The molten-carbonate oxidation (MCO) process shows promise as a means of safe disposal and/or recycling of solid waste. The MCO process is being developed for use in regenerative life-support systems in outer space, but may also prove useful in managing institutional, industrial, and/or municipal solid waste. The MCO process completely oxidizes wastes as diverse as polytetrafluoroethylene, polyvinyl chloride, polyethylene terephthalate, polyethylene, feces, wheat straw, and cellulose. An MCO system can operate at atmospheric pressure without flames and without direct feed of fuel into the oxidation chamber — all important safety features.
In the MCO process, a mixture of approximately equimolar proportions of sodium carbonate and potassium carbonate is melted and heated to a temperature between 800 and 900 °C. The waste to be oxidized is fed into the melt. Oxygen or air is also fed into the melt. The chemical environment in the carbonate melt favors the formation of superoxide ions (O2–), which catalyze oxidation of the waste. The result is a kinetically rapid three-dimensional homogeneous reaction in which the solid waste is converted to carbon dioxide and water vapor, which bubble away from the melt and can be reclaimed. Any inorganic materials in the waste are converted to minimal amounts of ashes and/or to inorganic salts, which can be removed by a commercial salt-splitting unit and reused.
Typically, the melt is contained in a stainless-steel or alumina tank. Both the oxidizing gas and the waste feed are introduced into the melt from the bottom: This practice forces the waste and gas to rise through the full depth of the melt, maximizing contact between the waste and the molten-salt mixture and thereby the degree and the overall rate of oxidation of the waste.
The melt is heated initially and thereafter maintained at the required high temperature by use of an electric furnace. Inasmuch as most wastes are low-grade fuels, the oxidation of the waste supplies some heat, thereby reducing the electric power needed to maintain the high temperature. The consumption of energy can be reduced further by efficient insulation of the tank and/or furnace and by regenerative recovery of heat from the reaction products.
The MCO equipment is relatively simple, with few moving parts. Pretreatment of waste is not strictly necessary, although some milling of the waste feedstock can be beneficial. The flow of oxidizing gas and the proportion of nonexothermic material in the waste feed can be adjusted to help keep the temperature of the melt in the desired range.
This work was done by G. Duncan Hitchens and Oliver J. Murphy of Lynntech, Inc., for Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Machinery & Automation category.