| OCR Text |
Show and outlet conditions that allow maximum destruction of the NO. A substoichiometric amount of oxygen relative to the CO and H2 present in products from the afterburner along with a more than stoichiometric ratio relative to the NO present (CO+H2>02>NO) must be maintained at the inlet to the reduction catalyst. Under these conditions, it is speculated that oxygen reacts with NO to form the more reactive N02 which then reacts with the CO/H2 to form N2, H20, and CO2 as illustrated by the following reaction steps: 2 N02 + 4 CO -. N 2 + 4 CO2 (5) (6) (7) (8) since the NO oxidation reaction to form N02 is reversible and equilibrium limited, the driving force for the overall mechanism is the depletion of the N02 by reactions (6) and (7) • The concentration of CO exiting the reduction catalyst must be maintained between 750 and 1500 ppmv. Below this range, N02 is not reduced across the reduction catalyst because of the lack of combustibles. Above this range, NOx reduction is achieved across the reduction catalyst, but bound nitrogen compounds such as HCN and NH2 are also formed. These compounds are oxidized across the oxidation catalyst to reform N02 and thus limit the overall reduction. The data further indicate that very high levels of CO exiting the reduction catalyst bed result in decreased NOx reduction. This is caused by the competition between CO and NO for reaction with the 02 that is present. The high concentrations of CO consume the 02 and prevent NO from oxidizing to N02. Discovery of this potential mechanism for the overall NO reduction reaction has led to an improved Hybrid Low NOx Process that yields maximum NOx reduction with minimum excess fuel. The key control parameters are the ratio of excess fuel to CO2 to NO entering the reduction catalyst bed and the excess combustibles exiting this bed. In addition, to achieve very low levels of NOx leaving the bed, 02 must be consumed across the reduction catalyst to ensure that it does not compete with the N02 in the CO/H2 oxidation reaction to consume N02. The data clearly indicate that all of the O2 is consumed across the reduction catalyst bed when an excess of at least 750 ppmv CO is maintained. The required reduction catalyst inlet and outlet conditions can readily be achieved by monitoring the CO concentrations into and out of the reduction catalyst bed with standard CO analyzers. The inlet CO concentration is used to control the amount of excess fuel to the afterburner and the outlet CO -7- |
