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Article
Effect of N2/CO2 Dilution Ratios on Explosion Characteristics of
Hydrogen/Propane Mixtures
Chengxu You, Songping Yang, and Chengcai Wei*
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ABSTRACT: With the advancement of hydrogen energy, hydrogen-blended fuels have gained widespread application in industrial
and energy sectors, drawing significant attention to the explosion
characteristics and safety risks associated with hydrogen/propane
(H2/C3H8) gas mixtures. To effectively mitigate these explosion
risks, this study investigates the inerting effects of various nitrogen
(N2) and carbon dioxide (CO2) dilution ratios on H2/C3H8 gas
mixtures. The CHEMKIN-Pro software was employed to simulate
the explosion and inerting properties of these mixtures, analyzing
parameters such as adiabatic explosion pressure, flame temperature,
concentrations of key radicals, heat release rate, and sensitivity of
elementary reactions. The results indicate that an increase in the
CO2 dilution ratio corresponds to a linear decrease in both the
adiabatic explosion pressure and the flame temperature. Furthermore, a higher CO2 dilution ratio leads to a decline in the heat
release rate and the generation rates of H, O, and OH radicals, with the generation rate of H radicals experiencing the most notable
reduction. Sensitivity analysis of elementary reactions reveals that reaction R1: H + O2 = O + OH has the most significant promoting
effect, while R410: C3H8 + H = H2 + iC3H7 exhibits a pronounced inhibitory effect. CO2 effectively suppresses and transforms key
intermediates through specific reaction pathways (such as R52: CH + CO2 = HCO + CO and R79: CH2 + CO2 = CH2O + CO),
thus reducing the overall heat release rate of the reactions. This study offers important theoretical insights into the inhibitory role of
inert gases in H2/C3H8 gas mixtures, providing a foundation for safety management and the advancement of clean energy
technologies.
1. INTRODUCTION
In the context of dual carbon goals and the continuous growth
of global energy demand, hydrogen energy has emerged as a
clean energy source receiving increasing attention, particularly
in industrial and energy sectors.1−3 However, because of its
low ignition energy and broad explosive range, hydrogen gas is
susceptible to spontaneous ignition or explosion.4−6 Hydrogen-blended fuels are an effective approach to ensuring the safe
application of hydrogen energy.7,8 Propane, a primary
component of liquefied petroleum gas,9 enhances the
combustion performance of hydrogen/propane (H2/C3H8)
gas mixtures.10−12 Nevertheless, the explosion characteristics
and safety risks associated with these mixtures have garnered
considerable scrutiny. H2/C3H8 gas mixtures can easily create
explosive environments under specific conditions,13 especially
in confined spaces where the risk of explosion significantly
increases. The high diffusivity and low ignition energy of
hydrogen,5 combined with the high energy density of propane,
present substantial safety hazards during the storage and
transportation of these gas mixtures.14,15
Therefore, studying the explosion characteristics and
inerting methods for preventing explosions in H2/C3H8 gas
© 2025 The Authors. Published by
American Chemical Society
mixtures is essential. Research indicates that the explosive
behavior of H2/C3H8 gas mixtures is influenced by several
factors, including gas concentration, temperature, and
pressure.16−18 To effectively mitigate the explosion risk,
inerting techniques serve as a valuable inhibition method.
Nitrogen (N2) and carbon dioxide (CO2), commonly used
diluents, can significantly diminish the intensity of explosive
hazards or completely suppress explosions by lowering oxygen
concentration and absorbing heat.19−22
Current research on the inerting effects of N2 and CO2 on
common flammable gases has yielded significant findings,23−26
particularly regarding natural gas (CH4), hydrogen (H2),
hydrogen−natural gas mixtures (H2NG), syngas (H2/CO),
and liquefied petroleum gas (LPG). Hu et al.27 studied how
Received: October 2, 2024
Revised: November 4, 2024
Accepted: January 24, 2025
Published: February 17, 2025
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CO2, N2, Ar, and He influences the laminar burning velocity of
methane−air mixtures, finding that the inhibiting effect of
these gases ranked as CO2 being the strongest, followed by N2,
Ar, and He. Zhong et al.28 investigated the flame instability of
CH4/O2/CO2 and CH4/O2/N2 mixtures in a closed pipe,
discovering that N2 dilution made oxygen-rich methane more
susceptible to Rayleigh−Taylor instability compared to CO2,
leading to distorted tulip flames. Wang et al.29 evaluated how
N2 and CO2 impacted the explosion pressure and deflagration
time parameters associated with hydrogen, calculating thermal
losses and thermodynamic state parameters during combustion. Their results indicated that CO2 had a more pronounced
impact on thermal diffusion rates. Liu et al.30 conducted
experimental research on how N2 and CO2 affected the
deflagration properties of H2NG within pipes. They
discovered that both explosion pressure and shock wave
velocity decrease exponentially as N2 and CO2 concentrations
increase, with CO2 showing a stronger suppression effect on
the deflagration of H2NG. Chen et al.31 investigated the
laminar combustion behavior of mixtures containing CH4, H2,
CO2, and N2, concluding that CO2’s inhibition of laminar
burning velocity was primarily achieved through dilution,
thermal effects, and chemical interactions. Shang et al.32
analyzed how N2 and CO2 dilution impacted the laminar flame
speed of H2/CO/air mixtures, demonstrating that CO2 exerted
a more significant influence through its thermodynamic and
chemical kinetic properties in lowering flame temperature and
radical concentrations compared to N2. Burbano et al.33
conducted experimental measurements on the laminar burning
velocity of H2/CO/air mixtures that were diluted with N2 and
CO2, indicating that increased dilution of both gases reduced
heat release and increased heat capacity, thereby decreasing
laminar burning velocity. Nair et al.34 investigated how CO2
and N2 dilution affected the laminar burning velocity of LPG−
air mixtures at elevated temperatures, finding that CO2 dilution
had a more pronounced effect on temperature compared to N2
dilution.
