Fuel-to-Oxidant Ratio

At still higher temperatures, when sufficient oxygen is present, combustion and "hot" flames are observed the principal products are carbon oxides and water. Key variables that determine the reaction characteristics are fuel-to-oxidant ratio, pressure, reactor configuration and residence time, and the nature of the surface exposed to the reaction 2one. The chemistry of hot flames, which occur in the high temperature region, has been extensively discussed (60-62) (see Col ustion science and technology). 

For some elements, especially those which tend to form thermally stable oxides, fuel-to-oxidant ratio may have a dramatic effect upon atomic absorbance signal. Figure 4, for example, illustrates the effect of increasing fuel flow upon aluminium determination. 

One point which is often overlooked when optimizing fuel-to-oxidant ratio is that the optimum fuel flow is sometimes matrix-dependent. For example, the determination of calcium in water using an air-acetylene flame is more sensitive if a fuel-rich flame is used. If, however, the samples contain dilute sulfuric acid, a more fuel-lean flame usually gives substantially improved sensitivity. As a general rule, it is best to find optimal conditions for the particular matrix which you are analysing. 

Both AAS and AFS with flame atomizers depend upon the stable and reproducible production of ground state atoms. Therefore atomizer parameters such as fuel-to-oxidant ratio or burner head position relative to the excitation beam will be equally important in both techniques. The major difference in AFS lies with the types of flames sometimes employed. 

The extent of chemical interferences in flame spectrometry varies with flame conditions and analyte concentration. Thus it is most unlikely that the same wrong answer will be obtained at two different heights in the flame or at two different fuel-to-oxidant ratios. Indeed it has been suggested that the former of these two options may provide automated detection of chemical interferences.2 The burner was moved up and down using a microprocessor-controlled stepper motor. Alternatively, results in air-acetylene and nitrous oxide-acetylene flames may be compared. Similarly, it is unlikely that the same wrong answer will be obtained at two different dilutions. Thus if it is thought that there might be a risk of chemical interference, the determinations on a selection of samples should be repeated under diverse conditions, either on the same or different instruments. 

Optimization in Flame AAS Source-related Parameters Effect of Lamp Current Effect of Lamp Warm Up Time Lamp Alignment Lamp Deterioration Choice of Lamp Atomizer-related Parameters Choice of Atomizer Effect of Fuel-to-oxidant Ratio Optimization of Burner Position Burner Design, Warm Up, and Cleanliness Gas Flow Stability Monochromator-related Parameters Choice of Slit Width Choice of Wavelength Optimization in Flame AFS Source-related Parameters Lamp Operating Parameters Lamp Alignment Atomizer-related Parameters Monochromator-related Parameters Optimization in Flame AES... 

Fig. 4 Stability loop (]) = fuel to oxidant ratio, 2 = loading factor...

FIGURE 32. Effect of height above burner on antimony absorption A, air-hydrogen flame, fuel-to-oxidant ratio 1.2 B, air-acetylene flame, fuel-to-oxidant ratio 0.7. Other conditions as for Figure 31. Reproduced from Reference 222 by permission of The Royal Society of Chemistry... 

EFFECT OF FUEL TO OXIDIZER RATIO ON BURN RATE... 

As the fuel to oxidizer ratio varies from the ideal ratio (that required for complete burning), burn rate generally decreases. 

In (c) and (d) the response AVto saturated hydrocarbons at 600°C as a function of equivalence ratio a is given, as well as the response to unsaturated hydrocarbons at temperatures of 100-400°C and concentrations well below the equivalence ratio. The equivalence ratio is defined as the ratio of the actual fuel-to-oxidizer ratio to the stoichiometric fuel-to-oxidizer ratio. Panels (a) and (b) are reprinted with permission from the Journal of Applied Physics 98 3 (2005), 034903. 2012 American Institute of Physics (Eriksson eta ., 2005). Panel (c) is reprinted with permission from Sensors and Actuators B43 (1997), 52-5. 1997 Elsevier (Baranzahi ef a/., 1997). Panel (d) is reprinted with permission from the Proceedings of the IEEE International Conference on Sensors, Atlanta, Georgia, USA, October 2007, 493-4. 2007 IEEE (Andersson etal., 2007)... 

