Popular Science Monthly/Volume 14/December 1878/Explosions from Combustible Dust
I WISH to demonstrate to you this evening, by a few simple experiments, the fact that all combustible material when finely divided forming a dust or powder, will, under proper conditions, burn with explosive rapidity.
If a large log of wood were ignited it might burn a week before being entirely consumed; split it up into cord-wood, and pile it up loosely, and it would burn in a couple of hours; again, split it into kindling-wood, pile loosely as before, and perhaps it would burn in less than an hour; cut it up into shavings and allow a strong wind to throw them into the air, or in any way keep the chips comparatively well separated from each other, and it might be entirely consumed in two or three minutes; or, finally, grind it up into a fine dust or powder, blow it in such a manner that every particle is surrounded by air, and it would burn in less than a second.
Perhaps you have noticed that shavings and fine kindlings will sometimes ignite so quickly in a stove that the covers will be slightly raised, the door forced open, or perhaps small flames will shoot out through the front damper. You have, in such a case, an explosion on a very small scale similar to that of the Washburn, Diamond, and Humboldt Mills of this city, on the night of May 2d—upon which occasion the rapid burning of hundreds of tons of flour, bran, etc., completely demolished the solid-masonry walls, six feet thick, of the mills, and threw sheets of iron from the roof of the Washburn so high into the air that they were carried two miles by the wind before striking the ground.
Let us see now why such explosions occur. Wood has in it a large amount of carbon, the material of which charcoal is composed, and the air is about one-fifth oxygen. Now, at the ordinary temperature, the carbon of the wood and the oxygen of the air do not combine; but, when they are heated, as by friction, concentration of the sun's rays, chemical action as from a match, or in any other way, they combine to form carbonic-acid gas. This chemical action produces a large additional amount of heat which keeps up the action as long as there is any carbon and oxygen left to unite, and also makes the temperature of the gas which is formed very high.
As the space occupied by the carbonic-acid gas and that occupied by the oxygen which entered into the combination is the same at the same temperature, there would be no bursting if, after combination, the temperature were the same as before; but it is a fact, which you have all observed, that fuel in burning produces heat; it is also a fact that heat expands a gas, and it is this great amount of heat, taken up by the carbonic acid formed, that produces the immense pressure in all directions.
Let us return to our log of wood. There is exactly the same amount of heat and carbonic acid produced when complete combustion takes place in each of the cases of burning, the only difference being as to time. In the first case, the explosion or pushing aside of the surrounding air occupies a week, in the last only a second.
Snow-flakes fall gently upon your shoulders, and you are required to perform an insensible amount of work to resist the crushing effect of each flake; but, suppose that all the snow that has fallen upon your head and shoulders for the last ten years was welded together in one solid mass of ice, weighing perhaps one hundred pounds, and that it should descend with the velocity of a snow-flake upon you, an immense effort would be required to prevent its crushing you, even if you were able to withstand the shock at all. The work of many days would be concentrated into an instant.
So it is with burning wood: four or five cords of wood, and a large stove, will give you a roaring fire all winter; the work done is manifested by the heat obtained, by the rushing of hot gases up the chimney, and of air from outside into the room through every crack. But, if the wood were ground into a powder and scattered through all the house, and burned instantly, the cracks, doors, windows, and flues, would not be sufficient to give vent to the hot gas, and the roof and sides of the house would be blown to pieces.
What is true of wood is also true of grains; also of vegetables, with their products when they contain carbon, with this exception: grain, either whole or ground, will not burn readily when in bulk. A fire could be built upon a binful of flour, and kept burning for half a day without igniting the flour; it would char upon the surface, but it lies in such a compact mass that the air does not get access to it readily, hence it does not burn.
I wish to show you now how combustible dust will burn when blown into the air by means of a pair of ordinary hand-bellows.
I have here two boards, about twelve by eighteen inches, nailed together, forming a V (see Fig. 1). Just outside of the V an ordinary Bunsen's gas-burner is placed, and within is a small handful of dust taken from a sash-and-blind factory. Upon blowing it smartly with the bellows a cloud is formed about fifteen feet high—extending, in fact, to the ceiling—which ignites from the lamp and produces a flash, very quick and exceedingly hot, resembling very much a gunpowder-flash. You will notice that a large amount of dust falls from all around the edge of the flame without burning; that is because it is not thick enough. Two things are necessary: first, that each grain of dust be surrounded with air, so that it can get the oxygen required instantly; and, secondly, that each grain shall be so near its neighbor that the flame will bridge over the space and pass the fire from particle to particle.
