FOSSIL FUELS

Q: What are fossil fuels?

A: Coal and petroleum and their derivatives are known as "fossil fuels" because they were formed from the decomposition of organic compounds (dead bodies of plants and animals) during the carboniferous period, 280 to 345 million years ago. High heat and pressure from the Earth's core caused the decaying matter to turn first into a spongy-textured substance called peat and then into either a hard, carbon-rich rock (coal) or a liquid (petroleum).

Natural Gas

Q: What is natural gas?

A: Natural gas is a very light fuel within the continuum of distillates: it is a mixture of hydrocarbons, mainly methane (CH4). Other gases in the mix typically include ethane and propane (also hydrocarbons), as well as nitrogen, water vapor, and carbon dioxide. Natural gas is used as fuel in the form of a gas but can be compressed into compressed natural gas (CNG) or liquefied natural gas (LNG) for transporting over long distances. In its gaseous form, natural gas is commonly transmitted through pipelines. Natural gas is used in the transportation, industrial, commercial, and residential sectors. It can be used for tasks such as cooking and residential heating or as a fuel to produce electricity.

Q: How is natural gas produced?

A: Natural gas occurs in abundant quantities in the United States, and it is mainly processed and used domestically. The gas is extracted from reservoirs (wells) or gas streams and refined to separate it from other accompanying petroleum products, such as crude oil. Natural gas, as methane, can also come from landfills or water treatment facilities and can be referred to as landfill gas (LFG) or "swampgas." After being separated from its component liquids, the remaining gas is refined further for specific sectors. It may be dehydrated further, "sweetened" (by removing any hydrogen sulfide and other sulfur compounds present), and sent through many other quality-control procedures.

Q: Pros and cons of natural gas:

Pros: Natural gas is in demand by many user groups in the United States because of its many positive features, which include the following:

§ Substantial domestic stock: Natural gas is a domestic resource and so does not necessitate U.S. dependence on other nations.

§ Clean-burning: Natural gas does not release the types or quantities of polluting particulates and air emissions that other fossil fuels, such as petroleum and coal, emit when combusted. Natural gas can be mixed with biofuels to produce an even cleaner transportation fuel.

§ Availability to end-use consumers: In addition to featuring in many industrial processes, natural gas is commercially available directly to end-users as a heating and cooking fuel, and for various other uses.

Cons: At present, demand for natural gas, especially from the industrial and electric power generation sectors, is growing at a faster rate than that which supply can meet. When used near the site of its production, natural gas is inexpensive to produce, but the process needed to compress and liquefy it is costly and energy-inefficient, and natural gas cannot easily and efficiently travel or overseas without being compressed or liquefied. Because this issue has arisen relatively recently, prices for natural gas have gone up at a faster rate than was predicted. Another issue of concern is the high flammability potential of natural gas and the frequent leaks occurring in natural gas pipelines.

Petroleum

Q: What is petroleum?

A: Petroleum is a liquid fossil fuel (composed of decayed organic matter ) that occurs naturally in underground reservoirs and is refined to produce natural gases, kerosene, diesel, aviation fuel, and many other derivatives. Petroleum and all of its byproducts are hydrocarbons, complex chains of hydrogen (H2) and carbon (C) atoms. Petroleum is extracted from subsurface reservoirs as a black and oily liquid called crude oil and then sent to a refinery to be separated into its various component fuels. Although these fuels are often categorized based on their boiling points and molecular weights, a more straightforward way to think of them may be on a scale of viscosity: the lightest and least viscous fuels are the gases, and the thickest are dense, slow-moving liquids such as heavy #6 heating oil.

At present, the United States relies on petroleum more than any other fuel source, and in the transportation sector petroleum accounts for 97 percent of the fuel used. In 2002, the United States imported 60 percent of its petroleum (11.7 million barrels per day, out of 19.3 million total barrels supplied per day). About 20 percent of total petroleum imports come from the Persian Gulf, an oil-rich but historically politically unstable area. Researchers in the United States are currently exploring methods of diversifying our fuel portfolio in order to be less vulnerable to price shocks and political volatility abroad.

