Microbubble Plasma Processing for N-Fertigation via Plasma Catalysis

Abstract

Plasma can use intermittent renewable energy and abundant use of air to produce N-fertilizers, with option of decentralized production "at-farm". Plasma can generate both nitrate and ammonia (NH3) as key platform molecules toward all N-fertilizers, including urea and ammonium nitrate. The most convenient way is to produce a "performance chemical" at the farm site. This is a fertigation solution that is ready to be pumped to the farm fields. This study reports about using a nonthermal plasma to generate nitrate and NH3 in presence of water, in which these are immediately absorbed; creating a fertigation solution, termed in literature "plasma activated water (PAW)". Both air and nitrogen plasmas have been used. The N-content has been optimized by a process-parametric study, yielding the second-best result reported in literature and the best when concerning scale-up ability. Strategic catalyst studies have been conducted, in terms of plasma-catalysis synergy, varying the type of catalyst and the location of catalyst. Silica-supported single/binary metal catalysts and gamma-alumina powder catalysts were used. Distinct catalyst placements inside the plasma electrode zone (glow or spark) and outside in the gas-liquid reactor were tested; with the aid of supports and without. We used proprietary NH3 and N-fixation catalysts besides specific commercial catalysts. Increasing the N-fixation performance by about 70% as compared to noncatalytic processing, the study used a recycling loop to further boost the N-fertilizer concentration. A plasma-catalyst synergy was determined by structural changes of the catalyst transiently (during plasma operation). This laboratory study achieved a result that has potential for commercial N-fertigation at farm, when multiplied by a factor of 10 via further process intensification, a factor of 10 by increase of plasma reactor scale, and a factor of 10 by reactor parallelization ("numbering-up"). Admittedly, this is ambitious and needs considerable process development, yet is a possible way to go without a road blocker.