Hydrocarbon fuels ratio

Fig. 3. Theoretical mole percent of the principal combustion products of hydrocarbon fuels for fuel hydrogen carbon ratios from 1, eg, to 4, eg, CH, ...

The metallic catalysts for exliaust pollution control are designed to perform three functions. The air/fuel ratio employed in combustion engines creates exhaust products which are a mixture of hydrocarbons, carbon oxides, and niU ogen oxides. These must be rendered environmentally innocuous by reactions on the catalyst such as... 

The second important parameter affecting efficiency is air/fuel ratio. For evei y hydrocarbon fuel, there is an air/fuel ratio that, in principle, causes all the hydrogen in the fuel to burn to water vapor and all the carbon in the fuel to burn to carbon dioxide. This chemically correct proportion is called the stoichiometric ratio. 

It has been reported by Lewis et al. [1] that the equivalence ratio where the minimum ignition energy has a minimum is dependent on the fuel property for hydrocarbon fuel and air mixtures, and that it moves to the rich side as the molecular weight of the fuel increases. This equivalence ratio dependency has been explained by the preferential diffusion effect. 

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. 

Ferguson, B. C Monitor Boiler Fuel Density to Control Air/Fuel Ratio, Hydrocarbon Processing, Feb. 1974. 

Figure 12 presents the results of these experiments. After exposure to rich exhaust for 1.0 s, the oxygen content of the catalyst bed decreased 36% of the way from its lean steady-state level to its rich steady-state level. This change in oxygen content could correspond to a conversion of 58% of the CO and in the 1.0 s pulse of rich exhaust (hydrocarbons are less reactive with the oxygen held by the catalyst than CO and I ). The difference in catalyst oxygen content between the rich and lean steady-states is sufficiently large to account for the extra conversion obtained in the CO step response experiment shown in Figure 9B. In addition, the rates of change of catalyst oxygen content shown in Figures 11 and 12 are sufficiently fast to affect the performance of the catalyst when the air-fuel ratio is cycled about the stoichiometric point at frequencies on the order of 1 Hz.

Losses of valuable components through waste streams The chemical analysis of various plant exit streams, both to the air and water, should indicate if valuable materials are being lost. Adjustment of air-fuel ratios in furnaces to minimize hydrocarbon emissions and hence fuel consumption is one such example. Pollution regulations also influence permissible air and water emissions. 

Reported flame speed results for most fuels vary somewhat with the measurement technique used. Most results, however, are internally consistent. Plotted in Fig. 4.21 are some typical flame speed results as a function of the stoichiometric mixture ratio. Detailed data, which were given in recent combustion symposia, are available in the extensive tabulations of Refs. [24-26], The flame speeds for many fuels in air have been summarized from these references and are listed in Appendix F. Since most paraffins, except methane, have approximately the same flame temperature in air, it is not surprising that their flame speeds are about the same (—45 cm/s). Methane has a somewhat lower speed (<40 cm/s). Attempts [24] have been made to correlate flame speed with hydrocarbon fuel structure and chain length, but these correlations... 

The stoichiometric molar (volumetric) fuel-air ratio is strictly proportional to the molecular weight of the fuel for two common hydrocarbon fuels that is,... 

FIGURE 8.13 Critical sooting equivalence ratios ( />c) based on CO and H20 stoichiometry of various hydrocarbon fuels as a function of calculated adiabatic flame temperature Tf. 

FIGURE 8.14 Critical sooting equivalence ratio l c at 2200K as a function of the number C—C bonds in hydrocarbon fuels. +, 0, and - indicate ethane/l-octane mixtures in molar ratios of 5 to 1, 2 to 1 and 1 to 2, respectively x, acetylene/benzene at a molar ratio of 1 to 3. The O symbol for 2 to 1, falls on top of the butene symbol. 

Following the conceptual idea introduced by Milliken [68], Takahashi and Glassman [53] have shown, with appropriate assumptions, that, at a fixed temperature, i/c could correlate with the number of C—C bonds in the fuel and that a plot of the log ipc versus number of C—C bonds should give a straight line. This parameter, number of C—C bonds, serves as a measure of both the size of the fuel molecule and the C/H ratio. In pyrolysis, since the activation energies of hydrocarbon fuels vary only slightly, molecular size increases the radical pool size. This increase can be regarded as an increase in the Arrhenius pre-exponential factor for the overall rate coefficient and hence in the pyrolysis and precursor formation rates so that the C/H ratio determines the OH concentration [12]. The 4>c versus C—C bond plot is shown in Fig. 8.14. When these... 

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. 

Figure 7-17 Conversions of hydrocarbons, CO (oxidation reactions), and NO (reduction reactions) versus air-fuel ratio. The engine should operate near the stoichiometric ratio to obtain maximimi conversions in all reactions, and cars are tuned to operate within this window of composition (dashed lines).

Recent emission control system development in the automotive industry has been directed mainly towards the use of three-way or dual bed catalytic converters, This type of converter system not only oxidizes the hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas but will also reduce the nitrous oxides (NO ). An integral part of this type of system is the exhaust oxygen sensor which is used to provide feedback for closed loop control of the air-fuel ratio. This is necessary since this type of catalytic converter system operates efficiently only when the composition of the exhaust gas is very near the stoichiometric point. 

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