Nickel based catalysts for hydrogen production from the pyrolysis/gasification of refuse derived fuel (RDF)

Hydrogen can be used as fuel for power generation; however current hydrogen production processes are not sustainable as they involve considerable CO2
emissions, and are mostly based on production from fossil fuels. Municipal solid waste (MSW) in the form of refuse derived fuel (RDF) can be subjected to thermal processes such as pyrolysis and/or gasification to produce a hydrogen rich syngas. Nevertheless some operational problems associated with tar formation arise, which significantly reduces the overall process yield.
In this work a two-stage reaction system was used for hydrogen production and tar reduction, during the pyrolysis/gasification of RDF, using different types of
catalysts. Firstly RDF was pyrolyzed at 600ºC, the pyrolysis gases were then passed through a second gasification stage where the catalytic steam reforming
process took place at 800ºC, in order to generate hydrogen and promote tar cracking reactions. Different analytical techniques were used in this work to
characterise RDF, product gases, tars/oils, and fresh/reacted catalysts.
Initially two different Ni/Al2O3 catalysts were prepared, and their catalytic activity towards hydrogen production and tar reduction were assessed during the pyrolysis/gasification of RDF. The results were compared with those obtained using a bed of sand. Using a 10 wt.% Ni/Al2O3 catalyst, about 45 vol.% of hydrogen in the syngas was obtained together with other gases: CO2, CO, CH4, C2-C4. Also the condensed tar fraction was analysed and was found to contain polyaromatic hydrocarbon (PAH) constituents included naphthalene, fluorene
and phenanthrene as the major components. Additionally, diverse Ni/SiO2 catalysts were prepared using different synthesis methods, including sol-gel,impregnation and homogenous precipitation. Among the catalysts tested for tar reduction, a 20wt.% Ni/SiO2 catalyst presented the highest activity resulting in a tar concentration of 0.15mgtar g-1RDF; PAH and oxygenated tar compounds were also identified within the analysed samples. For the catalysts tested in relation to hydrogen production, using a 10wt.% Ni/SiO2 catalyst prepared by homogeneous precipitation-sol-gel based method, resulted in a H2 concentration of 59 vol.%. Finally different Fe/SiO2 and Ni/SiO2 catalysts were prepared using nano-porous silica as the oxide support. Better catalyst activity in relation to H2 production was observed for the Ni/SiO2 catalysts. However the maximum H2 concentration obtained was around 44 vol.%. It was found that using calcination temperatures higher than 700ºC, both the surface area and the catalytic activity for hydrogen production was diminished for this series of catalysts.