Effects of Reactor Geometry on Plasma-Assisted Ammonia Decomposition in Coaxial DBD Reactors at Low Pressures

Plasma-assisted ammonia (NH3) decomposition is a promising strategy for hydrogen production. However, reactor geometry remains a key factor limiting its hydrogen yield per energy input (YH2). This study systematically investigates H2 production in outer-dielectric (OD), inner-dielectric (ID), and double-dielectric (DD) coaxial DBD reactors. The results show that the ammonia decomposition performance of OD- and ID-coaxial DBDs is significantly higher than that of the DD-coaxial DBD. OD- and ID-coaxial DBDs generate abundant micro-discharge pulses, enabling effective discharge energy deposition at lower peak voltages. Consequently, the reduced electric fields E/N are maintained within the optimal kinetic window for NH3 dissociation and H2 production. Moreover, by balancing residence time and energy density, the 8 cm length electrode achieves a peak YH2 of 1.22–1.24 gH2/kWh in the OD-coaxial DBD. For the ID-coaxial DBD, a 1 mm dielectric thickness yields a maximum capacitance of 86 pF, achieving a peak YH2 of ~1.35 gH2/kWh at the optimum E/N. In contrast, the DD-coaxial DBD exhibits the lowest YH2 (≤0.82 gH2/kWh) with minimal temperature rise. This is caused by the reduced current pulse numbers and the deviation of E/N from the optimal range with elevated operating voltages. This work provides guidance for the optimization of DBD reactors in plasma-assisted NH3 decomposition for efficient H2 production.