I think, after seeing the immense flame produced by such a small amount of fine saw and sand-paper dust, you will no longer wonder at the rapid spread of flames in furniture and similar factories. You know it is practically impossible to put out a fire after any headway is attained in these establishments; the draught produced will blow all the dust from walls and rafters into the air, and the building in an instant is a mass of flame. Perhaps many of you remember the fire in the EastSide Saw-Mills, a few years ago. Large masses of fine sawdust had probably collected upon the rafters, and the whole roof was perhaps filled with cobwebs loaded down with dust. A fire started from one of the torches used and shot through the mills with lightning-like rapidity, and, save for the fact that the ends and sides of the building were all open, there would have followed an explosion like that at the flour-mills. As it was, the men had very great difficulty in escaping with their lives, notwithstanding that a short run in any direction would have taken them out of the mill.
It is very evident that too great care cannot be taken to keep all such factories and mills as free from dust as possible.
I will now blow some ordinary starch into the air in the same way, and you notice the flame is more vivid than in the last experiment, and, if you were in my position, you would notice that the heat produced is much greater. Notice now that this powdered sugar burns in the same way.
You will see from the experiments further on that three-quarters of an ounce of starch' will throw a box, weighing six pounds, easily twenty feet into the air, and that half an ounce, burned in a box, will throw up the cover three inches with a heavy man standing upon it.
With these facts, which I have demonstrated before you, no one need regard as a mystery the Barclay Street explosion in New York, where a candy-manufactory, in which large amounts of starch and sugar might in many ways be thrown into the air by minor disturbances, took fire and completely wrecked a building and destroyed many lives.
I will now burn in the same way some buckwheat, which, as you will observe, gives a very large blaze; now some corn-meal, which is too coarse to burn as well; now some rye-flour, which burns much better
than the corn; now some oatmeal, the finer part of which only burns; and so I might continue with all sorts of finely-ground vegetable material.
Let us take up now the products of the manufacture of flour from wheat. There were between three and four hundred tons of these materials, upon which I am now to experiment, in the Washburn Mill at the time of explosion, and there was a corresponding amount in the Diamond and Humboldt Mills, which, by their sudden burning, produced the second and third shocks heard directly following the explosion of the larger mill.
The wheat is first placed in a machine where it is rattled violently and brushed. At the same time a strong draught of air passes through it, taking up all the fine dust, straw, etc., and conveying it through a spout to a room known as the wheat-dust room, or perhaps more commonly it is blown directly out of the mill.
You see some of this material here; it looks like the wood-dust of the first experiment, and, as you see, burns with a quick and sudden flash when subjected to the same conditions.
Here, then, we have the first source of danger in a flour-mill. A thick cloud of this dust, when conveyed through a spout by air, will burn in an instant if it takes fire; and, if there is any considerable amount of dust, as there would be if there were a dust-room, an explosion will follow which may become general if it stirs up a thick dust-cloud throughout the mill.
The wheat after it has been cleaned in this way goes to the crushers, which are plain or fluted iron or porcelain rollers, working like the rollers in a rolling-mill. The object of these rollers is, I believe, to break off the bran in as large pieces as possible, and to crush out or flatten the germ so that it can be separated with the bran from the rest of the meal.
The crushed wheat goes now to the stones, where so much heat is produced (average 135° Fahr.) that a large amount of steam is formed from the moisture in the materials. This steam would condense in the meal and interfere with bolting, etc, if it were not removed. To effect this another draught of air and another spout are employed, and, as might be expected, this current takes a large quantity of the very finest flour, called flour-dust, with it. To save this a room is provided near the end of the spout, called the flour-dust house. The spout conveying steam and dust enters this room on one side, and another spout opposite leaves it, passing to the open air. It is in this comparatively dead-air space that the dust settles, and can be collected from the floor. Here is some of this material, which, as you see, when blown into the air, produces a vivid flash, extending from the table to the wall.
The evidence taken before the coroner's jury shows very clearly that it was this material that started the great explosion of May 2d. Just how the mill took fire will probably never be known of course, but in all probability the stones either ran dry—that is, were without any meal between them—or some foreign substance, such as a nail, was in the feed, producing a train of sparks such as is produced by an emery-wheel, or a scissors-grinder's wheel. These sparks set fire to small wads of very hot dust, which, as soon as they were fanned into a blaze, communicated it to the spout and house full of dust. An eye-witness of the explosion first saw fire issuing from the corner of the mill where this flour-dust spout was situated, the end of the spout having probably been blown out. This fire was followed instantly by a quick flash, seen through all the windows of the floor upon which the flour-dust houses were situated, followed instantly by a flash in the second story, then the third, and, in rapid succession, fourth, fifth, and sixth stories; then followed the great report produced when the immense stone walls were thrown out in all four directions, and the roof and part of the interior of the mill shot into the air like a rocket.
It would seem that a blaze is necessary to ignite the mixture, for I have tried powerful electric sparks from a machine, and from a battery of Leyden-jars; also incandescent platinum wire in a galvanic circuit, and glowing charcoal, without producing any fire, however thick the dust might be. Perhaps, however, under more favorable conditions the dust would ignite directly from sparks, but it seems very improbable.
