PULVERIZED COAL 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 object of pulverizing the coal is to make a more complete mixture of the coal particles with the air in order that complete combus tion 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 adciinonal cost of pulverizing the coal is, 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: "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 stightly, 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 Y4-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." Although experiments in burningpulverized coal were begun as early as 1876, there has not as yet been any thoroughly satis factory method of taking care of the ash resulting from the burn ing 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 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 com paratively 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, pow dered 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, riveting and forging, there is likely to be a difference of opinion." In a recent article 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 watch ing 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.