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T a b l e 1. G e n e r a l D a t a of C o m b u s t ib le E le m e n t s a n d Com pounds From International-Critical Tables, 1927. HEATING VENTILATING AIR CONDITIONING GUIDE 1941 144 CHAPTER 7. COMBUSTION AND FUELS t on temperature before it will unite in chemical combination with '^en to produce combustion. The ignition temperatures for several of ^combustible constituents of fuels are presented in Table 1. HEAT OF COMBUSTION As previously stated, the process of combustion results in the evolution f heat The heat of combustion, or calorific value, of a fuel is the amount f heat generated by the complete combustion of a unit of the fuel and is Constant for a given combination of combustible elements and compounds. The heat of combustion of the several fuel elements and compounds in their pure state is given in Table 1. The reaction of the carbon in the fuel with oxygen may result in the formation of carbon monoxide or carbon dioxide. In burning to carbon monoxide, the carbon is not completely oxidized and, as shown by the data, the heat produced is considerably less than if it were completely oxidized. This fact is of greatest importance in considering the efficiency of combustion. The calorific value of a fuel is determined by direct measurement of the heat evolved during combustion in a calorimeter. As the calorific value, on a moisture and ash free basis, of coal from a given district or mine remains substantially constant the calculation of the calorific value of a particular lot of coal can be made if the lot is analyzed for moisture arid ash. From a known reliable calorific value for coal from the sarixe mine or district the calorific value on a moisture and ash free basis, often called the H value, is calculated from Formula (1). _. , . , ., 100 X Calorific value (as received) ,,. Calorific value, moisture and ash free = ------- -1-0--0--------- .(M..o.ist-u--r-e----+;--Ar--shrr)------ (1) If a dry or moisture free analysis is used it is necessary to correct for ash only to reduce to moisture and ash free basis. From the value ob tained by Formula (1) the calorific value for the sample under consider ation can be calculated as follows: ,, , ... , Calorific value (moisture and ash free) X [100 -- (Moisture + Ash)] Calorific value =----------------------- 5-------------------------- 100 ------1--'-- --------------------------- (2) In the above formulae moisture and ash are expressed in per cent. The H values for Illinois coals are published1 and it is to be expected that more data on H values for other coals will be available in the future. As practically all fuels contain.hydrogen they produce a certain amount of water vapor as one of the products of combustion. The amount of water vapor produced increases as the hydrogen content of the fuelincreases. When the calorific value of a fuel is determined in a calori meter the water vapor is condensed and the latent heat of vaporization that is given up during the condensation is reported as a portion of the heat value of the fuel. The heat value so determined is termed the gross or higher heat value and this is what is ordinarily meant when the heat value of a fuel is specified. In burning the fuel, however, the products of lStaie Geological Survey Bulletin, No. 62, Classification and Selection of Illinois CoaiU. 145