August 25, 192 1] 



NATURE 



813 



entering the larger volume of the collector is 

 deprived of its velocity and the coal dust drops. 

 With the screen separator the coal is elevated by 

 a bucket elevator and conveyed to the furnace by 

 a screw conveyor. 



There are a number of different forms of burn- 

 ing the dust in use, the fuel being driven into the 

 fire-box by means of either fans or compressed 

 air. In one system the air pressure is exerted in 

 the tank, which is in connection 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 system the coal dust is 

 drawn from the storage bin as required, mixed 

 with air, and carried in suspension through pipes 

 to the furnace at a velocity of 5000 ft. per minute. 

 Mixing air with the coal dust would appear to 

 increase the liability to explosion. In yet another 

 system the dust from the feed worms is blown 

 into the fire-box, the fuel and air passing as a 

 cloud into the fire-box and being ignited by a 

 piece of waste soaked in paraffin. 



One economy incidental to the use of pulverised 

 fuel under boilers has already been mentioned — 

 viz. reduction in the amount of fuel as compared 

 with lump coal to secure a given heat result. 

 Other economies may be mentioned, as follows : — 



(a) Ability to use low-grade coal. 



(b) Saving in labour of stoking. 



(c) Flexibihty of the operation, coal-dust firing 



being almost equal in this respect to oil 

 firing. 



(d) Elimination of "banking" and easier dis- 



posal of ashes. 



(e) Possibility of safely working the boilers at 



loads largely in excess of their normal 

 rating. 

 (/) Ease of control of furnace conditions in the 

 case of metallurgical furnaces. 



Against these advantages, however, must be 

 ranged the cost of preparing and conveying the 

 pulverised fuel and the interest and depreciation 

 on the capital ciutlay. These are very variable 

 items, dependent, as they are, on local conditions 

 in respect of labour, power, and fuel, but chiefly 

 on the output per day of the plant. For instance, 

 in the United Kingdom, under present conditions, 

 it is not considered a paying proposition to use 

 pulverised fuel in the case of stationary boilers 

 having a lower fuel consumption than 40 tons of 

 coal per diem. On the other hand, with a fuel 

 consumption of 200 to 300 tons per diem a hand- 

 some saving can usually be secured by the re- 

 placement of lump coal by pulverised fuel under 

 almost any conditions. As a rough guide it may 

 be taken that with almost any of the well-known 

 standard " systems " the cost of preparing, pulver- 

 isingj and burning in the form of dust i ton of coal, 

 will be about 55. in the case of a plant dealing 

 with 100 tons of coal per diem. Of course, the 

 higher the price of the raw fuel the greater the 

 saving by using it in pulverised form. 



Pulverised coal has been successfully applied to 

 NO. 2704, VOL. 107] 



almost every kind of heating work, with the pos- 

 sible exceptions of open-hearth steel furnaces, 

 steamships, and such furnaces as glass tanks, 

 where contamination of the charge from particles 

 of ash is to be avoided. The first really successful 

 application of dust-coal fuel was in respect of 

 rotary cement kilns, where the conditions are such 

 that the problem of the disposal of the ash does 

 not exist, and a large combustion volume is avail- 

 able with a free, unobstructed passage for the 

 flame. The next step in its application was in 

 the direction of various types of metallurgical 

 furnaces, more particularly reheating, puddling, 

 and similar furnaces, and complete success has 

 been obtained in most cases. 



The case of stationary steam boilers of the 

 water-tube type has been found more difficult of 

 treatment. In the early stages of the adaptation 

 of coal dust to firing, considerable trouble was ex- 

 perienced from the ash and the rapid wear of the 

 furnace lining and from irhperfect combustion. 

 Experience has shown the way of avoiding these 

 troubles, and it is now a fact that pulverised 

 fuel can with complete and permanent success be 

 applied in raising steam from any type of tube 

 boiler. In the case, however, of the cylindrical 

 internal flue type of boiler — as, for example, the 

 Lancashire boiler — the process of dust firing has 

 not, so far^ proved successful under continuous 

 operation ; but, seeing that firing with " straight " 

 oil and with " colloidal " fuel has succeeded in this 

 type, there seems no reason why the problem 

 should not in time be solved in respect of coal- 

 dust firing. 



Perhaps the most difficult conditions for the 

 successful application' of pulverised fuel were those 

 in respect of locomotives, owing to the small com- 

 bustion area available and the cramped conditions 

 generally ; yet recently it has been completely suc- 

 cessful, and locomotives equipped with this system 

 of firing are in use in the United States of 

 America. A fact of peculiar importance, as point- 

 ing to a means of utilising low-grade fuel, is that 

 on the Brazilian Central Railway some locomotives 

 are being worked with pulverised coal derived 

 from local deposits of inferior quality in place of 

 using high-grade imported lump coal. In 

 England a system for locomotive use has been 

 successfully developed and has been in service for 

 some time with very successful results. Enough 

 has been said to show that the preparation and 

 use of pulverised coal have been brought to a prac- 

 tical and economic stage, and where the conditions 

 are suitable its use constitutes a proposition 

 worthy of the serious attention of large consumers 

 of fuel. To readers who wish to pursue this matter 

 further the perusal is recommended of the report 

 in the Bulletin already named, the Report of the 

 Fuel Research Board, and Mr. C. F. Herington's 

 work on "Powdered Coal as a Fuel." 



As illustrative of recent developments, two 

 cases may be quoted. One is from the Bulletin, 

 in which M. Frion says : — 



" Nous ne citerons que I'exemple particulifere- 



