Atmospheric Pressure DBD Plasma Ammonia Synthesis and Separation Process Design and Environmental Impact Assessment

Abstract

Engineering more sustainable ammonia (NH3) production methods is of utmost importance since the conventional synthesis process utilizes high temperatures and high pressures. In this work, the substitution of the conventional NH3 reactor operating at 160 bar with a dielectric discharge barrier (DBD) plasma reactor operating at atmospheric pressure was performed using the overall process design approach and the corresponding environmental impact assessment was calculated using Life Cycle Analysis. In particular, a plasma DBD NH3 reactor was modeled to operate at near atmospheric pressure and 150 °C using a forming gas obtained in the conventional methane reforming unit to obtain ∼1600 tonnes/day of liquid NH3 for large-scale fertilizer production. The resulting gaseous effluent was compressed to 25 bar and expanded to cool down and condense gaseous NH3 product resulting in a significant pressure decrease across the process. However, state-of-the-art reported N2 per pass conversion of 5–10% used in the current model results in large recycle and high compressor work. The single limiting factor preventing the favorable process economics was very high-energy consumption to generate plasma requiring ∼1758 MWe at the reported 37.9 g/kWh, which is the highest NH3 yield reported in the literature for DBD plasma reactors. The model obtained suggested that $0.007/kWh electricity cost could result in a breakeven for such a process. Across the board, much larger environmental impacts in the process utilizing a plasma NH3 reactor were obtained due to the significant increase in electricity use when compared to the conventional process when modeled as coming from the typical U.S. grid mix. Utilizing only 100% wind-derived electricity to power a plasma NH3 reactor provided certain environmental benefits with the compression/separation section becoming the main greenhouse gas contributor. The data presented here suggest that, while the low-pressure DBD plasma-based NH3 production process carries a promise of enhancing the sustainability of the NH3 production, significant improvements in the reactor efficiency are needed for it to become cost-competitive or result in improved environmental impact.

Topics & Concepts