One of the most commonly used petroleum derivatives is diesel fuel, used as home heating oil and as a highway transportation fuel. Diesel fuel is thicker and less volatile than gasoline, and because it does not readily burst into flame when burned, it is combusted either through a compression system, as with diesel engines, or through aeration as with heating oil burners. In either method, the diesel does not require a flame catalyst. Engines that use diesel fuel have much higher compression ratios than gasoline engines, so they deliver roughly 30 percent more power and torque, making them ideal for large heavy-duty vehicles like trucks and buses. Many light-duty passenger vehicles can also run on diesel, and these are particularly popular in Europe. In the United States' transportation sector, all highway diesel is required to have a maximum sulfur content of 0.05 percent, and the Environmental Protection Agency (EPA) has set the date for implementation of an ultra-low standard stipulating a maximum sulfur content of 0.015 percent for 2007.

Although chemically the same as transportation diesel, home heating oil is dyed red to distinguish it at from transportation diesel because highway fuels are subject to different taxes and fees than heating oil. Most states require a maximum sulfur content of home heating oil of 0.5 percent, but due to mixing at the refinery the average sulfur content of heating oil is about 0.2 percent. Residential heating oil came into use around the 1940s, and at present it supplies heat to about 10 percent of houses in the United States. It is mainly used as a heat source in the Northeast, where it is combusted in furnaces or burners and stored on-site in tanks.

Q: What are the pros and cons of petroleum?

Pros: Petroleum is an energy efficient fuel source that is fairly inexpensive to refine and transport, and it can be processed to remove many impurities. As stated above, petroleum is currently one of the main fuel sources of fuel in the United States. It is employed in all sectors and is an essential part of the American energy landscape. Moreover, petroleum is a component of many common household products, such as petroleum jelly, plastics, and tires.

Cons: As with all other fossil fuels, petroleum exists in finite supply, and world oil demand is following a trajectory that may outpace existing stocks in the coming decades and drive high prices and fuel shortages. Furthermore, many of the oil-rich countries supplying petroleum to the United States are politically unstable or have used their status as oil providers to artificially raise prices, and in the recent past America has been subject to inconsistent pricing trends because of this instability. Since September 11th, 2001, focus on imports has intensified.

In addition to the above concerns surrounding supply, petroleum contains many toxic components and even after refining releases large quantities of air pollutants when combusted. Air pollutants include particulate matter (PM) in the forms of soot and ash, sulfur oxides (SOx), and nitrogen oxides (NOx), which cause environmental damage in the form of acid rain and health problems such as asthma and lung disease. Petroleum is also toxic if leaked into surface or groundwater supplies: for example, in 1989 the Exxon Valdez oil spill killed millions of animals and plants along the Alaskan coastline, and even now the area is not entirely free of oil contamination. Oil spills typically cause major damage by releasing a large amount of oil into a concentrated area, but far more oil leaks into water supplies each year from improper disposal of used motor oil. Another petroleum-related difficulty is storage: petroleum is an inherently unstable fuel that breaks down into sludge over time. Storage tanks can easily leach oil into groundwater, and contaminants such as water and other non-oil substances frequently become mixed with stored petroleum and impair its energy efficiency.

Coal

Q: What is coal?

A: The United States Department of Energy's Energy Information Administration (EIA) defines coal as "a readily combustible black or brownish-black rock whose composition, including inherent moisture, consists of more than 50 percent by weight and more than 70 percent by volume of carbonaceous material." Coal occurs naturally underground and must be extracted via mining; the United States contains plentiful natural reserves of coal and so has a large coalmining industry.

In addition to the element carbon (C), coal often contains components like hydrogen (H2), oxygen (O2), ash, nitrogen (N2), sulfur (S), chlorine (Cl), sodium (Na), and mercury (Hg). These can cause environmental problems when coal is combusted. Coal is categorized as lignite, anthracite, sub-bituminous and bituminous coal based on the amounts of C, O2, and H2, present. Lignite has the lowest carbon content and lowest combustion temperature, and it crumbles when transported over long distances. Lignite is mainly used to produce electricity, syngas (synthesis gas used in industrial processes), and fertilizer products. Large lignite deposits exist in North and South Dakota, Texas, and Montana.

The intermediary rank of coal in terms of carbon content is bituminous, or soft, coal. Bituminous coal is the most plentiful coal type, with a high combustion temperature but also containing high levels of sulfur. Over 80 percent of extracted bituminous coal is burned to generate electricity; the remainder is used in a large number of industrial processes to produce plastics, textiles, and many other common articles. The byproducts of bituminous coal can be converted into such derivatives as nylon and paint. Sub-bituminous coal is of a slightly lower grade (containing less carbon) than bituminous.

