Porous chromite spinels with nickel nanoparticle exsolutions and application in waste gasification

Biomass and mixed plastic waste are ideal source of energy and synthesis gas for producing chemicals in a circular economy. Currently these waste streams are principally landfilled or used for energy recovery by incineration and steam turbine generator, with an electrical efficiency of 20-30 %. Gasification offers a more flexible recovery route, where the produced gas can be used for electricity production (with a similar system efficiency). Moreover, the gas can be purified and stored, distributed in heating networks, or separated to obtain higher value components like ethylene and hydrogen. One problem with waste gasification is the range of by-products, spanning from heavy tars to volatile organic compounds. These complicate gas transport, separation and safety. To address this, broad-spectrum catalysts are sought to accelerate thermal cracking and steam reforming of said by-products. Chromite spinels are known oxidation catalysts which are here investigated for tar reforming, with a focus on self-supported porous monolithic forms. Investigations cover solid-state chemistry, sintering behaviour and shaping of this class of materials, building up the otherwise very limited body of literature on chromite spinel oxides. Additionally sealing with glass-ceramics is briefly explored. The reductive exsolution of nickel metal particles on the surfaces of chromite spinels was also investigated. While the particles produced were effective in steam reforming of light hydrocarbons, adherence to the spinels was poor and their effect on tar reduction was minimal. Finally, the products of gasification were analysed for a range of temperatures, space velocities and catalysts. Tars and volatile organic compounds could not be eliminated but only reduced. To that effect, nearly all resulted in a better tar gasification than uncatalysed gasification but no catalyst tested had a clearly outstanding performance. Simple magnesium-excess magnesium chromite alone was arguably the most effective, if not the simplest.