Despite numerous studies examining the individual inerting
effects of N2 and CO2 on flammable gases, a significant gap
remains in the systematic investigation of how different N2/
CO2 dilution ratios affect the explosion characteristics of H2/
C3H8 gas mixtures. In practice, CO2/N2 mixed gases are
significant components of industrial exhaust produced from
fossil fuel combustion,35 such as blast furnace gas, which
contains substantial amounts of CO2 and N2. As an effective
gas explosion inhibitor, N2/CO2 mixtures can significantly
reduce the costs associated with inert gases compared to using
pure CO2 or N2, while simultaneously achieving the dual goals
of environmental protection and lowering corporate safety
costs, thus facilitating the effective utilization of mixed gases.
Moreover, H2/C3H8 gas mixtures are crucial in the fields of
clean energy and chemical production, yet research on these
mixtures remains relatively limited. Therefore, studying the
inhibitory effects and mechanisms of different N2/CO2
dilution ratios on the explosion characteristics of H2/C3H8
gas mixtures is of great significance.
This study investigates the effects of varying N2/CO2
dilution ratios on the explosion characteristics of H2/C3H8
gas mixtures. By delving into the chemical reaction kinetics, we
aim to analyze the inhibitory effects on these characteristics
and uncover the microscopic mechanisms of inerting
suppression provided by N2/CO2 dilution gases. This research
Article
offers a theoretical foundation for optimizing safety design and
enhancing the safe utilization of H2/C3H8 gas mixtures.
2. CHEMICAL KINETICS SIMULATION AND RESULT
VERIFICATION
In this research, we used the CHEMKIN-Pro software to
simulate the explosion dynamics and inerting effects of H2/
C3H8 gas mixtures.36 This approach enabled the acquisition of
both macroscopic and microscopic deflagration characteristics
of H2/C3H8 under varying concentrations of inert gases and
different N2/CO2 dilution ratios. The simulated results are
adiabatic explosion pressures and flame temperatures (from 0D computations), concentrations of key radicals, heat release
rate, and sensitivity of elementary reactions (from 1-D
computations).
The chemical reaction mechanism utilized in this study is
the USC-Mech II, comprising 111 chemical species and 784
elementary reactions. This comprehensive mechanism is
specifically tailored to accurately model the chemical reactions
of C0−C4 hydrocarbon fuels, including their complex reaction
pathways and intermediates. The USC-Mech II mechanism has
undergone extensive validation against a broad and reliable
database of fundamental combustion experiments, ensuring its
accuracy and reliability in predicting combustion behavior.
This validation process has involved comparisons with
experimental data from a variety of sources, including shock
tube studies, laminar flame speed measurements, and ignition
delay times, among others.37 The robustness and precision of
the USC-Mech II mechanism make it an ideal choice for
simulating the combustion and deflagration characteristics of
the H2/C3H8 gas mixture in this study.
The simulations were performed under standard ambient
conditions, characterized by an initial temperature of 298 K
and an initial pressure of 1 atm. To initiate the reactions, an
ignition temperature of 1300 K was applied.38,39 The reaction
duration was set to 200 ms to ensure that the system reached a
steady state, allowing for a thorough analysis of the reaction
kinetics and the effects of the inert gases and diluents on the
explosion process. The equilibrium solver calculates chemical
equilibrium states and the homogeneous reactor models with
the Newton solver to address reaction kinetics. The number of
adaptive grid points was set to 100, with a solution gradient of
0.1, a solution curvature of 0.5, and a sensitivity relative
tolerance of 0.00001. The maximum ratios of adjacent cell sizes
were set between 1.1 and 1.3, and the maximum number of
iterations was set to 10.
To explore the effects of inerting on the explosion
characteristics, a series of inert gas concentrations were
systematically investigated, ranging from 5% to 30% in
increments of 5%. Additionally, the impact of different N2 to
CO2 dilution ratios was examined, encompassing a spectrum
from pure CO2 (0:10) to pure N2 (10:0), with intermediate
ratios of 1:9, 3:7, 5:5, 7:3, and 9:1. The specific simulation
cases are shown in Table 1. This comprehensive approach
allowed for a detailed analysis of how varying inert gas
concentrations and N2/CO2 dilution ratios affect the explosion
behavior of the H2/C3H8 mixtures.
To validate the reliability of the simulation results, we
conducted a detailed comparison with experimental data
obtained by Liu et al.16 using a 20 L spherical test apparatus.
The experimental setup did not include inerting and had a
hydrogen content of 40%. As illustrated in Figure 1, the
simulation results show a high degree of consistency with the
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Table 1. Simulation Cases
equivalent ratio of
H2/C3H8 mixture
gas
H2
volume
fraction
(%)
C3H8
volume
fraction
(%)
1.0
50
50
inert gas
concentrations
(%)
N2 to CO2
dilution
ratios
5
10
15
20
25
30
0:10
1:9
3:7
5:5
7:3
9:1
10:0
Figure 2. Trends in adiabatic explosion pressure as a function of inert
gas concentration and N2/CO2 dilution ratios.
instance, at a N2/CO2 dilution ratio of 5:5, the adiabatic
explosion pressure decreases from 9.16 bar at 0% inert gas
concentration to 6.52 bar at 30% inert gas concentration, a
reduction of 2.64 bar, or 28.8%. At a fixed inert gas
concentration of 30%, the adiabatic explosion pressure also
significantly decreases as the N2/CO2 dilution ratio shifts from
10:0 to 0:10. Specifically, the pressure drops from 7.13 bar at a
10:0 dilution ratio to 6.07 bar at a 0:10 dilution ratio, a
reduction of 1.06 bar, or 14.9%. In addition, through data
analysis, it is found that there is a linear relationship between
the increase in adiabatic explosion pressure and the CO2
dilution ratio when the volume fraction of inert gas is constant,
as shown in Figure 3. These results underscore the important
Figure 1. Comparison between chemkin simulation results and
experimental test results.
experimental data, particularly in capturing the trend of the
maximum explosion pressure as the equivalence ratio varies.
