Catalyst development and process design for CO2 methanation of biogas

Introduction As methane has been establishing itself as a primary energy source, to obtain it from renewable carbon feedstock rather than to extract it as natural gas is by far more appealing: biomass-generated methane is an efficient power generation mean with a virtually closed CO2 cycle, accompanying the transition towards a zero-carbon energy future. Biogas however contains large amounts of CO2, to be at least separated to exploit biomethane, and possibly valorised. A first option is CO2 hydrogenation to methane, also promising to transform an energy vector that is uneasy to handle (green H2) into a valuable and worldwide-distributed fuel and feedstock (CH4). A “power-to-gas” framework could then help to overcome the drawbacks of H2 as an energy storage medium and to increase the continuity and general availability of different intermittent renewable energy sources. This flexibility offers also additional possibilities for the downstream use of biogas, which may be richer in hydrogen or methane according to the process operation, even if these conditions might not fit the distribution networks nearer to the biomass-treating site. The aim of this work is to detail aspects of the methanation of CO2 as a method for Carbon Capture and Utilisation using green hydrogen. Different options for the efficient direct conversion of CO2 and H2 into CH4 (Sabatier reaction) are here explored both experimentally and through process design. Materials and Methods Ni-based catalysts (5-20 wt%) supported over CeO2, SiO2, Al2O3 and ZrO2 have been prepared by impregnation and co-precipitation. Testing has been done under practically relevant conditions at pressure up to 20 bar, with a stoichiometric H2/CO2 feed. Process design has been accomplished with Aspen Plus process simulator, considering the Sabatier reaction for the methanation of CO2. Results and Discussion The plant size considered is a small, delocalised plant scale, identified in a biogas production plant and based on the developed 20%Ni/CeO2 catalyst. H2 is considered as produced from water electrolysis fed with renewable power. A key issue is the strong exothermicity of the reaction. Our research explores the use of water vapour, added on purpose to the reactor as a thermal vector and later condensed. The simplest and most economical reactor arrangement is composed of a certain number of adiabatic beds (up to five) with intercooling. Alternative arrangement has been explored designing a fluidized-bed reactor, that allow better temperature control, but this led to incomplete conversion and was difficult to scale-up. The possibility to use the methane already present in biogas as diluent (i.e. thermal vector to control the exothermicity) was also considered, offering the additional advantage to eliminate the otherwise needed and expensive CO2 capture step. This option is intended to improve the CH4 yield and to meet the purity specifications for feeding the natural gas distribution grid. Possible poisons for the methanation catalyst, such as sulphides or nitrogen containing poisons, were considered and removed by proper pretreatment. Two options were further considered, one with preliminary CO2 separation from biogas and methanation of pure carbon dioxide, the other on with direct treatment of the biogas stream. At least 4 reactive stages for the methanation reaction were needed to get > 75% conversion. Either adiabatic or cooled catalytic beds were compared, operating at below 400°C, with an overall size of 105 Nm3/day of synthetic methane.