Recent progress in Green Ammonia: Production, applications, assessment; barriers, and its role in achieving the sustainable development goals

3.1. Power to ammonia synthesis (Water Electrolysis)

Hydrogen gas (H₂) and oxygen (O₂) are produced by the electrolysis of water (H₂O). The water-splitting process is done in electrochemical cells which can be categorized according to their configuration, i.e., polymer electrolyte membrane electrolyzers, alkaline electrolyzers, and solid oxide electrolyzers [48]. All cells are composed of an anode, cathode, electrolyte, power source (renewable), and sometimes a membrane. Upon passing an electric current through the electrodes (1.23 V in an ideal case), the covalent bonds will start to break down, allowing hydrogen and oxygen gases to be produced. In the case of an alkaline electrolyzer, potassium hydroxide is commonly used as an electrolyte to avoid acid corrosion; In terms of anodic and cathodic stability, nickel is the most commonly employed electrode [49]. The process of ammonia production using electrolysis includes four units: renewable source (solar or wind), electrolyzer, air separation unit, and Haber Bosch process (Fig. 1). The first electrolysis-based ammonia synthesis was introduced in 1920 based in Haber- bosh process with an energy consumption of 46 ± 2 GJ/tNH₃ [50]. However, this stopped by the 1990 s owing to the high cost of fossil fuels. Several projects for solar to ammonia electrolysis have been initiated in Europe and America. Today, the current trend toward green ammonia is seeking a direct method for ammonia production under mild operating conditions.

1. Download : Download high-res image (198KB)
2. Download : Download full-size image

Fig. 1. Process description for green ammonia production [58], https://www.man-es.com/discover/two-stroke-ammonia-engine/green-ammonia-production. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3.2. Emerging ways for ammonia synthesis

Even though the existing technologies for green ammonia synthesis offer many facilities due to the existing equipment’s from the last century, research and development are seeking a route for direct production of green ammonia without involving the intermediate step of hydrogen production. Many research themes imply a new synthesis concept to obtain a pilot-scale green ammonia production. Examples of these emerging methods are photosynthesis, electrochemical methods, heterogenous and homogenous catalysis, molten salt synthesis, solid-state synthesis, and non-thermal plasmatic synthesis. Such methods rely on mild operation conditions and open the way for direct ammonia production [59], [60].

3.2.1. Electrochemical ammonia production

The electrochemical production route of ammonia is distinguished from conventional NH₃ production by its reaction route, operating conditions, and energy source. The main aspect that distinguishes electrochemical synthesis from electrolysis is the direct synthesis of green ammonia from air and water utilizing renewable energy sources, such as tidal or solar energy. A common process involves oxidizing hydrogen or water at the anode, which releases protons that move over a solid or liquid electrolyte to the cathode side. Nitrogen reacts with the protons in the cathode under mild conditions to produce NH₃ [61], [62]. Typically, NRR “the nitrogen reduction reaction” and HER “the hydrogen evolution reaction” compete at the cathode as follows [63], [64], [65]:

4 In electrolytes with a pH < 6(2)" role="presentation" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 14.4px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0">${\mathit{�}}_2\left(\mathit{�}\right)+6{\mathit{�}}^{+}+6{\mathit{�}}^{-}\to 2{\mathit{�}\mathit{�}}_3\left(\mathit{�}\right)$(3)" role="presentation" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 14.4px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0">$2{\mathit{�}}^{+}+2{\mathit{�}}^{-}\to {\mathit{�}}_2\left(\mathit{�}\right)$

5 In electrolytes with a pH > 8(4)" role="presentation" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 14.4px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0">${\mathit{�}}_2\left(\mathit{�}\right)+6{\mathit{�}}_2\mathit{�}+6{\mathit{�}}^{-}\to 2{\mathit{�}\mathit{�}}_3\left(\mathit{�}\right)$(5)" role="presentation" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 14.4px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0">$2{\mathit{�}}_2\mathit{�}+2{\mathit{�}}^{-}\to {\mathit{�}}_2\left(\mathit{�}\right)+2{\mathit{�}\mathit{�}}^{-}$

The main issue facing this process is the thermodynamic stability of the N₂, which requires high energy to defeat the bonding energy (941 kJ/mol) of " role="presentation" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 14.4px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0">$�\equiv �$. Therefore, depending on the operating temperature, various electrochemical green ammonia production can be categorized, including low operating temperature up to 100 °C, mid-operating temperature between 100 and 350 °C, molten salt ammonia synthesis between 100 and 500 °C, solid-state ammonia synthesis > 500 °C, and high operating temperature 350 to 700 °C [66].

3.2.1.1. Low-temperature electrochemical ammonia synthesis

Low-temperature ammonia synthesis receives considerable attention among researchers with more than a hundred publications per annum [67]. A typical cell with low operating conditions consists of a platinum anode [66], Ru-based cathode [68], and a high conductive proton membrane, usually Nafion [69]. Most studies are performed at room temperature using liquid aqueous electrolytes such as water that can directly donate the H⁺ proton. Acidic electrolytes are preferable due to their high proton environment. In contrast, in basic electrolytes, there is a high competition between hydrogen evolution reactions and nitrogen reduction reactions, as shown in (equations 3 & 4) [70], [71]. Temperature plays a key role in enhancing NH₃ production rate (10⁻⁸- 10⁻⁵ mol s⁻¹ cm⁻²) at an operating temperature up to 90 °C [65], [72]. Another key factor affecting the overall performance of ammonia production is the type of catalyst. Today, numerous types of catalysts have been studied including noble metals [73], metal (oxides [74], sulfides [75], and nitrides [76]), organometallic complexes [77] etc. A study performed by Zhang et al. [78] involved using a bi-metallic oxide (CoMoO₄) for NH₃ synthesis at atmospheric pressure and room temperature. High faradic efficiency and ammonia yield of 22.86 % and 7.98 mol h⁻¹ g_cat⁻¹ are higher than those obtained using mono metallic oxides. Table 2 summarizes the progress done in preparing ammonia at low-temperature.

Table 2. Different experimental works on low-temperature ammonia synthesis.

T (°C)	Electrolyte	Cathode	Ammonia Yield (mol/s cm−2)	Ref.
50	AEM	Fe	380 × 10−12	[79]
80	Nafion	SmBaCuMO5+	870 × 10−10	[80]
80	Nafion	Sm1.5Sr0.5NiO4	105 × 10−10	[81]
80	SPSF	Sm1.5Sr0.5NiO4	103 × 10−10
Room T	Nafion	Fe/Fe3O4	111 × 10−10	[82]
Room T	AEM	Fe–CuS/C	686 × 10−12	[75]
50	AEM	RuPt	635 × 10−12	[83]
Room T	Nafion	Au nanorods	27 × 10−12	[84]
Room T	Nafion	Fe2O3-CNT	36 × 10−12	[71]

3.2.1.2. Molten-salt ammonia synthesis (Moderate Temperature)

The main challenge facing many aqueous systems for ambient ammonia synthesis is the high hydrogen evolution and slow kinetics. Molten salt electrolytes revealed a promising result with enhanced faradic efficiency. For moderate NH₃ synthesis, in the range of 100 to 400 °C, it utilizes ammonia using a molten state electrolyte. In a typical process, a strong competition of HER during NH₃ synthesis owing to the " role="presentation" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 14.4px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0">$�\equiv �$ bonding which can be migrated if H₂ at a gaseous state reacts with nitride ions. A typical cell consists of an electrolyte (molten salt), a porous cathode, and a permeable anode (Fig. 2).

1. Download : Download high-res image (254KB)
2. Download : Download full-size image

Fig. 2. Illustration of molten-salt ammonia synthesis, adapted from [85] with permission No. 5441990967119.

Murakami et.al [86] were the first investigators for the electrochemical molten-salt ammonia synthesis at atmospheric pressure. The hydrogen source for this type of synthesis could be gaseous hydrogen or water molecules. Depending on the electrolyte, there are three different configure of this type: molten chloride salts [87], [88], molten hydroxide salts [89], and composite electrodes [90]. Molten chloride salt started to receive great attention owing to the fast dissolution of Li₃N in molten electrolytes as a source of nitride ions. For this type of electrolyte, protons (H⁺) is favourable from water instead of hydrogen gas. However, the low faradic efficiency is observed owing to the possibility of hydroxide ions and carbon dioxide formation [87]. On the other hand, hydroxide molten operates at a lower temperature (200 °C) and is less corrosive with a faradic efficiency up to 35 % at low current density [91]. However, at a higher current density the faradic efficiency reveals a sharp decline owing to H₂ evolution. To tackle the lower efficiency problem, some researchers investigated the integration of a sub fuel cell to recycle the produced hydrogen; others suggested supporting activated carbons to inhibit H₂ evolution for metal-based catalysts [89], [92]. Table 3 shows the progress done in Molten-salt ammonia synthesis (@ Moderate Temperature), while Table 4 shows the progress done at high temperature.

Table 3. Molten-salt ammonia synthesis (Moderate Temperature).

T (°C)	Electrolyte	Cathode	Ammonia Yield (mol/s cm−2)	Ref
450	SDC-ternary carbonate composite electrolyte	La0.6Sr0.4Fe0.8Cu0.2O3−δ	539 × 10−11	[93]
450	LiAlO2 – (Li/Na/K)2CO3	Co3Mo3N-Ag	327 × 10−12	[94]
400	LiAlO2 – (Li/Na/K)2CO3	CoFe2O4-Ag	232 × 10−12	[95]
400	LiCl-KCl-CsCl + Li3N	Porous Ni/Ni	333 × 10−11	[86]
300	LiCl-KCl-CsCl + Li3N	Porous Ni/C	200 × 10−10	[87]
200	NaOH/KOH (Nano-Fe2O3)	Ni	100 × 10−10	[50]
400	LiCl, KCl, CsCl + Li3N	Al	333 × 10−10	[96]

Table 4. High-temperature ammonia synthesis.

