Ruggiero, Roberto (2025) Production of Synthetic Natural Gas from biomass derived syngas and through Power-to-Gas applications. [Tesi di dottorato]

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Abstract

Although fossil resources are currently the main source of natural gas, achieving the carbon neutrality target set by the European Union for 2050 requires a transition to alternative energy carriers. For this reason, the production of renewable methane (synthetic or substitute natural gas, SNG) is gaining increasing interest. In particular, when residual biomass is used as a feedstock, the amount of CO₂ emitted can be considered nearly equal to the CO₂ absorbed by the plants during their lifecycle, thus approaching the goal of carbon neutrality. After biomass gasification, syngas is obtained and can serve as the starting point for SNG production. Another possible route to the same final product is the conversion of carbon dioxide captured from industrial sources, combined with green hydrogen, following well-known Carbon Capture and Utilization (CCU) techniques.The objective of this project was to study, both from a modeling and experimental perspective, the main process steps required to obtain SNG as a valuable final product. In the first part of this work, a detailed thermodynamic analysis was carried out using Aspen Plus to investigate the cleaning stages required for the utilization of real syngas obtained from a gasification plant. Particular attention was given to the removal of tar compounds, which are among the most problematic for catalysts. An innovative wet-based cleaning method was proposed, achieving nearly complete removal of heavy tars and approximately 94% removal of lighter compounds such as benzene, toluene, and xylene. After the cleaning stage, a conditioning stage was simulated to achieve an H₂/CO ratio of 3 by means of a water–gas shift reactor. Several process configurations were then tested to maximize CO and H₂ conversion under less severe operating conditions, such as a maximum pressure of 10 bar and the addition of extra green hydrogen to increase CO₂ conversion. Although complete conversion of CO and CO₂ was not achieved in any of the studied configurations, an intermediate case with the injection of half the hydrogen required for full CO₂ conversion showed promising results, with both CO and H₂ conversion exceeding 95% at the highest operating pressure tested. The second part of this work focused on the methanation reaction conducted in a fluidized bed reactor, combining experimental and computational investigations. Experimentally, methanation was studied at relatively low temperatures compared to typical literature data. Conversions of approximately 12%, 22%, 35%, and 55% were achieved at operating temperatures of 200, 225, 250, and 300°C, respectively. Another set of experiments was performed to investigate the effect of adding a water sorbent during the methanation reaction. The introduction of 45 g of zeolite 3A enhanced conversion by removing water, a side product, thereby shifting the equilibrium toward methane formation. With the sorbent, conversions of around 20%, 31%, and 46% were achieved at 200, 225, and 250°C, respectively. Based on these experimental results, a bubbling fluidized bed model was developed in MATLAB, incorporating fluid-dynamic correlations and assuming the validity of the two-phase theory of fluidization. Commonly used kinetic expressions for methanation were implemented in the model, and new values for activation energy and pre-exponential factors were estimated to improve the agreement between experimental data and model predictions. In the final section, CO₂ methanation was studied in a fixed-bed reactor, representative of typical commercial applications. To evaluate the effects of renewable energy intermittency particularly variations in green hydrogen availability and fluctuations in total gas flow rate, the influence of parameters such as Gas Hourly Space Velocity (GHSV) and reactant molar ratio (H₂/CO₂) was investigated. Under conditions of hydrogen deficiency, the formation of undesired by-products (CO and solid carbon) was observed. Therefore, steam was introduced at volumetric fractions of 10% and 20% to assess its potential benefits under hydrogen-limited conditions. The results for 10% steam addition were remarkable: despite a moderate decrease in conversion in sub-stoichiometric conditions, nearly the same methane yield was achieved without additional hydrogen. However, at 20% steam, both conversion and methane yield decreased significantly.

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