(641c) Fundamentals of Multiphase Reaction Engineering for CO2 Free Hydrogen Production from Natural Gas

Hydrogen production is an essential component of the refining and petrochemical industries. Steam-methane reforming (SMR) is the primary method of hydrogen production in the U.S. Sorption enhanced steam-methane reforming (SE-SMR) is an attractive adaption of SMR that reduces operating temperature and down-stream processing. The primary drawbacks of SE-SMR materials are incomplete carbonation, slow carbonation kinetics, loss in activity upon cycling, and high regeneration temperature.

Nanofibrous calcium oxide, synthesized via electrospinning, was compared against calcium oxide synthesized via calcium acetate decomposition, hydrothermal synthesis with the aid of a surfactant, and natural sources. A thermogravimetric analyzer found that the sorption capacity of calcium oxide from nanofibers to outperform other samples, reaching stoichiometric capacity 0.79 g_CO2/gs_orbent after 1 hour at 600ºC, 1 atm, and 100% CO₂. The capacity of calcium oxide nanofibers and CaO derived from natural sources were measured over repeated carbonation and calcination cycles. Both calcium oxide nanofibers and natural calcium oxide lost ~33% of their initial capacity after ten cycles. To mitigate these capacity losses, various metal nitrates were added to the calcium nitrate electrospinning solution, at various compositional ratios, as dopants to increase the stability of the sorbent. The dopants studied comprised of metals from Groups 2, 3, 4, 12, and 13 on the periodic table of elements. These doped-sorbents were shown to have various positive and negative effects on capacity and stability. Generally, sorption capacity increased with the Tamann temperature of the additive metal, the most stable additives had a 3⁺ oxidation state, and various additive metals reacted with CO₂ to increase the sorption capacity compared to non-carbonating additives. Doped and undoped samples were characterized with SEM, BET, XRD, and TEM to identify the properties that influence capacity, kinetics, and stability.

SE-SMR studies verified that the samples with higher capacity indeed had longer breakthrough times, with calcium oxide nanofibers having the longest breakthough time. After 10 cycles additive modified calcium oxide outperformed the pure calcium oxide nanofibers due to loss of sorption capacity. Reactor studies were carried out to determine the transport effect on breakthrough time. In these studies, the of calcium oxide sorbent and nickel catalysts were varied from nanometer scale, using a single material, to millimeter scale, using catalyst pellets and sorbent pellets. The experimental results are complimented modeling the behavior based on the material sorption kinetics and capacity.