Investigation On Neat Ammonia Oxidation And Ammonia/n-heptane Co-oxidation

As global warming becomes an increasingly intractable global problem,reducing greenhouse gas emissions has become an urgent task for all countries.In order to achieve this goal,it is inevitable to replace traditional fuels with low-carbon or carbon-free fuels.Ammonia,a type of carbon-free fuel,has a broad development prospect in clean and efficient utilization and coupling substitution of fossil energy in the future.However,there are still many gaps and deficiencies in the understanding of the combustion properties of ammonia.To alleviate this problem,the oxidation characteristics of ammonia and its co-oxidation with n-heptane were investigated in this work.Firstly,to further understand the characteristics of ammonia as a fuel,the oxidation process of ammonia was explored based on a jet-stirred reactor（JSR）under a wide range of condition.Then based on a previous chemical kinetics model,an ammonia oxidation kinetic model with better prediction performance was proposed by replacing the rate constants of some important reactions.The results show that the ammonia oxidation activity is stronger under ultra-thin condition,with lower initial oxidation temperature and higher oxidation rate.And with the increase of equivalent ratio,the oxidation rate of ammonia decreases.This is due to the high oxygen concentration favoring the production of HO₂,which leads to a large amount of OH.Finally,OH promotes the dehydrogenation of ammonia.The modified kinetic model emphasizes the importance of HO₂ and its conversion path to OH.Secondly,to better understand the low-temperature oxidation characteristics of n-heptane,the low-temperature oxidation products of n-heptane and its properties were investigated based on CHEMKIN.The results show that n-heptane forms complex products after low-temperature oxidation.Among a series of oxidation conditions,i.e.oxidation temperature,pressure,equivalent ratio and residence time,the oxidation temperature has the biggest influence on the distribution of n-heptane oxidation products.Except the negative temperature coefficient（NTC）region,the addition of oxidation products obviously shortens the ignition delay time（IDT）of n-heptane.Besides,oxidation products from higher temperature,higher pressure and longer residence time have more obvious promoting effect on ignition.Finally,the oxidation of ammonia/n-heptane blends was investigated both on JSR.The oxidation products and intermediates were analyzed in detail.And the results and underlying reasons were explained with an improved kinetic model.During the low temperature region,the free radicals produced by n-heptane oxidation make ammonia exhibit oxidation behavior,while ammonia has little effect on n-heptane.This is because after entering the intermediate temperature region,the transformation of n-heptane oxidation pathway can provide rich OH radicals and promote ammonia oxidation,thus generating NOx.And NOx accelerates the formation of OH and weakens the original OH main source:R·→HO2→H2O2→OH.As such,a self-reinforcing looping cycle can be formed,leading to the rapid oxidation of ammonia/n-heptane blends.To sum up,the present work provides useful insights for ammonia oxidation,and the improved chemical reaction kinetics model has better simulation and prediction effect at a wider equivalent ratio than the previous work.At the same time,the experiments of ammonia and n-heptane blends oxidation also explore the possibility of using ammonia coupled hydrocarbon fuel.The analysis of the interaction mechanism between ammonia and n-heptane provides an idea for the oxidation of ammonia and hydrocarbon fuel,which is of great significance to reveal the co-oxidation mechanism of ammonia and large hydrocarbons.