735 Riddle Road Cincinnati, Ohio 45220
I am an atom of carbon. The tale I am about to unfold for you contains mystery and excitement. Before I begin to relate my story, may I describe myself. I was lying at my ease in a bed of coal. About 280 million years ago, during the Pennsylvanian Period within the Paleozoic Era, I helped in the formation of this coal. In all humility, I might say the birth of coal was a most 'carboniferous' time! By the Middle Pennsylvanian Period, swamps bordered a vast but shallow inland pond stretching across most of what later would become the United States of America. In those warm days, the plants growing in the bogs and mires across eastern Ohio and western Pennsylvania were buried in muck even before they decayed. By sinking in such stagnant water, this plant material did not rot, but slowly changed to peat. With more and more weight from more and more layers on top, the peat compacted into lignite, then into bituminous coal. Where I was located, the peat became compressed into a layer only 1/25 of its initial thickness.
I was part of a kind of coal about seventy percent of which was carbon like me. Five percent was hydrogen, six percent oxygen. The rest was incombustible, made up largely of the earthy matter on which the plants once grew. This moderately dense and fairly black coal glistened with a waxy luster.
Recently, workers mined the coal and me from the ground where we had been resting for a quarter of a billion years or so. Trucks hauled us for miles. An engineer loaded 820 pounds of this coal in what he called 'fuel bunkers,' and I found myself near a riveted seam. Naturally, I felt disoriented but that was nothing, compared to what I was soon to feel.
A few taps of the engineer's hammer struck off an egg-sized lump of the chunk of coal including me. He lifted this lump and several others on a scoop, swung open a small door, and scattered the coal just inside. The piece of coal which I was in dropped onto a thin layer of burning embers! The door clanged shut. Wow! Was it ever hot in there!
From my vantage point, I could see the engineer had spread the fresh coal over the thinnest part of the fire; where I was, the embers were only about three to four inches in depth. The engineer's quick but dexterous action had dropped the new coal precisely where holes were about to burn through, which would leave unwanted dead spaces in the fiery layer. Just beneath the embers were grates which the engineer could rock to remove ash and clinkers (pasty coal and ashes melted together in a fairly incombustible mass). The walls of the furnace were made of a special metal called 'firebox steel' and were 11/32 of an inch thick. The ceiling of the chamber was known as the crown-sheet; from above, stay bolts made of double-refined iron supported it. The height from the grates to the crown-sheet was thirty inches. Thirteen inches below the grates was an ash-pan. The length of the furnace was 39 inches, width 25 inches. The area of the grates was 6.9 square feet. I was sweltering inside of the firebox of an antique steam traction engine.
The flames licking just above the 'live' or burning coal brought the temperature of that part of the fire in excess of two-thousand degrees! Luckily, the crownsheet had water above it; otherwise, the intense heat concentrating on this firebox 'ceiling' might have melted the soft metal of the fusible plug, allowing the pressurized steam (and water from lower down in the boiler) to come rushing into the furnace, thereby extinguishing the fire and possibly doing damage. Were there an unreliable fusible plug and not water above the crown-sheet, the entire ceiling could blow down into the firebox and, perhaps, blast off the fire door or spawn a more menacing explosion.
Powerful wind currents puffed and whipped through, around, and above the coal. The five percent of hydrogen in the lump of coal containing me began to unite with some of my fellow carbon atoms. The temperature in the vicinity had risen to about two-thousand degrees. Soon, the hydrogen atoms had bonded with three times their weight of carbon and were becoming a gas. About one-fifth of my coal-lump was thus transformed into vapor. The bonding action of the hydrogen and carbon required a great deal of energy, and heat in the firebox was converted into this energy. In fact, this change of heat into energy to form chemical bonds partially cooled the fire!
The gusts of air contained oxygen, which began to combine with the hydrogen/carbon gas. This union meant that combustion was now occurring. Even though the fire had cooled somewhat by the transfer of heat into energy to form the hydrogen/carbon gas coming off the new coal, the fire in the live coal was still hot enough to ignite the gas. Because of this igniting temperature, the oxygen and gas could combine which is to say that combustion could happen. (At lower temperatures, a gas can flow along with air [containing oxygen], and no chemical combination takes place hence, no combustion.) The air also contained about eighty percent nitrogen, but nitrogen does not aid in combustion and, indeed, hinders it somewhat.
