OCR Text |
Show 1.6. U at 1225 K (1745 °F). The NO reductions that can be obtained in 1-2 ms are about 10 ppm, at best. The situation becomes even less attractive if account is taken of the remaining NH3-concentration. Fig. 13 shows the sum of [NH3] and [NO] as a function of time, for the same conditions as Fig. 10. At best, about 15 ms are needed to reduce this sum to the initial value of [NO] (100 ppm); about 50 ms are needed to reduce it to 40 ppm. In summary, application of the Thermal De-NOx process to postflame gases is subject to various requirements. The ratio A of initial NH3 to initial NO should be at least about 2, if the final NO-concentration is to be close to the average critical NO-concentration CNO]c. The residence time t after injection should be at least a few times the characteristic reaction time tc, if the final NH3-concentration is to be at an acceptable level. There are conflicting requirements on the OH-concentration: it must be low in order for [NO]c to be low; but it must be high in order for tc to be short. This conflict can be resolved by providing a sufficiently long residence time. Varying the OH-concentration changes the temperature range over which effective NO reduction occurs, but does not significantly change the maximum reduction that can be achieved in a given residence time. ACKNOWLEDGMENTS Helpful discussions with C.P.Fenimore, B.W. Gerhold and R.H. Johnson are gratefully acknowledged. |