Mitsubishi Heavy Industries Technical Review Vol. 60 No. 3 (September 2023)
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High-precision Combustion Simulation Technology
Supporting Energy Transition of Thermal Power Plants

ZHI ZHANG*1

AKIMASA TAKAYAMA*1

KOZO AOKI*1

YUSUKE TANAKA*1

KEIJIRO SAITO*2

KEIGO MATSUMOTO*2

Mitsubishi Heavy Industries, Ltd. (MHI) has been developing thermal power generation
systems burning carbon-free fuels, such as hydrogen, ammonia, etc., to achieve a carbon-neutral
society. For the transition to carbon-free fuels, the development of combustion equipment (e.g., gas
turbine combustors and boilers) is important. We have been utilizing high-precision combustion
simulation technology as a design support tool in addition to combustion tests using elemental
burners and actual combustion equipment. This report presents an overview of the high-precisioncombustion simulation technology that supports the energy transition of thermal power plants, aswell as its verification and application examples.

|1. Introduction
MHI has been promoting the energy transition of thermal power generation systems toward
the achievement of a carbon-neutral society, and has formulated a roadmap shown in Figure 1.
Specifically, we are developing gas turbines capable of co-firing or single-fuel firing of hydrogenor ammonia, and boilers capable of ammonia co-firing. Since these carbon-free fuels are
significantly different from existing fossil fuels in combustion characteristics such as burning rate,
the fuel transition for decarbonization requires combustion equipment design that achieves both
high reliability and low emissions. Development process based on combustion tests is expensive
and time-consuming, and also suffers from the difficulty in understanding the complicatedmulti-physics coupled phenomena. Therefore, the high-precision combustion simulation
technology developed and verified for natural gas-fired gas turbine combustors and coal-fired
boilers are further improved/extended considering the difference in combustion characteristics, tosupport the design of combustion equipment burning carbon-free fuels.
This report presents an overview of the combustion simulation technology as well as its
verification and application examples.

Figure 1

Roadmap for energy transition of thermal power generation systems*1

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

*2

Deputy Senior Director, Combustion Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 3 (September 2023)
2|2. Gas turbine combustor combustion simulation technology
Gas turbine combined cycle (GTCC) power generation, as the cleanest power generation
system burning fossil fuels, contributes to the reduction of both fuel consumption and emissions.
Currently, gas turbines for power generation mainly burn natural gas, and in recent years the fueltransition to hydrogen or ammonia is of a growing need to achieve a carbon-neutral society.
Although there are differences in combustion characteristics such as burning rate between the fuels,
the important evaluation items for the development of gas turbine combustors are the same in termsof ensuring stable combustion and reducing emissions. The development of combustors that
achieve both high reliability and low emissions requires the optimization of design parameters.
Since performance verification by combustion tests is highly time-consuming for the design,
fabrication, and testing cycles, there is a growing need for design support based on high-precisioncombustion simulation. This chapter presents an overview of combustion simulation technology
applied to gas turbine combustors and its verification and application examples.
The simulation of gas turbine combustors employs the Flamelet approach shown in Figure 2,
in which the controlling variables (mixing fraction, reaction progress, etc.) characterizing theflame structure are calculated directly in Computational Fluid Dynamics (CFD) simulation, and areused to parameterize all the other physical quantities such as reaction rate, temperature, species
mass fraction, etc., by referring to the database based on the detailed chemical reaction mechanism.
By simplifying the calculation of detailed chemistry, the Flamelet approach is considered to be a
combustion simulation model that is able to achieve both low computational cost and high accuracy(1).

Figure 2

2.1

Flamelet approach applied to gas turbine combustors

Natural gas combustion simulation technology

MHI's state-of-the-art JAC-series gas turbine, with turbine inlet temperature (T1T) as high as
1650℃, has achieved the world's highest power generation efficiency. Figure 3 shows an overview
of the natural gas-fired Dry Low NOx (DLN) combustor in JAC-series gas turbine. In thiscombustor the premixed combustion method in which fuel and air are premixed before combustion
is applied by utilizing the swirling flow.
To accurately predict the flame behavior affected by the highly unsteady turbulent flow in areal combustor (under the conditions of high temperatures, high pressures, and high Reynolds
numbers), a Large Eddy Simulation (LES) turbulence model is employed in the numericalsimulation. Specific simulation settings such as mesh size, time step, and numerical discretization
scheme are optimized based on the measured data of velocity distribution, concentration
distribution, temperature distribution, etc.

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Figure 3 Overview of DLN combustor in JAC-series gas turbine

Figure 4 shows the CFD simulation results of a natural gas-fired combustor. The CFD
simulation accurately reproduced the measured flame position (peak position of hydroxyl radical
OH* chemiluminescence intensity, bottom left of Figure 4) and temperature distribution (bottom
middle of Figure 4) (2). It also well reproduced the trend of changes in nitrogen oxide (NOx)emissions (bottom right of Figure 4) due to changes in the fuel nozzle design. This high-precisioncombustion simulation technology is currently being utilized as a design support tool for low NOx
natural gas-fired combustors.

