## 2 Features of Hydrogen Combustion Processes  
### Properties of Hydrogen and Flame Temperature  
The use of hydrogen in gas turbine engines and power plants is mainly considered a replacement for widely used natural gas (due to the similarity of their properties, in this paper, natural gas and methane are considered similar fuels). At the first stage of technological re-equipment, partial or complete replacement of natural gas with hydrogen implies no significant construction changes. This possibility can be assessed by comparing the physicochemical properties of hydrogen with the properties of natural gas. The comparison of the physicochemical properties of hydrogen and natural gas is shown in Table 1, where it can be seen that hydrogen has a significantly lower density (about eight times lower than methane), has wider ignition concentration limits, and is generally more reactive than methane. The net calorific value per unit mass is more than three times higher than that for methane, but the volume per unit is nearly half as much. Consequently, for the same thermal power, the required volumetric flow rate of hydrogen is more than twice the flow rate of methane.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Hydrogen** & **Natural Gas** \\ \hline Density at 273 K (kg/m\({}^{3}\)) & 0.09 & 0.72 \\ Molar mass (kg/kmol) & 2.016 & 16.04 \\ Ignition limits (\(\Phi\)) & 0.1-7.1 & 0.4-1.6 \\ Low calorific value (MJ/kg) & 120 & 50 \\ Low calorific value (MJ/m\({}^{3}\)) & 11 & 36 \\ Maximum laminar flame speed (m/s) & 3.25 & 0.45 \\ Adiabatic flame temperature at \(\Phi=1\) (K) & 2402 & 2216 \\ Stoichiometric coefficient & 34.2 & 17.2 \\ Melting temperature (K) & 14 & 91 \\ Boiling temperature (K) & 21 & 112 \\ Critical temperature (K) & 33 & 191 \\ Critical pressure (amt) & 12.8 & 45.2 \\ Autoignition temperature (K) & 850 & 810 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Properties of hydrogen compared to natural gas [4; 5; 6; 7].  
Figure 1: Analysis of fuel composition influence at different levels, adapted from [3].  
Hydrogen also has a higher adiabatic flame temperature, which can increase the probability of overheating the combustion chamber elements and the turbine while maintaining the excess fuel ratio in the combustion zone. The influence of the content of hydrogen in mixtures with natural gas on the flame temperature is shown in Figure 2. The flame temperature is mainly influenced by a specific heat of combustion and the flame speed. Since the flame temperature and specific heat of combustion are relatively high for hydrogen, during its combustion, while maintaining the stoichiometric coefficient, the flame temperature is higher. It is worth noting that the relative increase in the flame temperature of hydrogen compared to methane is greater for rich mixtures (about +15-17%), slightly less for lean mixtures (+10-12%), and the least of all for stoichiometric mixtures (about 7-8%). Therefore, a rational conclusion is that the heat load on the flame tube head of the combustion chamber can become 10-15% higher when switching to hydrogen fuel, which will require additional efforts to ensure thermal protection. While maintaining the total thermal power of the flame for the combustion chamber, the local thermal power of the flame increases. This is due to the fact that when enriched with hydrogen, the flame becomes more compact.