Chemical Engineering Journal 434 (2022) 134680
Contents lists available at ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
NOx emission reduction in ammonia-powered micro-combustors by
partially inserting porous medium under fuel-rich condition
Siliang Ni, Dan Zhao *
Department of Mechanical Engineering, College of Engineering, University of Canterbury, Private Bag 4800, Christchurch 8041, New Zealand
A R T I C L E I N F O
A B S T R A C T
Keywords:
Porous medium
Ammonia
NOx emission
Porosity
Chemical reaction
Micro-combustor
Based on the fact that NOx generation will be inhibited under the condition of high equivalence ratio, the microcombustors with double-rib structure partially inserted with porous medium (PM) are selected to examine the
NOx emission performances in fuel-rich combustion of ammonia. The effects of: 1) the inlet velocity, 2) the
porous material and 3) the porosity are evaluated on NOx emission reduction. Besides, the kinetic rate of key
reactions contributing NOx production are analyzed. It is found that, the PM can effectively reduce NO gener­
ation and emission under a larger inlet velocity. With the increased thermal conductivity of porous material, the
emission of NO is shown to be decreased by 99.98% from 2.70 × 104 ppm to 5.17 ppm, while N2O emission is
increased by 10000% from 24.8 ppm to 2.52 × 103 ppm. In addition, the porous material with a higher thermal
conductivity is beneficial to improving the uniformity of the combustor outer wall temperature. When the PM’s
porosity is increased from 0.5 to 0.9, NO emission is increased by 4120% from 3.53 × 102 ppm to 1.49 × 104
ppm, while N2O emission is decreased by 71.5% from 3.29 × 103 ppm to 9.38 × 102 ppm. The present work
reveals that the NOx emission can be effectively reduced, while increasing the thermal performance of microcombustors by adopting appropriate porous medium.
1. Introduction
Due to the heavy use of fossil fuels, the energy crisis and environ­
mental hazard are increasingly aggravated. Studies have shown that the
production of coal, oil and gas would need to be reduced by 6% to keep
the incensement of global temperature within 1.5 ℃ per year [1].
Therefore, it is imperative to seek alternative fuels to control carbon
dioxide emissions. One of the most environmentally friendly choices is
hydrogen. However, it is easy to cause explosions in the process of
transportation or use due to its extremely strong flammability and
volatility [2,3]. Another promising fuel is ammonia. As a renewable
energy, it has rich and extensive resources [4]. Besides, the production
cost is relatively low and the storage is safer [5]. Secondly, because of
the high hydrogen content (about 17.7 wt%) in ammonia, the power
released by the combustion is very considerable [6]. However, the
generation of NOx, such as NO, can lead to a series of health problems
including photochemical pollution and respiratory disease [7,8]. The
other major product is N2O, of which the global warming potential is
265–298 times that of CO2 [9]. Therefore, the emission of NOx must be
taken into account when evaluating the efficiency of ammonia micro-
combustion.
For the past few years, people have been able to produce a wide
variety of micro-electronic and micro-mechanical devices, such as
micro-satellite, micro-aircraft and micro-motors due to the burgeoning
advancement of machinery manufacturing [10–12]. Hence, there is an
increasing demand for energy supply equipment that have the advan­
tages of small size, high energy density and long duration [13,14]. As a
typical micro power supply system, MTPV system has been applied in
various fields [15,16]. As the core component, the energy conversion
efficiency of the micro-combustor will directly affect the total efficiency
of the system [17]. However, there are still many challenges to be
overcome before ammonia can be put into practical use in micro scale.
Because of the large ratio of surface area to volume, there is serious
heat loss for micro combustion, which leads to some unwanted char­
acteristics such as narrow blow-out limit [18,19]. Because the flame
propagation speed and chemical activity of ammonia are relatively low
comparing to other hydrocarbon fuels, the combustion condition for
ammonia is demanding [20]. In order to improve the flame stability
while prevent quenching and oscillation, many feasible methods are
proposed [21,22]. First, diluting ammonia with highly chemically active
fuels such as hydrogen and methane can effectively increase the
* Corresponding author.
E-mail address: dan.zhao@canterbury.ac.nz (D. Zhao).
https://doi.org/10.1016/j.cej.2022.134680
Received 27 October 2021; Received in revised form 20 December 2021; Accepted 10 January 2022
Available online 13 January 2022
1385-8947/© 2022 Elsevier B.V. All rights reserved.
