Interest
in sustainable nitrogen fertilizer production
using plasma
technology is rapidly growing, and this method is a promising way
to decentralize fertilizer synthesis and achieve carbon-free production.
However, improving efficiency remains a major challenge for the industrial
application of this technology. Herein, we present comprehensive experiments
using spark discharge to pinpoint the key factors affecting the efficiency
of this process and provide technical approaches to improve the energy
usage. We found that applying a bipolar voltage at high frequencies
greatly benefited the NO
x
yield and its
efficiency via the utilization of residual species. Measurements using
active cooling for the reactor and electrodes revealed that the chemical
loss process was not significant, but the energy loss during the heating
of the electrode was the dominant loss process, particularly at high
frequencies (>20 kHz) where an anchor state of the spark was reached.
However, the beneficial effects of higher frequencies compensated
for and mitigated the higher power loss resulting in slowing the increase
of the energy cost with the energy density. Moreover, enlarging the
plasma zone (reaction channel) by increasing the electrode gap was
an effective approach for enhancing the energy usage for the desired
reaction. Further, the limitations of enlarging the gap were resolved
by inserting a floating electrode in the large gap, yielding higher
NO
x
production (1.8%–3.0%) at a
lower energy cost (1.9–4.4 MJ/mol), and further enhancements
could be achieved by optimizing the reactor configuration. Therefore,
the results provide an important basis for the further development
and optimization of plasma reactors for efficient chemical reactions.