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rapid rate (Table 11),as to the large proportion of water employed. This assumption was verified by three experiments (Table V). I n each experiment the heating was done with the outlet valve closed for 30 min., after which a continuous blow-off was maintained for 90 min. I n each case the temperature was held within 1 degree of 180” C. for the 2 hrs., after which the flames were removed and the blow-off was completed.
Vol. 15, No. 5
The yields obtained are much higher than those in any of the experiments where the autoclave was kept closed throughout the heating period, except No. 5 in Table IV. Moreover, the ratios of cobs to water employed (1:5 and 1 : 6 ) are within the range of practical application. A ratio materially higher than l :4 is unsatisfactory, as it results in a much lower yield, which is evident from the very low yield in the case where the ratio was 1:2.5 (No. 1, Table IV).
Chemistry of Combustion in Coal-Fired Furnaces’ By W. I(.Lewis MASSACHUSETTS INSTITUTE
OF
TECHNOLOGY, CAMBRIDGE, MASS.
B
Y analyzing the gases
The combustion reactions in the fuel bed of coal-fired furnaces ( l a ) . Were the fuel bed hatre been studied by the Bureau of Mines.2 and the following disindefinitely thick, the cos a t varying Points in a COke-fired fuel bed, cussion is a resumi. of those results, fogether with an interpretation and c‘o curves would fiof their significance in the light of certain other work in this field. n a b reach equilibrium, as Curves of the type shown in the figure may be obindicated by the asymptained. Where petroleum totes. coke or some sort low in ash is used, the curves will start Where the coke is high in ash there exists a zone of ashes, along BC. The individual points will vary considerably from containing but little unburned combustible below the carthe curves, but this is due to the serious difficulties of Sam- bon itself, in which the air is largely preheated. I n this case pling the gases. If smooth curves are drawn among the points, the actual curves observed are the dotted ones starting on the the following conclusions may be drawn: line OA. A striking fact, not always appreciated, is that the rate of combustion of carbon on a grate is limited solely by The CO2, Curve 1, rises rapidly a t the start, and its initial slope varies but little with the air velocity. The oxygen, Curve 2 , the amountof air forced through the grate bars, and this in falls sharply a t the start practically independently of air velocity, turn is limited Only by the air velocity that will blow the fuel off the grate. Furthermore, a t a point somewhere between however great, and is of the logarithmic type-i. e., the rate of disappearance of oxygen is proportional to the amount present. 3 and 6 in. practically all the oxygen in the air has been Furthermore, in the initial stages the C02 curve is the invert of and this is true whatever the air velocity. the oxygen curve. The CO, Curve 3, starts out with practically no slope, rises more and more rapidly, passes through a EXPLANATION OF RESULTS point of inflection, and fades away to an asymptotic value. Analysis of the data will show that the slope of the CO curve The explanation of these facts is that the rate of diffusion of at any point is proportional to the height of the CO2 curve at oxygen through the gas film around the particles of incanthe same point, and its point of inflection is, therefore, directly descent carbon is proportiona1 to the gas since inbelow the maximum of Curve 1. Finally, temperature observathins down the film. Hence, a t double the tions prove that the temperature variation through the fuel bed creased is not great in any given case, and while temperature increases air velocity twice the oxygen diffuses in and reacts, and twice with increased air velocity this increase is relatively small for the GOz is formed, but since there is twice the air to dilute fuel beds of reasonable thickness. The curves as drawn are dia- it, the percentage compos~t~on of the gases remains unchanged. grammatic and not to scale. Therefore, the fuel burned per square foot of grate area per It seems impossible to explain these facts on other than the hour is ProPortional to the air velocity through the bed. Since the combustion rate is proportional to air velocity, following assumptions : The primary reaction of carbon with air is C + o2 = yet with formation of the same proportion of reaction products coz. The rate of this reaction is limited only by the rate of within the fuel bed, it follows that the rate of heat evolution diffusion of oxygen through the stationary gas film around in the bed is ProPortional t o COmbustion rate; thereby, a each particle of carbon, the rate of chemical combination of fraction of this energy leaves the fuel bed as sensible heat the oxygen with the carbon being indefinitely great in corn- in the gases; the rest is emitted by radiation, provided there parison with this diffusion. Were this not the case, the ini- is a heat-absorbing body “in sight” of the bed to take it UPslopes of Curves 1 and 2 would be small a t high velocities e. g., the steam boiler. Since radiation increases as the fourth and large a t low velocities, since the change in temperature of Power of the absolute temperature, the temperature of the the bed with velocity is not sufficient to account for the in- bed will rise only slightly to dissipate the additional heat creased reaction rate. CO is formed, not by direct combina- corresponding to a great increase in combustion rate. This tion of oxygen with carbon, but only by reduction of COZ, as is why the temperature of the bed changes SO little with air shown by the fact that its rate of formation is proportional to velocity. Low-grade fuels may have only enough heat to raise their t h e COZ present. Since Curve 3 is much flatter than Curves 1 and 2, it fol- own products of combustion to a reasonable combustion lows that the limiting factor in rate of co formation is not temperature (one a t which the combustion reactions progress In such case the diffusion but the specific rate of the chemical reaction. It a t a reasonable rate-i. e., a t least 900” therefore follows that a t high air velocity the CO curve will fuel bed must be protected from radiation, since otherwise be depressed (3a) and the last part of the C 0 2 curve raised its cooling effect will Put out the fire. This can be done by keeping the bed “out of sight” of any heat-absorbing body, Presented before the Section of G a s and Fuel Chemistry at the 64th as in Dutch Oven construc~ion~obviously, insulated walls, Meeting of the American Chemical Society, Pittsburgh, Pa., September 4 once they are brought up to furnace temperature, reflect back to 8, 1922. most of the radiation hitting them and do not cool the bed. a Kreisingcr, Ovitz, and Augustine, Bur. Mines, Tech. Paper 137.