Figure 4

2.2

High-precision simulation of natural gas combustion

Hydrogen combustion simulation technology

With support from the New Energy and Industrial Technology Development Organization(NEDO), we are developing gas turbine combustors capable of hydrogen firing with concentrationfrom 0 to 100%. Hydrogen burns about eight times faster than natural gas, which increases the riskof flashback. Therefore, for the development of the low-NOx emission hydrogen-fired combustor,
it is of great importance to guarantee stable combustion preventing the occurrence of flashback.
The hydrogen-fired combustor employs the "multi-cluster" concept shown in Figure 5(3). The
multi-cluster combustor does not use swirling flow for pre-mixing, but mixes air and hydrogen on asmaller scale, aiming for both high reliability and low emissions.
Due to the light molecular weight of hydrogen, the mass diffusion of the fuel is very fast
relative to the thermal conduction. The Lewis number (i.e., ratio of thermal diffusivity to massdiffusivity) of natural gas fuel is about 1, but it is much smaller than 1 under high hydrogenconcentration conditions. As shown in Figure 6, the calculation accuracy of the laminar flame
speed (combustion rate) of hydrogen cannot be ensured without considering the Lewis number
effect. Therefore, for hydrogen combustion simulation, the Lewis number effect is incorporated
into the Flamelet approach.

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Figure 5

Multi-cluster DLN combustor (hydrogen 0 to 100%)

Figure 6 Lewis number effect in hydrogen combustion

Figures 7 and 8 show CFD simulation results of a model burner cut from afull-scale hydrogen combustor. Similar to the natural gas combustion simulation, the
CFD simulation reproduced the flame position with a high accuracy. In addition, it
also well reproduced the trend of NOx emission changes due to changes in operating
conditions and design parameters. These results confirm that this high-precisioncombustion simulation technology is of the potential to be used as a design support toolfor hydrogen-fired combustors.

Figure 7

High precision prediction of flame position for hydrogen combustion

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Figure 8

2.3

High-precision prediction of NOx for hydrogen combustion

Ammonia combustion simulation technology

We have started the development of a small-to-medium-size ammonia single-fuel firing gas
turbine system. Ammonia burns at about 1/5 the burning rate of methane, which is the main
component of natural gas, making it difficult to ensure flame retention. In addition, if the flame isstretched (elongated) by a jet impinging on the wall or by the presence of vortices, the combustion
rate is further reduced. If the flame is highly stretched, local extinction may occur, which may leadto flame blowout. At atmospheric pressure, an ammonia flame is extinguished at a flame stretch
rate of about one-twentieth that of a methane flame(4), so the effect of flame stretch on stable flame
retention is significant. In addition to the difficulty of flame retention, direct ammonia combustion
also poses the problem of NOx control. Since NOx emissions can be suppressed when ammonia is
combusted in the appropriate fuel concentration range, a lower-NOx combustor can be achieved by
optimizing the fuel nozzle and injecting secondary air. If combustion stability and NOx emissions
can be predicted with high accuracy using combustion simulation during the combustor
optimization phase, the design process will be accelerated.
We have developed ammonia combustion simulation technology that takes into account theeffect of flame stretch to predict the stability of ammonia combustion and NOx emissions at theoutlet, as shown in Figure 9. This technology incorporates the effect of flame stretch on change inburning rate (reaction rate)(5) as a correction factor in the Flamelet approach. We are currently
verifying its accuracy in a literature model burner(6), and will apply it as a design support tool for
actual combustor in the future.

Figure 9

Ammonia combustion simulation considering effect of flame stretch|3. Boiler combustion simulation technology
To enhance exhaust gas performance of a boiler by improving burners and air ports andoptimizing operating conditions, it is necessary to understand the combustion conditions in the
boiler furnace in detail and accurately evaluate the effect of such improvements and optimizations.
It is also important to understand the characteristics of the metal temperature of each part toimprove the reliability. We have developed high-precision boiler combustion simulation
technology to evaluate the effects of various modifications on various boilers owned by ourcustomers in advance. This chapter introduces examples of verification and application of such

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boiler combustion simulation technology.

3.1

Coal-fired boiler simulation technology

Boiler combustion simulation is computationally demanding because of the need toincorporate submodels such as pulverized coal behavior and radiation heat transfer, and because of
the huge size of the furnace to be calculated. Therefore, in many cases, Reynolds-Averaged
Navier-Stokes (RANS) simulation, which is less computationally demanding and assumes atime-averaged field, is used for normal performance evaluation. On the other hand, one of the
difficult predictions to be made by RANS is the prediction of carbon monoxide (CO)concentration. One of the factors causing CO is the unsteady heterogeneity in the mixing ofpulverized coal and air, which limits the accuracy of prediction by RANS. This section introduces
an example of CO concentration prediction by LES, which can simulate non-steady state behavior.
Figure 10 shows an example of LES simulation of an actual boiler. It is observed that the LES canpredict the CO concentration at the furnace outlet accurately with respect to the actual
measurement, including the CO concentration distribution. The LES can also reproduce thebehavior of changes in CO concentration when the air distribution is changed from the condition(1) to the condition (2) with higher accuracy than RANS. This technology can be applied to thedevelopment of methods to reduce unburned fuel such as air nozzle modification.

