Assessment of a Sustainable Electrochemical Ammonia Production

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Assessment of a sustainable electrochemical ammonia production system using photoelectrochemically produced hydrogen under concentrated sunlight Yusuf Bicer, and Ibrahim Dincer ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b01638 • Publication Date (Web): 26 Jul 2017 Downloaded from http://pubs.acs.org on July 30, 2017

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ACS Sustainable Chemistry & Engineering

Assessment of a sustainable electrochemical ammonia production system using photoelectrochemically produced hydrogen under concentrated sunlight Yusuf Bicer and Ibrahim Dincer Clean Energy Research Laboratory, Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario L1H 7K4, Canada Emails: [email protected], [email protected] Abstract Intensive fossil fuel usage in ammonia production is considered non-sustainable, hence alternative ammonia synthesis options are under investigation. In this study, an environmental impact approach is performed to investigate the electrochemical ammonia synthesis at ambient pressure using photoelectrochemically produced hydrogen under concentrated solar light. The photoelectrochemical reactor consists of a membrane electrode assembly with copper oxide semiconductor on the stainless steel cathode plate. The electrolyte for ammonia synthesis is molten salt containing eutectic mixture of NaOH and KOH. The electrodes and wires are made of nickel. The life cycle assessment of the concentrated light photoelectrochemical hydrogen production is initially performed and integrated to molten salt-based electrochemical ammonia synthesis. The material and energy requirements of the life cycle assessment are taken from the experimental data. The results imply that electrochemical ammonia synthesis driven by solar energy can significantly reduce the total environmental impact corresponding to about 50% of current steam methane reforming-based ammonia production.

Keywords: Hydrogen; electrochemical synthesis; life cycle assessment; environmental impact; energy storage; Haber-Bosch.

Introduction There are multiple pathways under investigation for ammonia synthesis besides mostly used Haber-Bosch process. Since one of the major problem in Haber-Bosch process is the high operating pressure and temperature levels, the developing techniques propose low temperature and low pressure electrochemical ammonia synthesis. In the electrolytic routes, the required hydrogen can be sourced from natural gas similar to Haber-Bosch process or electrolysis of water, or even decomposition of an organic liquid such as ethanol and methanol. The greenness 1

ACS Paragon Plus Environment


level of each method depends on the energy resources. When hydrogen is produced from water electrolysis using a renewable energy source such as wind or solar, greenhouse gas (GHG) emissions could significantly be reduced during ammonia synthesis process. Water can also be directly utilized as a source of hydrogen inside the electrolytic cell through its reaction in the electrochemical process. There are four main types of electrolytes

1

commonly researched for

ammonia production as they are classified in Fig. 1. Proton conducting membranes (Nafion) ~80°C Solid state electrolyte Electrochemical ammonia synthesis electrolytes

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Oxygen ion conducting ceramic membranes ~650°C Proton conducting ceramic membrane 600-750°C

Composite membrane

(Na, K, Li) carbonate and LiAlO2 ~400-450°C YDC-Ca3(PO4)2-K3PO4 ~650°C

Molten salt

Eutectic and other salt mixtures ~180-500°C Organic solvents ~25°C

Liquid electrolyte

Ionic liquids ~25°C Aquesous solutions ~25°C

Fig. 1. Main electrochemical ammonia synthesis electrolyte types One of the major developments is the utilization of proton exchange membranes and specific combinations of anode and cathode materials for enhancement of operating conditions. In this respect, a brief review of the electrochemical ammonia synthesis methods is presented herein. Xu et al.

2

investigated electrochemical ammonia production under atmospheric pressure levels

and lower temperatures where they have used SFCN materials for cathode side and nickel-doped SDC (Ni-SDC) for the anode side. Nafion membrane is used as the electrolyte where current collector is made of silver-platinum paste. Ammonia with the maximum formation rates of 1.13 × 10−8 mol·cm−2·s−1 was synthesized at 80°C corresponding to current efficiency of about 90%. This study implied that the electrochemical reactions can also be carried out at lower temperatures when suitable membranes are used. 2


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When hydrogen is used as a reactant in ammonia synthesis, the source plays an important role in the life cycle. Ammonia is mainly produced from fossil fuels (natural gas and coal). Lan et al.

