(2) Gases or volatile materials which, escape from the coal when it is heated and which burn with a flame.
(3) Gases or volatile matter and water which escape from the coal when it is heated and which do not burn.
(4) Ash or mineral matter which does not burn and which remains as ashes after the coal is burned.
The bituminous coals of the central field (Illinois type) con tain from 40 to 55 per cent of fixed carbon, 10 to 25 per cent of combustible gas, 5 to 15 per cent of non-combustible gas, 8 to 15 per cent of moisture, and 8 to 15 per cent of ash. When improp erly fired or blamed in furnaces' not adapted to their use, central bituminous coals give off so large an amount of 'sooty, material that flues are often quickly clogged. These unconsumed volatile products also represent a direct loss of heat value. Coals of the Illinis, type ignite easily and burn freely. • The moisture arid non-combustible gases present in all coals are detected only by chemical analysis. They not only do not produce heat, but represent a definite loss because they absorb and carry off heat which would otherwise be available for useful purposes. The term moisture in coal does not mean the water adhering to the surface of the lumps, but that contained within the pores of the coal. A coal containing a high percentage of moisture by analysis limy appear perfectly dry.
The ash content of different coals varies greatly. Ash is non-combustible mineral matter, which not only has no heating value, and therefore, represents a portion of the coal from which no return is received, but it may hinder the free burning of the combustible components of the coal. If the ash contains certain mineral substances, it may by clinkering greatly interfere with the process of firing and with the cleaning of grates. The ash normally is removed through the ashpit into which often passes also a certain amount of unburned coal. For this reason the amount of ashes removed from the pit usually represents a larger percentage of the fuel fired than the analysis of the ash content indicates.
The eastern bituminous coals contain from 5 to 10 per cent of ash, from 25 to 35 per cent of combustible gases, from 2 to 5, per cent of moisture and non-combustible gases, and from 55 to 65 per cent of solid carbon. They are more generally of the coking variety than are the Middle West coals. In general, they are higher in heating value and lower in ash. They are more friable and are not so well suited for transportation and repeated hand ling as are many of the central bituminous coals.
Moisture in coal represents an appreciable loss in economy inasmuch as the coal may carry 16 per cent moisture and the heat required to evaporate it must be furnished by the coal itself, thus decreasing the amount available to heat water in the boiler. In excellent practice the per cent of the calorific value of coal as fired, which is lost by the evaporation of free' moisture, is given by Gebhardta as 0.5 per cent ; in average practice, 0.6 per cent;
and in poor practice, 0.7 per cent.
Fig. 14 is a chartb prepared by Mr. Joseph Harrington, com bustion engineer, showing the influence of, moisture in coal on its evaporative power as a fuel. With a moisture content of 30 per cent,• slightly more than 10,000 heat units out of a total of 15,000 are available.
It is obvious that ash is simply a diluting material, but never theless when slack coal is burned the ash content of the coal has been transported from the mine to the industrial plant, which may be hundreds of miles away. Freight for the ash and mois ture content of slack coal Must be paid for, although they are not only of no value, but actually are an added expense in operation. (See fig. 15.) The loss of coal through the grates is a serious item. The refuse from a fuel is that portion which falls into the pit in the form of ashes, unburned or partially burned fuel and cinders. In steam bOiler practice the unconsumed carbon in the ash pit ranges from 15 to 50 per cent of the total weight of dry refuse depending upon the size and quality of coal, type of grate and rate of driving. The loss resulting from this waste of fuel ranges from 1.5 to 10 per cent or more, of the heat value of the fuel. It is impossible to assign a minimum value because of the various influencing factors, but numerous tests of recent installa tions, equipped with mechanical stokers, indicate that actual loss ranges from 1.5 to 5 per cent of the heat value of the fuel at normal driving rates. Coal which necessitates frequent slicing is apt to give greater losses from this cause than a free burning coal.
The losses of atm.' due to the combustible matter in the refuse per pound of coal as fired may be calculated by the follow ing formula : A X 14600 (1 — C) Where A = chemical ash in coal C= percentage of combustible matter in the refuse. 14600 = calorific value in B.t.u. of one lb. of carbon burned to CO,.
At a coal-burning installation a continuous 24-hour full load test may show that 80 per cent of the heat of the coal is absorbed by the boiler, but when the heat represented by a month's evap oration is divided by the heat of the coal fed to the furnace during the same period the efficiency may drop to 70 per cent or lower. In an eight-hour day plant the fires must be banked at the con clusion of the day's run and this banking occasions a fuel loss which is obviated when oil is used. Table 13 gives the coal burned during banking periods : In hand-fired boilers another loss is occasioned by the opening of the fire box door, which admits a great inrush of cold air, reducing fire box temperatures and preventing the complete com bustion of carbon so that the loss of heat units through the stack is greately increased.