GREENHOUSE EFFECT: UNDERSTANDING THE KEY ISSUE

It is with increasing frequency that we read articles such as this one (TIME, October 2, 1995):

The article goes on to list the recommended strategies to avoid the 'apocalypse':

• switching from coal and oil to natural gas
• turning to nuclear and solar energy
• slowing deforestation
• altering land-use and traffic patterns
• curbing automobile use
• changing life-styles and employment patterns

Let's concentrate on the first one because it is the most important one. Why does switching from coal and oil to natural gas help reduce the greenhouse effect?

Natural gas is mostly methane, CH4, whose molecular weight is 16 and which has a heating value of 24000 BTU per pound. When methane burns, it combines with oxygen in air to produce carbon dioxide and water. From elementary chemistry, we know that one mole of methane (containing 6x10^23 molecules) combines with one mole of oxygen to produce one mole of carbon dioxide. Therefore, for every 16 pounds of methane, 44 pounds of carbon dioxide are produced. Thus, the amount of carbon dioxide produced per BTU of energy generated by burning natural gas is obtained as follows:

At the other extreme, coal is a very complex molecule consisting of hundreds of atoms of carbon, hydrogen, oxygen, nitrogen and sulfur. Fortunately we don't need to know how these atoms are distributed in the coal structure... All we need is the elemental composition of the fuel.

A low-rank coal, lignite (with a heating value of approximately 7000 BTU/lb), can be represented simply as follows: for every 100 carbon atoms in coal, there are 83 atoms of hydrogen, 23 atoms of oxygen and 1 atom of nitrogen. In other words, it contains (by weight) 72% carbon, 22% oxygen, 5% hydrogen and 1% nitrogen. Its greenhouse effect is obtained as follows:

A high-rank bituminous coal (with a heating value of about15000 BTU/lb) can be represented as follows: for every 100 carbon atoms in coal, there are 58 atoms of hydrogen, 9 atoms of oxygen, 2 atoms of nitrogen and 1 atom of sulfur. In other words, it contains (by weight) 82% carbon, 10% oxygen, 4% hydrogen, 2% sulfur and 2% nitrogen. Its greenhouse effect is obtained as follows:

Finally, a representative product of petroleum refining, gasoline (with a heating value of approx. 20000 BTU/lb), is a mixture of many compounds (for example, octane, C8H18 or CH2.25), but its molecular formula can be represented as CH1.9 (containing 190 hydrogen atoms for every 100 carbon atoms). Its greenhouse effect is obtained as follows:

For every unit of heat produced by burning fossil fuels, natural gas produces 2-3 times less carbon dioxide than coal. It also produces less carbon dioxide than oil. That's the good news. The bad news is that the world, and especially the U.S., has much less natural gas (and oil) than coal.

The other bad news is also illustrated above. Low-rank coals, which are abundant in the U.S., are increasingly attractive for power plants because they contain less sulfur, and therefore cause much less acid rain, than typical bituminous coals. But we see that their combustion produces much more CO2 than the combustion of high-rank coals.

This is why the TIME article mentions turning to nuclear and solar energy, as a second recommended strategy to avoid the 'apocalypse'. Turning to nuclear energy is more easily said than done, because it opens another 'can of worms'. Turning to solar energy is of course environmentally desirable, but quite a few technical and economic problems need to be resolved before this can be a viable alternative in the near future.

Here is the most recent update, based on the important speech of President Clinton to the National Geographic Society, in preparation for the Earth Summit in Kyoto, Japan (December 1997).

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