T (°C)	Electrolyte	Cathode	Ammonia Yield (mol/s cm-2)	Ref.
550	BaZr0.7Ce0.2Y0.1O3 − δ	Ni-BZCY72	286 × 10−11	[102]
550	BZCY72 “BaZr0.8Y0.2O3 − δ “	La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF)	850 × 10−13	[103]
500	BCY10 “BaCe0.9Y0.1O3 − δ“	Ag-Pd	300 × 10−13	[104]
620	BZCY72 “BaZr0.7Ce0.2Y0.1O3 – δ”	Ni-BZCY72	170 × 10−11	[105]
600	GDC “Ce0.9Gd0.1O2 − δ “	Pt	367 × 10−13	[106]

3.2.1.3. Solid-State ammonia production (High Temperature)

To increase the solid electrolyte's conductivity, ammonia synthesis utilizing solid-state electrolytes is typically carried out at high temperatures. As it is clear from (Fig. 3), a dense electrolyte is inserted between a porous anode and cathode electrodes with H⁺ or O^2–, depending on the ceramic type, migrating through the dense electrolyte [97], [98]. The merit of high operating temperature gives the advantage of higher reaction kinetics and better N₂ activation. However, the thermodynamic equilibrium at high temperatures might negatively impact the yield of ammonia yield and material degradation [99].

1. Download : Download high-res image (478KB)
2. Download : Download full-size image

Fig. 3. Direct ammonia synthesis (a) proton conductive ceramic membrane and (b) oxygen ion conductive ceramic membrane.

Different electrode materials were investigated for ammonia synthesis based on noble metals (Pt, Pd, Pd-Ag, Ru). For example, the first investigation involving solid-state ammonia synthesis used Pd as a porous cathode with a production rate of 450 × 10⁻¹¹ mol/s cm⁻². At 500 °C and ambient pressure, Yuan et al. [100] successfully produced NH₃ @ 307 × 10⁻¹¹ mol/s cm⁻² using yttrium-doped barium zirconate (BZY) as an electrolyte and α-Fe₂O₃/BZY as a cathode. A reasonable ammonia production rate was reported using SrCe_0.95Yb_0.05O_3−δ electrolyte and Pd electrodes at 570 °C [101]. A summary of different experimental works concerned with solid state NH₃ synthesis is shown in table 4, and the various electrolytes used for ammonia synthesis are shown in Fig. 4.

1. Download : Download high-res image (232KB)
2. Download : Download full-size image

Fig. 4. Different electrolytes used for electrochemical ammonia synthesis.

3.3. Assessment and comparison of different NH₃ technologies

With the increasing focus on green energies, ammonia has become a globally important energy source and energy carrier. Moreover, ammonia is used in preparing various products such as fertilizer, plastics, fibers, explosives, pharmaceuticals, and others. Ammonia is mainly produced from the reaction of nitrogen and hydrogen from natural gas (brown ammonia) using a conventional technique called the Haber-Bosch process. As it is clear from (Fig. 5), this process accounts for CO₂ footprint (releasing 1.9 tons CO₂ for each ton of green NH₃ produced and increased significantly in case of blue and brown ammonia). It is estimated that NH₃ production contributed to 1.8 % of the global CO₂ emissions [148].

1. Download : Download high-res image (114KB)
2. Download : Download full-size image

Fig. 5. Energy requirements and CO₂ footprint for brown, blue, and green Ammonia [150], [151], [152], [153], [154], [155]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The transition to green ammonia production through hydropower electrolysis was realized since 1920 in Norway. However, replacing the Haber process with electrolysis from renewable sources requires further consideration regarding the costs, production capacity, material development, and excess renewable energy source [149]. Theoretically, the energy needed for green ammonia production through conventional high-pressure synthesis is 30 ± 5 GJ/tNH₃. This has given priority to the transition toward direct NH₃ production under milder conditions.

Although all the synthesis routes still have considerable challenges, such as low efficiencies, high cost and negative environmental impacts. The indirect processes for ammonia production from hydrogen show promising results, and further research and development will accelerate green ammonia synthesis to achieve sustainable development. Investment, legislative laws and international/national policies also play significant roles in this matter.

4.1. Ammonia-based fuel cells

Renewable energy resources such as geothermal, hydro, solar, tidal and wind energies have been considered eco-friendly technologies. However, these resources are sporadic as they depend on geological parameters, solar availability and intensity, wind speed, altitude, etc. [171]. Efficient energy storage and conversion devices are the best choices to store and convert such energies for energy-deficient periods. Fuel cells are energy conversion systems that are eco-friendly, compact, efficient, available in different sizes, and demonstrated promising results in different applications [172], [173]. Various fuel cell types depend on the operating temperature, membrane type, applications, or fuel type. Due to its simple structure, carbon-free, and sustainable energy source, hydrogen is considered the best fuel for fuel cells [174], [175]. Several studies demonstrated the effectiveness of the integration of the fuel cell in renewable energy resources within periods of low energy output [176]. But there are two main challenges facing the utilization of hydrogen as fuel, consisting of its transportation and storage [14], [177]. Hydrogen carriers such as methanol, ethanol, ammonia, hydrazine, etc., have been proposed to overcome such challenges. Among them, ammonia is considered a carbon-free hydrogen carrier with high hydrogen content (17.6 wt%). In contrast to hydrogen, ammonia is easier to transport and store. Thus ammonia could be used directly or indirectly as fuel in Fuel cells. Indirect ammonia fuel cell depends on converting ammonia to hydrogen, which is fuel in the fuel cells. The electrolysis of ammonia at 250 °C produced high-purity hydrogen that can be used directly at the fuel cell's anode [178], [179]. A system that consists of a combustion engine fueled with ammonia and hydrogen (produced from the dissociation of ammonia) was used to power a vehicle [180]. The overall energy efficiency of that system reached 61.89 %. A Direct ammonia fuel cell is a system that transforms the chemical energy contained in ammonia directly to electrical energy with high efficiency. With the widespread production of green ammonia, the interest in ammonia-based fuel cells increases. Fuel cells are considered the most efficient device to extract ammonia's energy with low or no environmental impact [181], [182]. There are four categories of ammonia-based fuel cells, i.e., alkaline ammonia fuel cell (MAFC “Molten alkaline ammonia fuel cell”), ammonia-based SOFC-H “Proton conducting electrolyte based solid oxide fuel cell”, and ammonia-base SOFC-O “oxygen anion conducting electrolyte based solid oxide fuel cell”, AAEFC “ammonia-based alkaline electrolyte fuel cells”, and ammonia-based MFC “microbial fuel cell”.

4.1.1. Ammonia-based SOFC

Ammonia-based SOFC operates at high temperatures (500–1000 °C). At such high temperatures, ammonia cracking and power generation are consolidated [183], [184]. The cost of such FCs is relatively low as there is no need for separate ammonia cracking unit. Moreover, such high temperatures increase the catalytic activity, so a non-precious catalyst is used. Also, such a high temperature enhances the electrolyte's ionic conductivity [185]. Ammonia-based SOFCs are classified into two types according to the electrolyte type; SOFC-H “proton-conducting electrolyte SOFC” and SOFC-O “oxygen anion conducting electrolyte SOFC” [186].

4.1.1.1. ASOFC-O “Ammonia-fed oxygen anion conducting electrolyte-based SOFC”

In this type of fuel cell, ammonia is fed at the anode of the FC, where it thermally decomposes over the anode catalyst into hydrogen, while air or oxygen is fed on the cathode side. Oxygen reduction into oxygen ions occurs at the cathode/electrolyte interface. The oxygen ions transfer to the anode side via the electrolyte, where electrochemical reactions occur at the anode/electrolyte interface, producing water vapour. The nitrogen production during the ammonia decomposition reduces the reversible reaction as it dilutes the hydrogen concentration, as can be seen in Fig. 7.

1. Download : Download high-res image (189KB)
2. Download : Download full-size image

Fig. 7. Schematic diagram showing the principles of the asofc-o.

First, ammonia decomposed at the inlet of the anode surface as follows:(16)" role="presentation" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 14.4px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0">$\mathit{�}{\mathit{�}}_{\text{3}}\leftrightarrow \frac{3}{2}{\mathit{�}}_{\text{2}}+\frac{1}{2}{\mathit{�}}_{\text{2}}$

Then, the electrochemical reactions and overall reactions are shown in Eq. 17–19.

