Ammonia Production Plants

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This entry is adapted from the peer-reviewed paper 10.3390/fuels3030026

Ammonia (NH₃) is a fundamental manufacturing component and the cheapest compound combining nitrogen with raw elements, utilized in more than 76% of all nitrogen-based products.

ammonia fuel carbon

1. The Importance of Ammonia

A translucent product with a distinctive strong odor, NH₃ is mainly produced through the “harsh” reaction of N₂ and H at high temperatures and under compression in the presence of a proper catalyst. When created by this process, it is known as synthetic ammonia. NH₃ is also obtained as a byproduct in coal coking; however, this type of NH₃, referred to as byproduct ammonia, is generated in considerably lower amounts than the former type (synthetic ammonia) ^[1]. The Haber and Bosch technique is the most commonly used worldwide; however, it is also the most expensive. van Rooij ^[2] devised several improvements to the Haber and Bosch technique (including the operating conditions, catalysts, hydrogen and nitrogen generation, and storage). The nitrogen required for ammonia production mainly comes from the atmosphere, whereas the required hydrogen primarily comes from natural gas through steam-methane reformations.

Another consideration is that ammonia facilities are constructed by converting facilities, since both CH₃OH and NH₃ may generate similar facilities ^[3]. NH₃ is primarily employed to produce fertilizers (urea ammonium nitrate (UAN), di-ammonium phosphate (DAP), monoammonium phosphate (MAP) fertilizers). In addition, NH₃ is used as an energy vector in power plants, for utilization as a zero-carbon fuel ^[3]^[4]^[5]^[6]^[7]^[8]^[9]. Reference [] studied the potential to blend hydrogen with ammonia for utilization as fuel in a simple-cycle gas turbine and found that cycle efficiency could be increased compared to methane-powered gas turbines under lean combustion conditions (i.e., at an equivalence ratio of 0.75). This shows the potential to promote ammonia–hydrogen gas turbines to the power generation industry, demonstrating them to be efficient and sustainable. However, NO_x emissions should be carefully monitored and mitigated using cooling and dilution approaches ^[9].

In the commercial refrigeration industry, liquid ammonia is the most widely used refrigerant due to its low price and high thermal performance ^[4]. Liquified ammonia has been used as an inexpensive alkali in the stiffening of some steel-based materials ^[5], and in water purification ^[6]^[7]^[8]^[9]. Given the global scientific and industrial effort to use ammonia as a fuel to run power plants instead of natural gas, it is critical to objectively assess the literature before adjusting or proposing new techniques in ammonia plants, considering a variety of factors. As a result, this research evaluates the global effort to improve existing ammonia plants and identifies progress by evaluating the currently available datasets to identify knowledge gaps and highlight aspects that remain unresolved. This was conducted in the four sections of this research. Section 1 provides the reader with the essential background of ammonia plants, and covers aspects related to the modern ammonia production plants and presents the latest patents in the field of ammonia production. Section 2 provides a detailed description of one of the most widely used configurations of ammonia plant (Kellogg Brown and Root (KBR)). Section 3 evaluates the efforts to advance KBR models and presents a critical comparison between KBR plants and the Linde–Ammonia-Concept (LAC) plant. Section 4 addresses the safety issues related to ammonia production.

In addition, by evaluating the global scientific effort in advancing ammonia plants in terms of its contribution to enhancing ammonia production. The effort to advance ammonia plants is classified into six main categories in this research: (1) reducing ammonia-production-related energy consumption through renewable and sustainable approaches; (2) techno-economics of ammonia production; (3) proposing alternative approaches to supply nitrogen and hydrogen for the process; (4) advancing ammonia production catalysts; (5) altering the cycle configuration (design or/and operating conditions); (6) environmental aspects and ammonia-production-related carbon reduction. These categories were observed by evaluating the aims and the objectives of 130 research articles, published between 2015 and 2022.

