Development of a model of the combustion a single iron particle as a contribution to design of metal fuel burners

Development of a model of the combustion a single iron particle as a contribution to design of metal fuel burners

Subhedar, Nupur (TU Delft Mechanical, Maritime and Materials Engineering; TU Delft Process and Energy)

Roekaerts, D.J.E.M. (mentor)

Delft University of Technology

Mechanical Engineering | Sustainable Process & Energy Technology

2017-07-28

Recently an oxidation and reduction cycle of metals has been proposed as alternative for liquid fuel combustion. Out of several possible metals iron is an interesting candidate because the oxide does not form making it more easy to develop a cyclic process. As a first building block towards a computation or particle dust clouds here a computational model for the combustion of a single iron particle is developed. During high temperature oxidation of iron particle both iron and iron oxide remain in solid form and the combustion
occurs in the form of a reaction zone at the interface between iron core and iron oxide outer layer. Because of the difference in density of iron and iron oxide the particle grows in size during the combustion process. The conversion can be either oxygen diffusion limited or kinetically limited depending on the temperature. After ignition the process is mainly diffusion
limited.
The model is developed for the case of a spherically symmetric particle. It consists of the transient transport equations of oxygen and of energy with boundary conditions for oxygen concentration and radiative heat flux to the surroundings. These are solved using finite difference and finite volume methods on a one-dimensional computational domain with radial coordinate as independent variable. A submodel is developed for the evolution of the reaction
zone in the case of diffusion limited reaction. In this model the amount of oxygen available for reaction is obtained using penetration theory applied on the scale of the thin reaction layer.
Particle volume increase is also included. The model is implemented in Matlab.
Using the model combustion of particles with diameter in the range 2 tot 12 micrometer and for different levels of oxygen level and temperature of the surroundings is computed. The results show the development of oxygen concentration and temperature profiles over time and
lead to the prediction of particle degree of oxidation over time. The evolving particle peak temperature results from the competition between heat loss to the surroundings and heat gain from the oxidation reaction. The radial temperature profile is nearly flat due to the high thermal conductivity.
The predicted life-time of the particles is in agreement with experimental observations in the literature. The following observations are potentially important for process development: the combustion of the last 10% is slow due to the combined effects of particle cooling and oxygen diffusion limitation through an oxide layer of growing thickness. The energy released
by combustion ends up in the sensible heat of the oxide and the gaseous surroundings. Energy efficient heat recovery should take the balance between these two heat sources into account.

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Student theses

master thesis

© 2017 Nupur Subhedar