Feasibility Study of Plasma-Catalytic Ammonia Synthesis for Energy Storage Applications

catalysts

Article Feasibility Study of Plasma-Catalytic Ammonia Synthesis for Energy Storage Applications

Kevin H. R. Rouwenhorst * and Leon Leﬀerts *

Catalytic Processes & Materials, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands * Correspondence: [email protected] (K.H.R.R.); l.leﬀ[email protected] (L.L.)

Received: 10 August 2020; Accepted: 25 August 2020; Published: 1 September 2020

Abstract: Plasma catalysis has recently gained traction as an alternative to ammonia synthesis. The current research is mostly fundamental and little attention has been given to the technical and economic feasibility of plasma-catalytic ammonia synthesis. In this study, the feasibility of plasma-catalytic ammonia is assessed for small-scale ammonia synthesis. A brief summary of the state of the art of plasma catalysis is provided as well as a targets and potential avenues for improvement in the conversion to ammonia, ammonia separation and a higher energy efficiency. A best-case scenario is provided for plasma-catalytic ammonia synthesis and this is compared to the Haber-Bosch ammonia process operated with a synthesis loop. An ammonia outlet concentration of at least 1.0 mol. % is required to limit the recycle size and to allow for efficient product separation. From the analysis, it follows that plasma-catalytic ammonia synthesis cannot compete with the conventional process even in the best-case scenario. Plasma catalysis potentially has a fast response to intermittent renewable electricity, although low pressure absorbent-enhanced Haber-Bosch processes are also expected to have fast responses to load variations. Low-temperature thermochemical ammonia synthesis is expected to be a more feasible alternative to intermittent decentralized ammonia synthesis than plasma-catalytic ammonia synthesis due to its superior energy efficiency.

Keywords: plasma catalysis; ammonia; process design; electriﬁcation; scale-down

1. Introduction Renewable wind energy and solar power increasingly penetrate the electrical power grid, spurring the electriﬁcation of the energy landscape [1]. However, as these energy sources are intermittent, energy storage is required. A wide range of technology is available, including batteries and thermo-mechanical storage for short-term energy storage, typically up to a few days [2]. Chemical energy storage and pumped hydropower are the main alternatives for seasonal energy storage [2–4]. Even though pumped hydropower is a potential solution for low-cost energy storage in naturally suited areas [4], the energy density of such systems is low, and pumped hydropower heavily depends on the availability of large natural water formations. Chemical energy storage in the form of hydrogen or hydrogen carriers has been proposed to solve the intermittency challenge. Renewable hydrogen is produced from water via electrolysis using renewable electricity, producing oxygen as a by-product. Hydrogen can be combusted to water in a fuel cell or in a gas turbine, producing electricity again. However, hydrogen is not easily stored on the long-term. Therefore, hydrogen carriers are required and ammonia (NH3) is one of the options [3,5]. Ammonia is currently mainly produced for fertilizer applications [6–8]. However, ammonia may be a hydrogen carrier in the circular economy [5,9,10]. In this case, intermittent renewables such as solar, tidal and wind power are coupled with chemical plants to produce ammonia.

Catalysts 2020, 10, 999; doi:10.3390/catal10090999 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 999 2 of 19 Catalysts 2020, 10, x FOR PEER REVIEW 2 of 20

Currently, ammonia is mainly produced with the Haber Haber-Bosch–Bosch process,process, continuouslycontinuously operated at high temperature and and pressure, pressure, i.e. i.e.,, 400 400–500–500 °◦CC and and 100 100–300-300 bar bar [6 [–68,11–8,11]. ].The The high high temperature temperature is is required to activate the stable N N triple bond over the industrial catalyst, and a high pressure is required to activate the stable N≡N≡ triple bond over the industrial catalyst, and a high pressure is required to shift the equilibrium to ammonia. Typically, Typically, Haber Haber-Bosch–Bosch plants have high production 1 1 capacities of of up up to to 3300 3300 t- t-NHNH3 d3 −1d, −with, with a potential a potential increase increase to 5000 to 5000–6000–6000 t-NH t-NH3 d−1 in3 d the− innear the future near [12,13future] [. 12This,13 ].corresponds This corresponds to a few to a gigawatt few gigawatt (GW (GW)) in the in the case case of ofan an electricity electricity-driven-driven Haber Haber-Bosch–Bosch 1 plant. Such Such electrolysis-based electrolysis-based Haber-Bosch Haber–Bosch plants plants operate operate at an at energy an energy cost of cost about of about 26–40 GJ26– t-NH40 GJ3− t-, NHdepending3−1, depending on the on electrolysis the electrolysis technology technology used [used14]. About[14]. About 95% 95% of this of energythis energy is required is required for thefor thehydrogen hydrogen production production and and the the remaining remaining part part is is required required for for nitrogen nitrogenpurification purification andand ammonia synthesis. The The theoretical theoretical minimum minimum energy energy input input for for ammonia ammonia synthesis synthesis from from air and air andwater water is 21.3 is 1 GJ21.3 t- GJNH t-NH3−1 [7]3.− [7]. Upon scaling down the Haber Haber-Bosch–Bosch process, the energy losses increase and the energy consumption for a ammoniammonia synthesis increases ( (seesee Figure 11).). DownDown toto thethe 1–101–10 microwavemicrowave (MW)(MW) 1 scale (equivalent to 3–303–30 t-NH t-NH33 d−−1),), energy energy losses losses are are limited limited and and the the energy consumption for 1 ammonia synthesis synthesis is is about about 36–40 36–40 GJ GJt-NH t-NH3− 3[−115 [15]]. However,. However, upon upon further further scaling scal down,ing down, energy energy losses lossesbecome become increasingly increasingly severe, and severe, using and alternative using alternative technologies technologies becomes attractive. becomes Non-conventional attractive. Non- conventionaltechnologies typically technologies cannot typically compete cannot with Haber-Bosch compete with at Haber large-scale,–Bosch due at tolarge the- highscale, energy due to e thefficiency high energyof Haber-Bosch. efficiency However, of Haber alternative–Bosch. technologies However, alternative operating under technologies milder conditions operating can under be beneficial milder conditionsfor small-scale can production.be beneficial This for small is important-scale production. for intermittent This is operation important required for intermittent in the case operation of rapid variationrequired in thethe availabilitycase of rapid of solar variation and wind in the power. availability Small-scale of solar ammonia and synthesiswind power. may Small be relevant-scale ammoniafor energy synthesis storage, specificallymay be relevant for isolated for energy small storage, communities. specifically for isolated small communities.

Figure 1. Energy consumption of electrolysis electrolysis-based-based Haber Haber-Bosch–Bosch processes as function of ammoniaammonia production capacity. Adapted and modifiedmodified from [[16]16].. OriginalOriginal referencesreferences [[1717–2626]].. One microwave 1 (MW) corresponds toto approximatelyapproximately 100100 kgkg hh−−1 ammoniaammonia.. 1.1. State of the Art of Plasma-Catalytic Ammonia Synthesis 1.1. State of the Art of Plasma-Catalytic Ammonia Synthesis Plasma catalysis has recently gained attention for electrification of chemical processes and for Plasma catalysis has recently gained attention for electrification of chemical processes and for energy storage applications [27–31], and for ammonia synthesis in specific [32–34]. Plasma is the fourth energy storage applications [27–31], and for ammonia synthesis in specific [32–34]. Plasma is the state of matter, in which electrons, ions, molecules, radicals, and excited species exist in a quasi-neutral fourth state of matter, in which electrons, ions, molecules, radicals, and excited species exist in a state. In case of thermal plasmas, electrons and heavier species have the same temperature. On the quasi-neutral state. In case of thermal plasmas, electrons and heavier species have the same other hand, non-thermal plasmas, the electron temperature is significantly higher than that of heavier temperature. On the other hand, non-thermal plasmas, the electron temperature is significantly ions and neutral species. Non-thermal plasmas can be coupled with a catalyst. Excited species higher than that of heavier ions and neutral species. Non-thermal plasmas can be coupled with a generated in the plasma (for instance, vibrationally activated nitrogen, N2(v)) can have an enhanced catalyst. Excited species generated in the plasma (for instance, vibrationally activated nitrogen, N2(v)) can have an enhanced adsorption rate as compared to ground-state molecules. As N2 dissociation is

