2. Advantages and Disadvantages of Hydrogen
2.1 Physical Properties
Hydrogen is a colorless and odorless gas. Its density is 0.0899 g/l (air is 14.4 times as dense). Hydrogen boils at -252.77°C. Liquid hydrogen has a density of 70.99 g/l. With these properties, hydrogen has the highest energy to weight ratio of all fuels. 1 kg of hydrogen contains the same amount of energy as 2.1 kg of natural gas or 2.8 kg of gasoline. The energy to volume ratio amounts to about 1/4 of that for petroleum and 1/3 of that for natural gas. Water consists of 11.2% hydrogen by weight.
Hydrogen burns in air at concentrations in the range of 4 - 75% by volume (methane burns at 5.3 - 15% and propane at 2.1 - 9.5% concentrations by volume). The highest burning temperature of hydrogen of 2318 °C is reached at 29% concentration by volume, whereas hydrogen in an oxygen atmosphere can reach burning temperatures up to 3000°C (the highest reached burning temperature in air for methane is 2148°C and for propane 2385°C). The minimum required ignition energy required for a stoichiometric fuel/oxygen mixture is for hydrogen 0.02 mJ, for methane 0.29 mJ and for propane 0.26 mJ. Even the energy of a static electric discharge from the arcing of a spark is sufficient to ignite natural gas so it is largely irrelevant that hydrogen requires only a tenth of this energy for ignition. The temperatures for spontaneous combustion of hydrogen, methane and propane are 585°C, 540°C and 487°C respectively.
The explosive regions for hydrogen and methane lie in the ranges 13% - 59% and 6.3% - 14% respectively. The explosive range for hydrogen is clearly much greater, whereas methane is already explosive at a much lower concentration. The diffusion coefficient for hydrogen at 0.61 cm3/s is 4 times as high as that for methane. Hydrogen therefore mixes in air considerably faster than methane or petrol vapor, which is advantageous in the open but represents a potential disadvantage in badly ventilated interiors. Since both hydrogen and natural gas are lighter than air they rise quickly. Propane and petrol vapor are in contrast heavier than air and remain on the ground, leading to a higher likelihood of explosion.
2.2 Energetic Evaluation
The important combustion related properties of hydrogen, methane and propane are summarized in the following table:
Weight and volume related energy densities for hydrogen in comparison with other energy carriers
|Energy carrier||Form of Storage||Energy density by
|Energy density by volume
|Hydrogen||gas (20 MPa)||33.3||0.53|
|gas (24,8 MPa)||33.3||0.64|
|gas (30 MPa)||33.3||0.75|
|Natural gas||gas (20 MPa)||13.9||2.58|
|gas (24,8 MPa)||13.9||3.01|
|gas (30 MPa)||13.9||3.38|
|Electricity||Pb battery (chemical)||0.03||0.09|
To estimate total weights of energy carriers, it is also necessary to consider the weight of the corresponding storage device in each case. This weight is material and manufacturer specific and naturally depends on the size of the device. For a 100 l storage device, the following figures give rough estimates of weights: in the case of pressurized gas bottles (designed for natural gas or hydrogen), a 100 l steel bottle weighs about 100kg, an aluminum/composite bottle weighs about 65 kg and a modern full composite bottle around 30 kg. A 100 l liquid hydrogen tank weighs about 90kg. In the case of metal hydride storage, the device weight is already taken into account in the given energy density by weight figure, that is to say, the extra required cover weight is negligible. This is also true for a lead-acid battery. For gas and liquid storage devices, the weight rises in proportion to the volume to the power 2/3 (V2/3), reflecting that the storage volume increases with the third power, the storage device surface area however only with the second power. To be precise, this scaling factor is only valid in the case when the thickness of the storage device walls do not increase with the overall size. This is however only the case for a small range.
2.3 Environmental Advantages:
The burning of hydrogen with air under appropriate conditions in combustion engines or gas turbines results in very low or negligible emissions. Trace hydrocarbon and carbon monoxide emissions, if at all generated, can only result from the combustion of motor oil in the combustion chamber of internal combustion engines. Nitrous oxide emissions increase exponentially with the combustion temperature. These can therefore be influenced through appropriate process control. As hydrogen offers more possibilities than other fuels, a distinct reduction in NOx emissions is possible compared to mineral oil and natural gas, provided that a lower combustion temperature is achieved (e.g. with a high air to fuel ratio). Particulate and sulfur emissions are completely avoided apart from small quantities of lubricant remnants.
