Sorption enhanced steam reforming of model compounds of aqueous fraction of bio-oil using calcium-doped nickel and oxide catalysts and mineral residue for hydrogen production

Abstract

The present doctoral thesis evaluated the use of mining waste (MW) as an adsorbent in combination with Ni-Al2O3 and Ni-MgAl2O4 catalysts for the sorption enhanced steam reforming (SESR) of ethanol. Additionally, nickel catalysts supported on calcium and zirconium oxide, modified with alkali metals, as well as bifunctional nickel catalysts supported on CaO and alumina (NiCaAl) and those doped with ceria (NiCaAlCe), were employed in the SESR of model compounds of the aqueous fraction of bio-oil. Thermodynamic analysis by NASAS’s Chemical Equilibrium with Applications (CEA) and simulations using Aspen Plus software were developed to support the optimisation of the process. These catalysts were utilised to achieve high-purity H2 production and enhance the efficiency of the SESR process. Using MW as an adsorbent in the SESR process offers a sustainable method for reducing CO2 emissions and managing waste from processes. A thermodynamic analysis led to selecting operational parameters. Ni-Al2O3 and Ni-MgAl2O4 catalysts were prepared using incipient wetness impregnation and coprecipitation methods, respectively, and mixed with MW at a 1:1 mass ratio. Characterisation of the samples involved X-ray diffraction (XRD), temperature programmed reduction (TPR), thermogravimetric analysis (TGA). After 10 SESR cycles, both catalysts achieved over 99% bioethanol conversion. The Ni-Al2O3/MW catalyst produced a maximum H2 molar fraction of 82.2%, and the Ni-MgAl2O4/MW catalyst reached 75.8%. The pre-breakthrough (PB) stage duration remained stable over 10 cycles, indicating good material stability. In addition, SESR of model compounds of aqueous fraction of bio-oil fractions were also carried out using two series of catalysts. The first series included bifunctional catalysts based on nickel supported on calcium and zirconium oxide modified with alkali metals (Na and K). Thermodynamic analysis was conducted for conventional steam reforming (CSR) and SESR, focusing on temperature, adsorbent addition, and steam/carbon ratio. The SESR at 500°C and a steam/carbon ratio of 2.25 yielded a high H2 molar fraction. Nickel-based bifunctional catalysts on calcium and zirconium oxide, modified with alkali metals were prepared using the citrate sol-gel method. TPR, XRD, TGA, X-Ray Fluorescence (XRF), and N2 physisorption were performed. TGA revealed that the NiCaNaZr catalyst had the lowest CO2 capture capacity loss (21%). All catalysts achieved over 99% ethanol and 95% acetic acid conversion, with high H2 molar fractions (> 90 mol%) during the PB stage. Alkali metal doping extended the PB time significantly, with NiCaKZr and NiCaNaZr showing 30 and 40 min, respectively, compared to 18 min for the non-doped catalyst. Sintering was observed in spent catalysts, indicated by increased crystallite size of Ni0 and CaO. The second series consisted of bifunctional nickel catalyst supported on CaO and alumina, as well as nickel catalyst supported on CaO and alumina doped with ceria were prepared using the citrate method. XRD, XRF, TGA and N2-physisorption were performed. XRD identified CaO, Ca(OH)2, and Ni0 in both catalysts, with an additional CeO2 peak in NiCaAlCe. TGA showed NiCaAlCe had a smaller decrease in CO2 capture capacity (24%) than the ceria-free catalyst (31%) after 20 cycles. SESR achieved over 99% ethanol and 92% acetic acid conversion, with hydrogen molar fractions above 90%. Although ceria improved surface area, it didn’t significantly impact hydrogen yield or PB time. The simulations showed that SESR outperforms CSR with higher hydrogen yields (0.70 vs. 0.43 mol H2/mol bio-oil) and lower CO2 emissions in both the steam reforming process (10.34 vs. 27.14 kg CO2/kg H2) and overall plant scale (25.12 vs. 25.73 kg CO2/kg H2). However, SESR had a lower thermal efficiency (0.32 vs. 0.49) due to its higher energy demand, requiring more biomass burning.