H2 production from sorption enhanced steam reforming of biogas using multifunctional catalysts of Ni over Zr-, Ce- and La-modified CaO sorbents

Abstract

Sorption enhanced steam reforming of biogas is a promising process for H2 production by which biogas steam reforming is combined with simultaneous CO2 capture in a single unit operation. In the first part, Zr4+, Ce4+ and La3+ were doped on CaO and tested the performance for 10 CO2 sorption/desorption cycles. Zr-CaO and Ce-CaO offered a stable CO2 capture activity for at least 10 cycles (CO2 sorption capacity of 0.30 and 0.22 gCO2/gsorbent at the 1st cycle, and 0.29 and 0.23 gCO2/gsorbent at the 10th cycle for Zr-CaO and Ce-CaO, respectively). On the other hand, La-CaO indicated decreasing of CO2 sorption capacity about 16% over 10 cycle operations. This is due to the Zr-CaO and Ce-CaO presented oxygen storage at low temperature which is characterized by temperature program reduction oxidation (TPRO). This behavior promotes the CO2 diffusion from the surface to the bulk (sorption) and the bulk to the surface (desorption) of the modified CaO. In the second part, Ni2+ was loaded on the modified sorbents for H2 production. Ni-Zr-CaO and Ni-Ce-CaO can enable the production of H2 with purities of approximately 90% and 85%, respectively, with repeating cycles; however, the CH4 conversions decreased significantly over five cycles (the post-breakthrough period CH4 conversion of Ni-Zr-CaO and Ni-Ce-CaO decreased from 65.6 and 56.3% during the 1st cycle to 33.0 and 28.9% during the 5th cycle). This is because the Ni active sites at the surface of multifunctional catalysts were lost (as confirmed by XPS results) by pore closure (as shown in SEM images) which was possibly due to the CaCO3 formation.