Ammonia as a Renewable Energy Transportation Media - ACS

Sep 27, 2017 - The key advantage of ammonia is that it contains 40% greater hydrogen (17.75 wt %) than methanol (12.6 wt %) and can be produced from r...
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Ammonia as a Renewable Energy Transportation Media S. Giddey,* S. P. S. Badwal, C. Munnings, and M. Dolan CSIRO Energy, Private Bag 10, Clayton South 3169, Victoria, Australia ABSTRACT: Ammonia synthesized using hydrogen from renewable sources offers a vast potential for the storage as well as transportation of renewable energy from regions with high intensity to regions lean in renewable sources. Ammonia can be used as an energy vector for an emissionless energy cycle in a variety of ways. Ammonia at the point of end use can be converted to hydrogen for fuel cell vehicles or alternatively utilized directly in solid oxide fuel cells, in an internal combustion engine or a gas turbine. One ton of ammonia production requires 9−15 MWh of energy. However, its conversion back to useful form or direct utilization can lead to substantial energy losses. In this paper, we present an overview of the current processes and technologies for ammonia synthesis and its utilization as an energy carrier. We have performed an estimation of the round-trip efficiency of different routes for ammonia utilization at the point of end use along with some sensitivity analysis, and we discuss the outcomes resulting from the best and worst case scenarios. KEYWORDS: Hydrogen, Energy storage, Renewables, Solar, Wind, Power generation, Automotive, Emissions

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INTRODUCTION The renewable energy (RE) sources (solar, wind, geothermal, tidal) are dispersed on the earth and have varying degrees of intensity from one region to the other. Figure 1, for example, shows the world map of direct normal irradiation, clearly showing a large variation in solar intensity (kWh m−2) between different regions of the earth.1 With continuous improvement in RE based electricity generation technologies and a subsequent reduction in prices,2 it is logical to explore methods of transporting RE from regions of high RE intensity to regions lean in renewable energy in the form of a transportable and commercially viable commodity.3 Liquid fuels with high energy density such as liquid hydrogen, methanol, and liquid ammonia produced from RE sources, with hydrogen generated via water electrolysis as the feedstock, can be transported over long distances, unlike electrical energy storage options, and utilized as an energy source at end-use sites.4−6 Liquid hydrogen offers advantage in terms of easy reconversion to gaseous hydrogen for stationary and transport applications. However, liquefaction of hydrogen is very energy intensive (requiring more than 30% of the energy content of hydrogen in addition to boil-off losses during transportation). Furthermore, the necessary infrastructure for its shipping over long distances is yet to be developed. Alternatives to liquid hydrogen transportation include chemicals produced from renewable hydrogen, such as methanol and ammonia. Ammonia stays in a liquid form at room temperature at around 10 bar pressure. Production of ammonia as a transport vector for renewable energy and its subsequent reconversion to hydrogen are energy intensive steps but the handling and shipping infrastructure including regulations for transportation are already in place.7−10 The energy content of ammonia is 5.2 kWh kg−1 (or 5.2 MWh ton−1) based on the low heat value (LHV), and its hydrogen energy content is equivalent to 5.88 kWh per kg (based on LHV of H2) of ammonia. The key © XXXX American Chemical Society

advantage of ammonia is that it contains 40% greater hydrogen (17.75 wt %) than methanol (12.6 wt %) and can be produced from renewable hydrogen and nitrogen from air, without the involvement of carbon/CO species in the synthesis process. This makes ammonia an attractive choice as a renewable energy carrier as it can be colocated with low cost renewables. The only feedstocks required are energy, water, and nitrogen from air. Ammonia is the second largest chemical (after sulfuric acid) produced in the world (over 200 million tons per annum globally with >USD $60 billion market value). Ammonia production uses about 2% of the world’s fossil fuel energy and generates over 420 million tons of CO2 per annum. Approximately 80% of ammonia is utilized for fertilizers, with around 5% for explosives, and the balance for other chemical commodities. Little ammonia is used as an energy carrier. In the future, there is potential for ammonia to be used as a renewable fuel in fuel cells, internal combustion engines, or gas turbines with near zero carbon footprint. Ammonia is easy and safe to transport, has low ignition energy, low burning velocity, and a narrow flammability limit.11 This paper presents an overview of current processes and technologies for ammonia synthesis and its utilization as an energy vector, along with an estimation of the round-trip efficiency of different routes for ammonia utilization at the point of end use. We present the outcomes resulting from the best and worst case scenarios. Ammonia Synthesis. Ammonia is currently mostly produced by the highly energy and carbon-intensive Haber− Bosch process, which requires temperatures of 450−500 °C and pressures of up to 200 bar. The feedstock for this process is hydrogen from natural gas (NG), coal or oil, and nitrogen Received: July 4, 2017 Revised: September 25, 2017 Published: September 27, 2017 A

