The other important substances entering into the composition of coal are moisture and the refractory earths which form the ash. The ash varies in different coals from 3 to 30 per cent and the moisture from 0.75 to 45 per cent of the total weight of the coal, depending upon the grade and the locality in which it is mined. A large percentage of ash is undesirable as it not only reduces the calorific value of the fuel, but chokes up the air passages in the furnace and through the fuel bed, thus preventing the rapid combustion necessary to high efficiency. If the coal contains an excessive quantity of sulphur, trouble will result from its harmful action on the metal of the boiler where moisture is present, and because it unites with the ash to form a fusible slag or clinker which will choke up the grate bars and form a solid mass in which large quantities of unconsumed carbon may be imbedded. Moisture in coal may be more detrimental than ash in reducing the temperature of a furnace, as it is non-combustible, absorbs heat both in being evaporated and superheated to the temperature of the furnace gases. In some instances, however, a certain amount of moisture in a bituminous coal produces a mechanical action that assists in the combustion and makes it possible to develop higher capacities than with dry coal. Classification of Coal--Custom has classified coals in accordance with the varying content of carbon and volatile matter in the combustible. Table 36 gives the approximate percentages of these constituents for the general classes of coals with the corresponding heat values per pound of combustible. TABLE 36 APPROXIMATE COMPOSITION AND CALORIFIC VALUE OF GENERAL GRADES OF COAL ON BASIS OF COMBUSTIBLE +-------------------+----------------------------+--------------+ | Kind of Coal | Per Cent of Combustible | B. t. u. | | +------------+---------------+ Per Pound of | | |Fixed Carbon|Volatile Matter| Combustible | +-------------------+------------+---------------+--------------+ |Anthracite |97.0 to 92.5| 3.0 to 7.5 |14600 to 14800| |Semi-anthracite |92.5 to 87.5| 7.5 to 12.5 |14700 to 15500| |Semi-bituminous |87.5 to 75.0| 12.5 to 25.0 |15500 to 16000| |Bituminous--Eastern|75.0 to 60.0| 25.0 to 40.0 |14800 to 15300| |Bituminous--Western|65.0 to 50.0| 35.0 to 50.0 |13500 to 14800| |Lignite | Under 50 | Over 50 |11000 to 13500| +-------------------+------------+---------------+--------------+