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Microwave .] .] Microwave Microwave. r Microwaveeactors, Microwave dielectric (MW) (MW) (MW) (MW) (MW) and and barrier and and and discharge (DBD) An overview of reported energy consumptions for ammonia synthesis in various types of plasma radiofrequencyradiofrequencyradiofrequencyradiofrequencyradiofrequency (RF) (RF) (RF) (RF) (RF) plasmas reactorsplasmas plasmas plasmas plasmas have have have have have been been been been been studied studied studied studiedstudied studied in in in thein most inthe the the 1980sthe 1980s 1980s extensively1980s 1980s–1990s,––1990s,–1990s,–1990s,1990s, although although sincealthough although although the recently recently recently 2000srecently recently a [32,33,35 a fewa afew fewa few fewarticles articles articles articles ]articles. Microwave (MW) and reactors is shown in Figure2. Among the plasma reactors, dielectric barrier discharge (DBD) reactors havehavehavehavehave been been been been been published published published published published radiofrequency on on on onthis on this this this this subject subject subject subject subject (RF) as as as well as as plasmas well well well well [33] [33] [33] [33].[33] have. Glow . Glow. Glow. Glow Glow been discharges, discharges, discharges, discharges,studied discharges, in inspired inspired inspiredthe inspired inspired 1980s by by by– by1990s, the by the the the the commercial commercial commercialalthough commercial commercial recently a few articles have been studied most extensively since the 2000s [32,33,35]. Microwave (MW) and radiofrequency (RF) BirkelandBirkelandBirkelandBirkelandBirkeland–Eyde––Eyde–Eyde–EydeEyde process process process process processhave for for for forNO beenfor NO NO NO XNO productionX X publishedproduction XproductionX production production in on in inthe in thein thisthe the earlythe early early subjectearly early 20th 20th 20th 20th 20th century, as century, century, century, wellcentury, have[33] have have have have. been Glow been been been been studied studied discharges,studied studied studied in in inthe in thein the the1920s inspiredthe 1920s 1920s 1920s 1920s– –– –– by the commercial plasmas have been studied in the 1980s–1990s, although recently a few articles have been published on 1990s1990s1990s1990s1990s [32,33 [32,33 [32,33 [32,33 [32,33]. ]Other]. .]Other .]Other .Other Other technologies, technologies, Birkelandtechnologies, technologies, technologies,–Eyde such such such such such processas as as arcas asarc arc arc dischargesarc fordischarges discharges discharges dischargesNOX production and and and and andplasma plasma plasma plasma plasma in thejets jets jets jets earlyhavejets have have have have 20thonly only only only onlycentury, been been been been been reported reported reportedhave reported reported been in in in in studiedin in the 1920s– this subject as well [33]. Glow discharges, inspired by the commercial Birkeland-Eyde process for NO a afewa afew fewa few fewstudies, studies, studies, studies, studies, mostly mostly mostly mostly mostly with1990s with with with with an [32,33an an exploratoryan anexploratory exploratory exploratory ]exploratory. Other technologies,character character character character character [33,36 [33,36 [33,36 [33,36 such[33,36]. ]Various]. .]asVarious .]Various .Various arcVarious discharges authors authors authors authors authors have haveandhave have have discussed plasmadiscussed discussed discussed discussed jets plasma plasma plasmahave plasma plasma only been reported inX production in the early 20th century, have been studied in the 1920s–1990s [32,33]. Other technologies, reactorsrreactorseactorsrreactorseactors extensively extensively extensively extensively extensively [27,32,37 [27,32,37 a[27,32,37 [27,32,37 few[27,32,37 studies,]. ]]. .] .] . mostly with an exploratory character [33,36]. Various authors have discussed plasma such as arc discharges and plasma jets have only been reported in a few studies, mostly with an reactors extensively [27,32,37]. exploratory character [33,36]. Various authors have discussed plasma reactors extensively [27,32,37].

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The feasible region is discussed in Section 1.2.1. 1.2.1.2.1.2.1.2. 1.2.Comparison Comparison Comparison Comparison Comparison of of ofthe of the ofthe theS the mallS 1.2.S mallS mallSmallmall- ComparisonS-cale-SS-caleS-caleScale caleHaber Haber Haber Haber Haber– ofBosch––Bosch the–Bosch–BoschBosch Small-Scale P rocessesP P rocessesP rocessesProcessesrocesses and Haber-Boschand and and Sand tateS S tateS tateState-tateof--of-ofthe--of-the Processesofthe--the-Athe--ArtA-rtA- rtPlasmaA rtPlasma rtPlasma Plasma andPlasma State-of-the-ArtCatalysis Catalysis Catalysis Catalysis Catalysis Plasma Catalysis 1.2. Comparison of the Small-Scale Haber–Bosch Processes and State-of-the-Art Plasma Catalysis FigureFigureFigureFigureFigure 3 3shows3 3 shows3shows showsshows a acomparisona acomparisonacomparison comparisoncomparisonFigure3 ofshows of of reported of of reportedreported reportedreported a comparison state statestate statestate-of--of-of-theof--of-the ofthe--theartthe reported--artart- art-dataart datadata datadata on state-of-the-art onon onenergyon energyenergy energyenergy consumption consumptionconsumption consumptionconsumption data on of energy of of the of of thethe thethe consumption of the electrolysiselectrolysiselectrolysiselectrolysiselectrolysis-based--basedbased-based-based Haber HaberHaber HaberHaberelectrolysis-based–Bosch–Figure–Bosch–Bosch–BoschBosch process, process,3process, process,showsprocess, Haber-Bosch both botha both bothcomparison both at at at the at at the the the process, the10 10 10 kW10 10of kW kW kW reported kW and both and and and and 10 at 10 10 theMW10 10state MW MW MW 10 MW scale,- kWof scale, scale, scale,-the scale, and as - asart as 10well as as welldata well MW well well as as asonscale, plasmaas as plasma plasmaenergy plasma plasma as- well-- consumption-- as plasma-catalytic of the catalyticcatalyticcatalyticcatalyticcatalytic ammonia ammonia ammonia ammonia ammonia synthesis. synthesis. synthesis. synthesis. electrolysissynthesis.ammonia Electrolysis Electrolysis Electrolysis Electrolysis Electrolysis synthesis.-based -Haberbased--basedbased- Electrolysis-basedbased-based –hydrogenBosch hydrogen hydrogen hydrogen hydrogen process, production production production production production hydrogen both atis is requiredis the is required isproductionrequired required required 10 kW for for for andforboth for isboth both both requiredboth 10the the theMW the Haberthe Haber Haber Haber Haber scale,for– – both–– – as thewell Haber-Bosch as plasma- 1 process and plasma catalysis at an energy consumption of about 31.4−1 GJ−1−1−1−1 t-NH [16,66]. Nitrogen is BoschBoschBoschBoschBosch process process process processprocess and and and and andplasma plasmacatalytic plasma plasmaplasma catalysis catalysis catalysisammonia catalysis catalysis at at at ansynthesis.at at an an an energy an energy energy energy energy Electrolysis consumption consumption consumption consumption consumption-based of of of about ofhydrogen of about about about about 3 1 3 3.41 31 .4production 31.4 GJ1 .4 .4 GJ GJ GJ t GJ- tNH t-- NH tNH- tNH-3NH 3is 3[16,663 required3 [16,66 [16,66 [16,66[16,66]. 3]].− .] .]for . both the Haber– puriﬁed for all alternatives via pressure swing adsorption at an energy consumption of about 1.0 GJ NitrogenNitrogenNitrogenNitrogenNitrogen is is ispurified ispurified ispurified purified purified for Boschfor for for allfor all all alternativesall allalternatives processalternatives alternatives alternatives and via via via plasma via pressurevia pressure pressure pressure pressure catalysis swing swing swing swing swing adsorptionat adsorption adsorption adsorption anadsorption energy at at at anat consumptionatan an energyan anenergy energy energy energy consumption consumption consumption consumption consumption of about of 3 of1 of .4of of GJ t-NH3−1 [16,66]. t-NH [16]. At the 10 MW scale, the high pressure ammonia synthesis loop of the Haber-Bosch process aboutaboutaboutaboutabout 1.0 1.0 1.0 1.0GJ 1.0 GJ GJ tGJ - GJtNH t- -NHt NH-tNH-3NH [16].3 3[16]. 3[16]. 3[16].Nitrogen [16]. At At At theAt At 3the the the 10the 10is 10 MW 10 purified10MW MW MW MW scale, scale, scale, scale, scale, for the the theall the highthe high alternativeshigh high high pressure pressure pressure pressure pressure ammoniavia ammonia ammonia ammonia pressureammonia synthesis synthesis synthesis swingsynthesis synthesis loopadsorption loop loop loop loop of of ofthe of ofthe the the Habertheat Haber Haber anHaber Haber energy– –––– consumption of 1 has a limited energy loss and consumes about 3.6−1 GJ−1−1−1−1 t-NH [67]. However, at 10 kW, the energy BoschBoschBoschBoschBosch process process process process process has has has has hasa a limiteda alimited aboutlimiteda limited limited energy1.0 energy energy energyGJenergy t -lossNH loss loss loss loss3and [16].and and and andconsumes consumesAt consumes consumes consumes the 10 aboutMW about about about about scale, 3.6 3.6 3.6 3.6 GJ3.6 theGJ GJ tGJ - GJtNH hight- -NHt NH-tNH-NH pressure[67] [67] [67] [67] .[67] However,. .However, .However, .−However, However,ammonia at at at10 at synthesis10at 10 kW,10 10kW, kW, kW, kW, loop of the Haber– 1 consumption of the high pressure ammonia synthesis loop is as high as 45–50 GJ−1−1−1 t-NH−1−1 (see Figure3). thethethe theenergythe energy energy energy energy consumption consumption consumption consumption consumptionBosch of of ofthe of processofthe the the highthe high high high high pressurehas pressure pressure pressure pressurea limited ammonia ammonia ammonia ammonia ammonia energy synthesis synthesis losssynthesis synthesis synthesis and loop consumesloop loop loop loop is is isas is as is as highas ashigh abouthigh high high as as as 453.6as as45 45– 45 50GJ–45–50–50 –GJ50t 50-GJ NHGJ tGJ - GJtNH t- −1-NHt NH-t [67]NH-3NH3 3 .3 3 However, 3− at 10 kW, This is mainly due to large heat losses from the synthesis reactor operating at high temperatures. the energy consumption of the high pressure ammonia synthesis loop is as high as 45–50 GJ t-NH3−1

