Chemical looping combustion (CLC) is an emerging technology for clean energy-production from fossil and renewable fuels. In CLC, an oxygen carrier (typically a metal) is first oxidized with air. The hot metal oxide is then reduced in contact with a fuel in a second reactor, thus combusting the fuel. Finally, the reduced metal is transferred back to the oxidizer, closing the materials “loop”. In this way, CLC produces sequestration-ready CO2-streams without expensive air separation and hence without significant energy penalty. Combined with sequestration, CLC thus allows high-efficiency, CO2 emissions-free combustion of fossil fuels, or combustion processes with negative CO2-footprint from biomass-derived fuels. However, CLC suffers from insufficient stability of typical metal-based carrier materials at high-temperature redox conditions.

We previously demonstrated that the embedding of metal nanoparticles into a ceramic matrix can result in unusually active and sinter-resistant nanocomposite materials which combine the high reactivity of metals with the high-temperature stability of ceramics. The resulting materials show very fast oxidation and reduction kinetics, and were stable in multiple redox cycles in the temperature range of interest for chemical looping (~700 – 1000oC).

Here, we report on the effect of sulfur contamination in the fuel stream on the performance of different oxygen carriers in CLC. Most of the published work on oxygen carriers to-date has focused on their performance in “idealized”, contaminant-free fuel streams. However, realistic fuel streams contain significant amounts of contaminants, in particular sulfur (mainly in the form of H2S), which can significantly impact on the performance of CLC via sulfidation of the carrier material. Furthermore, metal sulfides often have lower melting points than the corresponding metals or metal oxides, and thus put an additional constraint on the operating temperature of a CLC process.

We studied the effect of H2S contamination on the performance of a nanostructured Ni-barium hexaaluminate carrier at different temperatures in CLC. Thermodynamic calculations were combined with experimental investigations. We found that in addition to sulfidation of the metal carrier, sulfidation of the support structure also needs to be considered. While metal sulfidation is a reversible process, the sulfidation of the support is irreversible and can result in structural changes in the carrier material. We are currently further investigating the effect of sulfur contaminants on the kinetic behavior of these carriers in cyclic-TGA (thermogravimetric analysis) experiments with coal-derived synthesis gas.

Preparation and characterization of the carriers before and after exposure to sulfur contaminants, as well as their performance in CLC will be discussed in detail in the presentation.