The use of hydrogen in fuel cell propulsion systems with low temperature fuel cells (Membrane fuel cells: PEMFC) completely eliminates all polluting emissions. The only by-product resulting from the generation of electricity from hydrogen and oxygen in the air is de-mineralised water. Use of hydrogen in fuel cells at higher temperature levels causes up to 100 times fewer emissions compared with conventional power stations. If the hydrogen is obtained from methanol however, then the reforming process itself will result in carbon dioxide emissions.
Furthermore hydrogen offers the possibility, depending on production method, to drastically reduce or avoid emissions, especially carbon dioxide (CO2), in the whole fuel cycle. Using hydrogen as secondary energy carrier would allow the flexible introduction of the most diverse renewable energies into the fuel sector.
Since hydrogen is a secondary energy carrier, the complete fuel cycle from primary energy source to final application must be considered when judging the environmental relevance.
2.4 Short overview regarding the state of hydrogen technology
Various hydrogen technologies have, to some extent, been tested and in use for decades.
For example there is an existing demand for hydrogen in the chemical and petrochemical industries for the synthesis of chemical raw materials (e.g. production of ammonia, ethylene and methanol), in other cases in the same industry hydrogen is sometimes inevitably produced as a by-product (e.g. through chlorine-alkaline electrolysis for production of Chlorine). A second and more important hydrogen producer and consumer is the processing of fuels in refineries (e.g. hydrogen production during thermocracking, hydrogen consumption for desulphurization and hydrogenation of fuels.)
As a result of this situation, several large scale processes have been developed for the production of hydrogen from fossil fuels and from water. These processes are the steam reforming of natural gas, the partial oxidation of hydrogen from heavy fuel oil or coal, chlorine-alkaline electrolysis and finally, the low pressure electrolysis of water.
Along with this, the related systems of hydrogen transport, storage and distribution have been built up. These systems include pressurized gas pipelines (Hüls, Germany, since 1938, 2.5 MPa, 215 km long, 168-273 mm diameter; Air Liquide, France & Belgium, since 1966, 6.5-10 MPa, 290 km long, various diameters), containerized liquid hydrogen transport in ISO 40 foot standard dimensions (for approx. 3 decades, 41-45 m3 geometr. volume), stationary storage of LH2 in large spherical tanks (NASA: 3,000 m3), underground storage in caverns (ICI, England), liquefaction plants of capacities of 4.4 t/d (Ger.), 5.5 t/d (NL), 10 t/d (Fra) and 10 t/d (CDN).
In terms of final applications, the industrial use of large H2 burners and gas turbines with H2/Natural gas mixtures is the most prominent.
A prototype fleet of 10 cars with internal combustion engine drivetrains and metal hydride storage was in service from 1984-1988 (Daimler-Benz in Berlin). Prototype cars with LH2 storage and internal combustion engine have been under test for 15 years (BMW). Since the 80's Mazda has been testing vehicles with Wankel engines and metal hydride storage. Industrial vehicles with combustion engines (VCST Hydrogen Systems, Bel; MAN, Ger; Daimler Benz, Ger; Hydrogen Consultants, Inc., USA) and with fuel cells (Ballard, CDN; Ansaldo/de Nora, Italy; H Power, USA; IFC, USA; Daimler Benz, Ger; MAN, Ger; Neoplan, Ger) are now moving into the prototype development and testing phase.
The following technologies are in the planning and development phases, and to some extent already in prototype testing: pressurized elektrolysers in the kW and MW range intended for fluctuating energy supplies, liquefaction plants with more than 100 t/d capacity (Jap, CDN), large LH2 tankers (Jap, Ger), advanced LH2 transport containers of 40 ft and 80 ft size, as well as containers with 300 m3, 600 m3, 3,600 m3, 24,000 m3 and 200,000 m3 volumes (CDN, Fra, Ger, Jap), LH2 refueling equipment for passenger cars and duty vehicles (Ger), aircraft combustion chambers and system layouts for LH2 (CDN, Ger, Rus), specialized fuel cell systems for passenger vehicles (CDN, Ger, Italy, USA, Jap), high temperature fuel cells (Ger, Den, Italy, Jap, Nor, NL, USA) and H2 gas turbines in the 100 MW class (Jap, USA).
There is also activity in the field of direct hydrogen production from biomass. Along with fuel cells as electricity generator , the use of gas turbines, a co-genenerator with combustion engine and an H2/O2 steam generator for electricity generation as spinning reserve are all being worked upon.
Presently, almost the only pure thermal use of hydrogen is that of catalytic burners and ceramic surface burners.
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