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Figure 1. World map of solar direct normal irradiation. (Reproduced with permission from ref 1. Copyright 2016 Solargis.)

million tons of CO2 emissions per annum, representing over 1% of global energy related emissions.13 The energy consumed for ammonia synthesis is mainly dependent on the region, plant size, and the source of hydrogen (feedstock). The best practice energy consumption values for NG, coal, and fuel oil are 7.8, 10.6, and 11.7 MWh per ton of ammonia, respectively, and the corresponding CO2 emissions are 1.6, 3.0, and 3.8 tons per ton of ammonia.14 In the past, a number of hydropower plants synthesized ammonia using hydrogen produced from electrolysis as the feedstock. However, due to falling NG prices and increasing local electricity demand, many of these plants have been closed or converted into NG feedstock based plants for ammonia synthesis.13 Figure 3 shows a schematic flowchart of ammonia synthesis by the Haber−Bosch process using fossil fuels as a feedstock (a) and using hydrogen produced by an electrolyzer operated with electricity from an RE source as a feedstock (b).15,16 The synthesis process using fossil fuels involves a number of reactorsa reformer/gasifier to convert fuel to syngas (H2 + CO), shift reactors to convert CO in syngas to hydrogen, CO2 removal, methanation to convert leftover CO to methane, compression of the H2 + N2 gas mixture to synthesis pressures, and finally ammonia synthesis in the Haber−Bosch reactor. Each

produced from air by cryogenic route or pressure swing adsorption (PSA). The share of NG, coal, and fuel oil feedstock for the global production of ammonia is 72%, 22%, and 4%, respectively (Figure 2),12 contributing to approximately 420

Figure 2. Feedstock used for global ammonia production (Redrawn from data in ref 12).

Figure 3. Flowchart of ammonia synthesis from different fossil feedstocks and renewable electricity. B

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Figure 4. Schematic of an integrated ammonia cracking−metal membrane hydrogen separation reactor with inbuilt retentate utilization in the process.

consumption as the electrolyzer/Haber−Bosch route (10−12 kWh per kg ammonia). Ammonia Cracking. The concept of using ammonia as a hydrogen carrier is not new, and ammonia decomposition, or “cracking”, has long been used in industry to generate forming gas (3H2 + N2). With interest now shifting to the generation of high purity H2 (>99.99%) for fuel cell vehicles, the ammonia cracking and hydrogen separation technologies require specific attention as NH3 will irreversibly damage the membrane in polymer electrolyte membrane fuel cells (PEMFC).21 This level of purification must be achieved either by near-complete conversion of NH3 or through postcracking purification. Nickel heterogeneous catalysts have been widely employed for NH3 cracking in the past22 but have required temperatures in excess of 900 °C to ensure complete NH3 conversion.9,10,23 In the past decade, catalysts based on ruthenium have shown significantly greater activity, allowing near-equilibrium conversion to be attained at 700 bar) for delivery to on-board hydrogen storage tanks. This can be achieved using a pressure swing adsorption process, or by a membrane process. The use of hydrogenselective, palladium alloy membranes to purify cracked ammonia were reported by Philpott in 1976.26 More recently the concept of integrating methanol or ammonia cracking catalyst with hydrogen permeable membrane based hydrogen separation has been proposed.27−29 The

