Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Alan W. Weimer

Second Advisor

J. Will Medlin

Third Advisor

Rich Noble

Fourth Advisor

Chris Perkins

Fifth Advisor

Frank Kreith

Abstract

Solar thermal gasification of biomass is a promising route to renewable fuel production. In order to design efficient solar biomass gasifiers, the kinetic rate of the char gasification step must be determined. The key advantages of solar thermal gasification are the ability to operate at temperatures significantly higher than those used in traditional gasifiers and to operate with steam instead of oxygen to produce a product stream with higher energy content. High temperature steam gasification kinetics are rarely studied in the literature, and the methods that are commonly used to measure low temperature gasification kinetics are often not applicable at high temperatures, for example due to heat and mass transfer limitations. The work presented in this thesis comprises studies designed to advance the state of the art in high temperature steam gasification by investigating char gasification kinetics, incorporating those kinetics into a CFD model, and facilitating gasification studies in aerosol flow through the development of a novel particle feeding system. A primary goal of this work was to develop a low-cost method to obtain an empirical rate expression for steam-char gasification. A modified fixed bed reactor was used to limit the effects of heat transfer, steam consumption, and hydrogen inhibition in order to ensure that the rate was measured at known conditions. After minimizing the above effects within the constraints of our laboratory system, the reaction rate was so rapid that our factory configured non-dispersive infrared analyzer could not provide high enough temporal resolution. In analyzing the data, we observed that the outlet flow meter could respond very quickly to changes in the gasification rate. After further analysis and testing, it was determined that the flow meter alone could be used to measure the rate of gasification within the fixed bed. This gas flow measurement technique was able to provide high resolution data with a very low cost and simple to use flow meter. Using the gas flow measurement technique, data were collected over a range of temperatures, steam concentrations, hydrogen concentrations and degrees of conversion. The results were used to develop an empirical rate expression based on the Random Pore Model for the dependence on conversion and a Langmuir-Hinshelwood type expression for the dependence on the reactor conditions. To demonstrate the power of an accurate kinetic rate expression, a simplified CFD model of a small fixed bed gasifier was developed using the commercially available Ansys Fluent software package. Validation experiments were performed in a laboratory scale fixed bed reactor. The model was able to accurately predict the overall reaction rate throughout time, and to lend insight into steam gasification in a fixed bed configuration. In addition to the investigation of high temperature steam char kinetics, a novel particulate feeding system was developed to aid in aerosol flow studies. Aerosol flow reactors are a valuable tool for measuring extremely fast reaction rates with very small particles, but they require the feedstock to be delivered pneumatically at a very consistent rate. The feeding system developed in this thesis is capable of feeding a variety of organic and inorganic particles with a diameter of less than 150 µm, and has successfully fed milled biomass containing a high fraction of hard to feed, high aspect ratio particles. It has been successfully used in several studies in our lab to measure gasification kinetics with a variety of feedstocks.

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