Catalysts 2020, 10, x FOR PEER REVIEW 4 of 20

(see Figure 3). This is mainly due to large heat losses from the synthesis reactor operating at high Catalyststemperatures.2020, 10, 999 4 of 19

Figure 3. Estimated energy consumption of state-of-the-art, small-scale, electrolysis-based Haber-Bosch, Figureand plasma 3. Estimated catalysis. energyFor the Haber-Bosch consumption process, of state estimates-of-the-art are, small based-scale, on reported electrolysis energy-based consumption Haber– Boschin Figure, and1. Forplasma plasma catalysis catalysis,. For thethe energy Haber– consumptionBosch process, for estimates gas recycling are based and ammonia on reported separation energy consumptionis based on estimates in Figure in 1 for. For low plasma pressure, catalysis low conversion, the energy systems consumption with solid for sorbents gas recycling as reported and ammoniain [16,67]. separation The energy is consumption based on estimates for plasma in for catalysis low pressure, is based low on dataconversion from Kim systems et al. [with43] over solid a sorbents as reported in [16,67]. The energy consumption for plasma catalysis is based on data from Ru-MgO/γ-Al2O3 catalyst at 0.2% conversion. The energy cost for recycling and separation with solid Kimsorbents et al. is [43] discussed over a Ru in Section-MgO/γ 3.1-Al.2O Technology3 catalyst at readiness 0.2% conversion. level (TRL). The Theenergy TRL cost levels for hererecycling apply and for separationthe complete with system. solid sorbents TRL 1 is is the discussed basic idea, in Section while TRL 3.1. 9Technology is a commercial readiness system. level For (TRL) more. The details, TRL levelssee ref. here [68]. apply for the complete system. TRL 1 is the basic idea, while TRL 9 is a commercial system. For more details, see ref. [68]. As shown in Figure3, the state-of-the-art system for a plasma-catalytic ammonia synthesis process has aAs considerably shown in higherFigure energy 3, the consumption state-of-the-art than system the electrolysis-based for a plasma-catalytic Haber-Bosch ammonia process, synthesis both at 10process kW and has at a 10 considerably MW. Plasma higher catalysis energy allows consumption for operation than at milder the electrolysis conditions-based than Haber conventional–Bosch process,catalysis both over at the 10 industrialkW and at iron10 MW. catalyst. Plasma However, catalysis allows plasma for catalysis operation is onlyat milder beneficial conditions when than the conventionalenergy consumption catalysis of over a plasma-catalytic the industrial iron ammonia catalyst synthesis. However, process plasma is atcatalysis least equal is only to the beneficial energy 1 whenconsumption the energy of the consumption electrolysis-based of a plasma Haber-Bosch-catalytic process ammonia at 10 synthesis kW (about process 80 GJ t-NHis at 3least− ). Theequal best to 1 thereported energy value consumption for plasma catalysisof the electrolysis is 95 GJ t-NH-based3− atHaber 0.2 mol.–Bosch % NH process3 [43], while at 10 most kW (about reported 80 values GJ t- −1 3 6 1 −1 varyNH3 in). The the rangebest reported between value 10 and for 10plasmaGJ t-NH catalysis3− at is various 95 GJ t- conversionNH3 at 0.2 levels mol. % (see NH Figure3 [43]2, )[while33]. 3 6 −1 mostFurthermore, reported thevalues energy vary consumption in the range ofbetween recycling 10 of and unconverted 10 GJ t-NH N32 andat various H2 for conversion the best reported levels plasma-catalytic(see Figure 2) [33] systems. Furthermore, is high, duethe energy to thelow consumption conversion of levels recycling (see Figureof unconverted2). The recycling N2 and costH2 for is theusually best notreported reported plasma for plasma-catalytic catalysis systems (like is inhigh, Figure due2), to but the is low significant conversion in case levels of low (see conversions Figure 2). Theto ammonia, recycling i.e., cost below is usually 1.0 mol. not %reported NH3 [67 for]. plasma catalysis (like in Figure 2), but is significant in case of low conversions to ammonia, i.e., below 1.0 mol. % NH3 [67]. 1.2.1. Targets and Strategies for More Energy-Efficient Plasma-Catalytic Ammonia Synthesis 1.2.1. Targets and Strategies for More Energy-Efficient Plasma-Catalytic Ammonia Synthesis It is estimated that plasma catalysis may be competitive with the Haber-Bosch at a target total 1 energyIt is consumption estimated that of 80plasma GJ tNH catalysis3− , based may on be the competitive comparison with with the electrolysis-based Haber–Bosch at Haber-Boscha target total 1 processenergy consumption at 10 kW, equivalent of 80 GJ to tNH about3−1, ~10based kg on t-NH the3 comparisonday− production. with electrolysis-based Haber–Bosch processThe at energy 10 kW, costs equivalent of hydrogen to about production ~10 kg t- andNH nitrogen3 d−1 production. purification is the same for both processes 1 and amountThe energy to about costs 32 of GJ hydrogent-NH3− . Ammonia production separation and nitrogen in the Haber-Bosch purification isprocess the same is achieved for both by processescondensation. and However,amount to underabout mild32 GJ pressure t-NH3−1. (

N2 and H2. In principle, the recycle cost is usually not an issue for industrial ammonia synthesis, as the 1 energy cost of recycling is below 1 GJ t-NH3− at NH3 concentrations above 1.0 mol. % in the reactor outlet [67]. However, for the current best-reported data for plasma catalysis, the ammonia concentration 1 is max 0.2% (see Figure3), implying recycling costs as high as 100 GJ t-NH 3− . Clearly, increasing the NH3 concentration in the outlet of the plasma reactor to above 1.0%, at a reasonable energy cost, is desirable. Another beneﬁt of increasing the conversion level and consequently the ammonia concentration is the reduction in capital investment for recycling, due to the smaller compressor. In the upcoming section, potential improvements for plasma catalysis are discussed.

2. Plasma Reactor and Catalyst Improvements to Plasma-Catalytic Ammonia Synthesis The plasma reactor forms the core of the plasma-catalytic ammonia synthesis process. In this section, the plasma reactor types are evaluated, as well as the necessary coupling with a catalyst.

2.1. Plasma Reactor Type The first choice to make is the plasma reactor type. While dielectric barrier discharge (DBD) reactors and glow discharge reactors operate at atmospheric pressure (or elevated pressures [69]), microwave (MW) reactors and radiofrequency (RF) reactors operate at sub-atmospheric pressures. In principle, operating at atmospheric pressures is beneficial, as vacuum conditions require expensive equipment and large volumes. Furthermore, separation of ammonia is difficult at low partial pressures. Metrics to evaluate the plasma reactor type is the energy consumption for ammonia formation (requirement about 40 GJ t-NH 1, see Figure3), as well as the conversion (requirement 1.0 mol. 3− ≥ % NH3). As shown in Figure2, no data in the literature have attained this so far. It should be noted that the data shown in Figure2 vary widely in temperature, pressure, H 2:N2 ratio, and catalyst choice. However, the apparent general trend is that the energy consumption for ammonia formation is lowest in dielectric barrier discharge reactors. Other reactor types (glow discharges, microwave reactors, and radiofrequency reactors) have an energy consumption of one to four orders of magnitude higher. This may be attributed to the more effective plasma-catalyst coupling for a DBD reactor. Therefore, a dielectric barrier discharge reactor is the preferred plasma reactor type, with the current knowledge.