The highest grade of coal is anthracite, which contains the most carbon and has the highest heating temperature. Anthracite is also very hard and so glossy that it appears metallic. It burns the longest and gives off the most heat with little soot byproduct, and its main use is to heat homes. Most of the anthracite in the United States is located in Pennsylvania.

Coal is extracted through either surface or underground mining. Surface mining involves the removal of coal deposits near the surface with heavy extraction equipment, and then reclamation, or restoration, of the area, as mandated by the U.S. Environmental Protection Agency. Surface mining usually enables the miner to recover 90 percent of the embedded coal. Underground mining involves drilling two openings in a mine to transport workers and equipment through to extract coal deep below the surface. This process can be dangerous and may cause leaching of toxic chemicals and heavy metals into the groundwater.

Coal is combusted to provide electricity generation and heat. In 2000, coal provided 51.8 percent of the United States' net electricity supply, and in some states, such as Kentucky, coal accounts for about 97 percent of the fuel used for electricity production. Coal is a plentiful resource, making electricity production relatively inexpensive and providing many jobs. In steam turbines, coal is pulverized, mixed with oxygen, and burned, with the heat producing steam that operates an electric generator. A side effect, however, is that this process releases many harmful and toxic pollutants into the air. Therefore, research is currently underway to pursue ways of making coal cleaner to combust. Below is an overview of some major clean coal technologies being studied and implemented today.

Clean Coal Technologies

Coal Benefication (pre-combustion cleaning): This process occurs "upstream," or before the coal is combusted, to remove pollutants: the coal is first crushed and screened to remove impurities, and then further cleaning techniques place the crushed coal in a liquid medium, where the solid impurities settle out. This process removes pyritic sulfur, which accounts for up to 30 percent of the sulfur content of coal.

Scrubbers (Fluegas Desulfurization Systems, or FGS): In this "end of pipe" cleaning procedure, after the coal is combusted the flue gas is sprayed with a slurry of water and an alkaline agent such as lime or limestone. The main impurity and one that accounts for the majority of acid deposition (acid rain) in the United States is sulfur dioxide (SO2), and this pollutant mixes with the slurry to form a pH-neutral compound such as calcium sulfate/sulfite, which is then eliminated in the form of a waste sludge.

Baghouses (electrostatic precipitators): Baghouse technology is another "end of pipe" method that operates after combustion to remove fly ash, the solid particulate ash emitted during coal combustion. As the fly ash passes through the baghouses, it is given an electric charge. The charge causes it to be attracted to a collector plate, and thus it is removed from the flue gas before it can enter the air. Coal plants often have a series of baghouse filters through which the gas passes before leaving the stack in order to remove as much of the particulate matter (PM) as possible.

Gasification (Integrated Gasification Combined Cycle, or IGCC): Gasification is a promising technology for future large-scale clean coal combustion. The technique for coal gasification is the same as that for biomass gasification: the coal is converted to form syngas, which is composed mainly of hydrogen (H2) and carbon monoxide (CO). In addition to biomass, petroleum coke, other petroleum residue, and industrial and municipal wastes can be used as feedstocks for gasification. The process takes place when the feedstock enters the gasifier and encounters oxygen (O2) and steam under high pressure and temperature conditions that allow the feedstock to be broken down into syngas and solid ash waste. The waste is removed, and the syngas undergoes heavy refining to eliminate particulates and other pollutants. Refining processes can also remove carbon dioxide (CO2) and recover hydrogen (H2).

Q: What are the pros and cons of coal gasification?

Pros: The process of coal gasification removes many of the most polluting components of coal, so that it releases fewer emissions into the air when combusted. In addition, many of the chemicals removed (which include ammonias, sulfur, and hydrogen) can be recovered and used in other industrial processes. The feedstock supply is flexible, with a large and varied resource base, and gasification can be used in processes to produce hydrogen power, fuels, chemicals, steam, electricity, and useful byproducts such as methanol.

Cons: Though gasification removes air pollutants more efficiently than scrubbers, the process is very expensive, and the technology is not sufficiently developed at this time to commence gasification on a substantial and economically viable scale. However, this may soon change, as coal gasification becomes one of the United States' top research priorities.