What are some useful uses of hydrogen
Lexicon> Letter W> Hydrogen
Definition: a flammable gas that can serve as an energy carrier
More general terms: fuel gas, energy carrier
Molecular formula: H2
Categories: Energy Sources, Renewable Energy
Author: Dr. Rüdiger Paschotta
How to quote; suggest additional literature
Original creation: 10/10/2010; last change: 01/09/2021
hydrogen (chemically H2) is a colorless, odorless, non-toxic and flammable gas at room temperature. It is a good 14 times lighter than air and can diffuse very quickly through porous materials or even through the smallest of leaks. Hydrogen can be burned in air, and mixtures of hydrogen with air or especially with pure oxygen are very explosive; hence the name Oxyhydrogen. The combustion (oxidation) of hydrogen produces pure water vapor (H.2O); At most, nitrogen oxides can also be produced during combustion with air, but otherwise no pollutants.
|density||0.090 kg / m3|
at 0 ° C, 1013 mbar
|Melting point||14.01 K = −259.14 ° C (at 1013 mbar)|
|boiling point||21.15 K = −252.0 ° C (at 1013 mbar)|
|calorific value||120.0 MJ / kg = 33.3 kWh / kg, |
10.8 MJ / m3 = 3.0 kWh / m3
at 0 ° C, 1013 mbar
|Calorific value||141.8 MJ / kg = 39.4 kWh / kg, |
12.7 MJ / m3 = 3.54 kWh / m3
at 0 ° C, 1013 mbar
(118% of the calorific value)
|combustion||2 H2 + O2 → 2 H2O |
Air requirement (for λ = 1):
2.39 m3 per m3 H2
H2O: 8.94 kg per kg of H2
(268 g / kWh in terms of calorific value)
The calorific value and calorific value of hydrogen (see the table on the right) are exceptionally high in relation to the mass in comparison with other fuels, while the volume-related values for normal pressure are relatively small due to the low density. Thus, an energy storage device with a high gravimetric energy density could be implemented as a hydrogen storage device, if the storage device could be built relatively easily. However, this is difficult with pressure accumulators (e.g. gas cylinders), since very high pressures are required; this problem can only be solved with liquefied hydrogen. On the other hand, the volumetric energy density of such storage is fundamentally much smaller than z. B. for natural gas.
Hydrogen is usually handled in a gaseous state - often at increased pressure and a correspondingly higher density. Hydrogen can also be liquefied by cooling it to very low temperatures. At atmospheric pressure, condensation only takes place at a temperature of 21.15 Kelvin = −252.0 ° C. The energy consumption for liquefaction is therefore significantly higher than that for the production of liquefied natural gas, for example. In addition, the technology must be elaborately designed so that practically no air and no moisture can get into the tank system.
Use of hydrogen
In principle, hydrogen could play a major role as a secondary energy carrier in the future; he would be the key element of one Hydrogen economythat enables the abandonment of fossil fuels. Hydrogen is not one Energy source, since (free, not chemically bound) hydrogen occurs in nature only in traces. However, hydrogen can be produced in several ways (see below) and also used as an energy carrier in different ways:
- Hydrogen can be used like z. B. burn natural gas directly with air, either to generate heat or in heat engines such. B. Otto engines. However, this type of use is not particularly energy efficient; the relatively low knock resistance of hydrogen makes a low compression ratio of the engine necessary, and this in turn reduces the efficiency compared to, for example, that which could be achieved with gasoline. The resulting exhaust gas mainly consists of harmless water vapor and remaining nitrogen in the air, but poisonous nitrogen oxides can also be formed in certain quantities (depending on the details of the combustion process).
- In fuel cells, hydrogen (and air or oxygen) can be used to generate electrical energy, in a relatively energy-efficient manner (sometimes with efficiencies well above 50%), quietly and without any pollutant emissions (i.e. also without nitrogen oxides). At the same time, waste heat is generated, which can sometimes also be used. Unfortunately, fuel cells are much more expensive than simple burners.
