they show the same features with a corresponding drop in quantity.
Manipulation
6
THISJOURNAL, 18,20 (1924).
February, 1925
INDVSTRIAL AND ENGINEERINQ CIIEMISTRY
Figure 5 , giving the weight of carbon dioxide evolved at the four temperatures studied, shows the behavior of the coal for these temperatures and obviously the values could he interpolated for all intervening temperatures. Charts showing the behavior of air instead of oxygen, on the same coal a t the same temperatures, have been plotted with similar results, excepting that the time is increased to approximately twice that for corresponding values with oxygen. Interpretation of Results The coals studied show an increase in weight throughout the period, seemingly approaching a saturation point at ahout 8 per cent of the original weight. The coals studied were all from Illinoiu. Moisture equivalent to from 4 to 8 per cent of the weight of the sample is evolved, varying in amount with the temperat.ure. The carbon dioxide evolved is equivalent to from 2 to Bper cent of the sample. Carbon monoxide, where determined, is present but in very small percentages, not exceeding 1 per cent a t 110”C . The preliminary stage of heating in nitrogen shows a small generation of both water and carbon dioxide, with a slight accession of weight by the coal, probably due to a small absorption of nitrogen. In the case of the water and carbon dioxide these would naturally result from the breaking down of the oxygen compounds of the coal. Bynotingthedischargeof oneconstituent, ascarbon dioxide, for the different temperatures employed, it is evident that the amount increases directly with the increase of temperature (Figure 5). Attention is specifically called to the fact that at ordinary atmospheric temperatures this value is very low hut consistent. The values for the other temperatures increase in an accelerated ratio with the increase in temperature. The loss of heat values is of especial interest. After 50 days a t 100’ C. the loss is of the order of 1200 in 14,700B.t. u., or approximately 8 per cent, while a t 150’ C. the drop is 3700 units, or 25 per cent, while for a continuous normal temperature the drop is insignificant, being substantially only that which would he indicated by an increase in weight due to the ahsorption of oxygen. The meaning of these facts from the standpoint of coal storage may he more concisely shown by a hypothetical graph based on these results and shown as Figure 6. Finally, there seems to he ground for the conclusion that oxygen is taken into the coal texture primarily in an adsorbed or surface condensation form. In this stage its presence is indicated by an increase in weight of the mass, and the loss in heat units corresponds to this weight increment; hence the heat loss is apparent and not real. But a t slightly elevated temperatures the adsorbed oxygen goes over into actual cbemical combination, with the evolution of water and carbon dioxide. Here there is an actual loss of heat, practically equip-alcnt in amount to the heat of combustion represented by the weight of carbon and hydrogen in the water and carbon dioxide evolved. These conclusions, if correct, will aid materially in explaining some of the wide discrepancies encountered, both in fuel literature and in practical experience, as to the lossor absence of loss in the heating value of stored coal. New Scientific Journal-The Rockeieller Foundation of New York has appropriated S50,OOO to finance a new scientific periodical, to be known as Inkrnalional Biological ifbslracls. It will give in condensed form a monthly summary of all nublicacover scientific territory- which at present ‘is neglected
117
Oxidation of Sulfur as a Factor in Coal Storage’ By S . W. Parr and E. R. Hilgard UNIYHBSmTY 011
IrLxnors, uaa*x*. ILL
ROM the investigations on coal oxidation of which this study is a part, the question is continually being raised-what are the conditions which furnish the initial incremenk of heat, assuming now the absence of extraneous sources, such as hot walls, steam pipes, sun’s rays, etc.? Obviously, under certain circumstances heat is generated within the coal mass independently of any external influence. The earlier references in the literature stress the presence of sulfur, whereas later investigations discredit that idea. Abundant proof is at hand showing that oxygen is
F
Marcasite Crystals
Concretion from Coal Showing Surlace Cryat&
of Marcasite
absorbed by coal and euters into heat-producing reactions at comparatively low temperatures, but this does not furnish evidence as to the source of the initial heat for starting the process. As studies proceed connected with the sulfur in coal, its distribution and forms, both with reference to crystallization and size of aggregates, it is evident that its possible role in the oxidation processes must he given further consideration. I
Reccived August 8,1924.
