Fuels for Fuel Cells | Fuel Types | Processing Required for Fuels
Fuel Types
Hydrogen
Hydrogen is the simplest element, composed of one proton and one electron. It makes up more than 90% of the composition of the universe. More than 30% of the mass of the sun is atomic hydrogen. It is the third most abundant element in the earth's surface, and is found mostly in water. Under ordinary (earthly) conditions, hydrogen is a colorless, odorless, tasteless, and nonpoisonous gas composed of diatomic molecules (H2).
Most of the industrial hydrogen in the United States is produced by steam reforming, or as a byproduct of petroleum refining and chemicals production. See the section on fuel reformation for other methods of extracting hydrogen from fuels.
Hydrocarbon Fuels
Hydrogen can be found in all hydrocarbon fuels and through the process of reformation, can be extracted and used in fuel cells. Hydrocarbon feedstocks include such fuels as natural gas, petroleum-based liquids (e.g. gasoline) or coal. Other significant sources of hydrogen are listed below. One of the major hurdles in using reformed hydrocarbons is the presence of sulfur, which must be removed to the single-digit part per million level before the fuel gas enters the fuel cell. This is required because sulfur is a "poison" that is known to significantly degrade the performance of fuel cell anodes and most reforming catalysts.
Methanol
Methanol is the simplest alcohol, containing one carbon atom. It is a colorless, tasteless liquid with a very faint odor and is commonly known as "wood alcohol." Methanol can be manufactured from a variety of carbon-based feedstocks such as natural gas, coal, and biomass (e.g., wood).
Methanol can be reformed to extract the hydrogen for use in fuel cells, or can be used directly without reforming in some fuel cells.
Methane
Methane is a colorless, odorless gas with a wide distribution in nature. It is the principal component of natural gas, a mixture containing about 75% CH4, 15% ethane (C2H6), and 5% other hydrocarbons, such as propane (C3H8) and butane (C4H10). It is also a byproduct of sewage treatment process (see Anaerobic Digester Gas below).
Ethanol / BioGas
Ethanol (ethyl alcohol, grain alcohol, ETOH) is a clear, colorless liquid with a characteristic, agreeable odor. Ethanol, CH3CH2OH, is an alcohol, a group of chemical compounds whose molecules contain a hydroxyl group, -OH, bonded to a carbon atom.
Today, ethanol is used as an automotive fuel by itself and can be mixed with gasoline to form what has been called "gasohol" - the most common blends contain 10% ethanol and 85% ethanol mixed with gasoline. Over 1 billion gallons of ethanol are blended with gasoline every year in the United States. Most industrial ethanol is denatured to prevent its use as a beverage. Denatured ethanol contains small amounts, 1 or 2 percent each, of several different unpleasant or poisonous substances. (Source: American Coalition for Ethanol)
Today, ethanol production facilities located in 20 states across the country have the capacity to produce more than 2.5 billion gallons of ethanol per year, and new facilities under construction and expansions will increase capacity to 2.8 billion gallons by the end of 2002. (Source: Renewable Fuels Association) (www.ethanolrfa.org)
Biomass
The term "biomass energy" is generally used to describe energy in the form of gaseous and liquid fuels, heat and electricity derived from organic matter of plant and animal origin. Traditional sources of biomass consist of agricultural and forestry residues and waste from animal husbandry. Industrial waste and specifically grown energy crops represent a more recent and growing biomass source. In some cases the concept is extended to municipal solid waste which contains a large organic fraction of domestic waste. Biomass has traditionally been used for domestic cooking and heating and such use is still widespread in developing countries.
Landfill Gas
Most of the waste we generate ends up in landfills, where it decomposes and produces landfill gas. Landfill gas released into the air smells bad, contributes to local smog, and is an explosion hazard. Additionally, landfill gas is about 50 percent methane, a potent greenhouse gas that contributes to global climate change. However, this methane is also a reliable and renewable fuel source that, if not collected, goes to waste.
Anaerobic Digester Gas
Anaerobic digestion is a biochemical process in which particular kinds of bacteria digest biomass in an oxygen-free environment. Several different types of bacteria work together to break down complex organic wastes in stages, resulting in the production of "biogas."
Controlled anaerobic digestion requires an airtight chamber, called a digester. To promote bacterial activity, the digester must maintain a temperature of at least 68° F. Using higher temperatures, up to 150° F, shortens processing time, allowing the digesterto handle a larger volume of organic waste.
The biogas produced in a digester (also known as "digester gas") is actually a mixture of gases, with methane and carbon dioxide (CO2) making up more than 90 percent of the total. Biogas typically contains smaller amounts of hydrogen sulfide, nitrogen, hydrogen, methylmercaptans and oxygen.
Methane is a combustible gas. The energy content of digester gas depends on the amount of methane it contains. Methane content varies from about 55 percent to 80 percent. Typical digester gas, with a methane concentration of 65 percent, contains about 600 Btu of energy per cubic foot.