It is customary in comparing the heating values of coals, to state the proportionate parts of fixed carbon, volatile matter, moisture and ash as well as the B.t.u. per pound of dry coal. The heat value in B.t.u. per pound of fuel is usually obtained by burning samples in a calorimeter from which the heat per pound is calculated. The heat value of fuels used in power plants are often determined by careful tests of the amount of power derived for each pound of fuel burned in the furnace. This is done by weighing the fuel burned and measuring the water evaporated. The ashes are weighed and this weight together with the weight of moisture present is subtracted from that of the coal to determine the amount of combustible of the fuel. The final results are expressed by the number of pounds of water evaporated per pound of combustible and also the weight to water evaporated per pound of coal burned.
Semi-bituminous coal
represents a class between the hard and soft grades. It contains less carbon and more volatile matter than hard coal. It burns with a short flame with very little smoke and is valuable as a furnace fuel. The Pocahontas coal of West Virginia is an example of this class. Semi-bituminous coal is often called smokeless coal, because in burning it produces relatively little smoke. It will be noted in the table of heat values on [page 192] that coal of this variety has high heat-producing properties. It is a very friable coal and for that reason is apt to contain considerable dust. As a furnace fuel it produces—when carefully fired—very satisfactory results.
Graphitic Anthracite.
—This is a type of coal found in Rhode Island and Massachusetts which resembles both graphite and anthracite coal. It is gray in color, very hard and burns with extreme difficulty.
Cannel Coal.
—This is a variety of bituminous coal, rich in hydrocarbons. It burns with a bright flame without fusing and is often used for open fires.
Lignite.
—This is a type of fuel that in point of geological formation represents the condition between true coal and peat. Lignite occurs in immense deposits throughout the middle portion of the western half of the United States, where beds 20 feet in depth are not uncommon. It varies in color from black to brown and in many localities is known as brown coal.
When newly mined, lignite contains a large percentage of water, sometimes as high as 50 per cent. On account of this large moisture content it has a relatively low calorific value, but when dry the amount of heat evolved per pound compares very favorably with soft coal.