Plasma Optimization Plasma optimization should be focused on maximizing the density of mildly activated molecular nitrogen species (N2(v) or N2(e)), rather than radical species, as the mildly activated molecular nitrogen species require less energy input than fully dissociated nitrogen radicals. Mild activation of N2 to N2(v) or N2(e) can primarily be achieved by a relatively low specific input energy (SIE). Upon increasing the SIE, the reduced electric field increases, thereby increasing the fraction of N2 dissociated in the plasma [27]. Furthermore, N2-rich feeds are beneficial for activating N2 rather than H2 [43]. Various authors have reported an improved energy efficiency upon using pulsed plasmas in a dielectric barrier discharge reactor, rather than a continuous AC plasma [43,53]. For AC plasmas, excited N2 molecules are readily dissociated due to the continuous presence of activated species. This leads to dissociation and the formation of N radicals, which recombine with other species and form heat. For pulsed plasmas, less of the activated N2 is dissociated in the plasma due to climbing along the vibrational ladder [70]. Thus, a pulsed plasma dielectric barrier discharge reactor is preferred as the plasma reactor. Further modifications can be made to the plasma properties, by changing the discharge frequency and the capacitive and discharge regimes. Plasma properties can be modified by the electrode material [45,54], as well as the dielectric constant in the reactor. Performance enhancement may be attained upon the physical mixture of the active catalyst with a dielectric material [40], which can be attributed to a change in electron number density and average energy distribution. Catalysts 2020, 10, x FOR PEER REVIEW 6 of 20 electrode material [45,54], as well as the dielectric constant in the reactor. Performance enhancement may be attained upon the physical mixture of the active catalyst with a dielectric material [40], which Catalysts 2020, 10, 999 6 of 19 can be attributed to a change in electron number density and average energy distribution.

2.2. Reaction M Mechanismsechanisms and C Catalystatalyst O Optimizationptimization Several mechanisms mechanisms towards towards ammonia ammonia synthesis synthesis in the in presence the presence of a plasma of a are plasma being arediscussed, being discussed,as previously as previou reportedsly in reported [38]. The in various[38]. The mechanisms various mechanisms for N2 activation for N2 activation are shown are in shown Figure 4in. 1 −1 −11 TheFigure bond-dissociation 4. The bond-dissociation energies energies of N2 and of N H2 and2 are H2 945 are kJ945 mol kJ mol− and and 436 436 kJ kJ mol mol− respectively,respectively, −11 which translates toto 66.1 GJ t-NHt-NH33− atat 100% 100% efficient efficient conversion conversion to to ammonia ammonia via via complete complete dissociation. dissociation. This implies implies that that plasma-induced plasma-induced dissociation dissociation and subsequent and subsequent radical reactionsradical reactions can never can be su neverfficiently be 1 −1 sufficientlyenergy efficient energy to attainefficient the to required attain the 40 required GJ t-NH3 40− GJas explainedt-NH3 as inexplained Section 1.2.1in Section. Thus, 1.2.1 a catalyst. Thus, isa catalystrequired is to required aid in the to aid dissociation in the dissociation of the molecules. of the molecules. As shown As in shown Figure in4, theFigure plasma 4, the can plasma activate can (ex) (ex) activatemolecular molecular N2 via vibrational N2 via vibrational or electronic or electronic excitation excitation (denoted as(denoted N2 in as Figure N2 4 in), whichFigure lowers 4), which the lowersbarrier the for Nbarrier2 dissociation for N2 dissociation over the catalyst. over the Typically, catalyst. catalystsTypically, which catalysts are able which to dissociateare able to N dissociate2 are also Nable2 are to also dissociate able to H dissociate2, while some H2, while mid-late some transition mid-late metaltransition catalysts metal such catalysts as Pd such and Ptas canPd and dissociate Pt can dissociateH2, but not H N2, 2but. not N2.

Figure 4. VariousVarious mechanisms for N 2 activationactivation by by the the pl plasmaasma and and subsequent subsequent dissociative dissociative adsorption adsorption

on a metalmetal surface.surface. Green: thermal N22 activation on aa catalyst.catalyst. Blue:Blue: plasma-activatedplasma-activated molecularmolecular NN22 dissociating overover thethe metalmetal with with a a barrier barrier over over the the catalyst. catalyst. Orange: Orange: complete complete dissociation dissociation of Nof2 byN2 theby plasma,the plasma, followed followed bybarrierless by barrierless adsorption adsorption onthe on metalthe metal surface. surface. 2.2.1. Plasma-Catalytic Ammonia Synthesis with Molecular Species 2.2.1. Plasma-Catalytic Ammonia Synthesis with Molecular Species Thus, plasma activation of N2 without complete dissociation is the required pathway for Thus, plasma activation of N2 without complete dissociation is the required pathway for plasmaplasma-catalytic ammonia synthesis with a low energy consumption. Mehta et al. [71] proposed that catalytic ammonia synthesis with a low energy consumption. Mehta et al. [71] proposed that plasmaplasma-activated molecular N2 can enhance the rate of ammonia formation, which was substantiated activated molecular N2 can enhance the rate of ammonia formation, which was substantiated by by Rouwenhorst et al. [38] for Ru-based catalysts at a low plasma power input of 83–367 J L 1. Rouwenhorst et al. [38] for Ru-based catalysts at a low plasma power input of 83–367 J L−1. Mehta −et Mehta et al. [71] postulated that the plasma activates the N2, thereby lowering the barrier for N2 al. [71] postulated that the plasma activates the N2, thereby lowering the barrier for N2 dissociation, dissociation, while subsequent hydrogenation steps are not inﬂuenced by the plasma. while subsequent hydrogenation steps are not influenced by the plasma. Furthermore, Mehta et al. [71] postulated that thermally less active metals can become good Furthermore, Mehta et al. [71] postulated that thermally less active metals can become good catalysts for converting plasma-activated N2 to ammonia. However, the importance of plasma-activated catalysts for converting plasma-activated N2 to ammonia. However, the importance of plasma- molecular N2 (either vibrational or electronic) has been experimentally substantiated exclusively for activated molecular N2 (either vibrational or electronic) has been experimentally substantiated Ru-based catalysts [38,72]. It remains an open question whether noble metals can become active for N exclusively for Ru-based catalysts [38,72]. It remains an open question whether noble metals can2 dissociation after vibrational or electronic excitation. Over Ru-based catalysts, the apparent barrier become active for N2 dissociation after vibrational or electronic excitation.1 Over Ru-based catalysts, for N2 dissociation over the catalyst decreases by about 40–70 kJ mol− upon plasma activation [38]. the apparent barrier for N2 dissociation over the catalyst decreases by about 40–70 kJ mol−1 upon The discussion hereafter focuses on Ru-based catalysts, as a mechanism with excited N has only been plasma activation [38]. The discussion hereafter focuses on Ru-based catalysts, as a mechanism2 with proven to be possible for this metal. However, the role of the support and alkali promoters should also excited N2 has only been proven to be possible for this metal. However, the role of the support and be valid for other metals [73]. alkali promoters should also be valid for other metals [73]. The support has a profound eﬀect on the ammonia synthesis activity over Ru-catalysts [74]. The ammonia synthesis rate can vary by multiple orders of magnitude, depending on the catalyst formulation. For Ru catalysts it was found that oxide supports with a low electronegativity show a Catalysts 2020, 10, 999 7 of 19

high ammonia synthesis activity for thermal catalysis [75]. A similar trend was conﬁrmed for plasma catalysis (see Figure5), suggesting that a pathway via excited molecular N 2 is indeed dominant. The introduction of promoters (alkali metals and alkaline earth metals) can enhance the activity further [74,76]. The nitrogen dissociation barrier decreases upon the introduction of an alkali metal due to electrostatic interactions [76]. A similar activity improvement upon the introduction of an alkali metal was also reported for Co catalysts [77]. For this mechanism, the temperature should be such that N2 dissociation on the catalyst and NH3 desorption from the catalyst are possible. Thus, the temperature Catalystsfor plasma 2020, catalysis10, x FOR PEER cannot REVIEW be much lower than for thermal catalysis. 7 of 20

