(98b) Comparison of Entrained-Flow and Fluidized-Bed Reactors for Production of Fast Pyrolysis Oils

The National Renewable Energy Laboratory (NREL) uses both entrained-flow and fluidized-bed reactors for the development and demonstration of biomass-derived fuel technologies. Fluid dynamic characteristics between the two types of reactors may result in differing heat transfer rates to the biomass particles, thereby changing the final pyrolysis oil quality when operated under similar conditions. The objective of this work was to understand the difference between the two types of reactors, so that product quality from the bench-scale systems can be reproduced at the pilot scale.

NREL has two bench-scale fluidized bed reactor systems and one entrained-flow pilot-scale reactor system from which pyrolysis data may be compared. A 2-inch diameter bench-scale fluidized bed reactor (0.5 kg/h feed rate) is used for feedstock selection and in- and ex-situ vapor-phase upgrading research. A larger bench-scale unit, attached to a Davison Circulating Riser used for catalyst development, employs a 2-inch diameter, fluidized bed reactor as the pyrolyzer (5 kg/h). Finally, a new, entrained-flow reactor for pyrolysis was installed in the existing Thermochemical Process Development Unit (TCPDU), a 0.5 ton/day (30 kg/h) biomass-to-fuels pilot plant used to demonstrate biomass conversion technologies at an industrially-relevant scale. The systems are similar in overall process design, control capability, pyrolysis oil collection, and analytical capabilities.

A simplified, 2D computational fluid dynamics (CFD) model was created to calculate heat transfer rates in the pilot-scale entrained-flow reactor and was validated using experimental data. The model shows that the entrained-flow reactor is capable of attaining the heating rates required for fast pyrolysis, despite the lack of heat transfer medium present in fluidized bed reactors.

Finally, data from the three reactor systems was used to compare the two reactor types at similar run conditions and similar feedstocks. Real-time analytical data included hot vapor analysis from Molecular Beam Mass Spectrometers (MBMS), and permanent gases were analyzed using online gas chromatography (micro-GCs) and non-dispersive infrared (NDIR) detectors. Raw pyrolysis oil was collected, and ultimate, proximate, ¹³C NMR, water content, viscosity, carbonyl, and acid analyses were conducted on several samples. Oil and light gas yields and mass balances from the systems were also evaluated.