3

reported an artificial ammonia synthesis bypassing N2 separation and H2 production

phases. A maximum ammonia production rate of 1.14×10-9 mol·cm-2·s-1 was realized when a voltage of 1.6 V was applied. They implied that in the future, other low cost ammonia synthesis catalysts such as Co3Mo3N and Ni2Mo3N41 can be used to exchange Pt for selective ammonia synthesis. Besides solid electrolytes, molten salt electrolytes at elevated temperatures are also preferred in electrochemical ammonia synthesis. Serizawa et al.

4

reported conversion rates as

high as 70% of Li3N into NH3 using a molten LiCl–KCl-CsCl electrolyte at temperatures between 360 and 390°C. These conversion rates have been achieved despite the side reactions where some portion of NH3 was dissolved in the form of imide (NH2-) and amide (NH2-) anions resulting in a lower NH3 yield. The required temperature levels are still high in this method. Licht et al.

5

obtained about 35% Faradaic efficiency where they supplied water and air to

produce ammonia in molten salt electrolyte. In their further research Li and Licht 6 reported that at 200 mA/cm2 , over 90% of applied current generated H2, rather than NH3 since they used water as hydrogen source. In this case, hydrogen was cogenerated but required higher potentials because of necessary water splitting voltage. Kim et al.

7,8

also performed electrochemical

synthesis of ammonia in molten LiCl-KCl-CsCl electrolyte using a mixture of catalysts nanoFe2O3 and CoFe2O4. Their maximum formation rate was 3×10−10 mol·cm-2·s-1 where they used water as hydrogen source and nitrogen for the reaction. Kyriakou et al.

9

recently reported extensive literature review on low temperature,

medium temperature and high temperature electrochemical NH3 synthesis routes showing that the synthesis rates can reach up to 3.3×10−8 mol·cm-2·s-1. Shipman and Symes

10

presented the

recent developments in electrochemical NH3 production options. They categorized the sources of proton as water, hydrogen and sacrificial proton donors and resulted that the techniques keeping the temperatures in the range of 100°C and 300°C such as molten salt may well demonstrate to be the most efficient. A number of different systems, based either on proton or mixed proton/oxygen-ion conducting solid electrolytes, are undergoing research and development for application in electrochemical ammonia synthesis. The key elements of the solid-state electrochemical system 3



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are two porous electrodes divided by a compact solid electrolyte, which permits ion transport of either protons or oxide ions and acts as a barrier to gas diffusion

11,12

. Solid-state proton

conductors (SSPC) denote a class of ionic solid electrolytes which have the ability to transfer hydrogen ions (H+)

13

. The proton conducting ceramic membrane, along with cathode and

ammonia synthesis catalyst, are most important components in these systems. These membranes are required to have substantial proton conductivity at temperatures above 400˚C 1. In a study where researchers used a proton-conducting solid electrolyte at 450°C to 700°C with Ru based catalyst, the conversion rates are observed to be lower compared to nitrogen or steam because of the low conductivity of the working electrode 14. The literature survey shows that the operating temperature and pressure are two critical parameters affecting the overall performance of the ammonia production which can be reduced using electrochemical pathways. Furthermore, the flexibility of using renewable energy sources for distributed ammonia production is one of the key advantages over Haber-Bosch process besides reduced environmental impact. Typically, electrochemical routes, investigated so far require operation at much lower pressures than those used in the Haber-Bosch process with operating temperatures from near room temperature for liquid and polymer electrolyte systems to between 400 and 800˚C for other electrolytic routes. The low temperature operation has the potential to decrease material and operating costs and increase life time of the electrochemical reactor by providing high ammonia production rates and high current efficiencies. However, one of the significant advantages of ammonia production by medium temperature electrolytic routes such as molten salt is that such systems can be integrated with excess heat from renewable or nuclear power plants thus reducing the overall energy input especially if water is used as the hydrogen source. The literature mainly focused on the formation rates and conversion efficiencies of the electrochemical ammonia synthesis options whereas the environmental impacts have not yet been assessed. We have previously performed various life cycle assessment studies for HaberBosch ammonia production methods using conventional and renewable resources

