Renewable and Sustainable Energy Reviews 114 (2019) 109339

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Renewable and Sustainable Energy Reviews

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Islanded ammonia power systems: Technology review & conceptual process design T

∗ Kevin H.R. Rouwenhorsta, , Aloijsius G.J. Van der Hamb, Guido Mulc, Sascha R.A. Kerstenb a University of Twente, Catalytic Processes and Materials Group, Enschede, Netherlands b University of Twente, Sustainable Process Technology Group, Enschede, Netherlands c University of Twente, PhotoCatalytic Synthesis Group, Enschede, Netherlands

ARTICLE INFO ABSTRACT

Keywords: Recent advances in technologies for the decentralized, islanded ammonia economy are reviewed, with an em- Ammonia economy phasis on feasibility for long-term practical implementation. The emphasis in this review is on storage systems in Hydrogen economy the size range of 1–10 MW. Alternatives for hydrogen production, nitrogen production, ammonia synthesis, Power-to-ammonia-to-power ammonia separation, ammonia storage, and ammonia combustion are compared and evaluated. A conceptual Chemical energy storage process design, based on the optimization of temperature and pressure levels of existing and recently proposed Decentralization technologies, is presented for an islanded ammonia energy system. This process design consists of wind turbines Islanded system Conceptual process design and solar panels for electricity generation, a battery for short-term energy storage, an electrolyzer for hydrogen production, a pressure swing adsorption unit for nitrogen production, a novel ruthenium-based catalyst for ammonia synthesis, a supported metal halide for ammonia separation and storage, and an ammonia fueled, proton-conducting solid oxide fuel cell for electricity generation. In a generic location in northern Europe, it is possible to operate the islanded energy system at a round-trip efficiency of 61% and at a cost of about 0.30–0.35 − € kWh 1.

1. Background Various chemical storage alternatives are discussed in the supplemen- tary information. A fossil carbon-free, circular economy is required to decrease the Ammonia can both be used to store energy in time (for islanded greenhouse gases emissions [1]. This can be accomplished by renew- systems) and in space (transportation from places with abundant wind able sources, such as wind power and solar power. However, these hours or solar hours to other places) [13]. Transportation costs are sources are intermittent and energy storage is required. Even though greatly reduced by adopting a decentralized, islanded energy economy batteries can provide energy storage for up to a few days, large scale [14]. Furthermore, political-economic factors influence energy prices chemical energy storage is required to accommodate the storage for the less in a decentralized energy economy. With small-scale ammonia remainder of the year. Currently, the world relies mostly on carbon- production gaining momentum in the upcoming decades [15], business based chemicals for electricity production in the absence of renewable models for the decentralized ammonia economy are currently under power, which is termed the carbon economy. A commonly suggested development [14]. alternative to the carbon economy is the hydrogen economy, which can Within this paper, current technological advances of the ammonia store renewable energy by the transformation of water (H2O) to hy- economy are reviewed, as well as their feasibility for long-term and drogen (H2) via electrolysis. However, storing and transporting hy- practical application on a local scale. A process, based on the currently drogen is difficult [2]. Therefore, the ammonia economy is proposed available literature is designed with the conceptual process design

[2–5]. Ammonia (NH3) is a carbon-free hydrogen carrier, which can methodology published by Douglas [16]. Power-to-ammonia-to-power mediate the hydrogen economy [1,4,6–8]. Thus, ammonia could be- technologies within the range 1–10 MW are reviewed. The proposed come the chemical ‘to both feed and power the world’ [9]. Especially solution constitutes of wind power and solar power, combined with a for long-term chemical energy storage (i.e., above 1 day), ammonia is battery for short-term energy storage (up to 1 day) and a power-to- more economically stored than hydrogen [10]. Among other chemical ammonia-to-power (P2A2P) process for long-term energy storage storage alternatives researched are methane [11] and methanol [12]. (above 1 day).

∗ Corresponding author. E-mail address: [email protected] (K.H.R. Rouwenhorst). https://doi.org/10.1016/j.rser.2019.109339 Received 1 April 2019; Received in revised form 24 July 2019; Accepted 11 August 2019 Available online 23 August 2019 1364-0321/ © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/). K.H.R. Rouwenhorst, et al. Renewable and Sustainable Energy Reviews 114 (2019) 109339

Abbreviations, units and nomenclature MJ Megajoule (energy unit) MW Megawatt (energy unit per time unit)

°C Temperature unit N2 Nitrogen AEHB Absorbent-enhanced Haber-Bosch NH3 Ammonia AEM Anion exchange membrane O2 Oxygen Ar Argon OpEx Operational expenditures Bar Pressure unit P2A2P Power-to-ammonia-to-power Barg Pressure unit PEM Polymer electrolyte membrane CapEx Capital expenditures SMR Steam methane reforming ΔH Heat of reaction SOFC Solid oxide fuel cell GJ Gigajoule (energy unit) SOFC-O Anion conducting solid oxide fuel cell

H2 Hydrogen SOFC-H Proton conducting solid oxide fuel cell H2O Water STP Standard Temperature and Pressure HHV Higher heating value T Tonne (mass unit) Kg Kilogram (mass unit) TDS Total dissolved solids Kt Kilotonne (mass unit) TRL Technology Readiness Level kWh kilowatt hour (energy unit, SI unit J) y Year (time unit)

LHV Lower heating value YNH3 Ammonia equilibrium mole fraction m3 cubic meter (volume unit)

An islanded energy system is analyzed, in which all electricity is From Fig. 1 it follows that in no case a near-complete conversion to generated from intermittent, renewable sources. Various alternatives ammonia is achievable under industrial conversion conditions. Thus, in for renewable electricity generation are available, but in the current commercial ammonia synthesis processes a significant recycle is re- case only wind power and solar power are considered due to the lo- quired. cation considered. In the case considered (Haaksbergen, a municipality The overall reaction from water and air is given by,equations − in the Netherlands with over 24000 inhabitants and an electricity (3)–(1) with a minimum theoretical energy consumption of 20.3 GJ t 1 − − consumption of 1.1*108 MJ y 1), wind power is mostly available in the ammonia (equivalent to 5.64 kWh kg 1 ammonia) [21]. The total en- winter months [17], while solar power is mostly available in the ergy consumption of ammonia from steam methane reforming (SMR) is − − summer months. The power generated by wind power and solar power about 28.0–32.6 GJ t 1 ammonia (equivalent to 7.78–9.06 kWh kg 1 consumed directly is about 55%, whereas 45% must be stored for later ammonia) [21,26]. If hydrogen can be produced at low overpotential, use. The case is elaborated upon in the supplementary information. electrolysis-based ammonia can be produced with a lower specific en- Nayak-Luke et al. [18] simulated similar scenarios for various locations ergy demand than SMR-based ammonia (see Table 1). It should be in the United Kingdom. Power scheduling is elaborated upon by Allman noted that the energy in case of SMR-based ammonia is fully provided et al. [19] and Nayak-Luke et al. [20]. in the form of heat, whereas the energy in the case of electrolysis-based ammonia is mostly provided in the form of electricity [21]. 1.1. Ammonia chemistry & thermodynamics 1.5HO2223+++→+ 0.641(0.78N 0.21 O 0.01 Ar ) NH 0.885 O2 + 0.0064Ar (3-1) The synthesis of ammonia from nitrogen and hydrogen is an exo- thermic reaction, favored by low temperatures and high pressures Ammonia is thermally decomposed according to equations (3)–(2), [21,22]. Due to the limited activity of industrially used iron-based an endothermic reaction favored by high temperatures and low pres- catalysts and the strong nitrogen-nitrogen bond of atmospheric nitrogen sures [27]. The energy requirement for producing 0.66 m3 (STP) ni- 3 (N2), industrial reactor conditions are typically 350–550 °C and trogen and 1.97 m (STP) hydrogen from 1 kg ammonia is 2.7 MJ − 100–460 bar [21,23,24]. The equilibrium content of ammonia in a (equivalent to 0.75 kWh kg 1 ammonia, excluding preheating and hydrogen-nitrogen mixture (H2:N2 = 3:1 M ratio) is shown in Fig. 1. evaporation) [27]. [21].

