Background on hydrogen
Here is a good, general background piece on hydrogen, borrowed from MANNYT on the MHTX board on RAGING BULL.
"Hydrogen Fuel
Since the early 19th century, scientists have recognized hydrogen as a potential source of fuel. Current uses of hydrogen are in industrial processes, rocket fuel, and
spacecraft propulsion. With further research and development, this fuel could also serve as an alternative source of energy for heating and lighting homes, generating
electricity, and fueling motor vehicles. When produced from renewable resources and technologies, such as hydro, solar, and wind energy, hydrogen becomes a renewable
fuel.
Composition of Hydrogen
Hydrogen is the simplest and most common element in the universe. It has the highest energy content per unit of weight—52,000 British Thermal Units (Btu) per pound (or
120.7 kilojoules per gram)—of any known fuel. Moreover, when cooled to a liquid state, this low-weight fuel takes up 1/700 as much space as it does in its gaseous state.
This is one reason hydrogen is used as a fuel for rocket and spacecraft propulsion, which requires fuel that is low-weight, compact, and has a high energy content.
In a free state and under normal conditions, hydrogen is a colorless, odorless, and tasteless gas. The basic hydrogen (H) molecule exists as two atoms bound together by
shared electrons. Each atom is composed of one proton and one orbiting electron. Since hydrogen is about 1/14 as dense as air, some scientists believe it to be the source of
all other elements through the process of nuclear fusion. It usually exists in combination with other elements, such as oxygen in water, carbon in methane, and in trace
elements as organic compounds. Because it is so chemically active, it rarely stands alone as an element.
When burned with pure oxygen, the only by products are heat and water. When burned with air, which is about 68% nitrogen, some oxides of nitrogen (or NOx) are formed.
Even then, burning hydrogen produces less air pollutants relative to fossil fuels.
Producing Hydrogen
Hydrogen bound in organic matter and in water makes up 70% of the earth’s surface. Breaking up these bonds in water allows us produce hydrogen and then to use it as a
fuel. There are numerous processes that can be used to break these bonds. Described below are a few methods for producing hydrogen that are currently used, or are under
research and development.
Most of the hydrogen now produced in the United States is on an industrial scale by the process of steam reforming, or as a byproduct of petroleum refining and chemicals
production. Steam reforming uses thermal energy to separate hydrogen from the carbon components in methane and methanol, and involves the reaction of these fuels with
steam on catalytic surfaces. The first step of the reaction decomposes the fuel into hydrogen and carbon monoxide. Then a "shift reaction" changes the carbon monoxide and
water to carbon dioxide and hydrogen. These reactions occur at temperatures of 392° F (200 ° C) or greater.
Another way to produce hydrogen is by electrolysis. Electrolysis separates the elements of water—H and oxygen (O)—by charging water with an electrical current. Adding
an electrolyte such as salt improves the conductivity of the water and increases the efficiency of the process. The charge breaks the chemical bond between the hydrogen and
oxygen and splits apart the atomic components, creating charged particles called ions. The ions form at two poles: the anode, which is positively charged, and the cathode,
which is negatively charged. Hydrogen gathers at the cathode and the anode attracts oxygen. A voltage of 1.24 Volts is necessary to separate hydrogen from oxygen in pure
water at 77° Fahrenheit (F) and 14.7 pounds per square inch pressure [25° Celsius (C) and 1.03 kilograms (kg) per centimeter squared.] This voltage requirement increases
or decreases with changes in temperature and pressure.
The smallest amount of electricity necessary to electrolyze one mole of water is 65.3 Watt-hours (at 77° F; 25 degrees C). Producing one cubit foot of hydrogen requires
0.14 kilowatt-hours (kWh) of electricity (or 4.8 kWh per cubic meter).
Renewable energy sources can produce elecricity for electrolysis. For example, Humboldt State University’s Schatz Energy Research Center designed and built a
stand-alone solar hydrogen system. The system uses a 9.2 kilowatt (KW) photovoltaic (PV) array to provide power to compressors that aerate fish tanks. The power not
used to run the compressors runs a 7.2 kilowatt bipolar alkaline electrolyzer. The electrolyzer can produce 53 standard cubic feet of hydrogen per hour (25 liters per
minute). The unit has been operating without supervision since 1993. When there is not enough power from the PV array, the hydrogen provides fuel for a 1.5 kilowatt
proton exchange membrane fuel cell to provide power for the compressors.
