Fuel rich

Hall R J and Boedeker L R 1984 CARS thermometry in fuel-rich combustion zones Appl. Opt. 23 1340-6... 

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. 

Control of SO is intrinsic to the MHD process because of the strong chemical affinity of the potassium seed in the flow for the sulfur in the gas. Although the system is operated fuel-rich from the primary combustor to the secondary combustor, the predominant sulfur compound in the gas is sulfur... 

The plant is designed to satisfy NSPS requirements. NO emission control is obtained by fuel-rich combustion in the MHD burner and final oxidation of the gas by secondary combustion in the bottoming heat recovery plant. Sulfur removal from MHD combustion gases is combined with seed recovery and necessary processing of recovered seed before recycling. 

The steam generator is a balanced draft, controlled circulation, multichamber unit which incorporates NO control and final burnout of the fuel-rich MHD combustion gases. The MHD generator exhaust is cooled in a primary radiant chamber from about 2310 to 1860 K in two seconds, and secondary air for afterburning and final oxidation of the gas is introduced in the secondary chamber where seed also condenses. Subsequent to afterburning and after the gas has been cooled down sufftciendy to soHdify condensed seed in the gas, the gas passes through the remaining convective sections of the heat recovery system. 

Third, design constraints are imposed by the requirement for controlled cooling rates for NO reduction. The 1.5—2 s residence time required increases furnace volume and surface area. The physical processes involved in NO control, including the kinetics of NO chemistry, radiative heat transfer and gas cooling rates, fluid dynamics and boundary layer effects in the boiler, and final combustion of fuel-rich MHD generator exhaust gases, must be considered. 

An oxygen storage component stores oxygen under lean operating conditions, ie, fuel-poor/air-rich, and releases it under fuel-rich, air-poor... 

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. 

In general, it has been found that much visible smoke is formed in small, local fuel-rich regions. The general approach to eliminating smoke is to develop leaner primary zones with an equivalence ratio between 0.9 and 1.5. Another supplementary way to eliminate smoke is to supply relatively small quantities of air to those exact, local, over-rich zones. 

The prompt mechanism predominates at low temperatures under fuel-rich conditions, whereas the thermal mechanism becomes important at temperatures above 2732 °F (1500 °C). Due to the onset of the thermal mechanism the formation of NOx in the combustion of fuel/air mixtures increases... 

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. 

Reburning is a process involving staged addition of fuel into two combustion zones. Coal is fired under normal conditions in the primary combustion zone and additional fuel, often gas, is added in a reburn zone, resulting in a fuel rich, oxygen deficient condition that converts the NO, produced in the primai y combustion zone to molecular nitrogen and water. In a burnout zone above the reburn zone, OFA is added to complete combustion. 

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. 

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. 

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 burner is operated on natural gas and air in a fuel-rich mode. Fluorine reacts with excess fuel to form HF and CF4. All of the air that goes through the containment cell (hood) is scrubbed with caustic before being released. (We considered destroying the fluorine by reaction with caustic, but our calculations suggested that this reaction was too slow.)... 

Typical examples of electrophilic reactions are the reduction of NO by ethylene on Pt32 and the CO oxidation on Pt under fuel-rich conditions.51,62... 

There is a third real reason for deviations from Eq. (5.18) in the case that a non-conductive insulating product layer is built via a catalytic reaction on the catalyst electrode surface (e.g. an insulating carbonaceous or oxidic layer). This is manifest by the fact that C2H4 oxidation under fuel-rich conditions has been found to cause deviations from Eq. (5.18) while H2 oxidation does not. A non-conducting layer can store electric charge and thus the basic Eq. 5.29 (which is equivalent to Eq. (5.18)) breaks down. 

On the fuel-rich side (kadPo2 kpc2H4) 0o is near zero, the oxygen adsorption step is the rls and equation (8.1) reduces to ... 

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

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