@ anode:(17)" role="presentation" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 14.4px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0">${\mathit{�}}_{\text{2}}+{\mathit{�}}^{2-}\to {\mathit{�}}_{\text{2}}\mathit{�}+2{\mathit{�}}^{-}$

@ cathode:(18)" role="presentation" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 14.4px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0">$\frac{1}{2}{\mathit{�}}_{\text{2}}+2{\mathit{�}}^{-}\to {\mathit{�}}^{2-}$

The overall reaction is:(19)" role="presentation" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 14.4px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0">${\mathit{�}}_{\text{2}}+\frac{1}{2}{\mathit{�}}_{\text{2}}\to {\mathit{�}}_{\text{2}}\mathit{�}+\mathit{�}\mathit{�}\mathit{�}\mathit{�}\mathit{�}\mathit{�}\mathit{�}\mathit{�}\mathit{�}\mathit{�}$

ASOFC-O is composed of the YSZ “Yittria stabilized zirconia” electrolyte doped with SDC “Samarium doped ceria”, SSC “Sm_0.5Sr_0.5Co_3-δ” cathode, and nickel anode. The ASOFC-O could produce 168.1 mW/cm² at 600 °C, compared with 191.8 mW/cm² in the case of hydrogen-based SOFC-O. With the slightly lower power output in the case of the ASOFC-O would be related to the dilution effect of the nitrogen produced ta the anode from the ammonia dissociation [187]. This power increased with the operating temperature and/or efficient cell components. For instance, ASOFC-O fabricated of SSC cathode, nickel oxide anode, and SDC electrolyte (24 µm thick), generated a 467 mW/cm² at 650 °C [188]. ASOFC-O with a nickel-based anode, BSCF “Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ” cathode, and SDC electrolyte (10 µm) operated at 650 °C [189]. This ASOFC-O showed high power densities of 1190 and 1872 mW/cm² using ammonia and hydrogen. YSZ electrolyte was also investigated in ASOFC-O but showed lower performance than those using SDC electrolyte. ASOFC-O with YSZ electrolyte, silver-based cathode and platinum-based anode was operated at 800 and 1000 °C, achieving 50 and 125 mW/cm² [190]. The thickness of the electrolyte also affected the cell performance; for instance, an ASOFC-O using YSZ electrolyte (15 µm), LSM cathode, and nickel anode showed a 202 mW/cm² at 800 °C [191]. Several studies have been done to check the effect of the electrolyte on cell performance [192], [193], [194].

4.1.1.2. ASOFC-H “ammonia-fed proton-conducting electrolyte-based solid oxide fuel cells”

Similar to the case of the ASOFC-O, ammonia decomposed at the inlet of the anode catalyst into hydrogen and nitrogen. However, in this case, the produced hydrogen is oxidized at the anode/electrolyte interface into hydrogen protons (H⁺), that migrate via the electrolyte membrane to the cathode side, as they combine with oxygen at the cathode/electrolyte interface producing water as can be seen in Eq. 20–22:

At the anode, the hydrogen generated from the dissociation of ammonia is oxidized, Eq. 20:(20)" role="presentation" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 14.4px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0">${\mathit{�}}_{\text{2}}\to 2{\mathit{�}}^{+}+2{\mathit{�}}^{-}$

These protons combine with oxygen at the cathode/electrolyte interface as follows, Eq. 21:(21)$\frac{1}{2}{\mathit{�}}_{\text{2}}+2{\mathit{�}}^{+}+2{\mathit{�}}^{-}\to {\mathit{�}}_{\text{2}}\mathit{�}$

The overall reaction is similar to that in the case of the ASOFC-O, Eq. 19. Fig. 8 shows the schematic of the operating principles of ASOFC-H.

1. Download : Download high-res image (211KB)
2. Download : Download full-size image

Fig. 8. Schematic diagram showing the principles of the asofc-h.

Compared to ASOFC-O, the formation of nitrogen oxides is avoided in the ASOFC-H, but the produced power is lower. BCG “gadolinium-doped barium cerate” is mainly used as the electrolyte in ASOFC-H. The performance of ASOFC-H with two different electrolytes BCG or BCGP “Gadolinium and Praseodymium-doped barium Cerate”, was investigated at 700 °C using platinum electrodes showing 25 and 35 mW cm⁻² for BCG and BCGP, respectively [195], [196]. A mixed ionic and electronic conducting cermet anode (Ni-BCE) was investigated in platinum-based cathode ASOFC-H with BCGP electrolyte [197]. Compared to the platinum anode, The Ni-BCE anode showed superior catalytic activity toward ammonia oxidation, achieving a peak power density of 28 mW cm⁻² (at 600 °C), while Pt anode showed 23 mW cm⁻² at the same operating temperature. ASOFC-H incorporated with Ni-BCE anode realized a very stable performance for>500 h. The performance of ASOFC-H with BCGO “BaCe_0.8 Gd_0.2 O_2.9” electrolyte, LCSO “La_0.5 Sr_0.5 CoO_{3– δ}” cathode and Ni-based anode was investigated at a temperature ranging from 600 to 750 °C [198]. The cell's power output increased from 96 mW cm⁻² @ 600 °C to 384 mW cm⁻² @ 750 °C. A thin BCGO electrolyte (30 µm) was tested in ASOFC-H operated @ 600 °C, using CGO (Ce_0.8Gd_0.2O_1.9)-Ni anode, and BSCFO “Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ” -CGO cathode attained an OCV of 1.12 V and peak power of 147 mW cm⁻² [199]. A BZCY “BaZr_0.1Ce_0.7Y_0.2O_3−δ” based electrolyte (35 μm) was also investigated in ASOFC-H using Ni-based anode and BSCF “Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ” cathode acquired 420 mW cm⁻² @ 700 °C [200]. The ASOFCs-H with the same electrolyte but using different electrolyte thicknesses and different cathodes were also examined. ASOFC-H with a thin electrolyte of 10 μm and GdBaCo₂O_5+x cathode achieved a 266 mW cm⁻² [201], ASOFC-H with electrolyte thickness of 50 μm and LSC “La_0.5Sr_0.5CoO_3−δ” cathode attained 330 mW cm⁻² [202], and ASOFC-H with electrolyte thickness of 65 μm and BPY “Ba(Ce_0.4Pr_0.4Y_0.2)O_3−δ” cathode attained 270 mW cm⁻² [203]. Moreover, BCNO “BaCe_0.9Nd_0.1O_3−δ” electrolyte of 20 µm thickness was tested in ASOFC-H using nickel oxide-based anode, and LCSO cathode @ 700 °C attained 315 mW cm⁻² [204].

4.2. Ammonia-based battery

S-TEGs “Solid state thermoelectric generators” are commonly used to transfer waste heat (thermal energy) to electrical power; however, the high cost of S-TEGs hinders their large-scale applications [213], [214]. Recently, TRBs “Thermally regenerative batteries” demonstrated an ability to convert heat (including waste heat resources) to electricity efficiently and at a reasonable price. TRAB “thermally regenerative ammonia-based battery” utilizes thermal distillation and redox reactions to transform the low-grade waste heat (<130 °C) to electricity using thermally regenerated electrolytes [215], [216]. Metal electrodes and ammonia combine to form a complex during the discharge process, which results in a potential difference that generates energy. During charging, the waste heat is utilized to extract ammonia from the anolyte and move it to the catholyte [217], [218]. There are two types of TRABs according to the electrode type, i.e., S-TRAB “single metallic” where both electrodes are made of the same metal [219], B-TRAB “bimetallic” where the two electrodes are made of different metals [220].

Cu-TRAB was constructed of copper mesh electrodes where 2 M ammonia was added to the anolyte using redox couples of “Cu(NH₃)₄²⁺/Cu and Cu[221]/Cu” [222]. Copper-based S-TRAB generated 115 ± 1 W m⁻², which increased to 136 ± 3 W m⁻² by increasing the ammonia concentration to 3 M. The power doubled when another cell was added. Cu-TRAB was also constructed and examined at different operating temperatures [216]. The Cu-TRAB generated 95 ± 5 W m⁻² and 236 ± 8 W m⁻² at 23 °C and 72 °C, respectively. The enhancement of the power at higher temperatures was related to the decreased electrode overpotentials and the adequate copper oxidation at the anode. Moreover, the effect of flow rate, reactor design, and electrode pore density on the performance of Cu-TRAB was investigated [223]. The various reactor designs demonstrated varying mass transfer effects and various powers. The Cu-TRAB with a flow-through electrode on both sides (Cu-TRAB-FT) showed the highest peak power of 22.9 W m⁻² due to the improved mass transfer of this electrode. Moreover, the optimum electrolyte flow rate and electrode pore density of Cu-TRAB-FT were 15 mL min⁻¹ and 100 PPI “pores per linear inch”. Also, Cu-AFB “Copper-based ammonia flow battery” was designed and operated at different temperatures [221]. The Cu-AFB demonstrated power densities of 204 W m⁻² and 280 W m⁻² at 25 °C and 55 °C, respectively. The influence of operating parameters, including membrane type, electrode distance, electrode type, electrode area, ammonia and NH₄NO₃ concentrations on the performance of S-TRAB was assessed [224]. Different membranes, i.e., cation and anion exchange membranes of the same thickness (0.45 ± 0.025 mm), were tested. Different electrode types including Cobalt, copper or nickel sheets of the same dimensions (20 × 8 × 1 mm), were utilized as electrodes using NH₄NO₃ solution (5 mol·L⁻¹ ) mixed with 0.1 mol·L⁻¹ of Co(NO₃)₂, Cu(NO₃)₂, or Ni (NO₃), respectively. Furthermore, different copper electrodes distances of 1.5, 4, 6, 8, 10, 12, and 14 cm, and different electrode areas of 1, 2, 6, 8, 10, 20, and 25 cm² were also tested. The results exhibited that among different electrodes, the copper electrodes achieved the highest peak power density of 40 W·m⁻². anion exchange membrane showed better Cu-TRAB performance than cation exchange membrane. The peak power density of Cu-TRAB increased with increasing electrode areas, ammonia and electrolyte concentrations over certain ranges. Additionally, definite concentrations of NH₃ created tradeoffs between energy and power densities [225]. Increasing the NH₃ concentration from 1 to 5 M decreases the energy density from 0.56 to 0.31 Wh/L, while the power increases from 11.2 to 28.5 mW cm⁻². The decrease in energy density was related to NH₃ crossover through the membrane during the Self-discharge of Cu-TRAB. A membrane-less microfluidic TRAB was also designed and operated at batch mode using pure copper electrodes or thick deposits of copper over compact graphite support electrodes [226]. Membrane-less microfluidic Cu- TRAB showed better power density than conventional Cu-TRAB operating under the same operating mode and conditions. The performance of membrane-less microfluidic TRAB was enhanced by replacing the pure copper electrodes with copper deposited on graphite electrodes that increased the power to 3.4 W m⁻² at 50 °C. Cu/Ni composite electrodes were synthesized by electrodeposition of copper on nickel foams and investigated in TRABs [227]. Compared to Cu-TRAB, Cu/Ni-TRAB achieved the same power of 6.5 mW, but with higher anodic coulombic efficiency, 94 %, and considerably extended electrode operation time (>55 h).