- In some chemical processes, hydrogen can be used as a reducing agent. For example, it can be used in the production of iron from iron ore, where coke is otherwise used in blast furnaces, which is oxidized to carbon monoxide and carbon dioxide in the process. In principle, climate-neutral steel production would be possible.
Hydrogen can also be stored before use (see below), i. H. it can also serve as an energy store.
A completely different energetic application of hydrogen would be Nuclear fusionwhich has so far only been used for nuclear weapons (specifically for the particularly effective thermonuclear atomic bombs). Here atomic nuclei of deuterium or tritium (heavier isotopes of hydrogen) are fused together to form helium. The energy yield is many orders of magnitude higher than with chemical reactions such as combustion. However, controlled nuclear fusion for the purpose of energy supply is extremely difficult to carry out technically, and the technical and, above all, economic usability for energy generation has not been proven.
Hydrogen is used in various other applications, for example in the chemical industry and in liquid form as a coolant (cryogen).
Production of hydrogen
Hydrogen can be produced in different ways:
Steam reforming and partial oxidation
A process that is already being used on an industrial scale today is Steam reforming of hydrocarbons such as methane (CH4) or methanol (CH3OH). As a rule, a fossil fuel such as natural gas is assumed, which is mainly made up of methane (CH4) consists. Steam (H.2O) is added and heat is supplied, so that a hydrogen-containing synthesis gas is produced. This high temperature heat is z. B. obtained by burning part of the natural gas, in the future possibly also by concentrated solar radiation or with high-temperature nuclear reactors. A catalyst usually accelerates the chemical reactions. The synthesis gas contains hydrogen and carbon monoxide, which can be further converted to carbon dioxide, with further hydrogen being generated. The disadvantages of hydrogen production from natural gas and other fossil hydrocarbons are that one remains dependent on fossil fuels and that CO2 is harmful to the climate2Emissions occur (except when the CO2 separated and permanently stored underground). The efficiency is z. B. when using natural gas a good 70%, with other raw materials rather lower; so significant amounts of energy are lost.
Partial oxidation is a variant. Here the raw material, e.g. B. methane or a heavy hydrocarbon, burned in a lack of oxygen, producing hydrogen and carbon monoxide. The carbon monoxide can be converted further to carbon dioxide, with further hydrogen being generated. In contrast to reforming, the hydrogen comes from the raw material alone (and not from water vapor), and the necessary heat is generated by the oxidation itself. In principle, the resulting CO2 also be sequestered to effective a CO2-to have a free (or poor) source of hydrogen.
Town gas, which is made from coal, also contains significant amounts of hydrogen, along with carbon monoxide. In principle, coal gasification could be used to produce hydrogen on a large scale, if the coal deposits are large enough for this (although some voices doubt this) and the CO2-Sequestration becomes possible at a reasonable cost (which is also uncertain).Biomass gasification could become an important source of hydrogen in the future. However, the methods used still need to be improved.
In the future, reforming could also be based on biomass such as B. wood, crop waste or sewage sludge can be used. For this, however, adapted biomass gasification processes have to be developed. Because of the more complex and inconsistent chemical composition of biomass, this is more difficult to optimize than the gasification of fossil fuels.
With the help of electrical energy, hydrogen (and oxygen) can be produced from water through electrolysis. (If energy from renewable sources is used, this hydrogen can be used as a RE gas This process has energy losses of typically 25 to 35%, similar to the reforming of natural gas. However, as long as natural gas is also converted into electricity (e.g. in combined cycle power plants), it does not make sense to produce hydrogen by electrolysis at the same time: It is much more efficient and cheaper to reform natural gas directly than converting it into electricity first and then electrolysis to operate. Electrical energy from renewable sources can also be used more sensibly to replace fossil fuels in electricity generation. At most, it would be conceivable to use short-term surpluses of electricity (e.g. when there is a strong supply of wind energy) to generate hydrogen. Then, however, the electrolysers would have a low capacity utilization, which leads to high costs and, because of the necessary cost optimization, to higher energy losses.
