when we burn hydrocarbon around a jet of air, the hydrogen burns, but the carbon is simply heated, and in time deposits as soot. To apply this to the case before us. By introducing above the fuel the air required for the gaseous portion, we burn the hydrocarbon in an atmosphere of air; while by drawing this air through the fuel, we to a great extent burn the hydrocarbon by jets of air. Any one may satisfy himself that the latter is the case generally in practice. The flame described by Gmelinwhite, yellow, red, and finally black-is a common phenomenon in most furnaces. And it may be accepted as an axiom that a flame which exhibits decreasing intensity of light, invariably ends in smoke. A flame in which the carbon is being converted into carbonic acid, can never exhibit a red appearance; when that occurs, the temperature has already fallen below the point of ignition. To return; we thus find that of the air required for Newcastle coal, 60 per cent. (660 lb.) should pass through the bars, and 40 per cent. (453 lb.) should be introduced above. And it appears to be most in accordance with the principle here involved, that if excess of air above the quantity chemically required be given, it should be above entirely. It is probable that a slight excess of air contributes to the combustion of the carbonaceous portion of hydrocarbon gases. Assuming the regulation of draught as practicable, how may we best obtain that regular and uniform combustion of both fixed and volatile ingredients which can alone agree with a regular and uniform supply of air above and below the bars? An automaton feeding apparatus, by which the fuel is constantly projected into the furnace in small quantities, seems most suitable. But as such apparatus requires a uniform motive power, and is inapplicable to the greater number of boiler furnaces, we shall confine our remarks to hand-fueling, and enumerate the points which tend to secure regular and uniform combustion, keeping previous remarks on minimum draught still in view. 1. Frequent fueling or firing. 2. Maintaining a uniform thickness or depth at all times, and in all parts of the furnace. The effect of this is to render the resistance to the passage of the air uniform. 3. Maintaining considerable depth of fuel, much greater than is commonly the case. By no other means can we diminish the draught and raise the initial temperature,-consume the gaseous carbon and economise fuel. 4. Another most important end served by having great depth of fuel, and at the same time sufficient area of bar surface, is, that the draught being diminished, fewer passages are required for the hot gaseous products rising from the cinder below. The upper layers of fresh fuel may thus lie close without prejudice to the draught, are subjected to a more gradual heat, and consequently give off the gaseous combustible vapours more slowly and uniformly. Influence of Excessive Draught on the joint Combustion of Carbon and Hydrogen. In applying the principles previously laid down to this part of our inquiry, we must keep in view the nature and succession of the phenomena which accompany the combustion of hydrocarbons. 1. Previous to combustion, they are decomposed, under the influence of heat, into their elements, hydrogen and carbon. Flame is simply carbon precipitated in a visible form, and raised to a high temperature by the surrounding caloric. 2. The hydrogen element burns first. A very low temperature is sufficient; at 600°, considerably under red heat, it commences. The carbon is necessarily freed from combination, and deposited at the same moment, and its colour will indicate the temperature present. 3. If the temperature produced by the combustion of the hydrogen is high, or if caloric from any other source is present, the precipitated carbon burns also. This does not occur, however, until the temperature reaches a certain intensity. At a red heat, 800° to 900°, combustion of carbon slowly commences, but does not go on rapidly, until a much higher temperature, at least a white heat, is attained. If this temperature is not attained, the carbon passes off unconsumed, becoming red, dull red, and black, as it cools. It appears evident, that in any ordinary furnace, the temperature may, and does, suffice to decompose the hydrocarbon, and burn the hydrogen; but whether the carbon thus separated be also burned, depends on the presence of a temperature of considerably greater intensity. And this temperature must be present, not on the hearth, but in the open space above in the body of the furnace, where the decomposition of the hydrocarbons takes place. In other words, the initial temperature must be high; and this can be effected only by the regulation of draught, as before indicated, and by presenting the fuel to its influence under proper conditions. The colour of the flame or precipitated carbon affords an indication of the temperature at any point of a furnace. For a few data on this point, see Table I, columns A and B. The occurrence of black or dark red in the flame will thus indicate excessive