Flame structures at the lower limit

In this section, the flame structure at the flammability limit (u_L = 15 cm/s) was analyzed. The simulation was performed at p = 40 bar, φ^′= 0.87, and a T_uwhich was adjusted to ensure a resulting flame speed of 15 cm/s (as shown in Figure 6.3). The required T_uto achieve a laminar flame speed of 15 cm/s for MeOH-Air, MeOH-EGR, R-EGR, SG50-Air and SG50-EGR cases respectively is 333 K, 398 K, 365 K, 317 K and 385 K. Figure 6.5 shows the temperature profile of the five mixtures as a function of the axial distance. The burned temperature T_bof the five mixtures is around 2120 - 2160 K. In the research of Flynn et al. [193], the end of combustion flame temperature is estimated using a constant pressure, adiabatic condition, at the point where 95% of the total heat release has occurred. The flame temperature at the point where 95% of the total heat released is calculated, was

REACTION FRONT PROPERTIES 141

around 1983 K, 1952 K, 1942 K, 1965 K and 1936 K for MeOH-Air, MeOH-EGR, R-EGR, SG50-Air and SG50-EGR, respectively (see symbols in Figure 6.5).

The flame temperature is lower with the dilution of EGR (same fuel) because of a higher heat capacity of the burned gases. These temperatures are similar to the data cloud in the research of Flynn et al. [193]. As can be seen, there is a very small difference between the temperature profiles of MeOH and SG50. Compared to the EGR-type dilution cases (R-EGR, MeOH-EGR and SG50-EGR), although the unburned gas temperature is lower, the air dilution cases have a higher burned gas temperature. The T_bof the R-EGR mixture is higher than that of the normal EGR dilution (MeOH-EGR and SG50-EGR) and lower than the air dilution cases.

This can be explained by the concentration of burned products like carbon dioxide (CO₂) and water vapour (H₂O), species which have higher heat capacity values.

In the air dilution cases, because no combustion products are present and there is a greater oxygen concentration, T_b is higher than for the EGR-type dilution cases. Compared to normal EGR dilution, the R-EGR case has a higher T_bdue to a smaller concentration of water vapour in the reactant because it was consumed partly for the reforming.

-0.002 0 0.002 0.004 0.006

Temperature(K)

Figure 6.5: Temperature profiles at lower flammability limit, p = 40 bar and φ^′= 0.87.

Symbols: the flame temperature and location where 95% total heat release occurred.

The concentrations of four main species (CH₃OH, O₂, CO₂and H₂O) are compared between MeOH-Air and SG50-Air in Figure 6.6. Because of its replacement by syngas, the mole fraction of CH₃OH in the unburned zone of the SG50-Air case is obviously smaller than for the MeOH-Air case. The stoichiometric air to fuel ratio (by mole) of syngas is much smaller than that of methanol (∼1.81 compared to 7.14), so the required amount of O₂to have the same φ = 0.87 when syngas is

added is lower than for pure methanol combustion. However, CH₃OH is almost consumed at the same location. CO₂is a product of the fuel reforming process, so the initial CO₂concentration is higher with the addition of syngas. In this study, syngas is a product of steam reforming, therefore the mole fraction of H₂O increases faster to a higher concentration for the enrichment with syngas. With a higher heat capacity of H₂O, the combustion temperature of SG50 is lower than methanol if EGR is used to dilute the combustion, as can be seen in Figure 6.5.

0

-0.002 0 0.002 0.004 0.006

Species mole fraction (-)

Figure 6.6: Comparison of species mole fraction between MeOH-Air and SG50-Air cases at φ^′= 0.87.

To compare the air and EGR dilution, the mole fraction of the mentioned species of the SG50-Air case was plotted together with the SG50-EGR case, as shown in Figure 6.7. In the EGR dilution case, the mole fraction of O₂in the unburned region is much lower than for the air dilution case. Although the simulation is carried out at a similar φ^′, the mixture for the EGR dilution cases is stoichiometric (φ = 1), so the O₂ in the burned zone is almost completely consumed. The combustion products concentrations, CO₂and H₂O, are higher in both the unburned and burned zones, leading to a lower combustion temperature.

