Plasma-catalytic ammonia decomposition using a packed-bed dielectric barrier discharge reactor

Plasma-catalytic ammonia decomposition as a method for producing hydrogen was studied in a packed-bed dielectric barrier discharge (DBD) reactor at ambient pressure and a fixed plasma power. The influence of packing the plasma zone with various dielectric materials, typically used as catalyst supports, was examined. At conditions (21 W, 75 Nml/min NH 3 ) where an NH 3 conversion of 5% was achieved with plasma alone, an improved decomposition was found when introducing dielectric materials with dielectric constants between 4 and 30. Of the tested materials, MgAl 2 O 4 yielded the highest conversion (15.1%). The particle size (0.3–1.4 mm) of the MgAl 2 O 4 packing was found to have a modest influence on the conversion, which dropped from 15.1% to 12.6% with increasing particle size. Impregnation of MgAl 2 O 4 with different metals was found to decrease the NH 3 conversion, with the Ni impregnation still showing an improved conversion (7%) compared to plasma-only. The plasma-assisted ammonia decomposition occurs in the gas phase due to micro-discharges, as evident from a linear correlation between the conversion and the frequency of micro-discharges for both plasma alone and with the various solid packing materials. The primary function of the solid is thus to facilitate the gas phase reaction by assisting the creation of micro-discharges. Lastly, insulation of the reactor to raise the temperature to 230 °C in the plasma zone was found to have a negative effect on the conversion, as a change from volume discharges to surface discharges occurred. The study shows that NH 3 can be decomposed to provide hydrogen by exposure to a non-thermal plasma, but further developments are needed for it to become an energy efficient technology. • The plasma was found to achieve an NH 3 conversion of 5% at ambient conditions. • Introduction of dielectric materials with ε = 4–30 improved conversion. • MgAl 2 O 4 (ε = 8.3) yielded the highest conversion found of 15%. • A linear correlation between conversion and number of micro-discharges was found. • The decomposition primarily proceeds in the gas phase by collision with electrons.