A novel cactus-shaped reactor design for ammonia cracking: Using an integrated numerical modelling and response surface optimisation method

• A novel cactus-inspired reactor geometry is proposed for ammonia decomposition. • The fractal-shaped design enhances heat transfer and reduces cold spots in the catalyst bed. • CFD–RSM optimisation increases conversion from 74% (conventional) to 88.12%. • The optimised reactor produces 2.93 kg H 2 /day with 22.36 kWh total energy consumption. Achieving maximum hydrogen production from ammonia through catalytic decomposition with minimal energy consumption remains a major challenge in this field. In the present numerical investigation, a novel reactor was designed and optimised. The reactor geometry was inspired by the Mighty Saguaro cactus. This fractal-shaped configuration enhances the heat transfer surface area and prevents cold spots within the reactor. The design parameters include geometrical characteristics such as the branch angle and diameter, along with operating conditions like ammonia flow rate, inlet temperature, and reactor wall temperature. Design points were generated using the design of experiments method, and reactor performance at each point was evaluated through simulations of the flow field, heat transfer, and mass transfer based on the finite volume method. The objective functions were defined as the maximum hydrogen mole fraction, the minimum operating temperature, and the minimum pressure loss. By coupling the numerical results with the response surface method, the optimum design point was predicted, and the accuracy of the predicted reactor performance under the suggested optimum condition was verified by numerical simulation. Sensitivity analysis at the optimum point indicated that hydrogen production shows the greatest sensitivity to inlet velocity (−83.73%) and the reactor wall temperature (9.57%). In the optimum cactus-shaped reactor design, conversion efficiency reaches 88.12%, whereas under the same operating conditions, the conventional reactor achieves only 74% efficiency. Although the cactus-shaped reactor increases the pumping power required to overcome the pressure drop, the corresponding rise in hydrogen production under optimum conditions makes this trade-off acceptable and justified. Overall, the current optimised novel reactor enables production of 2.93 kg of hydrogen per day using 22.36 kWh (including heating load and pumping power).