Towards a carbon independent and CO₂-free electrochemical membrane process for NH₃ synthesis

Abstract

Ammonia is exclusively synthesized by the Haber-Bosch process starting from precious carbon resources such as coal or CH4. With H2O, H2 is produced and with N2, NH3 can be synthesized at high pressures and temperatures. Regrettably, the carbon is not incorporated into NH3 but emitted as CO2. Valuable carbon sources are consumed which could be used otherwise when carbon sources become scarce. We suggest an alternative process concept using an electrochemical membrane reactor (ecMR). A complete synthesis process with N2 production and downstream product separation is presented and evaluated in a multi-scale model to quantify its energy consumption. A new micro-scale ecMR model integrates mass, species, heat and energy balances with electrochemical conversions allowing further integration into a macro-scale process flow sheet. For the anodic oxidation reaction H2O was chosen as a ubiquitous H2 source. Nitrogen was obtained by air separation which combines with protons from H2O to give NH3 using a hypothetical catalyst recently suggested from DFT calculations. The energy demand of the whole electrochemical process is up to 20% lower than the Haber-Bosch process using coal as a H2 source. In the case of natural gas, the ecMR process is not competitive under today's energy and resource conditions. In future however, the electrochemical NH3 synthesis might be the technology-of-choice when coal is easily accessible over natural gas or limited carbon sources have to be used otherwise but for the synthesis of the carbon free product NH3.

Recent progress towards the electrosynthesis of ammonia from sustainable resources

“…It has been estimated that such a system could be up to 20% more energy-efficient than a Haber Bosch process using coal as the hydrogen source, although with today's technology it would remain 30% less efficient than a Haber Bosch process using natural gas as its hydrogen source. 9 In this review, we shall examine recent progress towards electrochemical systems that produce ammonia by the reduction of nitrogen. We shall look in turn at systems where H 2 , sacrificial reagents and water are used as the proton sources, and we shall also explore systems where air can be used directly as the source of nitrogen.…”

Electrochemical Synthesis of Ammonia: A Low Pressure, Low Temperature Approach

“…One alternative approach to solve the ammonia problem is to use electricity to drive the ammonia production reaction, decreasing the need for high pressure and heat, [8][9][10] and reducing energy demand. The concept of using electricity to drive nitrogen reactions and fertilizer production is not new.…”

“…Recently, a multi-scale simulation model found that energy consumption of an electrolytic process could easily match the HaberBosch process. 10 The intrinsic design of electrochemical systems allows oxidation and reduction reactions to be separated, enabling a wider range of chemistries, 12 and potentially more selective catalysts that can be used for each reaction. This flexibility in chemistries and catalysts may eliminate the need to use highly purified inlet streams, allowing air to be the nitrogen source.…”

A membrane electrode assembly for the electrochemical synthesis of hydrocarbons from CO2(g) and H2O(g)

“…The advantage of this system over the L-L is believed to be higher mass transport of CO 2 in the electrode material. Main products of CO 2 reduction in the G-L ecMR are comparable to those formed in L-L type reactors [3,8,[10][11][12][13][14]. 3.…”

“…2. A second, "gas-liquid" (G-L) type ecMR utilises gaseous CO 2 in the cathodic compartment and a liquid electrolyte in the anodic compartment [3,10,11]. The advantage of this system over the L-L is believed to be higher mass transport of CO 2 in the electrode material.…”