Fig. 1. Ammonia equilibrium mole fraction (YNH3) for various temperatures and pressures. (H2:N2 = 3:1, no inert). Data reproduced from Ref. [25].

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Table 1 Specific energy demand of various large-scale ammonia synthesis technologies, and nitrogenase in nature.

23NH32→+ N H 2 (3-2) readiness level is a measure of the maturity of a technology, where Level 1 represents the fundamental discovery of a principle and Level 9 an industrially applied technology. A schematic overview of the major 2. Current developments in power-to-ammonia-to-power components in a power-to-ammonia-to-power is shown in Fig. 2. Power-to-ammonia-to-power is currently going from merely aca- Ammonia is currently widely researched as an energy vector demic research to pilot-scale plants. As of 2018, demonstration plants [26,28–30]. Especially electrochemical ammonia synthesis has gained were opened in Japan and the United Kingdom, whereas the University substantial attention in recent years [31]. Even though electrochemical of Minnesota operates its small-scale, wind-powered ammonia plant for ammonia synthesis can become a commercial technology after 2030 five years already [40–43]. The NFUEL® units of Proton Ventures B.V. [32], improvement on the existing Haber-Bosch process are likely to be (mini ammonia plants) have industrial references in Argentina, China the near-term alternative for sustainable ammonia synthesis [29]. Re- and Switzerland [44]. Furthermore, ammonia fueled SOFCs (solid oxide cent developments in ammonia synthesis are scale-up for enhanced fuel cells) are currently tested in pilot plants by IHI cooperation (1 kW energy efficiency and scale-down for smaller investments, whereas class power) [45]. medium-sized plants will be built less frequently in the upcoming decades [15]. Improvements on the existing Haber-Bosch process are 2.1. Ammonia synthesis processes the focus within this paper, with an emphasis on decentralization. Previously, Fuhrmann et al. [33], Bañares-Alcántara et al. [24], Ikä- Ammonia can be synthesized along various pathways, namely (1) heimo et al. [13], and the ISPT (Institute for Sustainable Process thermochemical synthesis (the Haber-Bosch process), (2) electro- Technology) [34] evaluated alternatives for renewable ammonia chemical synthesis, (3) non-thermal plasmas, (4) dense metallic mem- synthesis (and its combustion for electricity generation). For the am- branes and (5) solar thermochemical redox cycles [46]. The production monia-to-power part, both fuel cells and gas turbines have been eval- of ammonia was published and patented by Haber and Le Rossignol in uated within this paper. Valera-Medina et al. [35] and Yapicioglu et al. 1913 and 1916, which would be termed the Haber-Bosch process in the [36] recently evaluated the alternatives for ammonia-to-power. Afif years to follow [47–49]. Ammonia has been synthesized for over 100 et al. [37], Lan et al. [38] and Siddiqui et al. [39] reviewed the alter- years by the Haber-Bosch process, starting from 1913 at BASF in Oppau, natives for ammonia fed fuel cells. In the current paper, feasible al- Ludwigshafen (Germany) [50,51]. A typical modern ammonia synthesis ternatives are compared, but an extensive review on all possible tech- plant operates at around 400–450 °C and 100–150 bar [21]. The ap- nologies is not provided here. The selection of technologies presented proximate energy demand of various ammonia production routes is here is based on the technology readiness level (TRL) as well as the listed in Table 1. The best SMR-based Haber-Bosch processes already technological progress made over the past decade. The technology outperform nitrogenase in nature. It should be noted that the energy

Fig. 2. Schematic overview of major components for power-to-ammonia-to-power. Electricity streams are indicated by dotted lines, while material streams are indicated by full lines.

3 K.H.R. Rouwenhorst, et al. Renewable and Sustainable Energy Reviews 114 (2019) 109339 consumption of the conventional, carbon-based Haber-Bosch processes As shown in Fig. 4, the separation efficiency of ammonia at a given is fully composed of heat, whereas electrolysis-based Haber-Bosch temperature is a function of the ammonia partial pressure in the stream. processes mostly require electricity input. Thus, upon operating at reduced pressures (< 100 bar), cooling to low temperatures (< 0 °C) is required when condensation is used for se- 2.2. Decentralized production paration. However, a more complete separation of ammonia after the synthesis reactor may be attained by an affinity separation. In the small- Decentralizing ammonia synthesis is mainly conducted along two scale, medium-pressure plant simulation of Cussler et al. the energy pathways, namely (1) using conventional high-pressure technology, consumption for feed compression and the synthesis loop is decreased − and (2) using milder reaction and separation conditions with new down to 0.7–3.1 kWh kg 1 ammonia by utilizing supported metal ha- technologies. lides for ammonia separation (in a low-pressure ammonia synthesis Conventional technology operating at high-pressure is currently the loop operating at 10–30 bar, see Table 2)[58,62]. The energy con- prevalent alternative. Literature data from Brown [54], Morgan et al. sumption of the feed compression is close to non-existent (0.017 kWh − [56], Pfromm [57], and Palys et al. [58], as well as industrial data from kg 1 ammonia) [58]. This is the major driver to choose for an ab- the electrolysis-based ammonia synthesis systems of ThyssenKrupp sorption-enhanced process, as compressors are major cost factors in [59,60], the NFUEL® units (Proton Ventures B.V.) [10,61] and the chemical plants. A comparison of the electricity consumption and other Morris Plant (University of Minnesota) [43] are compared, as shown in characteristics of small-scale ammonia synthesis plants is listed in Fig. 3. The electrolyzer typically consumes 90–96% of the required Table 2. energy input [44,56,57]. For large-scale electrolysis-based conventional Haber-Bosch plants 2.3. Hydrogen production down to the scale of the NFUEL® units, the energy consumption is − nearly constant (about 9–11 kWh kg 1 ammonia, see Fig. 3). However, The usual source for zero-carbon hydrogen is water (i.e., by elec- upon further scale-down, heat transfer limitations become relevant for trolysis), with a net reaction given by equations (3)–(3). The reaction is − conventional high-pressure, electrolysis-based Haber-Bosch plants (see highly endothermic (ΔHo = 571.8 kJ mol 1), giving it a high energy Fig. 3). This can be dealt with by designing the process for less severe requirement. process conditions (i.e., lower operating temperature and lower oper- 2HOl ()→+ 2 H () g O () g (3-3) ating pressure than conventional Haber-Bosch plants). A solution pro- 222 posed by Cussler et al. is a low-pressure ammonia synthesis process (at Various alternatives for electrolysis are available. Some of these are 10–30 bar) [63,64]. This technology, ammonia synthesis combined already commercially available, such as alkaline electrolyzers and with an absorbent-enhanced process for ammonia removal, is elabo- proton exchange membrane (PEM) electrolyzers. Other technologies rated upon in next section and section 2.6. are on the brink of commercialization (solid oxide electrolyzers), whereas other systems are currently in the research & development 2.2.1. Synthesis pressure of small-scale ammonia synthesis plants (such as anion exchange membrane (AEM) electrolyzers) [69]. In the A small-scale, medium-pressure plant as investigated by Cussler current section, only (nearly) commercial technologies are considered − et al. (production rate of 1.5 kg h 1 ammonia, operating pressure (i.e., alkaline electrolysis, PEM electrolysis, and solid oxide electro- − 150 bar) uses 3.62 kWh kg 1 ammonia for syngas feed compression and lysis). − the ammonia synthesis loop (about 2.16 kWh kg 1 ammonia for the Electrolysis is achieved in an electrolysis cell composed of an elec- − synthesis loop and 1.46 kWh kg 1 ammonia for the feed compression) trolyte (the ionic conductor), active layers for redox reactions, and a [58]. These figures are similar to those of NFUEL® units (operating at current and material collector (the electronic conductor), which enables − 460 bar, as of 2010), for which 2 kWh kg 1 ammonia is used for am- the electricity supply, as well as the supply and collection of reactants − monia synthesis and 2 kWh kg 1 is used for utilities (for instance the and products [70]. Next to the electrolysis cell, an electrolysis system compressor and cooling fan) [65]. By increasing the pressure to above consists of gas cooling, purification, compression, and storage [71]. 300 bar, no refrigeration is required for ammonia separation (i.e., Furthermore, the power from the power source needs to be conditioned cooling water and cooling fans are sufficient, see Fig. 4). A benefitof for the electricity input system of the electrolyzer, and safety control small-scale operation is that high pressure equipment is more easily systems are installed [72]. Pre-treatment is performed by either eva- constructed than for large-scale operation. poration combined with mechanical vapor compression, reverse

