Optimal Energy Recovery from Ammonia Synthesis in a Solar Thermal Power Plant

This work estimates the optimal performance, and evaluates a process modification to approach optimal ammonia production and subsequent energy produced, in the ammonia synthesis heat recovery system for a 10 MW(e) solar thermal power plant which typically requires a 1500-MTD ammonia synthesis reactor. The one-dimensional steady-state pseudo-homogeneous plug flow model is considered for the simulation of ammonia conversion in a synthesis reactor. The mass and energy conservation equations, together with the thermodynamics and reaction kinetics, are numerically solved for the ammonia concentration and temperature profile in the reactor. The resulting gas temperature is compared with the optimal, and equilibrium, temperature profiles, determined from a variational formulation. It is found that with optimal performance, the ammonia production can increase by 15 % over that in the reference design. This requires an inlet temperature of the order of 900 K which exceeds industrially achievable inlet temperatures by approximately 200 K. We thus consider one process modification-non-uniform distribution in three catalyst beds to reduce the gap between the actual and optimal temperatures, and evaluate the effect of small changes in catalyst concentration. It is found that the improvement caused by a 50 and 25 % concentration increase in the first and second beds respectively, results in a 10 % increase in ammonia conversion. Thus, in an industrial synthesis reactor, an optimal configuration can be achieved by determining a variable catalyst concentration, in a large number of catalyst beds, to achieve enhanced ammonia conversion and energy recovery in a solar thermal power plant.