Cu-TRABs suffered from unbalanced cathode deposition and anode dissolution rates during discharging cycles. To overcome such challenges, a Cu-TRAB using ligands was developed to stabilize Cu(I) and Cu[221] ions·NH₃(aq) and Br⁻(aq) ligands were used in Cu-TRAB operated at 25 °C achieving a cell potential difference of 695 ± 2 mV, 350 W m⁻² and high Coulombic efficiency (>90 %) [228]. The system's energy storage density was twice that reported for alternated Cu-TRAB chemistries and can reach four times higher. A steady and reversible electrode reaction was also accomplished using inert carbon electrodes and silver salts throughout a number of cycles [229]. Under bath mode test, 23 W m⁻² was achieved, which is 64 % higher than that generated using Cu-TRAB. Furthermore, a 30 W m⁻² was stable over 100 cycles under continuous flow operation, realizing superior reversibility. Cu/Zn-TRAB, using copper and zinc electrodes was constructed and examined at different temperatures (10–45 °C) [218]. Peak power of 389 W m⁻² was achieved and increased to 723 ± 45 W m⁻² by increasing the temperature from 10 to 40 °C with linear slope of 12.25 W m − 2 °C − 1. However, further increases in the temperature resulted in decreasing the power output. Copper/zinc bimetallic TRA based flow battery (Cu/Zn-TRAFB) was also developed, realizing a voltage discharge of 1.38 V and 535 W m⁻² with redox couples [Cu[221]/Cu and Zn(NH₃)₄²⁺/Zn] [230]. A power of 1070 (535 W m⁻²) could be achieved by connecting two cells (parallel or series).

4.3. Ammonia as fuel in ignition engines (AIE)

The automobile industry is one of the most pollutant sectors of the environment where it is based on combustion engines powered by fossil fuels. The decarbonization of power generation, marine and automotive markets is compulsory for limiting global warming and meeting the emissions of greenhouse gases. Hydrogen is considered a carbonless fuel that could be implemented with diesel in IE to decrease carbon emissions. But hydrogen is compressed at high pressure, and thus, it is a dangerous and expensive approach. Therefore, hydrogen carriers are considered a good candidate for diesel-based engines. Ammonia is deemed a hydrogen carrier and could be used effectively as an alternative fuel in CIE. Ammonia could be burned with any fuels with low auto-ignition temperature or diesel (dual fuel combustion, DFC), thereby decreasing the carbon-based emissions. But NOx emissions and high unburned ammonia are the main demerits of the DFC approach, and thus the after-treatment system is needed. Consequently, compression ignition using ammonia could be a proper solution for heavy-duty, marine and power generation applications. marine.

In 2007, Ammonia and gasoline-powered vehicle were designed and tested from Detroit to San Francisco [231]. In 2013, Toyota “Marangoni GT86 ECO” developed the first ammonia-fueled racing car that operated with ammonia fuel up to 2800 rpm while gasoline fuel was used at a higher speed [161]. In 2014, a vehicle that was partly fuelled with 70 % ammonia “ port injection” and 30 % gasoline “direct injection” was designed and operated successfully by the Korean Institute of Energy Research [232]. In the same year, a carbon-free hydrogen ammonia tractor was evolved by a Nevada corporation, “HEC Inc.” [233]. Newly, ammonia/hydrogen-fueled vehicles were developed using carbon-free hybrid system as hydrogen was produced by the electrochemical ammonia splitting [234].

Ammonia is the perfect fuel for marine and power generation applications [235], [236]. Marine and power generation are not space-constrained compared to automotive applications as they could be implemented with auxiliary equipment to reduce NOx emissions. Ammonia has been proposed as a favourable power-to-liquid approach for establishing a hydrogen economy [237]. Moreover, A company, “MAN Energy Solutions multinational company” plans to replace the dual-fuel marine engine (fueled with liquefied petroleum gas and diesel) with an ammonia-based engine [238], [239].

5.1. Environmental barriers

The direct environmental effect of ammonia is related to its level in the environment. Ammonia has severe health issues for humans and other living creatures. Therefore, the exposure to ammonia, whether as a gas in the atmosphere, liquid, or fertilizer, must be within a definite limit. Another environmental impact of ammonia is related to its production method, usually associated with CO₂ emissions from fossil fuels being used for hydrogen production, N₂ separation from the air, and nitrogen reduction into ammonia NH₃ [98]. Furthermore, ammonia as a chemical has several environmental impacts that must be considered during handling and storage [240], [241], [242]. The environmental impacts of ammonia are seriously considered as there is rapid growth and a need for ammonia as an energy source that is competitive with fossil fuels. The different environmental impacts of ammonia are summarized as follows:

• •

  Green House Gases (GHGs) contribution: Industrial ammonia production has increased to reach>170,000 tons of produced ammonia, contributing as the second most produced chemical. However, most of the current ammonia production methods depend on conventional fuel-burning yielding ammonia, contributing to CO₂ emissions at around 1.8 % [243]. For instance, in the case of using conventional fuel for NH₃ production, the amount of CO₂ produced is around 2.2 kg CO₂/ kg NH₃ [98]. Therefore, CO₂ emissions plus the unreacted methane (source of hydrogen) will directly affect global warming by increasing the GHG. On the other hand, the main contribution of producing green ammonia from a direct and clean energy source could reduce the carbon footprint by around 90 %, contributing to about 1.7 % of the global carbon dioxide emissions footprint [45], [244], [245].

• •

  Marine and ground life contribution (disturbance of ecological balance): The release of ammonia into the atmosphere will react with moisture producing ammonium that fall back to the earth as rainfall. Such ammonium in the rainfall will be converted to nitrates by the soil's bacteria, causing an increase in the soil's osmotic concentration [246]. Another aspects of ammonia gas at the atmosphere is the formation of ammonium sulphate casued by the reaction of ammonia gas with sulfuric or nitric acid vapours at the atmosphere to form small particles known as aerosols [247]. This will cause an increase in the soil pH, soil acidification, and direct toxic damage to essential nutrients found in the soil. Moving to ammonia effect to the aquatic ecosystem if it is directly spilt to surface water or its vapor cloud is allowed to reach water (Fig. 10). A concentration of only 0.02 ppm is enough to kill sensitive marine creatures [248], [249]. This can be mitigated during the production of green ammonia by following proper safety strategies, including fast detection, leak detection, emission capture and treatment.

  

  1. Download : Download high-res image (225KB)
  2. Download : Download full-size image

  Fig. 10. A schematic diagram for the impact of ammonia on surface water and marine life.

• •

  Human life contribution: Ammonia is corrosive, and its impact on human health depends on the average weighted time and exposure dose (Table 8). Ammonia can significantly impact human health and the environment because it forms a strong basic solution that irritates the skin, the respiratory system, and the eyes. A 5 ppm of ammonia is the threshold limit for the pungent order; this can act as a warning of NH₃ presence in the environment. Below 50 ppm, there is no recorded effect on the human; however, severe noise and throat irritation are recorded at 134 ppm. Above 2000 ppm, the person might die within a minute owing to a lack of oxygen and severe skin blisters and burns [250]. The experimental findings by Franks et al. [251] shows that the broad use of dangerous toxic load for ammonia toxicity could be calculated from the following equation (Eq. 16):

  Table 8. Exposure effect on human beings [252], [253], [254].

  Exposure (ppm)	Body effect	Permissible Exposure
  50 ppm	No recorded effect	No effect
  134 ppm	Rhinitis symptoms	8 h
  700 ppm	Severe rhinitis symptoms and possible loss of sight	1 h
  1,700 ppm	Death if not treated urgently owing to severe lung damage	Not permitted
  2,000 ppm	Chemical burns	Not permitted
  5,000 ppm	Suffocation might die directly	Not permitted

Where c is the concentration of ammonia in ppm and t is the exposure time.

• •

  Land availability: the available land for new green ammonia production and storage is limited; therefore, expanding an existing plant is highly recommended. Hence, future studies should focus on developing and upgrading conventional ammonia production plants.

• •

  Landscape and visual impact: This requires a technical study to assess the potential impact on the visual environment that ammonia projects can cause on the local community. It is about studying different locational installations, visual characterization, and the social life of the place. For example, a simple development was suggested based on the method of the landscape and visual impact of ammonia at the Yara Pilbara ammonia plant [255].

