Fuel rich flames

Experimental evidence for this mechanism has been found in low pressure H2/ air flames, fuel-rich H2/02/N2 and CH4/02/N2 flames at atmospheric pressure and... 

Burning of the fuel is a stepwise degradation leading to many intermediate products and formation of radicals. Some of these are of interest because of their ability to interact with metallic atoms, notably atomic oxygen and the OH radical. If the available oxygen equals the theoretical amount necessary to bum the fuel completely, such a flame is called stoichiometric. Otherwise, depending on the amount of fuel, we speak of a fuel-rich or a lean flame. Where fuel and metallic atoms compete for O and OH radicals, the formation of metal oxides can be kept down by making the flame fuel-rich. Fassel et al. (F2, F3) have recently shown that satisfactory atomic vapor concentrations can be produced in fuel rich flames with a a iety of metals particularly prone to form refractory oxides in stoichiometric flames. 

Allan (A7) during his studies of ashed plant materials also investigated interferences. Using an air-acetylene flame, sodium, potassium, calcium, magnesium, and phosphate had no effect. In the air-coal gas flame, as employed by Elwell and Gidley (E2), recoveries of iron were only 80-90% when the test solutions contained an excess of calcium, copper, aluminum, titanium, and zirconium. With silicon added, iron recovery was 26%. Owing to incomplete vaporization of iron in the flame, sensitivities attained are higher in the air-acetylene flame and lower in the air-coal gas flame. Since iron is subject to oxidation in the flame, fuel-rich flames are preferable. 

Low concentrations of boron can be determined in water (0.4-0.6 mg dm ) by atomic absorption spectroscopy using nitrous oxide/acetylene flame (fuel rich) and measuring absorbance at 249.8 nm. 

Thermal energy in flame atomization is provided by the combustion of a fuel-oxidant mixture. Common fuels and oxidants and their normal temperature ranges are listed in Table 10.9. Of these, the air-acetylene and nitrous oxide-acetylene flames are used most frequently. Normally, the fuel and oxidant are mixed in an approximately stoichiometric ratio however, a fuel-rich mixture may be desirable for atoms that are easily oxidized. The most common design for the burner is the slot burner shown in Figure 10.38. This burner provides a long path length for monitoring absorbance and a stable flame. 

Many hydrocarbon flames are luminous because of the incandescent carbon particles formed in the flames. Under certain conditions, these particles are released from the luminous flames as smoke. Smoke from hydrocarbons is usually formed when the system is fuel rich, either overall or locally. 

Prompt NO Hydrocarbon fragments (such as C, CH, CH9) may react with atmospheric nitrogen under fuel-rich conditions to yield fixed nitrogen species such as NH, HCN, H9CN, and CN. These, in turn, can be oxidized to NO in the lean zone of the flame. In most flames, especially those from nitrogen-containing fuels, the prompt... 

Unbumed Hydrocarbons Various unburned hydrocarbon species may be emitted from hydrocarbon flames. In general, there are two classes of unburned hydrocarbons (1) small molecules that are the intermediate products of combustion (for example, formaldehyde) and (2) larger molecules that are formed by pyro-synthesis in hot, fuel-rich zones within flames, e.g., benzene, toluene, xylene, and various polycyclic aromatic hydrocarbons (PAHs). Many of these species are listed as Hazardous Air Pollutants (HAPs) in Title III of the Clean Air Act Amendment of 1990 and are therefore of particular concern. In a well-adjusted combustion system, emission or HAPs is extremely low (typically, parts per trillion to parts per billion). However, emission of certain HAPs may be of concern in poorly designed or maladjusted systems. 

Partially Premixed Burners These burners have a premixing section in which a mixture that is flammable but overall fuel-rich is generated. Secondary combustion air is then supplied around the flame holder. The fuel gas may be used to aspirate the combustion air or vice versa, the former being the commoner. Examples of both are provided in Figs. 27-33 and 27-34. 

Combustion behavior differed in some respects between continuous and instantaneous spills, and also between LNG and refrigerated liquid propane. For continuous spills, a short period of premixed burning occurred immediately after ignition. This was characterized by a weakly luminous flame, and was followed by combustion of the fuel-rich portions of the plume, which burned with a rather low, bright yellow flame. Hame height increased markedly as soon as the fire burned back to the liquid pool at the spill point, and assumed the tilted, cylindrical shape that is characteristic of a pool fire. 

Similar behavior was observed for LNG clouds during both continuous and instantaneous tests, but average flame speeds were lower the maximum speed observed in any of the tests was 10 m/s. Following premixed combustion, the flame burned through the fuel-rich portion of the cloud. This stage of combustion was more evident for continuous spills, where the rate of flame propagation, particularly for LNG spills, was very low. In one of the continuous LNG tests, a wind speed of only 4.5 m/s was sufficient to hold the flame stationary at a point some 65 m from the spill point for almost 1 minute the spill rate was then reduced. 

