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Graphitic coke in microwave-assisted catalytic pyrolysis to improve process efficiency and soil quality

University of British Columbia

Microwave pyrolysis is an effective method of converting wood waste into valuable biochar which can be added to agricultural soils to improve water retention, structural stability, and nutrient adsorption. When coupled with advantages of catalytic coke formation, the process is improved both by increasing the efficiency of microwave absorption and the retention of fertilizers in soil. In this exploratory study, sawdust is mixed with 30 or 150 wt% potassium phosphate (K₃PO₄) as a pyrolysis feedstock. The K₃PO₄ acts both as a reaction catalyst and as a fertilizer for soils. The K₃PO₄ was separated from the biochar post-reaction and analyzed for coke formation. Coke produced at a higher reaction temperature (550 °C) was found to have a greater ratio of graphitic to oxygenated coke, up to 4.5:1, while coke produced during a longer reaction time was found to increase the total coke yield. Combining the two (more graphitic coke, greater coke yield) by producing coke at 550 °C for 50 min produced coked K₃PO₄ that has the greatest microwave absorption with a loss tangent 30 times greater than fresh K₃PO₄. This improvement is likely due to greater amount of polyaromatic C=C bonds under which the ‘Maxwell-Wagner-Sillars’ effect takes place, releasing energy in the form of heat. The coke layer surrounding K₃PO₄ particles was tested in soil as a nutrient release barrier. In all cases, the coke slowed the leaching of both K⁺ and PO₄- ions, up to 10 and 18 %, respectively. The slowest release was observed with low temperature coke (350 °C) which likely has more oxygen functional groups which can electrostatically interact with the leaching ions. The coke produced over a longer reaction time of 50 min also showed an improvement in K and P retention, likely because of the increased fraction of coke on the K₃PO₄ surface. It is estimated that coke produced at 350 °C for 50 min would have an even better retention in K and P as it has both advantages of a higher content of oxygen functional groups and a higher yield of coke. This exploratory study suggests that coke has the potential to both improve microwave absorption during pyrolysis and act as a slow-release barrier for fertilizers in soil. To expand the boundaries and robustness of this study, the effects should be investigated under extended microwave power, different catalysts, and soil conditions in a larger scale experimental system.

Text

2021-03-09

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/77489

Chemical and Biological Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/