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How to Prepare Pulverized Coal > A Resume of the Crushing, Drying and Other Processes Involved in the Use of Powdered Fuel—May Be Used on Shipboard ULVERIZED-COAL firing has been so extensively adopted for boilers during the past few years that the methods of preparation of the fuel have become a matter of con- siderable interest to the engineering profession in general. It is the pur- pose of this paper to outline briefly such of these methods as have gained a field of use sufficiently wide to in- volve standardization of the major details in central pulverizing plants. Three operations, crushing, drying and pulverizing, are generally re- quired, and in addition proper means of transportation of the material are necessary. Crushing is eliminated when the coal as received is already small enough to be a satisfactory mill feed. Drying, likewise, may at times be eliminated when the moisture con- tent of the coal as received is very ‘low; but as this is largely a seasonal characteristic, elimination of the dry- ing equipment is not to be recom- mended. ° % Crushing Comes First The crushing operation is necessary, not only to reduce the coal to a size suitable for the pulverizers, but also to expose larger surfaces to the hot gases used for drying. Any stand- ard type of crusher can be used, but crushers having the ability to reduce the coal to proper size in one stage, and without excessive consumption of power, are preferable. To make a satisfactory mill feed, the product of the crusher should pass a 1-inch or 1%4-inch ring to suit the present standard sizes of mills. Coal may be stored before or after crushing. This does not effect the pulverizing, and is generaly determined by the plant layout. If stored out- side it is recovered by means of an overhead ‘or locomotive crane, and then delivered to a hopper located be- low the tracks. A feeder, usually of the apron type, is attached to the hop- per and transfers the coal directly to the bottom of the hopper and trans- fers the coal directly to the crusher, which discharges into an elevator de- > Presented at a joint meeting of the Cleve- land Engineering society and the Cleveland sections of the American Society of Heating and Ventilating Engineers and The American Society of Mechanical Engineers, Cleveland. The author, H. S. Barnhurst, is advisory en- gineer, Fuller-Lehigh Co., Fullerton, Pa. , refractory BY H. G. BARNHURST livering into a bin, from which it is fed to the drier. . The necessity of drying the coal be- fore pulverizing has been often called into question, particularly by oper- ators whose coal supply is normally of low moisture content. The facts are inescapable, however, that drying the coal considerably increases the capaci- Well Known Engineer on Powdered Coal Ernest H. Peabody, president of the Peabody Engineering Corp., in .an address before the _ tech- nical committee of the American Steamship Owners’ association, pointed out that pulverized coal had been successfully burned in shore plants for some time, not- ably in connection with the man- ufacture of cement. Land practice, however, had been accomplished through large furnace volumes which are impossible to secure in marine practice. Recently it was discovered that turbulence of the gases, which may be obtained by giving the air for combustion a rotary mo- tion, makes it possible to reduce the furnace volume well within the limits of marine water’ tube boilers. For Scotch boilers Mr. Peabody stated that while he be- lieved that they can be adapted to powdered coal, the first installa- tion would be more or less in the nature of an experiment. This question he declared would be settled by a thorough test in a marine Scotch boiler plant at the Philadelphia navy yard. The test will take place in January under the combined auspices of the bureau of mines, navy de- partment and shipping board, and then accurate data will be avail- able. The economy to be obtained by. ’ the use of powdered coal Mr. Peabody believed would far off- set any additional costs for equipment. ty and reduces the power consumption of the pulverizers, and makes, its handling after pulverizing a positive instead of a doubtful operation. Damp pulverized material is one of the most substances there is to handle, hanging up and blocking at every point possible, and its trans- portation is unreliable and a continuous annoyance. When dried, the | -28 fuel mixes and burns more readily in the furnace, and, as the moisture must be removed either before or after en- tering the furnace, the effect on fuel economy is almost negligible. Even when coal is very dry as normally re- ceived, there will be occasions when the weather will make drying es- sential, and the absence of proper dry- ing equipment may cause interruptions that will prove to be far more costly than its provision would have been. Drying Is Essential Two types of driers are available: the rotary, which has been stand- ard for many years, and the vertical stationary type, of comparatively re- cent development. The former con- sists of a rotating shell, slightly in- clined‘ and ribbed internally, through which the material passes by gravity in conjunction with the rotation of the drier. The hot gasses are usually passed around the outside of the shell before entering and flowing through it in direct contact with the material, which is picked up by the ribs and dropped across the path of the gases. This is known as the _ indirect-fired type of rotary drier. The vertical drier is a sectional box through which in the usual arrange- ment the coal flows by gravity on its way from the storage bin to the pul- verizer. Hot gases under fan pressure or suction pass through the interstices in the body of the material and carry off the moisture. These gases are pre- ferably taken from the boiler stack, in which case the term “waste heat dier” is usually used. Steam-heated surfaces may also be incorporated in this drier to assist the evaporative ac- tion of the gases. The fact that this drier is stationary and occupies less space than an equivalent rotary drier not to ‘mention its lower cost, makes it preferable under certain conditions. “Cyclones” are essential for remov- ing the dust in the discharge gases from a rotary drier, and are desir- able with a vertical drier, particular- ly when the coal contains a consider- able proportion of fines. A large volume of comparatively low-tempera- ture gases is generally desirable, and pyrometers or thermometers are used to permit proper control. In summer (Continued on Page 380)