Fig. 3. Energy consumption of various electrolysis- based Haber-Bosch processes at various ammonia capacity levels (at ≥100 bar operating pressure for conventional processes, absorbent-enhanced pro- cesses are denoted by AEHB). Data reproduced from Refs. [10,43,54,56,57,59–62]. The solid line re- presents the thermodynamic minimum energy con- sumption.

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Fig. 4. Left: Ammonia vapor pressure as function of temperature. Antoine equation parameters reproduced from Ref. [66]. Right: Ammonia fraction in gas phase as function of the condensation temperature, for various synthesis pressures. The 4.18 mol.% ammonia indication limit is based on the Krupp-Uhde process [27].

Table 2 trip efficiency (about 80%) than chemical storage (20–40%). A pres- Comparison of various small-scale ammonia synthesis loops. The electricity surized electrolyzer is preferred, because this omits the requirement of consumption covers the feed compression, energy input for separation, and pressurizing hydrogen for the ammonia synthesis loop. The Battolyser, recycle loop compression. technology developed by the Mulder et al. [76], combines an iron- Alternative Advantages Disadvantages nickel battery and an alkaline electrolyzer. Thus, this can reduce costs of the overall system. The capital cost of the Battolyser is projected to − High-pressure Haber-Bosch • Well-known • Very high be 370 € kW 1 (purchased equipment cost) [52]. process technology pressure and 400-550°C [24] • No sharp temperature 300-460 bar [24] separation • Operating safety 2.3.1. Water purification Electricity consumption required • High capital −1 Water can be purified by reverse osmosis, electro dialysis, and 4 kWh kg NH3 [65] • No refrigeration investment TRL 9 mechanical vapor compression [56]. Electrolyzers generally have pur- Medium-pressure Haber- • Well-known • Requires large ification equipment based on reverse osmosis modules, as these are Bosch process technology scale reliable and modular [56]. Alkaline electrolyzers require a TDS of 350-525°C [23] • No sharp • High pressure and ≤10 ppm, whereas this is even more stringent for PEM electrolyzers 100-200 bar [23] separation high temperature ≥ Electricity consumption required • Refrigeration ( 2 ppm) [56,71,72]. As the system is circular, water with a low TDS −1 3.62 kWh kg NH3 [58] required (high value can be recycled from the ammonia-to-power part. Mixing fresh TRL 9 OpEx) tap water with the recycled water results in low feed TDS values. The • Operating safety maximum TDS value from tap water is 500 ppm. Depending on the Absorbent-enhanced Haber- • Relatively low • Efficient Bosch process pressure separation application the value of deionized water from reverse osmosis is – 370-400°C [58,67] • Lower CapEx and required (low 0.1 400 ppm [77]. Thus, it is possible to achieve 10 ppm (required for 10-30 bar [58,67] OpEx ammonia partial alkaline electrolyzers) [71]. Reverse osmosis is not capable at produ- Electricity consumption • Safer operation pressure) cing the required water purity from sea water efficiently [56,73,78]. −1 0.7-3.1 kWh kg NH3 [58,62] • No refrigeration 4-5 Thus, the application of reverse osmosis is limited to tap water and sea TRL • Kinetics no longer fi rate limiting, water is usually puri ed using evaporation combined with mechanical recycle is rate vapor compression. − limiting [68] The levelized cost of reverse osmosis are as low as 1 € t 1 ammonia (including maintenance and capital expense charges) [57,79]. The en- − − ergy consumption is about 4.7 kWh t 1 ammonia (3 kWh t 1 water, osmosis, or electro dialysis [24,73]. In this case reverse osmosis is 97% water utilization) [57]. The cost of the demi-water unit for a 1k preferred (see section 2.3.1). NFUEL® unit of Proton Ventures B.V. costs about 50 k€ (purchased The main advantages and disadvantages of alkaline electrolysis, equipment cost) [80]. PEM electrolysis and solid oxide electrolysis systems are listed in Table 3. From this it follows that only alkaline electrolyzers and PEM electrolyzers are currently commercially available. Solid oxide elec- 2.4. Nitrogen production trolyzers show great promise, but these are not yet commercially available (on the MW scale) [71,74]. Various technologies may be employed for the production of pur- A main consideration is the cost of the electrolyzer, as these typi- ified nitrogen gas from air. Nitrogen is commercially separated from air cally make up for a major portion of the capital cost of electrolysis- by an air separation unit (cryogenic distillation), by pressure swing based ammonia plants. In this regard, alkaline electrolyzers are still adsorption, by membrane permeation, or by hydrogen combustion preferable over PEM electrolyzers, as the latter alternative is typically a [94]. Hydrogen combustion processes are not considered for the pro- few times as expensive as the former alternative. A combination of a duction of ammonia. Currently, cryogenic distillation is the prevalent battery for short-term storage with a conventional pressurized alkaline technology in large-scale industry [94]. − electrolyzer (operating at 3.8–4.4 kWh Nm 3 hydrogen [75]or The nitrogen production should be able to follow the patterns of the − 7.3–8.4 kWh kg 1 ammonia) for long-term storage is currently the most hydrogen production to some extent. This can be achieved by (1) a feasible available technology. A benefit of this is that day-night cycles fluctuating nitrogen production, or (2) by a storage tank that controls can be accounted for by the battery function, which has a higher round- the release of purified nitrogen gas. In case of the first (i.e., fluctuating nitrogen production), pressure swing adsorption and membrane

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Table 3 3 Advantages and disadvantages of the alkaline, battolyser, PEM and solid oxide electrolysis systems. 1 Nm H2 is equivalent to 0.090 kg H2.