pass in the synthesis reactor converts only approximately 15% of the reactants to ammonia. Thus, multiple passes are required to achieve near 100% conversion. Ammonia synthesis (N2 + 3H2 = 2NH3) is an exothermic reaction (ΔH = −46 kJ mol−1) and involves a decrease in entropy. The reaction is thus favored at lower temperatures and high pressures. However, due to the slow kinetics of the reaction, the synthesis reaction is carried out at high temperatures and high pressures (450 °C, 200 bar) in the presence of an iron oxide based catalyst. In the renewable ammonia synthesis process, hydrogen is produced by electrolysis. The required electricity is generated from a renewable source such as solar photovoltaics (PV), wind generators, or tidal power. The hydrogen and nitrogen from an air separation unit (ASU) are compressed to the required synthesis pressure and fed to the Haber−Bosch synthesis reactor. The cost of the ammonia and the energy input for this route are mainly dependent on the electrolyzer capital costs and operating (electricity input) costs. The energy consumption by this route has been suggested to be in the range of 10−12 kWh per kg ammonia.16−19 Note that although the theoretical electric input required to produce hydrogen by the electrolysis of water is ∼7 KWh for each kg of ammonia produced (7 MWh ton−1), the actual electrical input will be higher due to electrolyzer stack component resistive, polarization, and system losses. Current generation electrolyzers operating at 65−70% net efficiency require 48−55 kWh renewable energy input (LHV) to produce 1.0 kg of hydrogen, similar to the energy input when hydrogen is produced from natural gas (∼45−50 kWh kg−1 hydrogen). In one ton of ammonia, the net hydrogen energy content is equivalent to 5.88 MWh. In an alternative technology developed to only fundamental scale, called electrochemical synthesis, ammonia is synthesized in a single electrochemical reactor with water or steam and nitrogen as the feedstocks.15,20 The synthesis process occurs at a temperature between room temperature and 800 °C, depending on the type of electrolyte used in the electrochemical cell. Both aqueous (alkaline, ionic liquids, molten salts) and solid state (polymer and ceramic electrolyte membranes) electrolytes with different electrodes/catalysts are being investigated to achieve the required hydrogen conversion and synthesis rates. The ammonia synthesis rates obtained by this route are quite low at this stage (10−9−10−8 mol cm−2 s−1) and need to be improved at least by 1−2 orders of magnitude to allow for commercial use of this technology.15 However, the advantage of this technology is that the reactor allows the synthesis of ammonia using renewables at significantly milder process conditions and in a single reactor. This route is also expected to have similar energy C

DOI: 10.1021/acssuschemeng.7b02219 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 1. Characteristics and Technology Status of Major Commercial Fuel Cellsa fuel cell type

Top, °C

fuel and purity

electric efficiency (thermal), %

module size available

applications

PEMFC PAFC MCFC SOFC

RT-80 200−220 650 600−1000

H2 (>99.99) H2 (>99.9) H2, CO, CH4 H2, CO, CH4

40−48 (35−40) 40−45 (35−40) 45−55 (30−40) 50−55 (30−40)

0.5−200 kW 100−460 kW 300 kW−3 MW 1−250 kW

T, S S S S

a

PEMFC: polymer electrolyte membrane fuel cell. PAFC: phosphoric acid fuel cell. MCFC: molten carbonate fuel cell. SOFC: solid oxide fuel cell. S: stationary. T: transport.

