One of the urgent 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 will consider the issue of hydrogen combustion, the temperature and chemistry of this process.
What is hydrogen?
Before considering the question of what is the combustion temperature of hydrogen, 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 oC) it is present in the gaseous state. Its molecule (H2) 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 (H2)odorless, tasteless and colorless. It is not toxic, however, when its content in the atmospheric air is a few percent, then a person may experience suffocation due to lack of oxygen.
It is curious to note that although from a chemical point of view, all the H2 molecules are identical, their physical properties are somewhat different. It's all about the orientation of the electron spins (they are responsible for the appearance of a magnetic moment), which can be parallel and antiparallel, such a molecule is called ortho- and parahydrogen, respectively.
Combustion chemical reaction
Considering the question of the combustion temperature of hydrogen with oxygen, we present a chemical reaction that describes this process: 2H2 + O2=> 2H2O. 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 that after its passage, only pure water remains, which does not pollute the environment, as occurs during the combustion of fossil fuels (gasoline, alcohol).
On the other hand, this reaction is exothermic, that is, in addition to water, it releases some heat that can be used to drive cars and rockets, as well as to transfer it to other sources of energy, such as electricity.
Mechanism of the hydrogen combustion process
Described in the previousparagraph chemical reaction is known to any high school student, but it is a very rough description of the process that occurs in reality. Note that until the middle of the last century, mankind did not know how hydrogen burns in air, and in 1956 the Nobel Prize in Chemistry was awarded for its study.
In fact, if O2 and H2 molecules collide, no reaction will occur. Both molecules are quite stable. For combustion to occur and water to form, free radicals must exist. In particular, H, O atoms and OH groups. The following is a sequence of reactions that actually occur when hydrogen is burned:
- H + O2=> OH + O;
- OH + H2 => H2O + H;
- O + H2=OH + H.
What do you see from these reactions? When hydrogen burns, water is formed, yes, that's right, but it only happens when a group of two OH atoms meets an H2 molecule. In addition, all reactions occur with the formation of free radicals, which means that the process of self-sustaining combustion starts.
So the key to starting this reaction is the formation of radicals. They appear if you bring a burning match to an oxygen-hydrogen mixture, or if you heat this mixture above a certain temperature.
Initiating reaction
As noted, there are two ways to do this:
- With the help of a spark that should provide only 0,02 mJ of heat. This is a very small energy value, for comparison, let's say that the similar value for a gasoline mixture is 0.24 mJ, and for methane - 0.29 mJ. As the pressure decreases, 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 highly flammable.
- With the help of temperature. That is, the oxygen-hydrogen mixture can simply be heated, and above a certain temperature it will ignite itself. When this happens depends on the pressure and 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 oC. These are quite high values compared to a gasoline mixture, which begins to spontaneously ignite already at temperatures below 300 oC.
Percentage of gases in the combustible mixture
Speaking about the temperature of hydrogen combustion in air, it should be noted that not every mixture of these gases will enter into the process under consideration. It has been 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 no reaction will occur. However, the limits of the existence of a combustible mixture are 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 minimum for oxygen.
Ifconsider a pure oxygen-hydrogen mixture, then the limits here are even wider: 4, 1-94%.
Reducing the pressure of gases leads to a reduction in the specified limits (the lower limit rises, the upper one falls).
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 termination 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 can be controlled, or the process is explosive. What determines the burning rate? Of course, on the concentration of reagents, on pressure, and also 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 greatest amount of energy per unit volume is released.
The marked nature of combustion presents a big problem for using this reaction as a controlled source of energy.
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 oC for a mixture with 19.6% H2. That is, the combustion temperature of hydrogen in air is higher2000 oC (for other concentrations it can reach 2500 oC), and compared to a gasoline mixture, this is a huge figure (for gasoline about 800 oC). If you burn hydrogen in pure oxygen, the flame temperature will be even higher (up to 2800 oC).
Such a high flame temperature presents another problem in using this reaction as an energy source, since there are currently no alloys that can work for a long time in such extreme conditions.
Of course, this problem is solved by using a well-designed cooling system for the chamber where hydrogen combustion occurs.
Amount of heat released
As part 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 a hydrogen mixture is much higher than for gasoline, which is a huge plus for its use as a fuel for internal combustion engines. However, not everything is so simple here either. It's all about the density of hydrogen, it's too low at atmospheric pressure. So, 1 m3 of this gas weighs only 90 grams. If you burn this 1 m3 H2, then about 10-11 MJ of heat will be released, which is already 4 times less than when burning 1 kg gasoline (just over 1 litre).
The given 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 already creates additional difficulties, both in terms of technology and safety.
The use of a hydrogen combustible mixture in technology: problems
It must be said right away that at present the hydrogen combustible mixture is already used in some areas of human activity. For example, as an additional fuel for space rockets, as sources for generating electrical energy, as well as in experimental models of modern cars. However, the scale of this application is minuscule compared to that of fossil fuels and is generally experimental in nature. The reason for this is not only the difficulty in controlling the combustion reaction itself, but also in the storage, transportation and extraction of H2.
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 H2O. The whole problem is that this consumes more energy than can then be obtained by burning H2.
Another important problem 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" from anycontainers. In addition, getting into the metal lattice of alloys, it causes their embrittlement. Therefore, the most efficient way to store H2 is to use carbon atoms that can firmly bind the "elusive" gas.
Thus, the use of hydrogen as a fuel on a more or less large scale is possible only if it is used as a "storage" of electricity (for example, converting wind and solar energy into hydrogen using water electrolysis), or if you learn deliver H2 from space (where there is a lot of it) to Earth.