Figure 5.3 shows a moment of flame propagation in an unconfined propane cloud. On the left side, a flame is propagating through a premixed portion of the cloud its flame is characteristically weakly luminous. In the middle of the photograph, fuel-rich portions of the cloud are burning with characteristically higher flames in a more-or-less cylindrical, somewhat tilted, flame shape. 

As far as flame composition is concerned, it may be noted that an acetylene-air mixture is suitable for the determination of some 30 metals, but a propane-air flame is to be preferred for metals which are easily converted into an atomic vapour state. For metals such as aluminium and titanium which form refractory oxides, the higher temperature of the acetylene-nitrous oxide flame is essential, and the sensitivity is found to be enhanced if the flame is fuel-rich. 

Procedure (ii). Make certain that the instrument is fitted with the correct burner for an acetylene-nitrous oxide flame, then set the instrument up with the calcium hollow cathode lamp, select the resonance line of wavelength 422.7 nm, and adjust the gas controls as specified in the instrument manual to give a fuel-rich flame. Take measurements with the blank, and the standard solutions, and with the test solution, all of which contain the ionisation buffer the need, mentioned under procedure (i), for adequate treatment with de-ionised water after each measurement applies with equal force in this case. Plot the calibration graph and ascertain the concentration of the unknown solution. 

A double-beam atomic absorption spectrophotometer should be used. Set up a vanadium hollow cathode lamp selecting the resonance line of wavelength 318.5 nm, and adjust the gas controls to give a fuel-rich acetylene-nitrous oxide flame in accordance with the instruction manual. Aspirate successively into the flame the solvent blank, the standard solutions, and finally the test solution, in each case recording the absorbance reading. Plot the calibration curve and ascertain the vanadium content of the oil. 

The negative ion concentrations all show very definite maxima on the fuel-rich side of stoichiometric flame composition. 

Mere destruction of the original hazardous material is not, however, an adequate measure of the performance of an incinerator. Products of incomplete combustion can be as toxic as, or even more toxic than, the materials from which they evolve. Indeed, highly mutagenic PAHs are readily generated along with soot in fuel-rich regions of most hydrocarbon flames. Formation of dioxins in the combustion of chlorinated hydrocarbons has also been reported. We need to understand the entire sequence of reactions involved in incineration in order to assess the effectiveness and risks of hazardous waste incineration. 

For the analysis of the chemical structure of flames, laser methods will typically provide temperature measurement and concentration profiles of some readily detectable radicals. The following two examples compare selected LIF and CRDS results. Figure 2.1 presents the temperature profile in a fuel-rich (C/O = 0.6) propene-oxygen-argon flame at 50 mbar [42]. For the LIF measurements, 1% NO was added. OH-LIF thermometry would also be possible, but regarding the rather low OH concentrations in fuel-rich flames, especially at low temperatures, this approach does not capture the temperature rise in the flame front [43]. The sensitivity of the CRDS technique, however, is superior, and the OH mole fraction is sufficient to follow the entire temperature profile. Both measurements are in excellent agreement. For all flames studied here, the temperature profile has been measured by LIF and/or CRDS. 

Temperature profile measurement in a flat, premixed fuel-rich pro-pene low-pressure flame by LIF, using seeded NO, and CRDS, using naturally present OH radicals. (Adapted from Figure 3 in Kohse-Hoinghaus, K. et al., Z. Phys. Chem., 219,583,2005.)... 

Comparison of the C2 radical mole fraction profile measured by LIF and CRDS in a flat, premixed fuel-rich propene flame at 50mbar. 

Benzene profile in a fuel-rich propene flame (C/O = 0.77), measured independently with three different mass spectrometric techniques X(. mole fraction, h height above burner. 

Mole fraction profiles of C3H3 in a series of fuel-rich propene flames (C/O = 0.5) doped with ethanol, measured by EI-MBMS (left) and VUV-PI-MBMS (right). The inset shows the effect of ethanol addition on the peak mole fraction, normalized to the value in the undoped flame r, mole fraction, h. height. 

Intermediate species concentrations in fuel-rich flames (C/O = 0.5) of the two isomeric esters methyl acetate (left) and ethyl formate (right) burnt under identical conditions mole fraction, h height species named on the left side of each graph correspond to left y-axes, species on the right to right y-axes. 

Lamprecht, A., Atakan, B., and Kohse-Hoinghaus, K., Fuel-rich propene and acetylene flames A comparison of their flame chemistries. Combust. Flame, 122, 483, 2000. 

Kohse-Hoinghaus, K. et al.. The influence of ethanol addition on premixed fuel-rich propene-oxygen-argon flames, Proc. Combust. Inst., 31,1119, 2007. 

Hansen, N. et al.. Initial steps of aromatic ring formation in a laminar premixed fuel-rich cyclopentene flame, /. Phys. Chem. A, 111, 4081,2007. 

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