15–17

. On

contrast, in this study, we present the life cycle assessment of electrochemical ammonia synthesis which is integrated to photoelectrochemical hydrogen production system. Solar energy-based hydrogen and ammonia production arises as one of the most sustainable solutions of today’s critical energy, environmental and sustainability issues. Since solar energy cannot be directly stored or continuously supplied, it is required to convert solar 4


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energy to a storable type of energy. Hydrogen and ammonia is a significant candidate as a sustainable energy carrier. Specifically, for solar energy storage applications, H2 can act as shortterm storage whereas NH3 can serve as long-term storage medium which reduces the storage losses significantly. Alternative ammonia production methods are being investigated in which there are less environmental emissions and energy consumption. Electrochemical ammonia synthesis is currently under deep investigation as an alternative pathway. Using renewable energy resources to drive the electrochemical NH3 synthesis, the carbon footprint of current fossil fuel-based NH3 production industry can be lowered significantly. Electrochemical NH3 synthesis routes offer higher integrability to stand alone and distributed NH3 production which is a carbon free fuel for various sectors. Assisting electrochemical process with solar energy will create an environmentally friendly method. The absence of practical solar energy-based integrated hydrogen and ammonia production systems which are environmentally benign, low cost, efficient, and safe is one of the main complications for the transition to a solar energy-based economy. The main objective of this study is to investigate the environmental impacts of an electrochemical ammonia synthesis process combining photoelectrochemical hydrogen production system under concentrated and split solar spectra to increase the solar spectrum utilization and make the ammonia production process greener.

System Description In this study, photoelectrochemically generated H2 is directly used in the formation of electrochemical NH3. The electrochemical synthesis of ammonia using photoelectrochemically produced H2 in a molten salt ambient including the catalyst of nano-Fe3O4 is assessed in terms of environmental impact where N2 is supplied from air separation plant. The complete schematic diagram of the system is shown in Fig. 2. Nitrogen gas sent through the porous nickel cathode is reduced to nitride according to the following equation: Nଶ + 6eି → 2N ଷି

(1)

As it becomes N3-, the nitride moves to the other electrode where H2 is being supplied. Hydrogen ions combine with nitrogen ions and form NH3 at anode electrode expressed as follows: 2N ଷି + 3Hଶ → 2NHଷ + 6eି

(2)

The overall reaction is: 5



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(3)

3Hଶ + Nଶ → 2NHଷ

The pure alkali hydroxides NaOH and KOH each melt only at temperatures above 300°C. The individual melting temperatures of NaOH and KOH are 318°C and 406°C, respectively. The NaOH-KOH eutectic is of particular attention and melts at 170°C. Ammonia synthesis rates increase when the molten hydroxide (NaOH-KOH) electrolyte is mixed with high–surface area Fe3O4 to provide iron as a reactive surface and when nitrogen and hydrogen are present in the reactor. Electricity is supplied between two nickel anode and cathode electrodes in the molten salt medium. The mixture is prepared in the beginning by simply adding NaOH and KOH pellets in the reactor. The reactor body is heated up using heating tape positioned around the alumina crucible. After the salts melt, nano-Fe3O4 is added to the electrolyte and then stirred.

