C o2CO

It follows from the stoichiometry of these reactions that, per mole of 02 consumed and thus approximately per joule of energy produced, natural gas generates less carbon dioxide than does oil, which in turn is superior to coal, in a ratio of 1:1.33:2. (The actual ratio is computed in Problem 7-3.)

PROBLEM 7-3

Given the following thermochemical data, determine the actual ratio of COz per joule of heat released upon the combustion of methane, CH2, and elemental carbon (graphite). AHf values in k] mol"1: CH4, —74-9; C02(g), -393.5; H20(1), -285.8; C(graphite), 0.0; CH2, -20.6.

PROBLEM 7-4

The relative amounts of oxygen required to oxidize organic compounds to carbon dioxide and water can be deduced from calculating the change in the oxidation number (state) of the carbon atom in going from the fuel molecule to the product. Show that the ratio of oxygen required to combust C, CH2, and CH4 stands in the ratio of 2:3:4 according to such a calculation.

FIGURE 7-6 Releasing natural gas deposits from the Earth.

Fossil Fuels: Natural Gas

Petroleum and natural gas are essentially mixtures of hydrocarbons. They originated as the small fraction of marine organisms and plant matter that were buried and therefore cut off from the oxygen that was required for their complete oxidation. The high temperatures and pressures to which this buried material was later subjected decomposed it further, into liquid and gaseous hydrocarbons. Like petroleum, natural gas deposits are found in geological formations in which the gas mixture has been trapped by a mass of impermeable rock. Drilling a hole down through the rock releases the gas in a steady flow to the surface (Figure 7-6).

In terms of its hydrocarbon component, natural gas as it exits from the ground consists predominantly (60-90%) of methane, CH4. The other component alkanes—ethane, propane, and the two butane isomers—are gases present to varying extents depending upon the geographic origin of the deposit. (See Appendix I if you are unfamiliar with the terminology and numbering systems of organic molecules.)

Methane's boiling point is so low (-164°C) that it does not readily condense into a liquid, even at moderately high pressures. In contrast, the other gaseous alkanes possess substantially higher boiling points. This makes it possible to largely remove the other alkanes from natural gas by lowering the mixture's temperature and thereby condensing these other hydrocarbons to liquids.

Sulfur compounds are also important impurities in natural gas, as previously mentioned: Some deposits contain more H2S than CH4! The hydrogen sulfide is removed from the gas by the Claus reaction, as discussed in Chapter 3. After processing to remove the other alkanes and the sulfur compounds, the natural gas—which now is mainly methane—is transported under pressure by pipeline to consumers.

Unfortunately, as we have discussed in Chapter 6, a small fraction of the methane being transported from its source to the consumer is lost to the atmosphere when natural gas pipelines leak; the greenhouse-enhancing effect of this methane could override some of the advantage methane has in producing less C02 per joule upon combustion compared to oil and especially compared to coal (see Additional Problem 3).

The enormous quantity of natural gas held in methane hydrates (clathrates) in ocean sediments and permafrost, as mentioned in Chapter 6, would double the fossil-fuel reserves if they could be tapped. The technology to extract the clathrates, many of which are in dilute form and mixed with sediments that lie far below the seabed, does not yet exist.

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Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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