Mitsubishi Heavy Industries Technical Review Vol. 60 No. 3 (September 2023)
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Development Design of Ammonia Decomposition System
for Safe Operation of Ammonia Fuel toachieve Carbon Neutrality

RYOZO SASAKI*1

RYOTA BUN*2

The use of ammonia as a fuel is expected to expand with the trends such as thecommencement of demonstration of ammonia co-firing in the field of thermal power generation,and the development of ammonia-fueled engines in the field of marine vessels. The key factors forthe demand expansion are considered to be: the safety of operation because of the use of toxic
ammonia, the reasonable economic efficiency as an alternative to fossil fuel, and theenvironmentally conservative properties involving no generation of greenhouse gases (GHGs) or
by-products that can lead to environmental destruction. The ammonia decomposition system,
which is proposed by Mitsubishi Heavy Industries, Ltd. (MHI), employs its original catalytic
technology and will be one of the means to satisfy these factors. However, the operation conditionsof such systems and the requirement specifications have yet to be clarified. This report summarizes
our system that can satisfy the factors, in addition to the specifications and features. Even in a
project requiring the reduction of NOx by-products in the exhaust gas after ammonia isdecomposed, the increase in the system size can remain at a minimum. Our system was also
development designed in modules to make it transportable. We hope that this report will become aproposal for customers who are considering the use of ammonia as a fuel, and will lead them todetermine their own system operation conditions and specifications required for ammonia gas orwastewater detoxification.

|1. Introduction
In Japan, research and demonstration tests are in progress to expand the use of ammonia as afuel toward achieving carbon neutrality by 2050. The Ministry of Economy, Trade and Industry(METI) has set the targets for the total amount of ammonia domestically introduced as a fuel for
co-firing in thermal power generation and combustion in marine vessels: 3 million tonnes per yearby 2030, and a 10-fold increase by 2050, which means 30 million tonnes per year (Figure 1)(1).
As ammonia gas is toxic, decomposition system is required to be equipped with an ammonia
detoxifying system if the operation and maintenance are to be conducted safely. The system should
also be less costly and environmentally compatible.
The major technologies for ammonia decomposition are: absorption in water, biologicaltreatment, combustion of ammonia gas, and catalytic decomposition (Table 1). For absorption inwater, water is required in large quantities to prevent the re-liberation that is associated with theheat of absorption. There is another issue of the costs for post-absorption wastewater treatment andindustrial waste disposal. The biological treatment requires a large site area. Considering the costs
incurred as well, we consider this treatment system difficult to be introduced in a simple way.
When it comes to the combustion of ammonia gas, a pilot fuel for combustion is necessary, which
involves the risk of generating gaseous by-products including GHGs. It is also challenging tocompletely burn low-concentration ammonia.
MHI has adopted catalytic decomposition, which can decompose low-concentration
ammonia and generates no gaseous by-products or wastewater. In our design, the system will beequipped with a sub-system to treat ammonia-containing wastewater with the intention of allowing*1

Chief Staff Engineer, AQCS Engineering Department, Energy Systems, Mitsubishi Heavy Industries, Ltd.

*2

Chemical Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 3 (September 2023)
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ammonia to be quickly absorbed in water in the case of emergency.

Figure 1 Road map for introduction of ammonia as fuel and its expansion in use (Agency
for Natural Resources and Energy, METI)(1)
Table 1 Comparison of ammonia decomposition technologies
MethodAbsorption in
water

Summary
Water or an acid is sprayed
to allow ammonia gas to beabsorbed as ammonia water.

Merit
・High responsiveness.

Biologicaltreatment

The N content in water isnitrified/denitrified by
microorganisms to enable it
to be treated as sludge.
Ammonia gas is
decomposed by combustionin the burner.

・Commonly applied method
as the treatment of N content
in wastewater from thermal
power plants.
・The equipment is compact
and less costly.
・No wastewater.

Ammonia gas is
decomposed using acatalyst.

・Can be decomposed at low
concentrations.
・No wastewater or gaseous
by-products, and therefore
no need for secondary
treatment.
・Has past installation records
for power plants andfactories.