Fig. 2. The schematic diagram of the experimental setup used for life cycle assessment study In this section, we present the materials and energy requirements required for the experimental system. The ammonia electro-synthesis chamber comprises of a nickel mesh cathode and a nickel mesh anode immersed in molten hydroxide electrolyte containing 10 g suspension of the nano-Fe3O4 contained in alumina crucible which is sealed to allow gas inlet at the cathode and gas outlet from the exit tubes. The reactants, H2 and N2, are bubbled through the mesh over the anode and cathode, respectively. Although there is a stainless steel separator 6


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between the anode and cathode sides, the exiting gases can also contain unreacted gases. The combined gas products (H2, N2 and NH3) exit through two exit tubes in chamber head space. The product gases are bubbled in a dilute H2SO4 for NH3 capture. In this respect, firstly photoelectrochemical hydrogen production process is built in the LCA software. The material list required for the PEC reactor design is listed in Table 1. PEC hydrogen generation system in this work involves mainly a photoelectrochemical cell with a membrane electrode assembly, photovoltaic (PV) module, light source (concentrated sun light), electricity supply (from photovoltaic module) and optical tools such as Fresnel lens and spectrum splitting mirrors. The PEC system is an integration of solar concentrator, PV and PEC cells for both hydrogen and electricity production where the produced electricity by the PV module is used in the PEC cell. The upper wavelength spectrum (above 700 nm) is used for photovoltaics and lower wavelength spectrum (below 700 nm) is used for PEC cell to increase the solar energy utilization. The cathode plate of the PEC reactor has copper oxide photosensitive coating enhancing the hydrogen evolution as photocathode. This system has previously tested and published in our former studies

18,19

.

Table 1. The type and quantity of the materials used in the PEC reactor. Material Value Unit High density polyethylene (HDPE)

2

kg

Stainless steel electrodes

2

kg

Nafion membrane

930

cm2

Copper oxide

2.7

g

Platinum black

2.7

g

Washers

0.125

kg

Bolt/Nuts

0.125

kg

Acrylic or polycarbonate reactor window

0.4

kg

Plastic piping

100

g

Rubber gasket

0.2

kg

The amount of listed materials are employed in LCA software. The boundary of the LCA study for PEC hydrogen production is shown in Fig. 3. The main energy and material requirements of this PEC hydrogen production system are summarized in Table 2. The required 7



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electricity for both electrochemical ammonia and photoelectrochemical hydrogen system is supplied from photovoltaic cells.

Fig. 3. The boundaries of the conducted LCA for PEC hydrogen production. Table 2. Main energy and material flows in PEC hydrogen production system. Parameter Value Unit Hydrogen (product)

0.000004669

g

Electricity, production from photovoltaic

0.698

J

Water, deionized

0.000042021

g

Solar energy

9.418948327

J

Since the system uses concentrated light with a set of other structures such as Fresnel lens, support mechanism, dielectric mirrors and other equipment, the complete setup is also considered in the LCA analysis as listed in Table 3. This step includes only hydrogen production from PEC system. In this table, ammonia reactor is not taken into account, since it will be already included in the ammonia production step. In the second step, electrochemical ammonia synthesis process is simulated as shown in Fig 4.

8


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Table 3. The materials and quantities used in the integrated system for concentrated light PEC hydrogen production. Material Value Unit Plastic pipes for gases

300

g

Photovoltaic cell, multi-Si

625

cm2

Fresnel lens (Polycarbonate)

1

kg

Dielectric mirrors (6 in total) (Borosilicate glass)

0.5

kg

PEC hydrogen production reactor

1

item

Support structure (wood)

50000

cm3

Support mechanism (Aluminum alloy)

2

kg

Fig. 4. The boundaries of the conducted LCA for electrochemical ammonia synthesis process. The ammonia production reactor consists of the following materials listed in Table 4. The quantities are entered in the LCA software.

9



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Table 4. The quantities of the materials used in the ammonia reactor. Material Value Unit Nickel Wiring

2

m

Nickel Electrodes

200

cm2

Reactor casing Alumina Crucible (Al2O3)

500

mL

Reactor Lids (Stainless steel 316 Alloy)

2

kg

Washers

0.125

kg

Bolt Nuts

0.125

kg

Reactor tubes (Ceramic Round Single Bore Tubes Alumina 99.8%)

50

g

Piping (plastic)

2

m

Nitrogen production is the cryogenic air separation plant considering the plant construction, operation and maintenance where the inventory data are taken from SimaPro database 20. The electricity and material inputs required for the life cycle assessment are derived from the experimental results. The main energy and material requirements of this electrochemical ammonia synthesis which uses PEC hydrogen are summarized in Table 5. Here, the amounts of catalysts and electrolyte are calculated based on service time since they are not in fact consumed in the reaction. Furthermore, the heat is not taken as input in the LCA analysis since it is assumed that the synthesis reaction occurs at constant set temperature from any excess heat source.