5.2. Technical barriers

Technical barriers are not only referred to the capacity, feasibility, infrastructure, and viability but also the lack of technical expertise and labour. For instance, a capacity barrier- producing green ammonia at a large scale would create an inability to handle and store the large capacity of ammonia which can cause severe problems to the ammonia infrastructure. The potential of renewable energy, wind and solar, to produce green ammonia is the major contribution to its green synthesis. This will raise another technical barrier because of the need to implement renewable energy projects for ammonia synthesis. Some other barriers, such as efficiency for direct green NH₃ synthesis, availability, reliability, and maturity, are also worth mentioning. The technical criteria for green ammonia can be summarized as follows:

• o

  Efficiency: It is defined as the ratio of the input energy over the output energy. The current trends are moving toward producing green ammonia through renewable sources (solar, tidal, or wind) to power an electrolyzer and air separation unit. The process efficiency can reach up to 83 % by involving a hybrid system consisting of high temperature operating system with heat integrations [47]. However, unconventional methods for direct ammonia (photosynthesis, electrochemical methods, low-temperature synthesis, and non-thermal plasmatic synthesis) are preferable. The main challenge facing these methods is their low efficiency. Fortunately, research and development are progressing toward increasing their efficiency up to 60 % using electrochemical methods [256].

• o

  Availability: Green ammonia synthesis has existed from 1920s to the 1970s in Norway using a hydro-powered electrolysis plant. Nowadays, the switch to green synthesis is mainly concerned with the electrolyser's production cost and capacity [149]. However, the direct unconventional methods for green ammonia synthesis are still not viable for industrial applications, which requires further research.

• o

  Decentralization; a locality of renewable energies where renewable energy differs from one country to another and even in the same country from one region to another. This puts the burden on the feasibility and economic studies where the accuracy of each study is mandatory to investigate each plant independently.

• o

  Capacity barriers: Currently, the main barriers to green ammonia production are directly related to the production of green hydrogen because the available methods utilize hydrogen as an intermediate feedstock to produce green ammonia. Therefore, their capacity barriers would be concerned with electrolyzer capacity if power to ammonia synthesis was used.

• o

  Storage: This is one of the main barriers. Commonly, ammonia is transferred and stored as a liquified compressed gas in either compressed gas at atmospheric temperature, refrigerated at atmospheric pressure, or intermediate pressure and temperature (semi-refrigerated state). Most importantly, the storage areas should be free from flammable materials and oxidiser sparks and must be well ventilated [257]. There are three methods for storing ammonia:

  • 1-

    Storing ammonia under high pressure (15–18 bars) at room temperature using spherical or cylindrical vessels with a capacity up to 2000 tons.

  • 2-

    Storage of ammonia at low temperatures (– 33 °C) and a pressure around (1.1 to 1.2 bar) using insulated vertical cylindrical tanks with a capacity up to 50,000 tons.

  • 3-

    Intermediate pressure storage at 0 °C using insulated reduced-pressure spherical vessels with a capacity up to 2500 tons.

Compressed gas at atmospheric temperature and refrigerated at atmospheric pressure are the most common ways used to store ammonia. Initially, a high-pressure system (bullets and Horton spheres) was the dominant method to store ammonia, with a capacity reaching up to 2000 tons. However, low-pressure storage was later preferable to high-pressure for two reasons: 1- Capital requirements are lower per unit volume, and 2- Considerably safer than spherical ammonia storage under high pressure. Currently, the large industrial scale has shown a great interest in refrigerated ammonia storage at ambient pressure owing to the high-capacity storage [258], [259].

For pressure storage, cylindrical tanks can upstand up to 25 bar pressure. Compared to spherical tanks, maximum design pressure should not exceed 16 bar to prevent a wall thickness above 30 mm. Pressure storage is very economical for low quantities of ammonia needed in down streams unit processing ammonia and for services that require loading and unloading pressurized ammonia using tank cars, trucks, rail, and marine [260], [261].

Currently, refrigerated ammonia storage is preferred owing to its lower capital cost per unit volume compared to other methods and the higher volume capacity. Furthermore, this method is very convenient for loading and unloading refrigerated vehicles in ammonia synthesis plants. For refrigerated storage at –33 °C, many tanks configuration are available for a storage capacity up to 50,000 tons [262], [263]. The main types of atmospheric ammonia storage tanks are:

• 1-

  Single-wall steel tanks with external insulation (made of rockwool or foam insulation) are sometimes surrounded by concrete bunds to prevent pollution.

• 2-

  Steel tanks with double walls and perlite insulation in between the walls (double wall tanks). This type of tank can also be found in two different configurations depending on the insulation type. The first one is with insulation on the annular space that needs a full shutdown in case of an inner tank frailer. The second type, which contains outer insulation, offers a longer operational time in case of inside tank failure. Table 9 provides a summary of the different ammonia energy storage systems.

  Table 9. Summary of different storage techniques for ammonia [258], [259], [260], [261], [262], [263].

  Property	Comments
  Pressure Storage
  System suitability	1- Small amount of ammonia Entrance or exist from system pipelines. When it is used as an intermediate chemical stock
  Typical Pressure storage	16–18 bar
  Storage capacity	Up to 2000 tons
  Design temperature	25 °C
  Refrigerated Storage
  System suitability	1- Large scale ammonia storage. Marine transportation of large amount of ammonia. In facilities that require ammonia to be cooled.
  Typical Pressure storage	1.1–1.3 bar
  Storage capacity	Up to 50,000 tons
  Design temperature	–33 °C

• o

  Research and development Lack of comparative studies between green ammonia and conventional ones equipped with carbon capture and storage. Indirect research is also required, such as life cycle assessments of resources used in renewable energies, such as terbium and dysprosium in wind generation, germanium, tellurium, indium and selenium in solar energy, etc. Intensive studies are needed before transitioning from the conventional Haber-Bosch process to modern electrochemical synthesis. The thermodynamic potential to activate nitrogen gas needs efficient catalysts that do not simultaneously accelerate hydrogen evolution [188]. There is a research gap in ammonia technology, such as the difficulty in separating from unreacted materials and, thus, low yield of ammonia at low pressure and searching for unconventional, highly selective and efficient catalysts.

5.3. Economic Barriers:

Compared with other competing technologies, the economic barriers to green ammonia production are mainly concerned with its high capital and operational costs (CAPEX and OPEX). The capital cost includes the electrolysis for hydrogen production, air separation unit, renewable energy, purification units, Harber-Bosch ammonia facility, storage units, plant auxiliaries, and civil works. Operational cost includes the availability of water, power source, labor cost, maintenance, land leasing, etc. The following are the main points that are related to the economic barriers:

• o

  Investment cost: This includes CAPEX cost, electricity, labor, maintenance, land lease or purchase. Guerra and his research group [264] did a technical–economic analysis for green ammonia synthesis (123,400 t year⁻¹) using an electrolyzer stack of 164.21 MW. The investment cost for an ammonia production plant was estimated to be 144,375,000 €, plus OPEX of more than a million per year. However, 7.2 year payback period was estimated with a net present value of 88,300,000 €.

• o

  Energy cost: One of green ammonia's main challenges is the high cost of renewable sources (CAPEX of the renewable energy plant). This concern is seriously considered when considering the price of energy from conventional fuels. Sanchez and Martin [265] developed a model for green ammonia synthesis from air and water. Their study estimated that the project cost of green ammonia could be lower to 1.36 € kg⁻¹. However, the high CAPEX and OPEX of producing green ammonia are still higher, almost triple using conventional fuel; thus, using green ammonia in the energy sector is nearly-four times higher in price than conventional fuel [266]. Green ammonia production has to be trustable to receive funds from all over the world, not only limited to developing countries. Still, several countries consider the support of green ammonia projects is non-economical compared to those powered by fossil fuels. Luke and Alcantara [25] investigated the Levelized cost of electricity (LOCE) for green ammonia synthesis using electrolyzer power by renewable energy (wind and solar) in 534 locations in over 70 countries. They expected a lower LOCE to be achieved by 2030 up to 310 $ t⁻¹ with a price of 16.6 $ GJ⁻¹ compared to a current price of 25.4 $ GJ⁻¹. This study shows that green ammonia might be comparable to conventional fuels, i.e., Kerosene (8.7 – 18.3 $ GJ⁻¹). Public and private finance can significantly promote investment in green ammonia] [187].

5.4. Policies and regulations

Green ammonia is still at its earliest stages of satisfying the global need for ammonia and its application in the energy sector. Some of these barriers are economic, financial, technical, and social. Therefore, it needs policy support to help in removing/decreasing these barriers and thus transfer green ammonia into a viable production method.

The main barriers to green ammonia synthesis are the high investment cost and energy requirements. According to the International Renewable Energy Agency, by 2050, the demand for green hydrogen will significantly increase to (134 to 159 million tons) which will require 1,775 GW of offshore wind farms, 2,243 GW of onshore wind, 4,240 GW of solar PV to satisfy this amount of green hydrogen production [267]. This is also true for green ammonia production because the main feedstock for its synthesis is hydrogen, and NH₃ is considered a feasible way to store and transport green hydrogen. The high electrolyzers' CAPEX and renewable energy costs can be overcome by establishing a proper supporting policy. For example, tackling electrolysers' high costs can support research, economic scale, and innovation. The most important policy is closing the significant cost gaps between brown and green ammonia production. There are also many technical barriers affecting the infrastructure of green ammonia. One of the most significant barriers is direct ammonia synthesis without the need for the intermediate step for hydrogen production. Direct green ammonia production from water and air would be a significant step toward minimizing the production cost of ammonia. Another important policy is supporting green ammonia production in countries with plenty of renewable energy sources.

Ammonia is essential in energy as a zero-carbon fuel source, hydrogen carrier, and potential fuel in marine transport. According to the International Renewable Energy Agency, the insertion of green ammonia in the energy sector will significantly impact the global temperature rise by 1.5 °C and contribute toward a zero-carbon emission energy policy [268]. Like other sectors, cost plays the most obstacles to the development of the green ammonia sector. For instance, there is a gap between conventional and renewable fuels in terms of energy content and price. Unfortunately, green ammonia is almost four times higher than that grey ammonia (produced from natural gas). Therefore, there should be policies that consider the cost of the reserved CO₂ emissions in the case of using green ammonia. Furthermore, safe policy attention should be taken to satisfy the obstacles of shipping and storing green ammonia.