Both the simulation and experimental data indicate that the
maximum explosion pressure initially increases with the
equivalence ratio, reaches a peak, and then decreases.
However, it is observed that the simulation results are slightly
higher than the experimental values. This discrepancy can be
attributed to the ideal adiabatic conditions assumed in the
simulations, which do not account for heat loss that occurs in
real-world experimental setups. The slight overestimation in
the simulation results is expected due to the absence of heat
transfer and other nonideal factors that are present in the
experimental environment but not in the idealized simulation
model. Despite this minor difference, the overall agreement
between the simulation and experimental results provides
strong evidence for the accuracy and reliability of the
simulation approach, validating its use in predicting the
deflagration characteristics of H2/C3H8 gas mixtures under
various conditions.
Figure 3. Relationship between the increase in adiabatic explosion
pressure and the CO2 dilution ratio.
3. RESULTS AND DISCUSSION
3.1. Adiabatic Explosion Pressure and Flame Temperature. Adiabatic explosion pressure and flame temperature are
critical parameters that indicate the pressure and temperature
achieved under ideal conditions when a fuel reacts completely
with oxygen, reflecting the intensity of a gas explosion.40 Figure
2 illustrates the trends in adiabatic explosion pressure as a
function of inert gas concentration and N2/CO2 dilution ratios.
The results reveal a gradual decrease in adiabatic explosion
pressure with increasing concentrations of inert gases. For
role of inert gases in reducing explosion pressure, highlighting
their potential value in mitigating gas explosion risks in
practical applications.
Figure 4 illustrates the trends in adiabatic flame temperature
as a function of inert gas concentration and N2/CO2 dilution
ratios. The results indicate that the adiabatic flame temperature
decreases as both the concentration of inert gases and the CO2
dilution ratio increase. The primary reason for this reduction in
flame temperature is that the addition of inert gases dilutes the
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3.2. Key Radical Equilibrium Mole Fractions. In the
oxidation reactions of hydrocarbon fuels, H, O, and OH
radicals serve as the primary active intermediates.42−44 Figures
6−8 illustrate the trends in the equilibrium mole fractions of
H, O, and OH radicals as functions of inert gas concentration
and N2/CO2 dilution ratios.
Figure 4. Trends in adiabatic flame temperature as a function of inert
gas concentration and N2/CO2 dilution ratios.
concentrations of the flammable gas and oxygen, thereby
reducing the collision frequency between reactants.41 This
results in a slower combustion reaction rate and decreased
energy release.20 In particular, at high concentrations of inert
gases, the combustion reaction rate is significantly diminished,
leading to lower flame temperatures and reduced explosion
risks. For example, at a N2/CO2 dilution ratio of 5:5, the
adiabatic flame temperature decreases from 2654.6 K at 0%
inert gas concentration to 1811.2 K at 30% inert gas
concentration, a reduction of 843.4 K, or 31.8%.
Through data analysis, it is found that there is a linear
relationship between the increase in adiabatic flame temperature and the CO2 dilution ratio when the volume fraction of
inert gas is constant, as shown in Figure 5. At a fixed inert gas
Figure 6. Trends in the equilibrium molar fraction of H radicals as a
function of inert gas concentration and N2/CO2 dilution ratios.
As shown in Figure 6, the equilibrium molar fraction of H
radicals decreases with increasing inert gas concentration and
CO2 dilution ratio. This result highlights the significant role of
inert gases in suppressing chemical reactions. For instance,
when the N2/CO2 dilution ratio is 5:5, increasing the inert gas
concentration from 0% to 30% results in a remarkable 98.1%
reduction in the equilibrium molar fraction of H radicals. This
significant reduction suggests that adding inert gases effectively
dilutes the reactants, lowering the concentration of flammable
gases and inhibiting the formation of H radicals. Moreover, as
the N2/CO2 dilution ratio shifts from 10:0 to 0:10, the
equilibrium molar fraction of H radicals shows varying degrees
of reduction at different inert gas concentrations. Specifically,
at inert gas concentrations of 5%, 10%, 15%, 20%, 25%, and
30%, the equilibrium molar fraction of H radicals decreases by
34.2%, 56.3%, 28.1%, 17.2%, 10.1%, and 94.3%, respectively.
These findings suggest that an increase in the CO2 dilution
ratio significantly impacts the concentration of H radicals,
thereby affecting the reaction rate.
As illustrated in Figure 7, the equilibrium molar fraction of
O radicals decreases with increasing inert gas concentration
and CO2 dilution ratio. For instance, when the N2/CO2
dilution ratio is 5:5, raising the inert gas concentration from
0% to 30% leads to an impressive 99.9% reduction in the
equilibrium molar fraction of O radicals. Additionally, as the
N2/CO2 dilution ratio shifts from 10:0 to 0:10, the equilibrium
molar fraction of O radicals consistently declines across various
inert gas concentrations. Specifically, at inert gas concentrations of 5%, 10%, 15%, 20%, 25%, and 30%, the equilibrium
molar fraction of O radicals decreases by 22.2%, 42.2%, 65.3%,
82.9%, 92.9%, and 97.6%, respectively. This trend indicates
that increasing the CO2 dilution ratio significantly suppresses
the concentration of O radicals, reflecting the diminished
availability of oxygen in the reaction environment.
