Air-to-fuel ratios

The following provides a calculation method for determining the amount of air needed for perfect combustion of one cubic foot of any gaseous fuel. The following expression provides an estimate of the ratio of the volume of air needed to the volume of fuel (i.e., the air to fuel ratio, 0) ... 

The ACF is the actual cubic feet of gas measured at t, F and P, psig. SCF represents standard conditions at 70 F and 14.6 psia. The formulas provided require input information on the pressure and temperature of the fuel gas, the fuel gas analysis by volume (or mole percent if the pressures are sufficiently low), and the percent excess air. The calculation provides the air to fuel ratio required for complete combustion. 

Overfire air (OFA) is often used in conjunction with LNBs. As the name implies, OFA is injected into the furnace above the normal combustion zone. It is added to ensure complete combustion when the burners are operated at an air-to-fuel ratio that is lower than normal. 

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. 

Aleifres, P.G., Y. Hardalupas, A.M.K.P. Taylor, K. Ishii, and Y. Urata, Flame chemiluminescence studies of cyclic combustion variations and air-to-fuel ratio of the reacting mixture in a lean-bum stratified-charge spark-ignition engine. Combustion and Flame, 136 72-90, 2004. 

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). 

Cant et al [21] focused their attention on the concentration of N20 in the automotive exhaust gas, which are rather low (14 ppm) but quite dependent on the air-to-fuel ratio. Typically 60-80% of NO is converted into N20 below the light-off temperature on Rh and then the selectivity drops at relatively high temperature 370°C [21,22] when the partial pressures of NO tends below lOTorr [22-25]. 

Table 10 Composition, molecular weight, stoichiometric air-to-fuel ratio (s), net heat of complete combustion (Ahcj), and maximum possible stoichiometric yields of major products (yi max) for ordinary polymers... 

In an extended paper on ACC catalysts, Herz and Sell350 essentially confirm the findings that Schlatter and Mitchell344 observed previously. That is, in a lean-to-rich step response over Pt/Rh/Ce/Al203, Pt/Ce/Al203, and Pt/Rh/Al203 catalysts, the reduction of CO emissions could not be completely explained on the basis of stored O. This indicated an important role of water-gas shift in ACC catalysis. In cycling between the air to fuel ratio from 14.1 to 15.1 at a rate of 2.0 Hz, they reported CO conversions of Pt/Rh/Ce (97%) > Pt/Ce (95%) > Pt/Rh (92%). 

However, a change in the flight speed and/or the flight alhtude alters the airflow rate. Then, the air-to-fuel ratio in the combushon chamber is also altered, and the thrust produced by the ducted rocket is altered. Consequenhy the flight envelope of the projechle becomes highly limited. These operahonal characteristics of the fixed fuel-flow ducted rocket reshict its application as a propulsion system. 

Fig. 15.7 Specific impulse of gasgenerating pyrolants as a function of air-to-fuel ratio.

The specific impulse of each pyrolant is computed as a function of air-to-fuel ratio, as shown in Fig. 15.7. In the computations, the pressure in the ramburner is assumed to be 0.6 MPa at Mach number 2.0for a sea-level flight When GAP pyrolant is used as a gas-generating pyrolant, the specific impulse is approximately 800 s at e = 10. It is evident that AP pyrolant and NP pyrolant are not favorable for use as gas-generating pyrolants in VFDR. However, the specific impulse and burning rate characteristics of these pyrolants are further improved by the addition of energetic materials and burning rate modifiers. 

Fig. 15.9 Specific impulse and combustion temperature of GAP pyrolants as a function of air-to-fuel ratio ramburner pressure 0.6 MPa and Mach number 2.0 at sea-level flight.

Boron particles are incorporated into GAP pyrolants in order to increase their specific impulse.[8-i2] xhe adiabatic flame temperature and specific impulse of GAP pyrolants are shown as a function of air-to-fuel ratio in Fig. 15.10 and Fig. 15.11, respectively. In the performance calculation, a mixture of the combustion products of the pyrolant with air is assumed as the reactant. The enthalpy of the air varies according to the velocity of the vehicle (or the relative velocity of the air) and the flight altitude. The flight conditions are assumed to be a velocity of Mach 2.0 at sea level. An air enthalpy of 218.2 kj kg is then assumed. 

Boron is one of the essential materials for obtaining high specific impulse of a ducted rocket However, the combustion efficiency of boron-containing gas-generating pyrolants is low due to incomplete combustion of the boron particles in the ramburner.[i3-i l Fig. 15.20 shows the combustion temperature ofa boron-containing pyrolant with and without boron combustion as a function of air-to-fuel ratio, 8. A typical boron-containing pyrolant is composed of mass fractions of boron particles b(0-30), ammonium perchlorate ap(0.40), and carboxy-terminated polybutadiene ctpb(0-30). If the boron particles burn completely in the ramburner, the maximum combustion temperature reaches 2310 K at 8 = 6.5 and v = Mach 2 p =... 

Fig. 15.19 Combustion plumes of a VFDR subjected to an SF] test (a) with an optimum air-to-fuel ratio, and (b) with a fuel-lean air-to-fuel ratio.

Fig. 15.20 Theoretical flame temperature versus air-to-fuel ratio of a boron-containing pyrolantwith and without boron combustion.

When the airflow induced from the atmosphere is introduced through the singleport intake, the mixture formed in the forward part of the ramburner is fuel-lean because all the air induced from the single-port air-intake is introduced into the forward part. Thus, an excess-air mixture (fuel-lean mixture) is formed, the temperature of which becomes too low to initiate self-ignition. However, when a multi-port intake is used, the airflow is divided into two separate flows, entering at the forward part and the rear part of the ramburner. At the upstream flow, the air-to-fuel ratio can be made stoichiometric, which allows the mixture to ignite. At the downstream flow, the excess air is mixed with the combustion products and the temperature is lowered to increase the specific impulse. 

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