Flue gas pre-treatment for more accurate emissions measurements

For most gas analysis methods, we need to use appropriate equipment to extract part of the gas from the process gas, pre-treat it and then feed it to the analytical sensor unit to analyze and calculate the measurement results.

For flue gas analysis, pre-treatment of the flue gas is a very important step.

Emission measuring devices

More info

Importance of pre-processing

Definition of flue gas pre-treatment: i.e. the particles in the flue gas are first removed by filters and then the flue gas is cooled to a certain temperature for drying.

Reasons for the pre-treatment of exhaust gases:

Particles tend to contaminate the gas path and the sensors, which can cause the devices to malfunction;
If moisture is present in the form of water vapor, the flue gas is diluted and the dilution ratio fluctuates with the water vapor content and therefore the measurement results;
If the moisture is in liquid form, the liquid water forms compounds with certain water-soluble substances in the flue gas, which affect the actual concentration of the substance and the measurement results of the substance;
Moisture can lead to inaccurate measurements with some sensors, e.g. interference with non-dispersive sensors. For example, it can cause interference with non-dispersive infrared gas sensors and impair the permeability of the filter membrane of electrochemical sensors.

This is possible with a flue gas pre-treatment system:

Effective isolation of sensitive analytical sensor units from harsh operating conditions;
Conversion of extracted flue gases into clean, dry gases under actual operating conditions;
With the pre-processing unit to do some application expansion, flexible use of flue gas pre-processing unit, can be a sampling unit with more than one analytical sensor unit for use, but also an analytical sensor unit can be used with a number of sampling points (through the switch to the gas circuit) to use more than one analytical sensor unit to compare between the multiple sampling points between the sampling points of the rapid switching of sampling.

Working mechanisms for the extraction and pre-treatment of flue gas

The fume extraction and pre-treatment systems on the market are designed either as separate units or as a single unit, but regardless of the design, they are generally constructed and function as follows:

Sampling probes

First, the flue gas is extracted using sampling probes, which are made of different materials for different temperature ranges (e.g.: testo industrial probes).

A coarse filter is attached to the front of the probe, while a secondary filter on the handle or hose effectively removes the particles. The probes are also connected to pressure and temperature sensors to measure the flue gas temperature and pressure, which facilitates advanced calculations and adjustment of the sample gas pump performance.

Industrial probe set 1200 °C

Sample gas line

Sample gas lines, either in unheated mode (extremely high ambient temperatures or non-critical gases) or in heated mode, which is designed to keep the temperature of the gas above its dew point and thus avoid the formation of condensate and the loss of water-soluble gases.

Industrial probe set heated

Analysis sensor unit

The flue gas analysis housing has a third stage dust filter and condensate separator for fine filtration or gas-water separation without a palladium module to further pre-treat the flue gas.

Gas cooler

The gas cooler is located between the sampling probe and the analysis sensor unit. Flue gases and process gases always contain more or less moisture, which is present as water vapor at high temperatures (above the dew point) and as liquid droplets at low temperatures (below the dew point).

If the moisture in the flue gas is present in liquid form in the gas path, the water-soluble components of the flue gas react chemically with the water, which leads to falsified measurement results. In addition, the measuring devices can be eroded and damaged by corrosive solutions caused by chemical reactions (e.g. SO2 with liquid water).

The flue gas is cooled to a specific temperature (e.g. 4 °C) using a gas cooler (e.g. testo internal and external external gas conditioning), which removes most of the water vapor from the flue gas. The cooled flue gas is almost a dry gas, and the condensate produced by the cooler is drawn off and discharged using a peristaltic pump.

External gas conditioning

testo 3007 acid-filled, low-loss flue gas pre-treatment sampling system

In summary, flue gas pretreatment protects the sensitive analytical sensor units, reduces the possibility of equipment failure and provides the flexibility to extend the application of on-site detection and analysis. Most importantly, the output flue gas is analyzed by the sensor to ensure that very accurate gas concentration values are obtained.