2. Modern Ammonia Production Plants

To produce anhydrous ammonia, new NH₃-producing facilities supply H₂ to the process with steam-methane-reforming (SMR) methods to react with N₂ under harsh temperature and pressure (730 K, 20 MPa) conditions, and is accompanied by the presence of a catalyst to synthesize the compound. The Haber–Bosch (HB) process is the concept used to describe this stage. Currently, fossil fuels, air, and water are the stream supplies required to produce ammonia. Natural gas is the most commonly used fossil-fuel energy source, accounting for about 76% of all NH₃ power-generated globally. Coal-based power plants account for 24% of total capacity.

To prepare natural gas for the SMR process, it is blended with a relatively small portion of H_2~ and then preheated to approximately 730 K either within the built-in reforming furnace or within an external source of heat (heat exchangers, heaters, etc.). It is necessary to purify the preheated gas mixture of any sulfur-based contents (H₂S and organic sulfur compounds) to below 1% of molar fraction in a single or double-reactor series. The first of these contains a cobalt molybdenum (Co-Mo) catalyst and the second contains zinc oxide (Z_nO) adsorbent to eliminate any poisoning of the nickel-based catalyst in the latter. It is possible to divide the reformer unit into two stages. The primary reformer is a sub-unit of a methane-reforming plant, in which a heated steam–methane blend (1:4 by a molar fraction) is supplied through radiation-heated channels with a nickel-based reforming catalyst and partially converted to H₂, CO, and CO₂ (typically, 66% of the initial methane supply) []. The necessary heat for the first reformer is produced by gas-fueled burners, categorized as side-fired, top-fired, or bottom-fired burners. A convection bank is used to recycle the wasted heat produced in the furnace box (heat content of the flue gas) for use in other operations (such as supercritical steam heating and preheating process air). The partly converted gas is sent into the secondary reformer, where it is mixed with a regulated quantity of air (which has been warmed and compressed to 790 K and 4 million pounds per square inch). The temperature is increased from 1050 K to about 1490 K by partial combustion of the gas to complete the endothermic process.

Almost all CH₄ is adiabatically transformed in standard plants as it passes through the catalytic material, leaving an unreacted concentration below 0.6% ^[9]^[10]. All carbon oxides must be eliminated from the mixture to fulfill the criteria for NH₃ synthesis fuel. Traditionally, the water–gas-shift reaction has been used to transform CO into a form that can be removed from the atmosphere. To utilize the wasted heat used to increase the temperature of the superheated steam, the temperature of the gas products from the second reformer is reduced by heat exchange.

The superheated steam is then supplied to the high-temperature-shift reactor, filled with Fe₂O₃ and Cr₂O₃. At 600 K, the CO reacts with surplus H₂O to form H₂ and CO₂, with a 310 K-equilibrium approach to the reaction. A sufficient amount of H₂O is essential for this process to eliminate the Boudouard reaction (which is prevented using efficient Fischer–Tropsh catalysts) ^[10]. If a conventional plant is used, the reformer must have an S/C ratio of 3.0 at the very least to meet the high-temperature shift (HTS) requirements. A 2% or less of CO content is achieved at the outlet of the high-temperature-shift reactor; thus, the usage of the low-temperature-shift (LTS) reactor is necessary to transform the residual CO in the synthesis gas at 490 K. The water–gas shift (WGS) produces a large amount of CO₂ as its feed output. To effectively dissolve CO₂ at high pressure, the utilization of a solvent is common. Many different solvents are available at present, and they are classified as physical-based or chemical-based solvents depending on the amount of CO₂ that is present in the input stream. Chemical solvents, which are mostly generated from alkanolamine, are used in the ammonia synthesis pathway because they provide a high-mass CO₂ transfer while also requiring a high-energy input for regeneration.

