Catalyst deactivation in green Haber-Bosch process

摘要

The inherent features of renewable energy sources cause an imbalance between energy supply and demand.One way to deal with this problem is to convert the electrical energy produced from renewable sources into energy-dense Carbon-Neutral Liquid Fuels(CNFLs)and back into electricity or hydrogen on demand [1].Among CNFLs ammonia and its production from renewable sources(known as power-to-ammonia)has gained special attention in recent years.Ammonia is industrially synthesized through the Haber-Bosch process,in which hydrogen and nitrogen in the presence of an iron-based catalyst react in a packed-bed reactor at high pressure and temperature.In power-to-ammonia,hydrogen and nitrogen are produced from water and air,respectively,while their production rate will be fluctuating and intermittent [2].Model-based analysis has shown that the Haber-Bosch process in general is feasible for operating at strongly different ammonia production rates when modifying the inlet flow rate and composition [3].A solution to the problems caused by instabilities of the flow rate of the feed in the Haber-Bosch process is to use excess amounts of nitrogen,argon or ammonia in the synthesis loop [4,5].With this solution,however,some questions arise:To what extent is the reactor flexible to the changes of the composition of the inlet gas? Will the catalyst be deactivated with a higher rate? How will the ammonia production rate change? In order to investigate the flexibility of the Haber-Bosch process and its catalyst deactivation in power-to-ammonia,a multi-scale model is developed.As a feed to the ammonia synthesis reactor,we consider hydrogen and nitrogen produced from water electrolysis and Pressure Swing Adsorption(PSA),respectively,in which renewable sources are used as the source of energy.The outlet of both processes contains ppm amounts of H2O and O2,which are poisons to the ammonia synthesis catalyst.A microkinetic model of ammonia synthesis and catalyst deactivation predicts the ammonia production rate at the surface of the catalyst.This is then coupled with the reactormodel to predict macroscopic parameters of the process,e.g.conversation of ammonia.The results show how the catalyst activity changes as the composition of the gas mixture in the synthesis loop varies.