Oxygen-to-fuel ratio

The flame height is intimately related to the entrainment rate. Indeed, one is dependent on the other. For a turbulent flame that can entrain n times the air needed for combustion (Equation (10.34)), and r, the mass stoichiometric oxygen to fuel ratio, the mass rate of fuel reacted over the flame length, Zf, is... 

Decreasing the oxygen-to-fuel ratio, x, results in increasing demand for water (water-to-fuel ratio), with commensurate increases in the yield and concentration of hydrogen in the reformate gas. When x = 0, the equation reduces to the strongly endothermic steam reforming reaction. 

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 availability of oxygen is expressed in terms of an equivalence ratio, [21, 22, 53]. Equivalence ratio is the total mass of oxygen available for combustion per unit total mass of the polymer vapors burned, normalized by the mass stoichiometric oxygen-to-fuel ratio. For < 1, fires are fuel lean (well ventilated), and there is little effect of the amount of oxygen available for combustion. For... 

The primary function of methyl tert-butyl ether (MTBE) as an additive in gasoline is to enhance the octane level of unleaded gasoline. By virtue of the oxygen atom it contains, it increases the oxygen-to-fuel ratio in gasoline. It has also been shown that adding MTBE results in lower emissions of carbon monoxide and hydrocarbon as well as polycychc aromatic hydrocarbons (PAHs). MTBE oxidation, initiated by hydroxyl radicals, can yield a number of products such as tert-butyl formate, formaldehyde, methyl acetate, and acetone, depending on the pathway. 

At higher elevations the air is less dense—the volume of oxygen per unit volume of air is smaller. Most engines are designed to achieve a 14 1 oxygen-to-fuel ratio in the cylinder prior to combustion. If less oxygen is... 

Partial oxidation should be reacted so that the overall reaction is exothermic, but at a low oxygen-to-fuel ratio to favor higher hydrogen yields. 

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

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. 

Figure 10.2. Principle of the 2-probe oxygen sensor used to regulate the injection system to obtain the correct air-to-fuel ratio in the exhaust gas.

One of the most straightforward methods to reduce carbon dioxide emissions is to enhance the fuel efficiency of engines. The three-way catalyst, although very successful at cleaning up automotive exhaust, dictates that engines operate at air-to-fuel ratios of around 14.7 1. Unfortunately, this is not the optimum ratio with respect to fuel efficiency, which is substantially higher under lean-burn conditions at A/F ratios of about 20 1, where the exhaust becomes rich in oxygen and NOx reduction is extremely difficult (Fig. 10.1). 

A complete chemical reaction in which no fuel and no oxygen is left is called a stoichiometric reaction. This is used as a reference, and its corresponding stoichiometric oxygen to fuel mass ratio, r, can be determined from the chemical equation. A useful parameter to describe the state of the reactant mixture is the equivalence ratio, d, defined as... 

The quantity rTF o/To2,oo is the ratio of stoichiometric oxygen to fuel mass ratio divided by the available or supplied oxygen to fuel mass fractions. In contrast, for the premixed adiabatic flame temperature,... 

The chemical equation is represented in terms of the stoichiometric mass ratio of oxygen to fuel reacted, r. 

The maximum rate of fuel that can bum in the control volume dz in Figure 10.5(a) is that which reacts completely with the entrained oxygen or with a known stoichiometric ratio of oxygen to fuel, r. Thus, we can write... 

The maximum height and diameter for the fire occur when all of the fuel bums out at t = tb- The mass of the fireball at that time includes all of the fuel (mF) and all of the air entrained (me) up to that height. The mass of air entrained can be related to the excess air factor, n, the stoichiometric oxygen to fuel mass ratio, r, and the ambient oxygen mass fraction, To2j00. Thus the fireball mass at burnout is... 

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. 

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