Steam electrolysis is a variation of the conventional electrolysis process. Some of the energy needed to split the water is added as heat instead of electricity, making the
process more efficient than conventional electrolysis. At 2,500 degrees Celsius water decomposes into hydrogen and oxygen. This heat could be provided by a solar energy
concentrating device to supply the heat. The problem here is to prevent the hydrogen and oxygen from recombining at the high temperatures used in the process.
Thermochemical water splitting uses chemicals such as bromine or iodine, assisted by heat. This causes the water molecule to split. It takes several steps—usually
three—to accomplish this entire process.
Photoelectrochemical processes use two types of electrochemical systems to produce hydrogen. One uses soluble metal complexes as a catalyst, while the other uses
semiconductor surfaces. When the soluble metal complex dissolves, the complex absorbs solar energy and produces an electrical charge that drives the water splitting
reaction. This process mimics photosynthesis.
The other method uses semiconducting electrodes in a photochemical cell to convert optical energy into chemical energy. The semiconductor surface serves two functions,
to absorb solar energy and to act as an electrode. Light-induced corrosion limits the useful life of the semiconductor.
Biological and photobiological processes use algae and bacteria to produce hydrogen. Under specific conditions, the pigments in certain types of algae absorb solar energy.
The enzyme in the cell acts as a catalyst to split the water molecules. Some bacteria are also capable of producing hydrogen, but unlike algae they require a substrate to
grow on. The organisms not only produce hydrogen, but can clean up pollution as well.
Recently, research funded by the U.S. Department of Energy has led to the discovery of a mechanism to produce significant quantities of hydrogen from algae. For 60 years,
scientists have known that algae produce trace amounts of hydrogen, but have not found a feasible method to increase the production of hydrogen. Scientists from the
University of California (UC), Berkeley, and the U.S. DOE’s National Renewable Energy Laboratory found the key. After allowing the algae culture to grow under normal
conditions, the research team deprived it of both sulfur and oxygen, causing it to switch to an alternate metabolism that generates hydrogen. After several days of generating
hydrogen, the algae culture was returned to normal conditions for a few days, allowing it to store up more energy. The process could be repeated many times. Producing
hydrogen from algae could eventually provide a cost-effective and practical means to convert sunlight into hydrogen.
Another source of hydrogen produced through natural processes is methane and ethanol. Methane (CH4) is a component of "biogas" that is produced by anaerobic bacteria.
Anaerobic bacteria occur widely throughout the environment. They break down or "digest" organic material in the absence of oxygen and produce biogas as a waste
product. Sources of biogas include landfills, and livestock waste and municipal sewage treatment facilities. Methane is also the principal component of "natural gas" (a
major heating and power plant fuel) produced by anaerobic bacteria eons ago. Ethanol is produced by the fermentation of biomass. Most fuel ethanol produced in the United
States is made from corn.
The United States, Japan, Canada, and France have investigated thermal water splitting, a radically different approach to creating hydrogen. This process uses heat of up to
5,430°F (3,000°C) to split water molecules.
Potential Uses for Hydrogen
The transportation, industrial, and residential sectors in the United States have used hydrogen for many years. Many people in the late 19th century burned a fuel called
"town gas," which is a mixture of hydrogen and carbon monoxide. Several countries, including Brazil and Germany, still distribute this fuel. Airships (dirigibles and
blimps) used hydrogen for transportation. Currently, industries use hydrogen for refining petroleum, and for producing ammonia and methanol. The Space Shuttle uses
hydrogen as fuel for its rockets.
With further research, hydrogen could provide electricity and fuel for the residential, commercial, industrial, and transportation sectors in the United States.
