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Great description of H2 production and practical uses
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Great description of H2 production and practical uses
FOR THOSE OF US THAT ENJOY GRAPHICS AND PICTORIAL AIDS, THE COMPLETE ARTICLE IS HERE

Hydrogen
H2 CHEMICAL AND PHYSICAL PROPERTIES

Hydrogen is by far the most plentiful element in the uni-verse, making up 75% of the mass of all visible matter in stars and galaxies.

Hydrogen is the simplest of all elements. You can visualize a hydrogen atom as a dense central nucleus with a single orbiting electron, much like a single planet in orbit around the sun.

In most hydrogen atoms, the nucleus consists of a single proton, although a rare form (or “isotope”) of hydrogen con-tains both a proton and a neutron.

Most of the mass of a hydrogen atom is concentrated in its nucleus. In fact, the proton is more than 1800 times more massive than the electron. Neutrons have almost the same mass as protons. However, the radius of the electron’s orbit, which defines the size of the atom, is approximately 100,000 times as large as the radius of the nucleus! Clearly, hydro-gen atoms consist largely of empty space.

A proton has a positive electrical charge, and an electron has a negative electrical charge. Neutrons do not carry a charge. Together, the charges associated with the proton and elec-tron of each hydrogen atom cancel each other out, so that individual hydrogen atoms are electrically neutral.

Chemically, the atomic arrangement of a single electron orbiting a nucleus is highly reactive. For this reason, hydro-gen atoms naturally combine into molecular pairs (H2 in-stead of H).


COMPARISON WITH OTHER FUELS

It is natural for us to compare hydrogen to other hydrocar-bon fuels with which we are more familiar. All hydrocarbon fuels are molecular combinations of carbon and hydrogen atoms.

The simplest of all hydrocarbons is methane, which is the principal constituent of natural gas. Methane has the chemical formula CH4, which means that each molecule has four hydrogen atoms and one carbon atom. Other common hydrocarbons are ethane (C2H6), propane (C3H8) and butane (C4H10).

Gasoline is composed of a mixture of many different hydro-carbons, but an important constituent is heptane (C7H16). Gasoline, diesel, kerosene, and compounds found in asphalt, heavy oils and waxes, are considered heavy hydrocarbons as they contain many carbon atoms per molecule, and therefore have high molecular weight.

The lightest hydrocarbons are gases at normal atmospheric pressure and temperature. Heavier hydrocarbons, with 5 to 18 carbon atoms per compound, are liquid at ambient condi-tions and have increasing viscosity with molecular weight.

Other chemical fuels include alcohols whose molecules com-bine an oxygen/hydrogen atom pair (OH) with one or more hydrocarbon groups. Common alcohol fuels are methanol (CH3OH) and ethanol (C2H5OH). These may be blended with hydrocarbons for use in internal combustion engines.


HAZARDS

Usually people see hydrogen as a very dangerous gas, because of its high ignition probability. Other gases, such as methane, lpg or also gasoline vapour, normally used as fuels, have similar or even more dangerous caratheristics.

As an example let us take flammability range of the compound fuel-air. The flammability range of a gas is defined in terms of its lower flammability limit (LFL) and its upper flammability limit (UFL). The LFL of a gas is the lowest gas concentration that will support a self-propagating flame when mixed with air and ignited. Below the LFL, there is not enough fuel present to support combustion; the fuel/air mixture is too lean. The UFL of a gas is the highest gas concentration that will support a self-propagating flame when mixed with air and ignited. Above the UFL, there is not enough oxygen present to support combustion; the fuel/air mixture is too rich. Between the two limits is the flammable range in which the gas and air are in the right proportions to burn when ignited. One consequence of the UFL is that stored hydrogen (whether gaseous or liquid) is not flammable while stored due to the absence of oxygen in the cylinders. The fuel only becomes flammable in the peripheral areas of a leak where the fuel mixes with the air in sufficient proportions.

Hydrogen is flammable over a very wide range of concentrations in air (4 – 75%). As a result, even small leaks of hydrogen have the potential to burn or explode. Leaked hydrogen can concentrate in an enclosed environment, thereby increasing the risk of combustion and explosion.

As you can see, Methane has a smaller flammability range than hydrogen, as gasoline and diese fuels do. But gasoline and diesel starts to burn (if ignited) at a lower concentration than hydrogen. In effect, with diesel vapours you can have ignition with a concentration as small as 0.6% in air, about seven times that of hydrogen.

The molecules of hydrogen gas are smaller than all other gases, and it can diffuse through many materials considered airtight or impermeable to other gases. This property makes hydrogen more difficult to contain than other gases.

Leaks of liquid hydrogen evaporate very quickly since the boiling point of liquid hydrogen is so extremely low.

