Hydrogen production by methane cracking using Ni-supported catalysts in a fluidized bed

Nickel, supported on porous alumina (γAl ₂O ₃), non-porous alumina (αAl ₂O ₃), and porous silica, was used to catalyze methane cracking in a fluidized bed reactor for hydrogen production. The effects of temperature, P _CH4, and particle diameter, and their interactions, on methane conversion were studied with each catalyst. Temperature was the dominant parameter affecting the hydrogen production rate for all catalysts and particle diameter had the strongest effect on the total amount of carbon deposited. Maximum methane conversion as a function of support type followed the order Ni/SiO ₂ > Ni/αAl ₂O ₃ > Ni/γAl ₂O ₃. Nonetheless, better fluidization quality was obtained with Ni/γAl ₂O ₃. Methane conversion was increased by increasing temperature and particle size from 108 to 275 μm due to better fluidization achieved with 275 μm particles. Increasing the flow rate and methane partial pressure (P _CH4) caused a drop in methane conversion. Tests were also run in a fixed bed reactor, and at constant weight hourly space velocity (WHSV), higher conversion was achieved in the fixed bed, but at the same time faster deactivation occurred since a higher methane conversion led to increase in carbon filament and encapsulating carbon formation rates. A critical problem with the fixed bed was the pressure build-up inside the reactor due to carbon accumulation. Finally, a series of cracking/regeneration cycle experiments were carried out in the fluidized bed reactor. The regeneration was performed through product carbon gasification in air. Ni/αAl ₂O ₃ and Ni/γAl ₂O ₃ activity decreased significantly with the first regeneration, which is attributed to Ni sintering during exothermic regeneration/carbon oxidation. However, Ni/SiO ₂ was thermally stable over at least three cracking/regeneration cycles, but mechanical attrition was observed.