Art. IV.—On a Smokeless and Self-feeding Furnace for Lignites and other Fuels, and the Utilization of the Waste Heat.
[Read before the Otago Institute, 12th August, 1873.]
At a time when the material wealth of nations is recognised as being so intimately associated with those immense stores of power obtainable from the fuels occurring in the carboniferous deposits of past ages, any apology for treating on the economic consumption of these fuels would be superfluous.
It is evident that when fuel, of whatever kind, is consumed in such a manner that there is no smoke evolved, and no cinders and unoxidized portions of the fuel left among the ash, then the theoretical conditions leading to the evolution of all the heat force are present, so far as the furnace itself is concerned; while it belongs to the external arrangements, such, as form of boiler and flues, and perfection of engine employed, to determine how much of this force is actually available, or, in other words, how much duty can be obtained from a given weight of fuel.
Judging of the evaporating power of fuels by the amount of their fixed carbon, their analyses show that the combustion of 1.5 tons of Green Island or Clutha coal should produce the same amount of steam as one ton of Newcastle coal. If we take one ton of Newcastle coal and consume it in a given time—say twenty-four hours—in an ordinary furnace specially adapted for the combustion of this and other coals that coke and can be stirred, and then, in the same furnace, attempt the combustion of the equivalent 1.5 tons of Green Island coal in the same time, failure would be the probable result. For, in the first place, the bottom of the fire and fire-bars would soon be covered with) an
accumulation of ash and small fuel, which would prevent a proper supply of oxygen to the rest of the fuel and to the evolved gases—the fire, therefore, smoulders rather than burns; and, secondly, when the fire is stirred (and also when fresh fuel is added) a certain amount of the burning fuel is broken and falls through the bars to waste, and if the fire-bars are placed nearer to each other, with a view to prevent the fuel passing through them, so much more will the draught be impeded. Another evil arising from the stoppage of air through the fire will be that unburnt gases and smoke pass up the chimney. Now, if these sources of waste were removed, there would, of course, be the same quantity of heat evolved from the 1 ½ tons of Green Island coal as from the ton of Newcastle coal; but considerably more time would be required, first in getting up the fire, and then in consuming it, which is a serious drawback to its use for steam purposes, and determines the use of Newcastle and other coals even in those places where the brown coals may be said to be at our very doors.
It becomes a matter of importance, therefore, to consider whether a special construction of furnace could be devised in which the ashes are removed as soon as formed from all parts of the glowing fire, and all the evolved gases which are capable of uniting with the oxygen of the air thoroughly oxidized, while, at the same time, such a degree of intensity may be imparted to the combustion as to render it available for the generation of steam with the rapidity requisite for marine boilers and locomotives.
To effect this, I propose to do away with the fire-bars, and to use a certain fraction of the heat force of the furnace (when changed into its equivalent motive force), so that it shall send a gentle blast of air upwards through all parts of the fuel, and thence through a great number of small and thin copper tubes of the boiler till it reaches the smoke box. It now becomes necessary, before proceeding further, to consider the relative specific gravities of the contents of the furnace after the fire has been kindled some time and the blast in operation; and there is this remarkable property which must have struck everyone that has used these brown coals, with regard to their ashes, namely, their extreme lightness and the ease with which they are reduced to an impalpable powder.
|Specific gravity of Clutha coal||1.26|
|Specific gravity of its ash flake||0.04|
The ash is therefore about thirty times lighter than the fuel, and twenty times lighter even when the volatile gases are driven off from it. It is important that these bulky ashes be removed, not only from the bottom of the furnace, but from every part of the glowing fuel, for dust would not be more obstructive to the proper action of the human lungs than accumulations of ash to that of a furnace. The difference between the specific gravities of coal and its ash flake allows, therefore, of its removal as fast as it is formed, while the
clean and glowing fuel remains, and is by the same means supplied with a constant stream of oxygen; heat of any required intensity is thus produced, which can be adjusted by regulating the blast. If we take the experimental quantity of 1.5 tons of lignite, and consume it in twenty-four hours, then the furnace must be supplied with a charge of 14lbs. of fuel every six minutes, and as this fuel yields on analysis 5.5 per cent. of ash, it follows that only ¾lb. of ash requires removing and carrying along with the draught in the six minutes.
