Investigation On Fundamental Characteristics Of Oxygen And Hydrogen Enriched Ammonia Laminar Premixed Combustion

Global warming is increasingly severe due to the excessive greenhouse emissions,governments all over the world are seeking to reduce the carbon footprints.In recent years,ammonia was considered as a potential energy vector to the carbon-free economy,because ammonia has no carbon atom,can be easy to store,transport,and converse to hydrogen,and possess well-established infrastructures.Nevertheless,the low reactivity of ammonia causes a great challenge in ammonia fuelization.Oxygen and hydrogen enrichment are both well-developed methods to improve the reactivity of combustible mixtures.However,there are few systematic studies on the fundamental combustion characteristics of laminar premixed oxygen or/and hydrogen enriched combustion.The data and theory are incomplete to guide the engineering usage of oxygen and hydrogen enriched ammonia combustion.To solve these problems,the least points required in speed extrapolation by constant pressure spherical flame was determined,and the laminar burning velocity of oxygen and hydrogen enriched ammonia were measured using self-developed constant volume combustion vessel and sub-pixel flame image processing under the different oxygen/hydrogen fraction,equivalence ratio and initial temperatures.Thirteen ammonia oxidation kinetics mechanisms were compared with the data measure in this study and in literature in wide conditions to find the most accurate mechanism for oxygen and hydrogen enriched ammonia combustion.Then the mechanism was used in predicting the combustion properties and nitrogen oxides generation.Results showed that none of the thirteen mechanisms can accurately predict the laminar burning velocity of oxygen and hydrogen enriched ammonia flame at the same time.The reduced mechanism proposed by Okaford et al.(2019)and the detailed mechanism constructed by Stagni et al.(2020)is most accurate in predicting oxygen and hydrogen enriched ammonia combustion,respectively.However,according to the sensitivity analysis of reaction rates,species thermal,and transport parameters,the critical species are the same in both conditions,indicating that the accuracy of the ammonia oxidation mechanism is determined by the choice of reaction rate constants.Both oxygen and hydrogen enrichment can remarkably increase the laminar burning velocity,mixture heating value,and active radical concentrations of ammonia combustion.On the other hand,the combustion efficiency is decreased due to the water decomposition at high flame temperature.Oxygen enrichment can significantly reduce combustion efficiency which impedes practical application,but hydrogen enrichment is less influential on combustion efficiency.The first sharp then slow decrease of burned gas Markstein length of oxygen enriched ammonia flame is controlled by the decrease of flame thickness,while the first decreased then increased Markstein length of hydrogen enriched ammonia flame is dominated by the combined effect of Zel'dovich number and Lewis number.Oxygen and hydrogen enrichment will cause the super-adiabatic flame temperature and non-equilibrium,caused by the water decomposition and post-oxidation in burned gas,which causes uncertainties in burning velocity measurement.The heat release analysis showed that oxygen enrichment has little effect on the fuel oxidation pathway.However,hydrogen addition will shift the reaction pathway towards the hydrogen reacting system,which represents by the change of individual reaction contribution to heat release.Then the NOx production reaction pathway was analyzed.Results showed that both oxygen and hydrogen will obviously increase the NOx emissions.At most conditions,NO is the major component in NOx,but converses to NO₂ near the flammability limits due to the lower flame temperatures,leading to the ultra-lean combustion failure to control the fuel type NO generation.Under all conditions,H radical gradually plays a leading role in NO generation,so that the nonlinear variation of H mole fraction in flame causes the NO mole fraction to first increase then decrease,and reach the peak value at hydrogen fraction of 75%.Except for the pure hydrogen condition,the HNO reaction pathway contributes to NO generation more than50%,in contrast,the consumption of NO mainly depends on the reaction of NH_i(i=1,2,3).NO exhibits post-oxidation reduction in the burned gas for both oxygen and hydrogen enriched combustion,which makes the final NO emission is less than in the flame front.However,its low reaction rates cause the NO emission cannot to reach the equilibrium state.The post-reduction of NO in burned gas is controlled by the reactions in the thermal NO mechanism,which is sensitive to the flam temperature.Therefore,the post-reduction of NO is enhanced after oxygen enrichment and causes NO emission hardly changes at high oxygen fractions.N₂as the final product of NH₃ oxidation has negligible reactivity in the flame front.Nevertheless,the chemical effect of N₂ affects the post-reduction of NO,leading to the increase of final NO emission.At last,the laminar hydrogen enriched ammonia combustion was investigated under engine relevant high temperature and pressure conditions and compared with the stoichiometric methane,methanol,and ethanol air combustion properties.Considering the properties of hydrocarbon fuel combustion as a reference and the tradeoff between NOx and combustion efficiency,the potential working conditions for hydrogen enriched ammonia fueled engine are equivalence ratio of 1.0-1.05,hydrogen fraction of 40%-60%.Moreover,given that the ammonia/hydrogen blend has good knock resistance and better combustion performance at a high compression ratio,it is more appropriate to work at high compression ratios.