煤前駆体の生成シミュレーション

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Any soot formation model should be able to reproduce an experimentally observed bell-shaped dependence of soot yield on temperature. We start to construct a model on the shock-tube pyrolysis of acetylene by replacing soot with soot precursor and examine the rate parameters of reactions included in a model. Since Ogura found the maximum yield of C4H4 at l500K in his shock tube experiment carried out over the temperature range from 1000 to l700K, C4H4 was considered as an appropriate soot precursor reflecting the main feature of soot formation. The most serious points of modeling are the discrimination of reversible, elementary steps and the evaluation of the rate constants of the reverse reactions. To accomplish first of them, potential energy diagrams were examined based on the heat of formation of the intermediate species encountered on the reaction path. On the second problem, to check the reliability of the heats of formation, 5 cases shown in Table 2 were studied. To simplify the treatment we restrict the participation of reacting species to C2 and C4 hydrocarbons as listed in Table 2,and a model composed of 35 chemical reactions as shown in Table 3 was examined. The rate constants of the forward reactions are taken from our compiled file of various types of C-H-O reactions. The rate parameters referred by the largest number of authors are preferred, but when they are scattered widely, the rate expression near the center of the dispersion was selected. The computed results are compared with the experimental values of C4H4,and C4H2 obtained by shockheated sample gases of 5% in argon at total density = 2.35E-5 mole/cm3 and reaction time of 1 msec. To realize the reasonable coincidence with the experimental yield of C4H4 at l500K, some rate constants of sensitive reactions are modified by multiplying adjusting factors. Sensitivity coefficients of most of the reactions are small and suggest the existence of equilibrium. It was unable to obtain a consistent result in case A unless modifying adjusting factors more than two orders of magnitude. This means the value of 114 kcal/mole for the heat of formation of C2H is unsuitable. On the contrary, in the cases of B to E, it is possible to simulate the maximum C4H4 yield at l50OK by the slight change of the adjusting factors. As clearly shown in Fig. 3,the calculated profile of case B shifts to the lower value at the lower temperature side. According to the recent paper written by Kiefer, von Drasek on the same subject, an excellent coincidence was obtained in the whole temperature range. To reproduce this agreement we construct a simplified scheme F shown in Table 5. We changed the activation energies of Q39 and Q45 in our scheme B to the lower values of 74.8 and 76.l kcal/mole, respectively. The modified scheme is named B' and the computed results were considerably improved. The main difference between the cases of B' and F is the absence or absence of Q82,but inclusion of Q82 to scheme B' (case G) does not show the appreciable improvement. The rate constants of some chemical reactions of Table 3 and 5 are remarkably different, but they are not sensitive as shown in Table 7. As the heat of formation of C2H, the higher value of 132 or 135 kcal/mol is preferable to the lower value of 114 kcal/mole, but it is impossible from the present simulation to settle the choice between 154 and 186 kcal/mole for the heat of formation of C4H.
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煤前駆体の生成シミュレーション

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