Table 5. Main energy and material flows in electrochemical ammonia synthesis using PEC hydrogen. Parameter Value Unit Ammonia (product)

1.875

mg

Hydrogen from PEC integrated system

0.331

mg

Nitrogen, gas, at plant, US Grid

1.543

mg

Iron oxide, catalyst

2.7778×10-9

g

Sodium hydroxide (electrolyte)

2.7778×10-9

g

Potassium hydroxide (electrolyte)

2.7778×10-9

g

Electricity production from PV

105

J 10


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LCA is an instrument which helps engineers, scientists and policy makers to assess and compare energy and material use, emissions and wastes, and environmental influences for various products or processes. Some of the assumptions made for the LCA analysis are listed below: •

The nitrogen is considered as gas from cryogenic air separation unit.

•

The inputs used in calculations are feedstock, energy or electricity and emissions.

•

The processes for ammonia production contains production of hydrogen and nitrogen separately.

•

The stoichiometric mass balance is used to identify the amount of hydrogen and nitrogen required for unit ammonia production considering multiple passes of the unreacted gases.

•

The functional unit is one kg ammonia production and the process is from cradle to gate.

CML 2001 method is a technique developed by a group of scientists under the lead of CML (Center of Environmental Science of Leiden University) counting a set of impact categories and characterization methods for the impact assessment step in 2001 21. CML 2001 method presents the results based on the environmental impact categories such as global warming, human toxicity and abiotic depletion. In this method, the results are mostly presented per equivalent substance amount such as kg CO2 equivalent and kg SO2 equivalent. The following environmental impact categories are considered for the assessment: •

Depletion of Abiotic Resources: Abiotic resources are natural resources including energy resources, such as natural gas and crude oil, which are considered as non-living. The unit is expressed as kg Sb. (antimony) eq.

•

Human Toxicity: It is computed by the effects of toxic substances for 500 years time horizon. 1,4-dichlorobenzene equivalents/kg emissions is used to express each toxic substance.

•

Global Warming: This category is related to greenhouse gas emissions and climate change. The unit kg CO2 eq. is used to express the Global Warming Potential for time horizon 500 years (GWP500).

Results and Discussion

11



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The LCA results are obtained using SimaPro LCA software. PEC-based electrochemical ammonia production, which is the method experimentally tested, uses photovoltaic cells for electricity requirements of the system. PV electrolysis for ammonia production has higher environmental effects because of mainly the production phase of the PV cells and aluminum support mechanism. Therefore, for PEC-based electrochemical ammonia synthesis option, the environmental effects are higher than some renewable routes such as hydropower, municipal waste and wind. The LCA results obtained for PEC-based electrochemical ammonia production method using concentrated light are given in detail to reveal the contribution of various sub-processes. When reporting the contribution of different processes to overall impact category, 1% cut-off is applied. There are mainly three processes in the PEC-based ammonia synthesis namely; hydrogen production from photoelectrochemical reactor, nitrogen production from air separation and electricity production from PV cells for energizing the process. The main contributor in all categories is the electricity production from PV cell as shown in Tables 6 to 8. Approximately 7% of total human toxicity is caused by nitrogen production process whereas about 30% is due to hydrogen production from PEC system as listed in Table 6. The electricity production from PV is mainly responsible for the remaining part. There are numerous substances causing toxicity for human health such as arsenic and nickel as shown in Fig. 5. Arsenic and polycyclic aromatic hydrocarbons are the two fundamental toxic substances (about 64% in total) released to the environment in this process. These are mainly caused by copper and aluminum production processes for PV and support structures as shown in Fig. 6. Polycyclic aromatic hydrocarbons are released due to nitrogen production from air separation plant since electricity grid-mix is used. The total human toxicity value of the system is found to be 0.949 kg 1,4-DB eq. per kg of ammonia.

