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COAL

As coal is burned, the amount of heat that is released and the chemical byproducts of combustion depend to a large extent on the constituents of the coal. In St. Louis, the local electric utility burns mostly a Wyoming Powder Valley coal that is surface-mined and referred to as subbituminous coal. Following is an ultimate analysis of this coal:

• Carbon, 50.6 percent by weight.

• Oxygen, 33.0 percent by weight.

• Hydrogen, 6.1 percent by weight.

• Nitrogen, 1.2 percent by weight.

• Sulfur, 1.1 percent by weight.

The higher heating value is 8,700 Btu per pound.

The combustion of coal substantially is a combination of three active chemical reactions:

• The combination of carbon (C) and oxygen (O₂) to form carbon dioxide (C + O₂ = CO₂).

• The combination of hydrogen (H₂) and oxygen to form water vapor (H₂ + 0.5 O₂ = H₂O).

• The combination of sulfur (S) and oxygen to form sulfur dioxide (S + O₂ = SO₂).

In each case, the number of atoms on the left side of the equation is equal to the number of atoms on the right. Also, the weight in pounds of a mole of a gas is the same as the numerical value of the molecular weight of the element. Thus, the approximate weight of a mole of hydrogen (H₂) is 2 lb, a mole of oxygen (O₂) 32 lb, and so on (see table). The moles of constituents on the two sides of the equation do not necessarily balance. However, according to the law of conservation of mass, the weight of all of the constituents on the left side of the equation must equal the weight of all of the constituents on the right (the products of combustion). Thus, "C + O₂ = CO₂" could be restated as:

1 mole C + 1 mole O₂ = 1 mole CO₂

or:

12 lb C + 32 lb O₂ = 44 lb CO₂

Carbon makes up 50.6 percent, or 0.506 lb, of a pound of Wyoming Powder Valley coal. If 1 mole of carbon (12 lb) is combined with 1 mole of oxygen (32 lb), 1 mole (44 lb) of carbon dioxide is produced. For every pound of carbon burned, then, 3.66 lb (44 divided by 12) of carbon dioxide is produced. So, for every pound of Wyoming Powder Valley coal burned, 1.85 lb (0.506 times 3.66) of carbon dioxide is produced. (Similar calculations can be performed to show the amounts of water vapor and sulfur dioxide produced per pound of coal burned.)

To determine the carbon dioxide produced per unit of energy consumed in the combustion of Wyoming Powder Valley coal, the heating value of the coal must be considered:

(1,000,000 Btu) x (8,700 Btu per lb coal) = 114.94 lb coal

The carbon dioxide produced in burning this amount of coal would be:

(114.94 lb coal) x (1.85 lb CO₂ per lb coal) = 212.6 lb CO₂ per MMBtu coal