STEAM BOILERS 349 



The carbon and hydrogen unite readily with the oxygen 

 of the air, generating heat and light, but the mineral 

 matter remains and forms the ash. 



When the carbon of the coal mixes with the oxygen 

 of the air and the mixture is at or above the igniting 

 temperature, combustion takes place and either carbon 

 monoxide (CO) or carbon dioxide (CO 2 ) is formed, de- 

 pending upon the amount of air supplied. If the air is 

 insufficient in quantity to furnish enough oxygen to form 

 CO 2 , CO will be formed. If the mixture is not hot 

 enough to form complete ignition a great deal of free 

 carbon in the form of smoke is thrown off and is a loss. 



478. Heat of combustion. Carbon will not unite with 

 oxygen when in the free state until a certain temperature 

 is reached. This temperature is known as the igniting 

 temperature. When the igniting temperature has once 

 been reached and the carbon of the fuel combines with 

 the oxygen of the air, they in turn throw off heat. By 

 experiment it has been found that one pound of carbon 

 burned to carbon monoxide (CO) produces 4,400 

 B.T.U., and if burned to carbon dioxide (CO 2 ) 14,650 

 B.T.U. are produced. One pound of hydrogen united 

 with sufficient oxygen produces 62,100 B.T.U. 



479. Air for combustion.* By weight, 12 pounds of 

 carbon unite with 16 pounds of oxygen ; hence I pound of 

 carbon forms 



28 -r- 12 = 2Y 3 



pounds CO, or if it be burned to CO 2 it will require 

 twice as much oxygen for each pound of carbon ; hence 



12+ (2 X 16) -M2 = 3^ 

 pounds CO 2 for each pound of carbon. 



Since in the 3 2/3 pounds CO 2 there is one pound of 



*A good discussion of this will be found in Peabody and Miller's 

 "Steam Boilers." 



