COMBUSTION AND FUELS 81 



the fusing prevents the air from passing freely through the 

 bed of burning fuel. When caking coals are burned, the spongy 

 mass must be frequently broken up with the slice bar, in order 

 to admit the air needed for its combustion. Free-burning 

 coal is a class of bituminous coal that is often called non-caking 

 coal from the fact that it has no tendency to fuse together 

 when burned in a furnace. Cannel coal is a grade of bituminous 

 coal that is very rich in hydrocarbons. The large percentage 

 of volatile matter makes it valuable for gas making, but it is 

 little used for the generation of steam, except near the places 

 where it is mined. 



Lignite. Lignite, or brown coal, contains from 30 to 60% of 

 carbon, a small quantity of hydrocarbons, and a large amount 

 of oxygen. It occupies a position between peat and bituminous 

 coal, being probably of a later origin than the latter. It has 

 an uneven fracture and a dull luster. Its value as a steam 

 fuel is limited, since it will easily break in transportation. 

 Exposure to the weather causes it to absorb moisture rapidly, 

 and it will then crumble quite readily. It is non-caking and 

 yields but a moderate heat, and is in this respect inferior to 

 even the poorer grades of bituminous coal. 



Miscellaneous Fuels. Coke is made from bituminous coal 

 by driving off the volatile matter. It consists of from 88 to 

 95% of carbon, i to 2% of sulphur, and from 4 to 12% of ash. 

 It is little used for steam-boiler fuel. 



Wood is used for fuel in localities where it is plentiful. It 

 contains from 20 to 50% of moisture when cut, and this per- 

 centage is not reduced much below 20% by drying. Wood 

 has a calorific value of 6,000 to 7,000 B. T. U. per Ib. 



Peat consists of vegetable matter that is partly carbonized and 

 is found at the surface of the earth. It contains from 75 to 80% 

 water when cut, and must be dried before it can be used as fuel. 



Bagasse is the refuse left after the juice has been extracted 

 from the sugar cane by means of the rolls. It is used to some 

 extent in tropical and semitropical countries. Naturally, its 

 use is limited to the places where the sugar cane is grown. 



Dried tan bark, straw, slabs, and sawdust being refuse, their 

 use is local and usually confined to tanneries, planing and saw- 

 mills, and threshing outfits. 