The processes listed above occur in an environment that includes fuel and oxidant molecules and the products of the reactions between the fuel and the oxidant. The temperature of the flame, which is primarily responsible for the occurrence of these processes, is determined by several factors, including (1) type of fuel and oxidant, (2) the fuel-to-oxidant ratio, (3) type of solvent, (4) amount of solvent entering the flame, (5) type of burner, and (6) the region in the flame that is focused onto the entrance slit of the spectral isolation unit. 

Careful adjustment of fuel-to-oxidant ratio, as well as total fuel-oxidant volume, is required for best response of a flame source. Some elements are best determined in a fuel-rich environment and others in a fuel-lean flame. Control of fuels and oxidants normally is made by use of pressure-reducing valves. Two sets of valves are required, one on the tank for pressure reduction, and the other for close control of the volume of gases entering the burner or mixing chamber. Control by pressure-reducing valves is usually sufficient to produce good, steady flames but does not provide close adjustment or monitoring of the quantities of fuel and oxidant consumed. In some cases it may be advisable to install flow meters in the gas supply systems so rates of gas consumption can be monitored and controlled. 

Flame temperatures and fuel-to-oxidant ratios are two of the most important parameters to consider when using a flame as an atomic absorption sample cell. Table 10-2 lists maximum temperatures attainable with various combinations of fuel and oxidant. 

The entrance and exit slits of the monochromator also should be adjusted if variable slit widths are available. Proper slit width adjustments can increase the signal-to-noise ratio as well as sensitivity. Relatively narrow slits usually provide best signal-to-noise conditions. The fuel-to-oxidant ratio also should be optimized for each element. 

The effect of fuel to oxidizer ratio was investigated in SCS processes for synthesizing transition metal nanoparticles in aqueous solutiorrs. The thermodynamic calcirlatiorrs predict the possibility of synthesizing Ni, Cu and... 

Co using their respective metal nitrate and glycine mixture. Nickel was chosen as a model to successfully verify the thermodynamic predictions. It can be concluded from the results and discussion that the fuel to oxidizer ratio, (p, is an important parameter in SCS systems and significantly influences the synthesized nanoparticles. The q> value not only affects the combustion temperature but also the nature of the solid product (metal or metal-oxide), porosity and crystalhte size. It is anticipated that the other metal-systems (Cu and Co) will also follow a similar trend. The properties of the synthesized nanoparticles can be controlled and fine-tuned by adjusting the fuel to oxidizer ratio in SCS processes. 

The temperatures of several common analytically useful flames are given in Table 1. These are the so-called theoretical temperatures, calculated for stoichiometric fuel-oxidant gas mixtures by Snelleman. They are roughly one hundred degrees higher than most measured temperatures. Moreover, the stoichiometric mixtures do not give the highest attainable temperatures these are reached at somewhat higher fuel-to-oxidant ratios, especially for the air-acetylene flame, due at least in part to air entrainment. Fuel richness also alters rates and extents of chemical reactions in flames. In any case, the tabulated values show the relative temperatures of useful flames. 

As shown in Figure 9-2. important regions of a flame include the primary combustion zone, the interzonal region, and the secondary combustion zone. The appearance and relative size of these regions vary considerably with the fuel-to-oxidant ratio as well as with the type of fuel and oxidant. The primary combustion zone in a hydrocarbon flame is recognizable by its blue luminescence arising from the band emission of C-, CH, and other radicals. Thermal equilibrium is usually not achieved in this region, and it is, therefore, rarely used for flame spectroscopy. 

Fortunately, with flame atomization, spectral interferences by matrix products are not widely encountered and often can be avoided by variations in the analytical variables, such as flame temperature and fuel-to-oxidant ratio. Alternatively, if the source of... 

It is usually possible to obtain the desired burn rate with a pyrotechnic mixture by the appropriate selection of components as well as by varying particle sizes of the components and adjusting the fuel-to-oxidizer ratio. Interest in propellants has continued to focus on faster and faster burning materials, however, and hence the interest in catalysts has become signihcant. 

The parameters which influence the redox reaction include type of fuel, fuel to oxidizer ratio as well as water content [77-82]. In general, a good fuel should tract nonvio-lently, produce nontoxic gases, and acts as a complexant for metal cations [83]. 

The properties of the obtained oxide powders [such as crystalline or amorphous structure, oxidation state of the metal cations, size, surface area, particle clustering, and extent and nature (hard and soft) of agglomeration] are also strongly depended on adopted processing parameters [84]. They are primarily governed by the enthalpy or flame temperature generated during combustion, which is itself dependent on the nature of the fuel and fuel-to-oxidant ratio [85]. 

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