The monoethanolamine (MEA) system consists of regeneration stripping columns and HP absorption columns, which are typically approximately 5.3 MPa, with pressure losses of 3 KPa between each stage. Whether or not a reboiler is used, the total number of steps is generally between 10 and 15 ^[9]. One downside to the system is that carbonate salts build in the absorption solution, which is quite caustic. Newly discovered solvent additives, such as liquid NH₃- and Ca-based solvents that may prevent the production of carbonates, are being employed in the industry to reduce carbonate formation. It is necessary to perform final purification in the ammonia synthesis, since residuals range between 0.2–0.5% mol. of CO and 0.005–0.2% mol. of CO. The copper-based scrubbing approach was extensively used in early plants but has become obsolete due to the high energy consumption. Moreover, it is deemed ecologically unfavorable when the remaining carbon is removed. Methanation is the most common approach to lowering carbon content levels to below 10 parts per million. At the same time, the exothermic process is used to recover energy and recycle it back into the system. The reaction occurs on a nickel-containing catalyst at from 2.5 to 3.5 MPa ^[11]. When the immediate exothermic reaction occurs, temperatures may rise to between 500 and 1040 K. The Steam Rankine Cycle (SRK) uses the rejected heat to generate electricity for power regeneration. It is necessary to remove H₂ and CO₂ residuals from the gas produced by methanation by running the outputs in a drying process (i.e., pressure-swing-adsorption, cryogenic separation). It should be noted that most techniques are utilized to increase the purity of the H₂ and N₂ required for ammonia production. The synthesis of ammonia only occurs in the last block of the reaction. At this point, N₂ and H₂ are routed as a set of compression stages that are powered by steam turbines to complete the process. While centrifugal-based compression has a cheap initial investment, low maintenance costs, and good dependability, it has a lower efficiency than reciprocating compressors ^[12], which should be considered. Preheating and increasing the synthesis gas pressure to 15–25 MPa are performed before the requisite synthesis temperature is reached.

The converter where NH₃ production takes place is at the core of the synthesis system. The converter’s response rate and operational parameters impact the converter’s overall performance. When the pressure is raised, the ammonia yield dramatically rises due to the favorable equilibrium reaction and the reaction rate itself. The synthesis pressure in contemporary ammonia facilities varies between 15,000 and 25,000 kPa. In addition, maintaining the required temperature is critical because the pace of the production process varies dramatically as the temperature changes. The H₂:N₂ ratio in the incoming synthesis gas and the feed stream speed impact the converter’s performance when combined with the previously listed factors. The best conversion is achieved when the space velocity is high, and the H₂:N₂ ratio is two. On its way through the catalyst, the synthesis undergoes a partial conversion of 25–35%, according to ^[13]. After that, the ammonia that is created is separated from the unreacted gas before being returned to the converter. A variety of synthesis loop designs are feasible, and the position of the NH condensation determines which one is used. The separation of ammonia from the unreacted gas occurs in all contemporary facilities using refrigerated chilling. Ordinarily, the temperature is lowered to around 25 K, and the liquefied NH₃ in the elevated-pressure separators is flashed at 2000 kPa. Then, the key individual steps involved in ammonia production are summarized.

3. Ammonia Synthesis Patents

Several patents were granted in ammonia synthesis, as summarized in Table 1. Haber and Bosch ^[14] patented the NH₃ production process in 1916 (U.S. patent-1202995). Many other patents have been obtained for ammonia synthesis since then. Wright and colleagues ^[15] developed a set of equipment for ammonia synthesis, which consisted of two catalytic converters and was patented (U.S. patent-3721532). Several heat-exchanging means are linked to the converters’ intake and output ports on both sides of the system. The second converter’s input discharges a supply stream into the heat-exchanging means to be cooled. The patent describes the implementation of a support platform for the converters and the heat-exchanging means. To develop a new process, it is necessary to operate within a pressure of 10–31.3 MPa, while maintaining the temperature in the range of 477–320 K.

Table 1. Ammonia synthesis patents.

