Science Notes: Chemistry of Fossil Fuels

Combustion and Energy Release
The chemistry principles previously described can be used to describe the burning of methane (CH4, marsh or natural gas), or of carbon in coal. Combustion involves combinations of the fuel with oxygen.

Thus,

C + O2 CO2
CH4 +
2O2 CO2 + 2H2O

We can show that these reactions release energy. The basic reaction of the burning of C is the basis of our largest energy source -- fossil fuels of various types, including coal, natural gas, and oil. Recall the energy in these bonds came originally form the solar energy captured by plants and then "processed" for millions of year (transformed over millions of years) under the pressure in the Earth.

Burning of Coal
Coal is mainly carbon, water, some hydrogen, and oxygen. There are many different kinds of coal. In addition to H and O, coal also contains some small amounts of nitrogen, sulfur, and some other minerals.

Most of the carbon in coal is bound so that there is only one C-C bond for every C atom. Thus, for calculating the energy release of C + O2 CO2 in the case of coal, one assumes only the breaking of one C-C bond.

Following the previous example,

C (1 mole)
+
O2 (1 mole)
CO2 (1 mole)
C - C
+
O=O

O=C=O

Energy required to break bond:
83 kcal/mole + 119 kcal/mole
=
202 kcal/mole
Energy Released:
2 x 177 kcal/mole = 354 kcal/mole
net release = 152 kcal/mole of carbon
Thus 12 g of carbon yields 152 kcal of energy provided sufficient oxygen is available for complete combustion.

1 kg of carbon therefore gives approximately 11,000 kcal

152 kcal
12g
x
1000 g
=
12666 kcal
As coal contains other ingredients , it works out that the actual yield of 1 kg of coal is about 700 kcal.

Byproducts from Coal Combustion
As seen in the equation, CO2 is the main byproduct of coal combustion -- 44g of CO2 is produced for every 23g of C burnt. The contribution of CO2 to global climate change is one of the fundamental problems of our fossil fuel economy. Other products of coal burning originate from the sulfur and nitrogen present in coal. The nitrogen is usually released as N2 or NO2 gas. The sulfur forms SO2, which is one of the gases that causes acid rain (discussed in detail in the Atmospheric System).

Insufficient oxygen supply during combustion -- as for example, burning coal (or wood) in a closed environment such as a room without adequate ventilation or a fireplace without a proper chimney -- produces carbon monoxide. As the oxygen is depleted, the reaction C + CO2 2CO becomes possible. Carbon monoxide is a colorless, odorless, and very poisonous gas. When breathing in CO, the CO takes the place of O2 in the hemoglobin molecules in the blood supply in the lungs, causing asphyxiation.

 

 

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