Document Xpb5VRMG4qk2qqa72w5LLBmJ

T a b l e 1. G e n e r a l D a t a op C o m b u s tib le E le m e n ts a n d Compounds From International Critical Tables; 1927. HEATING VENTILATINC AIR CONDITIONING CUIDE 1943 166 CHAPTER 8. COMBUSTION AND FUELS ignition temperature before it will unite in chemical combination with oxygen to produce combustion. The ignition temperatures for several of the combustible constituents of fuels are presented in Table 1. HEAT OF COMBUSTION As previously stated, the process of combustion results in the evolution of heat. The heat generated by the complete combustion of a unit of fuel is constant for a given combination of combustible elements and com pounds, and is known as the heatof combustion, calorific value, or heating, value of the fuel. The heat of combustion of the several fuel elements and compounds iri 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. Although the ash and moisture content of coal from a given mine or locality may vary widely, the heating value of the coal, on a moisture and ashfree basis, remains relatively constant. It is therefore possible to approximate the heating value of a shipment of coal as received if its moisture and ash content are determined, and if the heating value of similar coal on a moisture and ash free basis is known. This may be calculated by Equation i. Heating value, as received = Heating value, moisture and ashfree X [100 -- (Moisture + Ash)] 100 ,, ,, W. where, moisture and ash are expressed in per cent. The heating values for Illinois coals are published1 and it is to be ex pected that 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 fuel increases. When the heating 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 heating value of a fuel is specified. In burning the fuel, however, the products of combustion are not cooled to the dew-point and the higher heating value cannot be obtained. FLAME The appearance of the flame or products of combustion may serve as an approximate measure of the temperatures developed in the combustion . 'State Geological Survey Bulletin; No. 62, Classification and Selection of Illinois Coafls.. 167 *