As illustrated in Figure 8, the equilibrium molar fraction of
OH radicals shows a significant decrease with increasing inert
Figure 5. Relationship between the increase in adiabatic flame
temperature and the CO2 dilution ratio.
concentration of 30%, the adiabatic flame temperature also
shows a significant decrease as the N2/CO2 dilution ratio shifts
from 10:0 to 0:10. Specifically, the temperature drops from
1978.4 K at a 10:0 dilution ratio to 1686.9 K at a 0:10 dilution
ratio, a reduction of 291.5 K, or 14.7%. These results
underscore the critical role of inert gases in lowering flame
temperature, further emphasizing their importance in mitigating explosion risks.
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3.3. Heat Release Rate and ROP of Key Radicals. The
heat release rate (HRR) is a critical indicator of the rate at
which heat is released during a combustion reaction, directly
reflecting the intensity and efficiency of the chemical processes
involved. Figure 9 illustrates the trend of HRR as the N2/CO2
Figure 7. Trends in the equilibrium molar fraction of O radicals as a
function of inert gas concentration and N2/CO2 dilution ratios.
Figure 9. Trend of heat release rate as a function of N2/CO2 dilution
ratios.
dilution ratio changes at a constant inert gas concentration of
30%. The figure shows that HRR gradually decreases with an
increasing proportion of CO2, indicating a significant inerting
effect. The addition of CO2 increases the inert components
within the reaction system, thereby reducing the concentration
of flammable gases. Furthermore, CO2 participates in some
chemical reactions, diluting the reactants and slowing the
reaction rate, which subsequently lowers the HRR. This result
underscores the role of CO2 in suppressing the explosion
process of H2/C3H8 mixtures, especially under high concentration conditions where its inerting effect is particularly
pronounced. Additionally, due to its endothermic properties,
the increasing proportion of CO2 also affects the reaction
temperature. A lower reaction temperature further inhibits the
reaction progress, leading to a decrease in HRR.23
The rate of production (ROP) represents the generation
rate of specific chemical substances per unit time during a
reaction, serving as a crucial indicator for understanding the
mechanisms of chemical reactions. It reveals the trends of
various species throughout the reaction, aiding in the analysis
of the dynamic processes and the formation of major
intermediate products.45−47 Figure 10 illustrates the changes
in the ROP of H, O, and OH radicals. The figure shows that, as
the CO2 dilution ratio increases, the ROPs of H, O, and OH
radicals all exhibit a downward trend, with the ROP of H
radicals decreasing the most significantly. This phenomenon
indicates that the addition of CO2 has a notable inhibitory
effect on H radicals during explosion suppression. As the CO2
proportion increases, the active components in the reaction
system are diluted, resulting in a marked reduction in the
generation rate of H radicals. Since H radicals are key reactive
species that trigger combustion and explosions, their reduction
can significantly diminish the intensity of the reaction.48 Thus,
this finding underscores the effectiveness of CO2 in inerting
and preventing explosions.
To further analyze the impact of elementary reaction steps
on the rate of production (ROP) of key radicals, Figures
Figure 8. Trends in the equilibrium molar fraction of OH radicals as a
function of inert gas concentration and N2/CO2 dilution ratios.
gas concentration and CO2 dilution ratio. Specifically, when
the N2/CO2 dilution ratio is set to 5:5, increasing the inert gas
concentration from 0% to 30% results in a remarkable 99.9%
reduction in the equilibrium molar fraction of OH radicals,
highlighting the substantial impact of inert gas concentration
on the suppression of these reactive species. Moreover, the
effect of varying the N2/CO2 dilution ratio on the equilibrium
molar fraction of OH radicals is also noteworthy; as the N2/
CO2 dilution ratio increases from 10:0 to 0:10, the equilibrium
molar fraction of OH radicals decreases progressively by
85.6%, 71.6%, 53.4%, 36.2%, 22.4%, and 12.4% at the
respective dilution ratios of 9:1, 7:3, 5:5, 3:7, 1:9, and 0:10.
These results demonstrate that an increase in the CO2 dilution
ratio significantly diminishes the concentration of OH radicals,
underscoring the critical role of CO2 in inhibiting radical
formation, which is a key factor in the suppression of the
combustion process. The data show that CO2 outperforms N2
in reducing the concentration of OH radicals, providing
valuable insights into the mechanisms of combustion inhibition
and the design of safer fuel mixtures, particularly in
environments where the control of deflagration and explosion
risks is paramount.
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Figure 10. Reaction rate of production (ROP) of key radicals.
Figure 11. Reaction steps affecting H radical ROP.
Figure 12. Reaction steps affecting O radical ROP.
11−13 illustrate the critical steps influencing the ROP of H, O,
and OH radicals, respectively.
As shown in Figure 11, the top five critical reaction steps
affecting the ROP of H radicals are R1: H + O2 = O + OH, R2:
O + H2 = H + H2O, R3: OH + H2 = H + H2O, R31: CO +
OH = CO2 + H, and R158: C2H2 + O = HCCO + H.
Specifically, R1 has the most significant impact on the
consumption of H radicals, while R3 is the primary step
influencing their production. At an N2/CO2 dilution ratio of
10:0, the contribution of R1 is negative, indicating that this
reaction predominantly consumes H radicals. In contrast, R2,
R3, R31, and R158 show positive contributions, suggesting
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Figure 13. Reaction steps affecting OH radical ROP.
Figure 14. Sensitivity analysis of elementary reactions.
they facilitate the production of H radicals. When the N2/CO2
dilution ratios change to 5:5 and 0:10, the contributions of R1
and R31 turn negative. This shift is attributed to changes in the
reaction environment, such as the increased CO2 dilution ratio,
which dilutes the reaction system and alters the kinetic
properties of the reactions. Although R2, R3, and R158
continue to positively impact H radical production, the
consumption effects of R1 and R31 become more pronounced
overall.
