Utilisation of residues from biodiesel production in fuel cells

Abstract

With the achievements recorded in the development of fuel cell technology, arguably hydrogen-driven systems would replace the combustion fossil fuel-based systems in the nearest future for safer and pollution-free environment. The much desired renewable and sustainable hydrogen infrastructure to replace or complement the fast-depleting fossil-based hydrogen fuel for the full commercialisation of fuel cell could be achieved through catalyst development and gasification of by-product glycerol glut from biodiesel production activity seen as a waste at the moment.

In this study, the development of catalysts for the conversion of biomass vegetable oil via transesterification reaction to biodiesel has been explored in parallel to the subsequent catalytic gasification of pure and by-product glycerol from biodiesel synthesis to generate hydrogen-rich gases for utilisation in fuel cells.

Reaction of tricalcium aluminate (C3A) with adsorbed water vapour under controlled hydration process at elevated temperatures was found to modify its surface morphology by formation of strongly basic hydroxide products. This was found to increase its surface basic strength and ability to catalyse transesterification reaction to biodiesel for the first time. Furthermore, basic alkaline earth metal oxides MgO, SrO and transition metal oxide ZnO that are known to catalyses transesterification reaction but suffered deactivation due to profuse leaching were doped and incorporated into the non-hydrated tricalcium aluminate (C3A) lattice structure. The doped catalysts were found to be not only active and selective to biodiesel formation but also resistant to deactivation by leaching of the doped active metals for the first time.

The rapid deactivation of the nickel-based catalyst Ni/Al₂O₃ due to carbon deposition; agglomeration and phase transformation especially during prolonged high temperature operations using feedstock glycerol in steam reforming was minimised through the use of promoters such as ceria (CeO₂) and LSCM (La₀.₇₅Sr₀.₂₅Cr₀.₅Mn₀.₅O[sub](3-δ)) and alternative supports such as samarium-doped ceria (Ce₀.₈Sm₀.₂O[sub](2-δ)) and zirconia-doped ceria (Ce₀.₇₅Zr₀.₂₅O₂). This led to the development of a new catalyst system NiLa₀.₇₅Sr₀.₂₅Cr₀.₅Mn₀.₅O[sub](3-δ)/

Ce₀.₇₅Zr₀.₂₅O₂ (Ni-LSCM/Ce-Zr) which was found to be very active and offered much better suppression of carbon deposition and agglomeration minimizing catalyst deactivation. However, the work revealed that, the ‘traditional’ wet impregnation method does not offer sufficient control over particle size, growth and distribution. It takes time, is costly and results in weak interaction between the active phase metal catalyst particles and support leading to agglomeration, instability and deactivation at times even where a promoter was used; hence this offered poor catalytic properties.

This study has demonstrated for the first time the use of a new phenomenon called redox lattice reorganisation and already known redox exsolution as alternative methods to wet impregnation in the preparation of oxide-supported nickel-based metal catalysts in glycerol steam reforming (GSR). The work has revealed that unlike what happens with the traditional wet impregnated catalysts where metal catalyst superficially interact with the oxide support resulting in catalyst deactivation due to agglomeration and carbon deposition or phase transformation. Redox lattice reorganisation in spinel has shown that metal catalyst particles can be grown out from the support itself and firmly anchored on the spinel oxide support leaving behind elaborate macro porous channels. That provides good surface area, strong metal support-interaction and reduced tendency for catalyst deactivation by agglomeration

and offered effective coking suppression and good catalytic behaviour. The work has

further

shown that particle size, population, metal-support interaction, size of the channels in redox lattice reorganisation can all be tailored for better catalytic behaviour by simple control of reduction temperature. The work revealed further that redox exsolution in perovskite; particle size and distribution, metal-support interaction and general morphology of the catalyst surface could be tailored for good catalytic performance through control of B-site doping, careful choice of dopant metals for both A-site and B-site cations and defect chemistry in glycerol steam reforming (GSR). The metal exsoluted catalyst systems were found to be not only active and selective toward the desired products but have also demonstrated great potentials to suppress carbon deposition.