Teledyne API Model 201A NH3 Analyzer Instruction Manual, 04689, Rev. B1
converts some of the ammonia to NO, thus the TN channel data contains a small ammonia signal. Secondly, the ammonia converter does not convert all of the ammonia or ambient NO2 to NO. Finally, the high temperature in the ammonia converter causes the oxygen and nitrogen in ambient air to react, creating a background of NOx which needs to be factored into computing the result. Fortunately, all of the above shortcomings have been overcome in the M201A by careful calibration, and calculation of the result. .
Periodically, the AutoZero valve switches allowing the analyzer to read zero background. The AutoZero readings are subtracted from all of the other readings. This improves zero baseline stability.
As with many chemical reactions the conversion of ammonia in the presence of other oxides of nitrogen is complicated.
In the molybdenum converter operating at 315oC, the following significant reactions are taking place:
Mo + NO2 → MoO3 + NO ~100% Eff
NH3 + Mo + O2 → MoO3 + NO <10% Eff
Notice in the second reaction some of the ammonia is converted to NO by the molybdenum metal.
The M501NH NH3 Converter operates at 825oC. At this high temperature, several reactions occur:
NO → NO | Efficiency = ~100 % |
NOx → NO | Efficiency = ~85% |
NH3 → NO | Efficiency = ~85% |
N2 + O2 → NO | Efficiency = <1% |
As can be seen from the above reactions, the calculation of the ammonia concentration and overall calibration of the instrument must be done carefully if accurate ammonia concentrations are to be measured. The calibration procedure is covered in Section 2.
1.1.1 Special Considerations for Ammonia Measurement
Ammonia is a difficult gas to measure due to its chemical characteristics. Because of its large dipole moment, the gas tends to adsorb on surfaces and diffuse into many materials. The following precautions should be observed when designing an ammonia sampling system and connecting the M201A analyzer to the system:
Materials:
Do NOT use TEFLON tubing or fittings in contact with NH3 sample or span gas.
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