Study and analyses of a CLC oxygen carrier degradation mechanism in a fixed bed reactor

•Determination of an oxygen carrier degradation mechanism.•Experimental study in fixed bed reactor.•Physical characterization of the material evolution.•Description of the causes of attrition and agglomeration phenomena.

This paper deals with the study, analysis and determination of the oxygen carrier degradation in the Chemical Looping Combustion of methane with NiO/NiAl2O4 as oxygen carrier. A fixed bed reactor fed alternatively with oxygen (5% in Argon) and a mixture of methane (5%) and steam (from 0.3% to 2% in Argon) has been used with. operating temperature ranging from 700 to 850 °C. Without fluidization, and with only 10 cycles of reduction/oxidation, a high degradation of the material has been observed using X-ray diffraction analysis, SEM observations and BET or PSD characterizations. It is shown that this degradation is caused by the carbon, produced by the methane cracking reaction catalyzed by reduced nickel, and deposited at the surface of the material. The contraction and expansion of the elementary particles composing the grains during the CLC reactions is also a cause of the observed degradations. These phenomena of contraction/expansion cannot be avoided, unless the reduction degree of the material is reduced during the cycles. Therefore, an increase of the operating temperature or of the feeding steam concentrations can lead to a decrease of the material degradation.