The hydrogen in the gas was more combustible than the carbon, so it burned first. Bluish flames began to leap near me. This bonding of hydrogen and oxygen formed water and, at the high temperature in the furnace, this water appeared in the form of steam. As the hydrogen in the gas was combining with the oxygen in the air, it was released from its bond with my carbon colleagues. We carbon atoms never allow ourselves to be in a gaseous state all alone; in these situations, we always prefer to be gaseously combined with other substances. As quickly as the hydrogen burned away from my fellow carbon atoms, they formed a fine soot. At the hot igniting temperature in the furnace, much of this soot combined with oxygen and glowed. Each particle radiated light for only an instant; then the carbon/oxygen combination was perfected, producing a transparent gas. Comprised of 'zillions' of twinkling carbon soot particles, the yellow flames only appeared continuous because new carbon particles, in their turn, were released from more of the burning hydrogen, were heated, were ignited, and glowed for a moment. Some of the soot did not meet up with oxygen and, therefore, did not catch fire. It was borne rapidly upward on the wind, was carried to the front wall of the furnace, and was forced through tubes which opened into that wall. (Smoke is nothing more than this unconsumed soot, the ineffectual nitrogen from the air, and the steam produced by the combination of hydrogen and oxygen.)
As the engineer but recently had opened the firedoor to scatter new lumps of coal (including me) on the fire, the inrush of cool air into the furnace had lowered the temperature somewhat especially along the edges of the firebox. In a few places, the temperature was just a little too low for all of the particles of carbon to ignite. Also, where the temperature was sufficiently high, the grates, now well covered with new coal, could not introduce enough oxygen to go around for the gases being released from all that fresh coal. Quite a high number of my carbon colleagues blew away as black soot not having met up with oxygen and, hence, not having undergone combustion or not having reached igniting temperatures. The smoke, thus, was thick at first.
May I pause in my narrative to say that many authorities ascribe to the idea that too much smoke is an indication of a poor engineer. In the extreme case of an engineer constantly opening the firebox door and dumping far too much coal on a fire far too thick, the dense smoke and the fact that the engineer can hardly keep up steam despite his continued efforts would indicate an engineer with little knowledge of firing a boiler. James H. Maggard put this matter in definite and memorable terms:
[A] good fireman doesn't make much smoke. We are now speaking of firing with coal. If I can see smoke ten miles from a threshing engine, I can tell what kind of a fireman is running the engine. If there is a continuous cloud of black smoke being thrown out of the smokestack, I make up my mind that the engineer is having all he can do to keep the steam up; and also conclude that there will not be much coal left by the time he gets through with the job. While, on the other hand, should I see at regular intervals a cloud of smoke going up and lasting for a few minutes, and for the next few minutes see nothing, then I conclude that the engineer of that engine knows his business. . . .We will first go to the engine that is making such a big smoke, and we will find that the engineer has a big coal-shovel just small enough to allow it to enter the firedoor. You will see the engineer throw in about two or perhaps three shovels of coal, and as a matter of course we will see a volume of black smoke issuing from the stack. The engineer stands leaning on his shovel watching the steam gauge, and he finds that the steam doesn't run up very fast, and about the time the coal gets hot enough to consume the smoke we will see him drop his shovel, pick up a poker, throw open the firedoor, and commence a vigorous punching and digging at the fire. This starts the black smoke again, and about this time we will see him down on his knees with his poker, punching at the underside of the grate bars; about the time he is through with this operation the smoke is coming out less dense, and he thinks it time to throw in more coal, and he does it. (87-88)
Maggard depicted an extremely incompetent engineer. In more moderate situations, however, the presence of smoke does not necessarily condemn the engineer. No matter how black the smoke may be, it is only between a half to one and a half percent of the carbon in the coal. To the person who assumes that smoke is evidence of inefficiency, I would say (from personal experience) that far greater losses occur which cannot be seen. For example, radiation of heat not transferred to the water in the boiler is a large waste. More significantly, much unconsumed hydrogen/carbon gas passes through the tubes and out the smokestack with the smoke. And these losses are by no means the only ones all of them invisible. The careful regulation of the locations and thick nesses of live coal and of new coal on the grates and the expert admission of air in the right amounts and at the right time have the most to do with efficiency.
