272 



NATURE 



[July 17, 1890 



point, but it is pretty generally admitted that cases of 

 heating and ignition are more frequent in coals shipped 

 from east coast ports than in South Wales shipments. 

 So much, however, depends upon the quantity of small 

 coal present, that a well-loaded cargo of any coal would 

 be safer than a cargo of Welsh steam coal in which a 

 quantity of dust had been produced during loading. 



4. The size of the coal (small coal being much more 

 liable to spontaneous ignition than large.) — This is due 

 to the increase of active absorbent surface exposed to the 

 air, a fact which is verified by the experience of large 

 consumers of coal on land ; gas managers recognizing 

 the fact that coal which has been stamped down or shaken 

 down during storage is more liable to heat than if it has 

 been more tenderly handled, the extra breakage causing 

 the extra risk. 



5 . Shipping coal rich in pyrites {or brasses) whilst wet. 

 — The effect of external wetting on coal is to retard at 

 first the absorption of oxygen, and so to check the action ; 

 but it also increases the rate of oxidation of the pyrites, 

 and they, when oxidized, swell and split the coal into 

 pieces, and this increases heating due to the exposure of 

 fresh dry surfaces. 



6. Ventilation of the cargo. — For ventilation to do any 

 good, cool air would have to sweep continuously and 

 freely through every part of the cargo — a condition im- 

 possible to attain in coal cargoes — whilst anything short 

 of that only increases the danger — the ordinary methods 

 of ventilation supplying just about the right amount of 

 air to create the maximum amount of heating. The 

 reason of this is clear. A steam coal absorbs about twice 

 its own volume of oxygen, and takes about ten days to do 

 it under favourable conditions, and it is this oxygen which, 

 in the next phase of the action, enters into chemical com- 

 bination, and causes the serious heating. Ventilation, such 

 as used to be sometimes arranged for by a box shaft along 

 the keelson with Venetian lattice up-shafts, supplies about 

 as much air as is necessary to produce the results which 

 end in spontaneous ignition. 



7. Rise in temperature in steam colliers, due to the in- 

 troduction of triple- expansion engines and high-pressure 

 boilers. — The increase in stokehold temperature, due to 

 this, is from 5" to 10° F., and this affects the temperature 

 of the adjacent parts of the vessel. 



In the coal bunker, the question of mass, which plays 

 so important a part in a hold laden with coal, is almost 

 entirely eliminated, as 50 to 400 tons would be about the 

 capacity of any ordinary bunker, and the cases of spon- 

 taneous ignition in masses of coal less than 500 tons do 

 not amount to more than \ per cent. The question of 

 initial temperature, therefore, becomes the one important 

 factor. Bunker fires are almost entirely confined to 

 vessels in which the bunker bulkheads are only separated 

 from the funnel by a narrow air-space, or are in close 

 proximity to the boilers themselves ; but where the 

 bunkers are stepped back from the funnel casing and 

 boilers, spontaneous ignition is a great rarity. If coal is 

 kept at a high temperature, even though it be far below 

 its igniting point, ignition is only a question of time, and 

 if the bunker coal next the bulkhead is kept at 120° F., 

 any coal with a tendency to absorb oxygen will run a 

 great chance of igniting within a few days. In order to 

 prevent spontaneous combustion of the coal under these 

 circumstances, all that is necessary is to reduce the 

 temperature of the bulkhead in contact with the coal, as 

 if this is kept at a temperature not exceeding 80° to 90° 

 F., there is little or no fear of the oxidation of the hydro- 

 carbons of the coal proceeding with such rapidity as to 

 cause ignition in such a quantity of coal as can be carried 

 in the bunkers, the iron decks, by subdividing the mass, 

 also helping to reduce any risk. In order to reduce the 

 temperature to the required extent, it would be necessary 

 to make the bulkheads close to any heating surface, such 

 as the funnel casing, double, and the side spaces six inches 



NO. 1 08 1, VOL. 42] 



apart, the inner wall being provided at intervals with 

 water-tight openings, through which the interior space 

 can be coated with protective compositions from time to 

 time. Through this double casing sea-water would be 

 allowed to circulate very slowly, and would effectually 

 prevent any undue rise of temperature, whilst to make 

 the arrangements complete a thermostat should be fixed 

 on the inner plate of each bulkhead, which, if the tempera- 

 ture rose to 100° F., would ring a bell in the captain's 

 room, when the rate of flow of water could be increased 

 until the required fall in temperature took place. Should 

 this arrangement prove impossible from any structural 

 cause, then a rapid current of air forced through the 

 bunkers by means of a fan, or even an up-current formed 

 by a good air-pump ventilator in the crown of the bunker, 

 would go far to keep the temperature within safe limits. 

 If such an arrangement were adopted in the fast liners, 

 bunker fires would become a thing of the past, whilst such 

 an arrangement of double bulkhead and water circulation 

 would also solve the still more important problem of how 

 to keep the magazines on board Her Majesty's ships at a 

 sufficiently low temperature to fit them for the storage of 

 E.X.E. and S.B.C. prism powders, and the still more 

 delicately constituted smokeless powders, none of which 

 could otherwise be kept in the auxiliary magazines of the 

 new programme ships ; as for safety they are placed 

 between the boilers, and must, of necessity, reach a tem- 

 perature far above that which any powder could stand 

 without losing moisture, and in consequence developing 

 far higher strains than the guns should properly be 

 subjected to. 



The question of explosions in coal bunkers and in the 

 holds of coal-laden ships is a subject totally distinct from 

 that of spontaneous ignition. During the conversion of 

 woody fibre derived from various forms of vegetation 

 into coal, considerable quantities of a gaseous compound 

 of carbon and hydrogen, called methane, marsh-gas, or 

 light carburetted hydrogen, is evolved, and as the action 

 has been spread over long ages most of this gas has 

 found its way to the surface of the coal seam and has 

 diffused itself through the superincumbent soil and has 

 escaped ; but a portion has been occluded (absorbed) in 

 the pores of the coal itself, and some also imprisoned in 

 small cavities and fissures in the coal. Marsh-gas, when 

 pure, is perfectly non-explosive, .and burns quietly with a 

 faint luminous flame, producing, as the products of its 

 combustion, carbon dioxide and water vapour, but when 

 mixed with ten times its own volume of air, and a light 

 applied, it explodes with a force equal to about 210 lbs. 

 on the square inch. Another cause which tends to in- 

 crease the danger of explosion is that if the air is charged 

 with fine coal-dust, less than one per cent, of marsh-gas 

 mixed with it gives an explosive mixture, and also ex- 

 tends the area of explosion. In both colliers and coal 

 bunkers the risk of explosion is greatest during the first 

 ten days after shipment. 



Marsh-gas is a non-supporter of combustion, so that 

 the presence of the gas, or a mixture of it with air, if 

 present, is a safeguard against spontaneous ignition ; and 

 if the precautions pointed out to prevent ignition were 

 carried out in conjunction with simple precautions against 

 explosion, explosions and fires in coal cargoes and bunkers 

 would soon be a thing of the past. 



The lecturer strongly advocated the adoption in the 

 bunkers of all new vessels of the double bulkhead, and 

 water circulation to such portion of the bunkers as im- 

 pinge upon any unduly heated portion of the hold, and 

 that all bulkheads should be made gas-tight ; whilst in 

 bunkers containing not more than 300 to 400 tons of coal, 

 as thorough ventilation as possible should be obtained 

 by fitting water-tight air-pump ventilators in the deck 

 above the surface of the coal, while inlets for as cool 

 air as possible should be provided at the bottom of 

 the bunkers, and, where necessary, air driven in from the 