Title of Invention	Summary	Patent Number/(Year)	Reference
Ammonia synthesis system	An integration of equipment and processes for the production of NH₃, consisting of two catalyst synthesis converters and numerous means of exchange, has been developed. Conditions from 10,000 to 31,300 kPa pressure, and from 770 to 610 K, should be met.	U.S.-3721532 (1973)	^[15]
Process for the production of ammonia	The synthesis of hydrogen and nitrogen results in the production of ammonia. A high-pressure electrolyzer was used in the generation of hydrogen. The primary compressor is removed from the equation. The pressure is 20,000 kPa, and the temperature is 650 K.	U.S.-4107277 (1976)	^[16]
Low energy ammonia synthesis process	Synthesis of ammonia at low pressures, ranging between 2 and 10 MPa, and temperatures ranging between 610 and 715 K. The separation of ammonia occurs by absorption in a liquid solution.	U.S.-4148866 (1978)	^[17]
Preparation of ammonia synthesis	The use of autothermal reformation to produce NH₃ from hydrocarbon fuel is described, with pressure and temperature conditions ranging from 2500 to 5000 kPa and 740 to 1000 K, respectively.	U.S.-4479925 (1983)	^[18]
Ammonia synthesis process	NH₃ synthesis based on varying pressure and temperature condition ranges from 4000 to 12,000 kPa and 590 to 740 K, respectively.	U.S.-4695442 (1993)	^[19]

Da Rosa ^[16] used high-pressure electrolysis to produce H₂ without the need for compression, which was a breakthrough in the field (U.S. patent 4107277). The concept also used oxygen at elevated pressures to liquefy ammonia in the refrigeration subsystem, which was the first in the industry. The exothermic nature of the ammonia synthesis process means that the steam recovered from the reactor may be utilized to produce energy, which can then be used to power the electrolyzer. The procedure was carried out at a pressure of 200 atm and temperatures ranging from 370 to 650 K. Becker registered a patent (U.S. 4148866) for the manufacture of ammonia with minimal energy usage, which was issued to Colman L. Becker ^[17].

It is necessary to initiate the ammonia production process at low pressure levels ranging between 2 and 10 MPa. A liquid–water-based combination separates the synthesized ammonia from the residual gases through absorption and stripping. A gas-based ammonia synthesis system (U.S.-patent-4479925) was designed by Shires et al. ^[18]. The syngas is then combined with H₂O, a reforming reactor, where it undergoes an endothermic process, resulting in hydrogen production. The effluent gas is then combined with air in an autothermal reformer before being sent into the synthesis converter for further processing. High temperatures ranging from 990 to 1190 K and low pressures ranging from 2500 to 5000 kPa are necessary for this procedure. The majority of ammonia-producing facilities at present depend on fossil fuels and natural gas for their energy. These facilities have a single train of gigantic reactors. Before the nitrogen and hydrogen can be introduced into the synthesis converter, the raw gas must pass through a purification process. As a result, many patents have been issued in connection with the purifying methods used for the natural gas supply. In the patent (U.S.-4695442), the adsorption characteristics of gases that are occupied throughout an acceptable range of raw gas composition are described in detail ^[19]. Carbon dioxide and hydrogen are present in sufficient quantities to bring the boiling point of N₂ into balance. N₂ and H₂ are then combined and supplied into the convertor, resulting in the production of ammonia gas with a high yield of hydrogen recovery while minimizing the need for additional adsorption bed volume and purge gas. This procedure operates at temperatures ranging from 640 to 790 K and pressures ranging from 2500 to 5000 kPa.

4. State-of-the-Art Ammonia Production

This section includes a survey of the literature on subjects and research related to the generation of ammonia. The section also provides an overview of the contemporary ammonia-related technologies that are used worldwide.