As shown in Figure 12, the top five critical reaction steps
affecting the ROP of O radicals are R1, R2, R89: CH3 + O =
CH2O + H, R157: C2H2 + O = CH2 + CO, and R158. Among
these, R1 primarily influences the production of O radicals,
while R2, R89, R157, and R158 mainly affect their
consumption. This pattern underscores the dual role of O
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decrease in the reaction rate.23 These results highlight that
CO2 concentration is a critical regulatory factor in the inerting
and explosion prevention process. By optimizing the CO2
dilution ratio, it is possible to effectively control the
concentrations of H, O, and OH radicals in the reaction,
thereby reducing the intensity of the chemical reaction and
enhancing safety measures in applications involving H2/C3H8
mixtures.
Figure 15 illustrates the reaction pathways in which CO2
participates in chemical reactions. Compared to the dilution
radicals in chemical reactions, acting as both products and
reactants. At an N2/CO2 dilution ratio of 10:0, R2 has the
most significant impact on the consumption rate of O radicals.
However, when the dilution ratio shifts to 0:10, the primary
reaction affecting O radical consumption changes to R158.
This shift reflects the influence of CO2 on reaction kinetics,
particularly under high dilution conditions. The increased CO2
concentration alters the reaction environment, enhancing the
prominence of specific steps, like R158, in their inhibitory
effects, which subsequently affects the production of O
radicals.
As shown in Figure 13, there are significant differences in the
critical reaction steps affecting the ROP of OH radicals under
varying N2/CO2 dilution ratios. At an N2/CO2 dilution ratio of
10:0, the top five reaction steps influencing the ROP of OH
radicals are R1, R2, R3, R4: 2OH = O + H2O, and R31.
Among these, R3 and R31 have negative contributions,
indicating that they promote the consumption of OH radicals
at this dilution ratio. Conversely, R1, R2, and R4 show positive
contributions, suggesting that these steps enhance the
production of OH radicals. When the N2/CO2 dilution ratio
is adjusted to 5:5, the critical reaction steps affecting the ROP
of OH radicals shift to R1, R2, R3, R31, and R92: CH3 + OH =
CH2 + H2O. Here, R3 and R92 exhibit negative contributions,
significantly promoting the consumption of OH radicals. R1,
R2, and R31 continue to contribute positively, supporting the
production of OH radicals. Further adjusting the N2/CO2
dilution ratio to 0:10, the top five reaction steps influencing the
ROP of OH radicals include R1, R2, R3, R31, and R84: CH2O
+ OH = HCO + H2O. In this case, R3 and R84 present
negative contributions, while R1, R2, and R31 remain positive.
These results indicate that the production and consumption
of H, O, and OH radicals are influenced by different reaction
steps. Under varying N2/CO2 dilution ratios, the intensity of
specific reaction steps changes with the reaction environment.
Understanding these reaction mechanisms is crucial for
optimizing inerting and explosion prevention technologies,49
as controlling the composition of the reaction environment can
effectively reduce the concentration of key radicals.
3.4. Elementary Reaction Sensitivity and Suppression
of Reaction Steps. Figure 14 presents the sensitivity analysis
of elementary reactions concerning H, O, and OH radicals, as
well as temperature. The sensitivity analysis reveals how each
reaction step influences the overall chemical reactions.
Specifically, the sensitivity values of R1, R276: C2H5 +
CH3(+M) = C3H8 + M, R96: CH3 + HO2 = CH3O + OH,
R254: C2H4 + OH = C2H3 + H2O, and R123: CH4 + H = CH3
+ H2 are all positive, indicating that these steps promote the
reaction process, with R1 exhibiting the most significant
promoting effect. In contrast, the sensitivity values of R369:
C3H6 + OH = aC3H5 + H2O, R360: C3H6 + H = aC3H5 + H2,
R409: C3H8 + H = H2 + nC3H7, R104: 2CH3(+M) =
C2H6(+M), and R410: C3H8 + H = H2 + iC3H7 are negative,
suggesting that these steps inhibit the reaction process, with
R410 demonstrating the most pronounced inhibitory effect.
Overall, as the CO2 concentration increases, the promoting
effects of the positive sensitivity reactions gradually weaken,
while the inhibitory effects become more pronounced. This
shift is likely due to CO2’s characteristics as an inert gas: in a
high CO2 concentration environment, the likelihood of
effective collisions between reactants decreases, and CO2
absorbs heat from the reaction process. Moreover, CO2 can
participate in some chemical reactions, leading to an overall
Figure 15. Pathways of CO2 participation in chemical reactions.
and heat absorption effects of N2, CO2 also participates in
certain chain reactions, thereby exerting an inhibitory effect on
the overall chemical reaction. Specifically, CO2 primarily
inhibits the reaction through two key steps: R52: CH + CO2
= HCO + CO and R79: CH2 + CO2 = CH2O + CO. In
reaction R52, CO2 reacts with the reactive intermediate CH to
form HCO and CO. As the CO2 concentration increases, the
active species in the reaction are converted into other
compounds, leading to a decrease in the reaction rate. This
is because the formation of HCO consumes active radicals,
further inhibiting the chain reaction and slowing down the
overall reaction process. In reaction R79, CH2 reacts with CO2
to form CH2O and CO. Similar to R52, this reaction also
consumes the active intermediate CH2, reducing the reaction
rate by converting active species into less reactive products.
To sum up, the addition of CO2 not only significantly affects
the reaction pathways but also highlights its importance in
inhibiting reactions. In practical applications, by adjusting the
N2/CO2 dilution ratios, it is possible to effectively control the
heat release and the concentration of generated radicals,
thereby reducing the risk of explosions.
