What gas preparation needs to be carried out in order to maintain accurate results?

Some flue gases are soluble in water. In most combustion processes the gases extracted from flues are saturated or become saturated as they cool in the extraction probe and its hose. At this point condensation occurs and water flows down the hose to the instruments water trap. During this period the extracted gases are in contact with the water for tens of seconds and some gases are absorbed into the condensate.

Two flue gases in particular are problematic. Nitrogen Dioxide is almost entirely absorbed in the condensate of a standard combustion analyser with say 5 metres of hose. Sulphur Dioxide loses around 10% of its concentration in similar circumstances.

To overcome these losses and maintain the accuracy of sampling ideally the temperature of the gas sample should never be allowed to fall below the dewpoint which is normally above 100°C. Unfortunately most sensors cannot survive at such temperatures and so a compromise must be adopted.

The approach is to maintain the sample above dewpoint and then to flash chill the sample to extract most of its moisture contents. Flash chilling takes place in a small volume chamber chilled to around +2°C. The residence time in the chilled chamber is in the order of 1 second and as the gas heats up to ambient its’ relative humidity rapidly falls and moves well away from the dewpoint.

In practice there are two approaches. The first uses a mains powered heated line and perhaps a heated probe to bring the sample to a mains powered unit at ground level. Peltier elements or a fridge unit are used for cooling. Condensate is immediately pumped from the chiller chamber using a peristaltic pump.

The second approach moves the chiller chamber to the end of the stack probe and thus eliminates the inconvenience and expense of a heated line.

Using either of the above techniques sample losses are typically restricted to less than 1% of the gas value.