The Ecology Question

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We have shown that there is an almost unavoidable trend toward increasing energy utilization. We have also pointed out that at present the energy used is at least 85% of fossil origin. Finally, we have observed that the fossil fuel reserves seem ample to satisfy our needs for a good fraction of the next millennium. So, what is the problem?

Most of the easily accessible sources of oil and gas have already been tapped. What is left is getting progressively more expensive to extract. Thus, one part of the problem is economical. Another is political—most of the fuel used by developed nations is imported (using the large American reserves is unpopular, and politicians hesitate to approve such exploration). This creates an undesirable vulnerability. The major problem, however, is ecological. Fossil fuels are still the most inexpensive and most convenient of all energy resources, but their use pollutes the environment, and we are quickly approaching a situation in which we can no longer dismiss the problem or postpone the solution.

By far, the most undesirable gas emitted is carbon dioxide whose progressively increasing concentration in the atmosphere (from 270 ppm in the late 1800s to some 365 ppm at present) constitutes a worrisome problem. It is sad to hear influential people (among them, some scientists) dismiss this problem as inconsequential, especially in view of the growing signs of a possible runaway ecological catastrophe. For instance, in the last few decades, the thickness of the north polar ice has decreased by 40% and in the first year of the current millennium, a summertime hole appeared in the polar ice. Since increased concentrations of CO2 can lead to global warming, some people have proposed increasing the emission of SO2 to stabilize the temperature because of the cooling effect of this gas. Even ignoring the vegetation-killing acid rain that would result, this proposal is equivalent to balancing a listing boat by piling stones on the other side.

Public indifference to the CO2 problem may partially be due to the focus on planetary temperature rise. Although the growth in CO2 concentration is very easily demonstrated, the conclusion that the temperature will rise, though plausible, is not easy to prove. There are mechanisms by which an increase of greenhouse gases would actually result in a cooling of Earth. For instance, increasing greenhouse gases would result in enhanced evaporation of the tropical oceans. The resulting moisture, after migrating toward the poles, would fall as snow, thereby augmenting the albedo of the planet and thus reducing the amount of heat absorbed from the sun.

The Kyoto Treaty aims at curbing excessive carbon dioxide emissions. It is worth noting that China, the world's major CO2 emitter,^ is exempt from the treaty's restrictions, as are numerous other countries, such as India and Brazil. The United States and Australia have never ratified the treaty and are, therefore, also exempt. There are many contributors to CO2 pollution, but by far the largest single culprit is the coal-fired power plant, which emits 30% of the amount of carbon dioxide dumped into the atmosphere.^ The problem will not go away unless the coal-to-electricicity situation is corrected.

Some scientists and engineers who are less concerned with political correctness are investigating techniques to reduce (or at least, to stabilize) the concentration of atmospheric carbon dioxide. This can, in principle, be accomplished by reducing emissions or by disposing of carbon dioxide in such a way as to avoid its release into the air. Emissions can be reduced by diminishing overall energy consumption (an utopian solution), by employing alternative energy sources, by increasing the efficiency of energy use, and by switching to fuels that yield more energy per unit amount of carbon emitted. It is known that 1 kmole of methane, CH4, when burned yielding liquid water and carbon dioxide, releases 889.6 MJ and emits 1 kilomole of carbon: it generates heat at a rate of 889.6 MJ per kilomole of carbon. n-heptane, C7H-6, which can represent gasoline, releases 4820 MJ of heat per kilomole burned and emits 7 kilomoles of CO2—a rate of 688.6 MJ per kilomole of carbon. Clearly, the larger the number of carbon atoms in the hydrocarbon molecule, the lower the ratio of the heat of combustion to the amount of carbon dioxide emitted because the ratio of hydrogen to carbon decreases. This is one reason for preferring methane to oil and oil to coal.

Renewable forms of energy are attractive but, at least for the present, they are too expensive to seriously compete with fossil fuels. Hence, methods for reducing carbon dioxide emission are under intense investigation. All these methods have two stages: carbon dioxide capture and carbon dioxide disposal or sequestration. The capture stage is described, superficially, in Chapter 10. In lieu of sequestration, the captured and purified gas gan be sold to, for instance, the carbonated drink industry. But this can only take care of a minute fraction of the total CO2 involved.

In order to select a technique, for carbon dioxide disposal, it is important to inquire where nature stores the existing carbon. Table 1.8 shows the estimated amount of carbon stored in different places.

Methods to dispose of CO2 could include the following.

^In mid-2007, China surpassed the United States as the major carbon dioxide emitter. tt It is somewhat surprising that a 1 GW coal-fired plant can emit 100 times more radioactive isotopes (because of the radioactive traces in common coal) than a nuclear plant of the same power.

Table l.8 Stored Carbon on Earth

Oceans Fossil fuels Organic matter Atmosphere

45 x 1015 kg 10 x 1015 kg 2.4 x 1015 kg 0.825 x 1015 kg

1.10.1 Biological

For photosynthesis to remove carbon dioxide from the air, the biomass produced must be preserved; it cannot be burned or allowed to rot. There is a limited capacity for this method of CO2 disposal. The current biological uptake rate of carbon is only 0.002 x 1015 kg year.

1.10.2 Mineral

CO2 is removed naturally from the air by forming carbonates (mainly of magnesium and calcium). The gas is removed by reacting with abundant silicates, a process too slow to cope with human-made emissions.

Ziock et al. (2000) propose the use of magnesium silicates to sequester carbon dioxide at the point where fossil fuels are burned. Enormous deposits of magnesium oxide-rich silicates exist in the form of olivines and serpentines.

For serpentine, the net reaction involved is

Notice that the end products are materials that already exist naturally in great abundance. Substantial additional research is needed to improve the proposed disposal system and to make it economical.

1.10.3 Subterranean

CO2 can be sequestered underground as the oil industry has been doing (for secondary oil recovery) for more than 50 years. The volume of the exhaust gases of a combustion engine is too large to be economically stored away. It is necessary to separate out the carbon dioxide, a task that is not easy to accomplish. One solution is proposed by Clean Energy Systems, Inc. of Sacramento, California. The suggested equipment extracts oxygen from air (a well-developed process) and mixes this gas with the fuel. Combustion produces steam and CO2 at high temperature and pressure and drives several turbines at progressively lower temperatures. The water in the final exhaust is condensed and recycled leaving the carbon dioxide to be pumped, at 200 atmospheres, into an injection well. At present, no turbines exist capable of operating at the high temperature (over 3000 C) of the combustion products. See Anderson et al. (1998).

1.10.4 Undersea

The Norwegian government imposes a stiff carbon dioxide emission tax that has made it economical to install disposal systems that pump the gas deep into the ocean. It appears that liquid carbon dioxide can be injected into the seas at great depth and that it will stay there for a long time. Again, more work is required to determine the feasibility of the scheme.

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