Table 6. The shares of different sub-processes in human toxicity category for concentrated light PEC-based electrochemical ammonia synthesis. Inflows Flow Unit Total

100

%

Electricity, production photovoltaic, multi-Si

63.2

%

Hydrogen, PEC integrated system, concentrated light

29.9

%

12


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Nitrogen, gas, at plant

6.9

%

Potassium hydroxide

0.000102

%

Sodium hydroxide

6.96×10-5

%

Iron oxide

7.06×10-9

%

Propylene oxide 2%

Benzene 1%

PAH, polycyclic aromatic hydrocarbons 1%

Remaining substances 1%

Copper 2% Cadmium 5%

Nickel 8% Arsenic 41% Chromium VI 16%

PAH, polycyclic aromatic hydrocarbons 23%

Fig. 5. The share of toxic substances for concentrated light PEC-based electrochemical ammonia synthesis.

13



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Copper, primary, at refinery Remaining processes Ferrochromium, high-carbon, 68% Cr, at plant Aluminium, primary, liquid, at plant Anode, aluminium electrolysis Hard coal, burned in power plant Disposal, uranium tailings, non-radioactive# Dipropylene glycol monomethyl ether, at plant Disposal, sulfidic tailings, off-site P-dichlorobenzene, at plant Copper, from imported concentrates, at refinery 0.

0.1 0.2 0.3 0.4 0.5 Human toxicity (kg 1,4-DB eq/kg ammonia)

Fig. 6. Contribution of various sub-processes to human toxicity potential of concentrated light PEC-based electrochemical ammonia synthesis. Table 7 shows the shares of main processes contributing to abiotic depletion potential. Almost half of the total abiotic depletion is because of PV electricity production whereas about 25% is due to hydrogen production from PEC reactor. The molten salt electrolyte and reaction catalyst have reasonably small shares in total impact.

Table 7. The shares of different sub-processes in abiotic depletion category for concentrated light PEC-based electrochemical ammonia synthesis. Inflows Flow Unit Total

100

%


51.7

%


24.5

%


23.8

%

Potassium hydroxide

0.000256

%

Sodium hydroxide

0.000141

%

Iron oxide

1.83×10-5

%

14


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Furthermore, as shown in Fig. 7, coal and natural gas are two main substances depleting in this method due to high electricity consumption in the PV cell manufacture factory and aluminum needed for support mechanism. Crude oil and brown coal have shares of 14% and 7%, respectively as shown in Fig. 8. The total abiotic depletion potential of the system is calculated to be 0.00822 kg Sb eq. per kg of ammonia. Tellurium, 0.5ppm in sulfide, Te 0.2ppm, Cu and Ag, in crude ore, in ground, 1.1951, 1%

Coal, brown, in ground, 7.3262, 7%

Remaining substances, 0.514, 1%

Oil, crude, in ground, 13.5654, 14% Gas, natural, in ground, 39.5717, 40%

Coal, hard, unspecified, in ground, 36.7749, 37%

Fig. 7. The share of depleting abiotic sources for concentrated light PEC-based electrochemical ammonia synthesis. Hard coal, at mine Natural gas, at production onshore Natural gas, at production offshore Hard coal, at mine Remaining processes Lignite, at mine Natural gas, unprocessed, at extraction Crude oil, at production onshore Crude oil, at production offshore Ethylene, average, at plant Anode slime, primary copper production 0. 0.0005 0.001 0.0015 0.002 Abiotic depletion (kg Sb eq/kg ammonia)

Fig. 8. Contribution of various sub-processes to abiotic depletion potential of concentrated light PEC-based electrochemical ammonia synthesis. 15



