Steam, Its Generation and Use

by Babcock & Wilcox Company

[TABLE 30] KINDLING TEMPERATURE OF VARIOUS FUELS Degrees Fahrenheit Lignite Dust 300 Dried Peat 435 Sulphur 470 Anthracite Dust 570 Coal 600 Coke Red Heat Anthracite Red Heat, 750 Carbon Monoxide Red Heat, 1211 Hydrogen 1030 or 1290

Combustibles—The principal combustibles in coal and other fuels are carbon, hydrogen and sulphur, occurring in varying proportions and combinations.

Carbon is by far the most abundant as is indicated in the chapters on fuels.

Hydrogen in a free state occurs in small quantities in some fuels, but is usually found in combination with carbon, in the form of hydrocarbons. The density of hydrogen is 0.0696 (Air = 1) and its weight per cubic foot, at 32 degrees Fahrenheit and under atmospheric pressure, is 0.005621 pounds.

Sulphur is found in most coals and some oils. It is usually present in combined form, either as sulphide of iron or sulphate of lime; in the latter form it has no heat value. Its presence in fuel is objectionable because of its tendency to aid in the formation of clinkers, and the gases from its combustion, when in the presence of moisture, may cause corrosion.

Nitrogen is drawn into the furnace with the air. Its density is 0.9673 (Air = 1); its weight, at 32 degrees Fahrenheit and under atmospheric pressure, is 0.07829 pounds per cubic foot; each pound of air at atmospheric pressure contains 0.7685 pounds of nitrogen, and one pound of nitrogen is contained in 1.301 pounds of air.

Nitrogen performs no useful office in combustion and passes through the furnace without change. It dilutes the air, absorbs heat, reduces the temperature of the products of combustion, and is the chief source of heat losses in furnaces.

[Pg 151]

Calorific Value—Each combustible element of gas will combine with oxygen in certain definite proportions and will generate a definite amount of heat, measured in B. t. u. This definite amount of heat per pound liberated by perfect combustion is termed the calorific value of that substance. [Table 31], gives certain data on the reactions and results of combustion for elementary combustibles and several compounds.

[TABLE 31] OXYGEN AND AIR REQUIRED FOR COMBUSTION AT 32 DEGREES AND 29.92 INCHES BY WEIGHT 1 2 3 4 5 6 7 8 9 10 Oxidizable Substance or Combustible Chemical Symbol Atomic or Combining Weight Chemical Reaction Product of Combustion Oxygen per Pound of Column 1 Pounds Nitrogen per Pound of Column 1. 3.32[23] × O Pounds Air per Pound of Column 1. 4.32[24] × O Pounds Gaseous Product per Pound of Column 1[25] + Column 8 Pounds Heat Value per Pound of Column 1 B. t. u. Carbon C 12 C+2O = CO2 Carbon Dioxide 2.667 8.85 11.52 12.52 14600 Carbon C 12 C+O = CO Carbon Monoxide 1.333 4.43 5.76 6.76 4450 Carbon Monoxide CO 28 CO+O = CO2 Carbon Dioxide .571 1.90 2.47 3.47 10150 Hydrogen H 12 H+O = H2O Water 8 26.56 34.56 35.56 62000 Methane CH4 16 CH4+4O = CO2+2H2O Carbon Dioxide and Water 4 13.28 17.28 18.28 23550 Sulphur S 32 S+2O = SO2 Sulphur Dioxide 1 3.32 4.32 5.32 4050 BY VOLUME 1 2 11 12 13 14 15 16 17 18 Oxidizable Substance or Combustible Chemical Symbol Volumes of Column 1 Entering Combination Volume Volumes of Oxygen Combining with Column 11 Volume Volumes of Product Formed Volume Volume per Pound of Column 1 in Gaseous Form Cubic Feet Volume of Oxygen per Pound of Column 1 Cubic Feet Volume of Products of Combustion per Pound of Column 1 Cubic Feet Volume of Nitrogen per Pound of Column 1. 3.782[26] × Column 15 Cubic Feet Volume of Gas per pound of Column 1 = Column 10 ÷ Column 17 Cubic Feet Carbon C 1C 2 2CO2 14.95 29.89 29.89 112.98 142.87 Carbon C 1C 1 2CO 14.95 14.95 29.89 56.49 86.38 Carbon Monoxide CO 2CO 1 2CO2 12.80 6.40 12.80 24.20 37.00 Hydrogen H 2H 1 2H2O 179.32 89.66 179.32 339.09 518.41 Methane CH4 1C4H 4 1CO2 2H2O 22.41 44.83 67.34 169.55 236.89 Sulphur S 1S 2 1SO2 5.60 11.21 11.21 42.39 53.60