FigureFigure 5. 5. AmmoniaAmmonia synthesis synthesis rate rate as as a a function function of of the the electronegativity electronegativity for for plasma plasma-catalytic-catalytic ammonia ammonia synthesis. Reproduced from [38]. synthesis. Reproduced from [38]. The lowest reported energy requirement for plasma catalysis so far is for a promoted Ru/Al O The support has a profound effect on the ammonia synthesis activity over Ru-catalysts [74]. The2 3 catalyst [43]. As listed in Table1, trends in thermal catalysis indicate that changes in the catalyst ammonia synthesis rate can vary by multiple orders of magnitude, depending on the catalyst may facilitate a lower energy cost for NH synthesis, given that the right plasma reactor is used. formulation. For Ru catalysts it was found that3 oxide supports with a low electronegativity show a Promoted Ru/MgO catalysts are about 25 times more active than promoted Ru/Al O catalysts for high ammonia synthesis activity for thermal catalysis [75]. A similar trend was confirmed2 3 for plasma thermal catalysis [78]. As similar trends are valid for plasma catalysis at low conversions (see Figure5), catalysis (see Figure 5), suggesting that a pathway via excited molecular N2 is indeed dominant. The an increased productivity and a lower energy cost may be attained upon using promoted Ru/MgO introduction of promoters (alkali metals and alkaline earth metals) can enhance the activity further catalysts. As listed in Table1, the enhancement for plasma catalysis upon using promoted Ru /MgO is less [74,76]. The nitrogen dissociation barrier decreases upon the introduction of an alkali metal due to profound at high conversions resulting in relatively high ammonia concentration (1.1–3.7 mol. % NH ). electrostatic interactions [76]. A similar activity improvement upon the introduction of an alkali metal3 was also reported for Co catalysts [77]. For this mechanism, the temperature should be such that N2 Table 1. Comparison of activity trends over Ru catalysts for thermal catalysis and for plasma catalysis. dissociation on the catalyst and NH3 desorption from the catalyst are 1possible. Thus, the temperature Thermal catalysis: data of Muhler et al. [78], 385 ◦C, 1 atm, 40 mL min− ,H2:N2 = 3:1, 138 mg catalyst. for plasma catalysis cannot be much lower than for thermal catalysis.1 Plasma catalysis: data of Ruan et al. [79], 180 ◦C, 60 mL min− ,H2:N2 = 3:1, 1.1-3.7 mol. % NH3. The lowest reported energy requirement for1 plasma catalysis so far is for a promoted Ru/Al2O3 Kim et al. [43], 250–300 ◦C, 1 atm, 1–4 L min− ,H2:N2 = 1:4, 17.1 g catalyst, 0.01–0.16 mol. % NH3. 1 catalystTarget [43] value. As listed for plasma in Table catalysis: 1, trends 40 GJ in t-NHthermal3− at catalysis 1.0% NH indicate3 at outlet. that “Relative changes act.” in the refers catalyst to the may facilitaterelative a lower ammonia energy synthesis cost for rate NH on3 synthesis, various catalysts, given withthat Ruthe/Al right2O3 plasmaas the base-case reactor (1.0).is used. Promoted Ru/MgO catalysts are about 25 times more active than promoted Ru/Al2O3 catalysts for thermal catalysisCatalyst [78]. As Thermal similar Catalysis trends are valid for plasma Plasma catalysis Catalysis at low conversions (see Figure 5), an increased productivityMuhler et and al. [ a78 lower] Ruan energy et al. cost [79] may be attained Kim et al. upon [43 ] using promoted Ru/MgO 1 catalysts. As listedRelative in Table Act. 1, the enha Relativencement Act. for plasma Relative catalysis Act. uponEnergy using Cost promoted (GJ t-NH Ru/MgO3− ) is less profound at high conversions resulting in relatively high ammoniaAC concentration Plasma Pulsed (1.1–3.7 Plasma mol. % NH3). Ru/Al2O3 1.0 1.0 1.0 1029–1800 - Ru/Al O 2 3 2.5 - 2.8–3.3 313–563 101–141 promotedTable 1. Comparison of activity trends over Ru catalysts for thermal catalysis and for plasma catalysis. RuThermal/MgO catalysis: data 9.2 of Muhler et al. [78] 1.5, 385 °C, 1 atm, 40 - mL min−1, H2:N2 -= 3:1, 138 mg catalyst. - RuPlasma/MgO catalysis: data of Ruan et al. [79], 180 °C, 60 mL min−1, H2:N2 = 3:1, 1.1-3.7 mol. % NH3. Kim et 62 3.3 - - - promotedal. [43], 250–300 °C, 1 atm, 1–4 L min−1, H2:N2 = 1:4, 17.1 g catalyst, 0.01–0.16 mol. % NH3. Target value for plasma catalysis: 40 GJ t-NH3−1 at 1.0% NH3 at outlet. “Relative act.” refers to the relative ammonia

synthesis rate on various catalysts, with Ru/Al2O3 as the base-case (1.0).

Thermal Catalyst Plasma Catalysis Catalysis Ruan et al. Muhler et al. [78] Kim et al. [43] [79] Relative Relative Act. Relative Act. Energy Cost (GJ t-NH3−1) Act. AC Pulsed

Plasma Plasma Ru/Al2O3 1.0 1.0 1.0 1029–1800 -

Catalysts 2020, 10, x FOR PEER REVIEW 8 of 20

Ru/Al2O3 2.5 - 2.8–3.3 313–563 101–141 promoted Ru/MgO 9.2 1.5 - - - Ru/MgO 62 3.3 - - - Catalystspromoted2020, 10 , 999 8 of 19

2.2.2.2.2.2. Best Best-Case-Case S Scenariocenario for for Plasma Plasma Catalysis Catalysis AsAs shown shown in in Figure Figure 33,, thethe energyenergy costscosts ofof thethe state-of-the-artstate-of-the-art systemsystem forfor plasmaplasma catalysiscatalysis andand recycling are high (about 197 GJ t-NH3−1). 1Furthermore, the energy cost for recycling scales with the recycling are high (about 197 GJ t-NH3− ). Furthermore, the energy cost for recycling scales with reciprocalthe reciprocal of the of the single single pass pass conversion. conversion. This This implies implies that that increasing increasing the the conversion conversion by by catalyst catalyst optimizationoptimization and and plasma plasma optimization optimization inherently inherently decreases decreases the energy the energy cost of cost the recycle. of the An recycle. ammonia An ammoniaconcentration concentration of about 1.0 of mol.about % 1.0is requiredmol. % is to required eﬀectively to effectivelyuse the reactor use the volume reactor for volume separation, for separation,as will be discussed as will be in discussed Section 3.1 in. Section 3.1. A target of 40 GJ t-NH3−11 at 1.0 mol. % ammonia outlet concentration was set for the plasma A target of 40 GJ t-NH3− at 1.0 mol. % ammonia outlet concentration was set for the plasma rreactoreactor (see (see Section Section 1.2.11.2.1).). The The lowest lowest energy energy consumption consumption reported reported so so far far is is for for a a pulsed pulsed plasma plasma DBD DBD reactors with an energy consumption of 95 GJ t-NH13−1 at 0.16 mol. % over a Mg-Ru/Al2O3 catalyst reactors with an energy consumption of 95 GJ t-NH3− at 0.16 mol. % over a Mg-Ru/Al2O3 catalyst [43]. [43]Thus,. Thus, an energy an energy consumption consumption enhancement enhancement by a factor by a factor 2.4 is required,2.4 is requir whereased, whereas the conversion the conversion should shouldincrease increase by a factor by a factor 6.3 to 6.3 attain to attain feasible feasible operation. operation. Based Based on on trends trends in in thermal thermal catalysis,catalysis, a rate rate enhancement is expected by changing the catalyst from promoted Ru/Al2O3 to promoted Ru/MgO enhancement is expected by changing the catalyst from promoted Ru/Al2O3 to promoted Ru/MgO (see Table 1). It should be noted that the plasma activation of NH3 is an inherent limitation to the (see Table1). It should be noted that the plasma activation of NH 3 is an inherent limitation to the energy efficiency at high conversions, as NH3 is increasingly activated by the plasma with increasing energy eﬃciency at high conversions, as NH3 is increasingly activated by the plasma with increasing NH3 concentration. NH3 concentration. InIn order order to to get get an an estimate estimate of the of best-case the best- scenariocase scenario (BCS) for (BCS) plasma-catalytic for plasma-catalytic ammonia ammonia synthesis, synthesis, we assume the barrier decrease for N2 dissociation equals the energy input for plasma- we assume the barrier decrease for N2 dissociation equals the energy input for plasma-catalytic catalytic NH3 synthesis. The decrease in the N2 dissociation barrier over Ru catalysts is about 0.7 eV NH3 synthesis. The decrease in the N2 dissociation barrier over Ru catalysts is about 0.7 eV [38], [38], which translates to about 4.0 GJ t-NH1 3−1 or 1.1 kWh kg1 −1. From Figure 6, it follows that an which translates to about 4.0 GJ t-NH3− or 1.1 kWh kg− . From Figure6, it follows that an ammonia outlet concentration of 0.35 mol. % is required to attain an energy cost of 40 GJ t-NH3−1. For1 ammonia outlet concentration of 0.35 mol. % is required to attain an energy cost of 40 GJ t-NH3− . 1.0 mol. % NH3 concentration at the reactor outlet, the total energy consumption for the plasma For 1.0 mol. % NH3 concentration at the reactor outlet, the total energy consumption for the plasma reactor (BCS) and the recycling is about 5.0 GJ t-NH3−1 1(see Figure 6). reactor (BCS) and the recycling is about 5.0 GJ t-NH3− (see Figure6).

