PULVERIZED FUEL. The use of coal in the form of dust for raising steam has been known from isolated experiments for about the last forty years, but the fact that it was one of the most difficult methods of burning coal long delayed the develop ment of the practice.
The theoretical advantages of burning crushed coal become obvious when one considers that if a cubic inch of coal, which has an exposed surface of six square inches, is crushed into cubes each of which has a side one-hundredth of an inch in length, the area of the exposed surface is multiplied one hundred times, and a more intimate mixing of the coal and air is rendered pos sible, and this without using a large excess of air. It is consid ered good practice under normal conditions with an average boiler furnace under conditions of mechanical stoking if i so to 200 per cent of excess air is being admitted to the furnace. Un der pulverized coal conditions of firing no difficulty is experi enced in working regularly with not more than 20 to 3o per cent of excess air.
Inasmuch as coal-dust cannot be burnt in the ordinary way, special burners have to be provided. These differ in design, but the principles underlying all of them are the same. The coal is dried, pulverized, and the dust passing to the furnace is conveyed to a burner, mixed with air and burnt in the form of a jet. A point common to all the systems is that of the fineness to which it is necessary to reduce the coal. It has to be so fine that all of it will pass through a ioo mesh screen (that is to say a screen containing IooX ioo=io,000 apertures to the square inch) and 85 per cent of it through a 200 mesh screen.
There are a number of different forms of burners, the dust being driven into the fire-box by means of either fans or com pressed air. In one system the air pressure is exerted in the tank, which is connected with the furnace by means of a pipe and the dust forced in a stream, unmixed with air, through the pipe to the furnace. In another it is drawn as required from the storage bin, mixed with air, and carried in suspension through pipes to the furnace at a high velocity. In yet another system the dust from the feed worm is blown into the fire-box.
Whilst the chief use of powdered fuel is in its application for raising steam, it is also largely used in cement works and in steel furnaces.
The economies claimed in respect of the use of coal in the powdered form over the lump coal are :— (I) A reduction (by reason of the more complete combus tion) in the amount of fuel necessary to secure a given heat result.
(2) Ability to use low-grade fuel.
(3) Saving in labour as stoking is rendered unnecessary.
(4) Flexibility of the operation, coal dust firing in this respect being almost equal to oil firing.
(5) Elimination of "banking" and easier disposal of ashes.
(6) Ease of control of furnace conditions in the case of metal lurgical furnaces.
Of late years oil has been taking the place of coal in the gen eration of power in the movement of ships. Thus of the ships under construction in the year 1927 only about 31 per cent were those in which coal will be used, 69 per cent being driven by oil either to raise steam or used in internal combustion (Diesel) en gines. The relative comparative efficiencies and costs of oil versus coal driven ships may be expressed as follows:— The high of oil engines gives i S.H.P. for lb. of oil and I lb. of oil costs of a penny, so that i S.H.P. in a Diesel engine costs of a penny. In an ordinary marine steam engine and boiler, I S.H.P. requires 1.7 lb. of coal, which (with coal at 24/– per ton) costs M of a penny, and one coal produced S.H.P. costs M of a penny. In the particular case of "King George V.," built in 1926 and using high pressure steam, this has been reduced to of a penny, so that it has been demonstrated that coal is a cheaper fuel than oil for ship propulsion where high pressure steam is used, i.e., 500 to 600 lb. per square inch. With pulverized fuel still further economies would be secured.
In America the development in the use of pulverized coal since 192o has been exceptionally rapid and probably the world's consumption of pulverized fuel at the present time exceeds 15o million tons annually. In the United States of America the use of pulverized coal per annum has been estimated as follows:—