Fuels limiting oxygen concentrations

Limiting Oxygen Concentration (LOC) Below the limiting oxygen concentration it is not possible to support combustion, independent of the fuel concentration. The LOC is expressed in units of volume percent of oxygen. The LOC is dependent on the pressure and temperature, and on the inert gas. Table 23-1 lists a number of LOCs, and it shows that the LOC changes if carbon dioxide is the inert gas instead of nitrogen. 

The limiting oxygen concentration (LOC) is the maximum oxygen concentration of a flammable dust with air and an inert gas for which there is no self-sustaining propagation of flames for any fuel concentration. Fuel concentration is varied by varying the concentration of inert gas. The determination takes place under standardized test conditions (vid. [34]). 

Flammability limits. A flammable gas will bum in air only over a limited range of composition. Below a certain concentration of the flammable gas, the lower flammability limit, the mixture is too lean to burn, i.e., lacks fuel. Above a certain concentration, the upper flammability limit, it is too rich to burn, i.e., lacks oxygen. Concentrations between these limits constitute the flammable range. 

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). 

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. 

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. 

It is further found that the adiabatic flame temperature is approximately 1300 °C for mixtures involving inert diluents at the lower flammable limit concentration. The accuracy of this approximation is illustrated in Figure 4.19 for propane in air. This approximate relationship allows us to estimate the lower limit under a variety of conditions. Consider the resultant temperature due to combustion of a given mixture. The adiabatic flame temperature (7f ad), given by Equation (2.22) for a mixture of fuel (Xp), oxygen (Xo2) and inert diluent (Xd) originally at 7U, where all of the fuel is consumed, is... 

For determination of concentration limits for combustion of fuel gases in air (or oxygen) burning can be conducted either in glass tubes open at one end or in apparatus illustrated in Fig of Ref 6 and Fig 28 of Ref 15, p 120, reproduced here as Fig A. For determination of lower limit, the concentration of combustible gases is decreased until flame ceases to appear, while for determination of upper limits, the concentr ation is increased. 

Frey and T ien estimated theoretically the extinction limits of the diffusion flaming combustion for a thermally thin fuel according to oxygen concentration. 

Flammability Limits. As the oxygen concentration in the oxidizer increases, the flammability limits for the fuel increase. Figure 1.19 shows the increase for CH4... 

Higher Turndown Ratio. As previously shown in Figure 1.19 and Table 1.2, as the oxygen concentration in the oxidizer increases, the flammability limits for the fuel increase. This leads to increased turndown ratio for the combustion system. A flame may exist under a wider range of conditions. For example, an air/CH4 flame could exist at stoichiometries between about 1.3 and 3.8. An 02/CH4 flame could exist at stoichiometries between about 0.7 and almost 18. This is a consequence of removing the diluent N2. 

The nature of intermediates formed in diffusion flames is similar to the premixed ones, albeit differences in the contacting pattern. In Fig. 11, the species concentration profiles in a laminar ethylene diffusion flame front are presented. The fuel and oxygen diffuse toward each other undergoing virtual annihilation within the flame zone concomitant with the establishment of a peak temperature of about 1600°C. Because premixed systems provide a better control of combustor temperature, and many practical combustion devices operate under diffusion limited conditions, considerable effort has been expended to ensure the rapid mixing of fuel and oxygen in combustion chambers and approach premixed conditions. 

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