(345a) Sustainable Ammonia Production with an Advanced Solar Thermochemical Cycle: Techno-Economic Analysis

Ammonia (NH₃) is one of the most produced industrial chemicals in the world, and more than 80% of its production is used in the manufacturing of fertilizers. Since it is as an energy-dense and can be a carbon-neutral chemical it is also a potential candidate for energy carrier. Currently, NH₃ is synthetized via the Haber-Bosch (H-B) process, which is based on the gas phase exothermic reaction of hydrogen (H₂) with nitrogen (N₂) in a high pressure (150-300 bar) catalytic reactor at a moderately elevated temperature (350-500 °C). The essential feedstocks (N₂ and H₂) for the synthesis stem from hydrocarbons: H₂ from CH₄ via steam reforming, and N₂ from stoichiometric air after O₂ removal via CH₄ combustion. In addition, more hydrocarbon combustion is necessary for the balance of plant, further increasing CO₂ emissions. Modern H-B processes (that use natural gas or CH₄) consumes about ~28.5 MJ and generates ~1.6 kg of CO₂ per kg of NH₃. In total, NH₃ production represents ~1.4% of global CO₂ emissions, which is ~1% of global greenhouse (GHG) emissions.

Here, we propose and design a sustainable pathway for NH₃ production that uses concentrated solar irradiation in place of the hydrocarbon resource and decreases pressure requirements by about one order of magnitude compared to H-B. The process uses an advanced solar thermochemical looping approach to produce (and store) N₂ from air for subsequent NH₃ synthesis/re-nitridation loop via an advanced two-stage process each consisting of two steps (see Figure 1a).

In the first stage, N₂ separates from air with a two-step redox-active metal oxide (MO_x) thermochemical cycle. The first step is thermal reduction of a redox-active MO_x in which oxygen releases from the lattice. Concentrated solar radiation provides the heat to promote this endothermic reaction. In the second step, the reduced MO_x is re-oxidized by consuming O₂ from an air stream, resulting in high purity N₂. After the second reaction step, the MO_x re-oxidizes and returns to the first step closing the loop. The second stage is the production of NH₃ at low pressure compared to H-B with a two-step metal nitride thermochemical cycle. The first step of the second stage is the synthesis of NH₃ by reduction with H₂ (removes nitrogen from) a metal nitride (MN_y). NH₃ and an N-deficient MN_y-γ are produced directly. In the second step, purified N₂ from the first stage re-nitrides the reduced metal nitride back to its original nitrogen composition. Upon completion of the second reaction, the MN_y returns to the first stage closing the second loop. Note that the H₂ production is outside the scope of this project, however, it is essential to source the H₂ from a carbon-neutral process.

Figure 2b shows one of the most promising designs for this process. Here, we envision the N₂ separation process to operate with two counter-current particle-air reactors, one for reduction and one for re-oxidation. The reduction reaction occurs on-sun at elevated temperature (~800 °C) while the reoxidation occurs off-sun at a lower temperature (~500 °C). The particles move from one reactor to the other in a closed loop. For the NH₃ synthesis subprocess, we propose two alternative fixed-bed reactors that in principle, we intend to operate at isothermal conditions (in the range of ~600-700 °C). These reactors operate simultaneously: while the NH₃ synthesis reaction takes place in one reactor, the re-nitridation reaction progresses in the other. When the reactions finish, the reactions and the inlet gases switch between the reactors. We designed an energy management system to allow continuous production of N₂ and NH₃ and to assist in heat recovery. This system includes two particle storage bins and a hot solid media storage that acts as a thermocline. The heat transfer medium between the N₂ production system and the NH₃ synthesis subsystem is air. The balance of the plant is completed with an electricity generator, a turboexpander and compressor system, and gas separation units for NH₃ (via condensation) and H₂/N₂ (via membrane or similar).

The development of the system model used four different software programs: SolarPILOT for the design and simulation of the solar field and receiver optical efficiency, OpenModelica to accelerate the annual simulations, Microsoft Excel for the visualization of the results, and Matlab for the software integration, design, and processing of the model. In this preliminary simulation, a 30 MW_th plant can produce 45.3 tonnes of NH₃ per year. This plant requires 7.58 MJ/kg NH₃ and delivers 0.13 MJ/kg NH₃ (~46.3 Wh/kg NH₃) of renewable electricity to the grid. This system configuration requires an additional heat input of 0.26 MJ/kg NH₃. The results show that the techno-economic analysis is highly sensitive to the NH₃ synthesis sub-system, NH₃ conversion yield, and extent of re-nitridation. Starting with Co₃Mo₃N as the nitride material, cycle times of one hour, 20% NH₃ product yield, and 50% of theoretical extent of reduction/re-nitridation, an estimate of the levelized cost of NH₃ without H₂ is 300 $/tonne. One of the main advantages of this process is that it can offer price stability, since it does not depend on highly volatile markets, especially if it can use earth abundant materials.

Acknowledgements

This material is based on work supported by the U.S. Department of Energy Solar Energy Technologies Office under Award No. DE-EE0001529. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government.