reports of 2.7 kWh (8.1% of H2 LHV) being achievable.35 For our purposes, we assumed the energy requirements for compression, dispensing and cooling to be between 12 and 17% of the energy content of the dispensed hydrogen. Ammonia Utilization at the Point of Delivery. Utilization of Ammonia in Fuel Cells. Fuel cell technology can now be considered to be fully commercial. Fuel cells are available in a range of sizes from sub-kilowatt to multi-megawatt and are used for residential, embedded, and large-scale central power generation, as well as for transport vehicles.36−38 A number of different fuel cell systems are available with a wide range of operating temperatures, materials, and efficiencies as summarized in Table 1. Most fuel cells operate on fuels derived from coal or natural gas (H2/CO) by gasification/reforming followed by hydrogen separation/purification for fuel cells requiring high purity hydrogen. Ammonia is currently under consideration as a potential fuel source,39 although at this stage there are no commercially available fuel cells which directly or indirectly utilize ammonia as a fuel. Due to the high operating temperature of solid oxide fuel cell (SOFC), ammonia can be utilized directly without pretreatment. Ammonia cracking can occur inside the fuel cell stack over the Ni/zirconia anode, with hydrogen being used in the electrochemical reaction. Staniforth and Ormerod for a tubular SOFC reported complete decomposition of ammonia to N2 and H2 with no NOX formation at 600 °C.40 For Ni/Y2O3−ZrO2 anode supported small button cells with ∼30 μm electrolyte layer and ammonia as the fuel Ma et al. reported peak power densities of 299 and 526 mW cm−2 and OCV of 1.07 and 1.03 V at 750 and 850 °C respectively.41 At these temperatures ammonia easily reduced NiO to Ni in the anode and produced power densities similar to those obtained for hydrogen as the fuel.40,41 A significantly higher power density of 1190 mW cm−2 at 650 °C with 10 μm Sm2O3−CeO2 (SDC) electrolyte on Ni/SDC anode has been reported by Meng et al.42 Fuerte et al. reported no serious degradation in SOFC performance over several hundred hours of operation.43 The low temperature fuel cells such as the PEMFC and phosphoric acid fuel cells (PAFC) have very little tolerance to ammonia (99.99%) as discussed above which can then be used to produce power for transport applications and both heat and power for stationary applications. Obviously this conversion from ammonia to hydrogen has cost and overall efficiency implications. However, ammonia can be used as a fuel in alkaline membrane fuel cells operating on a similar principle to alkaline fuel cells with the following reactions:44

hydrogen selective membrane can be based on a metal/alloy (such as Pd or Pd−Ag) or a microporous ceramic (such as silica). This concept is schematically depicted in Figure 4. Apart from lowering the ammonia cracking temperature to around 400−500 °C, the ammonia cracker efficiency is enhanced significantly due to the continuous removal of hydrogen through the hydrogen permeable membrane. Metallic membranes can achieve N2 and NH3 removal in a single step, but membranes must be free of defects in order to achieve the required NH3 threshold. Furthermore, the fragility of these membranes, and poor tolerance to thermal cycling, means their likely role will be in centralized H2 generation prior to compression and dispensing into fuel cell electric vehicles (FCEVs). Hydrogen Compression/Storage. In order to effectively crack ammonia, it is necessary to heat ammonia to a temperature where it will dissociate. In order to be thermodynamically favorable, this process must be carried out at modest pressures. This leads to a relatively low pressure stream of hydrogen gas (at 99.99% purity for use in polymer electrolyte membrane fuel cell (PEMFC) vehicles. Following transport of ammonia to the end-use site, it is cracked on a catalyst at 400 °C followed by an integrated hydrogen separation via a metal membrane reactor as discussed above. Energy losses considered in the ammonia cracker include: ammonia boil off (turning liquid ammonia at ∼8 bar/room temperature to gaseous ammonia/room temperature) based on heat of evaporation, energy required to raise the ammonia temperature to 400 °C, ammonia conversion to hydrogen in the catalyst bed, thermal energy losses in the ammonia cracking/ membrane reactor unit, hydrogen recovery via the membrane reactor, energy recovered from the retentate (uncracked ammonia and nonpermeated hydrogen) and thermal energy recovered from hydrogen from the cracker during cooling from the cracker temperature (400 °C) to the fuel cell temperature (70 °C) (Table 2). Table 2. Data (in Addition to Table 1) Used for Calculating the Round Trip Efficiency of Various Power Generation Scenarios Given in Figure 5 input parameters energy required for NH3 synthesisfrom hydrogen feedstock from RE/electrolysis (MWh per ton NH3) efficiency of ammonia cracking, % conversion of NH3 hydrogen recovery (permeated) from the membrane ammonia reactor, % of total H2 produced by dissociation energy recovered from retentate, % of heat of combustion of uncracked NH3 and nonpermeated H2 thermal heat recovered from hydrogen from the ammonia cracker, % of heat recovered while cooling from cracker to fuel cell temperature of 70 °C thermal energy losses from ammonia cracker, % of total hydrogen energy content available in ammonia energy losses for hydrogen compression from atm. to 880 bar (for 700 bar fill), % of H2 content available after cracking IC engine electric efficiency, % of ammonia energy content turbine combined cycle electric efficiency, % of ammonia energy content

