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Document Type

Open Access

Degree Program

Mechanical Engineering

Degree Type

Master of Science in Mechanical Engineering (M.S.M.E.)

Year Degree Awarded

2013

Month Degree Awarded

September

Keywords

mixing, fluidized bed, biomass, computational fluid dynamics, simulation, alternative energy

Abstract

Producing gasoline-type fuels from lignocellulosic biomass has two advantages over producing alcohol-type fuels from plant sugars: gasoline has superior fuel characteristics and plant lignin/cellulose does not compete with human food supplies. A promising technology for converting lignocellulose to fuel is catalytic fast pyrolysis (CFP). The process involves injecting finely ground biomass into a fluidized bed reactor (FBR) at high temperatures, which reduce the biomass to gases that react inside the catalyst particles. This entails complex hydrodynamics to efficiently mix a stream of biomass into a catalyst bed that is fluidized by a separate stream of inert gas. Understanding the hydrodynamics is complicated by the fact that the entire process occurs inside a heavily insulated, opaque, reactor vessel. Numerical simulations offer a promising approach to understanding, predicting, and optimizing hydrodynamic mixing in a CFP biomass reactor.

The purpose of this research is to understand the simulation techniques and statistical measures appropriate for quantifying mixing in a CFP biomass reactor. The methodology is validated against the canonical configuration of a non-reacting, single-inlet fluidized bed. A new finding is that the minimum bubbling velocity may be predicted by a significant increase in temporal variance of the pressure drop. The methodology is then applied to a non-canonical FBR in which biomass is injected into the catalyst bed via a vertical center tube. Since no hydrodynamic mixing data exist from laboratory experiments, mixing is inferred from the aromatics yield from the laboratory reactor. Flow configurations with which simulations demonstrate the best mixing have the highest aromatic yields in the experiments. The simulations indicate that when the bed is in the bubbling regime, the gasified biomass from the center tube is efficiently mixed radially throughout the catalyst bed. If the flow rate of inert gas is insufficient to bubble the bed, then the gasified biomass exits the center tube, reverses direction, and flows upward along the tube's outside wall. Provided the bed is bubbling due to the inert gas stream, the upper limit on the flow through the center tube, and thus the aromatic yield potential, has yet to be determined.

First Advisor

Stephen M de Bruyn Kops

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