4.1. Casale Small Ammonia Plant Concepts

Casale presents two concepts for micro-NH₃ plants: the A-60, which has a capacity of up to 3000 ton/month, and the A-600, with a capacity that ranges from 9000 to 30,000 ton/month. The synthesis loops of these two plant models vary to accomplish their designated production targets. The synthesizing loop operates at high pressure in the A-60 concept, which primarily reduces the number of equipment items (above 20,000 kPa). Consequently, ammonia is created in high concentrations and readily condenses using H₂O or atmospheric air. As a result, the refrigeration unit is omitted. The A-600 concept envisions a low-pressure synthesizing loop instead of the A500 concept. With this design, the goal is to make the production facility simpler while maintaining the primary compression stage. When using a low-pressure synthesis loop, the mass flow rate in the compression stage increases, which allows for the employment of a more dependable centrifugal machine. Casale’s ammonia plant designs depend on methane as an input supply to the plant. They are also applicable to alternate feedstock sources such as biomass fuel supply extracted from waste and H₂ produced through electrolyzers ^[20].

4.2. Casale NH₃ Plants with Biomass Feedstocks

Bio-methane is a biomass fuel supply that can be produced from renewable energy sources such as biofuel. The first-generation (organic waste) ^[21], second-generation (lignocellulosic biomass) ^[22], and third-generation (micro-algae) biofuels all depend on the organic matter used as the starting point ^[23]. Only a few energy-based and environmental-based research projects on NH₃ synthesis from biofuel gasification have been conducted in the literature. Andersson et al. carried out a techno-economic study on NH₃ synthesis by implementing a biofuel gasification of the leftover wood generated in pulp- and paper-production plants ^[24]. The study findings revealed that combining the pulp-production and paper-production plans improves economic sustainability and leads to a 9% boost in the performance of the cycle when compared to a standalone ammonia plant.

The price of the generated ammonia, on the other hand, is higher than the current market pricing for ammonia. Tock et al. conducted a thermo-environmental optimization evaluation of ammonia production using wood-based gasification using an energy integration approach, which they found to be effective ^[25]. The integration used CO₂ capture and storage technology to limit CO₂ emissions to the atmosphere. However, the research findings indicated that energy efficiency decreased due to the expensive CO₂-compression stage. Additionally, the plant’s energy efficiency was evaluated to be 51% and 1.78 ton CO₂/ton NH₃ through the biofuel produced by the crop.

In contrast, the system’s energy efficiency in natural gas-based ammonia production was rated at 65% and 0.78 ton CO₂/ton NH₃. According to ^[26], the authors investigated the feasibility of NH₃ synthesis using wood-based gasification for a system capable of producing 1200 tons NH₃/day. According to the study’s findings, up to 66% of the gas that contributes to global warming (CO₂, CO, NOx) reductions were accomplished.

The economically sustainable dry-based biomass gasification (61 USD/ton) contributed to reducing the cost of NH₃ synthesis to 501 USD/ton. Biomethane may be converted to syngas in a practical manner by utilizing Casale’s A-60 or A-600 NH₃ plant designs, depending on their required capacity. In the Casale A-60 design, the fuel supply is processed in a patented, partially oxidized (POX) reactor, which was developed by the company. Based on Casale’s sophisticated burner technology, Partial Oxidization (POX) enables soot-free operation at a low H₂O/C ratio when the feed gas composition varied. Furthermore, POX has a meager minimum turndown ratio, allowing for steady functioning in 21% of the average load without compromising performance. With these characteristics of Casale POX, it is possible to reduce the plant’s overall size while maintaining excellent durability, performance, and an extended life cycle. After being inspired by the notion that the ammonia plant should be simplified and reduced to several units, the A-60 proposes a single shift stage (at elevated temperature levels), followed by a purification stage. The latter procedure is conducted in the Pressure Swing Adsorption (PSA) facility, which is highly automated.

Furthermore, microbes such as cyanobacteria, which generate the enzymes responsible for nitrogen fixation and ammonia production ^[27], should be noted. These enzymes may also be used in conjunction with a chemical reaction to react and develop electron transfer in the presence of low potential, which serves as the driving force in a biochemical reactor ^[28]^[29]. Immobilizing enzymes on the electrode surface increases the biocatalytic potential for nitrogen fixation and ammonium production.