When properly stored, hydrogen as a fuel burns in either a gaseous or liquid state. Motor vehicles and furnaces can easily be converted to use hydrogen as a fuel. Since the
1950s, hydrogen has powered some airplanes. Automobile manufacturers have developed hydrogen-powered cars. Hydrogen burns 50% more efficiently than gasoline, and
burning hydrogen creates less air pollution. Hydrogen has a higher flame speed, wider flammability limits, higher detonation temperature, burns hotter, and takes less energy
to ignite than gasoline. This means that hydrogen burns faster, but carries the danger of pre-ignition and flashback. While hydrogen has its advantages as a vehicle fuel it still
has a long way to go before it can be used as a substitute for gasoline.
Fuel cells are a type of technology that use hydrogen to produce useful energy. In fuel cells, electrolysis is reversed by combining hydrogen and oxygen through an
electrochemical process, which produces electricity, heat, and water. The U.S. space program has used fuel cells to power spacecraft for decades. Fuel cells capable of
powering automobiles and buses have been and are being developed. Several companies are developing fuel cells for stationary power generation.
Hydrogen could be considered a way to store energy produced from renewable resources such as solar, wind, biomass, hydro, and geothermal. For example, when the sun is
shining, PV systems can provide the electricity needed to separate the hydrogen (as in the Humboldt State University’s Schatz Energy Research Center descibed abvoe). The
hydrogen could then be stored and burned as fuel, or to operate a fuel cell to generate electricity at night or during cloudy periods.
Storing Hydrogen
In order to use hydrogen on a large scale, safe, practical storage systems must be developed, especially for automobiles. Although hydrogen can be stored as a liquid, it is a
difficult process because the hydrogen must be cooled to -423° Fahrenheit (-253° Celsius). Refrigerating hydrogen to this temperature uses the equivalent of 25% to 30% of
its energy content, and requires special materials and handling. To cool one pound (0.45 kg) of hydrogen requires 5 kWh of electrical energy.
Hydrogen may also be stored as a gas, which uses less energy than making liquid hydrogen. Because hydrogen is a gas, it must be pressurized to store any appreciable
amount. For large-scale use, pressurized Hydrogen gas could be stored in caverns, gas fields, and mines. The hydrogen gas could then be piped into individual homes in the
same way as natural gas. Though this means of storage is feasible for heating, it is not practical for transportation because the pressurized metal tanks used for storing
hydrogen gas for transportation are very expensive.
A potentially more efficient method of storing hydrogen is in hydrides. Hydrides are chemical compounds of hydrogen and other materials. Research is currently being
conducted on magnesium hydrides. Certain metal alloys such as magnesium nickel, magnesium copper, and iron titanium compounds, absorb hydrogen and release it when
heated. Hydrides, however, store little energy per unit weight. Current research aims to produce a compound that will carry a significant amount of hydrogen with a high
energy density, release the hydrogen as a fuel, react quickly, and be cost-effective.
The Cost of Hydrogen
Currently the most cost-effective way to produce hydrogen is steam reforming. According to the U.S. Department of Energy, in 1995 the cost was $7.39 per million Btu
($7.00 per gigajoule) in large plant production. This assumes a cost for natural gas of $2.43 per million Btu ($2.30 per gigajoule). This is the equivalent of $0.93 per gallon
($0.24 per liter) of gasoline. The production of hydrogen by electrolysis using hydroelectricity at off peak rates costs between $10.55 to $21.10 per million Btu ($10.00 to
$20.00 per gigajoule).
Hydrogen Research in the United States
Recognizing the potential for hydrogen fuel, the U.S. Department of Energy (DOE) and private organizations have funded research and development (R&D) programs for
several years. The Federal government allocates an average of about $18 million each year for hydrogen R&D. Current work in the United States includes research at the
National Renewable Energy Laboratory (NREL), Texas A & M University, the Brookhaven National Laboratory, and the Hawaii Natural Energy Institute.
The Florida Solar Energy Center (FSEC) conducts a Hydrogen from Renewable Energy program. FSEC long-term goals under DOE sponsorship are to develop a dual-bed
reactor process to photocatalytically split water into hydrogen and oxygen and to chemically synthesize an electrolytic membrane for high temperature electrolysis. Another
DOE research project is to develop a process to reform natural gas to hydrogen for on-site production of hydrogen-methane blends that are applicable to automobiles.
In order to use hydrogen on a wider scale, researchers in the United States must develop more practical and economical ways to store and process hydrogen. " |
