15HPCI magazine Vol.

Challenging
Decarbonization:
Next-Generation
Ammonia FuelContributing to Burner Design throughAmmonia Combustion Simulation
Because ammonia does not emit carbon dioxide when burned, attention is turning
to its potential as a fuel for achieving carbon neutrality by 2050. However, ammonia
is generally a difficult substance to burn, and forcibly combusting it may result in theemission of harmful NOx exceeding environmental standards. To address thesechallenges through combustion methods, Dr. Yukihiko Okumura from Kagawa
University is advancing research through a dual approach of experimentation andsimulation. Obtaining insights unique to simulations that cannot be gained throughexperiments alone, Dr. Okumura arrived at a “tornado burner” that safely andefficiently burns pure ammonia.

Yukihiko Okumura

Professor, Faculty of Engineering and Design, Kagawa University01

15HPCI magazine Vol.

Ammonia Fuel: More Cost-Effective andEnergy-Efficient than Hydrogen
With an eye on achieving carbon neutrality by 2050, a movement isarising to realize a hydrogen society. Therefore, a method has beendeveloped in which hydrogen (H2) is converted into ammonia (NH3) andtransported from overseas, after which the hydrogen is extracted from theammonia and used. This is done to transport hydrogen, which liquefies atminus 253°C, in a cost-effective and energy-efficient manner. However,Dr. Okumura, explaining the motivation for ammonia burner research,says, “Rather than extracting hydrogen from ammonia and using it ashydrogen fuel, burning ammonia itself allows us to obtain high calories
without energy loss. Using additional energy for extracting the hydrogenis also unnecessary.”
However, ammonia is a fire-resistant substance that does not easily ignite
even when exposed to flame. Additionally, forcibly burning it releases
harmful nitrogen oxides (NOx). National environmental standards
stipulate that “NOx emissions should be 150ppm or less for boilers and600ppm or less for gas engines.” That is why, in 2017, Dr. Okumura
began researching how to efficiently combust ammonia with minimal NOxemissions. Three years later, the Japanese government selected
Hydrogen/Fuel Ammonia Industries as one of 14 fields in its GreenGrowth Strategy*1 in December 2020. Reflecting on the time he started
studying ammonia combustion, Dr. Okumura says, “Finding a way to use
inherently difficult-to-burn ammonia as fuel was a challenge itself. Little
was known about ammonia combustion at that time.”
Having conducted combustion research on various substances since his
student days, Dr. Okumura believed that understanding the combustion
mechanism and flame structure of ammonia was crucial for developing an02

ammonia burner. Therefore, he decided to pursue combustion researchusing both experimentation and simulation.

Simulation of Ammonia Combustion
To burn ammonia, a mixed combustion method is commonly used thatinvolves co-burning it with methane (CH4), the main component of natural

3m/s

Air

Highvelocity
air

3m/s

8m/s

3m/s

Laminar flow

3m/s

3m/s

Turbulent flow

Fig. 1 : Ammonia burner used in the research
The space between rims blows out ammonia, hydrogen and air. In the right diagram,
turbulence is created by high-speed air (8m/s) flowing faster than ammonia andhydrogen. Generally, turbulent flow results in higher combustion efficiency per unit
volume than laminar flow. To streamline the flow around the rim, students in theOkumura Laboratory laid cut straws and directed air at 3m/s. As a result, thecalculation of the surrounding airflow was simplified.

Challenging Decarbonization: Next-Generation Ammonia FuelContributing to Burner Design through Ammonia Combustion Simulationgas. However, burning methane produces CO 2 emissions. SoDr. Okumura initiated research on a burner with hydrogen flame stabilizer
to assist in the combustion of ammonia using hydrogen gas (Fig. 1). He
designed and fabricated the burner so that it could create both laminar flows,in which the flame calmly rises, and turbulent flows, where the fuel is forcibly
mixed with high-velocity air and becomes turbulent flame, by adjusting theamount of air sent along with the combustion gas. This allowed him to
conduct experiments and simulations under the same conditions.
In experimental studies, temperature was measured at various locationsin the flame. Gas measurements were also taken to determine theamounts of major chemical species (H2, O2, N2, NH3, NOx, H2O) at thoselocations. Meanwhile, simulation studies utilized fluid dynamics softwarecalled “Advance/frontflow/red” to calculate in detail the distribution
and reactions of all chemical species including radical species within theflame, determining how temperature changes with the progression ofreactions.