4.3. ThyssenKrupp’s Green Ammonia Concept

ThyssenKrupp Industrial Solutions (TKIS) is popular in the fertilizer business due to its Uhde NH₃ Production Technology ^[30]. Energy, transportation, agricultural sectors, and chemical industries are among the areas in which TKIS has created comprehensive and environmentally beneficial methods ^[31]. The alkaline–water–electrolysis (AWE) technology used by the company serves as the foundation for all these applications by supplying hydrogen to downstream technologies. TKIS’s AWE and downstream processes to “green” syngas, H₂, CH₃OH, and NH₃ now allow for renewable energy, created from renewable sources, to be stored, addressing the primary barrier and issues associated with renewable energy: fluctuation. Ammonia may also be converted into nitrogen fertilizer solutions by additional processing. As a result, a wide range of eco-friendly technology is accessible today. Unlike a traditional NH₃ plant, which generates H₂ via the SMR units, the AWE generates H₂ using electrolyzers. An Air-Separation Unit (ASU) generates the N₂ necessary for NH₃ synthesis in the concept proposed here. When developing its NH-production approach, TKIS concluded that having a standardized and modularized concept, without spending time and resources on tailormade engineering, was a critical prerequisite for providing possible clients with optimized solutions. The modularization and standardization of the green ammonia concept are required to increase the viability of this concept and make it more feasible.

Although the AWE and ASU sections of the plant were already modularized, TKIS had to undergo additional work to modularize the NH₃ synthesis section, which resulted in a comprehensive concept being developed from a single source. When modularizing the green ammonia plant, it is beneficial to have both hydrogen and nitrogen accessible at the same time under the same circumstances. Furthermore, from modularization, it is helpful to retain capabilities at a lower level to maintain facilities at a compact size. According to the evaluation of the TKIS report by references ^[30]^[31], a significant proportion of wind clients from offshore and onshore farms generally have at least 20,000 kW of power. NH₃ production of 1500 ton/month may be reached with a 20,000 kW input-power, according to the manufacturer. The key objective of TKIS for this 50 ton/day concept was to optimize the usage of economically sustainable equipment to avoid jeopardizing the viability of the project.

Moreover, TKIS has created a second concept based on a 120,000 kW of input power that will produce 300 ton/day ammonia. Due to the scale of the plant, TKIS decided that, to compete with traditional approaches, somewhat more emphasis should be placed on the energy-efficiency of this particular concept. In addition, since the plant operates on a higher economic scaling level, TKIS believes that it might be a realistic upgrading option in existing NH₃-production facilities, able to partially replace traditional NH₃ production with green ammonia production ^[7].

4.4. Electrolysis for Ammonia Production

The electrolysis of alkaline-water is based on the well-established Chlor-alkali electrode technique ^[32]. Power and water are necessary for AWE-based hydrogen generation. In contrast to the direct supply of electricity to AWE via a transformer rectifier, raw water must be demineralized before it can be given to AWE.

The oxygen and hydrogen produced by the AWE process are the primary byproducts. Both goods have been thoroughly cleaned. This procedure does not need oxygen, which might be saved for use in other methods further downstream. The hydrogen produced by AWE is compressed, deoxygenated, and dried before being used. The hydrogen has now been prepared for use in the ammonia production process. To prepare the synthesis gas that is to be fed into the synthesis gas compressor, the appropriate quantity of N₂ is mixed with H₂ at a stoichiometric ratio. Nitrogen is created in a cryogenic air separation unit.

The AWE can be set up in minutes and responds to load fluctuations in milliseconds. It is also very cost-effective. As a result, the AWE provides the necessary flexibility from renewable energy sources. The ammonia synthesis portion is a crucial source of concern. For this reason, intermittent hydrogen storage is being constructed upstream of the synthesis gas compression to address the problem. The capacity of the intermittent storage system may be adjusted in line with the availability of power resources. With these safeguards in place, the NH₃ synthesis unit will be capable of operating on a 24-h basis without interruption

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1. The Importance of Ammonia

2. Modern Ammonia Production Plants

3. Ammonia Synthesis Patents