We may now consider whether there is really any loss of power, and, if so, how much, by the use of a blast. By its use, the tall chimney is, of course, dispensed with. It may be thought that the draught caused by ordinary chimneys costs nothing; but is it not a fact that a certain amount of motive force, or its equivalent of heat force, is used in the act of causing a draught in common chimneys, and that if all the heat of the furnace were really utilized (as by evaporating the theoretical amount of water, for instance), there could then be no heat left to expand the column of air in the chimney to work the draught, for the gases evolved would be no hotter than the ordinary temperature of the atmosphere? The modern theory of heat shows that in whatever manner work is done, if work be done at all, then its equivalent of heat force is expended. Now, the column of ascending gases and air in a chimney is continually pushing away the atmosphere and making room for its passage through it. The furnace may be taken as so much colder by that amount, that is, there will be that amount of heat less that can be used for evaporating the water.
Experiment No. 1.—In a closed furnace, specially arranged so that no air could pass into it except through the burning fuel, a thermometer was attached so that the bulb projected into the furnace about 3 in. above the fuel, until it showed a constant temperature of 286°, with the damper open; then, on shutting the damper in the chimney, in seven minutes it rose to 358°; then opened it a very little, the, temperature at once fell proportionately to the amount that the damper was opened; on opening it wider, it fell to 285°; then completely closed it, when it rose in three minutes to 320°. This experiment was varied with like results. Also, on another occasion, when the thermometer was removed and a vessel of water placed on the top, it commenced boiling when the damper was shut, and immediately ceased when it was opened. This was repeated several times, which results may be ascribed principally to the fact of the gases above the fire being more easily heated under the extra pressure when the damper is shut; for when it is opened, then some of the heat can exert itself in expanding the gases in the chimney, and thus disappears. Now let the chimney be removed and the fire supplied with the same amount of air from a blast, and there can be no more
dynamic force required to work the blast than could be obtained from the heat which disappears in a chimney. It has been calculated that 1lb. of coals employed to raise steam, will do the work of 500lbs. expended in rarefying air, such air being discharged through a chimney 35 ft. high.
After proving that there is no real loss of power by the using of the blast, we will proceed to consider how it now opens up a way to utilize a large portion of the waste heat after it leaves either the tubes of the steam boiler or the super-heater and hot-air jackets of the engine. It has been calculated that 1lb. of coal should vaporize 14lbs. of water from 212° F., whereas about 10lbs. only are evaporated in Cornish, boilers; this, is ascribed mainly to the large amount of heat which passes up the chimney. Taking the temperature of the contents of the boiler to be 300° F., that of the gases leaving the tubes is considerably more than this, being in locomotives as high as 600° F., or about one-fourth of the total heat of the furnace. Now, in common boilers, if we cool this heated air in the chimney by attempting to utilize this heat, we at once impair the draught, but the use of the blast allows us to exhaust all the heat we can the moment that it leaves the boiler. The way that I propose to utilize this heat, is to cause it to raise the temperature of all the large volume of air which is required for the combustion of the fuel in the furnace. The apparatus—which we may call, for convenience, a thermo-convector—corresponds to the ordinary smoke-box somewhat enlarged, and divided horizontally into a series of narrow compartments, the connections of which alternate as in the figure (Plate V.), so that the discharged products of combustion pass along those spaces marked BB in the direction of the arrows, while the current of fresh air is conveyed in the opposite direction within the other spaces AA. These latter spaces also communicate with one another by broad lateral arches not shown in the figure. We have, now, virtually two broad and narrow tubes. The walls or partitions of these tubes should be of material specially selected, either for its transmitting power, or else for its conducting power, such as a metal with its surfaces so prepared as to facilitate the absorption and radiation of heat, as, for instance, thin unpolished sheet-iron, which is usually covered with oxide of iron, which oxide Tyndall has shown to be almost as effective an absorber and radiator for obscure heat as lampblack, which is, as we know, capable of absorbing nearly all the heat from any source, luminous or obscure.
