Comparison of different hydrogen vectors for the storage and distribution of green hydrogen: methanation of CO2 vs. ammonia synthesis

Comparison of different hydrogen vectors for the storage and distribution of green hydrogen: methanation of CO2 vs. ammonia synthesis Matteo Tommasi 1, Simge Naz Degerli 1, Gianguido Ramis 2, and Ilenia Rossetti 1* 1 Chemical Plants and Industrial Chemistry Group, Dip. Chimica, Università degli Studi di Milano, CNR-ISTM and INSTM Unit Milano-Università, via C. Golgi 19, 20133 Milan, Italy, ilenia.rossetti@unimi.it, presenting author * corresponding author 2 Dip. Ing. Chimica, Civile ed Ambientale, Università degli Studi di Genova and INSTM Unit Genova, via all’Opera Pia 15A, 16145 Genoa, Italy, gianguidoramis@unige.it INTRODUCTION Hydrogen production through water electrolysis is becoming a green practical alternative to exploit intermittent renewable sources, such as wind and solar power, storing them for unavailable periods in form of an energy vector. However, technologies for the transport, distribution and final usage of hydrogen are still lacking. Further vectors are therefore under study to assess store the green hydrogen into molecules that are readily produced and distributed through an already available infrastructure. Among these, ammonia, methanol and methane offer very large scale capacity and transport/distribution network. Also, the regulatory aspects for production, transport and storage are already well known. 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. The size considered is a small, delocalised plant scale, identified in a biogas production plant. This solution will be compared with an alternative method for exploitation of CO2 and green hydrogen, e.g. the synthesis of ammonia. EXPERIMENTAL/THEORETICAL STUDY Process design has been accomplished with Aspen Plus process simulator, considering the Sabatier reaction for the methanation of CO2. H2 is considered as produced from water electrolysis fed with renewable power. This option is intended to improve the CH4 yield and to meet the purity specifications for feeding the natural gas distribution grid. Possible poisons such as sulphides or nitrogen containing poisons for the methanation catalyst 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. An ammonia synthesis plant was designed based on the Haber Bosch synthesis with multibed configuration and it was associated with an ammonia cracking reactor for the delivery of recovered hydrogen for local use. RESULTS AND DISCUSSION 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 atmospheric pressure below 400°C, with an overall size of 105 Nm3/day of synthetic methane. The Haber-Bosch synthesis of ammonia was carried out at 200 bar and in a temperature range between 300 and 400 °C, using two catalysts: Fe (wustite) and Ru/C. 550 kg/day of NH3 at 98.8 wt% were obtained, demonstrating the proof of concept of this newly designed process for the production of ammonia from green hydrogen. This product was split into 10 ammonia cracking step for delocalised hydrogen use. CONCLUSIONS The methanation reaction requires higher investment and energy consumption than ammonia synthesis, mainly due to the preliminary CO2 capture step. If avoidable, the methanation reaction can be a valid option for green hydrogen storage. ACKNOWLEDGMENTS This study was carried out within the Agritech National Research Center and received funding from the European Union Next-Generation EU (PIANO NAZIONALE DI RIPRESA E RESILIENZA (PNRR) – MISSIONE 4 COMPONENTE 2, INVESTIMENTO 1.4 – D.D. 1032 17/06/2022, CN00000022).