Of the gaseous fuels available for steam generating purposes, the most
common are blast furnace gas, natural gas and by-product coke oven gas.
Blast furnace gas, as implied by its name, is a by-product from the
blast furnace of the iron industry. This gasification of the solid fuel
in a blast furnace results, 1st, through combustion by the oxygen of the
blast; 2nd, through contact with the incandescent ore (Fe_{2}O_{3} + C
= 2 FeO + CO and FeO + C = Fe + CO); and 3rd, through the agency of
CO_{2} either formed in the process of reduction or driven from the
carbonates charged either as ore or flux.
Approximately 90 per cent of the fuel consumed in all of the blast
furnaces of the United States is coke. The consumption of coke per ton
of iron made varies from 1600 to 3600 pounds per ton of 2240 pounds of
iron. This consumption depends upon the quality of the coal, the nature
of the ore, the quality of the pig iron produced and the equipment and
management of the plant. The average consumption, and one which is
approximately correct for ordinary conditions, is 2000 pounds of coke
per gross ton (2240 pounds) of pig iron. The gas produced in a gas
furnace per ton of pig iron is obtained from the weight of fixed carbon
gasified, the weight of the oxygen combined with the material of charge
reduced, the weight of the gaseous constituents of the flux and the
weight of air delivered by the blowing engine and the weight of volatile
combustible contained in the coke. Ordinarily, this weight of gas will
be found to be approximately five times the weight of the coke burned,
or 10,000 pounds per ton of pig iron produced.
With the exception of the small amount of carbon in combination with
hydrogen as methane, and a very small percentage of free hydrogen,
ordinarily less than 0.1 per cent, the calorific value of blast furnace
gas is due to the CO content which when united with sufficient oxygen
when burned under a boiler, burns further to CO_{2}. The heat value of
such gas will vary in most cases from 85 to 100 B. t. u. per cubic foot
under standard conditions. In modern practice, where the blast is heated
by hot blast stoves, approximately 15 per cent of the total amount of
gas is used for this purpose, leaving 85 per cent of the total for use
under boilers or in gas engines, that is, approximately 8500 pounds of
gas per ton of pig iron produced. In a modern blast furnace plant, the
gas serves ordinarily as the only fuel required. Table 49 gives the
analyses of several samples of blast furnace gas.