Combustion ofammonia gas

Catalytic
decomposition

Demerit・Requires large quantities of water
for high-concentration treatment.
・Requires wastewater collection
and secondary treatment.
・Requires the retention time, andthe site area needs to be larger
than other methods.
・Currently at the stage of assessing
if the low-concentration ammonia
can be decomposed.
・Requires a gas for combustion(e.g., LNG)
・Generates gaseous by-products(NOx and others).
・As catalysts are consumables,
they need to be replaced every
few years.

As described later, we have a good track record of delivering this decomposition system.
However, since we have not delivered it for ammonia co-firing at thermal power plants or formarine vessels, it is necessary to scale up the system capacity and change the specifications
according to the installation site and type of application. Thus, MHI carried out development
design to upgrade the whole system. We are also improving the catalysts, taking the reduction ofenvironmental impact into consideration.

|2. Characteristics of our catalyst for ammonia decomposition
MHI offers various catalyst products, including denitrification catalysts for power generation
or industrial plants. Its original catalyst technologies feature prominently in our ammonia

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decomposition system as well.
As a mechanism of ammonia decomposition using a catalyst, an ammonia-containing gas isoxidatively decomposed on a catalyst by the following chemical reactions.
4NH3 + 3O2→
2N2 + 6H2O
(1)
4NH3 + 5O2→
4NO + 6H2O
(2)
4NH3 + 7O2→
4NO2 + 6H2O
(3)
2NH3 + 2O2→
N2O + 3H2O
(4)
In the case of using general oxidation catalysts containing oxidizing components such as
platinum, decomposition of ammonia involves the production of not only nitrogen as shown inequation (1), but also by-products such as NOx and N2O as shown in equations (2) to (4). The moreammonia is decomposed, the more by-products are produced. When a large amount of ammonia isdecomposed using an oxidation catalyst, it is therefore necessary to add, downstream of theoxidation catalyst process, a means of purifying the exhaust gas containing by-product components
such as NOx and N2O. This results in the problems such as the increase in the system size andpower consumption.
To address these problems, MHI has developed a catalyst that can work for both NOx
removal (denitrification) and oxidative decomposition of ammonia (i.e., a multifunctional catalyst),
and have made it available commercially. The multifunctional catalyst was originally developed for
projects such as US heat recovery steam generators (HRSGs), whose exhaust gas had a high CO
concentration. Based on this catalyst that realizes both CO oxidation and denitrification, we have
improved to make it applicable to ammonia decomposition as well.
Figure 2 shows the ammonia decomposition reaction model of our multifunctional catalyst.
A pore in the catalyst contains the first component that participates in denitrification, and thesecond component that takes part in ammonia oxidative decomposition is distributed at intervals.
When ammonia enters the catalyst with such a structure, some of it is adsorbed onto the firstcomponent at the entrance of the pore, while some other of it advances further into the pore and isdecomposed on the second component, producing NO as shown in equation (2). The produced NO
collides with the adsorbed ammonia in the process of diffusing out of the catalyst, to be reduced to
non-toxic N2 as shown in equation (5).
NH3 + NO + 1/4O2 →N2＋3/2H2O(5)
As indicated by the mechanism above, ammonia can be decomposed on our multifunctional
catalyst to give N2 without generating NOx. Moreover, as ammonia is oxidized to N2 by thereaction written above, it is advantageously unlikely to produce intermediates of ammonia
decomposition such as N2O, as in equation (4).

Figure 2

Reaction model for our multifunctional catalyst (2)Figure 3 compares the results of our multifunctional catalyst and a typical oxidation catalyst.
It clearly indicates that, with the use of our multifunctional catalyst, the production rate ofby-products such as NOx and N2O can be decreased to practically zero, while the decomposition
rate of ammonia as high as that of the oxidation catalyst is maintained.
These characteristics of the catalyst allow the system size to be kept to a minimum even in a
project requiring the reduction of by-product NOx emissions in the exhaust gas after ammonia is

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 3 (September 2023)
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decomposed. We have also made a transportable version of the system available commercially, as
described in the later sections.
As more and more amount of ammonia is promoted for use as a fuel in society, the quantityof ammonia to be treated is expected to increase. As described earlier, since there is a correlation
between the amount of ammonia treated and that of by-products produced, more stableperformance is required if the concentrations of ammonia and by-products are to be kept low. In
recent years, despite our catalyst having already achieved high performance in both ammonia
decomposition and denitrification, we are further improving it to have the capability ofdecomposing the N2O by-product in trace amounts, and will obtain a patent for it. The global
warming potential of N2O is extremely high (about 300 times higher than CO2). With regard to theexhaust gas from the engines of NH3 fueled ships and transport ships, the importance of taking
measures is also emphasized. We will accelerate the development of our catalyst technologies torealize more superior properties of environmental conservation and further social contribution.