draught. The truth of this indication may be relied on, as the results were verified by actual calculation of the amount of air passing. The only exception occurs where close proximity of a metallic conducting medium, such as boiler plate, cools the flame prematurely, even with limited draught. Such furnaces are, it is feared, incurably smoky in most cases. We would recommend frequent attention to these appearances; and it will be found, with the above exception, that as the draught is reduced, and other arrangements altered to correspond, the flame will become reduced in size, and increased in brilliancy. Since combustion of the gaseous carbon, as precipitated, is the same thing as prevention or consumption of smoke, we may recapitulate the principal points involved. First, Proper regulation of draught, by maintaining a high temperature in the body of the furnace, renders the combustion of the carbonaceous particles therein suspended possible. Secondly, It diminishes the rush of hot air through the fuel, and thus tends to produce regular and uniform evolution of the gas. Thirdly, It lengthens the period during which the carbonaceous particles are subjected to the undiminished initial temperature. As a corollary, we may safely add, that every influence. tending to diminish initial temperature tends to produce smoke and waste fuel. Damp in the fuel has a very cooling effect, as it absorbs both latent and sensible heat at the moment when it can be least spared. But that which exerts the most powerful influence in reducing initial temperature is the practice, now so common, of completely enclosing the furnace in the boiler flue. The body of hot gaseous matter which it is desirable to maintain for a time, at a temperature of 2000° and upwards, in order that it may burn the carbonaceous particles suspended in it, is thus surrounded by a rapidly absorbing metallic surface, at a temperature of 250° to 300°. The consequence is, a rapid and premature diminution of the initial temperature, while the combustion of the precipitated carbon is suddenly arrested. Other things being equal, such furnaces produce more smoke than others, with every precaution as to fueling, and regulation of draught. This method has been highly recommended for this reason, that the furnace being limited by the boiler, its dimensions are not left to the discretion of parties considered. incompetent. But a large furnace is not in itself an evil, as the experience of the Cornish miners for generations proves; and only becomes so when a system of thin fires, with excessive draught, is persisted in. Cornish practice, the most economical and successful in the world, is "thick fires, extensive grate area, and slow draught." The consequence must be, high initial temperature, gradual and uniform combustion, prevention of smoke, and the utmost economy of heat, both in its production and application. Unless the furnace in which the heat is produced be removed from the boiler or other body to which the heat is to be applied, either entirely, or so far at least as to allow of a high initial temperature, we do not hope to see smoke generally consumed. In modern boiler-making, it seems to be forgot, that the fuel, both in its fixed and volatile constituents, is consumed by caloric while generating it. A style of furnace, which seems somewhat in accordance with the views here expressed, is mentioned in "Fuel and its Applications," by Drs Ronalds and Richardson, p. 264. It is there stated to be smoke-consuming. Supposing a regulated draught, and gradual and uniform combustion of both fixed and gaseous combustibles attained, it may be useful to inquire what appearance the waste products ought to present, as they issue from the chimney-top. Assume 100 lb. coal burned in ten minutes 10 lb. per minute; the product per minute should consist of This emission of steam, at the rate of 4 to 5 lb. per minute, will appear as a faint cloud, on contact with the cold atmosphere. The ordinary appearance is very different. For the first two or three minutes after fueling, we see a dense black cloud of soot mingled with vapour of water, which suddenly ceases, leaving nothing but invisible carbonic acid and nitrogen, until another fueling occurs, to be followed by the same results. As these sudden evolutions are always smoky, let us see what takes place in the furnace, supposing the gaseous evolution over in the first three minutes out of ten, with 100 lb. of fuel. The draught remains nearly uniform generally, whatever be the fluctuations in the combustion. This system of fueling thus places us in a dilemma. If the draught is so adjusted as to be equal to one equivalent on the whole quantity of fuel, one-half of the combustibles escape unconsumed during the first three minutes. If air is supplied sufficient for the first period, it is in excess for the remainder, and amounts in all to two equivalents. This causes loss of heat, as before shown. Besides, the probability is, that after all, smoke will be produced, as there is more hydrocarbon Tenth Minute. Air. |