So far (as of 2010) only a few percent of the hydrogen in Germany is produced by electrolysis, because the costs are considerably higher than z. B. in natural gas reforming. This could change in the future if, on the one hand, natural gas becomes much more expensive and, on the other hand, electrical energy from renewable sources is available in large quantities.
The Kværner process enables the production of hydrogen from natural gas, for example, whereby the carbon is obtained in elemental form instead of CO2. This significantly facilitates the climate-friendly production of hydrogen. On the other hand, considerable amounts of electrical energy are required - but at least significantly less than for electrolysis.
Possible future possibilities
In principle, hydrogen can be generated directly from water by adding heat (thermal water splitting). However, this requires extremely high temperatures of over 2500 ° C, and it is difficult to separate the hydrogen from the oxygen before the two react again to form water. That's why is on thermochemical cycle processes worked, in which the water splitting is divided into several individual steps. For example, metal oxides can be used which are further oxidized with water vapor, producing hydrogen. These oxides can later be reduced again by supplying heat (with the elimination of oxygen). The required high-temperature heat could be obtained by solar thermal energy, most likely in a solar tower absorber that is illuminated by many mirrors.
There are several methods of making biohydrogen, i. H. of hydrogen from biomass or with the help of biological organisms. Probably the most promising is biomass gasification, i. H. the thermochemical conversion of biomass, which is partly similar to coal gasification. What is particularly interesting is the fact that different types of biomass can be used for this and that a large part of the plant material can be converted, not e.g. B. only the fruits. However, further technological advances are necessary for widespread use.
Work is also being carried out on genetically modified algae, which emit hydrogen when irradiated with sunlight. Certain enzymes play a role here (Hydrogenases) play a role, and the energy for the reaction comes from photosynthesis. So far, however, efficiencies of only a few percent have been achieved, which is far less than is possible with electrolysis with electricity from solar cells.
Green, blue, turquoise and gray hydrogen
Although hydrogen gas is always colorless, of course, it is occasionally assigned colors depending on how it is made:
- “Green hydrogen” is obtained with renewable energy, e. B. with electrolysis and green electricity.
- “Blue hydrogen” is hydrogen produced by reforming natural gas, whereby the resulting climate-damaging CO22Emissions are avoided by reducing CO2-Deposition and storage (sequestration) practiced. This could be a temporary solution until enough “green” hydrogen is available. However, sufficient capacities for the CO2-Storage can be tapped.
- “Turquoise hydrogen” is hydrogen that is produced via methane pyrolysis.
- “Gray hydrogen” is hydrogen from fossil sources, which is produced with substantial CO2Emissions.
Storage of hydrogen
Like other gases (e.g. natural gas), hydrogen can be stored in pressurized gas cylinders. However, because of the low density of hydrogen, high energy densities cannot be achieved. Even at 500 bar, the density is only 44.4 kg / m3, so that the calorific value results in an energy density of only 6.35 MJ / l - over three times less than for natural gas at the same pressure, or 5 times less than for gasoline, which can be stored without pressure.Hydrogen is much less suitable than natural gas for being carried in pressurized gas cylinders in vehicles.
The compression of the hydrogen required when filling the bottles can be done, for. B. require more than 10% of the energy content of hydrogen in the form of electrical energy at 700 bar pressure. The gravimetric (weight-related) energy density achieved is indeed very high for the hydrogen itself. However, since the pressurized gas cylinders are very heavy, the energy density of the overall system is still several times smaller than for a petrol tank.Liquid hydrogen allows longer vehicle ranges, but means more technical effort and higher energy losses.
A significantly higher density, even without excess pressure, is achieved with liquid hydrogen. At normal pressure the boiling point is −252 ° C; a tank for liquid hydrogen must therefore be extremely cold and usually have strong thermal insulation. As heat enters the tank through the insulation, hydrogen evaporates. Since the pressure must not increase arbitrarily, hydrogen must be continuously released. This leads to additional energy losses if hydrogen cannot be used continuously (e.g. when a vehicle is idle for days). Liquefaction also requires a lot of energy.Metal hydride storage systems do not offer an optimal solution either.
