Methane Production from Pyrolysis of Mixed Solid Wastes

There has recently been an increased interest in using pyrolysis of mixed solid wastes in space or on planetary surfaces to produce methane for applications in propulsion and for power generation using fuel cells. This paper involves a review of previous pyrolysis results collected at Advanced Fuel Research, Inc. (AFR) and elsewhere to determine how the pyrolysis conditions and feedstock composition affect methane yields. In general, the production of methane from primary pyrolysis of most biomass materials is pretty modest, 0.1 to 2.5 wt. % for a wide range of materials, with an average slightly above 1.2 wt. % (dry, ash-free basis). The primary pyrolysis yield variations for methane (and other species) with biomass sample type are well described using a simple Neural Network model. In pyrolysis experiments that include significant secondary reactions (e.g., tar cracking), the methane yield can be increased by a factor of 2-3. The methane yield can also be increased significantly by increasing the plastic component of the mixed waste stream (e.g., by the addition of polyethylene), but would be unlikely to exceed 15 wt. % by conventional, low-pressure pyrolysis of a typical mixed waste stream. The use of high pressure (>500 psig) pyrolysis in pure hydrogen is one approach that could be used to increase the methane yield even further. However, this approach would require a much heavier reactor unit, high pressures, and the associated safety concerns. An alternative pathway to higher methane yields would be to oxidize the waste completely to carbon dioxide and water and use the Sabatier reaction to convert the carbon dioxide to methane.