469496 Effects of Recycling Regenerated Heat Carrier on the Performance of an Auger Pyrolysis Reactor

Thursday, November 17, 2016: 3:15 PM
Union Square 19 & 20 (Hilton San Francisco Union Square)
Tannon J. Daugaard and Mark Mba Wright, Mechanical Engineering, Iowa State University, Ames, IA

Auger pyrolysis reactors offer unique advantages over traditional fluidized bed pyrolyzers. Some advantages include the minimal requirement of an inert gas and the flexibility of heat carrier materials with varying thermophysical properties. Previous work used a lab-scale auger pyrolyzer to test the effect of heat transfer properties of various heat carriers such as stainless steel shot, silicon carbide and sand. While high liquid yields were achieved (~65 wt. %), the long-term performance of these heat carriers is not well understood. If auger pyrolyzers are to be commercially viable, the heat carrier must be recycled to minimize the cost of system inputs and solids handling. The objective of this research was to evaluate the effect of heat carrier regeneration and recyclability on the long-term performance of an auger pyrolysis reactor. The effect on product yields and changes to the thermophysical characteristics of the heat carrier materials were of specific focus.

A twin-screw, lab-scale auger reactor was used to pyrolyze up to 1 kg/hr of red oak at 515°C. The heat carriers of interest in this study included fine (250-600 µm) and coarse sand (600-1000 µm). Pyrolysis trials using recycled heat carrier were conducted at up to 5 recycles. The heat carrier was regenerated following each trial to remove any carbon residue. A cold gas quench system was used to collect the bio-oil into two fractions: a heavy phase and an aqueous phase. Product characterization included ultimate and proximate analysis of the bio-oil and char. Gas chromatography of the bio-oil fractions was also conducted. Product yields of bio-oil (~60 wt. %), char (~17 wt. %) and gas (~18wt %) were consistent across the limited number of recycle trials. Heat carrier attrition varied from 4 to 8 wt. %, and the mean particle size decreased following the recycled trials. These results indicate that heat carrier properties may change significantly over a short number of rotations, but their impacts on biomass pyrolysis yields is small. Long-term tests would be necessary to determine heat carrier replacement rates in order to avoid significant impacts on bio-oil yields and quality.

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