Figure 6. Energy cost of the recycle as a function of the ammonia concentration at the reaction outlet, Figureas well 6. as Energy the best cost reported of the recycle value for as plasmaa function catalysis, of the ammonia and a best-case concentration scenario at for the plasma reaction catalysis. outlet, asThe well energy as the cost best of recyclingreported isvalue estimated for plasma based catalysis on values, and reported a best by-case Smith scenario et al. [ 67for]. plasma The best catalysis reported. Thevalue energy for plasma cost of catalysis recycling is found is estimated in the data based of Kim on values et al. [ 43 reported]. by Smith et al. [67]. The best reported value for plasma catalysis is found in the data of Kim et al. [43]. 3. Ammonia Separation and Conceptual Process Design

An evaluation of various process alternatives for ammonia separation in a plasma-catalytic ammonia synthesis process is presented. The cost of such a small-scale process is compared to a small-scale Haber-Bosch synthesis loop. As hydrogen and nitrogen production occur in a similar manner, this is not discussed here. Catalysts 2020, 10, 999 9 of 19

3.1. Separation and Storage Ammonia is conventionally separated by condensation. However, this does not allow for efficient and complete separation at low operating pressures, as ammonia has a significant vapor pressure under ambient conditions (~7 bar). Therefore, an alternative method for ammonia separation is required for ammonia synthesis under mild pressures [67]. Solid sorbents can be used for this purpose and a wide range of materials has been tested [66]. Among these sorbents, metal halides (e.g., metal chlorides and metal bromides) and zeolites are most promising (see Table2). A benefit of metal halides over zeolites is the higher ammonia density of the storage material. Furthermore, ammonia can be separated at elevated temperatures using metal halides, minimizing the temperature difference between the plasma reactor and the separation step. This saves on the cost of heat integration between the plasma reactor (probably 200–300 ◦C) and the separation step, i.e., 150–250 ◦C for metal halides, as compared to 20–100 ◦C for zeolites. For low conversion systems such as plasma-catalytic ammonia synthesis, the investment cost of the recycle compressor and heat exchanger generally dominate the investment cost for the synthesis loop [67]. Therefore, metal halides are the preferred option in this case. However, it should be noted that zeolites are of interest if plasma-catalytic ammonia synthesis at room temperature is developed with plasma-activated N2 as the relevant species.

Table 2. Comparison of ammonia separation technologies. Based on references [16,80–83]. * The energy 1 consumption increases to 20–25 GJ t-NH3− at 20 bar [67].

Condensation Metal Halides Zeolites Separation temperature ( C) 20 to 30 150–250 20–100 ◦ − Desorption temperature (◦C) - 350–400 200–250 Pressure (bar) 100–450 10–30 10–30 1 Energy consumption (GJ t-NH3− ) 3–5 * 6–11 8 Ammonia at outlet (mol. %) 2–5 0.1–0.3 0.1–0.3 Ammonia capacity (wt. %) 100 5–30 5–15 3 Ammonia density (kg m− ) 680 100–600 30–90 Chemical stability - Low/Medium High Technology readiness level (TRL) 9 4–5 4–5

Ammonia can be stored inside the metal halides. Metal halides absorb ammonia via diffusion of ammonia into the lattice, forming an ammine complex. For instance, ammonia can be absorbed into CaCl2 and MgCl2, forming Ca(NH3)XCl2 and Mg(NH3)XCl2. Ammonia leakage risks are reduced as compared to liquefied ammonia, as the ammonia vapor pressure in equilibrium with the ammonia loaded metal halides is significantly lower as compared to the vapor pressure of pure ammonia [84]. This decreases safety risks substantially. Thus, ammonia is stored inside metal halides, until ammonia is required for combustion in for instance a fuel cell or engine [16]. A simplified process scheme of a plasma-catalytic ammonia synthesis loop is shown in Figure7. The adsorption-desorption cycle can be achieved by switching between multiple beds or by using moving bed reactors. Catalysts 2020, 10, 999 10 of 19 Catalysts 2020, 10, x FOR PEER REVIEW 10 of 20

Figure 7. Process scheme of a plasma-catalytic ammonia synthesis loop. Figure 7. Process scheme of a plasma-catalytic ammonia synthesis loop. 3.2. Synergy between Plasma Reactor and Ammonia Separation and Storage 3.2. Synergy between Plasma Reactor and Ammonia Separation and Storage As discussed above, synergy may be attained upon matching the operating temperature of the plasmaAs reactordiscussed and above, the ammonia synergy separator.may be attained The plasma upon matching reactor can the operate operating at temperatures temperature aboveof the plasma reactor and the ammonia separator. The plasma reactor can operate at temperatures above the light-off temperature (typically > 150 ◦C), where a higher temperature implies a higher ammonia the light-off temperature (typically > 150 °C), where a higher temperature implies a higher ammonia synthesis rate. On the other hand, upon increasing the temperature too much (i.e., above 300 ◦C), synthesisthe benefits rate. of On plasma the other catalysis hand, upon over increasing heterogeneous the temperature catalysis without too much a plasma(i.e., above are 300 negligible. °C), the benefits of plasma catalysis over heterogeneous catalysis without a plasma are negligible. Some Some catalysts show substantial thermal activity above 300 ◦C[85–88], which implies that the catalystsuse of plasma show issubstantial not necessary. thermal activity above 300 °C [85–88], which implies that the use of plasma is not necessary. Among metal halides, MgCl2 shows suitable absorption-desorption cycles. At an ammonia Among metal halides, MgCl2 shows suitable absorption-desorption cycles. At an ammonia partial pressure of 10 kPa and at 200 ◦C, two moles of ammonia can be absorbed in MgCl2, forming partial pressure of 10 kPa and at 200 °C, two moles of ammonia can be absorbed in MgCl2, forming Mg(NH3)2Cl2. Upon increasing the temperature to 300 ◦C, all ammonia can be released again. AsMg(NH ammonia3)2Cl2. absorptionUpon increasing is determined the temperature by the kinetic to 300 rate °C, ofall the ammonia surface can reaction be released [89], maximizing again. As ammonia absorption is determined by the kinetic rate of the surface reaction [89], maximizing the the available MgCl2 surface area is beneficial. Supporting MgCl2 on an inert oxide can increase the available MgCl2 surface area is beneficial. Supporting MgCl2 on an inert oxide can increase the available surface area of MgCl2. Furthermore, nano-pores are formed in the metal halide structure upon availableammonia surface desorption. area of Possibly, MgCl2. theFurthermore, mechanical nano stability-pores can are be formed better maintainedin the metal upon halide supporting structure upon ammonia desorption. Possibly, the mechanical stability can be better maintained upon MgCl2 on an inert oxide. Higher ammonia absorption capacities are obtained when loading metal supporting MgCl2 on an inert oxide. Higher ammonia absorption capacities are obtained when halides on oxide supports [82]. MgCl2/SiO2 (40 wt. % MgCl2) is the best among the tested materials loading metal halides on oxide supports [82]. MgCl2/SiO2 (40 wt. % MgCl2) is the best among the with an experimental sorbent capacity of about 5 wt. % ammonia at 200 ◦C, whereas the theoretical testedmaximum materials is 11 wt.with % an ammonia. experimental sorbent capacity of about 5 wt. % ammonia at 200 °C, whereas the theoreticalThe operating maximum conditions is 11 andwt. % the ammonia. energy consumption for the state-of-the-art plasma reactor and a best-caseThe operating scenario conditions (BCS) plasma and the reactor energy are consumption listed in Table for3 the, as state well-of as-the the-art operating plasma conditionsreactor and a best-case scenario (BCS) plasma reactor are listed in Table 3, as well as the operating conditions for for ammonia separation with a MgCl2/SiO2 sorbent. Due to the similar reaction conditions in the plasmaammonia reactor separation and in with the absorptiona MgCl2/SiO step,2 sorbent. heat integration Due to the cansimilar indeed reaction be minimized. conditions This in the simplifies plasma rtheeactor synthesis and in loop the substantiallyabsorption step, in caseheat ofintegration intermittent can operation. indeed be Fromminimized. Table3 Thisit follows simplifies that thethe synthesis loop substantially in case of intermittent operation. From Table 3 it follows that the main main improvement required is the energy consumption in the plasma reactor, as was also discussed improvement required is the energy consumption in the plasma reactor, as was also discussed in in Section 2.2.2. Section 2.2.2.

