Fluidized bed pyrolyzers are widely accepted as well-developed pyrolysis reactors. However, commercial fluidized bed reactors can have heat transfer limitations and require significant energy input from handling of the fluidization gas. These limitations have led to research in alternative reactors such as auger reactors. Auger pyrolyzers are gaining interest due to their minimal dependence on inert gas and their low operational complexity while still achieving similar yields to that of traditional fluidized bed pyrolyzers. Auger pyrolyzers can employ different modes of heat transfer such as conventional indirect heating or direct heating via heat carrier materials to provide higher heat transfer rates. Additionally, heat carriers allow for flexibility in the selection of reaction media with different thermophysical properties. However, the effects of heat carrier thermophysical properties are not well understood. The objective of this research was to determine how different heat carrier properties affect the yield and characteristics of pyrolysis products in an auger pyrolyzer.
We pyrolyzed up to 1 kg/hr of red oak in a twin-screw, lab-scale auger reactor at 515⁰C. Multiple heat carriers with varying specific heat capacities (500-830 J/kg-K) and thermal conductivities (0.2-120 W/m-K) were tested. The heat carriers tested included stainless steel, silicon carbide, fine sand (250-600μm) and coarse sand (600-1000μm). A cold gas quench system was used to collect the bio-oil into two fractions: a heavy phase and an aqueous phase. Proximate and ultimate analyses were conducted on both bio-oil fractions and char. Moisture and solids content was also determined for both bio-oil fractions. Identification of select bio-oil compounds was conducted. Bio-oil yields ranged from 59-66 wt. % on a dry basis across all heat carriers. Char and non-condensable gas yields ranged from 15-22 wt. % and 14-17 wt. % on a dry basis, respectively. Further fundamental studies are needed to optimize the yields and quality of pyrolysis products in an auger pyrolyzer.