Hydrogen leaks are dangerous in that they pose a risk of fire where they mix with air. However, the small molecule size that increases the likelihood of a leak also results in very high buoyancy and diffusivity, so leaked hy-drogen rises and becomes diluted quickly, especially out-doors. This results in a very localized region of flammability that disperses quickly. As the hydrogen dilutes with distance from the leakage site, the buoyancy declines and the ten-dency for the hydrogen to continue to rise decreases. In contrast, leaking gasoline or diesel spreads laterally and evaporates slowly resulting in a widespread, lingering fire hazard. Propane gas is denser than air so it accumulates in low spots and disperses slowly, resulting in a protracted fire or explosion hazard. Heavy vapors can also form vapor clouds or plumes that travel as they are pushed by breezes. Methane gas is lighter than air, but not nearly as buoyant as hydrogen, so it disperses rapidly, but not as rapidly as hy-drogen. For small hydrogen leaks, buoyancy and diffusion effects in air are often overshadowed by the presence of air currents from a slight ambient wind, very slow vehicle motion or the radiator fan. In general, these currents serve to disperse leaked hydrogen even more quickly with a further reduction of any associated fire hazard.

When used as vehicle fuel, the propensity for hydrogen to leak necessitates special care in the design of the fuel system to ensure that any leaks can disperse with minimum hin-drance, and the use of dedicated leak detection equipment on the vehicle and within the maintenance facility.


Hydrogen has the added property of low electro-conductivity so that the flow or agitation of hydrogen gas or liquid may generate electrostatic charges that result in sparks. Constant exposure to hydrogen causes a phenomenon known as hydrogen embrittlement in many materials. Hy-drogen embrittlement can lead to leakage or catastrophic failures in metal and non-metallic components.

Hydrogen flames are very pale blue and are almost invisible in daylight due to the absence of soot. Visibility is enhanced by the presence of moisture or impurities (such as sulfur) in the air. Hydrogen flames are readily visible in the dark or subdued light. A hydrogen fire can be indirectly visible by way of emanating “heat ripples” and thermal radiation, par-ticularly from large fires. In many instances, flames from a hydrogen fire may ignite surrounding materials that do pro-duce smoke and soot during combustion. Hydrogen fires can only exist in the region of a leak where pure hydrogen mixes with air at sufficient concentrations. For turbulent leaks, air reaches the centerline of the leakage jet within about five diameters of a leakage hole, and the hydrogen is diluted to nearly the composition of air within roughly 500 to 1000 diameters. This rapid dilution implies that if the turbulent leak were into open air, the flammability zone would exist relatively close to the leak. Therefore, when the jet is ignited, the flame length is less than 500 diameters from the hole (for example, for a 0.039 in/1 mm diameter leak, the flame length will be less than 19.7 in/0.5 m).

In many respects, hydrogen fires are safer than gasoline fires. Hydrogen gas rises quickly due to its high buoyancy and diffusivity. Consequently hydrogen fires are vertical and highly localized. When a car hydrogen cylinder ruptures and is ignited, the fire burns away from the car and the interior typically does not get very hot.

Gasoline forms a pool, spreads laterally, and the vapors form a lingering cloud, so that gasoline fires are broad and en-compass a wide area. When a car gasoline tank ruptures and is ignited, the fire engulfs the car within a matter of seconds (not minutes) and causes the temperature of the entire vehicle to rise dramatically. In some instances, the high heat can cause flammable compounds to off-gas from the vehicle upholstery leading to a secondary explosion.

Hydrogen emits non-toxic combustion products when burned. Gasoline fires generate toxic smoke.

As a conclusion we can say that hydrogen can become dangerous, in case of a leak, only if it reaches a concentration between flammability limits. Also if this happens, to burn hydrogen needs to be ignited (for instance, by a spark). In case of burning, the flame is, for many respects, safer than that generating from conventional fuels.


PRODUCTION

Despite its abundance in the universe, hydrogen does not occur freely on earth, as it reacts very readily with other elements. For this reason, the vast majority of hydrogen is bound into molecular com-pounds. To obtain hydrogen means to remove it from these other molecules. With respect to the energy required, it is easy to remove hydrogen from compounds that are at a higher en-ergy state, such as fossil fuels. This process releases energy, reducing the amount of process energy required. It takes more energy to extract hydrogen from compounds that are at a lower energy state, such as water, as energy has to be added to the process.

The process of extracting hydrogen from water is called electrolysis. In principal, electrolysis can be entirely non-polluting and renewable, but it requires the input of large amounts of electrical energy. Consequently, the total envi-ronmental impact of acquiring hydrogen through electrolysis is largely dependent on the impacts of the source power.

In electrolysis, electricity is used to decompose water into its elemental components: hydrogen and oxygen. Electrolysis is often touted as the preferred method of hydrogen production as it is the only process that need not rely on fossil fuels. It also has high product purity, and is feasible on small and large scales. Electrolysis can operate over a wide range of electrical energy capacities, for example, taking advantages of more abundant electricity at night.

