Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Engineering Science

Program

Chemical and Biochemical Engineering

Supervisor

de Lasa,Hugo

Abstract

The present MESc thesis reports the performance of a fluidizable CeO2 promoted Ni/γ-Al2O3 catalyst to be used in a post-gasification process for tar removal. The catalysts developed were prepared using the incipient wetness technique and characterized chemically and physically, using the following techniques: N2 Adsorption-Desorption, XRD, NH3 and CO2 TPD, Pyridine-FTIR, H2-TPR, and H2-Pulse Chemisorption. The catalysts were tested in a fluidized CREC Riser Simulator, in the 500°C-550°C temperature range, within 5 s-10 s reaction times, using both steam and steam-H2/CO2 atmospheres.A 2-methoxy-4-methlyphenol (2M4MP) compound was used as a biomass derived tar surrogate.

The CeO2 promoter helped to reduce the strong and very strong acidity of the γ-Al2O3 support, without significantly affecting the specific surface area of the alumina support. However, the specific surface area of the precursor catalyst decreased moderately, with the increase of the Ni loading. Using the developed Ni-CeO2-/γ-Al2O3 catalyst, runs with the 2M4MP were conducted in the CREC Riser Simulator. These runs showed a high overall 2M4MP conversion, comparable to those of the thermal non-catalytic runs. There was, at the same time, a significant desirable reduction of the C1+ hydrocarbons product species selectivity, with a simultaneously high CO2/CO and H2/CO ratios. These two high ratios are indicators of a strong water-gas shift reaction influence, with H2/CO larger than 2 yielding a valuable syngas for methanol synthesis.

Summary for Lay Audience

Greenhouse gas emissions from fossil fuels are responsible for climate change. Over the years, the scientific community, industries, and governments have been looking for ways to reduce their impact. One of the alternatives is the use of renewable sources of energy, such as wind, solar, or hydro. Another option is the use of biomass, which is considered carbon neutral. Gasification is a thermochemical process that can be implemented for the valorization of different types of biomasses, such as agricultural residues, to obtain biofuels, gases, and chemicals. However, the chemical reactions that take place during gasification can lead to the formation of tars, due to the complex chemical structures of the biomass constituents (cellulose, hemicellulose, and lignin). Tars are chemical compounds that reduce the quality of the gas and the efficiency of the process. There are different alternatives for tar removal. These options can be designated as primary (inside the gasifier) or secondary (as a downstream process).

In this present study, a set of catalysts, composed of Ni as the active phase, CeO2 to enhance chemical and physical properties and γ-Al2O3 as base, were synthesized to study their performance, and their possible use in a post-biomass gasification process. These catalysts were engineered for both tar removal and syngas enrichment, to produce a syngas ready for alcohol synthesis. Experiments were developed in a CREC Riser Simulator, bench-scale mini-fluidized reactor. Experiments were run at different operating conditions such as reaction time and temperature, showing the value of the catalysts developed given they promote efficiently the steam cracking and water gas shift reactions.

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