Stoichiometric value

Unreacted NH3 in the flue gas downstream the SCR reactor is referred to as NH3 slip. It is essential to hold the NH3 slip below 5ppm, preferably 2-3 ppm, to minimize the formation of (NH4)2S04 and NH4HS04, which can cause plugging and corrosion of downstream equipment. In order to avoid the ammonia slip, and to limit the direct oxidation of NH3 to N2, the NH3/NO ratio in the feed is typically maintained below the stoichiometric values, e.g. between 0.90 and 0.95. 

Another important parameter is the selectivity in conversion of NO with respect to ammonia, being always present as a side reaction of ammonia combustion, the minimization of which is a key factor for both process economics and NO efficiency. Related to this aspect, another important parameter is the efficiency of reduction of NO when the ammonia present is less than the stoichiometric value. 

The coefficient for mp(sloux,j, p) is derived from the stoichiometric values given in the SSN. 

Some measured values of hardness are given in Table 8.1 which shows how the hardness varies with stoichiometry (Qian and Chou, 1989). The values in the table are averages of 30 measurements for each composition. The stoichiometric value is 16X the yield stress (albeit from different authors). Since hardness numbers for metals are determined by deformation-hardening rates, the latter is very large for Ni3Al causing the hardness numbers to be 16X the compressive yield stress instead of the 3X of pure metals. 

No other products were detected in the gas phase. The amount of H2 produced from 85 pmol of m-C16H34 was 4.14 mmol, which is close to the stoichiometric value. One can note that reaction 2.72 stoichiometry resembles that of steam reforming of hexadecane. The authors proposed the following mechanism, which involves the initial generation of active species holes (p+) in the valence band and electrons (e ) in the conduction band of... 

The correlation between 02 fraction and stoichiometric value X is given with the equation... 

Another possibility to adjust the combustion air flow to the supplied fuel gas flow is to maintain a constant level of C02-content in the flue gas. Thus, a stoichiometric value can be adjusted for any type of fuel gas, which varies only slightly from the optimum value. 

Furthermore factors such as stoichiometric value, heat load and design of the burner as well as the combustion chamber have a significant impact on the emission of pollutant gases. Depending on the reaction of a combustion system to a changing equivalence ratio decisions can be made how to minimize the pollutant emissions by adapting the flow rate of air or gas. A combustion control system based on monitoring the CO fraction in the flue gas could thus be considered. 

Another method is the indirect measurement of gas concentrations in a mixture To detect a flame in household gas appliances it is usual to record the ionization current, which reveals more than just the existence of a flame. The junction between the ionization of a reacting gas and the stoichiometric value 7 of this combustion is one very promising possibility of controlling the combustion [8]. 

Figure 5.17. A laminar diffusion flamelet occurs between two regions of unmixed fluid. On one side, the mixture fraction is unity, and on the other side it is null. If the reaction rate is localized near the stoichiometric value of the mixture fraction st, then the reaction will be confined to a thin reaction zone that is small compared with the Kolmogorov length scale.

If a temperature is desired at an equivalence ratio other than that listed, it is best obtained from a plot of T versus for the given values. The errors in extrapolating in this manner or from the graph are trivial, less than 1%. The reason for separate Figs 1.4 and 1.5 is that the values for = 1.0 and 4> = 1.1 overlap to a great extent. For Fig. 1.5, = 1.1 was chosen because the flame temperature for many fuels peaks not at the stoichiometric value, but between = 1.0 and 1.1 owing to lower mean specific heats of the richer products. The maximum temperature for acetylene-air peaks, for example, at a value of = 1.3 (see Table 1.2). 

C02 increases and the differences diminish. At the highest reaction enthalpies, the temperature for many fuels peaks not at the stoichiometric value, but, as stated, between

mean specific heats of the richer products. 

The development for the gradients of flashback and blowoff suggests a more appropriate plot of gBF versus , as shown in Figs. 4.35 and 4.36. Examination of these figures reveals that the blowoff curve is much steeper than that for flashback. For rich mixtures the blowoff curves continue to rise instead of decreasing after the stoichiometric value is reached. The reason for this trend is that experiments are performed in air, and the diffusion of air into... 

