8l2 



NATURE 



[August 25, 192 1 



Pulverised Coal as a Combustible. 



By Sir R. A. S. Redmayxe, K.C.B. 



COAL, which has, ever since the growth of 

 modern industrialism, proved the main 

 source of artificial heat, power, and light in civil- 

 ised countries, is likely to continue to occupy that 

 position for very many years to come. In some 

 industries it constitutes the chief item of cost in 

 production ; in others it is second only to that of 

 labour. Its importance, therefore, as a factor in 

 the cost of living is very great indeed. That the 

 price of coal, at any rate for a long time, will be 

 maintained beyond a pre-war level cannot, I 

 think, be controverted. The higher wage de- 

 mands of labour incident to the advance in the 

 standard of comfort claimed are not likely to be so 

 abated as to bring wages down to a^ pre-war posi- 

 tion ; for the same reason the cost of the materials 

 so largely used in mining — e.g. timber, steel, 

 lubricants, and machinery — will remain at a high 

 rate. The chief hope of securing a reduction in 

 the cost of production must lie along the lines of 

 research. Similarly, also, the reduction in the 

 cost of our fuel bill must be sought in economy 

 in use — that is to say, in an endeavour to use 

 efficiently every calorie available in the fuel. 



In this connection the use of coal in the form 

 of dust has for some few years been occupying 

 the attention of engineers, particularly during the 

 last five years, and more especially in North 

 America. In the year 1919 the Fuel Research 

 Board published a brochure on the subject, and 

 the May number of the Bulletin de la Societe 

 d' Encouragement pour I'Industrie Nationale con- 

 tains a most interesting article by M. Prion en- 

 titled "Le Chauffage au Charbon pulverise," being 

 a report of the "Commission d'Utilisation des 

 Combustibles," in which it is stated that " le de- 

 veloppement devint assez rapide a partir de cette 

 ^poque, et actuellement les industries du fer et de 

 I'acier emploient environ 3 a 4 millions de tonnes 

 de charbon pulverise par an, et les industries du 

 cuivre un tonnage a peu pr^s ^gal. " 



The use of coal in the form of dust for raising 

 steam had, from isolated experiments, been known 

 for the last thirty to forty years, but the fact that 

 it is probably the most difficult method of burning 

 coal delayed the development of the practice until 

 means were discovered of surmounting the 

 obstacles in the way of its use. When it 

 is considered 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, and the 

 exposed surface of the crushed coal becomes 

 600 square Inches, the theoretical advantage of 

 burning crushed coal becomes obvious. A more 

 Intimate mixing of the fuel and air Is rendered 

 possible, and this without using a large excess of 

 air ; for example, with an average boiler furnace 

 fitted with mechanical stoking it is considered 

 good working practice under normal conditions 

 NO. 2704, VOL. 107] 



if 150 to 200 per cent, of excess air is being ad- 

 mitted to the furnace; on the other hand, under 

 pulverised-fuel firing there is no difficulty In work- 

 ing regularly with not more than 20 to 30 per 

 cent, of excess air. 



One of the diflSculties which originally lay in 

 the way of the widespread use of pulverised fuel 

 was the heat engendered in the grinding of the 

 coal to the requisite fineness, sometimes resul^ng 

 in combustion. Again, inasmuch as coal dust 

 cannot be shovelled into and burnt in an ordinary 

 furnace, special burners had to be provided. How- 

 ever, a number of well-tried and standard methods 

 for both the preparation and the burning of the 

 fuel are now in existence, the underlying prin- 

 ciple of all of them being the same, though 

 differing in the design of the various parts of the 

 equipment. The coal is dried, pulverised, and 

 the dust, passing to a furnace, is conveyed to a 

 burner, and then, mixed with air, burnt in the 

 form of a jet. Each system has its own peculiar 

 methods of performing these operations, some 

 systems being more suited to certain conditions 

 than others. A point common to all the systems, 

 however. Is that of the fineness to which it is 

 necessary to reduce the coal. It has to be ground 

 so fine that the dust will pass through a loo-mesh 

 screen {i.e. a screen containing 10,000 apertures 

 to the square Inch), and 85 per cent, through a 

 200-mesh screen (i.e. a screen having 40,000 aper- 

 tures per square Inch). In order to effect this 

 the coal must be dried so as not to contain more 

 than I per cent, of uncomblned moisture, the dry- 

 ness being necessary from the point of view of 

 manipulation, as the fuel must be capable of being 

 handled without clogging or sticking in the feed- 

 ing and burning equipment. In the process of 

 drying, care has to be taken against overheating, 

 which may result in loss of volatile hydrocarbons. 

 The cost of securing a higher degree of fineness 

 than that specified above is not justified by the 

 extent of the Increased efficiency obtained. On 

 the other hand, practice has shown that if the 

 degree of fineness is much below the standard 

 named above troubles arise due to deposits of ash 

 and slag and from Irregular burning. 



The separation of the coal ground to suitable 

 fineness from that which is not of sufficient fine- 

 ness is effected by screening or by air separation. 

 In the latter method a stream of air at constant 

 velocity carries away from the crushed coal par- 

 ticles of a certain definite size and so secures a 

 uniform product, but the use of an air separator 

 requires upwards of 50 per cent, more power to 

 work it than a screen to perform the same amount 

 of useful work, in addition to which the cost of 

 maintenance of the former is heavier, due to high 

 velocity and excessive strains. With air separators 

 the mixture pf air and coal dust is carried to a 

 cyclone dust collector, where the stream of air 



