Carbon formation on iron-based oxygen carriers during CH4 reduction period in Chemical Looping Hydrogen Generation process

- Carbon deposition process on Fe2O3/Al2O3 in methane reduction is investigated.
- The active phase Fe or Fe3C in the reduction results in the carbon deposition.
- Carbon deposition is affected by temperature and methane concentration.
- MgAl2O4 and alkali additive K2CO3 could alleviate the carbon deposition.
- Carbon deposited is in the form of iron carbide.

Chemical Looping Hydrogen Generation (CLHG) is promising hydrogen production technology with inherent capture CO2 and high-purity hydrogen production simultaneously. Although the reactivity and durability of oxygen carriers can be improved by the combination of inert materials. Carbon deposition over oxygen carrier in the reduction period is still a serious problem in the process of CLHG with CH4 as fuel, and it affects the purity of hydrogen in steam oxidation period and the carbon capture efficiency. In this study, oxygen carriers, Fe2O3/Al2O3 and Fe2O3/MgAl2O4, composed of 60 wt% Fe2O3, were prepared by mechanical mixing method. Moreover, different contents of K2CO3 were incorporated into the Fe2O3/Al2O3 oxygen carriers by using impregnation method. Experiments were performed in a fluidized bed reactor by exposing the oxygen carriers to alternating methane reducing, steam oxidizing and air oxidizing conditions. The effects of inert supports and alkali additive on the carbon formation performance of iron oxides with different methane concentration were investigated. Results indicate that carbon was formed on the oxygen carriers Fe2O3/Al2O3 during the reduction, the cracking reaction of CH4 on the surface of iron-based oxygen carrier starts as active phase Fe or Fe3C formed. However, the methane decomposition promoted the reduction rate of oxygen carriers and it increased with the temperature and CH4 concentration. Lower carbon deposition rate was found on Fe2O3/MgAl2O4 compared with Fe2O3/Al2O3. Furthermore, the addition of K2CO3 exhibited a beneficial effect on suppressing carbon formation but decreased the reduction activity of oxygen carrier.