As the important effect of temperature on NO formation is discussed in the following sections, it is useful to remember that flame structure can play a most significant role in determining the overall NOx emitted. For premixed systems like those obtained on Bunsen and flat flame burners and almost obtained in carbureted spark-ignition engines, the temperature, and hence the mixture ratio, is the prime parameter in determining the quantities of NOx formed. Ideally, as in equilibrium systems, the NO formation should peak at the stoichiometric value and decline on both the fuel-rich and fuel-lean sides, just as the temperature does. Actually, because of kinetic (nonequilibrium) effects, the peak is found somewhat on the lean (oxygen-rich) side of stoichiometric. 

However, in fuel-injection systems where the fuel is injected into a chamber containing air or an air stream, the fuel droplets or fuel jets bum as diffusion flames, even though the overall mixture ratio may be lean and the final temperature could correspond to this overall mixture ratio. The temperature of these diffusion flames is at the stoichiometric value during part of the burning time, even though the excess species will eventually dilute the products of the flame to reach the true equilibrium final temperature. Thus, in diffusion flames, more NO, forms than would be expected from a calculation of an equilibrium temperature based on the overall mixture ratio. The reduction reactions of NO are so slow that in most practical systems the amount of NO formed in diffusion flames is unaffected by the subsequent drop in temperature caused by dilution of the excess species. 

Continued addition of 02 beyond one-half the stoichiometric value with the hydrocarbons present encourages a net destruction of the hydrocarbon radicals. For the temperature range 1200-1300 K, production of the hydrocarbon radicals via hydrogen abstraction by 02 is rapid, even assuming an activation energy of 520kJ/mol, and more than adequate to provide sufficient radicals for NO reduction in the stay time range of 125 ms. 

A peculiarity of the processes described in the patents is that all of them use isobutane-rich conditions, with isobutane-to-dioxygen molar ratios between 2 (for processes that include a relatively low concentration of inert components) and 0.8, and so closer to the stoichiometric value 0.5 (for those processes where a large amount of inert components is present). This is shown in Figure 14.1, which reports in a triangular diagram the feed composition claimed by the various companies, with reference to the flammability area at room temperature. Low isobutane conversions are achieved in all cases, and recirculation of unconverted isobutane becomes a compulsory choice. For this reason, Sumitomo claimed the oxidation of CO to CO2 (contained in the effluents from the oxidation reactor) in... 

Based on experimental results and a model describing the kinetics of the system, it has been found that the temperature has the strongest influence on the performance of the system as it affects both the kinetics of esterification and of pervaporation. The rate of reaction increases with temperature according to Arrhenius law, whereas an increased temperature accelerates the pervaporation process also. Consequently, the water content decreases much faster at a higher temperature. The second important parameter is the initial molar ratio of the reactants involved. It has to be noted, however, that a deviation in the initial molar ratio from the stoichiometric value requires a rather expensive separation step to recover the unreacted component afterwards. The third factor is the ratio of membrane area to reaction volume, at least in the case of a batch reactor. For continuous opera-... 

Table 5.62 lists the chemical compositions of natural feldspars, taken from Deer et al. (1983). Feldspar chemistry is solved on a 32-oxygen anhydrous basis, corresponding to four formula units. The X cation summation is quite close to the stoichiometric value (4) the Z cations are also close to the theoretical value (16). 

At present the most effective available after-treatment techniques for NO, removal under lean conditions are ammonia selective catalytic reduction (SCR) [1-3] and NO, storage reduction (NSR) [4—6]. Indeed, three-way catalysts (TWCs) are not able to reduce NO, in the presence of excess oxygen, because they must be operated at air/ fuel ratios close to the stoichiometric value. Also, non-thermal plasma (NTP) and hydrocarbon-selective catalytic reduction (HC-SCR) are considered, although they are still far from practical applications. 

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