OCR Text |
Show Predicting the Limits of Fuel/Diluent Mixtures The flammability limits for any fuel are considered to be associated with a certain critical energy release and active species-production rates, and therefore essentially with a certain critical reaction temperature. This concept has been recognized by a number of researchers in the past for the lean limit, supported by experimental evidence [2, 4]. This can also serve as the basis for an approach to determine the flammability limit values for fuel /diluent mixtures in air, merely from a knowledge of the flammability limit value of the pure fuel in air under the same corresponding conditions. It is based on the assumption that, for a limiting mixture, the final temperature in the reaction zone is at approximately the same level under the same operating conditions, no matter whether the fuel was diluted or not. For simplicity, this temperature is based on the calculated adiabatic flame temperature, since the temperature of the reaction zone can be reasonably assumed to be proportional to such a temperature. Moreover, any diluent present in the mixture may be considered associated with either the air or the fuel. It is usually assumed that the diluent is associated with the fuel. The calculated temperature of the products of adiabatic combustion of the limiting mixtures of the fuel on its o w n in air may be considered to be approximately of the same value for all limiting mixtures of the fuel and diluent for the same initial conditions and mode of propagation. From the usually known limit of the fuel in air the corresponding final product adiabatic temperature for this mixture ( T f) can be evaluated simply from the energy equation using well established thermodynamic procedures [5]. The value of this temperature can then be used to evaluate the concentration of the fuel-diluent in air ( which is the corresponding fuel-diluent limit), that on combustion under the same initial conditions produces the same value of final temperature. This temperature is usually calculated on the basis that chemical equilibrium is attained. T o establish the time needed for achieving near-equilibrium conditions in combustion systems would require the consideration of full chemical kinetics of the system, which are usually unknown, especially for fuel rich mixtures. For such limiting mixtures the equilibrium calculations would unlikely yield a meaningful estimate of the reaction-zone temperatures. Accordingly, approximating methods are justified for establishing the associated apparent flame temperature of the rich limiting homogeneous mixtures. A c o m m o n approximating approach for general consideration of the overall oxidation of common hydrocarbon fuels is on the basis of two overall stages [6, 7]. In the first stage the oxidation of the fuel yields C O and H 2 0 , only to be followed by a slower limiting step in which the C O becomes converted to C 0 2 via its reaction with the available oxygen and specifically via its reaction with mainly the O H radicals [8, 9]. For the rich mixtures, the lack of oxygen and the slowness of the overall oxidation process does not result in the further oxidation of C O within the residence time available in the flame zone before quenching occurs. Hence, it can be assumed for the purpose of calculating the combustion temperature that the available oxygen in fuel-rich mixtures is consumed on a basis of priority [4, 8]: first to oxidize the fuel carbon to carbon monoxide; second to oxidize the fuel hydrogen to water vapour; and third to oxidize the carbon monoxide to carbon dioxide ( if there is still some unconsumed oxygen available in the mixture). The adiabatic flame temperature for rich-limiting fuel/air homogeneous mixtures can be estimated according to this priority approach. The lean and rich flammability limits of various fuels with different diluents were calculated by means of the constant flame temperature approach described. The fuels considered were methane, ethylene, ethane, propane, n-butane, hydrogen and carbon monoxide. The diluents were nitrogen, carbon dioxide, helium and argon. Calculations were made for a range of fuel compositions for which |