S. Ni and D. Zhao
Chemical Engineering Journal 434 (2022) 134680
Nomenclature
→
v
N
Tg
Yi
kg
T0
Cg
Pd
kmix
Ts
kw
Ai
hi
DdII
ks
ks,eff
l0
Tw
Tw
Ri
Vi
Wi
∇T
h0
Qloss
Di
Ti
P
hv
velocity (m/s)
total number of grids
temperature in fluid (K)
mass fraction for ith species
thermal conductivity of gas (W/(m⋅K))
ambient temperature (300 K)
specific heat of gas (J/(kg⋅K))
pressure drop (Pa)
thermal conductivity in porous region (W/(m⋅K))
temperature in porous region (K)
thermal conductivity of combustor surface (W/(m⋅K))
surface area of ith grid (m2)
molar enthalpy of ith species (kJ/mol)
thermal diffusivity due to dispersion (m2/s)
thermal conductivity in porous region (W/(m⋅K))
effective thermal conductivity porous region (W/(m⋅K))
the path length of light in porous medium (m)
mean surface temperature (K)
temperature of combustor surface (K)
specific gas constant (8.315 (J/(mol⋅K))
diffusion velocity of ith species (m/s)
molecular weight of ith species
temperature gradient (K/m)
natural convection (W/(m2⋅K))
ds
heat loss (W)
mass diffusion coefficient of ith species (m2/s)
surface temperature of ith grid (K)
pressure (Pa)
convective heat transfer coefficient of porous medium (W/
(m3⋅K))
fibre diameter (m)
Greek symbols
density in fluid domain (kg/m3)
porosity
φ
equivalence ratio
τ
viscous stress tensor (N)
ωi
molar rate of production of ith species [kmol/s]
)
(
σ
Stefan-Boltzmann constant (5.67 × 10− 8 W/ m2 ⋅K4 )
εs
emissivity of porous medium
εw
emissivity of outer surface
ρg
ε
Abbreviation
PM
porous medium
MWT
mean wall temperature
Dimensionless numbers
Nu
Nusselt number
Pr
Prandtl number
Re
Reynolds number
flammability [23]. Studies have shown that when methane is used as a
diluent, the flame can remain stable at a wide equivalence ratio of 0.79
≤ φ ≤ 1.24 [24]. In addition, the more methane is substituted by
hydrogen, the more often the flashback will occur. Secondly, a backflow
zone can be formed if the premixed/non-premixed inlet condition is
replaced by swirling flow which can highly promotes the mixing degree
of ammonia and air [25,26]. Furthermore, ammonia can be burned
steadily in air without any diluent for swirling flame [27]. Finally,
increasing the operating pressure can not only prevent the occurrence of
flashback, but also improve the flammability of swirling flame [28,29].
Low energy conversion efficiency caused by heat loss is another
problem that needs to be solved urgently. It has been proven that the
energy conversion efficiency can be effectively improved by using the
bluff body structure, inserting PM, coating the inner wall with catalytic
material, or changing the combustor shape [30–32]. One of the most
promising methods is to fill the fluid domain with PM. Due to the unique
loose pore channels, gases can be mixed intensively without blockage,
resulting in more adequate combustion [33,34]. Because metal is the
common option for porous materials, the high thermal conductivity and
emissivity are conducive to preheating the reactants, which promotes
the energy to be better transferred to the outer surface in forms of heat
and radiation [35]. According to our previous research, the emitter ef­
ficiency can be enhanced by 8.65% if PM is inserted for a T-shaped
micro-combustor [36,37].
A large number of experimental and numerical studies have been
conducted to control NOx emissions in combustion [38,39]. First, NOx
generation can be effectively reduced by changing the inlet boundary
conditions. It has been shown that NO generation can be effectively
suppressed by adjusting the equivalence ratio, for example, by using the
concepts of RQL and MILD [40]. Moreover, the adoption of SiC porous
medium can remarkably lower the NOx concentrations to a level of 35
ppm when equivalence ratio is higher than 1.3 [41]. Another approach
is to increase the operating pressure. Studies have shown that the NO
and NH3 emissions can be reduced from 700 PPM to 200 PPM when the
operating pressure increases from 0.1 MPa to 0.5 MPa for a gas turbine
like combustor [42]. Finally, a more commonly method in industry is to
use the catalytic reaction. Studies have shown that most exhaust NOx
can be converted to N2 and H2O by using Pt/TiO2/ZSM-5 and urea
catalysts [43].