At the heart of electrolysis is an electrolyzer. An electrolyzer is a series of cells each with a positive and negative elec-trode. The electrodes are immersed in water that has been made electrically conductive, achieved by adding hydrogen or hydroxyl ions, usually in the form of alkaline potassium hydroxide (KOH).

The overall reaction that occurs is very simple:

2H2O + Energy -> 2H2 + O2

The rate of hydrogen generation is related to the current density (the amount of current divided by the electrode area measured in amps per area). In general, the higher the cur-rent density, the higher the source voltage required, and the higher the power cost per unit of hydrogen. However, higher voltages decrease the overall size of the electrolyzer and therefore result in a lower capital cost. State-of-the-art elec-trolyzers are reliable, have energy efficiencies of 65 to 80% and operate at current densities of about 186 A/ft2 (2000 A/m2).

A fuel cell reverses the process of electrolysis. Electrolysis adds electrical energy to low-energy water to release two high-energy gases. A fuel cell allows the gases to react and combine to form water, releasing electrical power. Both proc-esses release heat, which represents an energy loss.

When viewed together with fuel cells, hydrogen produced through electrolysis can be seen as a way of storing electrical energy as a gas until it is needed. Hydrogen produced by electrolysis is therefore the energy carrier, not the energy source. The energy source derives from an external power generating plant. In this sense, the process of electrolysis is not very different from charging a battery, which also stores electrical energy. Viewed as an electricity storage medium, hydrogen is competitive with batteries in terms of weight and cost.

To obtain 1 m3 of hydrogen from electrolisys we need about 5 kWh of electrical energy.

To be truly clean, the electrical power stored during electrolysis must derive from non-polluting, renewable sources. If the power is derived from natural gas or coal, the pollution has not been eliminated, only pushed upstream. In addition, every energy transformation has an associate energy loss. Consequently, fossil fuels may be used with greater effi-ciency by means other than by driving the electrolysis of hydrogen. Furthermore, the cost of burning fossil fuels to generate electricity for electrolysis is three to five times that of reforming the hydrogen directly from the fossil fuel.

Non-polluting renewable energy sources include hydroelec-tric, solar photovoltaic, solar thermal and wind. These meth-ods of power generation are applicable only in specific geographic or climatic conditions. Furthermore, with the exception of hydroelectric, each of these power sources is intermittent. Despite growth in the use of these energy sources, they currently provide a very small amount of the power consumed today. Hydroelectric power generation uses the energy of moving water to turn turbines that in turn rotate generators. Hy-droelectric power is only feasible in areas with major rivers that undergo significant changes in height. Most suitable locations worldwide have already been developed. Hydro-electric power is a cheap source of clean power especially when utilizing excess, off-peak power. The efficiency of hy-droelectric power generation can top 80%. This is probably the optimum form of renewable energy although the environmental and ecological cost of dams is high.

As an example we can take the December 2003 report of the italian National Electrical Energy Manager (GRTN) which shows that, only in the month of December 2003, 927 GWh were used, in idro-electrical plants, to re-pump water back uphill during low-load periods. If such amount of energy should have been used to produce hydrogen by electrolisys, about 185.000.000 m3 should have benn produced only in the month of December.

REFORMING

The process of extracting hydrogen from fossil fuels is called reforming. Today, this is the principal and least expensive method of producing hydrogen. Unfortunately, reforming emits pollutants and consumes non-renewable fuels.

Reforming is a chemical process that reacts hydrogen-containing fuels in the presence of steam, oxygen, or both into a hydrogen-rich gas stream. When applied to solid fuels the reforming process is called gasification. The resulting hydrogen-rich gas mixture is called reformate. The equipment used to produce reformate is known as a reformer or fuel processor. The specific composition of the reformate depends on the source fuel and the process used, but it always contains other compounds such as nitrogen, carbon dioxide, carbon monoxide and some of the unreacted source fuel. When hydrogen is removed from the reformate, the remaining gas mixture is called raffinate.

In theory, any hydrocarbon or alcohol fuel can serve as a feedstock to the reforming process. Naturally, fuels with existing distribution infrastructures are the most commonly used. They are: Methane, Methanol, Gasoline and Diesel, Ammonia.

Reformers are of three basic types: steam reformers, partial oxidation reactors and thermal decomposition reactors. A fourth type results from the combination of partial oxidation and steam reforming in a single reactor, called an autother-mal reformer. Steam reformers are currently the most efficient, economical and widely used technique of hydrogen production.

The advantages of reforming fossil fuels are that they: uses existing fuel infrastructures; reduces the need to transport and store hydrogen; does not need the input of large amounts of energy as in electrolysis; is less expensive than other hydrogen production methods.

The disadvantages of reformers are that they: can have relatively long warm-up times; are difficult to apply to vehicle engines because of irregu-lar demands for power (transient response); are complex, large and expensive; introduce additional losses into the energy conversion process, especially those that have small thermal mass; use non-renewable fossil fuels; generate pollution.


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You can't skin a fish before you catch it !
08-02-2008 07:10 AM
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