The chemical reaction for ammonia combustion can be expressed as
4NH 3 + 3O 2 → 2N 2 + 6H 2O, which is an equation summarizing all thereactions. In fact, within the flame, highly reactive and unstable radicals
are produced, changing into other chemical species. There are 31 related
chemical species and 203 reaction equations (Fig. 2). Considering all ofthem is crucial for understanding the details of combustion. Furthermore,
the reactions with hydrogen and the surrounding air and flow fields also
add complexity to the overall flame picture.
[mm]5°

Area ofcalculation

burner nozzle

Fig. 3 : Simulation of a laboratory-scale ammonia burner
Fig. 2 : Example of elementary reaction equations forammonia combustion
The study utilized CRECK’s reaction mechanism (31 chemical species, 203 reaction
equations).

The area near the gas outlet (rim) employs a fine mesh with 0.1mm squares, while
the part away from the rim is coarsely divided to reduce computational load.
Assuming axial symmetry, the calculation range covers only 5 degrees rather thanthe entire circumference.
Fig. 3 - Fig. 6 : Reprinted from the Journal of the Combustion Society of Japan, Vol.64 No.208 (2022) 168-176,“Flame Structure and Reaction Analysis for Ammonia Turbulent Burner with Hydrogen Flame Stabilizer,”
Okumura, Y., et al., with permission from the Combustion Society of Japan.

0315HPCI magazine Vol.

Experiment

Simulation= 8.0m/s= 8.0m/s

800
600

250

400
200

2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
030

2010010203040h = 100mm
2500

2000
40302010010203040h = 50mm

1500

1000

30

20

100102030

300

401.0

3000

0.92700

Volume fraction [-]

2400

N20.6

2100
1800

Temperature

0.5

1500

0.4NOX

H20.3

1200
900

H 2O

0.2

600

O20.1

300

0.0

00

50

100

150

200

250

Distance from burner [mm]

04300

350

Temperature [K], NOx [ppm]Experiment

NH30

1000
800
600
400
200
302030203020

2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
4010010203040203040203040h = 100mm
10010

2400
2200
2000
1800
1600

1400120010008006004002000h = 50mm
4010010Radius from center [mm]Radius from center [mm]0.7501200

0100500400.8150h = 300mm

1400

040200Temperature [K]

2200
2000
1800
1600
1400
1200
1000
800
600
400
200040Temperature [K]Temperature [K]Temperature [K]0Temperature [K]300h = 300mm

1000

1600

Temperature [K]Temperature [K]1200

Fig. 4 : Comparison ofexperiment by using ammonia
burner and simulation(turbulent flow)
The upper part shows the temperature
distribution. The simulation closely aligns withexperimental results, indicating the accuracyand precision of the simulation. The lower part
illustrates the distribution of chemical species
within the flame. Consistency with the simulation
data is shown in the original paper. (In detail:the Journal of the Combustion Society of Japan,
Vol.64 No.208 (2022) 168-176.) Although
not shown here, simulations also match
experimental results well for laminar flow.

“We divided the space occupied by the flameand surrounding air into a grid, solving agoverning equations in reactive flow that
included mass conservation, energy
conservation, and momentum conservation,
as well as the state equation of gases and theequation of mass conservation for chemicalspecies (element conservation). Except for
the equation of mass conservation forchemical species, this is the same as theatmospheric simulations used for weather
forecasting. However, as soon as chemicalreactions (elementary reaction groups)a r e included in the calculations, thecomputational load becomes enormous,”
Dr. Okumura says.
To reduce computational load, Dr. Okumura
initially targeted laboratory-scale burners.
He explored options to coarsen the grid while
maintaining calculation accuracy, assumingaxial symmetry and time-averaging turbulence(Fig. 3). For later turbulent calculations forthe tornado ammonia burner and largeburners, he transitioned to “Fugaku”-classcomputational resources capable of handlinglarge-scale calculations (360° range andlarge spatial dimensions).
“At the beginning of the research, I had 69
elementary reaction equations for ammonia
combustion. However, calculations at thatChallenging Decarbonization: Next-Generation Ammonia FuelContributing to Burner Design through Ammonia Combustion Simulation]

300[200Furthermore, Dr. Okumura believes that simulationscould differentiate whether a nitrogen in a specificNH( × 105 )0.7NH2( × 104 )Temperature

OH × 1000.650

H20.4O( ×102 )1000

0.3O2H2O

0.10123456789 10 11 12500

0Radius [mm]4High air Velocity

3Air

H2NH301500

H × 100

0.5

0.0

100

2000

(3m/s)(3m/s)

8(3m/s)10Dr. Okumura explains the simulation study strengths asfollows. “There are limitations to the quantities that canbe measured experimentally. Especially, it is almost
impossible to determine the spatial distribution of
essential radicals for combustion, such as H radicals, Oradicals, and NH radicals. In contrast to that, simulation
allows us to clarify reactions occurring at every location
within the flame.” Simulation results have revealed thatOH radicals and H radicals move and mix due toturbulence, and that radicals and heat derived fromhydrogen fuel particularly promote and sustain thecombustion reaction of ammonia (Fig. 5).

