## 2 Features of Hydrogen Combustion Processes
### Ignition Delay Time and Autoignition Temperature  
One of the most important characteristics of the combustion process is the ignition delay time. In combustion chambers, this value has an effect on the amount of time that is available for pre-mixing the fuel with air before the ignition and combustion (when the air temperature after the compressor is higher than the auto-ignition temperature of the mixture). On the one hand, to achieve a low level of nitrogen oxide emissions, a high-quality fuel-air mixture preparation is required; on the other hand, the premix burner section (premixer) must be designed in such a way as to avoid undesirable ignition within it. Due to the high temperature and pressure at the inlet to the combustion chambers of gas turbines, this becomes a difficult task, and for highly reactive fuels such as hydrogen, it is even more difficult. Figures 5 and 6 show that the ignition delay time strongly depends on the temperature and composition of the fuel-air mixtures: the ignition delay time decreases by orders of magnitude with an increase in the proportion of hydrogen in the fuel. Similar results were obtained in [8]. Despite this, at typical air temperatures at the inlet to the combustion chamber (600-800 K), the ignition delay time for lean hydrogen mixtures with air is about 100 ms, which is quite sufficient to organize the mixing process. [PERSON], [PERSON], and [PERSON] [13; 14; 15] showed that an increase in pressure also leads to a decrease in the ignition delay time of the methane-hydrogen mixture, as well as of pure methane (Figure 7). Figure 5: Change in the ignition delay time of a natural gas mixture depending on the volume fraction of hydrogen [13]. According to our estimates (using a perfectly mixed reactor), the minimum temperature of hydrogen autoignition under atmospheric conditions with the residence time in the  
Figure 6: Change in the ignition delay time of a natural gas mixture depending on the volume fraction of hydrogen [14; 15]. Figure 7: Change in the ignition delay time depending on pressure (experiment \(\Phi\) =0.5, mixture of 40% H2 and 60% natural gas by volume) [14]. reactor of 10 s is about 780-810 K (Figure 8), whereas according to [PERSON] et al. [10; 16] (the autoignition induction time is 5 min) it is about 700 K, and for the stoichiometric mixtures of methane, it is about 850 K. The minimum autoignition temperature shifts to the region of rich mixtures and is approximately 70-150 degrees lower than that of methane [17; 18]. It should be noted that the lower autoignition temperature of hydrogen leads to limitations in the use of lean premix combustion technology for power plants with a high temperature after compressors. Thus, for engines with the air temperature T\({}_{\rm k}\) of the order of 700 K and above, it is essential to use another combustion system, for example, a diffusion burner or a microcluster. At the same time, for engines with T\({}_{\rm k}\) below 700 K, the lean burn technology can still be used with high efficiency. Experimental studies in shock tubes have revealed a certain relationship between the high-temperature ignition and the pressure or fuel composition [19]. In the studies by [PERSON] and [PERSON] [17; 18; 20], three characteristic systems (modes) of ignition were identified, depending on the composition of the fuel:  
1. [label=(0)]
2. MCDI (methane chemistry dominating ignition) system--for methane-hydrogen fuel mixtures containing less than 40% hydrogen, a decrease in the ignition delay time typical of hydrocarbons with increasing pressure is observed. 3. CCMHDI (combined chemistry of methane and hydrogen dominating ignition) system--for a 40% CH\({}_{4}\)/60% H\({}_{2}\) fuel mixture, there is no noticeable effect of pressure on the ignition time. In this case, the nature of ignition is determined neither by the kinetics of hydrogen nor by the kinetics of methane. 4.