A possibility to store hydrogen with a higher density, but without having to build up a high pressure, offer Metal hydride storage. So far it has not been possible to develop inexpensive metal hydride storage systems with high capacity. The gravimetric energy density is also low, as the mass of the metal is much higher than that of the stored hydrogen.Liquid hydrogen carriers offer interesting prospects.
Another approach is the binding of hydrogen to certain liquid hydrogen carriers. These are mostly organic liquids such as dibenzyltoluene, which can chemically bind substantial amounts of hydrogen to themselves with the help of a catalyst at elevated temperatures. The hydrogen can later be dissolved out again at an elevated temperature and with a catalyst. Since the binding energy is not too high, the associated energy losses are limited.
For more details see the article on hydrogen storage.
Transport of hydrogen
Hydrogen can be transported by transporting storage facilities for hydrogen (see above). For example, pressurized gas cylinders can be used in hydrogen cars or tanks for liquid hydrogen in ships. In this case, however, the mass of the storage facility is usually much higher than that of the hydrogen transported in it. In addition, a considerable amount of energy is required to bring the hydrogen into the form required for transport through compression or even liquefaction. In this respect, hydrogen is much more difficult to transport than liquid fuels and fuels such as heating oil and gasoline.
Some improvement is possible with the liquid hydrogen carriers mentioned above. In principle, the transport is very simple here, although only a small part of the transported weight comes from the hydrogen and the remainder has to be transported back again after the hydrogen carrier has been discharged in order to be reloaded.A pipeline of a given size can transport far less energy in the form of hydrogen than natural gas.
Hydrogen can also be transported in pipelines in a similar way to natural gas, although not all materials are suitable because they are e.g. B. could become brittle under the influence of hydrogen. The effect of the low energy density of hydrogen here is that the line capacity (i.e. the transportable power) is only a fraction of that of natural gas and that the energy losses due to the pumping effort are also significantly greater.
Because of its unfavorable storage and transport properties, it is also considered to transport hydrogen in chemically bound form, for example as methanol (an alcohol). This greatly facilitates storage and transport, but additional chemical processes are necessary before and after storage or transport, which also lead to considerable energy losses.
Dangers when handling hydrogen
If hydrogen escapes into the air, there is a high risk of explosion. Such an escape is promoted by the extreme diffusibility of hydrogen as well as by the high pressures that are often necessary. However, the risk of explosion is put into perspective by the fact that hydrogen also volatilizes very quickly. For example, if he leaves a vehicle tank, he rises quickly, which significantly reduces the potential for danger. Compare the situation with the situation when gasoline leaks and burns under the vehicle, or when liquid gas (heavier than air!) Collects on the ground and can then explode.
Additional dangers arise from liquid hydrogen, if only because of the extremely low temperatures, which can lead to serious injuries if touched. In any case, handling hydrogen requires appropriately optimized equipment and devices.
In terms of hazard, it is advantageous that hydrogen is completely non-toxic and that its use usually does not produce any pollutants.
Comparison with other energy sources
A meaningful comparison of the advantages and disadvantages of hydrogen compared to other energy sources is only possible in relation to a specific application. A comparison of hydrogen with gasoline and other fuels for gasoline engines is of particular interest, since hydrogen could in principle replace gasoline in large quantities in the distant future. This shows, however, that the various advantages of hydrogen (above all the production with the help of renewable energy without the consumption of scarce raw materials and the fact that its use is largely free of pollutants) contrasts with a whole series of more or less serious disadvantages. The significantly higher production costs can be put into perspective by taking into account the future sharp rise in costs for fuels from crude oil as well as falling costs for the production of renewable energy. During transport, however, there are considerable disadvantages due to the lower energy density; in particular, significantly higher relative energy losses for transport are to be expected. Hydrogen is also far from ideal as a fuel for internal combustion engines, even if these are specially optimized for hydrogen. In principle, a massive increase in efficiency enables the use of hydrogen in fuel cells instead of internal combustion engines; however, even then, the use of hydrogen produced from electrical energy is very inefficient compared to the use of batteries.
