Imaging apparatus characterises drops in 'dirty' laboratory environments

A high-fidelity spectrometric system for studying the behaviour of drops and particles in industrial flame reactors has been constructed by researchers at the University of Illinois at Urbana-Champaign in collaboration with researchers at the University of Arizona. The instrument was used to study the potential of thermal combustors for reducing the volume of liquid nuclear wastes for safe, long-term storage.

Vitrification of radioactive waste into glassy solids is the most popular approach for disposal. By spraying radioactive sludge into a high-temperature combustor, essentially all the water and other non-radioactive material could be removed, leaving only the radioactive metallic elements to be vitrified for burial. Under optimised conditions, up to 99.99 percent of the metal ions in a waste stream can be scavenged in the combustor.

"That kind of efficiency would be great for most applications, but it's not good enough when dealing with radioactive waste," said Alexander Scheeline, a professor of chemistry at Illinois. "Understanding the cause of the unscavenged fraction and devising a way to reduce it are essential if thermal processing is to be used for nuclear waste treatment."

One possible explanation is that large 'rogue' drops are responsible for the unscavenged metals, Scheeline said. These drops do not pass through the hottest zones in the combustor, resulting in only partial vaporisation.

To investigate the role of rogue drops in this process, Scheeline and his colleagues - Illinois postdoctoral researcher Jerry Cabalo, Arizona professor and head of chemical and environmental engineering Jost Wendt, and Arizona graduate student John Schmidt - developed an optical system to monitor drop sizes and trajectories at very high spatial resolutions.

"In the thermal waste destruction process, small particle formation is also very important," Scheeline said. "Metals released into the gas phase readily form small particles, so it was crucial that this system also have the capability of detecting small soot particles."

In operation, large drops of water or diesel fuel were injected into the furnace. An excimer laser sent a beam into the combustor, illuminating a plane through which the drops passed.

The scattered light was then passed to a CCD (charge-coupled device) camera and analysed.

In contrast to optical monitoring of typical combustion experiments performed in a reasonably clean environment, "these measurements took place in a coal combustion laboratory at the University of Arizona," Scheeline said. "Coal dust from experiments and sand from the desert were all-too-frequent visitors."

To protect delicate optical components, the researchers covered the optical system with plastic panels and pumped clean, dry air into the enclosure. "Despite months of experiments on coal dust combustion taking place in the laboratory - which left a thick layer of dust on the outside of the spectrograph and on the plastic housing - the optical path remained free of dust and other contaminants," Scheeline said.

In their initial studies, the researchers demonstrated that the optical system could track large drops and the resulting soot particles through the flame.

"To get these drops to break up and vaporise, we need a longer combustion zone, or we need to spray finer drops," Scheeline said.

The same kind of optical measurements and combustion research is relevant to designing cleaner-running automobile engines, studying combustion processes in rocket engines and developing alternative means for solid waste disposal.