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The global warming potential of PV electricity production is responsible for almost 50% of total GHG emissions where almost 76% of PV electricity is because of PV cell production process in the factory. The shares of main processes for global warming potential are tabulated in Table 8. PEC electrochemical route has considerably lower than any other fossil fuel based ammonia production options. Overall, concentrated light PEC-based electrochemical synthesis yields about 1.09 kg CO2 eq./kg ammonia. Table 8. The shares of different sub-processes in global warming category for concentrated light PEC-based electrochemical ammonia synthesis. Inflows Flow Unit Total

100

%


51.9

%


24.5

%


23.6

%

Potassium hydroxide

0.000248

%

Iron oxide

0.000143

%

Sodium hydroxide

0.000143

%

Methane, fossil, 1.7505, 2% Sulfur hexafluoride, 3.501, 3%

Methane, tetrafluoro-, CFC-14, 1.5016, 1%

Dinitrogen monoxide, 0.5952, 1%

Carbon dioxide, fossil, 92.1436, 93%

Fig. 9. The share of greenhouse gas emissions for concentrated light PEC-based electrochemical ammonia synthesis.

16


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Global warming category includes all greenhouse gas emissions however, CO2 is the main gas emitted to the environment corresponding to about 93% of total in this method as shown in Fig. 9. Sulfur hexafluoride (3%) and methane (2%) are the other gases contributing to total GHG emission. Sulfur hexafluoride emission is mainly due to magnesium production in the plant required for PV cell production. As shown in Fig. 10, electricity production in cogeneration plant and hard coal burned in power plant are mainly because of silicon production required for PV cells.

Remaining processes Hard coal, burned in power plant Electricity, at cogen 1MWe lean burn Natural gas, burned in power plant Flat glass, uncoated, at plant Lignite, burned in power plant Magnesium, at plant MG-silicon, at plant Heat, at cogen 1MWe lean burn Disposal, plastics, mixture, 15.3% water, to municipal# Heavy fuel oil, burned in power plant Natural gas, burned in industrial furnace >100kW Natural gas, burned in gas turbine, for compressor# Aluminium, primary, liquid, at plant Clinker, at plant

0.

0.05 0.1 0.15 0.2 0.25 0.3 Global warming (kg CO2/kg ammonia)

0.35

Fig. 10. Contribution of various sub-processes to global warming potential of concentrated light PEC-based electrochemical ammonia synthesis. LCA uncertainty analyses results Defining the uncertainties within the LCA study confirms the accuracy of the results. The uncertainty analyses are performed in SimaPro software using Monte Carlo technique. The presented results here are for concentrated light PEC-based electrochemical ammonia production method using the experimental system defined in the system description section. The confidence interval for the results is 95%. The number of runs performed for the results is 3224. The uncertainty analyses results are shown in Table 9 for the selected environmental impact categories. The mean of global warming value is 1.09 kg CO2 eq. and standard error of mean is 0.00301 kg CO2 eq. corresponding to 17.1% coefficient of variation

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which is the lowest among other categories. The highest coefficient of variance is found to be 43.9% for abiotic depletion category.

Table 9. Uncertainty analyses results of concentrated light PEC-based electrochemical ammonia production. Standard Standard Coefficient of Impact category Unit Mean Median error of Deviation Variation mean Abiotic depletion Global warming 500a

kg Sb eq.

0.0082

0.00746

0.00361

43.90%

0.00773

kg CO2 eq.

1.09

1.07

0.187

17.10%

0.00301

0.949

0.884

0.302

31.80%

0.0056

Human toxicity

kg 1,4-DB

500a

eq.

The probability distributions of the selected environmental impact categories are shown in Figs. 11 to 13. Fig. 14 shows the comparison of uncertainty ranges for different selected categories. This method is still in early investigation phase resulting in less reliable data for LCA inventory step, especially for the energy requirement. However, the results of this study imply a good indicator regarding the total environmental impact reduction in comparison with conventional Haber-Bosch process. Taking into account the uncertainties of LCA results for PEC-based electrochemical ammonia production method, this process can be more environmentally benign than other renewable routes.

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0.06 Global warming (kg CO2 eq/kg ammonia)

Probability

0.05 0.04 0.03 0.02 0.01 0.