Questions and comments from readers
The statement “Even as a fuel for internal combustion engines, hydrogen is anything but ideal, even if these are specially optimized for hydrogen” cannot stand as it is. Previous engines (e.g. at BMW) all came from the fossil fuel application. New designs for hydrogen operation cannot be compared with these designs.
In addition, thermal water splitting can be used, as it is possible to achieve water splitting at approx. 873 ° C instead of 2500 ° C (actually 2700 ° C). One often reads that the actual processes involved in splitting water have not been adequately researched, and the question arises: Is the scientific exchange actually being carried out incorrectly? A fission cycle can be achieved by taking into account the proportions of hydrogen peroxide if the proportion of oxygen is taken into account. The initial ignition must also be carried out with hydrogen and Oxygen, as well as hydrogen peroxide, take place, with the new storage systems and required small quantities accommodating the new developments. The development seems to be faster than the widespread research on the subject.
Answer from the author:
Maybe you're making a better engine, and the people who worked on it at BMW just weren't competent enough. So far I haven't seen anything like it. In any case, this approach is very inefficient in terms of energy.
The same applies to the splitting of water; let's just wait and see!
In the case of hydrogen production, I miss reference to important technologies such as wood gasification, pyrolysis, IGCC power plants and polygeneration plants, to H2 can also be obtained directly from renewable raw materials, sewage sludge or other waste (if you consider coal to be cheap but CO2- want to avoid critical raw material) ...
I think hydrogen technology is just about to be ready for the market: In addition to mass production of fuel cells (if possible also in Germany), the way for this growth engine should be paved with the rapid development of a powerful hydrogen industry with a sales network, fuel pumps and LOHC -Storage…
(various sources mentioned)
Even if you subtract a propaganda factor from these sources, there are still convincing arguments in favor of hydrogen technology, right?
Answer from the author:
I mentioned biomass gasification in the article. Unfortunately, I can hardly find the time to understand all developments in detail and to explain them in a well-prepared manner; that is a massive effort.
It is difficult to judge how the challenge really is with all of these approaches, especially since propaganda actually seems to play a major role in many communications: various allegations are made, while many details remain vague and much is simply incomprehensible.
Of course, it would be desirable if biomass gasification in particular could be used on a large scale for the production of hydrogen. Perhaps this will actually come at some point.
Your good overview of the possibilities of using hydrogen as an alternative to fossil fuels in various areas is recognizable - always well founded - characterized by skepticism.
Are there any recognizable technical developments that make you more confident about the use of hydrogen?
Ammonia is often mentioned in connection with the transport of green energy. What is there to say about it?
A lot of fossil energy is used in households in the form of gas heating. I see the question of whether the huge pipeline network that has been set up over many decades would be suitable for the transport of hydrogen as very important. You write that hydrogen can lead to embrittlement of gas pipes. Do you have any information on how to overcome the problem?
Regarding the thermal baths (gas burners) used in households, I heard that devices are already being designed that can burn both natural gas and hydrogen. Conversely, this means that the thermal baths currently in use are not suitable for hybrid use. What do you know about that?
Answer from the author:
There would be a wide range of uses for green hydrogen; I see the problem more in the fact that we would need very large amounts of green electricity for this, but first of all we have to replace the coal-fired power plants.
The hopes of politics seem to be based largely on someone else producing the hydrogen for us - for example in the Middle East or in Africa - and that we can import it on a large scale, as we have previously done with crude oil.
Ammonia is a possible hydrogen carrier for long-distance transport, unfortunately with a relatively low energy density. It's hard to say which approach will prevail best in the long run.
A certain amount of hydrogen added to natural gas (e.g. 5%) is certainly not a problem, but at some point you will reach limits with the pipes and the existing burners. However, we do not have so much “remaining” hydrogen that we would be limited by this problem.
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See also: hydrogen economy, energy sources, bio-hydrogen, hydrogen storage, natural gas, town gas, synthesis gas, steam reforming, electrolysis, fuel cells, renewable energy, biomass gasification, RE gas
as well as other articles in the categories of energy sources, renewable energy
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