OCR Text |
Show rapidly with combustion products. Practical combustors have highly turbulent regions where the fuel is rapidly mixed with combustion products. Some of the physical and chemical processes occurring in these regions may be simulated as a stirred reactor with chemical kinetics controlling the extent of fuel consumption and the production of intermediate products including the production of air toxics species. As described below, Lutz et a1.5 have used a turbulent model which includes two stirred reactors to model the production of pollutants in a turbulent, reacting jet. The stirred reactor model allows the examination of a wide range of operating parameters such as temperature, pressure, residence time, and equivalence ratio. All these operating parameters are easily-specified, input parameters. Chemical kinetic mechanism and numerical model The chemical kinetic model was based on a previous mechanism developed for the oxidation of hydrocarbon fuels which has been documented earlier.6,7 The mechanism used treats hydrocarbons through C3 hydrocarbons and consists of 73 species and 376 reversible reactions. The oxidation of the fuel was examined under conditions of a perfectlystirred reactor where the reactants, intermediate species and products were assumed to be perfectly mixed and react for a specified residence time. The PSR (Perfectly Stirred Reactor) code by Glarborg et al.8 and CHEMKIN'9 were used to perform the calculations. In the following examples, the temperature of the reactor was specified, although alternatively, the energy equation could have be solved and the temperature calculated. Results Calculations were performed over a wide range of temperature, residence time, and equivalence ratio to examine the effect of these parameters on the consumption of the fuel and the production of intermediate products that include air toxic species. The fuel used was one typical of fuel gas used at a petrochemical refinery. The assumed composition is given in Table I. The effect of residence time of the fuel-air mixture was examined, and the results are given in Fig. 1. A stoichiometric, fuel/ air ratio was assumed, and an average reactor temperature of 1200 K was chosen. Methane, the fuel species present in highest concentration, along with carbon monoxide and formaldehyde, which are both considered to be air toxics, are shown in Fig. 1. The concentrations shown are those exiting the reactor. The carbon monoxide and formaldehyde increase initially with residence time as they are formed and then decrease with increasing residence time as they are oxidized. The methane concentration decreases monotonically with residence time as it is consumed. The effect of equivalence ratio on the fuel and air toxic concentrations are shown in Fig. 2. Their final concentrations all decrease with decreasing equivalence ratio. The excess 02 leads to more complete oxidation. In an actual combustor, the -3- |