21.5) Mass concentrations of gas components

  flue gas analysers also make calculations of mass concentration expressed in [mg/m3] from the concentration expressed in [ppm]. Mass concentration of gas elements depends on the gas pressure and temperature. To make comparison of results possible, the idea of standard conditions was introduced, that is standard temperature and pressure values at which the mass concentration of the elements is calculated. The flue gas analyser uses standard conditions of 1000 hPa and 0^oC.

The screen displaying, amongst others, mass concentration of the measured gases is shown below. It should be noticed that the analyser indicates two different values expressed in [mg/m3], they are the so called absolute mass concentration and mass concentration in relation to oxygen. These values are often confused - in the next section the way they are calculated and the differences between them are explained.

21.5.1) Absolute mass concentration of gas components

where:

CO[mg/m3] -absolute CO mass concentration in combustion gas (at standard conditions).

CO[ppm] -absolute CO volume concentration in combustion gas (from measurement).

A_CO -correction factor from table 5.

Gas	A -
CO	1.250
NO	1.340
SO2	2.860
NO2, NOx	2.056
H2S	1.520
H2	0.089
Cl2	3.220
HCl	1.693
HCN	1.205

Table 5: Factors to convert concentration in [ppm] into mass concentration in [mg/m³] (at standard conditions 1000 hPa, 0^oC).

N o t e: mass concentration of nitrogen oxides (NO_x) is calculated by the analyser (according to the standards) using the nitrogen dioxide (NO₂) factor.

Mass concentration calculated by a portable flue gas analyzer is comparable with the results obtained by other methods (or different types of analysers) only when the calculations have been carried out based on the same standard conditions.

21.5.2) Mass concentrations in relation to the concentration of oxygen in combustion gases

As well as absolute mass concentration, the mass concentration in relation to the oxygen concentration in the combustion gases is calculated. The concentration of a given component in relation to the oxygen concentration is expressed by the following formula (as an example for CO value):

where:

COrel - CO concentration in relation to oxygen expressed in [mg/m³]

O_2ref -reference oxygen, conventional parameter (chosen by selecting fuel or entered independently from keyboard ) expressed in [% vol]

O_2meas - the measured concentration of O₂ in combustion gases expressed in [ % vol ]

20.95% - oxygen concentration in pure air

CO - the measured concentration of CO in combustion gases expressed in [ mg/m³ ] ( absolute mass concentration)

Using similar formulae, the concentrations in relation to oxygen concentration of sulphur dioxide SO₂, nitrogen oxides NO_x and other gases are calculated. The concentration related to oxygen concentration was introduced to make the evaluated concentration independent of the way the combustion process is carried out. The absolute value (expressed in [ppm]) can be lowered artificially in the combustion process with an increase in excess air (large amount of O₂ in the combustion gases). It does not have to be a decrease of the total emission. The formula which calculates concentration related to oxygen takes into account the oxygen concentration of the combustion gases, making the results independent of the excess air factor.

The parameter O_2ref - reference oxygen is a standard value. Standards recommend different values of this parameter for various types of fuel. In flue gas analyzers the value of reference oxygen can be accepted automatically in the process of fuel selection (the so-called automatic reference oxygen choice ) or entered by the operator from the keyboard (the so-called manual selection of reference oxygen). Relative mass concentration calculated from two different measurements are comparable only if the same reference oxygen and the same standard conditions have been used.

N o t e : If O_2meas < O_2ref then relative concentration CO_rel calculated from formula is less than absolute concentration. In such a case, the analyser replaces the value of relative concentration with the value of absolute mass concentration.