(117d) Process Improvement in the Energy Recovery Ammonia Synthesis Reactor of a Solar Thermal Power Plant

Abstract— Among renewable energy sources, solar energy has been harnessed from photovoltaic (PV) technologies as well as concentrated solar power (CSP) technologies, both with the drawback of not being able to operate as baseload plants in the same manner as conventional fossil-fueled or nuclear power plants. Thus, thermal storage during solar insolence is necessary to permit full 24-hour operation of a power plant. Among the storage options, liquid ammonia, a chemical available in abundance, has been investigated as a thermochemical storage medium. During solar insolence, the solar thermal energy is used for the endothermic cracking of ammonia to nitrogen and hydrogen; and recovered subsequently by exothermic synthesis in the well-known industrial Haber-Bosch Process.

This paper considers optimal process parameters of two power plants, 1 kW to 10 MWe, in which an ammonia loop is used for energy recovery during periods when solar insolence is not available. Two industrial designs viz a Kellogg Brown Root (KBR) horizontal reactor and a Haldor Topsøe type vertical reactor, are used in the analysis.

The models are based on the one-dimensional mass and energy conservation equations with the thermodynamics and reaction kinetics expressed by the Temkin-Pyzhev relation. The optimal requirements are obtained from a variational formulation seeking to maximize the heat of reaction from the synthesis of ammonia. A numerical solution is carried out to quantify the effect of system pressure, spatial catalyst concentration, and activation energy on the energy recovery system. The results indicate that significant process improvements, such as reduction in system pressure and close-to-equilibrium operation, are possible by varying the spatial catalyst distribution to take advantage of the ‘importance’ of the first catalyst bed in a series of beds. An ‘optimal’ catalyst loading in a reactor is shown to yield the ‘best’ temperature profile resulting in maximum ammonia conversion. The optimal distributions are compared with equilibrium, temperature and concentration, profiles. It is found that a close-to-equilibrium conversion can be achieved by placing a high-activity catalyst in the first bed resulting in enhanced heat recovery when high-activity, or spatially concentrated, catalysts are placed in ‘important’ zones of the synthesis reactor.