Chemistry lecture 32

FOSSIL FUELS

Since the beginning of the industrial revolution, fossil fuels have been important sources of energy. European industrialization began in the late 1700s in England, and coal soon became a major fuel. In 1850 wood was still the main energy source in the United States. During the latter half of the nineteenth century, the United States and other industrialized nations relied on coal (a fossil fuel) to provide the energy for industrialization. Coal remained the major fuel source for many years, and then, in the latter half of the twentieth century, oil and natural gas became the primary energy sources. The first oil well was drilled in Pennsylvania in 1859.

In 2000, fossil fuels accounted for almost 90 percent of the world's energy production.

Although coal combustion produces substantially greater air pollution problems than does oil or natural gas combustion, because of its great abundance in the United States and other countries (such as Russia), there has been renewed interest in developing technology to burn coal more cleanly. However, all fossil fuels consist mainly of hydrocarbons (compounds that contain only carbon and hydrogen), which, upon complete combustion, yield carbon dioxide, a major greenhouse gas.

It is widely accepted in the scientific community that fossil fuels (coal, oil, and gas) have a biological origin and are ultimately derived from the buried remains of plant and animal matter, although some still argue in favor of a nonbiological or inorganic source. It is believed that a small fraction (much less than 1%) of dead plant and animal matter accumulates as deposited matter, is removed from contact with atmospheric oxygen, is subject to elevated temperatures and pressures (inhibiting decomposition by bacteria), and over geological time, is transformed into fossil fuels.

COAL COMPOSITION
Type of Coal	%, C	%, H	%, O	%, N	%, Moisture	Heating Value (kcal/kg)
Anthracite	92–95	3–4	2–3	0–2	1–4	7500–8000
Bituminous	75–92	4–6	3–20	1–2	5–30	5000–8000
Lignite	60–75	4–6	17–35	1–2	30–50	3000–4500
Peat*	45–60	3–7	20–45	1–3	70–90	< 3000

 

Coke

Coal can be transformed into coke and other fuels by various industrial and experimental processes. Coke is produced by the pyrolysis (heating in the absence of air) of coal and is used in the production of iron and steel. The coking procedure removes moisture and other volatile components from coal, yielding an extremely carbon-rich material. Coal can also be transformed into relatively clean liquid and gaseous fuels (liquifaction and gasification). However, this is accomplished at high cost—in money and energy.

Petroleum

Petroleum is an extremely complex mixture of hydrocarbons, which can be separated into liquid (oil) and gas fractions. Compared to coal, petroleum being a liquid is easier to transport. It probably originated in marine sediments, in contrast to the terrestrial origins of coal.

Because petroleum varies greatly in composition and distribution throughout the world, elaborate systems of refining and transport have been developed. Major oil fields or giant petroleum fields ("giant" indicating oil fields capable of producing at least 500 million barrels of oil) are found primarily in the Middle East, North and South America, and countries that made up the former Soviet Union. Coastal waters are particularly vulnerable, not only to oil spills, but also to contamination by bilge water and tank-washing water from commercial oil tankers. Most plastics and other petrochemicals are made from petroleum, along with almost all gasoline, diesel fuel, jet fuel, heating oil, and lubricants. However, Earth's supply of petroleum is limited. Some experts estimate that world production of oil could climax as early as 2004.

 

Natural Gas  

The history of natural gas dates back to 900 B.C.E. (Before Common Era), when its use was mentioned in China. It was apparently unknown in Europe until 1659, when it was discovered in England. It was not discovered in the United States until 1815 in West Virginia. In the early twenty-first century, natural gas has become the favorite fuel of industrial nations.

Natural gas, which consists mainly of methane (CH₄), can contain up to 20 percent of other gases—mainly ethane (C₂H₆), and possibly propane (C₃H₈), butane (C₄H₁₀ ), pentane (C₅H₁₂), carbon dioxide (CO₂), and nitrogen (N₂). Some natural gases contain small amounts of hydrogen, argon, carbon monoxide, or even hydrogen sulfide. Certain gas wells in Oklahoma also contain helium. In fact, they are a major source of helium in the United States. Natural gas is also colorless, odorless, and nontoxic but very flammable. (The odor we associate with natural gas is because of a mercaptan added to make gas leaks detectable.) Most natural gas is burned as fuel; however, ethane and the higher alkanes can be separated out and cracked to ethylene and propylene for making plastics. Although it is considered a "clean" and environmentally friendly fuel, compared to oil and coal, it is itself a major greenhouse gas and upon combustion yields carbon dioxide, the other major greenhouse gas. Like carbon dioxide, methane is also a greenhouse gas. However, natural gas fuel is thought to be only a minor contributor to methane in the atmosphere. Methane is constantly being generated by marsh and swamp terrain and by certain animals. Some experts believe that animals are the main source of atmospheric methane.

 

Other Sources of Fossil Fuels

Oil shales and tar sands also contain significant amounts of hydrocarbon materials that might eventually prove to be important energy sources. Oil shales are fine-grained sedimentary rocks (shales) that contain hydrocarbons that are dispersed within the matrix of the rock. A ton of shale contains from 10 to 100 gallons of kerosene, a waxy material that breaks down to oils when heated in the absence of air. It is estimated that three states (Utah, Colorado, and Wyoming) contain shale bearing more oil than exists in all the proven reserves in the world. Tar sands are the extremely viscous petroleum deposits associated with sedimentary rocks. They are mixtures of clay, sand, and extremely viscous oils called bitumens. The utility of oil shales and tar sands is currently limited, because of problems having to do with hydrocarbon recovery and the disposal of large amounts of inorganic residues.