Fuel concentration

Inert Gas Dilution. Inert gas dilution involves the use of additives that produce large volumes of noncombustible gases when the polymer is decomposed. These gases dilute the oxygen supply to the flame or dilute the fuel concentration below the flammability limit. Metal hydroxides, metal carbonates, and some nitrogen-producing compounds function in this way as flame retardants (see Flame retardants, antimony and other inorganic compounds). 

Prevention of a flammable atmosphere may be accomplished using any of the alternatives presented in NEPA 69. in cases where fuel concentration cannot be limited, the most common technique (inerting) is to add a suitable inert gas such as nitrogen, so that the residual oxygen concentration is insufficient to support a flame. A safety factor is then applied. Eor most flammable gases and vapors this typically involves reducing the oxygen concentration to less than 5-8 vol% (see Chapter 2-7 of NEPA 69). 

I he direct-flame incinerator is the simplest type of thermal oxidation system. It comprises a combustion chamber and supplementarv fuel-injection system with no energy-recovery equipment. Direct-flame incineration is suitable only for gases that support combustion without requirements for auxiliary fuel (concentrated streams) or for intermittent use. 

Equivalence Ratio The ratio of fuel concentration in the actual fuel-air mixture divided by the fuel concentration in a stoichiometric mixture. 

A massive amount of propane is instantaneously released in an open field. The cloud assumes a flat, circular shape as it spreads. When the internal fuel concentration in the cloud is about 10% by volume, the cloud s dimensions are approximately 1 m deep and 100 m in diameter. Then the cloud reaches an ignition source at its edge. Because turbulence-inducing effects are absent in this situation, blast effects are not anticipated. Therefore, thermal radiation and direct flame contact are the only hazardous effects encountered. Wind speed is 2 m/s. Relative humidity is 50%. Compute the incident heat flux as a function of time through a vertical surface at 100 m distance from the center of the cloud. 

Lean mixture A mixture of flammable gas or vapor and air in which the fuel concentration is below the fuel s lower limit of flammability (LFL). 

Flanumbility limits (or explosion limits) for a flammable gas define tlie concentration range of a gas-air ini. ture witliin wliich an ignition source can start a self-propagating reaction. Tlie minimmn and maximmn fuel concentrations in air tliat will produce a self-sustaining reaction mider given conditions are called tlie lower Jlammability limit (LFL) and tlie upper Jlammability limit (UFL). (The abbreviations LEL and UEL, for lower and upper explosivity limits, are sometimes used.) The flanunability limits are functions of... 

Two limiting cases for gasification at the fuel surface were considered. In case 1, the fuel concentration was assumed constant and independent of time, i.e., f(Cf) = Cf and in case 2, it was assumed that the fuel mass flux was constant and independent of time or pressure, i.e.,/(Cy) = — D 8Cf/ dx = rfi. Case 1 was identified with a condensed phase behaving as a boiling liquid or subliming solid, and case 2 with a polymer undergoing irreversible decomposition at constant temperature. 

Schulz, C. and Sick, V., Tracer-LIF diagnostics Quantitative measurement of fuel concentration, temperature and fuel/air ratio in practical combustion systems, Prog. Energy Combust. Sci., 31, 75, 2005. 

Appearance of the flame propagating in an axially decreasing vortex flow in a tube (fuel propane, fuel concentration 7.7%, tube 31 nun in inner diameter and 1000mm long, injector 4 slits of 2mm x 20mm, mean axial velocity 3m/s). 

Displacement speed of tribrachial flames in 2D counterflow with fuel concentration gradient. (From Ko, Y.S., Chung, T.M., and Chung, S.H., /. Mech. Sci. Technol., 16,170,2002.)... 

Flame velocity versus fuel concentration for H2/air mixtures in the 10 m long tubes of 5, 15, and 30 cm internal diameter with obstacles (orifice plates) BR = 1 - d /D - blockage ratio, where d is the orifice diameter and D is the tube diameter. (From Lee, J.H., Advances in Chemical Reaction Dynamics, Rentzepis, P.M. and CapeUos, C., Eds., 246,1986.)... 

The chemical products from complete combustion of a hydrocarbon fuel are mainly C02 and H20 (vapor). Combustion of gaseous fuel in air can occur in two different modes - one where fuel and oxygen is mixed during the combustion process, and the other where fuel and air are premixed (gas condensing boilers) and the fuel concentration must be within the flammability limits. In general the premixed situation allows the fuel to burn faster, i.e. more fuel is consumed per unit time. 

In the TWA Flight 800 tragedy the accident is blamed on explosion of fuel vapors in the central fuel tank. The volume of the central fuel tank is 18,000 gal. a. If, at the time of the explosion, the fuel concentration in the tank is 1 % by volume and the pressure inside the tank is 12.9 psia, determine the equivalent energy of explosion for the vapor (in pounds of TNT). Assume a temperature of 80°F. Be sure to state carefully any assumptions. 

Figure 7-6 Estimating a target fuel concentration at point S for taking a vessel out of service. Point M is the intersection of the LFL line with the stoichiometric line.

OSFC is the out-of-service fuel concentration, that is, the fuel concentration at point S in Figure 7-6,... 

Another approach is to estimate the fuel concentration at point S by extending the line from point R though the intersection of the minimum oxygen concentration (M) and the stoichiometric combustion line. The analytical result is... 

Table 7-1 Experimental In-Service Oxygen Concentrations (ISOCs) and Out-Of-Service Fuel Concentrations (OSFCs)1...

Equations 7-16 and 7-17 are approximations of the fuel concentration at point S. Fortunately, they are usually conservative, that is, less than the experimentally determined OSFC value. For instance, for methane the LFL is 5.3% and z is 2. Thus Equation 7-16 predicts an OSFC of 10.7% fuel. This is compared to an experimentally determined OSFC of 14.5% (Table 7-1). By using an experimental LOC of 12%, an OSFC value of 14% is determined. This is closer to the experimental value but still conservative. For ethylene, 1,3-butadiene, and hydrogen Equation 7-17 predicts a higher OSFC than the experimentally determined value. For all other species in Table 7-1, Equation 7-16 estimates an OSFC that is less than the experimental value. 

ISOC is the in-service oxygen concentration in volume % oxygen, z is the stoichiometric coefficient for oxygen given in Equation 6-9, and LFL is the fuel concentration at the lower flammability limit, in volume percent of fuel in air. 

At the specific fuel concentration of jtFuel = LFL it follows from Equation AC-10 that... 

The out-of-service fuel concentration (OSFC) is the maximum fuel concentration that just avoids the flammability zone when a vessel is being taken out of service. It is shown as point S in Figure AC-6. 

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