270NH3.MAN.REVD.062019
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8
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17
GENERAL DESCRIPTION, AMMONIA CONVERSION SECTION
The NH
3
converter mounts to the right side of a standard 270 series probe enclosure. The probe enclosure is
fastened directly to a mounting flange on the stack. A heat shrink boot on the bottom of the probe enclosure
provides a sealed input location for all sample lines and electrical wiring for both units.
Tubing connects one of the heated probe filter outputs to the input on the NH
3
converter. This tubing is
wrapped with a heater to ensure that no condensate forms. This heater, along with the heater inside the
converter chamber, is terminated in the probe enclosure, where user power is provided.
Blowback is typically provided with the probe assembly to flush particulate matter out of the filter. When the
accumulator sphere is discharged, small amounts of air will also travel through the converter. Blowback air
does not harm the converter but will momentarily disturb both sample outputs. Clean and dry instrument
quality air is essential to avoid particulate buildup in the converter chamber. Typical blowback time intervals
are user controlled, as frequent as every 15 minutes, but not more` than 24 hours.
Calibration (cal) gas is injected into the probe chamber, ahead of the filter. This fulfills most EPA
requirements for cal gas insertion with respect to proximity of the sample source. A back-pressure check
valve, set to approximately 3 psi, is provided in the cal gas injection path to ensure that the gas does not leak
into the sample path during normal operation. After the filter, cal gas is drawn through both the converted and
unconverted sample outputs.
The converter canister provides a torturous gas path through 316 stainless steel disks and balls, with elevated
surface temperatures above 1200°F. The temperature of the chamber is user controlled via monitoring of the
pre-installed type K thermocouple which is welded to the chamber core. The oven temperature may be safety
controlled up to 1500°F; however, the stainless steel surfaces, along with the ammonia gases, oxidize at the
high end of the range, reducing the service life of the reactor.
As the sample passes through the reactor, all NO
X
and NH
3
are thermally oxidized into Nitric Oxide (NO).
Reaction of NH
3
proceeds according to this exothermic reaction:
4 NH
3
+ 5 O
2
→ 4 NO + 6 H
2
O
The controlled temperature, gas concentration, and flow rate through the converter should be optimized by
each user through empirical means. It is recommended to first test the conversion efficiency at 1250°F. Most
applications result in as high as 95% effectiveness at this temperature. Testing above 1500°F has shown to
reduce effectiveness in many instances.
For maximum conversion efficiency and long reaction life (several years), observe the following:
Ratio of O
2
to NH
3
should be greater than 1.25.
Flow through the reactor should be limited to 1 – 1.5 l/m.
Do not expose the reactor to ammonia concentrations > 20 ppm.
Minimize (NH
3
ppm * hours * flow rate) for longest reactor life.
