A Plasma-Water Droplet Reactor for Process-Intensified, Continuous Nitrogen Fixation at Atmospheric Pressure

Abstract

Current industrial production of ammonia via the Haber−Bosch process has a massive carbon footprint because the hydrogen gas feedstock comes from the reformation of fossil fuel which releases large amounts of carbon dioxide. One possible solution is to provide hydrogen gas by water electrolysis that is powered by renewable sources. A more radical solution is to use water as the hydrogen source in a process-intensified scheme whereby ammonia is produced by directly reacting nitrogen gas and water. Here, we investigated an atmospheric-pressure plasma process to continuously produce ammonia from nitrogen gas and atomized water droplets. The successful production of ammonia was carefully confirmed by several control experiments. We find that other forms of fixed nitrogen are synthesized as well, predominantly nitrate and some nitrite. To understand the role of water droplets, we characterized their lifetimes in the reactor and compared the ammonia production with a water vapor feed. The results show that the ammonia forms through gas-phase chemistry and the liquid phase does not appear to substantially contribute. Additional insight into the reaction mechanism was provided by replacing the water with hydrogen gas or oxygen gas as the feedstock. As expected, no nitrates or nitrites were synthesized with hydrogen gas, and the ammonia production was comparable, indicating similar mechanisms may be occurring that involve feedstock dissociation to atomic hydrogen. No ammonia or nitrates/nitrites were detected with oxygen gas, which suggests that water provides a unique reaction pathway for nitric and nitrous acid to be formed in the gas phase. Our study demonstrates that nitrogen gas and water droplets can serve as feeds in a plasma process for the sustainable and distributed production of ammonia or other fixed nitrogen compounds.

Direct Oxidative Nitrogen Fixation from Air and H2O by a Water Falling Film Dielectric Barrier Discharge Reactor at Ambient Pressure and Temperature

“…Thus, the small plasma‐liquid contact area limits the room to improve the synthesis rate and energy efficiency. Therefore, while promising, its energy consumption was found to be about 200 times higher than that of H−B according to the related research at the present stage [3] …”

“…Currently, NH 3 production is primarily dependent on the Haber‐Bosch (H‐B) process via the N 2 /H 2 reactions mediated by catalysts at high temperatures and pressures; and the demand for HNO 3 is satisfied by catalytic oxidation of H‐B‐generated NH 3 through the Ostwald process, which however raises energy cost compared with direct oxidative nitrogen fixation (Figure 1). [1,2] Both of the two processes consume a lot of fossil fuel and have a massive carbon footprint [3] . Therefore, greener and more sustainable routes towards NH 3 and HNO 3 production are actively investigated [4] …”

“…Therefore, while promising, its energy consumption was found to be about 200 times higher than that of H À B according to the related research at the present stage. [3] In stark contrast to the previous "pin-to-plate" configuration, herein, we for the first time propose a gas-liquid NTPNF process based on the water falling film dielectric barrier discharge (WF-DBD), as shown in Figure 2. Detailed description of the reactor and the product detection method are presented in the Supporting Information.…”

“…[1,2] Both of the two processes consume a lot of fossil fuel and have a massive carbon footprint. [3] Therefore, greener and more sustainable routes towards NH 3 and HNO 3 production are actively investigated. [4] Comparing with the various nitrogen-fixation methods, non-thermal plasma-assisted nitrogen fixation (NTPNF) was suggested to have the lowest theoretical energy consumption (~0.2 MJ mol À 1 ).…”

“…The intermediate products NO and NO 2 can continue to react with OH in the gas phase environment to form HNO 2 and HNO 3 , which then dissolved in the liquid film on the central electrode through diffusion, [3,8,16] NO þ OH ! HNOðgÞ…”