Generating Equipment - Gas Turbines
Catalytic Reduction (SCR)
reduction (SCR) systems selectively reduce NOx emissions by injecting ammonia
(NH3) into the exhaust gas stream upstream of a catalyst. Nitrogen oxides, NH3,
and O2 react on the surface of the catalyst to form N2 and H2O. The exhaust gas
must contain a minimum amount of O2 and be within a particular temperature range
(typically 450°F to 850°F) in order for the SCR system to operate properly.
range is dictated by the catalyst material, which is typically made from noble
metals, including base metal oxides such as vanadium and titanium, or zeolite-based
material. The removal efficiency of an SCR system in good working order is typically
from 65-90%. Exhaust gas temperatures greater than the upper limit (850°F)
cause NOx and NH3 to pass through the catalyst unreacted. Ammonia emissions, called
NH3 slip, may be a consideration when specifying an SCR system and are often limited
by air permitting.
in the form of liquid anhydrous ammonia, or aqueous ammonia hydroxide is stored
on site or injected into the exhaust stream upstream of the catalyst. Although
an SCR system can operate alone, it is typically used in conjunction with water-steam
injection systems or a lean-premix system to reduce NOx emissions to their lowest
levels (less than 10 ppm at 15 percent oxygen for SCR and wet injection systems).
flow diagram for gas engine system
SCR installations incorporate CO catalytic oxidation modules along with the NOx
reduction catalyst for simultaneous CO/NOx control. Carbon monoxide oxidation
catalysts are typically used on turbines to achieve control of CO emissions, especially
turbines that use steam injection, which can increase the concentrations of CO
and unburned hydrocarbons in the exhaust. CO catalysts are also being used to
reduce VOCs and organic HAPs emissions. The catalyst is usually made of a precious
metal such as platinum, palladium, or rhodium. Other formulations, such as metal
oxides for emission streams containing chlorinated compounds, are also used. The
CO catalyst promotes the oxidation of CO and hydrocarbon compounds to carbon dioxide
and water as the emission stream passes through the catalyst bed. The oxidation
process takes place spontaneously, without the requirement for introducing reactants.
The performance of these oxidation catalyst systems on combustion turbines results
in 90-plus percent control of CO and about 85-90% control of formaldehyde. Similar
emission reductions are expected on other HAP pollutants. This could become an
important control mechanism as the new MACT formaldehyde standard for new gas
turbines is 91 ppb (parts per billion) at 15% O2.
Catalytic Reduction Technologies
New catalytic reduction technologies have been developed and are currently
being commercially demonstrated for gas turbines. Such breakthrough technologies
include, but are not limited to, the SCONOX and the XONON systems, both of which
are designed to reduce NOx and CO emissions.
The SCONOX system is applicable to natural gas-fired turbines. It is
based on a unique integration of catalytic oxidation and absorption technology.
CO and NO are catalytically oxidized to CO2 and NO2. The NO2 molecules are subsequently
absorbed on the treated surface of the SCONOX catalyst. The system manufacturer
guarantees CO emissions of 1 ppm and NOx emissions of 2 ppm. The SCONOX system
does not require the use of ammonia, eliminating the potential of ammonia slip
conditions evident in existing SCR systems. Only limited emissions data are available
for a gas turbine equipped with a SCONOX system. This data reflects HAP emissions
and currently is not sufficient to verify the manufacturer's claims.
The XONON system is applicable to diffusion and lean-premix combustors
and is currently being demonstrated with the assistance of leading gas turbine
manufacturers. The system utilizes a flameless combustion system where fuel and
air reacts on a catalyst surface, preventing the formation of NOx while achieving
low CO and unburned hydrocarbon emission levels. The overall combustion process
consists of the partial combustion of the fuel in the catalyst module followed
by completion of combustion downstream of the catalyst. The partial combustion
within the catalyst produces no NOx, and the combustion downstream of the catalyst
occurs in a flameless homogeneous reaction that produces almost no NOx. The system
is totally contained within the combustor of the gas turbine and is not intended
as a process for cleaning the turbine exhaust. The catalyst manufacturer claims
that gas turbines equipped with the XONON catalyst emit NOx levels below 3 ppm
and CO and unburned hydrocarbons levels below 10 ppm.
programs with several turbine manufacturers are underway to market XONON and SCONOX
technologies. A rule change to exempt new gas turbines that use lean premix and
diffusion flame combustion from the MACT standard for formaldehyde is proposed.
due to their relatively small size and low operating temperatures have not been
required to add any post-combustion controls.