**2. Key-Driver of Ammonia Economy**

The issue of the storage and distribution of hydrogen creates opportunities for ammonia to be seen as alternative storage of renewable energy. The previous study on the potential hydrogen storage material by Kojima [38] has revealed that ammonia has the highest gravimetric densities with the highest volumetric densities, as shown in Figure 3 [16,38]. Consequently, attempts are being made to leverage ammonia over others to replace hydrogen as a central energy distribution block [16]. In addition, ammonia can be used to replenish soil nutrients, boosting the growth of crops and accelerating afforestation that will indirectly help to balance CO2 gas in the atmosphere through photosynthesis by plants.

**Figure 3.** The density of hydrogen in hydrogen carriers [38].

The sustainable future of renewable energy-driven ammonia production is not a novel idea but has never been widely embraced over methane or coal-based ammonia. By transforming into ammonia that can be liquefied under moderate pressure, renewable energy can be transported from places where renewable energy is cheap or excessive to where it is limited or expensive. This synergy could open up opportunities for exports and imports of renewable energy, similar to today's hydrocarbon fuel. In addition to being used directly in the form of ammonia, this may also be dissociated into its component for use as hydrogen fuel at a relatively low cost. Cost comparison of the hydrogen produced from a variety of feedstock is presented in Table 2.


**Table 2.** Cost comparison of hydrogen production via various feedstock [39–43].

In addition to the aforementioned factors, ammonia transportation and storage facilities already exist today, where around 18 million tonnes of ammonia are exchanged annually. Unlike hydrogen that cannot be liquefied under a pressurised tank, ammonia may be kept in liquid form when at least 8.58 bar is maintained. Under this load, carbon steel tanks are sufficient to be used. Ammonia may also be contained as a liquid in lower temperature storage if the temperature of −33 ◦C is preserved while hydrogen only can be liquefied under cryogenic cooling at −252.87 ◦C. The energy density in this form is roughly half that of petrol and more than ten times that of batteries. Ammonia also offers a much lower storage cost compared to hydrogen. Commonly, long-distance ammonia transport is accomplished by using carbon steel pipelines that are opposed to hydrogen, which now still has material issues with the pipeline. For transporting ammonia via pipeline over 1610 km, it requires 1119 kJ/kg-H2, which is much lower than that of hydrogen transport of 14,814 kJ/kg-H2. This disparity can be described by the state of the two fluids where hydrogen gas is distributed with the aid of compressors while the ammonia is carried as a liquid using a pump. The comparison between ammonia and hydrogen is given in Table 3.

**Table 3.** Difference between NH3 and H2 in terms of storage and distribution [39,44,45].


Ammonia also can be transported as a pressurised liquid via truck and rail. Trailers can transport 43,530 L ammonia at 2.07 MPa. Such a tank could hold up to 26 tonnes or 600 GJ of energy of ammonia [46]. In contrast, a hydrogen lorry only can be used to transport around 340 kg of hydrogen gas at 17.91 MPa, equivalent to 48 GJ of hydrogen energy content, while transporting in liquid form in a hydrogen trailer could hold around 3.9 tonnes of hydrogen, equal to 553 GJ of energy [47]. Rail transport uses a pressurised tank with a capacity of 126,810 L at 1.55 MPa, which indicates the capability to carry 77.5 tonnes of NH3 [46]. Ammonia can be transported by ship or barge using pressurised storage vessels. By using these vessels, existing ocean-going ships can transport 55,000 tonnes of ammonia [48]. The NH3 and H2 transport methods are summarised in Table 4.


**Table 4.** Differences in NH3 and H2 transport methods [44].

Note: HHV (higher heating value).

In the recent development, storing ammonia as metal ammine complexes, i.e., hexaamminemagnesium chloride, Mg(NH3)6Cl2 (Figure 4), also gives a beneficial advantage for storing and transporting since it has very low vapour pressure (0.002 bar at ambient temperature) [49,50]. Hexaamminemagnesium chloride is formed simply bypassing ammonia at room temperature over anhydrous magnesium chloride. Mg(NH3)6Cl2 can be formed into a small and dense solid without any void space. The amine has volumetric hydrogen content between 105 and 110 kg H2 m−<sup>3</sup> and gravimetric hydrogen content over 9 wt.% [49].

**Figure 4.** Mg(NH3)6Cll2 for ammonia storage [50].

#### **3. Ammonia-Based Energy Storage**

#### *3.1. Characteristics*

Ammonia is comprised of N2 and H2 with the chemical formula of NH3. A highly irritating, colourless gas with a distinctive pungent scent characterises the chemical. With a density of 0.769 kg m−3, ammonia is lighter than air. It also liquefies easily, which is caused by a resilient hydrogen bond among molecules. The boiling point and the freezing point at standard temperature and pressure are −33.35 ◦C and −77.65 ◦C, respectively. Ammonia in the air has a flammability limit between 15 and 25%. Ammonia burns with a light yellowish-green flame. Where a suitable catalyst and high temperature is present, ammonia is decomposed into the constituent elements.