Catalysts 2020, 10, 999 11 of 19

Table 3. Synergy between plasma and ammonia separation and storage. * This includes the energy consumption for plasma-catalytic ammonia synthesis (PC) and the energy consumption for recycling

(Rec). The energy consumption for recycling is based on the outlet NH3 concentration, based on interpolation of estimates in [16,67]. The state-of-the-art value for plasma catalysis is based on the data of Kim et al. [43], while the best-case scenario is based on an energy input of 0.7 eV due to a

decrease in the N2 dissociation barrier over Ru catalysts (see Section 2.2)[38]. The diﬀerence between the operating pressure of the plasma reactor and the separation step is to account for the pressure drop over the system.

State-of-the-Art Plasma Reactor BCS Plasma Reactor Separation Type DBD reactor (pulse) DBD reactor (pulse) Solid absorbent Material Promoted Ru/Al2O3 catalyst More active catalyst MgCl2/SiO2 Reaction temperature (◦C) 300 200 200 Desorption temperature (◦C) - - 300 Operating pressure (bar) 1.5 1.5 1.0 Outlet NH3 concentration (mol. %) 0.16 1.0 0.1 Outlet ammonia pressure (kPa) 1.6 10 0.3 1 Energy consumption (GJ t-NH3− )* 197 (PC:95, Rec:102) 5 (PC:4, Rec:1) 10 Syngas ratio (H2:N2) 1:4 1:4 1:4

3.3. Investment Cost Comparison Most of the discussion so far has focused on the energy consumption in the ammonia synthesis loop, as the energy cost is usually the major cost contributor for electricity-driven ammonia synthesis [90]. Given that the energy consumption of small-scale systems is larger, the electricity cost is expected to be a major cost contributor in small-scale systems as well. However, investment costs can become substantial cost factors as well. In this section, an estimate is provided on the capital investment for plasma-catalytic ammonia synthesis as compared to a decentralized Haber-Bosch synthesis loop. As hydrogen and nitrogen are required for both a small-scale Haber-Bosch process and a plasma-catalytic ammonia synthesis process, the costs for hydrogen production and nitrogen puriﬁcation are excluded in the analysis. The capital investment for the ammonia synthesis loop scales with a cost-scaling factor of 0.6 [14], while electrolyzers scale modularly. Thus, the contribution of the ammonia synthesis loop to the total cost increases upon scale-down. Figure8 shows a comparison between the capital investment for the Haber-Bosch synthesis loop, the state-of-the-art plasma-catalytic ammonia synthesis loop, and a best-case scenario for the plasma-catalytic ammonia synthesis loop. For reference, the capital investment for a low-temperature, absorbent-enhanced Haber-Bosch synthesis loop is also shown. 1 The equipment cost for the plasma generator is estimated to be 0.9 € W− , based on estimates of van Rooij et al. [91]. The equipment cost of all other components are estimated, based on cost-scaling relations described in [67]. The investment cost of the state-of-the-art plasma-catalytic ammonia synthesis loop is about ten times as high as that of the small-scale Haber-Bosch synthesis loop. The main reason for this is the large recycle, implying a large compressor is required. Furthermore, separation of ammonia is less eﬃcient at low ammonia concentrations, implying large equipment for ammonia absorption. Even for the best-case scenario with minimal heat exchange between the reactor and the sorbent, plasma-catalytic ammonia synthesis is equally expensive as the Haber-Bosch synthesis loop. The high-pressure Haber-Bosch synthesis loop requires a large feed compressor, as well as substantial heat integration between ammonia synthesis at 400–500 ◦C and ammonia separation at near-ambient temperature. On the other hand, the cost of ammonia separation and the recycle compressor are low due to high ammonia partial pressures and relatively high single pass conversions of about 15%. The best-case scenario plasma-catalytic synthesis loop operates at low pressures, thereby eliminating the requirement for a feed compressor. However, the recycle compressor is more expensive, due to low single pass conversions of about 1%. Furthermore, the low ammonia partial pressure also leads to more expensive ammonia separation equipment. Catalysts 20202020,, 10,, 999x FOR PEER REVIEW 12 of 20 19

Figure 8. Capital investment for the conventional Haber-Bosch synthesis loop, state-of-the-art plasma Figurecatalysis, 8. Capital a best-case investm scenarioent for for the plasma conventional catalysis, Haber and an–Bosch absorbent-enhanced synthesis loop, state Haber-Bosch-of-the-art synthesis plasma catalysis, a best-case scenario1 for plasma catalysis, and an absorbent-enhanced Haber–Bosch synthesis loop at 10 kg t-NH3 d− (~10 kW). Estimates for the conventional Haber-Bosch synthesis loop and the loopabsorbent-enhanced at 10 kg t-NH3 d Haber-Bosch−1 (~10 kW). Estimates synthesis for loop the are conventional based on [ 67Haber]. The–Bosch cost of synthesis the plasma loop reactor and the is absorbentassumed to-enhanced be the combination Haber–Bosch of a synthesis conventional loop reactor are based and aon plasma [67]. The generator cost of source.the plasma A cost-scaling reactor is assumedfactor of 0.6 to is be used the forcombination scale-down of of a equipment. conventional See reactor also Table and3 a. plasma generator source. A cost- scaling factor of 0.6 is used for scale-down of equipment. See also Table 3. 4. Plasma-Catalytic Ammonia Synthesis in Perspective 4. PlasmaAll in-C all,atalytic it can beAmmonia concluded Synthesis that plasma-catalytic in Perspective ammonia synthesis does not provide significant investmentAll in costall, it advantages can be concluded over the thatHaber-Bosch plasma-catalytic synthesis ammonia loop on a synthesis small scale does (about not provide 10 kW), significanteven in the investment best base scenario cost advantages (see Section over 3.3 the). The Haber energy–Bosch consumption synthesis loop of the on best-case a small scale scenario (about for plasma-catalytic10 kW), even in ammoniathe best synthesisbase scenario is lower (see than Section that of3.3). the Haber-BoschThe energy consumption process at 10 kWof the (see best Figure-case9). Atscenario larger for scale, plasma the Haber-Bosch-catalytic ammonia process synthesis is more energyis lower e ffithancient that in of any the case Haber (see– FigureBosch 3process), implying at 10 plasma-catalytickW (see Figure 9). ammonia At larger synthesis scale, the is Haber not a viable–Bosch alternative. process is more energy efficient in any case (see FigureIn 3 the), implying previous plasma sections,-catalytic the focus ammonia was on improvementssynthesis is not of a plasma-catalyticviable alternative. ammonia synthesis. However,In the a previous wide range sections, of technology the focus is was currently on improvements being researched of plasma as an alternative-catalytic ammonia to the Haber-Bosch synthesis. processHowever, on a a wide small range scale. of Electrochemical technology is synthesis,currently being photochemical researched synthesis, as an alternative homogeneous to the catalysis, Haber– Boschand chemical process looping on a small approaches scale. haveElectrochemical been reported. synthesis, The estimated photochemical state-of-the-art synthesis, energy homogeneous consumption catalysis,for these technologiesand chemical islooping shown approach in Figurees9. have An extensive been reported. account The on estimated these technologies state-of-the is-art given energy by consumptionRouwenhorst for et al.these [66 technologies]. Electrochemical is shown ammonia in Figure synthesis 9. An extensive is often proposedaccount on as these an alternative technologies to isammonia given by synthesis Rouwenhorst under mild et al. conditions. [66]. Electrochemical However, producing ammonia significant synthesis ammoniais often proposed concentrations as an alternativeat a low energy to ammonia cost has synthesis proven to under be diffi mildcult conditions. [92–94]. Even However, if ammonia producing is produced significant at a su ammoniafficiently concentrationslow energy cost, at thea low conversion energy cost levels has and proven separation to be difficult of ammonia [92–94] from. Even the if electrolyte ammonia is become produced key atissues a sufficiently [95]. None low of energy the other cost, technologies the conversion can levels be implemented and separation on aof near-term, ammonia from due to the high electrolyte energy becomecost and key low issues ammonia [95]. yields None obtainedof the other [66]. technologies can be implemented on a near-term, due to high Gradualenergy cost improvements and low ammonia to the Haber-Bosch yields obtained process [66]. have gained considerable research interest as well.Gradual The current improvements industrial multiple to the Haber promoted–Bosch iron process catalyst have has gained been developed considerable over research the past interest century withas well. minor The changes current inindustrial catalyst formulationmultiple promoted and optimization iron catalyst of catalysthas been preparation developed [ 96over,97 ].the On past the centuryother hand, with activated minor changes carbon in supported catalyst formulation Ru catalysts (Ruand/ AC)optimization have also of been catalyst implemented preparation in industry [96,97]. Onto a the lesser other extent hand, [7 activated]. The implementation carbon supported of Ru Ru/AC catalysts has been (Ru/AC) limited, have due also to abeen higher implemented catalyst cost in industryand a shorter to a lesser catalyst extent lifetime [7]. The than implementation iron-based catalysts. of Ru/AC For has the been industrial limited, iron-based due to a higher catalysts catalyst and costthe first and generation a shorter catalyst of Ru-based lifetime catalysts than iron (Ru/-AC,based Ru catalysts./Oxide), NFor2 dissociation the industrial is the iron rate-based limiting catalysts step, whichand the can first be generation enhanced by of theRu- introductionbased catalysts of alkali (Ru/AC, and Ru/Oxide), alkaline earth N2 promotersdissociation [74 is]. the In variousrate limiting cases, step, which can be enhanced by the introduction of alkali and alkaline earth promoters [74]. In various cases, ruthenium-based catalysts have been replaced by wüstite-based iron catalysts in ammonia

