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. TABLE 49 TYPICAL ANALYSES OF BLAST FURNACE GAS +----------------------------------------------------------------+ |+-----------------------+------+----+-----+----+------+--------+| || |CO_{2}| O | CO | H |CH_{4}| N || |+-----------------------+------+----+-----+----+------+--------+| ||Bessemer Furnace | 9.85|0.36|32.73|3.14| .. |53.92 || ||Bessemer Furnace | 11.4 | .. |27.7 |1.9 | 0.3 |58.7 || ||Bessemer Furnace | 10.0 | .. |26.2 |3.1 | 0.2 |60.5 || ||Bessemer Furnace | 9.1 | .. |28.7 |2.7 | 0.2 |59.3 || ||Bessemer Furnace | 13.5 | .. |25.2 |1.43| .. |59.87 || ||Bessemer Furnace[47] | 10.9 | .. |27.8 |2.8 | 0.2 |58.3 || ||Ferro Manganese Furnace| 7.1 | .. |30.1 | .. | .. |62.8[48]|| ||Basic Ore Furnace | 16.0 |0.2 |23.6 | .. | .. |60.2[48]|| |+-----------------------+------+----+-----+----+------+--------+| +----------------------------------------------------------------+ Until recently, the important consideration in the burning of blast furnace gas has been the capacity that can be developed with practically no attention given to the aspect of efficiency. This phase of the question is now drawing attention and furnaces especially designed for good efficiency with this class of fuel are demanded. The essential feature is ample combustion space, in which the combustion of gases may be practically completed before striking the heating surfaces. The gases have the power of burning out completely after striking the heating surfaces, provided the initial temperature is sufficiently high, but where the combustion is completed before such time, the results secured are more satisfactory. A furnace volume of approximately 1 to 1.5 cubic feet per rated boiler horse power will give a combustion space that is ample.