6.1. Impact of green ammonia on SDG2; zero hunger

All crops utilize nitrogen, with a special focus on wheat, maize and rice, where they account for higher than 50 % of the total demand for nitrogen-based fertilizer Globally (Fig. 12). Maize and rice have the widest application rates for the nitrogen-based fertilizer range, with an average of 98 and 96 kg N/ha, respectively. Some crops can directly fix nitrogen from the atmosphere, such as soybeans, peanuts, palm and clover, leading to less or zero fertilizer requirements [270].

1. Download : Download high-res image (132KB)
2. Download : Download full-size image

Fig. 12. Nitrogen-based fertilizer demand and application rate of the main crop, , . adapted from [270]https://iea.blob.core.windows.net/assets/6ee41bb9-8e81-4b64-8701-2acc064ff6e4/AmmoniaTechnologyRoadmap.pdf

Feeding ∼80 % of the population worldwide depends mainly on ammonia-based fertilizer (ABF). Recently, the focus has been to transform the production process from the Haber-Bosch process, which consumes intensive -capital cost and -energy and uses fossil feedstock such as coal, to green ammonia technology, which depends on renewable energy such as hydropower, solar, and wind. Production of green ammonia yields social and economic benefits, such as producing fertilizers locally using the localized resources of renewable energy instead of importing fertilizers. Additionally, green ammonia served as a buffer to balance the seasonal variations of renewable energies, yielding consistent power needed for the development foundation. The possibility of combining renewable electricity with Haber-Bosch could occur via developing hydrogen from water electrolysis and separating nitrogen from the air via pressure-based adsorption. Haber–Bosch process is the traditional route for ABF production at a large scale. Agricultural production is significantly dropped due to soil nutrient depletion (nitrogen, phosphorous, and potassium). For this reason, around 80 % of 160 million tons/y of ammonia is utilized in fertilizers production, contributing to feeding > 70 % of the world's population, which nearly represents 50 % of the nitrogen in the human body. On the other hand, green ammonia plays a significant indirect role in nutrition. Animal production is directly proportional to grasses and plants, whose productivity is related to ABF. However, increasing grass production has lower environmental effects and higher efficiency than feed concentration; owning to that feed concentration needs considerably higher resources than pasture to yield and generate higher emissions. It was estimated that the farm had the lowest carbon footprint when sequestration was included [271]. Fig. 13 concludes the main factors that control the impact of green ammonia on SDG2.

1. Download : Download high-res image (114KB)
2. Download : Download full-size image

Fig. 13. Inputs, outputs and the main factors for green ammonia contribution in SDG2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

6.2. Impact of green ammonia on SDG3; healthy life and well-being

The impact of green ammonia on human health could be assessed by evaluating its acidification, human toxicity, eco-toxicity, carcinogens, ozone depletion potential, and abiotic depletion impacts. Ammonia deposition yields eutrophication and acidification, and it is highly recommended to consider these values for accuracy purposes and subsequent mitigation strategies. For example, NH₃ contributes to ∼ 95.5 in farm acidification. However, intensification increased NH₃ emissions owing to the increase in fertilizer quantities. The eutrophication and NH₃ emissions were reported as 86 %–94 % and 41 %–42 %, respectively [271]. Ammonia emissions which act for 13 % of agricultural emissions, increase the population's exposure to particulate matter by 0.36 μg /m³ on average [272]. However, lowering air pollution yielded a reduction in the particulate matters-based avoidable deaths by 2,804–8,249 in the year 2010 and 9,870–23,100 in 2020, pollution control policies have numerous advantages that minimize PM -and O₃ -based mortalities by about 23 % during the period from 2015 to 2030 [273], [274]. It was reported that the potential global warming yields from coal gasification and steam methane reformation are 3.85 and 3.03 kg CO₂-eq/kg NH₃ produced, respectively. The minimum global warming (0.378 kg/kg ammonia) is recorded by biomass gasification, while steam methane reformation yields high abiotic depletion compared with other synthesis routes: 0.026 kg Sb-eq/kg NH₃ [275]. Replacing conventional ammonia with green ones will minimize these negative impacts since green technology is based on renewable energies.

6.3. Impact of green ammonia on SDG6; clean water and sanitation

The ammonia production could occur via a coal-based ammonia synthesis approach which involves coal mining, washing, transportation, and ammonia production. The system is similar to the first approach for the coke oven gas-based ammonia synthesis route, with an additional coal coking stage. The system includes natural gas exploitation and transportation and ammonia production for the natural gas-based ammonia synthesis approach. It is worth noting that, for all conventional approaches to ammonia synthesis, there is wastewater polluted with contaminants released into the ecosystem [181]. The eutrophication potential is calculated from electricity consumption, the coal-burning, which generates nitrogen oxide, and wastewater discharged from the coking process that involves numerous contaminants such as chemical oxygen demand, phosphorus, and nitrogen. All of these constituents increased the eutrophication potential. The eutrophication potentials of natural gas-, coal- and coke oven gas-based ammonia are 0.0012, 0.0012 and 0.0016 kg phosphate-eq. The photochemical ozone creation potentials of natural gas-, coal- and coke oven gas-based ammonia are 0.599, 0.871 and 1.10 g ethene-eq [181]. The emissions related to water pollution and acidification (kg SO₂-eq) are increased as more nitrogen-based fertilizers are applied in the agricultural sector [182]. Hence, consuming much more conventional ammonia should be replaced by green one to mitigate the negative impact on the ecosystem with a special focus on agricultural wastewater.

6.4. Impact of green ammonia on SDG7; green and affordable energy

Green ammonia is the synthesis of ammonia using renewable energy, air and water. This zero-carbon energy storage vector has significant energy applications owing to its availability in different geographies and low cost of transporting and storing compared to the requirements of underground hydrogen storage or geological storage of captured carbon, Fig. 14. Ammonia can be easily stored as a liquid via pressurizing at 9 bar ambient temperature or cooling to –33 °C; the process doesn’t involve significant losses. Ammonia can be easily and safely transported in carbon-steel pipelines, ships, rail cars, and trucks. Also, plants' conversion from natural gas and oil into green ammonia is easy since the latter is used in the same pipelines with minor modifications. Moreover, 50 % much more energy could be delivered in case of the replacement of natural gas with liquid ammonia owing to the higher volumetric energy density of ammonia [276]. The typical storage tank has a capacity of liquefied NH₃ up to 30,000 Mt, equivalent to 190 GW (H₂ reformation from NH₃), corresponding to 0.1 US$/kWh capital cost [277].

1. Download : Download high-res image (175KB)
2. Download : Download full-size image

Fig. 14. The main contributions of green ammonia in the energy sector. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The power2ammonia2power technology is based on multidisciplinary connections where the battolysers provide the grid with power. The grid can also provide the ammonia production plant with power when the battolysers are discharged or/and the wind is weak in order to maintain the system heated and pressurized. However, no backup system is required. The system is characterized by flexibility, where excess ammonia can be introduced from an external source [278].

Green ammonia can be directly combusted in gas turbines, reciprocating engines, or electrochemically dissociated in fuel cells. Although there is a limitation in its implementation in current Otto cycle engines owing to its narrow flammability range, the NH₃-based engine was designed and constructed for bus engines during the 2nd World War. To eliminate nitrous oxide formation during NH₃ combustion, catalytic cracking and sodium-amide processes are suggested for the NH₃ decomposition into its elements. The developed hydrogen can be efficiently combusted in gas turbines or fuel cells [277].

6.5. Impact of green ammonia on SDG8; economic growth

Transforming hydrogen into ammonia is essential to decrease the final cost of highly tumbling renewable electricity in an entirely green energy system [279]. By 2040, green ammonia will be an economic competitor fuel for zero-carbon electricity using efficient gas turbine power plants. The estimated Levelized cost of electricity (LCOE) for green ammonia at a power plant capacity of 25 % is 156–185 €/MWh in 2040, assuming a fuel price of 355 €/t. This cost of electricity is considerably less than bio-energy and coal plants coupled with carbon capture and storage and nuclear power. Also, it is comparable with natural gas-based power plants with post-combustion carbon capture and storage. The additional costs of 28 €/MWh owning to cracking of ammonia to hydrogen in gas turbines recommends the development of turbine technologies which are compatible with ammonia in the long term.

It was estimated that the Levelized cost of green ammonia ranges from 208 €/t to 450 €/t in 2040 according to the capital cost, solar PV cost, and electrolyzer costs. The local price of electricity is the main indicator of economic competitiveness for replacing traditional ammonia with green one. However, the economic impact of green ammonia could be clear by integrating its production plant with the national grid. This is attributed to the variation in prices of locally available or/and imported fuels. Also, some grids need varied capacity-utilization rates of dispatchable energy resources. Future research should focus on the considerations of these factors for the localized grid and regional scale as well as addressing other aspects such as a contradiction between electricity demand and cheap renewable supply such as in Japan, variation of seasonable renewable resources, the price of alternative approaches such as the low cost of gas in the USA which promote carbon capture and storage technologies, flexibility requirements, and legislative laws towards decarbonization.