Figure 3 Comparison of our multifunctional catalyst andtypical oxidation catalyst|3. Summary of our system and application
Let us present the system configuration and process flow diagram (Figure 4). In order tokeep the ammonia decomposition reaction going, it is necessary to keep the temperature high in thereactor (with a catalyst bed). Therefore, before the system is started, the heater is turned on toincrease the temperature in the system. Once ammonia gas, which needs to be detoxified, is fed, theheat generated by ammonia decomposition is recovered by the heat exchanger for the purpose ofheating the gas at the reactor inlet. In this way, the reaction can continue without operating theheater. During normal operation, therefore, there is almost no need to use power for anything otherthan the air fan, making energy-saving operation possible. Meanwhile, if an excessive
concentration of ammonia is fed into the reactor, the temperature in the system also increases
excessively because of the heat of the reaction, resulting in thermal degradation of the catalyst. To
avoid that, the ammonia concentration at the reactor inlet is required to be controlled appropriately.
Specifically, the concentration is kept under control using the air fan to provide dilution air. This
also leads the ammonia concentration in the system to be kept below the explosion limit, enablingsafe operation with long-term stability.
Next comes the application of the system (Figure 5). The expected settings in which thesystem is in operation include the thermal power plants introducing ammonia co-firing or
single-fuel firing, and the marine vessels equipped with an ammonia storage facility and/or anammonia-fueled engine. For example, it is necessary to safely purge and detoxify ammonia gas,Mitsubishi Heavy Industries Technical Review Vol. 60 No. 3 (September 2023)
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when a periodic inspection on the ammonia storage tank is performed. That is to say, the conduct ofammonia slip treatment of engine exhaust gas or boil-off gas (BOG) treatment is needed to detoxifyammonia. For marine vessels, we also expect the on-board operation of the system.
Meanwhile, in the event of an emergency in which leakage occurs because of a problem suchas a fire, it is necessary to quickly treat large quantities of ammonia gas. There is a concern that thecatalytic system cannot handle such sudden treatment, because it requires pre-heating before thesystem start-up. Although absorption in water is often employed for sudden ammonia treatment,
the work and cost of post-absorption wastewater treatment become an issue. In our system
configuration, a stripping tower is installed before the stage of the reactor system in whichammonia is detoxified. It therefore becomes possible to treat ammonia gas as well as
ammonia-containing wastewater.
In the case of large decomposition capacity, the system will also increase in size andtherefore, we offer a normal stationary type. Depending on the decomposition capacity of thesystem, however, we can arrange a transportable type. If the type of application does notnecessitate the constant operation of the system, the transportable system can be operated wherever
and whenever needed. The actual application examples are given in a later chapter.

Figure 4

Ammonia decomposition system configuration and process flow

Figure 5

Conceptual diagram of application to ammonia fuel plants

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 3 (September 2023)
6|4. System specifications and features
At this point in 2023, the commencement of demonstration of ammonia co-firing in the fieldof thermal power generation and the development of ammonia-fueled engines in the field of marine
vessels are in progress. The requirements for the ammonia abatement system to be installed as theirdecomposition additional equipment have, in large part, yet to be clarified.
In MHI, the development design of the system is advanced, assuming the specifications are
the same as the current ones. We will flexibly modify the system as needed, when new laws andregulations are set up or the customer narrows down the required specifications. To give a roughidea, Table 2 lists our (target) specifications.
Table 2 List of ammonia decomposition system basic specifications (for reference)
Item
Ammonia decomposition
capacity(Stripping tower inlet)
Ammonia concentration in
wastewater(Stripping tower outlet)
Ammonia concentration in thetreated water(Reactor inlet) Ammonia
concentration in gas(Reactor outlet) Ammonia
concentration in gas(Reactor outlet) NOx
concentration in gas

