Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Professor Paul A. Charpentier

Abstract

Gasification of waste biomass to form hydrogen, H2, is a promising new source of green energy; while providing the additional benefit of treating challenging and hazardous waste streams that pollute the environment. Gasification of biomass in supercritical water (SCW) offers an attractive alternative to avoid the energy intensive drying process. In this approach, biomass is hydrolyzed by water into smaller molecules in the presence of a suitable catalyst. This study was aimed at developing an alumina supported nickel based non-noble metal catalyst suitable for biomass gasification in SCW. A lack of detailed characterization on fresh and spent catalysts in SCW has held back progress in this field and is critical due to the highly unusual properties of SCW at high pressure and temperature compared to ambient water. Typically hydrogen rich gaseous product from gasification of biomass in SCW requires temperatures higher than 700 °C, while low temperature processes (300-500 °C) produce methane rich gases. Use of suitable catalysts can lower the activation energy of the reaction, and hydrogen rich gaseous products can be achieved at low temperatures thus lower the operating cost. Use of suitable catalysts also can reduce the formation of chars and tars formed during the gasification process in SCW. Moreover, non-noble catalysts could be beneficial in terms of availability and cost. A kinetic study of SCW gasification is still under development due to the numerous intermediate and final products and complex reaction pathways.

In this research, supercritical water gasification (SCWG) and partial oxidation (SCWPO) of a model biomass compound was studied to produce hydrogen rich syngas at lower temperatures (400-500 °C). In this respect non-noble nickel catalysts were synthesized, evaluated and characterized (fresh and spent) to study the catalyst role in SCWG. The catalysts studied were synthesized via incipient wetness impregnation of metal salts on synthesized θ-alumina nanofibers and commercial gamma alumina (converted to theta) pellets (3mm average diameter) as catalyst supports. To synthesize nano structured catalyst supports (alumina nanofibers); a one-pot sol-gel route in scCO2 was adopted without using any hazardous organic solvents, surfactants or other additives for the first time. Aerogel nano catalysts were also directly synthesized via a sol-gel technique using isopropanol as solvent and supercritical carbon dioxide (scCO2) as the drying agent.

In this research, it was found that introduction of oxidant after gasification is beneficial in terms of gaseous products and reducing the chemical oxygen demand (COD) in the liquid effluents. Another finding is that nickel (Ni) loading on alumina above 11 wt% consumed carbon dioxide with a simultaneous increase in methane attributed to hydrogen consumption by the methanation reaction. However, lanthanum (La) modified Ni/θ-Al2O3 enhanced production of hydrogen by retarding the methanation reaction and promoting the water gas shift (WGS) reaction. In addition, adsorption of CO2, one of the main products, by La was attributed to shifting the reaction equilibrium to the products and thus contributed to enhance hydrogen production.

Nano catalysts showed higher activity towards hydrogen production, carbon gasification efficiency and total organic carbon (TOC) destruction in the liquid effluent compared to coarser heterogeneous catalysts. However, hydrogen production using aerogel catalysts where metals were loaded directly through sol-gel reaction was found comparatively less than nanofiber catalysts where metals were impregnated on the nano support. This phenomenon was attributed to the formation of Ni-La-Al-O nano structure complex by direct addition of metals during sol-gel reaction. Unlike impregnated catalysts, incorporation of La to the main structure of the sol-gel derived catalysts could not contribute to enhance the WGS reaction.

The fresh and spent catalysts were characterized using different physicochemical techniques which revealed that the catalysts were active in SCW even though the metallic sites of nickel agglomerated when exposed to SCW conditions, oxidized and reacted with the support alumina. It was found that lanthanum retards the formation of graphitic coke, and adsorbed carbon dioxide during supercritical water gasification.

To our knowledge, hydrogen yield, total organic carbon destruction and gasification efficiency were significantly higher using La modified Ni/θ-Al2O3 nano catalyst fibers than that of any other reported results of SCWG of any biomass compound at moderate temperatures (~500 °C) and pressures (~28 MPa). However, exposing the nanofiber catalysts to the SCW environment led to disintegration of the fibrous structure.

A global kinetic model for TOC destruction in supercritical water was developed using non-linear regression, which convincingly fit with the experimental results.


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