30-10-2012, 01:51 PM
Hydrogen –The Green fuel
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Introduction
About Hydrogen
Hydrogen is an energy carrier, not an energy source — it stores and delivers energy in a usable form, but it must be produced from compounds that contain it.
Hydrogen can be produced using diverse, domestic resources including fossil fuels, such as coal (with carbon sequestration) and natural gas; nuclear; and biomass and other renewable energy technologies, such as wind, solar, geothermal, and hydroelectric power. Great potential for diversity of supply is an important reason why hydrogen is such a promising energy carrier.
Hydrogen can be produced at large central plants as far as several hundred miles from the point of end-use; semi-centrally, 25 to 100 miles from the point of end-use; or in small distributed units located at or very near the point of end-use, such as at refueling stations or stationary power sites.
Hydrogen is a gas at normal temperatures. It is highly reactive, combining
readily with a number of elements and compounds, the most familiar
example being oxygen to form water (H20). The 2H + O = H20 (hydrogen
plus oxygen equals water) combustion reaction is highly charged, explosive,
producing a great deal of heat as a by product, thus making hydrogen a true
competitor with fossil fuels as a source of power.
The same reactive quality that makes hydrogen a good fuel source, however,
also makes free hydrogen rare in nature—it is almost always found bound to
other chemicals. One of the challenges, then, of moving to a hydrogen
energy regime is to develop economical ways of freeing hydrogen from the
chemicals to which it is bonded so it can be used as a fuel, then returned to
nature.
PRODUCTION
The breakthrough means ethanol which comes from the fermentation of crops can be completely converted to hydrogen and carbon dioxide for the first time.
The hydrogen generated would be used to power fuel cells - devices which convert fuels into electricity directly without the need for combustion.
The new method - which has the potential to be used to power homes, buildings and cars in the future - is the result of a 10 year collaboration project between scientists from the University of Aberdeen alongside international partner laboratories.
Over 90% of the hydrogen currently generated across the globe is made using natural gas found in fossils fuels.
The main concern with this method is the generation of large amounts of carbon dioxide increasing the risk of global warming.
This new production method uses ethanol which is produced by the fermentation of crops and is therefore carbon neutral meaning any carbon dioxide produced is assimilated back into the environment and used by plants to grow.
Professor Hicham Idriss, Energy Futures Chair at the University of Aberdeen who has led the study said: "We have successfully created the first stable catalyst which can generate hydrogen using ethanol produced from crop fermentation at realistic conditions.
"Moreover, hydrogen generated using this method is very clean and therefore suitable for fuel cells because it also converts all carbon monoxide, which is poisonous, generated in the process to carbon dioxide at the same time.
Steam reforming
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 200oC or greater.
Steam Electrolysis
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 2,500oC 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
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
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.
STORAGE
Hydrogen can be stored using six different methods and phenomena: (1) high-pressure gas cylinders (up to 800 bar), (2) liquid hydrogen in cryogenic tanks (at 21 K), (3) adsorbed hydrogen on materials with a large specific surface area (at T<100 K), (4) absorbed on interstitial sites in a host metal (at ambient pressure and temperature), (5) chemically bonded in covalent and ionic compounds (at ambient pressure), or (6) through oxidation of reactive metals, e.g. Li, Na, Mg, Al, Zn with water. The most common storage systems are high-pressure gas cylinders with a maximum pressure of 20 MPa (200 bar). New lightweight composite cylinders have been developed which are able to withstand pressures up to 80 MPa (800 bar) and therefore the hydrogen gas can reach a volumetric density of 36 kg.m(-3), approximately half as much as in its liquid state. Liquid hydrogen is stored in cryogenic tanks at 21.2 K and ambient pressure. Due to the low critical temperature of hydrogen (33 K), liquid hydrogen can only be stored in open systems. The volumetric density of liquid hydrogen is 70.8 kg.m(-3), and large volumes, where the thermal losses are small, can cause hydrogen to reach a system mass ratio close to one. The highest volumetric densities of hydrogen are found in metal hydrides. Many metals and alloys are capable of reversibly absorbing large amounts of hydrogen. Charging can be done using molecular hydrogen gas or hydrogen atoms from an electrolyte. The group one, two and three light metals (e.g. Li, Mg, B, Al) can combine with hydrogen to form a large variety of metal-hydrogen complexes. These are especially interesting because of their light weight and because of the number of hydrogen atoms per metal atom, which is two in many cases. Hydrogen can also be stored indirectly in reactive metals such as Li, Na, Al or Zn. These metals easily react with water to the corresponding hydroxide and liberate the hydrogen from the water. Since water is the product of the combustion of hydrogen with either oxygen or air, it can be recycled in a closed loop and react with the metal