Let us continue now with the process through which the ground wheat is made to pass. From the stones it is conveyed to the bolting reels, where the very finest is sifted out first, and we obtain a grade of flour; after the finer material is sifted out it goes to a coarser bolt, where the "middlings," as it is called, passes through, leaving the bran which comes out at the end of the reel. The middlings, as it comes from the bolts, has fine bran and dust in it, and, to purify it, it is subjected to an operation similar to that of cleaning the wheat, that is, in the middlings purifiers it is subjected to a draught of air which takes away all the light bran and dust, leaving the heavier material (purified middlings), which goes again to the stones to be ground into flour.
Here is some of the dust from these "middlings-machines;" you observe it burns as the other materials burned, quickly, and with intense heat.
Here is some of the purified middlings; each grain is comparatively large and heavy, making it difficult to blow it well into the air, but, as the blaze produced by each particle is quite large, a flash is produced which does not differ materially from the others.
Here is some of the general dust of the mill, that is, dust swept up from the floors, walls, beams, etc. You will see it acts in all respects like the other substances.
And, finally, here is some of the flour taken this afternoon from the flour-sack at home; it burns, you observe, if possible with even more energy than the other kinds of dust.
I have performed a few experiments, which I will now repeat, which will illustrate to you the immense power that these materials exert when burned in a confined space.
This box (Fig. 2) has a capacity of two cubic feet; the cover has a strip three inches deep nailed around it, so that it telescopes into the box; there is in this lower corner an opening for the nozzle of the bellows, in this an opening for the tube to the lamp. I place now a little flour in the corner, light the lamp, and my assistant places the cover upon the box and steps upon it. Take notice that upon blowing through the hole, and filling the box with a cloud of flour, the cover comes up suddenly, man and all, until the hot gas gets a vent, and a stream of fire shoots out in all directions.
Here is a box (Fig. 3) of three cubic feet capacity, including this spout, nine inches square and fifteen inches long, coming from the top of it-at the ends doors are arranged closed like steam-boiler manholes; openings for light and bellows are arranged as in the previous box.
Here is a box, weighing six pounds, that will just slip over the spout; it has a rope lest it should strike the wall after the explosion. Placing now the lamp in the box, some dust in the corner, and the box over the spout, we are ready for another explosion. You observe, after blowing vigorously for a second or two, the dust in the box takes fire; the box over the spout is shot off, and rises until the rope (about twelve feet long) jerks it back; it strikes the stage with great force, rebounds and clears the foot-lights, and would strike the floor below were it not for the rope.
I have thrown a box similar to this in the open air twenty feet high, while, as we shall see presently, less than an ounce of flour is being consumed.
I have fastened over the top of the spout five thicknesses of newspaper; upon igniting a boxful of dust as before, the paper is thrown violently into the air, accompanied by a loud report as it bursts.
For the last experiment I have a box of four cubic feet capacity (Fig. 4); five sides are one and a half inch thick, the remaining side one-quarter inch. Upon igniting the dust in this box, filled as in the other cases, the quarter-inch side bursts, and a stream of fire shoots out half-way across the stage.
One pound of carbon and two and two-thirds pounds of oxygen, when they combine to produce carbonic acid, will evolve heat enough if it were applied through a perfect heat-engine, to raise 562 tons ten feet high; if, therefore, forty per cent, of flour is carbon, it would require two and a half pounds to accomplish this result, if an engine from which there would be absolutely no radiation, conduction, or loss of heat, in any way, were a practical possibility. Let us see how much air would be required to supply oxygen enough. Under ordinary conditions every 100 cubic inches of air contains 7.13 grains of oxygen, from which we find that 1512 cubic feet of air would be required for the 23 pounds of oxygen. Hence the 22 pounds of flour must be equally distributed as a dust through 1512 cubic feet of air, in order to produce the most powerful result.
If 41 ounces of flour requires 151 cubic feet of air for perfect combustion, one cubic foot of air will supply oxygen enough for 151 of an ounce of flour. Hence our box, which lifts the man so readily, burns one-half ounce of flour or less; and the other, which throws the box into the air, three-quarters of an ounce, unless, as I think quite probable, an additional amount of air is drawn in through the cracks as soon as the vent is opened at the top of the box. In fact, these experiments work better if a few small holes are made near the bottom of the boxes.
It may be worthy of mention here, as a point of interest to insurance companies that, in all dust-explosions, a fire precedes the explosion in every case. The dust must burn before the heat that produces the immense expansive force is generated.
Too great precaution cannot be taken in all kinds of manufactories, where combustible dust is produced, against fire, especially in those establishments where it is conveyed in thick clouds by air-draughts through spouts and rooms.
- Lecture delivered June 1, 1878, at Association Hall, Minneapolis, Minnesota, at the request of the millers of the city.