A Steam-Plasma Igniter for Aluminum Powder Combustion

Abstract

High-temperature ignition is essential for the ignition and combustion of energetic metal fuels, including aluminum and magnesium particles which are protected by their highmelting-temperature oxides. A plasma torch characterized by an ultrahigh-temperature plasma plume fulfills such high-temperature ignition conditions. A new steam plasma igniter is designed and successfully validated by aluminum power ignition and combustion tests. The steam plasma rapidly stabilizes in both plasma and steam jet modes. Parametric investigation of the steam plasma jet is conducted in terms of arc strength. A high-speed camera and an oscilloscope method visualize the discharge characteristics, and optical emission spectroscopy measures the thermochemical properties of the plasma jet. The diatomic molecule OH fitting method, the Boltzmann plot method, and short exposure capturing with an intensified charge coupled device record the axial distributions of the rotational gas temperature, excitation temperature, and OH radical distribution, respectively. The excitation temperature at the nozzle tip is near 5500 K, and the gas temperature is 5400 K.

Combustion of aluminum powder with steam entrained in a helium plasma using a 2.45 GHz microwave plasma torch

“…The best matched spectra were observed to be alpha alumina (corundum, α-Al 2 O 3 [18]) noted by black bars, delta alumina (δ-Al 2.67 O 4 , transient), gamma alumina (γ-Al 2 O 3 , transient), and elemental aluminum (Al) noted by red bars. A similar match in XRD profiles, specifically the XRD profile of corundum, was found in another experiment that used a steam plasma to ignite aluminum powder [4], which indicates that ignition and combustion of aluminum powder and steam was achieved through the use of a microwave-induced helium plasma without the need for high pressures generally necessary for auto-ignition and very hot (>5000 K) electrical discharges. The presence of delta and gamma alumina can be attributed to the existence of OH ions (hydroxyl), since both transition alumina are uniquely defined by the presence of hydroxyl in their crystalline structure [18].…”

“…aluminum oxide or alumina) inhibits reactivity by isolating the elemental metal from reactive oxidants and raising the effective melting point from 930 K to 2350 K. Although previous pilot-flame-based ignition mechanisms, such as a hydrogen-oxygen flame or methane torch, have been shown to be capable of igniting aluminum powder, the degree of ignition is not consistent, which is believed to be a consequence of the adiabatic flame temperature, which is below the melting point of aluminum oxide for micron-sized aluminum particles [3]. Alternatives to chemical pilot flames, motivated by the desire for a more complete ignition and combustion process, have recently focused on electrically-excited mechanisms, such as arc discharges for aluminum combustion with steam [4] and high-frequency plasmas for hydrocarbon fuels [5].…”