Fuel lean results

In addition, conservation of energy for the entire control volume (I + II) gives 

---

Before collecting data, at least two lean/rich cycles of 15-min lean and 5-min rich were completed for the given reaction condition. These cycle times were chosen so as the effluent from all reactors reached steady state. After the initial lean/rich cycles were completed, IR spectra were collected continuously during the switch from fuel rich to fuel lean and then back again to fuel rich. The collection time in the fuel lean and fuel rich phases was maintained at 15 and 5 min, respectively. The catalyst was tested for SNS at all the different reaction conditions and the qualitative discussion of the results can be found in [75], Quantitative analysis of the data required the application of statistical methods to separate the effects of the six factors and their interactions from the inherent noise in the data. Table 11.5 presents the coefficient for all the normalized parameters which were statistically significant. It includes the estimated coefficients for the linear model, similar to Eqn (2), of how SNS is affected by the reaction conditions. 

From the model results presented in Figure 11.14, it can be seen that the maximum NO conversion for a fixed lean fraction occurred at an intermediate cycle time. Similar results have been reported by Han et al. [85] and Kabin et al. [86], and it was concluded that, at very short cycle times, the catalyst responded as if the fuel rich and fuel lean feed gases were mixed. However, an additional explanation for the decrease in NO ... 

The early experiments of Bowman and Seery appeared to confirm this conclusion. Some of their results are shown in Fig. 8.3. In this figure the experimental points compared very well with the analytical calculations based on the Zeldovich mechanisms alone. The same computational program as that of Martenay [11] was used. Figure 8.3 also depicts another result frequently observed fuel-rich systems approach NO equilibrium much faster than do fuel-lean systems [12]. 

Based on conventional thinking, lower temperatures should result in a decrease of thermally produced NO, . However, despite the substantially lower temperatures in the PSR, as shown in Fig. 26.4a, the NO, mole fractions do not significantly differ between the two reactors, except for very fuel-lean mixtures, near extinction of self-sustained flames. Similar behavior was recently observed when radiation from... 

Figure 26.56 is the corresponding plot for 12% inlet H2 in air. In this case, there is an extinction at about 1000 K for both reactors. The qualitative features are similar to that of the PSR discussed above for 28% H2 in air. For such fuel-lean mixtures, the flame is attached to the surface. As a result, the thermal coupling between the surface and the gas phase is strong, and reduction in surface temperature affects the entire thermal boundary layer resulting in significant reduction of NOj,. These results indicate that the bifurcation behavior, in terms of extinction, determines the role of flame-wall thermal interactions in emissions. 

Another key factor determining the emission levels is the bulk equivalence ratio ((l>) in the furnace, a parameter that describes the relative amounts of fuel and air. Levendis and co-workers (1996, 1998fo) have shown that for both coal and tyre (and for various blends thereof) the specific emissions, that is, the mass of a certain pollutant relative to the mass of fuel burned, vary systematically when the combustion environment changes from fuel-lean (low ) to fuel-rich (high results document that, at a given temperature,... 

NOx reduction conversions met in the DOC are quite low. Excess of air in burned lean fuel mixture results in excess of oxygen in the exhaust. Under such conditions, the reducing components naturally present in diesel exhaust (CO, H2 and HC) are readily oxidized by the excessive oxygen and NOx remains unreduced. However, the unburned hydrocarbons still exhibit a certain activity for NO reduction on NM/y-Al203 and NM/zeolites catalysts under lean conditions (HC-SCR). Many efforts have been put into the investigation of different NM-based or alternative catalysts tailored for the HC-SCR reaction and the development of reliable reaction mechanisms—cf., e.g., Joubert et al. 

The theoretical and experimental results for a fuel-lean methane-air flame are given in Figures 5-7. These results include temperature and major species compositions. The experimental and theoretical results are compared by matching the abcissas of the temperature profiles. The model very accurately predicts the slope of the temperature profile but predicts a larger final flame temperature than is measured. This is a consequence of heat lost to the cooled, gold-coated burner wall that is 1.5 mm away from the positions where data were taken. 

Emissions of soot on the other hand represent a smaller fraction of the overall emission, but are probably of greater concern from the standpoint of visibility and health effects. It has been suggested that soot emissions from fuel oil flames result from processes occurring in the vicinity of individual droplets (droplet soot) before macroscale mixing of vaporized material, and from reactions in the bulk gas stream (bulk soot) remote from individual droplets. Droplet soot appears to dominate under local fuel lean conditions (1, 2), while bulk soot formation occurs in fuel rich zones. Factors which are known to affect soot formation from liquid fuel flames include local stoichiometry, droplet size, gas-droplet relative velocity and fuel properties (primarily C H ratio). 

From these correlations it would be natural to expect that the maximum blowoff velocity as a function of air-fuel ratio would occur at the stoichiometric mixture ratio. For premixed gaseous fuel-air systems, the maxima do occur at this mixture ratio, as shown in Fig. 56. However, in real systems liquid fuels are injected upstream of the bluff-body flame holder in order to allow for mixing. Results [60] for such liquid injection systems show that the maximum blowoff velocity is obtained on the fuel-lean side of stoichiometric. This trend is readily explained by the fact that liquid droplets impinge on the stabilizer and enrich the wake. Thus, a stoichiometric wake undoubtedly occurs for a lean upstream liquid-fuel injection system. That the wake can be modifled to alter blowoff characteristics was proved experimentally by Fetting et al. [65]. The trends of these experiments can be explained by the correlations developed in this section. 

---