- 40 years in service of the enviroment
OXYDAN Stack Gas Dilution Probe System
Instrumatic is a world leader in design and manufacture of exhaust gas dilution probe systems.
No matter how good or precise the analyzer/sensor are the result never better than the sample provided to the analyzer/sensor. Instrumatic has more than 35 years of experience in dilution sampling.
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The extractive dilution technique is the most widely used method for continuous emission monitoring of SO2/CO2 in USA (<95%). Dilution is ideal for use in monitoring NOx, CO, H2O among other gasses as well.
• The dilution probe system use dry contamination-free air (“dilution air”)
and an eductor (also referred to as a venturi pump) to extract a diluted
flue gas sample from the exhaust stack or duct.
• It is not necessary to remove the moisture from the flue gas sample
prior to analysis by the analyzers, because the sample is diluted
sufficiently to avoid condensation problems during analysis at normal ambient
• Dilution probe systems provide wet-basis concentrations that works
very well with flue gas flow rate monitor which also provides wet-basis
stack flow rate values, therefore no moisture analyzers are required for
Theory of Operation
The technique is based on extracting a small sample from the stack, removing particulates by a proprietary filter and then accurately diluting the sample with clean, dry zero air; thus reducing the dew point in order to prevent condensation of water vapor and acid mist in the sample lines. The diluted sample is then sent down unheated umbilical lines to the base of the stack where it is analyzed by high precision gas analyzers. Due to the high sensitivity of these analyzers, dilution doesn’t influence measurement accuracy. In fact, accuracy is improved over other systems since water vapour cannot condense and remove compounds such as SO2 & NO2.
Calibration of the system is achieved effectively by sending span or zero gas directly to the stack and directly into the centre of the probe filter. This effectively displaces the stack gas, so that the entire system samples the calibration gas. This scheme provides a true calibration of the full system under working conditions. Because excess calibration gas is vented to the stack, even stack pressure effects are tested. This method of calibration eliminates the need to know the dilution ratio as the analyzers are adjusted to read the span gas concentration directly.
Unheated Sample Lines
One of the many benefits of this dilution technique is to lower the dew point of the sample gas to below ambient temperature enabling it to be transported by an unheated umbilical cord to the analyzers located at the base of the stack. The use of unheated sample lines eliminates the costly maintenance associated with heated sample lines such as failures in the line heating system and cold spots, which cause condensation and even line plugging.
A non-dilution extractive method requires large amount of sample gas, typically 2-5 litres per minute, this sample gas requires filtering and the removal of moisture from it before it can be measured by the analyzers. However, the dilution extractive method requires only a small amount of sample gas, around 100 - 300 cc/min. This increases the lifetime of the filter in the probe and eliminates the need for using a chiller to remove moisture; thus further decreasing the level of maintenance required. Dilution of acidic stack gases substantially reduces the risk of damage to accurate analyzers.
Sample Gases aren't lost
The straight extractive method requires that a water removal system be used prior to the sample reaching the analyzers. This process of removing water also removes part of the very water-soluble sample gases, thus introducing an error into the measurement of SO2, NO2.
Direct "Wet Basis" Measurement
The removal of water from the sample gas (extractive systems) alters the ratio of gases in the sample and this requires that sample temperature and a % moisture curve for saturated conditions in order to determine moisture contents of the stack gas and correct the reading of this loss of water.
However, by using the extractive dilution method moisture is not removed from the sample and therefore the measurement can be classified as a "wet basis" measurement without further corrections being required.
Calibration gases last longer
An extractive dilution system only requires 100-300 cc/min of sample gas as opposed to the 2-5 litres required by extractive systems. Therefore, only 200-400 cc/min of calibration gas is required for a full calibration. Typically with daily calibrations a "D" size cylinder will last for up to two years.
Dilution Probe System
PLC controlled sampling, dilution air pressure, probe vacuum, sample pressure, remote or automatic calibration, web server, data logger & plotter.
Filter blow-back control (timer or sample pressure).
Automatic 5 point liniarity control/test data logged in PLC.
Sample dew point measurement.
Maintenance interval sample filter inspection or replacement once a year.
The systems contain a zero velocity filter and has a positive pressure in the sample line.
Fast response sample velocity 8 m/sec.
Dilution probe sampling systems do not require electrical sample pumps, heated sample lines or
numerous flow control valves as required by non-dilution sampling systems – therefore, this is another area of lower maintenance.
Using very clean dilution air high dilution rates eliminates analyzer internal corrosion.
Minimum connections and easy installation
Only needs connection of instrument air, and power.