4. CONCLUSIONS
Through chemical reaction kinetics simulations, the inhibitory
characteristics of H2/C3H8 gas mixtures under different N2/
CO2 dilution ratios were studied. The analysis focused on
adiabatic explosion pressure, flame temperature, key radical
concentrations, heat release rate, and elementary reaction
sensitivity. The following conclusions are drawn:
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mixtures explosion in a pipeline. Int. J. Hydrogen Energy 2024, 88,
462−476.
(2) Cai, C.; Su, Y.; Wang, Y.; Ji, W. Suppressive effects of potassium
salt modified dry water material on hydrogen/methane mixture
explosion. Int. J. Hydrogen Energy 2024, 79, 537−550.
(3) Diao, S.; Wen, X.; Guo, Z.; He, W.; Deng, H.; Wang, F. Flame
Propagation Characteristics of Syngas-Air in the Hele-Shaw Duct with
Different Equivalence Ratios and Ignition Positions. ACS Omega
2022, 7 (23), 20118−20128.
(4) Wu, Z.; Cheng, Q.; Gu, X.; Wang, Z.; Lu, L.; Pan, X.; Hua, M.;
Xiao, J.; Jiang, J. Experimental investigation on the development
characteristics of high-pressure hydrogen jet flame under different
ignition conditions. Int. J. Hydrogen Energy 2024, 79, 791−801.
(5) Ma, X.; Nie, B.; Wang, W.; Zhao, D.; Zhang, Y.; Yang, Y.; Ma,
C.; Hu, B.; Chang, L.; Yang, L. Effect of hydrogen concentration,
initial pressure and temperature on mechanisms of hydrogen
explosion in confined spaces. Combust. Flame 2024, 269, 113696.
(6) Sun, B.; Loughnan, T. Consequence analysis of vapour cloud
explosion from the release of high-pressure hydrogen storage. Int. J.
Hydrogen Energy 2024, 80, 1137−1150.
(7) Gao, Y.; Lu, Z.; Hua, Y.; Liu, Y.; Tao, C.; Gao, W. Experimental
study on the flame radiation fraction of hydrogen and propane gas
mixture. Fuel 2022, 329, 125443.
(8) Zhao, Z.; Liang, Y.; Zhong, X.; Song, S.; Guo, B.; Liu, Z. Study
on the effect of duct length and hydrogen on explosion characteristics
of LPG/H2 blended clean fuel. Fuel 2024, 365, 131209.
(9) Liang, H.; Wang, T.; Luo, Z.; Wang, X.; Kang, X.; Deng, J. Risk
assessment of liquefied petroleum gas explosion in a limited space.
ACS Omega 2021, 6 (38), 24683−24692.
̇ , M.; Tarhan, C. Effect of
(10) Yılmaz, I. .;̇ Taştan, M.; Ilbaş
turbulence and radiation models on combustion characteristics in
propane−hydrogen diffusion flames. Energy Convers. Manage. 2013,
72, 179−186.
(11) Wang, W.; Zuo, Z.; Liu, J. Experimental study of propane/air
premixed flame dynamics with hydrogen addition in a meso-scale
quartz tube. J. Energy Inst. 2020, 93 (4), 1690−1696.
(12) Tang, A.; Ni, Q.; Deng, J.; Huang, Q. Role of hydrogen
addition in propane/air flame characteristic and stability in a microplanar combustor. Fuel Process. Technol. 2021, 216, 106797.
(13) Giurcan, V.; Movileanu, C.; Mitu, M.; Razus, D. The impact of
H2-enrichment on flame structure and combustion characteristic
properties of premixed hydrocarbon-air flames. Fuel 2024, 376,
132674.
(14) Movileanu, C.; Giurcan, V.; Razus, D.; Musuc, A. M.; Hornoiu,
C.; Chesler, P.; Mitu, M. Hydrogen influence on confined explosion
characteristics of hydrocarbon-air mixtures at sub-atmospheric
pressures. Int. J. Hydrogen Energy 2024, 67, 150−158.
(15) Nan, F.; Luo, Z.; Cheng, F.; Xiao, Y.; Li, R.; Su, B.; Wang, T.
Research progress and development trends of hydrogen explosion
suppression materials and mechanisms. Process Saf. Environ. Prot.
2024, 184, 1318−1331.
(16) Liu, Y.; Zhang, Y.; Zhao, D.; Yin, J.; Liu, L.; Shu, C.-M.
Experimental study on explosion characteristics of hydrogen−propane
mixtures. Int. J. Hydrogen Energy 2019, 44 (40), 22712−22718.
(17) Li, Y.; Bi, M.; Gao, W.; Zhou, Y.; Huang, L. Interaction of flame
instabilities and pressure behavior of hydrogen-propane explosion. J.
Loss Prev. Process Ind. 2020, 64, 104078.
(18) Law, C. K.; Jomaas, G.; Bechtold, J. K. Cellular instabilities of
expanding hydrogen/propane spherical flames at elevated pressures:
theory and experiment. Proc. Combust. Inst. 2005, 30 (1), 159−167.
(19) Wang, J.; Liang, Y.; Zhao, Z. Effect of N2 and CO2 on explosion
behavior of H2-Liquefied petroleum gas-air mixtures in a confined
space. Int. J. Hydrogen Energy 2022, 47 (56), 23887−23897.
(20) Luo, Z.; Wei, C.; Wang, T.; Su, B.; Cheng, F.; Liu, C.; Wang, Y.
Effects of N2 and CO2 dilution on the explosion behavior of liquefied
petroleum gas (LPG)-air mixtures. J. Hazard. Mater. 2021, 403,
123843.
