"Wet-Basis" is analysis without removing the water from the flue gas
"Dry-Basis" is the analysis after the water has been removed (theoretically completely, but usually in part )
A typical "Wet-Basis" measurement would be an In-Situ or across the stack measurement
"Dry-Basis" measurement is performed with extractive system such as a portable flue gas analyser with condensation trap and /or built in gas cooler/dryer.
A look at a typical flue gas composition could show the following values.
| oxygen | 2.20 % | |
| carbon dioxide | 13.20 % | |
| nitrogen | 71.10 % | |
| water vapour | 13.34 % | |
| sulphur dioxide | 0.04 % | |
| carbon monoxide | 0.10 % | |
| oxides of nitrogen | 0.02 % |
If the water vapour is removed now, the volume of the gas decreases and the concentrations of the other components increase. If the sample is conditioned at +4°C the partial pressure of water vapour at 4°C should be used to determine the water content of the gas. However, we can ignore this concentration for our example, since this is an extremely small error well within normal measuring accuracy limits.
|
COMPOUND
|
WET BASIS
|
DRY BASIS
|
|
Oxygen
|
2.2 %
|
2.53 %
|
|
Carbon dioxide
|
13.2 %
|
15.23 %
|
|
Nitrogen
|
71.1 %
|
82.05 %
|
|
Water vapour
|
13.34 %
|
NIL
|
|
Sulphur dioxide
|
0.04 %
|
0.047 %
|
|
Carbon monoxide
|
0.1 %
|
0.115 %
|
|
Oxides of nitrogen
|
0.02 %
|
0.023 %
|
As we see, one can obtain different results depending on the methods used. This is not a problem as long as the methods used are stated and not compared with those made under another basis without correction.
Some authorities require results to be reported on a dry basis and others leave it open as long as the basis employed is stated clearly.