Fossil Fuels Coal
The main fossil-fuel reserve is coal, which is available in abundance in many regions of the world, including developing countries, and which is cheap to mine and transport. Five countries—the United States, Russia, China, India, and Australia—have 75% of the world's coal reserves. At today's rate of consumption, coal reserves are estimated to last another 200 years, much longer than oil or gas (see below). Indeed, some observers believe the world will return to a greater reliance on coal as the major fossil fuel later in this century. The 2100 coal-fired power plants in the world are collectively responsible for about a third of all anthropogenic C02 emissions. Currently, coal produces about half the electric power in the United States and 80% of that in China.
Although it is a mixture, to a first approximation coal is graphitic carbon, C. It was formed from the tiny proportion of ancient plant matter that was covered over by water and could not be recycled back to C02 at that time. This also accounts for the buildup of 02 in the atmosphere. Coal was formed from the highly aromatic, polymeric component of land-based woody plants called lignin; an approximate formula for lignin is C3H3O. Over long periods of time during which the material was subjected to high pressures and temperatures, both water and carbon dioxide were lost. The material polymerized further in the process to yield the very carbon-rich, hard material known as coal. Unfortunately, during its formation the coal also incorporated measurable quantities of virtually every naturally occurring element, so that when it is burned, it emits not only C02 and HUO but also substantial quantities of many air pollutants—notably sulfur dioxide, fluoride, uranium and other radioactive metals, and heavy metals including mercury. Thus coal has a reputation for being a "dirty fuel." The removal of some of these impurities, especially sulfur, by various modern technologies was discussed in Chapter 3.
The burning of coal domestically in stoves and furnaces produces a great deal of soot, and it therefore has been largely discontinued in developed countries. However, coal is still used in most developed and developing countries for electric power production. When burned in such plants, the soot problem is readily solved, although emissions of sulfur and nitrogen dioxides and of mercury require more sophisticated and expensive equipment to control, also discussed in Chapter 3 and the online case study associated with Chapter 8.
The heat produced in the combustion of the fossil fuel is used to generate high-pressure steam, which in turn is used to turn turbines and thereby produce electricity. As discussed below, however, the ratio of C02 to energy produced from coal is substantially greater than for the other fossil fuels. Coal can also be used to produce alternative fuels—as discussed in Chapter 8—but unfortunately, the conversion processes are not very energy-efficient. Although the emission of carbon dioxide is not reduced by such conversions, they do allow the removal of sulfur dioxide and other pollutants and so are "clean" ways to use coal.
To a first approximation, the amount of heat released when a carbon-containing substance burns is directly proportional to the amount of oxygen it consumes. From this principle, we can compare different fossil fuels by the amount of C02 released when they are burned to produce a given amount of energy.
Consider the reactions of coal (mainly carbon), oil (essentially polymers of CH2), and natural gas (essentially CH4) with atmospheric oxygen and written so that the amount of carbon dioxide produced is identical in each case:
Continue reading here: C o2CO
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