Alternative Advantages Disadvantages

− Alkaline • Mature technology [85] • Low current densities (0.2–0.45 A cm 2)[69,81,82] 60-90°C [69,81,82] • Non-noble, abundant catalysts (Ni, Ni-Mo alloys) [74,81,83] • Cross-over of gases [71,81] 0.05-30 bar [69,74,82] • Long term stability (60000–90000 h) [81,82] • High minimum load requirement (10–40%, although state of the art € −1 DC energy consumption • Low cost (current lowest: 450 kWel )[75,86] systems with 5% are available) [71,74,81] −3 4.2-6.6 kWh Nm H2 [69,82] • Stacks in MW range (up to 5.3 MW) [81,82,87] • Slow dynamics (seconds scale) [74] −3 ≤ μ −1 3.8-4.4 kWh Nm H2 [75] • Low water purity requirement ( 5 Scm )[71] • Low operating pressures (nowadays high pressure systems of up to − Electrolyte 20–40 wt% KOH • High stability (efficiency degradation 0.25–1.5% y 1)[69] 30 bar are available) [74,82] − [74,83] • Low maintenance cost (2–3% of capital cost y 1)[69] • Corrosive liquid electrolyte [71,81] TRL 9 [84] • Complicated system design, especially when high purity is required [71] Battolyser • (Most of the) advantages of alkaline electrolyzers • (Most of the) disadvantages of alkaline electrolyzers 30-60°C • Combined battery and electrolyzer [76] • Low operating pressures (however, operation at pressures up to 1-30 bar • Non-noble, abundant catalysts (Fe, Ni) [76] 30 bar (or above) is possible) − DC energy consumption • Low cost (about 370 € kW 1)[52] • New technology (currently commercialized in Battolyser B.V.) [88] −3 – 4.4 kWh Nm H2 • Operation near room temperature (30 60 °C) [76] Electrolyte 21 wt% KOH [76] TRL 4-5 − − PEM • High current densities (0.6–2.0 A cm 2)[81,82] • High cost of components (capital cost 1860–2320 € kW 1, − 50-80°C [69,81,82] • High voltage efficiency [81] possibly as low as 900 € kW 1)[71,81,82,89] 10-200 bar [69,74,82] • Low minimum load requirement (0–10% or 5–10%) [81,82] • Acidic corrosive environment [81] DC energy consumption • Fast dynamics (milliseconds scale) [74] • Low stack lifetime (however, nowadays the warranted lifetime is −3 4.2-6.6 kWh Nm H2 [69,82] • Compact system design [81] equal to that of alkaline electrolyzers) [85] Electrolyte Nafion [74,81] • High gas purity (99.9–99.9999%) [71,74,82] • Stacks below MW range (however, nowadays 1.1 MW size available) TRL 5–7 [84] • Flexibility in modes of operation [71] [82] − • Acceptable stability (efficiency degradation 0.5–2.5% y 1) • Use of scarce materials (Ir, Pd, Pt, Ru) [71,81] − [69] • High water purity requirement (≥1 μScm 1)[71,72] − • High maintenance costs (3–5% of capital cost y 1)[69] − Solid oxide • Reversible operation possible [93] • High cost (current capital cost 2000 € kW 1, potentially low cost 650-1000°C [69,90–92] • Potentially low energy requirement for splitting water due to low cost materials) [87] − 1-25 bar [69,74] [87,90,91] • Low practical current densities (0.3–0.6 A cm 2)[71] DC energy consumption • Potential for using low cost materials (current materials are • New technology (not commercial yet) [71,74] −3 – ffi – −1 > 3.7 (> 4.7) kWh Nm H2 NiO/YSZ and La0.8Sr0.2MnO3/YSZ) [90 92] • Low stack lifetime (e ciency degradation 3 50% y )[69,82] [85] • Potentially high energy efficiency [69] • High temperature operation [85] −3 3.7-3.9 kWh Nm H2 [69] • High minimum load requirement (> 30%) [85] Electrolyte YSZ [90,91] • Slow dynamics (seconds scale) [85] TRL 3–5 [84] • Stack sizes in kW range [69] permeation offer the highest flexibility (see Table 4). A significant are not removed, these can build up in the ammonia synthesis loop. drawback of membrane permeation is the low attainable nitrogen Oxygen containing compounds can poison ammonia synthesis catalysts, purity at low energy consumption (up to 95 wt% nitrogen purity), as discussed in section 2.5.1. which makes this alternative unfeasible when other options are avail- Sánchez et al. reviewed the cost of nitrogen production recently able [94]. [98]. For small-scale systems (< 1 MW scale), membrane permeation is As nitrogen is produced from air, oxygen containing compounds can the preferred alternative, while pressure swing adsorption is preferred be present in the purified nitrogen stream. Therefore, a deoxo unit for at intermediate scales (1–100 MW) [98]. At large scales (> 100 MW), deep oxygen removal is often installed as a last step in the nitrogen cryogenic distillation is preferred [98]. Concluding, the pressure swing production stage as a safety measure. If oxygen containing compounds adsorption unit is the preferred alternative for nitrogen production at

Table 4 Comparison of nitrogen purification alternatives.

Alternative Advantages Disadvantages

Cryogenic distillation • High purity nitrogen production (up to 99.999 wt% purity) • Requires continuous operation (Load range 60–100%) [73] Capacity range [12,94] • Slow dynamic response (order of hours) [73] − 250-50000 Nm3 h 1 [94] • Argon in oxygen product (up to 98 wt% purity oxygen) • Cryogenic conditions DC energy consumption [12,94] • Refrigeration compounds −1 0.11 kWh kg N2 [78] • (Pure oxygen product) TRL 9 Pressure swing adsorption • Flexible operation (Load range 30–100%) [73,96] • Nitrogen purity up to 99.8 wt% (however, oxygen can be as low as Capacity range • Ambient temperature 0.01 wt%) [94] − 25-800 Nm3 h 1 [94] • Little maintenance [24,97] • Argon in nitrogen product [94] DC energy consumption • Automated operation [97] • Additional deoxo unit required −1 , 0.22-0.31 kWh kg N2 [95] • Short start-up times [24,97] • (No pure oxygen product) TRL 9 • Compact unit design [24,97] • High degree of fire safety [97] Membrane permeation • Flexible operation (Load range 30–100%) [73] • Nitrogen purity up to 95 wt% (oxygen removal down to 0.5 mol.%) Capacity range • Ambient temperature [94,95] − 3-3000 Nm3 h 1 [94] • Argon in nitrogen product [94] DC energy consumption • Additional deoxo unit required − 0.22-0.63 kWh kg 1 [95] • (No pure oxygen product) TRL 8-9

