Green Haber-Bosch Process:A Small-Scale Ammonia Reactor System Design

The global energy transition from a fossil fuel base energy system to a renewable energy source base system is the key mission for a low-carbon future. The target of CO2 emission reduction by 2050, following the Paris Climate Agreement, is 90% compared to the CO2 level of 1990. Haber-Bosch process is the main industrial procedure for the production of ammonia today and about 80% of the global ammonia is consumed by the fertilizer industry. However, the century old Haber-Bosch process is normally energized by fossil fuel and itreleases about 3% of the global carbon footprint. In light of this fact, replacing the conventional fossil fueled Haber-Bosch process for manufacturing ammonia with renewable source powered ammonia production is the main goal of this study. Instead of obtaining H2 from steam-reformed CH4, H2 is produced from electrolyzed H2O. This transition enables the conventional ammonia manufacturing process transforming into a green Haber-Bosch production of ammonia. Two Dutch companies, TNO and Zero Emission Fuels, are cooperatingand developing a small scale of reactor system that can convert ammonia from air and water by using solar PV panels. In this work, a new design of ammonia reactor system is developed. Ammonia is typically formed at high pressure (150 - 250 bar) and high temperature (400 - 500oC) using a promoted iron base catalyst. High temperature ensures rapid reaction kinetics, and high pressure boosts the product yield. Here a reactor system, that is operated at lower pressure (≤100 bar) and uses condensation to remove ammonia, is kinetically simulated in ASPEN. The effect of different operation conditions - reaction temperature (300oC, 350oC and 400oC) , pressure (50, 75 and 100 bar) and feed gas (N2 : H2) ratio (1 : 3 and 1 : 5) - on the production rate in a small-scale ammonia reactor have been systematically computed. The mass flow rate of the single pass reactor is set to 50 g/h in this work. With a catalyst bed length of 15 cm and inner diameter of 3.6 cm, according to the simulation, reaction temperature of 400oC and operating pressure of 100 bar can lead to the highest conversion (40%) in a single pass reactor. The average heat transfer area of the reactor system is to a great extend less than 50 m2, therefore, the double pipe heat exchanger is a favorable heat exchange system for the proposed reactor system. In the reactor design validation section, the selected optimum operation conditions are tested in the same scale reactor laboratory setup. Experimental results show that the single pass conversion of nitrogen at 400oC and 100 bar in such a small-scale reactor can reach 15.4% which is in the range of the industrial one pass conversion level. For reaction operated at 50 bar, 6% of ammonia yield is obtained. It is clear that ammonia production in small-scale and in milder operation condition is possible and the results are promising. The techno-economic analysis has been performed based on above mentioned outcome. The reactor system is integrated with ZEF AEC, ZEF compressor system and a membrane nitrogen separation system. With current ammonia design production (350 g/day), the cost of ammonia per kilogram can be achieved in the range of €1.8 to €2 depending on the operation condition. This is about 5 times more than fossil ammonia prices, but it is very competitive with biomass ammonia. In accordance with the sensitivity analysis, increasing the capacity of feed gas production or reducing the cost in plant equipment can remarkably reduce the ammonia price to less than 1 €/kg NH3. Furthermore, recommendations in four categories are discussed in the last section of this work, which can lead to a further step towards a green ammonia plant in small-scale.Mechanical Engineering | Process and Energy Technolog