Boilers are used to generate steam from the otherwise waste heat of cement kilns, metallurgical furnaces, coke ovens, gas gen erators and other such processes where high exit temperatures of the products of combustion prevail. In the McEwen-Runge process of coal carbonization, the coke in pulverized form served as a fuel for boilers as did also the screened coke residue from the K.S.G. process which was burnt on forced draft chain grate stokers.
Combustion within the furnace of a steam generator consists in the chemical combination of the elements of the fuel with oxygen from atmospheric air supplied for that purpose. Corre sponding to the weights of carbon (C), hydrogen (H), oxygen (0) and sulphur (S) in the fuel fired, the weight of air theo retically required for combustion is given by since atmospheric air contains 23.2% of oxygen by weight.
It is necessary to supply "excess air" above this theoretical minimum, however, and "free oxygen" always occurs in the products of combustion. Even with excess air, some combustible will escape unburnt from the furnace and a small amount of carbon monoxide may simultaneously be found in the products of combustion. The unburnt combustible is generally carbon with a corresponding combustion loss of 14,600 B.T.U. per pound. The formation of carbon monoxide (CO) instead of carbon dioxide (CO2) is named "incomplete combustion." One pound of carbon burned completely to liberates 14,600 B.T.U. One pound of carbon burned incompletely to CO liberates only 4,400 B.T.U., hence the loss is the difference between these values or io,i6o B.T.U. per pound of carbon burnt to carbon monoxide. While these losses are reduced by increasing the percentage of excess air, the gain in combustion efficiency is more or less counteracted by the increase in another loss, namely, the heat content of the excess air at the temperature of the products of combustion escaping to the stack. Leakage of air through the walls of the furnace and boiler setting increases the excess air loss to the stack without effecting, in general, any corresponding gain in combustion efficiency. Considerable saving in fuel may often be made simply by reducing leakage through cracks or openings in boiler settings.
For each kind of fuel, type of furnace and method of firing, there is a certain percentage of excess air which results in a minimum loss. The most economical percentage of excess air is
determined for any given installation from analyses of the products of combustion, and it corresponds to that percentage of carbon dioxide at which the carbon monoxide amounts to only a few tenths of 1%. For hand-fired coal furnaces and older stoker installations with restricted furnace volume, the best carbon dioxide percentage is around 12% which represents about 5o% excess air. Larger combustion chambers were later employed because it was thereby possible to reduce the excess air below 5o% without producing much carbon monoxide. At high operat ing capacities, however, furnace temperatures became so high that slagging of refractory walls and formation of clinkers in fuel beds became excessive. Water-cooled walls have eliminated refractory troubles and permitted the most economical percentage of excess air to be used without reaching excessively high tem peratures, so that in some of the most modern steam generator furnaces as high as 15 to 16% of carbon dioxide is attained in daily operation.
With gases and oils, the most economical percentage of carbon dioxide will, in general, be less than mentioned above for coal, although the excess air percentage may be lower by reason of the more intimate mixture of fuel and air that can be obtained with proper burners and furnaces. This is due to the larger percentage of hydrogen in these fuels which combines with oxygen forming water that is not ordinarily included in analyses of products of combustion.
When burning pulverized coal, thorough mixing in the furnace of the fuel particles with the air is conducive to complete com bustion with low excess air. This may be secured by means of . turbulent firing where the coal and air streams impinge upon one another in the furnace; but while the volatile matter may be burnt within a small volume, a much larger furnace must be provided to give time for the fixed carbon particles to burn before they escape from the furnace. Finer pulverization will reduce the unburnt combustible loss at the expense of the use of more power for grinding the coal. For solid fuels burned on grates and stokers, the furnace shape as well as its size is im portant in securing thorough mixture.