6 K.H.R. Rouwenhorst, et al. Renewable and Sustainable Energy Reviews 114 (2019) 109339 the scale considered, because of its flexible operation, (near) ambient ammonia separation step during intermittent operation to keep the temperature operation and low cost. The hydrogen and nitrogen pro- synthesis loop operating in a stable manner. duction can be streamlined, and these can be operated at similar tem- peratures and pressures. The energy consumption for nitrogen pro- − − 2.5.1. Poisoning duction is about 0.32 kWh kg 1 nitrogen or 0.25 kWh kg 1 ammonia Iron-based catalysts have been the preferred alternative for con- (see Table 4)[95]. The energy consumption depends on the required ventional SMR-based ammonia synthesis plants, in which methane is nitrogen purity. A major portion of the energy consumption is required − present in the ammonia synthesis loop, while oxygen-containing com- for the feed air compression (about 0.18 kWh kg 1 ammonia). pounds are completely removed [27]. Carbon poisoning is not sig- nificant for industrial iron-based catalysts [21]. Carbon is a poison for 2.5. Ammonia synthesis catalyst ruthenium-based catalysts [21]. Decomposition of methane is an in- hibitor of using ruthenium-based catalysts in conventional Haber-Bosch Various catalytic systems have been researched for ammonia plants [111]. synthesis. Among these, iron-based catalysts and ruthenium-based In case hydrogen is produced via electrolysis rather than via steam catalysts are the prevalent alternatives in heterogeneous catalysis. methane reforming, oxygen compounds may be present in the synthesis Industrially, iron-based catalysts are used in ammonia synthesis for loop rather than carbon compounds. Industrial iron-based catalysts are over a century [27]. However, academic research focuses more on ru- poisoned by oxygen [21]. On the other hand, ruthenium-based catalysts thenium-based catalysts, because these catalysts are more active at are less poisoned by oxygen [105]. All in all, iron-based catalysts are lower temperature (and consequently lower pressure can be applied) suitable for streams containing carbon impurities, whereas ruthenium- and at high ammonia conversion levels [21,99]. Both iron-based cata- based catalysts are suitable for streams containing oxygen impurities, lysts and ruthenium-based catalysts were extensively discussed by Liu which is the situation in this case study. [21]. Bi-metallic catalysts have also been researched, but these have not found any industrial application so far [99]. A drawback is the high temperature nitrification process, which makes the production of cat- 2.6. Ammonia separation & storage alysts with high surface areas difficult for bi-metallic catalysts [100]. The activities of various catalysts at low temperature and low Conventionally, ammonia is separated by phase separation of liquid pressure are listed in Table 5. For comparison, the reaction rate of in- ammonia from gaseous nitrogen and hydrogen by cooling. As shown in dustrial catalysts at industrial conditions (i.e., ≥400 °C and ≥100 bar) Fig. 4, the condensation fraction of ammonia highly depends on the − − is typically ≥100 mmol ammonia g 1 h 1, based on the kinetic rate synthesis pressure. ffi expression of Sehested et al. [101]. Ru/Ba-Ca(NH2)2 is the most active Recent developments in a nity separation by absorption in metal catalyst in the low temperature regime. Other catalysts, such as Ru/ halides have made sharp separation at high temperatures feasible − – C12A7:e and Cs-Ru/SrZrO3 only exhibit similar activities when elec- (at > 100 °C and 5 30 bar) [67,68]. This makes low-pressure ammonia tric fields are employed [102,103] or when the temperature is increased synthesis feasible. Both temperature swing absorption and pressure [104]. The use of an electric field amounts to an additional energy swing absorption systems were proposed. Although the latter shows − consumption of 2–5 kWh kg 1 ammonia. promise with up to 32 wt% ammonia reversibly stored at room tem-

Ru/Ba-Ca(NH2)2 offers a solution to suppress hydrogen poisoning, perature in CaCl1.33Br0.67 [112], no stable systems with near-term re- as barium acts as an efficient electronic promoter, which forms a shell levance for practical application have been reported so far. Therefore, around ruthenium nanoparticles [105]. In this manner the nitrogen temperature swing absorption is discussed in section 2.6.1. dissociation is accelerated in such a manner, that the hydrogenation is Beach et al. [113,114] have developed a zeolite as adsorbent for the rate limiting step rather than the nitrogen dissociation for the Ru/ ammonia separation.at low conversions (down to 1 mol.% ammonia)

Ba-Ca(NH2)2 catalytic system [105]. In conventional catalysis (both for their fast-ramping reactor concept. Commercial molecular sieves iron-based and ruthenium-based), nitrogen dissociation is the rate (4A, 5A, and 13X) are used for the sharp removal ammonia [115]. The limiting step [21]. As hydrogenation is the rate limiting step for Ru/Ba- adsorbents can take up 9 wt% ammonia [113]. Zeolites can separate – Ca(NH2)2, hydrogen poisoning is less apparent due to fast consecutive ammonia at low temperatures (20 100 °C). The ammonia is desorbed at reactions with adsorbed nitrogen. elevated temperatures (> 200 °C) [116,117]. After desorption from the Conventional iron-based catalysts show a more or less linear in- zeolite, the ammonia is liquefied [113,114]. crease in activity upon increase in temperature (range 250–400 °C) A wide range of solid and fluid materials have been proposed for [21,105]. Conventional ruthenium-based catalysts are typically more ammonia separation, such as activated carbon, covalent organic fra- active than iron-based catalysts in the range 375–450 °C [21,105]. meworks, deep eutectic solvents, ionic liquids, metal organic frame- However, conventional ruthenium-based catalysts experience severe works, oxides, and porous organic polymers [115,116,118–123]. hydrogen poisoning below 320 °C [105]. Ru/Ba-Ca(NH2)2 is the pre- However, these materials are either still in the proof of concept phase or ferred catalyst for low temperature operation, having both little hy- − drogen poisoning and a high surface area (100 m2 g 1)[105]. By using Table 5 Comparison of ammonia synthesis catalysts at low temperature and pressure. Ru/Ba-Ca(NH2)2 rather than the conventional iron-based and ruthe- −1 −1 nium-based catalysts, the temperature can be decreased by about 100 °C Reaction conditions are 300 °C, 9 bar and 36 L g h (WHSV). The stability is – to achieve the same activity [105]. The Tokyo Institute of Technology represented with +, 0 and signs. ++ very stable for a very long time (> 10 years), + stable for a long time (about 10 years), 0 somewhat stable for a have teamed up with Ajinomoto and other companies for the com- reasonable time (5–10 years), - (or –) (very) unstable for a prolonged time (up mercialization of the technology [106]. Tsubame BHB aims to have the to 5 years). catalyst operational in an ammonia test facility by 2021 [106]. −1 Catalyst TRL rNH3 (mmol g NH3 yield Stability The choice for the catalytic system mostly depends on the reaction − h 1) (%) product separation and the operating pressure [21]. For small-scale processes with intermittent operation, low temperature (275–300 °C) Ru(10%)/Ba-Ca 3 23.3 [105] 3.16 [105] +/0 [105,107] and consequently low pressure operation (5–30 bar) is preferable over (NH2)2 high temperature (375–550 °C) and consequently high pressure opera- Industrial Fe catalyst 9 5.4 [105] 0.74 [105] +/0 [21,22,108] – tion (100–450 bar) due to increased heat losses to the environment Ru(10%)-Cs/MgO 6 7 0.6 [105] 0.08 [105]0[27] Ru(2%)/C12A7:e- 4–6 0.76 [105] 0.10 [105]0[109] upon scale-down [58]. Furthermore, it is desirable to have a small Cs-Ru/SrZrO3 3 ––--[103,110] temperature swing between the ammonia synthesis reactor and the