Basic specifications
15-60 kg/h (100% ammonia, transportable)
15-400 kg/h (100% ammonia, stationary)Note

30,000 mg/L

<125 mg/L as N-NH4

Effluent standards by MLIT (inthe case of discharging into thesewage treatment plants)(3)30,000 ppm<25 ppm

ABS and NK guidelines(4)(5)<10 ppm

Our plan is to provide two options of transportable or stationary, depending on the ammonia
decomposition capacity. Specifically, the capacity ranges will be about 15-60 kg/h (100%ammonia) for the transportable type and about 15-400 kg/h (100% ammonia) for the stationary
type, respectively. As for the transportable type, we plan to transport it as a single unit by truck, etc.
Economic evaluation of the system has been performed. With regard to the industrial waste
treatment of ammonia-containing wastewater, we compared the costs of initial investment,
operation and maintenance, when our system was introduced. Although the evaluation results varydepending on the scale of the system and the amount of wastewater, the investment can berecovered roughly in less than three years. As the amount of wastewater increases, a shorter period
for investment recovery can be expected. We omit the detailed calculations, because otherwise it
involves disclosing the customer’s system information. However, please feel free to contact us forinquiries as we can provide estimates according to the customer’s situation.

|5. System delivery records
Lastly, our delivery records are presented. A total of 26 units have been delivered, including
for ammonia denitrification tanks to be purged for periodic inspection, semiconductor factories andwastewater treatment facilities. Of these, two are shown below.

5.1

Delivery record (1): Power plant in Japan- Transportable-type system intended for purging of gas in the ammonia denitrification tankat a power plant- Gas treatment capacity: 670 Nm3/h (equivalent to 20 Nm3/h for 100% ammonia)- Gas treatment results: <5 ppm for ammonia and <10 ppm for NOx

Figure 6

Delivery record (1)Mitsubishi Heavy Industries Technical Review Vol. 60 No. 3 (September 2023)
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5.2

Delivery record (2): Sludge wastewater treatment compost center- Stationary-type system equipped with both stripping tower and reactor sub-systems for
wastewater treatment- Gas treatment capacity: 850 Nm3/h (equivalent to 4 Nm3/h for 100% ammonia)- Having a proven record of 5,000 Nm3/h or more gas treatment at a semiconductor factoryas well.

Figure 7

Delivery record (2)|6. Conclusion
In order to achieve carbon neutrality, the demand for use of ammonia as a fuel is growing.
Our proposal under such circumstances is MHI’s ammonia decomposition system. In this report,
we pointed out the three key factors for the system and described how our system handles each ofthem. With regard to safety, while the characteristics of our catalyst enable low-concentration gas
to be decomposed, the concentration in the system is kept below a certain level to make sure that itis below the explosion limit. For economic efficiency, as there is almost no need to use power foranything other than the air fan during normal operation, our system’s superiority in economic
efficiency has been confirmed when compared with other methods such as industrial waste
treatment and biological treatment. When it comes to environmentally conservative properties, ourdeveloped catalyst enables the formation of harmful by-products to be practically zero.
Although the market environment is expected to change considerably in years to come, wewill continue to contribute to providing a solution to global issues such as energy and theenvironment.

References
(1)
(2)

(3)
(4)
(5)

Energy White Paper 2021, Part 3, Chapter 8, Section 4, Ministry of Economy, Trade and Industry,
https://www.enecho.meti.go.jp/about/whitepaper/2021/html/3-8-4.html
Hirofumi Kikkawa et. al, (Former Babcock-Hitachi K.K., now Mitsubishi Heavy Industries, Ltd.),
Method and apparatus for treating liquid ammonia remaining in tank，Patent Applications Publication
2004-216300，Publication date, 2004.8.5
Effluent water, Ministry of Land, Infrastructure and Transport and Tourism,
https://www.mlit.go.jp/pubcom/01/pubcom23/pubcomt23-2_.html
American Bureau of Shipping (ABS) : Guide for Ammonia Fueled Vessels. (Sep.2021)
Class NK：Guidelines for alternative combustion lines (Methanol, Ethanol, LPG, Ammonia) (Aug.2021)