Processes for decentralized ammonia synthesis from water, air and renewable energy

Ammonia is one of the largest volume chemicals produced worldwide. Today, almost all ammonia is produced from hydrogen and nitrogen gas by the Haber-Bosch process. The majority is used as a precursor for the production of synthetic N-fertilizers, which have become indispensable in the global food chain. Unfortunately, current Haber-Bosch plants make use of fossil fuels for the production of hydrogen, which comes with the emission of vast amounts of CO2. The Haber-Bosch process can be electrified by using green hydrogen, produced via water electrolysis with renewable energy sources such as solar and wind. However, solar and wind energy are typically characterized by an intermittent energy supply and a decentralized nature. The Haber-Bosch process on the other hand is highly centralized and requires constant operating conditions due to high pressures and temperatures. In this thesis, new processes for small scale production of ammonia from water, air and renewable energy are explored. In this thesis, two novel process concepts for flexible small-scale ammonia synthesis are introduces. The SECAM (Solar ElectroChemical AMmonia) process is based on the electrochemical nitrogen reduction reaction (NRR). Current electrocatalysts typically suffer from a low selectivity towards ammonia and produce large amounts of hydrogen byproduct. The SECAM process solves this selectivity problem in a unique way through the use of two different modes of operation. When a large amount of energy is available, ammonia is produced from water and air. A stoichiometric mixture of N2 and H2 is produced as byproduct and temporarily stored. When a small amount of energy is available, the process uses the stored N2/H2 mixture and converts it to ammonia at lower energy cost. This way, ammonia is the only product. The PNOCRA process is based on a plasma reactor and a Lean NOx Trap (LNT), adapted from the automotive industry. The plasma reactor consumes electrical energy for the partial conversion of air to NOx. This NOx is separated by adsorption on the LNT. When the LNT is sufficiently saturated, a H2 containing gas is fed to the LNT and the adsorbed NOx is reduced to ammonia and released. The produced ammonia is separated from the H2 containing gas via absorption in water. The SECAM and PNOCRA processes are both able to produce ammonia from air, water and renewable energy, which makes them a viable solution for the vast amounts of CO2 emitted by the current ammonia production method. However, simply replacing the existing ammonia production with these green alternatives does not resolve the other problems related to a disrupted N-cycle, such as increasing atmospheric N2O concentrations, loss of biodiversity, eutrophication etc. To solve these issues, a more efficient use of N-nutrients is required. This can be achieved by increasing N uptake efficiency, for example by combining the SECAM or PNOCRA process with fertigation (a combination of irrigation and fertilization).

Jaar van publicatie:2023

Toegankelijkheid:Closed