Torrefaction Processing of Spacecraft Solid Wastes
Serio, Michael A.
Cosgrove, Joseph E.
Wójtowicz, Marek A.
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New technology is needed to collect, stabilize, recover useful materials, and store human fecal waste and other spacecraft solid wastes for long duration space missions. The system should also require minimal crew interactions, low energy demands, and tolerate mixed or contaminated waste streams. The current study addressed the technical feasibility of a torrefaction (mild pyrolysis) processing system that could be used to sterilize feces and related cellulosic biomass wastes (food, paper, wipes, and clothing), while simultaneously recovering moisture and producing small amounts of other useful products (e.g., CO2, CO, and CH4). This work was done using bench scale torrefaction processing units and examined different modes of heating (conventional and microwave) in laboratory studies. A fecal simulant was tested over a range of process conditions (temperature, holding time and atmosphere), along with selected runs with a sludge derivative (Milorganite), cotton fabric, and wipes. The results demonstrated that microwave heating allowed for careful control of torrefaction conditions for the fecal simulant. The net result was complete recovery of moisture, some additional water production, a modest reduction of the dry solid mass, and small amounts of gas (CO2, CO, and CH4) and hydrocarbon liquid production. The amounts of solid vs. gas plus liquid products can be controlled by adjusting the torrefaction conditions, especially the final temperature and holding time. The solid char product from the fecal simulant was a dry, free flowing powder that did not support bacterial growth and was hydrophobic relative to the starting material. The proposed torrefaction approach has potential benefits to NASA in allowing for solid waste sterilization and stabilization, planetary protection, in-situ resource utilization (ISRU) and/or production of chemical feedstocks and carbon materials. In particular, the torrefaction char residue has several potential applications in space. These include production of activated carbon, a nutrient-rich substrate for plant growth, construction material, radiation shielding, storage of elemental carbon, hydrogen, or oxygen, and fuel gas (CH4, CO, and H2) production.