Increase in bio-oil quality from improvements on fast pyrolysis fluidized beds

Date
2021-01-01
Authors
Plouffe, Colin
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Robert C Brown
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Mechanical Engineering
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Mechanical Engineering
Abstract

Through the pretreatment of lignocellulose, sugars can be major products of fast pyrolysis, most prominently the anhydrosugar levoglucosan. The analytical pyrolysis of pure cellulose can produce up to 60 wt.% yields of levoglucosan. However, in continuous pyrolysis trials, levoglucosan yields are significantly lower, suggesting that significant secondary reactions occur before the levoglucosan can be removed from the reactor and quenched. Previous research has revealed that biochar can catalyze the decomposition of levoglucosan at pyrolysis temperatures. Biochar has been shown to accumulate near the surface of a fluidized bed pyrolyzer, reaching a steady-state loading through which pyrolysis vapors must pass. However, it has not been determined whether gas-solid reactions of levoglucosan and biochar occur to an appreciable extent in a continuous fluidized bed pyrolyzer. We have performed experiments to test this hypothesis that limiting the gas-solid reactions would improve our overall bio-oil yield.Micropyrolysis experiments were performed to better understand the mechanism of sugar degradation. A significant loss in sugar yield was observed for cellulose overlain with biochar powder compared to the pure cellulose control sample. Further tests were performed in a micropyrolyzer to investigate the effect of iron sulfate pretreated biomass which is meant to passivate the biochar’s catalytic activity. In the worst case of both biochar mixed into the sample and overlain, there was a reduction of levoglucosan from 60% to 25% with untreated biochar. On the other hand, iron sulfate biochar only saw a drop down to 53% levoglucosan yield. Due to this drop to 25% levoglucosan in micropyrolyzer testing, the interaction between cellulose and biochar was tested using a mixture of 85 wt.% cellulose and 15 wt.% untreated corn stover biochar by continuously feeding in a fluidized bed reactor under both conventional and autothermal operation. Bio-oil produced in the reaction was recovered, and the yield of levoglucosan was determined through acid hydrolysis and HPLC analysis. Sugars decreased from 61.3 wt.% to 21.3 wt.% and 41.5 wt.% to 11.6 wt.% for conventional and autothermal operation, respectively. The significant drop in sugar yield due to biochar interaction encouraged changes in the design or operation of continuous pyrolysis reactors to reduce vapor-product interactions with the goal of diminishing secondary reactions responsible for loss of levoglucosan yield. The injection of biomass was changed to be above the bed rather than feeding directly into the bed. This change reduced the exposure of pyrolysis vapors to the biochar layer at the fluidized bed’s surface, increasing bio-oil and sugar yields by 9.3% and 9.1%, respectively.

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