OCR Text |
Show of both reactions becomes extremely low, the NO reduction falls off drastically, and part of the NH flows through unreacted. Use of hydrogen helps to reduce unreacted ammonia breakthrough and also shifts the temperature window to lower levels. Hydrogen does not widen the temperature range, but merely changes it. The magnitude of this shift is mainly a function of the amount of H injected relative to the NH3- At H2/NH3 ratios on the order of 2:1, the NOx reduction reaction can be forced to proceed rapidly at 1290 °F (700 °C). By judiciously selecting the H2/NH3 injection ratio, flue gas treatment can be accomplished at intermediate temperatures. The same effect produced by H can also be provided by other combustible gases, such as hydrocarbons or carbon monoxide. Use of these additives for temperature control, however, is not generally recommended because of the possible formation of undesirable byproducts. The effects of temperature and hydrogen on NO reduction are shown in Figures 1 and 2 for laboratory data from pilot plant tests . In these tests, as would normally be the case in commercial units, there is a distribution of temperature about the average, and consequently the observed variation of NO reduction with average temperature is more gradual than it is with the local temperature. In addition to temperature, the process is also sensitive to initial NOx and NH concentrations. The NH injection rate is generally expressed as a mole ratio relative to the initial NOx concentration. The reductions obtained with various initial NOx levels are shown as functions of this parameter in Figure 3. Other variables affecting performance are excess oxygen and available residence time at the reaction temperature. Minimizing excess air tends to enhance the NOx reduction, as does maximizing residence time. Minor flue gas species such as water, sulfur oxides, and ash have negligible effects. NOx emissions have been reduced by as much as 95% under laboratory conditions, but field applications are less ideal. Nonuniformities in temperature, gas composition, and flow conditions result in somewhat lower performance. No commercial installations currently exist in the U.S., but installations on six commercial boilers and furnaces firing gas and oil have 4-11 P-233 |