Abstract
The power-to-ammonia concept allows for the production of ammonia, one of the most produced inorganic chemicals, from air, water and (renewable) electricity. However, power-to-ammonia requires flexible operation for use with a directly intermittent renewable energy supply. In this paper, we systematically analyse the operating envelope for steady-state operation of the three bed autothermic Haber–Bosch reactor system for power-to-ammonia by pseudo-homogeneous model. Operational flexibilities of process variables, hydrogen intake and ammonia production flexibilities are analysed, along with maximum and minimum possible changes in recycle load and recycle to feed ratio for the following process variables: reactor pressure, inert gas percentage in synthesis loop, NH3 concentration, H2-to-N2 ratio, total flow rate and feed temperature. Among the six process variables, inert gas fraction and H2-to-N2 ratio provided very high flexibilities, ca. 255% operational flexibility for Ar, up to 51 to 67% flexibility in hydrogen intake, and up to 73% reduction and 24% enhancement in ammonia production. However, a decrease in ammonia production by H2-to-N2 ratio significantly increases recycle load. Besides inert gas fraction and H2-to-N2 ratio, the total mass feed flow rate is also significant for minimum hydrogen intake and ammonia production.
Highlights
Ammonia is the second most produced industrial chemical, and the production process has been intensively developed over a period of one century
It can be concluded that the autothermic reactor is viable for power-to-ammonia process, as it can be operated for a wide range of process variables while maintaining operational, hydrogen feed intake and ammonia production exibilities
Operating outside these boundaries leads to the shutdown of reactor system autothermic operation or damage to the catalyst due to overheating
Summary
Ammonia is the second most produced industrial chemical, and the production process has been intensively developed over a period of one century. The efficiency of power-to-ammonia is estimated between 50 and 60%, including hydrogen and nitrogen production,[22] which is lower than from the latest classical Haber–Bosch ammonia production plants i.e. between 60 and 64%.23 This is mainly due to higher energy requirements and energy losses in production of H2 from electrolysis of water by atmospheric alkaline, high pressure alkaline (16 bar) or proton exchange membrane electrolysis cells.[22]. Sanchez & Martın carried out complete system simulation and operation optimisation, including a kinetic approach for Haber–Bosch synthesis reactor Even so, they didn't consider an autothermic ammonia synthesis reactor, which is of high interest for realising stand-alone power-toammonia plants. The following section gives an analysis on the exact challenges a exible Haber–Bosch process faces, which will be analysed using modelling in later sections
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