Experiment No. 2.—A common thermometer at 60° was placed 1 in. distant from a heated mass of iron, forming a constant source of obscure radiant heat; in seven minutes it rose to 118°. A large piece of the oxidized iron was then interposed midway between, and the thermometer (previously cooled to 60°) was returned, when in seven minutes it rose to 104°. The thermometer was then removed to a place where the temperature was 60°;
then, in four minutes, it sank to 64°, and in six minutes to 63°. This indicates a loss of heat corresponding to only 14° F. by the interposition of the sheet of oxidized iron. This material was proved to heat very rapidly, and it also cools rapidly if a current of cooler air is passed over it.
From these considerations it is evident that the heat in the flattened tubes BB is continually radiating as long as there is a current of cooler air flowing through the other tube AA. Let us assume that the gases in both tubes are similar in quantity and properties, then it follows that all the air required by the furnace can be formed into a hot blast, having a temperature of at least 300° F. This is on the assumption that the fresh air is made no hotter than it would be if actually mixed with all the evolved gases; but it will be seen that, by a proper arrangement and selection of material for the tubes, the air will have been raised to nearly 300° when it has traversed only half the length of the tube, or at A 2; consequently, as it goes onward to a still hotter part, near the tube B, it is continually acquiring fresh accessions of heat until it reaches that part of the thermo-convector where the temperature is, as we have seen, 600°; and similarly it may be shown that the evolved gases are cooled down to near 300° when they reach B 2, and as they pass on they are rapidly cooled by imparting heat to the incoming current of cooler air. In the above apparatus, conduction and surface radiation only are alluded to; but let us consider if transmission through a diathermanous medium could not be employed to advantage. We are indebted to Melloni for the discovery of the almost perfect transparency of rock-salt for all kinds of radiant heat. It, moreover, does not appear to suffer by a heat approaching to redness. I am unable to ascertain if there is any difficulty in procuring it in large pieces, but as optical perfection would not be necessary, and plenty can doubtless be procured sufficiently transparent, small panes of this substance could be inserted at the top of each convolution of the air tube A. Then the hot gases from the furnace will instantaneously radiate heat into the air tube, and because the heat rays impinge on the surface of the sheet of oxidized iron at the bottom of the air tube, therefore they are immediately arrested and impart their vibrations to the contents of the air tube. Thus, as glass in our windows transmits all the rays of light, so do these plates of rock-salt form, ot doors only, but windows for radiant heat.
It is necessary for us now to consider the radiative property of the evolved gases in B, and the absorbent property of the air in the A tube A (premising that both these properties are always possessed equally by the same body). First, with reference to the evolved gases, Tyndall has shown that they possess these qualities in an eminent degree, because they are compound gases; but this cannot be said of the air in the other tube; indeed, if this air was quite dry and pure, all the radiant heat would merely pass through it without heating it at
all until it reached a surface formed by an absorbent body, such as our oxidized iron, which is capable of receiving the heat vibrations, which receptive surface can then heat the air above it.
Now, as the small amount of aqueous vapour in our atmosphere is the main absorber of radiant heat, thus allowing the diathermanous air to become heated, which heated air can then be conveyed to distant parts by winds; so also can a minute admixture of the compound molecules of various gases—such as those from coal—greatly promote the heating of air for blast furnaces.
It is evident that in this manner the heat vibrations alone can be transferred from one tube to the other, while their gaseous contents are prevented from mixing. If a revolving fan be the means adopted for circulating the gases, it can be applied to any convenient part of the circuit of either of the tubes A or B. In the figure it is connected with A, and therefore first draws the fresh and heated air, and then discharges it under pressure into a large and broad tube extending close along the bottom of the boiler, and thence into a reservoir directly below the furnace and boiler, which supplies the tuyères with the heated air. The thermo-convector and connections there-from are covered with felt, and then cased with tin, leaving an air space between.
The outlet of the tube B conveys the waste and cooled gases downwards over a shallow vessel of water (c), thus arresting the ashes, which speedily sink, and the gases can escape there for locomotives (which would be an important desideratum in tunnels and underground railways), or be conveyed over the ship's side in marine engines, or else made to go upwards through a funnel by the pressure of the blast. This funnel is useful in first getting up the fire, for which purpose the door D is lowered, which then closes the tube B.
As there is a considerable amount of water in some brown coals, and as this would have a tendency to condense in the tube B and thus arrest the ashes, it was necessary to determine the temperature at which its vapour would condense, and I find that if 10 per cent. of water exist in the coal, its vapour will not condense until it is lowered to 27° F., which gives a sufficient margin to ensure a dry exit to the ashes.
