One of the pressing problems is environmental pollution and limited energy resources of organic origin. A promising way to solve these problems is to use hydrogen as an energy source. In the article, we consider the issue of hydrogen combustion, temperature and chemistry of this process.
What is hydrogen?
Before considering the question, what is the temperature of hydrogen combustion, it is necessary to remember what this substance is.
Hydrogen is the lightest chemical element, consisting of only one proton and one electron. Under normal conditions (pressure 1 atm., Temperature 0 o C) it is present in a gaseous state. Its molecule (H 2 ) is formed by 2 atoms of this chemical element. Hydrogen is the 3rd most abundant element on our planet, and the 1st in the Universe (about 90% of all matter).
Hydrogen gas (H 2 ) is odorless, tasteless, and colorless. It is not toxic, however, when its content in the atmospheric air is several percent, then a person may experience suffocation due to a lack of oxygen.
It is interesting to note that although from the chemical point of view the entire H 2 molecule is identical, their physical properties are somewhat different. It is all about the orientation of the electron spins (they are responsible for the appearance of the magnetic moment), which can be parallel and antiparallel, such a molecule is called ortho- and parahydrogen, respectively.
Chemical reaction of combustion
Considering the question of the combustion temperature of hydrogen with oxygen, we give the chemical reaction that describes this process: 2H 2 + O 2 => 2H 2 O. That is, 3 molecules participate in the reaction (two hydrogen and one oxygen), and the product is two water molecules . This reaction describes combustion from a chemical point of view, and it can be judged from it that after its passage there remains only pure water, which does not pollute the environment, as occurs when fossil fuels (gasoline, alcohol) are burned.
On the other hand, this reaction is exothermic, that is, in addition to water, it emits a certain amount of heat that can be used to propel machines and rockets, as well as to transfer it to other energy sources, for example, electricity.
The mechanism of the hydrogen combustion process
The chemical reaction described in the previous paragraph is known to any high school student, but it is a very crude description of the process that is actually taking place. Note that until the middle of the last century, mankind did not know how the combustion of hydrogen in air occurs, and in 1956 the Nobel Prize in Chemistry was awarded for its study.
In fact, if the molecules of O 2 and H 2 collide, then no reaction will occur. Both molecules are fairly stable. In order for combustion to occur and water to form, the existence of free radicals is necessary. In particular, H, O atoms and OH groups. The following is the sequence of reactions that actually occur during the combustion of hydrogen:
- H + O 2 => OH + O;
- OH + H 2 => H 2 O + H;
- O + H 2 = OH + H.
What is evident from these reactions? When hydrogen burns, water is formed, yes, that's right, but this only happens when a group of two OH atoms meets the H 2 molecule. In addition, all reactions occur with the formation of free radicals, which means that the process of self-sustaining combustion is started.
Thus, the key to triggering this reaction is the formation of radicals. They appear if you bring a burning match to the oxygen-hydrogen mixture, or if you heat this mixture above a certain temperature.
Reaction initiation
As noted, there are two ways to do this:
- Using a spark, which should provide only 0.02 mJ of heat. This is a very small value of energy, for comparison we say that a similar value for a gasoline mixture is 0.24 mJ, and for a methane mixture - 0.29 mJ. With decreasing pressure, the reaction initiation energy increases. So, at 2 kPa it is already 0.56 mJ. In any case, these are very small values, so the hydrogen-oxygen mixture is considered to be flammable.
- With the help of temperature. That is, the oxygen-hydrogen mixture can simply be heated, and above a certain temperature it will ignite. When this happens depends on the pressure and the percentage of gases. In a wide range of concentrations at atmospheric pressure, the spontaneous combustion reaction occurs at temperatures above 773-850 K, that is, above 500-577 o C. These are quite high values compared to a gasoline mixture, which begins to self-ignite even at temperatures below 300 o C.
The percentage of gases in the combustible mixture
Speaking about the combustion temperature of hydrogen in air, it should be noted that not every mixture of these gases will enter into the process under consideration. It was experimentally established that if the amount of oxygen is less than 6% by volume, or if the amount of hydrogen is less than 4% by volume, then there will be no reaction. However, the existence of a combustible mixture is quite wide. For air, the percentage of hydrogen can range from 4.1% to 74.8%. Note that the upper value just corresponds to the required oxygen minimum.
