Lean fuels

Vehicle Emissions. Gasohol has some automotive exhaust emissions benefits because adding oxygen to a fuel leans out the fuel mixture, producing less carbon monoxide [630-08-2] (CO). This is tme both for carbureted vehicles and for those having electronic fuel injection. 

The gas turbine eombustors have seen eonsiderable ehange in their design as most new turbines have progressed to Dry Low Emission NOx Combustors from the wet eombustors, whieh were injeeted by steam in the primary zone of the eombustor. The DLE approaeh is to burn most (at least 75%) of the fuel at eool, fuel-lean eonditions to avoid any signifieant produetion of NOx. The prineipal features of sueh a eombustion system is the premixing of... 

Combustion instability only used to be a problem with eonventional eombustors at very low engine powers. The phenomenon was ealled rumble. It was assoeiated with the fuel-lean zones of a eombustor, where the eonditions for burning are less attraetive. The eomplex 3D-flow strueture that exists in a eombustor will always have some zones that are suseeptible to the oseillatory burning. In a eonventional eombustor, the heat release from these oseillating zones was only a signifieant pereentage of the total eombustor heat release at low power eonditions. 

Spar-gemisch, n. (of fuel) lean mixture, -kalk, m. = Estrichgips. -kapsel,/. (Ceram.) spacesaving sagger, economy sagger, -lampe, /. economy lamp, small lamp, spdrlich, a. sparse, scarce, scanty. 

Key L = fuel-lean R = fuel-rich AA= air/acetylene AP = air/ propane NA= nitrous oxide/acetylene AH = air/hydrogen Notes (1) If there are many interferences then NA is to be preferred. 

Connect the appropriate gas supplies to the burner following the instructions detailed for the instrument, and adjust the operating conditions to give a fuel-lean acetylene-air flame. 

Set up a double-beam atomic absorption spectrophotometer with a lead hollow cathode lamp and isolate the resonance line at 283.3 nm adjust the gas controls to give a fuel-lean acetylene-air flame in accordance with the operating manual supplied with the instrument. 

A typical example of an electrophobic reaction is the oxidation of C2H4 on Pt4,59 (Fig. 4.13), Rh50 and Ag11,12,49,77 under fuel-lean conditions.59... 

Nevertheless there are some reactions which never change. Thus NO reduction on noble metals, a very important catalytic reaction, is in the vast majority of cases electrophilic, regardless of the type of solid electrolyte used (YSZ or P"-A1203). And practically all oxidations are electrophobic under fuel lean conditions, regardless of the type of solid electrolyte used (YSZ, p"-Al203, proton conductors, even alkaline aqueous solutions). 

On the fuel-lean side (kadPo2 kpc2H4) the oxygen coverage is near unity and step (8.3) is the rate limiting step (rls). Thus equation (8.1) reduces to ... 

It turns out1,5 that varying Uwr and O cause dramatic (up to sixty-fold) increases in k but have practically no effect on kad. Thus NEMCA is much more pronounced on the fuel-lean side, i.e. when equation (8.7) is valid. This was shown in Fig. 4.24 which depicts the effect of the po2/pc2H4 ratio in the well-mixed reactor (CSTR) on the rate under open-circuit conditions and when Uwr is set at +1 V. There is a sixtyfold increase in the rate for high po2/pc2H4 values. 

In summary the oxidation of C2H4 on Pt is one of the most thoroughly studied reactions from the point of view of NEMCA and, in view of its rather simple mechanistic scheme, one of the most thoroughly understood systems. Under fuel-lean conditions the reaction is a classical example of global promotional mle Gl, i.e. electrophobic behaviour. 

Figure 11.5. Concentration profile for N02 and CO during a switch from fuel-lean to fuel-rich conditions for a typical NSR catalyst. N02 decreases and CO increases with time [61].

Fuel rich and fuel lean conditions are based on the molar ratio of oxidizing to reducing molecules as defined by the SR ... 

SR greater than 1 refers to fuel lean conditions, while SR less than 1 refers to fuel rich conditions. To convert from fuel rich to fuel lean experimentally, a portion of the helium in the reactant gases was replaced with an equal volume of 02 using a 4-way switching valve. The pressures of the switching valve outlets were balanced such that only the reactant concentration is changed while keeping the flow rates and the pressure constant. 

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 ... 

Figure 11.16. Initial NOx storage/reduction for different catalysts under fuel lean conditions [90].

Ozturk, S. and Senkan, S. (2002) Discovery of new fuel-lean NO reduction catalyst leads using combinatorial methodologies. Appl. Catal. B Environ., 38, 243. 

In fuel-lean premixed burners, the primary air ratio determines the quality of combustion changing the rotational speed of the flue gas fan has also some influence. An ionization probe is used to determine the quality of combustion. A dedicated system, developed at GWI, provides accurate detection and analysis of the ionization signal. This includes a metering device, which is provided with a rectangular supply voltage, thus warranting very accurate ionization signals. 

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