Fig. 11. Probability distribution of global warming potential for concentrated light PEC-based electrochemical ammonia production. 0.16 0.14 0.12 Probability

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Human toxicity 500a (kg 1,4-DB eq/kg ammonia)

0.1 0.08 0.06 0.04 0.02 0.

Fig. 12. Probability distribution of human toxicity potential for concentrated light PEC-based electrochemical ammonia production.

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0.25 0.2 Probability

Abiotic depletion (kg Sb eq/kg ammonia) 0.15 0.1 0.05 0.

Fig. 13. Probability distribution of abiotic depletion potential for concentrated light PEC-based electrochemical ammonia production. 250 Uncertainty range (%)

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200 150 100 50

Abiotic depletion

Global warming 500a

Human toxicity 500a

Fig. 14. Uncertainty ranges of the selected impact categories for concentrated light PEC-based electrochemical ammonia production. As the average GHG emission from commercial ammonia plants range between 2 to 2.5 kg CO2 eq. per kg ammonia, PEC-based electrochemical ammonia production can reduce the GHG emissions down to about 1 kg CO2 eq. The main reason of having higher environmental effects in PEC electrochemical route than some renewable routes is that this process still consumes higher energy because of operating conditions and specific materials used. Because, the pressure is the ambient pressure, the conversion rate is lower. However, the technology and materials are improving quite quickly which will decrease the energy required for electrochemical ammonia synthesis and eventually yield less environmental impact.

Conclusions 20


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Solar energy is abundant and clean which can be used in ammonia synthesis applications. The electrochemical ammonia synthesis brings the flexibility of utilizing renewable electricity that is an alternative to conventional energy intensive centralized ammonia production plants. This study performs a life cycle assessment for molten salt-based electrochemical ammonia production system where the required hydrogen is sourced from concentrated light-based photoelectrochemical process. Most of the inventory data are taken from the conducted experimental results in which ammonia is electrochemically generated at ambient pressure using H2 and N2 in a molten hydroxide medium with nano-Fe3O4 catalyst. The results show that 63.2% of human toxicity, 51.7% of abiotic depletion and 52% of global warming potential of the system is caused by the electricity production from photovoltaics. The total abiotic depletion potential of the system is found to be 0.00822 kg Sb eq./kg ammonia, the global warming potential is calculated as 1.09 kg CO2 eq./kg ammonia and the human toxicity potential is obtained as 0.949 kg 1,4-DB eq./kg ammonia which are considerably lower than conventional Haber-Bosch process.

Acknowledgement The authors acknowledge the support provided by the Natural Sciences and Engineering Research Council of Canada. Nomenclature CML Center of Environmental Science of Leiden University GHG Greenhouse gas HDPE High density polyethylene LCA Life cycle assessment LHV Lower heating value PEC Photoelectrochemical PV Photovoltaic SDC Samarium doped cerium oxide SFCN SmFeCuNi SSPC Solid-state proton conductors References 1. Giddey, S., Badwal, S. P. S. & Kulkarni, A. Review of electrochemical ammonia production technologies and materials. Int. J. Hydrogen Energy 38, 14576–14594 (2013). 2. Xu, G., Liu, R. & Wang, J. Electrochemical synthesis of ammonia using a cell with a Nafion membrane and SmFe0.7Cu0.3−x Ni x O3 (x = 0−0.3) cathode at atmospheric 21



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3. 4. 5. 6.

7.

8.

9.

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11.

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16.

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19.

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20. 21.

Ecoinvent v3 | High-Quality LCI Database Integrated in SimaPro. (2017). Available at: https://simapro.com/databases/ecoinvent/. (Accessed: 8th March 2017) SimaPro Life Cycle Analysis version 7.2 (software). (2017).

For Table of Contents Use Only

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Ammonia is a clean energy carrier which is electrochemicaly synthesized using photoelectrochemical hydrogen for sustainable development.

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Assessment of a Sustainable Electrochemical Ammonia Production

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