Figure 10

High-precision unburned carbon prediction

Next, we will explain our approach to reliability assessment through the combustion
simulation. The superheater at the top of a boiler furnace is composed of many heat transfer tubes,
and its metal temperature is affected by the bias of combustion gas, steam flow rate in the tubes,
and other factors. For creep life control, proper control of metal temperature is important, but anapproach based on measurement using thermocouples is not realistic in terms of cost, durability,
etc. Therefore, we have developed a method for predicting heat transfer tube metal temperaturesusing coupled simulation of heat transfer from combustion gas to steam inside the tubes. By
simulating heat transfer to steam flowing through each tube simultaneously with combustion gas
flow outside the tube, it is possible to predict the local metal temperature. Figure 11 shows anexample of a project commissioned by NEDO to obtain metal temperature data from an actual
superheater and verify the prediction accuracy. This simulation indicated the circumferential metal
temperature distribution in the tube cross-section. The highest metal temperature among themeasurement points was at the point (a) in both the actual measurement and simulation results, andthe simulation was able to predict with an error of +14.2°C relative to the actual measurement. The
order of high and low metal temperatures among the measurement points was also consistent
between the actual measurement and simulation results, confirming the effectiveness of thistechnology. We will apply this technology to further improve the reliability of boilers.

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Figure 11

3.2

High-precision prediction of heat transfer tube metal temperature

Coal-ammonia co-firing simulation technology

In recent years, there has been a demand for reducing carbon emissions by coal-ammonia
co-firing, and evaluation technology for NOx emissions in the co-firing process is needed. We
conducted combustion tests assuming ammonia co-firing in actual equipment using the two-stage
drop tube furnace (DTF)(7) shown in Figure 12, which consists of an upper stage for reductive
combustion of fuel in an air deficient environment and a lower stage for complete combustion withadditional air input and can simulate two-stage combustion behaviors in actual equipment, to obtain
combustion characteristics data. We developed a NOx model for coal-ammonia co-firing by
incorporating ammonia oxidation, pyrolysis, NOx formation, and reduction models into an NOx
model for coal firing(8), and verified the model in the two-stage DTF. Figure 12 shows theverification result. It was indicated that the model can accurately predict NOx characteristics evenin ammonia co-firing and single-fuel firing.

Figure 12 Acquisition of NOx characteristics data and verification of prediction model
for coal-ammonia co-firing

Next, this paragraph is introducing an example of the verification of this simulation
technology through a small burner ammonia co-firing test (Figure 13). In the test, coal andammonia were co-fired in a dedicated burner for each. The simulation showed that a large amountMitsubishi Heavy Industries Technical Review Vol. 60 No. 3 (September 2023)
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of NOx was generated near the ammonia burner, but that the reduction progressed thereafter andthe NOx generated at the burner section almost disappeared by the time additional air was added.
As indicated by the comparison of predicted NOx values at the furnace outlet, the simulation
accurately reproduced the NOx behavior in the test. The amount of air to the burner was changed
between the ammonia co-firing conditions (1) and (2). The NOx level in the condition (1) was
suppressed to the same level as that in coal single-fuel firing, and the test and simulation results
were in agreement. It is considered that optimization of air volume is important to suppress NOxlevels in co-firing. In the future, we will obtain data from large-scale burner combustion tests,
further improve the model and the accuracy, and apply this technology to performance evaluation
for modification of actual equipment.

Figure 13

Flame state of coal-ammonia co-firing and accuracy verification|4. Conclusion
This report presented an overview of high-precision combustion simulation technology that
supports the energy transition in thermal power plants, as well as its validation and application
examples. For gas turbines, the simulation technology verified for natural gas-fired combustors was
further improved considering the different characteristics of carbon-free fuels, and reproduced theflame positions and NOx emissions of hydrogen or ammonia combustion with a high accuracy. For
boilers, the simulation technology verified with actual coal-fired boilers was further extended basedon the basic data of ammonia combustion, and reproduced NOx emissions of ammonia co-firingand single-fuel firing with a high accuracy. Going forward, we will utilize these technologies in thedevelopment of combustion equipment, thereby contributing to the acceleration of energy
transition.
Acknowledgment: Some of the experimental data presented in this report were obtained as aresult of a project commissioned and supported by the New Energy and Industrial Technology
Development Organization (NEDO) (JPNP16002, JPNP14026). We would like to express our
gratitude to NEDO.

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