Nizhnyk N. Reduction of the formation of nitrogen oxides during the combustion of methane with the addition of hydrogen solutions

In the dissertation, an experimental laboratory stand was developed to solve the problems related to the study of combustion processes of mixtures with hydrogen added to methane, the determination of their heat of combustion, the study of the formation of nitrogen oxides, and the appropriate method of conducting experiments was also developed. The combustion process in the work was considered as a three-component reaction involving water vapor contained in the atmospheric air, the breakdown of water into components triggers the main processes of chain reactions that are inherent in fuel combustion. The theoretical study of combustion processes included a quantitative analysis of the energy spent on heating and evaporation of water contained in the atmospheric air, and estimates of its share in relation to the useful energy of the fuel, which depends on the hydrogen content in the fuel and the coefficient of excess air. It was determined that with a moisture content in the air of 20g/m3 and an excess air ratio of 1.15, the value of this share of energy ranges from 1.98% (natural gas) to 6.01% (hydrogen). In order to identify the effect of hydrogen impurities, which was supplied in the form of an aqueous solution of hydrogen peroxide, on the process of formation of nitrogen oxides and post-oxidation of NO to NO2 in combustion processes, experimental studies were conducted. The hydrogen peroxide solution of different concentrations (from 4% to 30%, which corresponded to the hydrogen content in the aqueous solution from 10.8% to 9.44%) was introduced as a drop of liquid into the flame front of the laminar methane torch. It was established that the nature of the obtained dependences is similar to the distribution of nitrogen monoxide concentration when a drop of distilled water is introduced into the flame front. An intense increase in the concentration of nitrogen oxides is already observed in the initial cross-sections of the flame front from the moment the mixture begins to heat up, even before the maximum temperature is reached. Moreover, the higher the concentration of the H2O2 solution introduced into the flame front, the more intensively the nitrogen oxide content increases. When a 30% H2O2 solution (which corresponds to a 9.44% H2 content in the solution) is introduced into the flame front, the level of NO concentration almost reaches the initial level, i.e. obtained without the introduction of droplet moisture (90-95 mg/m3), while introduction into the front the H2O flame (corresponding to the H2 content in the solution of 11.1%) leads to a decrease in the concentration of NO to 65 mg/m3, i.e. by 35-40%. This can be explained by the fact that H2O2 forms the OH- radical more intensively than H2O, which takes an active part in the oxidation of hydrocarbons. Thus, an increase in the concentration of H2O2 to 30% in the solution (corresponding to the concentration of H2 in the solution to the minimum in the experiments - 9.44%) led to an increase in the total yield of nitrogen monoxide by 35-45% under the conditions of the experiment due to the more intensive formation of OH- radicals, HO2- and an increase in the temperature level due to the afterburning of CO. Separate measurements of NO and NO2 showed that the dependence of the fraction of NO2 on the content of H2 in the solution introduced into the flame front is ambiguous and significantly depends on the place of sampling in the cross section of the flame front. A temperature above 800 ºC significantly affects the reaction rate and changes the kinetics of the processes taking place with the participation of hydrogen peroxide, and these changes are of a different nature in different sections of the front, which can be explained by a decrease in the time of existence of H2O2 in the flame (20 ms or less). The most favorable conditions for the reoxidation of NO to NO2 are the introduction into the torch of a solution with a hydrogen content of 10.0 - 10.5%, in the temperature range of 800 - 1000º C. These conditions are not met by the flame front, but by the flaming area. Therefore, with an increase in the concentration of hydrogen in the methane-hydrogen fuel mixture in the boiler furnace, the concentration of NO will increase accordingly, and the proportion of NO2 will decrease. The intensity of the post-oxidation of NO to NO2 has its extreme, which depends on the concentration of hydrogen in the injected solution. The maximum formation of NO2 in our experiments was observed for the H2 content in the solution of about 9.4%. An increase in the concentration of the H2O2 solution from zero to 30% (and, accordingly, a decrease in the hydrogen content in the solution from 10.8% to 9.44%) led to an increase in the total formation of nitrogen oxides by 35-45% and an increase in the temperature level, including due to afterburning CO.