So far, there are relatively few experimental studies on micro com­
bustion powered by ammonia. Due to the diverse transformation path­
ways of ammonia combustion, different oxides will be generated
according to the temperature of the fluid domain and OH radical,
leading to the change of energy conversion efficiency. Therefore, how to
reduce NOx emissions without sacrificing the efficiency is a problem
that needs comprehensive consideration. According to our previous
study, adopting the rib structure while reasonably adjusting the distance
between the rib and the inlet can reduce the NO emission by 47% when
φ = 1.1 [44]. Based on the previous finding, we select the optimal in­
ternal structure to test the effects of PM, porous material properties and
its porosity on NOx emission and thermal performances. Besides, the
production rates of NO and N2O under different porous materials are
analysed. The current work attempts to provide some ideas for opti­
mizing ammonia combustion in micro-scale with reduced emission of
NO and N2O.
2. Geometrical model and numerical method
2.1. Geometrical model
The combustion in PM is more stable with higher energy density,
which helps to improve the heat transfer. In order to further study the
impact of PM on the thermal performance and NOx emission in micro
combustion, the cylindrical micro-combustor partially inserted with PM
is selected. As shown in Fig. 1, the combustor chamber is divided into
two chambers with a ring-shaped rib. The first chamber is filled with PM
downstream by 2/3 of the volume, while the second chamber is fully
filled. The overall length of the combustor (L1) is 18 mm. The inlet
length (L2) and the length of the first chamber (L3) are 2 mm and 4 mm,
respectively. In addition, the rib length (L4) and the outlet length (L5) are
2
S. Ni and D. Zhao
Chemical Engineering Journal 434 (2022) 134680
Fig. 1. Structure diagram of micro-combustor with PM.
both 1 mm. In order to maximize the flame length and enhance heat
transfer at a constant inlet velocity, the inlet is designed to be stepped.
The diameter of the combustor (D1) and inlet (D2) are 4 mm and 2 mm
respectively. The inner diameter (D3) is 3 mm. The rib and outlet di­
ameters (D4) are both 1.4 mm. In this study, 5 porous materials are
selected to investigate the effect of physical properties of materials on
the combustion process and NOx generation. Physical properties are
summarized in Table 1. It is worth noting that all the material mentioned
are only used for numerical study. Some material with low thermal
conductivity may not be suitable for porous material due to the low
tolerance to thermal shock and low melting point.
T3
ks,eff = (1 − ε)ks + 16σεs l0 s
3
Energy equation for the solid wall:
Species conservation equation:
(
(
)
→)
∇ • ερg →
v Yi = − ∇ • ερg Yi Vi + εω̇i Wi
(3)
Energy equation for gas phase:
(
)
[ (
)
]
(
)
∇ • ερg →
v Cg Tg = ∇ • ε kg + ρg Cg DdII ∇Tg + hv Ts − Tg
(
)
N
N
∑
∑
→
+ε
hi ω̇i Wi − ∇ • ε
hi ρg Yi Vi
i=1
(4)
i=1
Energy equation for PM [47]:
(
)
(
)
∇ • ks,eff ∇Ts + hv Ts − Tg = 0
(7)
The total heat loss:
∑
∑ (
)
Qloss = h0
Ai (Ti − T0 ) + εw σ
Ai Ti4 − T04
(8)
(9)
The mathematical expression [47] for determining the key param­
eter hv (convective heat transfer coefficient of PM) in Eq. (5) is sum­
marized in Table 2. All the flow/combustion/heat transfer control
equations involved in the model are solved by Fluent 2020 R20. The
reaction mechanism includes 19 reactions and 63 different species. The
pressure–velocity coupling scheme and the implicit scheme are adopted.
Although the convergence rate of the implicit scheme is slower than that
of the discrete one, it has better computational performance for the
single-phase steady-state flow model [48]. The second-order upwind
scheme is used to discrete the space equations and the convergence
criteria for all residuals are less than 10-6. The density and specific heat
of mixed gas are calculated by ideal gas law and mixing law respectively
[13]. The thermal conductivity and viscosity of the mixture are evalu­
ated from a weighted average of the mass fraction of each species. Soret
effect is considered for its importance to the prediction of NO produc­
tion. However, the Dufour effect and the influence of pressure diffusion
are ignored as the model is small enough. [49]. According to previous
research, the production of NO2 is far less than that of NO and N2O [44].
Therefore, this study only focuses on the generation and transformation
of NO and N2O.
