Plasma-enhanced catalysis for biogas upgrading

Non-thermal plasma can be described as a partially ionized mixture of gases in which, due to its non-equilibrium nature, active species are present at a supposedly low temperature. This technology can be combined with a catalyst to activate chemical reactions at lower temperatures, with a positive effect on the product distribution for exothermic equilibrium reactions. This thesis investigated experimentally the potential contribution of non-thermal plasma in the catalytic upgrading of biogas to biomethane with H2. The experimental setup includes a packed bed reactor, with commercially available methanation catalyst, where non-thermal plasma was created using a high-voltage generator. The initial part of the experimental campaign led to the choice of CP1518 by Johnson Matthey for the catalytic bed, instead of the other commercial catalysts tested. The research on plasma resonance peaks in different gases revealed key insights into plasma stability and its dependence on electrical parameters. For pure gases, the ignition voltage is proportional to the gas dielectric strength. In the methanation mixture, helium helps in lowering the breakdown voltage. Resonance peak frequencies are more affected by the setup than the gas type, and the main peaks for the employed setup are located at 30 kHz and 45 kHz. The activity tests with the plasma-catalysis setup produced positive results, with performances higher than test conducted in conventional setups in similar conditions. The energy efficiency of the plasma-catalysis setup improves with higher flow rates. Increasing the gas flow there is a slight reduction in reactants conversions, which are around 93% for the test at 2000 h-1 and 89% at 15000 h-1, but this decrease is compensated by the increase in treated flowrate and thus by the quantity of methane produced. At GHSV equal to 15000 h-1, the non-insulated reactor became autothermic after the plasma was turned off, maintaining the same reactants conversions. The activity tests also show that the generated plasma is able to heat the catalytic bed, significantly affecting reaction performance. When the plasma-catalysis setup was thermally insulated the catalytic bed quickly exceeded the optimal temperature range for methanation at all tested space velocities. The best reaction performances were obtained in without insulation and at GHSV: 5000 h-1, with conversions over 93% and an outlet composition of 86.59% CH4, 10.71% H2 and 2.70% CO2. These values are above the Italian grid injection limits, suggesting that a second methanation stadium or an improvement of the setup are required to reduce the molar fractions of the unwanted species below the grid limits.