Date of Award
Doctor of Philosophy
Mechanical Engineering; Biorenewable Resources and Technology
Pyrolysis of lignocellulosic biomass is a promising conversion pathway for the production of renewable fuels and chemicals. The vital component of the pyrolysis conversion process and biorefinery is the pyrolysis reactor. Auger pyrolysis reactors have been gaining recent research interest for their advantages over fluidized bed reactors. While auger pyrolysis research continues to grow, voids in literature exist and need to be addressed to minimize risk in scale-up to potential commercialization. Research in this dissertation addresses some of these voids, specifically, to develop a fundamental understanding of the phenomena that constraints heat transfer and heat recovery in directly-heated auger pyrolyzers.
A laboratory-scale, twin-screw auger pyrolyzer with heat carrier for the pyrolysis of red oak was of specific focus throughout this work. First, the effect of thermophysical properties of heat carriers on the performance of an auger reactor was investigated. Heat carriers with a wide range of thermal diffusivities were tested. This included stainless steel shot, fine sand, coarse sand and silicon carbide. It was found that the heat carriers exhibited similar organic yields and composition of bio-oil. However, significant differences of reaction water, char and non-condensable gas yields were observed. It was also found that residual carbon contributed to as high as 20 wt.% of total char yield for some heat carriers. Attrition of heat carrier as high as 7 wt.% was present after as little as 2 hours of operation. The results from this study suggest tradeoffs may exist between physical performance, material cost, and product yields when selecting heat carrier materials for pyrolysis of biomass in an auger reactor.
The second study investigated the effect of recycling sand heat carrier on the long-term performance of a laboratory-scale auger reactor. Sand heat carrier with a particle size range of 600-1000 ÃÂ¯ÃÂ¿ÃÂ½m was used in pyrolysis trials and then subsequently regenerated and recycled at up to five times. Attrition as high as 8% on a mass basis and a decrease in mean particle size of the sand was evident after each recycle. This prompted further investigation into the effect of heat carrier particle size. A smaller fraction of sand (250-600 ÃÂ¯ÃÂ¿ÃÂ½m) was tested in comparison to the original. Significant differences in the yields of organic bio-oil, reaction water, char and non-condensable gases were observed between the two fractions of sand. The smaller sand fraction produced more char and reaction water at the expense of organic bio-oil and non-condensable gases. This study shows that heat carrier material selection and particle size plays an important role in the continuous operation of an auger pyrolyzer.
The third study investigated the effect of regeneration parameters on carbon burn-off times from biomass pyrolysis char. A laboratory-scale fluidized bed reactor was used to regenerate sand heat carrier and char from biomass pyrolysis. Regenerations were conducted with varying regenerator temperatures (450-750ÃÂ¯ÃÂ¿ÃÂ½C), varying superficial fluidization velocities (100-250% minimum fluidization), and varying oxygen sweep gas concentrations (13.6-28.5 vol.% O2). Carbon burn-off times increased with increasing temperature at the same state of fluidization, suggesting superficial fluidization velocity plays an important role in carbon burn-off times at these temperatures. Increasing the superficial fluidization velocity and oxygen sweep gas concentration, both significantly decreased carbon burn-off times. Furthermore, increasing regeneration reaction temperatures was shown to promote carbon dioxide production. The results from this study show the influence temperature, gas velocity, and oxygen concentration has on carbon burn-off times in biomass char regeneration unit.
Lastly, the potential impact of industry technology-learning rates was investigated on varying biorefinery capacities of advanced biofuel technologies. Predictions of learning-based economies of scale, S-Curve, and Stanford-B models were studied on the optimal plant capacities and production costs of biorefineries. Biofuel cost reductions of 55 to 73% compared to base case estimates were found using the Stanford-B model. The optimal capacities range from small-scale (grain ethanol and fast pyrolysis) producing 16 million gallons per year to large-scale production of 210 million gallons per year capacity for gasification facilities. Results from this study suggest there is an economic incentive to invest in strategies that increase the learning rates for advanced biofuel production, which could lead to the reduction of the optimal size and production costs of biorefineries.
Tannon Jeffrey Daugaard
Daugaard, Tannon Jeffrey, "Understanding heat carrier effects in a laboratory-scale auger pyrolyzer" (2018). Graduate Theses and Dissertations. 16803.