Gas Turbine Handbook : Principles and Practices

Gas Turbine Exhaust Treatment 141
dust loading, exhaust gas conditions, and the required NO x
removal
efficiency. The efficiency of the SCR can be defined as the quantity
of NO x
removed divided by the quantity of NO x
present in the inlet
stream.
The SCR reactor consists of the catalysts, housing, and ammonia
injection system (Figure 9-6). In most applications the reactor size
is larger than the duct size and therefore, diverging/converging transition
ducts are used before and after the reactor, respectively. Care
should be taken in preparing the physical design of the SCR reactor,
particularly with regard to the gas-stream pressure drop. Gas-stream
pressure drop in the order of 2 inches (50mm) to 4 inches (100mm)
water column are normally acceptable.
Anhydrous ammonia is undiluted, pure, liquid ammonia. In this
form it is toxic and hazardous. The liquid anhydrous ammonia is
expanded through a heater, mixed with air, and injected into the gas
turbine exhaust (Figure 9-7).
Aqueous ammonia (NH 4
OH) is a mixture of ammonia (30%) and
water (70%). Since it is diluted it is less hazardous than anhydrous
ammonia. Injecting aqueous ammonia is only slightly more complicated
than injecting anhydrous ammonia. A pump is required to move
the aqueous ammonia to a vaporizer tank where it is mixed with
hot [575°F (300°C) to 850°F (455°C)] air. There the ammonia/water
Figure 9-6. Selective catalytic reduction.