symbol

value range 10−12

ηC ηH2‑rec

90−95 80−85

ηCR‑rec

70−90

ηH2‑thm‑rec

70−90

ηC‑thm

2−4

Figure 6. Example of calculations made on RTE for Route 1 in Figure 5, for the best case scenario in Tables 1 and 2, showing the process, process efficiency, and hydrogen energy content at each stage of the route assuming 10 MWh energy required per ton of liquid ammonia produced from RE sources at the beginning of the route.

for this route. The three rows in the figure show the process, process efficiency, and hydrogen content at each stage of the route. The route assumes that 10 MWh of energy is required per ton of liquid ammonia produced from RE sources at the beginning. The hydrogen content thus at the beginning of the route is 5.88 MWh per ton of ammonia (58.8% process efficiency). Therefore, the net electric power obtained from RE ammonia in a PEM fuel cell car can be estimated by multiplying this value with the efficiency of the ammonia cracker, hydrogen compressor and PEM fuel cell car. This value is calculated to be 1.89 MWh and equates to 19% RTE. Figure 7 shows the net energy produced and the efficiency for the automotive

12−17

35−40 55−60

The overall losses in the ammonia cracker, thus, can be expressed as follows: Net energy losses in ammonia cracker, MWh per ton ammonia = (Ammonia boil‐off losses) + (Raising ammonia temperature to cracker temperature) + (Heat of ammonia dissociation) + (Heat losses from cracker) − (Heat recovered from H 2 in retentate) − (Heat recovered from ammonia in retentate) − (Heat recovered from hydrogen while dropping temperature from cracker to fuel cell temperature) = (Hev /3.6) + (C NH3(Tc − Ta)/3.6) + (EdisNH3ηC /3.6) + (E H2(MWH2/MWNH3)ηC ‐ thm /3.6) − (E H2ηC(1 − ηH2 ‐ rec) (MWH2/MWNH3)ηCR ‐ rec /3.6) − (E NH3(1 − ηC)ηCR ‐ rec /3.6)

Figure 7. Comparison of net energy produced and the efficiency for different end-use applications of ammonia. The changing color shade in each column shows performance from the worst to the best case scenario as per data in Tables 1 and 2. The numbers at the top of each column are for the best case scenario.

− (C H2(Tc − Tfc)ηCηH2 ‐ recηH2 ‐ thm ‐ rec(MWH2/MWNH3)/3.6) (3)

The net energy required for the ammonia cracking has been calculated to be 0.28 and 0.30 MWh per ton ammonia for the F

