Fuel-Oxidant Ratio

A fuel is a substance capable of burning the CH bonds (electrons acceptor). An oxidant is a substance that helps in burning, providing oxygen (electrons donor). Only when the oxidizer and fuel are intimately mixed in an appropriate proportion can an exothermic chemical reaction be initiated that generates substantial heat. The temperature reached when the reaction starts in the oxidizer and fuel solution, is called the ignition temperature. 

The ratio of fuel and oxidizer is considered one of the most important parameters in determining the properties of synthesized powders obtained by combustion. Product properties such as crystallite size, surface area, morphology, phase, degree and nature of agglomeration, are generally controlled by adjusting the fuel-oxidant ratio. 

The fuel-oxidant ratio determines the influence of gases on the morphology of the particles. The pore size depends on the fiiel-oxidant ratio, because the greater the amount of fuel, the larger the pore size of the particles. 

Recent research on SCS has investigated the role of fuel in the control of particle size and microstructure of the products under different fuel-oxidant ratios (Table 2.3). The fuel-oxidant ratio, however, is not always calculated using a thermodynamic modeling and/or theory of the propellants. 

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The erosion of graphite in nozzle appHcations is a result of both chemical and mechanical factors. Changes in temperature, pressure, or fuel-oxidizing ratio markedly affect erosion rates. Graphite properties affecting its resistance to erosion include density, porosity, and pore size distribution... 

The C-C and C-H BDEs for ethane are 377.0 and 423.0kJ/mol, respectively, and the H-0 BDE of hydroperoxyl (HO2 ) radical is only 207.5kJ/mol. The large reaction endothermicities for reactions 6.4 and 6.5 highlight the unlikeliness of their occurrence at lower temperatnres. In pyrolytic (heat, but no oxidant) or high fuel/oxidant ratio conditions, the endothermicities indicate that ethane and other alkanes will... 

Incomplete dissociation of alkaline-earth oxides and hydroxides often causes unwanted molecular bands to be stronger than atomic lines. This can often be cured by altering flame temperatures or fuel/oxidant ratio, to shift the equilibrium in the desired direction. 

However, the combustion process for methane requires no fewer than 325 individual mechanistic steps (elementary reactions) to be accurately described, rather than the one-step route shown above. As such, incomplete combustion is a common occurrence and ROS are pervasive byproducts of that phenomenon, affecting an engine s fuel efficiency and producing atmospherically detrimental emissions. Moreover, combustion varies with system temperature, as different oxidative pathways become accessible, as well as fuel/oxidizer ratio (equivalence ratio). By examining the representative cases of methane oxidation at high and low temperatures, this phenomenon becomes clearer. 

Any refractory material that does not decompose or vaporize can be used for melt spraying. Particles do not coalesce within the spray. The temperature of the particles and the extent to which they melt depend on the flame temperature, which can be controlled by the fuel oxidizer ratio or electrical input, gas flow rate, residence time of the particle in the heat zone, the particle-size distribution of the powders, and the melting point and thermal conductivity of the particle. Quenching rates are very high, and the time required for the molten particle to solidify after impingement is typically 10-4 to... 

In this reaction most of the energy released is derived from the formation of water, and the fuel-oxidizer ratio is adjusted to leave some of the hydrogen unoxidized to achieve an appropriate balance between the release of heat and the molecular weight of the combustion products. 

In direct aspiration FLAA, a sample in the aerosol form is aspirated into a flame fueled with acetylene. An oxidant (air or nitrous oxide) is mixed with the acetylene fuel to create the necessary temperature conditions. Depending on the fuel/oxidant ratio, the temperature of the flame may range from 2400 to 2800°C. Precise temperature control is critical in FLAA analysis, as the concentrations of ionized and unionized species of a vaporized element are sensitive to the temperature of the flame. 

Thus, it can be seen that there are several variables associated with the flame atomiser that must be optimised to achieve the best sensitivity and detection limit. The flame must be correctly positioned with respect to the light path. The fuel oxidant ratio should be investigated to establish the optimum chemical environment for atomisation. The nebuliser and impact bead (where fitted) must be optimised to produce, overall, the best signal-to-noise ratio. 