The temperature of inlet is fixed at 300 K with a uniform mass
fraction of species and a velocity profile ranging from 1.6 m/s to 2.4 m/
s. Pressure outlet (=0 gauge pressure) is adopted for the exit. Besides,
steel is chosen for the material of both inner wall and outer wall. In order
to simplify the process, the near-wall reaction and catalytic reaction are
not considered in this model. Both the side surface of the inlet and outlet
are set as adiabatic. The combustor inner wall is specified as coupled
thermal condition with no slip and 0 diffusion flux of any species. In
Knudsen number (Kn) is an important indicator to decide whether
the fluid can be regarded as continuous. Kn is first calculated under
different working conditions, and it is found that Kn is always less than
the critical value 0.001. Therefore, Navier-Stokes equations are still
applicable to current model [45]. Because the maximum Reynolds
number calculated for all conditions is much less than 500, the fluid
domain is treated as laminar flow [46].
The governing equations are summarized as follows:
Mass conservation equation:
(
)
∇ • ερg →
v =0
(1)
(2)
∇ • (kw ∇Tw ) = 0
Mean wall temperature:
∑
Ai Ti
Tw = ∑
Ai
2.2. Numerical methodologies
Momentum conservation equation:
(
)
∇ • ερg →
v→
v = − ε∇P + ∇ • (ετ) + Pd
(6)
(5)
The effective heat transfer coefficient of PM:
Table 2
Expression of convective heat transfer coefficient of PM.
Table 1
Porous materials and physical properties.
Porous material
M1
M2
M3
M4
M5
Density (kg/m3)
Specific heat (J/ kg⋅K)
Thermal conductivity (W/m⋅K)
8978
381
387.6
2719
871
202.4
8900
460.6
91.74
8000
503
12
2800
856
2.25
Property
Expression
hv
2(1 − ε)Nu • kg
d2s
]
[
4(1 − ε)
1
+ (1 − ε)1/2 Re3/5 Pr/3
Nu = 1 +
2
ε
Nu
3
hv =
S. Ni and D. Zhao
Chemical Engineering Journal 434 (2022) 134680
addition, a mixed thermal condition including heat conduction and ra­
diation is considered for the outer surface. In the following cases for
mesh independent study and model validation, the convective heat
transfer coefficient h0 and wall emissivity εw are set as 10 (W/(m2⋅K))
and 0.8 respectively according to the experiment [49] and our previous
work [44]. To avoid backflow, the mass fraction of oxygen is fixed to
23.2% at outlet.
[52] are selected for comparison. Both mechanisms are verified to be
effective in predicting NOx emission in micro combustion. In particular,
the mechanism from Okafor contains 42 species and 130 steps, which
provides a more comprehensive description of the NH3 kinetics. As
illustrated in Fig. 3(d), the laminar flame speeds under three different
reaction mechanisms show good agreement with a maximum relative
error of less than 1.6%. Therefore, current mechanisms is accurate
enough to simulate ammonia combustion in micro scale. The detailed
reaction mechanisms can be found in the supplementary file.
2.3. Mesh independence study and model validation
3. Effect of inlet velocity vin on NOx emission
The 3D models are established by structured grid using Hypermesh
14.0. A comparison is carried out under 3 different mesh densities:
coarse (847,000 cells), medium (2,169,000 cells) and fine (4,430,000
cells) to investigate the effects of mesh density on numerical results. The
temperature along the centreline of the combustors with or without PM
are examined under the vin of 2.0 m/s and φ of 1.1. As shown in Fig. 2,
refined meshes are adopted near the central axis and interface for both
the combustor and fluid domain to improve the computing accuracy.
As illustrated in Fig. 3 (a) and (b), the surface temperature of com­
bustors with/without PM in various mesh densities are compared. It can
be found that, the data shows good consistency between the inlet and the
first rib whether there is PM or not. Besides, the results of both medium
and fine mesh match well while the result of coarse mesh are obviously
different from the other two downstream. Hence, the model of medium
mesh is chosen in order to reduce computing cost.
The numerical and experimental results are compared in Fig. 3 (c).
Because of the absence of experimental data for ammonia combustion in
micro scale, the results of H2/Air micro combustion for the same model
is adopted for validation [50]. It can be seen that the changing trends of
both numerical and experimental results are the same. However, an
obvious discrepancy is observed near the region of inlet which can be
mainly attributed to 1) the experimental measurement errors, 2) the
modelled combustor is circular-shaped, while the experimental
combustor is rectangular one and 3) the assumptions that are made to
simplify the model. Because the maximum relative error is no more than
4%, the model is applicable and accurate enough for numerical
investigation.