7 K.H.R. Rouwenhorst, et al. Renewable and Sustainable Energy Reviews 114 (2019) 109339 insufficient ammonia separation capacities have been obtained. The pressurized storage [25]. A significant drawback of conventional sto- relevant ammonia separation technologies are compared in Table 6. rage methods is high ammonia vapor pressure of 7 bar at ambient Among the non-conventional technologies, absorption of ammonia in conditions (see Fig. 4). metal halides is currently most technologically advanced. Absorption- Another option for ammonia storage is the use of an absorbent as based ammonia separation by supported metals is capable of inter- storage material. This adds inherent safety to the system, because am- mittent operation, whereas condensation must be operated con- monia can only be desorbed upon heating or pressure decrease. Metal tinuously. Thus, absorption of ammonia in metal halides is the pre- halides are proposed for ammonia storage [4,126,128,129]. About ferred alternative. The absorption bed is regenerated when sufficient 5–10 wt% ammonia can be stored on CaCl2/SiO2 (40 wt% CaCl2 and ammonia is absorbed. 60 wt% SiO2)[125]. Energy is released upon combustion of ammonia for electricity 2.6.1. Absorption-based ammonia separation by metal halides generation (see section 2.7). This heat can be used for the regeneration The fundamentals of absorption-based ammonia separation can be of the absorbent bed (i.e., the CaCl2/SiO2). A synergy between am- understood by the atomic structure of ammonia. The hydrogen atoms monia separation and storage can be achieved by integrating the pro- are not equally distributed around the nitrogen atom (as is the case for cess steps into a single process step [129]. This saves on energy con- the hydrogen atoms around the carbon atom in methane). Therefore, sumption, as the ammonia storage system can be integrated with the ammonia is a polar molecule with a negative dipole at the nitrogen ammonia combustion step. fi atom, and with positive dipoles near the hydrogen atoms. The most practical con guration of this storage alternative is having – The ammonia capturing effect in metal halides can be understood a storage with many cylinders of about 0.5 1.0 m diameter and a few from the polarity of the ammonia molecule. Metal halides are composed meters in height. After desorbing modus at 350 °C, the cylinders are − fl of metals (such as Sr2+,Mg2+ and Ca2+) and halides (such as Cl and cooled with nitrogen ow from the pressure swing absorption unit. − Br ). Therefore, the polar properties of ammonia can be employed by the metal halide. The negative dipole of the ammonia molecule is lo- 2.7. Ammonia-to-power cated towards the metal ion, whereas the positive dipoles are located towards the halide ion, thereby forming ammine complexes. Depending Power generation from ammonia was systematically studied by on the combination of metal, halide and temperature, the coordination Tanner in the 1940s for the first time, even though the first known use number is determined (i.e., the number of ammonia molecules captured of ammonia for power generation in vehicles was in 1933 [28,130]. ffi per metal halide unit structure) [125,126]. The a nity for ammonia Power generation from ammonia can be performed directly or in- separation in various metal halides can be derived from electro- directly [131]. In the latter case, ammonia is first cracked into nitrogen negativities of cations and anions [127]. and hydrogen. At least 14% of ammonia is lost in the cracking process Silica (gel) supported calcium chloride (CaCl2/SiO2) is currently the for heating purposes [27]. Therefore, direct ammonia combustion is preferred alternative among the chlorides and bromides for scale-up preferred over an intermediate cracking step. In case of direct ammonia [125]. Its absorption capacity is lower than that of other alternatives fuel cells, ammonia is first decomposed into nitrogen and hydrogen (MgCl2, MgBr2 and CaBr2), but the low cost of CaCl2 and the high within the fuel cell (over the anode), after which the hydrogen is stability are reasons to choose for CaCl2. Among experimented supports combusted. Key advantages and disadvantages of various alternatives (alumina, kaolinite, diatomaceous earth, silica and zeolite), only silica for electricity generation from ammonia are listed in Table 8. Two main ff and zeolite are e ective supports [125]. Zeolite has a higher capacity alternatives are available for power generation, namely gas turbines than silica at high temperatures (i.e., at > 200 °C) [125]. However, the and fuel cells. It should be noted that the efficiency estimates in Table 8 higher absorption capacity of zeolite does not outweigh the additional represent the electrical efficiency. This is an important distinction from costs [125]. The reversible absorption capacity of ammonia on CaCl2 the total energy efficiency, which includes useful heat generation. −1 supported on silica is about 70 mgNH3 gCaCl2/SiO2 at 150 °C and 4 bar Another constraint on the system is intermittent operation, which operating pressure [125]. requires a fast switch on and off capability within the minutes range. At large scale power generation, a combined cycle gas turbine is the most 2.6.2. Storage electrically efficient power generation method. However, as system Ammonia is conventionally stored in liquid form [25]. The methods sizes decrease, heat losses increase. At small scale, fuel cells are gen- for liquid ammonia storage are listed in Table 7. The main factor de- erally the preferred alternative. So far, no ammonia-fueled systems are termining the type of storage is the required storage capacity. At large industrially employed. However, ammonia fueled SOFCs (solid oxide capacities, low-temperature liquid storage is economically feasible, fuel cells) are currently tested in pilots by IHI cooperation (1 kW class whereas at the smallest capacities, non-refrigerated pressure storage is power) [45], and ammonia fueled gas turbines are currently tested in most feasible [22]. There is a growing trend towards the use of pilots in the United Kingdom [42].

Table 6 Comparison of reaction product separation technologies.

Advantages Disadvantages

Condensation • Well-known technology • Large temperature swing in process (−20 °C to 250–400 °C, see Fig. 4) Separation temperature • High heat exchange and refrigeration costs −20°C to 30°C [124] • High pressure (100–460 bar) [23,80] TRL 9 • Not capable of intermittent operation • No sharp separation Absorption (metal halides) • Small temperature swing in process (150–250 °C to 250–400 °C) • Lab-scale/pilot plant scale [63,64,67,68] Separation temperature • Low pressure (5–30 bar) • Absorption material required 150-250°C [58,125] • Capable of intermittent operation • Temperature swing for regeneration (to 300–400 °C) [64] TRL 4–5 • Sharp separation Adsorption (zeolites) • Separation at low conversions (1 mol.% ammonia) • Lab-scale/pilot plant scale [114] Separation temperature • Low pressure (5–30 bar) • Adsorbent material required 20-100°C [115,116,119] • Capable of intermittent operation TRL 4 • Sharp separation

8 K.H.R. Rouwenhorst, et al. Renewable and Sustainable Energy Reviews 114 (2019) 109339

Table 7 Characteristics of ammonia storage methods. Data reproduced from Refs. [22,25,67,125].

Type TRL Typical pressure (bar) Design temperature (°C) t ammonia per t steel Capacity (tNH3) Refrigeration compressor

Non-refrigerated storage 9 16–18 20–25 2.8-6.5 < 270 or < 1500 None Semi-refrigerated storage 9 3–5 Ca. 0 10 450–2700 Single stage Low-temperature storage 9 1.1–1.2 −33 41–45 4500-45000 (< 50000) Two stage Absorption-based storage 3–41–30 20–250 –– None

