This work identifies a benchmark for the performance of a small-scale ammonia synthesis plant powered by wind energy. The energy used is stranded, far from urban centers but near locations of fertilizer demand. The wind energy drives the pressure swing absorption of air to make nitrogen and the electrolysis of water to make hydrogen. These are combined in the small-scale continuous Haber process to synthesize ammonia. The analysis of runs of the small plant presented in this article permits an assessment of how the current production rate is controlled by three resistances: catalytic reaction, ammonia separation by condensation, and recycling of unreacted gas. The measured catalytic reaction rates are consistent with separate experiments on chemical kinetics and with published reaction mechanisms. The condensation rates predicted are comparable with literature correlations. These rate constants now supply a rigorous strategy for optimizing this scaled-down, distributed ammonia plant. Moreover, this method of analysis is recommended for future small-scale, distributed manufacturing plants.

Making ammonia from renewable wind energy at a competitive price may be possible if the conventional ammonia condenser is replaced with an ammonia absorber. Such a process change requires an ammonia selective absorbent. Supported metal halide sorbents for this separation display outstanding dynamic capacity close to their equilibrium thermodynamic limits. Alkaline earth chlorides and bromides supported on silica and zeolite Y are the most promising. MgCl2 and CaBr2 at 40% loading on silica show capacities of 60−70 mgNH3/gsorbent at 150 °C and 4 bar. Overall, cations with smaller atomic numbers show more affinity to ammonia; bromides hold ammonia more strongly than chlorides. Different solvents and metal halide mixtures do not show significant changes in the absorption capacity. These absorbents can be incorporated into ammonia reaction-absorption syntheses to achieve faster production rates.

In this work, the techno-economic analysis of a 20,000 metric ton (MT) green ammonia production facility is presented. This facility is 30 times smaller than a large-scale conventional process, producing ammonia from totally renewable resources: hydrogen from water electrolysis and nitrogen from pressure swing adsorption. Two different configurations of the Haber–Bosch (HB) process are investigated: high-pressure reaction-condensation (RXN-CON) and low-pressure reaction-absorption (RXN-ABS). Process simulation was implemented using ASPEN Plus, where the reactor and absorber columns were designed as a custom model. The results obtained were then used to estimate the total capital and operating costs. The high-pressure processing improves the single-pass conversion and loop efficiency but relies on costly compression, whereas the low-pressure processing is more favorable for both capital and operating costs. The performance analysis of the HB process indicates that the operating pressure affects ammonia production costs. The levelized cost of ammonia (LCOA) from our small-scale Haber process was found to be about twice more expensive than the conventional commodity ammonia prices. Our sensitivity analysis suggests that inherently safer low-pressure RXN-ABS can be utilized for thermochemical energy storage of renewable resourcesfor scenarios that numerous small ammonia plants can be implemented in areas with local ammonia demand, with access to excess renewable electricity at the time of high penetration of renewable resources. Under such conditions, the LCOA from this plant can be comparable with the ammonia commodity prices. When the revenue from selling oxygen is considered into economics, small-scale all-electric ammonia can be profitable with an after-tax rate of return of 27.50% for RXN-ABS.

Renewable wind energy can be used to make ammonia. However, wind-generated ammonia costs about twice that made from a traditional fossil-fuel driven process. To reduce the production cost, we replace the conventional ammonia condensation with a selective absorber containing metal halides, e.g., calcium chloride, operating at near synthesis temperatures. With this reaction-absorption process, ammonia can be synthesized at 20 bar from air, water, and wind-generated electricity, with rates comparable to the conventional process running at 150−300 bar. In our reaction-absorption process, the rate of ammonia synthesis is now controlled not by the chemical reaction but largely by the pump used to recycle the unreacted gases. The results suggest an alternative route to distributed ammonia manufacture which can locally supply nitrogen fertilizer and also a method to capture stranded wind energy as a carbon-neutral liquid fuel.

Ammonia is normally made at high temperature and pressure using a promoted iron catalyst. High temperatures are needed to get fast kinetics; the high pressure is used to ensure high conversion. Alternatively, ammonia can be made at high temperature but lower pressure if the product ammonia is rapidly separated. Here, we have systematically studied the effect of temperature and pressure on the rates of reaction. We then have qualitatively investigated the absorptive separation of ammonia using calcium chloride in a reaction−separation process. Rapid separation reduces the constraint of reversible reaction and enables us to obtain appropriate reaction rates at relatively lower pressure. The effect of different operating conditionsreaction temperature, pressure, absorption temperature, and gas transporton production rates is carefully measured, and this elucidates the potential and the limits of this type of low-pressure ammonia synthesis.