Previous research [39] has shown that the detailed reaction mech­
anism GRI 3.0 tends to underestimate the quenching stretch rate of
ammonia flames due to the absence of some core ammonia oxidation
steps, such as the conversion of NH3 to NH2 and NO [39]. These re­
actions are critical for predicting NO production in highly concentrated
ammonia combustion. Therefore, Chemkin Pro R19.2 is used to verify
the reliability of the present reaction mechanism in micro combustor,
and the detailed setup is summarized in the supplementary file. In
addition the simplified mechanisms proposed by Okafor [51] and Drake
Although N2O and NO2 are both important contributors to NOx
emissions, their mole fraction is much lower than that of NO. Therefore,
controlling NO emissions is the main challenge to achieve low NOx in
NH3/O2 premixed flame. In fuel-rich combustion, NO production can be
greatly reduced, but unburned NH3 can also be increased. This phe­
nomenon indicates that there is a trade-off between reducing NO and
unburned NH3. Some studies have shown that when φ = 1.1, the total
emissions of NO and unburned NH3 are the minimum [39]. Therefore, in
this section, φ = 1.1 is selected as the main working condition to study
the emission of NO and NH3 in cylindrical micro-combustors with/
without porous medium under various vin . As shown in Fig. 4, the
changing trends of NO formation are opposite when with or without PM.
It can be found that when PM is absent, the production of NO increases
significantly with the increase of vin .
In contrast, when PM is inserted, the production of NO decreases
with the increase of vin . An interesting phenomenon is that when vin is
increased to 2.0 m/s, the region with large NO mole fraction in the
center of the cavity in front of the first rib moves to the side of the wall
when PM is inserted. In addition, on the central axis, the mole fraction of
NO starts to decrease rapidly downstream, and then increase slowly. The
main reason for this phenomenon is that the production of NO is closely
related to the density of O/H, HNO and NHi (i = 1 or 2), and the con­
centration of these radicals mainly depends on the position of the flame.
NO is mainly formed by the reaction of free radicals containing O/H and
HNO, or is converted into N2 or N2O by the reaction of free radicals
containing HNi. However, the position of flame center is changed due to
insertion of PM. Due to the large thermal conductivity of porous mate­
rial, the heat is transferred more efficiently from the flame to the outer
surface. As a result, the flame temperature drops rapidly when it touches
PM, resulting in the flame bifurcation and decrease of high temperature
volume. Because O/H radicals mainly concentrate in the center of the
flame while HNO and NHi radicals mainly concentrate in the flame front
[53], NO will be mostly generated in the high temperature region of the
flame front before the first rib when flame bifurcates. Therefore, the
Fig. 2. Mesh of medium density for solid and fluid domain.
4
S. Ni and D. Zhao
Chemical Engineering Journal 434 (2022) 134680
Fig. 3. Wall temperature of combustors (a) without PM and (b) with PM in various mesh densities; (c) wall temperature distribution of numerical and experimental
data; (d) laminar flame speed of micro combustion for various kinetic models under different φ.
Fig. 4. NO mole fraction distribution on y-z plane for micro-combustors without (left column) and with (right column) PM under: (a)vin = 1.6 m/s, (b)vin = 1.8 m/s,
(c)vin = 2.0 m/s, (d)vin = 2.2 m/s and (e)vin = 2.4 m/s when φ = 1.1.
mole fraction of NO in the downstream region along the central axis will
drop rapidly due to the blocking effect of the rib. This phenomenon
shows that under high vin , the production of NO can be effectively
inhibited by inserting PM. However, the thermal performance and the
total NOx emission under this working condition still need to be further
confirmed.
Previous studies show that the formation/destruction of N2O as an
intermediate is closely related to NO dynamics, and the two productions
are negatively correlated. In addition, N2O is known to be an oxidation
product of ammonia gas at relatively low temperatures. When the flame
temperature is high enough, any generated N2O will be decomposed
before the gas emits [54]. Therefore, how the emission of N2O changes is
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S. Ni and D. Zhao
Chemical Engineering Journal 434 (2022) 134680
worth paying attention to. In Fig. 5, NO and N2O emissions as well as
thermal performance of micro-combustors with or without PM are
analysed and compared under various vin . It can be seen from Fig. 5 (a)
that when vin = 1.6 m/s, whether PM is inserted or not has little influ­
ence on the MWT. When vin is increased to 1.8 m/s, the MWT of microcombustor without PM is 18.4 K higher than that with PM. However, the
effect of PM on improving MWT is more obvious with the increase of vin
when vin rises to 2.4 m/s, the MWT of micro-combustor with PM is 59.8
K higher than that without PM. Fig. 5 (c) compares the temperature
distribution of the outer surface when vin = 2.0 m/s. It can be seen that in
the upstream region, the wall temperature for combustors with PM is
significantly higher. However, the wall temperature drops more rapidly
than that without PM in the downstream region. The reason for this
phenomenon is that the flame bifurcates to both sides of the front end of
the first rib in the presence of PM, resulting in high temperature for outer
surface near the inlet. Although PM material has a higher heat transfer
coefficient, it prevents the wall temperature from decreasing too
quickly. However, as the high temperature zone of the flame shifted
towards the inlet, PM cannot prevent the dropping trend effectively.