0.8

0.2

150

Simulation Is the Only Way toUnderstand the Mechanism ofAmmonia Combustion in Detail

NH3Outer Wall[mm]

250h/R = 2.0

0.9

Temperature [K]1.0

Volume fraction

time, which used a scheme developed based onhydrocarbons, did not agree with the experimentalresults at all. Concurrently with my research, theelucidation of elementary reactions for ammonia
combustion progressed, and the number of reactions
related to ammonia increased. It was only when weincluded 203 elementary reaction equations that we
finally obtained calculation results close to experimentalresults (Fig. 4). Performing calculations with 203
elementary reaction equations was made possible by
the computational power of HPCI,” Dr. Okumura says.

13.5

80[mm]Fig. 5 : Radical concentrations in a turbulent burner with hydrogen
flame stabilizer analyzed using the supercomputer “Fugaku”
At left is the temperature distribution of the flame. The upper-right graph shows the temperature andchemical species amounts at the red circle position (flame height of 14mm), and the lower-right indicates thelocations where various gases are jetted. Looking at the upper-right graph, the highest temperature isobserved in the region of 7<r<9 [mm] (where air is mixed), and OH radicals (pink line) and H radicals (blueline) that promote combustion reactions are generated more abundantly due to the combustion ofsupplementary hydrogen. In addition, the heat generated by burning hydrogen is transferred to the ammonia
injection part, supplying energy to maintain the combustion chemical reactions of ammonia. The essential
OH radicals and H radicals for combustion are diffused through turbulence and forcibly mixed in the regionof 5<r<8 [mm], contributing to the stable maintenance of combustion.

05

15

HPCI magazine Vol.

NOx molecule formed from the air or the ammonia. He calculated thecombustion process with distinguishing nitrogen atoms derived fromammonia and nitrogen molecules in the air, revealing that the majority ofNOx emitted from the top of the flame into the atmosphere is derived fromammonia, with air-derived NOx existing only at the bottom of the flameand overwhelmingly minimal (Fig. 6 (1)).

(1)

(2)

200
[mm]

250[mm]Temperature
distribution.

180

200

160
140

Fuel No[ppm]

120

Thermal No
[ppm]

3500

100

2000

3000

80

2500
2000
1500
1000
500
060
20

1800
1600
1400
1200
1000
800
600

20400
200

00150
NOx forming
Reaction Rate
[mol/cm3 s]

Temperature[K]2500

1002000

1500

3

1.0E-450

1000

300

1.0E-3

Reflecting on the simulation, Dr. Okumura notes, “NOx did not reachextremely high levels.” The reason became apparent also through thesimulation. Nitrogen atoms are oxidized to form NOx molecules, and theyturn into N2 molecules, when reduced. Analyzing where and how much ofthis oxidation-reduction reaction occurs, it was found that NOx is primarily
generated in the slightly inner region of the high-temperature zone of theflame where fuel is abundant, and at almost
the same location, reduction reactions also
effectively eliminate NOx (Fig. 6 (2)). “It250[mm]became clear that the contribution of thereduction reaction is significantly large,
although the reaction rate of NOx generation200
is higher than that of NOx reduction. Thisknowledge, obtained through simulation,
explains why there was no extreme increase
150in NOx concentration. It underscores theDeNOx
significance of simulation research inReaction Rate
[mol/cm s]providing insights into reactions and flame
1.0E-3100structure,” Dr. Okumura says.

1.0E-5

1.0E-450

1.0E-60(a) NOx formation

1.0E-7

1.0E-5

1.0E-60

1.0E-7(b) DeNOX

Fig. 6 : Mechanism of NOx formation(1) The left half shows the distribution of NOx derived from ammonia (Fuel NO). The right half shows the distribution of NOxderived from nitrogen molecules in the air (Thermal NO).
(2) Reaction rates of NOx formation (a) and NOx reduction (b). NOx rapidly disappears in the region near its intense creation
at a rate comparable to the formation rate.