The ammonia molecule has a pyramidal trigonal form with an angle of the bond of 106.7 ◦C. The atom of nitrogen consists of five outer electrons with an additional three electrons from each hydrogen atom, giving eight electrons in total, or pairs of four tetrahedral arranged electrons. This shape gives a dipole moment to the molecule, which makes it polar. The polarity of the molecule and, in particular, its capability to create hydrogen bonds makes ammonia very soluble in water. The chemical is naturally a base and a

proton acceptor. Ammonia aqueous solution with a concentration 1.0 M has a pH of 11.6 at 298 K [51]. Even though accidental explosion or combustion is relatively low [16], the US-National Fire Protection Association (NFPA) has identified this as a hazardous material, putting it at high safety risk [52]. Safety concerns regarding ammonia storage for end-users arise due to the high vapour pressure of liquid ammonia. At high temperatures, ammonia can decompose, forming highly flammable hydrogen and toxic nitrogen oxide. Ammonia is also known to be corrosive with certain alloys such as copper, brass and bronze, as presented in Table 5.


**Table 5.** Material compatibility of ammonia and its derivative [16,53–55].

Note: A (excellent); B (good); C (fair); D (poor).

Ammonia is also fatal if inhaled, causing a lung injury. Some people may be somewhat irritated by 30 ppm for 10 min, while the remainder is sensitive to 50 ppm. At 500 ppm, the nose and throat get immediate and severe irritation. Short exposure to levels over

1500 ppm can lead to fluid accumulation in the lungs [56]. Immediately Dangerous to Life and Health (IDLH) is the degree whereby a healthy person can get 30 min of exposure without causing permanent health effects. Ammonia is also believed to be responsible for the depletion of ozone by an accumulation of nitrous compounds in the atmosphere [57]. Table 6 summarises the Health and Safety data, including their vapour pressure, IDLH and toxicity.


**Table 6.** Health and Safety data of ammonia and its derivatives [58–60].

Note: VP (vapour pressure); AT (apparent toxicity).

#### *3.2. Ammonia production Using Haber–Bosch Method*

Fritz Haber and Carl Bosch invented the Haber–Bosch (H–B) process in the middle of the 20th century, replacing the Birkeland–Eyde process and Frank–Caro process to synthesise ammonia [37]. Since its discovery, the Haber–Bosch process dominating the industrial process and has undergone many substantial improvements and optimisations. The minimum energy consumption in the mid-1950s is reduced from more than 60 GJ tNH3−<sup>1</sup> to 27.4–31.8 GJ tNH3−<sup>1</sup> today [61]. Such improvements represent an improvement in overall energy efficiency from 36 to 65%. The most significant increase in productivity was made by replacing coal with CH4 to produce H2.

In short, the production cycle of ammonia can be broken down into two major phases; the first is synthetic mixture production, and the subsequent is the synthesis of ammonia synthesis by the H–B process. In the first phase, hydrogen production takes place via two-stage steam methane reforming (SMR) reactors before being transferred for water-gasshifting (WGS) reaction, CO2 subtraction and then methanation [61]. The primary SMR reactor works at 850–900 ◦C and 25–35 bar that travels through the catalyst. In this process, the energy needed for the endothermal reactions generated by the external combustion of methane fuel through the furnace [61]. Water-saturated methane is then fed into a catalytic converter that converts methane to hydrogen and carbon monoxide before it is transferred to a second SMR. In the secondary SMR, ambient air is compressed and transferred at 900–1000 ◦C to partially oxidise reagents in the next reactor. In this process, air–oxygen and steam convert unreacted methane into hydrogen and carbon monoxide, besides providing the stoichiometric nitrogen required for Haber–Bosch downstream reactions [58].

The next process involves converting carbon monoxide to carbon dioxide by steam injection in the WGS reactor to prevent coke from forming on the catalyst and side reactions. The feed is then sent to a CO2 remover column, giving a nitrogen and hydrogen-rich feed with high purity for the next process [61]. The ammonia production then takes place on Haber–Bosch reactor when the process is usually performed in a reactor with two to four catalytic converter beds at 200 to 350 bar and 300 to 500 ◦C [58]. Since the process has low single-pass conversion efficiency (~15%), it is necessary to recycle the unreacted gas. The nitrogen fixation process for ammonia and the reaction process that occurs during the Haber–Bosch process is shown below:

$$\text{NH}\_2 + 3\text{H}\_2 \rightarrow 2\text{NH}\_3\\\text{ }\Delta\text{H}\_{298\text{K}} = -92\text{ kJ mol}^{-1} \tag{1}$$

CH4+H2O → CO + 3H2 (Primary Steam Reforming) (2)

$$4\text{CH}\_4 + \text{O}\_2 + 2\text{H}\_2\text{O} \rightarrow 10\text{H}\_2 + 4\text{CO (Secondary Steam Reforming)}\tag{3}$$

$$\text{CO} + \text{H}\_2\text{O} \rightarrow \text{CO}\_2 + \text{H}\_2 \text{ (Water} - \text{gas shift)}\tag{4}$$