It has been shown by Joule and Mayer that the mechanical force arising from heating 1lb. of water 1° F. is equal to raising 772lbs. one foot high. When we therefore consider the enormous quantity of air necessary to promote combustion of the fuel (one ton of coal requiring about 448,000 cubic feet, or more than 15 tons), and, further, that this immense volume of air requires its temperature raising to that of the burning fuel, it is evident that a very great saving can be effected by heating it before it enters the furnace, for it matters not on what part of the absolute scale this 1° increase occurs, let it only be
imparted to a substance at a lower heat requiring it and we utilize the obscure heat, whether it be 1° or 500°, while, at the same time, the luminous heat in the furnace is vastly augmented, and can act quicker by conduction and radiation upon the contents of the boiler.
The bottom of the furnace is made concave upwards, and is formed by the boiler-plate itself, so that the water is brought close to that part where ignition is the strongest, which could not be effected in a furnace with fire-bars. Steam will be generated at this part of the furnace with immense rapidity, for not only have we radiation of the heat, but conduction too. A number of tubes convey the hot blast upwards, in a convergent direction, through this part of the boiler into the furnace, and these tubes or tuyères, together with the bottom of the furnace, are kept from overheating by the water in the boiler. To prevent the possibility of the spheroidal condition being imparted to the water from the intense heat, this part of the boiler is roughened internally to facilitate the vaporization and agitation of the water; and, still further to insure safety against explosions and to prevent priming, a certain amount of the evolved gases from B, above the ash-pit, are conveyed into the boiler near this part of the furnace, either directly by a pump, or into the feed water, or else into the distilled water of the surface condenser, if one be used. From the repeated vaporizations and condensations of the water in the surface condenser it is deprived of its air, and its boiling point is thus raised from its increased cohesion; but when it is charged with these gases, which it quickly absorbs, then ebullition takes place from the bottom and from all parts of the boiler, and this at a less temperature and with more regularity.
Fresh fuel is added by placing it between doors in front of the furnace, one of which is shut when the other is opened, which serves to keep it sufficiently air tight when fed, at the same time preventing loss of heat by radiation, and rendering it cooler for the fireman; and sufficient fuel is added so that it falls in the proper form of heap of itself, and covers all the tuyères but one to a proper depth. The tuyère which is not covered delivers hot air among the evolved gases, and, therefore, instead of cooling them and thus forming smoke, as cold air would, it heats them and ensures their combustion; and this is effected without cooling the sides of the furnace and boiler. If the fuel be lignite, it will not need stirring, neither will there be any clinkers formed to need removing.
The use of the blast allows the furnace to be made smaller, and because heat varies inversely as the square of the distance, it is obvious that intensity of evaporation of the water is increased by being brought nearer to the centre of heat; and because the density or elasticity of the air is diminished one-half for an increase in temperature of 491° F., therefore it will require discharging
under double the pressure of a cold blast. From the increased diffusibilityof these gases, it follows that the boiler tubes can be reduced in diameter, and consequently made thinner with safety; and, as combustion is rendered complete, there will be no smoke to deposit soot in them.
The advantage to be derived from the use of a hot, in place of a cold, blast is clearly proved in a series of blow-pipe experiments made at the laboratory of the Geological Survey of New Zealand, and published in Vol. II. of the “Transactions,” where it is shown that such refractory substances as platinum, fire-clay, flint, pipe-clay, agate, and opal, were fusible if air at a temperature of 500° F. be employed. And with an exalted intensity of heat in the furnace, we are enabled to avail ourselves to a still greater extent of the economy arising from both the super-heating of the steam and its subsequent expansion in the cylinders of the steam engine.
Description of Plate.
The air tube, the top of which supplies the blower E.
The tube containing the gases of combustion.
Ash pan, containing water.
A door which can drop and close the tube B.
A tube, or space, the full width of the thermo-convector, through which the waste and cooled gases can be discharged upwards to the funnel.
The blowing apparatus supplied with the fresh heated air from the top of the tube A, by means of a space similar to G, and which fills in the front of the convector, but is removed in the figure in order that the interior can be seen.
Reservoir of hot air.
Water space of boiler perforated by the air tubes.
Fire doors, enclosing space holding a charge of fuel.