Sorption-Enhanced Steam Reforming process (SESR) is an auspicious technology for hydrogen (H2) production with simultaneous capture of carbon dioxide (CO2) derived from design modifications performed on the conventional Steam Reforming Process (SRP). Enhancing the reforming reaction in terms of kinetics, yield and purity through capturing in situ CO2 using high temperature solid sorbents brings advantages including reduction in energy requirements and lower investment capital. Even though SESR is a cost-effective route for energy generation based on organic volatile and gaseous feedstock including natural gas, its implementation implies overcoming challenges. Diminishing the sintering in CaO-based CO2 sorbents has become one of these challenges since the reactivity of these captors decreases significantly as the number of carbonation/calcination cycles proceeds.
This research work centres its efforts in the development of novel sintering resistant CaO-based CO2 sorbents enhanced by means of the incorporation of a refractory, high surface area, polycrystalline fibrous support (Saffil), with high-Al2O3 content, acting as structural stabilizer of CaO. Four families of CaO-based CO2 sorbents were prepared using wet impregnation and precipitation methods. Different variants such as CaO precursor, precipitant agents, pH, stirring, aging time, etc. were tested to optimize the synthesis parameters. The best preparation conditions were aimed at achieving a homogeneous deposition of CaO over the Saffil support as well as a morphology in CaO that might improve the durability of CO2 acceptors. In particular, the formation of nanoflakes, and particles with an octahedral shape were found to be two of the most promising morphologies.
Upon the determination of optimal synthesis parameters, the as-prepared CO2 sorbents were characterized in order to determine their physicochemical properties such as textural features (surface area and pore size distribution – N2 physisorption), real CaO content (XRF), dispersion of CaO over Saffil supports (SEM-EDX), phase identification (XRD), etc.
Carrying capacities and durability of CaO-based CO2 sorbents were assessed through multicycle carbonation/decarbonation tests under controlled conditions such as temperature and atmosphere. The dynamic/isothermal experiments conducted in a TGA system confirm an enhancement in reactivity when CaO grows over the periphery of the Saffil support. In addition to the use of a support, achieving a ‘clamping effect’ (to diminish lateral mobility of CaO, thus avoiding particle densification), in conjunction with the morphology adopted by CaO, is shown to provide thermal stability. SEM-EDX techniques applied on used CaO-based sorbents (30 carbonation-calcination cycles) corroborate that the enhancement in durability is due to the outstanding sintering resistance exhibited when CaO adopted the nanoflake or octahedral structure.
The viability of the as-prepared sorbents was also confirmed through a kinetic study in which kinetic parameters and mechanisms associated with both carbonation and calcination reactions were estimated. Concerning the CO2 uptake kinetics, the isothermal method was used to collect mass change data whilst model-based equations were employed to elucidate the kinetic triplet. For the calcination reaction, the kinetic study was performed using both isothermal and non-isothermal methods. Activation energies assessed for the carbonation and calcination reactions were compared among them and also in relation with other CaO-based sorbents available in the literature for reliability purposes.