This model assumes the critical ignition temperature to be equal to T, temperature at which the fuel is gasified in combustion. The temperature T is assumed to be equal to that of the adiabatic flame with a stoichiometric fuel/oxidant ratio. De Ris noted important features of flame spread along a material surface, but the fact that T, must be determined experimentally reduces the potential of the method for predicting the flame spread rates. 

The flame is responsible for production of free atoms. Flame temperature and the fuel/oxidant ratio are very important in the production of free atoms from compounds. Many flame fuels and oxidants have been studied over the years, and temperature ranges for some flames are presented in Table 6.1. In modem commercial instmments, only air-acetylene and nitrous oxide-acetylene flames are used. 

The loss of free atoms in the atomizer is also a function of the chemistry of the sample. If the oxide of the analyte element is readily formed, the free atoms will form oxides in the flame and the population of free atoms will simultaneously decrease. This is the case with elements such as chromium, molybdenum, tungsten, and vanadium. On the other hand, some metal atoms are stable in the flame and the free atoms exist for a prolonged period. This is particularly so with the noble metals platinum, gold, and silver. Adjusting the fuel/oxidant ratio can change the flame chemistry and atom distribution in the flame as shown in Fig. 6.17(b). Atoms with small ionization energies will ionize readily at high temperatures (and even at moderate temperatures). In an air-acetylene flame, it can be shown that moderate concentrations of potassium are about 50% ionized, for example. Ions do not absorb atomic lines. 

An important additional restriction on flame structure studies of kinetics is provided by the practical problem of maintaining one dimensional flame stability over a wide range of compositions, which limits the ranges of fuel/oxidizer ratios and percentage dilution by inert additives accessible to experimental study. The advantages to the kineticist of freedom in the selection of these quantities has been stressed above. The choice of experimental conditions in flames is also limited by the direct relationship between flame temperature and initial composition. In the shock tube where heating is provided by an outside source, i.e., the shock wave, these two variables can be selected independently of one another. The instability problem with exothermic reactions in shock waves is of a different character from that in flames. 

In all but the most extreme flames, HaO and Ha will be major constituents of the flame gases, whose ratio will not depend to any significant extent on whether or not the radicals are equilibrated, though it will depend on the fuel/oxidant ratio. Determination of the... 

Temperatures can vary slightly from the values given depending on the fuel oxidant ratio used. Acetylene-based flames are the most commonly used for analytical spectroscopy. 

In the case of the synthesis of alumina, chrome, nickel, iron and nanocrystalline cobalt oxides using the solution combustion technique, for example, we lack, so far, a deep understanding of the influence of the fuel-oxidant ratios well as a model of the thermodynamic variables associated with enthalpy, adiabatic flame temperature and the total number of moles of gas generated related to the powder characteristics, such as crystallite size and surface area. 

In fact, the reaction mechanism of the combustion is very complex. There are several parameters influencing the reaction such as the type of fuel, fuel-oxidizer ratio, use of excess oxidizer, ignition temperature, and amount of water contained in the precursor mixture. In general, a good combustion synthesis does not react violently, produces non-toxic gases and acts as a complexant for metal cations. 

The main parameters of combustion that have been widely investigated in the literature are type of flame, temperature, generated gases, air-fuel-oxidant ratio and chemical composition of the precursor reagents. 

Table 2.3 Study of the influence of fuel-oxidant ratio in obtaining ceramic oxides by SCS...

Early in the development of atomic absorption spectroscopy it was recognized that enhanced absorbances could be obtained if the solutions contained low-molecular-weight alcohols, esters, or ketones. The effect of organic solvents is largely attributable to increased nebulizer efficiency the lower surface tension of sueh solutions results in smaller drop sizes and a resulting increase in the amount of sample that reaches the flame. In addition, more rapid solvent evaporation may also contribute to the effect. Leaner fuel-oxidant ratios must be used with organic solvents to offset the presence of the added organic material. Unfortunately, however, the leaner mixture produces lower flame temperatures and an increased potential for chemical interferences. 

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