Chemical Kinetic Modeling of the Combustion of NH3/H2 and NH3/syngas Fuel Mixtures Using a Large Amount of Experimental Data

Abstract

Nowadays, due to the global climate change, it is extremely important to find alternative fuels to reduce the utilization of fossil fuels and the emission of air pollutants such as carbon dioxide (CO2), hydrocarbon pollutants, and soot. Ammonia (NH3) is a promising carbon free fuel candidate that can store and transport renewable hydrogen (H2) energy.
Ammonia has several advantages over hydrogen in practical applications, but the combustion characteristics of NH3 are different from traditional hydrocarbon fuels. A possible solution to improve the disadvantageous combustion properties of ammonia is to blend it with other fuels. For this purpose, two of the most usually used co fuels are hydrogen and syngas (H2/CO).
This study reports a collection of currently available chemical kinetic mechanisms from the literature that can be applied for modeling the combustion of NH3/H2 and NH3/syngas fuel mixtures. An indirect experimental data collection is also presented which can be used for testing the performance of these combustion mechanisms.
In this work, 19 detailed reaction mechanisms were investigated that had been published in the last 13 years. Their performance was quantitatively assessed based on how well they can reproduce the results of indirect experiments. Almost 5000 experimental data points were utilized in the mechanism comparison including ignition delay times measured in shock tubes, concentration measurements in jet stirred and flow reactors, and laminar burning velocity measurements. Based on the results, it can be concluded that there are significant differences between the performances of the different models, and the performance of a mechanism may also vary significantly with the type of experiments.
Local sensitivity analysis was carried out on the best performing mechanisms to identify the kinetic and thermodynamic parameters to which model outputs are most sensitive. Even though the investigated models are different, sensitivity analysis identified largely the same set of important reactions and thermodynamic data in these mechanisms.
Results presented in this work may serve as a good basis for further mechanism development for the combustion of NH3/H2 and NH3/syngas fuel mixtures.

Popular Abstract

In recent years, one of the most frequently heard terms in media has been global climate change because it raises concerns about the future of humanity. One cause of this phenomenon is the so-called greenhouse effect, which is due to the presence of greenhouse gases (GHGs) in the atmosphere. There are many GHGs, but based on quantity, the most important anthropogenic GHG is carbon dioxide (CO2), whose main anthropogenic source is the combustion of biomass and fossil fuels.
Combustion of fossil fuels is the primary way of energy production nowadays. These processes take place in industry, heating of households, transportation, etc. The main fuel used in combustion devices is a hydrocarbon or a mixture of hydrocarbons, such as methane, gasoline, diesel, or kerosene. During the combustion of these fuels, not only CO2 is released into the atmosphere, but also various carbonaceous air pollutants including carbon monoxide (CO), hydrocarbon pollutants, volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), and soot, which are harmful to human health.
Due to these reasons, it is crucial to find alternative carbon-free fuels that can be used efficiently for energy production. A promising candidate for this purpose is ammonia (NH3). Some of the advantageous characteristics of ammonia are that it can be produced using renewable energy sources in an entirely carbon-free process, and there exists an already established and reliable infrastructure for its storage and transportation. However, NH3 also has disadvantageous combustion properties as compared to conventional fossil fuels such as its low heat of combustion and high ignition energy, which must be improved to utilize ammonia as a fuel in practical combustion processes.
To design new NH3-based engines or power plants, detailed knowledge is necessary about the processes that take place during ammonia combustion. The rates of the occurring chemical reactions have to be described quantitatively, that is, accurate chemical kinetic mechanisms are needed for the combustion of ammonia under conditions relevant to industrial applications.
Several readily useable models exist in the literature that may be utilized for this purpose. This study aims to compare the performances of several such reaction mechanisms based on how accurately they can reproduce the results of experimental measurements. From the results, it can be concluded that none of the mechanisms can describe the investigated systems satisfactorily under all circumstances. From this, it follows that further mechanism development is needed to improve the predictive capabilities of the models. The study also reveals more detailed information about some selected models, which gives more insight into the chemistry of ammonia combustion, and most importantly, it can be used in future research to construct a reaction mechanism that can describe these systems better than any existing model.

• author: Szanthoffer, Andras
• supervisor: Tamás Turányi
• organization: Department of Chemistry
• course: KEMR30 20221
• year: 2022
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: Ammonia combustion, Detailed chemical kinetic mechanisms, Quantitative mechanism comparison, Local sensitivity analysis, Chemical Physics
• language: English
• id: 9082232
• date added to LUP: 2022-06-02 16:04:41
• date last changed: 2022-06-02 16:04:41