UCLA

This work focuses on improving material deficiencies in calcium oxide CO2 sorbents for applications in sorption-enhanced steam methane reforming (SE-SMR). Challenges such as low sorption conversion and low stability of calcium oxide sorbents were addressed with studies on material synthesis and material chemistry. Calcium oxide decomposed from electrospun PVP-Ca(NO3)2 nanofibers lead to higher conversions and higher carbonation kinetics compared to calcium oxide from marble and synthesized via decomposition and hydrothermal treatment. Favorable material properties such as large macropores, high surface area and porosity, and small agglomerate size were enabled by electrospinning and led to the improved material performance. Electrospinning led to calcium oxide reaching stoichiometric capacity, 0.79 gCO2 gCaO-1, at 600 and 650�C in 100% CO2.

In order to address the low stability of calcium oxide sorbents, metal additives were incorporated in the electrospinning solution so that upon nanofiber decomposition there was a mixture of additive metal oxide and calcium oxide. A systematic study was carried out on additive oxides using Al, Co, Cr, Er, Ga, In, La, Li, Mg, Nd, Y, and Zn additives to probe which properties impact the sorption capacity and stability of modified calcium oxide. Material properties such as molecular weight of the additive oxide or the formation of a mixed oxide (MxCayOz) and the M:Ca ratio directly impact the sorption capacity of the modified sorbents due to the effect of dilution and consumption of CaO in the mixed oxide. The Tammann temperature of the additive oxide was found to have a strong effect on the conversion of the modified sorbent. If the Tammann temperature was higher than the maximum treatment temperature the sorbent approached 100% conversion, however if the Tammann temperature was lower than the maximum treatment, conversion decreased with decreasing Tammann temperature. Further, Tammann temperature was found to be the primary material property influencing the stability of the modified sorbent and the formation of mixed oxides did not impact material stability. The stability of the modified sorbents increased with increasing Tammann temperature of the additive oxide. By optimizing the metal loading the alumina modified CaO-nanofibers, 1Al-20Ca-O, reached 0.75 gCO2 gCaO-1, while being an order of magnitude more stable during carbonation-regeneration cycling compared to unmodified CaO-nanofibers.

Calcium oxide was also tested in a packed bed reactor in CO2 breakthrough experiments and in bench-scale SE-SMR experiments. The CO2 breakthrough experiments show that calcium oxide sorbents are even less stable when they carbonate in the presence of steam. SE-SMR experiments showed that the sorption capacity determined in the TGA related directly to the CO2 breakthrough time during the reforming reaction, with CaO-nanofibers having the highest breakthrough time and CaO-marble having the lowest breakthrough time. Further, the modified 1Al-20Ca-O sorbent lost only 5% of its CO2 breakthrough time over 10 cycles, indicating that modified sorbents retain their stability benefits under realistic SE-SMR operating conditions.