Like all sample systems the requirements for flue-gas analysis follow certain basic rules. There are, however, some extra points which must be borne in mind if consistent and accurate results are to be obtained.
In all cases the probes should have built-in filters, preferably heated at the exit of the probe prior to the sample line.
The next item in the sample system is the sample line. If the dew-point is high, as is the case with most flue gases then it should be heated to about 120°C, (better 180°C for some flue gas measurements). For sulphur dioxide (SO2), nitrogen dioxide (NO2)and other acidic gases a sample transport line with a core tube of PTFE is preferable as this will not react with the aggressive gases in the sample line.
An unheated sample line may be used if the sample is already dry or deliberate cooling from the probe to the analyser desirable. In this case the sample line must fall to the analyser and incorporate a drain, usually automatic, at the lowest point. Entry of condensation into the analyser must be avoided at all costs, since this will cause damage to pumps and sensors. One larger continuous systems it is common practice to split the system, with the cooler being close to the stack and the analyser at a more cnvenient location. This avoids the expense and maintenance work involved with long runs of heated hose.
The sample is then rapidly cooled to + 4°C using a Peltier cooler. The sample may be either pumped to the cooler or preferably drawn by suction from the flue through the probe, sample line and cooler.
In either case the pump requires the protection of a filter. When the sample is pumped to the cooler the condensate has to be removed. Possible ways are a peristaltic pump or (less effectively) a catch-pot.
Relationship between the dew point of flue gases and the SO2 content
When sulfur dioxide is involved, the option of using a catch-pot or automatic drain is not really available. Sulfur dioxide is highly soluble in water and the presence of condensate will lead to grossly falsified results. This may produce the desired result for someone trying to keep an old system running where the levels of sulfur dioxide are above permitted maximum levels, but this is not really the purpose of gas analysis!
As the graph above shows, sulfur dioxide has a marked effect on the dewpoint of the gas, which is not only relevant for the measurement, but also places a lower limit on the temperature of the flue gas when it exits the stack. Temperatures below this limit will lead to the formation of sulfurous acid and sulfuric acid, which will corrode the inside of the stack extremely quickly. Measuring the concentration of sulfur dioxide to ascertain the dewpoint accurately is one of the important uses for the flue gas analyzer. It will also be necesary to mount a temperature sensor at the top of the stack to keep a watch on the dewpoint precisely. This is one of the points where a careful watch is essential to ensure that the dewpoint is not reached and that the efficiency does not become too low as a result.
The fuels used in many countries are now low in sulphur, but there are still many cases where the sulphur content cannot be reduced. The only real answer to this is to fit filters and scrubbers to the stack to remove the sulphur compounds that are formed. On large systems the sulfur can be collected and used for other purposes. It is an important chemical in use in many industries. Again, all the parts of the sulfur separation will have to be kept above the dewpoint to prevent droplet formation.
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