The economic impacts of developing green ammonia-based fertilizer with the aid of hydropower show that the process determines ∼216 M€, equivalent to ∼165 % investment return for a net present value of 30 years compared to imported fertilizers. For example, this yields a saving of at least 50 M US dollars for importing rice. Additionally, green ammonia can compensate for seasonal fluctuations of hydropower ranging from 900 to 50 MW and provide additional power up to 370 MW [280]. A techno-economic analysis showed that transforming from small-scale (10 kW) to large-scale (10 MW) reduces the minimum hydrogen selling prices from 7.03 USD/kg to 3.98 USD/kg. Moreover, sensitivity analyses showed that H₂ selling prices might be minimized by 50 %. The H₂ produced from green NH₃ reported significant CO₂ reduction by 78–95 % (kg CO₂/kg H₂) compared to traditional methods such as steam methane reforming, biomass gasification, etc. [278].

6.6. Impact of green ammonia on SDG9; industry and infrastructure

Ammonia is the second largest product in the chemical industry sector worldwide after sulfuric acid. Also, it is the main precursor for synthetic fertilizers (urea, ammonium phosphate, and ammonium nitrate). Moreover, it is one of the raw materials for the production of numerous nitrogen-based chemicals (hydrazine (N₂H₄), urea (CO(NH₂)₂), ammonia carbonate ((NH₄)₂CO₃), nitric acid, and ammonia borane (NH₃BH₃)), and capturing agent for acidic gaseous. Also, it is used in refrigerators and air-conditioners, manufacturing of acids, explosives, fibers, papers, plastics, polymers, and alternative fuel in ICE and FCs for power generation with/or without reforming. Ammonia production increased from 137 Mtons in 2012 to 140 Mtons in 2018; China, Russia, USA and India recorded the highest ammonia production with 31.4 %, 10 %, 8.9 %, and 7.8 %, respectively. To achieve sustainability, green hydrogen derived from renewable energy should be promoted to minimize greenhouse gas emissions and fossil fuel consumption [281], [282], [283]. Green hydrogen could be supplied from renewable energy-based water electrolysis and biomass gasification [13], [284]. A biomass-based ammonia production plant was compared to a natural gas-based. The exergy efficiencies of the biomass- and natural gas-based ammonia plants are 41.3 % and 65.8 %, respectively. Also, the elevated-temperature electrolysis achieved better integration with ammonia production facilities than low-temperature-based technologies owing to their high electrical efficiencies and heat integration. A techno-economic feasibility study of green ammonia production versus traditional one with a reference capacity 50 kton/y showed that biomass-to-ammonia is the most exothermic process with considerable limitations because of the immense heat required for acid gas scavenges. The power-to-ammonia recorded the maximum efficiency (74 %) compared to biomass-to-ammonia (44 %) and methane-to-ammonia (61 %). The production cost of one ton of ammonia using a biomass-based plant achieved 450 US $, equivalent to a payback higher than six years, while the cost of one ton of ammonia developed from a methane-based plant is 400 US $, with a payback of 5 years. The power-based plant wasn’t economically feasible because of high electricity and stack costs. However, the power-based ammonia production plant can be a competitor when the payback time is < 5 years; production of solid oxide could occur at full scale and increment of renewable power penetration [285].

The recent interest in using green ammonia for decarbonization shipping is a clear indicator of the numerous advantages of green ammonia that can be implemented in full-scale power generation involving scalable production at an economical cost, enhanced energy density, and safe use in industrial environments and simple storage needs. By 2050, the forecast for green ammonia as a fuel in the marine sector is 99 %. Although green methanol is also considered a significant fuel for the shipping sector, green ammonia is considered the first one [286]. Production of hydrogen from ammonia is favorable since the reformation process requires low energy (46.22 kJ/mol), and strong catalysts could enhance the efficiency of cracking. Ammonia is also a flexible energy barrier that can be utilized as green fuel in direct ammonia fuel cells and solid oxide fuel cells. The advantages of these two fuel cells are their low cost, medium operating temperature, high efficiency, very robustness, and they can be fueled with various fuels [287], [288].

6.7. Impact of green ammonia on SDG11; sustainable cities and communities

Implementation of ammonia in power plants and transportation in cities has been environmentally assessed by studying its impacts on acidification, global warming potential, ozone layer depletion and abiotic depletion. The results have proved a reduction in the GHGs, showing eco-friendly performance because of the absence of carbon in such fuel. The environmental impacts of replacing conventional fuels (diesel, gasoline and natural gas) with ammonia were investigated in power generation plants and transportation via wind energy-based ammonia production plants. The GHGs from ammonia implemented in transportation are 0.1 kg CO₂-eq/km, significantly less than diesel (0.230 kg) and gasoline (0.270 kg) driven vehicles. Moreover, ammonia implementation in power plants can substantially reduce the corresponding by 60 % compared to natural gas-based power plants [289]. Production of ammonia from wind-based water electrolysis using molten salt electrolyte in an electrochemical reactor has shown that clean and abundant wind energy can be applied in green ammonia production that significantly lower the environmental impacts. The ammonia-based car can suppress the GHGs to 0.1 kg/km compared to gasoline-based cars (0.27 kg/km). This yields a high lowering in the total GHG emissions in cities. The ammonia-based power plant generates about 83 g CO₂ eq.; in contrast, the natural gas-based power plant develops 130 g CO₂ eq. per MJ electricity production [289]. Such small amounts of the CO2 accompanied by ammonia can be significantly decreased or even eliminated by the use of green ammonia. Fig. 15 shows the contribution of green ammonia in sustainable communities.

1. Download : Download high-res image (113KB)
2. Download : Download full-size image

Fig. 15. contribution of green ammonia in green cities via transportation, power generation with zero or near-zero carbon emissions, agricultural, and industrial sectors. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

6.8. Impact of green ammonia on SDG12; responsible consumption/production

During the last 50 years, ammonia synthesis technology has been dramatically developed. However, there are significant changes in the consumed amounts of natural resources such as coal, oil, and natural gas (as shown in Fig. 16) [290]. Natural gas is consumed during conventional ammonia-fertilizer production, where it is consumed as energy supply and processing gas. It was estimated that producing one ton of NH₃ consumes 35.2 GJ of natural gas [277], [291]. Therefore, natural gas consumption is increased per ton of agricultural products. Still, there is an urgent need for responsible consumption of natural resources and energy conservation. Replacing traditional ammonia with green ones will significantly save these resources since green ammonia production is based on renewable energy resources. The lowest potential for abiotic depletion is recorded by ammonia-based cars (0.76 g Sb eq/km). In the life cycle of ammonia implemented in transportation, four materials are being depleted: hard coal, nylon, lignite, and natural gas by 41, 8, 7, and 6 %, respectively. The main source of abiotic depletion is ammonia-based production (44.7 %), which utilizes grid mix electricity. Around 29 % is developed during electricity production from the wind, with a particular focus on materials production [289]. The advantage of green ammonia as an energy vector is that the main feedstock, nitrogen, is locally abundant everywhere in the atmosphere, while methanol (the main competitor for ammonia) requires CO₂ for its production process. However, CO₂ can be supplied from different point sources if additional purification stages are applied to eliminate catalyst degradation [277].

1. Download : Download high-res image (314KB)
2. Download : Download full-size image

Fig. 16. The applications and the main natural resources consumed during the production of ammonia (a) [292], and feedstock sources of ammonia production worldwide as percentage (b) [276] (with permission No. 5444930989758), and energy requirements in ammonia production in various region worldwide (c) adopted from [293], open access.

6.9. Impact of green ammonia on SDG13; climate action

The major source of NH₃ emissions is the agriculture sector, with a contribution ranging from 55 to 56 % of the emissions worldwide. The climate change impact of N₂O emissions from artificial fertilizer spreading 10 % [271]. It was recorded that 94 % of NH₃ emissions in Europe are developed from the agriculture sector only. Synthetic fertilizers are responsible for 10 % of these values. An annual assessment of ammonia emissions is carried out to monitor the reduction in NH₃ emissions. Although there was a significant drop in these emissions by 23 % from 1990 to 2015, in Europe, more efforts are still needed. Not attributed only to the impact on humans and the ecosystems, these emissions also affect the generation of particulate matter, as mentioned above [294].

There is increasing motivation to address any change in the climate; however, nitrogen-based fertilizer is responsible for around 1 % of greenhouse gas emissions globally. Environmental legislation and investments are moving towards green technologies that support minimizing these harmful emissions. Lately, emissions from ammonia production plants (CO₂, SO_x and NO_x) recorded a significant reduction due to the optimization of the operating conditions, the feedstock mix, heat integration, energy conservation, enhanced CO₂ absorption systems, and legislative laws [290]. Synthesis of ammonia using coke oven gas recorded the maximum GHG emissions (3.97 kg CO₂-eq). This is attributed to the methane needing heat during the conversion process, which is supplied by burning fuel gas that is provided from the excess gas in pressure swing adsorption; it contains specific amounts of CO₂, CH₄, etc. This gas is one of the most significant causes of high carbon emissions. The GHG emissions were 0.197–0.211 kg CO₂-eq in the mining stage. However, the emissions during the transportation of natural gas in pipelines were 0.003 kg CO₂-eq [181], which is much less than that of coal railage (0.01687 kg CO₂-eq). Another investigation reported a global warming potential for coal gasification and steam methane reformation of 3.85 and 3.03 and kg CO₂-eq/kg ammonia developed [275]. The acidification potentials of natural gas-, coal- and coke oven gas-based ammonia are 0.008, 0.0105 and 0.0138 kg SO₂-eq [181]. The acidification potentials of gasoline-, diesel-, and ammonia-based cars are 0.7, 0.44 and 0.9 g SO₂ eq/km, respectively [289]. Combustion of ammonia causes a low global warming potential (6 %). On the other hand, the combustion of natural gas causes 97 % of GHG in natural gas-based power plants, and CO₂ act for 98.6 % of these emissions. Production of ammonia from wind-based water electrolysis using molten salt electrolyte in an electrochemical reactor has shown that clean and abundant wind energy can be applied in green ammonia production that significantly lower the environmental impacts. The ammonia-based car can suppress the GHGs to 0.1 kg/km compared to gasoline-based cars (0.27 kg/km). This lowers total GHG emissions in cities. The combustion process is responsible for 97 % of the global warming potential in the natural gas power plant. It has been shown that replacing conventional fuels with green ammonia, the significant effect of power generation and city transportation, could be extremely mitigated [289]. The carbon footprint of a hybrid energy production system consists of wind energy and ammonia energy storage system Power-to-Ammonia-to-Power (P2A2P) compared to coal-based, and gas-based energy production, which is represented in Fig. 17. The CO₂-footprint of the P2A2P is 0.03 kg/kWh, which is considerably lower than current technologies. Even without carbon capture storage (CCS), the P2A2P system is an economic competitor with traditional alternatives, and it is the most cost-effective option when CCS is introduced [278].

