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Catalysts
Selective Catalytic
Reduction (SCR) deNOx Catalyst
SCR catalysts are used to reduce NOx
from exhaust streams containing greater than one percent
O2. As the name implies, NOx is selectively reduced
by reacting it with a reagent, - usually ammonia (NH3)
or an ammonia-based reductant such as urea - across
an SCR catalyst which reduces the NO and NO2 to nitrogen
and water, as shown below.
| - 4NO + 4NH3 + O2 |
=> |
4N2 + 6H2O |
| - 2NO2 + 4NH3 + O2
|
=> |
3N2 + 6H2O |
Where there is mixture of NO and NO2
present in the gas stream, the following – somewhat
faster – reaction will occur in parallel:
| - NO + NO2 + 2NH3 |
=> |
2N2 + 3H2O |
Most SCR catalysts use base metal chemistry,
employing vanadium, tungsten, titania, silica and/or
zeolite-based materials depending on the required
temperature of operation. These can be extruded into
a honeycomb structure or coated directly onto a metallic
or ceramic honeycomb support.
The reduction of NOx is dependent on
the volume, reductant concentration, operating temperature
and activity of the SCR catalyst. The major factors
influencing catalyst selection include process gas
temperature and a thorough examination of potential
contaminants such as particulate and catalyst poisons.
The majority of the SCR catalysts in
use today are designed to function in the 315-425°C
temperature range where conversions in excess of 95%
are routinely achieved.
| Low: |
120-350°C |
Porous extrudates in bed reactor. |
| Medium: |
265-425°C |
V/Ti/W on high-density honeycomb. |
| High: |
345-590°C |
Zeolite on ceramic substrate. |
The reagent injection system should
be designed to ensure complete mixing of the reagent
with the flue gas as an aid to the conversion process.
Ammonia is typically used as the reductant at a 1:1
mole (or volume) ratio. Some unreacted ammonia will
pass or “slip” through the process and
this slip ammonia is usually limited by regulation
to less than 20ppm and, in some cases, less than 5ppm.
As a precaution, a slip catalyst may also be used
to remove any residual ammonia from the stream prior
to venting.
SCR/CO catalyst technology is a proven
technology for a wide range of industrial heat and
power generation applications, particularly in the
US, which has over twenty years’ experience
and several thousand stationary field installations
in service today.
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