Catalytic Aqueous-Phase Reforming of Biorefinery Water Fractions

Abstract

Biorefineries can produce renewable fuels and chemicals through processes such as pyrolysis of lignocellulosic biomass or Fischer-Tropsch (FT) synthesis using syngas derived from biomass gasification. Although commercial-scale plants exist, the production costs of renewable fuels and chemicals are usually higher than the production costs of fossil-based fuels and chemicals. To improve the competitiveness of biorefineries, this thesis proposes processing the water fractions derived from biorefineries to optimise the production of renewable fuels and chemicals. Biorefinery water fractions include diluted oxygenated hydrocarbons that can be processed using aqueous-phase reforming (APR) technology to produce hydrogen, which is the main desired product, and other gases such as carbon dioxide, carbon monoxide and alkanes. This study tested different Ni-based catalysts, first in the APR of model compounds representative of pyrolysis liquid aqueous fraction (PLAF) and FT waters, and finally, in the APR of a real water fraction derived from FT synthesis. The APR of model compounds representative of PLAF, i.e. solutions of acetic acid, ethanol, 1-hydroxypropan-2-one and benzene-1,2-diol, resulted in low hydrogen yields and significant deactivation of different Ni-based catalysts. The hydrogen yield was around 10% from ethanol and close to 1% from the other oxygenates. The APR of FT water model compounds, i.e. solutions of C1-C4 alcohols over Ni-based catalysts, yielded relatively high amounts of hydrogen, ranging from 13% to above 100%, and 11% in the APR of real FT water. A copper-doped nickel catalyst supported on ceria-zirconia was selected to conduct the APR of real FT water under different operation conditions. The results were utilised to develop a kinetic model that could be applied through concept development to integrate APR into a FT synthesis process.The hydrogen produced in the APR of FT water could be used in the production of renewable fuels and chemicals to improve their production efficiency. Nickel-copper/ceria-zirconia catalyst is a suitable catalyst to process FT waters. Nonetheless, the catalyst composition should be further optimised to increase the hydrogen yield.