Gas Turbine Handbook : Principles and Practices

Gas Turbine Exhaust Treatment 143
Aging Effect
The catalyst’s performance tends to deteriorate with time. The
rate of deterioration is high at the beginning of operation and becomes
moderate after initial settlement.
Ammonia Slip
In theory the amount of ammonia injected into the gas stream
should equal the amount of NO x
to be removed from the gas stream.
However, because the ammonia does not completely mix with the
NO x
, more ammonia must be injected. Ammonia slip is the excess
residual ammonia in the downstream gas. Also, based on the catalyst
selected, SO 2
can convert to SO 3
. When the ammonia, water vapor,
and SO 3
combine ammonia sulfates are formed as follows 12 :
SO 3
+ 2NH 3
+ H 2
O ⇒ (NH 4
) 2
HSO 4
(9-6)
SO 3
+ NH 3
+ H 2
O ⇒ (NH 4
)HSO 4
(9-7)
Ammonia sulfates are sticky substances that will deposit on
the equipment downstream of the SCR. These deposits will result in
reducing the cross-sectional area and increasing backpressure. For
each inch water column increase in backpressure power decreases by
0.25% and heat rate increases 0.25%. Ammonia sulfate starts forming
when ammonia slip is greater than 10 ppm and the SO 3
concentration
is greater than 5 ppm. This problem is less likely to occur when
exhaust gas temperatures are maintained above 400°F (205°C) and
natural gas or low sulfur fuel is used.
References
1. “Atmospheric Chemistry: Trace Gases and Particulates,” W.H. Fisher
(1972).
2. “The Sulfur Cycle,” W.W. Kellogg (1972).
3. “Ground Level Concentrations of Gas Turbine Emissions,” H.L. Hamilton,
E.W. Zeltmen (1974).
4. “Air Quality Standards National and International,” S. Yanagisawa,
(1973).
5. “Emissions, Concentrations, And Fate Of Gaseous Pollutants,” R. Robinson,
R.C. Robbins (1971).
6. “Abatement of Nitrogen Oxides Emission for Stationary Sources,” National
Academy of Engineers (1972).
7. “Gas Turbine Fuels,” W.S.Y. Hung, Ph.D., Solar Turbines, Inc, 1989.