1. Download : Download high-res image (700KB)
2. Download : Download full-size image

Fig. 17. (a) The first-in-the-world P2A2P proto-type at the UMN WCROC, Morris, MN [295], open access, (b) A schematic flow diagram for P2A2P where PSA. Pressure Swing Adsorption, HB. Haber-Bosch and an inset shows the carbon footprint and the levelized cost of electricity (LCOE) of P2A2P compared to coal and gas-based power with/without Carbon Capture, and heavy fuel [278], open access (c) Reduction in CO₂ owning to the use of green ammonia instead of ammonia [295], open access. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

CO₂ emissions are generated at different points of ammonia production and utilization, where they are emitted during energy implementation during production, transportation and on-land machinery work. It was recorded that half of the total CO₂ emissions are owned to nitrogen-based fertilizer. However, increasing the concentration of these fertilizers doesn’t guarantee equivalent productivity and yields higher CO₂ emissions rates. For N₂O emissions, the majority of these emissions (48 %), which are strongly dependent on productivity, are attributable to the generated nitric acid. The latter is part of ammonium nitrate generation and is attributed to the nitrification/denitrification process [291]. However, the over-implementation of ammonia will cause a burden on the air quality since the CO₂ emissions are discharged during ammonia production, and its usage as fertilizers will generate CO₂ and nitrous oxide emissions. Further contributions to climate change by N₂O “nitrous oxide” emissions are the production of nitric acid. Air pollution is associated with NH₃, NOx emitted from soils, and pollutants emitted from production facilities.[270] The production of green ammonia will eventually result in a significant decrease or minimization of the GHG emissions mentioned above.

6.10. Impact of green ammonia on SDG15: Life on land

The utilization of land for agricultural purposes is decreased as the implementation of green ammonia increases. Because implementing green ammonia as green fertilizer will result in a higher yield of agricultural products per unit area and thus lowers the land consumed per ton of agricultural products [296]. Moreover, green ammonia will conserve the main nutrients and minerals such as nitrogen, potash, phosphate, etc. The impact of green ammonia on land is not limited to the arable area but also via eutrophication. The effect of green ammonia on aquatic eutrophication is challenging to evaluate since the impact starts with the manufacturing stage and application and ends with discharging. There are interconnected factors, such as water bodies and land proximity structures. Conventional ammonia is also responsible for terrestrial ecotoxicity, which is expressed in kg 1,4-dichlorobenzene (1,4-DCB) equivalent, where increasing the fertilizer quantity increases the ecotoxicity [182], [297].

The ammonia production utilizing municipal solid waste recorded the minimum global warming potential (0.34 kg CO₂-eq/kg NH₃) compared to other routes, such as the nuclear-based route (0.84 kg CO₂-eq/kg NH₃) and biomass-based approach (0.85 kg CO₂-eq/kg NH₃). The municipal solid waste also reported the minimum impact (160 g 1,4-DB-eq/kg NH₃) on human health and toxicity, while nuclear high-temperature electrolysis-based ammonia (NHTEA) reported the maximum effect (950 g 1,4-DB-eq/kg NH₃) owing to the large amounts of nuclear waste and hazardous gases discharged from the nuclear plant. The abiotic depletion reached its maximum value in the NHTEA route, followed by hydropower electrolysis-based ammonia (HEA) as illustrated in Fig. 18. This is owing to the main energy resource in the first route, which is the uranium, resulting in large consumption of the limited uranium per unit mass of ammonia developed [276]. Moreover, the crops will supply the soil with nitrogen. Since fertilizer improves yields, more widespread utilization could decrease the land required to produce the same crops and thereby help decrease the need to transform natural ecosystems into agricultural production. Meanwhile, improving fertilizer use can reduce environmental impacts from air pollution [270]. The production of green ammonia will result in a significant decrease or depreciation of the GHG emissions.

1. Download : Download high-res image (326KB)
2. Download : Download full-size image

Fig. 18. (a) Green ammonia production via biomass in comparison to conventional Haber-Bosch process, (b) the corresponding greenhouse emissions, toxicity and abiotic depletion potentials of MWEA “municipal solid waste based electrolysis”, BEA “biomass electrolysis-based ammonia”, HEA “hydropower electrolysis-based ammonia”, NHTEA “nuclear high-temperature electrolysis-based ammonia” [276] reporoduced with permission No. 5444940362568, and (c) percentages of CO₂ emissions in different industrial sectors [270], open access. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

6.11. Impact of green ammonia on SDG17: Partnership for achieving the goals

Strengthening the global partnership for the sustainable development of green ammonia will occur via finance and technology development related to sustainable green ammonia production. The nations can improve sustainability via national legislative laws, technology transfer, and integrated investment.

6.12. Contribution of green ammonia in SDGs targets

As illustrated above. Green ammonia is strongly contributing to eight of the 17 SDGs. Table 10 illustrates the impact of green ammonia on SDGs targets.

Table 10. Direct contribution of green ammonia into the SDGs and the related targets.

SDGs	SDGs targets	Contribution of Green Ammonia
SDG2	2.3	• Promote the agricultural productivity
	2.4	• Sustainability of food production
	2.5	• Ensuring plants are cultivated and seeds diversity
SDG3	3.9	• Lowering deaths and illnesses yields from air/water/soil pollution with hazardous chemicals
SDG6	6.4 6.6	• Enhancement of hydropower-use efficiency in the production of green ammonia Protection of water-related ecosystems via responsible consumption of renewable energy from hydropower
SDG7	7.1	• Contribution of wind power and hydropower in the development of the production process energy services
	7.2	• By 2030, increase the contribution of renewable energy sources in the green ammonia industry
	7.3	• Enhancement of renewable energy efficiencies
SDG8	8.1	• Sustain economic growth per capita with a special focus on incomes of small-scale food production
	8.2	• Achievement of higher economic productivity via upgrading of production technologies
	8.3	• Supporting decarbonization policies that support creativity, innovation, and entrepreneurship and promote the formalization as well as the growth of enterprises at different scales
	8.4	• Improvement of resources efficiencies in consumption and production Compensation for economic decline resulting from environmental degradation,
SDG9	9.1	• Development of sustainable industrial infrastructure to support economic development with a special focus on food, energy industries
	9.2	• Ensuring sustainable industrialization
	9.3	• Increment in industrial projects in developing countries owning to the implementation of their local renewable energies
	9.4	• Upgrading industrial infrastructure via coupling with renewable energies facilities, with an increment of the efficiencies of renewable energies resource-use Adoption of green and eco-friendly technologies in industrial processes
	9.5	• Enhance of innovated public and private research
SDG11	11.6	• Reduction of environmental impact on cities, including to air quality and management of municipal and other waste
SDG12	12.1	• Sustainable consumption /production via a reduction in fossil fuel consumption
	12.2	• Sustainable management of renewable energies Efficient use of natural resources
	12.3	Compensate food losses via enhanced productivity
	12.4	• Significantly lowering the release of fossil fuel emissions into the air, subsequently minimising their negative impacts on the environment and human
	12.5	• Minimize waste generation via prevention or/and reduction of using fossil fuel
	12.6	• Promote companies to adopt new practices
	12.7	• Encourage public procurement practices that support policies
SDG13	13.1	• Lowering GHGs -related hazards Improved air quality
	13.2	• Promote climate change-based national policies
SDG15	15.5	• Minimizing the decline of natural habitats, promoting biodiversity
	15.9	• Developing and establishing local and national planning through the introduction of biodiversity and ecosystem principled
SDG17	17.1	• Supporting local resources mobilization
	17.2	• Technology transfer from developed countries to developing one
	17.3	• Widen the financial resources for developing countries
	17.4	• Production of green ammonia with the aid of integrated policies will reduce debt distress
	17.5	• Accelerate investment regimes for developed countries
	17.6	• Enhancement of international cooperation to share science and technology through a global science, and innovative technology facilitation mechanism
	17.7	• Green ammonia production will promote the dissemination, transfer, and development of eco-friendly technologies with a special focus on low-income countries.
	17.11	• Significantly increase the exports of developing countries, with a special focus on those own renewable energies such as wind and solar energies.
	17.13	• Supporting the macroeconomic stability worldwide.
	17.14	• Strengthen the policies for sustainable development
	17.16	• Transferring from conventional ammonia to green ammonia production will strengthen the global partnership for sustainable development through sharing the financial resources, expertise, knowledge, and technology
	17.17	• Encourage effective public and public/private partnerships based on experience and resource exchanges.