Combustion insights and emission reductions of iron powder
Aravind Ravi defended his PhD thesis at the Department of Mechanical Engineering on November 8th.
In light of the urgent need for renewable, carbon-neutral energy sources to address environmental concerns, metal fuels like micron-sized iron particles present a compelling alternative. These metal fuels boast high energy density, affordability, and availability, and they can be recycled using renewable energy. Notably, they have the potential to replace fossil fuels in existing coal power infrastructure with minimal modifications. The PhD research of Aravind Ravi delves into the burning dynamics of iron powder, revealing that specific particle sizes can accelerate burning velocity. Moreover, iron powder combustion produces significantly lower nitrogen oxide emissions鈥攁bout ten times less than fossil fuels or hydrogen. This research also uncovered distinct hybrid flame regimes that arise from varying concentrations of methane and iron powder, offering valuable insights into hybrid combustion processes and advancing our understanding of sustainable energy alternatives.
The combustion of iron powder is influenced by factors such as particle morphology, size, and concentration. In the first part of the thesis, a numerical investigation is conducted for mono-, binary-, and poly-dispersed iron aerosols to comprehend the impact of different particle sizes on burning velocity and flame structure. Initially, single iron particle modeling is validated against experimental results. Then, flame propagation in mono-dispersed iron aerosols is studied in different atmospheres. The flame speed of iron burning in a O2/He atmosphere is four times faster than O2/N2, and O2/Ar atmospheres near stoichiometric conditions due to the difference in the thermal diffusivity, and specific heat of carrier gas.
Crucial role of size distribution width
Considering binary dispersed powders, explores the effects of particle size and concentration on flame structure and burning velocity. Different concentrations of small and large particles result in separated or overlapping flame fronts. In-depth analyses of total iron mass concentration variations reveal that the particle size ratio dictates the fuel-air ratio at which maximum burning velocity occurs. Considering flames of poly-dispersed powders, flame speed and structure for narrow and broad distributions are investigated. Results show that the fuel-air ratio at which the maximum flame speed occurs, varies significantly with the characteristic width of the particle size distribution for the same average size. These findings highlight the crucial role of the size distribution width and the challenge of comparing experiments and simulations with different particle size distributions.
Producement of nitrogen oxides
While the burning of iron in air does not generate carbon dioxide (CO2) emissions, it may produce nitrogen oxides (NOx). A numerical study with detailed gas phase chemistry shows that the nitrogen oxide emissions of an iron flame using dry air are very small, due to the low amount of oxygen radicals. However, even for a small fraction of water vapor (volume fraction < 0.5%) in the mixture, hydroperoxyl (HO2) is formed, which accelerates the dissociation of molecular oxygen and increases the nitrogen oxide formation rate. This demonstrates that water vapor in air should be considered when studying nitrogen oxide emissions of iron dust combustors. Furthermore, the particle temperature can significantly exceed the gas temperature in iron-air flames, which may lead to high NOx formation rates. However, using an experiential analysis the researcher shows that nitrogen oxide formation inside the particle boundary layer is not a concern.
Stabilizing iron flames
The second part of the thesis focuses on hybrid methane-iron-air flames. Iron flames are difficult to stabilize in experimental setups and industrial burners as the burning velocity is only a few cm/s. A supporting fuel can help stabilize iron flames by enhancing the burning velocity. Numerical simulations are performed using methane as a supporting fuel as the combustion characteristics of methane burning in air are well known and experimental data for methane-iron-air flames are available. The study focuses on the effect of adding iron powder to a stoichiometric methane flame in air in one and three dimensions (1D and 3D). The burning behavior of iron powder is found to be influenced by particle size, and concentration. A critical concentration is identified as the concentration resulting in a sudden change in the flame structure. Below this critical concentration, iron particles burn individually without forming a flame front, which is confirmed by the 3D simulations. While above this concentration, a flame front is observed in which the iron particles are completely oxidized and form a flame front on their own, as seen in both 1D and 3D simulations. The laminar burning velocities obtained from the simulations are compared with experimental results, which adds validity to the numerical findings.
Title of PhD thesis: . Supervisors: Prof. Jeroen van Oijen, and Prof. Philip de Goey.