− 2.7.1. Solid oxide fuel cells (1190 mW cm 2 at 650 °C) [133]. Solid oxide fuel cells (SOFCs) are the preferred alternative for In the SOFC-H type, ammonia decomposition occurs at the anode electricity generation from ammonia in the 1–10 MW range. As com- side, and the protons subsequently pass through the electrolyte, to react pared to gas turbines, solid oxide fuel cells have a higher electrical with oxygen gas at the cathode side [37]. No NOx is formed in case of efficiency at the required scale (1–10 MW). Furthermore, solid oxide the SOFC-H type, because the oxidation occurs at the cathode side, fuel cells offer a higher electrical efficiency than low-temperature fuel rather than the anode side [37]. The theoretical efficiency of the SOFC- cells when operating each with air. Both the SOFC-O and SOFC-H type H type is higher than that of the SOFC-O type [134]. This is partially fuel cells are available [37,39]. due to the lack of hydrogen dilution of SOFC-H type, as protons pass In a SOFC system, ammonia is cracked to hydrogen and nitrogen at through the electrolyte [37]. However, the highest peak power density − the anode side of the fuel cell. The subsequent reaction depends on the obtained so far is 390 mW cm 2 at 750 °C [135], which is lower than type of SOFC used. The SOFC-O types are oxygen anion conducting power densities reported for SOFC-O systems. electrolyte-based solid oxide fuel cells and SOFC-H are proton con- A major drawback for SOFC-O systems is the formation of water ducting electrolyte-based solid oxide fuel cells [39]. The SOFC-H is also vapor on the anode side. In case of SOFC-H systems, the anode side only termed a protonic ceramic fuel cell (PCFC). consists of ammonia, nitrogen and hydrogen, making it possible to use In the SOFC-O type, the anode serves for both ammonia decom- this off-gas for desorption of the ammonia from the storage. Another position and electro-oxidation of hydrogen [132]. A drawback of this is benefit of SOFC-H types is the requirement for conducting the small the formation of NOx, due to the presence of both nitrogen gas at high protons rather than the larger oxygen ions, making operation of the temperature combined with oxygen anions. Upon decomposition of former feasible at lower temperatures [136]. Proton-conducting mem- ammonia over the Ni cathode, nitrogen gas dilutes the hydrogen gas, branes (such as BZCY) are stable against water and carbon dioxide, thereby reducing the reversible cell potential [37]. The highest peak making operation in air feasible [136,137]. Eguchi et al. operate their power densities obtained so far are for SOFC-O systems SOFC system at an efficiency of 55%(LHV) [138]. Thus, assuming a

Table 8 Comparison of ammonia-to-power alternatives for a 1–10 MW system.

Alternative Advantages Disadvantages

Alkaline fuel cells • Operation near room temperature (20 °C) [139] • Pure oxygen feed required [139] 20°C [139] • Fast start-up • Low lifetime (target 1 y) [143] (< 100°C [140,141]) • High electrical efficiency [140,141] • Reactivity of electrolyte [142] Electrical efficiency Hydrogen fuel specific: 60-65%(LHV) (Pure oxygen) [140,141] • Large temperature swing between ammonia decomposition and Electrolyte fuel cell NaOH/KOH [142] Ammonia fuel specific: – – ffi ffi TRL 8 9(H2-based), 1 3 (NH3-based) [38] • No su ciently e cient system known (in research) PEM fuel cells • Operation with air [139] • Cannot be fed with ammonia due to acidic environment of 60-80°C [139] • Operation near room temperature (60–80 °C) [139] PEM fuel cells (< 120°C [140,141]) • Fast start-up Hydrogen fuel specific: Electrical efficiency • Commercially available (hydrogen-based systems) • Large temperature swing between ammonia decomposition and 40-55%(LHV) [140,141] [139] fuel cell Electrolyte Nafion [142]

TRL 8–9(H2-based), N/A (NH3-based) Solid oxide fuel cells • Operation with air [139,144] • High operation temperature (700–775 °C) [138,144] 700-775°C [138,144] • Fast hot start-up (130 s) [138] • Minimum load of 7% required [144] (500–1000°C [139–141]) • High efficiency (> 50%(LHV) in commercial • Brittle ceramic components [142] Electrical efficiency application) [144] • Slow cold start-up [142] > 50%(LHV) [138,144,145] • Commercially available (hydrogen-based systems) Ammonia fuel specific: – TRL 8 9(H2-based), 4 (NH3-based) [144] • Not commercial (demonstration stage) [147] • Near ambient pressure operation [146] Gas turbines • Operation with (oxygen-enriched) air [150] • Minimum load of 10% required to suppress NOX emissions 900-1100°C [146] • Conventional combustion equipment [151] [155] Electrical efficiency • Demonstration stage with near-term use-case • Slow start-up (hours range) 25-30%(LHV) [148,149] [152–154] • Low electrical efficiency – – TRL 4 7(H2-based), 4 6 (NH3-based) • High pressure operation (> 5 bar) [146] Ammonia fuel specific: • Low laminar burning velocity [156–158] • Increased NOx formation [159] • High ignition temperature required • Slow burning speed (5 times smaller than methane) [153] • Low flame stability • Increased oxygen content required • Not commercial

9 K.H.R. Rouwenhorst, et al. Renewable and Sustainable Energy Reviews 114 (2019) 109339

55%(LHV) electrical efficiency for ammonia-based fuel cells, about 3.1. Energy consumption of power-to-ammonia-to-power − 2.84 kWh kg 1 ammonia electricity is produced. In order to understand how much energy is consumed during full load ammonia synthesis, the energy consumption of the subsystems is 3. Process proposal for power-to-ammonia-to-power storage quantified. This serves as an indication on the round-trip efficiency system after power generation. An increased energy consumption of 15% is assumed during low load operation [161]. Process alternatives were discussed previously, from which a con- The estimated energy consumption of the power-to-ammonia plant ceptual process design is constructed and the process was modelled in is listed in Table 10. As inefficiencies are taken into account in most Aspen Plus. The Aspen Plus model is elaborated upon in the supple- units, a reasonably accurate energy consumption is provided. The cal- − mentary information. An index flowsheet of the major components in culated energy consumption of 8.7–10.3 kWh kg 1 ammonia is com- the power-to-ammonia-to-power storage system is shown in Fig. 5. parable to that of a large-scale Haber-Bosch process with a low tem- − Technology pushes within the process design are the development of perature PEM electrolyzer (8.6–9.5 kWh kg 1 ammonia, see Table 1). new materials (the battolyser for a combined battery function and hy- The variation in energy consumption is due to the desorption energy of drogen generation, the Ru/Ba-Ca(NH2)2 catalyst for ammonia synthesis ammonia, which can be heat integrated with the SOFC, thereby re- −1 at relatively low temperature, and the CaCl2/SiO2 absorbent for am- sulting in a power-to-ammonia energy consumption of 8.7 kWh kg − monia separation and storage), enabling the decentralized production ammonia. The high value (10.3 kWh kg 1 ammonia) represents the of ammonia from renewables at a relatively low temperature and case in which heat must be provided externally. The SOFC is assumed to pressure (275 °C, 8 bar) [58,76,105,125]. By combining emerging have an energy efficiency of 55%(LHV), as discussed in section 2.7. The − technologies, the hydrogen production, nitrogen production and am- heat from the SOFC is used for the desorption (1.6 kWh kg 1 am- monia synthesis loop operate at the same pressure (8 bar), thereby monia). This results in a round-trip efficiency of 33% for power-to- simplifying the process and allowing for intermittent operation. Fur- ammonia-to-power. thermore, the development of ammonia-fueled solid oxide fuel cells All in all, small-scale ammonia synthesis can be close to or as energy (SOFCs) enables direct electricity generation from ammonia, rather efficient as large-scale ammonia synthesis, when assuming the heat of than from a cracked hydrogen and nitrogen feed stream [138,160]. This the SOFC can be used for desorption of ammonia from the absorbent. simplifies the process design significantly (no ammonia cracker is re- However, this requires the synthesis loop to be operated at a lower quired), and temperature swings within the process are decreased. The temperature and consequently a lower pressure to prevent heat losses synergistic effects are discussed in the supplementary information. associated with small-scale operation. It should be noted that conven-

