Numerical Predictive Combustion Model of Hydrogen Enriched Natural Gas

. Hydrogen was established as one of the main pillars of energy stability in the Europe Union. One of the ways how to achieve this goal is natural gas enriched with hydrogen. Due to this is very important to know the properties of this fuel and its behaviour during combustion. The main scope of the research is to provide a better understanding of the emissions, efficiency, and performance of the heat source when combusting hydrogen and gas fuel mixture. In this paper is described hydrogen characteristics, hydrogen fuel preparation, an overview of gas fuel combustion in gas a ppliances with the hydrogen additive, a mathematical model for the combustion process estimation. In the conclusion, multiple predictive models were compared. We can state that, based on calculations of a numerical predictive model, as hydrogen concentration raised emissions, as nitrogen, carbon dioxide, wet exhaust, and water, are decreasing.


Introduction
Due to the climate and energy crisis partially caused by the war in Ukraine and due to the effort of the European Union to reduce our demand for energy supplies from the Russian Federation. One of the possible ways how to reduce our dement on imported energy commodities and one hand reduce our impact on our climate is the combustion of enriched natural gas by green hydrogen. Green hydrogen which is produced from renewable energy sources such as electricity from wind and solar power plants, as directly produced from water by photolyze. As we can see combustion of hydrogen enriched natural gas is in these days very actual theme. In Europe is natural gas used in significant amounts in house heating systems and for their reliable and safe operation properties of this fuel is needed. Nowadays, the maximum concertation of hydrogen in natural gas varies from 0.05 % in Belgium up to 10 % in Germany. Some limitations aspect can be in specific applications like CNG pumps or in chemical processes. Based on previous research we can follow that hydrogen-enriched __________________________________ natural gas can be used up to 25 % of the volume concentration of hydrogen in standard gas boilers due to his wobe index (W25 = 47.52 MJ/m 3 ) which is the same group of gaseous fuels as natural gas. Regards emission impact works of other authors show emissions decreasing with rising hydrogen concentrations in the fuel mixture. Same trend we can observe in results in experimental works of other researchers, which were done on domestic boilers, industrial boilers or even on basic household cooktop burners. In this article, stoichiometric combustion numerical model was made, due to specify the properties of hydrogen-enriched natural gas. Based on the volume of hydrogen mixed in natural gas properties such as flame speed, flashback properties, flame head speed, wobe index, or calorific value is changed. If we want to change fuel for our gas devices, we must know its exact properties and how this fuel changes the combustion process. Due to the possibility of variation in the concertation of hydrogen was calculations in this article made with various concentrations of hydrogen in natural gas.

Hydrogen as natural resource
The hydrogen atom is the lightest and simplest from all elements. In state of the molecule as H2 is a colourless, non-toxic, highly combustible gas with no odours and with a low density, one-fourteenth of the density of air. To condensate gas hydrogen to liquid form at atmospheric pressure, the temperature has to be reduced to 20.3 K. [14] Released hydrogen is dispersing rapidly. Compared to other fuels, hydrogen has the highest combustion energy per unit mass. Hydrogen is used in gaseous, liquid, or slosh forms. Liquid hydrogen is transparent with a little light blue tint. Slush hydrogen is a mixture of liquid and solid at the temperature of triple point. [15] The conditions of hydrogen triple point, are, temperature at 13.8 K and pressure 7 200 Pa. The highest possible temperature, at hydrogen vapours can be liquefied, is 33.145 K which is critical temperature. The corresponding critical pressure is 1 300 kPa, density in this critical point is 31.263 kg/m 3 . Above this temperature is impossible to condense hydrogen into its liquid phase just by increasing the pressure. At normal conditions, temperature, pressure, etc., hydrogen is not very reactive substance, to accelerate reaction, catalysator or spark is needed, it proceeds with high rate an explosive violence. Hydrogen is an extremely flammable gas. It burns in air and oxygen to produce water. In air chlorine mixture, it can explode spontaneously just by spark, heat or even sunlight. [7,8] Flame visibility. There is common misconception about that hydrogen flames are not visible. Visible emissions from hydrogen flames are considerably weaker, than emissions from comparable hydrocarbon flames, however they are visible. [7] Flammability limits. The flammability range of hydrogen is from 4 % up to 75 % by volume in air at NTP (normal temperature and pressure). Hydrogen flammability range expand with temperature, hydrogen with temperature 100 °C at NTP has its down flammable limits 3 %. [7] Hydrogen adiabatic premixed flame temperature for stoichiometric mixture in air is higher compared to other fuels and it is 2403 K. [16] Stoichiometric hydro-genoxygen is composed from 66.66 % of hydrogen and 33.33 % of volume of oxygen. For conditions in normal air it is 29.59 % volume of hydrogen and 70.41 % of air volume. [9,10,11,12,13]

Predictive numerical model of hydrogen enriched natural gas combustion
Calculations physical properties of hydrogen enriched natural gas were make in terms of EN ISO 6976:2016.
For basic calculation and verifications of accuracy or eliminations of mistakes was for first calculation used standard natural gas with compositions as in ISO 6976:2016.
For determination of basic properties for our comparison was made calculations with properties of natural gas which is used in Slovakia.

Molar fracture
A molar fraction is defined as the ratio of the number of moles of a given component present in a solution to the total number of moles present in a given solution. The result of adding the molar fractions of all components is one, resp. hundred percent.

Compression factor
The compression factor, also known as the gas compressibility factor, tells us how much the actual gas differs from the ideal gas at a given pressure and temperature. (1) (2)

Gross calorific value
Combustion heat defines the amount of heat released by the complete combustion of a unit amount of gas with a stoichiometric amount of oxygen or air at a constant pressure and temperature. All the resulting combustion products cooled to the initial temperature remain in the gaseous state except for the water which condenses.

Net calorific value
The calorific value defines the amount of heat released by the complete combustion of a unit amount of gas with a stoichiometric amount of oxygen or air, at a constant pressure and temperature, all combustion products cooled to the initial temperature being in the gaseous state.

For mass based
For molar based Density Relative density Wobe index Wobe index is used as criterium for interchangeability of gaseous fuels. Wobe index is defined as the ratio of combustion heat (or calorific value) and square root of relative density.

Conclusion
One of the possibly way how to stabilize and reduce our demand from fossil fuels is blend of natural gas and hydrogen. Due to reality that natural gas grid is very complex system and likewise whole industry which depend on natural gas is very fragile on composition of natural gas is necessary to know as much detail as possible to start our trans-formation from pure natural gas to hydrogen enriched natural gas. Based on these we make numerical predictive combustion model of hydrogen enriched natural gas to better understand behaviour of these gaseous fuel which we want to use in future. In this article emission changes were evaluated and then compared with another author. As we can see on charts above data have the same trend and we can state that as hydrogen concentration raised emissions calculated by us are decreasing. In terms of emission reduction is hydrogen mix very potential fuel for our energy demands. However, for future using of hydrogen enriched natural gas is important to solve multiple problems for example flame stability or quenching distance. Even though some problems hydrogen enriched natural gas is fuel with very high potential for future.