Oxidation to fuel ratio

The nozzle of original design was fabricated from a niobium alloy coated with niobium silicide and could not operate above 1320°C. This was replaced by a thin shell of rhenium protected on the inside by a thin layer of iridium. The iridium was deposited first on a disposable mandrel, from iridium acetylacetonate (pentadionate) (see Ch. 6). The rhenium was then deposited over the iridium by hydrogen reduction of the chloride. The mandrel was then chemically removed. Iridium has a high melting point (2410°C) and provides good corrosion protection for the rhenium. The nozzle was tested at 2000°C and survived 400 cycles in a high oxidizer to fuel ratio with no measurable corrosion.O l... 

The explanation of the poor performance at high oxidizer to fuel ratios lies in the failure of the excess nitrogen tetroxide to decompose to molecular nitrogen and oxygen as would be predicted from equilibrium considerations. 

Another source of noise in a combustion system comes from the burner and is sometimes referred to as combustion roar. 33 This noise is a combination of the gas flow through the burner nozzles and also from the combustion process itself. There are many factors that affect the noise level produced by the combustion system. These include the firing rate, oxidizer-to-fuel ratio, turbulence intensity of the gas flows, combustion or mixing intensity, amount of swirl, preheat of the oxidizer or fuel, type of fuel and oxidizer, number of burners, geometry of the combustion chamber, insulation used in the combustor, and even the dampening effects of the material being heated. 

Deviate from the stoichiometric oxidizer to fuel ratio ( 2.7 1). 

Simultaneous oxidation and reduction can take place in a single catalytic bed, provided that the air-to-fuel ratio is adjusted precisely at the stoichiometric 14.7 =t 0.1. This precise metering is required for the redox or three-way catalyst as shown in Fig. 8. A narrow window exists for some catalysts where more than 80% conversion efficiency can be obtained on all three pollutants (46). This precise metering cannot be attained by... 

Fig. 9. Principle of single catalytic bed for simultaneous reduction and oxidation with oxygen sensor and feedback control on air-to-fuel ratio.

Most of the NO reducing catalysts in pellet or monolithic form begin to lose their activity at 2000 miles and fail to be effective at 4000 miles. This lack of durability may well be connected to the usage of the NO bed for oxidation purposes during the cold start, which exposes the NOx catalysts to repeated oxidation-reduction cycles. Better catalyst durability can be anticipated in the single bed redox catalyst with a tightly controlled air-to-fuel ratio, since this oxidation-reduction cycle would not take place. Recent data indicates that the all metal catalysts of Questor and Gould may be able to last 25,000 miles. 

Additionally, NO is reduced by H2 and by hydrocarbons. To enable the three reactions to proceed simultaneously - notice that the two first are oxidation reactions while the last is a reduction - the composition of the exhaust gas needs to be properly adjusted to an air-to-fuel ratio of 14.7 (Fig. 10.1). At higher oxygen content, the CO oxidation reaction consumes too much CO and hence NO conversion fails. If however, the oxygen content is too low, all of the NO is converted, but hydrocarbons and CO are not completely oxidized. An oxygen sensor (l-probe) is mounted in front of the catalyst to ensure the proper balance of fuel and air via a microprocessor-controlled injection system. 

The liquid propellant rocket combination nitrogen tetroxide (N204) and IJDMII (unsymmetrical dimethyl hydrazine) has optimum performance at an oxidizer-to-fuel weight ratio of 2 at a chamber pressure of 67 atm. Assume that the products of combustion of this mixture are N2, C02, H20, CO, H2, O, H, OH, and NO. Set down the equations necessary to calculate the adiabatic combustion temperature and the actual product composition under these conditions. These equations should contain all the numerical... 

The ratio of oxidizer to fuel will also affect the amount of heat and gas that are produced. A stoichiometric mixture of KCIO 3 and sulfur (equation 8.1) contains a 2.55 1 ratio of oxidizer to fuel, by weight. Colored smoke mixtures in use today contain ratios very close to this stoichiometric amount. The chlorate /sulfur reaction is not strongly exothermic, and a stoichiometric mixture is needed to generate the heat necessary to volatilize the dye. 

The ratio of oxidizer to fuel can be altered for a given binary mixture to achieve substantial changes in the rate of burning. The fastest burning rate should correspond to an oxidizer/fuel ratio near the stoichiometric point, with neither component present in substantial excess. Data have been published for the barium chromate /boron system. Table 6. 5 gives the bum time and heat output per gram for this system [4]. 

As mentioned earlier, the oxidation of carbon monoxide and hydrocarbons should be achieved simultaneously with the reduction of nitrogen oxides. However, the first reaction needs oxygen in excess, whereas the second one needs a mixture (fuel-oxygen) rich in fuel. The solution was found with the development of an oxygen sensor placed at exhaust emissions, which would set the air-to-fuel ratio at the desired value in real time. So, the combination of electronics and catalysis and the progress in these fields led to better control of the exhaust emissions from automotive vehicles. 

If nitrogen oxide control is one of the catalytic requirements, the stoichiometry of air-to-fuel ratio must be kept nearly stoichiometric to reduce NO then air must be added and CO and hydrocarbons oxidized in a second part of the catalyst bed. 

It is well known that freshly formed oxides have high surface areas and in addition, can be cata-lytically active,52 thereby promoting both carbon deposition and subsequent oxidation processes.53 The reduced combustion rate arising from the effects of the fire-retardant filler also contributes to lowering the rate of smoke evolution and, by improving oxygen to fuel ratios, further limits levels of smoke density.1... 

The higher the percentage of oxygen, or the higher the deposition temperature, the more complete is the combustion (oxidation) that occurs. The oxidant-to-fuel (solvent) ratio helps to control the flame temperature, size and velocity. Using pure oxygen versus air results in a more efficient and rapid combustion this in turn minimizes the formation of NO, carbon monoxide, and elemental carbon. 

In combustion calculations, one primarily wants to know the variation of the temperature with the ratio of oxidizer to fuel. Therefore, in solving flame temperature problems, it is normal to take the number of moles of fuel as 1 and the number of moles of oxidizer as that given by the oxidizer/fuel ratio. In this manner the reactant coefficients are 1 and a number normally larger than 1. Plots of flame... 

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