Where was the air coming from in the firebox in which I found myself? Front and rear draft doors permitted air to circulate upward through the grates. In the firedoor, a vent consisting of over half a dozen small apertures introduced air above the fire, and this source contributed thirty percent to the total air in the furnace. As the engine on top of the boiler was running, it was exhausting steam in rapid bursts through a nozzle pointing upward inside the smokestack; this action greatly enhanced the draft, which caused the currents of wind above the fire to be best described as 'violent.' Such strong wind helped to mix the air and the gases from the coal, and that mixing ensured that more of the gas would be ignited and consumed all for the purpose of generating heat to boil the water under pressure in the boiler so that the steam could power an engine.
Suddenly, the temperature in the lump of coal where I was located reached the point where I and many other carbon atoms were caught up with hydrogen in gaseous form. Just as quickly, the hydrogen dropped us and formed steam with oxygen, and, in virtually no time, I combined with two atoms of oxygen. Together, the three of us atoms had now become carbon dioxide. Nearby, a fellow carbon atom found only one oxygen atom to join; they formed carbon monoxide. They generated less than one-third as many heat-units as I and the two oxygen atoms created. In a firebox, then, carbon monoxide is wasteful. Complete combustion (like mine) is desired.
Although I had now been burned, I was not destroyed. I simply existed in a new combination with oxygen. Combustion does not destroy matter any more than it creates it. Combustion merely transforms matter! There is a hopeful lesson in that fact!
At the instant that I and a number of my colleagues a number so high as to be beyond imagination entered into combustion by uniting with oxygen to form carbon dioxide, we were in a swirl of volatile matter igniting into a long yellow flame; inside that flame was a great deal of uninvited hydrogen/carbon gas. Flames, so to speak, burn only on the outside. To borrow an analogy, I would explain that a flame is like a clear current of water flowing all around the outer surface with a muddy current going along within. Even though terrific gusts of wind were tearing through the furnace, they could not offer every hydrogen atom and every carbon atom enough oxygen to burn.
Let me linger at this moment in my story to share a few fascinating facts. It takes approximately four and a half pounds of air to furnish one pound of oxygen. It takes 2.66 pounds of oxygen to bring one pound of carbon into perfect combustion (forming carbon dioxide). The complete combustion of a pound of carbon yields 14,500 heat-units, but, when carbon monoxide is formed (imperfect combustion), only 4,400 heat-units result. The engineer must admit excess air into the firebox to ensure that perfect combustion takes place most of the time. This surplus air has to be heated to the temperature of the furnace, and part of it may not mix when combined with the hydrogen or carbon; certainly, a portion is wasted, carrying through the tubes and up the smokestack all those heat-units which were spent on raising the temperature of the unused air. Furthermore, the introduction of fresh air at, say, seventy degrees, lowers the overall temperature in the firebox and increases the volume of gases in the furnace and, hence, the velocity at which the gases escape through the tubes, thus giving the hot gases less time to communicate their heat through the tubes to the water on the other side. Yes, it is possible to admit too much air into the furnace. But, if the firebox were to have too little air, imperfect combustion would occur. It takes about twelve pounds of air to give enough oxygen for perfect combustion of one pound of carbon under ideal circumstances, but, as reality always differs from the ideal situation, much more air than that is required per pound of carbon in a firebox like the one I was occupying. A wise engineer balances the potential losses from unused air and from incomplete combustion. (By the way, the combustion of hydrogen yields a great heat; one pound of hydrogen generates approximately 62,000 heat-units. But remember that the coal with which I was associated contained only five percent hydrogen.)
Now, to return to my tale because the engineer was talented, he had the air well regulated. Much of the carbon monoxide which formed among the embers arose, met more air coming in above the fire, and became carbon dioxide by uniting with the additional oxygen higher up. This second combustion above the fire improved the efficiency.