In Fig. 5 (b) and (c), the emissions of NO and N2O under different
working conditions are further observed. It can be found that NO
emission is negatively correlated with that of N2O. For the combustors
without PM, NO emission remains at a high level and rises slightly with
the increase of vin . The mole fraction of N2O at the outlet is always lower
than 5.82 × 10-3. However, NO emission decreases rapidly with the
increase of vin for the combustors with PM. When vin increases from 1.6
m/s to 2.4 m/s, the mole fraction of NO at the outlet decreases from
2.54 × 10-2 to 4.67 × 10-3. In comparison, the mole fraction of N2O at
the outlet increases from 1.13 × 10-4 to 1.32 × 10-3. Besides, the utili­
zation efficiency of NH3 is calculated under different working
conditions. It is found that the efficiency is greater than 99.7% under all
working conditions. The main reason for this difference is that the ex­
istence of PM changes the position of flame centre and the volume of
high temperature region, which leads to the variation of the radical
concentration and distribution. These results indicate that the addition
of PM can affect the contents of various nitrogen oxides in NOx, thus
controlling the total NOx emissions.
4. Effect of porous materials with different thermal
conductivities
Considering the effects of porous material on heat transfer, the dis­
tribution of NO, N2O and temperature in the y-z plane of combustors
with PM using various materials under the same working condition are
studied. The thermal conductivity of the PM decreases from (a)-(e). By
comparing the left column of Fig. 6, it can be found that the production
of NO gradually increases from (a)-(e). In Fig. 6 (a)-(c), NO concentra­
tion shows obvious bifurcation phenomenon at the front end of the first
rib. As can be seen from the middle column in Fig. 6, the generation of
N2O is opposite to that of NO. It can be seen from Fig. 6 (a)-(c) that the
area with the highest concentration of N2O is on the central axis in front
of the first rib where the concentration of NO is the lowest. This
conclusion verifies the negative correlation between NO and N2O.
Further observing the right column of Fig. 6, it can be seen that the
volume of high temperature area (above 1600 K) increases from (a)-(e),
but the wall temperature decreases along the downstream direction,
indicating that the ability of PM to prevent the wall temperature from
dropping too fast is decreasing. This is due to the fact that the thermal
conductivity of porous medium will directly affect the flame tempera­
ture and the heat conduction of outer surface. When the thermal
Fig. 5. (a) Mean wall temperature, (b) NO mole fraction, (d) N2O mole fraction under various vin with/without PM; (c) outer wall temperature distribution with/
without PM whenvin = 2.0 and φ = 1.1.
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S. Ni and D. Zhao
Chemical Engineering Journal 434 (2022) 134680
Fig. 6. Contour of NO mole fraction (left column), N2O mole fraction (middle column), temperature contour of gas phase and solid wall (right column) on y-z plane
for micro-combustors with various porous material (M1-M5) for (a)-(e) whenvin = 2.0 m/s and φ = 1.1.
conductivity of the material is large, the heat released by combustion
will be quickly transferred downstream to the wall. Although the tem­
perature of the flame center drops, it is beneficial to maintain high wall
temperature. By contrast, the heat released by combustion cannot be
effectively transferred to the outer surface when the thermal conduc­
tivity of the material is low. As a result, the volume of the high tem­
perature zone of the flame is larger, and the wall temperature decreases
faster downstream.