(1) As the concentration of inert gas and the CO2 dilution
ratio increase, the adiabatic explosion pressure and flame
temperature significantly decrease. The increase in
adiabatic explosion pressure and adiabatic flame temperature are both linearly related to the CO2 dilution ratio.
Simultaneously, the equilibrium molar fractions of H, O,
and OH radicals are notably reduced. This indicates that
the addition of N2/CO2 diluent gases effectively lowers
explosion intensity and suppresses the generation of key
radicals.
(2) With an increase in the CO2 dilution ratio, the heat
release rate gradually decreases, leading to a reduction in
the intensity of combustion reactions. During this
process, the rates of production (ROP) of H, O, and
OH radicals all show a downward trend, with the ROP
of H radicals exhibiting the most significant decline. The
addition of CO2 effectively inhibits the generation of key
radicals, thereby reducing the reactivity of the system.
(3) Based on sensitivity analysis, reaction R1: H + O2 = O +
OH has the most significant promoting effect on the
reaction, while reaction R410: C3H8 + H = H2 + iC3H7
shows a pronounced inhibitory effect. As the CO2
concentration increases, the promoting effects of various
elementary reactions gradually weaken, while the
inhibitory effects become more pronounced. CO2 exerts
its inhibitory effect on the reaction rate through specific
reaction steps, such as R52: CH + CO2 = HCO + CO
and R79: CH2 + CO2 = CH2O + CO, effectively
suppressing and transforming key intermediates, thereby
reducing the overall heat release and reaction rate.
In summary, this study provides important theoretical
insights into the effects of N2/CO2 diluent ratios on the
explosion characteristics of H2/C3H8 gas mixtures, offering
valuable references for safety management and engineering
applications.
■
AUTHOR INFORMATION
Corresponding Author
Chengcai Wei − School of Resources and Safety Engineering,
Chongqing University, Chongqing 400044, P. R. China;
orcid.org/0009-0001-1673-4960; Email: weichengcai@
stu.cqu.edu.cn
Authors
Chengxu You − Chongqing Vocational Institute of Safety
Technology, Chongqing 404121, P. R. China
Songping Yang − Chongqing Vocational Institute of Safety
Technology, Chongqing 404121, P. R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsomega.4c08907
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the Science and Technology
Research Project of Chongqing Education Commission (grant
no. KJQN202304704).
■
Article
REFERENCES
(1) Qu, J.; Zhao, H.; Zhao, L.; Luo, Z.-M.; Wang, T.; Deng, J. Flame
propagation characteristics of non-uniform premixed hydrogen-air
7997
https://doi.org/10.1021/acsomega.4c08907
ACS Omega 2025, 10, 7989−7998
ACS Omega
http://pubs.acs.org/journal/acsodf
(21) Zhang, S.; Wen, X.; Guo, Z.; Zhang, S.; Ji, W. Experimental
study on the multi-level suppression of N2 and CO2 on hydrogen-air
explosion. Process Saf. Environ. Prot. 2023, 169, 970−981.
(22) Luo, Z.; Qi, H.; Su, B.; Wang, T.; Li, R.; Cheng, F.; Zhang, T. A
macroscopic and microscopic study of hydrogen-rich explosions
under inert gas conditions. Int. J. Hydrogen Energy 2024, 61, 316−328.
(23) Chen, X.; Zhao, T.; Cheng, F.; Lu, K.; Shi, X.; Yu, W.
Investigation of the Suppression of Methane Explosions by N2/CO2
Mixtures in Different Proportions. ACS Omega 2023, 8 (12), 10863−
10874.
(24) Khan, A. R.; Anbusaravanan, S.; Kalathi, L.; Velamati, R.;
Prathap, C. Investigation of dilution effect with N2/CO2 on laminar
burning velocity of premixed methane/oxygen mixtures using freely
expanding spherical flames. Fuel 2017, 196, 225−232.
(25) Liu, G.; Zhou, J.; Wang, Z.; Yang, W.; Liu, J.; Cen, K. Adiabatic
laminar burning velocities of C3H8-O2-CO2 and C3H8-O2-N2 mixtures
at ambient conditions-PART II: Mechanistic interpretation. Fuel
2020, 276, 117946.
(26) Rajesh, N.; Akash, M.; Prathap, C. Investigation on the effects
of steam/CO2/N2 on the flame suppression and flame stability of the
methane-oxygen mixtures at elevated thermodynamic conditions. Fuel
2022, 309, 121987.
(27) Hu, E.; Jiang, X.; Huang, Z.; Iida, N. Numerical study on the
effects of diluents on the laminar burning velocity of methane−air
mixtures. Energy Fuels 2012, 26 (7), 4242−4252.
(28) Zhong, F.; Zheng, L.; Wang, X.; Shao, X.; Jia, H.; Shi, Z.;
Zhang, J. Comparative study on flame instability and combustion
characteristics of CH4/O2/CO2 and CH4/O2/N2 mixtures. J. Energy
Inst. 2022, 102, 70−81.
(29) Wang, T.; Liang, H.; Luo, Z.; Yu, J.; Cheng, F.; Zhao, J.; Su, B.;
Li, R.; Wang, X.; Feng, Z.; et al. Thermal suppression effects of
diluent gas on the deflagration behavior of H2−air mixtures. Energy
2023, 272, 127146.
(30) Liu, Q.; Liu, L.; Liu, Z.; Peng, S.; Liu, C.; Zhang, H.; Liu, C.; Li,
P.; Fan, T. Effects of H2 blended ratio and N2/CO2 dilution fraction
on the deflagration shock wave of H2NG in slender closed pipelines.
Int. J. Hydrogen Energy 2024, 73, 451−461.