c.).
May, 1923
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Another striking fact emphasized by these curves is that Whenever coal is burned it must first be brought to incandescence, and in this preliminary heating its volatile mat- the gases leaving the fuel bed are not only deficient in oxygen, ter distils out and coke is left behind. The curves just re- but also rich in monoxide. It is of course impracticable to ferred to apply to the burning of this coke, which represents a operate a fuel bed with a thickness of less than 3 or 4 in., and large fraction of the combustible in the fuel. They are, usually more is required. Consequently, the fraction of the total carbon leaving the fuel bed in the form of CO is large, therefore, controlling in furnace operation. and there is not enough oxygen with it to burn it. FurtherWhen coal distils it evolves unsaturated hydrocarbons of high molecular weight, and when these are heated they more, the hydrocarbons formed on distillation of the fuel “crack,” with deposition of carbon or soot, This is the source must also have further oxygen mixed with them to insure of most of the smoke in the combustion of coal. Bone3 has complete combustion. Since this additional oxygen cannot shown that if hydrocarbons are heated in the presence of possibly come through the fuel bed, it is absolutely necessary oxygen a t temperatures below their ignition point, they com- to admit it as secondary air. Since excess air represents a bine with this oxygen and form additional compounds, serious heat loss up the stack, this secondary air should which on further heating decompose but, even in the absence be kept a minimum. On the other hand, the oxidation of of further oxygen, without deposition of carbon. If, there- these gaseous products of primary combustion and distilfore, one wishes to avoid smoke in combustion, the coal should lation is purely a gas reaction and relatively slow. Outbe distilled in a stream of air so that the hydrocarbons evolved standing problems of furnace design and operation are, will be formed in. the presence of oxygen, to which they can therefore, the admission of the right amount of secondary add so that subsequent cracking with carbon deposition will air, mixing this secondary air perfectly and quickly with the be impossible. This can be accomplished on a hand-fired gaseous products of primary combustion, provision of adequate grate by firing fresh coal on a small, clear area of grate ob- combustion space and, where possible, of hot catalytic surtained by pushing back the hot coke from the fire door. The faces to insure completion of the oxidation reactions. The coal will be coked by heat brought to it by conduction and main reason why oil-fired furnaces have a higher thermal than coal-fired ones is the greater ease of securing radiation from the surrounding fuel bed and furnace ~ ~ 1 1 s efficiency . in them perfect mixing of the air suppIy with the gaseous The coal must never be thrown on top the hot fuel bed, products of primary combustion and distillation, thus enabbecause the gas rising from it is oxygen-free and coal distilled ling the operator to reduce excess air. under such conditions will inevitably deposit soot in an exThe problems of combustion are satisfactorily met in the tremely finely divided form which cannot be separated from best modern boiler furnaces, in the construction and operathe gas. Furthermore, this soot will burn very slowly, tion of which practice is we11 standardized. On the other because if only a little air is admitted the oxygen concentrahand, in most chemical furnaces important special conditions tion is low, while if more air is used the temperature falls, due obtain which make these problems exceedingly difficult for to the dilution, to a point where combustion rate is negligible. In automatic stokers with chain or inclined grates the fuel the chemical engineer. Furthermore, unlike boiler practice, is f i s t heated and distilled in the presence of air, and this these special factors vary widely in different industries and explains why with these stokers such high combustion rates operations. To solve them, therefore, requires a peculiarly thorough appreciation of the fundamental facts outlined can be realized without smoke difficulties. above, that provision may be made to meet the conditions necessary for efficient combustion. J . Chcm. SOC.,85 (1904),1659; 87 (1905),1248. 89 (1906),660. J