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combustion engine, the RTE can vary from 15 to 21%, while for the combustion of ammonia in a combined cycle gas turbine, the RTE may vary from 24 to 31% (electric) of the input renewable energy depending on the hydrogen production system used (Figure 7). Comparison with Alternative Renewable Energy Export Vectors. In this study we have considered a series of scenarios that provide a greater understanding of the efficiency losses associated with the use of ammonia as a vector in renewable energy transport. There are clearly other vectors that can be considered (such as methanol or liquid hydrogen) and a wide range of alternative usage cases. The direct use of electricity would clearly be far more efficient with distribution losses in most electrical grids being less than 10%.53 Similarly the use of electricity to directly charge electric vehicles would also result in significantly better efficiency as the losses during charging of most modern electric vehicles are typically significantly less than 20%.54 Conversion of renewable energy to a fuel would only be considered viable if the energy is to be transported over long distances by ship, the energy needs to be stored for extended (months) periods of time or there is another engineering constraint that precludes the direct use of the generated electricity. Technologies that could be used as an alternative to ammonia as a transport vector include methanol and liquid hydrogen. These fuels offer similar benefits to ammonia in terms of the ability to transport energy. There have been a number of studies investigating the round trip efficiency of methanol and liquid hydrogen pathways.55−57 In general the efficiency losses associated with methanol or liquid hydrogen supply chains are typically of a similar magnitude to ammonia. Neither liquid hydrogen nor methanol offer a significant efficiency gain in most cases. Specht et al. compared methanol and liquid hydrogen supply chains (with hydrogen sourced from a hydroelectric power operated electrolyzer) for automotive uses and found that the overall efficiency was 9.3% for methanol and 8.7% for liquid hydrogen vectors.55 The large difference between Specht et al. and our results is mainly related to the lower efficiencies of liquid hydrogen (17.8%) and methanol (M85:23.5%) fueled cars, fuel shipment and distribution losses, and liquid hydrogen boil-off losses considered in their study. Similarly, Adamson et al. carried out analysis of a solar electrolysis chain with methanol and liquid hydrogen as the energy vectors.56 The analysis found that the overall efficiency of the process was 6% for methanol and 8% for liquid hydrogen. These values include the losses of the solar hydrogen production process (high temperature electrolysis). If this loss is removed to make this comparable with the efficiencies calculated in this study, the methanol cycle efficiency increases to 17% and the liquid hydrogen cycle efficiency is 32%. The Adamson study was conducted in 2000 before the advent of modern fuel cell vehicles and used a predicted efficiency of 58% for the fuel cell drive train rather than a more realistic efficiency of around 40%. Factoring this into the calculation gives an efficiency of around 22% before boil off losses for the liquid hydrogen case. The methanol example relies on an efficiency of 51% for the fuel cell. This is likely to have overestimated the chain efficiency but with no commercial data available it is hard to determine what the exact value is. Other studies have looked at the efficiency of transport fuels derived from alternative sources in more detail, but this is beyond the scope of this study.57

application as a column chart with changing color shades in each column showing the best and worst case values. Scenario (2): Ammonia Utilization for Stationary Applications via Fuel Cells. In this scenario, the retentate from the cracker is utilized to meet the thermal demands of the cracker and the entire thermal output from the fuel cell is used for the residential application. Note that the hydrogen will be directly utilized from the ammonia cracker, without the need for any compression (Route 2 in Figure 5). The net combined heat and power (CHP) efficiency for this route, considering worst and best case scenario data given in Tables 1 and 2 was 25−39%, as shown in Figure 7. Thus, starting with one MWh renewable energy input, the net energy delivered to the grid or a distributed site for the best case scenario would be 214 kWh in the form of electricity and 179 kWh low grade heat which can be used to produce hot water or for space heating. In a high temperature fuel cell such as a solid oxide fuel cell (SOFC) with an operating temperature in the 700−1000 °C range, ammonia cracking can be thermally integrated within the fuel cell stack. Here no separate ammonia cracker or membrane gas separation reactors are required, and ammonia cracking is considered to occur within the SOFC (Route 3 in Figure 5). This has the potential to reduce losses associated with a separate ammonia cracker and hydrogen separation unit. In this case, it has been considered that heat is supplied by the SOFC stack for ammonia boil off, heat ammonia to the fuel cell temperature, and crack ammonia (providing heat of dissociation) thus boosting the electric efficiency at the expense of some loss in the thermal efficiency. Therefore, the energy content of ammonia has been assumed to be equivalent to the hydrogen content in ammonia. We have taken into account the thermal heat captured from the exhaust gases from the anode chamber (nitrogen and steam) while the temperature drops to 80 °C (including the captured heat of steam condensation) when reporting thermal output from the fuel cell. The net CHP efficiency for this route, considering the worst and best case scenarios for RE ammonia synthesis (12 and 10 MWh per ton) and utilization in a SOFC was 36% to 50% (Figure 7). In the best case scenario, 1 MWh of renewable energy input would result in 326 kWh of electricity delivered to the grid or a distributed site. The thermal output based on the percentage of captured heat from the exhaust gases (70−90%) from the SOFC was between 135 and 171 kWh. The electric energy output can be boosted further with a steam bottoming cycle in a large scale plant. Scenario (3): Ammonia Combustion in a Turbine (Stationary) or Modified IC Engine (Transport). In this scenario, ammonia combustion takes place in an internal combustion engine or in a turbine (Route 4 in Figure 5). However, due to high ignition temperature and low flame velocity, there are several technical issues which remain to be solved. Ammonia is often used in conjunction with other fuels (diesel, kerosene, methanol, etc.) in an internal combustion engine, in order to start the combustion process. With ammonia as a fuel, NOX emissions in the IC engine exhaust can vary depending on the ammonia−diesel fuel ratio as reported by Reiter et al.48 Although these emissions decrease with increasing percentage of diesel; however, this defeats the purpose of using ammonia as the main constituent of the fuel mixture. Ammonia in the exhaust can also lead to the poisoning of the exhaust catalyst. The efficiency of ammonia combustion in an IC engine can reach 35−40%, while much higher efficiencies (55−60%) can be achieved in combined cycle (CC) ammonia gas turbines.16 Thus, for the combustion of ammonia in an internal G

