Barry Dellinger of the University of Dayton spoke on "High Temperature Photochemistry Induced by Concentrated Solar Radiation." Mr. Dellinger described his experimental studies of high temperature, photochemical, solar-induced reactions with potential applications in the destruction of toxic organic wastes. Mr. Dellinger finds that for a number of compounds, under conditions of thermal photolytic destruction, decomposition occurs at significantly lower temperatures than is the case for thermal decomposition alone. Also, under these conditions thermal decomposition by-products (products of incomplete combustion or PICs) are also destroyed at lower temperatures than is the case with purely thermal decomposition.
According to Mr. Dellinger, a potential advantage of using concentrated solar radiation is that rapid heating rates are possible, which suggests that high temperature photochemical processes may be induced. Knowledge of these reactions is in its infancy, and he recommends research to elucidate the fundamental mechanisms of the process and the influence of temperature.
In comparison to controlled thermal incineration, solar destruction offers several advantages, according to Mr. Dellinger. Lower temperatures provide better control of vaporization of toxic metals and formation of nitrogen oxides; increased destruction efficiency of the parent compound and its by-products; use of excess thermal energy for thermal desorption of solids and sludges; reduced production of carbon dioxide, carbon monoxide, and toxic organic emissions from conventional fuels; cost savings from lower fuel costs and lower capital costs due to increased materials lifetime and reduced need for air pollution control devices; and increased public acceptance through reduced pollution and use of nonincineration disposal technology. Apparent disadvantages include unreliability of solar radiation, cost of solar collection and concentration, lack of off-the-shelf technology to construct a working pilot-or full-scale system.
Dellinger suggests a hybrid two-stage system for detoxification of soils and other solids. Thermal desorption of organics from the solids would take place in the primary unit while a solar reactor would be used for thermal photolytic destruction of the desorbed organics.
Wing Tsang of the National Institute of Standards and Technology spoke on "Chemical Processes During Incineration and Implications of Detoxification of Hazardous Waste Using Solar Photons." Destruction of organic chemicals proceeds without thermodynamic hindrance, but kinetic effects can lead to failure in incineration. Like Dellinger, Tsang also called for more data on photochemical reactions particularly at high temperatures.
According to Tsang, photodecomposition may require an "optically thin" system, and this may be an important constraint on large systems. Similarly, large concentrations of hazardous wastes may lead to the formation of solids and increase the optical density of the media.
Tsang anticipates that the applications for solar powered destruction of hazardous wastes will include very dilute mixtures of photodecomposable materials in a variety of matrices.
John F. Cooper of the University of California's Lawrence Livermore National Laboratory (LLNL) presented a paper on "Molten Salt Processing of Mixed Wastes and Potential Solar-Thermal Applications., In molten salt processing, organic wastes are oxidized at temperatures of 700–900ºC in a bed of molten carbonate/halide salts. The alkaline properties of the salt prevent formation of the acid gases hydrogen chloride or sulfur dioxide which are tied up as chlorides and sulfates. Heavy metals are also chemically incorporated in the salt. For example, arsenic is converted to sodium arsenate.
According to Dr. Cooper, major benefits of molten salt processing when compared to incineration of radioactive wastes are that it has the potential to eliminate or greatly reduce the amount of radioactive material in fugitive form (free molecules or gas-levitated particulates) and the elimination of stack emissions of gases in favor of total retention. Lower temperatures (700ºC), instead of the 1200ºC typical of incineration, will inhibit formation and loss of volatile radionuclide molecules and aerosols.
The process under R&D at Livermore is similar to a process developed by Rockwell. Rockwell has demonstrated destruction of PCBs at temperatures as