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test may show that 80 per cent of the heat of the coal is absorbed by the boiler, but when the heat represented by a month's evaporation is divided by the heat of the coal fed to the furnace during the same period the efficiency may drop to 70 per cent or lower. In an eight-hour day plant the fires must be banked at the conclusion of the day's run and this banking occasions a fuel loss
FIG. 15.-Influence of ash on fuel value of dry coal.
which is obviated when oil is used. Table 13 gives the coal burned during banking periods:
In hand-fired boilers another loss is occasioned by the opening of the fire box door, which admits a great inrush of cold air, reducing fire box temperatures and preventing the complete combustion of carbon so that the loss of heat units through the stack is greately increased.
To overcome the obvious disadvantages in burning raw coal screenings, the idea was conceived of pulverizing the coal and introducing the pulverized coal into the furnace by air pressure. The early attempts to burn pulverized coal under stationary boilers were unsuccessful because the coal was not thoroughly dried and was not pulverized finely enough. In introducing the powdered coal into the furnace, too high pressures were used, resulting in a blow-pipe effect creating zones in the furnace in which the gases had high velocity. The impingement of these gases against the refractories caused a serious erosive action. Later experiments showed that seven feet per second is the maximum velocity which can be maintained without destruction of the refractories. The
(B) Coal fed to furnace during baking period including that required to put boiler into service at end of banking period.
(C) Coal fed to furnace to put cold boiler into service, pound.
object of pulverizing the coal is to make a more complete mixture of the coal particles with the air in order that complete combustion may be obtained with a low percentage of excess air. All grades of coal can be burned in pulverized form with high efficiency, regardless of the percentage of ash. The additional cost of pulverizing the coal is, however, an important item. In an address recently delivered before the American Society of Mechanical Engineers, Mr. H. B. Barnhurst, chief engineer of the Fuller Engineering Company, gave the following estimate of the cost of pulverizing coal:
a. Gebhardt, Steam Power Plant Engineering, p. 72.
"The following cost of pulverizing is made of a number of items as follows: Power, repairs, drier fuel and labor. The first two items are nearly constant. The drier fuel will vary slightly, according to the price at which coal is received. The cost of labor diminishes as the quantity of coal increases. In the following table the power is assumed as costing 4-cent per kw-hr. Repairs at 7 cents per net ton. The dried fuel is based on coal at $5 per net ton delivered with an average moisture content of 7 per cent, assuming that 6 per cent of moisture would be driven off per pound of coal in the drier. The furnace labor is assumed at 50 cents per hour."
COST OF PULVERIZING AND DELIVERING THE PULVERIZED FUEL TO BOILER FURNACES.
No interest, depreciation, insurance or taxes have been included in the above total.
Although experiments in burning pulverized coal were begun as early as 1876, there has not as yet been any thoroughly satisfactory method of taking care of the ash resulting from the burning of the coal. When a slack coal with a high ash percentage is pulverized, the pulverized coal still contains the same percentage of ash as did the slack. Under the high heat developed in a fire box which burns powdered coal this ash forms a pasty slag which adheres to the sides and bottom of the fire box. The removal of this slag is accomplished with great difficulty and unless the slag is removed at frequent intervals, draft is interfered with and heat radiation to the boiler is decreased. Mr. C. F. Herrington,
probably one of the highest authorities in the United States on the burning of powdered coal, makes in Engineering News the following comparison between oil and powdered coal:
"Of the three fuels, powdered coal, oil and water gas, fuel oil has come into use far more than any other. The U. S. Navy Yards have been consistent in their adoption of it. All now use fuel oil for heating operations, many to the complete exclusion of coal. Without a doubt, fuel oil is one of the easiest of fuels to handle; it can be carried in pipes anywhere so long as there is air pressure or pump pressure behind it. It requires only a comparatively small outlay for equipment-all that is necessary is a couple of storage tanks, a pump to fill the storage tanks from the cars, a piping system to the furnaces, and means to secure the necessary pressure. As a fuel for burning under boilers, powdered coal may some time be a success. The use of powdered coal in Portland cement manufacture has proven very economical and here it has come to stay. But when it is claimed that it is equally good for various heating operations, such as welding, shingling, annealing, riveting and forging, there is likely to be a difference of opinion."
In a recent article in an engineering paper the following advantages were claimed for powdered coal:
(1) "Complete combustion, doing away with losses due to the carbon contained in the ash and in the escaping volatile matter." This is not correct, for if one stands for an hour watching one of these furnaces working, as the writer did, he will be completely covered with fine, unburned powdered coal, which has escaped through the furnace doors. This has become such a nuisance to the surrounding machinery and workmen that attempts are now being made to relieve these conditions by placing a hood over the furnace door and connecting it into the furnace stack. This has not proven successful as yet, and probably will not until an exhaust fan is provided to discharge this unburned coal through the roof.
(2) "Total absence of smoke." Certainly this is not true. inside of the shop, for powdered-coal furnaces, due to their nonuniform feed, smoke worse than oil. Powdered coal, as is well known, must be very dry to be pulverized and, when pulverized and allowed to remain quiet for 48 hours, it cakes and requires. that a man knock on the bins to loosen it. This leads to uneven
combustion in the furnace with large quantities of smoke when there is a large amount of coal coming through the burner and no smoke when the coal is sticking back in the bins. No doubt this is largely due to inefficient handling of the feeder and burner ; even so, a total absence of smoke cannot be claimed when such conditions are met.
(3) "A cheaper grade of coal may be used." The best coal for powdered fuel has a volatile content of not less than 30 percent, not more than 8 percent ash, and 14 percent sulphur. I think the readers will agree that coal meeting these specifications is of no very cheap grade. Pulverized coal must be handled with great care, for if it is mixed with any quantity of air, it is highly explosive, as the records of accidents in cement plants will prove.
Another very serious objection to powdered coal, due to the incomplete combustion of all the coal ejected into the furnace, is that this coal lies on the work, and when the work is taken out of the furnace, if not cleaned off, it is apt to be hammered into the work and make flaws which later are likely to be more or less serious according to the nature of the work. This is a fact seen from personal observation and cannot be denied. Powdered coal is not good for small furnaces, as it requires too large a chamber for combustion, and from the experience of users of powdered coal it is not desirable to have a combustion chamber separated by a bridgewall from the working chamber. It is found that the lesser of two evils is to remove the bridgewall and blow the powdered coal directly upon the work, which aggravates the condition mentioned above. If the large furnaces are changed from fuel oil to powdered coal, there will remain the small furnaces, and especially the portable ones, which will have to work on fuel oil. Then there would be the expense of handling two kinds of fuel where before there was but one. The pulverizing plant is to be considered. When it is reported that it costs only 30 to 50 cents a ton to perform a multitude of operations, I feel that some one has misplaced the decimal points, as will be shown later on.
Now comes the debatable point of what is the efficiency of the furnace when using the different fuels. The powdered coal advocates will claim that the efficiency should be figured on the B.t.u. basis. That is, if a furnace burns, say 22 gallons of oil