06

Promoting Ammonia Burners
in Industrial Settings andAchieving Carbon Neutrality
by 2050
In the industrial sector, initiatives are alreadyunderway to replace burners, used inprocesses such as breaking down naphtha
obtained from petroleum refining for plastic
production and pottery kiln firing, withChallenging Decarbonization: Next-Generation Ammonia FuelContributing to Burner Design through Ammonia Combustion Simulation

ammonia burners. Dr. Okumura aims to popularize ammonia burners thatcan be used in industrial settings. To achieve industrial applications, it is
crucial to realize ammonia combustion that meets environmental
standards at low cost. Consequently, Dr. Okumura has also initiatedresearch on an “ammonia dedicated burner,” which can combustammonia without using expensive hydrogen.
Recognizing the importance of continuously generating active radicals tosustain ammonia combustion, Dr. Okumura devised the tornado burner.
This burner releases a pre-mixed gas of ammonia and air from the bottom,and additionally injects it from the side of the cylinder, swirling the flame.
In this research, a combination of simulations using Oakbridge-CX at theUniversity of Tokyo and experiments were employed to investigate theratios of ammonia to air (equivalent ratio), flow velocities, and otherfactors for the main jet and swirling jets (Fig. 7). Successful results wereachieved, showcasing the conditions for ammonia-dedicated combustion.
The NOx emission level was also lowered, nearly reaching environmental
standards due to the two-stage combustion method and tornado effect.
Furthermore, technology has been developed to reduce unburnedammonia below environmental standards even with a large flow ofammonia injection.
Dr. Okumura emphasized the strength of experimental research in being
able to cross-check simulation results, stating that experimental results
are undeniable facts even if the combustion process is unstable or does
not fit the theory. He highlighted the synergy between simulation andexperimental research, describing them as the “two wheels” of a vehicle.
Striving to introduce ammonia burners for industrial use promptly,
Dr. Okumura is accelerating the research, leveraging both aspects. He
expressed his aspirations, saying “My combustion technology (ammoniadedicated burner) has come very close to completion and I hope mytechnology will contribute to achieving carbon neutrality by 2050.”

Turbulentenergy

Radicals(O, H, OH)

Thermalenergy
Swirljet fuelSwirljet fuelMain jet fuelFig. 7 : Simulation and actual combustion oftornado ammonia dedicated burner
The tornado burner enables combustion of the flame-retardant ammonia without theneed for hydrogen. Simulation results revealed that the main jet and swirling jetmutually supply thermal energy, turbulent energy, and radicals. Numerous
improvements have been made to the presented burner, bringing it closer tomeeting the exhaust characteristics that comply with environmental standards.

*1 : Abbreviation for “Green Growth Strategy through Achieving Carbon Neutrality by2050.” This refers to Japan’s industrial policy to connect the challenge of achieving“Carbon Neutrality by 2050” announced in October 2020 with the “virtuous cycle ofthe economy and the environment.” Among the 14 sectors expected to grow i st h e Hydrogen, Fuel Ammonia Industry.
https://www.meti.go.jp/english/policy/energy_environment/global_warming/ggs2050/index.html

0715HPCI magazine Vol.

Challenging Decarbonization: Next-Generation Ammonia FuelContributing to Burner Design through Ammonia Combustion Simulation

About theResearcherDr. Okumura is a longtime science enthusiast, having enjoyed experimenting since he was a child.
He was fascinated by experiment kits that came with science magazines, often contemplating
questions like, “Why does burning charcoal glow a bright orange color? Why does it keep burning?”
Growing up during the period of Japan’s high economic growth, Dr. Okumura questioned
environmental aspects, wondering, “Is it okay to emit so much CO2? Can forests alone absorb it?
Is the Earth large enough to dilute all this CO2? Is it acceptable to leave Osaka’s rivers with their
unpleasant odor and stagnation as they are?” With such thoughts, he chose to major incomprehensive energy engineering, focusing on research into combustion that does not produce
pollutants. As he matured, Dr. Okumura learned about the adverse global effects of indiscriminate
CO2 emissions and, considering industrialization, dedicated himself to the research of CO2-freeburners. The background to his research approach reflects his childhood aspirations.
Associated Research Projects:・Development of CO2-free combustor for high intensity combustion and NOx reduction with simultaneous function (hp200033)
・Development of ammonia burner for high intensity combustion and NOx reduction with simultaneous function (hp200176)
・Optimization design of ammonia burner for CO2-free combustion (hp210108)
・Optimization design of ammonia burner for a carbon-free society (hp220009)
Principal Investigator: Yukihiko Okumura, Kagawa University

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