Performance of Na2CO3-CaO sorbent in sorption-enhanced steam methane reforming

Abstract

The addition of alkali molten salts has been reported as a strategy to overcome the sintering problem presented by calcium oxide in CO2 capture systems. In this work, the influence of sodium doping in a CaO sorbent was investigated in the sorption enhanced steam methane reforming process (SE-SMR). The goal was to increase the stability of the sorbent and, consequently, the efficiency of the process. For that, a Na-containing sorbent was prepared using the precipitation technique and a pure calcium oxide was obtained by calcination of CaCO3. The sorbents were physically mixed with 10 % Ni/Al2O3 catalyst and tested in 10 cycles of SE-SMR at 600 °C and CH4:H2O equals to 4. In general, both materials showed 100 % of CH4 conversion and H2 molar fraction of 93.5 vol.%. However, regarding the stability over the SE-SMR cycles, it was evidenced that the addition of sodium decreased the duration of pre-breakthrough comparing with the non-doped material. The XRD, SEM, and TGA results allowed us to observe an inverse relationship of particle diameter and CO2 capture performance. Na2CO3-CaO presented a larger average crystallite size compared to the pure CaO which led to a higher probability of the CaCO3 layers to inactivate the calcium oxide and, consequently, caused a strong sintering effect. Besides the presence of sodium, the precipitation method and the synthesis conditions could have favored the low initial CO2 uptake and poor stability of the Na2CO3-CaO sorbent.