Scaling up catalytic microwave-assisted pyrolysis for energy production from biomass and plastic wastes
2021-04
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Scaling up catalytic microwave-assisted pyrolysis for energy production from biomass and plastic wastes
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2021-04
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Our society currently faces the dual challenge of resource depletion and environmental pollution. Converting sustainable biomass and recycled plastic wastes into energy and fuel products provides an attractive solution to this challenge. Among the various conversion technologies, microwave assisted pyrolysis serves as a promising alternative to conventional pyrolysis technology due to several unique benefits inherent to its dielectric heating mechanism. Yet, scaled-up design and operation of this technology are still lacking. In order to improve feasibility and scalability of the microwave-assisted pyrolysis process, a novel system of continuous microwave-assisted pyrolysis (CMAP) featuring a mixing SiC ball bed was developed and first tested for fuel production from wood pellets. High quality syngas was produced from this process. Specifically, at temperature of 800oC, producer gas with a high energy content of 18.0 MJ/ Nm3 and a high syngas (H2+CO) content of 67 vol.% was obtained at a gas yield of 72.2 wt.% or 0.80 Nm3/kg d.a.f. wood pellets. Downstream condensation and physical adsorption lowered the tar concentration from 7.83 g/Nm3 at the exit of pyrolysis reactor to below the detection limit at the end of the process. Energy balance analysis showed that a cold gas efficiency of 73.3% was achieved at 800oC, which consumed 7.2 MJ electrical energy per kg of wood pellets. Further measures to improve the energy efficiency could potentially reduce the electricity consumption to 3.45 MJ/kg wood, enabling a net electricity production. Then, pyrolysis of different plastic wastes for fuel production was conducted in the CMAP system. Overall, plastic wastes, especially polyolefin base plastics, produced much higher heating value byproducts, and relatively simpler compositions, compared to biomass, thus making plastic wastes a more desirable feedstock to produce high quality fuels, energy-efficiently. At 560oC, the highest liquid product yield, 47.4%, was obtained for thermal pyrolysis of HDPE, together with 24.5% wax product. The PP with fillers, (i.e. the mineral, talc) acted as a catalyst and showed noticeable cracking activity. The application of catalysts in the CMAP process has shown a significant impact on product yields and composition. Under a temperature of 620oC, incorporating ZSM-5 catalysts in a secondary catalyst bed, enabled the elimination of wax product and an increase of liquid yield to 48.9%, and the liquid products contained considerably higher contents of gasoline-range aromatics (45.0%) and isomerized aliphatic (24.6% ) contents. However, ZSM-5 catalysts also showed a tendency of rapid deactivation, and loss of activity at a feedstock/catalyst ratio of 5. Energy balance analysis of the process showed that 5 MJ of electrical energy were required to process 1kg of HDPE with the CMAP system, giving a total energy efficiency as high as 89.6%Furthermore, 6.1 MJ of electrical energy could potentially be generated from the gas products alone, making the process energy self-sufficient. In order to address a series of issues facing the application of catalysts in scaled-up pyrolysis systems, a structured catalyst of SiC foam supporting ZSM-5, was developed and tested for ex-situ catalytic upgrading of biomass pyrolytic vapors. A hydrothermal synthesis method was used to synthesize the catalysts, which resulted in a thin layer of ZSM-5 crystals firmly attached to the structure of a macroporous SiC foam material. Results suggest that the structured catalyst was more active and stable, compared to the randomly packed bed of catalysts, and also had the advantages of reduced pressure drop and enhanced heat and mass transfer. Therefore, this structured catalyst may serve as a promising candidate for future catalysts applied in large scale pyrolysis operations.
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University of Minnesota Ph.D. dissertation. 2021. Major: Biosystems and Agricultural Engineering. Advisor: Roger Ruan. 1 computer file (PDF); 155 pages.
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Zhou, Nan. (2021). Scaling up catalytic microwave-assisted pyrolysis for energy production from biomass and plastic wastes. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/220581.
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