If a pure oxygen-hydrogen mixture is considered, then the limits are even wider: 4.1-94%.
A decrease in gas pressure leads to a reduction in the indicated limits (the lower boundary rises, the upper boundary drops).
It is also important to understand that during the combustion of hydrogen in air (oxygen), the resulting reaction products (water) lead to a decrease in the concentration of reagents, which can lead to the cessation of the chemical process.
Combustion safety
This is an important characteristic of a flammable mixture, because it allows you to judge whether the reaction is calm, and you can control it, or the process is explosive. What determines the burning rate? Of course, on the concentration of reagents, on pressure, as well as on the amount of energy of the "seed".
Unfortunately, hydrogen in a wide range of concentrations is capable of explosive combustion. The following figures are given in the literature: 18.5-59% hydrogen in the air mixture. Moreover, at the edges of this limit, as a result of detonation, the largest amount of energy is released per unit volume.
The marked nature of combustion is a big problem for using this reaction as a controlled energy source.
Combustion reaction temperature
Now we come directly to the answer to the question, what is the lowest temperature of hydrogen combustion. It is 2321 K or 2048 o C for a mixture with 19.6% H 2 . That is, the combustion temperature of hydrogen in air is above 2000 o C (for other concentrations it can reach 2500 o C), and in comparison with the gasoline mixture this is a huge figure (for gasoline about 800 o C). If you burn hydrogen in pure oxygen, the flame temperature will be even higher (up to 2800 o C).
Such a high flame temperature presents another problem in using this reaction as an energy source, since there are currently no alloys that could work for a long time under such extreme conditions.
Of course, this problem can be solved if you use a well-designed cooling system of the chamber where the combustion of hydrogen takes place.
Amount of heat generated
In the framework of the question of the combustion temperature of hydrogen, it is also interesting to provide data on the amount of energy that is released during this reaction. For different conditions and compositions of the combustible mixture, values from 119 MJ / kg to 141 MJ / kg were obtained. To understand how much this is, we note that a similar value for a gasoline mixture is about 40 MJ / kg.
The energy yield of the hydrogen mixture is much higher than for gasoline, which is a huge plus for its use as a fuel for internal combustion engines. However, here everything is not so simple. It's all about the density of hydrogen, it is too low at atmospheric pressure. So, 1 m 3 of this gas weighs only 90 grams. If you burn this 1 m 3 H 2 , then about 10-11 MJ of heat will be released, which is already 4 times less than when burning 1 kg of gasoline (a little more than 1 liter).
The above figures indicate that in order to use the hydrogen combustion reaction, it is necessary to learn how to store this gas in high-pressure cylinders, which creates additional difficulties, both in the technological issue and in terms of safety.
The use of hydrogen fuel mixture in technology: problems
It must be said right away that hydrogen fuel mixture is currently used in some areas of human activity. For example, as additional fuel for space rockets, as sources for generating electric energy, as well as in experimental models of modern cars. However, the scale of this application is scanty compared to those for fossil fuels and, as a rule, are experimental in nature. The reason for this is not only difficulties in controlling the combustion reaction itself, but also in the storage, transportation and production of H 2 .
Hydrogen on Earth practically does not exist in its pure form, so it must be obtained from various compounds. For example, from water. This is a fairly popular method at present, which is carried out by passing an electric current through H 2 O. The whole problem is that it consumes more energy than can then be obtained by burning H 2 .
Another important issue is the transportation and storage of hydrogen. The fact is that this gas, due to the small size of its molecules, is able to "fly out" of any containers. In addition, getting into the metal lattice of alloys, it causes their embrittlement. Therefore, the most effective way to store H 2 is to use carbon atoms that can firmly bind an "elusive" gas.
Thus, the use of hydrogen as a fuel on a more or less wide scale is possible only if it is used as a “conservation” of electricity (for example, to convert wind and solar energy into hydrogen using water electrolysis), or if you learn how to deliver H 2 from space (where there is a lot of it) to the Earth.