The ammonia stored in the CaCl2/SiO2 is released by heating the tional SMR-based ammonia synthesis plants require mostly heat input, adsorbent with the heat produced by the SOFC (see section 3.1), giving whereas the small-scale ammonia synthesis plants discussed here are an electrical round-trip efficiency of about 33% for P2A2P (power-to- electrolysis-based. As a reference, the energy consumption of the most ammonia-to-power). For comparison, the electrical round-trip effi- efficient, large-scale ammonia plants based on steam methane re- − ciency of a battery is about 80% [76]. The resulting round-trip effi- forming (SMR) is 7.78 kWh kg 1 ammonia [54]. ciencies of the case study (Haaksbergen) are listed in Table 9. About 55% of the electricity is provided by wind and solar power directly, 16% by the battery function of the battolyser (efficiency 80%) and 29% 3.2. Dynamic operation by the power-to-ammonia-to-power process (efficiency 33%, see section 3.1 and the supplementary information). The overall efficiency of the The electricity source for the ammonia synthesis plant is variable system becomes 61%. More details on the Haaksbergen case can be due to the intermittency of power from wind turbines and solar panels. found in the supplementary information. Therefore, the process must be capable of following this intermittent input electricity. Compressors typically have a load range of 55–115% [24]. Thus, it is desirable to minimize the use of compression within the process design, especially in the feed compression. As discussed

Fig. 5. Index flowsheet of major components for power-to-ammonia-to-power. Electricity and heat streams are indicated by dotted lines, while material streams are indicated by full lines.

10 K.H.R. Rouwenhorst, et al. Renewable and Sustainable Energy Reviews 114 (2019) 109339

Table 9 Fractions of various electricity inputs and outputs, and efficiencies.

7 −1 7 System Efficiency (%el) Total output (x10 kWh y ) Fraction of total output Total input electricity (x10 kWh Fraction of total input (%) − (%) y 1)

Wind turbines and solar panels N/A 1.68 55 1.68 34 Battery function (≤1 day) 80 0.49 16 0.61 12 Chemical storage (≥7 days) 33 0.89 29 2.69 54 Total 61 3.06 100 4.98 100

Table 10 Electricity consumption of power-to-ammonia plant. Electricity consumption for storage and controls are not taken into account.

−1 Unit Energy consumption (kWh kg NH3) Fraction (%) Note

Hydrogen production 8.36 81–96 See section 2.3 • Water purification 0.0047 • electrolysis 8.36 Nitrogen production 0.250 2.4-2.9 See section 2.4 Synthesis loop 0.1–1.7 0.9–16 Recycle compression from Aspen Plus simulation. Cooling data reproduced from Ref. [58]. • Recycle compression 0.05 Desorption data was estimated. • Cooling 0.062 • Desorption 0–1.6 − Total 8.7–10.3 100 154–183% of theoretical minimum (5.64 kWh kg 1 ammonia) −1 (31.3–37.1 MJ kg NH3)

Table 11 accomplished by (1) increasing the argon fraction, and (2) increasing Estimated costs of hydrogen and ammonia as an energy vector. Reproduced the nitrogen fraction. Although the latter is a reactant, it retards the from Ref. [163]. reaction rate upon decreasing the H2:N2 ratio to below 1:1. As nitrogen − − Hydrogen (€ kg 1H2) Ammonia (€ kg 1H2) production by pressure swing adsorption can only be ramped down to 30% load [73], changing the H2:N2 ratio is the primary strategy for Production 2.70 3.40 dynamic operation. Upon increasing the inert fraction (for operating at Storage period 1 day 0.71 0.03 10% load), the specific energy consumption for ammonia synthesis is 15 days 1.78 0.05 – 182 days 13.48 0.49 claimed to increase by only 10 15% as compared to full load operation [161]. This load variation is dynamically possible in the hour range [162]. Table 12 Comparison of the proposed process design (base case) to alternative process designs. 3.3. Comparison to other systems

System Relative energy Overall system input efficiency (%) The conceptual process design developed in this paper utilizes various novel technologies, thereby allowing for operation at a single Base case 1.00 61 pressure level and a synthesis temperature of 275 °C. The significance of Best electrolyzer without battery 1.07 57 these technological developments is discussed in the current paragraph Best electrolyzer with battery 0.93 66 Conventional electrolysis-based 1.08-1.14 53–56 by comparing the proposed process design to various alternative pro- Haber-Bosch process cess designs. However, before performing this analysis, hydrogen and Less efficient ammonia-to-power 1.20-1.45 42–51 ammonia as energy vectors are compared. system When comparing hydrogen and ammonia, the combined production & storage cost are important. The estimated production costs of hy- drogen and ammonia (in terms of hydrogen) and their storage cost are previously, the use of compressors has been minimized by operating the listed in Table 11, based on an estimate by Vrijenhoef. From this it hydrogen production, nitrogen production and ammonia synthesis loop follows that the total cost for hydrogen as a chemical storage medium at the same pressure (8 bar). increases significantly with storage duration. On the other hand, the Various control strategies can be employed for load variation, total cost of ammonia as a chemical storage medium is fairly constant. namely (1) pressure variation, (2) loop parallelization, and (3) inert For one day storage, hydrogen and ammonia are similar in terms of variation [162]. In case of pressure variation, the loop pressure is costs. However, for long-term storage, ammonia is the preferred alter- adapted to the load by regulating the duty of the recycle compressor. native in terms of production and storage costs. Short-term energy Pressure variation following load variation is not feasible due to fatigue storage (i.e., for about 1 day) can already be accounted for by batteries, stress occurring in processing equipment [162]. Loop parallelization which have a higher round-trip efficiency than chemical storage alter- can be employed by opening or closing parts of the equipment, thereby natives. increasing or decreasing the ammonia production capacity. A drawback In the process design a combination of short-term storage and long- of this is the requirement for an advanced control system, as well as a term storage is proposed with a battery and a power-to-ammonia-to- high capital cost for equipment [162]. Load variation by inert variation power process. The Battolyser was proposed within the process, be- is the most feasible alternative. Ammonia licensor Casale S.A. has pa- cause the combination of the battery functionality and electrolyzer tented this control strategy for load variation [161]. functionality can decrease costs. However, the Battolyser is not the best Inert variation is regulated by purge control. By decreasing the − electrolyzer available (about 4.4 kWh Nm 3 hydrogen). A case without purge fraction, the inert fraction within the process is increased and the a battery functionality, but with the best electrolysis efficiency avail- single pass conversion is decreased. The increased inert fraction can be − able (about 3.8 kWh Nm 3 hydrogen) is compared to the proposed

11 K.H.R. Rouwenhorst, et al. Renewable and Sustainable Energy Reviews 114 (2019) 109339

− process design. The system with the best electrolysis system gives a kWh 1, which includes wind turbines, solar panels, a battery, and a power-to-ammonia-to-power efficiency of 38%. However, due to power-to-ammonia-to-power storage system. omission of a battery function, the total energy input is increased to − 5.4*107 kWh y 1, which is an increase of 7% compared to the proposed Declarations of interest process design. As the overall system scales with the total energy input, the cost of electricity is also expected to be 7% higher for the system None. with the best available electrolysis and without a battery functionality. If a battery functionality is added to the best available electrolysis Appendix A. Supplementary data − system, the total energy input becomes 4.7*107 kWh y 1, which is 7% lower than the process proposed within this paper. The overall effi- Supplementary data to this article can be found online at https:// ciency of such an islanded system is 66%. 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