If hydrogen is burned in the air, heat is generated due to the chemical conversion with the oxygen from the air. It is therefore considered a fuel – and, because of its suitability for use in engines, also a fuel.[3] Because this type of combustion – unlike that by which engines generate energy – is based only on an electrochemical reaction, it is also known as "cold combustion." Fuel cell technology takes advantage of this principle: In the hydrogen fuel cell or direct methanol fuel cell, hydrogen reacts with atmospheric oxygen to form water again. Water, electricity and heat are generated simultaneously. The big advantage is that no harmful byproducts are produced when hydrogen reacts with atmospheric oxygen in the fuel cell.[5]
The heating value of hydrogen, but also its calorific value, are used to quantify its energy content. In most cases, the calorific value is somewhat higher than the heating value. The calorific value of a fuel indicates how much energy (i.e. heat) can be obtained during its combustion.[1]
Unlike the heating value of hydrogen, the calorific value assumes that the water vapor contained in the combustion gases condenses completely, i.e. is liquefied.[4] In the case of the heating value of hydrogen, on the other hand, it is assumed that the water vapor does not condense despite the cooling of the combustion gases to 25 degrees Celsius, but leaves the plant in gaseous form. The difference is that the heating value of hydrogen does not include the heat of condensation and is therefore generally lower than the calorific value. In other words, the heating value of hydrogen quantifies how much energy becomes usable as heat by simply burning hydrogen. The calorific value of hydrogen, on the other hand, describes how much energy is recovered in the form of heat if energy is also extracted from the combustion exhaust gases. The heating value of hydrogen is used when the reaction product, water, is gaseous. If it is liquid, we are talking about the calorific value. For example, most internal combustion engines emit the resulting water in gaseous form, which is why no condensation heat can be obtained.[2]
As already described, condensation releases additional heat, the so-called heat of condensation. This is the reason why the calorific value is usually higher than the heating value of hydrogen. This is not only the case with hydrogen, but with almost all fuels – for example, with natural gas, whose calorific value is about ten percent higher than the heating value. How large the difference between the values is depends on the fuel. In the case of lignite, for example, which contains a lot of water, the difference is as much as 20 percent. Non-hydrous fuels such as carbon monoxide, on the other hand, have almost identical calorific and heating values.[4]
The unit in which the calorific and heating value of hydrogen is given is joule per kilogram (J/kg) and megajoule per kilogram (MJ/kg), respectively. However, calculations can also be found in kilowatt hours per kilogram (kWh/kg). If you take the density of hydrogen (in kilograms per liter (kg/l)), you can also convert the heating value of hydrogen into the energy produced per liter.
Compared with other fuels such as coal or natural gas, the heating value of hydrogen and its calorific value are exceptionally high.[1] The heating value and thus the energy content of gasoline is 43.6 megajoules per kilogram, while the heating value of hydrogen is 120. The heating value of gasoline is 47 megajoules per kilogram, while that of hydrogen is 143.[7] The heating value of hydrogen, and thus its energy density, thus beats that of gasoline by about three times.[7]
It is difficult to determine the heating value of hydrogen directly. For this, one would not only have to transfer the heat generated during combustion to a certain amount of water and then measure the temperature increase, but also ensure that the generated water vapor does not condense. As a rule, therefore, this is how one proceeds to calculate the heating value of hydrogen: With the help of a calorimeter, one determines the hydrogen calorific value and includes the most complete condensation of the water vapor possible. On this basis, one then calculates the heating value of hydrogen by determining the amount of condensed water and then subtracting the energy required to evaporate the water produced during combustion.[2]
What else should you know about the heating value and calorific value of hydrogen?
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