The SG50-EGR case is then compared to the R-EGR one in Figure 6.8. At φ^′= 0.87, the mass fraction of methanol for fuel reforming and the EGR ratio for the SG50-EGR and R-EGR cases are identical. Therefore as can be seen in Figure 6.8, the molar concentrations of CH₃OH, O₂and CO₂are related for these two cases. In the SG50-EGR case, a slightly lower concentration of CH₃OH and O₂ can be observed in the unburned mixture because of their displacement by H₂O, which is not partly consumed for the fuel reforming as in the R-EGR concept. The H₂O concentration is smaller in the R-EGR case both in the unburned and burned

REACTION FRONT PROPERTIES 143

-0.002 0 0.002 0.004 0.006

Species mole fraction (-)

Figure 6.7: Comparison of species mole fraction between SG50-Air and SG50-EGR cases at φ^′= 0.87.

zones because the water vapour in the EGR mixture is consumed partly for the steam reforming of methanol. The higher H₂O concentration in the SG50-EGR case leads to a decrease in the burned temperature (see Figure 6.5).

Figure 6.9 shows the mole fraction of H₂and CO, two components of syngas, as a function of axial distance through the flame. In the combustion of methanol (MeOH-Air and MeOH-EGR cases), hydrogen only appears as an intermediate species. In these cases, the hydrogen concentration grows and reaches its peak at around the flame layer position. With the dilution of EGR-type mixture, the hydrogen concentration increases faster and reaches a peak at a higher concentration. The H₂mole fraction in the burned zone is also higher because the H atom is less consumed by H-abstraction reactions at a lower combustion temperature [201]. In the R-EGR and SG50 cases, H₂ is present as a fuel component, so going from left to right, its concentration falls and almost equals the concentration in the combustion of methanol. Three EGR-type dilution cases (R-EGR, MeOH-EGR and SG50-EGR) have a higher H₂ concentration in the burned zone. As can be seen, the consumption rate of H₂in the R-EGR case is a bit faster than that of the SG50-EGR case, which might be due to a smaller concentration of water vapour in the reactant.

The CO concentration is also plotted in Figure 6.9. Although CO is a component of the methanol steam reforming product, CO is still an intermediate species in the R-EGR and SG50 cases. Compared to the EGR-type dilution cases, the air dilution cases have a lower CO concentration, which agrees with the typical trend of CO

0 0.05 0.1 0.15 0.2 0.25

-0.002 0 0.002 0.004 0.006

Species mole fraction (-)

Axial distance (cm) SG50-EGR R-EGR O2

H2O

CO2

CH3OH ϕ' = 0.87

Figure 6.8: Comparison of species mole fraction between SG50-EGR and R-EGR cases at φ^′= 0.87.

0 0.01 0.02 0.03 0.04 0.05 0.06

-0.006 -0.004 -0.002 0 0.002 0.004 0.006

Species mole fraction (-)

Axial distance (cm) MeOH-Air

MeOH-EGR R-EGR SG50-Air SG50-EGR

CO

H₂

ϕ' = 0.87

Figure 6.9: Comparison of H₂and CO mole fraction between five cases at φ^′= 0.87.

REACTION FRONT PROPERTIES 145

emission with lambda. The difference in CO mole fraction is very small between methanol and SG50, in both the air and EGR dilution cases. This can be explained as follows. First, the higher gas temperature promotes the CO oxidation, so the case which has a lower T_b results in a higher concentration of CO. The burned temperature of methanol and SG50 are similar, so the CO emission is almost the same between the two fuels. Second, the ratio of hydrogen to methanol (or H/C ratio) increases when syngas is added, leading to the decrease of carbon-related emissions like CO [202]. This is similar to the observation in the research of Han et al. [203], where the CO concentration in exhaust gases was higher with EGR dilution compared to air dilution (at the same φ^′ = 0.8), and there was no clear change in CO emissions with varied syngas fraction.