Ammonia (NH
3
) is not only expected to be used as a hydrogen
energy carrier but also expected to become a carbon-free fuel. Methane
(CH
4
) can be used as a combustion enhancer for improving
the combustion intensity of NH
3
. In addition, it is important
to understand the flame characteristics of NH
3
–air
at elevated pressures and temperatures. The laminar flame speed of
NH
3
–CH
4
–air is numerically investigated,
where the mole fraction of CH
4
ranges from 0 to 50% in
binary fuels and the pressure and initial temperature are up to 10
atm and 1000 K, respectively. The calculated value from the Okafor
mechanism is in excellent agreement with experimental data. The CH
4
in the fuel affects the flame speed by changing the main
species of free radicals in the flame; the high pressure not only
increases the rate-limiting reaction rate in the flame but also reduces
the amount of H, O, and OH radicals in the flame, so as to restrain
the propagation of the flame. At a higher initial temperature, the
faster flame speed is mainly due to the higher adiabatic flame temperature.
The laminar flame speed correlation equation has a consistent trend
with the simulation results, though with a slight underestimation
at higher pressures and temperatures. It is a more effective way to
calculate the laminar flame speeds of NH
3
–air for
a given pressure and temperature.
The effect of CO 2 , which replaces part of N 2 present in air, on flame stability, laminar burning velocities (LBVs), and intermediate radicals (O OH) of CH 4 /O 2 /N 2 /CO 2 premixed flames has been analyzed using detailed experiments and numerical studies. The numerical simulations were conducted using the PREMIX code with a detailed chemical reaction mechanism (GRI-Mech 3.0) and a reduced mechanism (39 species and 205 reactions) based on GRI-Mech 3.0 over a wide range of equivalence ratios (Φ = 0.7−1.3) and CO 2 substitution ratios (0−30%). The reduced mechanism showed a good agreement with the other detailed mechanisms and experimental data. The experimental and numerical results showed that the substitution of CO 2 diminishes the stability of the flame, and the flame blow-out speed is significantly reduced (the substitution ratio is 0−30%, and the corresponding flame blow-out velocity is 5.2−2.5 m/s). In addition, CO 2 inhibits the LBV of the flame owing to the decrease of O and OH mole fractions. It not only accelerates the consumption of these two free radicals but also inhibits the generation of these two free radicals. Further analysis concluded that the substituted CO 2 has the greatest influence on the LBV sensitivity coefficient of the HO 2 + CH 3 = OH + CH 3 O reaction.
To study the laminar
premixed flame characteristics of methane
under an O2/CO2 atmosphere in high pressure,
a new simplified chemical mechanism (44 steps and 19 species) was
extracted from the GRI-Mech 3.0 mechanism. The sensitivity coefficient
analysis method is used to retain the elementary reactions that have
great influence on the target parameters and remove other secondary
elementary reactions. Meanwhile, the closeness of the simplified mechanism
initially obtained was verified. The results of the laminar burning
velocities, the distribution of the main species, and the ignition
delay time were compared between the two mechanisms for the premixed
flame and the ignition process. Overall, the simplified mechanisms
performed fairly well over a wide range of pressure, equivalence ratio,
and fuel mixture composition.
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