528
INDUSTRIAL AND ENGINEERING CHEMISTRY
contain. This generalization applies not. merely t o coals (which become devolatilized) and to cokes (which have a well developed system of cracks and open pores), but also in some measure t o the more highly compact,ed and crack-free forms of carbon such as artificial graphite. While it has long been recognized that exposure t o a mildly oxidizing atmosphere a t temperatures above 600" C. causes internal etching of carbonaceous particles-this being the familiar method for activating carbon adsorbents-it is less generally known that fuel particles exposed to a fast current of air a t furnace temperat,ure may also exhibit internal burning in some degree. Indeed, the assumption is often made (and is implicit in most calculations of the mass-transfer of oxygen to t,he fuel surface by processes of diffusion) that a t sufficiently high temperatures the partial pressure of oxygen a t the external surfaces of a particle is zero. This implies that each molecule of oxygen reacts at, or very near, t'he point where i t impinges. Though it is difficult t.0 present a clear picture of the phenomenon of internal burning in all its complexity, it is believed that this assumption is incorrect. Some of the evidence t o this effect is given below.
A solid cylinder of artificial graphite 1.0 cni. in diameter and of about equal length Tyas drilled axially to form holes of I-mm. diameter extending from upper face to center. A quartz capillary mas then cemented into the neck to form a gas-tight joint. The capillary led to a small U-gage the other end of which was open bo atmosphere. In carrying out an experiment the cylinder 1w.s mounted in a vertical electrical furnace (the U-gage being external to this) and its temperature raised to, say 850" C. in a brisk current of nitrogen. At t,his stage the silica-t,o-carhon joint was tested for leaks. After the temperature and pressure equilibrium were attained, the gas passing up through the furnace was changed from nitrogen to air. Therc \vas an immediate build-up of gas within thc cavity, the pressure rising in the course of some minutes to, sap, 3 cm. of water and thereafter remaining constant. When the gas was collected and analyzed i t consisted substantially of carbon monoxide and nitrogen. These experiments have been described in greater detail by Bowring and Crone ('7). A similar build-up of pressure occurs
Vol. 43, No. 2
with such fuels as charcoal and commercial coke, though reproducible results are more difficult t o obtain with the,%. TECHKICAL APPLICATION
The recognition that when air passes through a fuel bed a path length equivalent t o no more than a single particle diameter suffices to gasify sufficient carbon to convert all its oxygen into carbon dioxide ( W / W o-- 0.6) suggested a novel type of solid fuel burner in which a ribbon-shaped stream of air is directed downv a r d onto the free surface of the fuel bed. By introducing the air through a jet its velocity can be ~ont~rolled independently of its quantity; it is relatively easy t80achieve aerodpamic conditions so that t,he excess oxygen and the carbon monoxide in thc outgoing gases are both negligible. Alternatively, a mildly reducing gas-containing excess of carbon monoxide--or a mildly Oxidizing one can be produced a t will. The construction and operation of coke-tired downjet furnaces now in commercial operation have been described by Ross and Sharpe (IS), whose paper gives details of t,he declinkcring mechanism used. LITERATURE CITED
(1) Arthur, J. It.,,Yalure, 157, 732 (1916). (2) Arthur, J. R., Bangham, D. H., and Thring, M. W., J . Soc. Chem. I d . ,68, 1 (1940). (3) Arthur, J. It., and Bowing, J. I?., .J. Ciiem. Soc.: 1949, special
issue.
(4) Bangham, 1).H., and Bennett, J. G., F u e l , 19, 95 (1940). (5) Bangham, D. H., and Thring. 11.IT.,Cortl Research, p. 48 (1944). (6) Bangham, D. I$., arid Townencl, D. T. A , , J . chim. phys., 47, 31.5 11950). (7) Bowring, J. R., and Crone, H. C., Ibid., 47, 543 (1950). (8) Cannon, C. G., unpublished data. (9) Dufraisse, C., and Horclois, E., Compt. ?end., 192, 564 (1941). (10) Grodrovskir, M. I
I 2
I
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,
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Figure 2. Influence of Phosphorus Oxychloride and Phosphorus Trichloride on Composition of Combustion Products from an Electrode Carbon Tube at 850" C.
End-on observation of the burning carbon tubes showed, in the absence of the inhibitors, a blue glow which spread over the whole cross section of the tube. I n the presence of the inhibitors the glow was extinguished, and the effect was found t o be reversible. The curves of Figures 2 and 3 show clearly that the additives suppress reactions which consume both carbon monoxide and oxygen, and also that the greater effect occurs with the two phosphorus halides. The maximum retarding effect occurs at small concentrations (
