Abstract
Ammonia combustion is a promising energy source as a carbon free fuel without greenhouse gas emissions. However, since the auto-ignition temperature is 651 degrees Celsius and the range of flammability limit is not wide compared to other fuels, fundamental studies on ammonia fires have rarely been conducted so far. Therefore, this study aims to numerically estimate fire spread characteristics when ammonia fuel in a high-pressure state leaks to the outside, especially focusing on the flammability limit according to oxygen concentration. Three kinds of reaction mechanism for numerical analysis were adopted to compare the flame structure, flammability limit, and combustion characteristics. Plank-mean absorption coefficients of nitrogen species were taken for the radiation model, in addition to the optically thin model. The effect of radiation heat loss could be identified from the maximum flame temperature trend at a low strain rate. It was confirmed that the pyrolysis of ammonia in the preheated zone results in hydrogen production, and the generated hydrogen contributes to heat release rate in the flame zone. It is found that the contribution of hydrogen would be an important role in the flammability limit of ammonia combustion. Finally, Karlovitz and Peclet numbers showed well the extinction behaviors of ammonia combustion as a result of LOC (Limit Oxygen Concentration) analysis as a function of global strain rate.
Highlights
In most countries, multipronged efforts are being made to strengthen the global response to the climate change issue
In the case of low strain rate, the flame extinction occurs at flame sheet condition in 1-D similarity concept because the flame thickness can be inversely proportional to the strain rate and the radiation heat loss depends on the flame thickand pressure at global
Due to a large amount of hydrogen generated from the preheating zone in front of the flame, ammonia combustion undergoes a process of simultaneously performing a chemical reaction containing nitrogen hydrides and hydrogen
Summary
In most countries, multipronged efforts are being made to strengthen the global response to the climate change issue. In order to achieve the global demand, it is essential to replace present fossil fuels with renewable energy sources, such as wind, solar, and carbon-free fuels. Carbon-free fuel refers to fuel with zero carbon emission in the process of fuel production and consumption, such as biofuel, hydrogen, and ammonia. In the case of biofuel, since biomass used for fuel production absorbed carbon dioxide, carbon dioxide emissions from the perspective of the fuel life cycle becomes zero. Considering that most biomass is used as food, there is a disadvantage that it can cause food shortages in developing countries. Hydrogen is known as the most ideal fuel to achieve zero carbon emissions; the high-level technology of cryogenic liquefaction and the risk of explosion are considered as major difficulties in hydrogen utilization
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