(31) Chen, J.; Chen, G.; Zhang, A.; Deng, H.; Wen, X.; Wang, F.;
Mei, Y. Experimental and numerical study on the effect of CO2
dilution on the laminar combustion characteristics of premixed CH4/
H2/air flame. J. Energy Inst. 2022, 102, 315−326.
(32) Shang, R.; Zhang, Y.; Zhu, M.; Zhang, Z.; Zhang, D.; Li, G.
Laminar flame speed of CO2 and N2 diluted H2/CO/air flames. Int. J.
Hydrogen Energy 2016, 41 (33), 15056−15067.
(33) Burbano, H. J.; Pareja, J.; Amell, A. A. Laminar burning
velocities and flame stability analysis of H2/CO/air mixtures with
dilution of N2 and CO2. Int. J. Hydrogen Energy 2011, 36 (4), 3232−
3242.
(34) Nair, A.; Velamati, R. K.; Kumar, S. Effect of CO2/N2 dilution
on laminar burning velocity of liquid petroleum gas-air mixtures at
elevated temperatures. Energy 2016, 100, 145−153.
(35) Li, M.; Xu, J.; Wang, C.; Wang, B. Thermal and kinetics
mechanism of explosion mitigation of methane-air mixture by N2/
CO2 in a closed compartment. Fuel 2019, 255, 115747.
(36) Kee, R.; Rupley, F.; Miller, J.; Coltrin, M.; Grcar, J.; Meeks, E.;
Moffat, H.; Lutz, A.; Dixon-Lewis, G.; Smooke, M. J. U. R. D.
CHEMKIN Theory Manual, 2006.
(37) Wang, H.; You, X.; Joshi, A. V.; Davis, S. G.; Laskin, A.;
Egolfopoulos, F.; Law, C. K. USC Mech Version II. High-Temperature
Combustion Reaction Model of H2/CO/C1-C4 Compounds, 2007.
https://chemistry.cerfacs.fr/en/chemical-database/mechanisms-list/
usc-ii-mechanism/.
(38) Su, B.; Luo, Z.; Krietsch, A.; Wu, D.; Wang, T.; Zhou, S.; Deng,
J. Quantitative investigation of explosion behavior and spectral radiant
characteristics of free radicals for syngas/air mixtures. Int. J. Hydrogen
Energy 2024, 50, 1359−1368.
(39) Su, B.; Luo, Z.; Deng, J.; Wang, T.; Liu, L.; Zhou, S.; Lu, K.;
Zhang, J.; Liu, L. Comparative study on methane/air deflagration with
Article
hydrogen and ethane additions: Investigation from macro and micro
perspectives. Process Saf. Environ. Prot. 2023, 174, 561−573.
(40) Wei, C.; Li, H.; Luo, Z.; Yu, Y.; Wang, T.; Diao, S.; Cui, J.;
Wang, J.; Yu, M. Investigating the explosive characteristics of
hydrogen/ n-butane blended fuel: Experimental and kinetic insights.
Process Saf. Environ. Prot. 2024, 191, 698−709.
(41) Sahu, A.; Wang, C.; Jiang, C.; Xu, H.; Ma, X.; Xu, C.; Bao, X.
Effect of CO2 and N2 dilution on laminar premixed MTHF/air
flames: Experiments and kinetic studies. Fuel 2019, 255, 115659.
(42) Yamamoto, K.; Ozeki, M.; Hayashi, N.; Yamashita, H. Burning
velocity and OH concentration in premixed combustion. Proc.
Combust. Inst. 2009, 32 (1), 1227−1235.
(43) Singh, D.; Nishiie, T.; Tanvir, S.; Qiao, L. An experimental and
kinetic study of syngas/air combustion at elevated temperatures and
the effect of water addition. Fuel 2012, 94, 448−456.
(44) Butler, C. J.; Hayhurst, A. N. Measurements of the
Concentrations of Free Hydrogen Atoms in Flames from Observations of Ions: Correlation of Burning Velocities with Concentrations
of Free Hydrogen Atoms. Combust. Flame 1998, 115 (1), 241−252.
(45) Wei, C.; Luo, Z.; Yu, Y.; Wang, T.; Yang, Y.; Li, H.; Diao, S.;
Cui, J.; Yu, M. Experimental and kinetic investigation of the explosive
behavior and free radical spectroscopic characteristics of hydrogen/
propane mixed gas. Process Saf. Environ. Prot. 2023, 180, 893−906.
(46) Yu, M.; Zhai, F.; Li, H.; Han, S.; Li, S.; Zheng, K.; Yu, Y.
Comparative study on the effect of initial temperatures and pressures
on the laminar flame speed of the heavily carbonaceous syngas
containing water vapor via reaction kinetics simulation. Int. J.
Hydrogen Energy 2022, 47 (77), 32763−32775.
(47) Yu, M.; Li, S.; Zhai, F.; Lou, R.; Li, H.; Zheng, K. Effects of
volatile contents on the deflagration characteristics and soot
formation of methane/coal volatiles hybrid explosion using reaction
kinetics simulation. Fuel 2022, 319, 123767.
(48) Su, B.; Dong, H.; Luo, Z.; Deng, J.; Liao, P.; Cheng, F.; Wang,
T.; Liu, L.; Liu, L. Effects of H2/CO ratio and CO2 dilution on the
explosion behavior and flame evolution of syngas/air mixtures. Int. J.
Hydrogen Energy 2024, 69, 451−465.
(49) Nan, F.; Luo, Z.; Cheng, F.; Xiao, Y.; Su, B.; Li, R.; Wang, T.
Study on the instability and suppression mechanism of methane/air
deflagration flame by inert gas-halogenated hydrocarbons. Fuel 2024,
374, 132351.
7998
https://doi.org/10.1021/acsomega.4c08907
ACS Omega 2025, 10, 7989−7998