Hydrogen is increasingly recognized as essential in the shift to renewable and sustainable raw materials, especially for decarbonizing energy and industrial sectors. Currently, hydrogen production largely relies on fossil fuels, underscoring the need to shift towards renewable sources like biomass. The gasification of biomass with supercritical water (SCWG) is particularly promising for converting organic biomass into hydrogen, utilizing feedstocks from diverse origins with a high water content. This thesis presents the development and laboratory-scale testing of a novel process for maximizing hydrogen yield from the SCWG of waste biomass model compounds. The designed process integrates SCWG with the subsequent steam reforming of hydrocarbons across four reactors in series: the SCWG reactor, a pre-reformer, a main steam methane reformer (SMR), and a water-gas shift (WGS) reactor. In the SCWG reactor, actual wet waste biomass or model compounds such as ethanol are gasified in supercritical water. For the purposes of this thesis, ethanol has been used, since its use provides a constant operation with high gasification efficiencies, similar product gas compositions with several waste biomass resources and without the need of additional gas cleaning units and technical constraints, e.g., clogging. ... mehrThe pre-reformer then uses a heterogeneous Ni-based catalyst to reform the heavier hydrocarbons than methane, produced during SCWG. The SMR completes methane reforming at elevated temperatures, while the WGS reactor promotes additional hydrogen production by converting residual CO into CO2 and H2.
In the first part of the experimental study, the first two reactors, i.e., the SCWG and the pre-reformer were tested. Ethanol was fed into the SCWG reactor, and various temperatures, pressures, and gas hourly space velocities (GHSV) were tested in the pre-reformer. The remarkable findings of this study include achieving complete conversion of hydrocarbons at relatively moderate reforming temperatures and that the excess steam from the SCWG reactor minimized coke formation on the catalyst but caused Ni crystallite sintering, resulting in minor activity loss after a long-term experiment.
The main steam methane reformer (SMR) was then integrated to the system, downstream of the pre-reformer and tested under different conditions, with two Ni-based catalysts, a NiO(14wt.%)/CaAl12O9 and a NiO(18wt.%)/CaK2Al22O34 catalyst.
Both showed minor coke deposition due to excess steam, though Ni sintering occurred at high temperatures. The NiO(18wt.%)/CaK2Al22O34 catalyst showed higher activity due to its higher Ni loading. Although carbon formation was minimized, the high S/C ratio, together with the high temperatures applied and the rather weak support interactions accelerated the sintering of the active metal. The concentration of ethanol in the feed to the system and its effect on the final product gas was also tested here. Increasing the ethanol concentration in the SCWG feed from 5 to 20 wt.% lowered the steam-to-carbon (S/C) ratio, causing a rise in CH4 and other carbonaceous species, which shifted SMR and WGS reactions equilibria, significantly reducing CH4 conversion and H2 yield. However, despite the lower S/C ratio, coke formation remained minor, indicating the stability of this multi-step process in processing organics at high concentrations.
Following the successful lab-scale demonstration, the process was theoretically scaled up using a process simulation software, and a techno-economic analysis evaluated its profitability and competitiveness in hydrogen production. Results showed a remarkable profitability improvement by increasing the concentration of ethanol in the feed from 8 wt.% to 20 wt.% ethanol. A concentration of 15 wt.% is recommended, balancing technical challenges like poor gasification efficiency, coking and catalyst poisoning in downstream catalytic reactors. Scaling up to 50 t h-1 drastically reduced the costs of the produced hydrogen. When using waste biomass, which is the ultimate objective for this developed process, like sewage sludge, feedstock availability becomes crucial. For instance, a 50 t h-1 plant processing sewage sludge at a 15 wt.% concentration would require disposal capacity on the scale of a city like Hamburg, Germany.
A sensitivity analysis showed that the ethanol price had the most profound impact on the hydrogen price. Thus, a more detailed investigation of the influence of the feedstock price on the price of the produced hydrogen was carried out. Applying negative feedstock prices, i.e., considering a revenue from processing sewage sludge, significantly lowered the produced hydrogen price to 0–1.8 $ kgH2-1, making it competitive with natural gas steam reforming. Although real waste biomass processing needs additional equipment, reduced feedstock costs could offset these expenses, as indicated by the sensitivity analysis.
This thesis encompasses the development of a novel chemical process, followed by detailed laboratory experiments to prove its feasibility and offer insights into the activity and stability of heterogeneous Ni-based catalysts within a distinctive steam reforming reaction system, with the absence of similar systems in the literature. Finally, it addresses the process scale-up, including the design of all essential units, from reactors to heat exchangers, and concludes with an exergy and techno-economic analysis. This work thus covers a broad scope of research in chemical and process engineering.
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