Measurement and Modeling of Torrefaction Processing for Human Solid Waste Management in Space

This study involves a torrefaction (mild pyrolysis) processing system that can be used to sterilize feces and produce a stable, odor-free solid product that can be easily stored or recycled, and also to simultaneously recover moisture. It was previously demonstrated that mild heating (200-250°C) in nitrogen or air was adequate for torrefaction of a fecal simulant and other analogs of human solid waste (canine feces). The net result was a nearly undetectable odor (for the canine feces), complete recovery of moisture, some additional water production, a modest reduction of the dry solid mass, and the production of small amounts of gas and liquid. This Torrefaction Processing Unit (TPU) is designed to be compatible with the Universal Waste Management System (UWMS) now under development by NASA. A stand-alone TPU could be used to treat the waste canister from the UWMS, thus allowing the waste canister to be reused and significantly reducing the number of canisters required on board. The current paper is the first part of a series of parametric studies and modeling efforts to examine some of the key issues affecting the TPU design including: 1) the canister size and geometry (aspect ratio); 2) the canister materials of construction; 3) sample depth and heat conducting properties; and 4) carrier gas composition and pressure. Experimental data, along with modeling using the COMSOL Multiphysics package, are being used to further optimize and finalize the reactor geometry. A heat-transfer model of the TPU was developed and its predictions showed good agreement with experimental data. It was found that the model predictions were sensitive to the top boundary conditions, the heat of vaporization, the temperature dependence of the physical properties, and the salt concentrations. Future model improvements will include the incorporation of the effects of void volume and the ambient pressure.