OCR Text |
Show -7- of ammonia injected, the NOx removals are higher and the NH3 slip lower for the NO/NOx mixture. Again, similar effects were seen at other space velocities. The effect of NO!NOx mixtures can also be seen by calculating the catalyst activity. This is done by operating the reactor with an excess of ammonia and assuming that the SeR reactions are zero order in NH3 and first order in NO. With these assumptions, an overall catalyst activity constant (K) is given by sv= A= ~NO x = K= -S-V 1 n (1 - II NO) A space velocity (hr1 , stp) catalyst specific surface area, m2/m3 NOx removal at the operating space velocity Figure 5 shows the activity as a function of NO!NOx ratio at two different gas temperatures, 650°F and 70QoF. As expected from the results shown in Figure 3, the peak activity is in the vicinity of NO!NOx = 5.0, although there seems to be a slight shift of peak activity to higher NO/NOx ratios with increasing temperature. Also, note that the effect of N02 on activity diminished as the temperature increased. Finally, note that the value of the peak activity is not affected by temperature. The above results show that the presence of N02 in the combustion products will result in an increase in catalyst performance. The next issue to be addressed is the overall stoichiometry of the reactions, i.e., trying to determine if Reaction 2 or 3 dominates, or if there are other processes ongoing in the reactor, such as reduction of N02 to NO prior to reaction. To look at this issue, a general stoichiometric ratio (SR) can be defined as SR = __N _H_s _ NO+ a N02 where "a" is the stoichiometric ratio for the NHJN02 reaction. To determine "a", it is assumed that all of the injected NH3 either reacts with NO, N02, or is emitted as NH3 slip. |