Massa, Fiorella (2022) Sorption Enhanced Methanation in fluidized bed reactors. [Tesi di dottorato]

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Although fossil resources are the current largest source of natural gas, the production of renewable methane (synthetic or substitute natural gas) is gaining increasing interest due to the efforts made to achieve the energy transition. Methane is a fundamental energy vector that benefits of a well�developed infrastructure and social acceptance worldwide. The pathways for renewable methane production are numerous and, among them, processes also combining carbon-capture and utilization (CCU) techniques are very interesting. This pathway would enable the chemical storage of the surplus of renewable electric energy, providing concurrently the utilization of captured CO2 as reactant. Although with greatly different purposes, the methanation process is historically consolidated, showing as one of the main drawbacks the complex process control due to the high exothermicity of the reactions. For renewable methane production, being the reactants fed to the process not present in traces like in other established methanation applications, the issue of the temperature control becomes even more sensitive. Therefore, the use of fluidized bed reactors, known to be suitable for large-scale application especially in the case of highly exothermic reactions, is attractive. Lastly, in recent studies the concept of sorption-enhanced reaction has been investigated applied to methanation. In this case, the performance of the process would increase by simultaneously absorbing a methanation product, water vapor. Synthesizing these concepts, in this work, the use of dual interconnected fluidized beds is applied to sorption-enhanced methanation (SEM), achieving a chemical looping system where continuously the hydration and regeneration of a water sorbent takes place. Precisely for this reason, the first part of the experimental campaign concerned the choice of a proper sorbent to perform SEM. In particular, CaO and commercial zeolites were tested. The former gives a chemical absorption in the range of interest, but it can be consumed by the undesired carbonation reaction in an environment containing CO2. The latter, interested by a physisorption process, can act as a molecular sieve allowing only the water molecules to be captured. CaO showed a lower average asymptotic H2O capture capacity, being subject to deactivation along the cycles. However, the cost of the material is in favor of CaO. Still, if considering the effect of carbonation, the decrease in the capture capacity was less significant than expected, presenting comparable values of capture capacity for test with both high and low CO2 concentration. Further scientific advances towards better performing and less expensive materials appear to be necessary to perform SEM on industrial scale. Focusing specifically on methanation from renewable H2 and captured CO2, the thermodynamics of CO2 sorption enhanced methanation was analyzed. Calculations involved low pressure levels that are of interest to achieve a decrease of the energy duty for the gas compression work: one of the main reasons why SEM is attractive. The results pointed out that SEM conditions enhance methanation performance at all temperatures and pressures, but they can result in easier carbon generation, that must be avoided to prevent catalyst deactivation. Optimal SEM conditions with stoichiometric feed imply only a partial steam removal: these more flexible operations can be ensured by fluidized bed reactors rather than fixed bed ones. In these latter, in fact, until steam breakthrough from the bed, the whole H2O produced is captured by the sorbent. This makes the proposed dual interconnected fluidized beds application, a promising concept. Besides the carbon deposition issue, one of the main aspects analyzed in the literature is the suitability of the methanation outlet gas for a direct injection in the natural gas grid. The H2 concentration resulted to be the critical one whereas limitations regarding the maximum CO and CO2 content in the gas for grid injection could be reasonably overcome. CO2 SEM in the chemical looping configuration was also simulated with Aspen Plus software using calcium oxide as sorbent for the water. Considering again one of the main aspects analyzed by the thermodynamic analysis, the goal was to obtain final synthetic natural gas streams matching the network specifications. To produce such suitable methane streams, the amount of input sorbent was varied for different feed conditions. The analysis showed that the undesired sorbent carbonation has a significant influence on the unconverted amount of hydrogen at the outlet, which increases with the amount of CaO fed. However, it was found that optimal operating conditions in terms of sorbent, using a sub-stoichiometric gas supply with respect to H2, may lead to obtaining directly injectable streams. Unfortunately, in such conditions the possible carbon generation was not prevented: however, a fluidized bed process may, again, offer a significant advantage ensuring an efficient temperature control and, in such way, limiting the rate of carbon generation. Finally, real SEM was experimentally tested using the sorbents previously evaluated, in a lab-scale dual interconnected fluidized bed system, providing the proof of concept of the process during methanation/hydration and dehydration cycles at different operating conditions. The chosen catalyst for the reaction was a purposely prepared 10%wt nickel-based catalyst on alumina support, which showed to be active starting from 250°C, in accordance with literature. A narrow range of temperatures (300-350°C) was investigated, since such range was compatible with the physical and chemical constraints imposed by water adsorption and methanation kinetics. As expected, for the calcium oxide sorbent, CO2 capture strongly affected the CaO sorbent performance, but with the undesired effect vanishing along the cycles. Therefore, a clear SEM effect occurred in the last cycles, with a sensible increase in the produced CH4 with respect to traditional methanation. The same qualitative enhanced effect was experienced with the commercial zeolites 3A. This latter confirmed a stable sorption behavior along the cycles, presenting, however, a quantitative enhancement effect on methane productivity lower than CaO. This result highlighted even more the need of further research towards highly active catalytic materials at lower temperatures to aid the adsorption efficiency of performing materials such as zeolites.

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