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 t o 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. Loss in Efficiency
In actual operating efficiency the loss while burning storage coal is very much greater than is indicated by the heat value loss on a B. t. u. basis. Loss in efficiencyis due, not alone to loss in heat value of coal, but also to necessity of carrying extra boiler units with their radiation losses, losses from inability to keep heating surfaces clean, and most of all through necessity of carrying higher drafts, resulting in “carry-off” of a higher percentage of heat with the increased volume of stack gases. Apparently one of the surest ways to prove a coal has been stored is through the volatile combustible. I n many cases where volatile has been found low in samples tested, investigation has shown that coal had been shipped from storage. Many samples from storage piles belonging to this company have indicated volatile loss from 1to 5 per cent, depending on length of storage. A bulletin of the University of Illinois2 contains results of boiler tests using fresh coal and weathered coal, and gives conclusions that boiler capacity and efficiency are not affected by weathering. These tests were run using fresh screenings from Danville, Ill., District, whereas weathered coal used consisted of nut coal and screenings from Springfield and Southern Illinois Districts. Coals from these districts do not weather alike, and nut coal weathers at a different rate than screenings. Checks on boiler operat.ion when stored coal is being burned almost invariably show higher draft, lower carbon dioxide readings, and higher uptake temperature than when fresh screenings from the same mine are being burned, even though this coal has been in storage but a short time. Efficiency may not be greatly affected under ideal testing conditions, but observations covering a number of years indicate a loss of efficiency, including loss in storage piles through the double handling, plus losses in the plant covered by their local operating conditions, of a t least 10 per cent.
* Bull. 9 1 (May 23, 1917). See also Parr, “Spontaneous Combustion of Coal in Storage,” p. 120, this issue.
China Chemists’ and Druggists’ Review Trade Commissioner A. V. Smith, Shanghai, has submitted to the Chemical Division of the Department of Commerce a copy of the initial issue of the China Chemists’ and Druggists’ Review. This periodical has been started by H. Schloten, a German 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.
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, P A .
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 t o 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 conclusively that 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 a peroxide, which, owing to its ready formation and instability, is the chief cause of spontaneous heating. I n this discussion it will be shown that for temperatures up to 130” G. the rate of oxygen absorption of a bituminous coal is about ten times the rate of evolution of oxygen as gaseous products. It will be shown further that the absorbed oxygen is unstable, and that most of it may be easily removed as water and oxides of carbon by heating in an inert atmosphere at 160”C. What this compound or group of compounds which is sensitive to oxidation is chemically, we do not know, but it is reasonable to believe that spontaneous heating is directly caused by the amount and distribution of these substances in the coal. The rate at which coal absorbs oxygen is commonly taken as a measure of its tendency to heat spontaneously, and considerable experimental work done in the past has been based on that a~sumption.~The conditions influencing spontaneous heating of coal have also been studied with its affinity for oxygen as a basis, but nowhere has it been shown, so far as the writers know, that the rate of generation of heat is proportional to the rate of absorption of oxygen. I n the present paper the method used to study spontaneous heating is based on the rate at which heat is developed rather than on the rate of absorption of oxygen, although one typical coal waa tested by both methods for the sake of comparison.
A
Apparatus and Method There are many factors which are known to effect the spontaneous heating of coal, the most important perhaps being fineness of the coal, moisture content, air supply, and heat conductivity of the coal. I n the method employed by the writers these factors were either kept constant or eliminated, so that the heat development by the coal was a11 used to raise its temperature; the rate a t which the temperature rose under these conditions was therefore taken as a measure of its tendency to heat spontaneously. All coals tested were reduced to the same h e n e s s (100 mesh) as accurately as possible without undue exposure to air, all had the same previous exposure to air (except where noted), and all were previously dried at 140’ C. in a current of natural gas. The drying temperature was above any attained in the heating experiments, and the oxygen supplied during the tests was dried over sulfuric acid. 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
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