After much of the gas had been expelled from the coal, the ember was what is known as 'coke' (in a general sense); with the exceptions of incombustible sand, ashes, slate particles, and cinders, the coke was almost pure carbon, much of it directly uniting with oxygen to form carbon dioxide. Many expert firemen recommend spreading new coal just inside the firebox door, waiting for the gases to burn off, pushing (with a hoe) the coke toward the tube wall (called a fluesheet), and permitting the extra heat of this layer of coke to give more perfect combustion to the gases escaping overhead from the next scattering of fresh coal inside the fire-door. This process has scientific merit, but the firedoor must be left open a little longer to shove the coke to the back; it is doubtful whether the improved efficiency of combustion compensates for the air disturbances caused in this way. Also, the thickness of the coke layer eventually works against the even combustion of the carbon in that coke. Other authorities suggest spreading fresh coal on one side of the firebox, allowing it to coke, then adding new coal to the other side of the furnace; thus, a fair portion of the gases arising from the fresh coal may enjoy increased opportunity to burn thoroughly before being pulled through the tubes. Still others advise keeping a very thin layer of embers throughout the firebox (especially along the troublesome edges) and adroitly adding new coal in any spots which are about to burn through to the grates. This method begs for an engineer having considerable skill at 'reading' a fire and at handling a small scoop in the brief interval that the firedoor may be left open.
I had no time to ponder the coke and the pros and cons of various means of firing the boiler, for I was now part of carbon dioxide gas myself and was rushing toward the tubes. The uppermost tubes fostered the greatest draft, and my fortune was to be carried into the centermost tube at the top. Depending on the capriciously fluctuating and forceful winds in the firebox, I could have been thrust toward any of the thirty-eight tubes, although the draft ordinarily pulls powerfully toward the topmost tubes. After diving into the two-inch-diameter tube, whatever flame I had been in was extinguished by contact with incombustible gases and by being out of reach of sufficient air. The temperature within the tubes, surrounded as they were by water, was too low to sponsor any further igniting.
Down the 90 inches of tube I flew. As I was entering the smokestack, the temperature of the stream of smoke, steam, and gases which carried me along was just over four-hundred degrees. Where had all the heat gone? The firebox temperature had averaged about two-thousand degrees despite the facts that the quantity of air in the furnace was (as it should have been) practically double the amount theoretically necessary and that air took plenty of heat-units to get hot. It was a good thing that the engineer had so carefully controlled the vents admitting air above the fire and had so quickly opened and closed the firedoor when putting in more coal, or else the temperature could have dropped below the minimum levels needed for igniting! (Also, were he to have left the firedoor open any longer than essential, he could have injured the sheet of metal into which the tubes opened by allowing such a relatively cold blast of air to continue to strike against it.) Anyway, I had been very hot, and now, at the point of exiting the smokestack, I was much less hot than I had been. I and the other gases had yielded much heat to the firebox, crownsheet, and tubes, which had passed on a portion of that heat to the pressurized water in the boiler. I had done my part to generate steam in that water beyond the tubes, and that steam would run the engine. Each pound of coal had evaporated about six pounds of water. The knowledgeable engineer was using about four and a half pounds of coal per brake horsepower per hour; in other words, at a load of fifty horsepower, his engine was consuming 225 pounds in one hour.
As I was jettisoned from the smokestack, I looked down on the large crowd of people attending the reunion of the Richland County Steam Threshers Association at Malabar Farm State Park near Mansfield, Ohio. They were admiring the 1917 50 HP Case which I had just helped to run. I thought how my having been mined from that ancient bed of coal had been worthwhile, for I and my colleagues (my 'carbon copies,' you might say) had assisted in demonstrating the important history of mechanical contributions to the science of agriculture.
For factual information, this article relies upon many sources, notable among them:
Boss, William The Heath Book for Threshermen. Winnipeg: Heath, 1908. Case catalogues, parts lists, and manuals (all reprints).
Forney, Matthias N. Catechism of the Locomotive. New York: Railroad Gazette, 1897.
Holmes, George C. V. The Steam Engine. London: Longmans, Green, 1900.
Kent, William. The Mechanical Engineers' Pocket-Book. New York: Wiley, 1895.
Maggard, James H. The Traction Engine: Its Use and Abuse. Philadelphia: David McKay, 1915.
The Power Catechism. New York: McGraw, 1897.
Rose, P. S. Steam Engine Guide. Madison: American Thresherman, 1910.
Thurston, Robert H. A Manual of Steam-Boilers. New York: Wiley, 1904.