Catalysts 2020, 10, 999 13 of 19 ruthenium-based catalysts have been replaced by wüstite-based iron catalysts in ammonia converters, because these catalysts show similar activity [96]. A catalyst that is active at substantially lower temperatures is required to replace iron-based catalysts. Operation at lower temperatures can minimize heat loss to the surroundings during a small-scale operation and lower the required energy input. Over the past years, Ru-based catalysts with substantially improved activity have been developed by using new support materials, such as electrides, among others [77,85–87,98–105]. The C12A7:e− structure used is an electride, stable at ambient temperature, consisting of a positively 4+ charged framework with the chemical formula [Ca24Al28O64] and four extra-framework electrons, accommodated in the cages as counter ions [106]. It has been proposed that the N2 dissociation rate is enhanced on electride supported Ru catalyst, due to a small band gap between the valence band of the electride and the conduction band of the metal [100]. Furthermore, the hydrogen poisoning effect commonly encountered for Ru catalysts is thought to be suppressed [99]. For these catalysts, hydrogenation over the catalyst is the rate-limiting step, rather than the N2 dissociation step [101]. These catalysts show activity at 200–250 ◦C, similar to that of industrial Fe-based catalysts at 350–400 ◦C[85]. These highly active Ru-based catalysts are clearly a competitive alternative to a plasma reactor, as these catalysts are sufficiently active to decrease the reactor temperature similarly to plasma-catalytic ammonia synthesis. Furthermore, plasma catalysis can only be employed at a high energy efficiency (i.e., a low energy consumption), when ammonia conversions are low, as the plasma also activates the product and we expect NH3 concentrations should be limited to about 1.0 mol. % in order to limit plasma activation of the product. This problem is not present for the new generation of Ru-based catalysts, which no longer have the N2 dissociation as the rate-limiting step. Solid sorbents such as metal halides can also be combined with the new generation of Ru-based catalysts, thereby allowing to decrease the pressure of the ammonia synthesis loop to that of the hydrogen production and nitrogen purification pressure (about 7 bar) [16]. This is coined the absorbent-enhanced Haber-BoschCatalysts 2020, 10 process., x FOR PEER REVIEW 14 of 20

Figure 9. Estimated energy consumption of state-of-the-art small-scale, electrolysis-based Haber-Bosch, plasmaFigure 9. catalysis Estimated (also energy best-case consumption scenario), of absorbent-enhanced state-of-the-art small Haber-Bosch,-scale, electrolysis single-based pass processHaber– absorbent-enhancedBosch, plasma catalysis process, (also and best electrochemical-case scenario), ammonia absorbent synthesis.-enhanced Estimates Haber based–Bosch on [16, , single67,107 , pass108]. Forprocess plasma absorbent catalysis,-enhanced the energy process, consumption and electrochemical for gas recycling ammonia and ammonia synthesis. separation Estimates is based on estimates[16,67,107,108 in for]. low For pressureplasma (1.5catalysis bar),, low the conversion energy consumption (0.4 mol. % NH for 3 gas) systems recycling with and solid ammonia sorbents (seeseparation also Section is based 3.1 on)[16 estimates,67]. * For in electrochemicalfor low pressure ammonia(1.5 bar), low synthesis, conversion only (0.4 the energymol. % NH consumption3) systems ofwith ammonia solid sorbents production (see isalso included Section and 3.1)the [16,67 energy]. * For cost electrochemical of ammonia separation ammonia and synthesis, recycling only is notthe included.energy consumption TRL stands of forammonia technology production readiness is included level. The and TRL the levelsenergy here cost applyof ammonia for the separation complete system.and recycli TRLng 1 is is not the included. basic idea, TRL while stands TRL 9for is technology a commercial readiness system. level. For more The details,TRL levels see ref.here [ 68apply]. for the complete system. TRL 1 is the basic idea, while TRL 9 is a commercial system. For more details, see ref. [68].

5. Outlook Plasma-catalytic ammonia synthesis does not appear to be a feasible alternative to long-term energy storage in NH3. In case of N2 activation for ammonia synthesis, alternative technologies under development for ammonia synthesis appear to be more feasible in the short term and in the long term (see Section 4). Ammonia synthesis from hydrogen and nitrogen is an exothermic reaction and an exergonic reaction and under most conditions [73], implying plasma activation is, in principle, not desirable. However, plasma catalysis is an interesting alternative to electrifying in the chemical industry, due to the ability of plasma to activate strong chemical bonds such as CH4, CO2 and N2 [28]. For instance, the bond dissociation energy of the N≡N is 9.79 eV, while electrons in DBD reactors typically have energies in the range 2–4 eV. The total barrier for CH4 activation can be decreased upon vibrational excitation [109], thereby increasing the dissociative sticking probability [110]. Furthermore, plasma-assisted technologies have a fast response to electricity load variations. The plasma may provide very localized heating, thus limiting the heat requirement for the reactor. This can be beneficial for endergonic reactions. Lastly, plasma activation has the potential to provide process intensification. For instance, N2 activation with a plasma can be of interest for the direct synthesis of NOX [111,112], thereby eliminating the ammonia synthesis step altogether. As discussed in this article, the product outlet concentration should typically be above 1.0 mol. %, in order to limit the recycle size and to allow for efficient product separation.

Catalysts 2020, 10, 999 14 of 19

Even for the best-case scenario (BCS) for plasma catalysis with a 5.8 times improvement in energy consumption for plasma catalysis (see Section 2.2.2), the energy consumption for a plasma-catalytic ammonia synthesis process is higher than for a low temperature synthesis, absorbent-enhanced Haber-Bosch process (see Figure9). Furthermore, the capital investment for a plasma-catalytic process is expected to be higher than for the absorbent-enhanced Haber-Bosch process, because of the required larger heat exchanger and larger recycle compressor, due to a lower single pass conversion [67]. For reference, the electrolysis-based Haber-Bosch process at 10 MW has a power-to-ammonia efficiency of 52% (LHV), while the best-case scenario for plasma catalysis has a power-to-ammonia efficiency of 39% (LHV). The current energy consumption for plasma-catalytic ammonia synthesis is about 240 GJ 1 t-NH3− , leading to a power-to-ammonia efficiency as low as 8% (LHV).

5. Outlook Plasma-catalytic ammonia synthesis does not appear to be a feasible alternative to long-term energy storage in NH3. In case of N2 activation for ammonia synthesis, alternative technologies under development for ammonia synthesis appear to be more feasible in the short term and in the long term (see Section4). Ammonia synthesis from hydrogen and nitrogen is an exothermic reaction and an exergonic reaction and under most conditions [73], implying plasma activation is, in principle, not desirable. However, plasma catalysis is an interesting alternative to electrifying in the chemical industry, due to the ability of plasma to activate strong chemical bonds such as CH4, CO2 and N2 [28]. For instance, the bond dissociation energy of the N N is 9.79 eV, while electrons in DBD reactors ≡ typically have energies in the range 2–4 eV. The total barrier for CH4 activation can be decreased upon vibrational excitation [109], thereby increasing the dissociative sticking probability [110]. Furthermore, plasma-assisted technologies have a fast response to electricity load variations. The plasma may provide very localized heating, thus limiting the heat requirement for the reactor. This can be beneficial for endergonic reactions. Lastly, plasma activation has the potential to provide process intensification. For instance, N2 activation with a plasma can be of interest for the direct synthesis of NOX [111,112], thereby eliminating the ammonia synthesis step altogether. As discussed in this article, the product outlet concentration should typically be above 1.0 mol. %, in order to limit the recycle size and to allow for efficient product separation.

Author Contributions: Conceptualization, K.H.R.R.; methodology, K.H.R.R., L.L.; formal analysis, K.H.R.R.; investigation, K.H.R.R.; writing—original draft preparation, K.H.R.R.; writing—review and editing, K.H.R.R., L.L.; visualization, K.H.R.R.; supervision, L.L.; funding acquisition, L.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Toeslag voor Topconsortia voor Kennis en Innovatie (TKI) from the Ministry of Economic Affairs and Climate Policy. Conflicts of Interest: The authors declare no conflict of interest.

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