Development of a Waste Processing Incinerator for Life Support

John W. Fisher

As space missions become longer, waste treatment on a space craft transitions from storage to reclamation of items such as activated carbon and carbon dioxide. Activated carbon and carbon dioxide can be reclaimed from hydrocarbon wastes such as paper, food scraps, and inedible plant biomass. Inedible plant biomass is produced when plants are grown in space to produce food. Growing plants consume carbon dioxide, and burning the inedible parts of a plant produces carbon dioxide that can be used to grow more plants. Unfortunately the process of burning, combustion, produces some toxic byproducts. One of the objectives of Ames’ research on waste processing is to develop technology to burn waste and reclaim carbon dioxide without releasing toxic materials into the spacecraft.

The combustion process generally does well at completely oxidizing biomass to carbon dioxide and water. This is obvious from observation of the results of a typical wood fire. Only a small residue of inorganic substances, ash, is left in a fireplace after burning wood. The process of combustion of biomass in an incinerator operates in a similar way; the biomass is converted to gaseous products and inorganic ash. However, combustion in a fireplace typically takes place with wide fluctuations in temperature and composition as a function of time and position in the burning zone. Efficient combustors reduce the combustion fluctuation and achieve cleaner burning.

Fluidized combustion is a technology that provides good control of the combustion process and minimizes contaminants due to incomplete combustion. A fluidized bed consists of a bed of solid particles such as sand that behaves as a fluid. The fluidization occurs because a gas such as air is blowing up through the bed and causing the particles of the bed to float. Because sand is much denser than air, the bed holds much more heat energy than an equivalent amount of air. The heat energy held by the bed buffers the combustion process against the wide fluctuations in temperature that cause incomplete combustion.

Even in the best of combustors, however, some unoxidized material remains. In addition, some contaminants such as nitrogen and sulfur oxides are necessarily formed. In recent years Ames’ research has focused on means to eliminate these byproducts. One approach has been to use reductive catalytic systems to convert the nitrogen and sulfur oxides to nitrogen and elemental sulfur – innocuous materials at room temperature. Oxidative catalysts can then oxidize the remaining hydrocarbon contaminants to very low levels.

In collaboration with outside university and corporate organizations Ames has developed and tested an integrated incineration system that utilizes a fluidized bed combustor followed by a catalytic cleanup system. This system has demonstrated the ability to burn inedible biomass and produce a very clean carbon dioxide product. The concentration of contaminants in the gas exiting the incinerator is generally less than a few parts per million. Except for the carbon dioxide itself (toxic to humans at high concentrations), the exit stream from the incinerator will be able to meet the Space Maximum Allowable Contaminant (SMAC) standards for clean air in a spacecraft.

When this system has been optimized for reliability and energy efficiency, it will be ready for testing in an advanced life support system that "closes the loop" on carbon. Carbon will travel in the system from plant to person to incinerator and back to the plant without ever becoming a stored waste.