In Fig. 7, the thermal performance, NO and N2O emission of com­
bustors with different porous materials under the same working condi­
tions are further analyzed. As can be seen from Fig. 7 (a) and (d), the
MWT decreases and then rises slightly with the decrease of thermal
conductivity of materials. The MWT for M1 is 33.56 K higher than that
for M4. In addition, when the thermal conductivity of the material is
higher, the wall temperature near the entrance is significantly lower. But
at the same time, the wall temperature decreases more slowly down­
stream. The results indicate that the use of PM with high thermal con­
ductivity is beneficial to increasing the MWT and wall temperature
uniformity. It can be seen from Fig. 7 (b) and (d) that the emission of NO
increases with the decrease of thermal conductivity. When the porous
material changes from M5 to M1, the mole fraction of NO at the outlet is
observed a drop of 99.98% from 2.70 × 10-2 to 5.17 × 10-6. At the same
time, the mole fraction of N2O at the outlet is observed an increases of
more than 100 times from 2.48 × 10-5 to 2.52 × 10-3.
Fig. 7. (a) Mean wall temperature, (b) NO mole fraction, (c) outer wall temperature distribution and (d) N2O mole fraction for micro-combustors with different
porous material when vin = 2.0 m/s and φ = 1.1.
7
S. Ni and D. Zhao
Chemical Engineering Journal 434 (2022) 134680
NOx conversion and emission are related to flame temperature, local
radical concentration and other factors. For better understanding of the
chemical reaction in ammonia combustion process and the various
products related to NOx, Chemkin Pro R19.2 is used to analyze the re­
action pathways for the generation of NO and N2O under current
mechanism. As shown in Fig. 8, It can be found that NO is mainly
transformed from HNO, N and NH particles, among which HNO takes
the largest proportion [55]. However, N2O is mainly transformed by the
reaction containing NH and NO radicals, which plays a positive role for
the consumption of NO
main reaction.
The pathway
NH2 ⇒+NO NNH ⇒+O2 N2 contributes the most of NO consumption [56].
However, the consumption of N2O mainly depends on the concentration
of H radicals. Further observation shows that NHi (i = 1 or 2) are
involved in both NO production and NO consumption. Therefore, it is
necessary to promote the transformation of NHi in the direction of N2
generation for the control of NO emission.
In Fig. 9, we investigate the kinetic rate of key reactions contributing
NO2 and NO when two porous materials with significantly different
thermal conductivity are adopted. For NO, the top 5 generation and
consumption reactions are analyzed. When the materials are M5 and M1,
the total production rates of NO are 0.234 kgmol/m3⋅s and 0.214 kgmol/
m3⋅s, respectively. It can be seen that the porous material has a signifi­
cant effect on the net production of NO. As can be seen from Fig. 9 (a),
the reaction HNO + H<=>H2 + NO contributes the most to the NO
production, accounting for 38.8% and 35.1% when materials are M5 and
M1. The reaction HNO + M<=>H + NO + M contributes 16.3% and
20.4%, respectively. The other major reaction pathways mainly contain
NH and N radicals. The reaction that contributes most to NO con­
sumption is NH2 + NO<=>NNH + OH, accounting for 35.5% and
35.1% when materials are M5 and M1. Besides, the reaction N +
NO<=>N2 + O contributes 22.1% and 18.4%, respectively. It should be
noted that NHi (i = 1 or 2) are involved in the other reactions that ac­
count for the highest proportion of NO consumption. Although NHi (i =
1 or 2) are also involved in NO production, they contribute more to the
overall consumption of NO. Besides, the total production rates of N2O
are 0.0682 kgmol/m3⋅s and 0.0705 kgmol/m3⋅s respectively. As can be
seen from Fig. 9 (b), N2O is mainly formed through the reaction NH +
NO<=>N2O + H. In addition, N2O is mainly consumed through the
reaction N2O + H<=>N2 + OH, accounting for 59.6% and 64.3%
respectively when the materials are M5 and M1. Further observation
shows that H radical plays a key role in both the production and con­
sumption of N2O.
5. Effect of PM’s porosity
The porosity of porous medium is closely related to the position of
the flame center and the volume of the high temperature, which affects
the NOx generation. As shown in Fig. 10, NO and N2O emissions for
combustors with PM of different ε under the same working conditions
are investigated. It can be inferred from the left column of Fig. 10 that
NO production increases significantly with the increase of ε. Similar to
the phenomenon observed in the previous section, the region with the
largest concentration of NO is in the front of the first rib near the wall.
The concentration of NO decreases first and then increases along the
central axis. Besides, the concentration of NO increases significantly
downstream when ε increases to 0.9. It can be seen from the right col­
umn of Fig. 10 that the production of N2O decreases with the increase of
ε. The N2O concentration volume decreases obviously when ε increases
from 0.5 to 0.9. Especially, the N2O concentration area is in front of the
first rib along the centerline when the ε increases from 0.8 to 0.9. The
main reason for this phenomenon is that the PM has different blocking
effect on flame due to the change of ε. The blocking effect of PM on the
flame decreases with the increase of ε, resulting in the expansion of the
high temperature volume, thus changing the distribution of H, O, OH
and NHi radicals.