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CONCLUSIONS In order to assess the potential of ammonia as an energy carrier for transport of renewable energy, we have discussed various technologies and pathways. We have also provided an analysis on the RTE for different end use applications of ammonia. Under the best case scenario for residential applications, PEMFC and SOFC produce RTE of 39% (CHP) and 50% (CHP), respectively. For the automotive sector, PEMFC and IC engines produce the best RTE of 19% and 21%, respectively. A combined cycle turbine can produce the best electric RTE of 31%. The overall efficiency of the entire energy chain using ammonia as the renewable energy vector is somewhat low for PEMFC based systems. However, this is a more mature technology compared with the direct utilization of ammonia in SOFC, IC engine or a combined cycle turbine. Despite the low efficiencies and different levels of maturity of the chain of technologies discussed, ammonia remains an excellent proposition for converting RE to hydrogen and then to ammonia, transporting it to locations with low renewable energy intensity and converting the ammonia back to hydrogen for local consumption. The RTE of electrical energy storage (battery, supercapacitors) can be higher than 80%. However, the end use and generation locations have to be in close proximity. Liquid hydrogen and methanol, despite also being alternative energy vectors, have lower RTE values as estimated in previous studies. Further, the infrastructure required for liquid hydrogen transport is almost nonexistent and methanol is an emission producing fuel at the point of use; make these alternatives less attractive at this stage. Ammonia therefore provides an attractive option in terms of RTE, as well as being an emission-less energy carrier. There are currently many unknowns for realistic cost estimates for renewable energy to ammonia to power at this stage. However, the levelized cost of hydrogen production via water electrolysis is forecast to reduce to below USD $5−6 per kg hydrogen as the price of electricity from solar PV or wind generators reduces. This will have a significant impact on the cost of ammonia production and has the potential to create a new industry around transporting clean and carbon-free energy.

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IC: internal combustion MWNH3: molecular weight of ammonia, g mol−1 MWH2: molecular weight of hydrogen, g mol−1 NG: natural gas PAFC: phosphoric acid fuel cell PEMFC: polymer electrolyte membrane fuel cell ppm: parts per million PSA: pressure swing adsorption PV: photovoltaic RTE: round trip efficiency SOFC: solid oxide fuel cell SDC: samarium doped ceria Tc: ammonia cracker temperature, 400 °C Tfc: fuel cell operating temperature, 70 °C ηC: ammonia cracker efficiency, % ηC‑thm: thermal losses from the cracker, % ηH2‑rec: hydrogen recovery (permeated hydrogen) from the cracked ammonia, % ηCR‑rec: heat recovery from the retentate, % ηH2‑thm‑rec: thermal heat recovery from hydrogen from ammonia cracker, %

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

S. Giddey: 0000-0002-8342-2895 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Dr. HyungKuk Ju and Dr. Krystina Lamb for reviewing this article. NOMENCLATURE ASU: air separation unit CC: combined cycle CHP: combined heat and power CH2: heat capacity of hydrogen, kJ K−1 g−1 CNH3: heat capacity of ammonia, kJ K−1 g−1 EdisNH3: dissociation energy of ammonia, KJ g−1 ENH3: heat of combustion of ammonia (LHV), kJ g−1 EH2: hydrogen energy content (LHV), kJ g−1 FCEV: fuel cell electric vehicle Hev: heat of evaporation of ammonia, kJ g−1 H

DOI: 10.1021/acssuschemeng.7b02219 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acssuschemeng.7b02219 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX