This article first appeared in Practical Engineering 1940 Vol1 No23. The content of the article is accurate as of 1940. This article describes developments in Engineering at the time.
The use of pulverised coal as a fuel is not a new idea, and the principles of this system of boiler firing will be familiar to many readers. Briefly, it consists of reducing raw coal to fine powder and feeding it into the furnace under a continuous air blast.
The choice of any particular method of firing is governed by the conditions existing on each individual job, but it is frequently found that boilers with a very large unit capacity are arranged for pulverised fuel firing. An important consideration in this connection is that such a method of firing dispenses with a large live fuel bed, and in the event of failure of the feed-water supply the fires can be extinguished immediately. Questions of the type of steam cycle to be employed and the grade of coal available also have to be considered.
Types of Mill
From the mechanical engineer's point of view, the chief interest in this system lies in the design of the mill used for the reduction of the raw coal to its powder form and mixing it with the air stream.
Apart from high-speed pulverisers there are two types to be considered —the slow-speed ball or tube mill and the medium-speed ball or roller mill, such as the Raymond, Loesche, Rema and Lehigh mills. This article describes a new mill recently produced by Babcock and Wilcox, Ltd., which supersedes the Lehigh mill and is known as the type "E" mill.
Automatic Response to Load
The adoption of the direct system of firing simplifies boiler-room control to such a degree that the Type E mill has been likened to the carburetter of a motor-car engine. Quick response to load demand is obtained by changing the ratio of air passing through the throat of the pulveriser. The feeder motor is of the two-speed type, and the adjustment on the air-to-mill supply determines which of these two speeds will operate. In addition, there is a hand adjustment on the feeder plough, which is set to take a mean of the load, so that the automatic control takes care of a change in load each side of the mean by altering the feeder speed.
The design of the Type E mill is clearly illustrated in the accompanying illustrations. It will be seen that the . mill consists essentially of two horizontal grinding rings, between which are arranged a row of balls. The raw coal is fed into the middle of the top ring, the pulverising action taking place as it finds its way, aided by centrifugal force, between the rotating balls, where it drops over the edge of the bottom ring into an upward current of hot air, which at this point has a comparatively high velocity. Separation is effected by the rotating classifier, assisted by the drop in velocity of the coal and air in the upper portion of the pulveriser. The fine powder is carried on by the air stream and enters the furnace via the burners, while the oversize particles are returned for further grinding.
The automatic feeder control operates so that the output of material is governed by the quantity of air fed to the pulveriser and maintains the correct relation of coal to air over the entire operation range. At reduced outputs, there is, of course, a lower air velocity through the mill, and consequently the material is pulverized more finely, which is most desirable in order to maintain good furnace conditions, particularly when firing coal with a low volatile content.
Owing to the absence of any considerable coal-storage capacity, this type of pulveriser is very responsive, and one test undertaken by the makers showed an increased and maintained steam flow from 100,0001b. per hour to 276,0001b. per hour in less than three minutes.
Uniform Grinding Pressure
The internal drive of the pulveriser is assembled in the base, the gear chamber forming a sump for the oil used to lubricate the gears, pinion, and main-shaft bearings. Forced feed is provided, the lubricating-oil pump being external to the mill to facilitate examination and adjustment.
The grinding elements consist of one row of forged steel balls between the rotating" bottom grinding ring and the stationary top ring. The balls are propelled by the rotating bottom ring, which is driven from the main driving shaft. Grinding pressure between the balls and the rings is maintained at a uniform pressure by steel springs in the top section. These are arranged for independent external adjustment.
Design of the Grinding Element
The design of the grinding element is such that the balls retain their true spherical shape throughout their life. Throughout the mill, the bearings used are of the roller type and a special air seal is provided above the top bearing on the vertical shaft to prevent any infiltration of dust-laden air from the grinding zone. The top section of the mill is constructed of heavy steel plate and provided with large dust-tight doors to give easy access to the grinding elements, which, if necessary, can be removed and replaced in a few hours.
The air supply is by means of a fan operating on the clean-air side of the mill. This makes possible the use of a high-efficiency fan of steel-plate construction, as the impeller is not subjected to erosion by coal particles as would be the case if the fan were to be situated between the pulveriser and the burners.
The raw coal is introduced to the pulveriser via a table feeder of the low-speed rotating disc type, which is designed to give a regular feed over the full range of operation. The rotating table and scraper gear are enclosed in a mild steel housing, and special provision is made in the construction of the driving mechanism to prevent the infiltration of coal dust. In the majority of cases, the rotating table is operated by a two-speed ¾ / 3/8 h.p. electric motor.
One of the most interesting features of this mill is the automatic feeder controller. This automatically maintains a constant ratio between the rate of airflow and the resistance to the flow of air through the pulveriser. Changes in the rate of coal feed to the burners are made simply by adjusting the damper in the hot-air pipe, and a uniform mixture of pulverised coal and air in the correct proportions for efficient combustion is always delivered to the burners.
The mechanism of this control consists of two diaphragms working in opposition to each other. One is actuated by the air-pressure differential across the orifice plate, measuring the airflow through the pulveriser, and the other, actuated by the drop in air pressure through the pulveriser, which varies with the quantity of coal in the pulveriser. Any unbalanced relation between the two diaphragms changes the position of contact points and the position of the latter determines the speed of the feeder motor. The type "E" mill is commendably quiet in operation, as, indeed, it must be for installation in quiet boiler rooms where conversation is possible in a normal voice, i.e., a sound level of 83 to 87 decibels, but it should be stressed that this is a factor which is greatly influenced by the type of ground on which the foundations are installed and the immediate surroundings.