The emission of NO and N2O and the changing trend of outer wall
temperature under different ε are further analyzed in Fig. 11. As can be
seen from Fig. 11 (a) that the mole fraction of NO at the outlet increases
significantly with the increase of ε. The mole fraction of NO increases by
more than 41 times from 3.53 × 10-4 to 1.49 × 10-2 when ε changes from
0.5 to 0.9. However, N2O decreases slightly with the increase of ε, from
3.29 × 10-3 to 9.38 × 10-4. This indicates that the change of ε has a more
prominent impact on NO production. It can be inferred from Fig. 11 (b)
that the peak value of wall temperature increases with the increase of ε.
However, it decreases the fastest near the outlet when ε is 0.9. This is due
to the fact that a smaller ε is beneficial to obtaining a more uniform wall
temperature and preventing temperature from decreasing too fast.
6. Discussion and conclusions
In this study, the double-rib micro-combustors partially inserted with
porous medium are numerically investigated. For this, 3D numerical
model is developed to simulate premixed ammonia-oxygen combustion
and NOx emission from the micro-combustor under fuel-rich condition.
The effects of 1) the inlet velocity vin ; 2) porous materials and 3) the
porosity on NOx emission and thermal performance are evaluated one at
a time. The kinetic rate of reactions related to NO and N2O production is
also analyzed. The key findings are summarized below.
• For a large inlet velocity vin , PM can effectively inhibit NO generation
and emission. When vin is increased from 1.6 m/s to 2.4 m/s, NO
emission is decreased by 81.6% from 2.54 × 104 ppm to 4.67 × 103
ppm, while N2O emission is increased by 10.6 times from 1.13 × 102
ppm to 1.32 × 103 ppm.
• With the increased thermal conductivity of porous materials, the
MWT drops slightly and then rises obviously. Besides, the emission of
NO is decreased by 99.98% from 2.70 × 104 ppm to 5.17 ppm, while
the emission of N2O is increased by 100 times from 24.8 ppm to 2.52
× 103 ppm.
• The porous material with higher thermal conductivity is beneficial to
improving the uniformity of the micro-combustor outer wall
temperature.
Fig. 8. Reaction pathways of ammonia combustion.
8
S. Ni and D. Zhao
Chemical Engineering Journal 434 (2022) 134680
Fig. 9. Kinetic rate of key reactions contributing NO and N2O for NH3/O2 micro combustion with different porous material whenvin = 2.0 m/s and φ = 1.1.
Fig. 10. Contour of NO mole fraction (left column) and N2O mole fraction (right column) on y-z plane for micro-combustors with PM made from material M3 with ε :
(a) 0.5, (b) 0.6, (c) 0.7, (d) 0.8 and (e) 0.9 when vin = 2.0 m/s and φ = 1.1.
• The reaction HNO + H<=>H2 + NO contributes the most to NO
production, and NHi (i = 1 or 2) contributes more to NO consumption
than to NO production.
• When the porosity is increased from 0.5 to 0.9, NO emission is
increased by 41.2 times from 3.53 × 102 ppm to 1.49 × 104 ppm,
while N2O emission is decreased by 71.5% from 3.29 × 103 ppm to
9.38 × 102 ppm.
The present work has shown that porous medium (PM) can not only
help maintain a higher and more uniform combustor outer wall tem­
perature but also effectively mitigate NOx emission for ammonia-fuelled
combustion in micro-combustors. Although PM is not commonly adop­
ted for ammonia combustion, the optimization method is valuable for
further controlling NOx production and the feasibility studies on
burning ammonia as an alternative fuel in the future.
Fig. 11. (a) NO mole fraction and N2O mole fraction; (b) outer wall tempera­
ture distribution for micro-combustors with PM of various ε whenvin = 2.0 m/s
and φ = 1.1.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
9
S. Ni and D. Zhao
Chemical Engineering Journal 434 (2022) 134680
the work reported in this paper.
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Acknowledgements
We gratefully acknowledge the financial support provided by the
University of Canterbury, New Zealand (grant no. 452DISDZ) and Numi
Ltd (grant no. 2003002). Siliang Ni would like to thank to College of
Engineering, University of Canterbury for providing his PhD
studentship.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.cej.2022.134680.
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