# The Period of Ignition Delay for Methane-Air Mixture with Hydrogen and Ethylene Additives
## 1 Introduction  
The use of methane and natural gas as fuel is widespread in the energy and transport industries. The main reasons are the high energy intensity and manufacturability of methane, its relatively low cost, the presence of a large number of raw materials, and the ability to minimize environmental pollution compared to other hydrocarbon fuels in power plants. It is important to mention that the high ratio of the mass fraction of water vapor to the mass fraction of carbon dioxide gH\({}_{2}\)O/gCO\({}_{2}\) is realized in the products of methane combustion. This factor/feature reduces the formation of soot on the elements of power plants compared to the use of other hydrocarbons as fuel. Along with the advantages of methane, some difficulties can be highlighted in its effective application in practice. These disadvantages include a relatively low rate of oxidative chemical reactions and a narrow concentration range of stable combustion with air. Therefore, the energy-efficient use of methane is associated with the necessity of intensifying combustion by physical or chemical methods. One of the promising chemical methods for methane combustion intensifying is adding a small amount of highly active components to the main fuel, which makes it possible to accelerate chemical reactions and reduce the concentration of pollutants in the combustion products. To improve the methane combustion characteristics in air, substances such as hydrogen and ethylene can be used. The choice of these additives is based on high combustion temperatures, short times of ignition delay, and wide temperature and pressure ranges for stable ignition and combustion of the fuel-air mixture. However, theaddition of hydrogen or ethylene increases the explosiveness of the mixture. Therefore, in order to predict the reactivity, combustion efficiency, and explosiveness of mixtures of methane with hydrogen and ethylene, it is extremely important to determine the dynamic characteristics of ignition. One of the most important dynamic characteristics is the time of ignition delay, which depends on component concentration, temperature, and pressure. Some fundamental features of methane and air combustion with the addition of hydrogen are presented in [1; 2; 3; 4], where the flame of burning methane was studied with the addition of hydrogen into the fuel. The analysis [1] was carried out using high-resolution photographs and registration of OH radicals. The addition of hydrogen (up to 29% by volume) reduced the height of the flame and increased the intensity of combustion at the base of the flame due to an increase in the burning rate. An increase in the proportion of hydrogen led to an increase in the concentration of OH radicals and a decrease in CO and NOx radicals in the combustion products. At the same time, the amount of soot in the burner flame was significantly reduced. The presence of hydrogen may have beneficial effects by increasing laminar burning velocity [2]. Different blend compositions were researched at 300 K and 1 atm. The results showed that the values of the laminar burning velocity are always smaller than those obtained by averaging the corresponding values of the pure fuels in molar proportions. The authors found these results are also true for mixtures at pressure up to 10 atm and temperatures up to 400 K [2]. Hydrogen enrichment is also beneficial in increasing the resistance of the flame to strain-induced extinction [3; 4]. The authors [3] investigated lean mixtures with the hydrogen mole fraction in the fuel varying from 0 to 0.5. They found that the intensity of the flame vortex interaction increased when the hydrogen mole fraction in the fuel increased. The addition of hydrogen to methane leads to a more dynamic regime where the continuity of the front is broken locally by the vortex, and it creates the separation of small flame pockets from the main front (H\({}_{2}>\) 20% mole).